Surface reaction mechanisms which underly the growth of III-V semiconductors by chemical beam epitaxy have been investigated using a combination of surface spectroscopic techniques in conjunction with modulated molecular beam scattering techniques. Emphasis is placed on understanding the complex growth rate effects observed during the growth of Ga(Al,In)As and the origin of selected area epitaxy. These effects are shown to arise from the surface sensitive nature of the decomposition of the group III alkyl source chemicals used in CBE.

We report studies of the kinetics of thermal decomposition of triethylgallium (TEGa), trimethylgallium (TMGa), and trimethylindium (TMIn) adsorbed on GaAs(100) in ultrahigh vacuum. The adsorbed layers were prepared by dosing GaAs(100) at room temperature, to either saturated coverage or coverages below saturation. Subsequent heating leads to loss of adsorbed hydrocarbons. The relative coverage of carbon was monitored by X-ray photoelectron spectroscopy (XPS), and products were detected with a differentially pumped quadrupole mass spectrometer. The kinetic analysis also includes measurements of laser-induced, rapid thermal decomposition (heating rates of ∼1011°C/s).

TEGa dissociatively chemisorbs on GaAs(100). Heating the substrate results in desorption of diethylgallium radicals at low temperature and C2H4 (and some C2H5) at higher temperatures, after most of the diethylgallium has desorbed. TMGa decomposes to yield a Ga-alkyl desorption product (either dimethylgallium, or a mixture of dimethylgallium and TMGa) at low temperature and CH3 at higher temperature. TMIn undergoes a methyl exchange reaction on GaAs(100) where a Ga-alkyl desorbs with the same cracking pattern as in TMGa decomposition. Decomposition mechanisms for these group-III metal alkyls are proposed, Arrhenius parameters are presented, and some implications are discussed for growth of Ga-containing III-V compound semiconductor films from these precursors by chemical vapor deposition and molecular beam techniques.

Doping effects on semiconductor etching rates have been proposed to be associated with field effects in the near-surface region. Detailed investigations of the chemistry of nand p-type Si and GaAs indicate that the majority carrier can also play an important role in determining the reactivity of surface intermediates, providing an independent mechanism for influencing the etch rate. A microscopic picture of central driving forces in semiconductor etching deduced from the doping cffects is proposed.

Molecular chlorine is scattered from a GaAs(100) surface and the reaction products detected by a mass spectrometer. A careful analysis of the velocity and angular distributions of the reaction products as well as the vibrational energy dependence of the daughter ion fragmentation pattern shows that the steady state reaction is stoichiometric in the removal of Ga and As until the incongruent evaporation rate exceeds the etch rate induced by the incident chlorine flux. The depletion of the nonreactive molecular chlorine signal is accounted for by the flux of chlorinated reaction products. The absolute etch rates as well as the incident chlorine flux are determined from the known evaporation rate of GaAs at the incongruent evaporation temperature. Modulated molecular beam scattering is used to establish some necessary reaction steps.

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.

Optical in situ methods are becoming of increasing importance for studies of the mechanisms of growth and for real-time control of surface processes involved in epitaxial growth. We present data from studies, using the reflectance-difference (RD) method, of different processes related to epitaxial growth of GaAs (001) from arsine (AsH3) and from triethylgallium (TEG). RD studies make it possible to follow how the near-equilibrium, Ga-rich, configuration of the GaAs (001) surface changes with temperature and we show how the transformation to this state can be followed from As-saturated as well as from Ga-saturated surfaces. From the initial slope of the RD-transient, corresponding to the AsH3 reaction with a Ga-stabilized surface, the temperature dependence of the catalytic effect of this surface on the decomposition of arsine is measured. Of special significance for an atomic layer epitaxial process (ALE) is the demonstration of a self-limiting reaction of TEG. This process can be followed as the surface modifies itself from a state obtained by exposure to excess TEG to the near-equilibrium Ga-terminated reconstruction. Finally, we demonstrate how pulsing sequences for the realization of ALE can be followed and controlled in real time using RD as a diagnostic tool.

Low pressure (∼10−5 Torr) studies of the adsorption and decomposition of tertiarybutyl arsine (t-BAs) on three different GaAs(100) surfaces (Ga-rich, H-atom treated and high temperature t-BAs treated) are presented. Mass spectrometric studies indicate that the primary gas-phase products of t-BAs decomposition on GaAs(100) are the tertiary-butyl radical and incomplete arsenic hydrides (predominantly AsH2). Infrared spectroscopic studies show that on H-atom treated and high-temperature t-BAs treated surfaces, t-BAs decomposes leaving behind very small quantities of alkyl groups. On Ga-rich surfaces however, significant amounts of alkyl products are observed. The surface species resulting from the adsorption of trimethyl gallium (TMG) are shown to be different at room temperature and high temperature.

Oxide layers that are thick (>200 Å and uniform have been produced on GaAs (110) and (100) by reacting the substrate (Ts<160°C) with high translational energy (1-3 eV) neutral atomic oxygen at flux levels of∼50 monolayers/second. The Ga and As species are formed in their highest oxidation states, which implies formation of either Ga2O3 and As2O5 or GaAsO4. Raman spectroscopy indicates that there is no metallic (amorphous or crystalline) As in the oxide or at the interface between the oxide and substrate and that there is no appreciable oxidation induced disorder of the substrate as is seen in high temperature thermal oxidation processes.

Passivation of AlxGa1−xAs surfaces by ammonium sulfide treatment has been investigated. Enhancement of photoluminescence intensity and dramatic reduction of oxide peaks in spectra acquired by X-ray photoelectron spectroscopy on AlxGa1−x.As surfaces prepared by this treatment lead to the reduction of surface state density. It was found that sulfur atoms were mainly bonded to Al atoms in an Al-enriched specimen of AlxGa1−xAs(x=0.78). The surface compositions of Al-enriched specimen(x=0.43, 0.78) were also found to be As-rich as a result of both sulfur treatment and the preferential etching of metallic atoms by an acidic solution, which prevents the initial oxidation of the surface of A1xGa1−xAs.

Compounds of the type (L2MEL'2)x, where M and E are the group III and V elements, respectively, and L and L' are ligands which may be thermally eliminated are being studied as single source precursors to III-V semiconductors. An array of these compounds have been synthesized with various III-V combinations, and hydride and alkyl ligands. Film growth was studied over the temperature range of 450-600 °C and at 10−4 Torr. Growth rates of 1 μm/hr are typical when the compounds are maintained at 125-140 °C. (Me2Ga(μ-t-Bu2As) [2 led to films which were polygrained with the grains oriented in the (111) direction and retained the 1:1 stoichiometry of the precursor. Films from [Me2Ga(μ-i-Pr2As)]3 did not retain the 1:1 stoichiometry and did not show the preference for (111)- oriented growth.

Cyclo-trigallazane, [H2GaNH2]3, is known to form bulk powders of the new cubic phase of gallium nitride upon pyrolysis. An explanation for this unusual example where the molecular structure of the precursor controls the crystal structure of the solid state product is presented. In a hot-wall atmospheric pressure chemical vapor deposition (CVD) reactor, arsine was found to react with TMAG to form films of polycrystalline GaAs which were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The growth rates for smooth films was 1-4 μm/h. In a low pressure CVD reactor, elemental arsenic vapor was also found to react with the TMAG to give GaAs thin films.

The atmospheric-pressure chemical vapor deposition of gallium nitride films from hexakis(dimethylamido)digallium, Ga2(NMe2)6, and ammonia precursors at 200 °C with growth rates up to 1000 Å/min is described. The films were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Rutherford backscattering analysis showed that the N/Ga ratio was 1.12–1.17. The films were crystalline with ≃ 5–15 nm crystallites.

GaN thin films have been deposited on Si and sapphire substrates by metalorganic chemical vapor deposition (MOCVD) using diethylgalliumazide and ammonia. Films were grown in the temperature range of 500-750°C. Growth rates were monitored in situ using laser interferometry. The addition of ammonia enhanced the growth rate significantly. At high temperatures, gas-phase depletion of the precursor reduced the growth rate of GaN. Films grown on (0001)-oriented sapphire substrates at temperatures above 650°C were highly textured with smooth surface morphology. Optical and electrical properties of the films are discussed and compared to those of films grown using conventional Ga and N sources.

Chemical vapor deposition experiments using (Me3Si)3As with either GaCl3 or Me3Ga at ambient pressure have produced films of GaAs on Si and semi-conducting GaAs substrates. The films have been characterized by X-ray diffraction and Auger electron spectroscopy, and each have small amounts of C and O impurities. No desired films were deposited from (C6F5)3GaAs(SiMe3)3 at 500°C and low pressures.

We have investigated the use of tertiarybutylarsine (TBAs) to selectively grow GaAs and AlxGa1−x As in trenches on partially-masked GaAs substrates. Both SiyNz and Si02 masks have been used, with geometries ranging from 4 - 160 μm. By varying temperature and pressure, we have optimized selectivity and minimized “ridge” growth near masked areas. The results show TBAs may be a suitable alternative to arsine for selective regrowth of these materials.

We have investigated the use of tertiarybutylarsine (tBAs) as an As source in the surface-selective growth of GaAs and AlGaAs by low-pressure OMVPE. Deposition was carried out onto GaAs substrates partly masked with SiNx. Triethylgallium (TEGa) and trimethylaluminum (TMAl) were used as precursors along with either arsine or tBAs. Growth pressures were in the range 0.3 to 3.0 Torr. The usual trends of decreasing deposition on the mask with increasing temperature, decreasing group V/III ratio, and decreasing pressure were observed. At very low group V/III ratios, for either As precursor, single crystal material with a pitted surface was obtained. A reduction in the nucleation of polycrystalline material on the mask was observed when tBAs was used in place of AsH3.

Using tertiarybutylarsine and trimethylgallium a GaAs MESFET structure was grown and fabricated into half and quarter micron devices. The typical transconductance and unity current gain frequency (ft) for the half micron device were 360 mS/mm and 24 GHZ respectvely. The corresponding numbers for the quarter micron devices were 510 mS/mm and 55 GHZ respectively.

The decompositions of tertiary stibines (R3Sb, R = methyl, vinyl, isopropyl) were studied in an atmospheric pressure flow tube reactor, using D2 and He as carrier gases. D2 was used to isotopically label the byproducts in order to elucidate the pyrolysis mechanism. The exhaust products were analyzed by a time-of-flight mass spectrometer. The decomposition of these tertiary stibines in the presence of group Ill precursors was studied in order to simulate the conditions of organometallic vapor phase epitaxial growth. A comparison between the pyrolysis temperatures, decomposition mechanisms, and surface area effects of these Sb source compounds is presented.

A CVD reactor has been coupled to a molecular beam apparatus in order to study the gas phase decomposition of an organometallic precursor to AIN, tris-dimethylaluminum amide, [(CH3)2AINH2]3. The onset of decomposition occurs at a reactor temperature of 125°C. By 300°C, all mass spectral signals due to precursor have disappeared. With the addition of helium as a carrier gas in the CVD process, the temperature at which all precursor signals disappear is raised to 400°C. The evolution of methane accompanies the precursor decomposition. Mass spectra of the precursor and its deuterated analogue, [(CH3)2 AIND2]3, obtained between 50°C and 90°C, offer support for the existence of trimer-dimer-monomer equilibria in this temperature range.

The low pressure chemical vapor deposition (LPCVD) of silicon carbide from (CH3)HSiCH2SiCH2 (CH3)CH2 SiH2 (CH 3) on Si(100) has been investigated between 700 and 1100°C at ca. 1.0 torr total pressure using a flow of argon as a carrier gas in a cold-wall LPCVD system. The gaseous byproducts were determined using quadrupole mass spectrometry (QMS) and gas chromatography - Fourier transform IR (FTIR) spectroscopy. The coating surface morphology varied from smooth to a columnar structure with increasing substrate temperature. Film composition and crystallinity were monitored as a function of deposition temperature using Auger electron spectroscopy (AES) and powder XRD.