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The design and characterization of strained-layer quantum well infrared photodetectors

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The design and characterization of strained-layer quantum well infrared photodetectors
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Chu, Jerome T., 1968-
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vii, 129 leaves : ill. ; 29 cm.

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Bandwidth ( jstor )
Dark current ( jstor )
Electric potential ( jstor )
Energy bands ( jstor )
Photometers ( jstor )
Quantum efficiency ( jstor )
Quantum wells ( jstor )
Spectral bands ( jstor )
Superlattices ( jstor )
Wavelengths ( jstor )
Infrared detectors ( fast )
Optoelectronic devices ( fast )
Quantum wells ( fast )
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non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1998.
Bibliography:
Includes bibliographical references (leaves 125-128).
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Typescript.
General Note:
Vita.
Statement of Responsibility:
by Jerome T. Chu.

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THE DESIGN AND CHARACTERIZATION OF STRAINEDLAYER QUANTUM WELL INFRARED
PHOTODETECTORS














By

JEROME T. CHU














DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA 1998














ACKNOWLEDGMENTS



I wish to express my deepest thanks to the chairman of my supervisory committee, Dr. Sheng S. Li, for all his guidance, patience, and encouragement throughout the entire research process and for giving me the freedom to learn about all the aspects involved in research and development. I would also like to extend my gratitude to Dr. Gijs Bosman, Dr. Arnost Neugroschel, Dr. Ramakant Srivastava, and Dr. Tim Anderson for serving on my supervisory committee.

I would like to thank Drs. Yanhua Wang, Yun-Shan Chang, Jung-Chi Chiang and Daniel C. Wang for their extensive assistance both mentally and physically in semiconductor processing and design. Thanks are extended to my laboratory colleagues, Jung Hee Lee and Chia-Hua Huang for their friendship, support and assistance in all aspects of my work.

And I would like to extend the greatest appreciation and thanks to my parents Dr. and Mrs. Chaunccy C. Chu and my brother (Gary T. Chu fobr giving me the unconditional love and support throughout my entire academic career. Without their backing and the grounding that they have given me, I would not have been able to even consider undertaking the work necessary for a doctorate.

Finally, the financial support of ARPA and US Air Force Materiel Command is gratefully acknowledged.









ii













TABLE OF CONTENTS


Page

ACKNOWLEDGMENTS ........................................................................ii

TA BLE O F C O N TEN TS ........................................................................ ..................... iii

A B ST R A C T ................................................................................... ..............................vi

1 IN T R O D U C T IO N ................................................................................................ 1


2 THE THEORETICAL STUDY OF P-TYPE QUANTUM WELL
INFRARED PHOTODETECTORS .................................................................. 8

2.1 Introduction ............................................................................................ 8
2.2 P-QWIP Physics ....................................... 9.......... ...........9
2.2.1 Strain-Layer Growth Limitations and Theory ................................9
2.2.2 Strain Induced Energy Band Shifts ....................... 10
2.2.3 Energy Band Calculations ............................. 12
2.2.4 The Transfer Matrix Method for the Calculation of
Transm ission Probability....................... .............. 15
2.2.5 D)etermination of" Intersubband Transitions and Absorption
Coefficients............................ ............... 17
2.2.6 Photoconductive Detection Mode Operation ................................. 19
2.3 P-Q W IP Figures of M erit ....................................................... .............. 19
2.3.1 Spectral Responsivity........................................... ....................... 20
2.3.2 QWIP Collection Efficiency ...................... .....21
2.3.3 Dark Current Relationship in a QWIP ............................................ 22
2.3.4 Noise and Detectivity in QWIPs ................................................. 23












iii









3 AN INGAAS/ALGAAS ON GAAS P-QWIP WITH COMPRESSIVE
STRAIN LAYERS AND LWIR AND MWIR DETECTION ........................ 26

3.1 Introduction ...............................................................26
3.2 P-type Compressive Strained Layer QWIP Design..................................... 27
3.3 Results and Discussion ........................................................28
3.4 C onclusion ............................................................................ .................... 3 1



4 A COMPRESSIVELY STRAINED-LAYER P-TYPE
INGAAS/ALGAAS/GAAS STEP BOUND TO MINIBAND QWIP AT
10 .4 m ............................................................................................................... 3 9

4.1 Introduction ...............................................................39
4.2 Theoretical Considerations .......................................... .......40
4.3 Device Growth and Fabrication......................................... 41
4.4 QWIP Characterization and Results ...................................... ....... 42
4.5 C onclusion .......................................... ................................................... 44



5 A STACKED COMPRESSIVELY STRAINED P-QWIP WITH TWOBAND TWO-COLOR DETECTION ..................................... ......... 50

5.1 Introduction ................... ........... ... ...................50
5.2 Theoretical Considerations and Device Fabrication...................... ......... 50
5.3 Device Characterization and Results ...................................................51
5 .4 C o n clu sio n ............................................................................... 5 3



6 SUPERLATTICE INFRARED PHOTODETECTORS .................................... 60

6.1 Introduction ........................................ ...................... 60
6.2 Superlattice Infrared Photodetector Design and Processing..................... 60
6.3 19.2 pm SLIP Characterization and Results....................... .....................62
6.4 A Voltage Tunable Two-color SLIP ...................................... ....... 64
6.5 Layer Structure and Fabrication of the Unstrained Voltage Tunable
SLIP.......................................................64
6.6 Unstrained Voltage Tunable SLIP Characterization and Results ............. 65
6.7 Conclusion ........................................ ...................... 66







iv









7 TENSILE STRAINED QUANTUM WELL INFRARED
PHOTODETECTORS ....................................... 78

7.1 Introduction ...............................................................78
7.2 Device Layer Structure and Processing ...................................... ........... 79
7.3 Device Characterization .................................................... 79
7.4 C onclusion ........................................... ................................................... 80



8 BROADBAND QUANTUM WELL INFRARED
PHOTODETECTORS ......................................................... 86


8.1 Introduction ...............................................................86
8.2 Layer Composition and Device Processing............................ ...... 87
8.3 Characterization Results .................................................... 88
8.3.1 N-type broadband QW IPs................................................................. 89
8.3.2 P-type broadband QWIPs..................................... ....... 91
8.4 D iscussion .................................................................................................... 93
8.5 C onclusion ........................................... ................................................... 94



9 SUMMARY AND CONCLUSION ............................................ 111

APPENDICES

I LIST OF SYMBOLS..................................... 118
2 ACRONYMS ..................................................... .......... 123



R E F E R E N C E S ................................................................................................................ 12 5

BIO G RA PH ICA L SK ETCH .......................................................................................... 129















V











Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the Require ments for the Degree of Doctor of Philosophy


THE DESIGN AND CHARACTERIZATION OF STRAINEDLAYER QUANTUM WELL INFRARED PHOTODETECTORS

By

Jerome T. Chu

May 1998

Chairman: Sheng S. Li
Major Department: Electrical and Computer Engineering

Many different types of p-type strained-layer quantum well infrared photodetectors have been developed for normal incidence 3-5 pm mid-wavelength infrared (MWIR) and 8-14 tm long-wavelength infrared (LWIR) detection. The benefits possible in p-type QWIPs are: normal incidence detection without grating couplers, lower dark current and the ability to detect all incident infrared (IR) radiation polarizations.

Ihe first p-type compressive strained layer (CSL-) QWIP is composed of ln(iaAs/AIGaAs and grown on semi-insulating (S.I.) (100) GaAs. which exhibits twocolor two-band detection with an MWIR peak at 5.5 ,tm and an LWIR peak at 7.4 pm. The next p-type InGaAs/GaAs/AlGaAs CSL-QWIP uses a step-bound-to-miniband (SBTM) intersubband transition to detect IR radiation. This device was also grown on S.I. (100) GaAs and was found to have a peak detection wavelength at 10.4 pm. The third p-type CSL-QWIP design consists of two highly strained InGaAs/AlGaAs multiquantum well stacks in series, separated by a common central ohmic contact. The MWIR stack showed two detection peaks at 4.8 and 5.4 p.m, while a 10.0 pm peak was found for the LWIR stack.



vi









The next p-type CSL design is called a superlattice infrared photodetector (SLIP) and consists of a radiation sensitive superlattice of 3 or more periods surrounded by a blocking barrier. The p-type CSL-SLIP was found to have a very long-wavelength infrared (VLWIR) detection peak at 19.2 tlm and was grown on S.I. (100) GaAs with InGaAs/GaAs/AlGaAs layers. A second p-type SLIP grown and characterized was an unstrained design which showed voltage tuning, with the detection peak of this SLIP shifts from 9.3 ptm to 6.5 pm with the relative responsivity of the LWIR peak decreasing and the 6.5 pLm peak increasing with bias.

The final p-type QWIP investigated is the tensile strained-layer (TSL) InGaAs/InA1As design grown on S.I. (100) InP which uses the light-hole to heavy-hole intersubband transition for detection. The TSL-QWIP was found to have a detection peak in the MWIR band at 5.2 pm. As with all of the other strained and unstrained p-QWIPs, the device was found to be sensitive to normally incident IR radiation.

The broadband QWIPs (both n- and p-type) studied consist of three or four different quantum wells of varying thickness and composition combined in a unit cell separated by thick barriers. Each different quantum well is designed with a slightly different detection peak than an adjacent one. so that the individual spectra overlap to form a broader spectrum. 'These four devices were found to have detection peaks ranging from 9.3 to 10.3 Vpm and spectral bandwidths of 21 to 63%.
















vii














CHAPTER 1
INTRODUCTION


We humans depend on the visible portion of the spectrum of photons emitted from the sun all of lives. But unbeknownst to many of us, we are also constantly bathed in an intense sea of infrared photons emitted by our surroundings and ourselves. These photons are generally sensed in the form of heat, but specialized infrared detectors can shed a whole new light on our environment by detecting and imaging these photons.

Infrared detectors have been widely studied in the last one hundred years [1]. Typically, infrared detectors are divided into two categories: photodetectors and thermal detectors. In the case of photodetectors, photons directly interact with the carriers in a semiconductor material to generate a photocurrent; while thermal detectors are dependent on changes to specific properties--such as the conductivity--of a material due to a change in temperature arising from absorption of infrared photons. The two main detection mechanisms which have been investigated for photodetectors are the photovoltaic (PV) and the photoconductive (PC) modes. Although, the PV mode of operation is promising for some practical applications, since the suppression of the dark current strongly improves the noise properties, most current infrared detectors operate in the PC mode.

With the impressive development and maturity of epitaxial layer growth techniques such as molecular beam epitaxy (MBE) [2] and metalorganic chemical vapor deposition (MOCVD), similar gains and improvements have been made in the growth and design of semiconductor heterostructures. Significant progress has been made in the area of optoelectronic devices based on quantum wells or superlattices because of the gains in MBE and MOCVD. In general, a quantum well is formed when a layer of narrow









bandgap semiconductor is sandwiched between a set of wider bandgap semiconductors. The motion and energy of the carriers perpendicular to the semiconductor layers then becomes quantized so that the localized two-dimensional (2D) subbands of quantized states are formed in the quantum well [3]. The energy at which these quantized states are formed is dependent on the effective mass of the carrier and the thickness of the smaller bandgap semiconductor; thus by carefully choosing the right combination of semiconductor material and thickness, a wide range of intersubband energies can be obtained.

The idea of using optical intersubband transitions in quantum wells for infrared detection was made by Chang et al. [4], Esaki and Sakaki [5], and Coon and Karunasiri [6]. These optical intersubband transitions were first observed in GaAs quantum wells by West and Eglash [7] and who were then followed by Harwit and Harris [8]. The first GaAs quantum well infrared photodetector (QWIP) was demonstrated by Levine et al. [9] in 1987. Since then, QWIPs based on bound-to-bound (BTB) [9], bound-to-continuum (BTC) [10], and bound-to-miniband (BTM) [11] transitions have been widely investigated in the past ten years [12]. The various types of n- and p-type QWII' schematic energy band diagrams are shown in Figures 1.1 and 1.2.

Currently. most QWIPs are of the n-type variety [12]. The use of electrons as the carrier for intersubband transitions is a good choice due to the low effective mass and excellent transport and absorption qualities of n-doped III-V materials. But n-type QWIPs also exhibit higher dark currents and are not able to couple normal incidence IR radiation due to the quantum mechanical selection rules. P-type QWIPs, on the other hand, are able to couple normally incident radiation and have larger effective masses, which corresponds to lower dark currents. But the larger effective masses of the holes also leads to decrease absorption coefficients and smaller quantum efficiencies.

But the effective mass of the holes can be reduced with the addition of compressive or tensile strain in the quantum wells [13]. When compressive strain is used the ground

2









state in the quantum well is a heavy hole state. This case can be easily achieved with the InGaAs/GaAs material system, where the amount of compressive strain can be controlled by the mole fraction of indium in the InGaAs quantum wells. In contrast, when tensile strain is applied in the quantum wells, the ground hole state is then a light hole state, which is comparable in terms of effective mass and transport properties with the conduction band electrons. Tensile strain can be incorporated easily in the quantum wells when using the InGaAs/InAlAs/InP material system. Each approach to using strain to enhance the capabilities of the quantum well infrared photodetector attempts to overcome the deficiencies of the unstrained p-QWIPs, while retaining the beneficial qualities inherent in p-type QWIPs, such as the ability to couple normally incident infrared radiation without the need for grating couplers.

Until recently, the development of high performance QWIPs for tactical (high background) and space based (low background) surveillance, imaging, ranging and tracking systems, has mainly been centered around n-QWIPs, which have been mentioned as a possible rival to the dominance of mercury cadmium telluride based (MCT) devices used in recent decades for LWIR imaging. Because of the added processing steps necessary to incorporate grating structures on n-QWIPs for light coupling, and the need 1or high performance, high yield, focal plane arrays (FPAs) necessary in these systems, some of the newer research efforts have made attempts to look into strained-layer p-QWIPs as a possible competitor or complement to n-type quantum well infrared photodetectors. Thus the interest in exploring p-type strained layer quantum well infrared photodetectors.

In chapter 2, we will describe the fundamental physics behind the quantum well infrared photodetectors. These sections include, the calculation of the electronic states in the quantum well by using the transfer matrix method (TMM), the calculation of the energy bandgap with the effects of strain included, the basic theory of intersubband



3









transitions, and a discussion on how to evaluate and compare the performance of various QWIPs.

Chapter 3 discusses the development and performance of a new compressively strained InGaAs/AlGaAs/GaAs p-type QWIP with MWIR and LWIR responsivity. This device demonstrates the viability of p-type designs to achieve good detectivities at relatively high temperatures while exhibiting normal incidence response. The detection peaks for this device were found to be 5.6 and 7.4 pim, in the MWIR and LWIR bands, respectively.

Next, in chapter 4, we look into the characterization of a compressive strained-layer (CSL) InGaAs/AlGaAs/GaAs p-type QWIP with a step-bound-to-miniband (SBTM) intersubband transition, as seen in Figure 1.2. The purpose of using this type of intersubband transition is to decrease the dark current due to thermionic emission while still maintaining a large responsivity at longer wavelengths so that operating temperatures can be increased. The CSL SBTM p-QWIP showed a strong response peak at 10.4 pm.

A stacked CSL InGaAs/AlGaAs/GaAs p-QWIP is investigated in chapter 5. This pQWIP design uses two separate stacks of p-QWIPs connected by a common ohmic contact to be able to sense infrared radiation in both the LWIR and MWIR bands. This was done so that the tactically and strategically important wavelengths of 10 and 4.2 pim could be imaged on one focal plane array without a large increase in focal plane area. This device showed detection peaks at 4.8, 5.4 and 10.0 tm.

Chapter 6 explores the new idea of superlattice infrared photodetectors (SLIPs) to enhance the quantum efficiency and extended the detection wavelength of the device out past 19 pm. An unstrained SLIP design also exhibited true voltage tunability in the LWIR band, which allows us to discriminate incoming LWIR radiation by simply altering the applied bias of the device. The detection peak of the unstrained voltage tunable SLIP can be varied between 6.5 and 9.3 pm.



4









In chapter 7, we experiment with a tensile strained-layer (TSL) p-type QWIP. The use of tensile strain inverts the heavy- and light-hole states in the quantum well so that the light-hole state become the ground state. Using the light-hole state as the heavily doped ground state has the benefits of smaller effective mass, which translates into larger absorption coefficients and better transport characteristics. The TSL p-QWIP is found to be sensitive in the MWIR region of the IR spectrum.

Finally, in chapter 8, we look into the design and characterization of both n- and ptype QWIPs for broadband (BB) detection. These devices have been designed to sense most of the photons in the 8-12 p.m LWIR range. While the p-type QWIPs typically exhibited full-width half-maximum bandwidths on the order of 35%, the n-type BBQWIPs exhibited not only larger responsivities throughout the LWIR range at high applied biases, but they also were found to be able to sense normal incidence photons at about 40-50% of the 450 incidence value.

And in the last chapter, we summarize the results of the extensive study into p-type and normal incidence quantum well infrared photodetectors, comparing the relative strengths and weaknesses of each design with those published in the open scientific literature. In addition. we will give some recommendations as to what might be experimented with in future research.


















5


















TC N-QWIP BTC N-QWIP














BTQB N-QWIP




nnn nnnnrn nnn







BTM N-QWIP



Figure 1.1: Schematic energy band diagrams for the bound-to-continuum, bound-toquasi-bound and bound-to-miniband conduction band intersubband transistions due to infrared photons in n-type QWIPs.




6













LH1 HH1

HH2 HH3 TSL P-QWIP




HH1

HH2



HH3


CSL P-QWIP




HH1


SHH4






SBTM P-QWIP



Figure 1.2: The schematic energy band diagrams for the p-type valence band intersubband transitions for the tensile strained and compressively strained bound-tocontinuum and the step-bound-to-miniband.





7













CHAPTER 2
THE THEORETICAL STUDY OF P-TYPE QUANTUM WELL INFRARED PHOTODETECTORS


2.1 Introduction


With the advent of molecular beam epitaxial technologies in the last few decades, device structures utilizing heterostructure quantum wells have been heavily explored. Ntype quantum well infrared photodetectors (QWIPs) have been extensively studied in recent years [11,12,14]. These systems use GaAs/AlGaAs and InGaAs/InAlAs structures for detection in the 3-5 plm mid-wavelength infrared (MWIR) and 8-14 p.m longwavelength infrared (LWIR) atmospheric transmission windows. Since n-type GaAs/InGaAs and InGaAs/InA1As QWIPs have inherently low electron effective masses and high electron mobilities, they offer excellent infrared (IR) detection properties. Due to the quantum mechanical selection rules which prohibit normal incidence intersubband absorption. focal plane arrays (FPA) using n-type QWIPs must use either metal or dielectric gratings to couple normal incidence IR radiation into the quantum well [11,15,16]. In contrast, because of the mixing between the light hole and heavy hole states under either biaxial tension or compressive strain, normal incidence illumination is allowed for the intersubband transition in p-type QWIPs; thus eliminating the need for metal or dielectric grating couplers.












8






9


2.2 P-QWIP Physics



2.2.1 Strain-Layer Growth Limitations and Theory


P-type QWIPs using valence intersubband transitions have been demonstrated [1719] in lattice-matched GaAs/AlGaAs and InGaAs/InAlAs material systems. In general, intersubband transitions excited by normal incidence radiation in p-type quantum wells are allowed since a linear combination of p-like valence band Bloch states exists, which provides a nonzero coupling between the normal radiation field and valence band Bloch states. The strong mixing between the heavy hole and the light hole states greatly enhances intersubband absorption. The drawback of using lattice-matched systems is the fact that the intersubband transition occurs between the heavy hole ground states and the upper excited states. Because of the relatively large heavy hole effective mass when compared to the electron effective mass, relatively weak absorption and therefore similarly low responsivity is predicted in the IR wavelength range when compared to ntype QWIPs. In order to increase the absorption characteristics and responsivity of PQWIPs, biaxial stress is introduced into the well layers of the QWIP structure. If the intentionall introduced biaxial stress between the well layers and the barrier layers contained in the layer thickness (the total thickness of the wells and barriers) in the PQWIP structure is less than the critical thickness, then pseudomorphic or coherent heterointerfaces can be grown without the introduction of defects between the layers. Based upon the force balance model [12,20-22], the equilibrium critical layer thickness, Le, for an epilayer with the lattice constant, a, grown on a substrate with a lattice constant, a,, is given as
a 1-vcos2 2
= -v 1+ln(h, [2 / a)] (2.1)


where h, is the epilayer thickness, is the angle between the dislocation line and the Burges' vector, x is the angle between the slip direction and the layer plane direction, 8,





10


is the lattice-mismatch or the in-plane strain, and v, is the Poisson ratio. 8,, is defined as 6,=(as-a)/a where 8,,>0 for tensile strain and 6,,<0 for compressive strain. Similarly, v, is defined as v,=CJC,,. C.'s are the elastic constants and can be found in reference 23.

The strained-layers have the same effective in-plane lattice constant, a,, (i.e., a.y), and can store the excess energy due to the elastic strain within the layers. The in-plane lattice constant, a,,, can be expressed by [20]

a = ai1+ o 1+ L, ]jJ, (2.2) where a,,2 and L,2 are the individual layer lattice constants and thickness, respectively, and .2 are the shear moduli as described by (=C,,+C,,-2C,22/C,,, where the C,'s are elastic constants for the strained material. 6 denotes the lattice mismatch between layers and al,2 are the lattice constants of the strained well and the substrate (or barrier) respectively. When a1yas,, the coherently strained superlattice structure is no longer in equilibrium with the substrate. If the lattice constant of the barrier layers is equal to that of the substrate, the strain will be completely accommodated in the well layers with no strain in the barrier layers. However. Hull el al. [24] showed that if the individual layer thickness in the superlattice is less than its critical thickness, even though a a,, the loss of coherence onlV occurs at the interface bet\\weecn the whole superlattice and the substrate. while the superlattice itself remains coherent.


2.2.2 Strain Induced Energy Band Shifts

If the QWIP structure is grown along the [100] direction and the strained-layer is within the critical thickness, L, then a pseudomorphic or coherent heterointerface can be obtained and the components of the strain tensor [e] are simplified to the expressions given by
e = e,, = el. (2.3)









ezz = -ell CI (2.4) exy = eye = ezx = 0. (2.5) In addition to altering the physical parameters of the QWIP, lattice strain can also induce energy band shifts, which can be used to alter the absorption characteristics of the QWIP. The strain induced energy band shifts for the conduction band, the heavy hole subband, and light hole subband can be approximated as follows.
c,, -C,
AEc = 2c, 12 8 0 (2.6) c,,

AE = b 11 + C2 8 (2.7) C,I

(AEhh)2
AE, = -AE + (2.8) 2A0

where c, is the combined hydrostatic deformation potential which characterizes the splitting of the F, valence band under strain, and b is the shear deformation potential, and Ao is the spin orbit split-off energy [23]. The total hydrostatic deformation potential c+ V,., where V,. is the valence band deformation potential, can be expressed by [25]
1 dEco
c, + V,- ( (, + 2(12) (2.9) where dl"',/dP is the unstrained energy bandgap change with respect to the unit pressure.

The effect of strain on the energy band structure results in the splitting of the heavy hole and light hole band at the valence band zone center [26] (i.e., the in-plane wavevector l1=0), which is degenerate in the unstrained case. When tensile strain is applied between the quantum well and the barrier layers [27-29] along the superlattice growth z-direction, the strain can push the light hole levels upwards and pull the heavy hole levels downwards. We can therefore expect that heavy hole and light hole states are inverted at specific lattice strains and quantum well thickness. This phenomena will in turn cause the intersubband transitions in a QWIP structure to take place from the populated light hole ground state to the upper energy band states. Since the light hole has





12


a small effective mass (comparable to the electron effective mass), the optical absorption and spectral responsivity in p-type QWIPs can be greatly enhanced, as a result of introducing strain in the quantum well. In addition to the utilization of the light hole states for their small effective masses, etc., certain heavy hole states under compressional strain may also have similar characteristics, like high mobilities, small effective masses, and long mean free paths; which in turn favorably alter the intersubband absorption and transport characteristics, as shown by Hirose et al. [13]. This is achieved by distorting the heavy hole valence band at and near the zone center via the introduction of compressional strain.


2.2.3 Energy Band Calculations

To calculate the locations of the energy subbands, we can use the transfer matrix method (TMM) [28,30,31], based on the eight-band k-p model. This model is represented by the Luttinger-Kohn Hamiltonian [32,33], H,, which describes the unstrained semiconductor.

H, = H + '(z) (2.10) w1e1re

II, H2 HI H.
H21 H22 H,3 H24 (2.11) H,3 H32 H33 H34
H4, H42 H43 H44

with
HI,, =' +2 k 2 +k2)+ 71 ---2 kY1 IY2 (k 2+k)+ YI +Y2 k2 H22 = k + k) + 2 HI2 = 3Yk, -ik)k,





13


H21 = H*2' H3 = H3 H24= H,3

H34 = H 2 =2' H2 = H' H43 = H,2

H4 =H23 =H32 =H41 = 0

and V(z) is a step function where V(z) vanishes inside the well layers and equals Vo in the barrier layers. The effect of strain is included by adding the Bir-Pikus Hamiltonian [34], H,, to the general Luttinger-Kohn Hamiltonian. As shown below, the strain Hamiltonian for the well material is a diagonal matrix.
SAE A.Eh 0 0 0
0 AEc + AEhh 0 0 (2.12)
0 0 AEc + AEh, O
0 0 0 AEc + AEhh

Using the aforementioned techniques, we can numerically calculate the energy of the zone-center valence subband levels as a function of well width for any material system under tensile or compressional strain and also determine the change in the valence subband structures.

All of the previously described calculations are derived from the multiband effective mass k-p model for a coherently strained structure, which is based upon the perturbation approximation. In the k-p model, the interactions of S-P type coupling among conduction (C). light-hole (LH). heavy-hole (HI-I). and spin-orbit (SO) states combined with spin-orbit like coupling are taking into consideration to derive the band structures. This results in an 8x8 k-p Hamiltonian and momentum matrix elements. Using the perturbation approximation, a set of wave functions of S,12: 11/2,1/2>c; P3/2: 13/2,3/2>, 13/2,1/2>; and P,,2: 11/2,1/2> are used to represent the unperturbed and unstrained basis in the IJ,mj> presentation [35]. mj=l/2 represents either the electron or LH states, while mj= 3/2 denotes the HH or heavy particle states. A slightly simplified 6x6 k-p Hamiltonian can be used to roughly predict the P-like properties of the coherently strained layers by considering the S-like conduction band states as a





14


perturbation, if a large enough bandgap exists, like in InGaAs and GaAs layers. The wave functions of the coherently strained superlattice at the zone center (k=O) are given by [36]

13/2,3/2 > HH states (2.13) y 13/2,+1/2 > +P1 /2,1/2 > LH states (2.14)

1 13/2,1 /2 > +y 11/2,+1/2 > SO states (2.15) where y and p are constants which are dependent on the strain parameters. Note that the heavy-hole states, 13/2,3/2>, are still decoupled from the other valence band states even under biaxial stress at the zone center, while the light-hole and spin-orbit split off states are coupled at k=O. However, the HH, LH, and SO states are mixed [37,38] in the coherently strained superlattice at off zone center k0O. This mixing between the states with different mix's is due to the boundary conditions across the interface of the quantum well layers. By examining the k-p matrix, we can see that the interaction between the different mj states is proportional to the transverse components of the wave vector, kx,y, so that the HH states are decoupled when kx,=O. It is interesting to note that the k,,,'s are conserved across the interfaces since the interface potential depends only on z, the quantum well growth direction. Thus the band mixing can be significant if the Fbandgap is small. e.g.. with GaAs and InGaAs. and if the LIH- and SO bands involved in the transition have a large kz value [37].

Since the heavy hole and light hole valence subbands are non-degenerate following the introduction of strain into the QWIP structure, a simpler method can be used to determine the energies of the subbands. By using the parabolic band approximation near the valence band zone-center, and the energy band shifts for the conduction band minimum, heavy hole subband maximum, and light hole subband maximum, we can utilize the simpler two-band Hamiltonian for electrons just by finding the effective mass of the carriers (i.e., heavy-hole effective mass and light-hole effective mass) and the barrier heights for each carrier type. Although this does not simultaneously determine the





15


energy levels of both carriers, it does allow accurate predictions of the energy subbands. When compared to the direct calculation of the energy subbands, the two-band approximation yields accurate results when compared to the direct calculation results [28,34]. One limitation of the TMM is that this method cannot calculate the energy levels of the allowed energy subbands in the continuum states. In order to determine the transition energy from the ground state to the continuous state, we used the KronigPenney model to determine the locations of the allowed energy bands in the continuum states.

When a biaxial internal tension is applied to the well material, the strain pulls the LH subbands up with respect to the HH subbands for a given well thickness. While quantum confinement effects tend to push the LH subbands down with respect to the HH subbands. As the well width is increased above a certain value, the strain effect can overcome the quantum confinement effect and therefore induce the inversion of the heavy hole and light hole subbands at the ground state. In contrast, with the application of compressional strain on the well layers, the strain forces the LH subbands down with respect to the HH subbands for a given well thickness. For a schematic description of what happens to the conduction. heave-hole and light-hole bands. see Figure 2.1.


2.2.4 The Transfer Matrix Method for the Calculation of Transmission Probability


The transfer matrix method (TMM) [31] allows the calculation of the transmission probability through a superlattice. Like any typical quantum mechanical barrier or well, the carrier conduction in each layer of the superlattice consists of the superposition of two components propagating forwards and backwards. The complete wave function can be expressed as
=yie -e +W -ik'e +A (2.16) where





16


A, = A2 = 0

A, = ki (d2 +d3 +. .+di)

i = 3,4,...,N

k[ 2m (E -E,)/2


where ,i+ and W, represent the magnitudes of the wave functions propagating in the forward, or +z direction and the backwards, or -z direction, respectively. While N is the number of periods in the superlattice, d is the thickness of the i-th layer in the superlattice, m,* is the effective mass of the particle in the i-th superlattice layer, and Ei is the potential energy of the i-th layer. Since the wave function, y, and it's derivative, dy/dz, are continuous at the boundaries, the wave functions then become Yi = (e' +, + re-"'yI,-) / t, (2.17) y = (ret', ,+l +e +y,-)/ t (2.18) The recurrence relationship of the wave functions can be written in matrix form as 92 1 e re
r= e-'i e'8i) y+) (2.19) i- i e e yi+1/

where at normal incidence k, -k, (2.20) k -ki
2k
t= k,+k, (2.21) and

8i = kidi. (2.22) Which gives us the following form for determining the N+I-th wave functions

S=SS2 =SS2 ... SN (2.23) (2 Y 3S N+1 where

S e' rie I (2.24) re e





17


Since there is no backwards, or in the -z direction, propagation in the N+1-th layer, the magnitude of the wave function Y, N+=0. Thus we can find the y,i term of E1,, in the i-th layer (i = 2, 3, 4, --, N+1).

If we determine the quantity, ;i+/9+, as a function of E,, then we will know the locations of the resonant peaks. The transmission probability can be expressed as IT T| = .' (2.25)


2.2.5 Determination of Intersubband Transitions and Absorption Coefficients


In addition to the energy level and energy band locations, the calculation of intersubband and interband transitions are also of great interest. In order to determine the intersubband and interband transitions in a p-type strained layer QWIP, the usage of the 6x6 Hamiltonian which includes the previously mentioned k-p Hamiltonian [32,33,36] and the strain Hamiltonian [34]. Since the strain and the spin-orbit coupling terms do not lift the spin degeneracy, the 6x6 Hamiltonian matrix can then be factored into two 3x3 irreducible matrices. The assumption that the Fermi distribution function is equal to one for the confined ground state and equal to zero for the excited states in equilibrium is used to simplify the calculation without loss oft accuracy. The absorption coefficient for the intersubband or interband transition between the initial ground state, i, and the final continuum state,f is given by [39]

472 e2 2dk 2 1/2 (2.26)
()= ni J 1i 2 ( /4) (2.26)
2 ,cm,2m (2[ A,(k)-ho] +(2 /4)

where nr is the refractive index in the quantum well, m,, is the free electron mass, Ai. is the energy difference between the initial ground state, i, of energy E,(k) and the final state, f, with the corresponding energy of E/k). 8 and co are the unit polarization vector and the frequency of the incident IR radiation, respectively, f and f, are the Fermi distribution functions of the initial and final states, and F is the full width of level broadening. 1F





18


h/t, where ty is the lifetime between the initial, i, and final, f states. ^ -Pi are the optical transition elements between the quantum well valence subband ground states, i, and the continuum subband states,f in the HH, LH, and SO bands; which can be derived from the two 3x3 k-p matrix elements as shown below.

Using the following 3x3 optical matrix,
HH HL THS
m" TLH T, TI. (2.27) SL TSL TSS

the optical matrix elements, e -Pi, can be obtained. These matrix elements have the same form as the k-p matrix elements except that the kk,'s are replaced with k,+k; multiplied by a constant factor of m/h [39]. The T's are defined as follows:
THH = 2(A-B)~k +(2A+ B)( exk +sky), (2.28) TLL = 2(A+B)Eskz +(2A-B)(E k, -,ky), (2.29) Tss = 2Aexk, +syk, +k (2.30) T = N(, cosl-E sin)k
Ti 3= cosil 6 sini) NE
iB(E,k -, k,)cosy (2.31) + ( r, k + E, kjsin



+ iFB(Ekr- kEkc )cos (2.32) + 2 N(k, +Fk,)siny

TLs = i2i/2B + Ne cps(X -r)- Y sin(I )k,
V, (2.33)

-i1B(Exk, + yky) Nkj cos(X -21) T'H =T's, (2.34) Ts, = T', (2.35) TH = TH. (2.36)





19


Here A, B, N, 7, r are inverse mass band parameters [39].


2.2.6 Photoconductive Detection Mode Operation


When IR radiation impinges on a photoconductor, the photoconductive material undergoes a physical change characterized by a change in resistance, ARd. This change in resistance is due to the photo-excitation of carriers, forming mobile excess carriers in the photoconductor. The excess photogenerated carriers, An, can be expressed as An = A L (2.37) Vd

where, r, is the quantum efficiency, A( is the incident photon flux, tL is the excess carrier lifetime, and Vd is the volume of the detector. These photogenerated carriers are transported out of the detector under the influence of the applied external bias, which results in a photovoltage signal. The change in the output photovoltage, AV,,, due to the resistance change is given by
VA R, ARd
(R, + R,)2

where R, is the load resistance and its value is typically chosen to be about equal to Rd, the detector resistance, to match loads and to optimize the output signal.


2.3 P-QWIP Figures of Merit


Although our band structure and absorption calculations can be used to determine the positions of the subbands in the quantum wells, and hence determine the peak absorption wavelength of the QWIP, many other factors must be taken into account to design a QWIP with the correct detection peak. Generally, for a high-performance QWIP, the responsivity must be high, while the noise current, and hence the dark current, must be low.





20


2.3.1 Spectral Responsivity

The responsivity, R, for a photodetector may be expressed as [40] qMci q
R G = q (2.39) hc hv

where q is the electronic charge, X is the wavelength of the incident photon, h is the Planck constant, c is the speed of light, "1 is the quantum efficiency, re is the collection efficiency, v is the incident frequency, and the photoconductive gain is G. The quantum efficiency and photoconductive gain are described, respectively, by [40] q= A(1- R)[1-exp(-Bal,,)] (2.40)
L
G (2.41) tC

where A is a constant that is polarization dependent, a is the absorption coefficient of the quantum well, 1,q is the total width of all quantum well regions, L is the mean free path of the carrier, Rf is the reflection coefficient, and rc is the total width of all quantum well and barrier regions. B is a constant dependent on the number of passes IR radiation makes through the photodetector. For n-type QWIPs, A=0.5, while for p-type QWIPs A=I. The mean free path of the carrier may be expressed as [40] IL = -i T, t// F. (2.42) where is the well reCCapturC lifetime of the carrier. T,, is the transmission coefficient over the quantum well, e,,fi is the effective mobility of the carrier, and F is the electric field. The effective mobility for a two-band transport model is shown to be [40] APlh h + APhh hh
4 =l (2.43) where Aphh and Aph are the concentrations of optically induced heavy- and light- hole carriers, respectively, and Ap,, and Ap,h are the concentrations of optically induced heavyand light- hole carriers. When only the ground state is completely occupied, either Aph, or Ap,,,, the optically induced light holes or the optically induced heavy holes dominate, so that we may estimate ,, as the in-plane effective mass of the ground state carriers.






21

2.3.2 QWIP Collection Efficiency


A figure of merit that can be easily quantified by simple measurements is the collection efficiency, ric. The collection efficiency describes the ease in which the energy from the incident photon flux is converted into mobile carriers which are swept out of the QWIP by the applied bias and collected; and is defined as the product of the quantum efficiency, ri, and the photoconductive gain, G.

ic = rI G (2.44) In addition to being expressed as the mean free path over the total width of the quantum wells and barriers, G can be viewed as the ratio of the carrier transport lifetime, tL, to the transit time, -r, through the QWIP. Empirically, the photoconductive gain can be described in terms of the capture or trapping probability, p, [41-43],
1
G = Npc(1 + pc (2.45) and N is the number of wells. If p is small, then G can be approximated as, G= 1/Np,

Physically, the trapping probability is defined as the ratio of the escape time from the well region to the lifetime of the excited carriers from the confined ground state. If the excited states are in resonance with the top of the barrier potential energy, then the escape time will be greatly reduced. \\hich theoretically minimizes the trapping probability and maximizes the photoconductive gain. Therefore in all of our designs, we attempted to make the energy of the upper excited peak for the main detective peak in resonance with the top of the barrier potential energy.

If B lq,,<
S= A( R) exp(- Ba )] N (2.46) Ba 1.
A(1- R)- (2.47) Np,

where B is a constant dependent on the number of passes the IR radiation makes through the photodetector, A is a polarization dependent variable equal to 0.5 for n-type QWIPs





22


and 1 for p-QWIPs, R is the reflection coefficient, lq,, is the total width of all quantum well regions, and ca is the absorption coefficient.


2.3.3 Dark Current Relationship in a QWIP

Another important parameter to be considered in a QWIP design is the dark current density Jd, which can be expressed using the Richardson-Dushman equation [29] as Jd oC T2m* exp (2.48) where m" is the effective mass, AE is the difference in energy between the barrier height and the quantum confined state in the well, kB is the Boltzmann constant, and T is the temperature. This type of expression assumes that the dominant source of dark current is thermionic emission over the quantum well barrier.

In the low-field regime, the thermionic emission current is related to the density of mobile carriers, n, and the average drift velocity, vd, and can be expressed as [44] l,h = qAdVdn,, (2.49) where A,, is the active detector area. q is the electronic charge. and pF
v, (2.50)


n, =(m*k,T/ 7t h 2L)exp[-(Ef- E, )/ k,]. (2.51) In the above equations, [ is the mobility, F is the electric field, v, is the saturation velocity, E,, is the cut-off energy related to the cut-off wavelength k,, and m'/i h is the two-dimensional density of states. The Fermi energy, E,, can be obtained from the expression of N,:
m'k,T F (E-E,)
N T lnI + exp. l -)" (2.52) D 7 h2 L \kT j m*

m (E, E,,).2.53) r h L E (2.53)





23


Equation (2.52) for N, is valid when summed over the subband levels E,, below the Fermi level, and Eq. (2.53) is only valid at cryogenic temperatures.

Using the previous result in the cryogenic temperature regime, we see that the dark current due to thermionic emission is exponentially dependent on the doping concentration in the quantum well, i.e.,
EF ND
I, oc exp oc exp (2.54) Therefore, as the doping density in the quantum well increases, the dark current density due to thermionic emission also increases exponentially. In contrast to this, the intersubband absorption is directly proportional to the doping concentration. Therefore, a tradeoff between the dark current density and the intersubband absorption is required to optimize the QWIP performance. However, in the case of p-QWIPs, the Fermi level in the quantum well is pinned at or slightly above the ground state energy for highly doped quantum wells. We can increase the doping in the quantum well to increase optical absorption without increasing the dark current of the p-QWIP significantly because the thermionic emission pinned with the Fermi level.


2.3.4 Noise and Detcctivity in OWIPs


The noise in QWIP structures is mainly due to random fluctuations of thermally excited carriers. The noise is expressed as [17] i,,fios, = 4AdqGAfJd (2.55) where Ad is the detector area, and Af is the noise bandwidth. Finally, a figure of merit measurement used to compare detectors is the detectivity, D', which is shown to be [40] D' = Af R (2.56) If the dark current in a particular QWIP is lower than the 300 K background photocurrent, then the QWIP can be considered to be under background limited





24


performance (BLIP). In a BLIP QWIP, the dominant current is due to photon noise, since all the other sources are negligible by comparison. The photon noise is calculated from the arrival statistics of the incoherent photons. The background photon noise current, i,,, is given by [45,46]

i2 = 4Adq2rlg2 PB / (hv), (2.57) where Pb is the incident background optical power, B, is the QWIP bandwidth, ir is the absorption quantum efficiency, v is the incident photon frequency, and g is the noise current gain. The photocurrent, I, can be approximated by Ip = A(q / hv) gP,, (2.58) where P, is the incident optical signal power. The constant, A, in Eqs. (2.46), (2.47), (2.57), and (2.58), is due to the polarization selectivity for n-type QWIPs versus p-type QWIPs. As previously stated, for n-type QWIPs, A=0.5, while A=1 for p-type QWIPs. By setting the signal-to-noise power ratio equal to unity, the background limited noise equivalent power, (NEP)n,,, and the detectivity, D',Ii,,, for n-type QWIPs can be expressed as
(NEP) ,,,, = 2,2hv BI, /r (2.59) D8-11 B I/wt (2.60) where A,, is the active area of the detector, and Qh=Pb/(Ahv) is the incident photon flux from the background for a given spectral bandwidth, Av, and a peak wavelength, ,,. Q,, is defined as

rt v 2Av sin2'-, (2.61) Sc2 exp[hv / (k, T)]- 2

where, 0, is the field of view (FOV). For a p-type QWIP, a factor of 12- is used in the denominator of Eq. (2.60), D'i.i,, since it can absorb both optical polarizations of the incident IR radiation.






25

















k= k=O k=O CB CB k CB
I I I I I I I I

I I



HH HH HH


LH I LH I LH



Compressive Strain No Strain Tensile Strain




Fiurc 2.1: A schClnutic diagrallm sho\\iin tihe clfccts of comlpressive and tensile strain onil the conduction (CB), heavy-hole (HH) and light-hole (LH) bands in a semiconductor.













CHAPTER 3
AN INGAAS/ALGAAS ON GAAS P-QWIP WITH COMPRESSIVE STRAIN
LAYERS AND LWIR AND MWIR DETECTION


3.1 Introduction


In the last few years, n-type QWIPs have been extensively investigated using III-V semiconductor material systems [10]. Because of the small electron effective mass and high electron mobilities, n-type GaAs/AlGaAs QWIPs offer excellent IR detection properties. These n-type QWIPs have utilized the bound-to-continuum [10,47,48] and bound-to-miniband [11] transition schemes in the 8-14 ptm LWIR and 3-5 [pm MWIR bands, to achieve reasonable detectivities and dark current characteristics. However, quantum mechanical selection rules for intersubband transitions requires that the electric field of the incident IR radiation has a component perpendicular to the quantum well plane in order to induce intersubband absorption in the quantum wells. Therefore. for ntype QWIPs. it is necessary to use either planar metal or dielectric grating structures to couple the normal incidence radiation into absorbable angles in the quantum wells [16,49].
In contrast, p-type QWIPs allow the absorption of normal incidence IR radiation due to the band mixing between the heavy-hole and light-hole states. In p-type quantum wells, intersubband transitions under normal incidence illumination are induced by the linear combination of p-like valence band Bloch states which provides a nonzero coupling between these components and the normal radiation field. The strong mixing between light-hole and heavy-hole states for I10O greatly enhances the normal incidence intersubband absorption. However, in the unstrained lattice matched GaAs/AlGaAs and InGaAs/InA1As quantum well systems recently demonstrated [17-19], the intersubband



26





27


transitions occur between the heavy-hole ground state and the upper heavy-hole excited states. Due to the large heavy-hole effective mass, weak absorption and low responsivity are expected for the unstrained p-QWIPs. By utilizing biaxial strain confined in the quantum wells of the QWIP, we can increase the responsivity and the background limited photocurrent (BLIP) temperature, which offers more flexibility in the design and fabrication of p-QWIPs. Here we present a new normal incidence p-type compressively strained-layer (PCSL) In0.2Gao.sAs/Alo.15Gao.85As QWIP grown on S.I. GaAs by molecular beam epitaxy. In this QWIP structure, the intersubband utilizes a bound to continuum transition between the ground heavy-hole (HHI) state to the second extended heavy-hole (HH3) state for LWIR detection. The MWIR detection peak is due to the intersubband transition from the HH 1 state to the first continuum heavy-hole (HH4) state.


3.2 P-type Compressive Strained Layer QWIP Design


Compressive strain is introduced in the In0.2Gao.sAs quantum wells of the QWIP, while no strain is present in the Al0 ,sGaO.85As barrier layers, which is lattice matched with the semi-insulating (SI) GaAs substrate. The induced strain pushes the heavy hole states upwards and the light hole states downwards relative increasing electron energy in the InGaAs quantum wells. Thus the heavy- and light-hole bands are split in the quantum wells, but remain degenerate in the AIGaAs barrier regions at the Brillouin zone center.

The In2Gao.sAs/Al1 Gao.ssAs p-type CSL QWIP was grown on a (100) SI GaAs substrate by molecular beam epitaxy. The structure consists of twenty periods of 48 A In0.2Gao.sAs quantum wells spaced with 500 A wide Alo.s1Gao.8sAs barriers. The wells were Be-doped to a density of 2x10'8 cm-, while the barriers were undoped. A 0.3 ipm cap layer and a 1.0 pm thick buffer layer of GaAs each Be-doped to 5x10'8 cm-3 were also grown to serve as ohmic contacts. In addition, 600 A wide Al0.5sGao.8sAs barrier layers were grown between the contact layers and the multi-quantum well structure to reduce the





28


large tunneling current from the triangle potential formed by the heavily doped, large bandgap ohmic contact regions. The exact layer structure for the p-QWIP is shown in Figure 3.2. In our design, the barrier and substrate are lattice matched and the well regions are in biaxial compression due to a lattice mismatch of -1.4%. The ground subband energy levels confined in the quantum wells are the highly populated heavy hole states, EHHI. The mobility of the heavy hole is enhanced by the compressive strain in the InGaAs quantum wells by the reduction of the heavy hole effective mass [13]. Another improvement which results from the introduction of compressive strain in the quantum wells is the reduction in the density of states in the InGaAs layers. Because of this, more free holes will reside in higher energy states, which implies that the Fermi level is elevated when compared with the unstrained case. The elevation of the Fermi level will result in an increase of the number of off zone center (i.e., k0O) holes with less effective mass. Therefore, a larger intersubband absorption under normal incidence infrared radiation can be expected.

As seen in Figure 3.1, the intersubband transition occurs from the highly populated ground heavy hole state (E,,,) to the upper heavy hole bound state (E,11,3) and the tirst extended heavv hole state (EH,,,) for the 7.4 pm LWIR and 5.5 pm MWIR detection peaks. respectively. Since the upper heavy hole bound state (LI,,,3) is slightly below the barrier valence band maximum, we expect a maximum in the absorption oscillation strength; whereas the first extended heavy hole state (EHH4) is above the barrier, which predicts a weaker absorption.


3.3 Results and Discussion


To facilitate the characterization of this p-QWIP, a 216x216 ptm2 mesa was etched onto the wafer by wet chemical etching. After patterning with a contact mask, a thin film of 120 A of Cr was deposited by E-beam evaporation. This layer was topped off with a





29


1000 A layer of Au to create both the top and bottom ohmic contacts. The top ohmic contact consists of a ring type structure around the edge of the mesa with a 50x50 pm2 contact pad for electrical connection.

Figure 3.3 shows the dark I-V characteristics of the InGaAs/AlGaAs compressively strained p-QWIP. As seen in this figure, the device is under BLIP at temperatures below 63 K for applied biases between -3 V and +3 V. A BLIP temperature of 70 K can also be achieved when the applied bias is less than 1 V. Like all of the previously studied pQWIPs, the dark current characteristic is slightly asymmetric. This can be attributed to the doping migration effect of the Be dopant during layer growth [50].

The responsivity of the p-QWIP was measured under normal incidence illumination as a function of temperature, applied bias, and incident IR radiation wavelength by using a blackbody radiation source running through an automatic PC-controlled single grating monochrometer with the appropriate IR filters attached. The output of the QWIP was measured with a Princeton Applied Research 5210 lock-in amplifier and converted to responsivity by calibrating the output with a pyroelectric detector. A schematic layout of the experimental setup is shown in Figure 3.4. The same experimental setup is used throughout this study.

:igures 3.5(a) and 3.5(b) show the results of these measurements. A single LWIR peak was found at p,,=7.4 pim and T=77 K with an applied bias of 5 V. Given a rather broad LWIR peak and a cut-off wavelength of approximately at 10 tm; this corresponds to a half-peak spectral bandwidth of A/,,=30%. The responsivity was determined to be 37 mA/W at the 7.4 [tm peak wavelength. A single MWIR peak was also found at Xp2=5.5 pim under the same conditions previously mentioned. The MWIR peak has a bandwidth ranging from approximately 4 to 6 [pm. For a cut-off wavelength of 6 pm, we derive a spectral bandwidth of A?/kp2=27%, which is again a rather wide peak. The asymmetrical responsivity around the MWIR spectral peak is attributed to the long-pass filter characteristic which has a cut-on at -ON=6.7 m. As seen in Figures 3.6(a) and





30


3.6(b), the responsivity is linearly proportional to the applied bias and that variation with respect to device temperature is minimal for both detection peaks.

Noise characterization was also performed on the p-type CSL QWIP using standard noise measurement procedures [51]. A Brookdeal 5004 low noise amplifier (LNA) which has an input reference current noise, S,-4x 10.27 A2/Hz, was used to amplify the signal generated by the QWIP. The spectral density from the output of the LNA was measured using a HP 3561A spectrum analyzer which has a bandwidth of 100 kHz and allows for data collection via computer. In order to extract the device parameters, all the measurements were carried out at temperatures higher than the device BLIP temperature of 67 K.

The noise spectral density measured with an applied bias of Vh=1.0 V and T=81 K was found to be 6.5x10-28 A2/Hz. Given a device area of 216x216 JLm2, and a measured current responsivity, RA=12.5 mA/W under the previously mentioned conditions, we calculated a detectivity of, D*=1.06x100 cm-Hzl2/W at the 7.4 pm peak wavelength. As the applied bias is increased, the detectivity decreases due to the increase in dark current and the corresponding increase in noise spectral density, even though the current responsivitv increases linearly. The calculated D' values at V;,=2 and 3 V are 6.3xl10' and 3.2x 10" cm-Hz' i/W. respectively. The noise spectral densit\ of this PCSIL QWIP as a function of temperature and applied bias voltage is shown in Figure 3.7. As shown in this figure, at a low bias voltage, the number fluctuation noise translates into current fluctuation noise via the diffusion mechanism. As the applied bias increases, charge transport becomes drift dominant and the number fluctuation noise couples to current noise via the hole drift mechanism; which results in a strong current dependence [51]. The dashed lines in Figure 3.4 show the results predicted for diffusion dominated noise, while the solid lines show the results predicted for drift dominated noise.





31


3.4 Conclusion


We have demonstrated a new normal incidence p-type compressively strained-layer (PCSL) InGaAs/AlGaAs QWIP grown on SI GaAs for MWIR and LWIR two-band two color detection. Maximum LWIR and MWIR responsivities were found at 7.4 and 5.5 ptm of 37 and 8 mA/W, respectively. The intersubband absorption and photoresponse of this normal incidence PCSL QWIP were enhanced by using compressive biaxial strain in the InGaAs quantum well layers. Since the total layer thickness of this PCSL QWIP is greater than the strained layer critical thickness, certain strain relaxation might occur, which may result in a lower photoresponse and higher dark current characteristic than expected; even though the individual layer thickness are within the critical layer thickness criteria. Although the LWIR detection peak for this QWIP is shorter than that required for most staring focal plane array (FPA) applications and the MWIR detective peak is slightly longer than required, we can shift the MWIR and LWIR detection peaks into more useful regions in addition to maintaining or improving the responsivity and dark current characteristics the PCSL QWIPs for FPA applications by further optimizing the quantum well dopant density and the biaxial strain strength, changing the well and barrier thickness as well as the In and Al compositions oftlhe well and barrier layers.





32









InGaAs AIGaAs InGaAs




HH1
.................................. ... Ef



HH2






HH3 HH4




Figure 3.1 The schematic energy band diagram for the two-color two-band compressively strained InGaAs/AlGaAs p-QWIP.





33














GaAs 300 nm Be=4x10'8 cm-"

Alo.15Gao.85As 50 nm undoped

Ino.2Gao.8As 4.8 nm Be=2xl0'8 cm-3 Repeat x 20
Ala.i5Gao.85As 50 nm undoped

GaAs 500 nm Be=4x10'8 cm-3 SI GaAs (100)





Figure 3.2: The layer structure of the InGaAs/AlGaAs two-color two-band compressively strained p-QWIP.





34








10-3


10-4 r .........300 K BG photocurrent 10-5 T = 92 K 10-6 85 K 77 K o10-7
74 K




10- 63K 10-10 10-11 10-12
-3 -2 -1 0 1 2 3 Applied Bias (V)



Figure 3.3: Dark current characteristic of the InGaAs/A1GaAs CSL p-QWIP.







35




















---------------------------------------IR Filter


MonG meerBlackbody



Chopper &
45B Controller
Semiconductor
Parameter izer Closed-cycle Cryostat
I- Lock-In Amplifier Measurement I Trans-Impedance
Set-Up Amplifier
Ji gal ain Lock-in Out Ref In


Control &
Data ]




Spectral Measurement Set-Up





Figure 3.5: A schematic layout of the experimental setup.






36






0.012
T=77K 0.010, V=5V

0.008

l 0.006

S0.004

0.002 0.000
3 4 5 6
Wavelength (um)

(a)
0.040

V=5V
0.030
T =77 K 0.020


M 0.010


0.000,
6 7 8 9 10 Wavelength (um)

(b)



Figure 3.5: (a) The MWIR and (b) LWIR responsivity of the InGaAs/A1GaAs CSL pQWIP at 77 K under an applied bias of 5 V.





37






0.045
0 T=80K
0.040 .
-4++ 77 K 0.035 50 K
0.030
5 0.025 a 0.020 8 0.015
0.010
0.005 LWIR peak
0.000
0 1 2 3 4 5 Applied Bias (V)

(a)
0.010
T = 77 K
0.008


0.006

. 0.004

0.002
MWIR peak
0.000 .. .
0 1 2 3 4 5 Applied Bias (V)

(b)



Figure 3.6: (a) the LWIR and (b) MWIR responsivity as a function of bias and temperature of the InGaAs/AIGaAs CSL p-QWIP.





38








10-22


130K

10-24


8 10 -K

2 8 9K .,,----
0

81K ----


10-28 .....
10-2 10-1 100 101 Reverse bias voltage (V)




Figure 3.7: The experimental and theoretical current noise spectral density versus reverse bias voltage at various temperatures for the InGaAs/AlGaAs CSL p-QWIP.













CHAPTER 4
A COMPRESSIVELY STRAINED-LAYER P-TYPE INGAAS/ALGAAS/GAAS STEP BOUND TO MINIBAND QWIP AT 10.4 [pm


4.1 Introduction


In recent years, n-type quantum well infrared photodetectors (n-QWIPs) have been extensively investigated using III-V semiconductor material systems. These QWIPs have utilized the bound-to-continuum (BTC), bound-to-miniband (BTM) and step bound-tominiband (SBTM) transition schemes to achieve detection in the 8-14 ptm longwavelength infrared (LWIR) band with reasonable detectivities and dark current [52,12,53]. Unlike n-QWIPs, which are forbidden to absorb normal incidence infrared radiation due to the quantum mechanical selection rule, p-type QWIPs exhibit normal incidence intersubband absorption because of the mixing between the off-zone center (i.e., k 0) heavy hole and light hole states [37]. Because of the larger hole effective mass and hence lower hole mobilities, especially for the heavy holes, the absorption coefficient. qcy and spectral performance of the p-QWIPs are generally lower than n-QWIPs [28,12]. However, when compressive strain is introduced into the quantum well layers, the hole effective mass and in-plane density of states decreases [13]. Thus, more free holes will reside in higher energy states, which implies an elevation of the Fermi level. The elevation of the Fermi level increases the number of off-zone center holes (k#0), which increases the magnitude of the off-diagonal matrix elements, which in turn increases the absorption coefficient and the associated performance parameters [28].

We report a new p-type SBTM CSL-QWIP grown on a semi-insulating (SI) (100) GaAs by molecular beam epitaxy (MBE) using the InGaAs/AlGaAs/GaAs material system for the quantum well/superlattice barrier layer structure. As illustrated in Figure



39





40


4.1, the transition scheme for this p-QWIP is from the localized ground bound heavy hole state (HH1) in the wide In0.12Ga0.ssAs quantum well to the resonant coupled miniband of the GaAs/Al1035Gao.65As superlattice (SL) barrier. This structure creates a potential difference between the SL barrier region and the quantum well which blocks part of the undesirable tunneling dark current from the heavily doped heavy hole ground state, HH1 [53]. The physical parameters were chosen so that the ground state in the wide InGaAs quantum well is well above the top of the GaAs/AlGaAs SL barrier, and the third excited heavy hole state (HH4) is in resonance with the ground level of the superlattice miniband (SL1) to achieve a higher quantum efficiency. Since the superlattice consists of thin barriers, the photoexcited holes can easily tunnel through the superlattice barrier layer and transport along the aligned miniband to be collected by the ohmic contacts.


4.2 Theoretical Considerations


In order to characterize this detector, we performed theoretical calculations of the energy states in the quantum well and superlattice barrier regions along with the transmission coefficient, IT*7], by using the multiple layer transfer matrix method (TMM) 131] which is described in Chapter 2. The results of the TMM calculation are shown in Figure 4.2. Using linearly interpolated values for the heavy-hole and light-hole effective masses and the compound semiconductor bandgaps at 77 K for GaAs, A1035Gao.65As, and In0.12Ga.s88As we determined the intersubband transition for the LWIR absorption to occur at a peak wavelength of 10 pm, when the effects of biaxial compressive strain are considered.

Another characteristic of p-type QWIPs is the inherently larger quantum efficiency than that of n-QWIPs, which is given by
n = A(1- R)(l- exp[- c ]) (4.1)





41


Given similar absorption coefficients, ca, and well thickness, 1, the prefactor A is equal to 0.5 for n-QWIPs and 1.0 for p-QWIPs, which gives us a doubling of the quantum efficiency for p-QWIPs. Due to the inherently low absorption coefficients of p-type materials, and the large hole effective mass, compressive strain must be used to reduce hole effective mass in order to increase the absorption.

With the inclusion of compressive strain, the mobility of the heavy holes is enhanced by reducing the heavy hole effective mass [13]. Also associated with the presence of compressive strain is the reduction of the density of states in the InGaAs quantum well. Thus, with a significant lowering of the effective mass of the ground heavy holes, an increase in the absorption coefficient and the corresponding quantum efficiency is expected.


4.3 Device Growth and Fabrication

The p-type SBTM CSL-QWIP consists of 90 A thick In0.12Gao.88As quantum wells Be-doped to 3x1018 cm3 to populate the ground heavy hole (HHI) state. The quantum well layer is under compressive strain with a lattice mismatch of nearly -0.8%. Surrounding the quantum well layers are the superlattice barriers which are made up of 20 A thick superlattice barriers of undoped A10.35Ga0o.6As alternating with 27 A thick undoped GaAs quantum wells. The complete superlattice barrier structure is composed of ten periods of the GaAs/AlGaAs superlattice barrier structure, which is lattice matched to the SI GaAs substrate. The whole superlattice barrier/quantum well structure is then repeated 20 times to form the p-type SBTM CSL-QWIP. The p-type ohmic contacts for this QWIP are formed by a 0.5 jim thick cap layer and a 1.0 jim thick buffer layer of heavily Be-doped (5.0x 1018 cm3) GaAs on top of the SBTM QWIP stack and between the SBTM QWIP stack and the substrate, respectively. The complete layer structure of this device is illustrated in Figure 4.3.





42


To facilitate the characterization of this p-QWIP, a 216x216 tm2 mesa was etched onto the wafer by wet chemical etching. After patterning with a contact mask, a thin film of 120 A of Cr was deposited by e-beam evaporation. This layer was topped off with a 1000 A layer of Au to create both the top and bottom ohmic contacts. The top ohmic contact consists of a ring type structure around the edge of the mesa with a 50x50 pm2 contact pad for electrical connection. It should be noted that for this type of mesa and ring contact structure, the normal incidence IR radiation is only allowed one pass through the SBTM quantum well layers; which effectively reduces the quantum efficiency when compared to those QWIP structures which incorporate backside thinning to create a waveguide-like layer and a top reflector to increase the number of times the incident IR radiation passes through the SBTM QWIP structure.


4.4 QWIP Characterization and Results

Device characterization was performed in a closed cycle helium cryogenic dewar. An HP 4145B semiconductor parameter analyzer was used to measure the dark I-V characteristics and the 300 K background photocurrent. Under dark conditions, holes can be transfered out of the quantum wells and produce the observed dark current mainly due to two mechanisms, thermionic emisson out of the quantum wells and thermally generated carriers tunneling through the superlattice barriers. Given the high aluminum composition in the superlattice barrier layers, x=0.35, considerable indium content (12%) in the quantum well layers, and the effect of the compressive strain lowering the energy of the heavy hole states, the effective barrier seen by the ground heavy hole states was found to be 299 meV; which should suppress the thermionic emission out of the quantum wells. Because of the heavily doped ohmic contact regions, a large triangle potential might be formed which would effectively lower the barrier to thermionic emission and thus results in a higher dark I-V characteristic than expected. Additional contributions to





43


the higher dark current characteristics might also arise from the higher aluminum content used in the superlattice barrier layers, which has been attributed to the formation of DX centers. Figure 4.4 shows the measured dark I-V characteristic with 300 K background photocurrent superimposed.

Figure 4.5(a) shows the measured photoresponse of the p-type SBTM CSL-QWIP. A single peak was found at k,=10.4 pim, which is in good agreement with the theoretically calculated value of 10 pm (see Figure 4.2). With a half-peak value at 12 jpm, we derived a spectral bandwidth of AJX,=20%. This narrow responsivity bandwidth is consistent with that expected from a bound-to-miniband transition scheme. A maximum responsivity of 28 mA/W was found at T=65 K and Vb=+3.0 V. At an operating temperature of 65 K, the noise spectral density was measured as 4.0x10-26 A2/Hz at a bias of 1.0 V. Corresponding to this operating point, the measured responsivity at the 10.4 jpm peak was found to be 13 mA/W. From the above data, the spectral detectivity was then calculated at D'=1.4x109 cm-Hzl/2/W. Note that this is the detectivity achieved by a single pass of the incident radiation through the p-type SBTM CSL-QWIP. The quantum efficiency of this QWIP was found to be 3.8%. If the test structure is altered to include backside thinning and a reflective top contact, then the responsivity will increase substantially. The corresponding detectivity will also increase, since the dark current and the noise spectral density remain constant. The variation of responsivity with applied bias is plotted in Figure 4.3(b). As easily seen, the responsivity and hence the photoconductive gain increase linearly with the applied bias at a fixed operating temperature. A schematic diagram of the experimental setup can be seen in Figure 3.4.

Assuming that six reflections can be achieved before the incident radiation is either completely absorbed or the photon flux reflected back into the QWIP layers becomes insignificant, we determined the following improved performance parameters for the SBTM p-QWIP operating at T=65 K and Vb=1.0 V. First of all, the quantum efficiency





44


increases to 20.7%, which increases the current responsivity to 67 mA/W. The detectivity is also increased by a similar amount to 7.2x 109 cm-Hzla/W.


4.5 Conclusion


We have demonstrated that with the use of compressive strain and superlattice barriers, the step bound-to-miniband transitions can be achieved, which could be useful in creating new narrow bandwidth LWIR p-QWIPs. Given the inherent benefit of normal incidence detection without the use of grating couplers, the simplicity of the p-QWIP design deserves futher investigation. A maximum responsivity of 28 mA/W was found at 10.4 tm, with a corresponding detectivity of 1.4x 109 cm-Hzl2/W. By futher optimizing the quantum well doping density, biaxial strain strength, superlattice barrier parameters, and inserting a triangle potential blocking layer in the ohmic contact regions, high performance LWIR and MWIR p-QWIPs using the SBTM intersubband transition can be developed for focal plane array imaging sensor applications. Additional increases in responsivity and detectivity can be achieved when the substrate is thinned so that a waveguide-like region is formed when multiple reflections can take place, increasing the quantum m efficiency of the device.






45











InGaAs A1GaAs/GaAs ............. HH1
Ef
..... ....... HH2


HH3




HH4 & MB1 HH5




MB2





l:igure 4.1: Schematic energy band diagram for the InGaAs/GaAs/AlGaAs CSI. SBTM p-OWIP.






46













HH4 & MB1 HH6 HH5





HH3




HH2




0 HHI


0 50 100 150 200 250 300

Energy (meV)




Figure 4.2: The calculated transmission coefficients of the InGaAs/GaAs/AIGaAs CSL SBTM p-QWIP using the TMM.





47











GaAs 500 nm Be=5x10'cm3

Alo.3Gao.65As 2.0 nm undoped Repeat x 10 Repeat GaAs 2.7 nm undoped x 20 In0 12Gao.88As 9.0 nm Be=3x10'8cm" Alo.35Gao,65As 2.0 nm undoped Repeat x 10
GaAs 2.7 nm undoped

GaAs 1000 nm Be=5xl0'8cm"3 SI GaAs (100)







Figure 4.3: The complete layer structure of the InGaAs/GaAs/AIGaAs C'SL SBTM pQWIP.





48






10-1

10-2 T=40, 49, 61, 71, 77, 87 K

10-3

10-4 10-5


10-7
10-8

a 10-9 10-10 300 K BG Pho ocurrent

10-11 Device Area = 216x216 um 2
10-12

-3 -2 -1 0 1 2 3 Applied Bias (V)



Figure 4.4: The dark I-V characteristic of the InGaAs/GaAs/AIGaAs CSL SBTM pQWIP with the 300 K background photocurrent suLerimposed.






49 20

T =40 K, V =2.0 V 15




10



5



0 111
8 9 10 11 12 Wavelength (um)

(a) 50

T= 40 K 40

E n




0 20
o TI = 65 K

10



0 1 2 3 4 5 Applied Bias (V)

(b)



Figure 4.5: The measured responsivity (a) as a function of wavelength at T=40 K, V=2.0 V and (b) the variation of peak responsivity as a function of applied bias at T=40, 65 K.













CHAPTER 5
A STACKED COMPRESSIVELY STRAINED P-QWIP WITH TWO-BAND TWOCOLOR DETECTION


5.1 Introduction


Recently, interest in the development and characterization of multi-color or multispectral infrared detectors has grown to the point where many studies have been started [48,54]. In the area of QWIPs, most of the structures in development or under consideration have been n-type QWIPs [48,54]. Little work has been done with the development of multi-color QWIPs using p-type materials that are sensitive to normal incidence IR radiation without the need for complex gratings. Therefore, this chapter is focused on the development and characterization of a stacked compressively strained ptype QWIP with two-band, two-color detection.


5.2 Theoretical Considerations and Device Fabrication


The p-QWIP outlined in this chapter is a multicolor stacked p-QWIP for the MWIR and LWIR two-band detection. Figure 5.1 shows the energy band diagram of a stacked p-type compressively strained layer (PCSL) InGaAs/AlGaAs QWIP for the MWIR and LWIR detection. This multicolor QWIP consists of two distinct multi-quantum well stacks separated by a common ohmic contact layer and sandwiched between he two (top and bottom) ohmic contact layers. This stacked PCSL-QWIP was grown by the MBE technique on SI (100) GaAs substrate. The bottom contact consists of a heavily Bedoped GaAs contact layer. On top of the contact, the A0.3Ga0oAs/In0.2Gao.sAs QWIP layer structure was grown for the MWIR stack. The LWIR QWIP stack was formed by using Be-doped In0.s1Gao.,As quantum wells surrounded with the undoped Alo.Ga0oAs barrier



50






51


layers. The whole stack was sandwiched between two thin Alo.1Gao.,As blocking barriers. Finally, heavily Be-doped GaAs layers were grown for the top and middle ohmic contacts. Both the LWIR and MWIR quantum wells are in biaxial compression. This multicolor stacked QWIP uses the bound-to-quasi-bound (BTQB) intersubband transition scheme for detection of MWIR and LWIR radiation. The complete layer structure of this stacked device is shown in Figure 5.2.

In order to evaluate the performance of the QWIP, a mesa structure with area of 216x216 [tm2 was formed on the MBE grown QWIP wafer by wet chemical etching for radiometric and electrical characterization. A narrow ring of Cr/Au film was deposited by E-beam evaporation to create the ohmic contacts. It is noted that in this type of mesa and ring contact structures, the normal incidence IR radiation is allowed only one pass through the QWIP stack.


5.3 Device Characterization and Results


For this multicolor stacked QWIP, three mesa structures of different thicknesses were etched to allow separate characterization of the LWIR, MWIR, and the combined stacked QWIP devices. The LWIR QWIP mesa structure was formed using the top and middle ohmic contacts, while the MWIR QWIP mesa structure had the top LWIR stack etched away before the mesa formation. The combined stacked QWIP used the top and bottom ohmic layers for contacts.

The device characterization was performed in a closed cycle helium cryogenic dewar. An HP 4145B semiconductor parameter analyzer was used to measure the dark IV characteristics and the 300 K background photocurrent. Under dark conditions, holes can be transfered out of the quantum wells and produce the observed dark current mainly due to two mechanisms: the thermionic emisson out of the quantum wells and thermally generated carriers tunneling through the superlattice barriers.





52


The dark I-V characteristic for the MWIR, LWIR, and the combined stacked QWIP measured at T=77 K is shown in Figure 5.3(a). As expected the dark current of the LWIR QWIP stack is several orders of magnitude higher than the MWIR QWIP stack, due to the exponential dependence of the dark current on the barrier height. As clearly seen in Figure 5.3(a), most of the voltage drop is across the MWIR stack due to the much larger dynamic resistance of the MWIR stack. Figure 5.3(b) shows the measured I-V curves at T=40, 60, and 77 K for the LWIR QWIP stack. The asymmetry in the dark I-V characteristics observed in this device can be attributed to the dopant migration effect [50].

The spectral responsivity for the MWIR QWIP measured at V=5 V and T=77 K is shown in Figure 5.4. The responsivity measurements were performed with the device mounted in a closed cycle helium dewar and illuminated by a blackbody source running through a grating monochrometer and appropriate IR filters. The resulting photocurrent is amplified and detected by a lock-in amplifier. A schematic diagram of the experimental setup can be seen in Figure 3.4. Results of the measurements revealed that two photoresponse peaks were observed in the MWIR band at k,,,n,,,i=4.8 tm and 1,,,,,,,,=5.4 pm. The 4.8 pim peak is in excellent agreement with the ground heavy hole (HHI) to second bound heavy hole (HH3) transition calculated by the TMM, which predicts a detection peak at 4.7 tim. The detectivity for this MWIR peak was determined to be D'=3.3x10" cm-Hz"2/W at V,=1.0 V and T=77 K. A responsivity of 13 mA/W was found at T=77 K, Vh=5 V at this peak. The measured spectral bandwidth for the first MWIR peak was found to be AX/X ,,,,=21% and AX/,,,,=26% for the second peak. The second, longer wavelength peak is attributed to the transition from the HH1 states to the second bound light hole (LH2) states within the quantum well. The calculated responsivity peak for this transition is 5.6 i.m, which is also in good agreement with the measured value. The higher responsivity of the 5.4 ptm peak is attributed to the higher absorption coefficent inherent with the HH to LH transition. The spectral detectivity for





53


this MWIR peak was found to be 5.5x10" cm-Hz"n/W at V,=l.0 V and T=77 K, with a corresponding responsivity of 19 mA/W at the same peak when Vb=5 V.

The responsivity for the LWIR QWIP stack as a function of the wavelength is shown in Figure 5.5. A peak detection wavelength at ?,lw=10.0 jpm was found for the LWIR QWIP device, which is in excellent agreement with the calculated value of 10 tm from the TMM. The maximum responsivity measured at T=40 K and V=2.0 V was found to be 25 mA/W, with a detectivity of 1.1x100 cm-Hz'n/W under the same conditions. It is interesting to note that a very broad response with full width at half maximum bandwidth of 40% was achieved for this device. Figure 5.6 shows the relative spectral response of the combined MWIR and LWIR QWIP stack, displaying one dominant response band at MWIR and two smaller response peaks at LWIR bands.


5.4 Conclusion

We have demonstrated a stacked p-type CSL QWIP design that has the capability to sense infrared photon in both the MWIR and LWIR regimes. This detector exhibited two MWIR peaks at 4.8 and 5.6 pm and a single LWIR peak at 10 im, all of which agree closely to the detection peaks calculated fi-om TMM. By using a stacked design that allows three ohmic contacts, we can simultaneously sense two different colors in two different atmospheric windows in the same FPA. Detectivities of 5.5 x 10" and 1.1 x 10'0 cm-Hz"/2/W were found at 5.4 and 10 jim, respectively with corresponding maximum responsivities of 19 and 25 mA/W at those same wavelengths. This device can be optimized so that the MWIR stack is has a responsivity peak at closer to 4.2 pm by increasing the indium concentration in the quantum well of the MWIR stack and slightly narrowing the quantum well width.





54










A1GaAs
InGaAs
A1GaAs InGaAs Ef HH1
HH1
e...Ef


HH2 HH2




Ev

HH3 Ev

HH3

MWIR LWIR



Figure 5.1: The energy band diagrams and intersubband transition scheme for the twocolor stacked PCSL-InGaAs/AlGaAs QWIP for the MWIR and LWIR detection.





55









GaAs 500 nm Be=5xl0"' cm-3

Alo.Gao.gAs 60 nm undoped

Ino.sGao.ssAs 5.5 nm Be=5x10' cm Repeat

Alo.Gao.,As 50 nm undoped x 20

Alo.,Ga0.9As 10 nm undoped

GaAs 300 nm Be=5xl0'" cm3

Alo.3Gao.7As 45 nm undoped



In.2Gao.sAs 3.3 nm Be=5xl018 cm3 Repeat x 20
Alo.3Gao.As 35 nm undoped Alo.3Gao.As 10 nm undoped

GaAs 500 nm Be=5x10" cm3 SI GaAs (100)





Figure 5.2: The complete layer structure of the stacked CSL p-QWIP.






56



10-1
10-2
LWIR 10-4
10-5
0-6 10 -6 MWIR 10-7


10-9 STACKED 10-10 10-11 T= 77 K 10-12
-5 -3 -1 1 3 5 Applied Bias (V)


(a) 10-2 10-3 T=77 K 10-4 10-5 60 K
i-G 10-6 10-7 C 10-8 40 K
10-9


10-10 10-11 LWIR Device

-5 -3 -1 1 3 5 Applied Bias (V)

(b)




Figure 5.3: The dark I-V characteristics for (a) the stacked, MWIR, and LWIR PCSLQWIP, and (b) the LWIR QWIP for T=40, 60, and 77 K.





57










20

T=77 K, V=5 V 15



10



5



0
3 4 5 6 Wavelength (um)



Figure 5.4: The spectral responsivity versus wavelength for the MWIR PCSL-QWIP, measured at T=77 K and V=5 V. Two response peaks at 4.8 pm and 5.4 ptm were observed for this device.





58






30

T=40 K, V=2 V










20












10

7 8 9 10 11 12

Wavelength (um)



Figure 5.5: The spectral responsivity versus wavelength for the LWIR PCSL-QWIP, measured at T=40 K and V=2 V. One response peak at 10 Vpm was obtained for this device.





59













T=77 K, V=5 V






o 0.5









0
3 4 5 6 7 8 9 10 11 12 Wavelength (um)



Figure 5.6: The relative photoresponse versus wavelength for the combined stacked PCSL-QWIP. Three photoresponse bands were detected in this stacked QWIP.












CHAPTER 6
SUPERLATTICE INFRARED PHOTODETECTORS


6.1 Introduction


With the advances in molecular beam epitaxy (MBE) technologies, device structures using heterostructures or quantum wells have been extensively investigated in the last few decades. N-type quantum well infrared photodetectors (QWIPs) grown by MBE have been extensively studied in recent years [11,12,14]. These devices use either the GaAs based GaAs/AlGaAs or InGaAs/AlGaAs material systems for detection in the 3-5 p.m mid-wavelength infrared (MWIR) or 8-14 ptm long-wavelength infrared (LWIR) atmospheric transmission windows [15,16,55]. Since n-type GaAs/AlGaAs and InGaAs/AlGaAs QWIPs have inherently low electron effective masses and high electron mobilities, they offer excellent infrared (IR) detection properties. However, due to the quantum mechanical selection rules which prohibit normal incidence intersubband absorption, focal plane arrays (FPAs) using n-type QWIPs must use either metal or dielectric gratings to couple normal incidence IR radiation into the quantum well [11,15,16]. In contrast, due to the mixing between the light and heavy hole states under either biaxial tensile or compressive strain, normal incidence absorption is allowed for the intersubband transition in p-type QWIPs; thus eliminating the need for grating couplers and simplifying the design of p-type QWIPs.


6.2 Superlattice Infrared Photodetector Design and Processing


Little work has been done in QWIPs with peak detection wavelengths longer than 16 jim [55,56], since QWIPs operating in the very long-wavelength infrared (VLWIR) band are required to operate at very low temperatures due to higher dark current. Levine,


60





61


et al [56] reported a bound-to-continuum n-QWIP with a detection peak at 16.6 jm and a cut-off wavelength of 19 Jpm. Gunapala and Bandara [55] reported a 16 pLm QWIP focal plane array (FPA) with good imagery. We report a normal incidence p-type strain layer QWIP which utilizes four closely spaced In0.27Ga0.73As/A10.jGaO.85As (thickness: 3.5/3.2 nm) superlattice (SL-) absorber layers to effectively create a large absorption thickness while maximizing the oscillator strength by using the ground heavy hole (HH1) to first excited heavy hole (HH2) state intersubband transition, in contrast with most p-type QWIPs which utilize the HH1I to HH3 intersubband transition for optical absorption [57]. The SL- absorber layers are sandwiched between the wide (50 nm) GaAs barrier layers to reduce the tunneling dark current at the device operation temperatures. The device was formed with three repeats of this basic structure and will be referred to as the superlattice infrared photodetector (SLIP).

Due to strain relaxation [20-22] in the thin SL- layers, the effective barrier height and the corresponding energy spacing between the HH1 and HH2 hole states was reduced from 120 meV for the strained case to 65 meV, which shifts the response peak to a calculated value of 18 pm. The schematic band diagram for the SLIP and the transmission coefficient calculated by the transfer matrix method (TMM) [31] taking into account strain relaxation are shown in Figures 6.1(a) and 6.1(b), respectively. The complete layer structure of the strained p-type SLIP is shown in Figure 6.2.

In order to characterize the device, a wet chemical etch was used to create a 216x216 jm2 mesa structure for the test devices. Cr/Au was used to form the top and bottom ohmic contacts. To facilitate normal incidence illumination, a ring contact around the mesa edge was used to allow light to pass through to most of the mesa top surface with a 75% fill factor. The devices were then bonded onto 68 pin chip carriers and wired to the contact pads via ultrasonic wedge bonding. Moderate background performance measurements were made in a side-looking dewar with a KRS-5 window and are achieved via a 32 mil cooled pin-hole aperture located at a sufficient distance to give a





62


1.780 field-of-view. For all the measurements, the device temperature was maintained within 0.5K of the stated measurement temperature.


6.3 19.2 pm SLIP Characterization and Results Figure 6.3(a) shows the responsivity as a function of incident radiation wavelength and device temperature for V,=20 mV under high flux illumination (FOV>200). The FWHM spectral bandwidth of the 19.2 pm response peak was found to be AX,=12%. This narrow spectral bandwidth is in excellent agreement with the value predicted by the TMM calculation, since the FWHM energy spread of this VLWIR peak is only 8 meV. For the responsivity measurement, a blackbody source at 1243 K was used as the light source into an Oriel MS257 monochrometer with various grating and filters to allow testing from 3-21 p.m. The device under test (DUT) was held at cryogenic temperatures by a open cycle dewar using liquid helium as the coolant. The output from the DUT is then amplified by a Keithley 428 transimpedance amplifier with a variable gain, after which the output is sent to an Oriel Merlin digital signal processing (DSP) lock-in amplifier. The output from the DUT is normalized against a reference sensor (an Oriel 70129 pyroelectric detector) to determine the relative photoresponsivity. The output of the detector at 22 Hz was analyzed with a Stanford Research 770 fast Fourier transform (FFT) network analyzer to determine the integrated optical response. The blackbody is placed as close as possible to the dewar to ensure that the cooled pin-hole aperture indeed limited the signal and that the entire device was illuminated. The gain on the transimpedance amplifier (TIA) was kept low enough to ensure no signal attenuation at 22 Hz due to gain roll-off. For this measurement, the FOV was limited to 1.780 and the chopped blackbody source was set at 800 K. At T=40 K and Vb=20 mV, the peak absolute photoresponse at ,,=19.2 ptm was found to be R;=49.8 mA/W. From this responsivity, the quantum efficiency gain product (rig) was determined to be 0.317%.





63


The variation of peak absolute responsivity as a function of bias and operating temperature is shown in Figure 6.3(b). The decrease in responsivity with applied bias at 50 mV point was attributed to the breakdown of the resonance between the bound HH2 states, which results in the photoexcited heavy holes with a much lower probability of tunneling out of the SL- absorber layers and registering as photocurrent. As shown in Figure 6.3(b), a large photovoltaic (PV) response at zero bias was observed in this device (e.g., RI=39.1 mA/W at T=40 K), which is attributed to the large oscillator strength of the HH1 to HH2 intervalence band optical transition in conjunction with the built-in field from dopant migration.

Figure 6.4 shows the dark current as a function of applied bias voltage measured at 20, 30 and 40 K. The internal current of the device is measured by capping of the device and placing a cold-shield around the device mount. This ensures that the background flux in the dewar is negligible and the only contributions to the dark current are due to the internal mechanisms in the device. The slight asymmetry in the dark current can be attributed to band bending due to the effect of dopant migration [50].

Table 6.1 summarizes the measured responsivity, noise, quantum efficiency gain product (rfg) and calculated detectivitv for different biases and temperatures. For sufficiently high operating temperatures it is possible to measure the noise voltage of the device. The current output of the detector is converted to a voltage using a TIA. The noise voltage is measured using a spectrum analyzer, which measures the power spectral density of the resultant signal at the chopping frequency (22Hz). The increase of responsivity from 0 to 20 mV has been attributed to the offset of the built-in field caused by dopant migration during layer growth [50] and the resonant lining up of the HH2 states to form a miniband. The decrease in the responsivity as the bias is increased from 20 to 50 mV is attributed to the breakdown of the resonant miniband formed by the combination of the superlattice/quantum wells, which causes the tunneling probability of the carriers through the thin superlattice barriers to dramatically increase. Note that these





64


two trends are consistent throughout at each temperature from 20 to 40 K, and that the noise current and hg product also follow the same trend. The calculated detectivities as a function of bias and device temperature are also included in Table 6.1.


6.4 A Voltage Tunable Two-color SLIP


Because of the strain relaxation in the strained superlattice of the 19.2 jpm SLIP, a new unstrained SLIP was grown for fabrication and characterization. This device was found to exhibit true voltage tunability. Typically, methods for achieving multi-color or multi-band detection in QWIPs have ranged from the use of stacks of different QWIP layers to increase the number of wavelengths or wavebands which the pixel is sensitive [48,54,58] to a voltage tuning scheme which shifts the detection peak around in a single waveband as a function of applied bias [59,60]. We have designed and characterized a novel p-type superlattice infrared photodetector which maximizes the flexibility of the QWIP design by exhibiting normal incidence detection and true voltage tuning, where the changing applied bias changes the detection peak by allowing one peak and suppressing another. Uses for such devices range from single pixel two color imaging to two color temperature resolution.


6.5 Layer Structure and Fabrication of the Unstrained Voltage Tunable SLIP


This SLIP was designed with GaAs (3.0 nm) quantum wells and Al0.4Gao.6As (3.5 nm) superlattice barriers, as seen schematically in Figure 6.5(a). The thick (50 nm) barriers separating the sets of four GaAs quantum wells are also grown with Al0o3Gao.7As. Figures 6.5(b) and 6.5(c) illustrate the two intersubband transitions and how the applied bias is used to tune the detection peak. Note that the 9.2 jtm peak can be seen only when the superlattice miniband is resonantly lined up at moderate biases, while at higher applied biases, the breakdown of the miniband resonance limited the detection at this





65


peak. From the transfer matrix method, we calculated two responsivity peaks at 9.6 and 6.4 [tm, which is in excellent agreement with the measured results. The complete layer structure of this device is illustrated in Figure 6.6.

In order to electrically and optically characterize this device, a wet chemical etch of 8:1:1 H20:H202:H3PO4 was used to fabricate 216x216 plm2 mesas. Then, Cr/Au (100 A/1500 A) was deposited on top of and around the mesas to form the top and bottom ohmic contacts, respectively. Since these devices respond to normal incidence radiation, a ring contact around the mesa edge was used to allow light to pass through to most of the mesa top surface, which simplifies the processing requirements while still maintaining a 75% fill factor. The chips were then bonded onto 68 pin chip carriers and wired to the contact pads via ultrasonic wedge bonding. For cryogenic testing, the chips were mounted into open cycle dewars which were capable of being mounted with KRS-5 windows and a variety of apertures to limit the field of view (FOV) from 400 to 00.


6.6 Unstrained Voltage Tunable SLIP Characterization and Results

Figure 6.7 shows the responsivity of the p-type SLIP as a function of incident radiation wavelength and negative applied bias. For the measurement of the responsivity. a blackbody source at 1243 K is used as the light source into a 'A m Oriel MS257 monochrometer with various grating and filters to allow testing from 3-21 tm. The DUT is held at cryogenic temperatures by an open cycle dewar using liquid helium as the coolant. The output from the DUT is then amplified by a Keithley 428 TIA with a variable gain, after which the output is sent to an Oriel Merlin DSP lock-in amplifier. The output from the DUT is normalized against a reference sensor (an Oriel 70129 pyroelectric detector) to determine the relative photoresponsivity. Then, the output of the detector at a specific chopped frequency was analyzed with a Stanford Research 770 FFT network analyzer to determine the integrated optical response. For this measurement, the





66


field of view was limited to 1.780 and the chopped blackbody source was set at 800 K. As the bias is increased from 0 to -50 mV, the response of the LWIR peak (9.3 [tm) rapidly increases, while the 6.5 jpm peak is effectively suppressed. As the bias is increased to -100 mV and greater, the superlattice miniband on which the photoexcited LWIR hole transport depends, loses resonance which causes the loss of responsivity for the 9.2 p.m peak. This effect gives rise to the voltage tuning capability of this SLIP structure. Note that at these higher applied biases, the 6.5 pm peak dominates and saturates at Vb,-200 mV with a maximum absolute responsivity of 8 mA/W. The lack of LWIR photoresponse in the positive bias regime can be attributed to the built-in field in the superlattice region arising from dopant migration [50], which causes the breakdown of the miniband resonance when positive bias is applied and can be seen in Figure 6.8.

Figure 6.9 shows the measured dark current as a function of temperature and applied bias for the p-type SLIP. Overlaid on top of the dark I-V curves is the FOV limited (1.780) background photocurrent. This shows that the device is under background limited performance (BLIP) operation at T=35 K or lower under this narrow field of view for a broad range of applied bias. The slight asymmetry in the dark current can be attributed to band bending as an effect of dopant migration [50]. since the mobile p-type dopant beryllium was used in the quantum well layers.


6.7 Conclusion


In conclusion, we have demonstrated two novel p-type SLIPs which exhibits a peak detection wavelength at 19.2 lpm and voltage tunability in the LWIR band. Operation up to 40 K was obtained for both the photoconductive (PC) and photovoltaic

(PV) modes detection for the VLWIR SLIP. An absolute responsivity of 49.8 mA/W and an rlg=0.317% were achieved at T=40 K and V,=20 mV with an FWHM spectral bandwidth of AX/X =12%. Further refinements can be made to this structure to tailor the
P





67


peak responsivity wavelength and FWHM spectral bandwidths by altering the layer composition and material thicknesses. In addition, one can use a slightly strained quantum well layer (<8% Indium) to reduce the effective mass of the ground heavy hole states [13], increasing heavy hole mobility, which in turn will increase the intersubband absorption and improves the transport characteristics of the device. We have designed, processed and demonstrated a new novel p-type QWIP which exhibits true voltage tunability in the LWIR band. Further improvements in this device can be made in terms of increasing responsivity by shortening the detection wavelength of the bound-tocontinuum peak by means of increasing the superlattice barrier height, which also should lower the effective dark current by decreasing the tunneling probability through the superlattice slightly.





68






In0.2ao.73AS
HH1
Ef
GaAs
E,



A10.15Ga.85As
(a)








0 20 40 60 80 100 120 140 160 Energy (lmeCV)
(b)


Figure 6.1: The (a) idealized band diagram and (b) the calculated transmission coefficient from TMM for the p-type SLIP.





69













GaAs 300 nm Be=5x0"' cm3

GaAs 65 nm undoped

A1l0.sGao.ssAs 3.5 nm undoped Repeat

InO.27Gao.73As 3.2 nm Be=3xl018 cm3 4 Repeat

Al0o.sGao.ssAs 3.5 nm undoped x 3

GaAs 50 nm undoped GaAs 15 nm undoped
GaAs 500 nm Be=4x10'" cm3 SI GaAs (100)



Figure 6.2: The complete layer structure of the p-type SLIP.





70









60
Vb = 20 mV, normal incidence 40K





20 40



12 14 16 18 20 Wavelength (pm)

(a)


60

50

E 40

30
0
20 K 10 2 30 K 40 K

0 10 20 30 40 50 60 Applied Bias (mV)
(b)



Figure 6.3: (a) The absolute responsivity as a function of p-type SLIP detector temperature at V,=20 K and (b) the responsivity as a function of applied bias and device temperature.






71
















10

40 K

30K
O 10
b 20K



10

10.c
-0.5 0.0 0.5 Voltage Bias (V)



Figure 6.4: The dark I-V characteristic of the p-type QWIP as a function of applied bias and device temperature.





72










Table 6.1: Summary of the SLIP performance as a function of applied bias and device temperature.


Temp. & Bias RA (mA/W) noise (AIHz"2) 71g (%) D* (cm-Hz'2/W)

20 K
0 V 5.6 1.36 xl0I" 0.035 8.86 x 106
20 mV 29.7 2.47 x 10l" 0.192 2.58 x 10' 50 mV 22.1 2.51 x 10-" 0.142 1.90 x 107

30K
0 V 12.4 3.46 x 10-" 0.079 7.78 x 106
20 mV 24.4 3.89 x 10-" 0.157 1.33 x 107 50 mV 22.6 4.03 x 10~" 0.142 1.20 x 107

40 K
0 V 39.1 3.96 x 10' 0.249 2.13 x 107
20 mV 49.8 5.83 x 10-' 0.317 1.84 x 10 50 mV 41.3 6.90 x 10 0.263 1.29 x 107





73




GaAs

HHI
.. .. . .. .. . ....E




EV A10.3Ga 0.SI 11111 HH2


Al 0.4Ga 0.6As Continuum
(a)



















(b) (c)



Figure 6.5: The (a) schematic energy band diagram for the unstrained SL-QWIP and the transport mechanisms for the SL-QWIP at (b) high and (c) moderate biases.





74













GaAs 300 nm Be=4x10' cm-3

Alo.3Gao.,As 100 nm undoped

Alo.4Gao.6As 3.5 nm undoped Repeat

GaAs 3.0 nm Be=2xl0'" cm"3 Repeat

Al0.4Gao.6As 3.5 nm undoped X10

Al0.3Gao.7As 60 nm undoped Al0o3Gao.As 40 nm undoped

GaAs 500 nm Be=4xl08 cm-3 SI GaAs (100)



Figure 6.6: The complete layer structure of the unstrained p-type SLIP.





75









T=20 K Normal Incidence

6


5

V = -50 mV
b








1
-200 mV
0
3 5 7 9 11 13
Wavelength (tm)



Figure 6.7: Relative photoresponse of the unstrained SL-QWIP as a function of applied reverse bias at T=20 K.





76







1.4
0.3 V T=20K

1.2 Normal Incidence


1.0
1.0- 0.2 V

0.8


0.6

0.1 V
0.4

0.2


0.0
3 5 7 9 11 13 Wavelength (tim)



Figure 6.8: The relative photoresponse of the unstrained SL-QWIP as a function of applied forward bias at T=20 K.






77








10-2

10 j35 K BG 1.780 FOV
-4
10
-5 "- 30 K
106 10
los 20 K


Q 109 10-1
-109

10-11 10-12

10-13
1 0 3 = = = l * I I I*
-2 -1 0 1 2 Bias (V)



Figure 6.9: I)ark current as a function of device temperature with the 300 K background photocurrent superimposed (FOV=1.780).













CHAPTER 7
TENSILE STRAINED QUANTUM WELL INFRARED PHOTODETECTORS


7.1 Introduction


Over the last decade, a significant amount of research in the area of p-type QWIPs has been centered on compressively strained designs [47]. While some work has explored the realm of tensile strained (TS-) p-QWIPs, most of the work has been rather disappointing because of the difficulties with cross-hatching and other problems that crop up when growing highly tensile strained epitaxial layers [47,61]. We have designed a TS-QWIP that is slightly strained in the quantum well and unstrained in the lattice matched barrier, to try to minimize the cross-hatching in such devices and to demonstrate that the InP base TS-QWIPs are able to survive many thermal cycles without degradation.

The motivation for using a tensile strained quantum well is because of the theoretical improvement in linear absorption coefficient that can be achieved with the light-hole carrier. As mentioned earlier, when coherent tensile strain in applied in a quantum well, the excess energy inherent in the quantum well becomes expressed as an energetic inversion of the heavy- and light-hole subbands in the valence band along with a reduction of the conduction band minimum. Now, when the quantum well is heavily pdoped, with beryllium for example, the ground state is completely occupied and the carriers are available for intersubband transitions. Since the ground holes are now lightholes, and typically the light-holes have an effective mass that is about an order of magnitude less than the effective mass of heavy-holes, we expect the linear absorption coefficient to be increased over an unstrained QWIP with the same infrared response peak because of the inverse relationship between the effective mass and the linear absorption coefficient. Because of the theoretical increase in the absorption coefficient, we also


78





79


expect and increase in the quantum efficiency and the absolute responsivity; both of which are directly related to the linear absorption coefficient.


7.2 Device Layer Structure and Processing


The p-type TS-QWIP consists of 20 In0.4Gao.6As quantum wells, 70 A thick sandwiched by 500 A thick undoped In0.52Gao.4gAs barriers. The quantum wells are Bedoped to a density of 3x10'8 cm-3. This multiquantum well structure is then surrounded by a top and bottom ohmic contact layer, 0.3 and 0.5 lpm thick, respectively. In between the highly doped ohmic contacts and the multiquantum well structure are extra undoped In0.52A10.48As layers 150 A thick which act as blocking barriers to reduce the triangle potential formed by the ohmic contacts. The ohmic contact layers are lattice matched (to InP) In0.52A10.48As layers Be-doped to a density of 5 x 1018 cm-'. All of the layers are grown on semi-insulating (100) InP. The complete layer structure for this device is shown in Figure 7.1.

In order to characterize the devices, a wet chemical etch was used to create a 216x216 pLm2 mesa structure for the both test devices. Cr/Au was used to form the top and bottom ohmic contacts. To facilitate normal incidence illumination. a ring contact around the mesa edge was used to allow light to pass through to most of the mesa top surface with a 75% fill factor. The devices were then bonded onto 16 pin chip carriers (TO-8 cans) and wired to the contact pads via ultrasonic wedge bonding.


7.3 Device Characterization

QWIP-D is sensitive in the MWIR range at 5.1 Lm, with a estimated detectivity of 1.1 x 1010 cm-Hz"2/W under a bias of 2 V at T=77 K. This is about 10% of the theoretical maximum D* at this peak wavelength for a photoconductive device. As seen in Figure 7.2, the intersubband transition occurs from the heavily doped ground light-hole state to





80


the extended continuum state. The responsivity curve for this particular device is shown in Figure 7.3. For the responsivity curves, the DUT is at 77 K with a field of view of 1800. This device was tested in a closed cycle cryogenic dewar and illuminated by a blackbody source running through a 1/8m grating monochrometer. The output of the DUT was amplified by a biasing TIA and sensed with a Stanford Research 830 lock-in amplifier. The data from the lock-in amplifier was then compared against a standard pyroelectric detector to normalize and scale the data to obtain the absolute photoresponse. A schematic diagram of the experimental setup is shown in Figure 3.4 The data shows a rather broad MWIR peak with an FWHM bandwidth of A?/JX=37%, which is typical for p-type strained-layer QWIPs [61].

As expected for an MWIR device, the dark current for this device is rather low, as we can see in Figure 7.4. The large asymmetry seen in the I-V curve can be attributed to the dopant migration effect which occurs during layer growth [50]. This effect was also seen in previous p-type tensile strained devices [47].


7.4 Conclusion

We have characterized an InGaAs/lnAlAs on InP TS-QWIP with a peak responsivity at 5.1 [tm. The detector was found to be a stable tensile strained p-type quantum well infrared photodetector, which exhibited an intersubband transition from a light-hole ground state. The performance of the TS-QWIP was moderate, with a detectivity of 1.1xl0'0 cm-Hz"/2/W at T=77 K, V,=2.0 V at the MWIR peak of 5.1 rtm. This device also had an FWHM bandwidth of AL/,=37%. This device did not exhibit the typical visible crosshatching that had been previously reported for similar InGaAs/InAlAs on InP p-QWIPs [47], which should improve reliability of InP based pQWIPs. Further work into this type of QWIP can be performed, and improvements can be made by shifting the detection peak towards the LWIR band and by changing the






81


intersubband transition mode from a bound-to-continuum to a bound-to-quasi-bound, in order to improve responsivity and lower dark current, thereby increasing detectivity.






82














In.52A1 0.48As 300 nm p=5x10'8 cm3

Ino.52A1 0.48As 15 nm undoped

In.52A1 0.48As 50 nm undoped Repeat

Ino.4Gao.6As 7.0 nm p=3x10' cm3 x20

In.52A1 0.48As 65 nm undoped

In.52A1 0.48As 500 nm p=5x10'8 cnr3 SI (100) InP




Fiure 7. 1: The complete layer structure of the p-type MWIR TS-QWIP.






83















Ino.4Gao.6As Ino.52Al0.48As Ino.4Gao.6As


LHI HHI HH2 LH2 HH3




Figure 7.2: The schematic energy band diagram of the p-type MWIR TS-QWIP.





84









3

T=77 K
3.0 V=3 V

2.

E
2.





0.
0.


3.5 4.0 4.5 5.0 5.5 6.0 Wavelength (um)



Figure 7.3: The measured photoresponse of the MWIR p-type TIS-QWIP as a function ol applied bias and incident IR radiation wavelength.





85









10-3

10-4 Dark Current @ 77 K
10-5
10-6

S10-7
10-8
4 10-9
10-10 10-11

10-12
-5 -3 -1 1 3 5 Applied Bias (V)



Figure 7.4: Thec measured dark current of the p-type MWIR TS-QWIP at T=77 K.













CHAPTER 8
BROADBAND QUANTUM WELL INFRARED PHOTODETECTORS


8.1 Introduction

Given the intense study over the last decade into the physics and operational characteristics of quantum well infrared photodetectors, many types of devices with different performance characteristics have been designed and tested. Most of these designs have concentrated on the LWIR or MWIR bands with rather narrow bandwidths, typically AXAX,<20% for n-type QWIPs, and A-30% for p-type QWIPs [12]. But some applications, such as Fourier Transform Infrared Spectroscopy (FTIR), demand simple, robust yet wideband infrared detection capabilities. A considerable amount of time and effort has been recently spent on the development of multicolor infrared photodetectors based on quantum well infrared photodetectors (QWIPs)[54,59,61]. Given the large flexibility provided by III-V materials, such as GaAs, InGaAs, AlGaAs, InP and InAlAs, we can create QWIPs that cover much more of the LWIR and MWIR bands, simultaneously. This chapter will cover the two QWIPs (one n-type and one ptype) designed to exhibit broadband LWIR infrared detection.

Most devices have concentrated on using specific QWIP layers to sense more than one IR wavelength. These devices have been composed of various QWIP layers with different detection peaks in series; which are sometimes separated by ohmic contact layers[54,61]. In contrast, our new broadband (BB-) QWIP designs seek to sense the whole LWIR band simultaneously. This is achieved by using three or four quantum wells that have varying thickness or composition as a unit cell and repeating the unit cell to make up the BB-QWIP.




86






87


8.2 Layer Composition and Device Processing The three well n-type BB-QWIP consists of an In0.2Gao.sAs quantum well 6.5 nm thick, an Ino.s1GaO.85As quantum well 6.5 nm thick and an Ino.Gao.,As quantum well 7.0 nm thick each separated by a 45 nm thick undoped A10.07Gao.93As barrier. Each quantum well is Si doped to 7x10'7 cm"-. The complete unit cell of three quantum wells and barriers is then repeated 20 times and surrounded by an ohmic cap layer 300 nm thick of GaAs (Si doped to 3x10'8 cm"3) and a bottom buffer layer of similarly doped GaAs 500 nm thick. The complete layer structure and schematic band diagram for this QWIP is shown in Figures 8.1(a) and 8.1(b), respectively.

Figures 8.2(a) and 8.2(b) show the schematic energy band diagram and complete layer structure of the four well n-type BB-QWIP. This four well n-type BB-QWIP design consists of an Ino.3Gao.7As quantum well 6.5 nm thick, an In0.25Gao.75As quantum well 6.5 nm thick, an Ino.2Gao.sAs quantum well 7.5 nm thick and an Ino.17Gao.ssAs quantum well 8.5 nm thick all separated by 45 nm thick undoped barriers of GaAs. The quantum wells are doped with Si to a density of 7x1017 cm"3, and the complete structure is then surrounded by extra undoped GaAs barriers 35 nm thick (to form an 80 nm thick blocking barrier) and 0.3 tpm and 0.5 p mt thick GaAs ohmic contacts doped with Si to 3x108 cmn The four quantum well and barrier unit was repeated 20 times to create the whole stack.

While the variable composition p-type BB-QWIP is very similar in operating theory, the materials and layer thickness of the quantum wells and barriers vary. The ptype BB-QWIP consists of an Ino.3Gao.As quantum well 5.0 nm thick, an Ino.25Gao.75As quantum well 5.5 nm thick and an In02Gao.sAs quantum well 6.0 nm thick each separated by a 40 nm thick undoped GaAs barrier. Each quantum well is Be-doped to 4x10'8 cm3. The complete unit cell of three quantum wells and barriers is the repeated 20 times and surrounded by an ohmic cap layer 300 nm thick of Be-doped GaAs (p=4xl0'8 cm3) and a





88


bottom buffer layer of similarly doped GaAs 500 nm thick. The complete layer structure for both devices is shown in Figures 8.3(a) and 8.3(b).

Unlike the n-type devices, we were able to design another three well p-type BBQWIP was grown to explore the effects of just varying the quantum well layer thickness in the device design. This variable thickness p-type BB-QWIP design consists of three quantum wells 4.5, 5.5 and 6.2 nm thick Ino.25Gao75As Be doped to 4x10'8 cm"-. These were separated by 40 nm thick undoped GaAs barriers and then the whole unit was repeated 20 times. The whole stack was then surrounded by 0.3 Plm and 0.5 pm thick GaAs ohmic contacts Be doped to 4x10'8 cm"-. The complete layer structure and schematic energy band diagram is shown in Figure 8.4(a) and 8.4(b) respectively.

In order to characterize the devices, a wet chemical etch was used to create a 216x216 [pm2 mesa structure for the both test devices. Cr/Au was used to form the top and bottom ohmic contacts for the p-type BB-QWIP, while AuGe/Ni/Au annealed at 4500C for two minutes was used as the top and bottom ohmic contacts for the n-type BBQWIP. To facilitate normal incidence illumination for both the p-type and n-type BBQWIPs, a ring contact around the mesa edge was used to allow light to pass through to most of the mesa top surface with a 75% fill factor. A 450 polished edge was also ground into the n-type BB-QWIP so that 450 incident IR radiation could be used to test the device. Both devices were mounted on TO-8 chip carriers and then wire bonds were attached ultrasonically for electrical connection.


8.3 Characterization Results


First, we will discuss the results of the three well n-type BB-QWIP, followed by the four well n-type BB-QWIP. The next subsection will cover the two p-type BB-QWIPs in this order, the variable composition three well p-type BB-QWIP and then the variable thickness p-type BB-QWIP. The responsivity was measured using a blackbody source





89


set at 1273 K, running through a 1/8m monochrometer with the appropriate filters as the source of IR radiation, which is also chopped at a set frequency. The device is set in a closed cycle liquid helium dewar and electrical leads are attached. The output of the device is then amplified by a transimpedance amplifier (TIA) with a gain of 106 V/A. The output of the TIA is then sent to a lock-in amplifier to determine the phase and magnitude of the output signal. The relative responsivity curve is then normalized against the response of a pyroelectric detector to determine the absolute responsivity. A schematic diagram of the experimental setup can be seen in Figure 3.4.


8.3.1 N-type broadband QWIPs


As shown in Figure 8.5, the three well n-type BB-QWIP exhibits a large responsivity peak at 10 p.m. This is in excellent agreement to the peak estimated by TMM [31] of 9.7 p.m. As seen in this figure, as the bias is increased, the absolute responsivity also increases rapidly; with a maximum responsivity value of 1.90 A/W achieved at T=40 K, Vh=-6 V when the device is illuminated from the 450 facet. It is also interesting to note that when the applied bias is negative, the bandwidth of the responsivity curve increases. For example, the fuill-width half-maximum (FWHM) spectral bandwidth of the device at Vi,=+4 V is A2/ ,,=13%, while the bandwidth at V,=-4 V is A/k,,=18%.

Figure 8.6 shows the responsivity of the three well n-type BB-QWIP at lower applied biases. Here we see that the bandwidth of the device at IV 61<2 V, is very broad with a maximum value of A,/X=21% at V,=-2 V, with a reasonable responsivity of 58 mA/W at 10 pm. It is interesting to note that the relative responsivity of the shorter wavelength quantum well and the longer wavelength quantum well (versus the 10 p.m quantum well) is a larger proportion of the peak responsivity at the low bias levels than at higher (e.g., Vb>4 V) biases. Also note the very broad responsivity curve of the three well





90


n-type BB-QWIP at Vh=-l V. The calculated spectral bandwidth at Vb=-1 V is AX/XP=40%. The responsivity at ,P=10 tm is 5 mA/W. But because of the very low dark current (Idk=50 nA) at this bias, the detectivity is expected to be on the order of 1010 cmHz"2/W. A normal incidence response of approximately 50% of the 450 value was found for this device under all of the biases tested.

Figure 8.7 shows the dark current as a function of applied bias and device temperature, with the 300 K background photocurrent at a field-of-view of 1800 superimposed. Note that at a temperature of 60 K the device is background limited when the applied bias is between +5V, while at a device temperature of 70 K, the n-type BBQWIP is background limited for -1 V
Using the results of the responsivity and dark I-V measurements, the detectivity of the three well n-type BB-QWIP can be estimated using a photoconductive gain of 0.1. This results in a measured D* of 1.11xl10'0 cm-Hz"'/W at Vh=-2 V at T=60 K. Under a higher applied bias of Vh=-4 V, D* was found to be 3.12x100 cm-Hz'/W with a gain estimated at 0.15. Under normal incidence, the detectivity was found to be roughly half the 450 value or 5.2x10' and 1.5x10"' cm-Hz' /W at T=50 K and V,,=-2, -4 V respectively.

When observing Figures 8.8 and 8.9, the most striking feature of the results of the four well n-type BB-QWIP is that the FWHM bandwidth is significantly increased. As seen in Figure 8.8, the maximum responsivity of the four well n-type BB-QWIP is achieved at 1,=10.3 tim. When the device is at T=40 K and Vh=+4.5 V, a maximum responsivity of 2.32 A/W is found, with a corresponding FWHM bandwidth of A,/?,,=18%. This value is a little less than 50% higher than that achieved for the three well case under similar biasing conditions. It is interesting to note that the shape of the four well n-type BB-QWIP responsivity curve under positive bias is very similar to that of the three well n-type BB-QWIP under negative bias. In Figure 8.9, we find that A?/ ,,=29% with a k,=10.7 gm at V,=-4 V, which is red-shifted from the lower negative applied biases and the positive applied biases. The spectral bandwidth achieved is almost





91


50% more than that of the three well n-type BB-QWIP under the same operating conditions. It is also interesting to note the increased prominence of the 12 gtm peak at higher negative biases, which significantly contributes to the flatness of the responsivity curve at Vh,:-4 V. As shown in Figure 8.11, we also a normal incidence response of this device of 50% or more of the 45 degree values throughout the active bias range.

Figure 8.12 shows the measured dark I-V characteristic of the four well n-type BBQWIP. Note the almost complete symmetry between the negative and positive bias regimes. The larger dark current of the four well device when compared to the three well device is attributed to the lower barrier height of the longest wavelength well in the four well n-type BB-QWIP, when compared with the longest wavelength well of the three well BB-QWIP. Because of the extended responsivity of this device, when compared with the three well n-type BB-QWIP, the BLIP operating temperature is predicted to be in the 50 K range. Using the results of the dark I-V and responsivity measurements, the estimated D* of 1.65x10"' cm-Hz"2/W was found at T=50 K and V,=-2 V, using a gain of 0.1 for the four well n-type BB-QWIP. When an applied bias of -4 V and a gain of 0.15 is used, the detectivity of this device was found to be 2.34x10'" cm-Hz"2/W at T=50 K. Once again, the normal incidence responsivity was found to be about half that of the 450 incidence value or 8.1xl0' and 1.2x10' cm-Hz'/W at T=50 K and V,,=-2, -4 V, respectively.


8.3.2 P-type broadband QWIPs

Next, we will discuss the results of the measurement of the variable composition ptype BB-QWIP. Figure 8.12 shows the absolute responsivity as a function of applied bias and incident IR radiation wavelength. This p-type BB-QWIP has a response peak at 9.3 utm, with a maximum responsivity of 19 mA/W at T=40 K and V,=-1.5 V. Also under the previously stated operating conditions, the FWHM spectral bandwidth is Ak/,=48%.





92


The half peak range is from 7 to 11.2 p.m. The peak responsivity is seen to increase linearly with applied bias in the negative bias regime. In contrast to the responsivity curve exhibited by either n-type BB-QWIP, the p-type BB-QWIP does not seem to have a large variation of spectral bandwidth as a function of applied bias; since the bandwidth of the device at low (Vb=-0.5 V) biases is also AX/,=48%. But this device does not also exhibit the massive increase in responsivity at high biases like either n-type BB-QWIP. In the case of the p-type BB-QWIPs, all of the responsivity measurements are illuminated through the top ring contact for normal incidence, and the IR radiation is considered to only pass through the layer structure once. As seen in Figure 8.13, the dark current increases as the device temperature is increased.

Using the results of the responsivity and dark I-V measurements, we were able to estimate the detectivity of the variable composition p-type BB-QWIP by estimating a gain of 0.02. This gave us values of D* equal to 3.08x109 and 3.63x109 cm-Hz"a/W at T=50 K and V,=-1.0 and -1.5 V respectively. Note that this calculated detectivity is only for a single pass of radiation through the multi-quantum well stack.

In Figures 8.14(a) and 8.14(b) we see the variation of responsivity as a function of applied bias and incident radiation wavelength at 7'40 K for the variable thickness ptype BB-QWIP. This p-type BB-QWIP has a response peak at 1,=9.6 ptm with a corresponding FWHM bandwidth of AX?,/,=63% under all applied biases from 6.5 to 12.5 p.m. A maximum responsivity of 25 mA/W was obtained under normal incidence single pass illumination at V,=+l.1 V. As also seen in these figures, there is a slight PV response with a peak at 10.2 p.m. Figure 8.15 shows the dark I-V characteristic of the variable thickness p-type BB-QWIP as a function of device temperature. Note the dark current asymmetry evident, which has been attributed to dopant (Be) migration. When comparing the two p-type BB-QWIPs, we see that variable thickness BB-QWIP has higher responsivity with a longer peak wavelength and a significantly larger FWHM spectral bandwidth.





93


Again using the results of the responsivity and dark I-V measurements, and an estimated gain of 0.02, the calculated detectivity of the variable thickness p-type BBQWIP was found to be 7.68x109 and 9.54x109 cm-Hz/2/W at T=50 K and Vb=+1.0 and +1.1 V, respectively. Note that this detectivity result is achieved when the incident IR radiation is passed through the multi-quantum well stack only one time.


8.4 Discussion


Broadband detection by means of quantum well infrared photodetectors has been demonstrated for both p- and n-type devices. When comparing the relative strengths and weaknesses of each BB-QWIP, one striking fact is obvious. The n-type BB-QWIPs have a much higher responsivity whereas the p-type BB-QWIPs have much larger spectral bandwidths. When comparing the four devices, we see that if the responsivity bandwidth is the dominant factor then either p-type BB-QWIP would be the detector of choice, with the variable thickness p-type BB-QWIP the design that has the largest bandwidth, the higher responsivity and detectivity. But if the absolute responsivity was the most important characteristic, then the obvious choice would be the four well n-type BBQWIP. since it exhibits the highest detectivity and responsivity of all four designs, while still having a relatively wide spectral response when under negative bias.

One possible way of extending the bandwidth of the n-type BB-QWIPs is by tailoring a grating which enhances the shorter wavelength regime of the LWIR band and patterning this on to the n-type BB-QWIP mesa. This has been proposed by Gunapala et al. and seems like a viable method for broadening the responsivity of the n-type QWIPs. If we seek to increase the responsivity, and thus the detectivity of either the n- or p-type BB-QWIPs, we can use a combination of backside thinning to create a waveguide-like structure that traps a significant part of the IR radiation in the BB-QWIP stack so that multiple passes of the light can be absorbed, which increases the quantum efficiency. By




Full Text
81
intersubband transition mode from a bound-to-continuum to a bound-to-quasi-bound, in
order to improve responsivity and lower dark current, thereby increasing detectivity.


CHAPTER 8
BROADBAND QUANTUM WELL INFRARED PHOTODETECTORS
8.1 Introduction
Given the intense study over the last decade into the physics and operational
characteristics of quantum well infrared photodetectors, many types of devices with
different performance characteristics have been designed and tested. Most of these
designs have concentrated on the LWIR or MWIR bands with rather narrow bandwidths,
typically AAA/,<20% for n-type QWIPs, and AAA^-30% for p-type QWIPs [12]. But
some applications, such as Fourier Transform Infrared Spectroscopy (FTIR), demand
simple, robust yet wideband infrared detection capabilities. A considerable amount of
time and effort has been recently spent on the development of multicolor infrared
photodetectors based on quantum well infrared photodetectors (QWIPs)[54,59,61],
Given the large flexibility provided by III-V materials, such as GaAs, InGaAs, AlGaAs,
InP and InAlAs, we can create QWIPs that cover much more of the LWIR and MWIR
bands, simultaneously. This chapter will cover the two QWIPs (one n-type and one p-
type) designed to exhibit broadband LWIR infrared detection.
Most devices have concentrated on using specific QWIP layers to sense more than
one IR wavelength. These devices have been composed of various QWIP layers with
different detection peaks in series; which are sometimes separated by ohmic contact
layers[54,61]. In contrast, our new broadband (BB-) QWIP designs seek to sense the
whole LWIR band simultaneously. This is achieved by using three or four quantum wells
that have varying thickness or composition as a unit cell and repeating the unit cell to
make up the BB-QWIP.
86


18
h/xifi where xif is the lifetime between the initial, and final, / states. 8 P,7 are the
optical transition elements between the quantum well valence subband ground states, /,
and the continuum subband states,/ in the HH, LH, and SO bands; which can be derived
from the two 3x3 kp matrix elements as shown below.
Using the following 3x3 optical matrix,
h
T T T
1HH 1HL 1HS
Tlh Tll Tls
Tsl Tsl Tss
(2.27)
the optical matrix elements, e -P(/, can be obtained. These matrix elements have the
same form as the k-p matrix elements except that the k-kjs are replaced with k,zJJrkJzi
multiplied by a constant factor of mjh [39]. The T0's are defined as follows:
Tm = 2(A B)szkz+(2A + B)(exkx+eyky), (2.28)
Tll = 2(A + B)ezkz +(2A B)(exkx-eyky), (2.29)
Tss =2 A{exkx + zyky +£/.), (2.30)
Th, = i-^N{ex cosp ev sinr|)&_, i Nezkl{
(2.31)
T =
* LS
- j3B¡zxkx -
-e Ajeos x
+ 7fM8v^,
V J
+ £1/v)sinx
1 l
AMsv cosq
6 '
- 8,. sinrijA:. -
1
- Ne
6
+ ij6B¡Exkx
-s^Jcosx
+ j6N(Ek
+ 8v^)sinx
i2s2Be, +
Nexcps(x
-fi)-
-is[2B[ exkx
Ne:kt
(2.32)
T -T
1 SH 1 HS
T = T'
1 si. 11.S
T = T
11.H 1 HI. '
(2.33)
(2.34)
(2.35)
(2.36)


CHAPTER 4
A COMPRESSIVELY STRAINED-LAYER P-TYPEINGAAS/ALGAAS/GAAS STEP
BOUND TO MINIBAND QWIP AT 10.4 pm
4.1 Introduction
In recent years, n-type quantum well infrared photodetectors (n-QWIPs) have been
extensively investigated using III-V semiconductor material systems. These QWIPs have
utilized the bound-to-continuum (BTC), bound-to-miniband (BTM) and step bound-to-
miniband (SBTM) transition schemes to achieve detection in the 8-14 pm long-
wavelength infrared (LWIR) band with reasonable detectivities and dark current
[52,12,53]. Unlike n-QWIPs, which are forbidden to absorb normal incidence infrared
radiation due to the quantum mechanical selection rule, p-type QWIPs exhibit normal
incidence intersubband absorption because of the mixing between the off-zone center
(i.e., k^O) heavy hole and light hole states [37], Because of the larger hole effective mass
and hence lower hole mobilities, especially for the heavy holes, the absorption
coefficient, quantum efficiency and spectral performance of the p-QWIPs are generally
lower than n-QWIPs [28,12], However, when compressive strain is introduced into the
quantum well layers, the hole effective mass and in-plane density of states decreases [13].
Thus, more free holes will reside in higher energy states, which implies an elevation of
the Fermi level. The elevation of the Fermi level increases the number of off-zone center
holes (k*0), which increases the magnitude of the off-diagonal matrix elements, which in
turn increases the absorption coefficient and the associated performance parameters [28].
We report a new p-type SBTM CSL-QWIP grown on a semi-insulating (SI) (100)
GaAs by molecular beam epitaxy (MBE) using the InGaAs/AlGaAs/GaAs material
system for the quantum well/superlattice barrier layer structure. As illustrated in Figure
39


CHAPTER 1
INTRODUCTION
We humans depend on the visible portion of the spectrum of photons emitted from
the sun all of lives. But unbeknownst to many of us, we are also constantly bathed in an
intense sea of infrared photons emitted by our surroundings and ourselves. These
photons are generally sensed in the form of heat, but specialized infrared detectors can
shed a whole new light on our environment by detecting and imaging these photons.
Infrared detectors have been widely studied in the last one hundred years [1].
Typically, infrared detectors are divided into two categories: photodetectors and thermal
detectors. In the case of photodetectors, photons directly interact with the carriers in a
semiconductor material to generate a photocurrent; while thermal detectors are dependent
on changes to specific propertiessuch as the conductivityof a material due to a change
in temperature arising from absorption of infrared photons. The two main detection
mechanisms which have been investigated for photodetectors are the photovoltaic (PV)
and the photoconductive (PC) modes. Although, the PV mode of operation is promising
for some practical applications, since the suppression of the dark current strongly
improves the noise properties, most current infrared detectors operate in the PC mode.
With the impressive development and maturity of epitaxial layer growth techniques
such as molecular beam epitaxy (MBE) [2] and metalorganic chemical vapor deposition
(MOCVD), similar gains and improvements have been made in the growth and design of
semiconductor heterostructures. Significant progress has been made in the area of
optoelectronic devices based on quantum wells or superlattices because of the gains in
MBE and MOCVD. In general, a quantum well is formed when a layer of narrow


42
To facilitate the characterization of this p-QWIP, a 216x216 pm2 mesa was etched
onto the wafer by wet chemical etching. After patterning with a contact mask, a thin film
of 120 of Cr was deposited by e-beam evaporation. This layer was topped off with a
1000 layer of Au to create both the top and bottom ohmic contacts. The top ohmic
contact consists of a ring type structure around the edge of the mesa with a 50x50 pm2
contact pad for electrical connection. It should be noted that for this type of mesa and
ring contact structure, the normal incidence IR radiation is only allowed one pass through
the SBTM quantum well layers; which effectively reduces the quantum efficiency when
compared to those QWIP structures which incorporate backside thinning to create a
waveguide-like layer and a top reflector to increase the number of times the incident IR
radiation passes through the SBTM QWIP structure.
4.4 QWIP Characterization and Results
Device characterization was performed in a closed cycle helium cryogenic dewar.
An HP 4145B semiconductor parameter analyzer was used to measure the dark I-V
characteristics and the 300 K background photocurrent. Under dark conditions, holes can
be transfered out of the quantum wells and produce the observed dark current mainly due
to two mechanisms, thermionic emisson out of the quantum wells and thermally
generated carriers tunneling through the superlattice barriers. Given the high aluminum
composition in the superlattice barrier layers, x=0.35, considerable indium content (12%)
in the quantum well layers, and the effect of the compressive strain lowering the energy
of the heavy hole states, the effective barrier seen by the ground heavy hole states was
found to be 299 meV; which should suppress the thermionic emission out of the quantum
wells. Because of the heavily doped ohmic contact regions, a large triangle potential
might be formed which would effectively lower the barrier to thermionic emission and
thus results in a higher dark I-V characteristic than expected. Additional contributions to


25
HH
Compressive Strain
No Strain
Tensile Strain
Figure 2.1: A schematic diagram showing the effects of compressive and tensile strain on
the conduction (CB), heavy-hole (HH) and light-hole (LH) bands in a semiconductor.


31
3.4 Conclusion
We have demonstrated a new normal incidence p-type compressively strained-layer
(PCSL) InGaAs/AlGaAs QWIP grown on SI GaAs for MWIR and LWIR two-band two
color detection. Maximum LWIR and MWIR responsivities were found at 7.4 and 5.5
pm of 37 and 8 mA/W, respectively. The intersubband absorption and photoresponse of
this normal incidence PCSL QWIP were enhanced by using compressive biaxial strain in
the InGaAs quantum well layers. Since the total layer thickness of this PCSL QWIP is
greater than the strained layer critical thickness, certain strain relaxation might occur,
which may result in a lower photoresponse and higher dark current characteristic than
expected; even though the individual layer thickness are within the critical layer thickness
criteria. Although the LWIR detection peak for this QWIP is shorter than that required
for most staring focal plane array (FPA) applications and the MWIR detective peak is
slightly longer than required, we can shift the MWIR and LWIR detection peaks into
more useful regions in addition to maintaining or improving the responsivity and dark
current characteristics the PCSL QWIPs for FPA applications by further optimizing the
quantum well dopant density and the biaxial strain strength, changing the well and barrier
thickness as well as the In and A1 compositions of the well and barrier layers.


Responsivity (A/W)
103
6 7 8 9 10 11 12 13 14
Wavelength (pm)
Figure 8.8: The measured photoresponse of the four well n-type BB-QWIP as a function
of positive applied bias under 45 illumination.


87
8.2 Layer Composition and Device Processing
The three well n-type BB-QWIP consists of an Iii^Ga^As quantum well 6.5 nm
thick, an Irio uGa^As quantum well 6.5 nm thick and an In*, .Ga^As quantum well 7.0
nm thick each separated by a 45 nm thick undoped Al0 07Gao 93As barrier. Each quantum
well is Si doped to 7x1017 cm'3. The complete unit cell of three quantum wells and
barriers is then repeated 20 times and surrounded by an ohmic cap layer 300 nm thick of
GaAs (Si doped to 3xl018 cm'3) and a bottom buffer layer of similarly doped GaAs 500
nm thick. The complete layer structure and schematic band diagram for this QWIP is
shown in Figures 8.1(a) and 8.1(b), respectively.
Figures 8.2(a) and 8.2(b) show the schematic energy band diagram and complete
layer structure of the four well n-type BB-QWIP. This four well n-type BB-QWIP design
consists of an In^GaojAs quantum well 6.5 nm thick, an In0 25Ga0 75As quantum well 6.5
nm thick, an In^Ga^gAs quantum well 7.5 nm thick and an Ir^pGa^jAs quantum well
8.5 nm thick all separated by 45 nm thick undoped barriers of GaAs. The quantum wells
are doped with Si to a density of 7x1 O'7 cm'3, and the complete structure is then
surrounded by extra undoped GaAs barriers 35 nm thick (to form an 80 nm thick
blocking barrier) and 0.3 pm and 0.5 pm thick GaAs ohmic contacts doped with Si to
3x10IS cm'3. The four quantum well and barrier unit was repeated 20 times to create the
whole stack.
While the variable composition p-type BB-QWIP is very similar in operating
theory, the materials and layer thickness of the quantum wells and barriers vary. The p-
type BB-QWIP consists of an Iii^Ga^As quantum well 5.0 nm thick, an In^sGao^As
quantum well 5.5 nm thick and an In^Oa^As quantum well 6.0 nm thick each separated
by a 40 nm thick undoped GaAs barrier. Each quantum well is Be-doped to 4xl018 cm'3.
The complete unit cell of three quantum wells and barriers is the repeated 20 times and
surrounded by an ohmic cap layer 300 nm thick of Be-doped GaAs (p=4xl018 cm'3) and a


54
AlGaAs
InGaAs
AlGaAs
InGaAs
LWIR
Figure 5.1: The energy band diagrams and intersubband transition scheme for the two-
color stacked PCSL-InGaAs/AlGaAs QWIP for the MWIR and LWIR detection.


121
4
qw
m*
m
N
(NEP)fLW
nr
n,
4
Pe
p,
q
Qh
R
4
Rf
r¡
4
R,
S,
4
T
Tqw
Vd
vd
v.
layer critical thickness
total width of all quantum well regions
effective mass of the carrier in the z'-th layer
free electron effective mass
number of periods in a superlattice
background limited noise equivalent power
refractive index of the quantum well
mobile carrier density
incident background optical power
trapping probability
incident optical signal power
electronic charge
incident background photon flux
responsivity
spectral responsivity (in A/W)
reflection coefficient
reflection coefficient of the z-th layer
detector resistance
load resistance
noise spectral density
input reference current noise
temperature
transmission coefficient of the z'-th layer
transmission coefficient over the quantum well
volume of the detector
average drift velocity
saturation velocity


58
Wavelength (uni)
Figure 5.5: The spectral responsivity versus wavelength for the LWIR PCSL-QWIP,
measured at r=40 K and V-2 V. One response peak at 10 pm was obtained for this
device.


30
3.6(b), the responsivity is linearly proportional to the applied bias and that variation with
respect to device temperature is minimal for both detection peaks.
Noise characterization was also performed on the p-type CSL QWIP using standard
noise measurement procedures [51]. A Brookdeal 5004 low noise amplifier (LNA) which
has an input reference current noise, S'/0~4x 1 O'27 A2/Hz, was used to amplify the signal
generated by the QWIP. The spectral density from the output of the LNA was measured
using a HP 3561A spectrum analyzer which has a bandwidth of 100 kHz and allows for
data collection via computer. In order to extract the device parameters, all the
measurements were carried out at temperatures higher than the device BLIP temperature
of 67 K.
The noise spectral density measured with an applied bias of Vh=\.0 V and £=81 K
was found to be 6.5xl0'28 A2/Hz. Given a device area of 216x216 pm2, and a measured
current responsivity, 2?^=12.5 mA/W under the previously mentioned conditions, we
calculated a detectivity of, £>*=1.06x10' cm-Hzl/2/W at the 7.4 pm peak wavelength. As
the applied bias is increased, the detectivity decreases due to the increase in dark current
and the corresponding increase in noise spectral density; even though the current
responsivity increases linearly. The calculated D* values at Vh=2 and 3 V are 6.3x109 and
3.2xl09 cm-Hzl/2/W, respectively. The noise spectral density of this PCSL QWIP as a
function of temperature and applied bias voltage is shown in Figure 3.7. As shown in this
figure, at a low bias voltage, the number fluctuation noise translates into current
fluctuation noise via the diffusion mechanism. As the applied bias increases, charge
transport becomes drift dominant and the number fluctuation noise couples to current
noise via the hole drift mechanism; which results in a strong current dependence [51].
The dashed lines in Figure 3.4 show the results predicted for diffusion dominated noise,
while the solid lines show the results predicted for drift dominated noise.


94
increasing the quantum efficiency, we will also increase the BLIP operating temperature
because more of the background photons will be sensed and thus increasing the
background photocurrent, which defines the onset of BLIP operation.
Another possible way of increasing the bandwidth of the n-type BB-QWIPs is
easily seen when comparing the three and four well n-type BB-QWIPs. Note the
enhanced responsivity of the four well BB-QWIP when compared with the three well n-
type BB-QWIP. This has been attributed to the additional Ino^Gao^As quantum well
and the closeness of the peak wavelengths between the two longer wavelength peak
quantum wells. Using the same idea as discovered in the four well n-type BB-QWIP,
except in the shorter wavelength region of the LWIR band, we can use additional
quantum wells to enhance the shorter wavelength (~8 pm) IR radiation absorption. By
designing the shorter wavelength quantum wells so that there are more of them and so
that their peak wavelengths are closer together, we should be able to bring up the
responsivity of the shorter wavelength region of the LWIR band, thus extending the
bandwidth of the n-type BB-QWIP.
8.5 Conclusion
We have demonstrated four quantum well infrared photodetectors which exhibit
broad response bandwidths. The three well n-type BB-QWIP was found to have a
responsivity peak at 10.0 pm, with an FWHM bandwidth that varies as a function of
applied bias polarity and magnitude. The maximum bandwidth of AX/X=2\% was
obtained at Vh=-2 V, which corresponded to a peak responsivity of 58 mA/W; whereas
the minimum bandwidth of AXfk =12% was achieved at Vh=-6 V, which corresponds to a
very large responsivity of 1.90 A/W at this bias. It is interesting to note that the two other
peaks predicted and seen in this device do not seem to exhibit nearly as much
responsivity as the 10 pm peak at higher, |FA|>3 V, biases. The four well n-type BB-


112
transition to detect incoming IR radiation. A single peak was found at A./J= 10.4 pm with a
spectral bandwidth of AX/Xp=20%. This narrow responsivity bandwidth is consistent with
that expected from a bound-to-miniband transition scheme. A maximum responsivity of
28 mA/W was found at T= 65 K and Vh=+3.0 V. At an operating temperature of 65 K, the
noise spectral density was measured as 4.0x1 O'26 A2/Hz at a bias of 1.0 V. At this
operating point, the spectral detectivity was then calculated at D=1.4xl09 cm-Hz1/2/W.
This design showed us that the step-bound-to-miniband intersubband transition can also
be used in the p-type strained-layer QWIPs to achieve better dark current characteristics
and detectivities than a typical unstrained GaAs/AlGaAs p-QWIP.
The third p-type strained-layer QWIP designed and characterized was the two-stack
two-color InGaAs/AlGaAs compressively strained device which had two separate QWIP
stacks in series to sense both MWIR and LWIR radiation. A peak detection wavelength
at Xpl=\0.0 pm was found for the LWIR QWIP device. The maximum responsivity
measured at T 40 K and V=2.0 V was found to be 25 mA/W, with a detectivity of
1.1 x 1010 cm-Hzl/2/W under the same conditions. A very broad response with full width at
half maximum bandwidth of 40% was achieved for this device. Two photoresponse
peaks were observed in the MWIR band at A./umi.,=4.8 pm and A. =5.4 pm. The 4.8 pm
peak is in excellent agreement with the ground heavy hole (Mill) to second bound heavy
hole (HH3) transition calculated by the TMM. The detectivity for this MWIR peak was
determined to be D*=3.3xl0" cm-Hzl/2/W at FA=1.0 V and 7=77 K. The measured
spectral bandwidth for the first MWIR peak was found to be AXIXpnml=2\% and
AX/Xpmw2=26% for the second peak. The second, longer wavelength peak is attributed to
the transition from the HH1 states to the second bound light hole (LH2) states within the
quantum well. The higher responsivity of the 5.4 pm peak is attributed to the higher
absorption coefficent inherent with the HH to LH transition. The spectral detectivity for
this MWIR peak was found to be 5.5x10" cm-Hzl/2/W at Vh=\.Q V and T=ll K. This
design shows us that different p-QWIPs with different detection peaks can be combined


TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ii
TABLE OF CONTENTS iii
ABSTRACT vi
1 INTRODUCTION 1
2 THE THEORETICAL STUDY OF P-TYPE QUANTUM WELL
INFRARED PHOTODETECTORS 8
2.1 Introduction 8
2.2 P-QWIP Physics 9
2.2.1 Strain-Layer Growth Limitations and Theory 9
2.2.2 Strain Induced Energy Band Shifts 10
2.2.3 Energy Band Calculations 12
2.2.4 The Transfer Matrix Method for the Calculation of
Transmission Probability 15
2.2.5 Determination of Intersubband Transitions and Absorption
Coefficients 17
2.2.6 Photoconductive Detection Mode Operation 19
2.3 P-QWIP Figures of Merit 19
2.3.1 Spectral Responsivity 20
2.3.2 QWIP Collection Efficiency 21
2.3.3 Dark Current Relationship in a QWIP 22
2.3.4 Noise and Detectivity in QWIPs 23
iii


9
2.2 P-OWIP Physics
2.2.1 Strain-Layer Growth Limitations and Theory
P-type QWIPs using valence intersubband transitions have been demonstrated [17-
19] in lattice-matched GaAs/AlGaAs and InGaAs/InAlAs material systems. In general,
intersubband transitions excited by normal incidence radiation in p-type quantum wells
are allowed since a linear combination of p-like valence band Bloch states exists, which
provides a nonzero coupling between the normal radiation field and valence band Bloch
states. The strong mixing between the heavy hole and the light hole states greatly
enhances intersubband absorption. The drawback of using lattice-matched systems is the
fact that the intersubband transition occurs between the heavy hole ground states and the
upper excited states. Because of the relatively large heavy hole effective mass when
compared to the electron effective mass, relatively weak absorption and therefore
similarly low responsivity is predicted in the IR wavelength range when compared to n-
type QWIPs. In order to increase the absorption characteristics and responsivity of P-
QWIPs, biaxial stress is introduced into the well layers of the QWIP structure. If the
intentionally introduced biaxial stress between the well layers and the barrier layers
contained in the layer thickness (the total thickness of the wells and barriers) in the P-
QWIP structure is less than the critical thickness, then pseudomorphic or coherent
heterointerfaces can be grown without the introduction of defects between the layers.
Based upon the force balance model [12,20-22], the equilibrium critical layer thickness,
Lc, for an epilayer with the lattice constant, a, grown on a substrate with a lattice constant,
a is given as
(2.1)
where h, is the epilayer thickness, 0 is the angle between the dislocation line and the
Burges' vector, ap is the angle between the slip direction and the layer plane direction, 5tt


Detectivity (Jones)
116
Figure 9.1: A comparison of the performance (D*) of the p-type strained-layer QWIPs
studied with reported unstrained p-QWIPs.


107
Figure 8.12: The measured photoresponse of the variable composition p-type BB-QWIP
as a function of applied bias and incident 1R radiation wavelength at T=40 K. The results
were measured for a single pass at normal incidence.


47
GaAs
500 nm
Be=5xl0l8cm'3
^0.35^*0.65-^
2.0 nm
undoped
Repeat
GaAs
2.7 nm
undoped
x 10
Repeat
x 20
Ino.ndaoggAs
9.0 nm
Be=3xlOl8cm'3
^lo.35^^0.65^^
2.0 nm
undoped
Repeat
GaAs
2.7 nm
undoped
x 10
GaAs
1000 nm
Be=5xl018cm'3
SI GaAs (100)
Figure 4.3: The complete layer structure of the InGaAs/GaAs/AlGaAs CSL SBTM p-
QWIP.


40
4.1, the transition scheme for this p-QWIP is from the localized ground bound heavy hole
state (HH1) in the wide Irio 12Gao 8gAs quantum well to the resonant coupled miniband of
the GaAs/Al0^jGao 65As superlattice (SL) barrier. This structure creates a potential
difference between the SL barrier region and the quantum well which blocks part of the
undesirable tunneling dark current from the heavily doped heavy hole ground state, HH1
[53]. The physical parameters were chosen so that the ground state in the wide InGaAs
quantum well is well above the top of the GaAs/AlGaAs SL barrier, and the third excited
heavy hole state (HH4) is in resonance with the ground level of the superlattice miniband
(SL1) to achieve a higher quantum efficiency. Since the superlattice consists of thin
barriers, the photoexcited holes can easily tunnel through the superlattice barrier layer and
transport along the aligned miniband to be collected by the ohmic contacts.
4.2 Theoretical Considerations
In order to characterize this detector, we performed theoretical calculations of the
energy states in the quantum well and superlattice barrier regions along with the
transmission coefficient, \T*T\, by using the multiple layer transfer matrix method
(TMM) [31] which is described in Chapter 2. The results of the TMM calculation are
shown in Figure 4.2. Using linearly interpolated values for the heavy-hole and light-hole
effective masses and the compound semiconductor bandgaps at 77 K for GaAs,
Al0 35Gao 65As, and Ino^Ga^gAs we determined the intersubband transition for the LWIR
absorption to occur at a peak wavelength of 10 pm, when the effects of biaxial
compressive strain are considered.
Another characteristic of p-type QWIPs is the inherently larger quantum efficiency
than that of n-QWIPs, which is given by
(4.1)


46
Energy (meV)
Figure 4.2: The calculated transmission coefficients of the InGaAs/GaAs/AlGaAs CSL
SBTM p-QWIP using the TMM.


28
large tunneling current from the triangle potential formed by the heavily doped, large
bandgap ohmic contact regions. The exact layer structure for the p-QWIP is shown in
Figure 3.2. In our design, the barrier and substrate are lattice matched and the well
regions are in biaxial compression due to a lattice mismatch of -1.4%. The ground
subband energy levels confined in the quantum wells are the highly populated heavy hole
states, Ehh,. The mobility of the heavy hole is enhanced by the compressive strain in the
InGaAs quantum wells by the reduction of the heavy hole effective mass [13]. Another
improvement which results from the introduction of compressive strain in the quantum
wells is the reduction in the density of states in the InGaAs layers. Because of this, more
free holes will reside in higher energy states, which implies that the Fermi level is
elevated when compared with the unstrained case. The elevation of the Fermi level will
result in an increase of the number of off zone center (i.e., k*0) holes with less effective
mass. Therefore, a larger intersubband absorption under normal incidence infrared
radiation can be expected.
As seen in Figure 3.1, the intersubband transition occurs from the highly populated
ground heavy hole state (EHH1) to the upper heavy hole bound state (EHH3) and the first
extended heavy hole state (Emi4) for the 7.4 pm LWIR and 5.5 pm MWIR detection
peaks, respectively. Since the upper heavy hole bound state (EHII3) is slightly below the
barrier valence band maximum, we expect a maximum in the absorption oscillation
strength; whereas the first extended heavy hole state (EHH4) is above the barrier, which
predicts a weaker absorption.
3.3 Results and Discussion
To facilitate the characterization of this p-QWIP, a 216x216 pm2 mesa was etched
onto the wafer by wet chemical etching. After patterning with a contact mask, a thin film
of 120 of Cr was deposited by E-beam evaporation. This layer was topped off with a


53
this MWIR peak was found to be 5.5x10 cm-Hz1/2/W at FA=1.0 V and T= 77 K, with a
corresponding responsivity of 19 mA/W at the same peak when Vh=5 V.
The responsivity for the LWIR QWIP stack as a function of the wavelength is
shown in Figure 5.5. A peak detection wavelength at X.p/M,=10.0 pm was found for the
LWIR QWIP device, which is in excellent agreement with the calculated value of 10 pm
from the TMM. The maximum responsivity measured at T=40 K and V=2.0 V was found
to be 25 mA/W, with a detectivity of l.lxlO10 cm-Hzl/2/W under the same conditions. It
is interesting to note that a very broad response with full width at half maximum
bandwidth of 40% was achieved for this device. Figure 5.6 shows the relative spectral
response of the combined MWIR and LWIR QWIP stack, displaying one dominant
response band at MWIR and two smaller response peaks at LWIR bands.
5.4 Conclusion
We have demonstrated a stacked p-type CSL QWIP design that has the capability to
sense infrared photon in both the MWIR and LWIR regimes. This detector exhibited two
MWIR peaks at 4.8 and 5.6 pm and a single LWIR peak at 10 pm, all of which agree
closely to the detection peaks calculated from TMM. By using a stacked design that
allows three ohmic contacts, we can simultaneously sense two different colors in two
different atmospheric windows in the same FPA. Detectivities of 5.5 x 10" and 1.1 x
1010 cm-Hzl/2/W were found at 5.4 and 10 pm, respectively with corresponding maximum
responsivities of 19 and 25 mA/W at those same wavelengths. This device can be
optimized so that the MWIR stack is has a responsivity peak at closer to 4.2 pm by
increasing the indium concentration in the quantum well of the MWIR stack and slightly
narrowing the quantum well width.


Current (A)
102
Bias (V)
Figure 8.7 Measured dark I-V characteristics of the three well n-type BB-QWIP with the
300 K background photocurrent superimposed. The FOV=180 for the 300 K
background photocurrent.


Relative Responsivity (a.u.)
76
Wavelength (pm)
Figure 6.8: The relative photoresponse of the unstrained SL-QWIP as a function of
applied forward bias at 7-20 K.


72
Table 6.1: Summary of the SLIP performance as a function of applied bias and device
temperature.
Temp. & Bias
R v (mA/W)
In0ise (A/Hz/2)
Tig (%)
D* (cm-Hz1/2/W)
20 K
OV
5.6
1.36 xlO'11
0.035
8.86 x 106
20 mV
29.7
2.47x10-"
0.192
2.58 x 107
50 mV
22.1
2.51 x 10'"
0.142
1.90 x 107
30 K
0 V
12.4
3.46 x 10"
0.079
7.78 x 106
20 mV
24.4
3.89 x 10'"
0.157
1.33 x 107
50 mV
22.6
4.03 x lO'"
0.142
1.20 x 107
40 K
0 V
39.1
3.96 x 10 "
0.249
2.13 x 107
20 mV
49.8
5.83 x 10 "
0.317
1.84 x 107
50 mV
41.3
6.90 x 10 "
0.263
1.29 x 107


74
GaAs
300 nm
Be=4xl018 cm 3
Alo.3GaQ.7AS
100 nm
undoped
Al0 4Ga06As
3.5 nm
undoped
Repeat
GaAs
3.0 nm
Be=2xl018 cm"3
x 4
Repeat
Al0.4Ga0 6As
3.5 nm
undoped
x 10
Al^Ga^As
60 nm
undoped
Alo^Gao^As
40 nm
undoped
GaAs
500 nm
Be=4xl018 cm"3
SI GaAs (100)
Figure 6.6: The complete layer structure of the unstrained p-type SLIP.


APPENDIX 2
ACRONYMS
2D
two-dimensional
BB
broadband
BLIP
background limited performance
BTB
bound-to-bound
BTC
bound-to-continuum
BTM
bound-to-miniband
BTQB
bound-to-quasi -bound
CSL
compressive strained-layer
CSL-QWIP
compressive strained-layer quantum well infrared photodetector
DSP
digital signal processing
DUT
device under test
FFT
fast Fourier transform
FOV
field of view
FPA
focal plane array
FTIR
Fourier transform infrared
HH
heavy-hole
HH1
heavy-hole 1 state
HH2
heavy-hole 2 state
HH3
heavy-hole 3 state
HH4
heavy-hole 4 state
IR
infrared
123


115
type BB-QWIP was found to have a maximum responsivity of 25 mA/W at 7=40 K,
Fa=+1.1 V with a spectral bandwidth of A)Jkp=63% at a peak wavelength of 9.6 pm. For
both of these p-type BB-QWIPs, the spectral bandwidth was independent of the applied
bias magnitude or the polarity. The evidence of more than one spectral peak in both n-
type BB-QWIPs and the very wide FWHM bandwidth of both p-type BB-QWIPs show
that the concept of using various quantum wells with different peak responses in a stack
can be a viable approach to making broadband detectors.
The overall results of this study indicate that strained layer p-type QWIPs do exhibit
better performance under normal incidence illumination than unstrained p-QWIPs. This
is illustrated in Figure 9.1 and summarized in Table 9.1. Some work still needs to be
done in increasing the quantum efficiency of the p-QWIPs, so that the responsivity and
detectivity of p-QWIPs under normal incidence without grating couplers can achieve that
of n-type QWIPs with highly optimized gratings. This can be done through a number of
physical means, ranging from thinning down the substrate so that a waveguide-like
structure is created out of the QWIP stacks and the substrate so that incident IR light can
have multiple reflections in each pixel to using anti-reflection coatings to cut down on
reflection losses to microlens arrays with the same pitch as the FPA in order to increase
the fill factor.
More work needs to be done in the theoretical analysis of valence band
intersubband transitions so that we can use the light-hole transitions available in tensile
strained layer p-QWIPs. Another interesting, but yet unproved concept is that of the
heterojunction infrared phototransistor which consists of a heterojunction transistor with
a base region that is composed of a QWIP for IR detection. This two terminal device has
the possibility of achieving large optical gains while operating at liquid nitrogen
temperatures.


93
Again using the results of the responsivity and dark I-V measurements, and an
estimated gain of 0.02, the calculated detectivity of the variable thickness p-type BB-
QWIP was found to be 7.68xl09 and 9.54xl09 cm-Hz1/2/W at T= 50 K and Vh=+\.0 and
+1.1 V, respectively. Note that this detectivity result is achieved when the incident IR
radiation is passed through the multi-quantum well stack only one time.
8.4 Discussion
Broadband detection by means of quantum well infrared photodetectors has been
demonstrated for both p- and n-type devices. When comparing the relative strengths and
weaknesses of each BB-QWIP, one striking fact is obvious. The n-type BB-QWIPs have
a much higher responsivity whereas the p-type BB-QWIPs have much larger spectral
bandwidths. When comparing the four devices, we see that if the responsivity bandwidth
is the dominant factor then either p-type BB-QWIP would be the detector of choice, with
the variable thickness p-type BB-QWIP the design that has the largest bandwidth, the
higher responsivity and detectivity. But if the absolute responsivity was the most
important characteristic, then the obvious choice would be the four well n-type BB-
QWIP. since it exhibits the highest detectivity and responsivity of all four designs, while
still having a relatively wide spectral response when under negative bias.
One possible way of extending the bandwidth of the n-type BB-QWIPs is by
tailoring a grating which enhances the shorter wavelength regime of the LWIR band and
patterning this on to the n-type BB-QWIP mesa. This has been proposed by Gunapala et
al. and seems like a viable method for broadening the responsivity of the n-type QWIPs.
If we seek to increase the responsivity, and thus the detectivity of either the n- or p-type
BB-QWIPs, we can use a combination of backside thinning to create a waveguide-like
structure that traps a significant part of the IR radiation in the BB-QWIP stack so that
multiple passes of the light can be absorbed, which increases the quantum efficiency. By


ACKNOWLEDGMENTS
I wish to express my deepest thanks to the chairman of my supervisory committee,
Dr. Sheng S. Li, for all his guidance, patience, and encouragement throughout the entire
research process and for giving me the freedom to leam about all the aspects involved in
research and development. I would also like to extend my gratitude to Dr. Gijs Bosnian,
Dr. Amost Neugroschel, Dr. Ramakant Srivastava, and Dr. Tim Anderson for serving on
my supervisory committee.
I would like to thank Drs. Yanhua Wang, Yun-Shan Chang, Jung-Chi Chiang and
Daniel C. Wang for their extensive assistance both mentally and physically in
semiconductor processing and design. Thanks are extended to my laboratory colleagues,
Jung Hee Lee and Chia-Hua Huang for their friendship, support and assistance in all
aspects of my work.
And 1 would like to extend the greatest appreciation and thanks to my parents Dr.
and Mrs. Chauncey C. Chu and my brother Gary T. Chu for giving me the unconditional
love and support throughout my entire academic career. Without their backing and the
grounding that they have given me, I would not have been able to even consider
undertaking the work necessary for a doctorate.
Finally, the financial support of ARPA and US Air Force Materiel Command is
gratefully acknowledged.
11


45
InGaAs AlGaAs/GaAs
HH4& MB1
MB2
Figure 4.1: Schematic energy band diagram for the InGaAs/GaAs/AlGaAs CSL SBTM
p-QWIP.


Responsivity (A/W)
101
Wavelength (pm)
Figure 8.6 The responsivity of the n-type BB-QWIP at low (\Vh\<2 V) applied biases.


56
c

¡~
s-
s
U
In
<8
Q
Applied Bias (V)
(a)
Figure 5.3: The dark I-V characteristics for (a) the stacked, MWIR, and LWIR PCSL-
QWIP, and (b) the LWIR QWIP for T=40, 60, and 77 K.


10
is the lattice-mismatch or the in-plane strain, and vp is the Poisson ratio. 5 is defined as
5 ={a-a)la where 8>0 for tensile strain and 5<0 for compressive strain. Similarly, vp is
defined as v=Cl2/Cu. C0's are the elastic constants and can be found in reference 23.
The strained-layers have the same effective in-plane lattice constant, a (i.e., axy),
and can store the excess energy due to the elastic strain within the layers. The in-plane
lattice constant, a can be expressed by [20]
!! = £!, 1 + 8 0 / 1 +
(2.2)
where a, 2 and L,2 are the individual layer lattice constants and thickness, respectively,
and t,l 2 are the shear moduli as described by £¡=CII+Cl2-2CI22/Clh where the C¡¡s are
elastic constants for the strained material. 80 denotes the lattice mismatch between layers
and a, 2 are the lattice constants of the strained well and the substrate (or barrier)
respectively. When a^as, the coherently strained superlattice structure is no longer in
equilibrium with the substrate. If the lattice constant of the barrier layers is equal to that
of the substrate, the strain will be completely accommodated in the well layers with no
strain in the barrier layers. However. Hull el al. [24] showed that if the individual layer
thickness in the superlattice is less than its critical thickness, even though a the loss
of coherence only occurs at the interface between the whole superlattice and the substrate,
while the superlattice itself remains coherent.
2.2.2 Strain Induced Energy Band Shifts
If the QWIP structure is grown along the [100] direction and the strained-layer is
within the critical thickness, Lc, then a pseudomorphic or coherent heterointerface can be
obtained and the components of the strain tensor [e] are simplified to the expressions
given by
e
XX
e
yy
e
(2.3)


Dark Current (A)
110
Applied Bias (V)
Figure 8.15: The measured dark I-V characteristics of the variable thickness p-type BB-
QWIP as a function of device temperature.


23
Equation (2.52) for Nn is valid when summed over the subband levels En below the
Fermi level, and Eq. (2.53) is only valid at cryogenic temperatures.
Using the previous result in the cryogenic temperature regime, we see that the dark
current due to thermionic emission is exponentially dependent on the doping
concentration in the quantum well, i. e.,
I,h oc exp
( £ \
KTj
OC
exp
A^
kT.
(2.54)
Therefore, as the doping density in the quantum well increases, the dark current
density due to thermionic emission also increases exponentially. In contrast to this, the
intersubband absorption is directly proportional to the doping concentration. Therefore, a
tradeoff between the dark current density and the intersubband absorption is required to
optimize the QWIP performance. However, in the case of p-QWIPs, the Fermi level in
the quantum well is pinned at or slightly above the ground state energy for highly doped
quantum wells. We can increase the doping in the quantum well to increase optical
absorption without increasing the dark current of the p-QWIP significantly because the
thermionic emission pinned with the Fermi level.
2.3.4 Noise and Detectivity in OWIPs
The noise in QWIP structures is mainly due to random fluctuations of thermally
excited carriers. The noise is expressed as [17]
W = ViUGAiC (2.55)
where Ad is the detector area, and Af is the noise bandwidth. Finally, a figure of merit
measurement used to compare detectors is the detectivity, D\ which is shown to be [40]
D'=JaM-A-- (2-56)
l noise
If the dark current in a particular QWIP is lower than the 300 K background
photocurrent, then the QWIP can be considered to be under background limited


64
two trends are consistent throughout at each temperature from 20 to 40 K, and that the
noise current and hg product also follow the same trend. The calculated detectivities as a
function of bias and device temperature are also included in Table 6.1.
6.4 A Voltage Tunable Two-color SLIP
Because of the strain relaxation in the strained superlattice of the 19.2 pm SLIP, a
new unstrained SLIP was grown for fabrication and characterization. This device was
found to exhibit true voltage tunability. Typically, methods for achieving multi-color or
multi-band detection in QWIPs have ranged from the use of stacks of different QWIP
layers to increase the number of wavelengths or wavebands which the pixel is sensitive
[48,54,58] to a voltage tuning scheme which shifts the detection peak around in a single
waveband as a function of applied bias [59,60], We have designed and characterized a
novel p-type superlattice infrared photodetector which maximizes the flexibility of the
QWIP design by exhibiting normal incidence detection and true voltage tuning, where the
changing applied bias changes the detection peak by allowing one peak and suppressing
another. Uses for such devices range from single pixel two color imaging to two color
temperature resolution.
6.5 Layer Structure and Fabrication of the Unstrained Voltage Tunable SLIP
This SLIP was designed with GaAs (3.0 nm) quantum wells and Al^Ga^As (3.5
nm) superlattice barriers, as seen schematically in Figure 6.5(a). The thick (50 nm)
barriers separating the sets of four GaAs quantum wells are also grown with Al^Ga^As.
Figures 6.5(b) and 6.5(c) illustrate the two intersubband transitions and how the applied
bias is used to tune the detection peak. Note that the 9.2 pm peak can be seen only when
the superlattice miniband is resonantly lined up at moderate biases, while at higher
applied biases, the breakdown of the miniband resonance limited the detection at this


bandgap semiconductor is sandwiched between a set of wider bandgap semiconductors.
The motion and energy of the carriers perpendicular to the semiconductor layers then
becomes quantized so that the localized two-dimensional (2D) subbands of quantized
states are formed in the quantum well [3]. The energy at which these quantized states are
formed is dependent on the effective mass of the carrier and the thickness of the smaller
bandgap semiconductor; thus by carefully choosing the right combination of
semiconductor material and thickness, a wide range of intersubband energies can be
obtained.
The idea of using optical intersubband transitions in quantum wells for infrared
detection was made by Chang et al. [4], Esaki and Sakaki [5], and Coon and Karunasiri
[6]. These optical intersubband transitions were first observed in GaAs quantum wells by
West and Eglash [7] and who were then followed by Harwit and Harris [8]. The first
GaAs quantum well infrared photodetector (QWIP) was demonstrated by Levine et al. [9]
in 1987. Since then, QWIPs based on bound-to-bound (BTB) [9], bound-to-continuum
(BTC) [10], and bound-to-miniband (BTM) [11] transitions have been widely
investigated in the past ten years [12]. The various types of n- and p-type QWIP
schematic energy band diagrams are shown in Figures 1.1 and 1.2.
Currently, most QWIPs are of the n-type variety [12]. The use of electrons as the
carrier for intersubband transitions is a good choice due to the low effective mass and
excellent transport and absorption qualities of n-doped III-V materials. But n-type
QWIPs also exhibit higher dark currents and are not able to couple normal incidence IR
radiation due to the quantum mechanical selection rules. P-type QWIPs, on the other
hand, are able to couple normally incident radiation and have larger effective masses,
which corresponds to lower dark currents. But the larger effective masses of the holes
also leads to decrease absorption coefficients and smaller quantum efficiencies.
But the effective mass of the holes can be reduced with the addition of compressive
or tensile strain in the quantum wells [13]. When compressive strain is used the ground
2


95
QWIP is observed to have an even higher 45 incidence responsivitiy of 2.35 A/W at
A.p=10.3 pm at Vh=+4.5 V and T= 40 K. Under a negative bias of -4.5 V, a large
bandwidth of A7JX=29% is achieved, with a corresponding peak responsivity of 2.25
A/W at A,p-11.3 pm. As has been reported in other published works [62], there was also a
significant normal incidence response with both n-type BB-QWIPs. This has been
attributed to the use of compressively strained quantum wells in the design. The variable
composition p-type BB-QWIP also examined was found to have a very large FWHM
bandwidth of AAAp=48% at T=40 K and Vh=-1.5 V. Under the aforementioned operating
conditions, a maximum absolute spectral responsivity peak of 19 mA/W was found at 9.3
pm when the incoming radiation is normally incident and allowed only one pass through
the multiquantum well layers. The variable thickness p-type BB-QWIP exhibited an even
larger FWHM bandwidth of AX/Xp=63% with detection peak of 25 mA/W at \=9.7 pm
and Vb-+\.\ V, T= 40 K. This result was also obtained when the IR light is only allowed
one pass through the multi-quantum well layers.


96
ln 0.1Ga0.9As ln0.15Ga 0.85As ln 0.2Ga 0.8As
E1
11.8 um 9.7 um 8.2 um
(a)
GaAs
300 nm
Si=3xl018
cm'3
Ad o 07Ga0 93 As
45 nm
undoped
Ino.2Ga08As
6.5 nm
Si=7xl 017
cm'3
A1 0,0lG& 0 93 As
45 nm
undoped
In o.i;Gao ss As
6.5 nm
Si=7x 1017
cm'3
Repeat
x 20
A1 o.o7^a0 93As
45 nm
undoped
In 0, Ga09As
7.0 nm
Si=7xl 017
cm'3
A1 o.o7^Ja0 93 As
45 nm
undoped
GaAs
500 nm
Si=3xl018
cm'3
S.I. GaAs substrate
(b)
Figure 8.1: The (a) schematic energy band diagram and the (b) complete layer structure
for the three well n-type BB-QWIP.


Relative Responsivity
59
Figure 5.6: The relative photoresponse versus wavelength for the combined stacked
PCSL-QWIP. Three photoresponse bands were detected in this stacked QWIP.


67
peak responsivity wavelength and FWHM spectral bandwidths by altering the layer
composition and material thicknesses. In addition, one can use a slightly strained
quantum well layer (<8% Indium) to reduce the effective mass of the ground heavy hole
states [13], increasing heavy hole mobility, which in turn will increase the intersubband
absorption and improves the transport characteristics of the device. We have designed,
processed and demonstrated a new novel p-type QWIP which exhibits true voltage
tunability in the LWIR band. Further improvements in this device can be made in terms
of increasing responsivity by shortening the detection wavelength of the bound-to-
continuum peak by means of increasing the superlattice barrier height, which also should
lower the effective dark current by decreasing the tunneling probability through the
superlattice slightly.


CHAPTER 2
THE THEORETICAL STUDY OF P-TYPE QUANTUM WELL INFRARED
PHOTODETECTORS
2.1 Introduction
With the advent of molecular beam epitaxial technologies in the last few decades,
device structures utilizing heterostructure quantum wells have been heavily explored. N-
type quantum well infrared photodetectors (QWIPs) have been extensively studied in
recent years [11,12,14], These systems use GaAs/AlGaAs and InGaAs/InAlAs structures
for detection in the 3-5 pm mid-wavelength infrared (MWIR) and 8-14 pm long-
wavelength infrared (LWIR) atmospheric transmission windows. Since n-type
GaAs/InGaAs and InGaAs/InAlAs QWIPs have inherently low electron effective masses
and high electron mobilities, they offer excellent infrared (IR) detection properties. Due
to the quantum mechanical selection rules which prohibit normal incidence intersubband
absorption, focal plane arrays (FPA) using n-type QWIPs must use either metal or
dielectric gratings to couple normal incidence IR radiation into the quantum well
L11,15,16]. In contrast, because of the mixing between the light hole and heavy hole
states under either biaxial tension or compressive strain, normal incidence illumination is
allowed for the intersubband transition in p-type QWIPs; thus eliminating the need for
metal or dielectric grating couplers.
8


88
bottom buffer layer of similarly doped GaAs 500 nm thick. The complete layer structure
for both devices is shown in Figures 8.3(a) and 8.3(b).
Unlike the n-type devices, we were able to design another three well p-type BB-
QWIP was grown to explore the effects of just varying the quantum well layer thickness
in the device design. This variable thickness p-type BB-QWIP design consists of three
quantum wells 4.5, 5.5 and 6.2 nm thick In^Gao^As Be doped to 4xl018 cm'3. These
were separated by 40 nm thick undoped GaAs barriers and then the whole unit was
repeated 20 times. The whole stack was then surrounded by 0.3 pm and 0.5 pm thick
GaAs ohmic contacts Be doped to 4x10'8 cm'3. The complete layer structure and
schematic energy band diagram is shown in Figure 8.4(a) and 8.4(b) respectively.
In order to characterize the devices, a wet chemical etch was used to create a
216x216 pm2 mesa structure for the both test devices. Cr/Au was used to form the top
and bottom ohmic contacts for the p-type BB-QWIP, while AuGe/Ni/Au annealed at
450C for two minutes was used as the top and bottom ohmic contacts for the n-type BB-
QWIP. To facilitate normal incidence illumination for both the p-type and n-type BB-
QWIPs, a ring contact around the mesa edge was used to allow light to pass through to
most of the mesa top surface with a 75% fill factor. A 45 polished edge was also ground
into the n-type BB-QWIP so that 45 incident IR radiation could be used to test the
device. Both devices were mounted on TO-8 chip carriers and then wire bonds were
attached ultrasonically for electrical connection.
8.3 Characterization Results
First, we will discuss the results of the three well n-type BB-QWIP, followed by the
four well n-type BB-QWIP. The next subsection will cover the two p-type BB-QWIPs in
this order, the variable composition three well p-type BB-QWIP and then the variable
thickness p-type BB-QWIP. The responsivity was measured using a blackbody source


51
layers. The whole stack was sandwiched between two thin Al^Ga^As blocking barriers.
Finally, heavily Be-doped GaAs layers were grown for the top and middle ohmic
contacts. Both the LWIR and MWIR quantum wells are in biaxial compression. This
multicolor stacked QWIP uses the bound-to-quasi-bound (BTQB) intersubband transition
scheme for detection of MWIR and LWIR radiation. The complete layer structure of this
stacked device is shown in Figure 5.2.
In order to evaluate the performance of the QWIP, a mesa structure with area of
216x216 pm2 was formed on the MBE grown QWIP wafer by wet chemical etching for
radiometric and electrical characterization. A narrow ring of Cr/Au film was deposited
by E-beam evaporation to create the ohmic contacts. It is noted that in this type of mesa
and ring contact structures, the normal incidence IR radiation is allowed only one pass
through the QWIP stack.
5.3 Device Characterization and Results
For this multicolor stacked QWIP, three mesa structures of different thicknesses
were etched to allow separate characterization of the LWIR, MWIR, and the combined
stacked QWIP devices. The LWIR QWIP mesa structure was formed using the top and
middle ohmic contacts, while the MWIR QWIP mesa structure had the top LWIR stack
etched away before the mesa formation. The combined stacked QWIP used the top and
bottom ohmic layers for contacts.
The device characterization was performed in a closed cycle helium cryogenic
dewar. An HP 4145B semiconductor parameter analyzer was used to measure the dark I-
V characteristics and the 300 K background photocurrent. Under dark conditions, holes
can be transfered out of the quantum wells and produce the observed dark current mainly
due to two mechanisms: the thermionic emisson out of the quantum wells and thermally
generated carriers tunneling through the superlattice barriers.


Responsivity (A/W)
104
6 7 8 9 10 11 12 13 14
Wavelength (|im)
Figure 8.9: The measured photoresponse of the four well n-type BB-QWIP as a function
of negative applied bias under 45 illumination.


The next p-type CSL design is called a superlattice infrared photodetector (SLIP)
and consists of a radiation sensitive superlattice of 3 or more periods surrounded by a
blocking barrier. The p-type CSL-SLIP was found to have a very long-wavelength
infrared (VLWIR) detection peak at 19.2 pm and was grown on S.I. (100) GaAs with
InGaAs/GaAs/AlGaAs layers. A second p-type SLIP grown and characterized was an
unstrained design which showed voltage tuning, with the detection peak of this SLIP
shifts from 9.3 pm to 6.5 pm with the relative responsivity of the LWIR peak decreasing
and the 6.5 pm peak increasing with bias.
The final p-type QWIP investigated is the tensile strained-layer (TSL)
InGaAs/InAlAs design grown on S.I. (100) InP which uses the light-hole to heavy-hole
intersubband transition for detection. The TSL-QWIP was found to have a detection peak
in the MWIR band at 5.2 pm. As with all of the other strained and unstrained p-QWIPs,
the device was found to be sensitive to normally incident IR radiation.
The broadband QWIPs (both n- and p-type) studied consist of three or four different
quantum wells of varying thickness and composition combined in a unit cell separated by
thick barriers. Each different quantum well is designed with a slightly different detection
peak than an adjacent one, so that the individual spectra overlap to form a broader
spectrum. These four devices were found to have detection peaks ranging from 9.3 to
10.3 pm and spectral bandwidths of 21 to 63%.
vii


77
Bias (V)
Figure 6.9: Dark current as a function of device temperature with the 300 K background
photocurrent superimposed (FOV=1.78).


Irio 52A10.4gAs
300 nm
p=5xl018 cm'3
1^0.52^ 0.48^3
15 nm
undoped
In0.52Al 0.48-^S
50 nm
undoped
Repeat
x 20
Illo.4Gao.6AS
7.0 nm
p=3xl018 cm'3
lUo 52A1 o.4sAs
65 nm
undoped
IiIq 52A1 0.48-^^
500 nm
p=5xl018 cm'3
SI (100) InP
Figure 7.1: The complete layer structure of the p-type MWIR TS-QWIP.


75
Figure 6.7: Relative photoresponse of the unstrained SL-QWIP as a function of applied
reverse bias at r=20 K.


InGaAs
AIGaAs
InGaAs
Figure 3.1 The schematic energy band diagram for the two-color two-band
compressively strained InGaAs/AlGaAs p-QWIP.


120
B
Bf
c
C,
C>j
£>*
D\
d,
Ecf
Ef
E,
F
ff
f
G
g
h
hi
H,
Hs
up
4
Jd
ktt
K
L
E/,2
number of passes IR radiation makes through the photodetector
QWIP bandwidth
speed of light
combined hydrostatic deformation potential
elastic constants
detectivity
background limited detectivity
thickness of the z'-th layer
cut-off energy
Fermi energy
potential energy of the z-th layer
electric field
fermi distribution function of the final state
fermi distribution function of the initial state
photoconductive gain
noise current gain
Planck constant
epilayer thickness
Luttinger-Kohn Hamiltonian
strain Hamiltonian
background photon noise current
photocurrent
dark current density
Boltzmann constant
wavevector of the z-th layer
carrier mean free path
individual layer thickness


41
Given similar absorption coefficients, a, and well thickness, /, the prefactor A is
equal to 0.5 for n-QWIPs and 1.0 for p-QWIPs, which gives us a doubling of the quantum
efficiency for p-QWIPs. Due to the inherently low absorption coefficients of p-type
materials, and the large hole effective mass, compressive strain must be used to reduce
hole effective mass in order to increase the absorption.
With the inclusion of compressive strain, the mobility of the heavy holes is
enhanced by reducing the heavy hole effective mass [13], Also associated with the
presence of compressive strain is the reduction of the density of states in the InGaAs
quantum well. Thus, with a significant lowering of the effective mass of the ground
heavy holes, an increase in the absorption coefficient and the corresponding quantum
efficiency is expected.
4.3 Device Growth and Fabrication
The p-type SBTM CSL-QWIP consists of 90 thick In ^Ga^gAs quantum wells
Be-doped to 3xl018 cm'3 to populate the ground heavy hole (HH1) state. The quantum
well layer is under compressive strain with a lattice mismatch of nearly -0.8%.
Surrounding the quantum well layers are the superlattice barriers which are made up of
20 thick superlattice barriers of undoped Al0 35Ga0 65As alternating with 27 thick
undoped GaAs quantum wells. The complete superlattice barrier structure is composed
of ten periods of the GaAs/AlGaAs superlattice barrier structure, which is lattice matched
to the SI GaAs substrate. The whole superlattice barrier/quantum well structure is then
repeated 20 times to form the p-type SBTM CSL-QWIP. The p-type ohmic contacts for
this QWIP are formed by a 0.5 pm thick cap layer and a 1.0 pm thick buffer layer of
heavily Be-doped (5.0xl018 cm'3) GaAs on top of the SBTM QWIP stack and between the
SBTM QWIP stack and the substrate, respectively. The complete layer structure of this
device is illustrated in Figure 4.3.


89
set at 1273 K, running through a l/8m monochrometer with the appropriate filters as the
source of IR radiation, which is also chopped at a set frequency. The device is set in a
closed cycle liquid helium dewar and electrical leads are attached. The output of the
device is then amplified by a transimpedance amplifier (TIA) with a gain of 106 V/A.
The output of the TIA is then sent to a lock-in amplifier to determine the phase and
magnitude of the output signal. The relative responsivity curve is then normalized
against the response of a pyroelectric detector to determine the absolute responsivity. A
schematic diagram of the experimental setup can be seen in Figure 3.4.
8.3.1 N-tvpe broadband QWIPs
As shown in Figure 8.5, the three well n-type BB-QWIP exhibits a large
responsivity peak at 10 pm. This is in excellent agreement to the peak estimated by
TMM [31] of 9.7 pm. As seen in this figure, as the bias is increased, the absolute
responsivity also increases rapidly; with a maximum responsivity value of 1.90 A/W
achieved at T=40 K, V,=-6 V when the device is illuminated from the 45 facet. It is also
interesting to note that when the applied bias is negative, the bandwidth of the
responsivity curve increases. For example, the full-width half-maximum (FWHM)
spectral bandwidth of the device at Vh=+4 V is AX/Xp=13%, while the bandwidth at Vh=-4
V is A)JX=] 8%.
Figure 8.6 shows the responsivity of the three well n-type BB-QWIP at lower
applied biases. Here we see that the bandwidth of the device at \ Vh\<2 V, is very broad
with a maximum value of A7JX=21 % at Vh=-2 V, with a reasonable responsivity of 58
mA/W at 10 pm. It is interesting to note that the relative responsivity of the shorter
wavelength quantum well and the longer wavelength quantum well (versus the 10 pm
quantum well) is a larger proportion of the peak responsivity at the low bias levels than at
higher (e.g., Vh>4 V) biases. Also note the very broad responsivity curve of the three well


63
The variation of peak absolute responsivity as a function of bias and operating
temperature is shown in Figure 6.3(b). The decrease in responsivity with applied bias at
50 mV point was attributed to the breakdown of the resonance between the bound HH2
states, which results in the photoexcited heavy holes with a much lower probability of
tunneling out of the SL- absorber layers and registering as photocurrent. As shown in
Figure 6.3(b), a large photovoltaic (PV) response at zero bias was observed in this device
(e.g., R,=39.1 mA/W at T=40 K), which is attributed to the large oscillator strength of the
HH1 to HH2 intervalence band optical transition in conjunction with the built-in field
from dopant migration.
Figure 6.4 shows the dark current as a function of applied bias voltage measured at
20, 30 and 40 K. The internal current of the device is measured by capping of the device
and placing a cold-shield around the device mount. This ensures that the background flux
in the dewar is negligible and the only contributions to the dark current are due to the
internal mechanisms in the device. The slight asymmetry in the dark current can be
attributed to band bending due to the effect of dopant migration [50],
Table 6.1 summarizes the measured responsivity, noise, quantum efficiency gain
product (r^g) and calculated detectivity for different biases and temperatures. For
sufficiently high operating temperatures it is possible to measure the noise voltage of the
device. The current output of the detector is converted to a voltage using a TIA. The noise
voltage is measured using a spectrum analyzer, which measures the power spectral
density of the resultant signal at the chopping frequency (22Hz). The increase of
responsivity from 0 to 20 mV has been attributed to the offset of the built-in field caused
by dopant migration during layer growth [50] and the resonant lining up of the HH2
states to form a miniband. The decrease in the responsivity as the bias is increased from
20 to 50 mV is attributed to the breakdown of the resonant miniband formed by the
combination of the superlattice/quantum wells, which causes the tunneling probability of
the carriers through the thin superlattice barriers to dramatically increase. Note that these


THE DESIGN AND CHARACTERIZATION OF STRAINED-
LAYER QUANTUM WELL INFRARED
PHOTODETECTORS
By
JEROME T. CHU
DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA


21
2.3.2 OWIP Collection Efficiency
A figure of merit that can be easily quantified by simple measurements is the
collection efficiency, r\c. The collection efficiency describes the ease in which the energy
from the incident photon flux is converted into mobile carriers which are swept out of the
QWIP by the applied bias and collected; and is defined as the product of the quantum
efficiency, r|, and the photoconductive gain, G.
tic=tiG (2.44)
In addition to being expressed as the mean free path over the total width of the
quantum wells and barriers, G can be viewed as the ratio of the carrier transport lifetime,
iL, to the transit time, t7> through the QWIP. Empirically, the photoconductive gain can
be described in terms of the capture or trapping probability,/^ [41-43],
G =
1
NpM+Pc)'
(2.45)
and N is the number of wells. If pc is small, then G can be approximated as, G&\/Npc.
Physically, the trapping probability is defined as the ratio of the escape time from
the well region to the lifetime of the excited carriers from the confined ground state. If
the excited states are in resonance with the top of the barrier potential energy, then the
escape time will be greatly reduced, which theoretically minimizes the trapping
probability and maximizes the photoconductive gain. Therefore in all of our designs, we
attempted to make the energy of the upper excited peak for the main detective peak in
resonance with the top of the barrier potential energy.
If B algw\ and p<< 1, an approximate expression for can be written as
Ac = Ml R) 1 ~ exp(~ Ba 0
1 -Pe
NPc
~ A(\ R)
NPc
(2.46)
(2.47)
where B is a constant dependent on the number of passes the IR radiation makes through
the photodetector, A is a polarization dependent variable equal to 0.5 for n-type QWIPs


State in the quantum well is a heavy hole state. This case can be easily achieved with the
InGaAs/GaAs material system, where the amount of compressive strain can be controlled
by the mole fraction of indium in the InGaAs quantum wells. In contrast, when tensile
strain is applied in the quantum wells, the ground hole state is then a light hole state,
which is comparable in terms of effective mass and transport properties with the
conduction band electrons. Tensile strain can be incorporated easily in the quantum wells
when using the InGaAs/InAlAs/InP material system. Each approach to using strain to
enhance the capabilities of the quantum well infrared photodetector attempts to overcome
the deficiencies of the unstrained p-QWIPs, while retaining the beneficial qualities
inherent in p-type QWIPs, such as the ability to couple normally incident infrared
radiation without the need for grating couplers.
Until recently, the development of high performance QWIPs for tactical (high
background) and space based (low background) surveillance, imaging, ranging and
tracking systems, has mainly been centered around n-QWIPs, which have been
mentioned as a possible rival to the dominance of mercury cadmium telluride based
(MCT) devices used in recent decades for LWIR imaging. Because of the added
processing steps necessary to incorporate grating structures on n-QWIPs for light
coupling, and the need for high performance, high yield, focal plane arrays (FPAs)
necessary in these systems, some of the newer research efforts have made attempts to
look into strained-layer p-QWIPs as a possible competitor or complement to n-type
quantum well infrared photodetectors. Thus the interest in exploring p-type strained layer
quantum well infrared photodetectors.
In chapter 2, we will describe the fundamental physics behind the quantum well
infrared photodetectors. These sections include, the calculation of the electronic states in
the quantum well by using the transfer matrix method (TMM), the calculation of the
energy bandgap with the effects of strain included, the basic theory of intersubband
3


24
performance (BLIP). In a BLIP QWIP, the dominant current is due to photon noise, since
all the other sources are negligible by comparison. The photon noise is calculated from
the arrival statistics of the incoherent photons. The background photon noise current, i
is given by [45,46]
i2np=4Adq\g2PbBfl(hv), (2.57)
where Ph is the incident background optical power, Bf is the QWIP bandwidth, r\ is the
absorption quantum efficiency, v is the incident photon frequency, and g is the noise
current gain. The photocurrent, Ip can be approximated by
/ = 4?/Av)ng/> (2.58)
where Ps is the incident optical signal power. The constant, A, in Eqs. (2.46), (2.47),
(2.57), and (2.58), is due to the polarization selectivity for n-type QWIPs versus p-type
QWIPs. As previously stated, for n-type QWIPs, ,4=0.5, while A=1 for p-type QWIPs.
By setting the signal-to-noise power ratio equal to unity, the background limited noise
equivalent power, (NEP)mw and the detectivity, D*BLIP, for n-type QWIPs can be
expressed as
(NEP)llur=2j2hv BPJq
D'llur=jAJ3/(NEP)
1, f
nur
lj2hc
0
vft
1/2
(2.59)
(2.60)
where A,, is the active area of the detector, and Qh=Ph/(Ahv) is the incident photon flux
from the background for a given spectral bandwidth, Av, and a peak wavelength, Xp. Qh
is defined as
Q =
2n
v Av
sin
27
(2.61)
c exp hv /(A:flr)j-
where, 0, is the field of view (FOV). For a p-type QWIP, a factor of V2 is used in the
denominator of Eq. (2.60), D*rlip, since it can absorb both optical polarizations of the
incident IR radiation.


Current (A)
34
Applied Bias (V)
Figure 3.3: Dark current characteristic of the InGaAs/AlGaAs CSL p-QWIP.


CHAPTER 9
SUMMARY AND CONCLUSION
Many novel p-type quantum well infrared photodetector (QWIP) structures have
been investigated through the course of this research project. We have looked into the
performance of normal incidence p-type strained-layer in the 3-5 pm mid-wavelength
infrared (MWIR) and 8-14 pm long-wavelength infrared (LWIR) regimes and found that
as a whole, strained-layer p-QWIPs perform better in terms of detectivity, responsivity
and operating temperature than unstrained p-QWIP designs, as shown in Figure 9.1.
The first p-QWIP investigated was a compressively strained InGaAs/AlGaAs
QWIP grown on SI GaAs for MWIR and LWIR two-band two color detection. The
intersubband absorption and photoresponse of this normal incidence p-type QWIP were
enhanced by using compressive biaxial strain in the InGaAs quantum well layers. A
single LWIR peak was found at \>i 7.4 pm and r=77 K with an applied bias of 5 V, with
a corresponding spectral bandwidth of AAA,,,=30% and a responsivity of 37 mA/W. A
single MWIR peak was also found at A, ,=5.5 pm under the same conditions previously
mentioned. The MWIR peak has a bandwidth ranging from approximately 4 to 6 pm.,
with a spectral bandwidth of AAy9y,=27%, which is again a rather wide peak. The
responsivity measured for this peak was found to be 8 mA/W at T=77 K and Vh=5.0 V.
The LWIR detectivity of this device was determined through noise measurement to be
D*=1.06x10' cm-Hz'/2/W at 7=81 K and Vh=\.0 V. This device demonstrated that p-type
strained-layer QWIPs can exhibit high responsivity and detectivity at high operating
temperatures while under normal incidence illumination.
The next design studied was a compressively strained p-type InGaAs/AlGaAs/GaAs
QWIP grown on S.I. (100) GaAs which utilized the step-bound-to-miniband intersubband


LH1
HH1
TSL P-QWIP
HH3
CSL P-QWIP
SBTM P-QWIP
Figure 1.2: The schematic energy band diagrams for the p-type valence band
intersubband transitions for the tensile strained and compressively strained bound-to-
continuum and the step-bound-to-miniband.
7


92
The half peak range is from 7 to 11.2 pm. The peak responsivity is seen to increase
linearly with applied bias in the negative bias regime. In contrast to the responsivity
curve exhibited by either n-type BB-QWIP, the p-type BB-QWIP does not seem to have a
large variation of spectral bandwidth as a function of applied bias; since the bandwidth of
the device at low (Vb=-0.5 V) biases is also &.Wkp=4%%. But this device does not also
exhibit the massive increase in responsivity at high biases like either n-type BB-QWIP.
In the case of the p-type BB-QWIPs, all of the responsivity measurements are illuminated
through the top ring contact for normal incidence, and the IR radiation is considered to
only pass through the layer structure once. As seen in Figure 8.13, the dark current
increases as the device temperature is increased.
Using the results of the responsivity and dark I-V measurements, we were able to
estimate the detectivity of the variable composition p-type BB-QWIP by estimating a
gain of 0.02. This gave us values of D* equal to 3.08xl09 and 3.63xl09 cm-Hzl/2/W at
T=50 K and Vh=-\.0 and -1.5 V respectively. Note that this calculated detectivity is only
for a single pass of radiation through the multi-quantum well stack.
In Figures 8.14(a) and 8.14(b) we see the variation of responsivity as a function of
applied bias and incident radiation wavelength at T=40 K for the variable thickness p-
type BB-QWIP. This p-type BB-QWIP has a response peak at 7.p=9.6 pm with a
corresponding FWHM bandwidth of A777^=63% under all applied biases from 6.5 to 12.5
pm. A maximum responsivity of 25 mA/W was obtained under normal incidence single
pass illumination at Vh=+\.\ V. As also seen in these figures, there is a slight PV
response with a peak at 10.2 pm. Figure 8.15 shows the dark I-V characteristic of the
variable thickness p-type BB-QWIP as a function of device temperature. Note the dark
current asymmetry evident, which has been attributed to dopant (Be) migration. When
comparing the two p-type BB-QWIPs, we see that variable thickness BB-QWIP has
higher responsivity with a longer peak wavelength and a significantly larger FWHM
spectral bandwidth.


17
Since there is no backwards, or in the -z direction, propagation in the 7V+/-th layer,
the magnitude of the wave function \\iN+i=0. Thus we can find the vj/,+ term of E,+, in the
z'-th layer (i = 2, 3,4, , N+l).
If we determine the quantity, vy/Vt)//, as a function of Eh then we will know the
locations of the resonant peaks. The transmission probability can be expressed as
\T-1\=
Vl
(2.25)
2.2.5 Determination of Intersubhand Transitions and Absorption Coefficients
In addition to the energy level and energy band locations, the calculation of
intersubband and interband transitions are also of great interest. In order to determine the
intersubband and interband transitions in a p-type strained layer QWIP, the usage of the
6x6 Hamiltonian which includes the previously mentioned k p Hamiltonian [32,33,36]
and the strain Hamiltonian [34], Since the strain and the spin-orbit coupling terms do not
lift the spin degeneracy, the 6x6 Hamiltonian matrix can then be factored into two 3x3
irreducible matrices. The assumption that the Fermi distribution function is equal to one
for the confined ground state and equal to zero for the excited states in equilibrium is
used to simplify the calculation without loss of accuracy. The absorption coefficient for
the intersubband or interband transition between the initial ground state, i, and the final
continuum state,/ is given by [39]
r ti
,() = z
4712e2
ncmld) J,1Z l
r 2dk
/iZ (2n2)
(//"//)
A(/(k)-fao]2 +(r2 / 4)
(2.26)
where nr is the refractive index in the quantum well, m0 is the free electron mass, A(/ is the
energy difference between the initial ground state, i, of energy E¡(k) and the final state, /
with the corresponding energy of E/k). e and co are the unit polarization vector and the
frequency of the incident IR radiation, respectively, f and / are the Fermi distribution
functions of the initial and final states, and T is the full width of level broadening. T


BIOGRAPHICAL SKETCH
Jerome T. Chu was bom in Taiwan, the Republic of China on January 8, 1968. He
received a Bachelor of Science in Electrical Engineering degree from the University of
Florida in May of 1991. In June of 1993, he was awarded a Master of Science in
electrical engineering from Stanford University. Afterwards, he enrolled in the
Department of Electrical and Computer Engineering at the University of Florida and
began his Ph.D. research in August of 1993, on the design and development of p-type
strained-layer quantum well infrared photodetectors for mid- and long-wavelength
infrared imaging array applications.
129


79
expect and increase in the quantum efficiency and the absolute responsivity; both of
which are directly related to the linear absorption coefficient.
7.2 Device Layer Structure and Processing
The p-type TS-QWIP consists of 20 In^Ga^As quantum wells, 70 thick
sandwiched by 500 thick undoped In052Ga048As barriers. The quantum wells are Be-
doped to a density of 3xl018 cm'3. This multiquantum well structure is then surrounded by
a top and bottom ohmic contact layer, 0.3 and 0.5 pm thick, respectively. In between the
highly doped ohmic contacts and the multiquantum well structure are extra undoped
In,, 52A10 48As layers 150 thick which act as blocking barriers to reduce the triangle
potential formed by the ohmic contacts. The ohmic contact layers are lattice matched (to
InP) In0 52Al0 48As layers Be-doped to a density of 5 x 1018 cm'3. All of the layers are
grown on semi-insulating (100) InP. The complete layer structure for this device is
shown in Figure 7.1.
In order to characterize the devices, a wet chemical etch was used to create a
216x216 pm2 mesa structure for the both test devices. Cr/Au was used to form the top
and bottom ohmic contacts. To facilitate normal incidence illumination, a ring contact
around the mesa edge was used to allow light to pass through to most of the mesa top
surface with a 75% fill factor. The devices were then bonded onto 16 pin chip carriers
(TO-8 cans) and wired to the contact pads via ultrasonic wedge bonding.
7.3 Device Characterization
QWIP-D is sensitive in the MWIR range at 5.1 pm, with a estimated detectivity of
1.1 x 1010 cm-Hzl/2/W under a bias of 2 V at T=77 K. This is about 10% of the theoretical
maximum D* at this peak wavelength for a photoconductive device. As seen in Figure
7.2, the intersubband transition occurs from the heavily doped ground light-hole state to


48
-3-2-10123
Applied Bias (V)
Figure 4.4: The dark I-V characteristic of the InGaAs/GaAs/AlGaAs CSL SBTM p-
QWIP with the 300 K background photocurrent superimposed.


99
GaAs GaAs
In0.25 ^a0.75As In0.25^a0.75^s In0.25^a0.75^S
4.5 nm 5.5 nm 6.2 nm
GaAs
300 nm
Be=4xl018cm'3
GaAs
40 nm
undoped
^n0.2^^0.7fA^
4.5 nm
Be=4xlOl8cnv
GaAs
40 nm
undoped
Repeat
Ino.25Gao.75As
5.5 nm
Be=4xlO'W
x 20
GaAs
40 nm
undoped
In02/jao jsAs
6.2 nm
Be=4xl0l8cm'3
GaAs
40 nm
undoped
GaAs
500 nm
Be=4xl018cm'3
S.I. GaAs substrate
(b)
Figure 8.4: The (a) schematic energy band diagram and the (b) complete layer structure
of the variable thickness p-type BB-QWIP.


85
Applied Bias (V)
Figure 7.4: The measured dark current of the p-type MWIR TS-QWIP at T=ll K.


12
a small effective mass (comparable to the electron effective mass), the optical absorption
and spectral responsivity in p-type QWIPs can be greatly enhanced, as a result of
introducing strain in the quantum well. In addition to the utilization of the light hole
states for their small effective masses, etc., certain heavy hole states under compressional
strain may also have similar characteristics, like high mobilities, small effective masses,
and long mean free paths; which in turn favorably alter the intersubband absorption and
transport characteristics, as shown by Hirose et al. [13]. This is achieved by distorting
the heavy hole valence band at and near the zone center via the introduction of
compressional strain.
2.2.3 Energy Band Calculations
To calculate the locations of the energy subbands, we can use the transfer matrix
method (TMM) [28,30,31], based on the eight-band kp model. This model is represented
by the Luttinger-Kohn Hamiltonian [32,33], H which describes the unstrained
semiconductor.
H,=H + V(z)
(2.10)
where
(2.11)
with


44
increases to 20.7%, which increases the current responsivity to 67 mA/W. The
detectivity is also increased by a similar amount to 7.2x109 cm-Hzl/2/W.
4.5 Conclusion
We have demonstrated that with the use of compressive strain and superlattice
barriers, the step bound-to-miniband transitions can be achieved, which could be useful in
creating new narrow bandwidth LWIR p-QWIPs. Given the inherent benefit of normal
incidence detection without the use of grating couplers, the simplicity of the p-QWIP
design deserves futher investigation. A maximum responsivity of 28 mA/W was found at
10.4 pm, with a corresponding detectivity of 1.4xl09 cm-Hzl/2/W. By futher optimizing
the quantum well doping density, biaxial strain strength, superlattice barrier parameters,
and inserting a triangle potential blocking layer in the ohmic contact regions, high
performance LWIR and MWIR p-QWIPs using the SBTM intersubband transition can be
developed for focal plane array imaging sensor applications. Additional increases in
responsivity and detectivity can be achieved when the substrate is thinned so that a
waveguide-like region is formed when multiple reflections can take place, increasing the
quantum efficiency of the device.


113
in series to form multicolor QWIPs. The benefit here is since the p-QWIPs do not require
grating couplers, the design and processing of the stacked multicolor p-QWIP is much
simpler and therefore more robust than that of a similar n-type device.
The fourth p-QWIP design characterized was a pair of superlattice infrared
photodetectors (SLIPs). The compressively strained-layer SLIP exhibited peak detection
wavelength at 19.2 pm in the VLWIR band, while the unstrained SLIP possessed voltage
tunability in the LWIR band. Operation up to 40 K was obtained for both the
photoconductive (PC) and photovoltaic (PV) modes detection for the VLWIR SLIP. An
absolute responsivity of 49.8 mA/W and an r|g=0.317% were achieved at T=A0 K and
Vh=20 mV with an FWHM spectral bandwidth of AAA =12%. A maximum detectivity of
2.58x107 cm-Hzl/2/W was measured at T= 20 K, Vh=20 mV. The response peak of the
unstrained SLIP could be varied from 9.2 pm to 6.5 pm by simply altering the applied
bias on the device. An absolute responsivity of 8 mA/W was achieved under normal
incidence at the 6.5 pm peak at T 20 K, Vh=-200 mV. As predicted the responsivity of
the unstrained p-QWIP is much lower than that of the strained p-QWIP. These two
devices demonstrated that the SLIP is a device which can extend the response peak of the
QWIP out to the VLWIR range and can also exhibit voltage tunability all while under
normal incidence illumination.
The fifth design looked at was the InGaAs/InAlAs tensile strained p-QWIP grown
on InP. Previous attempts at using InP based technology have not been very successful
due to the inclusion of cross-hatching during material layer growth which destroys the
absorption characteristics of the QWIP. We found that the tensile strained p-QWIP is
sensitive in the MWIR range at 5.1 pm, with a estimated detectivity of 1.1 x 10' cm-
Hzi/2/W under a bias of 2 V at T=77 K. This is about 10% of the theoretical maximum
D* at this peak wavelength for a photoconductive device. The intersubband transition for
this QWIP is from the ground light-hole state to the extended heavy-hole state. Since this
device was devoid of cross-hatching, we predict that this p-type tensile strained QWIP


98
Illg 2G&Q 8As Ing 25^^0 75AS Ing 7AS
GaAs GaAs
6.0 nm 5.5 nm 5.0 nm
(a)
GaAs
300 nm
B e=4x 1 0"cm'3
GaAs
40 nm
undoped
In03Ga07As
5 .0 nm
Be=4x 1 0l8cm'3
GaAs
40 nm
undoped
Repeat
I a 0 25 ^ a0 75A s
5.5 nm
B e = 4x 1 0l8cm'3
x 20
GaAs
4 0 nm
undoped
ln02Ga08As
6.0 nm
Be=4x 1 0l!cm
GaAs
40 nm
undoped
GaAs
500 nm
Be=4x 1 018cm'3
S.I. GaAs substrate
(b)
Figure 8.3: The (a) schematic energy band diagram and the (b) complete layer structure
for the variable composition p-type BB-QWIP.


109
Wavelength (jam)
(a)
Wavelength (^m)
(b)
Figure 8.14: The measure photoresponse of the variable thickness p-type BB-QWIP as a
function of (a) positive and (b) negative applied bias.


Responsivity (AAV)
105
Applied Bias (V)
Figure 8.10: The measured photoresponse of the four well n-type BB-QWIP as a
function of applied bias and incident illumination. Note that up to 50% of the 45
incidence responsivity is achieved at normal incidence.


80
the extended continuum state. The responsivity curve for this particular device is shown
in Figure 7.3. For the responsivity curves, the DUT is at 77 K with a field of view of
180. This device was tested in a closed cycle cryogenic dewar and illuminated by a
blackbody source running through a l/8m grating monochrometer. The output of the
DUT was amplified by a biasing TIA and sensed with a Stanford Research 830 lock-in
amplifier. The data from the lock-in amplifier was then compared against a standard
pyroelectric detector to normalize and scale the data to obtain the absolute photoresponse.
A schematic diagram of the experimental setup is shown in Figure 3.4 The data shows a
rather broad MWIR peak with an FWHM bandwidth of AXA/)=37%, which is typical for
p-type strained-layer QWIPs [61].
As expected for an MWIR device, the dark current for this device is rather low, as
we can see in Figure 7.4. The large asymmetry seen in the I-V curve can be attributed to
the dopant migration effect which occurs during layer growth [50]. This effect was also
seen in previous p-type tensile strained devices [47].
7.4 Conclusion
We have characterized an InGaAs/InAlAs on InP TS-QWIP with a peak
responsivity at 5.1 pm. The detector was found to be a stable tensile strained p-type
quantum well infrared photodetector, which exhibited an intersubband transition from a
light-hole ground state. The performance of the TS-QWIP was moderate, with a
detectivity of 1.1x10' cm-Hz1/2/W at T= 77 K, FA=2.0 V at the MWIR peak of 5.1 pm.
This device also had an FWHM bandwidth of AA/A7,=37%. This device did not exhibit
the typical visible crosshatching that had been previously reported for similar
InGaAs/InAlAs on InP p-QWIPs [47], which should improve reliability of InP based p-
QWIPs. Further work into this type of QWIP can be performed, and improvements can
be made by shifting the detection peak towards the LWIR band and by changing the


11
(2.4)
(2.5)
In addition to altering the physical parameters of the QWIP, lattice strain can also
induce energy band shifts, which can be used to alter the absorption characteristics of the
QWIP. The strain induced energy band shifts for the conduction band, the heavy hole
subband, and light hole subband can be approximated as follows.
(2.6)
(2.7)
(2.8)
o
where c, is the combined hydrostatic deformation potential which characterizes the
splitting of the T* valence band under strain, and b is the shear deformation potential, and
A0 is the spin orbit split-off energy [23]. The total hydrostatic deformation potential
c,+V where Vr is the valence band deformation potential, can be expressed by [25]
(2.9)
where dEJc/P is the unstrained energy bandgap change with respect to the unit pressure.
The effect of strain on the energy band structure results in the splitting of the heavy
hole and light hole band at the valence band zone center [26] (i.e., the in-plane
wavevector k||=0), which is degenerate in the unstrained case. When tensile strain is
applied between the quantum well and the barrier layers [27-29] along the superlattice
growth z-direction, the strain can push the light hole levels upwards and pull the heavy
hole levels downwards. We can therefore expect that heavy hole and light hole states are
inverted at specific lattice strains and quantum well thickness. This phenomena will in
turn cause the intersubband transitions in a QWIP structure to take place from the
populated light hole ground state to the upper energy band states. Since the light hole has


CHAPTER 5
A STACKED COMPRESSIVELY STRAINED P-QWIP WITH TWO-BAND TWO-
COLOR DETECTION
5.1 Introduction
Recently, interest in the development and characterization of multi-color or multi-
spectral infrared detectors has grown to the point where many studies have been started
[48,54], In the area of QWIPs, most of the structures in development or under
consideration have been n-type QWIPs [48,54], Little work has been done with the
development of multi-color QWIPs using p-type materials that are sensitive to normal
incidence IR radiation without the need for complex gratings. Therefore, this chapter is
focused on the development and characterization of a stacked compressively strained p-
type QWIP with two-band, two-color detection.
5.2 Theoretical Considerations and Device Fabrication
The p-QWlP outlined in this chapter is a multicolor stacked p-QWlP for the MW1R
and LWIR two-band detection. Figure 5.1 shows the energy band diagram of a stacked
p-type compressively strained layer (PCSL) InGaAs/AlGaAs QWIP for the MWIR and
LWIR detection. This multicolor QWIP consists of two distinct multi-quantum well
stacks separated by a common ohmic contact layer and sandwiched between he two (top
and bottom) ohmic contact layers. This stacked PCSL-QWIP was grown by the MBE
technique on SI (100) GaAs substrate. The bottom contact consists of a heavily Be-
doped GaAs contact layer. On top of the contact, the Al0 jGao 7As/IiIq 2Gao 8As QWIP layer
structure was grown for the MWIR stack. The LWIR QWIP stack was formed by using
Be-doped In015Gao85As quantum wells surrounded with the undoped Al0 ,Gao9As barrier
50


37
Applied Bias (V)
(b)
Figure 3.6: (a) the LWIR and (b) MWIR responsivity as a function of bias and
temperature of the InGaAs/AlGaAs CSL p-QWIP.


52
The dark I-V characteristic for the MWIR, LWIR, and the combined stacked QWIP
measured at T= 77 K is shown in Figure 5.3(a). As expected the dark current of the LWIR
QWIP stack is several orders of magnitude higher than the MWIR QWIP stack, due to the
exponential dependence of the dark current on the barrier height. As clearly seen in
Figure 5.3(a), most of the voltage drop is across the MWIR stack due to the much larger
dynamic resistance of the MWIR stack. Figure 5.3(b) shows the measured I-V curves at
r=40, 60, and 77 K for the LWIR QWIP stack. The asymmetry in the dark I-V
characteristics observed in this device can be attributed to the dopant migration effect
[50].
The spectral responsivity for the MWIR QWIP measured at V=5 V and T=ll K is
shown in Figure 5.4. The responsivity measurements were performed with the device
mounted in a closed cycle helium dewar and illuminated by a blackbody source running
through a grating monochrometer and appropriate IR filters. The resulting photocurrent
is amplified and detected by a lock-in amplifier. A schematic diagram of the
experimental setup can be seen in Figure 3.4. Results of the measurements revealed that
two photoresponse peaks were observed in the MWIR band at A,/),./=4.8 pm and
A./,,.,=5.4 pm. The 4.8 pm peak is in excellent agreement with the ground heavy hole
(HH1) to second bound heavy hole (HH3) transition calculated by the TMM, which
predicts a detection peak at 4.7 pm. The detectivity for this MWIR peak was determined
to be D*=3.3x10" cm-Hzl/2/W at Vh=\.0 V and 7=77 K. A responsivity of 13 mA/W was
found at T=ll K, Vh=5 V at this peak. The measured spectral bandwidth for the first
MWIR peak was found to be AX/Xpmwl=2l% and AX/Xpmw2=26% for the second peak. The
second, longer wavelength peak is attributed to the transition from the HH1 states to the
second bound light hole (LH2) states within the quantum well. The calculated
responsivity peak for this transition is 5.6 pm, which is also in good agreement with the
measured value. The higher responsivity of the 5.4 pm peak is attributed to the higher
absorption coefficent inherent with the HH to LH transition. The spectral detectivity for


20
2.3.1 Spectral Responsivity
The responsivity, R, for a photodetector may be expressed as [40]
R = 1PG = ^-
(2.39)
where q is the electronic charge, X is the wavelength of the incident photon, h is the
Planck constant, c is the speed of light, r| is the quantum efficiency, rp is the collection
efficiency, v is the incident frequency, and the photoconductive gain is G. The quantum
efficiency and photoconductive gain are described, respectively, by [40]
r| = Rf )[l- exp(- Ba /J]
L
G =
t.
(2.40)
(2.41)
where A is a constant that is polarization dependent, a is the absorption coefficient of the
quantum well, lqw is the total width of all quantum well regions, L is the mean free path of
the carrier, Rf is the reflection coefficient, and xc is the total width of all quantum well and
barrier regions. B is a constant dependent on the number of passes IR radiation makes
through the photodetector. For n-type QWIPs, .4=0.5, while for p-type QWIPs A=\. The
mean free path of the carrier may be expressed as [40]
L = x T/irptl// F, (2.42)
where x is the well recapture lifetime of the carrier, Tqu is the transmission
coefficient over the quantum well, is the effective mobility of the carrier, and F is the
electric field. The effective mobility for a two-band transport model is shown to be [40]
Pc# = ... (2-43)
AP//, + AP
hh
where Aphh and Ap,h are the concentrations of optically induced heavy- and light- hole
carriers, respectively, and Aphh and Ap,h are the concentrations of optically induced heavy-
and light- hole carriers. When only the ground state is completely occupied, either Ap,h or
A.p/h, the optically induced light holes or the optically induced heavy holes dominate, so
that we may estimate as the in-plane effective mass of the ground state carriers.


Responsivity (A/W)
100
6 7 8 9 10 11 12 13
Wavelength (|am)
Figure 8.5 The measured absolute responsivity for the n-type BB-QWIP as a function of
applied bias and incident IR wavelength at T= 40 K. The responsivity is measured
through a 45 facet.


117
Table 9.1: Summary of the QWIPs studied, comparing detectivity, responsivity and
operating conditons.
Device
Detectivity
(cm-HaP/W)
Operating
Conditions
Responsivitj
(mAAV)
Response
Peak(s) (pm)
Quantum
Eff. x Gain
1.06xl01J
T=81 K, Vb=l V
37
7.4
0.62%
Ino.2G3o.8 As/ At. is Gao.85 As
6.3x109
T=81 K, V¡,=2 V
p-type CSL
3.2x109
2.3x109
T=81 K, Vb=3 V
T=81 K, Vb=l V
8
5.5
0.20%
Ino.12Gao.88As/GaAs/
Alo 35 Gao 65 As
p-type CSL SBTM
1.4xl09
T=65, Vb=l V
28
10.4
0.33%
Irio.2G3o.8 As/At.3G30.7As
3.3x10"
T=77 K, Vb=l V
13
4.8
0.34%
MW p-type CSL
5.5x10"
T=77 K, Vb=l V
19
5.4
0.43%
Irio. 15 G3o.85 As/At. i Gao.9 As
1.1x10'
T=40 K, Vb=2 V
25
10
0.31%
LW p-type CSL
Iri0.27G30.73 As/ At. 15 G3o.85 As
2.6x107
T=20 K,Vb=20 mV
49.8
19.2
0.33%
p-type SLIP
Afc 4Gao.6 As/At 3 Gao.7 As/
T=20 K,Vb=-50 mV
4
9.3
0.05%
GaAs unstrained SLIP
T=20 K,Vb=-50 mV
5
6.6
0.10%
Ino 4Gao 6 As/In) 52 Gao 48 As
p-type TSL
l.lxl O10
T=77 K, Vb=2 V
3
5.1
0.07%
Ino.2G3o 8 As/IauGao s As/
Ino.2G3o 8 As/At 07G30 93 As
3.12x10'
T=60 K,Vb=-4 V
1900
10
23.5%
n-type CSL BB
O
O
X
T=60 K,Vb=-2 V
In0 3 Gao 7 As/I rb 25 Gao 75 As/
Illo 2G30 8 As/Il\) 17G30 85 As/
2.34x10'
T=50 K,Vb=-4 V
2320
10.3
27.9%
GaAs 11-type CSL BB
1.65x10'
T=50 K,Vb=-2 V
I n0 3 G ao 7 A s/I itj. 25 G ao. 75 As/
Iiio 25G30 75 As/GaAs
3.63x10''
T=50 K,Vb=-1.5 V
19
9.3
0.25%
p-type CSL BB
Ino 2sGao 75 As/GaAs
9.54x109
T=50 K,Vb=+l.l V
25
9.6
0.33%
p-type CSL BB


19
Here A, B, N, % r\ are inverse mass band parameters [39].
2.2.6 Photoconductive Detection Mode Operation
When IR radiation impinges on a photoconductor, the photoconductive material
undergoes a physical change characterized by a change in resistance, ARd. This change in
resistance is due to the photo-excitation of carriers, forming mobile excess carriers in the
photoconductor. The excess photogenerated carriers, An, can be expressed as
where, r\, is the quantum efficiency, AO is the incident photon flux, x, is the excess
carrier lifetime, and Vd is the volume of the detector. These photogenerated carriers are
transported out of the detector under the influence of the applied external bias, which
results in a photovoltage signal. The change in the output photo voltage, AV, due to the
resistance change is given by
(2.38)
where R, is the load resistance and its value is typically chosen to be about equal to Rlh
the detector resistance, to match loads and to optimize the output signal.
2.3 P-OWIP Figures of Merit
Although our band structure and absorption calculations can be used to determine
the positions of the subbands in the quantum wells, and hence determine the peak
absorption wavelength of the QWIP, many other factors must be taken into account to
design a QWIP with the correct detection peak. Generally, for a high-performance
QWIP, the responsivity must be high, while the noise current, and hence the dark current,
must be low.


91
50% more than that of the three well n-type BB-QWIP under the same operating
conditions. It is also interesting to note the increased prominence of the 12 pm peak at
higher negative biases, which significantly contributes to the flatness of the responsivity
curve at Vh<-4 V. As shown in Figure 8.11, we also a normal incidence response of this
device of 50% or more of the 45 degree values throughout the active bias range.
Figure 8.12 shows the measured dark I-V characteristic of the four well n-type BB-
QWIP. Note the almost complete symmetry between the negative and positive bias
regimes. The larger dark current of the four well device when compared to the three well
device is attributed to the lower barrier height of the longest wavelength well in the four
well n-type BB-QWIP, when compared with the longest wavelength well of the three
well BB-QWIP. Because of the extended responsivity of this device, when compared
with the three well n-type BB-QWIP, the BLIP operating temperature is predicted to be
in the 50 K range. Using the results of the dark I-V and responsivity measurements, the
estimated D* of 1.65x10' cm-Hz1/2/W was found at 7=50 K and Vh=-2 V, using a gain of
0.1 for the four well n-type BB-QWIP. When an applied bias of -4 V and a gain of 0.15
is used, the detectivity of this device was found to be 2.34x10I() cm-Hz1/2/W at 7=50 K.
Once again, the normal incidence responsivity was found to be about half that of the 45
incidence value or 8.1x10 and 1.2x10' ctn-Hz':/W at 7=50 K and V,=-2, -4 V,
respectively.
8.3.2 P-tvpe broadband OWIPs
Next, we will discuss the results of the measurement of the variable composition p-
type BB-QWIP. Figure 8.12 shows the absolute responsivity as a function of applied bias
and incident IR radiation wavelength. This p-type BB-QWIP has a response peak at 9.3
pm, with a maximum responsivity of 19 mA/W at T40 K and FA=-1.5 V. Also under the
previously stated operating conditions, the FWHM spectral bandwidth is AA./A,/)=48%.


This dissertation was submitted to the Graduate Faculty of the College of
Engineering and to the Graduate School and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy.
May 1998
Winfred M. Phillips
Dean, College of Engineering
Karen A. Holbrook
Dean, Graduate School


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27
transitions occur between the heavy-hole ground state and the upper heavy-hole excited
states. Due to the large heavy-hole effective mass, weak absorption and low responsivity
are expected for the unstrained p-QWIPs. By utilizing biaxial strain confined in the
quantum wells of the QWIP, we can increase the responsivity and the background limited
photocurrent (BLIP) temperature, which offers more flexibility in the design and
fabrication of p-QWIPs. Here we present a new normal incidence p-type compressively
strained-layer (PCSL) Ir^ 2Gao gAs/Al0 ^Gao 85As QWIP grown on S.I. GaAs by molecular
beam epitaxy. In this QWIP structure, the intersubband utilizes a bound to continuum
transition between the ground heavy-hole (HH1) state to the second extended heavy-hole
(HH3) state for LWIR detection. The MWIR detection peak is due to the intersubband
transition from the HH1 state to the first continuum heavy-hole (HH4) state.
3.2 P-type Compressive Strained Laver QWIP Design
Compressive strain is introduced in the In02Gao8As quantum wells of the QWIP,
while no strain is present in the Aln ^Ga^^As barrier layers, which is lattice matched with
the semi-insulating (SI) GaAs substrate. The induced strain pushes the heavy hole states
upwards and the light hole states downwards relative increasing electron energy in the
InGaAs quantum wells. Thus the heavy- and light-hole bands are split in the quantum
wells, but remain degenerate in the AlGaAs barrier regions at the Brillouin zone center.
The Irio jGaogAs/Alo ^Ga^As p-type CSL QWIP was grown on a (100) SI GaAs
substrate by molecular beam epitaxy. The structure consists of twenty periods of 48
In^GaogAs quantum wells spaced with 500 wide Alo^Ga^As barriers. The wells
were Be-doped to a density of 2x1 O'8 cm"3, while the barriers were undoped. A 0.3 pm
cap layer and a 1.0 pm thick buffer layer of GaAs each Be-doped to 5xl018 cm"3 were also
grown to serve as ohmic contacts. In addition, 600 wide Al0uGa^As barrier layers
were grown between the contact layers and the multi-quantum well structure to reduce the


124
LH
light-hole
LNA
low noise amplifier
LWIR
long-wavelength infrared red
MBE
molecular beam epitaxy
MCT
mercury cadmium telluride
MOCVD
metallorganic chemical vapor deposition
MWIR
mid-wavelength infrared
n-QWIPs
n-type quantum well infrared photodetectors
PC
photoconductive
PCSL
p-type compressive strained-layer
p-QWIPs
p-type quantum well infrared photodetectors
PV
photovoltaic
QWIP
quantum well infrared photodetector
SBTM
step-bound-to-miniband
SBTM CSL-QWIP
step-bound-to-miniband compressive strained-layer quantum well
infrared photodetector
SL
superlattice
SL1
superlattice miniband
SLIP
superlattice infrared photodetector
SI
semi-insulating
SO
spin-orbit
TIA
transimpedance amplifier
TMM
transfer matrix method
TS
tensile strained
TSL
VLWIR
tensile strained-layer
very long-wavelength infrared
VLWIR


REFERENCES
1. R.B. Emmons, S. R. Hawkins and K. F. Cuff, Opt. Eng. 14, 21 (1975).
2. L. Esaki and T. Tsu, IBMJ. Res. Develop. 14, 61 (1970).
3. S. L. Serzhenko and V. D. Shadrin, Sov. Phys. Semicond. 25, 953 (1991).
4. L. L. Chang, L. Esaki and G. A. Sai-Halaz, IBM Tech. Disci. Bull. 20, 2019 (1977).
5. L. Esaki and H. Sakaki, IBM Tech. Disci. Bull. 20, 2456 (1977).
6. D. D. Coon and R. P. G. Karunasiri, Appl. Phys. Lett. 45, 649 (1984).
7. L. C. West and S. J. Eglash, Appl. Phys. Lett. 46, 1156 (1985).
8. A. Harwit and J. S. Harris, Jr., Appl. Phys. Lett. 50, 675 (1987).
9. B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker and R. J. Malik, Appl. Phys. Lett.
50,1092 (1987).
10. B. F. Levine, C. G. Bethea, G. Hasnian, V. O. Shen, E. Pelve. R. R. Abbot and S. J.
Hsieh, Appl. Phys. Lett. 56, 851 (1990).
11. L. S. Yu and S. S. Li, Appl. Phys. Lett. 59, 1332 (1991).
12. B. F. Levine, J. Appl. Phys. 74, R1 (1993).
13. K. Hirose, T. Mizutani, and K. Nishi, J. Cryst. Growth 81, 130 (1987).
14. B. F. Levine, R. J. Malik, J. Walker, K. K. Choi, C. G. Bethea, D. A. Kleinman, and
J. M. Vandenberg, Appl. Phys. Lett. 50, 273 (1987).
15. G. Hasnain, B. F. Levine, C. G. Bethea, R. A. Logan, J. Walker, and R. J. Malik,
Appl. Phys. Lett. 54, 2515 (1989).
16. J. Y. Andersson and L. Lundqvist, J. Appl. Phys. 71, 3600 (1992).
17. B. F. Levine, S. D. Gunapala. J. M. Kuo, S. S. Pei, and S. Hui, Appl. Phys. Lett. 59,
1864 (1991).
125


BTC N-QWIP
BTQB N-QWIP
nnn nnnnnn nnn
BTM N-QWIP
Figure 1.1: Schematic energy band diagrams for the bound-to-continuum, bound-to-
quasi-bound and bound-to-miniband conduction band intersubband transistions due to
infrared photons in n-type QWIPs.
6


69
GaAs
300 nm
Be=5xl018 cm'3
GaAs
65 nm
undoped
^0.15^*0.85 AS
3.5 nm
undoped
Repeat
In 0.27 ^<1 0.73
3.2 nm
Be=3xl018 cm'3
x 4
Repeat
-^lo.isGno.ssAs
3.5 nm
undoped
x 3
GaAs
50 nm
undoped
GaAs
15 nm
undoped
GaAs
500 nm
Be=4xl018 cm"3
SI GaAs (100)
igure 6.2: The complete layer structure of the p-type SLIP.


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Require ments for the Degree of Doctor of Philosophy
THE DESIGN AND CHARACTERIZATION OF STRAINED-
LAYER QUANTUM WELL INFRARED
PHOTODETECTORS
By
Jerome T. Chu
May 1998
Chairman: Sheng S. Li
Major Department: Electrical and Computer Engineering
Many different types of p-type strained-layer quantum well infrared photodetectors
have been developed for normal incidence 3-5 pm mid-wavelength infrared (MWIR) and
8-14 pm long-wavelength infrared (LWIR) detection. The benefits possible in p-type
QWIPs are: normal incidence detection without grating couplers, lower dark current and
the ability to detect all incident infrared (IR) radiation polarizations.
The first p-type compressive strained layer (CSL-) QWIP is composed of
InGaAs/AlGaAs and grown on semi-insulating (S.I.) (100) GaAs, which exhibits two-
color two-band detection with an MWIR peak at 5.5 pm and an LWIR peak at 7.4 pm.
The next p-type InGaAs/GaAs/AlGaAs CSL-QWIP uses a step-bound-to-miniband
(SBTM) intersubband transition to detect IR radiation. This device was also grown on
S.I. (100) GaAs and was found to have a peak detection wavelength at 10.4 pm. The
third p-type CSL-QWIP design consists of two highly strained InGaAs/AlGaAs
multiquantum well stacks in series, separated by a common central ohmic contact. The
MWIR stack showed two detection peaks at 4.8 and 5.4 pm, while a 10.0 pm peak was
found for the LWIR stack.
vi


valence band deformation potential
potential step function
122
Vy
V(z)


33
GaAs
300 nm
Be=4xl018 cm'3
Al().15^*0.85-^S
50 nm
undoped
InojGao.gAS
4.8 nm
Be=2xl018 cm'3
Repeat
AIq.i sGaQ g5 As
50 nm
undoped
x 20
GaAs
500 nm
Be=4xl018 cm'3
SI GaAs (100)
Figure 3.2: The layer structure of the InGaAs/AlGaAs two-color two-band
compressively strained p-QWIP.


71
Figure 6.4: The dark I-V characteristic of the p-type QWIP as a function of applied bias
and device temperature.


22
and 1 for p-QWIPs, R is the reflection coefficient, lqw is the total width of all quantum
well regions, and a is the absorption coefficient.
2.3.3 Dark Current Relationship in a OWIP
Another important parameter to be considered in a QWIP design is the dark current
density Jd, which can be expressed using the Richardson-Dushman equation [29] as
Jd oc T2rn exp
r-AE^
Tjj
(2.48)
where m is the effective mass, tsE is the difference in energy between the barrier height
and the quantum confined state in the well, kB is the Boltzmann constant, and T is the
temperature. This type of expression assumes that the dominant source of dark current is
thermionic emission over the quantum well barrier.
In the low-field regime, the thermionic emission current is related to the density of
mobile carriers, n, and the average drift velocity, vd, and can be expressed as [44]
4 = 94/vc/, (2-49)
where Ad is the active detector area, q is the electronic charge, and
1/2
1 + (ps / Vv)2
nt = [m kTIn h2 Ljexp £,.)/ kKT
(2.50)
(2.51)
In the above equations, p is the mobility, F is the electric field, vv is the saturation
velocity, Ecf is the cut-off energy related to the cut-off wavelength Xc, and m'/nh is the
two-dimensional density of states. The Fermi energy, E,,, can be obtained from the
expression of ND:
N
D ~
m kBT
nti2 L
I In
(
1 + exp
v
Ef ~ E,
kj
\
(2.52)
m
nh2l
£(£,-£)
(2.53)


61
et al [56] reported a bound-to-continuum n-QWIP with a detection peak at 16.6 pm and a
cut-off wavelength of 19 pm. Gunapala and Bandara [55] reported a 16 pm QWIP focal
plane array (FPA) with good imagery. We report a normal incidence p-type strain layer
QWIP which utilizes four closely spaced In0 27Ga073As/Al0,jGa,, 85 As (thickness: 3.5/3.2
nm) superlattice (SL-) absorber layers to effectively create a large absorption thickness
while maximizing the oscillator strength by using the ground heavy hole (HH1) to first
excited heavy hole (HH2) state intersubband transition, in contrast with most p-type
QWIPs which utilize the HH1 to HH3 intersubband transition for optical absorption [57].
The SL- absorber layers are sandwiched between the wide (50 nm) GaAs barrier layers to
reduce the tunneling dark current at the device operation temperatures. The device was
formed with three repeats of this basic structure and will be referred to as the superlattice
infrared photodetector (SLIP).
Due to strain relaxation [20-22] in the thin SL- layers, the effective barrier height
and the corresponding energy spacing between the HH1 and HH2 hole states was reduced
from 120 meV for the strained case to 65 meV, which shifts the response peak to a
calculated value of 18 pm. The schematic band diagram for the SLIP and the
transmission coefficient calculated by the transfer matrix method (TMM) [31 ] taking into
account strain relaxation are shown in Figures 6.1(a) and 6.1(b), respectively. The
complete layer structure of the strained p-type SLIP is shown in Figure 6.2.
In order to characterize the device, a wet chemical etch was used to create a
216x216 pm2 mesa structure for the test devices. Cr/Au was used to form the top and
bottom ohmic contacts. To facilitate normal incidence illumination, a ring contact around
the mesa edge was used to allow light to pass through to most of the mesa top surface
with a 75% fill factor. The devices were then bonded onto 68 pin chip carriers and wired
to the contact pads via ultrasonic wedge bonding. Moderate background performance
measurements were made in a side-looking dewar with a KRS-5 window and are
achieved via a 32 mil cooled pin-hole aperture located at a sufficient distance to give a


GaAs
500 nm
Be=5xl018 cm'3
Al01Gao9As
60 nm
undoped
lno.i5Gao.g5AS
5.5 nm
Be=5xl018 cm'3
Repeat
Al01Gao9As
50 nm
undoped
x 20
Al0 jGa^As
10 nm
undoped
GaAs
300 nm
Be=5xl018 cm3
AlojGaojAs
45 nm
undoped

In0.2Ga0.gAs
3.3 nm
Be=5xl018 cm'3
Repeat
Alo.3Gao.7As
35 nm
undoped
x 20
Alo.3Gao.7As
10 nm
undoped
GaAs
500 nm
Be=5xl018 cm'3
SI GaAs (100)
Figure 5.2: The complete layer structure of the stacked CSL p-QWIP.


CHAPTER 7
TENSILE STRAINED QUANTUM WELL INFRARED PHOTODETECTORS
7.1 Introduction
Over the last decade, a significant amount of research in the area of p-type QWIPs
has been centered on compressively strained designs [47], While some work has
explored the realm of tensile strained (TS-) p-QWIPs, most of the work has been rather
disappointing because of the difficulties with cross-hatching and other problems that crop
up when growing highly tensile strained epitaxial layers [47,61]. We have designed a
TS-QWIP that is slightly strained in the quantum well and unstrained in the lattice
matched barrier, to try to minimize the cross-hatching in such devices and to demonstrate
that the InP base TS-QWIPs are able to survive many thermal cycles without degradation.
The motivation for using a tensile strained quantum well is because of the
theoretical improvement in linear absorption coefficient that can be achieved with the
light-hole carrier. As mentioned earlier, when coherent tensile strain in applied in a
quantum well, the excess energy inherent in the quantum well becomes expressed as an
energetic inversion of the heavy- and light-hole subbands in the valence band along with
a reduction of the conduction band minimum. Now, when the quantum well is heavily p-
doped, with beryllium for example, the ground state is completely occupied and the
carriers are available for intersubband transitions. Since the ground holes are now light-
holes, and typically the light-holes have an effective mass that is about an order of
magnitude less than the effective mass of heavy-holes, we expect the linear absorption
coefficient to be increased over an unstrained QWIP with the same infrared response peak
because of the inverse relationship between the effective mass and the linear absorption
coefficient. Because of the theoretical increase in the absorption coefficient, we also
78


68
k10.2'Pa0.737^S
(a)
H
s
Energy (nieV)
(b)
Ev
Figure 6.1: The (a) idealized band diagram and (b) the calculated transmission
coefficient from TMM for the p-type SLIP.


29
1000 layer of Au to create both the top and bottom ohmic contacts. The top ohmic
contact consists of a ring type structure around the edge of the mesa with a 50x50 pm2
contact pad for electrical connection.
Figure 3.3 shows the dark I-V characteristics of the InGaAs/AlGaAs compressively
strained p-QWIP. As seen in this figure, the device is under BLIP at temperatures below
63 K for applied biases between -3 V and +3 V. A BLIP temperature of 70 K can also be
achieved when the applied bias is less than 1 V. Like all of the previously studied p-
QWIPs, the dark current characteristic is slightly asymmetric. This can be attributed to
the doping migration effect of the Be dopant during layer growth [50],
The responsivity of the p-QWIP was measured under normal incidence illumination
as a function of temperature, applied bias, and incident IR radiation wavelength by using
a blackbody radiation source running through an automatic PC-controlled single grating
monochrometer with the appropriate IR filters attached. The output of the QWIP was
measured with a Princeton Applied Research 5210 lock-in amplifier and converted to
responsivity by calibrating the output with a pyroelectric detector. A schematic layout of
the experimental setup is shown in Figure 3.4. The same experimental setup is used
throughout this study.
Figures 3.5(a) and 3.5(b) show the results of these measurements. A single LW1R
peak was found at A. ,=7.4 pm and T=ll K with an applied bias of 5 V. Given a rather
broad LWIR peak and a cut-off wavelength of approximately at 10 pm; this corresponds
to a half-peak spectral bandwidth of AX/A.p,=30%. The responsivity was determined to be
37 mA/W at the 7.4 pm peak wavelength. A single MWIR peak was also found at
A.p2=5.5 pm under the same conditions previously mentioned. The MWIR peak has a
bandwidth ranging from approximately 4 to 6 pm. For a cut-off wavelength of 6 pm, we
derive a spectral bandwidth of A7Ap2=27%, which is again a rather wide peak. The
asymmetrical responsivity around the MWIR spectral peak is attributed to the long-pass
filter characteristic which has a cut-on at A.ON=6.7 pm. As seen in Figures 3.6(a) and


15
energy levels of both carriers, it does allow accurate predictions of the energy subbands.
When compared to the direct calculation of the energy subbands, the two-band
approximation yields accurate results when compared to the direct calculation results
[28,34], One limitation of the TMM is that this method cannot calculate the energy
levels of the allowed energy subbands in the continuum states. In order to determine the
transition energy from the ground state to the continuous state, we used the Kronig-
Penney model to determine the locations of the allowed energy bands in the continuum
states.
When a biaxial internal tension is applied to the well material, the strain pulls the
LH subbands up with respect to the HH subbands for a given well thickness. While
quantum confinement effects tend to push the LH subbands down with respect to the HH
subbands. As the well width is increased above a certain value, the strain effect can
overcome the quantum confinement effect and therefore induce the inversion of the heavy
hole and light hole subbands at the ground state. In contrast, with the application of
compressional strain on the well layers, the strain forces the LH subbands down with
respect to the HH subbands for a given well thickness. For a schematic description of
what happens to the conduction, heave-hole and light-hole bands, see Figure 2.1.
2.2.4 The Transfer Matrix Method for the Calculation of Transmission Probability
The transfer matrix method (TMM) [31] allows the calculation of the transmission
probability through a superlattice. Like any typical quantum mechanical barrier or well,
the carrier conduction in each layer of the superlattice consists of the superposition of two
components propagating forwards and backwards. The complete wave function can be
expressed as
v|/(. =V|/;e+*'iTA' +v|/ ~e-ik' e+A' (2.16)
where


73
GaAs
HH1
(b) (c)
Figure 6.5: The (a) schematic energy band diagram for the unstrained SL-QWIP and the
transport mechanisms for the SL-QWIP at (b) high and (c) moderate biases.


Dark Current (A)
108
Figure 8.13: The measured dark I-V characteristics of the variable composition p-type
BB-QWIP as a function of device temperature.


128
60. L. C. Lenchyshyn, H. C. Liu, M. Buchanan and Z. R. Wasilewski, J. Appl. Phys.
79(10), 8091 (1996).
61. J. Chu and S. S. Li, IEEE J. Quant. Elect. 33(7), 1104 (1997).
62. J. C. Chiang, S. S. Li and A. Singh, Appl. Phys. Lett. 71(24), 3546 (1997).


7 TENSILE STRAINED QUANTUM WELL INFRARED
PHOTODETECTORS 78
7.1 Introduction 78
7.2 Device Layer Structure and Processing 79
7.3 Device Characterization 79
7.4 Conclusion 80
8 BROADBAND QUANTUM WELL INFRARED
PHOTODETECTORS 86
8.1 Introduction 86
8.2 Layer Composition and Device Processing 87
8.3 Characterization Results 88
8.3.1 N-type broadband QWIPs 89
8.3.2 P-type broadband QWIPs 91
8.4 Discussion 93
8.5 Conclusion 94
9 SUMMARY AND CONCLUSION 111
APPENDICES
1 LIST OF SYMBOLS 118
2 ACRONYMS 123
REFERENCES 125
BIOGRAPHICAL SKETCH 129
v


CHAPTER 3
AN INGAAS/ALGAAS ON GAAS P-QWIP WITH COMPRESSIVE STRAIN
LAYERS AND LWIR AND MWIR DETECTION
3.1 Introduction
In the last few years, n-type QWIPs have been extensively investigated using III-V
semiconductor material systems [10]. Because of the small electron effective mass and
high electron mobilities, n-type GaAs/AlGaAs QWIPs offer excellent IR detection
properties. These n-type QWIPs have utilized the bound-to-continuum [10,47,48] and
bound-to-miniband [11] transition schemes in the 8-14 pm LWIR and 3-5 pm MWIR
bands, to achieve reasonable detectivities and dark current characteristics. However,
quantum mechanical selection rules for intersubband transitions requires that the electric
field of the incident IR radiation has a component perpendicular to the quantum well
plane in order to induce intersubband absorption in the quantum wells. Therefore, for n-
type QWIPs. it is necessary to use either planar metal or dielectric grating structures to
couple the normal incidence radiation into absorbable angles in the quantum wells
[16,49].
In contrast, p-type QWIPs allow the absorption of normal incidence IR radiation
due to the band mixing between the heavy-hole and light-hole states. In p-type quantum
wells, intersubband transitions under normal incidence illumination are induced by the
linear combination of p-like valence band Bloch states which provides a nonzero
coupling between these components and the normal radiation field. The strong mixing
between light-hole and heavy-hole states for k|^0 greatly enhances the normal incidence
intersubband absorption. However, in the unstrained lattice matched GaAs/AlGaAs and
InGaAs/InAlAs quantum well systems recently demonstrated [17-19], the intersubband
26


APPENDIX 1
LIST OF SYMBOLS
a absorption coefficient
ap angle between slip direction and layer plane direction
80 lattice mismatch or in-plane strain
AO incident photon flux
AAAp full-width half maximum bandwidth
Av spectral bandwidth
Aphh concentration of optically induced heavy-hole carriers
Aplh concentration of optically induced light-hole carriers
A spin orbit split-off energy
AE energy difference between the barrier and the quantum confined
state in the well
AEc strain induced conduction band shift
AEhh strain induced heavy-hole band shift
AE, strain induced light-hole band shift
Af noise bandwidth
A¡j energy difference between the initial ground state and the final
transition state
An excess photogenerated carriers (electrons)
Aphh concentration of optically induced heavy-hole carriers
APih concentration of optically induced light-hole carriers
ARtl change in detector resistance
AVn change in output photovoltage
118


In chapter 7, we experiment with a tensile strained-layer (TSL) p-type QWIP. The
use of tensile strain inverts the heavy- and light-hole states in the quantum well so that
the light-hole state become the ground state. Using the light-hole state as the heavily
doped ground state has the benefits of smaller effective mass, which translates into larger
absorption coefficients and better transport characteristics. The TSL p-QWIP is found to
be sensitive in the MWIR region of the IR spectrum.
Finally, in chapter 8, we look into the design and characterization of both n- and p-
type QWIPs for broadband (BB) detection. These devices have been designed to sense
most of the photons in the 8-12 pm LWIR range. While the p-type QWIPs typically
exhibited full-width half-maximum bandwidths on the order of 35%, the n-type BB-
QWIPs exhibited not only larger responsivities throughout the LWIR range at high
applied biases, but they also were found to be able to sense normal incidence photons at
about 40-50% of the 45 incidence value.
And in the last chapter, we summarize the results of the extensive study into p-type
and normal incidence quantum well infrared photodetectors, comparing the relative
strengths and weaknesses of each design with those published in the open scientific
literature. In addition, we will give some recommendations as to what might be
experimented with in future research.
5


126
18. J. Katz, Y. Zhang, and W. I. Wang, Electron. Lett. 28, 932 (1992).
19. W. S. Hobson, A. Zussman, B. F. Levine, and J. deJong, J. Appl. Phys. 71, 3642
(1992).
20. J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 32, 265 (1976).
21. J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974).
22. J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 29, 273 (1975).
23. O. Madelung, M. Schulz and H. Weiss, eds., Landolt-Bomstein: Numerical Data
and Functional Relationships in Science and Technology, Group III, vol. 17a, 22a,
Springer-Verlag, Berlin (1986).
24. R. Hull, J. C. Bean, F. Cerdeira, A. T. Fiory, and J. M. Gibson, Appl. Phys. Lett. 48,
56(1986).
25. G. Ji, D. Huang, U. K. Reddy, T. S. Henderson, R. Houre, and H. Morko?, J. Appl.
Phys. 62, 3366 (1987).
26. T. P. Pearsall, Semiconductors and Semimetals, 32, 55 (1990).
27. H. Asai, and Y. Kawamura, Appl. Phys. Lett. 56, 746 (1990).
28. H. Xie, J. Katz, and W. I. Wang, Appl. Phys. Lett. 59, 3601 (1991).
29. R. T. Kuroda and E. Garmire, Infrared Phys. 34, 153 (1993).
30. L. R. Ram-Mohan. K. H. Yoo. and R. L. Aggarwal. Phys. Rev B 38. 6151 (1988).
31. A. K. Ghatak, K. Thyagarajan, and M. R. Shenoy. IEEE J. Quantum Electron. 24,
1524 (1988).
32. J. M. Luttinger and W. Kohn, Phys. Rev. 97, 869 (1956).
33. J. M. Luttinger, Phys. Rev. 102, 1030 (1956).
34. G. L. Bir and G. E. Pikus, Symmetry and Strain-Induced Effects in
Semiconductors, Wiley, New York (1974).
35. E. O. Kane, Semiconductors and Semimetals, eds. R. K. Willardson and A. C.
Bear, vol. 1, 75 (1966).
36. F. H. Pollack, Semiconductors and Semimetals, ed. T. P. Pearsall, vol. 32, 17
(1990).
37. Y. C. Chang and R. B. James, Phys Rev. B 39, 672 (1989).


66
field of view was limited to 1.78 and the chopped blackbody source was set at 800 K.
As the bias is increased from 0 to -50 mV, the response of the LWIR peak (9.3 pm)
rapidly increases, while the 6.5 pm peak is effectively suppressed. As the bias is
increased to -100 mV and greater, the superlattice miniband on which the photoexcited
LWIR hole transport depends, loses resonance which causes the loss of responsivity for
the 9.2 pm peak. This effect gives rise to the voltage tuning capability of this SLIP
structure. Note that at these higher applied biases, the 6.5 pm peak dominates and
saturates at Vh<-200 mV with a maximum absolute responsivity of 8 mA/W. The lack of
LWIR photoresponse in the positive bias regime can be attributed to the built-in field in
the superlattice region arising from dopant migration [50], which causes the breakdown
of the miniband resonance when positive bias is applied and can be seen in Figure 6.8.
Figure 6.9 shows the measured dark current as a function of temperature and
applied bias for the p-type SLIP. Overlaid on top of the dark I-V curves is the FOV
limited (1.78) background photocurrent. This shows that the device is under background
limited performance (BLIP) operation at T=35 K or lower under this narrow field of view
for a broad range of applied bias. The slight asymmetry in the dark current can be
attributed to band bending as an effect of dopant migration [50], since the mobile p-type
dopant beryllium was used in the quantum well layers.
6.7 Conclusion
In conclusion, we have demonstrated two novel p-type SLIPs which exhibits a
peak detection wavelength at 19.2 pm and voltage tunability in the LWIR band.
Operation up to 40 K was obtained for both the photoconductive (PC) and photovoltaic
(PV) modes detection for the VLWIR SLIP. An absolute responsivity of 49.8 mA/W and
an r|g=0.317% were achieved at T=40 K and Vh=20 mV with an FWHM spectral
bandwidth of AAA =12%. Further refinements can be made to this structure to tailor the



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36
(a)
(b)
Figure 3.5: (a) The MWIR and (b) LWIR responsivity of the InGaAs/AlGaAs CSL p-
QWIP at 77 K under an applied bias of 5 V.


57
Wavelength (uni)
Figure 5.4: The spectral responsivity versus wavelength for the MWIR PCSL-QWIP,
measured at T=ll K and V=5 V. Two response peaks at 4.8 pm and 5.4 pm were
observed for this device.


Dark Current (A)
106
Applied Bias (V)
Figure 8.11: The measured dark I-V characteristics of the four well n-type BB-QWIP.


14
perturbation, if a large enough bandgap exists, like in InGaAs and GaAs layers. The
wave functions of the coherently strained superlattice at the zone center (k=0) are given
by [36]
|3 / 2,3 / 2 >
HH states
(2.13)
y |3/2,l/2>+p |l/2,l/2>
LH states
(2.14)
P |3 / 2,1 / 2 > +y |l/2,l/2>
SO states
(2.15)
where y and p are constants which are dependent on the strain parameters. Note that the
heavy-hole states, |3/2,3/2>, are still decoupled from the other valence band states even
under biaxial stress at the zone center, while the light-hole and spin-orbit split off states
are coupled at k=0. However, the HH, LH, and SO states are mixed [37,38] in the
coherently strained superlattice at off zone center k*0. This mixing between the states
with different m-s is due to the boundary conditions across the interface of the quantum
well layers. By examining the kp matrix, we can see that the interaction between the
different mj states is proportional to the transverse components of the wave vector, k^, so
that the HH states are decoupled when 1^=0. It is interesting to note that the k conserved across the interfaces since the interface potential depends only on z, the
quantum well growth direction. Thus the band mixing can be significant if the T-
bandgap is small, e.g., with GaAs and InGaAs, and if the LH and SO bands involved in
the transition have a large kz value [37],
Since the heavy hole and light hole valence subbands are non-degenerate following
the introduction of strain into the QWIP structure, a simpler method can be used to
determine the energies of the subbands. By using the parabolic band approximation near
the valence band zone-center, and the energy band shifts for the conduction band
minimum, heavy hole subband maximum, and light hole subband maximum, we can
utilize the simpler two-band Hamiltonian for electrons just by finding the effective mass
of the carriers (i.e., heavy-hole effective mass and light-hole effective mass) and the
barrier heights for each carrier type. Although this does not simultaneously determine the


I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a
dissertation for the degree of Doctor of Philosophy.
Sheng S. Li, Cha
Professor of Electrical and Computer Engineering
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a
dissertation for the degree of Doctor of Philosophy.
Gijs Bos
Professor of Electrical and Computer Engineering
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a
dissertation for the degree of Doctor of Philosophy.
Amost Ndugroscnel
Professor of Electrical and Computer Engineering
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a
dissertation for the degree of Doctor of Philosophy.
t L
Ramakant Srivastava
Professor of Electrical and Computer Engineering
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a
dissertation for the degree of Doctor of Philosophy.
Timothy J. Andprson
Professor of Chemical Engineering


65
peak. From the transfer matrix method, we calculated two responsivity peaks at 9.6 and
6.4 pm, which is in excellent agreement with the measured results. The complete layer
structure of this device is illustrated in Figure 6.6.
In order to electrically and optically characterize this device, a wet chemical etch of
8:1:1 H20:H202:H3P04 was used to fabricate 216x216 pm2 mesas. Then, Cr/Au (100
/1500 ) was deposited on top of and around the mesas to form the top and bottom
ohmic contacts, respectively. Since these devices respond to normal incidence radiation,
a ring contact around the mesa edge was used to allow light to pass through to most of the
mesa top surface, which simplifies the processing requirements while still maintaining a
75% fill factor. The chips were then bonded onto 68 pin chip carriers and wired to the
contact pads via ultrasonic wedge bonding. For cryogenic testing, the chips were
mounted into open cycle dewars which were capable of being mounted with KRS-5
windows and a variety of apertures to limit the field of view (FOV) from 40 to 0.
6.6 Unstrained Voltage Tunable SLIP Characterization and Results
Figure 6.7 shows the responsivity of the p-type SLIP as a function of incident
radiation wavelength and negative applied bias. For the measurement of the responsivity.
a blackbody source at 1243 K is used as the light source into a 'A m Oriel MS257
monochrometer with various grating and filters to allow testing from 3-21 pm. The DUT
is held at cryogenic temperatures by an open cycle dewar using liquid helium as the
coolant. The output from the DUT is then amplified by a Keithley 428 TIA with a
variable gain, after which the output is sent to an Oriel Merlin DSP lock-in amplifier.
The output from the DUT is normalized against a reference sensor (an Oriel 70129
pyroelectric detector) to determine the relative photoresponsivity. Then, the output of the
detector at a specific chopped frequency was analyzed with a Stanford Research 770 FFT
network analyzer to determine the integrated optical response. For this measurement, the


13
Hu=H2i = H32=H4i=0
and V(z) is a step function where V(z) vanishes inside the well layers and equals V0 in the
barrier layers. The effect of strain is included by adding the Bir-Pikus Hamiltonian [34],
Hs, to the general Luttinger-Kohn Hamiltonian. As shown below, the strain Hamiltonian
for the well material is a diagonal matrix.
0
- AEC + AEhh
0
0
^hh
0
0
0
0
0
0
0
(2.12)
- AEc + AEhh 0
0 AEc + AEhh
Using the aforementioned techniques, we can numerically calculate the energy of
the zone-center valence subband levels as a function of well width for any material
system under tensile or compressional strain and also determine the change in the valence
subband structures.
All of the previously described calculations are derived from the multiband
effective mass kp model for a coherently strained structure, which is based upon the
perturbation approximation. In the k p model, the interactions of S-P type coupling
among conduction (C), light-hole (LH), heavy-hole (HH), and spin-orbit (SO) states
combined with spin-orbit like coupling are taking into consideration to derive the band
structures. This results in an 8x8 kp Hamiltonian and momentum matrix elements.
Using the perturbation approximation, a set of wave functions of S1/2: |1/2,1/2>C; P3/2:
|3/2,3/2>, |3/2,l/2>; and Pl/2: |l/2,l/2> are used to represent the unperturbed and
unstrained basis in the |J,m> presentation [35], m=\/2 represents either the electron or
LH states, while m¡= 3/2 denotes the HH or heavy particle states. A slightly simplified
6x6 k p Hamiltonian can be used to roughly predict the P-like properties of the
coherently strained layers by considering the S-like conduction band states as a


84
Figure 7.3: The measured photoresponse of the MWIR p-type TS-QWIP as a function of
applied bias and incident IR radiation wavelength.


38
Reverse bias voltage (V)
Figure 3.7: The experimental and theoretical current noise spectral density versus reverse
bias voltage at various temperatures for the InGaAs/AlGaAs CSL p-QWIP.


90
n-type BB-QWIP at Vh=-l V. The calculated spectral bandwidth at Vh=-\ V is
AAAp=40%. The responsivity at ?y=10 pm is 5 mA/W. But because of the very low dark
current (7^=50 nA) at this bias, the detectivity is expected to be on the order of 1010 cm-
Hzi/2/W. A normal incidence response of approximately 50% of the 45 value was found
for this device under all of the biases tested.
Figure 8.7 shows the dark current as a function of applied bias and device
temperature, with the 300 K background photocurrent at a field-of-view of 180
superimposed. Note that at a temperature of 60 K the device is background limited when
the applied bias is between 5V, while at a device temperature of 70 K, the n-type BB-
QWIP is background limited for -1 V Using the results of the responsivity and dark I-V measurements, the detectivity of
the three well n-type BB-QWIP can be estimated using a photoconductive gain of 0.1.
This results in a measured D* of 1.1 lxl010 cm-Hzl/2/W at Vh=-2 V at T=60 K. Under a
higher applied bias of Vh=-4 V, D* was found to be 3.12x10' cm-Hzl/2/W with a gain
estimated at 0.15. Under normal incidence, the detectivity was found to be roughly half
the 45 value or 5.2x109 and 1.5x10' cm-Hz' 3/W at 7=50 K and V,=-2, -4 V respectively.
When observing Figures 8.8 and 8.9, the most striking feature of the results of the
four well n-type BB-QWIP is that the FWHM bandwidth is significantly increased. As
seen in Figure 8.8, the maximum responsivity of the four well n-type BB-QWIP is
achieved at A,p=10.3 pm. When the device is at 7=40 K and Vh=+4.5 V, a maximum
responsivity of 2.32 A/W is found, with a corresponding FWHM bandwidth of
AX/X=\S%. This value is a little less than 50% higher than that achieved for the three
well case under similar biasing conditions. It is interesting to note that the shape of the
four well n-type BB-QWIP responsivity curve under positive bias is very similar to that
of the three well n-type BB-QWIP under negative bias. In Figure 8.9, we find that
AA/A.^29% with a 2^=10.7 pm at Vh=-4 V, which is red-shifted from the lower negative
applied biases and the positive applied biases. The spectral bandwidth achieved is almost


43
the higher dark current characteristics might also arise from the higher aluminum content
used in the superlattice barrier layers, which has been attributed to the formation of DX
centers. Figure 4.4 shows the measured dark I-V characteristic with 300 K background
photocurrent superimposed.
Figure 4.5(a) shows the measured photoresponse of the p-type SBTM CSL-QWIP.
A single peak was found at ^=10.4 pm, which is in good agreement with the
theoretically calculated value of 10 pm (see Figure 4.2). With a half-peak value at 12
pm, we derived a spectral bandwidth of AAA/,=20%. This narrow responsivity bandwidth
is consistent with that expected from a bound-to-miniband transition scheme. A
maximum responsivity of 28 mA/W was found at T=65 K and Vb=+3.0 V. At an
operating temperature of 65 K, the noise spectral density was measured as 4.0x10"26
A2/Hz at a bias of 1.0 V. Corresponding to this operating point, the measured
responsivity at the 10.4 pm peak was found to be 13 mA/W. From the above data, the
spectral detectivity was then calculated at Z)*=1.4xl09 cm-Hz1/2/W. Note that this is the
detectivity achieved by a single pass of the incident radiation through the p-type SBTM
CSL-QWIP. The quantum efficiency of this QWIP was found to be 3.8%. If the test
structure is altered to include backside thinning and a reflective top contact, then the
responsivity will increase substantially. The corresponding detectivity will also increase,
since the dark current and the noise spectral density remain constant. The variation of
responsivity with applied bias is plotted in Figure 4.3(b). As easily seen, the responsivity
and hence the photoconductive gain increase linearly with the applied bias at a fixed
operating temperature. A schematic diagram of the experimental setup can be seen in
Figure 3.4.
Assuming that six reflections can be achieved before the incident radiation is either
completely absorbed or the photon flux reflected back into the QWIP layers becomes
insignificant, we determined the following improved performance parameters for the
SBTM p-QWIP operating at T= 65 K and Vh=\.Q V. First of all, the quantum efficiency


114
will remain stable after many thermal cycles. This design demonstrated that higher
performance tensile strained devices can be grown without cross-hatching, and therefore
proves the usefulness of the light-hole to heavy-hole intersubband transition.
The final devices designed and characterized were the broadband QWIPs, both n-
type and p-type. The impetus for these designs is that most QWIPs reported to date have
rather narrow spectral bandwidths and many applications require the detector to be able to
sense radiation over a large spread of wavelengths. The three well n-type BB-QWIP was
found to have a responsivity peak at 10.0 pm, with an FWHM bandwidth that varies as a
function of applied bias polarity and magnitude. The maximum bandwidth of
A)J\p=-21 % was obtained at Vb=-2 V, which corresponded to a peak responsivity of 58
mA/W; whereas the minimum bandwidth of A7JX=\2% was achieved at Vh=+6 V, which
corresponds to a very large responsivity of 1.90 A/W at this bias. There was also a
significant normal incidence response with the variable composition n-type BB-QWIP.
This has been attributed to the use of compressively strained quantum wells in the design.
A four well n-type BB-QWIP was also designed, grown, fabricated and characterized. It
was found to have a response peak at 10.3 pm, with an FWHM bandwidth that again
varied as a function of applied bias polarity and magnitude. A maximum bandwidth of
A)JXp=29% was found for all negative applied biases, and a responsivity peak of 2.21
A/W was found at T= 40 K. Vh~4.5 V. Under positive bias, the maximum responsivity
was found to be 2.32 A/W at T-40 K, FA=+4.5 V, with a corresponding bandwidth of
AX/Xp=21%. The four well n-type BB-QWIP was also found to have a normal incidence
response of 50% or greater than the 45 incidence value.
The variable composition p-type BB-QWIP also examined was found to have a
very large FWHM bandwidth of AX/Xp=48% at 7=40 K and Vh=-\.5 V. Under the
aforementioned operating conditions, a maximum absolute spectral responsivity peak of
19 mA/W was found at 9.3 pm when the incoming radiation is normally incident and
allowed only one pass through the multiquantum well layers. The variable thickness p-


49
(a)
E
£
>
t/2

c.
C/i

a
Applied Bias (V)
(b)
Figure 4.5: The measured responsivity (a) as a function of wavelength at T=40 K, V=2.0
V and (b) the variation of peak responsivity as a function of applied bias at T=40, 65 K.


70
Figure 6.3: (a) The absolute responsivity as a function of p-type SLIP detector
temperature at FA=20 K and (b) the responsivity as a function of applied bias and device
temperature.


3 AN INGAAS/ALGAAS ON GAAS P-QWIP WITH COMPRESSIVE
STRAIN LAYERS AND LWIR AND MWIR DETECTION 26
3.1 Introduction 26
3.2 P-type Compressive Strained Layer QWIP Design 27
3.3 Results and Discussion 28
3.4 Conclusion 31
4 A COMPRESSIVELY STRAINED-LAYER P-TYPE
INGAAS/ALGAAS/GAAS STEP BOUND TO MINIBAND QWIP AT
10.4 pm 39
4.1 Introduction 39
4.2 Theoretical Considerations 40
4.3 Device Growth and Fabrication 41
4.4 QWIP Characterization and Results 42
4.5 Conclusion 44
5 A STACKED COMPRESSIVELY STRAINED P-QWIP WITH TWO-
BAND TWO-COLOR DETECTION 50
5.1 Introduction 50
5.2 Theoretical Considerations and Device Fabrication 50
5.3 Device Characterization and Results 51
5.4 Conclusion 53
6 SUPERLATTICE INFRARED PHOTODETECTORS 60
6.1 Introduction 60
6.2 Superlattice Infrared Photodetector Design and Processing 60
6.3 19.2 pm SLIP Characterization and Results 62
6.4 A Voltage Tunable Two-color SLIP 64
6.5 Layer Structure and Fabrication of the Unstrained Voltage Tunable
SLIP 64
6.6 Unstrained Voltage Tunable SLIP Characterization and Results 65
6.7 Conclusion 66
IV


transitions, and a discussion on how to evaluate and compare the performance of various
QWIPs.
Chapter 3 discusses the development and performance of a new compressively
strained InGaAs/AlGaAs/GaAs p-type QWIP with MWIR and LWIR responsivity. This
device demonstrates the viability of p-type designs to achieve good detectivities at
relatively high temperatures while exhibiting normal incidence response. The detection
peaks for this device were found to be 5.6 and 7.4 pm, in the MWIR and LWIR bands,
respectively.
Next, in chapter 4, we look into the characterization of a compressive strained-layer
(CSL) InGaAs/AlGaAs/GaAs p-type QWIP with a step-bound-to-miniband (SBTM)
intersubband transition, as seen in Figure 1.2. The purpose of using this type of
intersubband transition is to decrease the dark current due to thermionic emission while
still maintaining a large responsivity at longer wavelengths so that operating temperatures
can be increased. The CSL SBTM p-QWIP showed a strong response peak at 10.4 pm.
A stacked CSL InGaAs/AlGaAs/GaAs p-QWIP is investigated in chapter 5. This p-
QWIP design uses two separate stacks of p-QWIPs connected by a common ohmic
contact to be able to sense infrared radiation in both the LWIR and MWIR bands. This
was done so that the tactically and strategically important wavelengths of 10 and 4.2 pm
could be imaged on one focal plane array without a large increase in focal plane area.
This device showed detection peaks at 4.8, 5.4 and 10.0 pm.
Chapter 6 explores the new idea of superlattice infrared photodetectors (SLIPs) to
enhance the quantum efficiency and extended the detection wavelength of the device out
past 19 pm. An unstrained SLIP design also exhibited true voltage tunability in the
LWIR band, which allows us to discriminate incoming LWIR radiation by simply
altering the applied bias of the device. The detection peak of the unstrained voltage
tunable SLIP can be varied between 6.5 and 9.3 pm.
4


97
In o.l 7Gao.83 As Ino.2Gao.8As Ino.25 Gao.75 As InQ jGaQjAs
(a)
GaAs
300 nm
Si=3xl018 cm'3
GaAs
35 nm
undoped
GaAs
45 nm
undoped
In03Gao7As
6.5 nm
Si=7xl017 cm'3
GaAs
45 nm
undoped
In 0 25Ga0 75As
6.5 nm
Si=7x 1017 cm'3
Repeat
x 20
GaAs
45 nm
undoped
In01Ga0gAs
7.5 nm
Si=7xl017 cm
GaAs
45 nm
undoped
O.I7^a0 83 AS
8.5 nm
Si=7xl017 cm'3
GaAs
80 nm
undoped
GaAs
500 nm
Si=3xl018 cm'3
S.l. GaAs substrate
(b)
Figure 8.2: The (a) schematic energy band diagram and the (b) complete layer structure
of the four well n-type BB-QWIP.


35
HP4145B
Semiconductor
Parameter
Anaylizer
l-V
Measurement
Set-Up
IR Filter
DUT
Grating
Monochrometer
Closed-cycle
Cryostat
-e
Trans-Impedance
Amplifier
Control &
Data
Acquisition
Blackbody
Chopper &
Controller

Lock-In Amplifier
Signailn Lock-In Out Ref In
Spectral Measurement Set-Up
Figure 3.5: A schematic layout of the experimental setup.


62
1.78 field-of-view. For all the measurements, the device temperature was maintained
within 0.5K of the stated measurement temperature.
6.3 19,2 pm SLIP Characterization and Results
Figure 6.3(a) shows the responsivity as a function of incident radiation wavelength
and device temperature for Vb=20 mV under high flux illumination (FOV>20). The
FWHM spectral bandwidth of the 19.2 pm response peak was found to be AX/Xp=12%.
This narrow spectral bandwidth is in excellent agreement with the value predicted by the
TMM calculation, since the FWHM energy spread of this VLWIR peak is only 8 meV.
For the responsivity measurement, a blackbody source at 1243 K was used as the light
source into an Oriel MS257 monochrometer with various grating and filters to allow
testing from 3-21 pm. The device under test (DUT) was held at cryogenic temperatures
by a open cycle dewar using liquid helium as the coolant. The output from the DUT is
then amplified by a Keithley 428 transimpedance amplifier with a variable gain, after
which the output is sent to an Oriel Merlin digital signal processing (DSP) lock-in
amplifier. The output from the DUT is normalized against a reference sensor (an Oriel
70129 pyroelectric detector) to determine the relative photo responsivity. The output of
the detector at 22 Hz was analyzed with a Stanford Research 770 fast Fourier transform
(FFT) network analyzer to determine the integrated optical response. The blackbody is
placed as close as possible to the dewar to ensure that the cooled pin-hole aperture indeed
limited the signal and that the entire device was illuminated. The gain on the
transimpedance amplifier (TIA) was kept low enough to ensure no signal attenuation at
22 Hz due to gain roll-off. For this measurement, the FOV was limited to 1.78 and the
chopped blackbody source was set at 800 K. At 7=40 K and Vb=20 mV, the peak
absolute photoresponse at ^=19.2 pm was found to be R= 49.8 mA/W. From this
responsivity, the quantum efficiency gain product (r|g) was determined to be 0.317%.


127
38. P. Man and D. S. Pan, Appl. Phys. Lett. 61, 2799 (1992).
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(1984).
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42. H. C. Liu, Appl. Phys. Lett. 61, 2703 (1992).
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59.


83
LHl
HH1
HH2
LH2
HH3
Iiio^GaoAs In0 52A10 48As
In0.4Ga0.6As
Figure 7.2: The schematic energy band diagram of the p-type MWIR TS-QWIP.


CHAPTER 6
SUPERLATTICE INFRARED PHOTODETECTORS
6.1 Introduction
With the advances in molecular beam epitaxy (MBE) technologies, device
structures using heterostructures or quantum wells have been extensively investigated in
the last few decades. N-type quantum well infrared photodetectors (QWIPs) grown by
MBE have been extensively studied in recent years [11,12,14], These devices use either
the GaAs based GaAs/AlGaAs or InGaAs/AlGaAs material systems for detection in the
3-5 pm mid-wavelength infrared (MWIR) or 8-14 pm long-wavelength infrared (LWIR)
atmospheric transmission windows [15,16,55]. Since n-type GaAs/AlGaAs and
InGaAs/AlGaAs QWIPs have inherently low electron effective masses and high electron
mobilities, they offer excellent infrared (IR) detection properties. However, due to the
quantum mechanical selection rules which prohibit normal incidence intersubband
absorption, focal plane arrays (FPAs) using n-type QWIPs must use either metal or
dielectric gratings to couple normal incidence IR radiation into the quantum well
[11.15,16], In contrast, due to the mixing between the light and heavy hole states under
either biaxial tensile or compressive strain, normal incidence absorption is allowed for the
intersubband transition in p-type QWIPs; thus eliminating the need for grating couplers
and simplifying the design of p-type QWIPs.
6.2 Superlattice Infrared Photodetector Design and Processing
Little work has been done in QWIPs with peak detection wavelengths longer than
16 pm [55,56], since QWIPs operating in the very long-wavelength infrared (VLWIR)
band are required to operate at very low temperatures due to higher dark current. Levine,
60


unit polarization vector
full width of level broadening
wavelength of the incident photon
cut-off wavelength
peak wavelength
quantum efficiency
collection efficiency
well recapture lifetime of the carrier
total width of all quantum well and barrier regions
lifetime between initial and final states
excess carrier lifetime
excess carrier transit time
mobility
carrier effective mobility
incident radiation frequency
Poisson ratio
angle between dislocation line and Burges vector
field of view
frequency
shear moduli
h/2n
polarization dependent constant
epilayer lattice constant
in-plane lattice constant
individual layer lattice constants
active detector area
substrate lattice constant


16
A, = A2 =0
A; kj (dj + dy H hi/( )
i = 3,4,...,
2m,
h
H E-E,)
1/2
where v|/,+ and v|// represent the magnitudes of the wave functions propagating in the
forward, or +z direction and the backwards, or -z direction, respectively. While N is the
number of periods in the superlattice, d, is the thickness of the z'-th layer in the
superlattice, m* is the effective mass of the particle in the z'-th superlattice layer, and E, is
the potential energy of the z'-th layer. Since the wave function, v|/, and it's derivative,
d\\i/dz, are continuous at the boundaries, the wave functions then become
(2.17)
(2.18)
The recurrence relationship of the wave functions can be written in matrix form as
(2.19)
v'Me'N',
e'V+i +e'8'M/-1
),
|V1
1
V*'
-ih \
r¡e '
iTT
W/"J
" t,
^,e&'
/8,
e 7
where at normal incidence
r =
v/+i
/. =
k, ~ k,
2k.
k.+k
(2.20)
(2.21)
i+i
and
6, = kidi. (2.22)
Which gives us the following form for determining the N+l-th wave functions
(2.23)
V,+N
= 5,
+ ^ i
-
Vf
= sts2.
SN
vj+r
WrJ
where
S. =
1
r.e
-iS,\
* /
1, \r,e
/6;
e y
(2.24)