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March 1997

MATERIALS LETTERS

ELSXIER Materials Letters 30 (1997) 255-263

Materials Update

MOCVD technology for semiconductors

Alan G. Thompson

EMCORE Corporation 394 Elizabeth Amme, Somenet. NJ 08873, USA

Received 26 July 1996; accepted 17 August 1996

Abstract

This article commences with a brief review of the MOCVD process as it applies to semiconductors. The various precursors used, including metalorganic compounds, hydride gases, and dopants are discussed, together with the basic deposition process. Typical MOCVD systems used for R&D and manufacturing are described, including the constraints imposed by safety and environmental requirements. Recent advances in the R&D arena, such as growth mechanisms, new metalorganic materials, heteroepitaxy, the use of alternative carrier gases, and reactor modeling are then covered. In the manufacturing arena, large scale reactors, cost of ownership (COO) models, and in situ controls are detailed. We conclude with a look at newer applications for materials prepared by MOCVD, including multi-junction solar cells, high brightness LEDs covering the visible spectrum, and laser diodes.

Kqvwords: MOCVD; Semiconductors

1. Introduction

Metal Organic Chemical Vapor Deposition (MOCVD) is a form of CVD utilizing metalorganic compounds for one or more of the precursors. Alter- native names include OM (OrganoMetallic) CVD, MOVPE (Vapor Phase Epitaxy), and OMVPE. All mean essentially the same, except epitaxy [l] is a special case of thin film deposition where the layer replicates the crystal structure of the substrate (al- though there are exceptions to this definition). Due to space limitations this Update will be restricted to the case of semiconductors, but many other materials have also been prepared by MOCVD. However, the fundamentals are similar, and the interested reader is referred to two important classes, metals for IC fabrication [2], and ferroelectrics for IC memory and optical applications [3]. In the IC industry, the CVD

00167-577X/97/$17.00 Copyright 0 1997 Elsevier Science B.V.

PII SOl67-577X(96)00215-7

technique is widely used to deposit films of Si, dielectrics such as SiO, and Si,N,, and metals such

as W, TIN, and intermetallic compounds. CVD has been used to deposit other semiconductors, but the development of MOCVD led to much improved uniformities and a wider variety of materials, en- abling bandgap engineering [I] to become practical. Essentially all III-V and II-VI semiconductors and most of their alloys have been successfully grown using MOCVD, making this possibly the most versa- tile growth technique for compound semiconductors. The precursors used are discussed in the next sec- tion. Thermodynamics, fluid dynamics, and gas and surface reactions all play a part in the deposition process. A “cold” wall reactor is typically used for MOCVD, with the precursors being delivered to the heated substrate by a carrier gas. Typical reactors for R&D and manufacturing will be covered next, along

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256 A.G. Thomp.wn/Muterials Letters 30 (1997) 255-263

with safety and environmental concerns, since many of these materials are hazardous. The following sec- tion describes recent advances in research, develop- ment, and manufacturing. Lastly, applications for the materials prepared using MOCVD will be covered briefly, in order to give the reader a feel for the commercial possibilities.

2. The MOCVD technique

In this brief article we can only touch on the theory behind the MOCVD technique. For more

complete discussions, there are several excellent re- view articles and books that go into depth on this complex topic [4-61. For a comparison with other growth techniques, see Ref. [l]. However, as back-

ground for the following sections, we will give an overview of the MOCVD technique.

2. I. Metalorganic (MO) compounds

These are typically metal atoms with a number of alkyl radicals attached, with methyl, ethyl, and iso- propyl being the most common for compound semi- conductors. Table 1 shows some of the more widely used MOs and their key properties. There is a vast array of these compounds available, with some being used in large quantities by industry. For semiconduc- tor applications, the volume is relatively small and they have to be highly purified, both of which con- tribute to the high cost (compared to an elemental metal of similar purity). Desirable properties include a reasonable vapor pressure at room temperature or

Table I The names and properties of some of the more commonly used metalorganic (MO) compounds for III-V and II-VI MOCVD. “Common”

imnlies a widelv used material. For more details, see Refs. r4.71

Element Name Symbol Vapor pressure

(Torr at “C)

Liquid or Solid Comments

aluminum trimethylaluminum

antimony

arsenic

trimethylantimony

tertiarybutylarsine

trimethylarsenic

cadmium diethylcadmium

dimethylcadmium

gallium triethylgallium

triisopropylgallium

trimethylgallium

indium ethyldimethylindium

trimethylindium

iron

magnesium

phosphorus

tellurium

zinc

biscyclodipentadienyl iron

biscyclodipentadienylmagnesium

tertiarybutylphosphine

diethyltelluride

diethylzinc

dimethylzinc

TMAl

TMSb

TBAs

TMAs

DECd

DMCd

TEGa

TIPGa

TMGa

EDMIn

TMIn

CP, Fe

Cp,Mg

TBP

DETe

DEZn

DMZn

9 at 20

50 at IO

125 at 20

300 at 25

I at30

28 at 20

3 at 20

I at25

65 at 0

2 at 20

2 at 20

I at 20

0.1 at 35

250 at 20

7 at 20

12at20

120at0

L

L

L

L

L

L

L

L L

L

s

S

s

L

L

L L

see text

high C

common

common

common

doping InP

Sp-dopant

see text

A.G. Thompson/Materials Letters 30 (1997) 255-263 251

below, high purity, low cost, a low affinity for oxygen and water vapor, and low toxicity. Unfortu- nately, many of the MOs that meet the vapor pres- sure and purity needs best are also highly reactive with oxygen (some are pyrophoric). They are there- fore contained in stainless steel bubblers, and great effort is made to avoid their contamination by air. Highly purified hydrogen (or an inert gas such as nitrogen) is bubbled through the liquid to transport the material into the reactor. A knowledge of the vapor pressure (at the bubbler temperature and pres- sure) and the carrier gas flow rate is sufficient to determine the transport rate. Equilibration of this rate is quite rapid for liquids, but solid materials (such as

TMIn) take longer and may vary with time as the surface area changes. Various proprietary schemes are used to mitigate this effect, and putting two bubblers in series is quite effective. For further information on MO compounds, see Ref. [7].

2.2. Hydride gases

For the III-V materials, the trihydrides (ASH,, PH,, NH,) are typically used, in spite of the fact that they are extremely toxic (ASH, has a TLV, the highest “safe” level allowed, of 50 ppb), since they are relatively inexpensive and give high purity lay- ers. Alternative materials are alkyl based, such as TBAs, and have now reached equivalent purity lev- els to the hydrides (although TBP is still not widely used due to possible oxygen contamination or affin- ity compared to PH,). The alkyl substitutes are much more expensive than the hydrides, but are also much less toxic, and have some other advantages. Increased production volumes will decrease the cost, but many users will not switch while the cost is high, a classic chicken and egg situation! Antimony based compounds are usually grown from an alkyl based material, since the Sb hydrides are very unstable.

2.3. Dopants

Convenient dopants are available in the form of MOs in bubblers (for example, Cp,Mg), MOs di- luted by a gas under pressure (DETe in hydrogen), or a gas (SiH, in hydrogen). The latter two cases are convenient since the concentration may be varied over a wide range by the gas supplier. In the former

case, the reactor gas panel has to adjust the concen-

tration, typically with a dilution network.

2.4. Growth mechanism

Simplistically, a typical MOCVD reaction is as follows, using GaAs prepared from TMGa and ASH, as an example:

(CH,),Ga + ASH, = GaAs + 3CH,. (1)

In practice, the surface is intimately involved, and there are many steps between the precursors entering the reactor and GaAs growing on the heated sub-

strate. The incoming materials partially decompose and are then adsorbed on the surface. Here they decompose further or are desorbed. The atoms and radicals move around on the surface with growth occurring at steps for smooth, two-dimensional lay- ers, replicating the structure of the substrate. For a good review of current understanding of this process, see Refs. [5,6]. In Eq. (l), it looks like C could be incorporated easily, leading to p-type doping of the GaAs. This is indeed the case, but to avoid it, excess hydride can be supplied; the atomic H from the decomposing ASH, scavenges the CH, products from the surface. V/III ratios of 100 are typically employed for devices prepared with methyl based MOs, and ratios of 10,000 have been used for the III-nitrides. Although this technique leads to good optical properties, it is a substantial contributor to the cost of the epitaxial structure.

3. MOCVD reactors

Problems common to all MOCVD reactors in- clude large temperature gradients (which can result in convective loops), high gas flows (which can lead to turbulence rather than laminar flow), and the need to have good wafer temperature uniformity (for dop- ing and compositional uniformity). Some precursors react with one another in the gas phase, creating particles and low deposition rates. This effect can sometimes be overcome by operating at low pressure (typically = 0.1 at), and although this results in additional system complexity, it has other beneficial effects and has become widely accepted, particularly for manufacturing. Keeping atmospheric constituents

258 A.G. Thompson /Materids Letters 30 C 1997) 255-263

out of reactors is a perennial problem, especially if they have to be opened to atmosphere for frequent cleaning. Long purges and bakes can be used in R&D, while nitrogen filled glove boxes and high vacuum load locks are used for more advanced systems (see Ref. [4] for basic information on reactor

requirements). The gas panel is the system which meters the

reactants and delivers them to the reactor. Fig. 1 shows a simplified system for III-V materials. The hydrides are fed to an injection block through valves and mass flow controllers. The MOs are transported by bubbling a controlled flow of hydrogen through the bubblers (which are also controlled at a selected pressure, not shown in Fig. I), and fed to their own injection block. At each block a push flow is added and the combined flows are directed either to the reactor or a vent line. The absolute and differential pressures are all dynamically controlled, so that when switching of flows occurs there are no pressure transients which could upset the transition from one layer to the next. This is particularly important for quantum well based devices, which often require monatomically abrupt interfaces. A main carrier gas

flow is also directed to the reactor, with combination of all flows preferably taking place in the reactor.

3.1. Research scale reuctors

These typically process a single wafer up to 3 ‘I in diameter, and are used for research, development, and limited volume manufacturing of some devices such as lasers. Both vertical and horizontal geome- tries have been used (see Fig. 2). The horizontal is the most common, since many early systems were home-built, and this is the easiest to construct pro- vided a good glass blower is available. The gases flow parallel to the wafer, from one side to the other. Reactant depletion effects result in a decreasing growth rate across the wafer, which can be mitigated by tilting the wafer, increasing the carrier gas flow, or rotating the wafer. The latter approach is the most complex. but has been the most successful at de- creasing the longitudinal depletion effect. A lateral depletion effect becomes important for wider tubes,

making scale-up of this geometry difficult. Since deposits occur on all heated surfaces, a quartz liner is used which has to be removed and cleaned fre-

H, In Inject Block To Reactor I

Hydride Push c

To Vent Line

Alkyl Push

& To Vent Line

T TMGa

l-lo Reactor

x =VALVE Main Shroud Flow

CD = MASS FLOW CONTROLLER

Fig. I, A simplified schematic for a III-V MOCVD gas system. showing how hydrides and alkyls are metered and fed into the reactor. Note

particularly the use of vent/run switching and pressure balancinp.

A.G. Thompson /Materials Letters 30 (1997) 255-263 259

r Wafer

Process - Exhaust

4 L Susceptor

Process Gases

Wafer ih Susceptor

(b)

Fig. 2. (a) Horizontal and (b) vertical MOCVD reactor geometries.

Virtually all MOCVD systems are based on one of these two

schemes. See text for explanation of gas flows.

quently. In the vertical geometry, the gases flow perpendicular to the wafer surface. It is more com- plex mechanically than the horizontal, since it is desirable to rotate the wafer to average out uneven heating effects. It is also difficult to scale up, be- cause of worsening uniformity, and is subject to

convective recirculation. The latter can be overcome by good reactor geometry and careful choice of flow conditions. A special case, where the substrate holder is rotated rapidly (the high speed rotating disk reac- tor, or RDR), successfully overcomes these disad- vantages. The disk rotation pulls the incoming gases to the wafer surface, and providing the reactants are fed in uniformly across the inlet results in a uniform deposit over the entire disk surface.

3.2. Manufacturing scale reactors

There are two main types of reactors used in MOCVD manufacturing, the pseudo-horizontal and the RDR. The former category encompasses all sys- tems where the gases flow across the wafers with depletion, and includes the barrel [81 and planetary [9] geometries. The barrel reactor has been widely used for AlGaAs solar cell structures, but suffers from temperature and thickness uniformity problems that have never been successfully overcome. The

planetary reactor is quite complex, relying on air

bearing type levitation and rotation of wafer carriers, which need frequent and careful cleaning. A glove box is used for atmospheric isolation, which affects throughput adversely. It has given excellent uniform- ities for a variety of materials, but it is maintenance intensive. The RDR [lo] relies on a magnetic fluid sealed bearing to isolate the high speed rotation from the atmosphere, and this approach works well when implemented correctly. This system has also given excellent uniformities for many materials, and has a higher throughput than the planetary system (for a given capacity) thanks to a high vacuum loadlock and lower maintenance requirements. Maximum wafer capacities for these manufacturing systems range from a few to several tens of 2 ‘1 wafers (or equivalent areas of larger wafers).

3.3. Safety and environmental concerns

As mentioned earlier, the hydride gases used in MOCVD are often highly toxic, and most of the metalorganics are pyrophoric. Hydrogen is typically used for a carrier gas. Therefore as much of an MOCVD system as possible should be constructed of stainless steel, and the system cabinets exhausted and fitted with gas detectors linked to the control system. The byproducts of the reaction include unre- acted gases and solids, all of which must be effi- ciently trapped and safely disposed of. There are a variety of chemical and thermal techniques available, and which is used depends on the system operator’s preference, the material being run, and local safety codes. Today, a well engineered system emits no hazardous gases to the atmosphere at any time, while generating only a low volume of solids for hazardous waste disposal. This topic is reviewed in Ref. [l 11.

4. Recent advances in MOCVD - R&D

4. I. Growth mechanism studies

The explanation of how growth proceeds given above is oversimplified. Until recently, most of the mechanisms postulated have been inferred from indi- rect evidence, such as optical studies, mass spec- trometry of the byproducts, and process behaviour

260 A.G. Thompson / Materiuls Letters 30 C 1997) 255-263

[4]. Kisker and co-workers [12] have made grazing incidence X-ray scattering measurements on GaAs wafers in a special MOCVD reactor using realistic growth conditions. They found that under some con- ditions, islands form initially that then merge to- gether, giving layer by layer growth. At higher tem- peratures, growth occurs at steps (step-flow growth), and this is the typical case for most MOCVD pro- cesses. Reflectance difference spectroscopy per- formed on the same reactor was correlated with the X-ray measurements to yield additional information on the process, particularly the surface reconstruction

]131.

4.2. New metalorganic compounds

Much of the development work of MO com- pounds in the last decade has gone into increasing their purity and consistency, and commonly used MOs such as TMGa, TEGa, TMAI, and TMIn are now well established. Work has also gone into ex- ploring other compounds, particularly for group V hydride replacement. As mentioned above, TBAs has now reached parity with ASH, so far as purity is concerned [14]. Other As compounds are either less pure or have too low a vapor pressure to be useful for mainstream MOCVD. There is a strong need for a P source to replace PH,; TBP is a leading candi- date but apparently gives deep level impurities that adversely affect minority carrier devices such as LEDs and laser diodes. Whether this is an intrinsic property or not is the subject of current speculation. Other P compounds either have low vapor pressure or suffer from C or impurity incorporation. The search for an N compound to replace NH, in GaN growth and hopefully overcome the need for very high V/III ratios has been unsuccessful so far; TBN was expected to behave like TBAs does in GaAs growth, but has not yet lived up to expectations. The issues of cost and safety continue to drive MO producers’ research efforts.

4.3. Heteroepitaq

The discovery [15] that very efficient and reliable blue LEDs could be fabricated from III-N material grown on highly mismatched substrates was a sur-

prise to many in the field. It has triggered extensive research into low temperature buffer layer growth and characterization. In the case of GaN, sapphire substrates are first cleaned in HZ at = 1 IOO’C, and then a thin (few 10’s of nm) buffer layer (of GaN or AlN) is grown at between 500 and 600°C. This layer is almost amorphous, but becomes crystalline while the temperature is ramped up to = 1050°C for the growth of the main device structure. The low tem- perature buffer layer thickness, growth conditions, and ramp time are all critical to the morphological and electrical properties of the upper layers. Current research is centered on elucidating the mechanisms involved through AFM and TEM measurements, and searching for compliant buffer layers for other mate- rials. The long term hope is that many materials may be grown on the most convenient substrate (large, low cost, strong, conducting, etc.> if suitable buffer layers can be grown.

4.4. Alternatice carrier gases

Hydrogen has been the carrier gas of choice for CVD since the earliest days for reasons of purity and its ability to clean heated reactor internals. Diffusing HZ through a palladium membrane results in a very pure gas, with particularly low levels of O2 and H,O. In MBE, UHV pumping and liquid nitrogen cooled shrouds are needed to achieve adequately low levels of these contaminants, but these are unneces- sary for MOCVD. The advent of effective nitrogen purifiers some 5 years ago led some researchers to explore if hydrogen could be replaced by nitrogen in the MOCVD process, and this effort has met with success recently [ 161. Their goal was to improve safety, be able to use MOs that react with HZ, and to decrease the incorporation of H2 in the layers. Reac- tor conditions had to be modified to allow for the different physical properties of nitrogen compared to hydrogen (higher density and viscosity, lower ther- mal conductivity), but they were able to achieve results at least meeting those for an optimized hydro- gen based process for GaAs, AlGaAs, InGaP, and InGaAIAs. The uniformity of the films was actually superior for their horizontal reactor. One can expect others to switch to nitrogen based on these promising results.

A.G. Thompson/Materials Letters 30 (1997) 255-263 261

4.5. Reactor modeling 5. Recent advances in MOCVD - manufacturing

The modeling of fluid flow in CVD reactors has advanced steadily from the early days of trying to explain results obtained with experimental reactors. Today, more sophisticated models are being used to design reactors and predict optimum process condi- tions. In particular, the RDR is relatively straightfor- ward to model compared to other geometries, and a group at Sandia has performed extensive calculations [ 171. Their work was recently successfully used to fine tune the design of two new large RDRs and establish the process conditions needed to replicate the results obtained in smaller systems [ 181. The group at MIT has modeled many different types of reactors and has recently published a good review of this topic [ 191. The next step, already under way, is to incorporate the chemistry and surface processes into the models. When successful, this would enable the researcher to predict the most appropriate precur- sors and process conditions for any given material.

5.1. Manufacturing scale reactors

In response to market demand and growing vol- umes of MOCVD based devices, larger capacity reactors have been developed and introduced in the last couple of years. The modeling of RDRs dis- cussed above led to scaling predictions that have now been proven [ 181. The newest RDR system (Fig. 3) has a 42 cm diameter disk capable of holding 38 X 2 ” or 9 X 100 mm wafers, and is engineered for continuous operation with minimal maintenance. Up to ten growth runs may be made sequentially without human intervention. This system has the

highest throughput of any commercial MOCVD sys- tem, and has already been placed in manufacturing in the US, Japan, and Europe. Hughes Spectrolab sub- sidiary, which produces advanced solar cells for space applications, has been running four such sys- tems for over a year and will take delivery of three

Fig. 3. A photograph of a current MOCVD production tool, the EMCORE Enterprise E400. This has the highest throughput of any commercially available system.

262 A.G. Thompson / Materid:, Lrttrrs 30 (I 997) 255-263

860 0.6% variatm

,g- 840

5

5 820

P a, 800

? 3 780

0.6% variation

.- r--o-

t

lished recently [20]. This model can be used to calculate system throughput, and to examine the contributions to product cost, optimize staffing levels and maintenance tasks, and to allocate production to the most appropriately sized tool. We expect in- creased use of COO models as production volumes grow.

760

740 I ., //' ' 'i/

/k..,,-

1070 1111 1116 1162 1167

Run Number

For many years, high vacuum growth techniques

such as MBE have enjoyed the advantage of using in situ diagnostic tools such as RHEED to determine substrate surface conditions and measure growth rates (albeit on stationary substrates). Now MOCVD has responded with a variety of optical techniques to perform diagnostics on the layers as they grow, and Aspnes has reviewed these recently [21]. Two of these techniques show promise as manufacturing tools to enhance reproducibility and minimize the number of calibration and test runs that have to be made before the full device structure is grown. They are optical retlectance and ellipsometry, and both have recently proven to work well under real growth conditions in multi-wafer reactors [22]. Accurate growth rates are obtained within the first 50 nm or so of layer growth, and also compositional information after suitable calibration constants have been mea-

Fig. 4. Long term reproducibility of the wavelength from a

complex VCESL structure grown by MOCVD using reflectivity

for in situ calibration (data courtesy of Sandia National Laborato-

ries).

more in 1996, making this the largest MOCVD installation in the world.

5.2. COO models

Cost of Ownwership models have become very important in the Silicon industry as a tool for predict- ing throughput and processing costs for a wide vari- ety of equipment. The compound semiconductor in- dustry is starting to follow suit, with the first pub-

DAR coating

&FL T’-contact

I II - Ge Sub. I I

Top Cell

Tunnel Junction

Middle Cell

Tunnel Junction

Bottom Cell

Voltage (V)

The I-V curve shows an AM0 efficiency of 25.7%

Metal-contact

Fig. 5. (a) Structure of a monolithic triple-junction solar cell grown by MOCVD on a Ge substrate, and (b) its I-V characteristics and

efficiency (data courtesy of Spectrolabs Inc., a Hughes Electronics subsidiary).

A.G. Thompson/Materials Letters 30 (IY97) 255-263 263

sured. Both may be used to either establish growth rates during buffer layer growth (this information is then used to set layer times), or direct control of layer thicknesses can be implemented for all but the thinnest layers. Using the reflectance technique on buffer layers, Sandia [22] was able to achieve a remarkable *0.3% reproducibility for a complex VCSEL structure over several weeks, as shown in Fig. 4. Widespread use of this technique can be expected for the manufacturing of difficult structures in particular.

6. Applications

MOCVD is increasingly being applied to devices whose volumes are growing rapidly. One such appli- cation is advanced multi-junction solar cells for satellites, which are grown on large Ge substrates. No other growth technique can come close to meet- ing the cell efficiency (Fig. 5), low manufacturing cost and high throughput of the MOCVD technique. In the LED arena, newer high brightness (HB) LEDs made from InGaAlP outshine earlier devices, and so are finding new applications in automobile stop lights, outdoor displays, and traffic lights. Here the user looks at the lifetime cost of the device, so a municipality is willing to pay much more up front for a traffic light that does not need annual bulb replacement and that uses less electricity. Bright III-nitride based LEDs are now available emitting in the blue and green, with applications in outdoor displays and traffic lights. Laser diodes have always been an important area for MOCVD, with long wavelength telecommunication devices being fabri- cated almost exclusively by MOCVD. Now MOCVD grown laser diodes for CD players have largely displaced those made by MBE for cost and perfor- mance reasons, while newer visible (red) devices made from InGaAlP are finding applications for advanced CD-ROM and DVD systems. These are only a few of the applications for MOCVD material, but bear in mind that there are many other optoelec- tronic and electronic applications also.

Acknowledgements

I would like to thank my colleagues at EMCORE Corporation, our customers, and others in the indus- try who have helped and educated me during the preparation of this article.

[3] B. W. Wessels, Annu. Rev. Mater. Sci. 25 (1995) 525.

[4] G.B. Stringfellow, Organometallic vapor-phase epitaxy

(Academic Press, New York, 1989).

[51 D.W. Kisker and T.F. Kuech, Handbook of crystal growth,

Vol. 3A (Elsevier Science, Amsterdam, 1994) ch. 3.

[6] G.B. Stringfellow, Handbook of crystal growth, Vol. 3B

(Elsevier Science, Amsterdam, 1994) ch. 12.

[7] A.C. Jones, P. O’Brien, CVD of compound semiconductors

(VCH, Weinheim, 1996).

ls1

[91

[lOI

N. Tomesakai, M. Suzuki and J. Komeno, J. Electrochem.

Sot. 140 (1993) 2432.

P.M. Frijlink, J. Crystal Growth 93 (1988) 207.

G.S. Tompa, W.J. Kroll, C. Chern, H. Liu, P.A. Zawadzki,

A. Gurary, A.G. Thompson, M. McKee and R.A. Stall, III-V

Review 7 (1994) 12.

llll l121

1131

Alan G. Thompson, Proc. Adv. Mater. 4 (1994) 181.

D.W. Kisker, G.B. Stephenson, J. Tersoff, P.H. Fuoss and S.

Brennan, J. Crystal Growth 163 (1996) 54.

I. Kamiya. L. Mantese, D.E. Aspnes, D.W. Kisker, P.H.

Fuoss, G.B. Stephenson and S. Brennan, J. Crystal Growth

163 (1996) 67.

t141

[I51

(161 1171

lt81

[I91

BOI

t211

1221

H.C. Chui, B.E. Hammons, N.E. Harff, J.A. Simmons and

M.E. Sherwin. Appl. Phys. Lett. 68 (1996) 208.

S. Nakamura, T. Mukai and M. Senoh, J. Appl. Phys. 76

(1994) 8189.

H. Hardtdegen, Electrochem. Sot. Proc. 96-2 (I 996) 49.

W.G. Breiland and G.H. Evans, J. Electrochem. Sot. 138

(1991) 1806.

A.G. Thompson, R.A. Stall, P. Zawadzki and G.H. Evans, J.

Electron, Mater. 25 (1996) 1487.

D.I. Fotiadis, S. Kieda and K.F. Jensen, J. Crystal Growth 102 (1990) 441.

Alan G. Thompson, W. Kroll, M.A. McKee, R.A. Stall and

P. Zawadzki, III-V Review 8 (1995) 14.

D.E. Aspnes, IEEE J. Select. Topics Quant. Elect, I (1995)

1054.

Alan G. Thompson, R. Karlicek, E. Armour, W. Kroll, P.

Zawadzki and R.A. Stall, III-V Review 9 (1995) 12.

References

[I] A.G. Thompson, R.A. Stall and B. Kroll, Semiconductor

Intern. (July 1994) 172.

[2] A.V. Gelatos, A. Jain, R. Marsh and C.J. Mogab, Mater. Res.

Sot. Bull. (August 1994) 49.