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The growth of undoped GaAs films from Ga(C2H5)3 (TEG) and AsH3 by Metalorganic Chemical Vapor Deposition (MOCVD) has been studied in a low-pressure reactor operating at 3 Torr. This precursor combination is known to produce GaAs films with very low carbon incorporation when compared to films grown from Ga(CH3)3 (TMG) and AsH3. A kinetic model of the growth process has been developed that includes both gas-phase and surface reactions based on reported decomposition mechanisms of TEG and AsH3. The kinetic model was coupled to a transport model describing flow, heat and mass transfer. Finite element simulations were performed to determine the rate constants of the growth reactions that provide the best fit between predicted and observed growth rates. Under typical operating conditions the surface reactions were found to dominate the growth process and a reduced surface kinetic model was identified by sensitivity analysis. The proposed reaction-transport models can successfully predict observed growth rates of GaAs films and they can be used for identifying optimal operating conditions in MOCVD reactors.

The diffusion rate of Zn in AlxGa1−xAs alloys has been determined, for values of x up to 1.0, as a function of temperature. At 625°C the diffusion coefficient shows a strong dependence on x, reaching a maximum at about 65% Al. A similar rapid increase in diffusion coefficient with x is seen at 700°C. The zinc concentration profiles are similar for all values of x, showing a very abrupt diffusion front. The activation energy for diffusion decreases with increasing x.

All of the low-pressure techniques have been developed to the point that high-purity films suitable for many device applications can be grown. The continued investigation of growth mechanisms should lead to further improvements in film quality and optimized reactor designs. Reducing the operating pressure of OMVPE offers improvments in interface abruptness and uniformity and reduced concentrations of impurities other than carbon when compared with the atmospheric pressure process. Molecular beam methods offer a further improvement in interface abruptness but must still contend with a significant amount of carbon incorporation when AlxGa1−xAs films are grown from organometallic sources. The optimum technique will probably depend on the purity requirements, structures, and materials required for a particular application. At present low-pressure reactor design is based on a combination of historical precedent and intuition. In the future reactors which are not simply modified OMVPE or MBE systems can be expected. Hopefully they will combine the advantages of both parent technologies. Because of the low growth rate required for the very thin layers used in most devices these systems will have to provide for growth on several wafers simultaneously. Multiple beams or very diffuse sources are required in multi-wafer molecular beam systems. Improved uniformity over a single substrate has been achieved by using multiple-orifice plates over the reactant supply cells in CBE (180). Vacuum epitaxy, in which a combination of multiple group III beams and viscous flow of the group V element are utilized, comes closest to applying the molecular beam approach to a multi-wafer system (144). A reliable model for the OMVPE process at a low pressure is far from complete. This is because both gas-phase and surface chemical processes are important. Surface chemical reactions become increasingly important at pressures below 103 Pa, although even at pressures down to 130 Pa gas-phase reactions such as adduct formation may still be significant (117). At very low pressures unimolecular gas-phase reactions such as AsH3 or TEGa decomposition are in the “fall-off” region in which the rate is a function of pressure and also of the specific diluent gas used (204). The degree of adduct formation and the extent of thermal decomposition reactions are influenced by reactor design. On the other hand a model of CBE growth is well-developed because the application of the RHEED technique is leading to a good understanding of the important surface chemical processes.

Transparent single crystals of AgNiF 3 up to 3 mm on an edge have been grown by slow cooling from AgF flux. The crystals are tetragonal, space group D 6 4 -p4 2 2 1 2, with cell constants a = 5.56 A and c = 7.86 A . The material has an optical absorption spectrum typical of divalent nickel in octahedral coordination with fluoride ions. A noncollinear antiferromagnetic transition at 197°K is observed.

The epitaxial growth of GaAs from trimethylgallium and arsine at 1–5 Torr was investigated. At deposition temperatures of 580–750°C the residual donor concentration decreased with increasing arsine excess, not increased as has been reported for higher pressures. Increases in growth rate also markedly reduced the rate of incorporation of the residual donor, and at high growth rates the films became p-type. Under optimum conditions the electron mobility and the residual impurity concentrations obtained were comparable to the best values reported for films grown using metalorganics.

Indium-tin oxide (ITO) and cadmium-tin oxide (CTO) transparent contacts have been used on GaAs 0.6 P 0.4 planar, red-light-emitting diodes. The entire emitting area of the diode can be contacted without blocking any of that area with opaque metal. Furthermore, the low sheet resistance of the transparent conducting films (2.5 to 3.5 ω/□) makes the spatial distribution of the emitted light more uniform. The spatial uniformity of the light output of LED's fabricated with transparent contacts is compared with that of conventional LED's. The current-voltage characteristics of the two types of LED are also compared. For an average current density of 25 A/cm2, an undesirable excess voltage (> 1.2 V) is developed across the reverse-biased (n-transparent conductor)-(p-GaAs 0.6 P 0.4 ) heterojunction. Procedures are described for reducing and, in some cases, completely eliminating this voltage by introducing a thin, transparent metal film between the transparent conductor and the GaAs 0.6 P 0.4 . The resulting devices have optical and electrical efficiencies comparable to those of conventional LED's.

The development of a process for chemical vapor deposition of polycrystalline α-phase silicon carbide onto carbon substrates is described. This process is based on the use of dichlorosilane and methane as reactants in nitrogen at atmospheric pressure. Deposits of fine-grained and hard material up to 12 mm thick have been obtained. The deposits are nonporous, relatively free of stress, free of cracks over large areas, and suitable for some structural applications

The resistivity and Hall coefficient of cubic zinc selenide crystals have been measured at temperatures up to 1000°C in zinc vapor. The crystals are n-type, and the electron concentration increases with the 1 2 power of the zinc pressure. This behavior indicates the presence of a singly ionized native donor defect. The enthalpy of formation of this donor and its concentration along the zinc-rich solidus have been determined.

Hall coefficient and resistivity measurements have been made on undoped ZnTe single crystals at temperatures up to 1000°C under controlled partial pressures of either zinc or tellurium. All samples are p -type. The Hall mobility of holes has been measured, and the equilibrium hole concentrations along both the zinc-rich and tellurium-rich solidus curves have been determined. At compositions close to the zinc-rich solidus, the dependence of hole concentration on Zn(g) partial pressure supports the conclusion, of Thomas and Sadowski that the hole concentration is controlled by the presence of a doubly ionized native acceptor defect. At compositions close to the tellurium-rich solidus the situation is apparently more complicated. The dependence of the hole concentration on Te 2 (g) partial pressure is considerably weaker than expected if the only defect of importance were the acceptor defect found close to zinc saturation. At high temperatures in this composition region there are appreciable concentrations of a neutral defect which precipitates on cooling to room temperature. We suggest that the acceptor defect is a zinc vacancy, as proposed by Thomas and Sadowski, and that the neutral defect is a tellurium interstitial. Evidence has been obtained for the association of the acceptor with a donor during slow cooling of tellurium-saturated specimens.

The slow photoconductive kinetics associated with Cl and Ga donors in CdTe are examined by Schottky-barrier capacitance methods. The results indicate that a nonelectronic mechanism is the rate-limiting step in the photodecay process.

Hall coefficient and conductivity measurements have been made at 450–1000°C on zinc telluride crystals heavily doped with aluminum, while the crystals were held in either zinc-rich or tellurium-rich atmospheres. At tellurium pressures close to saturation the crystals are p -type with hole concentrations not much reduced from those in undoped crystals. At zinc saturation on the other hand the crystals are n -type with an electron concentration in excess of 2 × 10 16 cm −3 at the highest temperature. The equilibrium electron concentrations at zinc saturation decreases rapidly with decreasing temperature.

Hall coefficient and conductivity measurements have been made on cadmium selenide single crystals, in cadmium atmospheres, at temperatures up to 950°C. The crystals are n-type, and the electron concentration increases with the + 1 3 power of the cadmium pressure. This behavior indicates the presence of a native donor, probably either a cadmium interstitial or selenium vacancy, whish is doubly ionized at temperatures above 600°C. The enthalpy of formation of this donor and its concentration along the cadmium-rich solidus have been determined.

A kinetic model describing the growth of GaAs films from triethyl-gallium (TEGa) and arsine by low-pressure metalogranic chemical vapor deposition (LP-MOCVD) has been developed. This precursor combination produces GaAs films with very low carbon contamination, when compared to films grown from trimethyl-gallium (TMGa) and arsine. The kinetic model includes both gas-phase and surface reactions based on reported decomposition mechanisms of the two precursors. Growth experiments were performed in a LP-MOCVD reactor operating at 3 Torr, a pressure that minimizes background levels of both ionized donors and acceptors in the film. The kinetic model was coupled to a transport model describing flow, heat and mass transfer in the experimental reactor. Finite element simulations were performed to estimate unknown rate parameters of surface growth reactions by fitting predicted to observed growth rates. The model predicts that the growth rate is limited by site-blocking effects due to slow desorption of adsorbed ethyl radicals at low temperatures and by the fast desorption of adsorbed gallium at high temperatures. The predicted Ga* coverage is high at intermediate temperatures suggesting the absence of a diffusion-limited growth regime, which is common in atmospheric pressure MOCVD. It appears that in LP-MOCVD, the temperature uniformity of the susceptor (and not gas-phase transport processes) controls the thickness uniformity of the film at operating conditions maximizing the growth rate. The proposed process model can evolve into a useful tool for reactor design and scale-up by minimizing the large number of experimental trial and error runs typically required to identify optimal operating conditions in MOCVD reactors.