• TABLE OF CONTENTS
HIDE
 Title Page
 Dedication
 Acknowledgement
 Table of Contents
 Abstract
 Introduction
 Theoretical analysis of photod...
 Fabrication of schottky barriers...
 Development of a high-speed GaAs...
 Development of a high-speed schottky...
 Development of a high-speed schottky...
 Summary, conclusions, and...
 Appendix
 Reference
 Biographical sketch
 Copyright














Title: Development of a high-speed gallium arsenide and indium gallium arsenide Schottky barrier photodetector for millimeter-wave optical fiber communications
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 Material Information
Title: Development of a high-speed gallium arsenide and indium gallium arsenide Schottky barrier photodetector for millimeter-wave optical fiber communications
Alternate Title: Schottky barrier photodetector for millimeter-wave optical fiber communications
Physical Description: ix, 184 leaves : ill. ; 28 cm.
Language: English
Creator: Kim, Jae-Hoon, 1952-
Publication Date: 1987
 Subjects
Subject: Optical communications   ( lcsh )
Fiber optics   ( lcsh )
Gallium arsenide   ( lcsh )
Electrical Engineering thesis Ph. D
Dissertations, Academic -- Electrical Engineering -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis (Ph. D.)--University of Florida, 1987.
Bibliography: Bibliography: leaves 175-183.
Statement of Responsibility: by Jae-Hoon Kim.
General Note: Typescript.
General Note: Vita.
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Bibliographic ID: UF00082414
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 - 000947028
oclc - 16865588
notis - AEQ9008

Table of Contents
    Title Page
        Page i
    Dedication
        Page ii
    Acknowledgement
        Page iii
        Page iv
    Table of Contents
        Page v
        Page vi
        Page vii
    Abstract
        Page viii
        Page ix
    Introduction
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    Theoretical analysis of photodetectors
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    Fabrication of schottky barriers and ohmic contacts on GaAs
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    Development of a high-speed GaAs schottky barrier photodiode for optical fiber communications
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    Development of a high-speed schottky barrier photodiode for infrared photodetection
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    Development of a high-speed schottky barrier photodiode for infrared photodetection
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    Summary, conclusions, and recommendations
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    Appendix
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    Reference
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    Biographical sketch
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    Copyright
        Copyright
Full Text













DEVELOPMENT OF A HIGH-SPEED GALLIUM ARSENIDE AND
INDIUM GALLIUM ARSENIDE SCHOTTKY BARRIER PHOTODETECTOR
FOR MILLIMETER-WAVE OPTICAL FIBER COMMUNICATIONS

















By

JAE-HOON KIM


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

UNIVERSITY OF FLORIDA


1987



















THIS THESIS IS DEDICATED TO MY LATE PARENTS.


















ACKNOWLEDGMENTS


I wish to express my sincere appreciation to my thesis adviser,

Professor Sheng S. Li, for his guidance, encouragement, and support

throughout the course of this work and to co-adviser, Professor Luis

Figueroa, for his invaluable discussions, encouragement, and help in

device characterization. I would also like to thank Professors Arnost

Neugroschel, Gijs Bosman, William R. Eisenstadt, and Paul H. Holloway

for their participation on my supervisory committee.

I am grateful to Professor P.K. Bhattacharya of the University of

Michigan, Dr. E.A. Rezek of TRW/Electro-Optics Research Center, and

Dr. G.H. Olsen of Epitaxx Inc., for growing the epitaxial layers,

Mr. R.S. Wagner of Los Alamos National Laboratory for impulse response

measurements, Drs. M.L. Timmons and P.K. Chiang of Research Triangle

Institute for their assistance in device fabrication, and Mr. Paul

Sierak of U.S. Air Force RADC/DCLW and Mr. J.J. Pan of E-Tek Dynamics

Inc., for their interests and support. Thanks are extended to my

friends and colleagues, Dr. Kwyro Lee, Dr. Hyung-Kyu Lim, Dr. Tae-Won

Jung, Dr. Mike Trippe, Mr. Won-Pyo Hong of the University of Michigan,

and Mr. Sang-Sun Lee for their helpful discussions and encouragements.












I would especially like to thank my former professors Heung-Suk

Yang, Hyung-Joo Woo, Young-Moon Park, and Hong-Sik Min of the Seoul

National University for their unceasing encouragements and guidance

throughout my graduate study.

I am greatly indebted to my late parents, my wife and son, and my

elder brothers for their endless love, patience and confidence in the

successful completion of this work, and support during all the years

of this study. The financial support of the U.S. Air Force RADC/DCLW,

E-Tek Dynamics Inc., and Microfabritech of the University of Florida

and the fellowship from the Korea Electric Association Scholarship

Foundation are gratefully acknowledged.


















TABLE OF CONTENTS



PAGE

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

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

CHAPTER

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

1.1. Development of High-Speed Photodetectors:
Motivation and Overview............................. 1
1.2. Synopsis of Chapters................................ 8

TWO THEORETICAL ANALYSIS OF PHOTODETECTORS .................... 10

2.1. General Requirements for A Photodetector............ 10
2.2. Spectral Response................................... 11
2.3. Response Speed..................................... 12
2.3.1. Drift Time............................. 13
2.3.2. Diffusion Time............................. 13
2.3.3. RC Time Constant........................... 14
2.4. Dark Current......................... ........... 15
2.4.1. Thermionic-Emission........................ 15
2.4.2. Generation-Recombination................... 16
2.4.3. Tunneling.................................. 17
2.5. D.C. Parameters..................................... 17
2.5.1. Junction Capacitance........................ 18
2.5.2. Overlay Capacitance ........................ 18
2.5.3. Series Resistance......................... 19
2.5.4. Lead Inductance............................ 21
2.6. Noise-Equivalent Power (NEP)........................ 21
2.7. Device Packaging................................... 25
2.7.1. Transmission Line Structure................ 27
2.7.2. Transmission Line Materials................. 28
2.7.2. Microstrip Transmission Line Design......... 32
2.8. Response Speed Measurement.......................... 35
2.8.1. Impulse Response.......................... 35
2.8.2. Sampling/Correlation...................... 38
2.8.3. Electro-Optic Sampling...................... 43
2.8.4. Optical Heterodyning........................ 46












THREE FABRICATION OF SCHOTTKY BARRIERS AND OHMIC CONTACTS
ON GaAs, InxGal_xAs, AND InP .............................. 48

3.1. Introduction....................................... 48
3.2. Schottky Barrier Contact Formation.................. 49
3.2.1. Schottky Barrier Height..................... 49
3.2.2. Barrier Height Enhancement.................. 52
3.2.3. Barrier Height Measurement.................. 56
3.3. Ohmic Contact Formation............................. 62
3.3.1. Ohmic Contact Technology.................... 62
3.3.2. Specific Contact Resistance Measurement..... 65
3.4. Device Fabrication................................. 74
3.4.1. Schottky Gate Formation..................... 74
3.4.2. Ohmic Contact Formation..................... 76
3.5. Experimental Results and Discussion................. 85
3.6. Summary and Conclusions.............................. 89

FOUR DEVELOPMENT OF A HIGH-SPEED GaAs SCHOTTKY BARRIER
PHOTODIODE FOR OPTICAL FIBER COMMUNICATIONS............... 91

4.1. Introduction....................................... 91
4.2. Theoretical Analysis ............................... 91
4.2.1. Quantum Efficiency.......................... 93
4.2.2. Response Speed.............................. 94
4.3. Device Fabrication.............................. .. 99
4.4. Experimental Results and Discussion................ 102
4.4.1. Current-Voltage Measurement................ 102
4.4.2. Capacitance-Voltage Measurement............. 102
4.4.3. A.C. Admittance Measurement.. ................... 104
4.4.4. Spectral Response Measurement............... 107
4.4.5. Response Speed Measurement................. 107
4.5. Summary and Conclusions............................. 111

FIVE DEVELOPMENT OF A HIGH-SPEED Au/p-In0.53Ga0.47As/P+-InP
SCHOTTKYBARRIER PHOTODIODE FOR INFRARED PHOTODETECTION... 113

5.1. Introduction....................................... 113
5.2. Theoretical Analysis................... ............. 113
5.2.1. Quantum Efficiency.................. ........ 113
5.2.2. Response Speed.............................. 114
5.3. Device Fabrication................................ 116
5.4. Experimental Results and Discussion................. 119
5.4.1 Current-Voltage Measurement................ 119
5.4.2. Capacitance-Voltage Measurement............ 119
5.4.3. A.C. Admittance Measurement................ 124
5.4.4. Spectral Response Measurement............... 124
5.4.5. Response Speed Measurement.................. 127
5.5. Summary and Conclusions............................ 133













SIX DEVELOPMENT OF A HIGH-SPEED Au/p+-n-In0.53Ga0.47As/n+-InP
SCHOTTKY BARRIER PHOTODIODE FOR INFRARED PHOTODETECTION... 135

6.1. Introduction........................................ 135
6.2. Theoretical Analysis............................... 136
6.2.1. Quantum Efficiency.......................... 136
6.2.2. Response Speed.............................. 139
6.3. Device Fabrication................................. 139
6.4. Experimental Results and Discussion................ 145
6.4.1. Current-Voltage Measurement...................... 145
6.4.2. Capacitance-Voltage Measurement............. 147
6.4.3. A.C. Admittance Measurement................. 147
6.4.4. Spectral Response Measurement............... 147
6.4.5. Response Speed Measurement................. 149
6.5. Summary and Conclusions............................ 149

SEVEN SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS................ 153

7.1. Summary and Conclusions ............................ 153
7.2. Recommendations for Further Study................... 155
7.2.1. Packaging Optimization of Photodiodes....... 155
7.2.2. A High-Speed In0Z53Ga0 47As Schottky Barrier
Photodiode on Semi-insulating InP Substrate
for Monolithic Integration.................. 155
7.2.3. A Low Noise and High Gain-Bandwidth Product
Quantum-Well Avalanche Photodiode .......... 156
7.2.4. Monolithic Optoelectronic Integration....... 160

APPENDIX

A General Model for Schottky Barrier Height................ 161
B Schottky Barrier Height Enhancement...................... 164
C Schottky Barrier and Ohmic Contact Formation.............. 167
D Lift-Off Photolithography............. .................... 170
E Mesa Etch and Metal Etch.................................... 172
F Surface Passivation........................................ 174

REFERENCES......... ....... ......................... ....... 175

BIOGRAPHICAL SKETCH........................................... .... 184
















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

DEVELOPMENT OF A HIGH-SPEED GALLIUM ARSENIDE AND
INDIUM GALLIUM ARSENIDE SCHOTTKY BARRIER PHOTODETECTOR
FOR MILLIMETER-WAVE OPTICAL FIBER COMMUNICATIONS



By



JAE-HOON KIM



MAY 1987



Chairman: Sheng S. Li
Major Department: Electrical Engineering

This dissertation describes the development of high-speed GaAs

and InP-based In0.53Ga0.47As Schottky barrier photodetectors operating

in near-infrared regime, especially close to the dispersion minimum of

optical fibers, i.e., 1.30-1.55 m for millimeter-wave optical fiber

communications. Novel high-speed Au/p+-n-In0 53Ga0 47As/n -InP and

Au/p-In0.53Ga0.47As/p+-InP Schottky barrier photodiodes as well as a

GaAs Schottky barrier photodiode have been developed in this study.

The results show that the GaAs Schottky barrier photodiode has

a responsivity of 0.41 A/W and a quantum efficiency of 62 % at 820 nm.

The impulse response to the pulse laser with a FWHM of 110 ps yields

a risetime of 27 ps and a FWHM of 94 ps.


viii












The GaAs Schottky barrier photodetector is attractive for the

short optical links where the maximum modulation frequency is not

limited by optical fiber dispersion but only limited by detectors,

lasers, and electronics. The InP-based In0.53Ga0.47As Schottky

barrier photodiode using p+-n-In0.53Ga0.47As or p-In.053Ga0.47As

structure has first demonstrated. To develop a p+-n-In0.53Ga.047As

Schottky barrier photodiode, the barrier height enhancement on

n-In0.53Ga0.47As epilayer has been studied.

The Au/p-In0.53Ga0.47As/p+-InP Schottky barrier photodiode has a

responsivity of 0.43 A/W and a quantum efficiency of 40.8 % at 1.3 m

without antireflection coating. The impulse response measurements

yield a risetime of 85 ps for Au/p-In0.53Ga0.47As/p+-InP photodiode

and 180 ps for Au/p+-n-In0.53Ga0.47As/n+-InP photodiode. The results

show that the Au/p-In0.53Ga0.47As/p+-InP Schottky barrier photodiode

is a very promising candidate for high-speed and high-frequency

detector applications, while the Au/p+-n-In0.53Ga0.47As/n+-InP

Schottky barrier photodiode needs more study to obtain reproducibility

and reliability of the barrier height enhancement on n-In.053Ga .47As

epitaxial layer inspite of its promising potential.

To improve photodetector performance and frequency response,

the future efforts should be directed towards the development of the

photodetectors on semi-insulating substrates suitable for monolithic

integration improving surface dielectric passivation and reducing the

undesired packaging parasitics.


















CHAPTER ONE
INTRODUCTION



1.1. Development of High-Speed Photodetectors-Motivation and Overview

The main motivation of this thesis is to develop a high-speed

photodetector capable of demodulating the optical signals up to 20 GHz

for millimeter-wave optical fiber links. Lightwave communications

require a high-speed and high sensitivity photodetector in order to

achieve a high data rate at low signal level. Most of the high-speed

photodetectors have been fabricated mainly on GaAs using a Schottky

barrier structure for 0.80-0.90 pm and on In0 .53Ga0.47As using a p-i-n

structure for 1.30-1.65 pm.

With the introduction of femtosecond laser pulse technology and

lightwave modulation in the several tens of gigahertz frequency range,

the development of photodetectors with a high-speed and broad

bandwidth is necessary for millimeter-wave optical fiber

communications operating in the infrared regime, especially close to

the dispersion minimum of optical fibers [1-5]. For this purpose a

novel high-speed InxGal_xAs (x=0.53) Schottky barrier photodiode for

1.30 1.55 pm photodetection has been developed.

Figure 1.1 shows the block diagram of millimeter-wave optical

fiber links using an electro-optic modulator (EOM). For these

applications, the photodetectors must satisfy several requirements































1 Km Single-Mode
Fiber Cable


Low Noise
Post-Detector
Amplifier


D.C. : Demountable Connector

I Laser Diode and EOM Interface

P : Single-mode Fiber Pigtail

T : Waveguide to Microstrip or
Fin Line Transition

W : Input/Output Waveguide Interface


Block diagram of millimeter-wave fiber optic link using an electro-optic modulator (EOM).


Output


Signal
Input


I
I
I
I
I
I
I
I
I
I
I
I
1
I
I
I
I
I
I
1
I
I


Figure 1.1.












such as high response speed, high sensitivity, and low noise at the

operating wavelength. The design goal of a high-speed photodetector

is listed in Table 1.1. The promising candidates are GaAs Schottky

barrier [6-11], AlGaAs/GaAs or GaAs p-i-n [12,13], InxGal_xAs p-i-n

[14-20], InxGal_xAs Schottky barrier [21-24], avalanche photodiode

including quantum-well structure [25-30], and photoconductive detector

[31-40]. The major structures for high-speed photodetectors and the

institutions that performed the pioneering work on these structures

are listed in Table 1.2. The III-V compound semiconductors have shown

a great potential for use as high-speed optoelectronic device

materials because of their high electron mobility, high saturation

velocity, and good lattice-match to the InP substrate [41-43].


Table 1.1. DESIGN REQUIREMENTS FOR

Millimeter-Wave Frequency

Modulation Bandwidth

Input/Output Power Level

Input/Output Impedance

Input Signal Type

Optical Source

Quantum Efficiency

Optical Fiber Type


HIGH-SPEED PHOTODETECTORS

20-27 GHz

10 % of Center Frequency

0 dBm

50 ohm

Analog Signal

Laser Diode

50 %

Single-mode


InxGal_xAs, whose composition is determined by the intersection

of two important III-V compound semiconductor alloys (i.e., Inl_xGaxAs

and InlxGaxAsyPy : y=2.2x), is one of the most promising materials














DEVELOPMENT OF HIGH-SPEED PHOTODETECTORS. (1980-1986)


Photodetector Structure Institution

GaAs Schottky Barrier Photodiode HP, Hughes

GaAs p-i-n Photodiode MIT Lincoln Lab.

AIGaAs/GaAs p-i-n Photodiode CALTEC

Interdigital GaAs Photoconductive Detector HUGHES, TUA

InGaAs/lnP p-i-n Photodiode Bell Lab, TRW/EORC

InGaAs/lnP Photoconductive Detector TUA (W.Germany)

Interdigital InGaAs/GaAs

Photoconductive Detector TRW/EORC

Avalanche Photodiode AT&T Bell Lab.

Quantum-Well Avalanche Photodiode AT&T BellLab.

InGaAs/lnP Schottky Barrier Photodiode U. Florida, TUA

Interdigital InGaAs Schottky

Barrier Photodiode U. Florida


Table 1.2.












for long wavelength photodetectors because its energy bandgap can be

tailored to the wavelength of 0.95-1.65 pm. Figure 1.2 shows a

detailed behavior of the energy bandgap vs. lattice parameter of an

Inl-xGaxAsy Ply as a function of alloy composition. Figure 1.3

illustrates the energy bandgap vs. lattice constant for several III-V

compound semiconductors. To take advantage of these excellent

physical properties, the parasitic RC components and the high power

consumption in parasitic resistances should be minimized. The low

specific contact resistance is required for high performance

photodiode applications. Several new types of high-speed Schottky

barrier photodiodes capable of demodulating the optical signals at

1.30-1.55 pm will be discussed in this thesis.

To develop a high-speed photodetector for millimeter-wave optical

fiber communications, the Schottky barrier structure has been chosen.

Schottky barrier photodiode has many advantages such as simplicity of

fabrication, reliability, absence of high-temperature diffusion

processes which can degrade a carrier lifetime, and high response

speed. Unfortunately, Schottky barrier contacts on n-InGaAs yield a

low barrier height (oBn = 0.2-0.3 eV) [44-46], which makes Schottky

contacts too leaky to be useful for photodetector applications.

Therefore, the effective barrier height needs to be increased to

overcome the problem associated with the low Schottky barrier height.

The significance of p -n-InGaAs/n -InP Schottky barrier lies in its

ability to enhance the barrier height and to reduce the dark current

for high performance photodetector applications.





















GaAs


1.30
1.20
1.10
1.00
0.90
0.80

0.70

0.60

0.50


0.40

InAs




Figure 1.2.


2.10 eV


2.00


1.90

1.80

1.70


\ \1.60

N. 1.50

'\ \ 1.40

z \ \ \ inP
z r----- >"P-




Energy bandgap vs. lattice parameter of an In xGa As P1-
quaternary compound semiconductor as a function of alloy
composition, x and y.















































0 1 I l i I I I
5.4 5.6 5.8 6.0 6.2 6.4


LATTICE CONSTANT d(A)


Figure 1.3.


Energy bandgap vs. lattice constant for III-V compound
semiconductors. Note that the solid lines represent a
direct bandgap material while the dotted lines represent
an indirect bandgap material.












However, Schottky barrier contacts on a moderately doped p-InGaAs

epilayer are expected to yield a good barrier height (gBp = 0.5-0.7

eV) for the proposed high-speed photodiode when a suitable metal and

good surface preparation are provided [47,48]. The results have shown

that Au/p-InGaAs/p -InP Schottky barrier photodiode is a promising

candidate for millimeter-wave optical fiber links.


1.2. Synopsis of Chapters

High-speed and high-sensitivity photodetectors are indispensable

for the gigabit rate lightwave communications as well as for the

integrated optoelectronic applications. This dissertation deals with

high-speed GaAs and InxGal_xAs photodetectors using Schottky barrier

structure for millimeter-wave optical fiber links. In chapter two, a

theoretical analysis of photodiode parameters relating to the general

design requirements for a photodetector is reviewed. For device

characterization the microstrip transmission line, on which the

photodetector is mounted, has been fabricated on a Cr-Au coated

alumina (A1203) substrate.

In chapter three, the formation of Schottky barrier and ohmic

contact on III-V compound semiconductors such as GaAs, InxGal_xAs, and

InP is discussed. The Schottky barrier height enhancement of n-InGaAs

to reduce the dark current associated with the low barrier height is

described. The optimum conditions for low resistance ohmic contact

and the ohmic contact measurement are discussed. In chapter four, the

fabrication of a high-speed GaAs Schottky barrier photodiode has been

discussed and the characterization of the photodiode has been












described by the current-voltage (I-V), capacitance-voltage (C-V),

a.c. admittance, spectral response, and impulse response measurement.

In chapter five, a picosecond response Au/p-InxGalxAs/p+-InP

Schottky barrier photodiode for the infrared detection is presented.

The Schottky barrier contact on p-InxGal-xAs epitaxial layer provides

the desired barrier height for the proposed photodetector depending on

the surface preparation conditions such as sputter etch or chemical

etch. In chapter six, a novel Au/p+-n-InxGal_xAs/n+-InP Schottky

barrier photodiode operating in the infrared regime is proposed. This

modified Schottky barrier structure requires the deposition of a thin

surface layer of p -InxGalxAs on n-InxGalxAs epitaxial layer. The

barrier height for such a photodiode can be tailored to its optimum

value via the properly selected thickness and the dopant density of

the ultra-thin surface layer.

In chapter seven, summary and conclusions are presented and

recommendations for further study are discussed, which include:

(1) photodetector packaging optimization, (2) development of an InGaAs

Schottky barrier photodiode on a semi-insulating substrate, and (3)

development of a photoreceiver module for monolithic optoelectronic

integration. In the appendix a general model for Schottky barrier

height, barrier-height enhancement, Schottky and ohmic contact

formation, lift-off photolithography, chemical etch including mesa

etch and metal etch, and surface dielectric passivation procedures are

described.


















CHAPTER TWO
THEORETICAL ANALYSIS OF PHOTODETECTOR PARAMETERS


2.1. General Requirements for A Photodetector

The general design requirements for a high-speed photodetector

include (1) high quantum efficiency, (2) low dark current, (3) low

capacitance and resistance (for high-speed and low noise), and (4) low

excess noise (especially for avalanche photodiode). These requirements

for photodetectors are tied to the particular material requirements

which include [15]

(1) Energy bandgap with a high absorption coefficient should be

smaller than the photon energy to be detected.

(2) Direct energy bandgap material must be used so that optical

radiation can be absorbed in a short distance in order to

minimize transit time effects (for high response speed).

(3) High-quality low-defect density and high-purity (especially

for long wavelength photodetector) material must be used so

that Zener-tunneling and dark current can be minimized.

(4) The material must be doped properly so that the depletion

layer width, which is determined by a trade-off between high

speed and high quantum efficiency, can be optimized.

(5) The epilayer material should be lattice-matched to the

substrate material for a long wavelength photodetector.












2.2. Spectral Response

For short wavelength (0.50-0.85 pm) detection, photons are

absorbed near the semiconductor surface. The photogenerated excess

carriers are separated in the depletion region close to the surface

of a photodiode. It is advantageous to use a metal-semiconductor

Schottky barrier structure with a thin (~ 100 R) semi-transparent

metal film. In this detection mode an extremely high response speed

and high quantum efficiency can be obtained, if the depletion region

is small and comparable to the light penetration depth. For long

wavelength (0.95-1.65 pm) detection, light penetrates deeply into the

material. Therefore, a high quantum efficiency requires the material

with a wide depletion layer width. For these photodiodes a trade-off

exists between a quantum efficiency and a response speed.

The external quantum efficiency of a Schottky barrier photodiode

is determined mainly by the transmission loss in the metal film and

the reflection loss at the metal-semiconductor interface as well as

the recombination loss in the diode. To reduce the reflection loss an

AR coating is usually incorporated in the photodiode fabrication. This

can be achieved by depositing a thin dielectric film such as

Ta205, SiO2or Si3N4 with its thickness equal to the quarter

wavelength of the incident radiation at the selected wavelength. The

thickness of a single layer AR coating film is given by [49]


dl (X/4 n) tan-1[2nlks/(nl2-ns2-ks2)] (2.1)


where Xo is the wavelength of a selected incident light, nl the index












of refraction of the dielectric film, and ns the complex index of

refraction of the semiconductor. In the case of a weakly or

nonabsorbing substrate, Eq. (2.1) can be reduced to the well-known

quarter wavelength design formula, i.e., di = Xo/4nl. The minimum

reflection loss with a quarter wavelength anti-reflection (AR) coating

is given by [50,51]


Rmin = [(n12-non2)/(n12+non2)]2 (2.2)

where no, nl, and n2 are the index of refraction of air, AR coating

film and semiconductor substrate, respectively.


2.3. Response Speed

The response speed of a photodetector can be determined primarily

by three parameters: transit time (ttr) across the depletion region,

diffusion time (tdiff) in the quasi-neutral region, and RC time

constant (tRC) required to discharge the junction capacitance (Cd)

through a combination of internal and external resistances. The

total risetime of a photodiode, which is defined as the response time

from 10 % to 90 % of a pulse height, is essentially equal to the

largest of the three. The total risetime can be expressed by


r = (ttr2 + tdiff2 + 'RC2)1/2 (2.3)


We can relate the risetime to the 3-dB cutoff frequency (fc) given by


f = 0.35/tr (2.4)


where the 3-dB cutoff frequency, fc is often regarded as the bandwidth












of the photodiode. For high speed operation, the carriers are being

excited within the depletion region of the junction or close to the

junction so that the diffusion time is shorter than or at least

comparable to the drift time and the photogenerated carriers are

collected across the junction at scattering limited velocity (Vsat)j

The depletion width is a trade-off between the fast transit time

requiring a narrow depletion region and the combination of quantum

efficiency and low capacitance which requires a wide depletion region.


2.3.1. Drift Time
*
When the carriers are generated in the depletion region, they

should be collected by traversing across the depletion region. The

carrier drift (transit) time across the depletion region is given by

ttr = W/2.8 vs [52], where vs is the saturation drift velocity of the

carriers, and W is the depletion layer width. This expression is

strictly true only for a constant junction field and injection of the

electrons into the junction, but it remains a reasonable approximation

for carriers created in the junction field. For high mobility

materials, the transit time is limited by a saturated drift velocity.


2.3.2. Diffusion Time

The carriers which are generated in the quasi-neutral base region

will diffuse to the drift region. This carrier diffusion will result

in a time delay of the carriers reaching the drift region. The

diffusion time is given by tdiff = Wp,n2/(243 Dnp) [52], where Wp,

is the thickness of the quasi-neutral epilayer.












2.3.3. RC Time Constant

The lumped circuit constant of a photodiode also limits its

response speed. Assuming that the drift time and the diffusion time

can be greatly reduced by optimizing the device configuration, the

response speed is mainly determined by the depletion capacitance (Cd),

the series resistance (Rs), and the load resistance (RL). The

bandwidth of the photodiode, and hence its response speed depend on

the load. For high frequency performance the load should be small.

The shunt resistance (Ri) is generally very high, but is included to

account for the relatively low leakage resistance of the photodiode.

The RC time constant is given by


tRC = (Rs + RL)Cd (2.5)

The bandwidth of a photodiode is usually characterized by the 3-dB

cutoff frequency, which is given by


fc = 1/[27T(Rs+RL)Cd] for (Rs+RL)/Ri << 1 (2.6)


It should be noted that practical detection systems usually have

lower cutoff frequency because of the finite load resistance, the

parasitic capacitance and the lead inductance of the photodiode. For

high speed operation, the series resistance should be small (usually

less than 10 ohm for a well-designed diode) and the load resistance

should be low (usually 50 ohm). The load resistance may be reduced in

a high-speed circuit. However, it should be as high as possible for a

low-noise detection circuit. Therefore, the depletion capacitance

should be minimized.












2.4. Dark Current


The dark current depends strongly on the barrier height of a

Schottky barrier photodiode. The dark current depends also strongly

on diode material, geometry, and surface passivation. The use of

InxGal_xAs/InP heterostructure and surface passivation could reduce

the dark current of a photodiode. The reduction of dark current is

important for the improvement of the minimum detectable power.

The dark current of a Schottky barrier photodiode consists of

thermionic-emission current, generation-recombination current via

traps in the depletion region, tunneling current due to carriers

tunneling across the bandgap, and surface leakage current or interface

current due to traps at the metal-semiconductor interface. Tunneling

current can be neglected for low impurity doping concentration (less

than 1017 cm-3). Surface leakage current is not a fundamental device

characteristic and in most cases can be eliminated by careful

processing and passivation techniques. The total current consists of

thermionic-emission current over the Schottky barrier and the

generation-recombination current in the depletion region.


2.4.1. Thermionic-Emission Current

The dark current in the forward bias direction of a Schottky

barrier diode is determined mainly by thermionic-emission of majority

carriers from the semiconductor into the metal for doping levels less

than 1017 cm-3 [53,54].


Ith= SA*T2exp[-q( Bgn)/kT][exp(qV/nkT)- 1]


(2.7)












I
The effective barrier height, I Bn = Bn 6m can be determined from

the measured value of the saturation current.


2.4.2. Generation-Recombination Current

At zero bias the depletion region of the Schottky barrier is in

thermal equilibrium and the rate of electron-hole pair generation is

balanced by the rate of recombination. In the presence of an applied

voltage, there will be a net generation or recombination current

depending on the polarity of the bias. The generation-recombination

current through the midgap traps in the depletion region, which is

dominant at low voltage, is given by [53]


Igr = qniAW/teff[exp(qV/2kT) 1] (2.8)


where teff = (tntp)1/2 is the effective carrier lifetime in the

depletion region. This current is added to the thermionic-emission

current and may cause deviations from ideal behavior in a Schottky

barrier diode. Note that the current is a generation current when the

junction is reverse biased and is a recombination current when the

junction is forward biased. The total current can be expressed by [53]


Itot = Iths[exp(qV/kT) 1] + Igrs[exp(qV/2kT) 1] (2.9)

The ratio of thermionic-emission current to generation-recombination

current increases with a bias voltage, energy-gap, effective carrier

lifetime, and temperature and decreases with the barrier height. The

recombination current is important in high barrier, in low lifetime

material, at low temperature, and at low forward bias voltage [53,54].











2.4.3. Tunneling Current

Tunneling current, either band-to-band or via deep-level traps

dominates the dark current at high voltage (and then low capacitance),

resulting in the soft breakdown characteristics. For heavily doped

semiconductors the dominant process changes from thermionic-field

emission to field emission and the contact states to behave like an

ohmic contact with a sufficiently small contact resistance. In

addition, the exponential dependence of the current changes from qV/kT

to qV/Eo [55].


J = Jsexp{qV/[Eoocoth(Eo,/kT)]} (2.10)


Eoo = (qh/2)(ND/m*s)/2 (2.11)


= 1.85x1e-14(ND/mrer)1/2 (2.12)


wnere m (= mrmo) is the effective mass of electron and Es (= eoer) is

permittivity of the semiconductor. Eoo is a very useful parameter in

predicting the relative importance of thermionic-emission or

tunneling. For Eoo/kT << 1, the thermionic-emission process dominates

and the contact behaves as a Schottky barrier. For Eoo/kT >> 1,

field emission dominates and the contact exhibits ohmic

characteristics. For Eoo/kT = 1, a mixed mode of transport occurs.


2.5. D.C. Parameters

The total capacitance of a packaged Schottky barrier photodiode

is given by CT = Cj + C + Cp, where Cj is the metal-semiconductor

junction capacitance, Co is the overlay capacitance across the












dielectric passivation layer, and Cp is the package parasitic

capacitance. The overlay and package parasitic capacitance should be

minimized.


2.5.1. Junction Capacitance

The junction capacitance is simply given by the one-sided abrupt

junction analysis. Measurements of junction capacitance can be used

for determining the background shallow impurity profile of a Schottky

barrier diode or a one-sided abrupt junction diode. The background

dopant density is given by


NB = (2/qoesA2){d(VR + VD)/dCj-2} (2.13)


where


Cj = A{qerND/2(VD V)}1/2 (2.14)


where Ng is the dopant density of the light-doped side. The diffusion

potential VD of a Schottky barrier diode is determined from the

intercept of C2 vs. VR curve, and the barrier height of a Schottky

diode can be calculated using the following equation.


CBn = VD + (kT/q)ln(Nc/ND) (2.15)


2.5.2. Overlay Capacitance

The capacitance due to the metal contact overlaying the

passivating dielectric layer in a Schottky barrier diode may be

important. Assuming negligible space-charge penetration (a realistic

assumption for SiO2 on the semiconductor), the overlay capacitance is












co = eoerA/Wo (2.16)


where Wo is the thickness of the dielectric layer, and A[=2(Ri+ A)A ]

is the area of the dielectric layer. This parasitic capacitance must

be kept to a minimum particularly at X-band frequency or higher.

Overlay contacts are not generally used above 40 GHz frequencies

because they degrade the overall performance of a diode [55]. Figure

2.1. shows the different values of the overlay capacitance for

different values of Ri, A and Wo. The thick dielectric layer (e.g.,

2 pm) can substantially reduce the overlay capacitance of a Schottky

barrier diode.


2.5.3. Series Resistance

The series resistance is composed of the lead resistance, the

spreading resistance of the base material, and the sheet resistance of

the epilayer and the substrate. The series resistance is due mainly to

the sheet resistance of the semiconductor substrate and the undepleted

epilayer. This resistance is distributed, depending on the contact

geometry, and is frequency dependent. The series resistance of an

epilayer is given by


Rsl = 2W/qpnNDA (2.17)


where W is the thickness of the epilayer, and Nd is the donor density

of the epitaxial or active layer. The resistance contributed by the

substrate may be modeled by using the resistance of a contact dot on a

semi-infinite semiconductor substrate.





















1.0









LL.
a-

f .10












.01
.02


Figure 2.1.


Overlay capacitance of the Si02 passivation
layer with a thickness of 0.1 pm.


.04 .06 0.1 0
R, MILSS)


0.4 0.6












Rs2 = 2P(A/7 )1/2 (2.18)

where ps is the substrate resistivity. The total series resistance

shown in Fig. 2.2. consists of Rs1 due to the epilayer and Rs2 due to

the semiconductor substrate.


2.5.4. Lead Inductance

The intrinsic response speed of a photodiode is usually degraded

by the extrinsic circuit elements, such as junction and parasitic

capacitances, series and load resistances, and inductances. The

inductance is due primarily to the bond wire. If the round wire is

used as a bond wire, the inductance Lb (nH) can be estimated by [56]


Lb = 5.08x10-3L[ln(L/d) + 0.386] (2.19)


where d (mil) is the diameter of the bond wire. If the metal ribbon

of width W, thickness t, and length L is used, the inductance due to

the bond wire can be obtained by [56]


Lb = 5.08xl0-3L[ln(L/W+t) + 1.19 + 0.022(W+t)/L] (2.20)


The inductance due to the round wire and ribbon is practically less

than 2 nH. The inductance is 0.14 nH for the bond wire with a diameter

of 1 mil and a length of 10 mil.


2.6. Noise-Equivalent Power (NEP)

The minimum detectable optical power of a photodiode is limited

by its noise performance. The noise generated in a photodiode

operating under reverse bias condition is a combination of shot noise,
















ric Layer
as SiO2)





Metal Back
Contact


RS = RS + RS2


Figure 2.2.


Structure and equivalent circuit of the
Schottky barrier diode.


WO
T












1/f noise (or flicker noise), and thermal noise (or Johnson noise).

The shot noise is due to the photogenerated currents of the signal,

background illumination, and the reverse-bias dark current. The

thermal noise arises from a random motion of the carriers within any

resistive materials including semiconductors, and is always associated

with a dissipative mechanism. The flicker noise (or 1/f noise) has a

current-dependent power spectrum which is inversely proportional to

the frequency existing in all devices when a current flows.

At low frequency 1/f noise dominates, and at intermediate

frequency the generation-recombination noise dominates. At high

frequency, the infrared photodetectors exhibit a white (frequency

independent) noise which include thermal, generation-recombination,

and shot noise. The transition points vary with semiconductor

material, doping concentration, and processing technology. However,

for infrared detectors these transition frequencies are roughly at 1

KHz and 1 MHz, respectively. In the wavelength region of interest for

optical communication, the detection is limited either- by thermal or

shot noise. The schematic diagram to characterize the photodetector

noise is shown in Fig. 2.3. The effect of noise on the signal

transmission is measured by the signal-to-noise ratio (SNR) for analog

signals. The signal-to-noise ratio (SNR) is the signal power at an

output of the detection circuit divided by the average noise power.


SNR = (1/2) s2R ff/- 2Reff (2.21)


where i2 is the sum of the square of all noise source currents and i
nl s






























MMW
AMPLIFIER MIXER


DEVICE
UNDER TEST


OSCILLATOR


Figure 2.3.


Schematic diagram for millimeter-wave photodetector noise measurement.
The detector noise can be measured at both dark and illuminated condition.











is the signal noise current amplitude obtained from a sinusoidally

modulated optical signal,is = ( ?qXPLq/hv), where PL is the average

optical signal power, which is assumed to be 100 % intensity

modulated. Therefore, SNR is given by [5]


(1/2)( 1q1XPL/hV)2
SNR = (2.22)
2q Af(Is+IB+ID)+(4kT Af/Reff)F


The optical power PL which generates a signal amplitude equal to the

noise amplitude ( i.e., SNR = 1 ) at the output is called the minimum

detectable power Pmin' which is given by


Pmin = (2hc/X7 )(Af)1/2[(Af)1/2+{ Af+(IB+ID)/q+(2kTF/q2Reff)}1/2]

(2.23)
The noise-equivalent power is obtained by dividing Pmin by (Af)1/2


NEP = (2hc/Xt )[(Af)1/2+{Af+(IB+ID)/q+(2kTF/q2Reff }/2] (2.24)


The noise due to the background illumination can be neglected

because it can be made vanishingly small in optical fiber

communication circuits. Figure 2.4 shows the theoretical calculation

of NEP vs. effective resistance, Reff, with a parameter of a dark

current for the photodetectors.


2.7. Device Packaging

The photodetector should be packaged to protect them from

mechanical or other possible damages and to allow their incorporation

into electrical and optical circuits. However, packaging introduces

additional parasitic effects and may attenuate or distort both

















1-10
10


-11
10


-12
10

-13
10


-14
10


-15
10


1 103 10 109 1012

EFFECTIVE RESISTANCE ()


Figure 2.4.


Noise-equivalent power (NEP) vs. effective
resistance as a parameter of a dark current.












electrical and optical signals, if not properly designed. The package

size determines the parasitic impedance added to the diode impedance,

predominantly the lead inductance and package capacitance. For low

noise and large bandwidth the capacitance should be extremely small.

Therefore, small packages are preferable for low noise-equivalent

power and high modulation frequency of photodiodes for optical fiber

communications.

In packaging a specially designed photodetector for optical fiber

communications, a fiber with a relatively large numerical aperature

and diameter is positioned close to the top contact above the

illumination window of the diode. This kind of package allows the

maximum quantum efficiency at minimum background illumination

obtainable for a fiber-diode connection. Electrical contact to a

photodiode is most easily achieved by soldering the wireleads or bonds

into the electrical circuit. If a microstrip transmission line is

used, the parasitic capacitance can be reduced. To allow a

demountable connection to a commercial 50 ohm amplifier, the

photodiode should be incorporated into a miniature coaxial cable. The

latter approach may limit the system performance because of the low

input impedance (thermal noise).


2.7.1. Transmission Line Structure

Microstrip transmission lines have been extensively used for

microwave and millimeter-wave hybrid integrated circuits. The passive

lumped elements can be fabricated on the same substrate and chip or

beam-lead active devices can be bonded directly to the strips.












Microwave integrated circuits (MIC) using microstrip line can be

designed for frequencies ranging from a few GHz up to many tens of

gigahertz (GHz). At higher frequencies, particularly into the

millimeter wavelength ranges, losses increase greatly and higher-order

modes become a considerable problem [57]. The frequency limit for 'the

use of microstrip transmission line is probably around 100 GHz.

Coplanar waveguide (CPW) structure is a good alternative to the

microwave device packaging. CPW has an important advantage over

microstrip transmission line in that the signal conductor and the

ground plane conductor share the same surface of the dielectric

substrate. The photodiode can be strapped across the intermetallic

gaps without drilling the substrate. This waveguiding structure

offers convenient incorporation of the lumped devices and short

circuits, which is more difficult in the microstrip line [57]. This

means that circuit connections to ground can be made simply with

short, low parasitic bond wire connections. A variety of transmission

line structures for MIC applications are shown in Fig. 2.5. Each type

of the transmission line structure has potential advantages for

various applications and therefore their characteristics are

summarized in Table 2.1.


2.7.2. Transmission Line Materials

The substrate materials should have the following characteristics

[58]: (1) high dielectric constant, (2) low dissipation factor (or

loss tangent), (3) frequency and temperature independent dielectric

constant, (4) high thermal conductivity, (5) high resistivity and

















Strip




Substrate Meta
SMetal
(A) MICROSTRIP LINE


Dielectric




SMetal


(C) FINLINE


(B) SUSPENDED STRIPLINE

) I D M OST ubstrate






(D) INVERTED MICROSTRIP LINE


Metal


LINE


SSubstrate
(F) SLOTLINE


Substrate


(G) COPLANAR WAVEGUIDE


Metal
Substrate

(H) DIELECTRIC IMAGE GUIDE


MIC transmission line structures for the photodetector package.


Slot













(E)


Figure 2.5.










PROPERTIES OF MIC TRANSMISSION LINE STRUCTURES.


TRANSMISSION CHARACTERISTIC FREQUENCY RANGE DEVICE/COMPONET COMPATIBILITY
LINE STRUCTURES IMPEDANCE (0) OF OPERATION (GHz) LOSS DISPERSION INTEGRATION LEVEL
IN SERIES IN PARALLEL
Microstrip 25-125 upto 100 Medium Low Easy Difficult
Line

Coplanar 30-150 up to 60 High High Easy Easy
Waveguide

Slot 60-200 High Non Difficult Easy
Line TEM Mode

Suspended
Stplined 40-150 >100 Low High Easy Difficult
Stripline

Fine 10-400 30-100 Low Low Easy Easy

Inverted
Microstrip Line 25-130 >100 Low Low Unknown Unknown

Trapped Inverted
Microstrip Line 30-140 up to 100 Low Low Unknown Unknown

Dielectric Image
Waveguide 20-30 >100 High High Difficult Difficult


Table 2.1.












dielectric strength, (6) high purity and constant thickness, and (7)

high surface smoothness. The conductor materials should have the

following properties: (1) high conductivity, (2) low temperature

coefficient of resistance, (3) good adhesion to the dielectric

substrate, (4) good etchability and solderability, and (5) easily

deposited or electroplated. The metal conductor stripe is deposited

by thin film technology on a dielectric substrate of relative

dielectric constant. The properties of dielectric films should have

(1) reproducibility, (2) capability of withstanding high voltages, and

(3) low RF dielectric loss.

Typical substrate materials are alumina, GaAs, Duroid, sapphire,

quartz, and fused silica with er varying from 2 to 12 and typical

conductor materials are gold, silver, aluminum, and copper.

Substrate properties show that good electrical conductors have poor

substrate adhesion, whereas poor electrical conductors have good

substrate adhesion. Aluminum has relatively good conductivity and

good adhesion. It is possible to obtain good adhesion with high

conductivity materials by using a very thin film of one of the poorer

conductors between the substrate and the good conductor. Some typical

combinations are Cr-Au, Cr-Cu, and Ta-Au. The choice of conductors is

usually determined by compatibility with other materials required in

the processes. For small losses, the conductors should be of the

order of 3 to 5 skin depths thick. That is, thick films of the good

conductor are required. The commonly used dielectric films are Al203,

SiO, SiO2, Si3N4, and Ta205 [58].












2.7.3. Microstrip Transmission Line Design

The parameter analysis of microstrip lines can be obtained by

numerical approximations such as conformal mapping method, variational

method, or relaxation method if the transverse electro-magnetic (TEM)

mode is assumed to be dominant. The electrical parameters are

characterized by characteristic impedance, attenuation factor, and

wavelength. The most important dimensional parameters are the

microstrip width, W, and height, H. The relative permittivity of the

dielectric substrate is also important.

The microstrip width can be calculated iteratively for a required

line impedance using the substrate dielectric constant and thickness.

The operation frequency is then used to obtain the guide wavelength

and phase velocity. The conductor loss and dielectric loss [58,59]

can be calculated using the substrate dissipation factor and

metallization resistivity and thickness. The microstrip lines are

assumed to be propagated in only quasi-TEM mode or can be approximated

as such at the operating frequency. Therefore, the operating frequency

must be lower than the cutoff frequency fc of the lowest transverse

electric surface wave.


fc = 75/H(er-1)1/2 (GHz) (2.25)


where H is the substrate thickness (mm). In transmission lines used

for interconnection purposes, the relative magnetic permeability of

the substrate is unity to prevent the propagation delay time for a

transmission line in a nonmagnetic medium.











The characteristic impedance to determine the width of microstrip

lines is given by [58]


Zo = /27T (pbeffeo)1/21n(8H/W + W/4H) (2.26)


= 60/(eeff)l/2ln(8H/W + W/4H) for W/H < 1 (2.27)


where /o = 4 xl0-7 H/m and eo = 8.854xl0-12 F/m and W,H are the width,

thickness of microstrip lines, respectively. The effective relative

dielectric constant eeff for a microstrip line depends on the ratio

W/H, the relative dielectric constant er, and the geometrical factors

of the boundary between air and dielectric substrate material [56].


eeff = (r+)/2 + (er-1)/2 [(l+12H/W)-1/2+0.04(1-W/H)2] (2.28)

The zero thickness (t=0) formulas given above can be modified to

consider the thickness of the microstrip when W is replaced by an

effective strip width W' as follows (t

W' = W + (t/R)[l + ln(47W/t)] (2.29)


The attenuation constant of the dominant microstrip mode depends on

geometrical factors, electrical properties of the substrate and

conductor, and the frequency. For a nonmagnetic dielectric substrate,

the two sources of dissipative loss in microstrip lines are conductor

loss ohmicc loss) in the strip conductor and ground plane and

dielectric loss in the substrate. The sum of these losses may be

expressed as losses per unit length in terms of an attenuation factor.

The conductor loss, ac for W/H < 1 is given by [58,59]












ac = (20Rs/Inl0)[l-(W'/4H)2]/27rZH[1+H/W'+H/7W'{ln(47rW/t)+t/W}}

(2.30)
where the surface resistivity Rs is given in terms of the free space

permeability Ao and the conductivity, of the strip metal as


Rs= (Tf/o/ )1/2 (2.31)


The dielectric loss, ad with loss tangent, tanS is given by [58,59]


ad = (207T/1nl0)(Qr/eeffl/2)(eef f- )/(er-) tan /X (2.32)


where the loss tangent, tan8 is the substrate dissipation factor.

In addition to the conductor and dielectric losses, the

microstrip line has radiation loss. The radiation loss depends on the

substrate thickness and dielectric constant as well as the geometry.

The radiation loss decreases when the characteristic impedance

increases. For lower dielectric constant substrates, radiation is

significant at higher impedance levels. For higher dielectric

constant substrates, radiation becomes significant until very low

impedance levels are obtained. The wavelength and the phase velocity

of the microstrip transmission line can be determined in terms of the

effective relative dielectric constant, eeff"


p = 29.980/(eeff)1/2f (cm) (2.33)

The Cr-Au coated alumina (A1203) film is used for the microstrip

line substrate. The design parameters and the structure of a

microstrip line are given in Table 2.2 and Fig. 2.6, respectively.












The microwave test fixture was constructed for the optical

response measurenent of the photodiode in this study. The external

connections from microstrip line to bias tee were made using

commercial OSSM subminiature coaxial connectors as shown in Fig. 2.7.


Table 2.2. DESIGN PARAMETERS FOR MICROWAVE
TRANSMISSION STRIPLINE

Alumina (A1203) Substrate lxlxO.025 "

Dielectric Constant er = 9.8

Substrate Thickness H = 0.025 "

Gold Film Thickness t = 0.0002 "

Output Impedance Z = 50 ohm

Microstrip Line Width W = 0.02425 "

W/H Ratio W/H = 0.97

Effective Strip Width W' = 0.02478 "


2.8. Response Speed Measurement

The typical response speed measurement techniques for an impulse

response of the photodetector are (1) impulse response technique using

a sampling oscilloscope or a microwave spectrum analyzer [60,61], (2)

sampling and cross-correlation technique using two photodetectors

[62,63], (3) electro-optical sampling technique [64-66], and (4)

optical heterodyne technique [8,67].


2.8.1. Impulse Response

This method requires an optical source generating the ultrashort

pulses preferably shorter than the impulse response of the device





































(Not in Scale)


Zo = 50 ohm, W/H = 0.97,


Figure 2.6.


Structure of the microstrip transmission
line fabricated in this study.


Er = 9.8
































V r~P~~~' ~r-.-~*~- -. !74 '-~. -


Figure 2.7.


Microstrip microwave test fixture for
characterization of high-speed photodetectors
fabricated in this study.


iLO- ~-












under test. For broadband characterization either a synchronously

pumped mode-locked dye laser or a diode laser driven by a comb

generator (or a step-recovery diode) at an operating wavelength are

necessary. The schematic diagram for the impulse response measurement

is shown in Fig. 2.8. The impulse response is measured by either a

sampling scope in a time domain or a microwave spectrum analyzer in a

frequency domain.

Note that the measured response is a convolution of the true

photodetector impulse response and the measurement system response

including the sampling gate width, the laser pulse width, the pulse

broadening due to transmission lines and connectors. Since most of

these are not known accurately, it is difficult to estimate the true

impulse response of the photodetector by deconvolution. A more

accurate estimate can be obtained by an electrical correlation

measurement using two identical photodetectors [61].


2.8.2. Sampling/Correlation

The standard technique for measuring the duration of picosecond

optical pulses is to make a nonlinear optical auto-correlation of two

identical optical pulses by delaying one with respect to the other and

then mixing them in a second harmonic generating crystal. This

technique requires two photodetectors, each of which is activated by a

picosecond optical pulse. One of the photodetectors has a dc bias,

and the output signal from the first photodetector is used to bias the

second one. The photodetectors are not necessary identical, in which

case the measurement is referred to a cross-correlation [62].




















r --------- -- ------------- ----------
Short Optical
Comb Pulse Train
*. A Generator
Microwave Wide-Band R Step a-- Diode Photodode
i Oscillatorue 2 Amplifier r i e RecoveryBspoe asurement of te p ete Photodlode
I Diode I
L------------------ ------------------- ----------------- -- j, -

DC Bias





Sampling J,
Scope "




Figure 2.8. Schematic diagram for impulse response measurement of the photodetector.












The photodetectors are connected in order that the device under

test (DUT) launches a waveform onto a transmission line by an incident

short optical pulse and the second photodetector is a sampling gate on

the transmission line that is probed by the same optical pulse with a

variable delay time. The schematic diagrams are shown in Fig. 2.9 and

Fig. 2.10. The variable delay is conveniently introduced by varying

the relative timing of the optical pulses absorbed at each

photodetector. The experimental quantity measured is the total charge

(or average current for a repetitive train of pulses) sampled at the

output of the second photodetector, as a function of the relative

delay between the two optical pulses.

The measurement correlates the arrival of the signal from the

first photodetector with the response of the second, which acts as a

sampling gate. If the two photodetectors are identical, the measured

charge is proportional to the auto-correlation function given by [63]


Q(T) = g(t)g2(t + 7)dt (2.34)


where g(t) is the signal produced by a single photodetector with a dc

bias and T is the delay time. The signal output from the second device

is given by a correlation of the signal from the first photodetector

with the response of the second photodetector with a delay time. Note

that each signal is a convolution of the impulse response of the

device, the optical pulse width, and the circuit effects such as the

transmission line. The time resolution of the measurement is

determined by the response of the two photodetectors and the





























V b g, (t)

+00
Q )v b Zjf g,(t


12 (t + ')
92 (t +T)


t) g2 (t +T) dt


-00


Schematic diagram for sampling/correlation measurement.


I (t)


V (t)


Figure 2.9.

















Vb

()


Photodetector


Sampling Scope

Photodetector


Q(T)


Figure 2.10.


Schematic diagram for sampling/double-gap
correlation measurement.












interconnecting circuit, and does not require any high-speed external

circuitry. Since each photodetector is used in a linear response

region so that the convolution integral can be readily interpreted,

the precise response can be obtained by a deconvolution technique.


2.8.3. Electro-Optical Sampling

This technique shown in Fig.2.11 requires the microstrip

transmission line deposited on a linear electro-optic crystal such as

LiTaO3 and used as an active element in a lithium tantalate

traveling-wave Pockels cell amplitude light modulator. A train of

picosecond pulses from a mode-locked dye laser is split into two

beams. One beam strikes the photodiode and launches a signal onto the

modulator transmission line. The other beam passes transversely

through the crystal and its intensity is modulated by the electric

field under the transmission line sampling the signal. By varying the

relative delay between the two beams, the temporal resolution of the

photodetector response is obtained [64-66].

The voltage waveform on the transmission line is a convolution of

the photodiode response, the laser pulse response, dispersion in the

transmission line. The subsequent sampling is a cross-correlation of

the laser pulse with the voltage waveform. The operations of

convolution and correlation are associative and the sampler output is

therefore equivalent to the convolution of the photodiode impulse

response with the auto-correlation of the laser pulse. The output

pulse of the photodetector and sampler to the narrow incident light

pulses are shown in Fig.2.11. Since the auto-correlation of the laser













































Figure 2.11.


Schematic diagram for electro-optical sampling measurement.













































Figure 2.12.


Typical output pulse of the photodetector
and sampler to the narrow incident light pulses.


Light Pulses
Light Pulses












pulse is independently measured, its contribution can be deconvolved

to extract the photodiode impulse response. Then the equivalent time

representation of the photodiode response is obtained. The temporal

resolution is determined by several factors; the sampling light beam

spot size, the optical transit time, and the laser pulse duration.


2.8.4. Optical Heterodyning

This technique can characterize the bandwidth of a photodetector

accurately using two CW lasers. The limitation in the accuracy is the

bandwidth of the transmission line and the microwave spectrum

analyzer. The simple system shown in Fig.2.13 consists of two

semiconductor diode lasers whose frequency is temperature tuned and

the combined beam is incident on the photodiode. To avoid

instabilities in the frequencies of the two lasers, it is necessary to

reduce optical feedback by the incorporation of an optical isolator

[8]. The outputs of both lasers are coupled into a short length of

single mode fiber, the output of which is incident on the photodiode.

Since the photocurrent is proportional to the square of the

electric field of the laser, the product of two laser fields at a

different frequency will produce a difference or beat frequency

detected by a photodiode. The longer wavelength laser is mounted on a

thermoelectric cooler. By operating the longer wavelength laser at a

heat sink temperature, the frequency of the cooled laser can be tuned

through the frequency of the uncooled laser. With thermoelectric

cooler/heater and feedback electronic circuit, the temperature of the

laser diode can be controlled to within a few tenths of a millidegree.



























Single Mode
Fiber


Figure 2.13.


Schematic diagram for optical heterodyning measurement.

















CHAPTER THREE
FABRICATION OF SCHOTTKY BARRIERS AND
OHMIC CONTACTS ON GaAs, InGaAs, AND InP



3.1. Introduction

The Schottky barrier contacts are used in most high-speed III-v

compound semiconductor electronic and optoelectronic devices such as

MESFETs, MODFETs, photodetectors, lasers, and LEDs. In0 53Ga0.47As

material is most suitable for optical fiber communications operating

in the 1.30-1.55 pm wavelength regime because of its energy bandgap

(i.e., Eg = 0.75 eV at 300 K), high electron mobility, high saturation

velocity, and lattice-match to the InP substrate [42,43]. However, the

technology of InGaAs(P)/InP material and device systems is still in

need of considerable development. The main reason for this is due to

the difficulty of achieving the Schottky contact with a sufficiently

high barrier height necessary for the development of MESFETs and the

lack of a suitable dielectric insulating layer with a low interface

state density required for the development of MISFETs.

The Schottky barrier height enhancement is a promising compromise

between MESFET's and MISFET's technology even though more study is

needed to have a good reproducibility. On the other hand, Schottky

barrier contacts on a moderate doped p-type InGaAs and InP can yield

good barrier heights (i.e., dBp = 0.76 eV for InP and 0.55 eV for

In0.53Ga0.47As) when a suitable metal and good surface preparation are












provided. For III-V compound semiconductors the electrical properties

of Schottky contacts depend strongly on the Fermi level pinning, which

results when metal is deposited on the semiconductor surface [68].

The reproducibility and reliability for the ohmic contacts still

need to be developed although ohmic contacts with low contact

resistance are essential for high performance and reliable operation

in most III-V compound semiconductor devices. New studies on the

ohmic contacts have been reported recently because of the needs for

good ohmic contacts on III-V compound devices and the availability of

more sophisticated high vacuum surface analytical instruments required

to understand the metallurgical properties of the ohmic contacts.

In this chapter, the optimum conditions for low resistance ohmic

contact and Schottky barrier height enhancement on n-InGaAs epilayer

are described using p+-n-In.53Ga0.47As/n+-InP structure. The

significance of this structure lies in its ability to increase the

barrier height, and hence to reduce the large dark current commonly

observed in the n-In0.53Ga0.47As Schottky barrier diodes. In addition,

the Au/p-InGaAs/p+-InP as well as Au/p-InP/P+-InP Schottky barrier

diode has been fabricated and characterized in this study.


3.2. Schottky Barrier Contact Formation


3.2.1. Schottky Barrier Height

The barrier height of metal-semiconductor system is determined by

both the metal work function and the interface traps. The general

expression[69,70] ofabarrier height can be obtainedfromthecharge












neutrality condition of a metal-semiconductor system shown in Fig. 3.1

by designating the oxide charge Qox = -qNox in the region of oxide

close to the oxide-semiconductor interface,


QM + Qsc + Qit + ox = 0 (3.1)


Qox = Qf + Qm + Qot (3.2)

where QM is surface charge on metal, Qsc space charge in the

depletion layer of semiconductor, Qit interface trap charge, Qox oxide

charge, Qf is oxide fixed charge, Qm mobile ionic charge, and Qot

oxide trapped charge. Assuming that the energy distribution of the

interface trap can be expressed by [70]


Dit(E) = Dit{exp[(E qSo)/Es] + exp[-(E qmo)/Es]} (3.3)


then the barrier height can be expressed as


aBn = C2(m X) + (1-C2)fn (l-C2)Nox/qDit A (3.4)


where do represents the position of the neutral level for interface

traps from the top of the valence band and Dit is the density of

interface traps per unit area per electron volt. The two limiting

cases considered previously can be obtained from Eq.(3.4). The

detailed derivation of a general model for the barrier height is given

in appendix A. The two limiting cases can be obtained as follows:

(1) Mott Limit (Dit ->0, C2 -> 1, and Ifn -> Vn)


dBn = (m X) Am


(3.5)


















qcM


qA



qx



+ +
+ .qAO qbi
sc + E i
sc "-! ^- 11 c


Work Function of Metal
Barrier Height of Metal-Semiconductor Barrier
Asymptotic Value of (Bn at Zero Electric Field
Energy Level at Surface
Energy Difference Between FERMI Level and Valence
Band at the Surface
Image Force Barrier Lowering
Potential Across Interfacial Layer
Electron Affinity of Semiconductor
Built-In Potential*
Permittivity of Semiconductor
Permittivity of Interfacial Layer
Thickness of Interfacial Layer
Space-Charge Density in Semiconductor
Interface Trap Density on Semiconductor
Surface-Charge Density on Metal


Energy-band diagram of a metal-n semiconductor contact.


(IM
(Bn
(Bo
(Do
b*

AcD
A

Vbi
ES
EI
8
Qsc
Qlt
QM


Figure 3.1.












which is the barrier height for an ideal Schottky barrier contact

where surface state effects are neglected.

(2) Bardeen Limit (Dit -> oo, C2 -> 0, and Ifn -> E /q MO)


"Bn = (Eg/q 0o) Am (3.6)

The Fermi level at the interface is pinned by the interface traps at

the value qao above the valence band. The barrier height is

independent of the metal work function and determined entirely by the

surface properties of the semiconductor.


3.2.2. Barrier Height Enhancement

The barrier height of an ideal Schottky contact is determined

primarily by the difference of metal work function and electron

affinity of the semiconductor. However, for a practical Schottky

diode the property of metal-semiconductor interface such as interface

trap density plays an importment role in determining the effective

barrier height of the Schottky contact. Since there are only limited

numbers of metals which are suitable for good Schottky contact, the

control of the Schottky barrier height is essential for specific

electronic circuit application. A low barrier height makes the

Schottky contact too leaky to be useful for MESFET and photodetector

applications. Thus, the effective barrier height need to be increased

in order to overcome the problem associated with low barrier height.

The Schottky barrier height enhancement can be achieved by the

use of (1) a thin insulating layer (i.e., MIS Schottky diode) [71],

(2) an oppositely doped thin surface layer to that of an active layer












(i.e., Quasi-Schottky diode) [72-75], and (3) a wide-bandgap

materials such as InP or AlInAs (i.e., Heterojunction Schottky diode)

[76-78]. The effective barrier height for a MIS Schottky barrier

structure, which consists of a thin interfacial insulating layer

between metal and semiconductor, can be increased and resulted in a

low reverse leakage current. However, high interface state density,

oxide breakdown, and charge storage effects are some of the problems

need to be overcome in III-V semiconductor Schottky contacts [71].

Schottky barrier enhancement is a promising technique for

formation of a Schottky contact on InGaAs/InP material system, which

employs a very thin p-In.053Ga.047As surface layer grown on the

n-InGaAs epitaxial layer. Schottky barrier contacts on n-InGaAs

usually yield very low barrier height (OBn = 0.2-0.3 eV), which makes

Schottky contacts too leaky to be useful for photodetector

applications. The barrier height enhancement can be achieved by

depositing a thin p -In0.53Ga0.47As layer on the n-In0.53Ga0.47As

epilayer as is shown in Fig. 3.2.

The effective barrier height can be increased by band bending due

to the space charge in the p -In0.53Ga0.47As surface layer provided

that the dopant density and the thickness of the surface layer are

selected to an optimum value and the layer is fully depleted at

thermal equilibrium. The thickness and the dopant density of

p -In0.53Ga.047As layer can be related to the barrier height

enhancement, A Bn given by [72]


A Bn = qNAxm2/2eoer


(3.7)






















p+H4-n-lnGaAs -4 n InP Substrate


Figure 3.2.


p Wn Ev



Energy-band diagram for Schottky barrier
height enhancement by energy-band bending
due to the space charge in the p+-InGaAs
surface layer.


- - - ---- - E


Metal












The enhanced barrier potential will reach a maximum value at

x = xm inside the p -In0.53Ga0.47As surface layer provided that

NAWp > NDWn.


Xm = (1/NA)(NAWp NDWn) and Em (q/eeor)(NAWp NDWn) (3.8)

The effective barrier height 6'Bn obtained at x = xm is given by


a'Bn = %n + Emxm qNAxm2/2eoer (3.9)

By substituting Eq. (3.8) into Eq. (3.9) the effective Schottky

barrier height can be obtained.


'Bn = "Bn + (q/2eoerNAN AAWp NDWn)2 (3.10)

For NA >> ND and NAWp >> NDWn, the barrier height enhancement of a

metal-p-n Schottky barrier diode, AiBn due to the p surface layer may

be simplified to


ABn = qNAWp2/2eoer (3.11)

It can be shown that Eq. (3.11) holds only for AgBn >> VDND/NA-

Note that NA and ND denote the dopant density of the p+- and

n-In0.53Ga. 47As layers, respectively. W is the thickness of the

p+-In0.53Ga0.47As layer, and VD is the built-in potential of the p+-n

junction. Therefore, the effective barrier height will increase as

the product NAWp increases. The thickness and dopant density of the

p -In0.53Ga0.47As surface layer should be determined in order to

satisfy the condition of A6Bn > VDND/NA.












The detailed derivation of Schottky barrier height enhancement is

given in appendix B. The depletion layer width of the n-InGaAs

epilayer is given by


= -Wp + Wp2 + (NA/ND)Wp2 + 2eodr(m n V)/qN]/2 (3.12)

where n = Xs + (kT/q)ln(Nc/ND) (3.13)


The effective barrier height for the proposed photodetector can

be tailored to its optimum value via properly selected thickness and

dopant density of the surface layer. Theoretically the effective

barrier height equal to the bandgap energy of In.053Ga0.47As can be

achieved by the proposed structure. Figure 3.3 shows the effective

barrier height vs. dopant density as a function of the thickness of

p+-In0.53Ga0.47As surface layer. Figure 3.4 shows the theoretical

saturation current density of the Schottky barrier diode on n-InGaAs.


3.2.3. Barrier Height Measurement

The effective barrier height V'Bn of Schottky barrier diode can

be determined by the following methods: (1) current-voltage (IF-VF or

IF-T), (2) capacitance-voltage (C-VR), (3) photoresponse (Iph-E)

measurement using Eqs.(3.14) through (3.17). The values of the

effective barrier height obtained different measurements often do not

agree. Therefore, an understanding of the inherent assumptions in each

technique as well as the practical limitations of each measurement is

very useful in interpretation of the experimental data.


BnV = (kT/q)ln(A*T2/J )
Bn


(3.14)




















Reverse-BI
SNumbers Ii
C of p+ InG,
.m
e

0.80 -
0.15 0.1
Q3 0.75

0.2
- 0.60 0.
w


m 0.40 -
w

I--
U 0.20 -
LL
U--
LLL

I 16 1 11
3.0x1016 1017


1018


1019 3.0x1019


DOPING CONCENTRATION OF p+- InGaAs LAYER (cmn3)


Figure 3.3.


Effective barrier height vs. dopant density of
p -In0.53Ga0.47As surface layer as a parameter
of the thickness of the p -In0.53Ga0.47As layer.

















103


cm
E







0
v,



U)

c-
z







0


cn
cc


101






10"1


10-7 L
0.2


Figure 3.4.


0.4 0.6 0.8
EFFECTIVE BARRIER HEIGHT(Dn(eV)


Theoretical saturation current density vs. effective
barrier height at T = 300 K.












'Bn = (kT/q)ln(Js/A*T2) (3.15)


'BnC-= VD + (kT/q)ln(NC/ND) (3.16)


SnI-E = hv/q (kT/q)(J/A*T2) (3.17)


where A (= 47rqm*k2/h3) is the effective Richardson constant for the

thermionic-emission neglecting the effects of optical phonon

scattering, quantum mechanical reflection, and tunneling of carriers

at the metal-semiconductor interface, and Js is the saturation current

density which is the extrapolated value of the current density at zero

voltage.

The current-voltage measurement can be used to determine the

barrier height of a Schottky barrier diode by several different

methods. The simplest method is to measure the forward current

density at a fixed temperature. The barrier height at zero bias can

be determined from Eq.(3.14) if A is known. Note that reliable

results can be obtained only if the plot of InJ vs. V is linear over

at least three orders of magnitude and n is low (i.e., n < 1.1). For

large values of n, or nonlinear plot of InJ vs. V, the diode is far

from ideal probably due to the presence of a thick interfacial layer

or recombination in the depletion region, and the barrier height is

not clearly defined [53,54].

An alternative method to determine the barrier height, if A is

not known, is to measure the saturation current density as a function

of temperature at a fixed forward-biased voltage. The barrier height

can be determined from the slope of the Richardson plot (i.e.,












ln(Js/T2) vs. 1/T) for a given forward bias. The intercept at 1/T = 0

yields the effective Richardson constant, A The capacitance-

voltage measurement can be used to determine the deep impurity levels

as well as the barrier height of a Schottky barrier diode. For a

uniformly doped semiconductor, the plot of 1/C2 vs. VR yields a

straight line and its intercept on the voltage axis gives the

diffusion potential, VD. Therefore, the barrier height can be

determined from Eq.(3.16).

In practical Schottky barrier diodes for both elemental

semiconductors and III-V compound semiconductors, it is not unusual

to find that the barrier height obtained from C-V measurement is

larger than the barrier height determined by I-V measurement, i.e.,

BC-V > II-V [53]. One reason for BC-V > I-V is due to a thin

compensated layer formed adjacent to, the metal during barrier

formation and hence the potential energy barrier is reduced at the

interface. Another reason is the lateral nonuniformity of the barrier

height across the metal-semiconductor interface. The current

measurement would emphasize the lower value of the barrier height,

while the capacitance measurement would provide a value of the barrier

height averaged over the interface.

The photoresponse measurement shown in Fig.3.5 is the most

accurate and direct method of determining the barrier height. When a

monochromatic light is incident on a Schottky barrier diode through a

metal contact, the photocurrent will increase sharply for q@Bn < hV<

E resulting from photoexcitation of electrons from the Fermi level of

























SEMICONDUCTOR

. BACK
ILLUMINATION


-OHMIC CONTACT














Ec
--- ---EF

S^hyV(1)


METAL SEMICONDUCTOR


Figure 3.5.


Photoresponse measurement. (a) Schematic
diagram; (b) Energy-band diagram for
photoexcitation process.


METAL


FRONT




















hV(
(1,2)












the metal to the conduction band of the semiconductor, and for h > E

the photocurrent will increase even more rapidly as a result of

band-to-band excitation. The resulting photocurrent Iph is given by

Fowler's theory for classical photoemission as


Iph = C(hV- qEBn)2 (3.18)


which is valid provided that hV qgBn > 3kT and qgBn < hV < E .

Thus, the plot of the square root of the photocurrent as a function of

photon energy gives a straight line and the extrapolated value on the

energy axis should give directly the barrier height.

In practice, the incident light is chopped to avoid edge effects

and other contributions to the diode leakage current, and if the diode

is illuminated from the front side, the metal is made thin enough to

allow adequate transmission of the light to the metal-semiconductor

interface. The photoresponse technique has been used not only to

determine the barrier height directly, but also to measure the voltage

dependence of image-force lowering, the temperature dependence of the

barrier height, and the direct and indirect bandgap energy in several

ternary compound semiconductors.


3.3. Ohmic Contact Formation


3.3.1. Ohmic Contact Technology

A practical way to obtain low resistance ohmic contacts [79-85]

is to increase the dopant density near the metal-semiconductor

interface (ND > 1019 cm-3) so that the depletion layer caused by












Schottky barrier becomes very thin and the current transport through

the barrier is enhanced by tunneling. Nearly all methods of making

ohmic contacts depend on depositing a thin layer of metal alloy on a

relatively oxide-free clean semiconductor surface and on heat

treatment during or after deposition in vacuum or in an inert

atmosphere. Generally, it is preferred that the metal deposited on

semiconductor should be heat treated at a temperature higher than the

alloying temperature because a heavily doped contact layer is often

formed between metal and semiconductor during the cooling cycle.

The selection of metals for ohmic contact to a particular III-V

compound semiconductor depends on several factors [55,81]. The

primary factor is that the metal used for contact should be such an

element that it can be acted as a dopant to the semiconductor so that

a heavily doped surface layer can be formed. For example, the

possible materials are Si, Ge, Sn, Se, or Te for contacts on n-type

semiconductors and Zn, Cd, Be, or Mg for contacts on p-type

semiconductors. In addition to this factor, there are a number of

other factors need to be considered before selecting a particular

contact metal: (1) easy deposition, (2) good adhesion, (3) low

alloying temperature, (4) minimum interface reaction, (5) minimum

thermal mismatch, (6) no surface tension effects during alloying, (7)

good electrical and thermal behavior, and (8) adaptability to thermo-

compression or ultrasonic wire bonding. The most widely used metal for

ohmic contact on III-V compound semiconductors are Au, Ag, or In base

alloys. The final factor for choosing a particular metal system is the













eutectic temperature of the alloy metal (i.e., Au, Ag, or In) with the

semiconductor and its correlation to the temperature for which the

semiconductor can be safely heated. After suitable choice of contact

metal, an appropriate technique has to be selected for depositing the

contact metal on the semiconductor surface. A number of techniques,

e.g., evaporation, sputtering, and electrolytic or electroless plating

in a chemical solution have been reported for this purpose.

The evaporation technique is by far the most widely used for the

deposition of contact metal systems on III-V compound semiconductors.

Sputtering technique has rarely been used for depositing the contact

metal system on III-V compound semiconductors because of low

sputtering rates, surface damage, and difficulty in accurate

monitoring of the metal film thickness [55,81]. The electroless

plating technique has been frequently used for depositing overlayers

of Au, Ni, etc., on ohmic contacts as well as Schottky contacts.

Such overlayers are required for bonding thin wires with contact metal

systems without any change in the properties of the contacts. The

wire bonding is usually carried out by either the thermocompression or

the ultrasonic bonding technique.

Finally, the requirement of a buffer layer (e.g., n+-layer on

n-type semiconductor) between the contact metal system and

semiconductor in order to ensure good ohmic contact can also be

satisfied by using epitaxial technique. Recently, the molecular beam

epitaxial (MBE) technique has been used for growing such buffer

layers. Ion implantation also promises to be a desirable alternative












to the epitaxial technique for obtaining submicron layers without

introducing any undesirable interface states. However, thermal

annealing is usually required after ion implantation to remove damages

and crystal defects. The annealing can be carried out by using

thermal, laser, or electron beam annealing. Therefore, it can be

predicted that ion implantation (for producing a buffer layer)

followed by evaporation (for depositing the contact metal system) may

be a good combination method for making good reproducible ohmic

contacts on III-V compound semiconductors.


3.3.2. Specific Contact Resistance Measurement

For III-V compound semiconductors, the specific contact

resistance can be determined from the following methods: (1) Cox and

Strack [86], (2) four-point [87-89], (3) Shockley extrapolation [90],

and (4) transmission line method [91-94]. For a homogeneous contact

of area A having uniform current density, the contact resistance Rc is

simply given by Rc = rc/A. The measured resistance R will be

approximately equal to Rc for most sample geometries when rc > 10-2
2
ohmcm2. However, for small values of rc, the spreading resistance of

the semiconductor Rb and the series resistance Ro of the semiconductor

substrate and the connecting wires should be taken into account, i.e.,

R = Rc + Rb + Ro.

The Cox and Strack method can be used to determine the specific

contact resistance of a circular contact of radius a on epitaxial or

bulk layers in the structure of Fig. 3.6. The spreading resistance of

the layer is given by



























Figure 3.6.


--a- /}--F(T)- Ro]
P Rb R
ROc




Rc= ca (R F(A) -Ro]


Specific contact resistance measurement by the Cox-Strack
method. The circular ohmic contact has radius of a.












Rb = (P/a)F(a/t) (3.19)


where F is a function of the ratio a/t and was found experimentally by

Cox and Strack to have the approximate form


F(a/t) = (1/7 ) tan-1(2t/a) (3.20)


Then with F(a/t) known, the contact resistance can be obtained by


rc = a2[R ( P/a)F(a/t) R]o (3.21)


In practice, the resistances of an array of contacts with different

areas are measured and the spreading resistance is calculated for each

contact using Eq.(3.19). The specific contact resistance can be

obtained from the slope of the plot of R-Ro vs. 1/a2, where Ro is

provided by the intercept on R-Rb axis.

The four-point method requires metallization of only one surface

of the sample as shown in Fig.3.7. The spreading resistance should be

first calculated and subtracted from the total measured resistance.

The spreading resistance Rb for radial current flow from a circular

contact of radius a is given in the form of an infinite series by Fang

et al.[88]. In the four-point measurement, the voltage V1 and V2 are

measured for a known current I. Assuming that the resistance of the

semiconductor film between the contacts is the same everywhere, then


V1 V2 = I(Rc + Rb) (3.22)


which gives


rc = 7a2[(Vl/I) (V2/I) Rb) (3.23)












































rc = 2 [V V2
SI I


if p a2 < rct


Figure 3.7.


Specific contact resistance measurement by
the four-point method.


3S 1

2 In 2











Kuphal has recently pointed out that the potential distribution

in the plane of the semiconductor layer is logarithmic rather than

linear, and the correct expression for the specific contact resistance

is given by [89]


rc = 7a2 (Vl/I) -(V2/I)ln{(3s/2a)-1/2}/21n2] (3.24)


provided that ra2 < rct, a << s, and t << s.

The Shockley extrapolation method can be used to determine the

specific contact resistance of the thin semiconductor layers on the

non-conducting substrates. As shown in Fig.3.8 this technique

consists of measuring the voltage drop V(x) along the surface of the

semiconductor film with coplanar ohmic contacts and using the

extrapolated voltage Vo appearing across the contacts, then rc can be

determined. Assuming that the sheet resistance Rs is laterally

uniform and the epilayer is infinitely thin, the potential

distribution under the contacts is given by [90]


V(x) = Voexp(-x/Lt) (3.25)


where Lt = (rc/Rs)1/2 is the transfer length. By extrapolating the

linear voltage drop measured between the two contacts to obtain Lt,

the specific contact resistance can be determined from


rc = RLt2 (3.26)


Another method to determine the resistance of ohmic contacts to a

thin III-V compound semiconductor layer on a non-conducting substrate













I~HHDI D


T

1


+ +
Va + -
n n


e-x/L T


ST -


2
rc= R LT
csT


Figure 3.8.


Specific contact resistance measurement by the Shockley
method. The linear voltage distribution between contacts
is extrapolated to obtain the transfer length Lt.












use the transmission-line model (TLM). In the transmission-line model

as shown in Fig. 3.9, the planar contact is treated as a resistive

transmission line with an uniform sheet resistance R, and a specific

contact resistance rc. The total resistance Rc of the contact and

the epitaxial layer under the contact is given by [91,92]


Rc = [(rcRsc)1/2/W]coth(d/Lt) (3.27)


where Lt = (rc/Rsc)1/2 is the transfer length of the Shockley method.

The current in the semiconductor film under the contact decays with

distance as exp(-alx), where the attenuation factor al is related to

the inverse transfer length, i.e., al = 1/Lt. In order to measure the

specific contact resistance using TLM, the total resistance R should

be first measured experimentally. This can be accomplished using the

arrangement of Fig. 3.10, where three identical ohmic contacts are

spaced at unequal distance L1 and L2 along the surface of the layer.

If R1 and R2 are the resistance measured, the total resistance is

easily given by


Rc = (- R2L1 + R1L2)/2(L2 Ll) (3.28)


The total resistance in Eq.(3.27) can be simplified


Rc = (rcRsc)1/2/W = RscLt/W for Lt << d (3.29)


The specific contact resistance can be determined from Eq.(3.29) with

a measured known value of Rc. The TLM has been extended to the

transmission line of circular geometry [93] and arbitrary shape [94].


































I ii-


Figure 3.9.


if d 2/c
Rsc


Specific contact resistance measurement by the
transmission line model (TLM) method. Rc is the
total resistance of the metal-semiconductor
interface and the epilayer under the contact.


R2 W2
R sc


















-ADK--L, DDL2ID K


R 2
'2


Figure 3.10.


Method of determining the total resistance
(Rc) using a linear array of unequally spaced
ohmic contacts. The shaded regions are the ohmic
contact areas.


T
w
4


A9


R 11












This theory can also be used to analyze the source and drain

ohmic contacts for field-effect transistors fabricated with GaAs and

other III-V compound semiconductors. The use of the TLM avoids the

necessity of measuring the potential distribution V(x) of the Shockley

method and hence is somewhat simpler to implement. However, both

methods assume that the sheet resistance between and under the

contacts is identical and that the semiconductor layer is infinitely

thin. In the alloyed ohmic contacts, this is not generally true.


3.4. Device Fabrication

3.4.1. Schottky Gate Formation

The Au/p+-n-In0.53Ga0.47As/n+-InP and Au/p-Ino.53Gao.47As/p+-InP

Schottky barrier diodes have been fabricated as is shown in Fig.3.11

using a standard lift-off process [95,96]. A lift-off photolithography

gives an excellent resolution available with positive photoresist and

avoids incompatability problem of many metal etchants with InP in a

two-metal system [55]. The p+-n-InO.53Ga0.47As epitaxial layers were

grown on n+-InP substrates by MBE technique. The thicknesses of the

p -In0.53Ga0.47As epilayer were chosen to be 0.03 pm 0.15 pm with

corresponding dopant densities of 5.5x1016 9.0x1017 cm-3, and the

thickness of an n-In.053Ga0.47As and p-In0.53Ga.047As epilayer with a

dopant density of 3.0x1015 cm-3 is 1.5 pm. A 100 a gold film was

deposited on the p -In0.53Ga0 47As layer at a deposit rate of 2 R/sec

and at a pressure of 5.0x10-7 Torr for the transparent Schottky

contact and Cr/Au (60/1,000 R) was deposited for the bonding pad. The

Cr provides contact adhesion to the semiconductor and Au reduces the





























---- Schottky Barrier Contact

-Bonding Pad (Cr/Au)


I


pt.- In0 53Ga0.47As


n In0 53Gao.47As


n+- InP


I


4-


Passivation (Polyimide)
Depleted Layer


--- Active Layer
(Epilayer)

- Substrate


-- Ohmic Contact (Au-Ge)


Figure 3.11.


Structure of the InGaAs Schottky barrier diode
using the barrier height enhancement technique.












contact resistance and provides a surface suitable for bonding or

probing. Just before evaporation, a wet chemical etching was

performed to remove native oxides from surface of the contact area.

Wafers are dipped in buffered HF (HF:H20 = 1:5) or etching solution

(NH40H:H202:H20 = 20:7:150).

For p-InP/p+-InP Schottky barrier diode shown in Fig. 3.12, the

(100) oriented Zn-doped InP substrates with a dopant density of

NA = 5.0x1018 cm-3 were used. The p-InP epitaxial layer with doping

concentration of NA = 1.0-2.0x017 cm-3 was grown on p -InP substrate

by Vapor Phase Epitaxy. Aluminum was used as the gate metal because

of low resistivity and low work function. The contact has a circular

shape with a diameter of 200-800 um, which gives a contact area of

3.0xl0-4 to 5.0xl0-3 cm2.


3.4.2. Ohmic Contact Formation

We investigated the properties of the ohmic contact using several

metal alloys at a different alloy temperature and alloy time to obtain

the optimum ohmic contact condition. The optimum conditions are

summarized in Table 3.1. The ohmic contact pattern was fabricated

using the lift-off process as shown in Fig. 3.13. The typical

I-V characteristics are shown in Fig.3.14 and Fig.3.16. The specific

contact resistance was calculated for vertical and planar structures

as is shown in Fig. 3.15 and Fig. 3.17, respectively. The specific

contact resistance at a different alloy time is shown in Fig. 3.18.

For ohmic contact on n+-InP, Au-Ge (88-12 %) alloy (1,500 R) was

deposited and alloyed at 400 0C for 30 sec in H2-N2 (5-95 %) forming






















Transparent Schottky
Barrier Contact (Au)


Absorption Layer


Ohmic Contact


Ohmic Contact: Mn (100 A)/Au (900 A)


Figure 3.12.


Structure of the InP Schottky barrier diode.
Mn and Au are deposited sequentially for
p-type ohmic contact.

















OPTIMUM CONDITIONS FOR OHMIC CONTACTS.


Metal Alloy Thickness (A) Alloy Time (sec) Special Comment



Au-Ge 1,200 30 at 400 oC Good Ohmic
Bonding Problem


Au-Ge/Ni 1,200/350

and/Cr/Au /400/1,000 120 at 450 oC Good Ohmic



Au-Ge/Ni/Au 1,200/500/1,000 90-120 at 450 C Good Ohmic


Au-Zn 1,500 30-45 at 400 OC Contamination
Adhesion Problem


Cr/Au-Zn/Au 50/500/1,500 60-90 at 450 oC Good Ohmic


Mn/Au 100/900 30 at 460 oC Good Ohmic


* This condition was done by Research Triangle Institute.


Table


3.1.












































Figure 3.13.


The linear array of unequally spaced
ohmic contacts. Contact spacing: 10,20,
30, 40 pm, Contact pad: 100x200 pm.






















































Figure 3.14.


Current-voltage characteristics.
(a) Au-Ge/Ni/Au ohmic contact;
(b) System (probe to ground chuck).































RTest= 2R+ R= Total R System = 0.27 []l

R = pt/Wd = (0.0013) (0.038)/(200) (100) (10'4) = 0.248 [Q]

Rc= 1/2 (R Test- R ) = 1/2 (0.27 0.248) = 0.011 [Q]

Thus,

rc= RcWd = (0.011) (200) (100) (10"8) = 2.2 x 10-6[Qcm2]


Figure 3.15.


Method of determining the specific contact
resistance by the transmission line model
method in the vertical structure.
























































Figure 3.16.


Current-voltage characteristics of
Au-Ge/Ni/Au ohmic contact (planar
structure). (a) Alloy at 450'C for
2 min; (b) Alloy at 450'C for 1.5 min.


































Rc =Rsc Lt /W= 1/2 (R Total RSystem ) = 1.2 [Q]

Rsc = (1.2) (200)/1= 240 [1] assuming Lt = 1 [pm]

Thus,


rc=R=R L 2
C sc t


Figure 3.17.


= (240) (1)2 = 2.4 x 106 [Qcm2]


Method of determining the specific contact
resistance by the transmission line model
method in the planar structure.




















Ohmic Contact:
1:
2:


Au-Ge/Ni/Cr/Au
380"C 3:
400C 4:


425"C
450"C


..J


0~


II I I I I I


ALLOY TIME (min.)


Figure 3.18.


Specific contact resistance
of alloy temperature.


vs. alloy time as a parameter


8.0 -


6.0 -


4.0 1-


2.0 -


0.0


2.0


3.0


I i I -












gas ambient. Zn, Be, and Mg are usually incorporated in epitaxial InP

layer as acceptors for the ohmic contact on p-InP. After cleaning

wafer with TCE, acetone, methanol, and D.I. water followed by blowing

dry with N2, an ohmic contact was formed on the back surface of the

p+-InP substrate by depositing Au-Zn (84%-16%) metal alloy (1500 a) in

E-beam evaporator at a pressure of 7.0x10-7 Torr. The Au-Zn ohmic

contact was annealed at 450 C for 2 min in H2-N2 (10-90%) forming gas

environment in an alloy furnace. The adhesion of Zn is not always good

with peeling off occurred during the lift-off process [89].


3.5. Experimental Results and Discussion

The current-voltage characteristics of Au/p+-n-InGaAs/n+-InP

Schottky diode with a p+-InGaAs layer of 1,500 R and 500 R thick show

a large reverse leakage current as is shown in Fig. 3.19. The reason

for the large leakage current may be attributed partially to the

surface leakage which stemmed from the poor surface morphology and

partially to the existence of the thin neutral region between p-InGaAs

Schottky barrier and p-n junction, consisting of a Schottky barrier

contact in series with a p-n junction diode due to a thick surface

layer. The capacitance-voltage characteristic of the Schottky diode

with a p+-InGaAs layer of 1,500 R thick shows a strong possibility of

this reason as shown in Fig. 3.20.

However, the leakage current was greatly reduced in Schottky

barrier diodes with a p+-InGaAs layer of 300 R as is shown in

Fig. 3.21. The leakage current density is given by 1.5x10-3 A/cm2 at

VR = 5 V. The effective barrier height of 0.52 eV is obtained by





















































Figure 3.19.


Current-voltage characteristics for
Au/p -n-In0.53Ga0.47As/n -InP Schottky
barrier photodiode. (a) 500 A,(b) 300 A.



















































1.0 2.0 3.0 4.0
REVERSE-BIASED VOLTAGE VR(V)


Figure 3.20.


Reverse-biased junction capacitance of the p -n-InGaAs
Schottky barrier photodiode with the surface layer of
the thickness of 1,500 A.


8.0



7.0


u-
IQ.

OL

0
z

O
CL
uJ







z

0
z
Z
,-


6.0



5.0



4.0



3.0



2.0


1.0



0.0-
0.0











































Figure 3.21.


Current-voltage (I-V) characteristics for
+ +
Au/p -n-In0 53Ga0 47As/n -InP Schottky barrier
photodiode with the Po-In0.53Ga 047As layer of
the thickness of 300 A. "












using A = 4.92 A/cm2/K2 for an electron effective mass of 0.041 mo.

The capacitance was found to be 0.3 pF at VR = 5 V for Schottky

barrier diode with a contact area of 2.0x10-5 cm2. The effective

barrier height, 'BnI-V = 0.52 eV and 'BnC-V = 0.55 eV is obtained,

which indicates the barrier enhancement of 0.32-0.35 eV.

Most of the p-InP/p+-InP Schottky barrier diodes have a

breakdown voltage of 15-20 V. The junction capacitance is C = 0.18 pF

at VR = 0 V and C = 0.16 pF at VR = 5 V for an Al/p-InP/p+-InP

Schottky barrier diode with a contact area of 5.0x0-3 cm2 as is shown

in Fig. 3.22. The background doping concentration determined from the

slope of C-2 vs. VR in Fig. 3.23 is 1.4x1017 cm3. The built-in

potential is obtained from the intercept of C-2, i.e., VD = 0.65 eV.

Thus, the barrier height for Al/p-InP/p+-InP Schottky barrier diode

can be determined by Eq.(3.16), which shows IBp = 0.77 eV.


3.6. Summary and Conclusions

The Au/p+-n-In0.53Ga.47As/n+-InP Schottky barrier diodes with a

different thickness of the p+-InGaAs surface layer have been

fabricated and characterized. The results show that our modified

Schottky barrier diodes have the total capacitance of 0.2-0.3 pF, the

series resistance of 11.8 ohm, and the effective barrier height of

0.52-0.55 eV. We have also fabricated Al/p-InP/p+-InP Schottky

barrier diode using a lift-off photolithographic process on p-InP

epilayer grown by Vapor Phase Epitaxy (VPE). The direct measurement

of the barrier height shows Bp CV = 0.77 eV.
















T-
co



< '--
o
x











I--
LU
0




0.


z


0








U-
0
IU





CO
LU~
>,
Zn


0.8 0.4 0 0.4 0.8


REVERSE-BIASED VOLTAGE VR(V)


Figure 3.22.


Method of determining the Schottky barrier height of the
p-InP Schottky barrier diode from the capacitance vs.
voltage measurement.


36.0


I I I I I I I /

Bp =VD +VT In (Nv/NA)
NV =1.28 x 1019 cnr3
NA = 2.0 x 1017 cm-3
VD = 0.65 eV














Au/p-Inp/p+-InP
Schottky Barrier Diode
/ Area = 5.0x10"3 cm2





0.65
/
/
/
/
/
/
/


10"651/ I I I I I I


35.0 -


34.0 I-


33.0




32.0


31.0 -


30.0


_

















CHAPTER FOUR
DEVELOPMENT OF A HIGH-SPEED GaAs SCHOTTKY BARRIER
PHOTODETECTOR FOR MILLIMETER-WAVE OPTICAL FIBER LINKS



4.1. Introduction

Lightwave communication systems require a high-speed and high

sensitivity photodetector in order to achieve high data rate at low

signal level. The GaAs Schottky barrier photodetector is very

attractive for short optical links where the maximum data rate is not

limited by fiber dispersion and the maximum modulation frequency is

only limited by photodetectors, lasers, and electronics. GaAs-related

photodetectors need to use ternary and quaternary materials made using

GaAs because it is difficult to design the photodetectors at long

wavelength. The physical parameters of a GaAs material are summarized

in Table 4.1. In this chapter, we describe a high-speed GaAs Schottky

barrier photodiode capable of detecting optical signals up to 20 GHz.


4.2. Theoretical Analysis

The main considerations in the design of photodetectors are

response speed and responsivity. For high speedoperation the GaAs

Schottky barrier photodiode requires a narrow depletion region for

short transit time and a wide depletion region and small area for low

junction capacitance. Therefore, the geometry and dimension of the

photodiode and the dopant density of the epilayer should be optimized.




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