N.I. Buchan

Carbon incorporation in metalorganic vapor phase epitaxy grown GaAs using CHyX4 − y, TMG and AsH3

Abstract The incorporation of carbon into GaAs grown by metalorganic vapor phase epitaxy has been studied through the addition of the halomethanes CCl 4 , CHCl 3 , CH 2 Cl 2 , CBr 4 , CI 4 , CHI 3 , CH 2 I 2 , CH 3 I and CH 4 . Growth temperatures of 600–750°C, V/III ratios of 30–100, and halomethane mole fractions of 2 × 10 -8 -1 × 10 -4 were investigated. The incorporation of carbon for all the halomethanes decreases at high V/III ratios and also decreases at high growth temperatures with an exponential Arrhenius fitting parameter of -30 to -60 kcal/mol. The carbon incorporation dependence of the chloromethanes on halomethane mole fraction conforms to a first order power function. However, carbon incorporation with CH 2 I 2 conforms to a second order power function. The trend in the halomethanes is to incorporate carbon more efficiently in more highly substituted halomethanes, following the series CH 3 X 2 X 2 3 4 throughout the range V/III ratios and growth temperatures investigated. good correlation in this series of low average bond strength to carbon incorporation is also observed. The dependence of carbon incorporation on V/III ratio and growth temperature are explained by a simple reaction scheme based on the competition at the growing interface between the decomposition of the halomethane to incorporate carbon and the desorption of the decomposing halomethane from the interface.

High carbon doping efficiency of bromomethanes in gas source molecular beam epitaxial growth of GaAs

Carbon tetrabromide (CBr4) and bromoform (CHBr3) have been studied as carbon doping sources for GaAs grown by gas source molecular beam epitaxy (GSMBE) with elemental Ga and thermally cracked AsH3. Hole concentrations in excess of 1×1020 cm−3 have been measured by Hall effect in both CBr4‐ and CHBr3‐doped GaAs, which agrees closely with the atomic C concentration from secondary‐ion mass spectrometry, indicating complete electrical activity of the incorporated carbon. The GaAs growth rate is unaffected by the CBr4 and CHBr3 fluxes over the range of dopant flow investigated. The efficiencies of carbon incorporation from CBr4 and CHBr3 are, respectively, 750 and 25 times that of trimethylgallium (TMG), which is commonly employed as a carbon doping source in metalorganic MBE (MOMBE). The sensitivity of carbon incorporation to varying substrate temperature and V/III ratio has been observed to be significantly reduced with CBr4 and CHBr3 from that obtained under similar growth conditions with TMG in MOMBE.

Carbon incorporation in metal-organic vapor phase epitaxy grown GaAs from CH x I 4-x , HI, and I 2

The incorporation of carbon into GaAs grown by metal-organic vapor phase epitaxy has been studied through the addition of CH2I2, CH3I, HI and I2 to the growth ambient. The epitaxial GaAs was grown using Ga(CH3)3 and AsH3 in a low pressure reactor. The addition of CH2I2 to the growth ambient resulted in the controlled incorporation of carbon. The addition of CH3I, HI and I2 did not result in any additional carbon incorporation. The decomposition of CH2I2 may result in the formation of methylene, CH2, on the growth surface. A reaction mechanism that explains the incorporation of carbon by CH2I2 and the implication of these results during conventional growth are discussed.

β-Hydride elimination reaction of triethylgallium on GaAs(100) surfaces

Abstract We have used a pulsed molecular beam and time-resolved mass spectrometry to study the pyrolysis of triethylgallium (TEGa) on GaAs(100) surfaces from room temperature to 450°C. The β-hydride elimination pathway which produces ethylene and hydrogen competes with the direct desorption of the ethyl radicals. We have made a quantitative measure of the branching ratio and found that the β-hydride elimination reaction is promoted by increasing the Ga/As stoichiometric ratio of the GaAs(100) surface, but becomes less important and independent of Ga coverage at higher temperatures. The β-hydride elimination process is the rate limiting step in the desorption of ethylene and is first order in the ethyl group coverage.

Calculation of unimolecular rate constants for common metalorganic vapor phase epitaxy precursors via RRKM theory

Abstract A mechanistic understanding of metalorganic vapor phase epitaxy (MOVPE) requires an understanding of fundamental parameters such as unimolecular gas phase (homogeneous) reaction rate constants. Using a large body of existing and estimated thermochemical data, Rice, Ramsperger, Kassel and Marcus (RRKM) theory was used to calculate the pressure dependence of the reaction rate constants for the MOVPE precursors Al(CH 3 ) 3 (TMAl), Ga(CH 3 ) 3 (TMGa), In(CH 3 ) 3 (TMIn), As(CH 3 ) 3 (TMAs), AsH 3 , and PH 3 . In addition, the pressure dependence of NH 3 decomposition was calculated. The reaction paths for the group V hydrides involving the elimination of molecular as well as atomic hydrogen are discussed. The fall-off pressures, P 1 2 , for TMGa and AsH 3 in a hydrogen carrier gas at 900 K are approximately 2 Torr and 1 × 10 5 Torr, respectively.

Selective epitaxy of MOVPE GaAs using diethyl gallium chloride

Abstract Selective epitaxy is the laterally controlled growth of epitaxial material on a substrate. We have demonstrated the highly selective epitaxial growth of GaAs using diethyl gallium chloride or (C 2 H 5 ) 2 GaCl. No GaAs growth is observed on the masking material over a wide range of growth temperatures, 600 ⩽ T ⩽800°C. The edges of the selectively grown GaAs are bounded by the slow growth planes typical of the inorganic based growth techniques. By appropriate pre-growth treatment the electrical properties of the interface between the selectively grown material and the underlying substrate can be made to be very good with no interfacial potential barrier.

The effect of pressure on the band-gap energy in ordered GaInP and AlGaInP grown by MOVPE

Abstract Photoluminescence (PL) measurements on Ga 0.52 In 0.48 P and Al 0.18 Ga 0.34 In 0.48 , P alloys, grown by metalorganic vapor phase epitaxy (MOVPE) on GaAs, have been made as a function of pressure up to about 4.5 GPa at 77 K. The substrates are oriented on the (100) and off the (100) plane toward [011] and [011] by up to 25 °. The band-gap energy anomaly in the CuPt-type ordered structure has been systematically investigated at high pressures. With increasing pressure the E 0 band gap shows a sublinear shift toward higher energies. For some samples, this shift tends to saturate, of the PL peak shows a decrease in energy with pressure. The pressures needed for these observations are found to be significantly smaller in AlGaInP than in GaInP. The observed behaviors strongly depend on substrate misorientation, and hence reflect the degree of ordering in these alloys. These results clearly demonstrate the importance of the effects of repulsion between the Γ-folded energy states on the optical spectra in the CuPt-type ordered structure. Possible explanations for some of the trends in high-pressure behavior of the E 0 direct band gap in GaInP and AlGaInP samples having different degrees of ordering are discussed, including the modification of the ordinary Γ-X crossover upon ordering.

Epitaxial growth of GaAs with (C2H5)2GaCl and AsH3 in a hot-wall system

Abstract The MOVPE growth of GaAs in a hot-wall system has been investigated using diethylgallium chloride ((C 2 H 5 ) 2 GaCl, DEGaCl), AsH 3 and H 2 . Recent measurements have determined the vapor pressure of DEGaCl, and have also determined that DEGaCl is dominantly associated as multimers in the gas phase. The growth temperature range of 350 ⩽ T ⩽ 700°C was investigated at reactor pressures of 0.10–10 Torr. The substrates were mounted parallel to the average gas flow direction throughout the study. The very low temperature at which the growth rate is reaction rate-limited in the hot-wall reactor may be attributed to the conversion of DEGaCl to a species that is more readily incorporated as GaAs, such as GaCl. At higher temperatures, the growth rate typically demonstrates a monotonic decrease with downstream location that may result from both the depletion of GaCl in the reactor, and/or a lower supersaturation of GaCl in the gas phase due to increasing concentrations of HCl down the length of the reactor. At even higher temperatures a drastic drop in growth rate may also be caused by the depletion of GaCl and/or the lower supersaturation of GaCl in the gas phase. The carrier concentration in the grown material is below 2.5s10 14 cm -3 at 450°C and increases at higher temperatures to 5s10 15 cm -3 n-type at 650°C.

The use of azo‐compounds as probes of carbon incorporation of nominally undoped metalorganic vapor phase epitaxy grown GaAs

Secondary ion mass spectroscopy has been used to quantitatively determine the carbon concentration in nominally undoped GaAs grown by metalorganic vapor phase epitaxy from TMG (13C 99%) and AsH3. Both an increase in the V/III ratio and the addition of supplemental gas phase radicals reduced the carbon incorporated from the TMG. Higher V/III ratios are proposed to increase the surface concentration of AsHx species. Supplemental gas phase t‐butyl radicals, produced from the decomposition of azo‐t‐butane, are proposed to attack AsH3, also resulting in an increase in the surface concentration of AsHx species. Higher surface concentrations of AsHx are then proposed to reduce carbon incorporation by enhancing the desorption of carbon‐containing species.

Surface Chemistry of CVD Reactions Studied by Molecular Beam/Surface Scattering

A molecular beam/surface scattering experiment in an ultrahigh vacuum is conceptually a simulation of a CVD reactor without the interference from gas phase and wall reactions. The surface chemistry can be studied in real-time during the deposition reaction at the desired temperature. In our experiment, we used pulsed molecular beams of the reactants and a mass spectrometer to monitor In real-time the reaction products evolving from the substrate surface. With this arrangement, the reaction probability of the molecules can readily be determined by measuring the unreacted fraction of the molecular beam. The reaction pathways can be deduced from the Identification of the reaction products, while their time-evolutions give the kinetic parameters. We shall illustrate this technique by our study on the reactions of trimethylgallium and triethylgallium on GaAs as related to the metalorganic CVD and atomic layer epitaxy of GaAs.