GaAs Crystal Growth Research Articles

The minimisation of thermal stresses during the growth of GaAs crystals

Undesirably high dislocation densities in Czochralski grown gallium arsenide crystals are generally accepted to be due to high thermal stresses generated by temperature gradients in the crystal during growth. Detailed numerical calculations are presented, using finite element methods, for partially and fully grown crystal geometries to calculate the resulting stresses. By varying such parameters as the ambient temperature, heat transfer coefficient and crystal geometry the dislocation density can be minimised and hence optimum growth conditions predicted. It is shown that the optimum conditions during the initial stages of crystal growth are radically different to those required for the mature crystal geometry.

Crystal growth of GaAs and AlGaAs by OMVPE using triethylarsenic as arsenic source

Triethylarsenic (TEAs), which is much less toxic than arsine, was used as an arsenic source for organometallic vapor-phase epitaxy (OMVPE) of GaAs and AlGaAs. The lower growth rates of GaAs at higher growth temperatures were remarkably increased by a low pressure growth. AlGaAs with a mirror-like smooth surface was obtained for the first time over a wide range of growth temperatures 600–710°C, V/III ratios 3–10, and Al compositions 0—0.5. Purity of TEAs was found to be an important key for good quality AlGaAs epilayers. The growth rate of AlGaAs became lower and the Al composition higher, as the growth temperature was increased. Room temperature, pulsed operation of double heterojunction laser diodes, which were fabricated with the grown layers of GaAs and AlGaAs doped with Si or Zn, was achieved at the first trial.

Uniform (Al)GaAs crystal growth and microwave HIFETs grown by barrel-reactor MOCVD

A barrel-reactor MOCVD system accommodating seven 2 inch wafers was developed. The growth rate uniformity of (Al)GaAs was ±2% in a wafer and ±1% at the same position in seven wafers. An (AlAs)5-(GaAs)2 monolayer-scale superlattice were grown and their structure were verified by Raman scattering and transmission electron microscopy. The sheet resistance of the two-dimensional electron gas in the AlGaAs/GaAs system was distributed within ±1% on one wafer and ±1.5% among seven wafers. The standard deviation of the threshold voltage (the gate pinch-off voltage) for fabricated HIFETs (so-called HEMTs) was 9.7 mV in a 16 mm × 16 mm square, which represents the VLSI-level uniformity. The average noise figure of 0.5 μm gate HIFETs at 12 GHz was 0.96 dB with a 0.06 dB standard deviation.

Patterned Crystal Growth of GaAs by Laser Assisted Atomic Layer Epitaxy

ABSTRACTAtomic layer epitaxy (ALE) of GaAs is achieved by metalorganic vapor phase epitaxy (MOVPE) under irradiation by Ar ion laser. The self-limiting mechanism for gallium deposition at one atomic layer on an arsenic atomic surface is realized by the site-selective decomposition of triethylgallium on the arsenic surface. A patterned growth of ALE is obtained by scanning a laser beam on a substrate.

Temperature gradients, dopants, and dislocation formation during low-pressure LEC growth of GaAs

It was widely accepted that in the presence of high thermal gradients thermoelastic stress is responsible for the dislocation structure observed in GaAs crystals grown from the melt. Recent studies suggest that another mechanism must be operative when growth takes place in the presence of sufficiently low thermal gradients. In this work we will review the current status of low pressure, low thermal gradient GaAs crystal growth. Our previous studies have shown that undoped GaAs crystals grown under low pressure by the liquid encapsulated Czochralski technique from melts with axial temperature gradients of 6°C/cm at the solid/liquid interface have dislocation densities of ∼ 3x10 3 cm -2 throughout the boule. Our recent work shows that growth at an order of magnitude slower pull rate has no effect on this dislocation density, but does result in more extensive polyganization. Increased pull rates, however, increase the dislocation density. Growth of crystals on the 〈111〉 axis results in dislocation densities 2.5 times greater than observed in crystals grown on the 〈100〉 axis. For GaAs, LEC growth using Dash-type necking is ineffectual for reducing dislocation densities. Doping with indium, silicon or tellurium at concentrations above “critical” values significantly reduces the dislocation density. Doping with tin, phosphorus, zinc or germanium has no observable effect. Also, there is no effect of dopant segregation on dislocation density. These observations together with others reported in the literature strongly support a mechanism wherein dislocations are formed as a result of microscopic stress fields (exceeding the critical resolved shear stress) that arise from inhomogeneous vacancy incorporation at the solid/liquid interface. The dislocation density is determined, then, by the Ga and As vacancy concentrations, and therefore can be affected by either stoichiometry or dopant additions.

Dislocation bundle formation during liquid encapsulated Czochralski growth of GaAs crystals

Residual dislocations in a P-alloyed and an undoped GaAs crystal grown by the liquid encapsulated Czochralski method were investigated by transmission electron microscopy. In the undoped GaAs, dislocation bundles constructing cell walls and/or lineages consist of at least three different Burgers vectors. In the P-alloyed GaAs with low dislocation density, individual dislocation reactions were observed. Sessile dislocations were found to be formed by an interaction between two kinds of dislocations. The sessile dislocation formation may be a first step for the complicated dislocation structure in the undoped crystal.

Magnetic field effect on residual impurity concentrations for LEC GaAs crystal growth

A horizontal magnetic field applied LEC (MLEC) apparatus, for mass-producing large diameter undoped LEC GaAs, has been newley developed. The detailed magnetic field effect on residual impurity concentrations has been studied. The carbon and boron concentrations have been found to be decreased by the MLEC technique, related to the applied magnetic field strength. Carbon and boron impurity concentrations in MLEC GaAs crystals decreased as the gallium molar ratio in molten GaAs decreased. The carbon concentration in MLEC GaAs decreased by decreasing the argon pressure in a high pressure chamber. The thermochemical mechanism for impurity incorporation into GaAs single crystals has been clarified through this study. The oxidation rate of free gallium and deoxidation rate of carbon oxide are shown to be the rate limiting reactions determining the impurity incorporation rate into GaAs crystals. The obtained results suggest that applying a magnetic field to molten GaAs suppresses the chemical reaction closely related to the impurity incorporation into resultant GaAs single crystals. The experimental results show good agreement with this predicted thermochemical mechanism.

Influence of temperature conditions in horizontal Bridgman furnace on crystal growth of GaAs

AbstractThe experimental investigation of the boat shape and boat position in the furnace in the case of unseeded horizontal Bridgman growth of GaAs single crystals was carried out. The relation between axial and radial temperature gradients in the furnace was also studied. On the basis of experimental results the optimum conditions for the growth of low dislocation density GaAs single crystals were derived.

Bulk GaAs crystal growth by liquid phase electroepitaxy

Bulk GaAs crystals (up to 4 mm thick and 20 mm in diameter) were grown by liquid phase electroepitaxy on melt-grown GaAs substrates. They were found to be of high quality (essentially dislocation-free) comparable to that of thin GaAs epitaxial layers; when undoped they exhibited a very high electron mobility (about 7000 cm 2/V·s at room temperature) low carrier concentration ( <10 15 cm -3), and no measurable electron traps. The thickness of the crystals and maximum growth rate were found to be limited primarily by convection in the solution. In view of the limitations imposed by convection, the growth of commercial size bulk GaAs crystals of epitaxial quality is being designed under microgravity conditions.

Growth of semi-insulating GaAs crystals with low carbon concentration using pyrolytic boron nitride coated graphite

It has been determined that the incorporation of carbon into GaAs crystals grown by the liquid encapsulated Czochralski method occurs as the result of a reaction between the GaAs melt and gaseous oxides of carbon, rather than carbon particles. The incorporation occurs during growth as well as during synthesis. The incorporation of carbon was particularly large when a commercially available large puller was used because of the greater number of carbon components in the hot zone which act as sources. Crystals with a low carbon concentration of 1×1015 cm−3 have been successfully grown by employing pyrolytic boron nitride coated graphite components in the large puller.

Low indium-doped LEC GaAs crystal growth employing thermal stress analysis

Recently, dislocation-free LEC GaAs crystals have been able to be grown by indium doping at the 10 20 cm -3 level. However, striations and In inclusions are still problems in In-doped LEC GaAs. We have overcome these problems by reducing the doped In concentration. The ingots have been grown so that the calculated stress value in any part of the ingot may not exceed 0.07 kg/mm 2, which was the critical resolved shear stress estimated by the thermal stress analysis. Employing this technique under the same growth conditions as those for the low dislocation, (4–5) x 10 3 cm -2 etch pit density, undoped GaAs crystal, dislocation-free 2 inch diameter wafers have been obtained at (1–4) x 10 19 cm -3 In doping level.

Growth and properties of large-diameter indium lattice-hardened GaAs crystals

The liquid-encapsulated Czochralski (LEC) growth of In-doped GaAs from 3 kg melts in a Melbourn high-pressure puller has resulted in low-dislocation, large-diameter crystals. Post-growth boule annealing at 950°C for 18 h is found to be an effective stress-relief treatment. This allows high wafer yields, comparable to those from undoped GaAs crystals to be obtained from In-doped boules, with the added advantage of greatly improved uniformity in electrical properties. These substrates are semi-insulating, thermally stable ( ρ ≥ 10 7 ω cm and μ H ⩾ 5000 cm 2/V · s with a ± 10% or better radial uniformity), and contain low residual impurity concentrations ( ⩽ mid-10 15 cm -3) as determined by secondary ion mass spectroscopic (SIMS) analysis. In this study the effectiveness of various concentrations of indium in reducing the dislocation density in GaAs have been explored. A comparison of the thoretically calculated thermal stress experienced by a crystal during LEC growth, with observed reductions in dislocation etch pitch density, indicate an apparent 28-fold increase in the critically resolved shear stress (CRSS) of In-doped over undoped GaAs for 8 x 10 19 cm -3 In in the solid. Polished substrates obtained from these crystals show minimal subsurface damage, believed to be related to the increased hardness of this material, and are now approaching high-quality silicon wafers in this respect.

In-situ observation of LEC GaAs solid-liquid interface with newly developed X-ray image processing system

A new LEC GaAs crystal growth monitoring system for in-situ real-time observation of the shape of the meniscus and the solid-liquid interface has been developed using an X-ray image processing technique. The system consists of an in-house modified high pressure puller and X-ray imaging apparatus linked to a real-time image processor which supresses the background noise and enhances the X-ray image. The relationship between the shape of the meniscus and crystallinity was investigated in the seeding procedure. The region of stable crystal growth is indicated by a meniscus angle in the range 18° to 45°.

Growth of GaAs by switched laser metalorganic vapor phase epitaxy

Crystal growth of GaAs by switched laser metalorganic vapor phase epitaxy (SL MOVPE) is reported. This growth technique is achieved by combining laser MOVPE and atomic layer epitaxy. The growth process in SL MOVPE can be explained by a model which assumes that trimethylgallium adsorbed on the crystal surface is decomposed in a photocatalytic reaction and that the decomposition rate depends on the surface species present on the substrate.

Homogeneous semi-insulating GaAs crystal growth by a new LEC technique with as injection into melt during growth

We have succeeded in growing homogeneous semi-insulating GaAs crystals reproducibly by a new and advanced high pressure LEC growth technique using As injection to control the melt composition during crystal growth. More uniform properties than those of conventional LEC GaAs were obtained reproducibly in not only longitudinal but also radial directions, including resistivity, C-V profile, infrared transmission image and FET performance. It is suggested that such homogeneity is due to the effect of controlling the melt composition at the near stoichiometric (or congruent) point.

Growth of silicon-doped dislocation-free gaas crystals by the LEC technique for optical device applications

Silicon-doped (100) GaAs single crystals up to two inches in diameter, dislocation and striation free, were grown by a low temperature gradient LEC technique. The crystals were characterized by etching technique, x-ray topography, SIMS, and Hall measurements. LED and LD devices fabricated on the crystal performed as well as those fabricated on silicon doped boat grown crystals. These LEC GaAs crystals have been demonstrated to be suitable for LED and LD device applications.

GaAs crystal growth from coordination compounds using the organometallic chemical vapor deposition process for solar cells

GaAs thin layers have been obtained from coordination compounds (adducts) using the organometallic chemical vapor deposition process (O.M.C.V.D.). That is the first step of a study going on to realize GaAlAs/GaAs solar cells. The preparation of the compounds has shown that the bond strength energy between the Ga and As atoms in the molecule is very important for use in a chemical vapor deposition process. The layers were p type and the use of As Et 3 occurs in the chemical equilibrium of the vapour phase to obtain n type material. The crystallographic orientation of the substrates can influence the growth mechanisms of the layers.

Research aiming for future optoelectronic integration the optoelectronics joint research laboratory

Progress in semiconductor optoelectronic devices in the past 20 years has been significant. With the progress of optical fibres, a new era of communication has been established by overcoming major difficulties in device technologies. However, if one considers the present status of optoelectronic technologies, one finds that a large potential advantage of semiconductor technologies, that of &apos;integration&apos;, has been left unrevealed. The monolithic integration of optical and electrical devices, called optoelectronic integrated circuit (OEIC), will result in a big improvement in optoelectronic technology.The Optoelectronics Joint Research Laboratory (OJL) has been established to develop new generic technologies for optoelectronic integration. Significant progress in the material process technologies has been obtained in bulk GaAs crystal growth, a new maskless dry process using focused ion beam doping, MBE and dry etching for GaAs. The future evolution of optoelectronic technology and OEIC markets is also discussed.

半導体素子―材料とプロセス技術 ＧａＡｓ結晶の育成と加工

半導体素子―材料とプロセス技術 ＧａＡｓ結晶の育成と加工

Molecular beam epitaxial growth of GaAs using trimethylgallium as a Ga source

Crystal growth of GaAs using trimethylgallium (TMG) as a Ga source in a molecular beam epitaxial system has been studied. Mirror-like monocrystalline epitaxial layers of GaAs were easily obtained on a GaAs substrate at the substrate temperature of 530–630°C. Epitaxial layers were p type and the carrier concentration of these films was about 1018–1019 cm−3. In particular, no deposition was observed on a SiO2 film in this TMG-As4 system. This feature showed the possibility of selective epitaxy of GaAs.