Title Page
 Table of Contents
 Electrical characterization...
 A generalized theory for determining...
 An accurate determination of dx...
 Proton and electron irradiation...
 Grown-in defects in te-doped ALxGA1-xAS...
 Defect characterization and electrical...
 Development and demonstration of...
 Conclusions and recommendation...
 Biographical sketch

Group Title: Defect characterization of GaAs/AlGaAs materials and silicon-on- insulator devices by separation-by-implant-of-oxygen
Title: Defect characterization of GaAsAlGaAs materials and silicon-on- insulator devices by separation-by-implant-of-oxygen
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00082280/00001
 Material Information
Title: Defect characterization of GaAsAlGaAs materials and silicon-on- insulator devices by separation-by-implant-of-oxygen
Physical Description: viii, 103 leaves : ill. ; 28 cm.
Language: English
Creator: Choi, Chung Gyune, 1955-
Publication Date: 1988
Subject: Gallium arsenide semiconductors -- Defects   ( lcsh )
Layer structure (Solids) -- Defects   ( lcsh )
Epitaxy   ( lcsh )
Integrated circuits -- Design and construction   ( lcsh )
Solid state electronics   ( lcsh )
Silicon   ( lcsh )
Electrical Engineering thesis Ph. D
Dissertations, Academic -- Electrical Engineering -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis (Ph. D.)--University of Florida, 1988.
Bibliography: Includes bibliographical references.
Statement of Responsibility: by Chung Gyune Choi.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00082280
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 001103097
notis - AFJ9190

Table of Contents
    Title Page
        Page i
        Page ii
        Page iii
    Table of Contents
        Page iv
        Page v
        Page vi
        Page vii
        Page viii
        Page 1
        Page 2
        Page 3
    Electrical characterization methods
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
    A generalized theory for determining the filed-enhanced thermal emission rate by reverse pulsed deep level transient spectroscopy
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    An accurate determination of dx center density and free carrier density in ALxGA1-xAS
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
    Proton and electron irradiation induced deep level defects in algaas
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
    Grown-in defects in te-doped ALxGA1-xAS grown by liquid-phase epitaxy vs growth temperature
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
    Defect characterization and electrical properties of simox based soi devices
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
    Development and demonstration of an accurate forward-bias capacitance spectroscopy
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
    Conclusions and recommendations
        Page 91
        Page 92
        Page 93
        Page 94
        Page 95
        Page 96
        Page 97
        Page 98
    Biographical sketch
        Page 99
        Page 100
Full Text











I wish to express my great gratitude to Professor Sheng S. Li for his guidance and

encouragement throughout the research and preparation of this dissertation. I would also

like to thank professors D. E. Burk, Arnost Neugroschel, Gijs Bosman, and Murab Rao for

their participation on my supervisory committee.

I am grateful to Dr. Bob Loo in Hughes Research Laboratories for his fabrication of

AlGaAs devices. Thanks are extended to my friends and colleagues, Dr. Tae-Won Jung,

Dr. Kerwin Teng, Ju Sung Park, Dr. Jong-Sik Park, Dr. Sang-I Lee, Doo Hwan Lee,

Dr. Jae-Hoon Kim, Hong Shin Chen, Jin Young Choi, Hang Geun Jeong, Young Jun Yu,

Sang Sun Lee, Soon Young Huh, and Young-Suk Kim for their helpful discussions and


I am greatly indebted to my parents, my lovely wife and son, and my younger brothers

for their patience and endless love throughout this work.


ACKNOWLEDGMENTS .............................................. iii

ABSTRACT ............................................................. vi


ONE INTRODUCTION .............................................. 1


2.1. Current-Voltage Characteristics ............................ 4
2.2. Capacitance-Voltage Measurement ......................... 5
2.3. Thermally Stimulated Capacitance Method ................ 5
2.4. Deep-Level Transient Spectroscopy ........................ 6
2.5. Forward-bias Capacitance Spectroscopy .................... 8
2.5.1. Forward-biased Schottky Diode ...................... 8
2.5.2. Dominant Exchanges with Semiconductor Conduction
Band ................................................ 9
2.5.3. Exchanges with Conduction and Valence Bands ....... 10
2.5.4. Exchanges with Conduction Bands of Semiconductor
and Metal .............................................. 12

SPECTROSCOPY ............................................. 13

3.1. Introduction ................... ........ .... ..... .. .. .......... 13
3.2. Development of A Generalized Theory ..................... 14
3.3. Determination of Temperature-dependent Capture Cross
Section ......................... ................. .............. 18
3.4. Summary ....... ................................... 19

4.1. Introduction ....................... ....................... 22
4.2. Theory ........ ...................................... 23
4.3. Summary .................. ................................. 30

LEVEL DEFECTS IN ALGAAS ............................... 32

5.1. Introduction ................. ............................. 32
5.2. Theorectical Review of Defect Creation by Partical
Irradiation .......................... ............ .......... 33
5.2.1. Dynamics of Collision ............................... 33
5.2.2. Differential Scattering Cross Section ................ 36
5.2.3. Primary Displacements ............................. 36
5.2.4. Secondary Displacements ........................... 37
5.2.5. Formation of Amorphous Layer by Irradiation ......... 38
5.2.6. Range of Particle .................................... 39
5.3. Results and Discussion ................................... 40

....................... .. .. ..... .................... 50

6.1. Introduction ............................................. 50
6.2. Experimental ............................................. 51
6.3. Results ............................................. 51
6.4. Conclusions .................................... ........ 56

OF SIMOX BASED SOI DEVICES .......................... 59

7.1. Introduction .... ....................................... 59
7.2. Fabrication Process ........................................ 60
7.3. Oxygen Related Defects in Simox Based SOI Devices ....... 60
7.4. Device Characteristics versus Process Parameters ........... 74
7.4.1. Characterization Methods of SIMOX SOI Device ...... 74
7.4.2. Epilayer Thickness .................................. 75
7.4.3. Annealing Time .................................... 75
7.4.4. Lateral Isolation .................................... 76
7.4.5. Ion Dose and Annealing Temperature ............... 76
7.4.6. Defects and Electrical Properties of P-well ............ 77
7.4.7. N-epilayer/p-well Interface .......................... 77
7.5. Summary and Conclusions ................................. 77


8.1. Introduction ................ .. .. ........................ . 83
8.2. Accurate Forward-bias Capacitance Spectroscopy ........... 84
8.3. Results and Discussion ................................... 88


APPENDIX ........................................................ .... 94

REFERENCES ......................................................... 95

BIOGRAPHICAL SKETCH ............................................ 99

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



Chung Gyune Choi

April 1988

Chairman: Sheng S. Li
Major Department: Electrical Engineering

This dissertation describes the defect characterization of GaAs/AlGaAs epitaxial lay-

ers and Silicon-On-Insulator (SOI) devices by Separation- by-Implant-of-Oxygen technique.

A novel experimental theory was developed for this study. A generalized theory for de-

termining the field-enhanced thermal emission rates from the deep level defects using the

Reverse-pulsed Deep-Level Transient Spectroscopy (RDLTS) has a potential as a powerful

characterization method for the decision on the potential-well types of deep-level defect

states. This method enables us to measure the field-enhanced thermal emission rates at

very high electric-fields. The previous methods cannot give those experimental data.

An experimental theory for determining the temperature-dependent capture cross sec-

tion was developed as the by-product of a generalized theory. This method is simpler and

easier than the previous experimental methods.

An accurate determination method for the DX center and shallow center densities was

developed. This method utilizes the thermally stimulated capacitance (TSCAP) technique.

Under the carrier freeze-out condition the Deep-Level Transient Spectroscopy measurement

cannot give the exact DX center density.

To make progress in the area of defect characterization, a generalized theory for de-

termining the field-enhanced thermal emission rates must be applied. This method makes

use of the fast part of capturing transient and hence the DLTS system must have the fast

circuitry. The computerized DLTS system with the fast circuit option is commercially avail-

able. If this fast DLTS system utilizing a generalized theory is applied, it gives a great deal

of flexibility in defect characterization.

An accurate forward-bias capacitance spectroscopy was developed for the study of the

Schottky interface state. This method is very accurate and efficient compared the previous

methods. An accurate forward-bias capacitance spectroscopy is implemented and is fully

demonstrated by measuring the real Schottky diode of small area. This method makes a

great breakthrough in the area of the metal-semiconductor interface state study.


The GaAs and its related compound semiconductor materials have shown promise in

the applications of faster and denser integrated circuit designs. The epitaxial growth tech-

nologies such as molecular beam epitaxy (MBE), metal-organic vapor epitaxy (MOCVD)

and liquid phase epitaxy (LPE) have become the major fabrication methods for the GaAs

and its related compound semiconductor devices. The device and materials characterization

must be carried out for quality control.

Defect plays the dominant role in determining the overall device performances. The

grown-in defects should be controlled below the tolerable levels. The process-related defects

should be traced back to their origins in order to develop a new process step of evading

those defect levels. Nowadays the fabricated devices are required to have the radiation-hard

properties because the devices might operate in the radiation environment such as in the

communication satellite. The irradiation-induced failure mechanisms in the GaAs integrated

circuits should be investigated as part of the process development for the radiation-hard

devices. Intensive study on the proton- or electron- induced defects in GaAs has been

carried out. However, very few experimental data concerning the radiation-induced defects

in the AlGaAs layers are available.

Many theoretical papers about the potential-well types have been published because

the decision on the potential-well type of a defect helps us to find out the defect origin. In

order to determine the potential-well type of a defect one must measure the field-enhanced

thermal emission rates versus electric field. The electric field range should be from very low

to very high. However, the published data were limited to rather low electric field. This

was caused by the absence of adequate experimental theories and techniques. A generalized

theory for determining the field-enhanced thermal emission rate has been developed by

the author. Using this theory, the data of very high electric field is, for the first time,

successfully obtained. This generalized theory enables us to determine the potenital-well

type of defect.

The determination method for temperature-dependent capture cross section is devel-

oped as the by-product of the generalized theory. The activation energy of capture cross

section can be determined in a much easier and simpler way than the previous methods.

The measured capture cross section can be used for determining the field-enhanced thermal

emission rate self-consistently.

An accurate determination method for the DX center and shallow donor densities,

especially in AlxGai-xAs (x > 0.3), is developed. In addition, an accurate determination of

free carrier concentration is performed by considering the Hall electron concentration and

the ratio of Hall to drift mobility. The previous method usually underestimates the shallow

donor density in the AlxGai-xAs of Aluminum composition over 30 %.

The epitaxial growth of GaAs on silicon substrate has become the important research

subject. The excellent devices with GaAs/AlGaAs epitaxial structures grown on Si have

been fabricated [1-7]. The microwave performance of GaAs metal-semiconductor field-

effect transistors on Si has been shown to be as good as MESFET's on GaAs substrate.

The GaAs integrated circuit on Si substrate is considered as a cost-effective substitute for

GaAs bulk grown wafers, especially for digital or low-frequency microwave applications.

Silicon is a much better thermal conductor than GaAs, and thus GaAs on Si can be applied

to the various applications such as power FETs. As mentioned earlier, one of the failure

mechanisms in GaAs digital integrated circuits in the radiant environment is the conversion

of semi-insulating substrate into semi-conducting layer. In order to prevent this problem,

GaAs on SOI (silicon on insulator) substrate is proposed as the future device stucture.

The presence of the oxide layer nullifies the necessity of the semi-insulating layer and it is

possible to fabricate the less thick epitaxial layer because the buffer layer is probably not

necessary in this new device structure. It also improves the device switching speed because

the parasitic capacitance reduces. The SOI substrate structure can be easily fabricated by

the SIMOX (Separation by Implant of Oxygen) technique. The SIMOX-based SOI devices

were fabricated and the electrical characterization was performed by using the Deep Level

Transient Spectroscopy, current-voltage, and capacitance-voltage methods. The process-

related defect levels such as divacancy plus oxygen complex were observed in the epilayer

grown on the superficial layer which went through high temperature annealing steps.

An accurate forward-bias capacitance spectroscopy was developed for the study of the

Schottky interface state study. This method is very accurate and efficient compared to

the previous methods. They had the difficulty to measure the forward-bias capacitance

under the high current flow condition. An accurate forward-bias capacitance spectroscopy

is implemented by the author and if fully demonstrated by measuring the real Schottky

diode of small area. This method makes a great breakthrough in the area of the metal-

semiconductor interface state study.


2.1. Current-Voltage Characteristics

Current-voltage characteristics of p-n junction under forward-bias condition is given


Jt = Jp(x.) + J(-xp) (2.1)

= qn( D + )(ea/kT 1) (2.2)
NdLp NaLn
= Jo(e v/kT 1) (2.3)

where Jp(xn) is the minority carrier (hole) current in the n-region and Jn(xp) is the minority

carrier (electron) current in the p-region, respectively; xn and Xp are the depletion layer

edges in the n-region and in the p-region, respectively; Jo is the saturation current density

and Va is the applied forward-bias voltage; Dn(Dp) is the diffusion constant for electrons

(holes); and Ln (Lp) is the diffusion length for electrons (holes), and Na(Nd) is the shallow

acceptor (donor) density. If the recombination current component is considered in the

space-charge region, the recombination current Jr can be expressed by

qxdni Va
Jr = exp( V (2.4)
2r. 2kT
where ro is the lifetime associated with the recombination of excess carriers in a region with

a density of Nt of recombination centers. The lifetime 7o is expressed by

To = N- (2.5)
The total forward current can be approximated by the sum of Eqs. (2.1) and (2.2) for Va

> kT/q:

F qDpn qVa qxdni / Va
JF = q dexp(-) + exp( (2.6)
F7p Nd kT 2ro 2kT

This total forward current can be represented by the empirical form,

JF = Joexp(qVa) (2.7)

The factor n is equal to 2 if the recombination current dominates and n is equal to 1 if the

diffusion current dominates. When both current components are comparable, the ideality

factor n has a value between 1 and 2.

2.2. Capacitance-Voltage measurement

The depletion layer capacitance is defined as C = dQ/dV, where dQ is incremental

charge increase according to an incremental change of the applied voltage dV. For the

one-sided abrupt junction or Schottky barrier diode, the capacitance is given by

dQ As\
C A (2.8)
dV W
Sb(Vb V 2kT)-1/2 (2.9)
= 2 q

where c, is the dielectric constant of semiconductor; Vbi is the built-in potential and V is

the applied voltage where signs are for the reverse- and forward-bias conditions, respec-

tively. NB is the background doping concentration. One can deduce the background doping

concentration from Eq. (2.6) and using the relationship,

d(/C2) 2 dC10)
= -( (2.10)
dV C3 dV

so that,

N(xd) 2 (2.11)
N(X) = q,[d(1/C2)/dV]A2 (2

2.3. Thermally Stimulated Capacitance Method

The TSCAP measurement is carried out by first cooling the junction device to 77 K and

then zero biasing it to fill the majority carrier traps. The junction is again reverse-biased

and then temperature is raised from 77 K to 400 K [8]. The thermal scan of capacitance

versus temperature plot is then taken by using an x-y recorder. A capacitance step is

proportional to the trap density. The trap density can be deduced by the relationship

Nt = Nd (2.12)

where Co is the depletion layer capacitance and AC is the capacitance change due to the

majority or minority carrier emission. Eq. (2.12) is valid only for the case when Nt is much

less than Nd. For the case of large trap density, a more exact expression should be used [9].

C2 A2qEOEs NT (2.13)
2 1 2(VD + VR)

This expression will be explained in detail in Chapter Four.

2.4. Deep-Level Transient Spectroscopy

The Deep Level Transient Spectroscopy (DLTS) is a high frequency (20 MHz) transient

capacitance technique which is first introduced by Lang in 1974 [10,11]. The DLTS scan

displays the spectra of deep level traps in the forbidden gap of a semiconductor as positive

or negative peaks depending on minority trap or majority trap. From this measurement it is

possible to measure the following parameters such as trap density, activation energy, defect

concentration profiling, electron and hole capture cross sections. Though the capacitance

transient were made use of in the conventional DLTS, the modified DLTS methods such

as current DLTS, optical DLTS, and RDLTS (Reverse-pulsed DLTS) have been developed

according to their specific needs subject to the test device structures and characteristics

[12-16]. For simplicity, the theory of conventional DLTS will be explained briefly. The

transient capacitance can be expressed as

C(t) = Co[1 -exp(-t/r)]1/2 (2.14)

where Co = C(Vr) is the depletion layer capacitance and r is decay time constant. This

time constant will be shown to be the inverse of emission rate as follows. Eq. (2.9) reduces

to a simpler form by using binomial expansion if the defect density is less than one tenth

of background doping, Nd

C(t) = Co[1 Texp(-t/r)] (2.15)
2 Nd

Here, Eq. (2.10) can be rewritten as

Ntexp(-t/r)= C(t)Nd (2.16)

where AC(t) = Co C(t). From the DLTS measurement, AC(0) can be obtained. The

junction capacitance Co and the background doping concentration, Nd, can be deduced

from the C-V measurement. Thus, the defect density Nt, can be calculated by using Eq.

(2.11). Eq. (2.11) can be rewritten as

C(t) = AC(0)exp(-t/r) (2.17)

Then, the capacitance difference can be defined by using two different time points, tl and


S(r) = C(0)[exp(-ti/r) exp(-t2/r)] (2.18)

The maximum or minimum value of S(r) is obtained by differentiating S(r) with respect to

r, i.e. dS(r)/dr = 0, which yields

(tl t2)
S= max ( (2.19)

This means that S(r) reaches to a maximum or minimum value at a given window rate

(i.e. rmax) and this peak point corresponds to a temperature point in the DLTS spectrum

which is a function of S(r) vs temperature. This temperature corresponds to an emission

rate which is the function of temperature. The emission rate is given by

en = Tma.x
=, Tma


The emission rate can be expressed by

Ec Et
en = (an < Vth > Nc/g)exp( kT ) (2.21)

where Et is the trap activation energy; g is the degeneracy factor; an is the electron capture

cross section which is temperature- dependent as [17]

n = axp(- (2.22)

where aoo is the capture cross section at very high temperature; AEb is the activation

energy of the capture cross section. The emission rate of Eq. (2.16) can be rewritten as

Ec (Et + AEb)
en = BT2exp (2.23)

Arrhenius plot of ln(en/T2) versus 1/kT can be carried out from several DLTS spectra. The

slope of this plot is the apparent activation energy (i.e., Et + AEt).

2.5. Forward-bias Capacitance Spectroscopy

2.5.1. Forward-biased Schottky diode

In a forward-bias Schottky diode, the charge of the interface states can be electrically

modulated. The modulation of charge state variation with frequency, bias, and temperature

appears as the capacitance and conductance change [18,19]. It is generally accepted that the

quasi-Fermi level of majority carriers is flat throughout the space-charge region of a forward-

biased Schottky junction. The position of the quasi-Fermi level at the interface varies with

bias [20]. On the other hand, in reverse bias, the quasi- Fermi level of majority carriers

remains pinned on that of metal. In forward-bias condition, the relationship between the

quasi-Fermi level and that of metal at the interface is given by

EFNS EFM = qV (2.24)

and the free carrier density at the interface is expressed by

n, = Ncexp(- kT (2.25)

In a forward-bias condition, the conductance due to the thermionic current Gsc = qI/kT

is very high and entirely obliterates Gi, due to the interface charge variation [18,20]. On

the other hand, Cc, ( capacitance due to charge modulation on the edge of the space-charge

area) varies only slowly with bias and Cis (due to the interface charge variation) exceeds

Cos to a large amount. If a small signal v = vo exp(iwt) with Vo << kT/q, is superimposed

on the direct bias voltage V, the admittance due to the state density Nis can be expressed

by [19]

Yis = Gis + iwCis (2.26)


Cis= q 2 N j(1 F)cnns (2.27)
1 + W272 kT

T-1 = Cnn, + Cnnl + Cpps + Cppl + rT1 (2.28)

The cut-off frequency will be

fc = (2.29)

Now we can derive the important parameters such as the interface state density activation

energy and capture cross section using the above equations in low frequency domain and at

constant temperature.

2.5.2. Dominant exchanges with the semiconductor conduction band

(cnni >> Cpps, rT).

The occupancy F and carrier exchange time constant can be simplified as

F= n (2.30)
n, + ni

r 1 = Cnn, + cnni


If q4 qV, = Es(V = V,) holds, then the electron quasi-Fermi level EFNS merges with the

interface states E.. The interface capacitance will be expressed as

Ciso = F(1 F) (2.32)

The maximum peak condition of dCio/dF = 0 gives F = 0.5. At F = 0.5, Ciso has a peak

value. The activation energy E, of interface state can be determined as shown in Fig. 2.1.

The cut-off frequency is given by

Scn(n + n) (2.33)
fe= (2.33)

Therefore, the cut-off frequency is proportional to Cnni (if V < Vs) and is proportional to

Cnns (if V > Vs). If the interface state is a continuous band states, then the spectrum of

Ciso will be broad.

2.5.3. Exchanges with the Conduction and Valence Bands

In this case, the occupancy F and the carrier exchange time constant can be simplified


F cnns (2.34)
cnn, + Cpps

r-1 = cnns + Cpps (2.35)

Likewise as in the first case, at F = 0.5 the spectrum of Ciso has a maximum value located

at the demarcation energy level Edn is defined by

Ec Edn = cBp kln(cN) (2.36)

The behaviour of a band state depends on its relative location with respect to the demar-

cation level Ed, If E,2 > Es > Ed, the direct observation is possible. If Edn > Es2 > Esl,

the apparent spectrum is centered on Edn. If Es2 > Edn > Esl, the apparent spectrum is

truncated and is reduced to the range from Ed, to Es2.



Figure 2.1.

E (qVF)

Esl Es2

Interface capacitance spectrum with the position of the quasi-Fermi
level of majority carriers at the interface: (a) Discrete interface state
and (b) Band of interface state.

E (qVF)

2.5.4. Exchanges with the Conduction Bands of Semiconductor and Metal

F = c(2.37)
Cnns + TT

r-1 = cnn, + rf (2.38)

Ec Edn = kTln(cnNcrr) (2.39)

The peak of Ciso spectrum occurs at Edm. The actual activation energy of state can be

obtained from the slope of log fc vs E curves. The re-emission of electrons to metal is

dominant if the Ciso peak moves with temperature, whereas the re-emission of electrons

to the conduction band of the semiconductor is dominant if Ciso is independent of the



3.1. Introduction

A generalized theory for determining the field-enhanced thermal emission rates and

carrier capture cross section of deep level defects at very high field and for large trap

density by the reverse pulsed deep level transient spectroscopy technique is developed in

this chapter. Using this new theory the field-enhanced emission rates for the DX center in

a liquid-phase epitaxy grown Sn-doped Alo.2Gao.sAs were determined for field strength of

up to 7 x 105 V/cm.

In this study we report the derivation of a generalized theory for determining the field-

enhanced thermal emission rates at very high field and for arbitrary ratios of NT/ND using

the RDLTS technique. It is well-known that determination of potential well for a deep level

defect is useful for identifying its physical origin. The theories for determine the types of

potential well for the deep level traps in a semiconductor from the field-enhanced emission

rate have been published in the literature [21-25]. However, the experimental data on the

field-enhanced emission rates reported so far have been limited to rather low field due to the

lack of adequate theory for describing the field-enhanced rates at very high field and large

trap density. As a result, the field-enhanced emission rate data for field strength greater

than 2 x 105 V/cm have not been reported in the literature.

Recently Li and Wang developed the RDLTS technique assuming that the transient

capacitance signal comes mainly from the narrow junction space-charge region at the in-

tersection of ET = EF, and its amplitude is proportional to the density of deep-level trap

[26]. However, as will be explained later in the text, the RDLTS signal is not a simple

exponential transient; instead it composes the fast and slow capacitance transients. In fact,

the capturing process occurs in a broader region from the intersection of ET = EF to the

depletion-layer edge. In addition, in the case where the defect density is comparable to the

shallow dopant density, the relationship, NT/ND = 2 AC/Co, is no longer valid. Therefore,

a rigorous theorectical model is needed in order to obtain the data of field-enhanced emis-

sion rates at high fields and arbitrary trap density from the RDLTS technique. A general

expression for the charge state of a deep-level trap under the transient condition will be

derived next. From this equation the field-enhanced emission rate at very high field and for

arbitrary trap density can be determined.

3.2. Development of A Generalized Theory

For the convenience of discussion, a Schottky barrier diode will be used for deriving

the RDLTS theory. Figure 3.1 shows the energy band diagram of a Schottky diode in

thermal equilibrium and under reverse-bias conditions in the RDLTS measurement. W1 is

the depletion width under steady-state condition, and W1x is the distance from x = 0 to

the point where EF coincides with ET; Wi(t) and Wlx(t) are the corresponding parameters

under transient condition. If the defect density is comparable to the shallow donor density,

then the initial depletion width W1(0) in the transient state will be drastically reduced from

W1 due to the increase of ionized traps during the reverse-biased pulse. W2 and W2x are

the depletion width and the distance from x = 0 to the point where ET = EF, respectively,

when a reverse-biased pulse of V2 is applied. During this reverse-biased pulse of width tp,

the occupied traps emit electrons to the conduction band and unoccupied traps captrue

electrons from the conduction band. Following the reverse-biased pulse of duration tp, the

applied voltage is returned to the quiescent voltage of V1 and the transient state follows.

The charge state of the trap in the transient state can be expressed by [27];

= -r1N + en(NT N+) (3.1)

o ~0 0

L- w
I I-


0 W2x I

S* *0 *

(b) i o

1 0 E

-I Ev
*i O -- ,,
0I o0 E c0

S 14---dW W1 W( Ec

- EF

I Ev

-wP( I
1w 1x M



Figure 3.1.

Energy band diagram of a Schottky barrier diode under different
reverse-biased pulse conditions: (a) at reverse-bias voltage Vi under
steady-state conditions, (b) at V2 during the reverse-biased pulse tp,
and (c) at V1 following the reverse-biased pulse and in the transient




t t


d + (7-r + e^)N+ = enNT, (3.2)


r,- = anVthn(x), (3.3)

n(x) = noexp-[Wl(t)- x]2/2LD, (3.4)

en = UnVthnoexp[(ET EF)/kT], (3.5)

and LD is the extrinsic Debye length.

In Eq. (3.1), since both rT-' and W1(t) vary with time, NT+(x,t) cannot be expressed

by a simple exponential function. Instead, r-'(x,t) is a fast varying function of position and

time when the defect density is comparable to the shallow donor density and the depletion-

layer edge changes rapidly. Thus, by multiplying Eq. (3.2) by I(x,t), one obtains

d Ixt)] = I(x,t)enNT, (3.6)


I(x,t) = exp (r-1 + en)dr. (3.7)

Solving Eq. (3.6) yields

N+(, I(x,t) 1 I(x, r)enNTdr + I(,t (3.8)
N I(x, t) oxt) (3.8)

Eq. (3.8) is an exact expression for the ionized deep-level defect density. Note that the value

of C1 in Eq. (3.8) can be determined from the initial conditions as follows. In the region

[W1x(O) < W1x], the defects remain totally ionized since ET is greater than EF during the
reverse-biased pulse period tp.

N+(x,O) = NT for W1x(O) < x < Wlx.


In the region (Wx < x < Wx,), the deep-level defects experience the field-enhanced thermal

emission during the reverse-biased pulse of pulse width tp,

N+(x,0) = NT- 1 exp(-entp)] for W1x < x < W1 (3.10)

From Eqs. (3.8), (3.9) and (3.10) one can obtain the expressions

N+(x,t) _x) 1 ft d NT (3.11)
I(x,t) (xr)enN I(x,t)

for W1x(0) < x < Wx, and

N(x, t- I(x,r)enNTdr + NT (3.12)
NI(x,t) oI(x,t)

for W1, < x < W1.

In Eqs. (3.11) and (3.12), the steady-state condition (i.e., t = oo) is also satisfied, and

N (x,)= en NT. (3.13)
T (r'-1 + en)

Eq. (3.13) can also be derived by setting dN+/dt = 0 in Eq.(3.1). Thus, it is clear that

Eqs. (3.11) and (3.12) are the exact expressions for the density of the empty state defect

in the transient as well as steady- state conditions.

To derive a generalized expression for determining the field-enhanced emission rates by

the RDLTS measruement at very high field and for arbitrary trap density, one can start

from the Poisson equation by using Eqs. (3.11) and (3.12). This is given by

C W1. (0)
q(V Vb) (ND + NT)xdx
/ix 1 t t NT
+ (ND + I(x, r)enNTdT + )xdx
w1. (0) I(x, t) o I(x, t)
w'ix(t) 1 t NT[1 exp(-entp)]
+ (ND + x t I(x, )eNTd + xt) )xdx (3.14)
Wix I(x,t) J I(x,t)

In Eq. (3.14), en and En are different in that en denotes the thermal emission rate at low

field and in the transient state conditions, and En is the field-enhanced thermal emission rate

during the reverse-biased pulse. Since the region [W1 < x < W2] can be neutralized within

the dielectric relaxation time, tD, this region is not considered in Eq. (3.14) assuming that

tD/Tc = EanVn/q/Pn is much less than unity [28].
Equation (3.14) can be differentiated with respect to time by using the Reibniz rule,

Op u2 Of du2 dul
Ot U dx + f(u2, t) f(ux, t)- (3.15)


p(t) = f(x, t)dx, (3.16)

and the result is given by

a -
( (V + Vbi)) = 0
Ot q
wx(t) (r-'1+ en) t NT(T + en)
7c I(x, r)enNT d + enNT )xdx
JW.(o) I(x, t) o II(x, t)
S1 fo NT[1 exp(-entp)] dW1
+ (ND + I(xtf I(x, r)enNTdr + -(t) )W(t)
I(xt) I(xt) dt
+ W NT(r -1 + en) --exp(-entp)xdx. (3.17)
JWix I(x, t)
For t = 0, Eq. (3.17) reduces to

fr (0) NTrc-lxdx (ND + NT)WI(0)(W )lt=o
exp(-entp)= WI.() dt (3.18)
ww f (0) NT(r,1- + en) x dx NTWI(0)(dIt )lt=o
In Eq. (3.18), W1(0), W1x(0), W1(0), and dWi/dt can be measured and f r-lxdx can
be solved analytically as shown in the Appendix. Therefore, by using the relationships,

Wi(0) = cA/Ci(0) and Wi(t)/dt = -(cA)2/C1 3dC1/dt, one can obtain the plot of en
versus electric field. The corresponding electric field with en can be determined by using

the depletion approximation with the reverse-biased voltage V2.

3.3. Determination of Temperature-dependent Capture Cross Section

If entp approaches infinity, then exp(-entp) goes to zero. This can be achieved under
a very large reverse-biased pulse condition for a long duration pulse. This would allow the
traps to be totally ionized, and hence the transient capacitance signal can reach saturation.

From Eq. (3.18) and using the relation that exp(-entp) = 0, the following expression is


/Wi(0) ND dW1
,(o) r dx = ( + 1)WI(0)--t=o, (3.19)
JWix(o) NT dt

1 [Wi x]2
T, avthnoexp ]2 (3.20)


1 w wl(o) [(W(t) x]
a [ (0) exp( t) )xdx]- (3.21)
vthno JW1ix() 2L
ND dW1
X (T + 1)W1(0) d t=o.
NT dt

Here one can measure the capture cross section at a fixed temperature by using Eq. (3.21).

From measurements of the capture cross sections at various temperatures one can plot

a versus 1/kT. In this plot, both aoo and EB can be determined by using the following

expression for the temperature-dependent capture cross section [29].

a = aoexp(- ). (3.22)

If this capture cross section is applied to Eq. (3.18), then the field-enhanced thermal

emission rate at any temperature can be determined self-consistently.

3.4. Summary

We have performed the RDLTS measurement on the LPE-grown Sn-doped Alo.2Gao.sAs

sample, and the field-enhanced emission rates for the DX center have been determined

from the theory presented in this text, as shown in Fig. 3.2. In the experiment the fast

capacitance transient was observed in the 1 p, second time frame [(r,-1 = (avthno)-1 = 3.42

x 10-6 s]. The observed value of capture cross section, a is in good agreement with the

value reported by Bhattacharya et. al [30]. From the C-V and DLTS measurements ND =

2.92 x 1017 cm-3 and NT = 3.10 X 1016 cm-3 were also determined for this sample.


A generalized theory for determining the field-enhanced thermal emission rate at very

high electric field and for arbitrary NT/ND ratio by the RDLTS technique has been derived

in this chapter. The theorectical expressions for the field-enhanced emission rates were

derived by using the fast capacitance transient and its slope in the RDLTS measurement. In

addition, it is shown that the capture cross section can be determined by using the saturated

capacitance transient which is obtained by applying a high reverse-biased saturation pulse.

The presented theory is applied to determine the field- enhanced thermal emission rates for

the DX center in the LPE-grown Alo.2Gao.sAs sample.

I I I I I I -

Sn- doped

T=100 K


, *'






1 2 3 4 5 6 7 8 9




The field-enhanced thermal emission rates versus electric field for
the Ec 0.20 eV (i.e., DX center) in the LPE-grown Alo.2Gao.sAs
as determined by the RDLTS measurement at 100 K.



Figure 3.2.



4.1. Introduction

Accurate determination of the densities of DX center and free carriers in AlxGalxAs

(for x > 0.3) grown by liquid-phase epitaxy (LPE) has been made by using the deep

level transient spectroscopy (DLTS), thermally-stimulated capacitance (TSCAP), constant

temperature capacitance-voltage (C-V) and Hall-effect measurements. An anomalously high

density of DX center is determined by the TSCAP and low-temperature C-V techniques

for AlxGal-xAs under the condition that majority of carrier freeze-out occurs at 77 K. The

apparent carrier density determined by the C-V method at room temperature is found to

be equal to the sum of the DX center density and the shallow donor density.

It is well known that the dominant deep-electron traps known as the DX center plays

a major role in controlling the electrical characteristics of AlxGal-xAs alloy system for x

> 0.2. Caswell et al. [31] showed recently that the ionized donors were not observable

below 150 K in the MBE grown Si-doped Alo.35Gao.65As. A significant decrease of the Hall

mobility and free carrier concentration as well as a marked carrier freeze-out are observed

if the Al content is increased beyond 0.25 [32,33]. Since the overall concentration of deep

electron traps is greatly enhanced near the direct-to-indirect bandgap cross-over point [34],

it is important to study the effect of deep electron traps of n-type AlxGal-xAs with x range

between 0.30 and 0.50. Mizuta et al. [35] and Bhattacharya et al. [36] showed that the DX

center is a substitutional deep-donor state in which its activation energy is associated with

the L conduction band minima; Kunzel et al. [37] pointed out that for x = 0.35 the DX

center governs the electrical properties of AlxGai-xAs. Nevertheless, for x > 0.3, the density

of DX center may exceed the background density and the Hall electron concentration should

include the contribution of three different valleys (i.e., F, L, and X bands). Therefore, an

accurate determination of the DX center density and the free carrier concentration becomes

extremely difficult for x greater than 0.30.

4.2. Theory

In this dissertation, an accurate method of determining the densities of DX centers

in LPE-grown Te-doped and Sn-doped AlxGal-xAs (0.2 < x < 0.4) using the combined

DLTS, TSCAP and constant temperature C-V method is presented. In addition, an accurate

determination of the free carrier concentration is performed by considering the Hall electron

concentration and the Hall to drift mobility ratio due the three conduction valleys. It

should be noted that the density of DX center increases dramatically as the aluminum

mole fraction ratio x is greater than 0.3. Our low-temperature C-V measurements revealed

that a complete carrier freeze-out occurred at 77 K for the Sn-doped Al0.33Gao.67As and

Al0.4Gao.6As samples as shown in Fig. 4.1 and Fig. 4.2. This result does not necessarily

mean that the shallow donor centers are not available at 77 K. Still, a good portion of

the shallow donor centers can exist. This can be explained in terms of the deepening of

the shallow donor activation energy. This will be shown later in this paper. As for the

room-temperature C-V measurement, the junction capacitance is contributed both from

the shallow and the deep-donor centers because the electron capture and emission at the

DX center can follow the 1 MHz small signal at room temperature. However, in the neutral

region the deep donors are mostly occupied since the Fermi-level is located close to but

slightly above the DX center level.

In the case that the concentration of DX center is very large, the simple relation NT =

2[AC(0)/C]ND used in the conventional DLTS analysis is no longer valid. Thus, for high

Al composition, it is impossible to determine the exact density of DX center by the DLTS

measurement due to the carrier freeze-out at low temperature. To provide an accurate

30 -r-PuJJ.1 LL -I10"33 T0"67.Mb

U 20

I '^300 K

10 160 K

120 K

77 K

0 2 4 6 8 10


Figure 4.1. The constant-temperature capacitance-voltage measurements of
Alo.33Gao.67As and the complete freeze-out at 77 K.

2 4 6 8


The constant-temperature capacitance-voltage measurements of
Alo.4Gao.6As and the complete freeze-out at 77 K.

Figure 4.2.

determination of the density of DX center, the TSCAP method can be used and a new

equation has been derived, which is given by [38,39]

c2 c2 A2qCo's
A2qOES NT (4.1)
Scl 2(VD + VR)

where C1 and C2 are the values of capacitance taken from the capacitance step observed in

the TSCAP scan as shown in Fig. 4.3. The TSCAP curve clearly shows two capacitance

steps for the Alo.33Gao.67As sample, and the locations of capacitance steps coincide well

with those DLTS peaks for the DX centers shown in Fig. 4.4.

The free electron density for the Te-doped Alo.38Ga0.62As determined by the C-V

method is 1.54 x 1017 cm-3 which is much higher than the electron density nh of 1.5

x 1016 cm-3 determined by the Hall effect measurement at room temperature. The reasons

for this discrepancy can be attributed to : (1) The DX center, whose density is 1.41 x 1017

cm-3 as determined from our new method, plays a major role, and (2) the Hall electron den-

sity should be corrected by the Hall to drift mobility ratio including muliti-conduction band

structure. The Hall electron density and the total free electron density can be expressed

respectively by [40,41]

nr[1 + + X n L1-]2
nh = n[1 n+nr rur )2 (4.2)
h JX+ PCX(J1)2 4+ n.L L/)2J
nr -r 'r nr nr r

nx nL Ph
nt = nr + nx + nL = nr[1 + + -] = nh (4.3)
nr nr Pd
where Pr, px and PL are the mobilities in r, X, L conduction band minima, respectively.

Assuming that the Boltzman statistics is valid for the present study, the following relation-

ships also hold

nr = Nce kT = (2 eT )3 (4.4)

m* = 0.067 + 0.083x (4.5)
nL,x (mxErx ,r
n( X)2 exp(- (4.6)
nr mr kT






The thermally-stimulated capacitance of the Sn-doped A10.33 Gao.67As
with two capcitance steps which means two DX centers.




77 100

Figure 4.3.





The DLTS spectra of the Sn-doped Alo.33Gao.67As with two
DX centers.

77 100

Figure 4.4.


nr + nx + nL + X = NSD + NDX NA (4.7)
1 + e kT ,

where EFL and Erx are the F-X and F-L intervalley separation. mdX,L is the density of state

effective mass in the X and L bands. The computer simulation for the ratio of Hall to drift

mobility can be executed by taking into account of all the possible scattering mechanisms in

this alloy system using the published physical parameters [40,41]. The calculated value of nt

at 300 K in the Te-doped Alo.3sGao.62As is 2.2 x 1016 cm-3 based on Eq.(4.3). In Eq.(4.7),

the right-hand side term can be determined from the room-temperature C-V measurement.

Also, the DX center density NDX can be accurately determined from Eq.(4.1). Thus, the

shallow donor density can be determined from the difference between the room-temperature

C-V measurement and the DX center density determined by Eq.(4.1). The measured shallow

donor density is 1.3 x 1016 cm-3. This means that both the shallow donor centers and the

DX centers comparably supply the free electrons to the three conduction bands at room

temperature. Thus, the widespread understanding that the free electron densities at the

multi-conduction bands are supplied by the DX centers, is probably incorrect. The location

of the Fermi-level can be determined from Eq. (4.7). The Fermi-level is located close to

but slightly over the DX center level at room temperature. The results are summarized in

Table 4.1.

In addition, the validity of using C-V method for determining the shallow donor density

and the DX center density must be reviewed. This is discussed as follows. The C-V data

at 300 K gives the sum of the shallow and DX center densities, and the 77 K C-V data

gives the shallow donor density. The difference between them is equal to the DX center

density. This method is based on the assumption that the Fermi-level is located below the

shallow donor level at 77 K and the shallow donor centers are completely ionized at 77 K.

However, this assumption is incorrect in the AlxGalxAs for x > 0.3 in which the shallow

donor energy becomes deeper based on the hydrogenic model. Using Eqs. (4.4) and (4.5),

the Fermi-level location can be determined at 77 K. Since the shallow donors are associated

with the r band and other deep donors are totally frozen at 77 K, the relationship nt =

nr holds. If it is assumed that the shallow donor density is in the range of 1014 to 1016

cm-3, then the corresponding location of the Fermi-level, which is calculated by Eq.(4.4),

will be in the range of 0.0107 eV < Ec EF < 0.0413 eV. Based on the published data,

the activation energy of the shallow donor center reaches 17 meV at x = 0.31 [42]. This

means that at 77 K the Fermi-level is located above or close to the shallow donor energy

and the shallow donor centers cannot be completely ionized in the AlxGai_xAs system for

x > 0.3. Therefore, the C-V measurement at 77 K probably underestimates the shallow

donor density, and overestimates the DX center density.

4.3. Summary

The electrical characterization of the Sn- and Te-doped AlxGa_-xAs (0.3 < x < 0.4)

grown by LPE technique has been carried out using the combined DLTS, TSCAP, constant

temperature C-V and Hall effect measurements. The combined TSCAP and C-V measure-

ments enable the accurate determination of the densities of DX center and shallow donors

in AlxGal-xAs for x > 0.3. For x > 0.3, the dominant carrier freeze-out occurs and accord-

ingly the accurate determination of DX center density becomes impossible by the DLTS

measurement. The ratio of Hall to drift mobility due to multivalley conduction should be

incorporated in the calculation of total electron concentration at room temperature. This

ratio is multiplied to the Hall electron concentration determined by the Hall-effect measure-

ment, and the total electron concentration is then determined. Both the shallow donors

and the DX center comparably supply the free electrons to the multi- conduction bands of

the AlxGal-xAs at room temperature.

Table 4.1. Defect parameters, shallow donor and free carrier densities
determined by the DLTS, TSCAP, C-V, and Hall-effect measurements.

Sample ND (cm-3) NSD n (at 300 K) EDX (eV) NDX
Alo.33Gao.67As 5.91E16 1.74E16 Ec-0.20 3.58E16
(Sn-doped)____ Ec-0.30 5.95E15
Alo.4Gao.6As 9.95E16 9.2E15 Ec-0.20 7.44E16
(Sn-doped) Ec-0.30 1.59E16
Alo.38Gao.62As 1.54E17 1.3E16 2.2E16 Ec-0.30 1.41E17


5.1. Introduction

Particle irradiation such as ion, electron, proton, and neutron induces the defects in the

active region of GaAs/A1GaAs device. Irradiation can takes place, intentionally or uninten-

tionally, during the growth process or in the operating condition. Ion implantation is now

an established method for doping control in the GaAs device, for which a diffusion doping

technology has not been developed. Ion implantation doping offers a number of advantages

in the fabrication of GaAs/AlGaAs devices such as independent control of the doping level,

its depth profile, uniformity and reproductibility of doping. However, radiation-induced

defects are observed to produce effective compensating centers which are stable at room

temperature and convert the doped GaAs layer into a semi-insulating layer [43-47]. An-

nealing steps are needed to remove the compensating center and to activate the doping


The concentration and distribution of irradiation-induced defect can be controlled to

some extent. The defect concentration is proportional to the dose of irradiation; the distri-

bution of defect is the function of the irradiant particles, their energy, and the impurities

contained in semiconductor; the impurities have the abilities to trap the intrinsic defects

originally produced by irradiation.

We can compare the energy spectrum of defects introduced by neutron, proton, and

heavy ion irradiation. In general, one expects heavier particles to create more complex

damage; either closely spaced point defects which mutually perturb each other's energy

levels, or extended defects where point defects totally lose their identity [48]. In terms

of the DLTS spectra, it is the general trend that as the mass of the high-energy particle

increases, the broader and deeper DLTS spectrum is shown. The low-fluence proton damage

is quite similar to 1 MeV electron damage, with a general trend towards a relatively larger

proportion of the damage being the E4 (Ec 0.76 eV) and E5 (Ec 0.86 eV) levels.

The Te-doped A10.38Gao.62As cells grown by Liquid-Phase-Epitaxy were irradiated by

300 keV, 1 MeV proton with fluences of 1011 cm-2, 1012 cm-2. The 1 MeV electrons were

also irradiated onto the Te-doped Alo.38Gao.62As with fluences of 1015 cm-2, 1016 cm-2.

The irradiation-induced defects were investigated by the Deep-Level-Transient-Spectroscopy

(DLTS) and Thermally-Stimulated-Capacitance (TSCAP) methods.The measured defect

parameters were correlated with the results of Current-Voltage (I-V) and Capacitance-

Voltage (C-V) methods.

5.2. Theoretical Review of Defect Creation by Particle Irradiation

5.2.1. Dynamics of Collision

The incident energetic particle irradiated into the solid lattice interacts with the elec-

trons and the nucleis. It loses its energy by several processes depending on the nature of the

particle and its energy. The defects are usually introduced by the collisions of particles with

the nucleis which cause the atomic displacements. The concentration of produced defects

per unit time, v, is expressed by

v = aN (5.1)

where a is the cross section which is characteristic of the interaction leading to atomic

displacements; N is the nuclei concentration per unit volume; 0 is a flux of particles. The

flux, 0 is defined by

S= nv (5.2)

where n is the particle density and v is the velocity of incident particle. The probability

that the particles penetrate and make collisions with nucleis at the distance, x, is given by

the exponential term

p(x) = exp(-oNx) (5.3)

Then, the average penetration depth of the particle is defined as

S= fo xp(x)dx (5
fo p(x)dx
The figure 5.1. shows the dynamics of elastic collision. Let E be the kinetic energy of an

incident particle of mass, m. It strikes an atom (of mass M) of a solid. The kinetic energy

T transmitted to an atom depends directly on the angular deflection, 0, of the incident

particle The conservation of kinetic energy and the conservation of momentum must

fulfilled. Then, the ratio of transmitted energy to particle energy is expressed by [49,50]

T M 1 1(0)
S= 2 (5.5)
E m (1+ M/m)2

where cos0 is given by

1 + (M/m)7?
cos0 = + (M/m)+ / (5.6)
/1 + 2(M/m) + (M/m)2
Then the maximum energy Tm is transmitted for 0 = 0 such that

T 4Mm E. (5.7)
(M + m)2
For a neutron, whose mass m is very small compared to M, the transmitted energy is given

by [49]

T = E(1- cosO) for 0 < 0 < r. (5.8)

For an energetic electron (m < M) relativistic corrections are necessary [51] and the kinetic

energy transmitted to the nucleus can be expressed by

T = Tmsin2 (5.9)

where the maximum transmitted energy is given by

m E
Tm = 2 IE(2 + E (5.10)
i mc2


m E 0
- --- - -


Figure 5.1. Dynamics of the collision between an incident particle of mass m
and energy E and an atom of mass M.

5.2.2. Differential Scattering Cross Section

In the case of irradiation of a solid by energetic particles, the interaction between the

nuclei of the solid and the incident particles takes place. The differential scattering cross

section is a function of the interacting potential. The elastic scattering is not adequately

described by the total scattering cross section. The differential scattering cross section da(O)

is more adequate to describe the elastic scattering because the angular distribution of the

scattered particles is necessary. For charged particles the interaction can be represented by a

Coulombic potential (Rutherford scattering); for neutral particles (neutrons) the interaction

is rather equivalent to collisions between rigid spheres (hard-core potential). For the hard-

core potential, the differential scattering cross section is given by

da = -27(Ri + R2)2cosOsin0 dO (5.11)

where R1 and R2 represent the radii of two rigid shares. For the Rutherford potential, the

differential scattering cross section is given by

irZ2e4 MdT
da mT (5.12)
Em T2

where Z represents the atomic number of lattice atom.

5.2.3. Primary Displacements

The number of total displacements produced by the incident particles is given by the

integration of differential cross section

Tm da
a(E) = dT dT. (5.13)

This integration must be carried out independently for each type of irradiating particles

since da/dT depends on the type of interaction and the nature of the particle.

For heavy charged particles (ions), the number of displacements is given by

SrZZ-e4 M1 1 1
a(E) = ) (5.14)
E M2 Td Tm

where Td is defined as the threshold energy. The minimum energy to displace an atom from

its substitutional site is called the threshold energy.

For neutrons the interaction is described by the hard sphere collision and the number

of displacement is given by

o(E) = 7r(R2 + R )2(1 Td). (5.15)
1 2 Tm
For electrons, electrons must have a considerable energy (in the range of 0.1 10 MeV)

to transmit energy of order of threshold energy, Td because they have very small mass

compared to the atomic mass (M). In this energy range, electrons are relativistic and the

number of displacements is given by [52,53]

(E) = 7( Ze m 1) (5.16)
mc2(72 1) Td
where 7 is given by

7 = 1- v2/c2. (5.17)

This equation can be approximated by McKinley-Feshbach's formula [54].

5.2.4. Secondary Displacements

A primary knock-on atom, to which the energy T has transmitted by the incident

energetic particle, can in turn displace other atoms when T is large enough. The number of

secondary defects can be estimated by the cascade model proposed by Kinchin and Pease

[55]. This model assumes hard-sphere collisions between atoms and the kinetic energy of a

knock-on atom is shared equally with the stationary lattice atom since they have the same

mass. The probability of displacements in cascade is expressed by

p(T) = T for T > 2Td. (5.18)

For the range of Td < T < 2Td, It is obvious that only one displacement can take place.

Thus the number of secondary displacements is given by the cross section
Tm T do
o(T) = dT. (5.19)
2Td 2Td dT

For more realistic calculations the differential cross section of displacement da can be used,

depending on the interaction potential.

If the particles have the kinetic energy of the order of Td, only single displacement

takes place and it causes the formation of vacancy-interstial pair. When Tm > 2Td, the

displacement of two neighboring atoms is possible and it leads to the formation of diva-

cancy. For example, the defects produced by the electron irradiation are divacancies and

their interstitials. These defects are, however, difficult to observe because vacancies and

interstitials are mobile at low temperature; they can be observed as the complex of them-

selves or complex with impurities. These defects are uniformly distributed in depths of the

order of ym with high-energy electron irradiation.

For particles which transmit very high energy to the lattice atoms compared to Td, the

total cross section for displacement can be expressed by

a = up + as (5.20)

where ap and as denote the cross sections for the primary displacement and for the secondary

displacement, respectively.

5.2.5. Formation of Amorphous Layer by Irradiation

The number of displacements produced by a single primary collision can be high if the

incident particle has the kinetic energy higher than the threshold energy. Then a cascade of

displacements .occurs and the region containing a large concentration of vacancies, i.e., the

heart of cascade, is formed and surrounded by a region containing interstials. The heart

of cascade contains divacancies ,trivacancies, etc. The irradiation is usually performed at

temperature above which interstitials and vacancies are mobile; therefore, after irradiation

the interstitials disappear and the size of the heart of cascade decreases due to the vacancy

out-diffusion. The heart of cascade is now surrounded by the point defects formed by the

association of the escaping vacancies with impurities such as E or A center. The inner part

of the cascade heart is composed of vacancy clusters since moving vacancies are agglomerate.

When a material is uniformly irradiated by ions, isolated damaged regions are first

created. Depending on the size of the damaged regions and of the dose of irradiation, these

regions start to overlap until a continuous disordered layer is formed. The disorder is found

to increase as a function of dose until it saturates at the critical dose [56]. This critical dose

depends on several parameters, in particular, (1) the nature and energy of the incident ions

and (2) temperature, because the size of the disordered region is the direct function of the

nature and the energy of the incident ions and vacancy annealing occurs. When the critical

dose is reached, the disordered layer is often said to be amorphous.

5.2.6. Range of Particle

Particles move through the solid and atomic displacements occur along their path.

They are slowed down due to the energy transfer to the electrons of the solid. This result

causes the ionization of atom. For the electron-particle scattering the rate of energy loss,

or stopping power, -dE/dx can be written in terms of the differential cross section do(E,T)


dE Tm
dx Ne T da(E, T) (5.21)
T [ITMrnin
where Ne is the density of electrons in the solid and Tnn and Tm are the minimum and

maximum energy transferable to an electron (of mass m) by the incident particle (of mass

M and energy E), respectively. The range R(E) of the particle is obtained directly from

this stopping power;
oE dE
R(E) = (5.22)
The minimum energy Tm is usually considered to be the ionization energy of an electron from

its atom because energy transfer smaller than the ionization energy does not appreciably

contribute to the slow-down of the incident particles. For incident charged particles, the

Rutherford scattering law gives

dE f 4((/M)E TrZ2e4 M dT
-d = E mT2' (5.23)

that is

dE rZ2e4 M 4mE
-Tx = NIE mn( (5.24)
dx Em MI

where I is the ionization energy of atom. Actually, the ionization energy of atom is different

depending on the electron states.

The notion of range of a particle is particularly important for ion irradiation since

it determines the depth distribution of the implanted ions. Because of the fluctuation in

energy loss and scattering angle at each collision, a range must be defined in terms of

probability distribution of penetration depth. The important parameters are;

1. the projected range Rp, i.e., the projection of the total range on the incident direction

of the ion,

2. the range straggling, ARp, i.e., the mean square fluctuation in range which charac-

terizes the depth distribution of implanted ions in the direction of implantation,

3. the range straggling perpendicular to the direction of implantation.

The numerical calculation has been performed by Lindhard, Scharffand Schrott [58]. Details

of the theory of ion range is available [59].

5.3. Results and Discussion

The proton and electron irradiations produced different defect states and the different

energy of irradiant rendered the variation of final defect state. The observed defect states

were the Ec- 0.28 eV, Ec- 0.39 eV, Ec- 0.55 eV, Ec- 0.76 eV, Ec- 0.86 eV and Ec 0.99

eV levels as are shown in Fig. 5.2., 5.3., 5.4., 5.5., 5.6., and Fig. 5.7. No observable hole

trap levels were detected. The Ec- 0.28 eV level is known as DX center and this level exists

dominantly both in the irradiated and unirradiated A10.38Gao.62As cells in terms of defect

density. This level (E3) was observed in the irradiated GaAs but the measured activation

energy is wide-spread from 0.31 eV to 0.45 eV [60-62].

The Ec 0.55 eV level is very stable at room temperature. This level was observed in

the Alo.38Gao.62As irradiated by 300 keV proton with the fluence of 1012 p/cm2 and in the

one irradiated by 1 MeV with the fluence of 1016. This level was observed only in the device

of high dose. It is definitely irradiation-induced level. This level was reportedly observed in

the Be+-implanted n-type AlGaAs and it was assigned as the Be-complex [63]. It is possible

that the irradiation induced vacancies are associated with Be impurities which are used as

the p-type dopant in this sample.

It should be noticed that the Ec-0.76 eV and Ec-0.86 eV, which seem like EL2 group,

were observed. The Ec-0.86 eV level was also observed in the pre-irradiated Alo.38Gao.62As

cells. The density of the Ec-0.86 eV level was significantly increased after proton and/or

electron irradiation.

The difference between 300 keV and 1 MeV proton irradiation is that the 300 keV

proton irradiation produces the higher overall density of irradiation-induced defects but the

1 MeV protons induce the midgap level of Ec-0.99 eV with the big capture cross section

of 1.37E-9 cm2. This level probably causes the major degradation of the A10.38Gao.62As

cells after the 1 MeV proton irradiation. This level showed the long tail of emission from

low temperature and the broad DLTS spectrum as shown in Fig. 5.2. and in Fig. 5.6.

The similar phenomena was observed in the neutron-irradiated AlGaAs [64]. The neutron-

induced level (assigned as the defect clusters) showed the almost same activation energy as

the Ec 0.99 eV and the long tail of broad DLTS spectra which comes from the enhanced

thermal emission at low temperature due to the electric field associated with the defect

clusters. The neutron and the proton have the identical mass. However, the transmitted

energy is quite different because the energy transfer to atom by protons is due to Rutherford

scattering, whereas the energy transfer by neutron is due to elastic scattering. Therefore,

the neutron irradiation can cause the heavier damage to the lattice. It is plausible that the

high-energy (1 MeV) proton irradiation gives the similar effects as the neutron irradiation

1 MeV Proton

Fluence: 1011 cii-2

Ec 0.99 eV

e, = 34.4 S-1

Ec 0.28 eV





Figure 5.2. The DILTS spectra of the Te-doped A10.3asGC .62As irradiated
by 1 MlV proton with fluence of 10o" c(11-2




_ __ ___ __

_ _~__ __ ___ __

1 MeV Electron

Fluence: 1010 cm-2

Ec 0.55 eV

en = 34.4 s-1





Ec 0.28 eV

150 200 250 300 350


Figure 5.3.

The DLTS spectra of the Te-doped Alo.38Gao.62As irradiated
by 1 MeV electron with fluence of 1016 cm-2.

EC 0.86 eV


Ec 0.86 eV

en = 34.4 s-1

1 MeV Electron

Fluence: 1015 cm-2




150 200 250 300 350

The DLTS spectra of the Te-doped Alo.38Gao.62As irradiated
by 1 MeV electron with fluence of 1015 cm-2.

EC 0.28 eV

Figure 5.4.

300 KeV Proton

Fluence: 1012 cm-2

Ec 0.55 eV

EC 0.86 1
SEc 0.76 eV
Qen = 34.4 s-1

200 250 300 350 401

Figure 5.5. The DLTS spectra of the Te-doped Alo.38Ga0.62As irradiated
by 300 KeV proton with fluence of 1012 cm-2.






Figure 5.6.

The DLTS spectra of the Te-doped Alo.38Gao.62As irradiated
by 1 MeV proton with fluence of 1012 cm-2

250 300 350 400


300 KeV Proton

Fluence: 1011 cm-2

U) e,, = 34.4 s-1

SEc 0

Ec 0.28 eV

100 150 200 250 300
Figure 5.7. The DLTS spectra of the Te-doped Alo.sGao.62As irradiated
by 300 KeV proton with fluence of 1011 cm2.

).76 eV


if the fluence and the energy of protons is increased. It must be noted that the Ec 0.99

eV level is not observed in the low-energy proton- irradiated A1GaAs and in the electron-

irradiated one. However, its density is quite small and it is in the order of 1014 cm-3 in the

high-energy proton irradiated samples. The estimated lifetime (7n = aooNTVth) is in the

order of 10-13 sec but the real lifetime rn = anNtvth is in the order of 10-11 second if the

multi-phonon emission is considered as an = a, exp(-E).

The C-V measurement at room temperature will give the total ionized impurity density

which composes of the sum of shallow level and DX center densities since the DX center at

the edge of space-charge region follows the 1 Mhz small signal and contributes to the mea-

sured junction capacitance. The total free carrier at the conduction bands were calculated

by [65]:

n = ND NT (5.25)

The I-V measurement showed that ideality factor is in the range of 2.16 to 2.55. The device

# 43 (300 keV proton, fluence of 1011 cm-2) has the ideality factor of 2.16 to 2.25. The

device # 44 (300 keV proton, fluence oflO12) has the ideality factor of 2.55. The device

# 46 (1 MeV proton, fluence of 1011 cm-2) showed the large variation of 2.20, 2.26, and

2.53 in ideality factor from sample to sample. The device #47 (1 MeV proton, fluence of

1012 cm-2) has the ideality factor of 2.22. The deveces # 41, 42 (1 MeV electron, fluences

of 1015, 1016 cm-2) have the ideality factors of 2.21 and 2.22, respectively. The measured

defect parameters were summarized in the Table 5.1.

Table 5.1. High Energy Proton and Electron Irradiation Induced Defect Parameters
Determined by DLTS, TSCAP, C-V, and I-V Method in Te-doped Alo0.38Gao.62As Cells.

Energy Fluence nt ND ET Nt ooo rn
(MeV) (cm-2) (cm-3) (cm-3) (eV) (cm-3) (cm2) (sec)
0.3 Ell 1.3E17 1.48E17 Ec-0.28 1.35E17 3.6E-14 2.02E-11
Ec-0.76 1.10E15 8.43E-14 1.08E-9
E12 7E15 1.41E17 Ec-0.28 1.34E17 3.66E-14 2.04E-11
Ec-0.55 4.46E15 4.54E-14 5.17E-10
Ec-0.76 7.99E15 8.43E-14 1.48E-10
Ec-0.86 7.05E15 6.73E-14 2.11E-10
1 Ell 2.8E16 1.54E17 Ec-0.28 1.26E17 3.66E-14 2.17E-11
Ec-0.99 7.87E14 1.37E-9 9.27E-14
E12 3.6E16 1.41E17 Ec-0.28 1.05E17 3.66E-14 2.60E-11
Ec-0.39 7.36E14 1.24E-16 1.10E-6
Ec-0.86 6.13E14 6.73E-14 2.42E-9
Ec-0.99 4.91E14 1.37E-9 1.49E-13
1 E15 1.9E16 1.33E17 Ec-0.28 1.14E17 3.66E-14 2.40E-11
Ec-0.86 7.60E14 6.72E-14 1.96E-9
E16 3.6E16 1.63E17 Ec-0.28 1.27E17 3.66E-14 2.15E-11
Ec-0.55 8.54E14 4.34E-14 2.70E-9
Ec-0.86 1.16E15 6.73E-14 1.28E-9
0 2.5E16 1.05E17 Ec-0.28 8.0E16 3.66E-14 3.42E-11
Ec-0.86 5.67E14 6.73E-14 2.62E-9


6.1. Introduction

The compound semiconductor, AlxGal-xAs, is widely used for the optoelectronic de-

vices such as double heterostructure lasers and heterface solar cells prepared by liquid-

phase-epitaxy technique. Recently, much interest has been focused on this material because

of its application in fabricating highly efficient photovoltaic devices. In particular, multi-

junction cascade solar cells using the GaAs-AlGaAs system have been reported to have a

potential efficiency of as high as 30 % at high solar concentration [66]. For the fabrication on

p-type A1GaAs, Zn and Ge have been commonly used. However, they have the shortcom-

ings as follows. Zn is not suitable for the p-type dopant because of its high diffusivity for

those multijunction structures, in which sharp impurity profiles are required and additional

epilayers have to be grown on the top of the Zn-doped layer [67]. Furthermore, the high

vapor pressure of Zn causes serious contamination in growing multilayers in an open tube

liquid phase epitaxial (LPE) system. Ge has shown to be difficult to obtain high carrier

concentration in AlxGax-lAs with x > 0.3 because of its strong dependence of the acceptor

ionization energy on the Al composition and its compensation effect due to amphoteric

behaviour [68,69]. In contrast, Be has the desirable characteristics such as low diffusion

coefficient, low vapor pressure, and high distribution coefficient.

The behavior of Ge and Zn as a dopant in AlxGalxAs has been well investigated. It

is well-known that besides the presence of the dopant- related centers (DX centers), deep

hole traps have been detected at both sides of p-n junction. The physical origin of such

hole traps, present in moderately large concentrations, is discussed in terms of Zn-related

or Ge-related complexes located at the Ev + 0.40 eV [70-74]. However, very few Be-related

acceptor levels have been reported in the literature.

6.2. Experimental

Te-doped AlGaAs homojunction mesa diodes grown by LPE have been used in this

study. The Al composition has been controlled by the growth temperatures and the char-

acteristics curve of Al composition versus growth temperature are shown in Fig. 6.1. The

relationship between the Al composition and growth temperature is linear. This relationship

was used for the determination of Al composition in LPE AlxGal-xAs. Growth tempera-

ture is in the range of 7020 C to 7800 C and it corresponds to the Al composition range of

0.23 to 0.41. The substrate used for the mesa A1GaAs homojunction is n+ GaAs doped by

Si. The samples of #6 and #7 have the same growth temperature but the device #7 has an

undoped AlxGal-xAs buffer layer between n+ GaAs substrate and Te-doped AlxGal-xAs

active layer as shown in Fig. 6.2. The epilayer thickness varies from 2.4 to 5.4 um and

the device area is 4.153 x 10-4 cm-2. The capacitance-voltage technique was used for the

determination of background doping concentration. The I-V measurements were used for

the determination current component under low forward-bias condition. The DLTS mea-

surements were performed in order to determine the physical parameters of grown-in defects

in AlxGal-xAs homojunction devices.

6.3. Results

One dominant electron trap (DX center) and two hole traps were observed from the

DLTS measurements. The results are summarized in Table 6.1. The Te-related center (DX

center) has an activation energy from 0.28 eV to 0.31 eV below the conduction band. The

density of DX center increases as Al composition increases but near the crossover point of

direct-indirect bandgap it becomes saturated and slightly decreases. This results generally

agrees well with the reported data indicating that the concentration of DX center decreases

as Al composition goes far beyond the direct-indirect band-gap crossover point [75].






0.2 -

700 720 740 760 780 800


Fig. 6.1. Characteristic curves of Aluminum composition versus growth temperature.

Device # 1, 2, 3, 4, 6


n (Te)-AIlGai_,As (2.4 5.4 pm)


Device # 7


n (Te)-AlxGaixAs (2.4 pm)

undoped AlxGaixAs


Figure 6.2. Test device structure of LPE grown AlxGalxAs mesa diodes.

As for the hole traps, their activation energies become deeper as the Al composition

increases. The shallow one (Ev + 0.25 to 0.28 eV) has the almost comparable densities

(about 4 x 1016 cm-3) except for device #7, which has the undoped AlGaAs buffer layer

between substrate and active layer. The device #7 has no considerable hole trap at around

0.25 eV above the valence band. It has been reported that in Sn-doped GaAs grown by

liquid-phase-epitaxy under arsenic-deficient conditions, compensation due to the possible

occupation of As sites by Si may exist [76,77]. A broad photoluminescence band is observed,

and it suggests that the Si acceptor level is located at the Ev + 0.20 eV. Therefore, the

measured value of the Ev + 0.25 to 0.28 eV is likely to be reasonable, assuming that the

Si-related acceptor level becomes deeper as the Al composition increases. It looks likely

that the buffer layer deters the diffusion of Si from the n+ GaAs substrate which is heavily

doped by Si. In comparison of #6 and #7, it is clear that the buffer layer slows down the

diffusion of Si from the substrate. The new level at the Ec 0.32 eV looks like the Si-related

DX center. Otherwise, another Te-related DX center is a possible candidate because Te and

Se can have two DX center in the Al composition range of 0.3 to 0.6. Actually, every sample

showed a small step at the right hand side of DLTS spectrum of the Te-related DX center

suggesting the presence of another level of smaller density.

As for the deeper hole trap, it becomes deeper with increasing the Al composition;

its activation energy varies from Ev + 0.68 eV to Ev + 0.93 eV with corresponding Al

composition range of 0.27 to 0.41 as shown in Fig. 6.3. In Alo.23Gao.77As, however, this

level was not observed. It is worthwhile to review the behaviour of other p-type dopant

materials such as Zn and Ge in AlxGalxAs for the purpose of comparison. Ruling out

occasional contamination, there is strong evidence that for Al compositions near bandgap

crossover, donors and acceptors tend to form both shallow and deep centers [78]. In fact,

dopants incorporate significantly into the deeper positions as aluminium content is close to

0.4 and goes beyond 0.4. Ge introduces two shallow acceptors and one deep acceptor in LPE


Ec- 0.31 eV


150 200 250 300


The DLTS spectra of Alo.31Gao.69As grown by Liquid Phase Epitaxy.

Figure 6.3.

AlGaAs, with the dominant one of larger activation energy increasing its activation energy

to 160 m eV at x = 0.4 [70,71,79]. It has been observed that Ge introduces Ge-comlex at

about 0.4 eV from the valence band [70,71]. Also, for Zn in AlxGal-xAs grown by LPE and

by MOCVD, the presence of a Zn-complex center (proposed as ZnGa-VAs) at the Ev + 0.4

eV was confirmed, only with the Al composition range of near or beyond the direct-indirect

bandgap crossover point with its density strongly depending on the Al composition [72-74].

This fact qualitatively agrees with our results. The deeper hole trap at the Ev + 0.68

eV to 0.93 eV ascribed to Be-related complex (proposed as BeGa-VAs) were observed only

in the Al composition range of 0.27 to 0.41, not in A10.23Ga0.77As. Also, its density has

a large variation from sample to sample; the device #3 of Alo.31Gao.69As has a relatively

small density of 1.42 x 1015 cm-3 whileas the device #4 of Alo.27Gao.73As has the highest

density of 2.12 x 1017 cm-3. One of the possible explanations is that at about x = 0.31

the Be-related complex (BeGa-VAs) yields the smaller density due to the unknown reasons

compared as the samples of x = 0.41, 0.36, and 0.27. An almost comparable observation

was reported by Calleja et. al. in Zn-doped LPE AlxGal-xAs [77]: The sample of Al

composition of 32 % showed the smaller concentration of the Ev + 0.40 eV center with the

highest external quantum efficiencies as compared to other samples. This fact qualitatively

agrees with our I-V measurements in which the sample of x = 0.31 has the best ideality

factor as compared to other samples of 0.27 < x < 0.41. Generally, the results of I-V

measurements are consistent with the data of DLTS measurements. For the samples #6

and #7, there exists a considerable discrepancy in the ideality factor probably due to the

two-dimensional conductivity in device #7 at the interface between undoped AlxGalxAs

and substrate.

6.4. Conclusions

In Te-doped LPE AlxGal-xAs homojunction diode, the grown-in defects have been

investigated by the DLTS, I-V, and C-V measurements. The grown-in defects are related to

the n- or p-typed dopant materials. The density of DX center increases as the Al composition

increases. However, near the crossover point of direct-indirect bandgap it becomes slightly

decreased. Two hole traps have been observed related to the Be and Si dopant materials

which diffuse from the p+(Be) AlxGalxAs layer and from the n+ (Si) GaAs substrate,

respectively. The BeGa-VAs complex has shown the characteristics similar to the ZnGa-VAs

and GeGa-VAs complexes in LPE and MOCVD AlxGal-xAs. The Beca-VAs complex has

been detected only in the Al composition range of 0.27 < x < 0.41, not in Alo.23Gao.77As.

This result is consistent with the features of Zn- and Ge-related complexes in AlxGai-xAs.

At x = 0.31, the I-V characteristics has shown the best ideality factor of 2.06 among those

samples in the range of 0.27 < x < 0.41.

Table 6.1. Defect parameters determined by DLTS, I-V and C-V measurements.

Device ND(cm-3 ET(cm-3) NT(cm-3) an,p(cm2) Possible origins
#1 2.40E17 Ec-0.30 1.65E17 3.41E-14 DX center(Te)
T=7600C Ev+0.26 4.08E16 5.56E-16 Si-related
x=0.36 n=2.07 Ev+0.88 1.88E16 7.13E-13 BeGa-VAs
#2 "2.18E17 Ec-0.28 1.00E17 1.09E-14 DX center(Te)
T=7800C Ev+0.28 3.60E16 5.86E-15 Si-related
x=0.41 n=2.11 Ev+0.93 4.27E16 1.16E-11 BeGa-VAs
#3 2.13E17 Ec-0.31 1.63E17 2.52E-13 DX center(Te)
T=7400C Ev+0.25 4.39E16 1.49E-15 Si-related
x=0.31 n=2.06 Ev+0.73 1.42E15 1.32E-14 BeGa-VAs
#4 2.95E17 Ec-0.31 3.12E16 6.02E-13 DX center(Te)
T=7210C Ev+0.25 5.55E15 5.56E-16 Si-related
x=0.27 n=2.13 Ev+0.68 2.12E17 8.32E-15 Beca-VAs
#6 3.41E17 Ec-0.28 2.23E16 3.12E-14 DX center(Te)
T=702C Ev+0.24 4.72E16 2.60E-13 Si-related
x=0.23 n=1.64
#7 3.97E17 Ec-0.31 1.58E16 9.16E-13 DX center(Te)
T=702C Ec-0.33 1.26E16 1.01E-13 DX center(Si)


7.1. Introduction

Recent development in Silicon-On-Insulator (SOI) technology has shown promise for

providing a viable technique for the fabrication of fully isolated transistor structures. Sev-

eral SOI approaches have been attempted. Among them, the Separation by IMplanted

OXygen (SIMOX) is a promising technology for complementary metal-oxide-semiconductor

large scale integration. In SIMOX technology, a high-dose of oxygen ion- implantation is

applied to the silicon substrate at high temperature. The effect of oxygen implantation

on the top silicon layer over the buried oxide is the main topic of research interests and

concern for the SIMOX based SOI devices. Since the top silicon layer is heavily damaged

during oxygen implantation, annealing process is used in all SIMOX based materials and

devices. The morphologies of the silicon/buried oxide/substrate have been studied using

various experimental methods such as Rutherford backscattering spectroscopy (RBS) and

cross-sectional transmission microscopy (XTEM). However, very few experimental data

concerning the electrically active defects in the SIMOX materials and devices have been

published by using the DLTS method. The DLTS technique is a powerful tool for direct

observation of electrically active defect levels in SOI materials and devices induced by the

oxgen implantation and the subsequent annealing process. The I-V and C-V methods can

also be applied for electrical characterization of the SIMOX devices and their respective

results can be correlated to the DLTS results.

7.2. Fabrication Process

The buried oxide structure was produced by using 150 KeV 0+ ion- implantation onto

the 3-5 ohm-cm n-type silicon substrate. The substrate temperature was kept at 500 OC.

The resultant buried oxide layer has a thickness of about 0.4 pm. The superficial silicon layer

is heavily damaged due to the 0+- ion implantation and hence annealing process should be

used in order to recrystallize the damaged top silicon layer over the buried oxide. This step

is a prerequisite for the silicon epilayer growth. Different annealing conditions were applied

to study their effects on the defects and electrical characteristics of the SIMOX based SOI

devices. Annealing temperature was varied between 1150 and 1350 o C and annealing time

varied between 2 and 24 hours. The dopant density of n- epilayer was 1 x 1015 cm-3 and its

thickness varied between 0.8 and 2.5 pm. The p-well region was formed by boron implant

and subsequently drive-in diffusion into the n- epilayer. The doping density of p-well region

is 2 x 1016 cm-3. The p+/n and n+/p junctions were formed by implantation of boron and

phosporous, respectively. The p+/n diodes were used to study the defects and electrical

properties of the n-epilayer and the n+/p diodes were used to study the defects in the p-well.

The lateral isolation was incorporated in batch #3 and #4 SOI devices by using LPCVD

growth of thermal oxide and undoped polysilicon. The test structure is shown in Fig. 7.1.

The process parameters of batch #1, #2, #3, and #4 are listed in Table 7.1.

7.3. Oxygen Related Defects in SIMOX Based SOI Devices.

The DLTS measurements on batch #1 through #4 SIMOX devices revealed various

defect levels. The measured defect parameters and the tentative assignment for the origins

of these defects are summarized in Table 7.2 and Table 7.3. The Ev + 0.65 eV hole level

was observed in all the devices studied. It is anticipated that the oxygen implantation

and the subsequent annealing processes will generate the oxygen-vancancy complex and

other related defects in the SIMOX based SOI devices. The implantation-induced vacancies


Lateral Isolation

Lateral Isolation

(1) Epi Thickness; 2.0 pm

(2) Oxide Thickness; 0.4 pm

(3) n- epi; 3 x 1015 cm-3

(4) p-; 2 x 1016 cm-3

(5) Substrate; 3 5 ohm-cm

Figure 7.1. Test structure of SOI device fabricated by SIMOX
(Separation by Implant of Oxygen) technique.

enhance the impurity diffusion. Therefore, the creation mechanism for the impurity related

defects is rather complex in the fabrication of SOI devices.

Aside from the stable defects observed in the tested SOI devices, some anomalously

unstable defects which are probably due to the mobile ions such as alkali metal impurities

were also observed in some SIMOX devices with lateral isolation (batch # 3 and #4).

The unstable defect levels were also observed in the control devices of batch #3 and #4,

indicating that unstable defect levels may be attributed to the metallic contamination during

lateral isolation and other high temperature processes applied during the device fabrication.

The oxygen ion implantation onto the silicon substrate creates the buried oxide layer

and induces vacancy and oxygen related defect clusters in the heavily damaged silicon

surface layer. Prior to the high temperature annealing, the top silicon layer and the substrate

region adjacent to the buried oxide layer were heavily damaged. It has been shown that a

three- hour annealing at 1150 OC on the SIMOX processed silicon film, the damaged surface

layer will be recrystallized to form a single crystal silicon [80]. High densities of oxygen

precipitates and dislocations can be observed in both silicon film and the substrate before

annealing [81,82]. When the annealing temperature increases to 1250 OC or higher, the

entire top silicon layer becomes precipitate-free [80,83]. The oxygen content in the single

crystal region is at least an order of magnitude higher than the solid solubility limit [84].

During high temperature annealing, possible sinks for the supersaturated oxygen atoms are

the oxygen precipitates, the surface and the top silicon/buried oxide interface.

In the SIMOX based SOI devices studied an anomalous capacitance step was observed

in the C-V curve of the p+/n junction diode formed on the n-epilayer. This capacitance (a

reduction of net dopant density) step may be due to the electric-field shielding effect of the

oxygen precipitates [85,86]. This phenomenon seems unique in the SIMOX devices studied,

which occurs when the depletion edge reaches the epi/oxygen-precipitates or the epi/oxide

interfaces. This phenomenon was not observed in the p-well region or in the control devices.

The reason that this anomalous capacitance step was not observed in the p-well region can

be explained as follows. The p-well fabrication step consists of boron implantation and

subsequent drive-in process at 12000C. Since boron diffusion is much faster than oxygen,

the p-well region is not much encroached by the diffusion of oxygen. Another explanation

for this capacitance step shown in the C-V plot of SIMOX devices is due to the increase

of positive fixed charge at the epi-oxide interface. The observed anomalously unstable

defect levels by the DLTS measurements are attributed to the contamination of alkali metal

impurities during the lateral isolation process. However, the increase of positive oxide fixed

charge actually reduces the width of depletion region next to the oxide layer in n-type

material. Thus the increase of positive fixed oxide charge will make the capacitance step

in the C-V plot occurred closer to the epilayer/buried oxide interface in the n-type epilayer.

This fact is contrary to the observed results of batch #3 and #4 SIMOX devices. The

presumption that anomalous capacitance step is due to the extension of oxide depletion

region into the n-epilayer is questionable. It is noted that the capacitance step occurs closer

to the surface if higher annealing temperature and longer annealing time are used, as is

shown in Fig. 7.9. This can be explained by the oxygen out-diffusion from the buried

oxide layer. For examples, for devices from S-16 and S-19, the annealing time (2 hrs) is

shorter than from S-12, and their capacitance step was found closer to the surface due to

the thinner epilayer. The main difference observed in the SIMOX devices with and without

lateral isolation is that the capacitance step is closer to the surface for devices with lateral

isolation, as is shown in Fig. 7.10. Even if annealing temperature and annealing time were

identical, the devices with lateral isolation showed that the capacitance step was closer to

the surface as compared to the devices without lateral isolation. In fact, for devices from

batch # 3 and # 4, which have lateral isolation, additional defects and the oxygen related

precipitates may come from the thermal oxide and LPCVD polyslicon used in the lateral

isolation. A comparison of the SIMOX devices with control devices shows clearly that the

Batch # 1

S 10 p+n

ET = Ec 0.29 eV an = 1.40 x 10-14 cm2

NT/ND = 6.30 x 10-2





Figure 7.2. The DLTS spectra of the n-epilayer of SIMOX-based SOI p+/n
junction diode (batch #1).




Batch # 1 S 10 p+n

ET = Ec 0.33 eV

a,, = 2.00 x 10-15 cm2

NT/ND = 1.73 x 10-'

200 250 300

The DLTS spectra of the n-epilayer of SIMOX-based SOI p+/n
junction diode (batch #1).




Figure 7.3.

Batch # 2


EA = Ev + 0.55 eV, op = 3.94 x 10-6 cm2

EB = Ev + 0.50 eV, op = 1.29 x 10-17 cm2

Ec = Ev + 0.63 eV, up = 2.91 x 10-16 cm2

I I I L-







Figure 7.4. The DLTS spectra of the n-epilayer and p-well of SIMOX-based SOI
devices (batch #2).

100 150 200 250 300 350

Figure 7.5.

The DLTS spectra of the n-epilayer of SIMOX-based SOI device
(batch # 3).

S 2 n+p

Batch # 3


en = 34.4 s-1

Ev + 0.65 eV

Up = 1.35 x 10-16 cn12

NT/NA = 2.91 x .10-2

100 150 200 250 300 350
Figure 7.6. The DLTS spectra of the p-well region of SIMOX-based SOI device
(batch # 3).


Batch # 4

Ec 0.20 eV

al, = 1.28 x 10-18 cm2

NT/ND = 0.10




The DLTS spectra of the n-epilayer of SIMOX-based SOI device
(batch # 4).



Figure 7.7.


I I I I -

Ev + 0.13 eV

op = 2.1 x 10-18 cm2

NT/ND = 7.2 x 10-2

Batch # 4

S- 16


eV NT/ND = 1.2 x 10-2

x 10-18 cm2

100 150 200 250 300 350

Figure 7.8.

The DLTS spectra of the n-epilayer of SIMOX-based SOI device
(batch # 4).

VR (Voltage)

The C-V characteristics of the n-epilayer of SIMOX-based SOI
device (batch # 3).

Figure 7.9.

0 5 10
VR (Voltage)

Figure 7.10. The C-V characteristics of the n-epilayer of SIMOX-based SOI
device (batch # 4).

abrupt capacitance step observed in the C-V curves of the lateral isolated SOI devices is

a direct result of the oxygen-precipitates enhanced diffusion from the recrystallyzed silicon

layer into the n-epi region. The growth of lateral isolation gives additional thermal annealing

or diffusion step for the oxygen precipitates to diffuse closer to the surface region of the

epilayer. Fig. 7.10 verifies this fact.

The electrical properties (e.g., leakage current) for the SIMOX devices with lateral

isolation and the electric-field-shielding effect are believed to be closely related to the oxygen

contained in the undoped polycrystalline layer above the buried oxide [85,86].

The results of I-V measurements support our assignment of the anomalous capacitance

step of the C-V curve to the oxygen precipitates and oxygen related defects. Furthermore, it

was observed that the diode ideality factor for the SIMOX devices which had the capacitance

step of C-V plots closer to the surface, was significantly higher. This arises from the

recombination current produced at the oxygen precipitates. The results of I-V and C-

V measurements are summarized in table 7.4, table 7.5, and table 7.6. The oxygen and

Si02 precipitates diffuse out to the surface or migrate close to the interface region of top

silicon and buried oxide during both annealing and lateral-isolation growth process at high

temperature. In other words, the transition part of the C-V curve corresponds to the oxygen

precipitates or Si02 precipitates. For example, the observed defects levels, the Ec 0.20 eV

and Ec -0.28 eV by the DLTS, are ascribed to the vacancy plus oxygen complexes, V30 and

V20, respectively, for the batch #4 devices (S-6, S-16) [87]. The applied reverse-bias pulse

was 0 to -3 V, of which the voltage range covered the anomalous capacitance step of the C-V

curve. It is expected that the oxygen and vacancy complexes can be found in the region

corresponding to the capacitance step part of the C-V curve in the SIMOX devices. The

location of the oxygen or SiO2 precipitates may be decided based on the prolonged annealing

process including the growth of lateral isolation and the epilayer growth at 11000C over the

recrystallized top silicon layer. Due to the difference in solubility at 1250 OC and at 875

OC [88], the silicon layer is still supersaturated with oxygen at 875 OC during the growth

of thermal oxide for the lateral isolation, and hence the oxygen precipitates may still be

formed but with a much lower density.

The final stopping location of oxygen precipitates is quantitatively predictable as a

function of effective annealing temperature, time, and epilayer thickness. Here, the effective

annealing temperature and time are defined as including the growth temperature and time of

thermal oxide and polysilicon. A computer model can in principle be developed to simulate

the SIMOX device fabrication using different process parameters.

In order to deter or slow down the diffusion of oxygen into the epilayer of SIMOX

devices, a two- step annealing process at annealing temperatures of 1250 and 1150 C is

suggested. Annealing time of at least 3 hours is required at each temperature. After the

two-step annealing, the surface of silicon layer, which contains high percentage of oxy-

gen precipitates after post- implantation annealing, must be removed prior to the epilayer

growth. The second annealing performed at 1150 OC is required in order to getter the su-

persaturated oxygen (at least one order higher than the maximum solid solubility) in the

single crystallized silicon layer over the buried oxide since the subsequent growth of epilayer

and the thermal oxide growth were processed near 1150 C. This approach will reduce the

oxygen precipitates significantly in the active region of the SIMOX device.

7.4. Correlation of Electrical Properties with Process Parameters

7.4.1. Characterization Methods of SIMOX SOI Devices

In this study, the DLTS, I-V and C-V methods were used to characteristize defects and

electrical properties of SIMOX devices with different process parameters. Oxygen dose, epi-

thickness, and post-implantation annealing condition were changed in order to investigate

the effect of individual process parameters on the defects and electrical properties of the

SIMOX based SOI devices fabricated at Harris Semiconductor. Correlation between process

history and electrical characteristics and defects observed in these devices were discussed

for batch #1 through #4 so that process parameters can be optimized.

7.4.2. Epilayer Thickness

In batch #4 devices, the epi-thickness varies from 1.7 to 2.5 pm. Experimental results

show that small change in epi-thickness has little influence on the electrical characteristics

( except batch #3 S-8 and #4 S-16 ). In general, the thicker the epilayer, the better the

electrical characteristics. The variation in leakage current is too small to tell the difference

between different devices, whereas the ideality factor and breakdown voltages show good

agreement with the above statement. The only exception is for devices in slice S-6 and S-12.

The difference may be due to the long annealing time and will be discussed later.

7.4.3. Annealing Time

From the published papers [80], it has been shown that high temperature heat treat-

ment on SIMOX materials for six hours produce a precipitate-free silicon layer. Shorter

annealing time may cause degradation of quality in Si/Si02 layer. It seems that during

epilayer growth, oxygen may diffuse into the epilayer and degrade the electrical properties.

The layer between the oxygen-free epilayer and the buried oxide may consist of a mixture

of polycrystaline silicon and silicon oxide. This layer has the electric-fieid-shielding (EFS)

effect and can be used to improve the breakdown voltages of MOSFETs fabricated using

SIMOX technology [81].

A comparison of batch #2, #3, and #4 reveals that the effective annealing time,

which includes post-implantation annealing time, epilayer growth time, and lateral oxide

growth time, and the process temperatures, principally decides the electrical characteristics

of the SIMOX devices. The main reason of degradation in electrical characteristics of batch

#3 and #4 devices is that the extra step for the lateral isolation causes the alkali metal

contamination and the diffusion of oxygen precipitates into the device active layer. The

significant increase of breakdown voltage of n-epi of the SIMOX devices is closely related to

the formation of the electric-field-shielding layer (EFS layer) due to the oxygen precipitates.

On the other hand, the breakdown voltage of p-well diodes for the SIMOX materials is equal

to or less than the breakdown voltage of the control devices. We have already discussed

that the diffusion of the oxygen precipitates into the p-well region of the SIMOX device is

blocked due to the fast diffusion of boron from the surface of epi-layer instead of oxygen

diffusion from the recrytallyzed silicon layer during the boron drive-in process at 12000C.

The formation of EFS layer significantly increases the breakdown voltage of SIMOX devices.

The I-V measurements are entirely consistent with the C-V measurements. The SIMOX

devices of batch #3 and #4 show that the large ideality factor is attributed to the recombina-

tion current component at the interface region of epi/oxygen-precipitates which corresponds

to the region where capacitance step occurred. The SIMOX devices with capacitance step

occurred closer to the surface consistently show a higher diode ideality factor.

7.4.4. Lateral Isolation

Lateral isolation is believed to be the main reason for the degradation of the I-V and

DLTS characteristics observed in batch #3 and #4 devices. Incorporation of lateral iso-

lation can cause great degradation in electrical characteristics, especially, leakage current

and ideality factor. Breakdown voltage is also closely related to the lateral isolation. Incor-

poration of lateral isolation increase breakdown voltage significantly.

7.4.5. Ion Dose and Annealing Temperature

The similarity of electrical characteristics between batch #4 S-16 and S-19 shows the

compensation effect of ion dose and epi-thickness. The correlation between ion dose and

electrical properties still need more study. The effect of 0+ dose can be investigated more

clearly if the other process parameters such as annealing time and temperature are identical.

7.4.6. Defects and Electrical Properties of P-well

In most SOI devices studied, the electrical characteristics of p+/n junction diodes

are superior than the n+/p junction diodes. This phenomenon can be attributed to the

contamination in the boron implanted p- well. The origins of the defects appeared to be

related to foreign..impurities introduced during the implantation and driven-in diffusion


7.4.7. N-epilayer/p-well Interface

The experimental results of batch #1 devices showed that in p/n- junction diodes,

which were intended for characterizing the epilayer in the lateral direction, several new

traps were found. These traps may be related to the n-epilayer/p-well interface related


7.5. Summary and Conclusions

1. Traps A, B, C, I, and M appeared to be introduced during the boron-implant or

drive-in process since these traps were found only in p-well region.

2. Most of traps found in n-epilayer have smaller activation energy than traps found in

the p-well. These traps are believed to be related to the foreign impurities introduced

during oxygen implantation or due to outdiffusion of oxygen atoms.

3. The density of most defects increases with depth in the n-epilayer. It may be related

to the outdiffusion of oxygen precipitates from the buried oxide to epilayer.

4. Lateral isolation process has the most significant influence on degrading electrical

properties of SOI devices studied. This is caused by the undesirable extra annealing

time due to the lateral isolation growth. This step gives rise to the outdiffusion of

oxygen into the n-epi layer.


5. Epi-thickness, ion dose, lateral isolation, annealing time and temperature are closely

related to the outdiffusion of oxygen precipitates. Optimal match between these

process parameters may be obtained by using process simulation.

6. The control of the outdiffusion of oxygen precipitates into the device active region is

the main concern for the fabrication of SIMOX based SOI devices.

Table 7.1. Process parameters of SIMOX based SOI devices.

Batch no. Slice no. O+- dose Anneal Anneal Epilayer Lateral

[cm-2] Temp. [OC] Time [hrs] thickness [im] Isolation
1 S-4 (simox) 1.8 to 1250 24 1.0 no
(1977-BI) S-5..(simox) 2.2E18 1350 2 2.0 no
S-9 (simox) 1250 24 1.5 no
S-10 (simox) 1150 3 1.5 no
2 S-19 (control) no
(3185-2) S-1 (simox) 1.8E18 1250 16 2.0 no
S-4 (simox) 1.8E18 1250 16 2.0 no
3 S-2 (control) yes
(3185-1) S-8 (simox) 1.8E18 1250 16 2.0 yes
S-15 (simox) 1.8E18 1150 3 2.0 yes
S-16 (simox) 2.1E18 1250 24 2.0 yes
S-2 (control) yes
4 S-6 (simox) 1.8E18 1250 16 1.8 yes
(3185-4) S-7 (simox) 1.8E18 1150 3 0.8 yes
S-12 (simox) 2.0E18 1250 16 2.5 yes
S-16 (simox) 2.2E18 1250 2 1.9 yes
S-19 (simox) 1.8E18 1250 2 1.7 yes

Table 7.2. Defect parameters in the n-epilayer of SOI devices.

Et [eV] Nt/ND an [cm2] Possible batch batch batch batch

ap Origin #1 #2 #3 #4
Ec 0.20 1.0E-1 1.28E-18 V30 complex *
EC 0.30 1.2E-2 1.58E-18 V20 complex *
Ec 0.12 .6.2E-2 5.74E-19 Mg, Fe *
Ec 0.65 1.4E-2 2.76E-11 Ge, Be *
Ev + 0.13 7.2E-2 2.1E-18 *
Ev + 0.19 3.7E-2 1.81E-18 Cu, Mn *

Table 7.3. Defect parameters in the p-well of SOI devices.

Et [eV] Nt/ND Op [cm2] Possible batch batch batch batch
Origin #1 #2 #3 #4
Ev + 0.45 2.91E-17 Ag, Fe *
Ev + 0.50 1.29E-17 Fe,Se, Ge *
Ev + 0.55 3.94E-16 Cd, Mn, Cu *
Ev + 0.65 7.75E-2 5.96E-16 Fe, Cd, 0 *

Table 7.4. Summary of the results for batch-2 devices.

Slice Diode n Leakage VB C-V Electron Hole
No. [A]at 5 V [V] trap Nt/ND trap Nt/NA
S-19 p+/n 1.01 3E-12 35 normal 2.6E-3 none
control n+/p 1.08 3E-12 19 normal 3.3E-2 D:3.77E-2
S-1 p-/n 1.15 9E-12 34 step 2.35E-3 none
SIMOX n+/p 1.39 2E-10 14 normal 3.16E-2 A:l.5E-2
S-4 p+/n 1.16 1E-10 35 step 2.86E-3 none
n+/p 1.39 2E-10 13 normal 3.1E-2 A:1.5E-2

Table 7.5. Summary of the results for batch-4 devices.

Slice Diode n Leakage VB C-V Electron Hole
No. [A]at 5 V [V] trap Nt/ND trap Nt/NA
S-2 p+/n 1.05 8.5E-12 38 normal -- -
n+/p 1.27 5.3E-10 14.5 normal M:4.3E-2 C:7.75E-2
S-6 p+/n 2.52 2.6E-7 44 abrupt J:1.0E-1 --
n+/p 2.05 1.0E-7 9.3 abrupt -- -
S-7 p+/n 1.74 1.9E-9 57 abrupt 7.1E-2 --
n+/p 1.99 1.6E-9 9.5 abrupt -- -
S-12 p+/n 1.52 2.4E-9 86 step 1.19E-1
n+/p 1.52 2.6E-8 10.5 normal --
S-16 p+/n 1.54 2.1E-9 68.2 step K:1.2E-2 L:7.2E-2
n+/p 1.54 8.2E-9 10.6 normal -- -
S-19 p+/n 1.54 1.9E-9 72 step < 1.4E 2 -
n+/p 1.61 1.3E-8 11.5 normal i 3.8E-3 3.03E-3

Table 7.6 Electron and hole traps observed in batch- 4 devices.

Trap Et [eV] a [cm2] Observed in
C Ev + 0.65 5.96E-16 p-well
J Ec 0.20 1.28E-18 n-epilayer
K Ec 0.28 1.58E-18 n-epilayer
L Ev + 0.13 2.00E-18 n-epilayer
M Ec 0.53 7.12E-17 p-well

Table 7.7 List of possible defect origins for the SIMOX based SOI devices.

Trap level Et [eV] Donor/acceptor Possible Origin
A Ev+0.53 D Mn
Ev+0.53 A Cu
Ev+0.54 Te
Ev+0.55 A Cd
B Ev+0.50 D Fe, Se, Sr
C Ev+0.61 D 0, Fe
I Ev+0.40 D Fe, Sn
Ev+0.45 D Be, Ag
E Ec-0.11 A Mg, Fe
F Ec-0.62 D Ge, Sr
Ec-0.67 D Be,Rb
H Ev+0.18 A Cu, Mn
Ev+0.21 A Na
J Ec-0.20 D V3O, VO
to 0.22__ Cd,Ti,W
K Ec-0.27 D V20
Ec-0.30 D Au,Ge
M Ec-0.52 D S
Ec-0.55 D Fe

* "D" denotes "donor-like" trap (i.e. a = 10-15 10-17).

"A" denotes "acceptor-like" trap (i.e. a = 10-12 10-15 cm2).


8.1. Introduction

Measurement of the forward-bias capacitance has been suggested as the nondestruc-

tive electrical technique for the investigation of interface state in the metal-semiconductor

contact. This technique has been called by various names such as Schottky spectroscopy,

accurate phase capacitance spectroscopy, and differential voltage capacitance spectroscopy

[18,19,20,89,90]. The principal idea is that sweeping the majority-carrier quasi-Fermi level

throughout the bandgap enables the modulation of charge states of interface defect lev-

els. This can be accomplished by applying the dc forward-bias to the Schottky diode with

the small ac signal superimposed on the dc forward-bias. The charge state modulation of

interface state contributes to the junction capacitance if the applied ac signal is in the low-

frequency regime. The theory for the analysis of forward-bias capacitance is well-developed

[18,19]. The problem in the application of this theory arises from the difficulty to measure

the capacitance under the large current flow.

Admittance spectroscopy for studying the MOS devices is not directly applicable to

the forward-bias capacitance measurement of a Schottky diode because the conductance

component due to thermionic emission overwhelms the susceptance (wC). A bridge method

using three equal registers was suggested by Barret and Vapaille [91]. This method is te-

dious and time-consuming even if only one data point is being obtained since the bridge

balance is obtained by changing the bias. During this procedure, one can easily overload the

lock-in amplifier or current-sensitive preamplifer. Greve modified Barret's method using the

current-sensitive preamplifier and floating ac signal source [92]. His biasing method has the

problem of overloading the current-sensitive preamplifier. The floating signal source easily

pick up extra noise. Wu, Evans and Yang developed the accurate phase capacitance spec-

troscopy (APCS) and the differential voltage capacitance spectroscopy (DVCS) [20,89,90].

Both systems still have the weak points. The small error of phase setting causes the large

error in measuring the susceptance in the APCS technique and the real value of dc-bias

to the Schottky diode is unclear in the DVCS technique. Also, the possibility of the large

differential dc-unbalance input into the differential preamplifier is high. Even if the differen-

tial preamplifier is used, the large dc-unbalance input into the differential preamplifier still

causes the overload of lock-in amplifier and it brings about the decrease of its sensitivity.

The best way to implement the forward-bias capacitance measurement system is to null

out both the in-phase component of a bridge circuit and the unbalanced dc differential

voltage imposed on the differential preamplifier at the same time. The circuit design and

implementation of an accurate forward-bias capacitance spectroscopy will be presented next.

8.2. Accurate Forward-bias Capacitance Spectroscopy

The prime concept of the accurate forward-bias capacitance spectroscopy is obtained

from the careful observation of current-voltage characteristics of a Schottky diode. One

must keep it in mind that the dc resistance, Rda and the ac resistance, Rac of a Schottky

diode are different. Let us discuss this matter more theoretically. One must note that the

current-voltage characteristics of a Schottky diode can be empirically expressed by

I = Is[exp(qVF/nkT) 1] (8.1)


I = Isexp(qVF/nkT) for V > kT/q. (8.2)

The dc bias of a Schottky diode is related with its dc resistance as

Rdc = VF/Idc.


If the imaginary part of the ac impedance of a Schottky diode is neglected, the ac resistance

can be expressed by

0I nkT

R.C = (5- )- -- (8.4)

S R for VF > kT/q. (8.5)
q VF

From the above equation, the following relationship can be derived as

Rac < Rdc for VF > kT/q (8.6)

Now one can conclude that even if the Rac is balanced, the Rdc can be unbalanced in the

previous bridge systems attempted by other groups. This kinds of dc unbalanced voltage

input into the differential preamplifier easily drives the lock-in amplifier into the overload

condition or nonlinear regime such that the sensitivity of measurement becomes instrumen-

tally limited.

An accurate forward-bias capacitance spectroscopy can be accomplished as in Fig.

8.1. A small signal is supplied from the built-in signal generator in the PAR 124A lock-

in amplifier and is superimposed on a de-bias voltage, which is applied by the HP 6112A

dc power supply. A variable resistor is connected in parallel to the Schottky diode. The

resistance value of variable resistor, Rp, will be determined from the maximum conductance

of a Schottky diode. This means that the parallel resistor will have Rac of which value is

close to the inverse of a Schottky conductance at the maximum forward-bias unless the

majority carrier quasi-Fermi level sweep over the conduction band edge. Since the resistor

has the same value in dc and in ac, the overall resistance value of a Schottky diode plus

the variable resistor is close to the resistance of a variable resistor. Thus, the bridge circuit

will be balanced in the terms of both ac and dc if the another variable resistor, R1, is

adjusted to null out the in-phase current. Then, the Schottky diode will be forward-biased

by one half of the applied voltage. The sensitivity of a lock-in amplifier can be set at

its maximum position. From low to high bias, one can do fine adjustments continuously

Schematic diagram of the accurate forward-bias capacitance spectroscopy.

Figure 8.1.

measuring as many data points as one wants without overloading the lock-in amplifier

because the adjusted value, R1, of the potentiometer is close to the value of Rp. Thus,

only a small adjustment is necessary at each different bias step. This means that the AFCS

measurement does not necessiate the wide range of potentiometer. This greatly improves

the efficiency of measurement.

The voltage output of the lock-in amplifier can be expressed by

Vo = [(1/R G)coswt + wCsinwt] (8.7)

where A is the voltage gain of the lock-in amplifier and Va is the magnitude of a small signal.

When the phase is set to 900 with a small error of Awt, which arises from the instrumental

limit of a lock-in amplifier, after the in-phase component is nulled out, the output becomes

Vo(Awt) = [(l/R G)Awt + wC] (8.8)

and the measurement error of wC is

A(wC) = -(G 1/R)Awt. (8.9)

If the conductance component is nulled out, the error will be very small even in the presence

of a significant Awt.

The measurement procedure can be summarized as follows:

1. The barrier height of a Schottky diode must be decided from the C-V and the I-V

measurements. Make sure the applicalbe maximum forward-bias from the barrier


2. The turn-on voltage must be decided from the I-V characteristics of a Schottky diode.

3. The conductance at the maximum forward-bias voltage must be determined and the

value of a parallel resistor, Rp, to the schottky diode can be determined in the range

of Rdc > Rp V Rac.

4. The measurements of the forward-bias capacitance can be carried out up to the max-

imum forward-bias which is equal to q(B.

As one expects from the measurement procedures described above, we can do the very

fine adjustments of a bride circuit using the precision potentiometer.

8.3. Results and Discussion

The accurate forward-bias capacitance spectroscopy was applied to the Al/n-type GaAs

Schottky diode fabricated by the metal-organic chemical vapor deposition (MOCVD) tech-

nique. The measurements were carried out at room temperature with changing the small-

signal frequency from 2 HIz to 500 Hz. Figure. 8.2. shows the result of an accurate forward-

bias capacitance spectroscopy measurement. It clearly shows the variation of forward-bias

capacitance, Cis versus frequency in the low-frequency regime. The forward-bias capaci-

tance, Ciso makes a peak at the bias of 0.65 V corresponding to the descrete interface state

located at the Ec 0.22 eV because the barrier height of Al/n-GaAs is 0.87 eV. To know

whether this level is a real descrete interface state or a demarcation level, further investi-

gation is needed. The relationships such as cut-off frequency versus temperature and time

constant of carrier exchange mechanism can be used for this purpose. The detailed steps

for this purpose are well explained in the reference [18,19]. If it is assumed that the Ec

0.22 eV is a real interface state defect, the estimation of the physical parameters such

as defect density, time constant of overall carrier exchange, and cut-off frequency can be

executed as follows. One can deduce the interface state capacitance, Ciso using the following


is = 1+ w2r2 (8.10)

First, using Eq. (8.9), one can estimate the time constant of carrier exchange mechanism,

r because one knows the forward-bias capacitances, Ci,(at 2 Hz) and Cis(at 5 lIz). The

time constant, r, will be 5.3 x 10-2 second at room temperature. The cut-off frequency is

- !
-- I
S 1
- I


/ U

. *

2 Hz i*O.




5 Hz,





20 Hz
S...o....- -**- *--*-*- .. 50 Hz
*I. -l -J-e- *-o *-- *-*-o -e--. e t I I

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

VF (V)

Figure 8.2. Forward-bias capacitance of the Al/n-GaAs Schottky diode of a small

area versus forward-bias with varying the small-signal frequency.

12001 1 1 I 1 I 1 1" i 1 1 i1 -

defined as

fe r. (8.11)

The estimated cut-off frequency, fc, is exactly 3 Hz. The peak value of Ciso is estimated

as 1625 nF. The interface state density can be easily deduced from Eq. (2.32). For the

convenience, this equation is rewritten as

Ciso = (1 F). (8.12)

Since the Ciso makes a peak at F = 0.5, the estimated interface state density, Nis, is 1.06

x 1012 cm-2eV-1. This value is very reasonable. Furthermore, considering both the time

constant, r, and the Ec 0.22 eV, this interface state level can be concluded not as a

demarcation level but as a real descrete interface state defect because the demarcation level

is located far below the Ec 0.30 eV [18,19]. The demarcation level, Edn, is expressed by
Edn = I)Bp -kTln(CN) (8.13)

where the energy Edn means that the demarcation level is located below the conduction

band; and 4Bp means the barrier height for hole carriers and its value is equal to Eg 4Bn,

i.e., 0.55 eV; and Cp (CN) denotes the capture coefficient of holes (electrons). From Eq.

(8.13), one can easily conclude that the demarcation level is located far below the Ec 0.22

eV level because the last term in Eq. (8.13) cannot be physically over 0.33 eV.

In this study, an accurate forward-bias capacitance spectroscopy (AFCS) was used to

measure the interface state capacitances in the Al/ n-type GaAs Schottky diode grown by

the MOCVD technique. It is fully demonstrated that the accurate forward-bias capacitance

spectroscopy is very powerful and flexible for the study of interface states in the metal-

semiconductor system. The AFCS method is more exact and more efficient than the other

techniques such as Schottky capacitance spectroscopy, accurate phase capacitance spec-

troscopy, and differential voltage capacitance spectroscopy. The AFCS is very useful for the

nondestructive study of the interface states in the real Schottky diode of a small area.


The major accomplishments of this study are:

(1) A generalized theory for determining the field-enhanced thermal emission rate by reverse-

pulsed deep level transient spectroscopy (RDLTS) is developed. The developed theory was

applied to measure the field enhancement at the real high electric field up to 7 x 105 V/cm.

The previous published data were limited to less than 105 V/cm. This method enables us

to measure the field- enhancement at the real high electric field just before the junction

breakdown voltage.

(2) A determination method for temperature-dependent capture cross section is developed.

This method is simple and direct. The previous methods are very complicated and undirect

because a lot of approximations are used in the procedure of theory development.

(3) An accurate determination method for DX center and shallow center densities is de-

veloped using thermally stimulated capacitance (TSCAP) technique. The free electrons on

the conduction bands of AlxGal-xAs (0.3 < x < 0.4) are supplied comparably by both the

shallow center and DX center at room temperature.

(4) Proton, electron, and neutron irradiation produce the different features of the deep level

defects in AlGaAs. The high-energy proton and neutron irradiation rendered the similar

damage to the lattice, whereas the low-energy proton and electron produced the same deep

level defects. The Ec 0.99 eV level assigned as the defect clusters, which had a big capture

cross section, was observed in the neutron and high-energy proton irradiated AlGaAs. The

Ec 0.55 eV level ascribed as the Be-vacancy complex was observed in the low-energy

proton and electron irradiated AlGaAs.

(5) The EL2-like group (Ec 0.76 eV and Ec 0.86 eV) were observed in the Te-doped

Alo.3sGao.62As irradiated by 300 keV and 1 MeV proton and 1 MeV electron. The Ec-

0.86 eV level was also observed in the pre-irradiated specimen but its density was increased

significantly, by the order, after irradiation. The Ec 0.86 eV level is assigned as AsGa and

the Ec 0.76 eV level is assigned as ASGa-VAs.

(6) An accurate forward-bias capacitance spectroscopy (AFCS) is developed for the Schottky

interface state study. This method is really applicable to the real Schottky diode of small

area. This approach is a powerful method for the nondestructive characterization of the

metal-semiconductor contacts in the VLSI integrated circuits.

The following topics are suggested as useful future efforts based on the developed methods

and approaches of the present study:

(1) A generalized theory for determining the field-enhanced thermal emission rate by RDLTS

must be applied to the GaAs/AlGaAs materials and devices characterization to decide on

the potential well type of the deep level defects. The flexibility of this theory will be limited

by the capabilities of DLTS system such as the applicable short pulse width and sensitivity

because the fast part of capturing transient capacitance is made use of in this theory. The

computerized DLTS system with the fast circuit option in connection with the excellent

pulse generator is recommended for this study.

(2) More fundamental study about the DX center is necessary in connection with the new DX

model that the DX center is not the donor + vacancy complex but the substitutional donor

itself. The DX center is related to the multiconduction band structure of AlGaAs. From

this study, the DX center has the polarization potential well. Based on the assumption that

the substitutional donor can communicate with the direct conduction band (F-conduction

band) and the indirect conduction bands at the same time, it is worthwhile to investigate

whether the polarization well can be assigned to the DX center.

(3) A determination method for the temperature-dependent capture cross section must be

applied to various deep level defects in the GaAs and AlGaAs epitaxial layers. The previous

University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs