• TABLE OF CONTENTS
HIDE
 Title Page
 Acknowledgement
 Table of Contents
 Abstract
 Introduction
 Metal organic chemical vapor...
 P-type doping of MOCVD InP: experiments...
 Epitaxially grown interference...
 Conclusions and recommendation...
 Appendix
 Reference
 Biographical sketch
 Copyright














Title: Growth and modeling of III-V compound semiconductor optoelectronic materials with device applications
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
        Page iii
    Table of Contents
        Page iv
        Page v
    Abstract
        Page vi
        Page vii
    Introduction
        Page 1
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    Metal organic chemical vapor deposition
        Page 13
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    P-type doping of MOCVD InP: experiments and modeling
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    Epitaxially grown interference filters
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    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











GROWTH AND MODELING OF III-V COMPOUND
SEMICONDUCTOR OPTOELECTRONIC MATERIALS
WITH DEVICE APPLICATIONS







By

ARNOLD JOHN HOWARD


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


1990














ACKNOWLEDGEMENTS

The author would like to thank his advisor and committee

chairman, Dr. T. J. Anderson, for his guidance and

encouragement throughout this educational program. Also, many

thanks go to the other members of his graduate committee, Dr.

M. E. Orazem, Dr. G. Bosman, Dr. S. S. Li and Dr. A. J.

SpringThorpe, for their advice and useful discussions.

The author wishes to extend his deepest thanks to Dr.

Balu Pathangey, Dr. Yasuhiro Hayakawa, Mrs. Vesna Jovic and

Mr. Pete Axson for their friendship and assistance in the

construction and operation of the MOCVD system. Special

thanks are in order for Dr. Pathangey for his assistance with

the dopant modeling portion of this work. The author is also

indebted to Mr. Keith Rambo, Mr. Dean Heinz and Mrs. Cheryl

Heinz, Dr. Shiro Sakai and Mr. Whitey Herrlinger for their

assistance at the University of Florida Surge Area.

The author had the good fortune to have been a visitor at

Bell Northern Research (BNR) in Ottawa, Canada, and is

grateful for the hospitality and advice of Dr. A. J.

SpringThorpe, Dr. C. Blaauw, Dr. M. N. Svilans, Dr. C. J.

Miner and Dr. I. C. Bassignana and several other members of

the scientific staff at BNR. The author also gratefully








acknowledges the financial support of BNR, DARPA and

Microfabritech for portions of this study.

Warm, personal thanks go to the author's family,

especially his wife Robin, and his mother Mary, for their

moral support and patience throughout the course of his

graduate studies. This work is dedicated to them and also to

his father, the late Peter V. Howard.


iii













TABLE OF CONTENTS


ACKNOWLEDGEMENTS ................................. ...... ii

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

CHAPTERS

I INTRODUCTION....................................... 1

1.1 III-V Semiconductors........................... 1
1.2 Epitaxial Growth Techniques................... 6

II METAL ORGANIC CHEMICAL VAPOR DEPOSITION ........... 13

2.1 A Brief History of MOCVD..................... 13
2.2 MOCVD Systems................................ 15
2.3 A Review of the Literature on InP Based MOCVD 22
2.3.1 InP Homoepitaxy....................... 22
2.3.2 GaInAs/InP ...... ..................... 30
2.3.3 GaInAsP/InP.......................... 33
2.3.4 InP Based Devices...................... 37
2.4 A Description of the MOCVD System. ........... 40
2.4.1 Introduction......................... 40
2.4.2 Gas Delivery System................... 43
2.4.3 Reactor and Heating System............ 47
2.4.4 Exhaust/Scrubbing System.............. 50
2.4.5 Safety ................................ 51
2.5 Determination of Optimum Growth Conditions
Based on Thin Film Characterization .......... 52
2.5.1 Experimental Approach................. 52
2.5.2 InP................................... 56
2.5.3 Growth of GaInAs Lattice-Matched to
InP .............................. .. 90
2.5.4 Growth of GaInAsP Lattice-Matched to
InP ................................... 108

III P-TYPE DOPING OF MOCVD INP: EXPERIMENTS AND
MODELING ......................................... 118

3.1 A Review of the Literature on p-Type Doping
of InP........................................ 118
3.1.1 Introduction.......................... 118
3.1.2 Bulk Crystal Growth................... 119
iv








3.1.3 Liquid Phase Epitaxy.................. 121
3.1.4 Molecular Beam Epitaxy................ 122
3.1.5 Chemical Vapor Deposition ............. 123
3.2 MOCVD Growth and Characterization of Mg-Doped
InP Using bis-(Methylcyclopentadienyl)
Magnesium as a Dopant Source................ 130
3.2.1 Introduction.......................... 130
3.2.2 MOCVD Growth.......................... 130
3.2.3 Results and Discussion ................ 131
3.2.4 Conclusions........................... 145
3.3 Experimental DMCd and DEZn for p-Type Doping
of InP by MOCVD.............................. 145
3.3.1 Introduction.......................... 145
3.3.2 DMCd Results.......................... 146
3.3.3 DEZn Results.......................... 148
3.4 Modeling of p-Type Doping of InP using DEZn.. 156
3.4.1 Introduction.......................... 156
3.4.2 Point Defect Structure................ 164
3.4.3 Discussion of Results................. 173

IV EPITAXIALLY GROWN INTERFERENCE FILTERS............ 177

4.1 Theory of Interference Filters............... 177
4.2 MBE Grown AlGaAs/GaAs Devices ................ 182
4.2.1 Introduction.......................... 182
4.2.2 Electrical Theory..................... 184
4.2.3 Experimental.......................... 185
4.2.4 Electrical Testing.................... 193
4.2.5 Conclusions........................... 204
4.2.6 Addendum.............................. 205
4.3 MOCVD Grown GaInAsP Devices ................... 209

V CONCLUSIONS AND RECOMMENDATIONS ................... 216

APPENDICES

A THEORY AND OPERATION OF THE LOW TEMPERATURE HALL
EFFECT SYSTEM .................................... 220

B OPERATION OF THE QUATERNARY MOCVD SYSTEM.......... 234

C DOPANT MODEL COMPUTER PROGRAM..................... 248

REFERENCES ............................................. 252

BIOGRAPHICALSKETCH....................................262














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

GROWTH AND MODELING OF III-V COMPOUND
SEMICONDUCTOR OPTOELECTRONIC MATERIALS
WITH DEVICE APPLICATIONS

By

ARNOLD JOHN HOWARD

December 1990

Chairperson: Timothy James Anderson
Major Department: Chemical Engineering

Several topics have been undertaken during the course of

this degree which are associated with understanding and

improving semiconductor processing. The growth, modeling and

characterization of III-V compound semiconductor materials and

optoelectronic devices has been emphasized. Epitaxial layers

of GaxIn1.xAsyP1.y with lattice-matched alloy compositions over

the range from x=0, y=0 (InP) to x=0.47, y=1 (Ga47In53As)

have been grown by metal organic chemical vapor deposition

(MOCVD) on InP substrates. Both the MOCVD system, used to

grow these layers, and a low temperature Hall effect system,

used to characterize these layers, were designed and

installed. The results from several other analytical

techniques were used to determine the optimal growth

conditions for high quality epitaxial layers.








The use of diethylzinc (DEZn), bis-(methylcyclo-

pentadienyl) magnesium (MCp2Mg) and dimethylcadmium (DMCd) as

p-type dopant sources for MOCVD InP was investigated at BNR in

Ottawa, Canada. It has been experimentally observed that the

carrier concentration dependence on dopant partial pressure in

the MOCVD reactor is different for each of these three

dopants. A novel model of the p-doping process of MOCVD InP

using DEZn has been developed that incorporates an equilibrium

boundary condition between the gas phase and solid phase point

defects. The results of this model indicate that at high DEZn

gas phase mole fractions, which results in low solid-phase

electrical activity, the dominant electrically inactive point

defects are intersticial zinc and zinc completed with a

phosphorous divacancy.

A novel optoelectronic device has been fabricated and

modeled which contains p-n heterojunctions in an optical

interference filter. Structures were grown by molecular beam

epitaxy at BNR using the GaAs/AlGaAs material system and by

MOCVD at the University of Florida using the InP/GaInAsP

material system. Structures with peak reflectivities at 1.3

and 1.40 microns were grown and good crystalline quality were

confirmed. Electrical bistability was observed in a forty-

layer device which has never been reported before in a

structure of this size.


vii













CHAPTER I
INTRODUCTION


1.1 III-V Semiconductors


Since the invention of the transistor in 1948 by

Shockley, Brattain, and Bardeen, there has been a revolution

in the electronics industry. Up to that time the vacuum tube

diode and triode were the most used electronic devices, but

then the transistor device using a semiconductor crystal as

its starting material was fabricated. The microchip, which is

the fundamental building block of present day computers,

contains a large number of tiny semiconductor transistors

using typically single crystals of silicon as a starting

material. Silicon has been the "workhorse" for the

electronics industry primarily due to its availability in high

single crystalline purity, ease of use in device fabrication,

and of course its good electrical properties. But, the

relatively low electron mobility and fixed indirect bandgap of

silicon makes it not suitable for present-day optoelectronic

device applications. As a consequence of these new demands,

research into the development of semiconductors with variable

electrical and optical properties has flourished.

Compound semiconductors such as GaAs, InP and others

composed of elements from group IIIA (Al, Ga, In) and group VA

1








2

(P, As, Sb) columns of the periodic table have electrical and

optical properties superior to those of silicon for certain

modern-day device applications. III-V materials have a wide

range of bandgap energies (0.18 to 2.4 eV), where the bandgap

energy is defined as the energy difference between the lowest

electron state in the conduction band and the highest hole

state in the valence band allowed in the semiconductor. Some

compound semiconductors have direct bandgaps, meaning that the

conversion of photons (light) to electrons (energy) or vice

versa, does not involve a third particle, such as a phonon.

The direct bandgap III-V compounds also have large electron

mobilities where mobility is defined (at low electric fields)

as the ratio of absolute electron velocity to the magnitude of

the electric field. A listing of these parameters and the

lattice constants of silicon and binary III-V semiconductors

is shown in Table 1[1]. As shown in this table, as much as a

two order of magnitude increase in electron mobility is

possible by using III-V compound semiconductors instead of

silicon for electronic devices. It is also significant to

note that a wide range of compound semiconductors can be

formed by creating solid solutions of the individual

semiconductors; hence, a wide selection of compound

semiconductors exists with a wide range of electrical and

optical properties.










Table 1

Properties of Silicon and III-V Binary
Semiconductors at 300 K


Bandgap Bandgap Electron Lattice
Type Energy(eV) Mobility Constant
(cm2/V-s) (Angstroms)
Si indirect 1.12 1350 5.43
InSb direct 0.18 100000 6.48
InAs direct 0.36 22600 6.06
GaSb direct 0.70 5000 6.09
InP direct 1.28 4000 5.87
GaAs direct 1.43 8500 5.65
AlSb indirect 1.60 200 6.14
AlAs indirect 2.16 180 5.66
GaP indirect 2.26 300 5.45
AlP indirect 2.45 80 5.46


Source: Streetman[1].








4

Since III-V compound semiconductors present a wide range

in values of direct bandgap energy, mobility, and lattice

constant, semiconductor devices have wider ranges of

application. The bandgap energy (Eg) of a semiconductor is

related to the cut-off wavelength (Ag) by the following

equation: Eg(eV) = 1.24/Ag (gm). The cut-off wavelength of a

semiconductor is the longest wavelength to which a detector

fabricated from this same semiconductor will respond. Another

degree of freedom available is the ability to form completely

miscible substitutional solid solutions independently on both

the group III and group V sublattices. In other words, not

only simple binary III-V compounds, but also III-III'-V or

III-V-V' ternary and III-III'-V-V' quaternary single

crystalline semiconductors such as AlxGa1. As, GaxInl-xAs,

GaASyP,1. and GaxIn_1.As P1. can be created. By using ternary

and quaternary semiconductors, it is possible to vary the

physical and electrical properties of these materials

continuously between the property limits of the constituent

binary compounds listed in Table 1. A plot of the lattice

constant versus the bandgap energy (at 300 K) for III-V

compound semiconductors is shown in Figure 1. Solid dots

indicate binary compounds, solid lines connecting dots

represent direct bandgap ternary solid solutions, and dashed

lines connecting dots represent indirect bandgap ternary solid

solutions.








5



6.6

In b


6.4 i




6.2 i
SA1 Sb
S\ GaSb
S \InAs

6.0

I-n


S5.8-\-


SA1As
GaAs
5.6 --


GaPS- -Al P

5.4 I

0 0.5 1.0 1.5 2.0 2.5

Bandgap (eV)




Figure 1: Lattice parameter and bandgap energy of various
III-V semiconductors








6

Basically, the entire area bounded by the solid and dashed

lines is available for use in the design of new III-V compound

semiconductor devices.

The cross-hatched area shown in Figure 1 is the lattice

parameter-bandgap energy space of the quaternary material

GaxInl.xAsyP1.y. This material has many optoelectronic device

applications due to its wide range of bandgap energy (0.36 to

2.26 eV) and possible lattice constants. Most semiconducting

single crystalline ternary and quaternary materials are

epitaxially grown on a substrate of nearly the same lattice

constant. Hence, the two compositional degrees of freedom

available with the GaInAsP system are important because

presently only GaAs, GaSb, GaP, InP, InAs and InSb are

available for use as substrate materials. For GaInAsP on InP,

or any other heteroepitaxial materials system, a difference of

lattice constant (lattice-mismatch) of greater than 0.1%

between the grown film and substrate leads to, for film

thicknesses greater than the critical thickness, the formation

of structural defects which can degrade device performance.

This problem puts strict demands on the epitaxial growth

technique employed.

1.2 Epitaxial Growth Techniques

The word "epitaxy" is derived from Greek and means

"arranged upon." Epitaxial films of III-V materials are

usually grown or arranged upon substrates with equivalent

crystalline structure and lattice constant. The two most








7

common substrate materials used today for III-V homoepitaxy,

(i.e., growth on a crystal of the same composition), or

heteroepitaxy, (i.e., growth on a crystal of different

composition or crystal structure), are GaAs and InP. The

substrates are cut from bulk crystals along a particular

crystal orientation from two to four inch diameter boules

which are sometimes created by withdrawing a seed crystal from

a heated liquid melt. Epitaxial growth on these substrates is

accomplished by exposing the heated surface to a flux of group

IIIA metals and group VA non-metals. The flux can be supplied

from a liquid, vapor, or molecular beam source. This

distinction defines the three primary methods for growing

epitaxial III-V films: liquid phase epitaxy (LPE), vapor

phase epitaxy (VPE), (also known as chemical vapor deposition

(CVD)), and molecular beam epitaxy (MBE). Each technique has

its own advantages and drawbacks, which will be discussed in

the following paragraphs.

LPE is a growth technique which can be used to deposit

thin single crystal layers of III-V compound semiconductors

from a heated liquid solution by decreasing the temperature of

the substrate relative to the solution. It is a relatively

simple, inexpensive, near equilibrium (reproducible) growth

technique that is well understood. The growth rate can be

high and a wide range of both p- and n-type dopants are

available and their incorporation is controllable. LPE has

been used to grow InP[2] and high quality GaInAs[3] on InP for








8

laser applications. However, problems such as surface

defects, poor thickness and compositional uniformity, and

difficulty in growing abrupt heterojunctions have made the LPE

technique unsuitable for present-day device fabrication

demands. For simple layer structures such as the AlGaAs solid

state laser used in compact disc players, LPE is perfectly

adequate.

There are three distinct VPE or CVD chemistries: chloride

CVD, hydride CVD and metal organic CVD. The chloride (or

sometimes referred to as halide) CVD process for GaAs growth,

as an example, uses AsC13 and metallic Gallium as sources in

an open tube system with H2 (as a carrier gas) to transport

reactants from the source zone, through a temperature gradient

zone to the deposition zone. The chloride CVD process is a

surface-kinetically limited process requiring careful source

composition control and accurate temperature control

throughout the system for reproducibility. Also, it is

difficult to vary the V/III ratio and transients are long so

abruptness is bad in chloride CVD. GaInAsP has been grown by

the chloride CVD method[4] but other CVD techniques are more

convenient and flexible for growing ternary and quaternary

III-V compounds. Hence, the chloride process is usually only

used to grow high purity epitaxial GaAs.

The hydride CVD process for growth of III-V compound

semiconductors differs from the chloride process by replacing

column V chlorides such AsC13 or PCl3 with column V hydrides







9

like AsH3 or PH3. For InP growth, HC1 gas is first reacted

with liquid indium metal in the source zone. The gaseous

product InCl is then carried by H2 to mix and react with PH3

to deposit InP in the growth zone. Similar to the chloride

process, accurate temperature control is required for this

three-zone process which is also surface-kinetically limited

in the low-temperature growth regime. The hydride process is

currently widely used for light-emitting diode (LED)

applications using GaAs1l.Px. It has also found application

in the growth of III-V GaInAsP and GaInAs for LED's, lasers

and detectors[5]. One major advantage the hydride system

provides over the chloride system is the ability to vary the

vapor phase V/III molar ratio by adjusting the inlet flow

rates of hydrides and HC1. One drawback of both the hydride

and chloride systems is that they are hot-wall systems;

interaction between the gas and heated SiO2 reactor wall

occurs which results in unintentional silicon incorporation

into grown layers. Due to its successful use, especially in

LED fabrication, hydride CVD will continue to have a

significant role in the growth of III-V materials.

The third type of CVD or VPE process is metal organic

chemical vapor deposition (MOCVD). The MOCVD process involves

an irreversible pyrolysis reaction of vapor-phase mixtures

usually of group IIIA metal organic sources and group VA

hydride sources. For InP, as an example, trimethylindium

(TMIn) and PH3 diluted in H2 would flow into an open cold-wall







10

quartz tube, decompose in the presence of a heated substrate,

and then deposit an epitaxial layer. Under normal deposition

conditions, the MOCVD process is kinetically limited by mass

transport of the column III source through a stagnant layer

near the growing surface. The MOCVD process is capable of

growing a wide variety of films with excellent abruptness

uniformity over large substrate areas. The principal device

area where MOCVD has made an impact is optoelectronics. A

thorough review of the MOCVD literature has been written by

Ludowise[6] and a brief history of MOCVD with emphasis on InP

based materials and device applications is presented in

section 2.1.

Molecular beam epitaxy (MBE) is a technique capable of

growing epitaxial films one atomic layer at a time. MBE makes

use of controlled evaporation from one or more thermal sources

to direct beams of atoms or molecules onto a heated substrate

under ultra-high vacuum conditions. During a MBE growth the

substrate temperature is generally kept relatively low (500-

6000C for GaAs). MBE growth rates are typically slow (0.1 -

2iLm/hr) which in combination with low growth temperatures

permits precise layer thickness, doping and compositional

control[7]. For GaAs, the As4 beam flux is much greater than

the Ga beam flux, and both fluxes are dependent upon the

temperature of the effusion oven, molecular weight of the

emitted atom, orifice area, and source cell to wafer distance.

With a properly placed two-inch rotating wafer, nonuniformity








11

of the growth film may be reduced to less than ten percent.

Numerous devices such as LEDs, lasers, FETs and HBTs have been

fabricated with MBE. One unique bonus with MBE is that in-

situ monitoring devices such as RHEED, mass spectrometers,

Auger spectrometers and ion gauges are feasible and

commonplace. However, large expense and limited throughput

restrict MBE usage.

During the last decade, several novel deposition

techniques for III-V compound semiconductors have emerged.

Each one is a spin-off of either MOCVD, MBE, or a combination

of the two and has some relative advantages and disadvantages;

none of these new techniques are widely used. One such

technique is called atomic layer epitaxy or ALE. For III-V

compound semiconductors, ALE proceeds by depositing a

monolayer of a group III metal followed by depositing a

monolayer of group V atoms, in a layer-by-layer sequence[8].

In ALE, grown layer thickness is determined by the number of

cycles rather than the time of growth. Another relatively new

technique is called chemical beam epitaxy (CBE) or metal

organic molecular beam epitaxy (MOMBE). In MOMBE, all of the

group IIIA and VA sources are metal organic; TMAs replaces As4

and AsH3, TEP replaces P2 and PH3[9]. The remaining aspects

of MOMBE are essentially the same as conventional MBE. A

final technique which is similar to ALE but is performed in a

conventional low pressure MOCVD system is called flow

modulation epitaxy or FME. FME has been used to grow InP at








12

temperatures as low as 3300C by alternately pulsing PH3 and

TMIn with intervening H2 purge steps into the reactor with

each step lasting on the order of several seconds[10]. So, it

is evident that there are a wide variety of epitaxial growth

techniques for III-V compound semiconductors. Most of the

experimental work presented in this dissertation made use of

a low pressure MOCVD system. One study made use of a MBE

system for a GaAs/AlGaAs device, but the rest of the

literature review will focus on the use of MOCVD for growth of

GaInAsP/InP materials for optoelectronic device applications.













CHAPTER II
METAL ORGANIC CHEMICAL VAPOR DEPOSITION


2.1 A Brief History of MOCVD


MOCVD refers to the deposition of multiconstituent films

using one or more metal organic compounds as sources, and the

term was originated by Manasevit[11]. He demonstrated that

single crystalline GaAs could be deposited using TEGa and AsH3

in an open tube reactor. Shortly after this report, it was

discovered that by mixing metal organic and hydrides of

different elements, binary and ternary III-V compounds such as

GaP, GaAsP, GaAsSb, A1N, GaN, InAs, GaInAs, InAsP, and InP

could be formed in a manner similar to GaAs[12-16]. Practical

information was also reported in these early efforts such as

the observation that GaAs film growth rate was mass transport

limited by the metal organic group III source and independent

of temperature below 8000C. Also, n-type doping using H2S,

H2Se and p-type doping using DEZn and DMCd were achieved.

Soon after the demonstration of high quality MOCVD grown

material, all MOCVD grown devices such as FETs,

photocathodes[17,18] and GaAs/AlGaAs DH laser diodes[19] were

reported. These milestones resulted in a rapid increase in

MOCVD research and development, and as a result, it has been

demonstrated that MOCVD has the capability of growing a wide

13








14

variety of device quality III-V compound semiconducting

materials.

The MOCVD process has been used for the epitaxy of most

III-V compound semiconductors. The basic overall reaction is

M11,(Alk)3(g) + XVH3(g) ----> M,11X(s) + 2 Alkane(g). In this

reaction, the organometallic and the hydride typically are

irreversibly pyrolyzed by the heat of the susceptor and

substrate to form molecular (or atomic) intermediates which

may react in the gas phase or on the substrate surface[20].

Some researchers say that the breaking of metal-carbon bonds

occur on the semiconductor surface[21]. Contradicting reports

such as these may be related to differences in test conditions

or configurations from team-to-team, but the only conclusion

that can be made is that the complete MOCVD process is not

well understood at this time.

Each step of the MOCVD process is not known, but there

are several practical trends that are generally agreed upon.

Some trends for GaAs and InP are: (1) III-V MOCVD growth

between 550 and 7500C is mass transport limited in the column

III source (below 5500C growth is reaction limited); hence

growth rates are determined by metal organic fluxes and are

temperature independent; (2) for ternary and quaternary films,

group III solid phase compositions are linearly related to gas

phase group III compositions, however, solid phase group V

compositions have a non-linear dependence on gas phase group

V compositions due to large differences in cracking








15

temperatures (PH3 is more difficult to decompose than AsH3);

(3) gas phase V/III ratios have a strong effect on background

carrier concentrations and p- and n-type doping levels; (4) in

some cases, group III metal organic react with group V

hydrides to form adducts and polymers that may be involatile

liquids or solids; and (5) group II alkyls act as p-type

dopant sources, group VI hydrides act as n-type dopant

sources, but depending on growth conditions, group IV hydrides

can act as donor or acceptor sources in III-V's. The extent

to which these trends apply to all III-V materials does vary,

but they are definitely useful in designing an MOCVD system or

in optimizing material specific growth conditions.

2.2 MOCVD Systems

Most MOCVD systems use quartz reactors oriented either

vertically[ll] where gas flow is usually down, or horizontally

[22] where gas flow is usually over a wedge shaped susceptor.

Other less common systems incorporate barrel reactors[23]

where growth can occur on multiple wafers with reactants

flowing from top to bottom, or "chimney" reactors[24] where

gas flow is up and wafers and susceptor are held vertically.

In a vertical reactor, uniform growth rates are more difficult

to achieve than in a horizontal reactor because with the

geometry of a vertical reactor, nonlaminar, turbulent gas flow

can more easily occur. In a horizontal reactor, a boundary

layer zone forms above the susceptor where the flow rate is

lower than that of the bulk gas above. The stagnant layer








16

thickness can increase along a flat susceptor in the gas flow

direction which in combination with the depletion of reactants

due to deposition, can result in nonuniform grown layer

thicknesses. This variation can be nullified by either

tilting the susceptor at an angle of 5 to 100 or inserting a

baffle into the reactor at an angle positioned above the

horizontal susceptor. This gradually reduces the cross-

sectional area in the reactor above the wafer resulting in a

gradual increase in linear gas velocity causing the boundary

layer to have a uniform thickness profile. With a constant

boundary layer thickness and constant diffusion coefficient of

group III source molecule, the flux of the mass transfer

limited reactant to the growing surface will be constant. As

a result of this, uniform semiconductor films can be grown

over large area substrates.

When MOCVD growth of mixed crystals involves the use of

more than one group III compound, GaInAs for example, solid

phase compositional non-uniformity can result. This can

result from concentration gradients in the gas flow direction

due to slight differences in the magnitude of the diffusion

coefficients. One way to avoid this problem is by selecting

group III metal organic sources with similar molecular weights

and correspondingly similar diffusion coefficients. Another

way of avoiding this problem is by using low pressure (= 0.1

atm) operation instead of atmospheric pressure. At reduced

pressures, the linear gas velocity increases and accordingly








17

the stagnant layer thickness decreases. Also, at lower

pressures, diffusion coefficients increase, making more abrupt

heterojunctions possible. Low pressure operation also reduces

the occurrence probability of unwanted parasitic reactions.

Because of all of these advantages, low pressure MOCVD growth

has become widely used especially for multi-wafer scale-up

production applications.

Another problem with the traditional horizontal "Bass

type"[17] reactor is due to the fact that having a cold dense

gas above a hot, less dense gas is unstable because of

gravity. This can cause natural convection which results in

closed stream-line gas flow patterns. This problem can

usually be minimized by operating at reduced pressures[25].

The best solution, however, appears to be the use of an

inverted reactor geometry[26] which completely eliminates

thermal buoyancy effects. In this geometry, the susceptor is

located at the highest and hottest point of a horizontal

reactor with the wafer mounted upon it facing downwards.

Another obvious benefit with this design is the elimination of

the problem of particles falling on the substrate before and

during epitaxial growth which can lead to structural defects.

With this design, improved GaInAs compositional uniformity and

a complete elimination of parasitic deposition on the quartz

wall opposite the susceptor have also been reported[26].

Another final technique that has been used to improve both

thickness and compositional uniformity is the use of moving








18

substrate holders in both circular and even planetary motion

configurations[27]. These techniques can greatly improve

uniformity, but also, unfortunately, greatly add to machine

complexity and expense.

Aside from the reactor, the other major parts of an MOCVD

system are the gas delivery, heating exhaust/scrubbing and

safety systems. Most gas handling systems are constructed

from high purity stainless steel tubing, valves (air-operated

and manual), regulators, electronic mass flow controllers and

filters or purifiers. The system typically delivers metal

organic (from bubblers), hydrides (from high pressure gas

cylinders) and most often hydrogen (from a palladium-alloy

purifier) to a fast switching manifold which directs gases to

the reactor or to a vent line. Gas manifolds should be

situated as close to the reactor inlet as possible to minimize

tube length and improve interface abruptness capabilities.

Most systems use manifolds with a linear valve arrangement,

but only in a radial manifold arrangement is the length from

each valve to the reactor potentially the same for all of the

gases[28]. In the "vent/run" type system discussed above, the

vent line and reactor line are "pressure balanced" so that

transient times associated with gases adjusting to and flowing

from a high pressure to a low pressure line, or vice versa,

can be eliminated.

For heating systems, most MOCVD reactors are heated by

inductively coupling RF power to a graphite susceptor. This







19

is ideal because it is a non-contact method, it selectively

heats only the graphite, and it is easy to configure by

arranging a copper coil around the susceptor portion of the

reactor. The RF generator size required depends on susceptor

size, gas velocity, coupling efficiency, and reactor wall

cooling mechanism. Infrared heating from quartz-halogen lamps

has also been used but, as wall deposition increases, non-

uniform heating may occur. A resistance heater embedded into

the graphite is another option but deposition on electrical

feedthroughs complicates reactor cleaning. Most heating

systems use an embedded thermocouple feedback system to

control temperature. Optical pyrometers have also been used

but wall deposition can result in false readings.

After passing through the heated zone of the MOCVD

reactor, some of the toxic gas sources still remain uncracked

and undeposited. These gases have to be neutralized before

being discharged into the atmosphere. Hazardous gases such as

AsH3, PH3 and SiH4 are commonly used in the MOCVD of III-V

semiconductors and are difficult to neutralize or "scrub"

especially when they are used in combination. There are four

different types of scrubbing systems commercially available;

depending on the application one alone may be inadequate. The

four types are liquid scrubbers, thermal crackers, dry powder

scrubbers, and incinerators. Liquid based scrubbers are most

commonly used and work by bubbling the toxic gas through a

basic (pH > 10) solution of sodium hypochlorite and sodium








20

hydroxide diluted in water where neutralized salts and acids

are products. Some gases are nearly insoluble in water,

though. Thermal crackers basically operate by heating the

exhaust stream to approximately 9500C to thermally decompose

toxic gases into less toxic compounds. Clogging and also

insufficient heat transfer at high flow rates are problems

with this technique. Dry scrubbers use powders such as

activated carbon or diatomaceous earth mixed with iron

chlorides to react with the toxic gases. This technique also

has problems associated with efficient gas solid contacting

and disposal of toxic corrosive powders. A final technique,

the incinerator or "burn box," operates in such a way that

gases are mixed with a fuel gas and oxygen and then ignited by

a pilot flame or electric igniter[29]. All of the four

scrubbing techniques have their individual problems. The

scrubbing system should ideally be a combination of two or

more of the individual systems in case one system fails.

As discussed, MOCVD of III-V compound semiconductors

presently involves the use of highly toxic and explosive

source gases. There has been some work on the use of less

toxic sources for MOCVD such as tertiarybutylphosphine (TBP)

[30] instead of phosphine (PH3), (the threshold limit value

(TLV) of PH3 is 0.3ppm while that of TBP is greater than

1000ppm), but the material grown with these new sources is

generally inferior. In any event, there must always be an

integrated safety component to all MOCVD systems.








21

Several papers have been published on the important topic

of MOCVD safety[31,32]. These papers are very useful when

designing the layout of an MOCVD machine and laboratory.

First, all MOCVD systems must have both toxic gas and hydrogen

sensors in and around them connected to an alarm. These

sensors must be capable of shutting down the machine in the

event of a detected leak. In order to shut down quickly, all

gas lines must be equipped with normally closed air-operated

valves. Toxic gas lines should also be double contained

(which is extremely expensive) and equipped with pressure

sensors that sound an alarm for abnormally high pressures.

Compressed air, hydrogen and nitrogen lines should have

sensors for abnormally low pressure which can, if activated,

shut the machine down. The reactor and pump exhaust line

should also have similar pressure sensors. Of course, smoke,

fire, and cooling water flow detectors and sensors are

necessary. Other things such as micro-switches on panel doors

and available supplied air masks are also required. Finally,

SCBAs and trained users should always be available outside of

the facility for emergencies. All of the above mentioned

safety issues/design features are important for the design and

operation of a modern MOCVD system. Of course, the most

important thing for lab safety is to provide adequate operator

training focusing on the nature of the toxic sources and how

to treat them.








22

2.3 A Review of the Literature on InP Based MOCVD

2.3.1 InP Homoepitaxv

The first reported growth of indium phosphide (InP) by

MOCVD was in 1969 by Manasevit and Simpson[12]. This work as

well as other early efforts[33-35] used triethyl indium (TEI)

and phosphine (PH3) as indium and phosphorus sources. For

several years, problems such as low, nonuniform growth rates

and high impurity levels were encountered. One source of

these problems was found by relating the observation of a

white smoke at certain growth conditions to the uncontrolled

gas phase reaction between metal organic indium sources and

phosphine. This reaction occurs at low temperatures <1000C,

is parasitic in nature, and produces a non-volatile liquid

polymer.

One method used to minimize this problem was the use of

low pressure reactor systems[36-38] to decrease the residence

time of unreacted species upstream of the growth region.

Another method involves the use of adducts such as TMI-TMP

[39] or TMIn-TEP[40] as indium sources which will not complex

with PH3. Another technique is to keep the reactants apart

and only let them mix just prior to the growth region. This

method can, however, lead to uniformity problems. The most

recent improvement is the use of TMIn (a solid powder at room

temperature which melts at 880C) instead of TEIn (a liquid at

room temperature), as the indium source[41-43]. TMIn also

decomposes at a much higher temperature (>3000C) than TEIn







23

(<1000C) and therefore is less likely to react upstream of the

heated growth zone. Also, TMIn has a much higher vapor

pressure than TEIn which is experimentally convenient because

heated gas lines would no longer be necessary.

Other techniques such as the use of hydrogen-nitrogen

mixtures as the carrier gas[41] and phosphine pre-crackers[44]

have been tried with varying degrees of success and merit. A

final conclusion is that proper reactor geometry, system

design, and growth conditions are very important for avoiding

parasitic gas phase reactions and obtaining superior InP thin

film quality. Currently, the reaction at certain growth

conditions of TMIn and PH3 in a properly designed MOCVD system

can yield uniform, high quality epitaxial InP with no evidence

of indium prereaction problems.

The deposition of high quality layers of InP for device

applications requires precise control of their unintentionally

introduced (undoped) and intentionally introduced (both p- and

n-type doped) impurity concentrations. An important condition

for obtaining reproducible p- and n-type doping levels is the

ability to grow undoped material with a reproducibly low

background carrier concentration. To obtain low background

levels, one needs a contamination free MOCVD system equipped

with high purity sources and optimized growth conditions.

Most conventional MOCVD systems are constructed from

ultra-high purity components such as electropolished welded

316 stainless steel and semiconductor-grade low-sodium content







24

quartzware. Coupled with the use of palladium-alloy diffused

hydrogen as a carrier gas, these precautions usually eliminate

the system as a source of high background impurity level

problems. In addition to high purity equipment, ultra high

purity sources contained in stainless steel bubblers and

corrosion resistant coated cylinders are required. For InP,

phosphine with five nines purity (99.999%) and diphos purified

(doubly sublimed) trimethylindium are both commercially

available. As purification technologies advance, then

progressively lower background doping levels surely will

follow.

The most important material's issues in the InP growth

area are the effect of growth conditions, substrate quality,

substrate orientation and substrate wafer cleaning techniques

on material quality. Several papers have been published on

each topic and the basis of comparison presented usually

involves characterization results of thin films such as room

temperature (300 K) and/or liquid nitrogen temperature (77 K)

mobilities and undoped carrier concentrations (ND-NA) cm3, etch

pit densities (EPD), photoluminescence (PL) intensities and

occasionally device performances.

The effect of growth conditions on properties of InP

grown by MOCVD has been studied by several research teams[45-

48]. Razeghi and Duchemin[45] showed that the growth rate of

InP is linearly dependent on indium metal organic reactor

partial pressure and independent of the phosphine partial







25

pressure. They also compared the growth rate of undoped InP

on (100), (111), and (115) InP oriented substrates and they

reported excellent film quality on (100) 20 towards (110) and

(115) 20 towards (111). Eguchi et al.[46] studied the effect

of V/III (phosphine to metal organic indium) ratio on EPD and

electrical properties and reported superior material at high

V/III ratios (>300). This result was in agreement with

Kasemset's[47] earlier work on both V/III ratio and growth

temperature effects. However, a survey of the effect of

growth temperature on layer quality is less conclusive.

Kasemset[47] indicates that a decrease in background carrier

concentration results upon increasing growth temperature,

while Scott et al.[48] report the opposite trend. This

discrepancy is probably due to different dominant impurities

in each group's TMIn source with correspondingly different

incorporation mechanisms. Most teams report high quality

MOCVD InP grown at temperatures between 5500C and 6750C.

Below 5500C, growth rates drop and material quality degrades.

Above 7000C, background carrier concentrations increase.

Presently, high quality two inch diameter wafers of InP

are commercially available as both doped (p and n-type) and

semi-insulating. Variation of results from team to team in

early research efforts and even today may be due in part to

the lack of reproducibility of substrate properties from batch

to batch and vendor to vendor. A recent paper from Knight et

al.[49] reports this problem. They observed a correlation







26

between leakage current of p-i-n InP based photodiodes and

substrate quality. Consequently, they set up a nondestructive

PL wafer-mapping system to evaluate grown film quality before

investing further processing time. Non-destructive techniques

such as PL mapping will remain essential unless wafer quality

control improves.

Proper wafer cleaning is also very important for the

growth of high-quality InP. Tuck and Baker[50] in 1973

published work on the chemical etching of (111) and (100) InP.

They compared the merits and disadvantages of using the

following four etching solutions: (1) 1HCL:1HNO3; (2)concen-

trated HC1; (3) 0.4N Fe3+; and (4) 1% bromine in methanol,

based on etching rate and hillock delineation. Nishitani and

Kotani[51] presented the use of H202-H2SO4-H20 solutions for

etching (100) and (111) oriented InP. Recently, studies have

been reported using sulphur to chemically passivate the

surfaces of InP and GaAs[52-53]. The goal of this work is to

reduce the substrate surface recombination velocity in order

to improve device performance. Another interesting study

compared several wafer cleaning methods using the surface

science techniques ISS, ESCA and AES[54]. This report states

that using a 5:1:1 mixture of H2SO4:H202:H20 in combination

with solvent degreasing step yields an InP surface with the

least amount of absorbed carbon and oxygen relative to the

other methods tested. Most crystal growth teams develop their







27

own technique of wafer preparation using a combination of the

methods reviewed above.

Once an MOCVD system has been optimized for growing high

purity InP and a proper substrate vendor, orientation and

cleaning procedure have all been selected, most authors report

that the source purity of both the metal organic indium and

phosphine have the strongest influence on background carrier

concentrations and mobilities. The initial work on InP growth

[12,33-34] reported room temperature carrier concentrations of

n=0.17 to 1.4 x 106cm'3 and 300 K and 77 K electron mobilities

of 3500-4200 and 16,000-36,000cm2/volt-sec, respectively.

After two decades of technological advancement in purification

techniques and machine design, the highest reported 77 K

mobility for undoped InP is now 305,000cm2/volt-sec with a

corresponding carrier concentration of n= 5 x 1013cm'3[55].

Based on low temperature PL it appears the dominant residual

acceptor in MOCVD InP is zinc[55]. For many years both carbon

and manganese(47] have also been reported as compensating

acceptors and silicon has been reported as the dominant

donor[56]. The recent work of Bose et al.[55] caution against

identifying PL peaks as carbon since the transverse optical

phonon replicas of the free-exciton recombination occur at the

same energy as carbon. For most teams, however, 300 K and

77 K mobilities of 4,700 and 80,000cm2/volt-sec, respectively,

and a carrier concentration of n= 1 x 1014cm"3 are typical for







28

undoped MOCVD InP. So, it is evident that device quality

unintentionally doped InP can be grown by MOCVD.

Most semiconductor devices require a junction of some

type in the host material where two materials with either

different electrical or optical properties meet. An

electrical junction can be created by post growth processing

techniques such as ion-implantation or diffusion of a donor or

acceptor into the host crystal. Another way is to just create

the junction in-situ during the MOCVD growth by adding a small

quantity of a donor or acceptor source into the inlet gas

stream. High quality undoped InP is usually n-type with a

background carrier concentration of n = 1 x 1014- 1015cm3.

The carrier concentration n or (ND-NA) can be increased by

adding an InP donor species to the inlet gas stream of the

MOCVD reactor. InP can be doped n-type by using H2S[45],

H2Se[43] and SiH4[43] or Si2H6[57] as sources. For each

source, the free carrier concentration is essentially

proportional to the dopant source mole fraction in the reactor

inlet stream. Controllable n-type doping from 015cm'3 to

1020cm'3 can be achieved without a significant decrease in

material quality by using a combination of these sources for

different parts of this wide incorporation range. Doping

levels and diffusion rates of these dopants are affected to

varying degrees by changes in growth conditions such as

temperature, V/III ratio, and indium mole fraction. For H2S,

the free carrier concentration in deposited InP layers







29

decreases when the growth temperature increases. For SiH4,

the opposite trend is observed, because the incorporation of

silane is reaction limited whereas H2S is adsorption limited.

Si is amphoteric in InP, acting as an acceptor or donor,

depending on site selection (adjusted by changes in the V/III

ratio used during the growth). H2S does not compensate itself

in InP but the diffusion coefficient of S is greater than that

of Si in InP. Depending on the device application, a suitable

n-type dopant source for InP is apparently available.

InP can also be doped p-type by adding an InP acceptor

species to the inlet gas stream of the reactor. The gas

stream must contain enough of an acceptor species to increase

the electrically active extrinsic acceptor level above the

electrically active intrinsic donor level. The metal organic

(MO) compounds DESn[45], DMZn[58], DMCd[59], Cp2Mg[60] and

MCp2Mg[61] act as sources for acceptors in InP with varying

degrees of success. There is not one single MO acceptor

source which dopes InP p-type over a wide doping range (1015-

1019cm3) without extended diffusion or surface morphology

degradation. This is the reason that so many different

sources have been investigated for InP as suitable p-doping

sources. This dilemma, in combination with the observation

that the electrical activation of some p-type dopants in InP

is much less than unity, was a driving force for the extensive

literature review and model development for p-doping of MOCVD

InP in Chapter III of this dissertation. The most commonly








30

used p-dopant source is DEZn which can be used to dope InP

over the range p = 1015-1018cm3. DEZn is sufficient for most

device applications, but its relatively high diffusion

coefficient at typical growth temperatures, D = 3 x 10'13

cm2/sec[58], does make it unsuitable for some device

applications. For a more extensive discussion on p-doping of

InP, the reader is referred to Chapter III of this

dissertation.

2.3.2. GaInAs/InP

The ternary compound GaxIn1.xAs can be grown lattice-

matched to InP by MOCVD. Unlike the AlGaAs/GaAs material

system, the GaInAs/InP system is not lattice-matched for all

compositions. Ga47In53As has an energy gap of 0.75 eV (Ag =

1.67 pm) and is the only composition which is lattice-matched

to InP. This ternary film can be grown by carefully

controlling the gallium to indium metal organic source

composition of the gas inlet to an MOCVD reactor. This is one

of the most severe heteroepitaxial growth scenarios possible

as the group V sublattice must be changed from pure phosphorus

to pure arsenic.

The first reported MOCVD growth of GaxIn1lxAs was on a

GaAs substrate[61] and hence was lattice-mismatched. The

early efforts provided useful information such as

compositional uniformity, merits of methyl versus ethyl MO

sources, gas phase reactions and purity for later GaInAs/InP

work. One conclusion that was useful for GaInAs/InP work was







31

the improvement in material quality observed upon using TEGa

instead of TMGa as the gallium source. Another useful

observation was that the solid phase composition is controlled

by and almost equal to the gas phase ratio [TMIn]/([TMIn] +

[TEGa]). Finally, the GaInAs growth rate is proportional to

the sum of the metal organic gas phase concentrations.

Lattice-matched Ga47In53As grown on InP by low pressure

MOCVD using TEGa and TEIn was first reported by Hirtz et al.

[62] in 1980. As stated previously, the choice of group III

alkyl sources used for GaInAs growth is critical. Using TMGa

and TEIn results in poor surface morphology, whereas using

TEGa and TEIn results in nearly featureless material over the

composition range 0.4 < x < 0.6 (Gax). This phenomena has

been attributed to a TMGa-InP substrate steric hindrance to

the heterogeneous decomposition of TEIn[63]. When growing on

InP, the initial stage of growth of GaInAs is also complicated

by the incongruent evaporation of phosphorus from the InP

substrates upon heating. It has been shown that the

morphological, optical and electrical properties of the GaInAs

epitaxial layer depend heavily on minimizing InP substrate

damage during the transition from PH3 to AsH3[64]. The best

approach is using an InP buffer layer and then allowing the

indium flow to continue while rapidly switching phosphine to

the vent and TEGa and AsH3 to a low pressure reactor.

As is the case for growth on GaAs substrates, the

composition of GaInAs is linearly dependent on the flow rate







32

of TEGa for a fixed TEIn or TMIn flow. Both the composition

and growth rate are independent of AsH3 flow for fixed metal

organic flow. The growth rate is independent of growth

temperature (500-6500C), but the composition can be slightly

affected due to slight differences in gallium and indium

source cracking efficiencies. If the composition of a layer

is different from the lattice-matched value, this layer is

mismatched. The lattice-mismatch between layer and substrate

is defined as Aa/a = (aL a)/ a when aL is the measured

room temperature (strained) lattice parameter and a is the

lattice parameter of the substrate. Razeghi et al.[38] have

reported that the mobility of a semiconducting layer is

dependent on the amount of mismatch in the layer relative to

the substrate. At optimum growth conditions, a lattice-

matching of Aa/a < 0.04% has been achieved resulting in Hall

mobilities of 12,000 (300 K), 100,000 (77 K) and 260,000cm2/

volt-sec (2 K) and background carrier concentrations of 0.7 -

1.0 x 1015cm'3[65]. Such high mobilities at 2 K are explained

by the existence of a two-dimensional electron gas formed at

the interface between undoped InP buffer and GaInAs layers and

are indicative of superior material quality.

It is evident that MOCVD can be used to produce undoped

Ga 47In.53As lattice-matched to InP, with very high quality

electronic properties. Intentionally doped both p- and n-type

Ga47In.53As on InP is also producible. Razeghi[65] presented

data on p-type doping using DEZn and n-type doping using H2S







33

of GaInAs by MOCVD. Zinc doped GaInAs carrier concentrations

are reported to decrease with increasing growth temperatures

over the range p = 1017 018cm-3. The opposite behavior is

observed for sulphur doped GaInAs and this trend is confirmed

by Logan et al.[66]; a carrier concentration of 1020cm'3 is

reported for a growth temperature of 5250C and at nearly

identical conditions except Tg = 6250C, n = 8 x 017cm'3. Wide

ranges of both p- and n-type doping are attainable for GaInAs

on InP by MOCVD which is useful for device applications.

2.3.3 GaInAsP/InP

The quaternary solid solution GaxIn.xAAs P1., is an alloy

semiconductor which can be lattice-matched to InP and GaAs

substrates. GaInAsP is a direct bandgap semiconductor (when

lattice-matched to GaAs or InP) which can be a very efficient

light emitter over the wavelength range 0.65 0.87gm

(lattice-matched to GaAs) and 0.92 1.65gm (lattice-matched

to InP). Very little work has been reported on quaternary

growth on GaAs substrates[67,68] due to greater interest in

GaInAsP alloys lattice-matched to InP substrates for optical

fiber device applications. Optoelectronic devices operating

at 1.3/m or 1.55Am wavelength regions have immediate

commercial applications because light transmission through

silica fibers exhibits low loss at 1.3/m and low dispersion at

1.55gm.

The MOCVD growth of GaInAsP alloys on InP substrates was

first reported using ethyl alkyls in a low pressure







34

system[63]. This same team later reported growth of nearly

the entire quaternary composition range lattice-matched to InP

and presented device test results of broad area and stripe

lasers fabricated using six different quaternary

compositions[69]. After these initial reports, numerous

quaternary related papers have been published on novel growth

techniques, relationship between gas phase growth conditions

to solid phase compositions, electrical and optical material

properties, and device applications. Each of these topics

will be reviewed in the following paragraphs.

Growth techniques used for depositing quaternary alloys

are direct extensions of techniques used for growing InP and

GaInAs. The growth rate is similarly proportional to the sum

of the partial pressure of TEGa and TMIn, and is independent

of the phosphorus and/or arsenic partial pressure. Similar to

the growth of GaInAs, the solid phase group III composition is

essentially equal to the gas phase metal organic composition

introduced to the MOCVD reactor. However, the behavior of

incorporation of group V elements is different and much more

difficult to control since both arsine and phosphine are

required and they do not incorporate with the same

probability. It is much more difficult to incorporate

phosphorus than arsenic at a fixed growth temperature since

the cracking temperature of PH3 is higher than AsH3. To

alleviate this problem, some workers have used precracked

PH3[38,70] or in-situ adduct formation techniques[71] (which







35

is a more easily cracked species) both with varying degrees of

success. Most teams use the low pressure MOCVD technique

which makes PH3/AsH3 ratios as high as 200 (which is required

for 1.0pm wavelength quaternaries) more safely attainable.

The operating parameters inlet partial pressures, total

pressure, deposition temperature, and V/III ratio have an

effect on the growth rate and composition of deposited

quaternary materials. Because of this, numerous papers have

been written relating the gas phase growth conditions to solid

phase material quality and composition. Razeghi[65] has

published graphs on which are plotted the relationship between

growth conditions ratios and bandgap wavelengths for the full

range of quaternary materials which are lattice-matched to

InP. The three ratios are: (1) Rg = PH3 / (PH3+AsH3) ; (2) R3

= TEGa/(TEGa + TEIn); and (3) R5/R3. Using these graphs,

which are only valid for a growth temperature of 6500C and

total flow rate of 7 liters/min, one can estimate growth

conditions for any lattice-matched quaternary composition.

Similarly, Fujii et al.[72] and Sugou et al.[70] present

quaternary compositions as a function of (In/Ga) and (P/As)

ratios in the gas phase for fixed V/III ratios. Koukitu and

Seki[73] use a thermodynamic approach to compute the solid

composition as a function of input mole ratio for several

quaternary III-V alloy systems. They also compute the

equilibrium partial pressures of gaseous species over

GaInAsP/InP as a function of temperature, V/III ratio and







36

Ag(Am). Using one of the above mentioned techniques, good

estimates of optimum gas phase growth conditions for the full

range of solid phase quaternary compositions can be predicted.

Several papers have been written presenting optical and

electrical property data as a function of composition for

GaxInl.AsyP1. lattice-matched to InP. One important

realization is that for lattice-matching, y is related to x by

the following simple relation: y = 2.16 x. This greatly

facilitates presenting data as it can be plotted as a function

of y or x under the assumption that the lattice-matching

condition is realized. A paper by Nahory et al.[74] presents

useful experimental lattice constant and bandgap values as a

function of composition relative to lattice constant values

predicted using Vegard's Law. Vegard's Law states that for a

lattice-matched system, the lattice parameter of the

quaternary can be deduced from those of the constituent

binaries. This team also presents the empirical relation for

bandgap variation (Eg(eV)) with composition.

Eg(y) = 1.35 0.72y + 0.12y2 (1)

Another group presented undoped electron and hole mobilities

as a function of composition, y. For undoped GaInAsP, room

temperature electron mobilities range from 4000 to

11,000cm2/volt-sec (y = 0 to y = 1) and room temperature hole

mobilities range from 130 to 200cm2/volt-sec (y=0 to y=l) [75].

Both p- and n-type doping of GaInAsP/InP grown by MOCVD

have been reported. Extensive doping studies as a function of








37

quaternary composition has not, however, been reported.

Saxena et al.[76] present data on 1.3Am p-GaInAsP doped by

DEZn over the range 1018-1019cm',and n-GaInAsP doped by DETe

and H2S. Using tellurium, n-type doping from 3x1017 to

5x109cm'3 is reported and, with sulphur a lower range 5x1016

to 3x1018cm'3 is reported. Meyer et al.[77] report n-doping

of 1.3 and 1.55Am GaInAsP using H2S over the range 1016 to

1019cm3. It is evident that p- and n-doping of GaInAsP is

possible over a wide range of doping levels which is

significant for device applications.

2.3.4 InP Based Devices

The wide range of compounds that can be grown with large

area uniformity by MOCVD make it suitable for fabrication of

long wavelength opto-electronic device structures. Several

different types of electronic, optical and opto-electronic

devices have been grown by MOCVD in the GaxInlxASyP1.y on InP

material systems. Manasevit et al.[35] in 1978 showed that

solar cells in which an InP active region grown by MOCVD can

perform as well as ones fabricated by other techniques. Other

devices such as lasers, field effect transistors, photo

detectors and waveguides have been grown by MOCVD and will be

discussed in the following paragraphs.

Some of the first devices grown using MOCVD GaInAsP were

broad area and stripe double heterostructure (DH) lasers which

lased at wavelengths between 1.15 and 1.54jm[69]. Distributed

feedback (DFB) lasers have also been successfully grown by








38

MOCVD. With MOCVD, it is possible to overgrow onto sub-micron

diffraction gratings which is vital to the operation of these

devices[79]. A typical laser structure composed of these

materials would start with a nr-InP substrate, followed by a

21m thick n* InP layer, then a 0.2Am active layer of

lattice-matched GaInAsP (undoped), then a 2Mm thick p-InP

layer ending with a p* InP contact layer of 0.2Am thickness.

The lasing wavelength is determined by the composition of the

active layer. Surface emitting semiconductor diode lasers,

which emit light perpendicular to the grown layer surface have

also been fabricated in an array form using MOCVD grown

GaInAsP[80].

Microwave devices such as Gunn diodes and metal to

semiconductor field-effect transistors (MESFET's) have been

grown using MOCVD InP. For Gunn effect devices, even though

the mobility of InP is lower than GaAs, other characteristics

such as cut-off frequency, acceleration-deceleration time,

relaxation time and peak-to-valley ratio are better in InP.

These devices require a three-layer structure of n+-n-n+-InP

grown on n+-InP and have been successfully grown by MOCVD for

60 GHz[36], and 94 GHz[81] operation. MESFET's have also

successfully been fabricated by the use of undoped InP grown

by MOCVD on Fe-doped substrates. The electrical properties of

Au-InP Schottky diodes are reasonable and comparable to other

crystal growth techniques[82].







39

The use of the ternary material GaInAs lattice-matched to

InP for long-wavelength photodetectors is well established.

Traditionally, LPE and hydride VPE are used but, MOCVD grown

p-i-n photodiodes have also been prepared[65]. The most

important device characteristics required of detectors are low

capacitance, low dark field leakage current and high quantum

efficiency. To attain these goals, low background doping

levels, accurate lattice-matching and an abrupt p-n junction

are required and all of these are possible with MOCVD.

Actually, with MOCVD the need for a post-growth zinc diffusion

processing step can usually be eliminated since in-situ p-

doping is possible. Several teams have reported improvements

in p-i-n photodiode performance by adjusting MOCVD growth

conditions[76], layer structure[65], and Schottky barrier

height enhancement[83]. MOCVD grown structures with leakage

currents as low as 3 pA at -10V using a 100/m device diameter

have been fabricated on two inch diameter InP substrates[84].

In addition to the above structures, a number of other

optoelectronic devices have been fabricated using MOCVD grown

InP based materials. Two dimensional electron gas (2DEG) and

multiple quantum well structures have been grown making use of

the extremely high mobilities, (in excess of 180,000cm2/volt-

sec at 9.2 K) possible with these materials[85]. Guided wave

devices such as optical waveguides and phase modulators have

also been grown[86]. Finally, GaInAsP/InP interference

filters have recently been grown by MOCVD[87]. The theory








40

and results of low pressure MOCVD grown GaInAsP/InP and MBE

grown AlGaAs/GaAs electrically tuneable interference filters

are presented in Chapter IV of this dissertation. More

extensive reviews of the wide range of MOCVD grown opto-

electronic devices using InP based materials are available in

the literature[65,88].

2.4. A Description of the MOCVD System

2.4.1 Introduction

The experimental apparatus used for the growth of

epitaxial layers of GaxIn1.AsyP1.y on InP substrates is a

commercial MOCVD system custom built for the University of

Florida by Nippon Sanso K.K. (Japan Oxygen Inc.). A

photograph of the front and a simplified schematic of the

Japan Oxygen MOCVD System are shown in Figures 2 and 3. The

complete operating procedures for performing epitaxial growths

and maintenance (e.g., such as reactor cleaning), are

presented in Appendix B of this text. The four basic parts of

the MOCVD system which are described in the following

paragraphs are: (1) the gas delivery system; (2) the reactor

and heating system; (3) the exhaust/scrubbing system; and (4)

the safety system. The gas delivery system, reactor, exhaust

and safety system are all integrated inside the MOCVD system

which is shown in the photograph in Figure 2. The heating

system is a separate unit (20 kW RF generator) as is the

scrubbing system which is located outside the building for

ease of maintenance reasons.
























































Figure 2: Photograph of the quaternary MOCVD system.














































p-dopant Vacuum pump


Figure 3: Quaternary MOCVD simplified flow diagram.









2.4.2.Gas Delivery System

The gas delivery system connects the sources to the

reactor and provides a method of transporting them in a

controlled fashion. Since impurity levels must be kept to a

minimum, all components of the gas delivery system are

constructed from electropolished 316L stainless steel and

connected with metal-gasket leak-tight couplings. Also for

improved purity, 0.2Am particle filters are installed at all

gas inlet points. All lines were wrapped with electrothermal

heating tape and aluminum foil and are heated during standby

mode to 500C to help desorb any of the sources or impurities

adsorbed on the inner walls of the stainless steel tubing.

The flow of gases is controlled by a combination of

manual valves, needle valves, pneumatic valves, check valves,

electronic mass flow controllers and regulators. The range of

possible flow rates for each source and the carrier gases

(hydrogen and nitrogen) are given in Table 2. The house

nitrogen gas which is mainly used for purging the MOCVD system

before a reactor or source change, passes through a molecular

sieve cartridge (Matheson Model 451) before entering the

machine. The house hydrogen, which is the carrier gas in the

system, is purified by diffusing it through a heated (4000C)

palladium-alloy membrane which is part of a 0-20 liter/min

hydrogen purifier system (Matheson, Series 8370V) that was

installed inside the Japan Oxygen machine.








44

Table 2

Flow Rate Ranges of Sources, Vendors and Purity


Source Flow Rate Range Vendor(Purity)

H2 0-20 SLM Gator Oxygen

(Alloy Diffused)

N2 0-10 SLM Linde(LN2 Boil-off)

AsH3 0-50 sccm Matheson (ULSI Grade)

PH3 0-200 sccm Solkatronic

(Micropure Grade)

1000ppm H2S 0-50 sccm Matheson

(in H2) (ULSI Grade)

TMIn 0-300 sccm Air Products

(Diphos Grade)

TEGa 0-100 sccm Akzo

(Electronic Grade)

DEZn 0-50 sccm Morton Thiokol

(Electronic Grade)







45

The gas delivery system for the metal organic sources

trimethylindium, triethylgallium and p-dopant diethylzinc

(which are held in stainless steel bubblers in temperature

controlled baths) consists of pneumatic, needle and manual

valves which are attached to the inlet and outlet ports of the

bubblers. Each metal organic line also has a pressure sensor

attached to it just before the inlet of the bubbler. The

metal organic sources are solids or liquids at room

temperature with fairly low, temperature dependent equilibrium

vapor pressures. By varying the temperature of the bubbler

bath, the hydrogen flow rate through the bubbler, and the

pressure of the bubbler region of the gas delivery system (by

opening or closing the needle valve), a controllable range of

metal organic source flow rates can be attained.

Since the hydrides (arsine, phosphine) and n-type dopant

(1000 ppm H2S diluted in H2) are highly toxic, combustible and

at high pressure, the gas delivery system for these sources is

slightly more complicated than for the metal organic. Each

line has a regulator, air operated valve and a manifold

attached to it. The manifold contains a high-pressure, high-

purity nitrogen purge line, a hydrogen purge line, and a third

line which can be used to evacuate the hydride line or vent

the hydride source directly to the scrubber. During normal

"standby" operation, palladium-alloy diffused hydrogen is

purging the hydride line to the vent. Only during toxic gas

flow is the hydrogen purge interrupted. Check valves on the








46

hydrogen and nitrogen lines prevent back flow of toxic gas

through the manifold to the rest of the system.

The "heart" of the gas delivery system is the fast-

switching "vent/run" manifold mounted just prior to the inlet

of the reactor. The status of the "vent/run" valves (opening

and closing) and the timing of this sequence can be manually

or automatically controlled. Automatic control is made

possible by using a process sequencer which is capable of

storing 150 valve patterns and the corresponding times for

each pattern or layer. The MOCVD manifold is also "pressure

balanced" which means that it is possible to adjust the

pressure difference between the two vent lines and the two

reactor lines to almost zero. This is accomplished by

carefully adjusting tube lengths and by installing a dead

volume in the vent line to counter balance the volume of the

reactor. Pressure balancing is especially important for MOCVD

growth of superlattice or multiple quantum well structures.

To keep the pressure difference between the reactor and vent

lines equal to zero (which is measured using the two

differential pressure indicators) each main source line has a

hydrogen compensation line. It is necessary for the MOCVD

growth of InP and related materials to keep the metal organic

separated from the hydrides to prevent parasitic gas phase

prereactions from occurring, hence the reason for the two

vent/two reactor line design.








47

2.4.3 Reactor and Heating System

The Japan Oxygen MOCVD system has a horizontal 4 inch

I.D. reactor which is equipped with a water cooling jacket

(see Figure 4) to minimize side wall deposition. At the inlet

of the high purity quartz reactor, two inlet lines exist to

keep the group III metal organic separated from the group V

hydrides at low temperatures. The dilute group III sources

enter the reactor through a 6 mm O.D. high purity quartz tube

10cm downstream from the inlet of the group V sources. These

two gas streams should ideally be well mixed and flowing under

fully developed laminar flow conditions before the mixture

encounters the high purity quartz deflector/silicon carbide

coated graphite susceptor heat source. The deflector -

susceptor unit is tapered at an angle of 170 with respect to

horizontal to improve growth rate uniformity.

The graphite susceptor is heated by radio frequency

inductive heating. A Lepel series T-15-3-KC-TL 15 kilowatt RF

generator is used to generate radio waves over the frequency

range 80-900 KHz. Some of these radio waves are picked up by

a 3/8" copper coil that is wrapped around the reactor. A

platinum/rhodium "R-type" thermocouple sealed in a high purity

quartz tube is embedded inside the graphite wedge. This

thermocouple is connected to a West series 2070 microprocessor

based temperature controller which was installed on the MOCVD

system and sends DC current to the RF generator to control















H2 and Hydride
Gas Inlet


Sample
Position


Quartz
Wafer Tray


Fork
Mechanism


/


Quartz Graphite Tnermocouple
Deflector Susceptor Tube


Figure 4: MOCVD reactor and loading mechanism.


H2 and MO
Inlet


,, __________,








49

its power output. Automatic temperature control from room

temperature to growth temperatures (550-7000C) to bakeout

temperatures (900-9500C) is possible with this elaborate

heating system.

As shown in Figure 4, a high-purity quartz sample tray

sits on top of part of the graphite susceptor. This tray

holds substrates as large as 1/4 of a two inch diameter wafer.

The tray and substrate are placed onto and removed from the

susceptor by an electro-mechanical fork which is capable of

precise x-y-z motion. The fork can move 90 cm horizontally

from the load lock area (where the sample and tray are loaded

and unloaded) through a gate valve and shutter valve, by the

gas exhaust port to directly above the susceptor. Precise

mechanical sample loading makes it possible to use the reactor

even after side wall deposition has obstructed view of the

susceptor.

To prevent the introduction of "dirty" room air to the

reactor during each loading of a substrate, the air in the

load-lock is evacuated by a rotary pump (Edwards model E2M2)

to a roughing pressure of 10-2 torr. Then a turbomolecular

pump (Balzers model TPH050) is turned on and evacuates the

load-lock to a pressure of 10"7 torr. At this point the load

lock is isolated and backfilled with the ultra high purity

hydrogen. The sample is now ready for loading and the ultra

high vacuum gate valve (VAT Ltd. model MSS4) is opened.







50

2.4.4 Exhaust/Scrubbing System

The waste products from the reactor flow through an

exhaust port in the water cooled flange which is bolted to the

exit of the reactor to a particle filter (Fuji Ltd.) which is

made out of glass fibers. The gas can then either flow at

atmospheric pressure directly to the scrubber or it can be

evacuated from the system by a rotary pump (Edwards model E2M

18) and then continue onto the scrubber. The rotary pump

makes it possible to grow films at low pressures (0.05 to 0.2

atm) which generally improves thickness uniformity.

The scrubber which was added to the Japan Oxygen MOCVD

system was built by Advanced Concepts (model 9625). It is a

liquid based scrubber which is designed to scrub toxic gases

and exhaust clean gas to atmosphere. For safety, the exhaust

was connected by a fireproof duct to the building's room air

scrubber. The scrubbing solution consists of a 80:5:2 by

volume mixture of water, 15% sodium hypochlorite, and 50%

sodium hydroxide. The pH and oxidation reduction potential

(ORP) of the scrubbing solution are constantly monitored to

evaluate the solution's scrubbing potential (pH > 10.0 and ORP

> 200 mV). The scrubber has an efficient gas-liquid venturi

contactor and a packed bed which can handle higher than

required toxic gas flow rates. For arsine and phosphine the

overall chemical neutralization reactions are:

AsH3(g)+3NaOC1(1)+H20(l) = H3AsO4(l)+3NaCl(s)+H2(g) (2)

PH3(g)+3NaOC1(1)+H20(l) = H3PO4(1)+3NaCl(s)+H2(g) (3)








51

The products of these reactions are less hazardous and for

added safety, when the system requires draining, the solution

flows to the building's neutralization pit.

2.4.5 Safety

Arsine, phosphine and hydrogen sulfide are highly toxic

combustible gases, therefore, several safety features are

installed in the MOCVD system, and several safety practices

must be followed in the facility. First of all, the facility

has a rule that at least two competent people must be present

in order to do any work in the building. The facility also

has an alarm system and card reader to deny access to

unauthorized personnel. The clean room has an eight point

toxic gas monitor (MDA, Inc.) which has a resolution of 1 ppb

for all hydrides. In the unlikely event of a toxic gas

detection anywhere in the facility, the MOCVD system will

completely shut down (all air-operated valves are normally

closed). The MOCVD system also has a four point hydrogen gas

detector (Matheson) connected to it so that if hydrogen levels

exceed 50 ppm, the machine will shut down. Also the facility

has a helium leak detector (Varian) which is used to find

actual leak points before the MOCVD system is used and to

check connections after valve replacements or reactor changes.

Not only is the facility well equipped with safety

features and practices, but the Japan Oxygen MOCVD System

itself has several integrated safety systems. There are two

types of alarms, facility failure and machine failure. If







52

either power fails, compressed air pressure drops, cooling

water pressure drops or temperature increases, the machine

will automatically alarm and all air operated valves will

close. There are smoke detectors in the machine and fire

detectors in the room. There are pressure sensors on the

hydride lines, in the reactor, and on the exhaust line and for

each, if a certain pressure value is exceeded, an alarm will

sound and the machine will shut down. There is also a pH, ORP

and temperature sensor on the scrubber which triggers an alarm

in the clean room if any of these values are out of the safety

range. Finally, there is a compressed breathable air supply

always on hand for reactor or source changes and two SCBAs

available for the emergency response team. It is evident that

safety is a big concern and since the machine was constructed

in Japan, there is even an earthquake sensor attached to it.



2.5 Determination of Optimum Growth Conditions Based on Thin
Film Characterization

2.5.1 Experimental Method

There are several experimental parameters that must be

determined before performing a MOCVD growth. Using the

simplest case as an example, undoped InP on InP, the first

thing that must be decided is what type of substrate is

required. For all of the GaxInl.xASyP1.y on InP experiments

performed in the Japan Oxygen MOCVD system, InP oriented (100)

20 towards (110) purchased from Sumitomo Inc. were used. Both

semi-insulating (iron doped) and n-type (n 8x1018cm'3, sulfur







53

doped) InP were used together or by themselves depending on

the purpose of the experiment. Next, the proper substrate

cleaning procedure must be decided. Also, experimental growth

conditions such as: growth temperature, growth pressure,

total hydrogen flow rate, trimethylindium mole fraction,

phosphine mole fraction and finally the length of time of the

planned deposition, must be determined. From the literature,

estimates of optimum growth conditions and proper wafer

cleaning procedures can be obtained, but these values or

procedures are system specific and hence had to be

experimentally optimized for the Japan Oxygen MOCVD system at

the University of Florida.

The procedure for calculating growth conditions for

undoped InP on InP by MOCVD is fairly straight forward. The

growth pressure, temperature and time must be chosen based on

knowledge of the material and capabilities of the system. It

should be noted that for convenience, some simplifications

were made in the derivation of the equations used to calculate

the growth conditions. Specifically, the ideal gas law was

used and it is assumed that the total flow rate to the reactor

is equal to the hydrogen flow rate and the bubbler pressure

was much greater than any metal organic vapor pressure. The

TMIn and PH3 flow rates are commonly expressed as TMIn mole

fraction (MFTmI) and PH3 mole fraction (MFPH3) or their ratio,

the V/III ratio, which in this case is defined as:










V FPH3 (4)
III MFTHIn

where:


F2
MFpH3 (5)





FH2,THIn VPTIn (T)
MFTMIn = (6)
FH2 Pb,TMIn



and FpH3 is the total pure phosphine flow rate (cm3/min), FH2

is the total hydrogen flow rate (cm3/min), FH2,TMIn is the

hydrogen flow rate through the TMIn bubbler (cm3/min),

VPTMIn(T) (mm Hg) is the bubbler temperature dependent TMIn

vapor pressure (torr), and Pb,TMIn is total pressure at the

TMIn bubbler (torr). All volumetric flow rates are measured

at standard conditions (300 K and 760 torr). The temperature

dependent vapor pressure equations for the metal organic

installed in the MOCVD system which were supplied by their

vendors (see Table 2) are shown in Table 3. It is therefore

possible with the use of the above equations to determine

conditions for a MOCVD experiment. Of course some initial

values must be known and others can be based on literature

values.










Table 3

Metal Organic Vapor Pressure Equations


Source Vapor Pressure Equation



log1oP(mm Hg) = 10.52 3014
TMIn T(K)





log,0P(mm Hg) = 8.224 2222
TEGa T(K)




2190
logo1P(mm Hg) = 8.28 2190
DEZn T(K)









2.5.2 InP

The method for determining the optimum growth conditions

for undoped InP on InP, n-type InP on InP and p-type InP on

InP will be discussed in the following sections. Before

presenting the results of this operating parameter sensitivity

and optimization study, the criteria for good epitaxial layers

are: mirror-like low defect density surfaces, uniform growth

rates over a large area substrate, less than n = 1x1015cm'3

background (ND-NA) doping levels for undoped InP with

corresponding high room temperature electron mobilities (j 2

3000cm2/volt-sec), experimentally convenient growth rates of

l-2jm/hour, and for doped layers and heterostructures,

atomically abrupt junctions and interfaces, respectively. All

of the above criteria have been determined by using various

thin film characterization techniques available on the campus

of the University of Florida.

2.5.2a Undoped InP

An extensive optimization study has been performed on the

MOCVD growth of undoped InP on InP. The results of this study

have been applied to aid in the determination of optimum

growth conditions for n- and p-type InP, undoped, n- and p-

type GaInAs, and undoped, n- and p-type GaInAsP on InP; these

optima will be discussed later. The timing and sequence of

events for a typical MOCVD growth are presented in the

Appendix B of this text.







57

After a growth has been performed, the first property

studied is the surface morphology of the thin film viewed

under an optical microscope. The thickness of the film can

also be determined by cleaving a sample and etching it for two

minutes in a 6:4:50 solution (measured by weight) of potassium

hydroxide, potassium ferricyanide, and deionized water to

delineate the grown layer from the substrate and then viewing

it under a microscope. A Nikon Optiphot microscope equipped

with Nomarski phase contrasting was used to view the surface

and delineated layer of the third growth in the machine, Q003.

Polaroid photos of the images observed at 2000X magnification

are shown in Figure 5. As shown, the surface morphology has

a ripple in it and this is sometimes referred to as an "orange

peel" surface. The side view photo of the sample was taken at

the leading edge of the sample with respect to the gas flow

direction. As shown, the layer thickness is clearly greater

(1.5Am) near the corner compared to the center (1.Om)

probably due to the lower growth rate right on the corner

which is at a different crystalline orientation, the (111)

orientation.

Several experimental parameters have been varied in an

attempt to improve the surface morphology of undoped InP grown

on InP. The growth temperature was varied from 450 to 7500C

and it was found that layers grown at temperatures between 575

and 7000C had better morphologies than those grown at higher

or lower temperatures. At lower temperatures lack of












































'1


Figure 5: Nomarski photographs of an InP plane view surface
and stained edge cross section taken at 2000X
magnification.







59

adequate phosphine decomposition and at higher temperatures,

slightly higher growth rates were the causes for inferior

material quality. The total hydrogen flow rate and growth

pressure were also varied but had little to no effect on

surface morphology. The TMIn mole fraction was varied and at

values greater than 1xl0"4 resulting material quality degraded

probably due to increased growth rates. The V/III ratio had

the most pronounced effect on layer surface morphology. This

effect can be clearly seen in Figure 6 which shows two InP

surfaces grown at identical conditions, except that the V/III

ratio was almost tripled by increasing the phosphine flow rate

to the reactor from 35cm3/min to 100cm3/min. This is in

agreement with the growth temperature study which concluded

that poor material results from insufficient phosphine

decomposition.

Another study performed with the goal of improving the

surface morphology of undoped InP on InP was varying the wafer

preparation procedure employed. Substrates were prepared

using five different techniques and then viewed under the

microscope. The procedures for each are as follows: (1)

filtered nitrogen blow off, five minutes each of warm acetone,

warm propanol, warm methanol, and then filtered nitrogen

blowoff; (2) procedure (1) followed by a 1 minute etch in 20%

nitric acid in methanol, methanol rinse, DI rinse, five minute

etch in room temperature 5:1:1 (sulfuric acid: hydrogen

peroxide: DI water), DI rinse, methanol rinse, filtered








60



I. .- i.





















































ratio on InP surface morphology (top: V/III=50,
bottom: V/III= 41).







61

nitrogen blowoff; (3) procedure (2) without the nitric acid in

methanol etch; (4) procedure (2) followed by a 1% bromine in

methanol etch for 6 minutes, methanol rinse, filtered nitrogen

blow dry; and (5) 2 minutes of surface treatment with the

UV/ozone cleaning system (UVOCS Inc).

Under a microscope at 200X magnification, particles were

observed on the surfaces prepared using procedures (1) and

(2). Procedure (4) resulted in a wavy surface probably due to

uneven bromine etching. Samples prepared using procedures (3)

and (5) had the best surfaces. Fresh substrates were cleaned

using procedures (3) and (5), loaded into the reactor, and

undoped InP was grown on both of them at the same time. This

experiment was performed several times and layers grown using

procedure (3) were consistently equal to or better than layers

grown using procedure (5). Procedure (3) was chosen as the

optimum and the exact details of this procedure are given in

Table 4.

The effect of growth conditions on the uniformity of

undoped epitaxial InP on InP was also studied. The total

volumetric flow rate was the only parameter that had any

significant effect on layer uniformity. Layers grown using

total hydrogen flow rates of 3, 5, 7 and 9 standard liters per

minute (SLM), had corresponding thickness variations of +/-10,

8, 6 and 6%. The increased variation at the lower flow rates

probably is associated with an increased boundary layer








62

Table 4

Optimum InP(100) Wafer Preparation Procedure





1. Cleave wafer and blow off with filtered N2.

2. Degrease teflon beakers and tweezers with warm methanol.

3. Place substrates into beaker with warm acetone for 5

minutes.

4. Place in warm propanol for 5 minutes.

5. Place in warm methanol for 5 minutes.

6. Rinse in running DI water for 1 minute.

7. Etch in a 5:1:1 solution of H2SO4:H202:DI at RT for 5 min.

8. Rinse in running DI for 1 minute.

9. Place in room temperature methanol for 1 minute.

10. Blow off with filtered nitrogen.

11. Load into reactor on wafer tray from oven.

12. Anneal wafer at growth temperature for ten minutes with H2

and PH3 flowing.







63

thicknesses resulting in depletion of TMIn near the growing

surface. This theory is confirmed by the observation that the

growth rate was lower, 0.95 and 1.2gm/hour for 3SLM and 5SLM,

than it was at higher flow rates (1.4 im/hour for 7SLM and

9SLM). The uniformity of undoped InP grown using a total

hydrogen flow rate of 7SLM is presented in Figure 7. A total

flow rate of 7SLM for hydrogen was chosen as optimum over 9SLM

because with 9SLM at a fixed TMIn mole fraction almost 30%

more TMIn material would be required.

The effect of several parameters on InP growth rate and

interface quality was also studied. As already stated the

total hydrogen flow rate has an effect on the growth rate.

The growth pressure and V/III ratio appear, however, to have

little to no effect. The growth temperature has a nonlinear

effect on the growth rate; at 450C the growth rate was

0.8Mm/hour, but going from 550 to 7500C the growth rate only

changed from 1.4 to 1.5Am/hour. This can be explained as

follows: at low temperatures (below 5500C), InP growth is

kinetically limited and consequently temperature dependent.

At substrate temperatures above 5500C, InP growth is TmIn

transport limited. This is confirmed by Figure 8 which shows

the linear relationship between InP growth rate and TMIn mole

fraction grown at 6000C. Some of the data for Figure 8 was

taken from the SEM micrograph shown in Figure 9. Figure 9 is

a scanning electron micrograph of a cross section from growth















m N 0



5
DISTANCE


10
FROM


15
LEADING


20
EDGE (mm)


n


-I


-20


S S


-10 0
DISTANCE FROM


10
CENTER (mm)


Figure 7: MOCVD thickness uniformity study.
variation, bottom: radial variation
the leading edge.


Top: axial
at 12 mm from


25


U U
Em..





















































0.0 f
0.00000


0.00005 0.00010 0.00015



TMIn Hole Fraction


0.00020


Figure 8: The effect of TMIn mole fraction on InP growth
rate.










* -


'Ul


Figure 9: SEM micrograph of growth Q054 at 20,000X.
layers are InP, dark are GaInAs.


Light


IEH








67

Q054 which shows InP layers grown with different MFTMIn for

different lengths of time separated by thinner dark GaInAs

marker layers. Figure 9 also shows the excellent abruptness

of interfaces between two different materials grown at 6000C

and 7SLM.

Another analytical technique used to evaluate material

quality is an electrochemical C-V profiler. The C-V profiler

(BioRad, Model PN4200) is thoroughly explained by Blood[89].

It uses a Schottky and ohmic contacts in an electrochemical

cell filled with .0.1M HC1 (for InP) to slowly etch away

material while it measures the change in capacitance as a

function of changing applied voltage. From this C-V data, the

net carrier concentration can be calculated (n = ND-NA or p =

NA-N,, where NA and ND are the number of acceptors and donors

per cm3) and plotted as a function of depth. A C-V profile of

growth Q056 which is undoped InP on n-type (8x1018cm'3)InP is

shown in Figure 10. From this figure one can get a rough

estimate of 1.2gm for the layer thickness. Also, the

epilayer-substrate interface abruptness can be assessed. Good

uniformity of the carrier concentration throughout the layer

can also be observed. Finally an average value of the

background carrier concentration for undoped InP of

n=3x1015cm'3 can be obtained. It is evident that a lot of

useful information can be obtained from an electrochemical C-V

profile, the only drawback to this technique is that it is

destructive.







































.5 1.0


X (um)


Figure 10: C-V profile of growth Q056 (undoped InP on n+InP).


15 L
0.


0


i i I r I I I


L


i








69

The effect of growth conditions on the background carrier

concentration of undoped InP has also been studied. The

growth pressure, total hydrogen flow rate, and TMIn mole

fraction apparently have little effect on the background

carrier concentration. The growth temperature and V/III

ratio, however, both have a strong effect. The background

carrier concentration measured by the C-V profiler with

respect to growth temperature is plotted in Figure 11. At a

low growth temperature, high carrier concentrations may be

associated with phosphorus vacancies in the InP crystal

(impurities are completing with these vacancies). At higher

temperatures, electrically active source impurities are

incorporating more efficiently. It appears that based on

background concentrations, the optimum growth temperature

should be between 5750C and 6250C for undoped InP. The

undoped InP carrier concentration is also strongly affected by

the V/III ratio as shown in Figure 12. Based on this figure,

high purity material can be grown with high PH3 flow rates.

Another analytical technique which is commonly used to

evaluate material quality is PL. The technique is explained

well by Dean[90]. Briefly, it typically uses laser light

energy incident upon a sample to stimulate the emission of

photons at discreet energy levels. These discreet energy

levels can be detected and related to impurities, defects, or

host crystalline energy level transitions. The resolution of

closely spaced transitions can be improved by reducing the










































300 400 500 600
TC


700 800 900


Figure 11: The effect of growth temperature, Tg, on undoped
(n-type) InP background carrier concentration.


10"


E
0




010

a
0



0
o
L.


(10


0


10


Ha=7.0 sl, P =80 torr
xi=1x 10-, V/f1=50


SI I I


I


















o 10
rj )
I H2=7.0 sin, P,=80 torr
E X,,=lx10-, T,= 600C



c--
I
C
0
r
10
O.
C \

i
0)

0 \







0.00 40.00 80.00 120.00 160.00
V/Ill Ratio








Figure 12: The effect of V/III ratio on C-V background carrier
concentration in undoped (n-type) InP.







72

temperature of the sample. The PL spectra of two different

InP samples grown at different V/III ratios and measured at

4.2 K are shown in Figures 13 and 14. The peaks labelled

(D,x) and (FE) are host crystalline donor to bound exciton

and free exciton transitions. The other peaks maybe due to

impurities such as carbon or crystalline defects like

phosphorus vacancies (Vp). Based on the reduction in area

under the peaks labelled (Vp), it is evident that increasing

the V/III ratio or phosphine flow rate definitely has a

positive effect of the material quality of undoped InP.

In addition to the C-V profiler, the Hall effect can also

be used to determine electrical properties of semiconductors.

A low temperature (room temperature to 7 K) Hall effect system

was set up at the University of Florida by this researcher.

The theory, and a manual explaining the use of the Hall effect

system is given in the Appendix A of this text. Using data

from the Hall effect system, an average value of the mobility,

resistivity and carrier concentration can be calculated if the

thickness of the measured film is known. These values are

usually measured at 300 and 77 K (liquid nitrogen temperature)

and sometimes plotted as a function of reciprocal temperature

to 4.2 K. The liquid nitrogen temperature mobility (p.K) is

widely used as a measure of purity of undoped semiconducting

films. In Figure 15, p A is plotted as a function of growth

temperatures for undoped InP. The highest mobility for InP























(Dox)

0.8752 eV
0.8750 eV

(vp) ??


d -
cu
ts ^
^ r


0.8746 eV

0.8744 eV


(FE)


10600 100oo 11600 112oo 1 o00 11600 1100oo


12000


Wavenumbers (cm-1)


Figure 13:


PL spectrum of an InP sample grown with a V/III
ratio of 140 measured at 4.2 K.






















(D, x)


(Vp)??

>o

I S gS

M o a

(, R), Carbon -
,m- I


0.8751 eV


10600 10800 11000 00 i 0 11400 11600 11800 12000
Wavenumbers (cm-1)















Figure 14: PL spectrum of an InP sample grown with a V/III
ratio of 219 measured at 4.2 K.























70000












50000
03
N









30000
1 40000-
-I








30000-





20000
500 550 600 650 700



Tg (c)













Figure 15: The effect of growth temperature on 77 K mobility
of undoped InP (bars indicate the range of data) .







76

material from in the Japan Oxygen MOCVD was measured to be

61,800cm2/volt-sec for a sample grown at a temperature of

6000C. There is a lot of scatter in the mobility data. The

scatter may be due to the fact that different material sources

with different impurity levels were used to grow these layers.

Also, the reactor was not changed/baked-out after each growth.

However, the average mobility values indicate that higher

quality material can be grown at 6000C and this is in

agreement with previously discussed characterization results.

Based on all of the observed characterization trends,

apparent optimum MOCVD growth conditions for undoped InP on

InP have been determined. These growth conditions are

presented in Table 5 along with the room temperature and 77 K

electrical properties of undoped InP grown at these

conditions. These optimum conditions are significant because

they are the basis for estimates of growth conditions for

intentionally doped InP, unintentionally and intentionally

doped GaInAs and GaInAsP, both lattice-matched to InP.

2.5.2b Growth of n-type InP Using H2S

The Japan Oxygen MOCVD system was designed with the plan

of using dilute H2S as an n-type doping source. A cylinder

containing a mixture of 1000ppm H2S and the balance ultra high

purity hydrogen (Matheson, ULSI grade) is connected to the

MOCVD system. The flow rate range measured at room

temperature and 760 torr for this mixture, as shown in Table

2, is 0-50cm3/min (sccm). Epitaxial layers of InP were grown








77

Table 5

Undoped InP Growth Conditions and Electrical Properties


Total H2 Flow Rate

Growth Temperature

TMIn Mole Fraction

V/III Ratio

Growth Pressure


= 7 SLM

= 6000C

= 0.7-10'4

= 419

= 80 Torr


at 300 K :


ND

ND

11300K

11300K


= 2.1014 cm'3 (lowest)

= 2*1015 cm'3 (average)

= 3461 cm2/volt-sec(highest)

= 2800 cm2/volt-sec(average)


at 77 K :


ND NA

ND N

I77K

977K


= 11014 cm'3 (lowest)

= 2*1015 cm'3 (average)

= 61800 cm2/volt-sec (highest)

= 40500 cm2/volt-sec (average)







78

with H2S mixture flow rates of 2 to 50 sccm, which corresponds

to gas phase mole fractions of H2S of 2.8x10"7 and 7.16x106,

respectively. A C-V profile of a S-doped InP grown layer is

shown in Figure 16. As one can see, a roughly l.lgm thick

layer of 2.5x1018cm"3 n-type material was deposited on a n*-InP

substrate, and this was achieved using a H2S mole fraction of

2.43x10'6. The "hump" in the profile in Figure 16 is a C-V

profiler error which occurs at interfaces. The relationship

between the measured C-V carrier concentration and H2S mixture

flow rate for several n-type samples is presented in Figure

17. As shown, a wide linear incorporation rate of sulfur in

InP is possible resulting in doping levels from 5x1017 to

2.5x1019cm'3. The surface morphology of the grown material

when viewed under the Nikon microscope at 2000x appeared

unaffected by the presence of the sulfur atoms even at the

highest n-type doping level. Room temperature Hall effect

measurements were also performed on the sulfur doped samples.

Hall carrier concentrations agreed with C-V measurements and

Hall mobilities (A300K) ranged from 498cm2/volt-sec at the

lowest doping level to 1064cm2/volt-sec at the highest level.

2.5.2c Growth of p-type InP Using DEZn

P-type conversion of MOCVD InP was achieved by mixing the

metal organic source diethylzinc (DEZn) with the standard gas

mixture used to grow undoped InP. In order to get a wide

range of p-type doping, the temperature of the DEZn bubbler

was varied from -20 to 200C resulting in a change in the




































E



0













iB
0.0


X (uml


Figure 16: C-V profile of a H2S doped InP film.









80









1020-





0








0
U El
o X
0






u l
H 7















10 "5 10 4 0
iol7---------------- .I




Partial Pressure (Torr)













Figure 17: The effect of H2S partial pressure on InP carrier
concentration.







81

vapor pressure of DEZn in the bubbler from 1.39 torr to 14.00

torr. The hydrogen flow rate through the bubbler ranged from

5 to 50 sccm and by carefully adjusting the opening of the

DEZn needle valve it was possible to keep the bubbler at a

pressure of 500 torr.

Characterization of the p-type InP material included

thickness and surface morphology measurements using the

optical microscope, C-V profiles, Hall measurements and

secondary in mass spectroscopy (SIMS). The SIMS technique,

which is explained elsewhere[91], was used to determine the

total atomic zinc concentration incorporated into several p-

type InP layers as a function of depth. This total zinc level

can be compared to the carrier concentration to determine the

percentage of electrically active zinc atoms. Also, based on

the depth of the atomic zinc profile, the extent of zinc

diffusion can be assessed relative to the epitaxial layer

thickness measurements. A C-V profile of growth Q080 which

was deposited at Tg=6000C, Pg=80 torr and V/III=50.0 is shown

in Figure 18. During the growth, a DEZn mole fraction of

4.06x10'5 was used which resulted in a C-V measured hole

concentration of 1.6x108cm"3, for the layer grown on the n+-

InP substrate. A room temperature Hall effect measured hole

concentration of 2.5x1018cm'3 was calculated for the layer

grown on the semi-insulating substrate. The room temperature

Hall mobility and resistivity for this p-type sample were

61.3cm2/volt-sec and 0.0402 ohm-cm, respectively.









































X (um)


Figure 18: A C-V profile of a DEZn doped InP film.







83

The relationship between the C-V measured hole concentration

and the corresponding DEZn partial pressure used for the

growth is plotted in Figure 19. As indicated, controllable p-

type doping from 2x1017cm3 to 3x1018cm3 was realized. Hall

measurements were attempted on all of the p-type layers grown

on semi-insulating InP substrates, but the alloyed indium

contacts were generally not ohmic in nature. Hence, only the

C-V carrier concentration results from films grown on n-type

substrates were used. The room temperature Hall carrier

concentrations of semi-insulating samples with ohmic contacts

agreed quite well with the C-V carrier concentrations on n-

type substrates.

The effect of several growth conditions on the p-type

doping of MOCVD InP was investigated. The following growth

parameters: DEZn mole fraction, growth pressure, V/III ratio,

growth temperature, and total hydrogen flow rate, were varied

during individual experiments. It was later discovered that

the effect of the extended diffusion of atomic zinc in InP

during growth influenced the spatial variation of zinc's

incorporation. Basically, any variation in incorporation that

might have existed due to variations in growth conditions

during the deposition, were negated by the rapid diffusion

rate of zinc. For example, Figure 20 shows a C-V profile of

growth Q105 during which the DEZn mole fraction was varied

from 0.2 to 1.0x10-5 and the C-V hole concentration of















19


10


z3 -
I-

T 0 "-
z


T%= 6000C
P p3=0.4 Torr
P9.n=.008 Torr


10 "17-
0.0001


I I I I I I I I I
0.001

PODEZn(Torr)


I I


0.01


Figure 19: The effect of DEZn partial pressure on InP hole
concentration.


- - --- - -


II


















20




n-Type


19


E DEZn Flowrate
2 Adjusted


\ I,/

p-Type





i7
0 1 2 3 1
X (um)












Figure 20: C-V profile of InP:Zn grown with different DEZn
partial pressures.







86

9xl017cm-3 as shown, is essentially constant throughout the

grown film. The large "dip" in the profile shown in Figure 20

is commonly observed at p-n electrical interfaces. A SIMS

measurement was also performed on this sample (see Figure 21)

and the atomic zinc concentration profile yielded essentially

the same result. Similar results were observed when comparing

the SIMS and C-V profiles of samples grown to study the effect

of the other system parameters on zinc incorporation.

One interesting and useful conclusion that can be derived

from the SIMS and C-V profiles of growth Q095, during which

the growth pressure was varied from 38 to 760 torr, is the

extent of zinc diffusion into the InP substrate. As shown in

Figure 22, the grown layer thickness of this sample is about

1.4Am and the zinc doping level is unaffected by the change in

growth pressure. In Figure 23, the atomic zinc level (from

SIMS) is essentially constant throughout the profile, but a

"spike" occurs in the zinc profile right at a depth of 1.4Mm.

The SIMS operators at BNR, where the data was taken, say that

this "spike" is due to silicon-zinc complex which results from

Si on the surface of the InP substrate wafer after cleaning

and it reproducibly indicates the position of the grown layer-

substrate interface. These "spikes" were observed in several

of the SIMS profiles done at BNR. Based on the depth of the

zinc profile in Figure 23, zinc has diffused approximately 0.5

microns into the InP substrate. Using this diffusion length,
























PROCEsBOE DATA
5 Aug ea Ca


0.0 0.5


BNR
PILE: 0105


1.5
DEPTH (micronr)


Figure 21: Atomic zinc profile of sample Q105 measured by
SIMS.








88












20




n-Type



19




z
C /
o p-Type












0 2 3
X (um)














Figure 22: C-V profile of growth Q095 during which the growth
pressure was varied.
























PROCESSED DATA
5 Aug a8 CE


4n-24--


o 1 "spike"



101






10 1



-1> Cc+Zn


0.0 1.5 2.0


1.5 2.0
DEPTM (microns)


Figure 23: Atomic zinc profile of sample Q095 measured by
SIMS.


BNR
FILE: 009o


0.0







90

L = 0.5 microns, and the length of time that the sample was at

growth temperature, t = 90 minutes, a rough estimate of the

diffusion coefficient, D, of zinc in InP can be calculated

using this equation:


D(cm2/sec) = L2(cm)/t(sec) (7)

A value of D = 4.6x1013cm2/sec is calculated which agrees very

well with the range of values that were reported by Nelson and

Westbrook[58], D = (1-6)x 10"3cm2/sec, for zinc in InP.

A review of the literature on the topic of p-type doping

of InP by several growth techniques, extensive data on zinc,

magnesium and cadmium p-type doping of InP by MOCVD, (which

was acquired when this investigator was a visitor at BNR) and

a theoretical model of the p-type doping process of MOCVD InP

is all presented in Chapter III of this text. The reader is

therefore referred to Chapter III for a more detailed and in-

depth discussion on p-type doping of InP.

2.5.3 Growth of GaInAs Lattice-Matched to InP

The mixed crystal Ga47In.53As which is lattice-matched to

InP, has been grown using the Japan Oxygen MOCVD system. The

growth conditions used were similar to the optimum conditions

for InP growth. Based on the results of several different

characterization techniques, optimum growth conditions for

undoped Ga47In.53As were determined. Whenever p- and n-doped

GaInAs was required for device applications, test layers were

grown to calibrate for the required DEZn and H2S gas phase







91

mole fractions. The timing and sequence for a typical MOCVD

growth of GaInAs on InP is presented in Appendix B of this

text.

Since GaxInl.xAs has only one composition (x=.47) which

is lattice-matched to InP, precise control of both the TMIn

and TEGa flow rates to the reactor is crucial. A small change

in the TMIn to TEGa gas phase molar ratio is approximately

equivalent to the change in deposited solid phase molar ratio.

Unfortunately, small changes in the solid phase composition

dramatically affect the material's quality and both electrical

and optical properties. When the lattice-mismatch (Aa/a),

where a is the lattice constant, is greater than approximately

0.5% and layer thickness is greater than the critical

thickness of the material (= 1000A for GaInAs), the strain in

the epitaxial layer is enough to form cracks or dislocations

which can propagate throughout the grown film. Dislocations

appear as a "cross-hatched" pattern and are clearly visible in

the surface of a grown layer as viewed under an optical

microscope. The surface morphology of GaInAs deposited on InP

is directly related to the degree of mismatch in the thin film

relative to the substrate.

Using X-ray diffraction (XRD), it is possible to

determine the lattice constant of a deposited thin film

relative to that of the substrate. With the lattice constant,

one can determine the lattice-mismatch and composition, x, of

a mixed crystal such as GaxInl-xAs. The XRD technique is







92

explained by Cullity[92]. It relies on the periodic structure

of a crystal to scatter incident X-rays in such a way that

some of the scattered beams will be in phase and reinforce

each other to form diffracted beams. Scattered rays will be

in phase if Bragg's law is satisfied:

nl = 2dsin0 (8)



where:


d= (9)
(h2 +k2 +12 )1/2



and I = 1.54051A (for copper Kal), (hkl) are miller indices

(usually (004)), n is an integer (equal to 1 for first order

reflection) and 9 is the angle of incidence of X-rays. Figure

24 shows the XRD spectra from two different GaxIni-xAs/InP

growths. The plot on the left is the spectrum for growth Q015

which is lattice matched. The reason there are two peaks is

because two different X-ray wavelengths close to each other

(Kal and Ka2 from copper) were incident upon the sample. The

spectrum on the right is from growth Q007 and it has a third

broad peak at a lower 28 angle relative to the InP substrate

peaks which when inserted into Bragg's law yields a lattice

constant of a = 5.897A. This is the lattice constant of

Ga 40In 6As (see Table 1 for the lattice constants of InAs and

GaAs), which has a lattice-mismatch of Aa/a = 4.6x10-3 or

0.46% on InP. If a GaInAs/InP sample had an X-ray spectrum













Ga47 In.As / InP


InP


I Q007
y 1
0015 Ga. In, As

C

L cL
4 2 0 Angle (deg)








Figure 24: X-ray diffraction patterns of GaInAs samples grown
on InP (left: lattice-matched, right: lattice-
mismatched) .




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