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Ridge waveguide mid-infrared InGaAsSb quantum well lasers fabricated with pulsed anodization etching

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Ridge waveguide mid-infrared InGaAsSb quantum well lasers fabricated with pulsed anodization etching
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Yoon, John
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Thesis (Ph. D.)--University of Florida, 2000.
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Includes bibliographical references (leaves 109-114).
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Printout.
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Vita.
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by John Yoon.

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RIDGE WAVEGUIDE MID-INFRARED InGaAsSb QUANTUM WELL LASERS
FABRICATED WITH PULSED ANODIZATION ETCHING















By

JOHN YOON












A 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 2000



























Copyright 2000

by

John Yoon














TO THE MEMORY OF MY MOTHER
AND
TO THE MEMORY OF MY BOY, ANDREW YOON














ACKNOWLEDGMENTS



I would like to express my sincere thanks to the chairman of my supervisory

committee, Dr. Peter S. Zory, Jr. for his guidance and encouragement during my studies. He has instilled in me not only knowledge but also professionalism.

I would like to extend my special thanks to the members of my supervisory

committee: Dr. Gijs Bosman, Dr. Sheng Li, Dr. Ramakant Srivastava, and Dr. David B. Tanner for their extraordinary guidance and support both professionally and personally. I would also like to thank Dr. John O'Malley, who had been my supervisor for lab courses that I taught as a teaching assistant during my master's program.

I would like to recognize Sarnoff corporation, which has funded this research project. Particularly, I'm indebted to Dr. Ray Menna and Dr. Hao Lee who provided laser materials and measurement data.

I would like to thank my laboratory colleagues and friends that I have met and worked with at the University of Florida. Additional thanks go to the department staff who have provided administrative and technical support.

I owe a debt of gratitude to all my friends in Korea and U.S. for their friendship all throughout the past years. Also, I would like to extend a special note of thanks to my aunt's family who has helped me to come back to U.S. and settle down.






iv








I would like to extend my greatest appreciation and thanks to my grandma, father, brother, and mother-in-law for their prayers, love and support. I can offer here only an inadequate acknowledgement of my appreciation.

Although my Mom and my boy, Andrew, are not here today to witness my academic accomplishment, I strongly believe that they in heaven feel proud of me.

My wife, Mejae Lee Yoon, deserves my utmost thanks for her unconditional love and support. She has been with me all throughout the hardships in the past years and inspired me to continue on my study. The completion of this work would never be possible without her companionship, forbearance and encouragement.

Our baby, who soon arrives in July, will think me as born with a doctorate.






























v















TABLE OF CONTENTS

pae

ACKNOW LEDGM ENTS ................................ ............................ .................... iv

A B S T R A C T ............................................................................................................. ix

CHAPTERS

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

1.1 Review of GaSb-based Semiconductor Laser Development ..................................4
1.2. D issertation O verview ................................................................................. 7


2 STRAINED QUANTUM WELL OF GaSb-BASED QUATERNARY ALLOYS...... 10

2.1 Introduction ...................................................... ....... .............. 10
2.2 Band Lineup of Strained Quantum Well ................... ..........................11
2.3 Quantized Energy Levels in Quantum Well ............................. .................. 17
2.4 Material Parameters for Quaternary Alloys ............... .... ............ 19
2.5 Optical Gain Spectrum in QW Structure ..............................................21
2.6 Calculation Results ............................ . ............................25
2.6.1 Material Parameters and Emission Wavelengths................. .....25
2.6.2 Optical Gain Spectra of InGaAsSb/AlGaAsSb Strained QW's ......................33


3 RIDGE WAVEGUIDE LASER DESIGN ........................................................ 39

3.1 Introduction ................. ............... .... ....... ....................... 39
3.2 Design of Optical Waveguide Structure of Semiconductor Laser ..... ......40
3.2.1 Control of Transverse Modes .............................................. 40
3.2.2 Control of Lateral M odes ........................... ....................... ... 41
3.2.3 General Principles for Design of Semiconductor Laser.............................42
3.3 Thick P-Cad RWG InGaAsSb QW Laser Design ................... ...................43
3.4 Thin P-Clad RWG InGaAsSb/AlGaAsSb QW Laser Design ..... ................46








vi









4 PULSED ANODIZATION ETCHING............................... .......... 51

4.1 Introduction ................................... ................ ........ .......... 51
4.2 Fundamentals of Pulsed Anodization Etching ...................................................52
4.3 PAE Technique for GaSb-Based Laser Materials ......................................... 60
4.4 Detection of Layer Interfaces .................................................................. 65
4.5 Optimization of the GWA-BOE Electrolyte System ..........................................68


5 DEVICE FABRICATION ............................................................... ... ...............72

5 .1 In tro du ctio n ...................................................................................... 7 2
5 .2 P hotolithog raphy .................. ......................................................................... 73
5 .3 . M etallu rg ies .................................................................................................. 7 4
5.3.1 Pulsed Electroplating ......................... ......... ................ 74
5.3.2 E-beam Evaporation/Annealing ..................................... ................78
5.4 Fabrication Procedure for RWG Lasers .............................................................78


6 CHARACTERIZATION OF InGaAsSb/AlGaAsSb QUANTUM WELL LASERS... 83

6.1 Introduction ................................... ............................................ 83
6.2 Characterization of Thick P-Clad InGaAsSb MQW Lasers ..................................84
6.3 Characterization of Thin P-Clad InGaAsSb DQW Lasers .................. .................91


7 SUMMARY AND FUTURE WORK ................................................................ 98

7.1 Dissertation Summary.................................................98
7.2 Future W ork ............................ ..... .............................. ................... 100


APPENDIX

POLARIZATION-RESOLVED OPTICAL GAIN IN QW STRUCTURE................... 102


LIST OF REFERENCES ........................................ 109


BIOGRAPH ICAL SKETCH ...................................................... ............. ..... 115










vii














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

RIDGE WAVEGUIDE MID-INFRARED InGaAsSb QUANTUM WELL LASERS
FABRICATED WITH PULSED ANODIZATION ETCHING By

John Yoon

August, 2000


Chairman: Peter S. Zory, Jr.
Major Department: Electrical and Computer Engineering

Mid-infrared quantum well lasers based on the InGaAsSb/AlGaAsSb material system are designed and demonstrated. Various aspects of the strained quantum well laser diode in this material system were extensively studied: the development of an etching technique, the design of a thin p-clad ridge waveguide structure, and the characterization of ridge waveguide lasers fabricated with the newly developed etching technique. Theoretical modeling also confirmed experimental results.

Deep etching of p-cap GaSb/p-clad AlGaAsSb layers was demonstrated using a single step, pulsed anodization etching for the first time. A new electrolyte composed of glycol, water, and two acids was used. An important feature of this technique is the realtime, electrical detection of layer interfaces in the diode laser structure.

Ridge waveguide lasers fabricated with this technique have demonstrated low threshold currents at room temperature in both pulsed and continuous-wave operation




viii








with emission wavelengths of 1.8-1.9 pm. Such experimental results imply that the pulsed anodization etching with the new electrolyte system is a viable process for fabricating high quality ridge waveguide InGaAsSb quantum well diode lasers.

In order to possibly raise the continuous-wave operating temperature and output power capability, an asymmetric cladding structure was designed in the InGaAsSb quantum well laser by decreasing the thickness of the p-clad layer. This design requires a shallow etching step in fabricating ridge waveguide lasers and could lead to a scheme for fabricating distributed feedback lasers without need of an epitaxial regrowth process. The pulsed anodization etching technique was used to fabricate the first thin p-clad InGaAsSb quantum well lasers that operated continuous-wave at room temperature at an emission wavelength of 2.2 ptm.

Theoretical modeling was conducted to compare with experimental data. The emission wavelengths were in good agreement with theoretical calculations that involve strain effects on the band lineups and several interpolation schemes for material parameters of InGaAsSb and AlGaAsSb quaternaries.




















ix













CHAPTER 1
INTRODUCTION



The observation of high efficiency electroluminescence in GaAs p-n junctions [Bla62] inspired the development of the first semiconductor lasers in late 1962 [Hal62], [Nat62]. After merely four decades of development, semiconductor lasers are now found in a number of applications such as compact disc (CD) players, bar-code readers, laser printers, fiber optic communications, and many defense-related applications.

The major surge in the role played by semiconductor lasers has been driven by continued improvements in various performance characteristics: low-threshold current, continuous wave (CW) operation at room temperature, high optical power, low cost, low electrical power consumption, high wall plug efficiency, and long life time. Joint progress in material growth and device fabrication technologies and theoretical understanding of semiconductor lasers have contributed to those improvements.

The structure of the semiconductor laser has evolved along with theoretical and technological improvements. In the late sixties, a double heterostructure (DH), shown in Figure 1-1(a), was introduced to achieve charge carrier and photon confinement and led to the first continuous wave (CW) lasing at room temperature [Alf68], [Kre69], [Hay69]. DH laser diodes prevailed through the seventies and eighties.

In the late eighties and early nineties, a new generation of semiconductor laser, the quantum well (QW) laser, emerged. In the seventies, theoretical studies had suggested that the dimension of an active layer, where the radiative recombination takes


1





2


place, be reduced in order to reduce the threshold current. It was not until new crystal growth technologies capable of growing very thin layers had been developed that high quality QW lasers could be made.

In the QW laser, the thickness of the active layer is reduced to -10 nm from -100 nm in the DH laser diode. Quantum effects due to the small dimension of the active layer greatly affect the laser performance features such as radiation polarization and lasing wavelength. With such thin active layers, the DH structure cannot confine the optical field very well and a different structure called the separate confinement heterostructure (SCH) is used, as shown in Figure 1-1(b). The injected carriers are confined in an active layer whose thickness is on the order of the electron wavelength, while the optical field is confined in a region with a thickness comparable to the photon wavelength. In the original QW lasers, all layers in the multi-layered structure were lattice-matched to the substrate. It was found subsequently that QW laser performance could be improved by introducing strain into the QW active layer. The strain modifies the valence band structure, leading to lower threshold current density and improved efficiency.

Recently, progress in engineering of new laser diode materials has paved the way for the development of semiconductor lasers with wavelengths covering the range from the blue to the mid-infrared [Haa91], [Oku92], [Nak96], [Cho92], [Gar99a], [Cho95]. This expansion of operating wavelength range has led to the replacement of gas and solid state lasers in applications such as displays, optical storage, medical surgery, and chemical sensing.

Active research efforts have been made recently to realize semiconductor lasers that emit in the wavelength range between 2 and 3 tm, which operate continuously at





3







Bulk DH structure




d


Optical field


Refractive index


(a)




Jit

Bulk DH structure






Optical field



Refractive index


(b)

Figure 1-1. Schematic diagrams of energy bands, refractive index profile, and optical field distribution for (a) double heterostructure (DH) and (b) separate confinement heterostructure (SCH) quantum well (QW) lasers.





4


room temperature. The laser diodes in this infrared (IR) range have received increasing attention because of their potential use in diode-pumped solid state lasers, medical welding and surgery, low-loss fluoride-based optical fiber communications, and spectroscopic applications. The development of GaSb-based IR semiconductor lasers is reviewed in the next section.




1.1 Review of GaSb-based Semiconductor Laser Development

Various material systems have been studied for use in fabricating 2 - 3 ptm IR

semiconductor lasers. They include compounds and alloys from the III-V, II-VI, and IVVI groups. Lead-salt-based lasers (IV-VI group) were the first to be adopted for commercial applications, but recent approaches for the development of 2 - 3 p.m IR lasers are based on the use of III-V materials.

Depending on materials used in forming heterojunctions, band line-ups are of two types, I and II. The heterojunctions of type I and II are illustrated in Figure 1-2. In type I, the potential wells for both electrons and holes are established in the narrow band gap side, as shown in Figure 1-2(a). In type II, a potential well for electrons is formed in one side, whereas the potential well for holes is in the other side, as shown in Figure 1-2(b).

Two different transition schemes for obtaining the stimulated emission are

currently being investigated. Interband-transitions, for which the stimulated emission occurs between the conduction band (CB) and the valence bands (VB), have been utilized in conventional laser diodes since the first demonstration of semiconductor lasers in 1962. On the other hand, intersubband transitions have been recently utilized to make unipolar semiconductor lasers [Loe91], [Kas91], [Fai94]. In this case, the optical gain








results typically from a population inversion between the first and second energy subbands in the CB part of the quantum well.




E AlGaAsSb GaSb E GaSb InGaAsSb










- z -> z


(a) (b)

Figure 1-2. Schematic diagrams of energy band structures for (a) type I and (b) type II heterojunctions.


Of the various material systems and lasing mechanism schemes discussed above, the research activities in this study are based on type-I, GaSb-based (III-V group) quaternary alloys utilizing interband-transitions. The active layer(s) in the semiconductor laser are made of InxGal-xAsySbl.-y alloy, lattice-matched to a GaSb substrate. The AlxGal-xAsySbl.y alloy is used to make the guide and clad layers.

The InGaAsSb/AlGaAsSb laser diode structure is well suited for stable room temperature operation, compared to other structures such as InGaAsSb/GaSb. Unlike GaSb, AlGaAsSb has a lower refractive index than InGaAsSb and provides a potential barrier high enough to confine both electrons and holes in the active layer(s).





6


The first InGaAsSb/AlGaAsSb diode lasers were demonstrated by Kobayashi et al. [Kob80]. A DH structure grown by liquid phase epitaxy (LPE) was used and room temperature laser action at 1.8 ptm wavelength demonstrated.

Further development of InGaAsSb/AlGaAsSb lasers were, however, restricted by material growth problems associated with the LPE growth technique. First, higher Al content in the confining layers was desired in order to reduce threshold current densities. An increase of Al of more than -30% requires higher growth temperatures, leading to a reproducibility problem [Cho9la], [Mor91]. Second, there exists a miscibility gap in the InGaAsSb system for compositions lattice matched to GaSb with In exceeding 20-25 % (emission wavelengths corresponding to greater than 2.4 [tm) [Cho9a], [Gar99a], [Eli99]. Such unstable alloys can be prepared by a nonequilibrium technique, not by a near-equilibrium technique such as LPE. Finally, thickness and composition controls during the LPE process are very limited. Poor thickness reproducibility makes the LPE technique unsuitable for quantum well structure preparations [Cho9a], [Eli99].

There have been attempts to use metal organic vapor phase epitaxy (MOVPE) to grow AlSb-containing alloys. The problem with this growth technique is that the high levels of carbon, a p-type impurity, are introduced, making it difficult to obtain n-type materials [Wan96a], [Wan96b].

Molecular beam epitaxy (MBE), a non-equilibrium growth technique, has proved successful in growing the high Al content alloys and the unstable InGaAsSb alloys in the miscibility range [Cho9lb]. The first InGaAsSb/AlGaAsSb QW laser was grown by MBE and demonstrated by Choi et al. [Cho92]. Recently, considerable progress in achieving longer wavelength operation at room temperature has been made both by





7


employing a broad waveguide structure to reduce high optical losses associated with pcap and p-clad layers and by using quasi-ternary heavily-strained InGaSb(As) QWs with In compositions chosen outside the miscibility region for InGaAsSb compounds [Gar99], [Mai99].




1.2. Dissertation Overview

This work describes theoretical analyses and experimental results of CW, room temperature operation of InGaAsSb/A1GaAsSb compressive-strained multiple quantum well (MQW) lasers emitting in the 1.8 - 2.3 p.m wavelength range, fabricated with pulsed anodization etching.

Goals in this study were two fold: one was to develop a reproducible, reliable

etching technique for GaSb/AlGaAsSb material systems and the other was to design and fabricate an aymmetric cladding structure in InGaAsSb/AlGaAsSb strained QW laser diodes. Also, theoretical calculations were performed to verify emission wavelengths of the devices and to obtain their optical gain spectra. Since experimental data of material parameters for Sb-based quaternaries are limited, the device parameters necessary for the theoretical calculations were found by extensively employing theoretical models.

In Chapter 2, the emission wavelengths and optical gain spectra of compressively strained InGaAsSb/AlGaAsSb QW laser diodes are theoretically obtained. Band offsets at the strained heterojunctions are determined using the "model-solid theory." Due to a lack of knowledge of the material parameters for the Sb-based quaternary alloys, several interpolation schemes are employed to obtain various material parameters. Energy levels





8

allowed in a QW structure are found using the transfer matrix method. The optical gain spectra are then calculated as a function of carrier density.

In Chapter 3, design issues of ridge waveguide (RWG) InGaAsSb QW lasers

based on two different transverse waveguide structures are discussed. On the one hand, the ridge height is determined to support single lateral mode operation in conventional, thick p-clad InGaAsSb MQW diode lasers. On the other hand, an RWG configuration is proposed in a semiconductor laser of which the transverse optical waveguide structure is modified. An asymmetric cladding structure is first designed by reducing the thickness of a p-clad layer. Potential advantages of this thin p-clad structure are briefly discussed. Finally, an RWG thin p-clad GaSb-based laser is designed.

In Chapter 4, the development of an etching technique for Sb-based materials is described. The chapter begins with a review of the experimental setup and basics for pulsed anodization etching (PAE) and then presents the first demonstration of deep etching for GaSb/AlGaAsSb materials using PAE with a new electrolyte system. Next, an important feature of this technique is described: real-time electrical detection of layer interfaces in diode laser structures during the PAE process. Progress in the optimization of the PAE technique with the new electrolyte system is then presented.

In Chapter 5, the processing methods used in fabricating the RWG devices are

first described. In particular, photolithography and electroplating procedures are adjusted for GaSb-based laser materials. The fabrication procedure is presented step-by-step with figures illustrating the cross section of the wafer in process.

In Chapter 6, experimental results of the mid-IR InGaAsSb/AlGaAsSb QW lasers fabricated with PAE are presented. Conventional thick p-clad lasers with both shallow-





9

ridge wide-stripe and high-ridge narrow-stripe structures are characterized in both pulsed and CW operation at room temperature. Measurement data include the voltage-current, optical power-current characteristics, and lasing spectra. Finally, the first thin p-clad GaSb-based QW lasers in both pulsed and CW operation at room temperature are reported with the characterization data.

A summary and future work are presented in Chapter 7.













CHAPTER 2
STRAINED QUANTUM WELL OF GaSb-BASED QUATERNARY ALLOYS



2.1 Introduction

Design and modeling of semiconductor diode lasers require a knowledge of

various material parameters such as lattice constant, energy band gap, effective mass, and refractive index, to name a few. Those parameters are essential to constructing the energy band structure of the heterojunction in the laser diode, which contains information vital to theoretical analyses.

Recently, new diode laser materials (mostly, alloys) have been utilized to expand the emission wavelengths of semiconductor lasers and successfully demonstrated [Haa91], [Nak96], [Cho92], [Cho95]. Despite the experimental progresses, however, the theoretical modeling of those semiconductor lasers have been limited by a lack of knowledge of the material parameters for the new alloy systems.

In addition, the inclusion of strains into a QW active layer has been pursued recently to enhance performance in IR semiconductor laser diodes [Cho92], [Gar96], [Gar99]. Strain effects on the band gap and effective mass of the QW active layer material should also be considered in simulations of the IR semiconductor lasers.

This chapter describes theoretical models used in calculating the various material parameters, emission wavelengths and optical gain spectra of InGaAsSb/AlGaAsSb strained QW diode lasers. Calculation results along with theoretical material parameters for the QW's with two different In content (15% and 25%) are presented at the end of the


10





11

Chapter. The diode lasers based on the two different QW's will be fabricated and characterized, to compare with the theoretical predictions. Those aspects are discussed in Chapter 5 and 6.

In Section 2.2, the "model-solid theory" is presented to calculate the band offsets, which determine the confinements of electrons and holes, of the heterojunction under strain. In Section 2.3, the transfer matrix method is described to find energy levels allowed in the QW structure, which is composed of a QW layer and two barrier layers. In Section 2.4, three different interpolation schemes are presented to calculate material parameters of the GaSb-based quaternaries. All the parameters necessary for theoretical calculations are then obtained. In Section 2.5, optical gain spectra of strained QW structures are analyzed as a function of carrier density. Finally, all the calculation results of the emission wavelengths and QW gain spectra are presented in Section 2.6.




2.2 Band Lineup of Strained Quantum Well

If the native lattice constant of a QW layer is different from that of surrounding barrier layers, the lattice-mismatched QW layer experiences distortions in crystal lattice due to either biaxial compressive or tensile strain, as illustrated in Figure 2-1. The distortion causes the strained QW layer to have a different lattice constant perpendicular

(I) and parallel (11) to the plane of the interface in order to keep the volume of each unit cell of materials the same. Closely following notations used in [Wal89], the lattice constants of the strained layers are given as: a, Gw hw +ab Gb hb
a G h +G(2.1) G, h, +Gb hb






12


aj, = a, [1 - Dj (all a, - 1)], (2.2)










barrier









barrier < ab

ab
(a)




barrier





aw


barrier all ab

ab >aw a < aw
(b)



Figure 2-1. Schematics of crystal lattice deformation under (a) comressive strain when ab < a, and (b) tensile strain when ab > a, [Co195].





13


where all is resulted from a biaxial strain and the same throughout the structure, a, and ab, denote the equilibrium lattice constant of the QW and the barrier layer materials, respectively, andj denotes the QW or barrier material, with

G, = 2(Cl1 +2 C2)(1-D,/2), (2.3)


D0' = 2 C12, (2.4) C,,

The equilibrium lattice constants and the elastic constants, CI and C12, of III-V binary semiconductors relevant to InGaAsSb and AlGaAsSb quaternary alloys are listed in Table 2-1. The constant D defined in Eq. (2.4) is for the case of strain along [001].






Table 2-1. Material parameter for selected III-V binary semiconductors. Lattice constants ao, band gaps Eg at F, and spin-orbit splitting A,o at room temperature from [Ada87]. Elastic constants Cll and C12, valence-band average Ev,av, hydrostatic deformation potentials ac and a, and shear deformation potential b from [Wa189].
Material ao C11 C12 Eg Aso Ev,av ac av b
(.) (dyn/cm2) (dyn/cm2) (eV) (eV) (eV) (eV) (eV) (eV)
AlAs 5.6611 1.25x1012 5.34x10" 2.95 0.28 -7.49 4.09 2.47 -1.50
GaAs 5.6533 1.22x10'2 5.71x10" 1.42 0.34 -6.92 -7.17 1.16 -1.70
InAs 6.0584 8.33x10" 4.53x10" 0.36 0.4 -6.67 -5.08 1.00 -1.80


AISb 6.1355 8.77x10" 4.34x10" 2.3 0.72 -6.66 3.05 1.38 -1.40
GaSb 6.0959 9.08x10" 4.13x10" 0.72 0.74 -6.25 -6.85 0.79 -2.00
InSb 6.4794 6.59x10" 3.56x10" 0.18 0.81 -6.09 -6.17 0.36 -2.10





14


The lattice-mismatch parameter, , is defined for the QW layer as follows.

a
s =- 1, (2.5) a,


6w = -a- 1, (2.6) a,

For biaxial compressive and tensile strain, e.I < 0 and el > 0, respectively. The strain breaks the cubic symmetry of the semiconductor lattice, causing the energy shifts of the CB and VB. Two components of the strain contribute to the energy shifts.

One is the hydrostatic component of the strain that is proportional to the volume change of the crystal lattice due to the strain. This strain component shifts the average valence-band energy E,,, = (Ehl, + El + E,,o) /3, i.e. the average of the energies of the heavy-hole (Ehh), light-hole (Eh ), and spin-orbit split-off (Eso) bands as follows: AE , = a,(2611 + ), (2.7) and similarly for the CB energy

AEh"y = a, (2 ell + sE), (2.8) where av and ac are referred to as the hydrostatic deformation potentials for the VB and CB, respectively. Eqs. (2.7) and (2.8) imply that the hydrostatic strain shifts the energy levels of the CB and VB either up or down depending on if the volume change is positive or negative.

The other component is referred to as the shear component of the strain that is

proportional to the asymmetry in the strain parallel and perpendicular to the stress plane. This component removes the band edge degeneracy of the heavy-hole (HH) and lighthole (LH) bands.






15


C

C
cy



LH AE
HH, LH
~HH m HH, LH
AE 5AE, HH HH, LH LH

SO



Tensile Unstrained Compressive Strain Strain


Figure 2-2. Schematic diagram of band energy shifts of the conduction band (C) and three valence bands (HH, LH, and SO for the heavy-hole, light-hole, and spin-orbit splitoff bands, respectively) for biaxial compressive and tensile strain. The magnitude of the energy shift is indicated next to each shift by using notations defined in the text [Cold95].



The energy shifts of the three VB due to the shear strain with respect to the HH bandedge energy, Ehh = E,, + A/3 (Ao is the unstrained spin-orbit splitting), are expressed:


AE'- 1 SEsh, (2.9)
2


AE1 =_1-Ao +1SEsh" + 2[A2 + AoSEh +9(3E'h)2 1/2, (2.10)
2 4 2 4

1 = 1 1 9 + (E)21/2 ,AE'" =1 Ao + SEh 2 0E 9Sh21/2, (2.11)
2 4 2 4 with

E'01,sh = 2b(E - 11), (2.12)





16


where b is the shear deformation potential for strain along [001]. The CB at F is not affected by shear strain shifts, but only subject to hydrostatic strain shifts. The band energy shifts due to the strains are illustrated in Figure 2-2.

In the model-solid theory [Wal89], the band positions are expressed in terms of E,,, on an absolute scale, which does not carry any physical meaning by itself, but only meaningful relative to other semiconductors. The HH, LH, and CB edge energies, i.e. Ev,hh, E,,,,, and Ec, respectively, are obtained on an absolute scale: Ev,,,h = Eav + o + AE, + AEshh, (2.13)
3 " '


Evvh = Ev,, + o+ AEhy + AEh (2.14)


3
c v'+ AE*j, (2.15) where E,,,, Ao, and E, refer to unstrained material properties. The separation between the HH and LH band edge energies, S, is then defined as S = Ev,hh - Ev,h, (2.16) The topmost VB edge energy, E,, is found by E, = max(Ev,,,, E,, ), (2.17) Consequently, the strained band gap is given as: Eg" = E, - E, (2.18) Finally, the VB and CB offsets for a strained QW are obtained simply by

AEv = E - E, (2.19) AE, = Eb -E;, (2.20)





17

where b and w denote barrier and QW materials, respectively, and vj indicates the HH or LH band. Figure 2-3 illustrates the band lineups of strained QW structures. Material parameters for III-V binary semiconductors, used in the calculation of strained-band lineups, are also listed in Table 2-1.



A A AA

AE AE
C C

V C V C
A A Eg(barrier)
E E
HH LH :
A A A A
AE S AE S
(=AE,) ' (=AE) A S LH A 'AE HH AE

(a) (b)

Figure 2-3. Schematics of band lineups of QW structures under (a) compressive strain and (b) tensile strain.



2.3 Quantized Energy Levels in Quantum Well

Energy levels allowed in a finite square well potential are found by solving the time-independent Schrodinger equation [Col95], [Man92]. The transfer matrix method [Gha88] was chosen here as a numerical technique to find energy eigenvalues.

In the one-dimensional potential well of width I and of height Vo, as shown in Figure 2-4, each of three separate regions of uniform potential has the solution to the Schrodinger equation as the sum of two counter propagating plane waves.





18


i = A4 eJkIz + B, e-jkz (2.21) with


k, = n h-2 (2.22) where i indicates the i th region in the QW structure, m jj is the effective mass in the growth direction for the material in the i th region, and AE, is the potential barrier height in the i th region.



E

Vb

Region 1 Region 2 Region 3

0

Z1 Z2 Z3 Z (= 0)


Figure 2-4. Schematic energy diagram of a simple square potential well of depth Vo.


By applying proper boundary conditions at each interface, the recurrence relationship of the wavefunction can be written in matrix form as ( = S(, (2.23) where the total transfer matrix S is defined as

3
S = fls, (2.24)
i=2





19


with


S, (2.25)


where


= 1� - '-_ , (2.26)
2 ki _l/m�,

For a physical reason, the outward-going components of the wavefunctions in the barrier regions (i.e. region 1 and 3) must be equal to zero at an infinite distance. Those boundary conditions result in the characteristic equation for one of the elements in the total transfer matrix S as follows.

Soo = 0, (2.27) where the matrix elements in the total transfer matrix S are defined as S = ( I 1 (2.28) From Eqs. (2.24), (2.25), and (2.27), the energy eigenvalues are numerically determined.




2.4 Material Parameters for Quaternary Alloys

The quaternary alloys used in QW structures of this study are InxGal.-AsySbl., and AlxGal-xAsySbl.y. However, experimental data of material parameters for these quaternary alloys are extremely limited at the present time. In order to predict the various material parameters of the quaternaries over the entire range of alloy composition, interpolation schemes should be used. Although the schemes are still open to experimental verification, they are known to be useful in reliably estimating material





20

parameters of quaternary alloys.

In a linear interpolation scheme, experimental (or theoretical) material parameters for the four possible binary constituents (AC, AD, BC, and BD) of quaternary compound AxB,-xCyDJy are utilized to estimate some material parameter Q(x,y) as follows [Ada87].

Q(x,y)= xyAC + x(1- y)AD + (1-x)yBC +(1-x)(1- y)BD, (2.29) The lattice constant, deformation potential, effective mass, and Luttinger parameter in the quaternary alloy are known to vary linearly with composition as described in Eq. (2.29).

For the band gap, spin-orbit splitting, and average VB energy, an interpolation

scheme takes into account four possible ternary alloys as well as the four binary materials in a given quaternary compound. The energy-related parameters of the quaternary alloy are by [Gli78], [Dew85].

Q(x, y) = xyA C + x(1 - y) AD + (1 - x)yBC + (1 - x)(1 - y)BD
(2.30)
- x (1 - x)[y CABC +(1 - y)CABD] - y (1 - y)[xCAD +(1 -C) CD

where the constant C is referred to as a bowing parameter. Table 2-2 lists the bowing parameters for various band gaps of III-V ternary alloys [Ada87].

The bowing parameter for the average VB energy of ternary alloy AxB-xC is to be found from the following expression [Car88], [Wal89].

Aa
E,,,, (x) = xE(AC) + (1- x)E(BC)- 3 x(1 - x)[a, (AC) - a, (BC)] , (2.31) a0

where a is referred to as a deformation potential and ao as a lattice constant with ao = xao(AC) + (1- x)ao(BC) and Aa = ao(AC)-ao(BC). Consequently, the bowing parameter for E,,, of the ternary alloy is:

3
C,,,, = -[a, (AC) - a, (BC)][ao(AC) - ao (BC)], (2.32) ao





21


Table 2-2. Bowing parameters of III-V ternary compounds at room temperature [Ada87].
Co C, CL CA (AI,Ga)As 0.370 0.245 0.055 0.070 (Ga,In)As 0.600 1.400 0.720 0.200 (AI,Ga)Sb 0.470 0.000 0.550 0.300 (Ga,In)Sb 0.420 0.330 0.380 0.100 AI(As,Sb) 0.000 0.000 0.000 0.000 Ga(As,Sb) 1.200 1.090 1.090 0.610 In(As,Sb) 0.580 0.590 0.570 1.200






2.5 Optical Gain Spectrum in QW Structure

The linear gain in a semiconductor laser structure can be defined from Fermi's Golden rule and is given as [Asa93]


g(E.) = 1 rcm + 2 red(E) c(E) + f(Egv)-l], (2.33) where e is the electron charge, e0is the permittivity of free space, c is the light speed in vacuum, mo is the electron rest mass, n is the effective index of the guided mode (for unguided optical modes, n is just equal to the refractive index of the QW material), /T 2 is the transition matrix element, 2 is the average transition matrix element, Pred is the reduced density of states, f is the Fermi-Dirac distribution for the electrons in the CB, and f, (= 1- f,) is the distribution for the holes in the VB. E, is an energy corresponding to the photon energy by





22


E, = Eg + Ej1I + E,,1, (2.34) with E,,,, and E11I given by h2k2 h2k2 EJll = E, + II El = E + (2.35)
2 mill 2 m,,ll where mzI and m.11 are the in-plane effective masses of the CB and relevant VB, respectively. Figure 2-5 shows schematic energy band diagrams of the QW structure with the aforementioned energy terms defined.






E E




h 2mE
E . - ------------------------- E- . ............{ . .. Ec.ll
B edge .................. ... ............................





V B edge -- -- - - - -- - - - -- - - - - -- - - - ,
v ege.............................- -----. -1-- - -. vj||


> z k

(a) (b)

Figure 2-5. Schematic energy band diagram (a) Energy versus position z (b) Energy versus wavevector k.





23


For QW, the effective masses in the growth and in-plane directions are not same, unlike for a bulk case. The growth and in-plane effective masses (mh, m , , mhll, and mlh|l) are defined by using Luttinger parameters as

1 h2 1 h2 mih, =- , ntl = (2.36) Y1 -22 2 71 +272 2

1 h2 1 h2 mhh l , nflhlI - (2.37) Y1 +r2 2 71- -2 2 The transition matrix element ' 12 describing the interaction between the CB and VB states is given as


-+-cos 20 forC-HHTEgain
2 2
T 2 1 -COS2 0 for C - HH TM gain 5 _ cos20 for C - LH TE gain
6 2

+ COS2 0 for C - LH TM gain
3

where


2= o -1) +Am E, (2.39) nic 2 E +-A


m61
(EC + Ej )
cos2 1 7, (2.40) (E,-E, -E -E ,)+(E , +E) m


with spin-orbit splitting energy A and reduced effective masses in the growth and inplane directions, mll_ and mn,., respectively, defined as





24


1 1 1 1 1 1
- = +- and - + , (2.41) rni ncjl� nm reil n mvll The reduced density of states Pred , with spin not included, for QW is given by uill for E, _ E.
Pred (Ev)= 2r h2d, , (2.42)
0 for E, < Es,

with the energy gap between thejth conduction and valence subbands defined as EJ = Eg + Eo + Evi (2.43) The Fermi-Dirac functionsf, andfh are defined as fc(E1 ) expE - )/kT) (2.44) 1+exp( I I -Ejfc/kBT)'


fh(E111) = 1 (2.45) 1+f(Eexp((E,,I -E,)/kT) (2.45) where Ef, and Ef, are the quasi-Fermi levels in the CB and VB, measured with respect to the CB and VB edges, , respectively, kE is the Boltzmann constant, and T is the temperature.

For parabolic CB and VB, the quasi-Fermi levels Ef, and Ef, are related to the electron density N and hole density P in the quantum well as follows.

N= kBT m ln(1+exp[- (Ec -Ej/kT), (2.46) P = mkT ,,In(1+exp[- (E, -E)/kT]), (2.47)
A2d3





25


The summation in Eq. (2.47) is over all subbands in the HH and LH bands for a given value ofP. Assuming the charge neutrality in the quantum well, N = P. Then, Ef, and E. can be found from Eqs. (2.46) and (2.47) for a given value of N at a given T.

If the subbands in the CB and VB are indexed by the quantum numbers n, and n,, the spectra gain associated with each subband transition pair at a photon energy E, can be denoted as gub(E,,,n~,). Finally, the total gain at E, is found by summing over all possible subband transition pairs: g(Ec) = g,, b(E,,n, n.) (2.48)
nc n ,




2.6 Calculation Results

Theoretical models presented in Section 2.2 through 2.5 are implemented to

simulate two different QW laser diodes: an Ino.12Ga88Aso. o2Sbo.98/Alo.25Gao.75Aso.02Sbo98 QW in a thick p-clad structure and an Ino.25Gao.75Aso.o2Sbo.98/Al0.3Gao.7Aso.02Sbo98 QW in a thin p-clad structure.



2.6.1 Material Parameters and Emission Wavelengths

Material parameters of those four GaSb-based alloys are obtained as described in Section 2.4. The parameters for the QW alloys, InxGa-xxAso.02Sbo.9s (x = 12 and 25%), are summarized in Table 2.3 and those for the barrier alloys, AlyGal-yAso.o2Sbo.9s (y = 25 and 30 %) in Table 2.4. Energy band gap for each QW structure is illustrated in Figure 2-6(a) and 2-7(a).





26

Lattice constants of quaternaries for the QW and the barrier layers are slightly

different and thus strains are introduced into the QW's. The band edges of the QW's are accordingly shifted, as described in Section 2.2. The strain-induced band shifts in the InxGal-xAso.02Sbo.9g QW's (x = 12 and 25 %) are illustrated in Figure 2-6(b) and 2-7(b), respectively. Finally, the "model-solid theory" is used to construct the band lineups at InxGal-xAso.02Sb.98 /AlyGal yAso.02Sbo.98 interfaces (x = 12, 25 % and y = 25, 30 %, respectively), as shown in Figure 2-6(c) and 2-7(c), respectively. Finally, the band offsets, which determine the confinement of electrons and holes, are obtained.

The lattice mismatch and the band edge shifts of both the CB and HH band are plotted as a function of In content in the QW's for the thick and thin p-clad laser diode structures in Figure 2-8 and 2-9, respectively. The strain strength, which is proportional to lattice mismatch, is increased with In composition in the QW. The minus signs of the lattice mismatch values indicate compressive strains.

The shifts of the CB and HH band edges are shown as a function of In content in the two different QW's in Figure 2-10 and 2-11, respectively. The shift of the HH band edge is more sensitive to In composition in the QW than that of the CB edge is. This is because the VB are affected by both hydrostatic and shear strains, while the CB is only by hydrostatic strain.

The resultant band gaps of the strained InGaAsSb/AlGaAsSb QW's decrease with In content in the QW, as shown in Figure 2-12 and 2-13. That is, the longer wavelength operation is achieved with higher In content in the QW.





27


Table 2-3. Theoretical strain-related material parameters of InxGalxAso.o2Sbo.98 quaternaries (x = 12 and 25 %) used in the QW active layer of the thick and thin p-clad laser diode structures with barriers of Alo.25Gao.75Aso.0o2Sbo.98 and Alo.3Gao.7Ao.o2Sbo.98, respectively.
Ino. 12Gao.88Aso.o2Sbo.98 Ino.25Gao.75Aso.o2Sbo.98 ao () 6.1331 6.1830
C11 (dyn/cm2) 8.84x10" 8.51x10"
C12 (dyn/cm2) 4.09x10" 4.02x10"
Eg (eV) 0.63 0.55 Aso (eV) 0.72 0.72 Ev,av (eV) -6.23 -6.21
ac (eV) -6.77 -6.68 av (eV) 0.75 0.69 b (eV) -2.00 -2.02







Table 2-4. Theoretical material parameters of AlyGal-yAso.o2Sbo.98 quaternary (x = 25 and 30 %) used in the barrier layers of the thick and thin p-clad laser diode structures. Deformation potentials ac, a,, and b are not obtained here because the barrier layers are assumed not too experience strains.
Alo.25Gao.75Aso.02Sb0.98 Alo.3Gao.7Aso.02Sbo.98 ao (1) 6.0968 6.0987
C1, (dyn/cm2) 9.07x10" 9.05x10"
C12 (dyn/cm2) 4.21x10" 4.22x10"
Eg (eV) 1.02 1.09 ASo (eV) 0.66 0.66 Ev,av (eV) -6.37 -6.39





28





E = 1019.3 meV E = 601.2 meV



In0.12 Gao88 AsO.02 Sb0 98 Al0 25 Gao.75 Aso.02 Sb0.98

(a)


CB
41.7 meV In. 12Ga88AsO.02Sb0.98 4 .... HH
Bulk 4.6 meV 22.2meV 20.7 meV V -LH


Hydrostatic Deformation Shear Deformation

(b)



225.9 meV X C

625.3 meV 1019.3 meV HH
\42.9 meV 168 meV LH , 125.1 meV


(c)

Figure 2-6. Schematic energy diagrams for (a) bulk materials (b) Ino.12Ga.88ssAso.o2Sbo.98 QW under strains, and (c) strained band lineup between Ino.12Ga.88Aso.o2Sbo.98 QW and Alo.25Ga.75Aso.o2Sbo.98 barrier.





29





E = 496.5 meV Eg = 1088 meV



In0.25 Gao.75 Aso02. Sb0.98 Alo3 Gao.7ASO.02 Sb0.98

(a)


--- CB
90.6 mev CB In0.25Gao.75Ao.02Sbo.98 I...... HH
Bulk 9.4 meV 50.4 meV 42.8 meV LH

Hydrostatic Deformation Shear Deformation

(b)



300.1 meV X c

546.1 meV 1088 meV HH
XA
- 93.2 meV 241.8 meV I LH
: 148.7 meV


(c)

Figure 2-7. Schematic energy diagrams for (a) bulk materials (b) Ino.25Ga.75Aso.02Sb0.98 QW under strains, and (c) strained band lineup between Ino.25Ga.75Aso.02Sbo.98 QW and Alo.3Ga.7Aso.0o2Sbo.98 barrier.






30



-0.4 i I

-0.

.c -0.6

-0.



--o.



10 12 14 16 18 Indium (%)



Figure 2-8. Lattice mismatch as a function of Indium content in InxGalxAso.o02Sbo.98/ Alo.25Ga.75Aso.02Sbo.98 QW structure (10% < x < 18%). The widths of the QW and barrier are 10 nm and 400 nm, respectively.






-1. 0



-1.2
-1.4







20 25 30 35 Indium (%) Figure 2-9. Lattice mismatch as a function of Indium content in InxGal-xAs.o2Sbo.98/ Alo.3Ga.7Aso.0o2Sbo.98 QW structure (20% < x < 35%). The widths of the QW and barrier are 20 nm and 300 nm, respectively.





31



25






200





150 i I 10 12 14 16 18 Indium (%) Figure 2-10. Shifts of the CB edge (squares) and the HH band edge (circles) due to strains as a function of Indium content in InxGal-xAso.o2Sbo.9s /Alo.25Ga.75Aso.0o2Sbo.98 QW structure (10% < x < 18%). The widths of the QW and barrier are 10 nm and 400 nm, respectively.





320 -300 28 "260

240

220,

18 20 22 24 26 28 30 32 34 36 Indium (%) Figure 2-11. Shifts of the conduction band edge (squares) and the heavy-hole band edge (circles) due to strains as a function of Indium content in InxGalxAso.o2Sbo.98 /Alo.3Ga.7Aso.02Sbo.98 QW structure (20% < x < 35%). The widths of the QW and barrier are 20 nm and 300 nm, respectively.


31






32



660 I 640


620


600"


580 I I I I 10 12 14 16 18
Indium (%) Figure 2-12. Energy band gap as a function of Indium content in InxGal-xAso.02Sbo.98 / Alo.25Ga75Aso.02Sbo.98 strained QW structure (10% < x < 18%). The widths of the QW and barrier are 10 nm and 400 nm, respectively.





580


560


540


4 W





20 25 30 35 Indium (%) Figure 2-13. Energy band gap as a function of Indium content in InxGal-xAso.o2Sbo.98 / Alo.3Ga.7Aso.o2Sbo.98 strained QW structure (20% < x < 35%). The widths of the QW and barrier are 20 nm and 300 nm, respectively.




32





33

Figure 2-14 (a) and (b) show theoretical emission wavelengths of the thick and

thin p-clad InxGal-xAso.02Sbo.9s /AlyGal.yAso.o2Sbo.9s8 strained QW lasers (x = 12, 25 % and y = 25, 30 %, respectively). Since multi-subbands exist in each QW structure, the emission wavelengths are computed for transitions between the first subbands in the CB and HH band (C1-HH1) and between the second subbands in the CB and HH band (C2HH2). In content (x) in the QW (that is, strain effect) affects the wavelength of the C1HH1 transition more significantly than QW width (that is, quantum effect), while the opposite holds true for the wavelength of the C2-HH2 transition. Therefore, the design of the QW structure in the GaSb-based laser diode should consider strain or quantum effects appropriately, based on the transition responsible for the emission wavelength.



2.6.2 Optical Gain Spectra of InGaAsSb/AlGaAsSb Strained QW's

Figure 2-15 shows the calculated optical gain for TE and TM modes as a function of photon energy in the InxGal.-Aso.o2Sbo.98 /AlyGal-yAs.o2Sbo.98 QW (x = 12 % and y = 25 %) at a carrier density N = 1.2 x 1018 cm3 and T= 300 K, with intraband relaxation ignored. The parameters used in the gain calculation are listed in Table 2-5. The table lists two subbands in each of CB, HH, and LH bands. There are actually more than two HH subbands allowed in this QW structure, while two subbands are allowed in the CB and LH band, respectively. HH's in the subbands higher than the second are meaningless in the gain calculation because they cannot participate in band-to-band transitions that are dictated by the k-selection rule [Cas78a], [Cor93].








33






34



2100
---- In=18%
2050- -A- In-15%
-0- In-12% 2000 --I n=10



U
t oo 1900- -a 1850

1800 10 15 20 QW width (nm)

(a)



1900
- In=180
-A- In= 15 , S 180 -0- In=120 *
- In1004 A S(n 170


0 160


150a

10 15 20 QW width (nm)

(b)

Figure 2-14. Emission wavelengths of InxGal-xAso.02Sbo.98/Al.25Ga.75Aso.o2Sbo.98 strained QW laser as a function of QW width for (a) the C1-HH1 transition and (b) the C2-HH2 transition. Indium content (x) in the QW varies from 10% to 18 % and the barrier layer is 400-nm thick.





34






35



3000
TE
2500 - -----... TM

2000

� 1500

1000

500

0
600 620 640 660 680 700 720 740 760 780 800
Energy (meV)

Figure 2-15. Calculated material gain for TE and TM modes in InxGal-xAso.o2Sbo.98/ Alo.3Gao.7Aso.o2Sbo.98 strained QW (x = 12 %) as a function of photon energy at a carrier density N= 1.2 x 1018 cm3 and T= 300 K. The second peak at E= 736 meV corresponds to the C1-LH1 transition.






Table 2-5. Material parameters used in the gain calculation for InxGal-xAso.02Sbo.98/ AlyGayvAso.02Sbo.98 QW (x = 12 % and y =25 %). The QW width is 10 nm.
Strained band gap (meV) 625.5 Energy levels in CB (meV) 37.7, 151.4

CB offset, AEc (meV) 225.6 Energy levels in HH (meV) 9.7, 38.5

VB offset, AEv (meV) 168.2 Energy levels in LH (meV) 29.3, 110.4

Splitting energy, S (meV) 43.2 Luttinger parameter, y, 12.574 SO band energy, A (meV) 717.7 Luttinger parameter, 72 4.426

Refractive index, n 3.5261 Electron mass (x mo) 0.042










35





36


Since the QW is compressively strained (- 0.6 %), the gain peak at a photon

energy of 673 meV (i.e., wavelength of 1.84 ltm) corresponds to the C1-HH1 transition and the TE gain is dominant over the TM gain, as expected. The second peak at 736 meV (i.e., wavelength of 1.69 rlm) is due to the C1-LH1 transition, which explains the reason for the dominance of the TM gain over the TE gain.

It is interesting to note that the TM gain curve in the QW is smooth like the bulkcase, not sharp as the gain characteristic of the 2-dimensional structure should appear. This is due to the natures of the transition matrix element and Fermi-Dirac distributions involved in the gain mechanism. The analysis of the polarization-resolved gains for all possible transition pairs between the conduction, HH, and LH subbands in a QW are discussed in Appendix A, to explain the behavior of the TM gain.





3500
N = 1.2 x 108 cn3 4
3000 -------.. N = 1.7 x 10 cm / 2500 N= 2.2 x 101 cm. . 2000 ..

1500




0 . . . .
1.4 1.5 1.6 1.7 1.8 1.9 2.0 Wavelength ([tm)

Figure 2-16. Calculated TE gain in InxGal-xAso.o2Sbo.98/Alo.25Gao.75Aso.o2Sbo.98 strained QW (x = 12 %) as a function of emission wavelength for several carrier densities (N= 1.2, 1.7, and 2.2 x 10'8 cm3 ) and T= 300 K. The second peak at 1.68 pm is due to the C1-LH1 transition.


36





37


Figure 2-16 shows TE gain as a function of the emission wavelength for several carrier densities N= 1.2, 1.7, and 2.2 x 10'8 cm3. At N = 1.2 x 1018 cm3, two gain peaks occur at 1.84 pm and 1.69 pLm corresponding to the C1-HH1 and C1-LH1 transition, respectively. The gain peaks increase with the injected carrier density. Gain from the C2-HH2 transition is also observed at 1.52 pm for an injected carrier density of 2.2 x 1018 cm3. However, the emission wavelengths from the C1-LH1 and C2-HH2 transitions are unlikely to be observed because their gain values even at very high levels of carrier densities are not high enough.

In Figure 2-17, TE gain in InxGalxAso.02Sbo.98 /AlyGa1.yAso.02Sbo.98 strained QW (x = 25 % and y = 30 %) is plotted as a function of emission wavelength for different carrier densities N= 5.5, 7.5, 9.5 x 1017 cm3. Table 2-6 lists the parameters used in the computation.

At low carrier densities (e.g. N= 5.5 x 1017 cm3), the maximum gain occurs at 2.2 pm corresponding to the C1-HH1 transition. As the carrier density is increased, the gain peak increases. The second gain peak is shown at 2 pm, corresponding to the C2-HH2 transition, for N= 7.5 x 1017 cm3. When the injected carrier density is increased to N= 9.5 x 1017 cm3, the gain peaks are almost the same at 2 and 2.2 plm. This implies that a laser with this QW structure can possibly operate at two simultaneous IR wavelengths if the required gain threshold is appropriately adjusted by changing a cavity length or using a different metal contact [Wu94]. Such phenomenon was previously demonstrated for InGaAs QW lasers in the near-IR regime (900 - 965 nm) [Wu94].







37






38





2000 . . . . .
N=5.5x 1017 cm"3
......---- N= 7.5x 1017 cm"3
1500
----- N = 9.5 x 1017 c Cd

1000 -/ *


500


0 * * , , I * * � * . .. . . . * . . . .
1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 Wavelength (p.m)


Figure 2-17. Calculated TE gain in InxGal-xAso.o2Sbo.98 /Alo.3Gao.7Aso.o2Sbo.98 strained QW (x = 25 %) as a function of emission wavelength for several carrier densities (N =
5.5, 7.5, and 9.5 x 1017 cm3 ) and T= 300 K. The second peak at 2 p.m is due to the C2HH2 transition.






Table 2-6. Material parameters used in the gain calculation for InxGal xAso.o2Sbo.9s/AlyGal-yAso.o2Sbo.98 QW (x = 25 % and y =30 %). The QW width is 20 nm.
Strained band gap (meV) 546.1 Energy levels in CB (meV) 15.3, 62.0

CB offset, AEc (meV) 300.1 Energy levels in HH (meV) 3, 11.9

VB offset, AE, (meV) 241.8 Energy levels in LH (meV) 12.4, 50.2

Splitting energy, S (meV) 93.2 Luttinger parameter, y, 13.759 SO band energy, A (meV) 723.0 Luttinger parameter, 72 5.014

Refractive index, n 3.5277 Electron mass (x mo) 0.038








38













CHAPTER 3
RIDGE WAVEGUIDE LASER DESIGN




3.1 Introduction

In semiconductor lasers, the light is confined and guided through dielectric

waveguiding where each layer has a different refractive index. The allowed modes and resulting optical fields in the laser structure are found by solving the wave equation. The pertinent mathematical description is found in various texts on the electromagnetic fields theory of guided waves [Col60], [Mar72], [Cas78a], [Tho80], [Agr93].

For low-threshold room-temperature operation of diode lasers, a single mode

operation is desired and its optical field needs to be confined in the active region where the lasing takes place. The epitaxial structure of the laser diode is designed such that these requirements for the laser mode are satisfied.

The design of the waveguide structure of the semiconductor laser considers mode behaviors in two different directions, transversely and laterally, that is in the direction perpendicular and parallel to the junction plane, respectively. In this chapter, the optical field distributions in the transverse and lateral directions are analyzed for two different designs of RWG InGaAsSb QW diode lasers that utilize a thick p-clad and a thin p-clad layers, respectively.

In Section 3.2, general design issues of semiconductor diode lasers are discussed in view of transverse and lateral mode behaviors. Also, the differences of the design



39





40

issues between the thick and thin p-clad structures are addressed. In Section 3.3, the design of the RWG InGaAsSb QW diode laser that incorporates a thick p-clad layer is described. Section 3.4 is devoted to the design of the RWG InGaAsSb QW diode laser that employs a thin p-clad layer.




3.2 Design of Optical Waveguide Structure of Semiconductor Laser

3.2.1 Control of Transverse Modes

The transverse modes are dependent on the refractive indices and thicknesses of the layers used in the semiconductor laser. The refractive index is determined by the material composition of the layer and the operating wavelength of the semiconductor laser. Once the refractive indices have been found, the layer thicknesses are adjusted to control the mode distributions.

In semiconductor lasers, the optical field confinement should be great in the active region and small elsewhere to reduce the required threshold gain. This is important particularly for semiconductor lasers in the GaSb-based material system because of high optical losses associated with the materials that are used as the p-cap and p-clad layers.

In the mid-infrared wavelength regime, p-type materials used in the GaSb-based semiconductor lasers are very lossy. At a wavelength of 2.2 rtm, for example, the optical absorption value of highly p-doped GaSb (p - 2 x 1019 cm3) is extrapolated as much as 2000 cm-1 [Bra59], and that of p-AlGaAsSb (p = 5 x 1018 cm3) is estimated about 70cm1 by interpolating published values of binary GaSb [Bra59] and AlSb [Bra62]. Therefore,





40





41

a careful design should take into account the high optical absorptions associated with the two top layers of the GaSb-based semiconductor laser.

Design issues are different for the thick and thin p-clad laser structures. In the

thick p-clad GaSb-based laser design, the mode loss associated with the high intervalence band absorption in the p-clad layer is kept small by broadening the guide layers [Gar96], [Gar98], [Gar99a]. The minimization of the transverse mode confinement in the lossy pcap and p-clad layers is a key issue to achieve low threshold current density.

In the thin p-clad design, the fractional mode power in the p-clad layer can also be kept small, but this must be balanced with the requirement that a reasonable amount of mode power must exist at the contact surface in order to obtain the desired coupling coefficient for distributed feedback (DFB) lasers.



3.2.2 Control of Lateral Modes

For low threshold operation and single spatial mode lasing, a strong guiding

geometry in the lateral direction is desired. Figure 3-1 shows the RWG structure that is used to control the lateral mode behavior of semiconductor diode lasers. From symmetric three-layer planar waveguide theory, the lateral index difference An between the ridge region and the outside-ridge region that allows only the fundamental mode to exist is given by [Hun91]


n f 2 (3.1) 8ngf w2

where 2o is the free space wavelength, nff is the effective refractive index of the fundamental mode in the ridge region, and w is the stripe width.




41





42


In designing RWG lasers, the ridge height is determined to provide the lateral index difference An found from Eq. (3.1).



p-contact metal

oxide layer


active p-confining
la er layer


n-confining layer & substrate




Figure 3-1. End view of a ridge waveguide laser structure





3.2.3 General Principles for Design of Semiconductor Laser

In conventional thick p-clad structures, the thicknesses of the p-cap and p-clad layers are set typically to 0.05 and 2 pm, respectively. The thickness of the p-clad layer is usually the same as that of the n-clad layer, comprising a symmetric waveguide semiconductor laser structure. The resulting transverse mode allowed in the structure is a fundamental mode. Based on the given transverse waveguide structure, the height is to be determined to provide the sufficient lateral index difference for single lateral mode operation. That is, the ridge height is a primary design parameter.

On the other hand, the thicknesses of the p-cap and p-clad layers as well as the

ridge height are design parameters for thin p-clad laser structures. The thicknesses of the p-cap and p-clad layers are to be adjusted to enhance transverse mode confinement in the 42





43

p-cap and p-clad layers as a resultant modal loss is maintained as small as possible. In addition, the ridge height is to be determined for single lateral mode operation. These complicated two-dimensional design issues can be conveniently configured in a design procedure developed in this study. Using the design scheme, the transverse and lateral structures are simultaneously determined to satisfy the requirements for the mode in each direction.

In following sections, optical mode analysis is performed using MODEIG

dielectric waveguide simulation software, which computes optical mode profiles and complex propagation constants with complex refractive indices.




3.3 Thick P-Cad RWG InGaAsSb OW Laser Design

Figure 3-2 shows the epitaxial structure of the 2.2 jim wavelength InGaAsSb/ AlGaAsSb double-quantum well (DQW), SCH laser diode used in the calculation.

The p-cap thickness top and the p-clad thickness tol were predetermined as 0.05

and 2 jlm, respectively. Using the simulation software MODEIG, the effective refractive indices of the fundamental modes in the ridge region and the outside-ridge region were computed. In calculations, it was assumed that the ridge waveguide was formed by removing the p-cap layer and part of the p-clad layer in the outside-ridge region and then being covered with a 100-nm thick native oxide and the whole structure is metallized with Au. The refractive index values used in the calculations are summarized given in Table 3-1.

Figure 3-3 shows An as a function of etch depth in the p-clad layer. According to Eq. (3.1), An for single lateral mode operation with a ridge width of 5 pm must be


43





44


smaller than - 6.8 x 10" . From the plot, the corresponding etch depth in the p-clad layer is about 1.8 rpm. In Figure 3-4, the refractive index distribution and the calculated optical intensity distribution in the transverse direction are shown together.








p-cap GaSb It p p = 2 x 1019 cm3 p-clad Alo.9Gao.lAso.07Sbo.93 tcl p = 5 x 1018 cmr3



waveguide Al0.3Gao.7ASO.02Sbo.98 320 nm In.25GaAsSbo.98 (2 x 20 nm) 240 nm DQW AlI.3GaAsSbo (200 nm) 240 nm

waveguide Alo.3Gao.7ASo.02Sb098 320 nm



n-clad Alo.Gao. Aso.o7Sbo.93 1.5 p.m n = 5 x 1017 cm3



substrate GaSb n = 1 x 1017 cm-3



Figure 3-2. Structure of the 2.2 lm wavelength thick p-clad InGaAsSb/AlGaAsSb DQW SCH laser diode used in the calculation for the effect of a ridge height on the effective refractive index.










44





45






Table 3-1. Refractive index values used in the calculation.
Materials n (Refractive Index) GaSb 3.76 Alo.9Gao.1Aso.02Sbo0.98 3.29 Al0.3Gao.7Aso.o02Sbo.98 3.6 Ino.15Gao.ssAso.02Sbo.98 3.9











0.030

0.0250.020

0.015

0.010

0.005

0.000
1.4 1.5 1.6 1.7 1.8 1.9 2.0
p-clad etch depth (Gm) Figure 3-3. Calculated lateral effective index difference as a function of etch depth in the p-clad layer for the InGaAsSb/AlGaAsSb DQW SCH laser shown in Figure 3-2 (tcp = 50 nm, tjl= 2 itm).











45






46







1.6 4.0 1.4 - - mode ......... index 3.9

1.2 3
-3.8
1.0
3.7
0.8
.3.6
0.6
-3.5
0.4
0.2 3.4
0.2 - n-clad p-clad
0.0 3.3 -0.2 . . . . . . JI . . . . . * I 3.2
-1 0 1 2 3 4 5
transverse position (pmn)


Figure 3-4. Profiles of refractive index and optical intensity as a function of position for the InGaAsSb/AlGaAsSb DQW SCH laser shown in Figure 3-2 (tp = 50 nm, tel = 2 pm).









3.4 Thin P-Clad RWG InGaAsSb/AlGaAsSb QW Laser Design

In thin p-clad RWG laser designs, tp, tcl, and the ridge height are all design


parameters. Since tj is to be much smaller than the conventional thickness (-2 tm), the mode overlap in the lossy p-cap layer becomes significant. In order to avoid a high mode loss, the p-cap thickness should be small enough to minimize the mode power absorbed in the p-cap layer. At the same time, however, material growth consideration dictates the minimum cap thickness. To satisfy these two requirements, the cap section incorporates


a three-layer-cap region which consists of a 30-nm p-GaSb (p=2x 1019 cm3) layer, a 100nm undoped Alo.3GaAsSb0.35 layer, and another 30-nm p-GaSb layer, as shown in Figure





46





47


3-5. The three-layer cap configuration was chosen in view of ohmic contact schemes for surface-modulated distributed feedback (DFB) laser designs in future. The p-GaSb layer underlying the undoped Alo.3GaAsSbo.35 layer will be served as a contact layer for the bottom region of the gratings that are to be made on the wafer surface by etching.








GaSb 30 nm - ---cap Alo.Ga o.ASO.0 Sb.9100m :::: p = 2 x 1019 cm-3
section
GaSb 30 nm

p-clad Alo.9Gao. Aso.oSb .93 l p = 5 x 1018 cm-3

waveguide Alo.3Ga.7Aso.0o2Sbo.98 320 nm Ino 25GaAsSbo098 (2 x 20 nm) 240 DQW Al .3GaAsSbo.98 (200 nm)

waveguide Alo.3Gao.7ASO.02Sbo.98 320 nm



n-clad Alo.9Gao.1Aso.0Sbo.93 1.5 p.m n = 5 x 1017 cm-3



substrate GaSb n = 1 x 1017 cm-3


Figure 3-5. Schematic diagram of the 2.2- pm wavelength thin p-clad RWG InGaAsSb/AlGaAsSb DQW laser with a three-layer-cap configuration.










47





48



35
30
25

20

15

S10
5
0 * * * * * *i .* *
0.0 0.5 1.0 1.5 2.0 p-clad thickness (gm)

Figure 3-6. Fundamental mode loss vs. p-clad thickness for the ridge region of the laser diode structure shown in Figure 3-5.



Figure 3-6 shows the variation of the fundamental mode loss with t, in the ridge region of the semiconductor laser structure illustrated in Figure 3-5. Since the mode loss is very high for t < 250 nm and saturates at tc - 600 nm, te in the range between 250 and 600 nm will be chosen.

The p-clad thickness to1 is then studied in relation to various ridge heights. Figure 3-7 shows An as a function of the remaining p-clad layer thickness, d, for different values of the p-clad etch depth, h, which were defined in Figure 3-5. For a desired value of An, different combinations of d and h are allowed and the choice of a combination of d and h serves as a starting point for a final design. For the first design of a thin p-clad RWG GaSb-based laser, tc, is chosen as 300 nm because a low-ridge configuration is desired for the convenience of ridge waveguide fabrication. For t11= 300 nm, the p-clad etch depth for single lateral mode operation is found to be about 120 nm from Figure 3-8. The





48






49


transverse optical intensity for the final thin p-clad laser structure is plotted in Figure 3-9,

along with the refractive index distribution.



0.009 .... .... .... .... .... .... ........
-h = 50nm
0.008 ------- h= 100nm
-----h = 150 nm 0.007 .--.... --............. h=200nm

.... ..............



0.005 "


0.004 *** 160 165 170 175 180 185 190 195 200
d(nm)

Figure 3-7. Calculated An as a function of the remaining p-clad layer thickness d for different values of the p-clad etch depth h for the structure shown in Figure 3-5.





0.020
0.0180.0160.014
0.0120.0100.0080.006
0.0040.002 . . . 50 100 150 200 250
p-clad etch depth (muu)


Figure 3-8. Plot of An versus p-clad etch depth for the structure shown in Figure 3-5, with tcl = 300 jtm.




49







50





1.6 ,I 4.0

1.4 -- mode ......... index
. p-clad - 3.9
1.2
S3.8
1.0
- 3.7
0.8
- 3.6
- 0.6
3.5
& 0.4
3.4
0.2 n-clad

0.0 - ...... - 3.3

-0.2 . I -- 3.2
-1 0 1 2 3
transverse position (Wm)


Figure 3-9. Profiles of refractive index and optical intensity as a function of position for the thin p-clad InGaAsSb/AlGaAsSb DQW SCH laser shown in Figure 3-5 (tl = 300 nm).












































50













CHAPTER 4
PULSED ANODIZATION ETCHING



4.1 Introduction

Etching and oxidation is an important procedure in defining an electrical contact region on the p-side of a semiconductor diode laser. In the simplest procedure, oxidation is only needed: oxides such as SiO2 are deposited by evaporation in the regions between stripes, which are consequently isolated from metal contacts. The stripe structure defined by this simple procedure, however, allows for a considerable amount of lateral current leakage. Therefore, threshold currents tend to be very high.

The leakage currents can be reduced by etching the highly doped p-cap region between the stripes and then depositing oxides for the isolation from the metal contact. The threshold current for this gain-guide structure of the semiconductor diode laser is greatly reduced [Hud93].

Conventionally, chemical etching or dry etching technique is used to remove semiconductor layers. However, those techniques are very time-consuming or require very expensive and sophisticated equipment.

Two separate process steps, etching and oxidation, can be achieved in a single step and in a very inexpensive way if PAE is used. The PAE technique has been fully developed for GaAs-based materials and used in fabricating their laser diodes [Hud93], [Gro94a], [Gro94b], [Wu94], [Wu95].




51





52

On the other hand, it has been reported that the current PAE technique has a limit of etch depth for GaSb-based laser materials [Lar95]. Consequently, the PAE technique was used to fabricate only low-ridge GaSb-based semiconductor lasers, not RWG lasers for which a high-ridge configuration is required. This chapter presents the development of a PAE technique that is capable of any arbitrary etch depth for the GaSb-based laser material system.

In Section 4.2, basic features of PAE are discussed. The experimental setup, process mechanism, and typical process results of the PAE are reviewed for GaAs materials. In Section 4.3, the development of the PAE technique for GaSb-based laser materials is presented. Discussed are a new electrolyte system used in the experiments and effects of the chemical composition in the electrolyte on the etch rate. Also, etch profiles resulted from the PAE with the new electrolyte system are addressed. In Section 4.4, the current pulse vs. time data is examined. It is found that the peaks in the currenttime plot correlate to layer interfaces in the semiconductor laser structure. In Section 4.5, progress in optimizing the PAE technique for different etching specifications in GaSbbased laser materials is described. The etch rate was reduced by adjusting chemical mixing ratios in the electrolyte, to demonstrate (1) a uniform deep etch depth with reduced roughness of sidewalls over a large wafer area and (2) a uniform shallow etch depth over a small wafer area.




4.2 Fundamentals of Pulsed Anodization Etching

A PAE technique has been shown to be a simple, reliable procedure for the rapid formation of ridges in GaAs-based material systems [Gro94]. In order to develop a new



52





53


PAE technique for GaSb-based laser materials, it is first necessary to understand the basic features of the PAE process. This section reviews the PAE technique that has been established for GaAs-based laser materials.





Pulse
Generator






resistor

oscilloscope









wafer (anode) electrolyte cathode

Figure 4-1. Experimental setup used in Pulsed Anodization Etching (PAE)



The PAE experimental setup is shown in Figure 4-1. Two electrodes, which are immersed in an electrolyte, and a pulsed voltage source are connected in a series with a load resistor of 10 0. A semiconductor wafer with photoresist ridge masking serves as the anode and a platinized titanium grid as the cathode. Two different schemes to mount





53






54




tweezers cover Al foil Swax


wafer




(a)


+

Vacuum
pump











wafer electrolyte

(b)

Figure 4-2. Schematic diagrams of wafer-mount schemes for PAE experiment. (a) waxcombo scheme: side view of sample preparation scheme. (b) vacuum-held scheme. wafers are shown in Figure 4-2. One of them, shown in Figure 4-2(a), is so called a "wax combo," where the sides of the wafer are completely covered with wax so that only the



54





55


top surface area (p-side) is exposed to the electrolyte. The anode contact is made by attaching an aluminum foil piece with silver epoxy onto the back surface (n-side) of the wafer. The other scheme, shown in Figure 4-2(b), is referred to as "vacuum-held" where the wafer and the metal tube are in direct contact by vacuum pressure. The p-side of the wafer is in contact with the top surface of the electrolyte and the n-side with the tube that is conducting the positive polarity. The former scheme allows one to observe a change of anodic oxide color, which correlates to oxide thickness, during PAE. The feature of the visual inspection of a wafer in process is useful to calibrating a new PAE technique in preliminary experiments. The latter scheme is greatly simple to use, but does not allow for observation of the wafer surface during PAE. The vacuum-held setup is therefore used in PAE experiments that have been well established for a certain specification.

Various electrolytes have been investigated for uniform, stable, and reproducible anodization of GaAs and AlGaAs since the first anodization of GaAs was reported in 1963 [Rev63]. A thorough review of the history of anodization on those materials is given in [Hud93]. As the successful electrolyte for GaAs-based materials, a mixture of ethylene glycol, water, and acid (GWA) has been chosen [Gro94a], [Gro94b], [Has76] [Hud93]. The water supplies ions required for the anodization, the ethylene glycol controls the relative diffusion rates of ions in the electrolyte, and the acid helps to increase the conductivity of the solution. The chemical reactions associated with the anodization process are described in [Gro94a], [Hud93].

In the PAE experiments for GaAs-based materials, pulse parameters used are a 80-V pulse amplitude, a 700-pts pulse width, and a 100-Hz repetition rate. The electrical






55





56



Photoresist p-cap layer
p-clad layer active layer
n-clad, Substrate

(a)

native oxide






(b)
oxide growth and dissolution







(c)










(d)


Figure 4-3. Schematic diagram of the PAE process illustrating a traveling oxide phenomenon: (a) semiconductor wafer with photoresist pattern (b) native oxide formed when a pulse is on (c) oxide dissolution and growth occurred subsequently (d) final ridge structure.




56





57


parameters, which in part determine the thickness of the native oxides resulted from PAE, have been adjusted to produce blue anodic oxides.

Since the GWA solution is a slow etchant for the native oxides of GaAs and

A1GaAs, native oxides grow when the pulsed voltage is on, and dissolve when it is off. The alternation of oxide growth and dissolution makes a native oxide layer move through the semiconductor material, which is referred to as a "traveling oxide" phenomenon [Gro94]. The process of this phenomenon is illustrated in Figure 4-3. In this manner, PAE results in oxide growth of arbitrary thickness up to about 2000 A and ridge formation of any arbitrary height simultaneously.

Current flow during PAE is monitored by measuring the voltage drop across the load resistor (10 K or 100 9) in the circuit. Figure 4-4 shows the typical current pulse traces at different times (to, ti, tf).



to

..... ..... 1 0 .
leading edge t,> t
current
trailing edge current
((Ir)


time

Figure 4-4. Typical shape of a current pulse monitored across a load resistor during PAE.



The amplitude of the leading edge of the current pulse remains constant from

pulse to pulse, but the amplitude of the trailing edge current (It,) decreases with time, as



57





58


shown Figure 4-5. The variation of the current pulse shape with time can be understood if the capacitor effect of the anodic oxide layer is considered. At the beginning of each pulse, the capacitance of the oxide layer shorts out the oxide resistance. Once the capacitor charges, the current decreases within a pulse as the oxide grows. It should be noted that the oxide thickness at this phase is determined by a competition between the oxide growth and a simultaneous oxide dissolution into the electrolyte. The Ith amplitude keeps decreasing from pulse to pulse (time ti, tf) until a steady-state is reached between the oxide growth and dissolution rates. When the two rates are balanced, the oxide thickness becomes constant, leading to the constant It, (time tf). From this point on, the oxide layer with the constant thickness is traveling in the material (traveling oxide phenomenon).


voltage
Voltage pulses






- time
oxide growth oxide dissolution current
Current pulses





t= t t= t t =tf Time

Figure 4-5. Time evolution of voltage and current pulsed during PAE.





58





59


Figure 4-6 shows the typical variation of the ht, amplitude with time for the PAE of bulk GaAs. The initial rapid current drop represents a dominating oxide growth (i.e., anodization) over an oxide dissolution into an electrolyte because oxides do not exist on the wafer surface in the first place and the bias voltage pulse is used mostly to drive the anodization. As the oxide becomes thicker, the voltage drop across the oxide increases, leaving less voltage available to the anodic reaction. The oxide growth rate consequently slows down and becomes comparable to the oxide dissolution rate, leading to the steadystate between the two processes that is represented by the constant current. The final oxide thickness is measured about 100 nm (blue oxide) in this case.





100 ,, ...... ...... ' ' ' ' .......
90
80
70
60




20 *
10*
0
0 30 60 90 120 150 180 210 240 270 300 330 Time
(sec)
Figure 4-6. Plot of Ir vs. time for the PAE of a GaAs sample using a GWA electrolyte, which consists of 40 parts ethylene glycol, 20 parts deionized water, and 1 part 85%diluted phosphoric acid. Pulse parameters used were a 80-v pulse amplitude, a 700-ps pulse width, a 100-Hz repetition rate.







59





60


4.3 PAE Technique for GaSb-Based Laser Materials

It was reported that the electrolyte used in the PAE of GaAs-based materials did not dissolve the native oxide of the InGaAsSb/AlGaAsSb material system [Lar95]. Since the traveling oxide effect did not occur, a single-step PAE cannot produce high ridges required for thick p-clad RWG GaSb-based lasers. This section presents a new electrolyte system that enables to produce a traveling oxide and form high ridges in GaSb-based laser materials.

The wafer structure used in experiments was grown on an n-GaSb substrate by molecular beam epitaxy (MBE), consisting of InGaAsSb/AlGaAsSb MQW between an undoped broadened Al0.3Gao.7Aso.07Sbo.93 waveguide layer. The two top layers are a 50nm thick p-GaSb cap layer (p = 2 x 1019 cm3) and a 2-1am thick p-Al0.9Gao.iAs0.07Sbo.93 cladding layer (p = 5 x 1018 cm3). The targeted ridge height of the structure is 2 tm and thus etching should stop in the vicinity of the interface between the p-clad layer and the underlying waveguide layer.

Preliminary experiments were first conducted to study the solubility of native oxides of GaSb and AlGaAsSb in the GWA-based electrolyte that has been established for GaAs and AlGaAs materials. A series of pulsed anodization and oxide removal steps were performed with a GWA mixture of 40 parts ethylene glycol, 20 parts deionized water, and 1 part 85%-diluted phosphoric acid (GWA40201) and with oxide etchants such as diluted KOH and H3PO4. Pulse parameters used were a pulse amplitude of 80 V, a pulse width of 700 pls, and a repetition rate of 100 Hz. The amplitude of the trailing edge current was recorded as a function of time during PAE. The surface profile was measured with a Dektak after each anodization and oxide removal. The first 2-minute



60





61

PAE process converted 80 nm of p-GaSb/p-Alo.gGao.Aso.07Sbo0.93 into 100 nm of greenish native oxides. The oxide was then removed by using a diluted KOH solution. However, as the experiment was repeated, the oxide was only partially removed by the KOH solution and other oxide etchant such as H3PO4. This problem resulted in very nonuniform etched profiles and limited the ridge height to a shallow depth (less than 150 nm). From the experiment results, it was concluded that the native oxide of GaSb readily dissolves in the GWA electrolyte and other oxide etchants, but the oxide of AlGaAsSb is extremely difficult to remove. Therefore, the ridge height produced by the PAE with the GWA electrolyte is limited to be shallow.

In order to form high ridges by utilizing the traveling oxide effect, a new

electrolyte should be made to dissolve native oxide of AlGaAsSb. After a number of experiments with various chemicals, a mixture of a GWA solution and a diluted BOE (GWA-BOE) was created as an electrolyte formula for GaSb and AlGaAsSb materials. Various mixing ratios of ethylene glycol, water, phosphoric acid, and diluted BOE were tested. The electrolyte composition should be adjusted to give uniform PAE results depending on the desired etch rate and time. This aspect is discussed in Section 4.5.

An electrolyte for GaSb-based laser materials is produced by first preparing a

GWA solution, consisting of 4 parts ethylene glycol, 8 parts deionized water, and 1 part 85%-diluted phosphoric acid (GWA481). This solution is volatile and needs to settle for about 24 hours before the next mixing procedure in order to accomplish stable and reproducible anodization results. The new electrolyte is completed by adding 9 ml of 6:1 diluted BOE to 650 ml of the GWA481 solution (GWA-BOE9). For pulse parameters, the pulse amplitude was adjusted to 60 V, while the pulse width and the repetition rate



61





62


remained same as those for GaAs-based materials, i.e. 700 ps and 100 Hz, respectively. Using this solution, 2 minutes of PAE successfully produced 1.9 ptm high ridges, which was measured from top of the cap layer to bottom of oxides, for both wide (50 plm) and narrow (5 pm) stripes. The native oxide was removed by using 6:1 diluted BOE before the measurement of the ridge height with a Dektak.




p-cap 450 pm 50 ptm



p-c.ad etchedm etched amount f2 Lm
layer

waveguide layer
active layer

(a)





245 p.m :25 pm' :'25 p.m 7-pm wide ridge etched 2 piim p-clad amount layer waveguide layer
active layer


(b)

Figure 4-7. Etched amount of PAE in fabricating 2-pm high ridges with (a) 50 pm and
(b) 5 pm wide stripes.




62





63

It should be noted that two different masks were used to define photoresist stripes depending on their widths in order to avoid undercutting, of which the amount increases with an etching amount. In defining the 5-ptm narrow stripe geometry, even a small amount of undercutting can give rise to a significantly different stripe width than the intended width. Taking into account such undercutting effect, the mask used has a pattern to allow for the least amount of lateral etching in achieving 2- pm high ridges: a 7plm wide line with 25-1pm wide channels on both sides, a 300-ptm center spacing. In defining the 100- pm broad strip geometry, the undercutting effect is not significant as much. The mask used has a pattern of a 50-p.m line width and a 500-p.m center spacing. The etching amounts corresponding to the two mask patterns are illustrated in Figure 4-7

(a) and (b), respectively.

The consumed rate of GaSb/Alo.9Gao.lAso.07Sb0.93 materials is very sensitive to the mixing ratio of BOE to electrolyte. Using an electrolyte that consists of 650-ml GWA481 solution and 6-ml 6:1 diluted BOE (GWA-BOE6), the same etch depth (-1.9 plm) was achieved in 3 minutes. Figure 4-8 shows the effect of the mixing ratio of BOE to electrolyte on the PAE time required to achieve 1.9-ptm high ridges. The required PAE time decreases monotonically with the ratio of BOE to electrolyte.

Figure 4-9 shows the consumed amount of Alo.9Gao.1Aso.07Sb0.93 as a function of PAE time when the GWA-BOE6 solution was used as an electrolyte. At the initial stage, the consumed amount of a p-Alo.9Gao.iAso.o7Sbo.93 cladding layer increases rapidly and then linearly with the PAE time between 1 and 3 minutes. Beyond 3 minutes, the PAE process slows considerably. This can be attributed to a change in the etch rate since the oxide front is now moving through different materials, i.e. the guide and QW layers.



63







64





5




4- 0 3




2




1 1 I I I I
0.08 0.10 0.12 0.14 0.16 0.18 0.20 Volume ratio of BOE to electrolyte (%) Figure 4-8. Effect of mixing ratio of BOE to electrolyte on PAE time required to consume 1.85 ptm of Alo.9Gao.lAso.oTSbo.93.






2.5



S 2.0 o 1.5 1.0



0.5 0.0
0 60 120 180 240 Time (see) Figure 4-9. Consumed amount of Alo.9Gao.jAso.7Sbo.93 as a function of PAE time for GWA-BOE6 electrolyte.











64





65


Figure 4-10 shows a scanning electron micrograph (SEM) picture of the ridge that was formed by the 2-minute PAE with the GWA-BOE9 solution. The picture indicates that the etched bottom surface and sidewall are somewhat rough. This is probably because of the very high etch rate (- 0.95 lpm/min) used. Similar profile characteristics were also observed for the 3-mintue PAE with the GWA-BOE6 solution. To reduce this roughness, the electrolyte composition has been adjusted and this study will be presented in Section 4-5.
















Figure 4-10. SEM picture of the 5-ptm wide, 1.9-p.m high ridge formed in p-GaSb cap and p-clad AlGaAsSb materials by the 2-minute PAE with the GWA-BOE9 electrolyte, followed by oxide removal with the 6:1 diluted BOE.




4.4 Detection of Layer Interfaces

In Section 4.2, it was mentioned that the t, amplitude correlates to the oxide

thickness during PAE. Another important feature of the tr, has been found in this study. The slope change in the Itr vs. time characteristics corresponds to layer interfaces in a semiconductor laser diode structure. Pertinent experimental results are presented in this section.



65





66


The trailing edge current ltr as a function of anodization time for InGaAsSb/

A1GaAsSb MQW laser diode wafers is plotted in Figure 4-11. The GWA-BOE9 solution was used as electrolyte. tr dropped very rapidly in less than first 5 seconds before a climb. The depth measurements implied that at this point, the p-GaSb cap layer was completely removed and the p-AlGaAsSb clad layer was about to be etched. As another peak began to appear, the PAE experiments were stopped and the resultant etch depths were consistently 1.8-1.9 ltm, indicating most of the p-AlGaAsSb clad layer was consumed. The results indicate that a change in slope in the I,r curve may be used to detect a layer interface in the multi-layered structure.






60


40


. 20-m


0
0 15 30 45 60 75 90 105 120 135 Time (sec)

Figure 4-11. Plot of trailing edge current vs. PAE time for InGaAsSb/AlGaAsSb MQW laser diode wafer. The GWA-BOE9 solution is used as electrolyte.


The same correlation of the slope change in the I, curve to the layer interfaces is found for the slower PAE process with the GWA-BOE6 solution. The second peak in the Itr curve occurred in 3 minutes and the etch depth was measured 1.9 p.m, corresponding to 66






67


the removal of most of the p-AlGaAsSb layer. Figure 4-12 compares the trailing edge currents Itr as a function of PAE time when the GWA-BOE9 and GWA-BOE6 solutions were used, respectively. The behavior of ,, is outstandingly consistent regardless of an etch rate.







80

70- -*- GWA-BOE9
-o- GWA-BOF6

40


o
*0 0 20

10 1 ;, 0 30 60 90 120 150 180 210 Time (sec)

Figure 4-12. Comparison of trailing edge current vs. PAE time characteristics for the GWA-BOE9 electrolyte (solid circles) and the GWA-BOE6 electrolyte (open circles).



From the Itr curve, it can be deduced that the oxide thickness goes through

different phases (decrease and then increase) as the process front passes the interface. This implies that the net oxide formation rate changes with the semiconductor material composition (from one layer to another).









67





68

4.5 Optimization of the GWA-BOE electrolyte system

Despite the successful etching demonstration for GaSb-based laser materials, the applicability of the PAE technique using the GWA-BOE electrolyte system is limited by its high etch rates (600 - 900 nm/min). The etch quality tends to suffer when the etch rate is too high. Efforts have been made in lowering the etch rate in order to accomplish following goals.

First, surface roughness should be reduced in order to avoid scattering losses

associated with rough bottom and sidewalls of ridges in semiconductor lasers. Secondly, an etch profile should be uniform over a large wafer area in order to apply this PAE technique to the fabrication of commercial diode lasers. The wafer size in a commercial fabrication line is greater by a factor of 20 than that used in this study and the uniformity is difficult to achieve in processing such a large wafer size. Thirdly, an etch rate should be low enough to produce uniform flat etched surfaces even in shallow etchings (150 300 nm) that are required for thin p-clad devices. A high etch rate results in a severe undercut during the shallow etching.

It was pointed out in Section 4.3 that the etch rate of the PAE depends on the ratio of BOE to an GWA-based electrolyte, as shown in Figure 4-8. Since a slower process results in a uniform flat etch profile with less undercut, chemical elements in the GWABOE electrolyte system has been varied to slow down the etch speed and to assure good etch quality.

One of optimization tasks was aimed for 2-p.m high, 5- pm wide ridges in the p-GaSb cap and p- AlGaAsSb clad material structure over a quarter size of the 5-cm diameter wafer. Previous GWA-BOE solutions were developed to process very small



68





69

sizes of wafers (0.3 - 1 cm2). For the big wafer size, the etch rate of an electrolyte should be significantly reduced.

As the etch rate is lowered, the PAE time required for a deep etch is increased, leading to undercutting problem more seriously. Therefore, the mask pattern used in defining ridges has been modified to reduce undercutting as much as possible. The width of dual channels outside the ridge region is now decreased to 10 pLm each (from 25 p.m).

A wafer sample has been PAE processed for 25 minutes by using a GWA-BOE electrolyte that consists of 300 ml of GWA40201 and 1 ml of 10:1 diluted BOE. The oxide from PAE has been removed by dipping the wafer sample in the 10:1 diluted BOE for 15 seconds. The pulse amplitude was set to 80 V, pulse width to 500 jIs, and the repetition rate remained 100 Hz.

Figure 4-13 shows measurements of etch depths at different points on the wafer. The etch depth is consistent over the wafer area when a very slow etch rate (- 100 nm/min.) is used. However, SEM pictures indicated that the ridge sidewalls were still somewhat rough.

Another optimization task was aimed for 300-nm high, 5-p.m wide ridges in the thin p-clad wafer sample of a small size (approximately 1 cm2). The electrolyte used in the PAE for a large wafer area resulted in rough channel bottoms in the shallow etching case.

A good result has been obtained by using a GWA-BOE electrolyte that was

prepared as follows. The GWA481 solution is first prepared and left settled for about 24 hours. The BOE is now more diluted by mixing 10 ml of 10:1 diluted BOE with 100 ml





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70


of deionized water (120:1 diluted BOE). The new GWA-BOE electrolyte is completed by adding 3 ml of the 120:1 diluted BOE to 300 ml of the GWA481.

The pulse parameters used were a pulse amplitude of 60 V, a pulse width of 700 uts, and a repetition rate of 100 Hz. The mask used in defining ridges has a pattern of a 7p.m wide line with dual channels of a 25-p.m width each.

After the PAE, the native oxide was stripped by dipping the sample in the 120:1 diluted BOE for 15 seconds before the depth measurement using a Dektak. The trailing edge current was recorded as a function of PAE time and the PAE was stopped at 2 minutes as the second peak appeared, shown Figure 4-14. The ridge height was measured as 280 nm, implying that most of the three-layer-cap region was removed and the p-clad layer was about to etch out. Again, the peaks in the trailing edge current curve consistently correspond to the layer interfaces in the thin p-clad structure.




3


4

5



1 2


Figure 4-13. Locations of depth measurement points. A quarter of the 5-cm diameter wafer was PAE processed for 25 minutes using an electrolyte that consisted of 300 ml of GWA40201 and Iml of 10:1 diluted BOE. The pulse amplitude was set to 80 V, the pulse width to 500 pLs, and the repetition rate to 100 Hz. The measured etch depth are: Point #1= = 2345 nm, #2 = 2315 nm, #3, 2340 nm, #4 = 2315 nm, and #5 = 2340 nm. The optical measurement of the cross section at point #5 indicates the channel depth of
2.2 � 0.2 p.m.


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71






15


10- *


5



0 20 40 60 80 100 120 140 Tine (sec) Figure 4-14. Variation of trailing edge current with PAE time for a shallow etching (280 nm) of the thin p-clad InGaAsSb/AlGaAsSb DQW laser diode wafer. The electrolyte used consists of 300 ml of the GWA481 and 3 ml of the 120:1 diluted BOE.































71













CHAPTER 5
DEVICE FABRICATION



5.1 Introduction

The as grown wafers are converted to laser devices through device processing steps such as photolithography, etching, and metal deposition. To maximize device performance, the optimization of the fabrication procedure is as important as that of the design.

The laser devices in this study incorporate InGaAsSb/AlGaAsSb QW structures and process technologies for GaSb-based laser materials are still under development. In order to assure the fabrication of high quality GaSb-based lasers, processing steps used conventionally in fabricating GaAs-based lasers should be verified for this relatively new material system.

For single lateral mode operation, the ridge structure has been utilized in the

devices of this study and the structural requirements call for a reliable and reproducible etching technique. Chapter 4 has discussed the development of a novel PAE technique to produce ridges of any height in the GaSb-based laser material system. In this chapter, the focus will be made on other processing steps such as photolithography and metal-contact formation.

In Section 5.2 and Section 5.3, the photolithography and metallurgy procedures used in fabricating RWG InGaAsSb QW diode lasers are described, respectively. In Section 5.4, the fabrication procedure for the RWG laser is summarized.


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5.2 Photolithography

UV photolithography is generally used to define a stripe region on the contact layer of the laser. The stripe geometry from a mask is transferred in a photoresist layer that is uniformly coated on the wafer surface with a thickness of about 1 p.m. UV light with a wavelength between 350 and 410 nm exposes the photoresist layer and a developer solvent is used to remove the exposed region, leaving the desired pattern on the photoresist layer.

Positive photoresist AZ1512 is spun at 4500 rpm for 30 seconds on the wafer

surface, forming a 1.2-pm thick photoresist layer. The time of the UV exposure typically increases with an intended stripe width. Upon exposure, the wafer is placed in AZ 312 MIF Diluted 1:1.3 developer. This procedure has been used in the photolithography for GaAs-based laser devices.

In contrary to GaAs-based materials, it was found that the aforementioned

developer etches a GaSb p-cap layer. In other words, the GaSb cap layer of the wafer could be etched while removing the exposed region of the photoresist, leading to nonuniform, rough profiles in a following etching-step.

A number of photolithography has been performed to find the optimum procedure for a given intensity of the UV light of the mask aligner used. The developing time should be less than 10 seconds and accordingly the exposure time has been increased to completely dissolve the radiated photoresist in such a short developing time. Finally, the photolithography was conducted with an exposure time of 8 seconds and a developing time of 7 seconds in fabricating laser devices in this study.





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74

5.3. Metallurgies

5.3.1 Pulsed Electroplating

Electroplating uses electrochemical reactions to deposit Au on the intended region of the wafer section. This technique is very simple to use and its deposition rate is much higher than other metallization technique such as electron-beam evaporation. A uniform and thick gold (Au) layer can be formed in a relatively short period of time without requiring complex apparatus.






Pulse
Generator
+






resistor oscilloscope








wafer (anode) Au plating cathode solution Figure 5-1. Experimental setup of pulsed electroplating.





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75





tweezers cover Al foil


- wax



p-side




(a)



+

tweezers cover glass Al foil



4 n-side



photoresist on the p-side




(b)

Figure 5-2. Schematic diagrams of wafer-mount schemes for pulsed electroplating experiment. (a) a side view of the wax-combo scheme for the p-side plating: the n-surface and sides of the wafer are sealed in wax and isolated from the gold solution. (b) a side view of the wafer/cover glass scheme for the n-side plating: the photoresist is coated on the p-side of the wafer for the isolation. The tweezers is used to grab the cover glass and wafer together.





76


Figure 5-1 shows the experimental setup used in pulsed electroplating. This setup is basically same as that for pulsed anodization etching (PAE) except three things. (1) The variable resistor is added to control the current flowing in the circuit; (2) The electrolyte is now an Au plating solution; and (3) The bias polarity is reversed: the wafer is now the cathode in order to attract the positive Au ions in the plating solution.

The schemes for mounting a wafer are illustrated in Figure 5-2. The "waxcombo" scheme, as described in Chapter 4, is used to electroplate the p-side of the wafer, shown in Figure 5-2(a). For n-side electroplating, the wafer is not sealed in wax, but simply placed p-side onto the cover glass after the p-side is covered with the photoresist. The wafer and cover glass are grabbed together by the tweezers, connecting the negative bias to the n-side only.

The electroplating rate increases with the current density of the wafer sample. For uniform electroplating on GaAs-based wafers, a low current density (-20 mA/cm2) is applied for the first 1 or 2 minutes of electroplating and the current density is increased to

-100 mA/cm2 afterwards. If the current density is too high, bubbles are formed on the wafer surface and result in non-uniformity. Therefore, the current density should be carefully controlled. The pulse parameters used are typically a pulse frequency of 100 Hz and a pulse width of 300 ps.

During electroplating, the color of the electroplated gold changes from shiny bright yellow to brownish yellow and then to rusty brown. The gold beyond the brownish-yellow phase is so thick that it can be easily peeled off during the cleaving into laser bars. A typical electroplating time for GaAs-based wafers is 4-6 minutes, with a gold-deposition rate of less than 100 nm/minute if the aforementioned pulses are used.



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77

In preliminary experiments for GaSb-based wafers, it has been found that the current densities should be higher than those for the GaAs-based in order to make the same bright-yellow gold within 10 minutes or so. It is recommended to complete electroplating in that order of time if a cyanide-based Au plating solution is used because the longer plating tends to result in non-uniformity or damage native oxides that were deposited for the isolation by PAE.

In addition, the current density needs to be adjusted depending on the ratio of the gold-plating area to the wafer area, which is dictated by the mask pattern used in defining a desired stripe geometry. For the masks used in electroplating the p-side of the wafer sample with a wide-stripe (>50 tm) and narrow-stripe (<10 lm) geometries, the area ratios are 0.1 and 0.0023, respectively. The electroplating procedure for each stripe geometry of the InGaAsSb QW laser is given as follows.

For the wide-stripe (>50 rtm) plating case, a current density of about 50 mA/cm2 is applied for the first minute and then set to 100 mA/cm2 for the following 4-5 minutes. If the bright-yellow gold has not been formed yet, the current density needs to be further increased up to 500 mA/cm2 for additional 2-3 minutes.

For the narrow-stripe (<10 pm) plating case, the first minute of electroplating is performed at a current density as high as 200 mA/cm2. Then, the current density is increased to 1 A/cm2 for the following 4 minutes.

The electroplating technique has been used in forming contacts on the p-side only. For an n-side contact, metal alloys were deposited by vacuum evaporation and then heated as presented in the following section.





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78


5.3.2 E-beam Evaporation/Annealing

Electron-beam evaporation and heat treatment (annealing) of metal alloys are

used for blanket metallization of the p- and n-sides of laser devices in this study. On the p-side, Ni (25 nm)/Au (150 nm) were evaporated on top of the electroplated gold for the convenience of laser testing, as addressed in the following section. On the n-side, Au-Ge (50 nm)/Ni (15 nm)/Au (200 nm) were evaporated and then annealed at 250 oC for 1 minute in a gas flow consisting of N2 (96%) and H2 (4%).

Deposited metal alloys and annealing conditions (e.g. heating temperature and gas-flow composition) are very important for the formation of low-resistance ohmic contacts. Poor contacts cause high thermal resistance of devices, leading to higher operating voltage and device heating. Consequently, the temperature characteristics of diode lasers and their reliability will suffer. Considerations for the metallurgy technique include (1) low contact resistance; (2) ease of fabrication; (3) good adhesion; (4) low temperature for contact formation; (5) thermal stability, etc [Sue94]. Further study on these subjects is left for future work.




5.4 Fabrication Procedure for RWG Lasers

The fabrication procedure for the RWG InGaAsSb/AlGaAsSb MQW diode laser is presented as follows. The figures are accompanied to envision the pertinent process.

1. A wafer section is cleaned by placing it in boiling TCA, acetone, and methanol for 5

minutes each. If dirt or residue is still found on the wafer section, repeat the

aforementioned cleaning procedure. Finally, the wafer is rinsed in deionized (DI)

water.



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79


2. AZ1512 photoresist (PR) is spun on the p-side of the wafer section at 4500 rpm for 30

seconds. The wafer is then baked at about 85 C for 10 minutes.

3. Photolithography will be done using a mask that has a pattern of a 7-ptm wide stripe

with 25-ptm wide channels on both sides of the stripe, spacing 300-ptm apart between

the stripes. UV light is radiated for 10 seconds and then the wafer is placed in AZ

312 MIF Diluted 1:1.3 developer for 7 seconds, followed by completely rinsing in DI

water. The wafer is then baked at about 110 C for 10 minutes. The cross section of

the developed wafer is shown below (not in scale).



25 p.m 25 pLm 243 ptm 25 pLm 25 pm photoresist (PR)

7 prm 7 pm wafer





4. The PR-patterned wafer is PAE processed to make ridges of a desired height. If

necessary, additional PAE is performed to reinforce native oxides in the channels.

For the purpose of illustration, the schematic diagram from this point on depicts only the portion of the front view of the wafer, corresponding to the area of a ridge and its

adjacent channels.









79





80


243 pm :26 .m: 26 pmrn 5-i.tm wide ridge P R ......_ ........................... .........
. . ................................... . . . . . ..



p-clad -2 pm waveguide layer native oxides native oxides




5. The wafer is rinsed in acetone to remove the PR.



243 pm :26 .trn, 26 lm: 5-p.m wide ridge p-clad -2 pm waveguide layer
native oxides




6. Procedure 2 is repeated. Then, Procedure 3 is done again with another mask to cover

the top and sides of the ridge with the PR. The mask used here has a pattern of a 30pm wide stripe with a spacing of 300 plm between the stripes.




243 5-m :26 pm :26 m : 5-tm wide ridge p-cap : ' " p-clad ~2 pm

waveguide layer
native oxides PR




80





81


7. The PR-patterned wafer is once again PAE processed. The purpose of this additional

PAE step is to replace with oxides a cap layer outside the ridge region.

8. The photoresist is removed by rinsing the wafer in Acetone.


243 [tm 26 tm: :26 [m:
native oxides

p-clad

waveguide layer 5 pm



9. The ridge stripe region is Au electroplated.


243 .tm 26 im: :26 pm: Au

: native oxides p-clad

waveguide layer 5 jim



10. Ni (25 nm)/Au(150 nm) is evaporated onto the p-side of the sample. This blanket

metallization is done for the convenience of probe testing and making contacts when

a completed device is mounted onto a heat sink.


Evaporated Ni/Aunative oxides


p-clad

waveguide layer 5 jim





81





82


11. The p-side of the wafer is coated with PR and baked at 85 C for 10 minutes. The

PR will protect the metallized p-side of the wafer during the lapping.

12. The wafer is thinned down to a thickness of -100 p.m and cleaned by rinsing in TCA,

acetone, and DI water, respectively.

13. Au-Ge (50 nm)/Ni (15 nm)/Au (200 nm) is evaporated on the n-side of the wafer. 14. The wafer is then annealed at 250 oC for 1 minute in a gas flow consisting of N2

(96%) and H2 (4%).

15. The wafer is cleaved into bars and then chips. To remove any residue, the bars or

chips are rinsed in acetone before the probe testing or the mounting.

































82













CHAPTER 6
CHARACTERIZATION OF InGaAsSb/AlGaAsSb QUANTUM WELL LASERS



6.1 Introduction

Experimental results of InGaAsSb/AlGaAsSb MQW diode lasers with symmetric (thick p-clad) and asymmetric (thin p-clad) cladding structures are presented in this chapter. Laser chips were fabricated from wafers, as described in Chapter 5, with facets uncoated. Each laser chip was then mounted on a heat sink and characterized.

Two different schemes of laser packaging were used. One of the schemes utilized so-called a "chop-on-block (COB)" packaging where the laser chip was mounted p-side down on a copper block with a gold wire bonded to the n-side using EPO-TEK H20E silver epoxy. To cure silver epoxy, the COB was baked for at least 10 minutes at 110 C. In the other packaging scheme, the laser chip was mounted (p-side down) on a T046 Header 3-Lead using Indium solder.

The heat sink capacities of the copper block and the TO46 Header are expected to be comparable. However, thermal conductivities of silver epoxy and indium solder are considered quite different. Silver epoxy is known to have a low thermal conductivity. Moreover, the baking of the COB to cure silver epoxy could damage the InGaAsSb QW laser chip because the semiconductor laser in this material system appears to be affected seriously at relatively low temperatures since their annealing temperature requires as low as 250 C. When the two different packaging schemes were used in mounting chips from




83





84


the same processed wafer, the Indium-soldered packaging resulted in a higher yield of laser operation.

The laser diodes were characterized in either pulsed or CW operation. The

temperature of the heat sink was set to 15 C. A large-area thermopile detector and a PbS photodetector were used for the measurement of pulsed and CW P-I characteristics, respectively. A Lock-In Amplifier was used to amplify small signals from the PbS detector.

In Section 6.2, characterizations are presented for symmetric cladding (thick pclad) lasers of both low-ridge wide-stripe and high-ridge narrow-stripe structures that were fabricated with PAE. Performance characteristics of the lasers are comparable to that of devices fabricated using conventional chemical etching techniques. In Section

6.3, experimental results of the first mid-infrared asymmetric cladding (thin p-clad) Sbbased QW diode lasers are presented. PAE using the standard GWA electrolyte was performed to fabricate low-ridge lasers with either wide or narrow stripes. Lasing wavelengths of the devices are compared with theoretical calculations that were presented in Chapter 2.




6.2 Characterization of Thick P-Clad InGaAsSb MQW Lasers

The thick p-clad laser diode grown by MBE consists of n-GaSb substrate, a 500nm thick n-GaSb buffer layer (n = 1 x 1018 cm-3), a 40-nm thick AlxGal-xAsySbli. graded layer (x = 0 - 0.9, y = 0 - 0.07, n = 1 x 1018 cm-3), a 1.5-gm thick n-Alo.9Gao.iAso.07Sb0.93

cladding layer (n = 2 x 1017 cm3), a 400-nm thick Al0.25Gao.75Aso.02Sb0.98 guiding layer, a MQW active region composed of five 10-nm thick, 0.6% compressively strained


84






85





p-cap GaSb 50 mn p = 2 x 1019 cm-3 grading GaSb/A ,,Ga, ,As,Sb, 1 40 nm p = 2 x 1019 cm-3 p-clad Alo.9Gao.1As0o.0Sbo.93 1.35 ptm p = 5 x 1018 cm"3 p-clad Alo.9Gao.lAsO.02Sb.98 150 nm p = 1 x 1018 cm-3


waveguide Alo0.25Ga.75Aso.0o2Sb.98 380 nm In .15GaAsSb98(5 x 10 nm) 150nm 5-QW Alo.25GaAsSbo 98 (5 x 20 nm)

waveguide Al0.25Gao.75AsO.02Sbo.98 400 nm




n-clad Al.9Gao.1Aso.0Sbo.93 1.5 pm n= 2 x 1017 cm-3 grading GaSb/Ao ,Ga, ,As, ,;Sbn, 140 nm n = 1 x 108 cm3 buffer GaSb 500 nm n = 1 x 1018 cm-3


substrate GaSb


Figure 6-1. Structure of the 1.8 p.m wavelength thick p-clad InGaAsSb/AlGaAsSb MQW SCH laser diode.



Ino.isGao.ssAso.o2Sbo.98 QWs separated by four 20-nm thick Alo.25Gao.75Aso.o2Sbo.98 barrier layers, a 400-nm thick Alo.25Gao.75Aso.o2Sbo.98 guiding layer, a 150-nm thick p-type Alo.9Gao.lAso.07Sbo.93 cladding layer (p = 1 x 1018 cm3), a 1.35- pm thick p-type 85





86


Alo.9Gao.lAso.07Sbo.93 cladding layer (p = 5 x 1018 cm-3), a 40-nm thick AlxGalxAsySbl-y graded layer (x = 0.9 - 0, y = 0.07 - 0, p = 2 x 1019 cm-3 ), and finally a 50-nm thick pGaSb cap layer (p = 2 x 1019 cm3), as illustrated in Figure 6-1. Fabry-Perot (FP) lasers with a stripe width of 50 jtm were fabricated using PAE with the GWA 40201 solution. The p-GaSb cap layer outside the stripe region has been etched out and covered with anodized oxides to minimize current spreading. Also, RWG lasers with 5-jtm stripe, 2jim high ridges were fabricated using the GWA-BOE9 electrolyte, as described in Chapter 4.

Table 6-1 summarizes probe-test results of low-ridge wide-stripe (80-nm high and 50-jtm wide) and RWG (2-jtm high and 5-jim wide) thick p-clad lasers in bar form under pulsed operation at room temperature. Pulses used were a pulse width of 1 s and a repetition rate of 1kHz. The threshold current density decreases with a cavity length since the mirror loss is reduced with a longer cavity length.




Table 6-1. Pulsed probe-test results for thick p-clad InGaAsSb/AlGaAsSb MQW lasers at room temperature. Pulse parameters used were a pulse width of 1 jim and a repetition rate of 1 kHz. The p-cap layer between stripes was etched out in wide-stripe devices. The ridge height of RWG devices is 2 pim.
Device Stripe width (jtm) Cavity length (im) Ith (mA) Jth (A/cm2)
Wide-stripe 50 500 130 520 Wide-stripe 50 1000 220 440

RWG 5 500 26 1040 RWG 5 1000 42 840






86






87


It should be noted that the threshold currents of the RWG lasers are comparable to the lowest values demonstrated previously by similar devices that were fabricated with conventional, wet chemical etching techniques. The low thresholds operation imply that the PAE with the GWA-BOE electrolyte system is a reliable process for fabricating high quality RWG InGaAsSb/AlGaAsSb QW lasers.

Figure 6-2 shows the voltage vs. current characteristic for a thick p-clad RWG

laser with a 1000 ptm cavity length at T = 15 C. The forward voltages at thresholds (50 to 75 mA depending on the cavity length) were higher than 1.3 V. Those voltage values are considered too high for mid-infrared laser diodes and can be attributed to poor contacts and wafer quality. As mentioned in Chapter 5, the metallurgy for GaSb-based laser diodes needs to be studied for achieving a smaller contact resistance. Moreover, the wafer quality has not been characterized and a poor doping quality, for instance, can be responsible for high operating voltage characteristics.


2.0





1.6- *
o
>*
1.4


1.2 . . . . .
60 80 100 120 140
I (mA)

Figure 6-2. Voltage versus current for thick p-clad RWG laser with a 1000p.m cavity length.


87






88


Figure 6-3 shows the CW P-I characteristics for a typical 1000-jim long RWG laser with uncoated facets at T = 15 C. As shown, the threshold current is about 50 mA, and the slope efficiency 0.114 W/A. Figure 6-4 shows that the near-threshold spectral emission from this laser has a peak wavelength of 1.836 ptm.



8


6


4


2


0 .... .... .... .... .... I.... .... ....
0 20 40 60 80 100 120 140 160 I (mA)


Figure 6-3. CW output power-current characteristic for thick p-clad RWG laser with a cavity length of 1000 Jim, at T = 15 C. The ridge is 5 lpm wide and 2 lm high.



The reason for the deviation from the expected peak wavelength near 2 p.m can be found from comparison between the theoretical and experimental emission wavelengths of this device. The calculation of the emission wavelength for an InGaAsSb/AlGaAsSb QW structure has been performed as a function of In content in the QW, as presented in Chapter 2. According to the computation results, the 10-nm thick InxGalxAs.02Sbo0.98/ 20-nm thick Al0.25Gao.75Aso.02Sbo.98 QW structure with x = 15% results in an emission wavelength of 1.891 p.m for the C1-HH1 transition. Based on the measured 1.833 p.m lasing wavelength, the actual content of In in the QW is estimated less than 15 %. For 88





89


the case of x = 12 %, the theoretical lasing wavelength is 1.842 ltm. The MBE technique for GaSb-based quaternary alloys has not been completely calibrated and thus a slight variation of In content in the QW is expected.

Figure 6-5 shows the CW emission spectra of the same RWG laser above the

threshold. As the injection current was increased, the lasing wavelength shifted to longer



















*1-~ iI







I \



1ato 1e 1 05 1 Q 14B 0 1845 18 1855 1,08 ........................ . .


Figure 6-4. CW emission spectrum of the thick p-clad RWG laser with 1000 jim cavity length near threshold (I = 50 mA) and T = 15 C. The ridge width is 5 jim.


89






90


wavelength. The red shift of the lasing wavelength can be explained by device heating and reduction of the band gap.






4. i
1840.3

4












a"




1 '









- ..... ..... ..... . .. . ...... ..... ....... .. ..,, . .



Figure 6-5. CW lasing spectrum of the thick p-clad RWG laser with 1000 plm cavity length above threshold (I = 70 mA) and T = 15 C. The ridge width is 5 lpm.





90





91

6.3 Characterization of Thin P-Clad InGaAsSb DQW Lasers

The first thin p-clad laser diode grown by MBE consists of n-GaSb substrate layer (n = 1 x 1017 cm3 ), a 1.5-ptm thick n-Alo.gGao.1Aso.o7Sbo.93 cladding layer (n = 5 x 1017 cm-3), a 320-nm thick Alo.3Gao.7Aso.o2Sbo.98 guiding layer, a DQW active region composed of two 20-nm thick, 1.3% compressively strained Ino.25Gao.75Aso.o2Sbo.98 QWs separated by a 200-nm thick Alo.3Gao.7Aso.o2Sbo.98 barrier layer, a 320-nm thick Alo.3Gao.7ASo.02Sbo.98 guiding layer, a 300-nm thick p-Alo.9Gao.1Aso.07Sbo.93 cladding layer (p = 5 x 1018 cm-3), a cap region composed of a 30-nm thick p-GaSb cap layer (p = 2 x 1019 cm3), an undoped 100-nm thick Alo.3Gao.7Aso.o2Sbo.98 layer, and finally a 30-nm thick p-GaSb cap layer (p = 2 x 1019 cm-3), as illustrated in Figure 6-6.

It should be noted that the cap region of the thin p-clad laser structure consists of three layers instead of a conventional, single p-type layer. This cap design was chosen in view of ohmic contact issues for surface-modulated DFB lasers in the future. The p-type GaSb layer below the undoped Alo.3Gao.7Aso.o2Sbo.98 layer in the 3-layer cap region will be served as a contact layer for the bottom region of gratings that will be made on the pside surface of the modulated-cap thin p-clad DFB lasers, which is planned to fabricate once performance of thin p-clad FP lasers have been optimized.

Low-ridge, thin p-clad lasers with two different widths (50 p.m and 5 pIm) were

fabricated using PAE with the GWA 40201 solution. The ridge height was about 140 nm, implying that most of the three-cap region between the ridges have been replaced









91




Full Text
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It should be noted that the threshold currents of the RWG lasers are comparable to
the lowest values demonstrated previously by similar devices that were fabricated with
conventional, wet chemical etching techniques. The low thresholds operation imply that
the PAE with the GWA-BOE electrolyte system is a reliable process for fabricating high
quality RWG InGaAsSb/AlGaAsSb QW lasers.
Figure 6-2 shows the voltage vs. current characteristic for a thick p-clad RWG
laser with a 1000 pm cavity length at T = 15 C. The forward voltages at thresholds (50 to
75 mA depending on the cavity length) were higher than 1.3 V. Those voltage values are
considered too high for mid-infrared laser diodes and can be attributed to poor contacts
and wafer quality. As mentioned in Chapter 5, the metallurgy for GaSb-based laser
diodes needs to be studied for achieving a smaller contact resistance. Moreover, the
wafer quality has not been characterized and a poor doping quality, for instance, can be
responsible for high operating voltage characteristics.
2.0
1.8
g
V 1.6
O
>
1.4
1.2
Figure 6-2. Voltage versus current for thick p-clad RWG laser with a 1000pm cavity
length.
87


42
In designing RWG lasers, the ridge height is determined to provide the lateral
index difference A/7 found from Eq. (3.1).
p-contact metal-
oxide layer
active
layer
insFifipwi
p cap
p-confining BBBB,IB
layer
n-confining layer & substrate
Figure 3-1. End view of a ridge waveguide laser structure
3,2,3 General Principles for Design of Semiconductor Laser
In conventional thick p-clad structures, the thicknesses of the p-cap and p-clad
layers are set typically to 0.05 and 2 p.m, respectively. The thickness of the p-clad layer
is usually the same as that of the n-clad layer, comprising a symmetric waveguide
semiconductor laser structure. The resulting transverse mode allowed in the structure is a
fundamental mode. Based on the given transverse waveguide structure, the height is to
be determined to provide the sufficient lateral index difference for single lateral mode
operation. That is, the ridge height is a primary design parameter.
On the other hand, the thicknesses of the p-cap and p-clad layers as well as the
ridge height are design parameters for thin p-clad laser structures. The thicknesses of the
p-cap and p-clad layers are to be adjusted to enhance transverse mode confinement in the
42


69
sizes of wafers (0.3 ~ 1 cm2). For the big wafer size, the etch rate of an electrolyte should
be significantly reduced.
As the etch rate is lowered, the PAE time required for a deep etch is increased,
leading to undercutting problem more seriously. Therefore, the mask pattern used in
defining ridges has been modified to reduce undercutting as much as possible. The width
of dual channels outside the ridge region is now decreased to 10 pm each (from 25 pm).
A wafer sample has been PAE processed for 25 minutes by using a GWA-BOE
electrolyte that consists of 300 ml of GWA40201 and 1 ml of 10:1 diluted BOE. The
oxide from PAE has been removed by dipping the wafer sample in the 10:1 diluted BOE
for 15 seconds. The pulse amplitude was set to 80 V, pulse width to 500 ps, and the
repetition rate remained 100 Hz.
Figure 4-13 shows measurements of etch depths at different points on the wafer.
The etch depth is consistent over the wafer area when a very slow etch rate (~ 100
nm/min.) is used. However, SEM pictures indicated that the ridge sidewalls were still
somewhat rough.
Another optimization task was aimed for 300-nm high, 5-pm wide ridges in the
thin p-clad wafer sample of a small size (approximately 1 cm2). The electrolyte used in
the PAE for a large wafer area resulted in rough channel bottoms in the shallow etching
case.
A good result has been obtained by using a GWA-BOE electrolyte that was
prepared as follows. The GWA481 solution is first prepared and left settled for about 24
hours. The BOE is now more diluted by mixing 10 ml of 10:1 diluted BOE with 100 ml
69


12
aji = aj[\-Dj{aJaJ-\)],
(2.2)
barrier
QW aw
barrier
ab
(a)
II(=*)< barrier
QW
barrier
+
(b)
1 Figure 2-1. Schematics of crystal lattice deformation under (a) comressive strain when
ab < aw and (b) tensile strain when ab > aw [Col95],


15
HH
LH
Tensile
Strain
Unstrained
Compressive
Strain
Figure 2-2. Schematic diagram of band energy shifts of the conduction band (C) and
three valence bands (HH, LH, and SO for the heavy-hole, light-hole, and spin-orbit split-
off bands, respectively) for biaxial compressive and tensile strain. The magnitude of the
energy shift is indicated next to each shift by using notations defined in the text [Cold95],
The energy shifts of the three VB due to the shear strain with respect to the HH band-
edge energy, Ehh = Evav + A/3 (Ao is the unstrained spin-orbit splitting), are expressed:
a e:'; = -Ee-\ (2.9)
A E* = -i A +i A£f =-A0 +i with
SEm,sh =2b(e -£,,)
(2.12)


27
Table 2-3. Theoretical strain-related material parameters of InxGai.xAso.o2Sbo.98
quaternaries (x = 12 and 25 %) used in the QW active layer of the thick and thin p-clad
laser diode structures with barriers of Alo.25Gao.75Aso.02Sbo.98 and Alo.3Gao.7Aso.02Sbo.98,
respectively
Ino.12Gao.88Aso.02Sbo.98 Ino.25Gao.75Aso.02Sbo.98
do
0
(A)
6.1331
6.1830
Cn
(dyn/cm2)
8.84xl0n
8.51xl0n
C12
(dyn/cm2)
4.09xl0n
4.02xl0u
Eg
(eV)
0.63
0.55
Ajo
(eV)
0.72
0.72
Ev'dV
(eV)
-6.23
-6.21
ac
(eV)
-6.77
-6.68
av
(eV)
0.75
0.69
b
(eV)
-2.00
-2.02
Table 2-4. Theoretical material parameters of AlyGa1.yAso.02Sbo.98 quaternary (x = 25 and
30 %) used in the barrier layers of the thick and thin p-clad laser diode structures.
Deformation potentials ac, av, and b are not obtained here because the barrier layers are
assumed not too experience strains.
Alo.25Gao.75Aso.02Sbo.98 Alo.3Gao.7Aso.02Sbo.98
a0
O
(A)
6.0968
6.0987
Cn
(dyn/cm2)
9.07xl0u
9.05xl0n
C12
(dyn/cm2)
4.21X1011
4.22xlOu
Eg
(eV)
1.02
1.09
Aio
(eV)
0.66
0.66
Ev,av
(eV)
-6.37
-6.39


82
11. The p-side of the wafer is coated with PR and baked at 85 C for 10 minutes. The
PR will protect the metallized p-side of the wafer during the lapping.
12. The wafer is thinned down to a thickness of-100 pm and cleaned by rinsing in TCA,
acetone, and DI water, respectively.
13. Au-Ge (50 nm)/Ni (15 nm)/Au (200 nm) is evaporated on the n-side of the wafer.
14. The wafer is then annealed at 250 C for 1 minute in a gas flow consisting ofN2
(96%) and H2 (4%).
15. The wafer is cleaved into bars and then chips. To remove any residue, the bars or
chips are rinsed in acetone before the probe testing or the mounting.
82


6
The first InGaAsSb/AlGaAsSb diode lasers were demonstrated by Kobayashi et
al. [Kob80], A DH structure grown by liquid phase epitaxy (LPE) was used and room
temperature laser action at 1.8 pm wavelength demonstrated.
Further development of InGaAsSb/AlGaAsSb lasers were, however, restricted by
material growth problems associated with the LPE growth technique. First, higher A1
content in the confining layers was desired in order to reduce threshold current densities.
An increase of A1 of more than -30% requires higher growth temperatures, leading to a
reproducibility problem [Cho91a], [Mor91], Second, there exists a miscibility gap in the
InGaAsSb system for compositions lattice matched to GaSb with In exceeding 20-25 %
(emission wavelengths corresponding to greater than 2.4 pm) [Cho91a], [Gar99a],
[E199], Such unstable alloys can be prepared by a nonequilibrium technique, not by a
near-equilibrium technique such as LPE. Finally, thickness and composition controls
during the LPE process are very limited. Poor thickness reproducibility makes the LPE
technique unsuitable for quantum well structure preparations [Cho91a], [E199],
There have been attempts to use metal organic vapor phase epitaxy (MOVPE) to
grow AlSb-containing alloys. The problem with this growth technique is that the high
levels of carbon, a p-type impurity, are introduced, making it difficult to obtain n-type
materials [Wan96a], [Wan96b],
Molecular beam epitaxy (MBE), a non-equilibrium growth technique, has proved
successful in growing the high A1 content alloys and the unstable InGaAsSb alloys in the
miscibility range [Cho91b]. The first InGaAsSb/AlGaAsSb QW laser was grown by
MBE and demonstrated by Choi et al. [Cho92], Recently, considerable progress in
achieving longer wavelength operation at room temperature has been made both by


16
where b is the shear deformation potential for strain along [001], The CB at T is not
affected by shear strain shifts, but only subject to hydrostatic strain shifts. The band
energy shifts due to the strains are illustrated in Figure 2-2.
In the model-solid theory [Wal89], the band positions are expressed in terms of
Ev av on an absolute scale, which does not carry any physical meaning by itself, but only
meaningful relative to other semiconductors. The HH, LH, and CB edge energies, i.e.
Ev hh, Evlh, and Ec, respectively, are obtained on an absolute scale:
E = £ +^ + AEhy +AEt,
v,hh v,av ^ ^^v,av
(2.13)
f fr + + AFhy + AF5h
^v,lh *-v,av + ^ + aJZ'v,av + lSCjlh
(2.14)
Ec =^VflV+ + E+AE?,
c v,av g c 5
(2.15)
where Ev av, A0, and Eg refer to unstrained material properties. The separation between
the HH and LH band edge energies, S, is then defined as
S=EvM-EvJh, (2.16)
The topmost VB edge energy, Ev, is found by
£v =max(£v (2.17)
Consequently, the strained band gap is given as:
ESJ =EC-EV, (2.18)
Finally, the VB and CB offsets for a strained QW are obtained simply by
Ae^e^-e;, (2.19)
AEc=Ebc-E:,
(2.20)


17
where b and w denote barrier and QW materials, respectively, and vj indicates the HH or
LH band. Figure 2-3 illustrates the band lineups of strained QW structures. Material
parameters for III-V binary semiconductors, used in the calculation of strained-band
lineups, are also listed in Table 2-1.
(a) (b)
Figure 2-3. Schematics of band lineups of QW structures under (a) compressive strain
and (b) tensile strain.
2.3 Quantized Energy Levels in Quantum Well
Energy levels allowed in a finite square well potential are found by solving the
time-independent Schrodinger equation [Col95], [Man92], The transfer matrix method
[Gha88] was chosen here as a numerical technique to find energy eigenvalues.
In the one-dimensional potential well of width / and of height Vo, as shown in
Figure 2-4, each of three separate regions of uniform potential has the solution to the
Schrodinger equation as the sum of two counter propagating plane waves.


108
structure is attributed to the combination of the natures of the polarization-dependent
transition matrix elements and Fermi-Dirac distributions involved in the gain mechanism.
Figure 5. Plot of the difference between Fermi-Dirac functions fc and fhh for the C-HH
transition in terms of photon energy.
108


2
place, be reduced in order to reduce the threshold current. It was not until new crystal
growth technologies capable of growing very thin layers had been developed that high
quality QW lasers could be made.
In the QW laser, the thickness of the active layer is reduced to ~10 nm from -100
nm in the DH laser diode. Quantum effects due to the small dimension of the active layer
greatly affect the laser performance features such as radiation polarization and lasing
wavelength. With such thin active layers, the DH structure cannot confine the optical
field very well and a different structure called the separate confinement heterostructure
(SCH) is used, as shown in Figure 1- 1(b). The injected carriers are confined in an active
layer whose thickness is on the order of the electron wavelength, while the optical field is
confined in a region with a thickness comparable to the photon wavelength. In the
original QW lasers, all layers in the multi-layered structure were lattice-matched to the
substrate. It was found subsequently that QW laser performance could be improved by
introducing strain into the QW active layer. The strain modifies the valence band
structure, leading to lower threshold current density and improved efficiency.
Recently, progress in engineering of new laser diode materials has paved the way
for the development of semiconductor lasers with wavelengths covering the range from
the blue to the mid-infrared [Haa91], [Oku92], [Nak96], [Cho92], [Gar99a], [Cho95],
This expansion of operating wavelength range has led to the replacement of gas and solid
state lasers in applications such as displays, optical storage, medical surgery, and
chemical sensing.
Active research efforts have been made recently to realize semiconductor lasers
that emit in the wavelength range between 2 and 3 pm, which operate continuously at


88
Figure 6-3 shows the CW P-I characteristics for a typical 1000-pm long RWG
laser with uncoated facets at T = 15 C. As shown, the threshold current is about 50 mA,
and the slope efficiency 0.114 W/A. Figure 6-4 shows that the near-threshold spectral
emission from this laser has a peak wavelength of 1.836 pm.
8
6
4
2
0
0 20 40 60 80 100 120 140 160
I (mA)
Figure 6-3. CW output power-current characteristic for thick p-clad RWG laser with a
cavity length of 1000 pm, at T = 15 C. The ridge is 5 pm wide and 2 pm high.
The reason for the deviation from the expected peak wavelength near 2 pm can be
found from comparison between the theoretical and experimental emission wavelengths
of this device. The calculation of the emission wavelength for an InGaAsSb/AlGaAsSb
QW structure has been performed as a function of In content in the QW, as presented in
Chapter 2. According to the computation results, the 10-nm thick InxGai.xAso.o2Sbo.98/
20-nm thick Alo.25Gao.75Aso.02Sbo.98 QW structure with x = 15% results in an emission
wavelength of 1.891 pm for the C1-HH1 transition. Based on the measured 1.833 pm
lasing wavelength, the actual content of In in the QW is estimated less than 15 %. For
88


22
Ecv Eg + E (2.34)
with and Evjn given by
VII
n2e
F F j I
* 2 m
2 ,2
VII
h k,
F -F + !
2m
(2.35)
VII
where mm and mvy1| are the in-plane effective masses of the CB and relevant VB,
respectively. Figure 2-5 shows schematic energy band diagrams of the QW structure
with the aforementioned energy terms defined.
(a) (b)
Figure 2-5. Schematic energy band diagram (a) Energy versus position z (b) Energy
versus wave vector k.


66
The trailing edge current / as a function of anodization time for InGaAsSb/
AlGaAsSb MQW laser diode wafers is plotted in Figure 4-11. The GWA-BOE9 solution
was used as electrolyte. I,r dropped very rapidly in less than first 5 seconds before a
climb. The depth measurements implied that at this point, the p-GaSb cap layer was
completely removed and the p-AlGaAsSb clad layer was about to be etched. As another
peak began to appear, the PAE experiments were stopped and the resultant etch depths
were consistently 1.8-1.9 pm, indicating most of the p-AlGaAsSb clad layer was
consumed. The results indicate that a change in slope in the Itr curve may be used to
detect a layer interface in the multi-layered structure.
Figure 4-11. Plot of trailing edge current vs. PAE time for InGaAsSb/AlGaAsSb MQW
laser diode wafer. The GWA-BOE9 solution is used as electrolyte.
The same correlation of the slope change in the Itr curve to the layer interfaces is
found for the slower PAE process with the GWA-BOE6 solution. The second peak in the
Itr curve occurred in 3 minutes and the etch depth was measured 1.9 pm, corresponding to
66


106
(a)
Energy (meV)
(b)
Figure 4. Plot of -T-\- as a function of photon energy for (a) the C-HH TM-case and
M
(b) the C-HH TE-case.
106


28
IK
E = 601.2 meV
g
ii
^0.12 ^a0.S8 ^S0 02 ^^0.98
(a)
^n0.12^a0.88^S0.02^^0.98
Bulk
41.7 meV
.v..
4.6 meV
A'"
22.2meV
20.7 meV
CB
HH
LH
Hydrostatic Deformation Shear Deformation
(b)
7V
225.9 meV :
X
625.3 meV
1019.3 meV
168 meV
X
HH
^42.9 meV
LH
A :
: 125.1 meV
V V
(c)
Figure 2-6. Schematic energy diagrams for (a) bulk materials (b) In0.12Ga.88As0.02Sb0.98
QW under strains, and (c) strained band lineup between Ino.12Ga.88Aso.02Sbo.98 QW and
Alo.25Ga.75Aso.02Sbo.98 barrier.


50
Figure 3-9. Profiles of refractive index and optical intensity as a function of position for
the thin p-clad InGaAsSb/AlGaAsSb DQW SCH laser shown in Figure 3-5 (c¡ = 300
nm).
50


CHAPTER 3
RIDGE WAVEGUIDE LASER DESIGN
3.1 Introduction
In semiconductor lasers, the light is confined and guided through dielectric
waveguiding where each layer has a different refractive index. The allowed modes and
resulting optical fields in the laser structure are found by solving the wave equation.
The pertinent mathematical description is found in various texts on the electromagnetic
fields theory of guided waves [C0I6O], [Mar72], [Cas78a], [Tho80], [Agr93],
For low-threshold room-temperature operation of diode lasers, a single mode
operation is desired and its optical field needs to be confined in the active region where
the lasing takes place. The epitaxial structure of the laser diode is designed such that
these requirements for the laser mode are satisfied.
The design of the waveguide structure of the semiconductor laser considers mode
behaviors in two different directions, transversely and laterally, that is in the direction
perpendicular and parallel to the junction plane, respectively. In this chapter, the optical
field distributions in the transverse and lateral directions are analyzed for two different
designs of RWG InGaAsSb QW diode lasers that utilize a thick p-clad and a thin p-clad
layers, respectively.
In Section 3.2, general design issues of semiconductor diode lasers are discussed
in view of transverse and lateral mode behaviors. Also, the differences of the design
39


CHAPTER 2
STRAINED QUANTUM WELL OF GaSb-BASED QUATERNARY ALLOYS
2.1 Introduction
Design and modeling of semiconductor diode lasers require a knowledge of
various material parameters such as lattice constant, energy band gap, effective mass, and
refractive index, to name a few. Those parameters are essential to constructing the
energy band structure of the heterojunction in the laser diode, which contains information
vital to theoretical analyses.
Recently, new diode laser materials (mostly, alloys) have been utilized to expand
the emission wavelengths of semiconductor lasers and successfully demonstrated
[Haa91], [Nak96], [Cho92], [Cho95], Despite the experimental progresses, however, the
theoretical modeling of those semiconductor lasers have been limited by a lack of
knowledge of the material parameters for the new alloy systems.
In addition, the inclusion of strains into a QW active layer has been pursued
recently to enhance performance in IR semiconductor laser diodes [Cho92], [Gar96],
[Gar99], Strain effects on the band gap and effective mass of the QW active layer
material should also be considered in simulations of the IR semiconductor lasers.
This chapter describes theoretical models used in calculating the various material
parameters, emission wavelengths and optical gain spectra of InGaAsSb/AlGaAsSb
strained QW diode lasers. Calculation results along with theoretical material parameters
for the QWs with two different In content (15% and 25%) are presented at the end of the
10


86
Alo.9Gao.1Aso.07Sbo.93 cladding layer (p = 5x 1018 cm'3), a 40-nm thick AlxGai.xAsySbi.y
graded layer (x = 0.9 ~ 0, y = 0.07 ~ 0, p = 2 x 1019 cm'3), and finally a 50-nm thick p-
GaSb cap layer (p = 2x 1019 cm'3), as illustrated in Figure 6-1. Fabry-Perot (FP) lasers
with a stripe width of 50 pm were fabricated using PAE with the GWA 40201 solution.
The p-GaSb cap layer outside the stripe region has been etched out and covered with
anodized oxides to minimize current spreading. Also, RWG lasers with 5-pm stripe, 2-
pm high ridges were fabricated using the GWA-BOE9 electrolyte, as described in
Chapter 4.
Table 6-1 summarizes probe-test results of low-ridge wide-stripe (80-nm high and
50-pm wide) and RWG (2-pm high and 5-pm wide) thick p-clad lasers in bar form under
pulsed operation at room temperature. Pulses used were a pulse width of 1 ps and a
repetition rate of 1kHz. The threshold current density decreases with a cavity length
since the mirror loss is reduced with a longer cavity length.
Table 6-1. Pulsed probe-test results for thick p-clad InGaAsSb/AlGaAsSb MQW lasers
at room temperature. Pulse parameters used were a pulse width of 1 pm and a repetition
rate of 1 kHz. The p-cap layer between stripes was etched out in wide-stripe devices.
The ridge height of RWG devices is 2 pm,
Device
Stripe width (pm)
Cavity length (pm)
Ith (mA)
Jth (A/cm2)
Wide-stripe
50
500
130
520
Wide-stripe
50
1000
220
440
RWG
5
500
26
1040
RWG
5
1000
42
840
86


45
Table 3-1. Refractive index values used in the calculation.
Materials
n (Refractive Index)
GaSb
3.76
Alo.9Gao. i Aso.o2 Sbo.98
3.29
Al0.3Ga0.7As0.02Sb0.98
3.6
Ino. 1 sGao.ss Aso.02Sbo.98
3.9
Figure 3-3. Calculated lateral effective index difference as a function of etch depth in the
p-clad layer for the InGaAsSb/AlGaAsSb DQW SCH laser shown in Figure 3-2 (tcp = 50
nm, tci = 2 pm).
45


59
Figure 4-6 shows the typical variation of the Itr amplitude with time for the PAE
of bulk GaAs. The initial rapid current drop represents a dominating oxide growth (i.e.,
anodization) over an oxide dissolution into an electrolyte because oxides do not exist on
the wafer surface in the first place and the bias voltage pulse is used mostly to drive the
anodization. As the oxide becomes thicker, the voltage drop across the oxide increases,
leaving less voltage available to the anodic reaction. The oxide growth rate consequently
slows down and becomes comparable to the oxide dissolution rate, leading to the steady-
state between the two processes that is represented by the constant current. The final
oxide thickness is measured about 100 nm (blue oxide) in this case.
Time
(sec)
Figure 4-6. Plot of Itr vs. time for the PAE of a GaAs sample using a GWA electrolyte,
which consists of 40 parts ethylene glycol, 20 parts deionized water, and 1 part 85%-
diluted phosphoric acid. Pulse parameters used were a 80-v pulse amplitude, a 700-ps
pulse width, a 100-Hz repetition rate.
59


9
ridge wide-stripe and high-ridge narrow-stripe structures are characterized in both pulsed
and CW operation at room temperature. Measurement data include the voltage-current,
optical power-current characteristics, and lasing spectra. Finally, the first thin p-clad
GaSb-based QW lasers in both pulsed and CW operation at room temperature are
reported with the characterization data.
A summary and future work are presented in Chapter 7.


33
Figure 2-14 (a) and (b) show theoretical emission wavelengths of the thick and
thin p-clad InxGai.xAso.o2Sbo.98 /AlyGai.yAso.o2Sbo.98 strained QW lasers (x = 12, 25 % and
y = 25, 30 %, respectively). Since multi-subbands exist in each QW structure, the
emission wavelengths are computed for transitions between the first subbands in the CB
and HH band (C1-HH1) and between the second subbands in the CB and HH band (C2-
HH2). In content (x) in the QW (that is, strain effect) affects the wavelength of the Cl-
HH1 transition more significantly than QW width (that is, quantum effect), while the
opposite holds true for the wavelength of the C2-HH2 transition. Therefore, the design of
the QW structure in the GaSb-based laser diode should consider strain or quantum effects
appropriately, based on the transition responsible for the emission wavelength.
2,6.2 Optical Gain Spectra of InGaAsSb/AlGaAsSb Strained OWs
Figure 2-15 shows the calculated optical gain for TE and TM modes as a function
of photon energy in the InxGai.xAso.o2Sbo.98 /AlyGai.yAso.o2Sbo.98 QW (x = 12 % and y =
25 %) at a carrier density N= 1.2 x 1018 cm3 and T= 300 K, with intraband relaxation
ignored. The parameters used in the gain calculation are listed in Table 2-5. The table
lists two subbands in each of CB, HH, and LH bands. There are actually more than two
HH subbands allowed in this QW structure, while two subbands are allowed in the CB
and LH band, respectively. HHs in the subbands higher than the second are meaningless
in the gain calculation because they cannot participate in band-to-band transitions that are
dictated by the k-selection rule [Cas78a], [Cor93],
33


3
Bulk DH structure
Optical field
Refractive index
(a)
Bulk DH structure
Jth
Optical field
Refractive index
(b)
Figure 1-1. Schematic diagrams of energy bands, refractive index profile, and optical
field distribution for (a) double heterostructure (DH) and (b) separate confinement
heterostructure (SCH) quantum well (QW) lasers.


107
where 1 T
K
N
for the C-HH TM-case is plotted. The graph is similar to the plot of
reflection coefficient vs. the incident angle for a TM mode in the electromagnetic theory.
The C1-HH1 transition energy is analogous to the Brewsters angle. Figure 4(b) shows
the plot of
K
M
for the C-HH TE-case for a comparison. It is gradually decreasing
with energies beyond the C1-HH1 transition energy. On the other hand,
K
K
for the
C-HH TM-case is an increasing function for photon energies beyond the C1-HH1
transition energy.
To completely explain the smooth shape of the TM gain for the C1-HH1
transition, the difference between relevant Fermi-Dirac functions, Afc_HH (- fc fhh),
has been computed and plotted in Fig. 5. The plot shows that Afc_HH is decreasing in the
energy range of interest. The multiplication of the decreasing function A/C_HH and the
increasing
K
H
for the C-HH TM-case results in the bulk-like gain spectrum with a
peak at an energy other than the C1-HH1 transition energy.
In conclusion, the theoretical TM gain spectrum for the C-HH transition in a QW
is found to be smoothed like the bulk-case gain spectrum, instead of a step-like shape.
Moreover, the TM gain is equal to zero at the C-HH transition energy, while all the other
polarization-resolved gains for the C-HH and C-LH transitions have peaks at the relevant
transition energies. The anomalies of the TM gain for the C-HH transition in a QW
107


103
Energy (meV)
Figure 1. Calculated TE and TM gain spectra for an 8 nm Ino.15Gao.85As/GaAs QW at a
carrier density N = 5.8 xlO18 cm'3 and T = 300K. Intraband relaxation broadening was
ignored in the computation.
Polarization-resolved gains for each transition pair of C1-HH1 and C1-LH1 are
plotted in Figure 2. For a better illustration of each component of the gain spectra, they
are replotted in Figure 3(a) and (b) with plot ranges adjusted. As expected, the TE gain is
dominant over the TM gain for the transition involving the HH state. The TM gain is
greater than the TE gain, when the LH state is involved in the transition. It should be
noted however, that the behavior of the TM gain curve for gCH TM (TM gain for Cl-
HH1), is different from the behaviors of the other three gain components g_CH TE (TE
gain for C1-HH1J, g_CL TE (TE gain for C1-LH1), and g CL 'I'M(TM gain for Cl-
LH1).
103


58
shown Figure 4-5. The variation of the current pulse shape with time can be understood
if the capacitor effect of the anodic oxide layer is considered. At the beginning of each
pulse, the capacitance of the oxide layer shorts out the oxide resistance. Once the
capacitor charges, the current decreases within a pulse as the oxide grows. It should be
noted that the oxide thickness at this phase is determined by a competition between the
oxide growth and a simultaneous oxide dissolution into the electrolyte. The Ilr amplitude
keeps decreasing from pulse to pulse (time ti, tf) until a steady-state is reached between
the oxide growth and dissolution rates. When the two rates are balanced, the oxide
thickness becomes constant, leading to the constant Itr (time tf). From this point on, the
oxide layer with the constant thickness is traveling in the material (traveling oxide
phenomenon).
voltage
Voltage pulses
current
- time
oxide growth
t = t0 t = t,
- time
oxide dissolution
: Current pulses
t = t
Figure 4-5. Time evolution of voltage and current pulsed during PAE.
58


TABLE OF CONTENTS
page
ACKNOWLEDGMENTS iv
ABSTRACT ix
CHAPTERS
1 INTRODUCTION 1
1.1 Review of GaSb-based Semiconductor Laser Development 4
1.2. Dissertation Overview 7
2 STRAINED QUANTUM WELL OF GaSb-BASED QUATERNARY ALLOYS 10
2.1 Introduction 10
2.2 Band Lineup of Strained Quantum Well 11
2.3 Quantized Energy Levels in Quantum Well 17
2.4 Material Parameters for Quaternary Alloys 19
2.5 Optical Gain Spectrum in QW Structure 21
2.6 Calculation Results 25
2.6.1 Material Parameters and Emission Wavelengths 25
2.6.2 Optical Gain Spectra of InGaAsSb/AlGaAsSb Strained QWs 33
3 RIDGE WAVEGUIDE LASER DESIGN 39
3.1 Introduction 39
3.2 Design of Optical Waveguide Structure of Semiconductor Laser 40
3.2.1 Control of Transverse Modes 40
3.2.2 Control of Lateral Modes 41
3.2.3 General Principles for Design of Semiconductor Laser 42
3.3 Thick P-Cad RWG InGaAsSb QW Laser Design 43
3.4 Thin P-Clad RWG InGaAsSb/AlGaAsSb QW Laser Design 46
vi


52
On the other hand, it has been reported that the current PAE technique has a limit
of etch depth for GaSb-based laser materials [Lar95], Consequently, the PAE technique
was used to fabricate only low-ridge GaSb-based semiconductor lasers, not RWG lasers
for which a high-ridge configuration is required. This chapter presents the development
of a PAE technique that is capable of any arbitrary etch depth for the GaSb-based laser
material system.
In Section 4.2, basic features of PAE are discussed. The experimental setup,
process mechanism, and typical process results of the PAE are reviewed for GaAs
materials. In Section 4.3, the development of the PAE technique for GaSb-based laser
materials is presented. Discussed are a new electrolyte system used in the experiments
and effects of the chemical composition in the electrolyte on the etch rate. Also, etch
profiles resulted from the PAE with the new electrolyte system are addressed. In Section
4.4, the current pulse vs. time data is examined. It is found that the peaks in the current
time plot correlate to layer interfaces in the semiconductor laser structure. In Section 4.5,
progress in optimizing the PAE technique for different etching specifications in GaSb-
based laser materials is described. The etch rate was reduced by adjusting chemical
mixing ratios in the electrolyte, to demonstrate (1) a uniform deep etch depth with
reduced roughness of sidewalls over a large wafer area and (2) a uniform shallow etch
depth over a small wafer area.
4,2 Fundamentals of Pulsed Anodization Etching
A PAE technique has been shown to be a simple, reliable procedure for the rapid
formation of ridges in GaAs-based material systems [Gro94], In order to develop a new
52


ACKNOWLEDGMENTS
I would like to express my sincere thanks to the chairman of my supervisory
committee, Dr. Peter S. Zory, Jr. for his guidance and encouragement during my studies.
He has instilled in me not only knowledge but also professionalism.
I would like to extend my special thanks to the members of my supervisory
committee: Dr. Gijs Bosman, Dr. Sheng Li, Dr. Ramakant Srivastava, and Dr. David B.
Tanner for their extraordinary guidance and support both professionally and personally. I
would also like to thank Dr. John OMalley, who had been my supervisor for lab courses
that I taught as a teaching assistant during my masters program.
I would like to recognize Sarnoff corporation, which has funded this research
project. Particularly, Im indebted to Dr. Ray Menna and Dr. Hao Lee who provided
laser materials and measurement data.
I would like to thank my laboratory colleagues and friends that I have met and
worked with at the University of Florida. Additional thanks go to the department staff
who have provided administrative and technical support.
I owe a debt of gratitude to all my friends in Korea and U.S. for their friendship
all throughout the past years. Also, I would like to extend a special note of thanks to my
aunts family who has helped me to come back to U.S. and settle down.
IV


35
Energy (meV)
Figure 2-15. Calculated material gain for TE and TM modes in InxGai.xAso.o2Sbo.98/
Alo.3Gao.7Aso.02Sbo.98 strained QW (x = 12 %) as a function of photon energy at a carrier
density N= 1.2 x 1018 cm3 and T= 300 K. The second peak at E = 736 meV corresponds
to the C1-LH1 transition.
Table 2-5. Material parameters used in the gain calculation for InxGai.xAso.o2Sbo.98/
AlyGa1.vAs0.02Sb0.98 QW (x = 12 % and y =25 %), The QW width is 10 nm.
Strained band gap (meV)
625.5
Energy levels in CB (meV)
37.7, 151.4
CB offset, AEc (meV)
225.6
Energy levels in HH (meV)
9.7,38.5
VB offset, AEv (meV)
168.2
Energy levels in LH (meV)
29.3, 110.4
Splitting energy, S (meV)
43.2
Luttinger parameter, yl
12.574
SO band energy, A (meV)
717.7
Luttinger parameter, y2
4.426
Refractive index, n
3.5261
Electron mass (x mo)
0.042
35


92
GaSb
30 11m p = 2 x 10|y cnr3
caP I Al03Ga07As002Sb098
100 nm
1 GaSb
; 30 nm p = 2 x 10iy cnv3
p-clad Al 9Ga0 j As0 07Sb0 93
300 nm p = 5xl018cnr3
waveguide Al0 3Ga0 7As0 02Sb0 9g
320 mu
nnw Ino.25GaAsSbo.98 (2 x 20 13m)
DC2W Al0 3GaAsSb098 (200 nm)
240 nm
A1 Ga A <5 Sb
waveguide 0.3 0.7 0.02 o.98
320 nm
n-clad Al09Ga0|As007Sb093
1.5 pm 11 = 5 x 1017 cm-3
substrate GaSb n = 1 x 1017 cnr3
Figure 6-6. Structure of the 2.2 pm wavelength thin p-clad InGaAsSb/AlGaAsSb DQW
SCH laser diode.
with anodized oxides. This ridge height is actually smaller than the calculated value for
single lateral mode operation as presented in Chapter 3. The low-ridge configuration was
chosen due to the limit of the etch depth associated with the GWA-based PAE technique.
Table 6-2 summarizes pulsed measurements using 1-pm pulses at a repetition rate
of 1 kHz for low-ridge thin p-clad lasers with wide (50 pm) and narrow (5 pm) stripes.
They were probe-tested in bar form at room temperature. The threshold current densities
of the thin p-clad lasers are much higher than those of the thick p-clad lasers because of
the higher modal loss as a result of the thin p-clad layer thickness.
92


84
the same processed wafer, the Indium-soldered packaging resulted in a higher yield of
laser operation.
The laser diodes were characterized in either pulsed or CW operation. The
temperature of the heat sink was set to 15 C. A large-area thermopile detector and a PbS
photodetector were used for the measurement of pulsed and CW P-I characteristics,
respectively. A Lock-In Amplifier was used to amplify small signals from the PbS
detector.
In Section 6.2, characterizations are presented for symmetric cladding (thick p-
clad) lasers of both low-ridge wide-stripe and high-ridge narrow-stripe structures that
were fabricated with PAE. Performance characteristics of the lasers are comparable to
that of devices fabricated using conventional chemical etching techniques. In Section
6.3, experimental results of the first mid-infrared asymmetric cladding (thin p-clad) Sb-
based QW diode lasers are presented. PAE using the standard GWA electrolyte was
performed to fabricate low-ridge lasers with either wide or narrow stripes. Lasing
wavelengths of the devices are compared with theoretical calculations that were
presented in Chapter 2.
6.2 Characterization of Thick P-Clad InGaAsSb MOW Lasers
The thick p-clad laser diode grown by MBE consists of n-GaSb substrate, a 500-
nm thick n-GaSb buffer layer (n = 1 x 1018 cm'3), a 40-nm thick AlxGai.xAsySbi.y graded
layer (x = 0 ~ 0.9, y = 0 ~ 0.07, n = 1 x 1018 cm'3), a 1,5-p.m thick n-Alo.9Gao.1Aso.07Sbo.93
cladding layer (n = 2 x 1017 cm'3), a 400-nm thick Al0.25Ga0.75As0.02Sb0.98 guiding layer, a
MQW active region composed of five 10-nm thick, 0.6% compressively strained
84


34
(a)
Figure 2-14. Emission wavelengths of InxGa1.xAso.02Sbo.98/Alo.25Ga.75Aso.02Sbo.98 strained
QW laser as a function of QW width for (a) the C1-HH1 transition and (b) the C2-FEH2
transition. Indium content (x) in the QW varies from 10% to 18 % and the barrier layer is
400-nm thick.
34


62
remained same as those for GaAs-based materials, i.e. 700 (as and 100 Hz, respectively.
Using this solution, 2 minutes of PAE successfully produced 1.9 pm high ridges, which
was measured from top of the cap layer to bottom of oxides, for both wide (50 pm) and
narrow (5 pm) stripes. The native oxide was removed by using 6:1 diluted BOE before
the measurement of the ridge height with a Dektak.
active layer
(a)
(b)
Figure 4-7. Etched amount of PAE in fabricating 2-pm high ridges with (a) 50 pm and
(b) 5 pm wide stripes.
62


75
(a)
(b)
Figure 5-2. Schematic diagrams of wafer-mount schemes for pulsed electroplating
experiment, (a) a side view of the wax-combo scheme for the p-side plating: the n-surface
and sides of the wafer are sealed in wax and isolated from the gold solution, (b) a side
view of the wafer/cover glass scheme for the n-side plating: the photoresist is coated on
the p-side of the wafer for the isolation. The tweezers is used to grab the cover glass and
wafer together.


CHAPTER 5
DEVICE FABRICATION
5.1 Introduction
The as grown wafers are converted to laser devices through device processing
steps such as photolithography, etching, and metal deposition. To maximize device
performance, the optimization of the fabrication procedure is as important as that of the
design.
The laser devices in this study incorporate InGaAsSb/AlGaAsSb QW structures
and process technologies for GaSb-based laser materials are still under development.
In order to assure the fabrication of high quality GaSb-based lasers, processing steps used
conventionally in fabricating GaAs-based lasers should be verified for this relatively new
material system.
For single lateral mode operation, the ridge structure has been utilized in the
devices of this study and the structural requirements call for a reliable and reproducible
etching technique. Chapter 4 has discussed the development of a novel PAE technique to
produce ridges of any height in the GaSb-based laser material system. In this chapter, the
focus will be made on other processing steps such as photolithography and metal-contact
formation.
In Section 5.2 and Section 5.3, the photolithography and metallurgy procedures
used in fabricating RWG InGaAsSb QW diode lasers are described, respectively. In
Section 5.4, the fabrication procedure for the RWG laser is summarized.
72


CHAPTER 1
INTRODUCTION
The observation of high efficiency electroluminescence in GaAs p-n junctions
[Bla62] inspired the development of the first semiconductor lasers in late 1962 [Hal62],
[Nat62], After merely four decades of development, semiconductor lasers are now found
in a number of applications such as compact disc (CD) players, bar-code readers, laser
printers, fiber optic communications, and many defense-related applications.
The major surge in the role played by semiconductor lasers has been driven by
continued improvements in various performance characteristics: low-threshold current,
continuous wave (CW) operation at room temperature, high optical power, low cost, low
electrical power consumption, high wall plug efficiency, and long life time. Joint
progress in material growth and device fabrication technologies and theoretical
understanding of semiconductor lasers have contributed to those improvements.
The structure of the semiconductor laser has evolved along with theoretical and
technological improvements. In the late sixties, a double het ero structure (DH), shown in
Figure 1-1 (a), was introduced to achieve charge carrier and photon confinement and led
to the first continuous wave (CW) lasing at room temperature [Alf68], [Kre69], [Hay69],
DH laser diodes prevailed through the seventies and eighties.
In the late eighties and early nineties, a new generation of semiconductor laser,
the quantum well (QW) laser, emerged. In the seventies, theoretical studies had
suggested that the dimension of an active layer, where the radiative recombination takes
1


79
2. AZ1512 photoresist (PR) is spun on the p-side of the wafer section at 4500 rpm for 30
seconds. The wafer is then baked at about 85 C for 10 minutes.
3. Photolithography will be done using a mask that has a pattern of a 7-pm wide stripe
with 25-pm wide channels on both sides of the stripe, spacing 300-pm apart between
the stripes. UV light is radiated for 10 seconds and then the wafer is placed in AZ
312 MIF Diluted 1:1.3 developer for 7 seconds, followed by completely rinsing in DI
water. The wafer is then baked at about 110 C for 10 minutes. The cross section of
the developed wafer is shown below (not in scale).
25 pm 25 pm 243 pm 25 pm 25 pm
<>
: :::::
photoresist (PR)
<->
7 pm
7
o
pm
wafer
4.The PR-patterned wafer is PAE processed to make ridges of a desired height. If
necessary, additional PAE is performed to reinforce native oxides in the channels.
For the purpose of illustration, the schematic diagram from this point on depicts only
the portion of the front view of the wafer, corresponding to the area of a ridge and its
adjacent channels.
79


21
Table 2-2. Bowing parameters of III-V ternary compounds at room temperature [Ada87],
Co Cx CL Ca
(Al,Ga)As
0.370
0.245
0.055
0.070
(Ga,In)As
0.600
1.400
0.720
0.200
(Al,Ga)Sb
0.470
0.000
0.550
0.300
(Ga,ln)Sb
0.420
0.330
0.380
0.100
Al(As,Sb)
0.000
0.000
0.000
0.000
Ga(As,Sb)
1.200
1.090
1.090
0.610
In(As,Sb)
0.580
0.590
0.570
1.200
2,5 Optical Gain Spectrum in QW Structure
The linear gain in a semiconductor laser structure can be defined from Fermis
Golden rule and is given as [Asa93]
g(Ecv) =
1
ne2h
Ktico Je0 cm\n
= |Mr f (E) \f'(Em) + l], (2.33)
where e is the electron charge, s0 is the permittivity of free space, c is the light speed in
vacuum, mo is the electron rest mass, n is the effective index of the guided mode (for
unguided optical modes, n is just equal to the refractive index of the QW material),
\mt\2 is the transition matrix element, |MT | is the average transition matrix element,
pred is the reduced density of states, fc is the Fermi-Dirac distribution for the electrons in
the CB, and fh (= 1 fv) is the distribution for the holes in the VB. Em is an energy
corresponding to the photon energy by


65
Figure 4-10 shows a scanning electron micrograph (SEM) picture of the ridge that
was formed by the 2-minute PAE with the GWA-BOE9 solution. The picture indicates
that the etched bottom surface and sidewall are somewhat rough. This is probably
because of the very high etch rate (~ 0.95 pm/min) used. Similar profile characteristics
were also observed for the 3-mintue PAE with the GWA-BOE6 solution. To reduce this
roughness, the electrolyte composition has been adjusted and this study will be presented
in Section 4-5.
Figure 4-10. SEM picture of the 5-pm wide, 1.9-pm high ridge formed in p-GaSb cap
and p-clad AlGaAsSb materials by the 2-minute PAE with the GWA-BOE9 electrolyte,
followed by oxide removal with the 6:1 diluted BOE.
4,4 Detection of Laver Interfaces
In Section 4.2, it was mentioned that the/ amplitude correlates to the oxide
thickness during PAE. Another important feature of the Itr has been found in this study.
The slope change in the Itr vs. time characteristics corresponds to layer interfaces in a
semiconductor laser diode structure. Pertinent experimental results are presented in this
section.
65


48
Figure 3-6. Fundamental mode loss vs. p-clad thickness for the ridge region of the laser
diode structure shown in Figure 3-5.
Figure 3-6 shows the variation of the fundamental mode loss with tc¡ in the ridge
region of the semiconductor laser structure illustrated in Figure 3-5. Since the mode loss
is very high for tci < 250 nm and saturates at tci = 600 nm, tc¡ in the range between 250 and
600 nm will be chosen.
The p-clad thickness tci is then studied in relation to various ridge heights. Figure
3-7 shows An as a function of the remaining p-clad layer thickness, d, for different values
of the p-clad etch depth, h, which were defined in Figure 3-5. For a desired value of An,
different combinations of d and h are allowed and the choice of a combination of d and h
serves as a starting point for a final design. For the first design of a thin p-clad RWG
GaSb-based laser, tci is chosen as 300 nm because a low-ridge configuration is desired for
the convenience of ridge waveguide fabrication. For tci 300 nm, the p-clad etch depth
for single lateral mode operation is found to be about 120 nm from Figure 3-8. The
48


49
transverse optical intensity for the final thin p-clad laser structure is plotted in Figure 3-9,
along with the refractive index distribution.
Figure 3-7. Calculated An as a function of the remaining p-clad layer thickness d for
different values of the p-clad etch depth h for the structure shown in Figure 3-5.
Figure 3-8. Plot of An versus p-clad etch depth for the structure shown in Figure 3-5,
with tci =300 pm.
49


29
7K
E = 496.5 meV
g
4k
^*0.25 ^a0.75 ^S0 02 ^^0.98
(a)
A'
90.6 meV
.v..
^n0.25^'a0.75J^S0.02^^0.98
Bulk
9.4 meV
...
50.4 meV
v...
^ 42.8 meV
CB
HH
LH
Hydrostatic Deformation Shear Deformation
(b)
300.1 meV
X
546.1 meV
241.8 meV
X
HH
93.2 meV
LH
1088meV
A
: 148.7 meV
-V V
(c)
Figure 2-7. Schematic energy diagrams for (a) bulk materials (b) In0.25Ga.75As0.02Sb0.98
QW under strains, and (c) strained band lineup between Ino.25Ga.75Aso.02Sbo.98 QW and
Alo.3Ga.7Aso.02Sbo.98 barrier.


74
5.3. Metallurgies
5,3,1 Pulsed Electroplating
Electroplating uses electrochemical reactions to deposit Au on the intended region
of the wafer section. This technique is very simple to use and its deposition rate is much
higher than other metallization technique such as electron-beam evaporation. A uniform
and thick gold (Au) layer can be formed in a relatively short period of time without
requiring complex apparatus.
wafer (anode) Au plating cathode
solution
Figure 5-1. Experimental setup of pulsed electroplating.
74


61
PAE process converted 80 nm of p-GaSb/p-Alo.9Gao.iAso.o7Sb0.93 into 100 nm of greenish
native oxides. The oxide was then removed by using a diluted KOH solution. However,
as the experiment was repeated, the oxide was only partially removed by the KOH
solution and other oxide etchant such as H3PO4. This problem resulted in very non-
uniform etched profiles and limited the ridge height to a shallow depth (less than 150
nm). From the experiment results, it was concluded that the native oxide of GaSb readily
dissolves in the GWA electrolyte and other oxide etchants, but the oxide of AlGaAsSb is
extremely difficult to remove. Therefore, the ridge height produced by the PAE with the
GWA electrolyte is limited to be shallow.
In order to form high ridges by utilizing the traveling oxide effect, a new
electrolyte should be made to dissolve native oxide of AlGaAsSb. After a number of
experiments with various chemicals, a mixture of a GWA solution and a diluted BOE
(GWA-BOE) was created as an electrolyte formula for GaSb and AlGaAsSb materials.
Various mixing ratios of ethylene glycol, water, phosphoric acid, and diluted BOE were
tested. The electrolyte composition should be adjusted to give uniform PAE results
depending on the desired etch rate and time. This aspect is discussed in Section 4.5.
An electrolyte for GaSb-based laser materials is produced by first preparing a
GWA solution, consisting of 4 parts ethylene glycol, 8 parts deionized water, and 1 part
85%-diluted phosphoric acid (GWA481). This solution is volatile and needs to settle for
about 24 hours before the next mixing procedure in order to accomplish stable and
reproducible anodization results. The new electrolyte is completed by adding 9 ml of 6:1
diluted BOE to 650 ml of the GWA481 solution (GWA-BOE9). For pulse parameters,
the pulse amplitude was adjusted to 60 V, while the pulse width and the repetition rate
61


80
5. The wafer is rinsed in acetone to remove the PR.
6. Procedure 2 is repeated. Then, Procedure 3 is done again with another mask to cover
the top and sides of the ridge with the PR. The mask used here has a pattern of a 30-
pm wide stripe with a spacing of 300 pm between the stripes.
80


CHAPTER 6
CHARACTERIZATION OF InGaAsSb/AlGaAsSb QUANTUM WELL LASERS
6.1 Introduction
Experimental results of InGaAsSb/AlGaAsSb MQW diode lasers with symmetric
(thick p-clad) and asymmetric (thin p-clad) cladding structures are presented in this
chapter. Laser chips were fabricated from wafers, as described in Chapter 5, with facets
uncoated. Each laser chip was then mounted on a heat sink and characterized.
Two different schemes of laser packaging were used. One of the schemes utilized
so-called a chop-on-block (COB) packaging where the laser chip was mounted p-side
down on a copper block with a gold wire bonded to the n-side using EPO-TEK H20E
silver epoxy. To cure silver epoxy, the COB was baked for at least 10 minutes at 110 C.
In the other packaging scheme, the laser chip was mounted (p-side down) on a T046
Header 3-Lead using Indium solder.
The heat sink capacities of the copper block and the T046 Header are expected to
be comparable. However, thermal conductivities of silver epoxy and indium solder are
considered quite different. Silver epoxy is known to have a low thermal conductivity.
Moreover, the baking of the COB to cure silver epoxy could damage the InGaAsSb QW
laser chip because the semiconductor laser in this material system appears to be affected
seriously at relatively low temperatures since their annealing temperature requires as low
as 250 C. When the two different packaging schemes were used in mounting chips from
83


104
Figure 2. Polarization-resolved gain spectra for the C-HH and C-LH transitions.
Since the spectrum broadening has not been considered in the gain calculation and
the nature of the reduced density of states for a 2 dimensional structure (i.e. QW) is step
like, the gain spectra should have sharp features like g_CH TE, g_CL_TE, and
g CL JIM. In order to account for the smooth shape of gCHTM, components
constituting the gain expression given in Eq. (2.33) need to be examined.
From Eq. (2.38), it should be noted that \Mt |2 can be equal to zero depending on
the polarization and the value of cos2 6¡. For the C-HH TM-case, |Mr|" becomes zero
when cos2 dj l. This condition is satisfied if Eis equal to the transition energy Ea
from Eq. (2.40). That is, \Mt | for the C-HH TM-case is zero for a photon energy equal
to the C1-HH1 transition energy (1307.8 meV). This is clearly shown in Figure 4(a),
104


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 Philosophy.
August 2000
.
M. J. Ohanian
Dean, College of Engineering
Winfred MPHillips
Dean, Graduate School


112
[Hud93] D. A. Hudson, Operating Characteristics of Shallow Ridge Diode Lasers
Fabricated with Pulsed Anodization, Masters Thesis, University of Florida,
Gainesville, FL, 1993.
[Hun91] R. G. Hunsperger, Intergrated optics: theory and technology, Springer-
Verlag, 1991.
[Kas91] A. Kastalsky, V. J. Goldman, J. H. Abeles, Possibility of infrared laser in a
resonant tunneling structure, ,4/?/?/. Phys. Lett., vol. 59, pp. 2636-2638, 1991.
[Kob80] N. Kobayashi, Y. Horikoshi, and C. Uemura, Room temperature operation of
InGAAsSb/AlGaAsSb laser at 1.8 urn wavelength, Jpn. J. Appl. Phys. vol.
19, L30-32, 1980.
[Kre69] H. Kressel and H. Nelson, Close confinement gallium arsenide PN junction
lasers with reduced optical loss at room temperature, RCA Rev., vol. 30, p.
106, 1969.
[Kre77] H. Kressel and J. K Butler, Semiconductor Lasers and Pieter junction LEDs,
Academic Press, New York, NY, 1977.
[Kri91] M. P. C. M. Krijn, Heterojunction band offsets and effective masses in III-V
quaternary alloys, Semicond. Sci. Technol., vol. 6, pp. 27-31, 1991.
[Lar95] C. C. Largent, M. J. Grove, P. S. Zory, H. K. Choi, and G. W. Turner, Pulsed
anodization technique for fabricating GaSb-based laser, Proc. SPIE 95, vol.
2382, pp. 244-249, 1995.
[Loe91] J. P. Loehr, J. Singh, r. K. Mains, and G. I. Haddad, Theoretical studies of
the applications of resonant tunneling diodes as intersubband laser and
interband excitonic modulators, Appl. Phys. Lett., vol. 59, pp. 2070-2072,
1991.
[Mai99] M. Maiorov, J. Wang, d. Baer, H. Lee, G. Belenky, r. Hanson, J. connolly,
and D. Garbuzov, New Room Temperature CW InGaAsSb/AlGaAsSb QW
Ridge Diode Lasers and Their Application to CO measurements near 2.3 p.m,
Proc. SPIE, vol. 3855, pp. 62-70, 1999.
[Man92] F. Mandl, Quantum Mechanics, John Wiley & Sons, 1992.
[Mar72] D. Marcuse, Light Transmission Optics, Van Nostrand Reinhold, New York,
1972.
[Mar95] R. U. Martinelli, d. Z. Garbuzov, H. Lee, P. K. York, R. J. Menna, J. C.
Connolly, and S. Y. Narayan, InGaAsSb/AlGaAsSb mid-infrared diode
lasers for gas sensing, Proc. SPIE, vol. 2382, pp. 250-261, 1995.


64
Figure 4-8. Effect of mixing ratio of BOE to electrolyte on PAE time required to
consume 1.85 (am of Alo.9Gao.1Aso.07Sbo.93.
Figure 4-9. Consumed amount of Alo.9Gao.1Aso.07Sbo.93 as a function of PAE time for
GWA-BOE6 electrolyte.
64


4
room temperature. The laser diodes in this infrared (IR) range have received increasing
attention because of their potential use in diode-pumped solid state lasers, medical
welding and surgery, low-loss fluoride-based optical fiber communications, and
spectroscopic applications. The development of GaSb-based IR semiconductor lasers is
reviewed in the next section.
1,1 Review of GaSb-based Semiconductor Laser Development
Various material systems have been studied for use in fabricating 2-3 pm IR
semiconductor lasers. They include compounds and alloys from the III-V, II-VI, and IV-
VI groups. Lead-salt-based lasers (IV-VI group) were the first to be adopted for
commercial applications, but recent approaches for the development of 2 3 pm IR lasers
are based on the use of III-V materials.
Depending on materials used in forming heterojunctions, band line-ups are of two
types, I and II. The heterojunctions of type I and II are illustrated in Figure 1-2. In type I,
the potential wells for both electrons and holes are established in the narrow band gap
side, as shown in Figure l-2(a). In type II, a potential well for electrons is formed in one
side, whereas the potential well for holes is in the other side, as shown in Figure l-2(b).
Two different transition schemes for obtaining the stimulated emission are
currently being investigated. Interband-transitions, for which the stimulated emission
occurs between the conduction band (CB) and the valence bands (VB), have been utilized
in conventional laser diodes since the first demonstration of semiconductor lasers in
1962. On the other hand, intersubband transitions have been recently utilized to make
unipolar semiconductor lasers [Loe91], [Kas91], [Fai94], In this case, the optical gain


18
yi=Ale*l'+Bie-*t
(2.21)
with
2 mLJ(E-Vt)
(2.22)
where i indicates the i th region in the QW structure, is the effective mass in the
growth direction for the material in the i th region, and AEq\s the potential barrier height
in the i th region.
Vu
0
A
Region 1
4 1
Region 2
1
Region 3
>
(=0)
Figure 2-4. Schematic energy diagram of a simple square potential well of depth Vo.
By applying proper boundary conditions at each interface, the recurrence
relationship of the wavefunction can be written in matrix form as
V-o7
= 5
[a2
J
(2.23)
where the total transfer matrix S is defined as
s-ffo
1=2
(2.24)


68
4.5 Optimization of the GWA-BOE electrolyte system
Despite the successful etching demonstration for GaSb-based laser materials, the
applicability of the PAE technique using the GWA-BOE electrolyte system is limited by
its high etch rates (600 ~ 900 nm/min). The etch quality tends to suffer when the etch
rate is too high. Efforts have been made in lowering the etch rate in order to accomplish
following goals.
First, surface roughness should be reduced in order to avoid scattering losses
associated with rough bottom and sidewalls of ridges in semiconductor lasers. Secondly,
an etch profile should be uniform over a large wafer area in order to apply this PAE
technique to the fabrication of commercial diode lasers. The wafer size in a commercial
fabrication line is greater by a factor of 20 than that used in this study and the uniformity
is difficult to achieve in processing such a large wafer size. Thirdly, an etch rate should
be low enough to produce uniform flat etched surfaces even in shallow etchings (150 ~
300 nm) that are required for thin p-clad devices. A high etch rate results in a severe
undercut during the shallow etching.
It was pointed out in Section 4.3 that the etch rate of the PAE depends on the ratio
of BOE to an GWA-based electrolyte, as shown in Figure 4-8. Since a slower process
results in a uniform flat etch profile with less undercut, chemical elements in the GWA-
BOE electrolyte system has been varied to slow down the etch speed and to assure good
etch quality.
One of optimization tasks was aimed for 2-p.m high, 5-p.m wide ridges in the
p-GaSb cap and p- AlGaAsSb clad material structure over a quarter size of the 5-cm
diameter wafer. Previous GWA-BOE solutions were developed to process very small
68


20
parameters of quaternary alloys.
In a linear interpolation scheme, experimental (or theoretical) material parameters
for the four possible binary constituents (AC, AD, BC, and BD) of quaternary compound
AXBi-xCyD¡-y are utilized to estimate some material parameter Q(x,y) as follows [Ada87],
Q(x,y) = xy AC + x(1 y) AD + (1 x)y BC + (1 x)(1 -.y)BD, (2.29)
The lattice constant, deformation potential, effective mass, and Luttinger parameter in the
quaternary alloy are known to vary linearly with composition as described in Eq. (2.29).
For the band gap, spin-orbit splitting, and average VB energy, an interpolation
scheme takes into account four possible ternary alloys as well as the four binary materials
in a given quaternary compound. The energy-related parameters of the quaternary alloy
are by [GU78], [Dew85],
Q(x, y) = xy A C + x (1 y) AD + (1 x) y B C + (1 x) (1 y) B D
- x(l x)[yCABC +(\-y)CABD]-y(\-y)[xCACD+(\-x)CBCDX
where the constant C is referred to as a bowing parameter. Table 2-2 lists the bowing
parameters for various band gaps of III-V ternary alloys [Ada87],
The bowing parameter for the average VB energy of ternary alloy AXB¡.XC is to be
found from the following expression [Car88], [Wal89],
Ev (x) = xE(AC) + (1 x)E(BC) -3x(l- x)[a, (AC) a, (BC)], (2.31)
0
where a, is referred to as a deformation potential and a0 as a lattice constant with
a0 = xa0(AC) + (\-x)ci0(BC) and A a = a0(AC)-a0(BC).
Consequently, the bowing parameter for Ev av of the ternary alloy is:
C
v,av
[a, (AC) a, (SC)][a0 (AC)- a0(BC)],
an
(2.32)


46
transverse position (pin)
Figure 3-4. Profiles of refractive index and optical intensity as a function of position for
the InGaAsSb/AlGaAsSb DQW SCH laser shown in Figure 3-2 (tcp = 50 nm, tc¡ = 2 pm).
3.4 Thin P-Clad RWG InGaAsSb/AlGaAsSb OW Laser Design
In thin p-clad RWG laser designs, tcp, tc¡, and the ridge height are all design
parameters. Since tci is to be much smaller than the conventional thickness (~2 pm), the
mode overlap in the lossy p-cap layer becomes significant. In order to avoid a high mode
loss, the p-cap thickness should be small enough to minimize the mode power absorbed
in the p-cap layer. At the same time, however, material growth consideration dictates the
minimum cap thickness. To satisfy these two requirements, the cap section incorporates
a three-layer-cap region which consists of a 30-nm p-GaSb (p=2xl019 cm'3) layer, a 100-
nm undoped Alo.3GaAsSbo.35 layer, and another 30-nm p-GaSb layer, as shown in Figure
46


76
Figure 5-1 shows the experimental setup used in pulsed electroplating. This setup
is basically same as that for pulsed anodization etching (PAE) except three things. (1)
The variable resistor is added to control the current flowing in the circuit; (2) The
electrolyte is now an Au plating solution; and (3) The bias polarity is reversed: the wafer
is now the cathode in order to attract the positive Au ions in the plating solution.
The schemes for mounting a wafer are illustrated in Figure 5-2. The wax-
combo scheme, as described in Chapter 4, is used to electroplate the p-side of the wafer,
shown in Figure 5-2(a). For n-side electroplating, the wafer is not sealed in wax, but
simply placed p-side onto the cover glass after the p-side is covered with the photoresist.
The wafer and cover glass are grabbed together by the tweezers, connecting the negative
bias to the n-side only.
The electroplating rate increases with the current density of the wafer sample. For
uniform electroplating on GaAs-based wafers, a low current density (-20 mA/cm2) is
applied for the first 1 or 2 minutes of electroplating and the current density is increased to
-100 mA/cm2 afterwards. If the current density is too high, bubbles are formed on the
wafer surface and result in non-uniformity. Therefore, the current density should be
carefully controlled. The pulse parameters used are typically a pulse frequency of 100
Hz and a pulse width of 300 p.s.
During electroplating, the color of the electroplated gold changes from shiny
bright yellow to brownish yellow and then to rusty brown. The gold beyond the
brownish-yellow phase is so thick that it can be easily peeled off during the cleaving into
laser bars. A typical electroplating time for GaAs-based wafers is 4-6 minutes, with a
gold-deposition rate of less than 100 nm/minute if the aforementioned pulses are used.
76


CHAPTER 4
PULSED ANODIZATION ETCHING
4.1 Introduction
Etching and oxidation is an important procedure in defining an electrical contact
region on the p-side of a semiconductor diode laser. In the simplest procedure, oxidation
is only needed: oxides such as SO2 are deposited by evaporation in the regions between
stripes, which are consequently isolated from metal contacts. The stripe structure defined
by this simple procedure, however, allows for a considerable amount of lateral current
leakage. Therefore, threshold currents tend to be very high.
The leakage currents can be reduced by etching the highly doped p-cap region
between the stripes and then depositing oxides for the isolation from the metal contact.
The threshold current for this gain-guide structure of the semiconductor diode laser is
greatly reduced [Hud93],
Conventionally, chemical etching or dry etching technique is used to remove
semiconductor layers. However, those techniques are very time-consuming or require
very expensive and sophisticated equipment.
Two separate process steps, etching and oxidation, can be achieved in a single
step and in a very inexpensive way if PAE is used. The PAE technique has been fully
developed for GaAs-based materials and used in fabricating their laser diodes [Hud93],
[Gro94a], [Gro94b], [Wu94], [Wu95],
51


RIDGE WAVEGUIDE MID-INFRARED InGaAsSb QUANTUM WELL LASERS
FABRICATED WITH PULSED ANODIZATION ETCHING
By
JOHN YOON
A 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
2000


57
parameters, which in part determine the thickness of the native oxides resulted from PAE,
have been adjusted to produce blue anodic oxides.
Since the GWA solution is a slow etchant for the native oxides of GaAs and
AlGaAs, native oxides grow when the pulsed voltage is on, and dissolve when it is off.
The alternation of oxide growth and dissolution makes a native oxide layer move through
the semiconductor material, which is referred to as a traveling oxide phenomenon
[Gro94], The process of this phenomenon is illustrated in Figure 4-3. In this manner,
o
PAE results in oxide growth of arbitrary thickness up to about 2000 A and ridge
formation of any arbitrary height simultaneously.
Current flow during PAE is monitored by measuring the voltage drop across the
load resistor (10 Q or 100 Q) in the circuit. Figure 4-4 shows the typical current pulse
traces at different times (to, ti, tf).
Figure 4-4. Typical shape of a current pulse monitored across a load resistor during PAE.
The amplitude of the leading edge of the current pulse remains constant from
pulse to pulse, but the amplitude of the trailing edge current (7,r) decreases with time, as
57


67
the removal of most of the p-AlGaAsSb layer. Figure 4-12 compares the trailing edge
currents Itr as a function of PAE time when the GWA-BOE9 and GWA-BOE6 solutions
were used, respectively. The behavior of 4-is outstandingly consistent regardless of an
etch rate.
Figure 4-12. Comparison of trailing edge current vs. PAE time characteristics for the
GWA-BOE9 electrolyte (solid circles) and the GWA-BOE6 electrolyte (open circles).
From the Itr curve, it can be deduced that the oxide thickness goes through
different phases (decrease and then increase) as the process front passes the interface.
This implies that the net oxide formation rate changes with the semiconductor material
composition (from one layer to another).
67


8
allowed in a QW structure are found using the transfer matrix method. The optical gain
spectra are then calculated as a function of carrier density.
In Chapter 3, design issues of ridge waveguide (RWG) InGaAsSb QW lasers
based on two different transverse waveguide structures are discussed. On the one hand,
the ridge height is determined to support single lateral mode operation in conventional,
thick p-clad InGaAsSb MQW diode lasers. On the other hand, an RWG configuration is
proposed in a semiconductor laser of which the transverse optical waveguide structure is
modified. An asymmetric cladding structure is first designed by reducing the thickness
of a p-clad layer. Potential advantages of this thin p-clad structure are briefly discussed.
Finally, an RWG thin p-clad GaSb-based laser is designed.
In Chapter 4, the development of an etching technique for Sb-based materials is
described. The chapter begins with a review of the experimental setup and basics for
pulsed anodization etching (PAE) and then presents the first demonstration of deep
etching for GaSb/AlGaAsSb materials using PAE with a new electrolyte system. Next,
an important feature of this technique is described: real-time electrical detection of layer
interfaces in diode laser structures during the PAE process. Progress in the optimization
of the PAE technique with the new electrolyte system is then presented.
In Chapter 5, the processing methods used in fabricating the RWG devices are
first described. In particular, photolithography and electroplating procedures are adjusted
for GaSb-based laser materials. The fabrication procedure is presented step-by-step with
figures illustrating the cross section of the wafer in process.
In Chapter 6, experimental results of the mid-IR InGaAsSb/AlGaAsSb QW lasers
fabricated with PAE are presented. Conventional thick p-clad lasers with both shallow-


14
The lattice-mismatch parameter, s, is defined for the QW layer as follows.
a,,
F 1
fcw|| 1
a...
(2.5)
a
tvl
w
CL.
(2.6)
For biaxial compressive and tensile strain, < 0 and > 0, respectively.
The strain breaks the cubic symmetry of the semiconductor lattice, causing the energy
shifts of the CB and VB. Two components of the strain contribute to the energy shifts.
One is the hydrostatic component of the strain that is proportional to the volume
change of the crystal lattice due to the strain. This strain component shifts the average
valence-band energy Evav (Ehh +Elh+Eso)/ 3, i.e. the average of the energies of the
heavy-hole (Ehh), light-hole (Elh), and spin-orbit split-off (Eso) bands as follows:
A£v* =a(2£l+£i), (2-7)
and similarly for the CB energy
A£c* =ac(2sl + sL), (2.8)
where av and ac are referred to as the hydrostatic deformation potentials for the VB and
CB, respectively. Eqs. (2.7) and (2.8) imply that the hydrostatic strain shifts the energy
levels of the CB and VB either up or down depending on if the volume change is positive
or negative.
The other component is referred to as the shear component of the strain that is
proportional to the asymmetry in the strain parallel and perpendicular to the stress plane.
This component removes the band edge degeneracy of the heavy-hole (HH) and light-
hole (LH) bands.


96
2150 2160 2170 2180 2190 2200 2210
2220
2235
2240
226
Figure 6-9. CW lasing spectrum of the low-ridge thin p-clad laser with a 1000-pm cavity
length at above threshold (I = 1 A) and T = 15 C. The ridge width is 50 pm.
The differential quantum efficiency was calculated as about 9 %. This value is
consistent with the previous report that the differential quantum efficiency normally does
not exceed 10% for the InGaAsSb lasers with 25-38% Indium in the QW [Mai99],
96


100
step in fabricating RWG lasers and could lead to a scheme for fabricating DFB lasers
without need of an epitaxial regrowth process.
The design of the thin p-clad structure considered high optical losses associated
with p-GaSb cap and p-AlGaAsSb cladding materials. The PAE technique was used to
fabricate the first thin p-clad InGaAsSb QW diode lasers that operate CW at room
temperature at an emission wavelength of 2.2 pm [YooOOc],
(4) Modeling
Interpolation schemes have been extensively used to obtain various material
parameters for InGaAsSb and AlGaAsSb quaternaries, of which the experimental data are
extremely limited at the present time. The emission wavelengths were then calculated
considering strain effects on the band lineup. They were in good agreement with the
experimental data. Optical gain spectra were also computed and the wavelength shifts
with the injected carrier density were discussed.
7.2 Future Work
Following subjects are suggested as future work to improve InGaAsSb QW laser
diode performance.
1. Metallurgies for GaSb-based laser materials should be studied in order to
reduce high contact resistances and consequent high operating voltages of InGaAsSb QW
diode lasers. To form good ohmic contacts, both metal alloys for evaporation and
annealing procedures should be optimized. A number of experiments are required to test
various types of metal alloys, annealing temperature and periods.
100


43
p-cap and p-clad layers as a resultant modal loss is maintained as small as possible. In
addition, the ridge height is to be determined for single lateral mode operation. These
complicated two-dimensional design issues can be conveniently configured in a design
procedure developed in this study. Using the design scheme, the transverse and lateral
structures are simultaneously determined to satisfy the requirements for the mode in each
direction.
In following sections, optical mode analysis is performed using MODEIG
dielectric waveguide simulation software, which computes optical mode profiles and
complex propagation constants with complex refractive indices.
3 3 Thick P-Cad RWG InGaAsSb OW Laser Design
Figure 3-2 shows the epitaxial structure of the 2.2 pm wavelength InGaAsSb/
AlGaAsSb double-quantum well (DQW), SCH laser diode used in the calculation.
The p-cap thickness tcp and the p-clad thickness tci were predetermined as 0.05
and 2 pm, respectively. Using the simulation software MODEIG, the effective refractive
indices of the fundamental modes in the ridge region and the outside-ridge region were
computed. In calculations, it was assumed that the ridge waveguide was formed by
removing the p-cap layer and part of the p-clad layer in the outside-ridge region and then
being covered with a 100-nm thick native oxide and the whole structure is metallized
with Au. The refractive index values used in the calculations are summarized given in
Table 3-1.
Figure 3-3 shows An as a function of etch depth in the p-clad layer. According to
Eq. (3.1), An for single lateral mode operation with a ridge width of 5 pm must be
43


Copyright 2000
by
John Yoon


BIOGRAPHICAL SKETCH
John Yoon received his B.S. degree in physics at Yonsei University, Seoul,
Korea, in 1992. Between 1992 and 1994, he was enrolled in post-bachelors program in
the department of electrical engineering at Texas A&M University, College Station. He
received his masters degree in electrical engineering at the University of Florida in 1996
and has since been working toward his Ph D. His research has involved the design,
modeling, fabrication, packaging, and characterization of semiconductor diode lasers in
the wavelength range from visible (650 nm) up to infrared (8 (am).
115


105
Energy (meV)
(a)
Energy (meV)
(b)
Figure 3. Polarization-resolved gain spectra for (a) the C-HH and (b) the C-LH
transitions.
105


81
7. The PR-patterned wafer is once again PAE processed. The purpose of this additional
PAE step is to replace with oxides a cap layer outside the ridge region.
8. The photoresist is removed by rinsing the wafer in Acetone.
9. The ridge stripe region is Au electroplated.
10. Ni (25 nm)/Au(150 nm) is evaporated onto the p-side of the sample. This blanket
metallization is done for the convenience of probe testing and making contacts when
a completed device is mounted onto a heat sink.
Evaporated Ni/Au \
f-)
waveguide layer 5 p,m
native oxides
81


113
[Mor91] M. B. Z. Morosini, J. L. Herrera-Perez, M. S. S. Loural, A. A. G. Von Zuben,
A. C. F. da Silveira, and N. B. Patel, Low-threshold GalnAsSb/GaAlAsSb
doubleheterostructure lasers grown by LPE, IEEE J. Quantum Electron.,
vol. 29, pp. 2103-2108, 1991.
[Nak96] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, and T. Uamada, T.
Matsushita, H. Kiyoku, and Y. Sugimoto, InGaN-based multi-quantum well-
structure laser diodes, Jpn. J. Appl. Phys. vol. 35, L74-L76, 1996.
[New98] T.C. Newell, L. F. Lester, X. Wu, Y. Zhang, and A. L. Gray, Gain and
threshold current density characteristics of 2 micron GalnAsSb/AlGaAsSb
MQW lasers with increased valence band offset, Proc. SPIE, vol. 3284, pp.
258-267, 1998.
[Nat62] N. I. Nathan, W. P. Dumke, G. Burns, F. H. Dill, Jr., and G. Lasher,
Stimulated emission of radiation from GaAs p-n junctions, Appl. Phys.
Lett., vol. 1, pp. 62-64, 1962.
[0Re94] E. P. OReilly and A. R. Adams, Band-structure engineering in strained
semiconductor laser, IEEE J. Quantum Electron., vol. 30, pp. 366-379, 1994.
[Oku92] FI. Okuyama, T. Miyajima, Y. Morinaga, F. Hiei, M. Ozawa, and K.
Akimoto, ZnSe/ZnMgSSe blue laser diode Electron. Lett., vol. 28, pp.
1798-1799, 1992.
[Rev63] A. G. Revesz and K. H. Zaininger, An amorphous modification of Gallium-
Arsenic (V) oxide, J. ofAmer. Cer. Soc., vol. 46, No. 12, pp.606, 1963.
[Sue94] Y. Suematsu and A.R. Adams (ed), Handbook of Semiconductor Lasers and
Photonic Intergrated Circuits, Chapman & Hill, London, 1994.
[Tho801 G. H. B. Thompson, Physics of Semiconductor Laser Devices, John Wiley,
New York, 1980.
[Wal89] C. G. Van de Walle, Band lineups and deformation potentials in the model-
solid theory, Phys. Review B, vol. 39, pp. 1871-1883, 1989.
[Wan96a] C. A. Wang, K. F. Jensen, A. C. Jones, and H. K Choi, n-AlGASb and
GaSb/-AlGaSb double-heterostructure lasers grown by organometallic vapor
phase epitaxy, Appl. Phys. Lett., vol. 68, pp. 400-402, 1996.
[Wan96b] C. A. Wang and H. K. Choi, Lattice-matched GaSb/AlGaAsSb double
heterostructure diode lasers grown by MOVPE, Electron. Lett., vol. 32, pp.
1779-1781, 1996.


71
Time (sec)
Figure 4-14. Variation of trailing edge current with PAE time for a shallow etching (280
nm) of the thin p-clad InGaAsSb/AlGaAsSb DQW laser diode wafer. The electrolyte
used consists of 300 ml of the GWA481 and 3 ml of the 120:1 diluted BOE.
71


55
top surface area (p-side) is exposed to the electrolyte. The anode contact is made by
attaching an aluminum foil piece with silver epoxy onto the back surface (n-side) of the
wafer. The other scheme, shown in Figure 4-2(b), is referred to as vacuum-held where
the wafer and the metal tube are in direct contact by vacuum pressure. The p-side of the
wafer is in contact with the top surface of the electrolyte and the n-side with the tube that
is conducting the positive polarity. The former scheme allows one to observe a change of
anodic oxide color, which correlates to oxide thickness, during PAE. The feature of the
visual inspection of a wafer in process is useful to calibrating a new PAE technique in
preliminary experiments. The latter scheme is greatly simple to use, but does not allow
for observation of the wafer surface during PAE. The vacuum-held setup is therefore
used in PAE experiments that have been well established for a certain specification.
Various electrolytes have been investigated for uniform, stable, and reproducible
anodization of GaAs and AlGaAs since the first anodization of GaAs was reported in
1963 [Rev63], A thorough review of the history of anodization on those materials is
given in [Hud93], As the successful electrolyte for GaAs-based materials, a mixture of
ethylene glycol, water, and acid (GWA) has been chosen [Gro94a], [Gro94b], [Has76]
[Hud93], The water supplies ions required for the anodization, the ethylene glycol
controls the relative diffusion rates of ions in the electrolyte, and the acid helps to
increase the conductivity of the solution. The chemical reactions associated with the
anodization process are described in [Gro94a], [Hud93],
In the PAE experiments for GaAs-based materials, pulse parameters used are a
80-V pulse amplitude, a 700-p.s pulse width, and a 100-Hz repetition rate. The electrical
55


with emission wavelengths of 1.8-1.9 pm. Such experimental results imply that the
pulsed anodization etching with the new electrolyte system is a viable process for
fabricating high quality ridge waveguide InGaAsSb quantum well diode lasers.
In order to possibly raise the continuous-wave operating temperature and output
power capability, an asymmetric cladding structure was designed in the InGaAsSb
quantum well laser by decreasing the thickness of the p-clad layer. This design requires a
shallow etching step in fabricating ridge waveguide lasers and could lead to a scheme for
fabricating distributed feedback lasers without need of an epitaxial regrowth process.
The pulsed anodization etching technique was used to fabricate the first thin p-clad
InGaAsSb quantum well lasers that operated continuous-wave at room temperature at an
emission wavelength of 2.2 pm.
Theoretical modeling was conducted to compare with experimental data. The
emission wavelengths were in good agreement with theoretical calculations that involve
strain effects on the band lineups and several interpolation schemes for material
parameters of InGaAsSb and AlGaAsSb quaternaries.
IX


APPENDIX
POLARIZATION-RESOLVED OPTICAL GAIN IN QW STRUCTURE
The relaxation-free optical gain of a compressive-strained QW structure is
calculated for TE and TM modes. The diode laser structure used in the computation is
same as given in [Hsu97], consisting of an 8 nm undoped Ino.15Gao.85As compressively
strained single QW surrounded by GaAs barrier layers. The numerical values used in the
calculation were taken from [Hsu97] and listed in Table 1.
Fig. 1 shows the calculated relaxation-free gain spectra for TE and TM modes at a
carrier density N = 5.8 x 1018 cm'3 and T = 300 K. The high carrier density was chosen
such that sharp features in the gain spectra are observed at photon energies corresponding
to both the C1-HH1 and the C1-LH1 transitions.
Table 1. List of material parameters for strained Ino.15Gao.85As/GaAS QW used in the
calculation.
Strained band gap (eV)
1.267
Energy levels in CB (meV)
31.6
CB offset, AEc (meV)
86.5
Energy levels in UH (meV)
9.2,35.4,
69.2
VB offset, AEv (meV)
70.7
Energy levels in LH (meV)
68.9
Splitting energy, S (meV)
62.3
Luttinger parameter, yx
8.773
QW width (nm)
8
Luttinger parameter, y2
3.041
Refractive index, n
3.6
Electron mass (x mo)
0.061
102


36
Since the QW is compressively strained (~ 0.6 %), the gain peak at a photon
energy of 673 meV (i.e., wavelength of 1.84 pm) corresponds to the C1-HH1 transition
and the TE gain is dominant over the TM gain, as expected. The second peak at 736
meV (i.e., wavelength of 1.69 pm) is due to the C1-LH1 transition, which explains the
reason for the dominance of the TM gain over the TE gain.
It is interesting to note that the TM gain curve in the QW is smooth like the bulk-
case, not sharp as the gain characteristic of the 2-dimensional structure should appear.
This is due to the natures of the transition matrix element and Fermi-Dirac distributions
involved in the gain mechanism. The analysis of the polarization-resolved gains for all
possible transition pairs between the conduction, HH, and LH subbands in a QW are
discussed in Appendix A, to explain the behavior of the TM gain.
Figure 2-16. Calculated TE gain in InxGa1.xAs0.02Sb0.98/Al0.25Ga0.75As0.02Sb0.98 strained
QW (x = 12 %) as a function of emission wavelength for several carrier densities (N=
1.2, 1.7, and 2.2 x 1018 cm3) and T= 300 K. The second peak at 1.68 pm is due to the
C1-LH1 transition.
36


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25
The summation in Eq. (2.47) is over all subbands in the HH and LH bands for a given
value of P. Assuming the charge neutrality in the quantum well, N = P. Then, Efc and
can be found from Eqs. (2.46) and (2.47) for a given value of A at a given T.
If the subbands in the CB and VB are indexed by the quantum numbers nc and
nv, the spectra gain associated with each subband transition pair at a photon energy Ecv
can be denoted as gsubEcv,nc,nv). Finally, the total gain at Emis found by summing
over all possible subband transition pairs:
g(Ecv) = yZyZgsub(Ecv,nc,rtv) (2.48)
nc tly
2,6 Calculation Results
Theoretical models presented in Section 2.2 through 2.5 are implemented to
simulate two different QW laser diodes: an In0.12Ga0.88As0.02Sb0.98/Al0.25Gao.75As0.02Sb0.98
QW in a thick p-clad structure and an Ino.25Gao.75Aso.02Sbo.98/Alo.3Gao.7Aso.02Sbo.98 QW in
a thin p-clad structure.
2,6,1 Material Parameters and Emission Wavelengths
Material parameters of those four GaSb-based alloys are obtained as described in
Section 2.4. The parameters for the QW alloys, InxGai.xAso.o2Sbo.98 (x = 12 and 25%),
are summarized in Table 2.3 and those for the barrier alloys, AlyGa1.yAso.02Sbo.98 (y = 25
and 30 %) in Table 2.4. Energy band gap for each QW structure is illustrated in Figure
2-6(a) and 2-7(a).


77
In preliminary experiments for GaSb-based wafers, it has been found that the
current densities should be higher than those for the GaAs-based in order to make the
same bright-yellow gold within 10 minutes or so. It is recommended to complete
electroplating in that order of time if a cyanide-based Au plating solution is used because
the longer plating tends to result in non-uniformity or damage native oxides that were
deposited for the isolation by PAE.
In addition, the current density needs to be adjusted depending on the ratio of the
gold-plating area to the wafer area, which is dictated by the mask pattern used in defining
a desired stripe geometry. For the masks used in electroplating the p-side of the wafer
sample with a wide-stripe (>50 pm) and narrow-stripe (<10 pm) geometries, the area
ratios are 0.1 and 0.0023, respectively. The electroplating procedure for each stripe
geometry of the InGaAsSb QW laser is given as follows.
For the wide-stripe (>50 pm) plating case, a current density of about 50 mA/cm2
is applied for the first minute and then set to 100 mA/cm2 for the following 4~5 minutes.
If the bright-yellow gold has not been formed yet, the current density needs to be further
increased up to 500 mA/cm2 for additional 2~3 minutes.
For the narrow-stripe (<10 pm) plating case, the first minute of electroplating is
performed at a current density as high as 200 mA/cm2. Then, the current density is
increased to 1 A/cm2 for the following 4 minutes.
The electroplating technique has been used in forming contacts on the p-side only.
For an n-side contact, metal alloys were deposited by vacuum evaporation and then
heated as presented in the following section.
77


93
Table 6-2. Room temperature pulsed-probe test results for thin p-clad InGaAsSb/
AlGaAsSb DQW lasers. Pulse parameters used were a pulse width of 1 pm and a
repetition rate of 1 k.
4z.
Device
Stripe width (pm)
Cavity length (pm)
Ith (mA)
Jth (A/cm2)
Wide-Stripe
50
500
400
1.6 K
Wide-Stripe
50
1000
550
f;l K
RWG
5
500
400
16 K
RWG
5
1000
800
16 K
Figure 6-7. Voltage versus current for the low-ridge (140 nm), wide-stripe (50 pm), thin
p-clad laser with a cavity length of 1000 pm.
The voltage-current characteristics of a thin p-clad laser is shown in Figure 6-7.
The forward voltages were as high as 3.5 to 4.0 V for injection currents of 400 to 800
mA. The high operating voltage for the thin p-clad laser diode could be attributed to the
3-layer cap configuration. The presence of the undoped layer sandwiched between two
93


94
highly p-doped layers has probably changed the doping profile in the whole 3-layer cap
region. The possible doping migrant from the highly doped layers to the lowly doped
layer will give rise to a poor ohmic contact between the top p-GaSb layer and metals.
Figure 6-8 shows the CW P-I characteristic for a low-ridge (140 nm), 50-p.m wide
stripe, lOOO-pm long, thin p-clad laser with uncoated facets at T = 15 C. The threshold
current and the slope efficiency were about 750 mA and 0.05 W/A, respectively. The
output power per facet was measured as 15 mW at I = 1 A. The threshold current density
was as high as 1.6 kA/cm2 and still operated CW at RT.
Figure 6-8. CW output power versus current for the 50-pm wide stripe, low-ridge (140
nm), thin p-clad laser with a cavity length of 1000 pm at T = 15 C.
The threshold current of the thin p-clad FP laser diode is considered too high for a
surface-modulated DFB laser to possibly operate CW at room temperature. The high
threshold current can be explained by following three reasons.
94


5
results typically from a population inversion between the first and second energy
subbands in the CB part of the quantum well.
E
A
AlGaAsSb GaSb
E
A
GaSb InGaAsSb
A
"A
: AEc
A
AE
AE
.V
-> z
AE
7\
Ty.V.
-> z
(a) (b)
Figure 1-2. Schematic diagrams of energy band structures for (a) type I and (b) type II
heterojunctions.
Of the various material systems and lasing mechanism schemes discussed above,
the research activities in this study are based on type-I, GaSb-based (III-V group)
quaternary alloys utilizing interband-transitions. The active layer(s) in the semiconductor
laser are made of InxGai.xAsySbi.y alloy, lattice-matched to a GaSb substrate. The
AlxGai.xAsySbi.y alloy is used to make the guide and clad layers.
The InGaAsSb/AlGaAsSb laser diode structure is well suited for stable room
temperature operation, compared to other structures such as InGaAsSb/GaSb. Unlike
GaSb, AlGaAsSb has a lower refractive index than InGaAsSb and provides a potential
barrier high enough to confine both electrons and holes in the active layer(s).


110
[Cho92] H. K. Choi and S. J. Eglash, High-power multiple-quantum-well GalnAsSb/-
AlGaAsSb diode lasers emitting at 2.1 pm with low threshold current
density, ,4/?/?/. Phys. Lett.,\ol. 61, pp. 1154-1156, 1992.
[Cho95] H. K. Choi and G. W. Turner, InAsSb/InAlAsSb strained quantum-well
diode lasers emitting at 3.9 pm, Appl. Phys. Lett., vol. 67, pp. 332-334,
1995.
[Cho91b] H. K Choi and S. J. Eglash, Room-temperature cw operation at 2.2 pm of
GalnAsSb/AlGaAsSb diode lasers grown by molecular beam epitaxy, Appl.
Phys. Lett., vol. 59, pp. 1165-1166, 1991.
[Col60] R. E. Collins, Field Theory of Guided Waves, McGraw Hill, 1960.
[Col95] L. A. Coldren and S. W. Corzine, Diode lasers and Photonic Integrated
Circuits, John Wiley 8c Sons, Inc., New York, NY, 1995.
[Cor93] S. W. Corzine, R. Yan, and L. A. Coldren, Quantum Well Lasers, P.S. Zory,
Ed. Academic Press, New York, 1993, ch.l.
[Dew85] J. C. DeWinter, M. A. Pollack, A. K. Srivastava, and J. L. Zyskind, Liquid
phase epitaxial Gai.xInxAsySbi.y lattice-matched to (100) GaSb over the 1.71
to 2.33 pm wavelength range, J. Electron. Mater., vol. 14, pp. 729-747,
1985.
[EH99] P. G. Eliseev, Semiconductor Lasers II: Materials and Structures, E. Kapon,
Ed. Academic Press, 1999, ch.2.
[Fai94] J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho,
Quantum cascade laser, Science, vol. 264, pp. 553-556, 1994.
[Gar95] D. Z. Garbuzov, R. U. Martinelli, R. J. Menna, P. K. York, H. Lee, S. Y.
Narayan, and J. C. Connolly, 2.7-pm InGaAsSb/AlGaAsSb laser diodes with
continuous-wave operation up to -39 C, Appl. Phys. Lett., vol. 67, pp. 1346-
1348, 1995.
[Gar96] D. Z. Garbuzov, R. U. Martinelli, H. Lee, P. K. York, R. J. Menna, J. C.
Connolly, and S. Y. Narayan, Ultra-low broadened-waveguide high-power 2
pm AlGaAsSb/InGaAsSb/GaSb separate-confinement quantum-well laser,
Appl. Phys. Lett., vol. 69, pp. 2006-2008, 1996.
[Gar98] D. Z. Garbuzov, H. Lee, V. Khalfin, L. DiMarco, R. Martinelli, R. Menna, and
J. C. Connolly, 2.3-2.6 pm CW high-power room temperature broaden
waveguide SCH-QW InGaAsSb/AlGaAsSb diode laser, IEEE/OSA Conf.
Lasers and Electro-Optics, SanFrancisco, CA, postdeadline paper CPD 16-2,
1998.



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TO THE MEMORY OF MY MOTHER
AND
TO THE MEMORY OF MY BOY, ANDREW YOON


31
Figure 2-10. Shifts of the CB edge (squares) and the HH band edge (circles) due to
strains as a function of Indium content in InxGai.xAso.o2Sbo.98 /Al0.25Ga.75Aso.o2Sbo.98 QW
structure (10% < x < 18%). The widths of the QW and barrier are 10 nm and 400 nm,
respectively.
Indium (%)
Figure 2-11. Shifts of the conduction band edge (squares) and the heavy-hole band edge
(circles) due to strains as a function of Indium content in InxGai.xAso.o2Sbo.98 /-
Alo.3Ga.7Aso.02Sbo.98 QW structure (20% < x < 35%). The widths of the QW and barrier
are 20 nm and 300 nm, respectively.
31


Ill
[Gar99] D. Garbuzov, R. Menna, M. Maiorov, H. Lee, V. Khalfin, L. DiMarco, D.
Capewell, R. Martinelli, G. Belenky, and J. Connolly, 2.3-2.7 pm room
temperature CW-operation of InGaAsSb/AlGaAsSb broad-contact and single
mode ridge-waveguide SCH-QW diode lasers, Proc. SPIE 99, vol. 3628, pp.
124-129, 1999.
[Gat98] C. Gatzke, S. J. Webb, K. Fobelets, and R. A. Stradling, In situ Raman
spectroscopy of the selective etching of antimonides in GaSb/AlSb/InAs
heterostructure, Semicond. Sci. Technol., vol. 13, pp. 399-403, 1998.
[Gha88] A. K. Ghatak, K. Thyagarajan, and M. R. Shenoy, A novel numerical
technique for solving the one-dimensional Schroedinger equation using matrix
approach-application to quantum well structures, IEEE J. Quantum
Electron., vol. 24, pp. 1524-1531, 1988.
[Ghi93] A. Ghiti and E. P. OReilly, Antimony-based strained-layer 2-2.5 pm
quantum well lasers, Semicond. Sci. Technol, vol. 8, pp. 1655-1661, 1993.
[G178] T. H. Glisson, J. R. Hauser, M. A. Littlejohn, and C.K. Williams, Energy
bandgap and lattice constant contours of III-V quaternary alloys, J. Electron.
Mater., vol. 7, pp. 1-16, 1978.
[Gro94a] M. J. Grove, On Pulsed Anodic Oxidation and Its Use in Fabricating Diode
Lasers, Ph.D. dissertation, University of Florida, Gainesville, 1994.
[Gro94b] M. J. Grove, D. A. Hudson, P. S. Zory, R. J. Dalby, C. .M. Harding, and A.
Rosenberg, Pulsed anodic oxides for III-V semiconductor device
fabrication, J. Appl. Phys., vol. 76, pp. 587-589, 1994.
[Haa91] M. A. Haase, J. Qiu, J. M. DePuydt, and H. Cheng, Blue-green laser diodes,
Appl. Phys. Lett., vol. 59, pp. 1272-1274, 1991.
[Hal62] R. N. Hall, G. E. Fenner, J. D. Kingsley, T. J. Soltys, and R. 0. Carlson,
Coherent light emission from GaAs junctions, Phys. Rev. Lett., vol. 9, pp.
366-368, 1962.
[Has76] H. Hasegawa and H. L Hartnagel, Anodic Oxidation of GaAs in mixed
solutions of Glycol and Water, J. Electrochem. Soc., vol. 123, No. 5, pp.
713-723, 1976.
[Hay69] I. Hayashi, M. b. Panish, P. W. Foy, A low threshold room temperature
injection laser, IEEE J. Quantum Electron., vol. 5, pp. 211-212, 1969.
[Hsu97] C. Hsu, P. S. Zory, C-H. Wu, M. A. Emanuel, Colomb enhancement in
InGaAs-GaAs quantum-well laser, IEEE J. Selected Topics in Quantum
Electronics, vol. 3, pp. 158-165, 1997.


97
Theoretical emission wavelengths for the InxGai.xAso.o2Sbo.98 /AlyGai-yAso.o2Sbo.98
strained QW structure (x = 25 % and y = 30 %) were obtained as 2.2 pm for the C1-HH1
transition, which is in excellent agreement with the measured 2.2-pm peak wavelength.
As mentioned earlier, however, a variation of In composition in the InGaAsSb QW layer
might also have occurred during the growth of the thin p-clad wafer. Then, the emission
wavelength could result from a transition between higher subbands in the CB and VB.
Calculation results showed that 35 % In in the QW also results in the lasing wavelength
of 2.17 jam from the C2-HH2 transition. The C1-HH1 transition in this case corresponds
to 2.42 pm. Exact information on the epitaxial structure is essential to analyzing
performance of InGaAsSb/AlGaAsSb QW diode lasers.
97


1 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.
/
Peter S. Zory, Chairman (
Professor of Electrical ancLCompufyr 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 Bosman<
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.
Sheng S. Li
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.
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.
David B. Tanner
Professor of Physics


90
wavelength. The red shift of the lasing wavelength can be explained by device heating
and reduction of the band gap.
4.5
A'
3.5
3 :
2.5
intensity ^
15 :
1
.5 .
t
0-
1640.3
I
1620 1625 1630 1835 1840 1845 1850
1855
1850
Figure 6-5. CW lasing spectrum of the thick p-clad RWG laser with 1000 gm cavity
length above threshold (I = 70 mA) and T = 15 C. The ridge width is 5 |am.
90


CHAPTER 7
SUMMARY AND FUTURE WORK
7.1 Dissertation Summary
RWG InGaAsSb/AlGaAsSb MQW lasers were successfully demonstrated in the
wavelength range between 1.8 and 2.3 pm in CW mode at room temperature. Sb-based
laser diodes have been developed recently for infrared applications such as spectroscopy.
Due to their short history, processing technologies for this material system are still under
development. Also, the laser design should be improved to enhance device performance
and meet the application requirements. Moreover, very few experiments have been
conducted on the material parameters for InGaAsSb/AlGaAsSb QWs which limits
capabilities of theoretical analyses. All those aspects were investigated in this study.
Pertinent research accomplishments are summarized as follows.
(1) Development of Processing Technology for GaSb-Based Materials
Deep etching of GaSb/AlGaAsSb materials has been demonstrated by using a
single step, PAE for the first time [YooOOa], A new electrolyte system was developed,
consisting of ethylene glycol, deionized water, phosphoric acid, (GWA) and BOE.
Electrical parameters for PAE have also been adjusted. The PAE technique with the new
electrolyte system is simple, quick, and reproducible in fabricating ridges in GaSb-based
laser diode wafers.
An important feature of the PAE technique with the GWA-BOE electrolyte
system is the real-time electrical detection of layer interfaces in the laser diode structure.
98


4 PULSED ANODIZATION ETCHING 51
4.1 Introduction 51
4.2 Fundamentals of Pulsed Anodization Etching 52
4.3 PAE Technique for GaSb-Based Laser Materials 60
4.4 Detection of Layer Interfaces 65
4.5 Optimization of the GWA-BOE Electrolyte System 68
5 DEVICE FABRICATION 72
5.1 Introduction 72
5.2 Photolithography 73
5.3. Metallurgies 74
5.3.1 Pulsed Electroplating 74
5.3.2 E-beam Evaporation/Annealing 78
5.4 Fabrication Procedure for RWG Lasers 78
6 CHARACTERIZATION OF InGaAsSb/AlGaAsSb QUANTUM WELL LASERS... 83
6.1 Introduction 83
6.2 Characterization of Thick P-Clad InGaAsSb MQW Lasers 84
6.3 Characterization of Thin P-Clad InGaAsSb DQW Lasers 91
7 SUMMARY AND FUTURE WORK 98
7.1 Dissertation Summary 98
7.2 Future Work 100
APPENDIX
POLARIZATION-RESOLVED OPTICAL GAIN IN QW STRUCTURE 102
LIST OF REFERENCES 109
BIOGRAPHICAL SKETCH 115
vii


19
with
f eJ(ki~ki-i)zt-\
e~j(.kl+kl-l)l-l
£+ e-Kki~kt-l)zl-l
(2.25)
where
£ =
1
1fel
(2.26)
y
For a physical reason, the outward-going components of the wavefunctions in the barrier
regions (i.e. region 1 and 3) must be equal to zero at an infinite distance. Those boundary
conditions result in the characteristic equation for one of the elements in the total transfer
matrix S as follows.
SoO=0;
where the matrix elements in the total transfer matrix S are defined as
(2.27)
S =
Sn
V 21 22 y
(2.28)
From Eqs. (2.24), (2.25), and (2.27), the energy eigenvalues are numerically determined.
2,4 Material Parameters for Quaternary Alloys
The quaternary alloys used in QW structures of this study are InxGai.xAsySbi.y
and AlxGai-xAsySbi.y. However, experimental data of material parameters for these
quaternary alloys are extremely limited at the present time. In order to predict the various
material parameters of the quaternaries over the entire range of alloy composition,
interpolation schemes should be used. Although the schemes are still open to
experimental verification, they are known to be useful in reliably estimating material


91
6.3 Characterization of Thin P-Clad InGaAsSb DOW Lasers
The first thin p-clad laser diode grown by MBE consists of n-GaSb substrate layer
(n = 1 x 1017 cm'3), a 1.5-pm thick n-Alo.9Gao.1Aso.07Sbo.93 cladding layer (n = 5 x 1017
cm'3), a 320-nm thick Alo.3Gao.7Aso.02Sbo.98 guiding layer, a DQW active region
composed of two 20-nm thick, 1.3% compressively strained lno.25Gao.75Aso.02Sbo.98 QWs
separated by a 200-nm thick Al0.3Ga0.7As0.02Sb0.98 barrier layer, a 320-nm thick
Al0.3Ga0.7As0.02Sb0.98 guiding layer, a 300-nm thick p-Alo.9Gao.1Aso.07Sbo.93 cladding layer
(p = 5 x 1018 cm'3), a cap region composed of a 30-nm thick p-GaSb cap layer (p = 2 x
1019 cm'3), an undoped 100-nm thick Alo.3Gao.7Aso.02Sbo.98 layer, and finally a 30-nm
thick p-GaSb cap layer (p = 2 x 10iy cm'3), as illustrated in Figure 6-6.
It should be noted that the cap region of the thin p-clad laser structure consists of
three layers instead of a conventional, single p-type layer. This cap design was chosen in
view of ohmic contact issues for surface-modulated DFB lasers in the future. The p-type
GaSb layer below the undoped Alo.3Gao.7Aso.02Sbo.98 layer in the 3-layer cap region will
be served as a contact layer for the bottom region of gratings that will be made on the p-
side surface of the modulated-cap thin p-clad DFB lasers, which is planned to fabricate
once performance of thin p-clad FP lasers have been optimized.
Low-ridge, thin p-clad lasers with two different widths (50 pm and 5 pm) were
fabricated using PAE with the GW A 40201 solution. The ridge height was about 140 nm,
implying that most of the three-cap region between the ridges have been replaced
91


11
Chapter. The diode lasers based on the two different QWs will be fabricated and
characterized, to compare with the theoretical predictions. Those aspects are discussed in
Chapter 5 and 6.
In Section 2.2, the model-solid theory is presented to calculate the band offsets,
which determine the confinements of electrons and holes, of the heterojunction under
strain. In Section 2.3, the transfer matrix method is described to find energy levels
allowed in the QW structure, which is composed of a QW layer and two barrier layers.
In Section 2.4, three different interpolation schemes are presented to calculate material
parameters of the GaSb-based quaternaries. All the parameters necessary for theoretical
calculations are then obtained. In Section 2.5, optical gain spectra of strained QW
structures are analyzed as a function of carrier density. Finally, all the calculation results
of the emission wavelengths and QW gain spectra are presented in Section 2.6.
2,2 Band Lineup of Strained Quantum Well
If the native lattice constant of a QW layer is different from that of surrounding
barrier layers, the lattice-mismatched QW layer experiences distortions in crystal lattice
due to either biaxial compressive or tensile strain, as illustrated in Figure 2-1. The
distortion causes the strained QW layer to have a different lattice constant perpendicular
(1) and parallel (||) to the plane of the interface in order to keep the volume of each unit
cell of materials the same. Closely following notations used in [Wal89], the lattice
constants of the strained layers are given as:
aGw hw + ah Gb hb
Gw +Gb hb
a
(2.1)


38
Figure 2-17. Calculated TE gain in InxGa1.xAso.02Sbo.98/Alo.3Gao.7Aso.02Sbo.98 strained
QW (x = 25 %) as a function of emission wavelength for several carrier densities (N=
5.5, 7.5, and 9.5 x 1017 cm3) and T= 300 K. The second peak at 2 pm is due to the C2-
HH2 transition.
Table 2-6. Material parameters used in the gain calculation for InxGai.xAso.o2Sbo.98/-
AlyGa1.vAs0.02Sb0.98 QW (x = 25 % and y =30 %). The QW width is 20 nm,
Strained band gap (meV)
546.1
Energy levels in CB (meV)
15.3, 62.0
CB offset, AEc (meV)
300.1
Energy levels in HH (meV)
3, 11.9
VB offset, AEv (meV)
241.8
Energy levels in LH (meV)
12.4, 50.2
Splitting energy, S (meV)
93.2
Luttinger parameter, yx
13.759
SO band energy, A (meV)
723.0
Luttinger parameter, y2
5.014
Refractive index, n
3.5277
Electron mass (x mo)
0.038
38


54
(a)
i-
Vacuum
pump
electrolyte
(b)
Figure 4-2. Schematic diagrams of wafer-mount schemes for PAE experiment, (a) wax-
combo scheme: side view of sample preparation scheme, (b) vacuum-held scheme.
wafers are shown in Figure 4-2. One of them, shown in Figure 4-2(a), is so called a wax
combo, where the sides of the wafer are completely covered with wax so that only the
54


24
1 1 1
1 and
1 1
+
mn mcji mqi
mm mci\\
(2.41)
The reduced density of states pred with spin not included, for QW is given by
Predi
m.
In h2dz
0
for Em >Eg.
for (2.42)
with the energy gap between the /th conduction and valence subbands defined as
Ea ~ Eg + Eq + Eyj
(2.43)
The Fermi-Dirac functions fc and //, are defined as
1 + exp((E ,-Ej/kj)
(2.44)
fh(Eyj\\)
(2.45)
where Efc and E,v are the quasi-Fermi levels in the CB and VB, measured with respect
to the CB and VB edges,, respectively, kB is the Boltzmann constant, and T is the
temperature.
For parabolic CB and VB, the quasi-Fermi levels .E/c and Efi, are related to the
electron density N and hole density P in the quantum well as follows.
kj
N =
nti2d
- 2 ln(1 + exp[- (Ec]-Efc)/kj}),
(2.46)
z J
p =
kj_
7th1 d,
2X||ln^ + exp[~ iEq-Efi)/k j\),
(2.47)


26
Lattice constants of quaternaries for the QW and the barrier layers are slightly
different and thus strains are introduced into the QWs. The band edges of the QWs are
accordingly shifted, as described in Section 2.2. The strain-induced band shifts in the
InxGai.xAso.o2Sbo.98 QWs (x = 12 and 25 %) are illustrated in Figure 2-6(b) and 2-7(b),
respectively. Finally, the model-solid theory is used to construct the band lineups at
InxGai.xAs0.o2Sbo.98 /AlyGai.yAso.o2Sbo.98 interfaces (x = 12, 25 % and y = 25, 30 %,
respectively), as shown in Figure 2-6(c) and 2-7(c), respectively. Finally, the band
offsets, which determine the confinement of electrons and holes, are obtained.
The lattice mismatch and the band edge shifts of both the CB and HH band are
plotted as a function of In content in the QWs for the thick and thin p-clad laser diode
structures in Figure 2-8 and 2-9, respectively. The strain strength, which is proportional
to lattice mismatch, is increased with In composition in the QW. The minus signs of the
lattice mismatch values indicate compressive strains.
The shifts of the CB and HH band edges are shown as a function of In content in
the two different QWs in Figure 2-10 and 2-11, respectively. The shift of the HH band
edge is more sensitive to In composition in the QW than that of the CB edge is. This is
because the VB are affected by both hydrostatic and shear strains, while the CB is only
by hydrostatic strain.
The resultant band gaps of the strained InGaAsSb/AlGaAsSb QWs decrease with
In content in the QW, as shown in Figure 2-12 and 2-13. That is, the longer wavelength
operation is achieved with higher In content in the QW.


7
employing a broad waveguide structure to reduce high optical losses associated with p-
cap and p-clad layers and by using quasi-ternary heavily-strained InGaSb(As) QWs with
In compositions chosen outside the miscibility region for InGaAsSb compounds [Gar99],
[Mai99],
1.2. Dissertation Overview
This work describes theoretical analyses and experimental results of CW, room
temperature operation of InGaAsSb/AlGaAsSb compressive-strained multiple quantum
well (MQW) lasers emitting in the 1.8 2.3 p.m wavelength range, fabricated with pulsed
anodization etching.
Goals in this study were two fold: one was to develop a reproducible, reliable
etching technique for GaSb/AlGaAsSb material systems and the other was to design and
fabricate an aymmetric cladding structure in InGaAsSb/AlGaAsSb strained QW laser
diodes. Also, theoretical calculations were performed to verify emission wavelengths of
the devices and to obtain their optical gain spectra. Since experimental data of material
parameters for Sb-based quaternaries are limited, the device parameters necessary for the
theoretical calculations were found by extensively employing theoretical models.
In Chapter 2, the emission wavelengths and optical gain spectra of compressively
strained InGaAsSb/AlGaAsSb QW laser diodes are theoretically obtained. Band offsets
at the strained heterojunctions are determined using the model-solid theory. Due to a
lack of knowledge of the material parameters for the Sb-based quaternary alloys, several
interpolation schemes are employed to obtain various material parameters. Energy levels


101
2. The threshold current densities of the RWG thin p-clad laser diodes were too
high for reliable CW, room temperature operation. In order to reduce the thresholds,
improvements in the following four areas are suggested. First, the effect of p-contact
metals on the performance of thin p-clad lasers in the Sb-based material system should be
studied to find its significance and optimization if needed. Secondly, the growth quality
of the 3-layer cap section needs to be improved to reduce scattering losses associated
with possibly rough interfaces between the layers in this section. Thirdly, the effect of
facet coating on device performance should be studied to prevent the facet degradation.
Finally, the device design should be improved to minimize the effect of high optical
losses of p-cap and p-clad layers. Concerned issues are the design of the contact layer
region and the thickness of the p-clad layer.
3. Once RWG thin p-clad laser performance has improved through developments
in the design and fabrication, RWG DFB lasers are to be fabricated with gratings formed
onto the p-side surface of the wafer. Among task issues are the design of gratings (e.g.
depth and period) and the development of an etching technique to generate the desired
gratings on the wafer surface.
101


70
of deionized water (120:1 diluted BOE). The new GWA-BOE electrolyte is completed
by adding 3 ml of the 120:1 diluted BOE to 300 ml of the GWA481.
The pulse parameters used were a pulse amplitude of 60 V, a pulse width of 700
ps, and a repetition rate of 100 Hz. The mask used in defining ridges has a pattern of a 7-
pm wide line with dual channels of a 25-pm width each.
After the PAE, the native oxide was stripped by dipping the sample in the 120:1
diluted BOE for 15 seconds before the depth measurement using a Dektak. The trailing
edge current was recorded as a function of PAE time and the PAE was stopped at 2
minutes as the second peak appeared, shown Figure 4-14. The ridge height was
measured as 280 nm, implying that most of the three-layer-cap region was removed and
the p-clad layer was about to etch out. Again, the peaks in the trailing edge current curve
consistently correspond to the layer interfaces in the thin p-clad structure.
Figure 4-13. Locations of depth measurement points. A quarter of the 5-cm diameter
wafer was PAE processed for 25 minutes using an electrolyte that consisted of 300 ml of
GWA40201 and 1ml of 10:1 diluted BOE. The pulse amplitude was set to 80 V, the
pulse width to 500 ps, and the repetition rate to 100 Hz. The measured etch depth are:
Point #1= = 2345 nm, #2 = 2315 nm, #3, 2340 nm, #4 = 2315 nm, and #5 = 2340 nm.
The optical measurement of the cross section at point #5 indicates the channel depth of
2.2 0.2 pm.
70


32
Figure 2-12. Energy band gap as a function of Indium content in InxGai-xAso.o2Sbo.98 /
Al0.25Ga.75As0.02Sb0.98 strained QW structure (10% < x < 18%). The widths of the QW
and barrier are 10 nm and 400 nm, respectively.
Figure 2-13. Energy band gap as a function of Indium content in InxGai.xAso.o2Sbo.98 /
Alo.3Ga.7Aso.02Sbo.98 strained QW structure (20% < x < 35%). The widths of the QW and
barrier are 20 nm and 300 nm, respectively.
32


60
4.3 PAE Technique for GaSb-Based Laser Materials
It was reported that the electrolyte used in the PAE of GaAs-based materials did
not dissolve the native oxide of the InGaAsSb/AlGaAsSb material system [Lar95], Since
the traveling oxide effect did not occur, a single-step PAE cannot produce high ridges
required for thick p-clad RWG GaSb-based lasers. This section presents a new electrolyte
system that enables to produce a traveling oxide and form high ridges in GaSb-based
laser materials.
The wafer structure used in experiments was grown on an n-GaSb substrate by
molecular beam epitaxy (MBE), consisting of InGaAsSb/AlGaAsSb MQW between an
undoped broadened Alo.3Gao.7Aso.07Sbo.93 waveguide layer. The two top layers are a 50-
nm thick p-GaSb cap layer (p = 2 x 1019 cm'3) and a 2-pm thick p-Alo.9Gao.1Aso.07Sbo.93
cladding layer (p = 5 x 1018 cm'3). The targeted ridge height of the structure is 2 pm and
thus etching should stop in the vicinity of the interface between the p-clad layer and the
underlying waveguide layer.
Preliminary experiments were first conducted to study the solubility of native
oxides of GaSb and AlGaAsSb in the GWA-based electrolyte that has been established
for GaAs and AlGaAs materials. A series of pulsed anodization and oxide removal steps
were performed with a GW A mixture of 40 parts ethylene glycol, 20 parts deionized
water, and 1 part 85%-diluted phosphoric acid (GWA40201) and with oxide etchants
such as diluted KOH and H3PO4. Pulse parameters used were a pulse amplitude of 80 V,
a pulse width of 700 ps, and a repetition rate of 100 Hz. The amplitude of the trailing
edge current was recorded as a function of time during PAE. The surface profile was
measured with a Dektak after each anodization and oxide removal. The first 2-minute
60


78
5.3.2 E-beam Evaporation/Annealing
Electron-beam evaporation and heat treatment (annealing) of metal alloys are
used for blanket metallization of the p- and n-sides of laser devices in this study. On the
p-side, Ni (25 nm)/Au (150 nm) were evaporated on top of the electroplated gold for the
convenience of laser testing, as addressed in the following section. On the n-side, Au-Ge
(50 nm)/Ni (15 nm)/Au (200 nm) were evaporated and then annealed at 250 C for 1
minute in a gas flow consisting of N2 (96%) and H2 (4%).
Deposited metal alloys and annealing conditions (e.g. heating temperature and
gas-flow composition) are very important for the formation of low-resistance ohmic
contacts. Poor contacts cause high thermal resistance of devices, leading to higher
operating voltage and device heating. Consequently, the temperature characteristics of
diode lasers and their reliability will suffer. Considerations for the metallurgy technique
include (1) low contact resistance; (2) ease of fabrication; (3) good adhesion; (4) low
temperature for contact formation; (5) thermal stability, etc [Sue94], Further study on
these subjects is left for future work.
5.4 Fabrication Procedure for RWG Lasers
The fabrication procedure for the RWG InGaAsSb/AlGaAsSb MQW diode laser
is presented as follows. The figures are accompanied to envision the pertinent process.
1. A wafer section is cleaned by placing it in boiling TCA, acetone, and methanol for 5
minutes each. If dirt or residue is still found on the wafer section, repeat the
aforementioned cleaning procedure. Finally, the wafer is rinsed in deionized (DI)
water.
78


89
the case of x = 12 %, the theoretical lasing wavelength is 1.842 pm. The MBE technique
for GaSb-based quaternary alloys has not been completely calibrated and thus a slight
variation of In content in the QW is expected.
Figure 6-5 shows the CW emission spectra of the same RWG laser above the
threshold. As the injection current was increased, the lasing wavelength shifted to longer
1820 1B2$ 1830 1836
1840
184$
1860
18$$
I860
Figure 6-4. CW emission spectrum of the thick p-clad RWG laser with 1000 pm cavity
length near threshold (I = 50 mA) and T = 15 C. The ridge width is 5 pm.
89


114
[Wu94] C. H. Wu, P. S. Zory, and M. A. Emmanuel, Contact Reflectivity Effects on
Thin p-Clad InGaAs Single Quantum-Well Lasers, IEEE Photon. Technol.
Lett., vol. 6, pp. 1427-1429, 1994.
[Wu95] C. H. Wu, P. S. Zory, and M. A. Emmanuel, Characterization of Thin p-Clad
InGaAs Single-Quantum-Well Lasers, IEEE Photon. Technol. Lett., vol. 7,
pp. 718-720, 1995.
[YooOOa] J. Yoon, P. S. Zory, R. Menna, and H. Lee, Pulsed anodization etching for
the fabrication of ridge-waveguide structures in mid-infrared InGaAsSb
quantum well lasers, submitted to IEEE Electrochemical and Solid-State
Letters.
[YooOOb] J. Yoon, P. S. Zory, R. Menna, and H. Lee, Ridge waveguide InGaAsSb
quantum well diode lasers fabricated with pulsed anodization etching, to be
published in Proc. LEOS 2000.
[YooOOc] J. Yoon, P. S. Zory, R. Menna, and H. Lee, 2.2 pm room temperature CW
operation of thin p-clad InGaAsSb-AlGaAsSb broad waveguide SCH-QW
diode laser, to be submitted to SPIE 2001.
[Zor93] P. S. Zory, Jr. (ed), Quantum Well Lasers, Academic Press, New York, 1993.


53
PAE technique for GaSb-based laser materials, it is first necessary to understand the basic
features of the PAE process. This section reviews the PAE technique that has been
established for GaAs-based laser materials.
wafer (anode) electrolyte cathode
Figure 4-1. Experimental setup used in Pulsed Anodization Etching (PAE)
The PAE experimental setup is shown in Figure 4-1. Two electrodes, which are
immersed in an electrolyte, and a pulsed voltage source are connected in a series with a
load resistor of 10 £1. A semiconductor wafer with photoresist ridge masking serves as
the anode and a platinized titanium grid as the cathode. Two different schemes to mount
53


30
Figure 2-8. Lattice mismatch as a function of Indium content in InxGai.xAso.o2Sbo.98/
Alo.25Ga.75Aso.02Sbo.98 QW structure (10% < x < 18%). The widths of the QW and barrier
are 10 nm and 400 nm, respectively.
Figure 2-9. Lattice mismatch as a function of Indium content in InxGai-xAso.o2Sbo.98/
Alo.3Ga.7Aso.02Sbo.98 QW structure (20% < x < 35%). The widths of the QW and barrier
are 20 nm and 300 nm, respectively.


99
The current pulse vs. time characteristics shows a peak when the etch front moves
through an interface between semiconductor layers. This allows for the precise timing of
the PAE process in achieving a desired depth.
Efforts have also been made to achieve uniform etching of large size wafers for
industry practice of ridge formation. By changing mixing ratios of the chemicals in the
electrolyte, the etch rate of the PAE has been decreased. The slower PAE technique has
reduced the roughnesses of ridge sidewalls and demonstrated uniform etching depth over
a large wafer area (a quarter size of 2-inch diameter wafer).
(2) Low-Thresholds of RWG InGaAsSb QW Lasers Fabricated with PAE
High ridges (5-pm wide, 2-pm high) were formed in the conventional, thick p-
clad InGaAsSb/AlGaAsSb MQW laser diode structure by using the aforementioned PAE
technique. The RWG devices operated near 1.84 pm in CW mode at room temperature
[YooOOb],
The pulsed thresholds of the RWG laser diodes with cavity lengths of 500 and
1000 pm were 26 mA and 46 mA, respectively. Those values are comparable to the
lowest thresholds of RWG InGaAsSb QW diode lasers fabricated with conventional, wet
chemical etching techniques. The experiment results imply that the PAE with the GWA-
BOE based electrolyte system is an outstandingly reliable process for fabricating high
quality RWG InGaAsSb/AlGaAsSb QW diode lasers.
(3) Thin P-Clad InGaAsSb/AlGaAsSb QW Laser
An asymmetric cladding structure has been implemented in InGaAsSb QW lasers
by decreasing the thickness of the p-clad layer. This structure requires a shallow etching
99


44
smaller than ~ 6.8 x 10'3. From the plot, the corresponding etch depth in the p-clad layer
is about 1.8 ¡am. In Figure 3-4, the refractive index distribution and the calculated optical
intensity distribution in the transverse direction are shown together.
p-cap GaSb
t p = 2 x 1019 cm-3
p-clad AIq 9GaQ j As0 07 SbQ 93
t cl p = 5 x 1018 cm'3
waveguide Al03Ga07As0 02Sb098
320 nm
Inn,,GaAsSbnos (2 X 20 nm)
r\r\\\r 0.25 o.s/8 v '
uOW Al03GaAsSbQ98 (200 nm)
240 nm
waveguide ^*o.3^ao.7^so 02^^098
320 nm
n-clad Al09Ga0 j As007SbQ93
1.5 pm n = 5 x 1017 cm'3
substrate GaSb n = 1 x 1017 cnr3
Figure 3-2. Structure of the 2.2 pm wavelength thick p-clad InGaAsSb/AlGaAsSb DQW
SCH laser diode used in the calculation for the effect of a ridge height on the effective
refractive index.
44


85
P-cap GaSb
50 mn p = 2xl019cm3
gradmg GaSb/AI,, 0Ga(1 ,Asnn7Sbll(
40 nm p = 2xl019cm'3
p-clad Al0 9GaQ jAs0 07SbQ 93
1.35 pm p = 5 x 1018 cm'3
p-clad ^^0.9^a0.1^S0.02^^0.98
1501U11 p = 1 x 1018cm'3
waveguide ^o.25Gao.75AsQ Q2Sb0 9g
380 nm
, nw ln015GaAsSb09g(5xlOnm)
Al 25GaAsSb0 98 (5 x 20 nni)
150 nm
waveguide ^Iq 25Gao 75Asq 02SbQ gg
400 mn
n-clad Al09Ga0 iAs0 07Sb0 93
1.5 pm n = 2xl017cm'3
grading GaSb/AI, 0Gan ,Asn n7Sbn Q, '
40 nm n=lxl018cm'3
buffer GaSb
500 nm 11 = 1 x 1018 cm-3
substrate GaSb
Figure 6-1. Structure of the 1.8 pm wavelength thick p-clad InGaAsSb/AlGaAsSb MQW
SCH laser diode.
Ino.15Gao.85Aso.02Sbo.98 QWs separated by four 20-nm thick Alo.25Gao.75Aso.02Sbo.98 barrier
layers, a 400-nm thick Alo.25Gao.75Aso.02Sbo.98 guiding layer, a 150-nm thick p-type
Alo.9Gao.1Aso.07Sbo.93 cladding layer (p = 1 x 1018 cm'3), a 1.35-pm thick p-type
85


41
a careful design should take into account the high optical absorptions associated with the
two top layers of the GaSb-based semiconductor laser.
Design issues are different for the thick and thin p-clad laser structures. In the
thick p-clad GaSb-based laser design, the mode loss associated with the high intervalence
band absorption in the p-clad layer is kept small by broadening the guide layers [Gar96],
[Gar98], [Gar99a], The minimization of the transverse mode confinement in the lossy p-
cap and p-clad layers is a key issue to achieve low threshold current density.
In the thin p-clad design, the fractional mode power in the p-clad layer can also be
kept small, but this must be balanced with the requirement that a reasonable amount of
mode power must exist at the contact surface in order to obtain the desired coupling
coefficient for distributed feedback (DFB) lasers.
3,2,2 Control of Lateral Modes
For low threshold operation and single spatial mode lasing, a strong guiding
geometry in the lateral direction is desired. Figure 3-1 shows the RWG structure that is
used to control the lateral mode behavior of semiconductor diode lasers. From symmetric
three-layer planar waveguide theory, the lateral index difference An between the ridge
region and the outside-ridge region that allows only the fundamental mode to exist is
given by [Hun91]
An <
K
(3.1)
where Xo is the free space wavelength, neff is the effective refractive index of the
fundamental mode in the ridge region, and w is the stripe width.
41


I would like to extend my greatest appreciation and thanks to my grandma, father,
brother, and mother-in-law for their prayers, love and support. I can offer here only an
inadequate acknowledgement of my appreciation.
Although my Mom and my boy, Andrew, are not here today to witness my
academic accomplishment, I strongly believe that they in heaven feel proud of me.
My wife, Mejae Lee Yoon, deserves my utmost thanks for her unconditional love
and support. She has been with me all throughout the hardships in the past years and
inspired me to continue on my study. The completion of this work would never be
possible without her companionship, forbearance and encouragement.
Our baby, who soon arrives in July, will think me as born with a doctorate.
v


95
For the thin p-clad structure, the contact metal significantly affects optical mode
loss and consequently threshold current [Wu94], Previously, the effect of p-contact
metals on the thin p-clad laser performance has been studied for InGaAs/GaAs QW
structures with AlGaAs cladding layers [Wu94], [Wu95], The same study for the thin p-
clad InGaAsSb/AlGaAsSb QW laser structure will be of great benefit to reduce threshold
current.
Also, the complicated 3-layer cap configuration might cause poor interface
quality. Rough interfaces between the layers result in scattering losses, which raise the
required threshold gain. The material growth for the 3-layer cap configuration, where the
undoped Alo.3Gao.7Aso.02Sbo.9s layer is surrounded by the highly p-doped thin GaSb
layers, has not been done before and thus the interface qualities are open to question.
Finally, it should be noted that the threshold for the InGaAsSb QW diode laser
increases substantially if their facets are left uncoated [New98], In their work, the
average threshold was increased by as much as 50% several months after the devices with
the uncoated facets were stored in a nitrogen chamber. The facets of the devices in this
study were left uncoated. Moreover, several months have passed from the wafer growth,
to the device fabrication, and finally to the device characterization. Therefore, the device
facets must have been degraded by the time that the measurement was made and led to a
reduction in the mirror reflectivity, that is, higher mirror loss.
Figure 6-9 shows the emission spectrum of a low-ridge (140 nm), 50-pm wide
stripe, 1000-pm long, thin p-clad laser at T = 15 C above the threshold. As shown, the
peak wavelength is centered at about 2.2 pm for a drive current of 1 A. The spectrum
indicates that the thin p-clad laser operated in multi-longitudinal modes.
95


63
It should be noted that two different masks were used to define photoresist stripes
depending on their widths in order to avoid undercutting, of which the amount increases
with an etching amount. In defining the 5-jim narrow stripe geometry, even a small
amount of undercutting can give rise to a significantly different stripe width than the
intended width. Taking into account such undercutting effect, the mask used has a
pattern to allow for the least amount of lateral etching in achieving 2-pm high ridges: a 7-
pm wide line with 25-p.m wide channels on both sides, a 300-gm center spacing. In
defining the 100-pm broad strip geometry, the undercutting effect is not significant as
much. The mask used has a pattern of a 50-p.m line width and a 500-pm center spacing.
The etching amounts corresponding to the two mask patterns are illustrated in Figure 4-7
(a) and (b), respectively.
The consumed rate of GaSb/Alo.9Gao.iAso.o7Sbo.93 materials is very sensitive to the
mixing ratio of BOE to electrolyte. Using an electrolyte that consists of 650-ml GWA481
solution and 6-ml 6:1 diluted BOE (GWA-BOE6), the same etch depth (~1.9 p.m) was
achieved in 3 minutes. Figure 4-8 shows the effect of the mixing ratio of BOE to
electrolyte on the PAE time required to achieve 1,9-|am high ridges. The required PAE
time decreases monotonically with the ratio of BOE to electrolyte.
Figure 4-9 shows the consumed amount of Alo.9Gao.1Aso.07Sbo.93 as a function of
PAE time when the GWA-BOE6 solution was used as an electrolyte. At the initial stage,
the consumed amount of a p-Alo.9Gao.1Aso.07Sbo.93 cladding layer increases rapidly and
then linearly with the PAE time between 1 and 3 minutes. Beyond 3 minutes, the PAE
process slows considerably. This can be attributed to a change in the etch rate since the
oxide front is now moving through different materials, i.e. the guide and QW layers.
63


73
5.2 Photolithography
UV photolithography is generally used to define a stripe region on the contact
layer of the laser. The stripe geometry from a mask is transferred in a photoresist layer
that is uniformly coated on the wafer surface with a thickness of about 1 pm. UV light
with a wavelength between 350 and 410 nm exposes the photoresist layer and a developer
solvent is used to remove the exposed region, leaving the desired pattern on the
photoresist layer.
Positive photoresist AZ1512 is spun at 4500 rpm for 30 seconds on the wafer
surface, forming a 1,2-pm thick photoresist layer. The time of the UV exposure typically
increases with an intended stripe width. Upon exposure, the wafer is placed in AZ 312
MIF Diluted 1:1.3 developer. This procedure has been used in the photolithography for
GaAs-based laser devices.
In contrary to GaAs-based materials, it was found that the aforementioned
developer etches a GaSb p-cap layer. In other words, the GaSb cap layer of the wafer
could be etched while removing the exposed region of the photoresist, leading to non-
uniform, rough profiles in a following etching-step.
A number of photolithography has been performed to find the optimum procedure
for a given intensity of the UV light of the mask aligner used. The developing time
should be less than 10 seconds and accordingly the exposure time has been increased to
completely dissolve the radiated photoresist in such a short developing time. Finally, the
photolithography was conducted with an exposure time of 8 seconds and a developing
time of 7 seconds in fabricating laser devices in this study.
73


13
where ay is resulted from a biaxial strain and the same throughout the structure, aw and
ab denote the equilibrium lattice constant of the QW and the barrier layer materials,
respectively, and j denotes the QW or barrier material,
with
Gl=2(Cn+2CW-Dl/2), (2.3)
D?'=2^, (2.4)
'-'ll
The equilibrium lattice constants and the elastic constants, Cu and C12, of III-V binary
semiconductors relevant to InGaAsSb and AlGaAsSb quaternary alloys are listed in
Table 2-1. The constant D defined in Eq. (2.4) is for the case of strain along [001],
Table 2-1. Material parameter for selected III-V binary semiconductors. Lattice
constants cio, band gaps Eg at T, and spin-orbit splitting Aso at room temperature from
[Ada87], Elastic constants Cu and C12, valence-band average Ev,av, hydrostatic
Material
Clo
Cu
c12
Es
Aso
Ev,av
ac
av
b
0
(A)
(dyn/cm2) (dyn/cm2)
(eV)
(eV)
(eV)
(eV)
(eV)
(eV)
AlAs
5.6611
1.25xl012
5.34x10
2.95
0.28
-7.49
4.09
2.47
-1.50
GaAs
5.6533
1.22xl012
5.71x10
1.42
0.34
-6.92
-7.17
1.16
-1.70
InAs
6.0584
8.33x10
4.53x10
0.36
0.4
-6.67
-5.08
1.00
-1.80
AlSb
6.1355
8.77x10
4.34x10
2.3
0.72
-6.66
3.05
1.38
-1.40
GaSb
6.0959
9.08x10
4.13x10
0.72
0.74
-6.25
-6.85
0.79
-2.00
InSb
6.4794
6.59x10
3.56x10
0.18
0.81
-6.09
-6.17
0.36
-2.10


23
For QW, the effective masses in the growth and in-plane directions are not same,
unlike for a bulk case. The growth and in-plane effective masses (mhhl, mlhl, mm, and
mlhy) are defined by using Luttinger parameters as
m
1
tv
hhL
Y\~2Y2 2
mihL =
1
tv
Y,+2Y2 2
(2.36)
mm =
_j r.
Y\+Yi 2
mm =
_J
Yx~Y2 2
(2.37)
The transition matrix element |Mr| describing the interaction between the CB
and VB states is given as
M2
11 2a
Icos 9
2 2 '
1 -cos2 9,
5 1 2 /j
COS <9,
6 2 J
1 2*
+ COS 9
13
for C-HHTE gain
for C-HHTM gain
for C LH TE gain
forC-LHTM gain
(2.38)
where
H =
mn
\c
fc+A)
m0Eg,
V+1\
(2.39)
(E, +EJ
m
d'-L
cos 9j =
m
dll
(E-£,-£4-£j+r£4+V
mrjl
(2.40)
m
dll
with spin-orbit splitting energy A and reduced effective masses in the growth and in
plane directions, mrjL and respectively, defined as


56
oxide growth
and dissolution
(d)
Figure 4-3. Schematic diagram of the PAE process illustrating a traveling oxide
phenomenon: (a) semiconductor wafer with photoresist pattern (b) native oxide formed
when a pulse is on (c) oxide dissolution and growth occurred subsequently (d) final ridge
structure.
56


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
RIDGE WAVEGUIDE MID-INFRARED InGaAsSb QUANTUM WELL LASERS
FABRICATED WITH PULSED ANODIZATION ETCHING
By
John Yoon
August, 2000
Chairman: Peter S. Zory, Jr.
Major Department: Electrical and Computer Engineering
Mid-infrared quantum well lasers based on the InGaAsSb/AlGaAsSb material
system are designed and demonstrated. Various aspects of the strained quantum well
laser diode in this material system were extensively studied: the development of an
etching technique, the design of a thin p-clad ridge waveguide structure, and the
characterization of ridge waveguide lasers fabricated with the newly developed etching
technique. Theoretical modeling also confirmed experimental results.
Deep etching of p-cap GaSb/p-clad AlGaAsSb layers was demonstrated using a
single step, pulsed anodization etching for the first time. A new electrolyte composed of
glycol, water, and two acids was used. An important feature of this technique is the real
time, electrical detection of layer interfaces in the diode laser structure.
Ridge waveguide lasers fabricated with this technique have demonstrated low
threshold currents at room temperature in both pulsed and continuous-wave operation
Vlll


47
3-5. The three-layer cap configuration was chosen in view of ohmic contact schemes for
surface-modulated distributed feedback (DFB) laser designs in future. The p-GaSb layer
underlying the undoped Alo.3GaAsSbo.35 layer will be served as a contact layer for the
bottom region of the gratings that are to be made on the wafer surface by etching.
GaSb
; 30 ran
caP < Al03Ga(l7 As002Sb098
100 um
l GaSb
: 30 ran
p-clad A1q 9Ga0 jASq 07Sb0 93
t ,
h
cl
d
waveguide Al03Ga07As002Sb09g
320 nm
Inn ,,GaAsSbn (2 X 20 ran)
U(')W Al03GaAsSb098 (200 ran)
240 nm
waveguide Al0 3Gao.7As0 02Sb0 9g
320 nm
n-clad Al0 9Ga0 jAs0 07Sb0 93
1.5 pm
substrate GaSb
p = 2 x 1019 cm-3
p = 5 x 1018 cnr3
n = 5 x 1017 cm'3
n = 1 x 1017 cm'3
Figure 3-5. Schematic diagram of the 2.2-pm wavelength thin p-clad RWG InGaAsSb/-
AlGaAsSb DQW laser with a three-layer-cap configuration.
47


37
Figure 2-16 shows TE gain as a function of the emission wavelength for several
carrier densities N= 1.2, 1.7,and2.2x 1018cm3. AtN= 1.2 x 1018 cm3, two gain peaks
occur at 1.84 pm and 1.69 |im corresponding to the C1-HH1 and C1-LH1 transition,
respectively. The gain peaks increase with the injected carrier density. Gain from the
C2-HH2 transition is also observed at 1.52 pm for an injected carrier density of 2.2 x
1018 cm3. However, the emission wavelengths from the C1-LH1 and C2-HH2 transitions
are unlikely to be observed because their gain values even at very high levels of carrier
densities are not high enough.
In Figure 2-17, TE gain in InxGai.xAso.o2Sbo,98 /AlyGai-yAso.o2Sbo.98 strained QW
(x = 25 % and y = 30 %) is plotted as a function of emission wavelength for different
carrier densities N= 5.5, 7.5, 9.5 x 10 cm Table 2-6 lists the parameters used in the
computation.
At low carrier densities (e.g. N= 5.5 x 1017 cm3), the maximum gain occurs at 2.2
pm corresponding to the C1-HH1 transition. As the carrier density is increased, the gain
peak increases. The second gain peak is shown at 2 pm, corresponding to the C2-HH2
transition, for N= 7.5 x 1017 cm3. When the injected carrier density is increased to N =
9.5 x 1017 cm3, the gain peaks are almost the same at 2 and 2.2 pm. This implies that a
laser with this QW structure can possibly operate at two simultaneous IR wavelengths if
the required gain threshold is appropriately adjusted by changing a cavity length or using
a different metal contact [Wu94], Such phenomenon was previously demonstrated for
InGaAs QW lasers in the near-IR regime (900 965 nm) [Wu94],
37


40
issues between the thick and thin p-clad structures are addressed. In Section 3.3, the
design of the RWG InGaAsSb QW diode laser that incorporates a thick p-clad layer is
described. Section 3.4 is devoted to the design of the RWG InGaAsSb QW diode laser
that employs a thin p-clad layer.
3.2 Design of Optical Waveguide Structure of Semiconductor Laser
3,2.1 Control of Transverse Modes
The transverse modes are dependent on the refractive indices and thicknesses of
the layers used in the semiconductor laser. The refractive index is determined by the
material composition of the layer and the operating wavelength of the semiconductor
laser. Once the refractive indices have been found, the layer thicknesses are adjusted to
control the mode distributions.
In semiconductor lasers, the optical field confinement should be great in the
active region and small elsewhere to reduce the required threshold gain. This is
important particularly for semiconductor lasers in the GaSb-based material system
because of high optical losses associated with the materials that are used as the p-cap and
p-clad layers.
In the mid-infrared wavelength regime, p-type materials used in the GaSb-based
semiconductor lasers are very lossy. At a wavelength of 2.2 pm, for example, the optical
absorption value of highly p-doped GaSb (p = 2 x 1019 cm'3) is extrapolated as much as
2000 cm'1 [Bra59], and that of p-AlGaAsSb (p = 5 x 1018 cm'3) is estimated about 70cm'1
by interpolating published values of binary GaSb [Bra59] and AlSb [Bra62], Therefore,
40