Decomposition Of Trimethylgallium Research Articles

In‐Situ mechanistic studies of the reaction of trimethylgallium and phosphine in zeolite H‐Y

AbstractCoherent anti‐Stokes Raman scattering (CARS) has been utilised to monitor the decomposition of trimethylgallium (TMGa) on the reactive internal surface of the protonated form of zeolite Y under metal organic chemical vapour deposition (MOCVD) conditions designed to produce GaP deposits that exhibit quantum‐size effects. The CARS data clearly reveal the facile room temperature decomposition of TMGa in the zeolite, as evidenced by the detection of increasing levels of methane with increasing reaction time. In addition, the zeolite samples were analysed by solid state nuclear magnetic resonance with magic angle spinning (MAS NMR) before reaction, after reaction of TMGa and after subsequent reaction with phosphine at 300°C. These data reveal that TMGa reacts with acid sites in the zeolite 12‐rings, resulting in species containing methyl groups. Subsequent exposure to phosphine removes these methyl group protons, leaving various, as yet not fully characterised, phosphorus species. Exposure to phosphine does not appear to regenerate the acid sites on the zeolite.

Surface photo-absorption study of the laser-assisted atomic layer epitaxial growth process of GaAs

Thermal and laser-assisted decomposition of trimethylgallium (TMG) and triethylgallium (TEG) on GaAs surfaces under atomic layer epitaxy (ALE) processing conditions were monitored in real time using the (SPA) surface photo-absorption reflectivity technique to elucidate the mechanism of self-limiting deposition. When As-terminated (001) GaAs surfaces held between 340 and 390°C were exposed to TMG, the SPA reflectivity decreased to a steady state, indicative of alkylgallium on the surface. Upon laser irradiation, this surface changed to an ‘intermediate’ steady state surface that was found to be in equilibrium with gas-phase alkylgallium species. Alkyl species then desorb from this surface when the TMG flow is stopped, leaving a self-limited deposit of Ga on the surface. No self-limiting deposition of Ga could be detected using TEG in laser-assisted ALE or thermal ALE. In this case, the observation of self-limiting alkylgallum deposition could explain successful ALE growth using TEG.

Growth kinetics and critical temperature measurements in MOMBE growth of GaAs with TMGa by RHEED

The decomposition process of trimethylgallium (TMGa) during the MOMBE growth of GaAs with As 4 has been studied from the behaviour of the growth rate and critical temperature measurements using RHEED intensity oscillations. Our results conclusively demonstrate the importance of surface As sites and the nature of the step edges in determining the decomposition of TMGa, as well as a greater anisotropy in the critical temperature compared to MBE growth using elemental Ga and MOMBE growth using triethylgallium (TEGa).

Tertiaybutyldimethylantimony for GaSb growth

Antimony-containing alloys have the lowest energy bandgaps of the conventional III-V semiconductors. Organometallic vapor phase epitaxy (OMVPE) has proven to be an effective and convenient technique for the production of the antimony alloys, normally using trimethylantimony (TMSb) as the antimony source. However, TMSb has several problems: (a) it decomposes at relatively high temperatures due to the strong CH3-Sb bond, and (b) its use results in carbon contamination from the active methyl radicals, especially for the growth of aluminum-containing alloys. Recently, tertiarybutyldimethylantimony (TBDMSb) has been successfully used to grow high quality InSb epitaxial layers at temperatures as low as 325°C with low V:III ratios. This paper reports the use of TBDMSb to grow GaSb layers at temperatures from 500° to 650°C. Group V:III ratios close to unity are required for good surface morphologies. The optimum V:III ratio decreases slightly as the growth temperature is lowered due to the incomplete decomposition of trimethylgallium (TMGa). The growth efficiency is about 1 × 104 μm/mole, indicating that there are no significant parasitic reactions between TMGa and TBDMSb. The as-grown layers are p-type. The background carrier concentration is nearly independent of growth temperature. Low temperature (10K) photoluminescence (PL) intensities are also nearly independent of temperature in this range. The PL spectra consist of two peaks, a bound exciton peak at about 792 meV and a native acceptor complex (VGa, GaSb) peak at about 775 meV. At every growth temperature, the ratio of the PL intensity of the bound exiton peak to the native acceptor peak is always higher for samples grown using optimum conditions. The results show that TBDMSb can be used for the OMVPE growth of antimony-containing materials over a wide range of growth temperature.

In-Situ generated arsine radicals for gallium arsenide homoepitaxy

Removed from the deposition region, an upstream hydrogen microwave plasma generates arsenic hydrides by etching the surface of solid arsenic. The hydrides are transported to the deposition region and mixed with trimethylgallium to achieve low temperature (350°-400°C) and low pressure (750 mtorr) homoepitaxial GaAs films. Low precursor V:III ratios are used to achieve homoepitaxial films with high levels of carbon dopants (l019 to mid 1020 cm−3). No active or afterglow plasma exists in the growth region. The observed homo epitaxial growth activation energies of 54 kcal/mole and 66 kcal/mole for films deposited with V:III ratios of 1:1 and 1:4, respectively, are in the range of those reported for the heterogeneous decomposition of trimethylgallium in the absence of arsine. The films are found to be of good crystalline quality via double crystal x-ray rocking curves. The majority carriers are holes and have hole concentrations that correlate to the carbon doping, as determined by room temperature Hall effect measurements and secondary ion mass spectroscopy. Carrier mobility versus carbon concentration is also presented.

MOMBE GROWTH OF CARBON DOPED p-TYPE GaAs AND InGaAs

The growth and electrical properties of carbon doped p-type GaAs and InGaAs grown by metal-organic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG), solid arsenic and solid indium as source materials have been studied systematically. The experimental results show that the growth rate and hole concentration of the samples are affected strongly by growth temperature and molecular beam fluxes,especially for InGaAs epilayers. The mechanism of carbon incorporation and its influence on the properties of the samples were also analysed based on the experimental results. It is shown that the changes in growth rate and hole concentration are mainly due to the dependence of TMG decomposition process on the growth condition.

Reactive chemical intermediates in metalorganic chemical vapor deposition of GaAs

Reactive chemical gas-phase intermediate species important in the metalorganic chemical vapor deposition growth process of GaAs have been identified for the first time using in situ ultraviolet absorption spectroscopy. Spectral features from Ga, GaH, and GaCH3 were observed during thermal decomposition of trimethylgallium (TMGa) in hydrogen gas. Evidence for free radical reactions involving CH3 and H in the pyrolysis of TMGa is presented. The addition of arsine to the mixture is found to change the reaction pathways as evident from the production of AsH2 and the disappearance of Ga and GaH. Gas-phase decomposition of the metalorganic species in hydrogen can produce gallium under the conditions of high pressure atomic layer epitaxy.

Stoichiometry- and bond-structure-dependent decomposition of trimethylgallium on As-rich GaAs(100) surfaces

The decomposition of trimethylgallium (TMG) on an As‐rich GaAs(100) surface associated with the surface stoichiometry has been investigated using mass spectrometry and reflection high‐energy electron‐diffraction observations. Surfaces with different stoichiometries are prepared by depositing metal Ga on well‐defined GaAs surfaces. It has been found that the TMG decomposition strongly depends on the surface stoichiometry; even such a low Ga coverage difference as 0.01 monolayer induces a surface decomposition enhancement. The TMG decomposition rate is lowest on a surface that shows the most distinct 2×4 reconstruction, which is the most stable structure. The role of the surface structure in TMG decomposition is also discussed.

Adsorption and decomposition of TMGa on GaAs(100)

The adsorption and decomposition of trimethylgallium (TMGa) on the gallium‐rich GaAs(100) surface has been studied using electron‐energy‐loss spectroscopy (EELS) and Auger electron spectroscopy (AES). It was found that the first layer of TMGa already decomposes at 100 K. Further dosing the surface resulted in condensation of TMGa. At room temperature only chemisorbed TMGa was found, with a saturation coverage of 0.3±0.1 monolayer gallium. We found the adsorbate species to be monomethylgallium and/or dimethylgallium. We also studied the adsorption and decomposition of iodomethane, as well as the adsorption of hydrogen. Coadsorption experiments showed that only TMGa could replace or react with the adsorbed hydrogen atoms. In the case of CH3I almost no coadsorption or reaction occurred.

Surface reaction mechanism in Si and GaAs crystal growth

In the field of applied crystal growth we have investigated the mechanism of crystal growth on semiconductors in each reaction step: gas-phase reaction, adsorption, surface reaction, and sticking to a stable site of the substrate. In this review mainly the investigations and the results of the reactions or the processes occurring on the substrate surface are described. In the case of epitaxial growth of Si, the formation of SiCl 2 was ascertained both by IR-absorption spectroscopy in the gas phase of the atmospheric pressure system and by mass spectroscopy in the surface reaction of molecular layer epitaxy. Migration of the adsorbate is important to form a high-quality crystal of Si. From the observation of pyramidal hillocks and the deformation of tables which were artificially formed on the substrate, surface migration is discussed. In GaAs epitaxy the gas-phase reaction was investigated by means of IR-absorption spectroscopy. An intermediate species which showed an absorption peak at 2080 cm -1 was detected. Molecular layer epitaxy was achieved by alternate injection of trimethyl gallium (TMG) as Ga source and AsH 3 as arsenic source. The formation of monolayer adsorption species closely depends on the surface reaction producing GaCH 3. In another case that multilayer growth occurs, the dominant surface reaction is the decomposition of TMG into Ga.

A Mass Spectrometric Study of the Decomposition of Trimethylgallium (TMGa) with Trimethylarsine (TMAs)

Trimethylgallium (TMGa) pyrolysis with Trimethylarsine (TMAs) is studied. I measure the decomposition rate constant by the sampling gas method with a quadrupole mass spectrometer. The decomposition rate constant of TMGa depends on the TMGa initial partial pressure. However, the rate constant of TMGa with TMAs is almost independent of the TMGa initial partial pressure. I propose a model in which TMAs on the reactor surface inhibits the adsorption and decomposition of TMGa on the surface. The TMAs decomposition rate constant increases until TMGa/TMAs=1. It remains constant from 1 to 10. I hypothesize a model for the decomposition of TMGa with TMAs. TMAs molecules adsorb almost all of the adsorption sites. They inhibit the TMGa from adsorption on the sites. TMGa does not decompose on the surface. On the other hand, TMGa catalyzes TMAs to decompose in the gas phase through TMGa·TMAs adducts.

A high resolution electron energy loss spectroscopy study of trimethylgallium decomposition on gallium rich GaAs(100)

The adsorption and decomposition of trimethylgallium (TMGa) on the gallium rich (4×6) and (1×6) GaAs(100) surfaces have been studied by high resolution electron energy loss spectroscopy (HREELS), in conjunction with temperature-programmed desorption, x-ray photoelectron spectroscopy, and low energy electron diffraction. TMGa adsorbs molecularly at 108 K and dissociates above 200 K to form monomethylgallium (MMGa) and, possibly, some dimethylgallium surface species. Above 400 K, MMGa dominates. Surface MMGa remains stable until 650 K, above which temperature methyl radicals desorb. HREELS results showed unambiguously that methyl groups remain intact throughout the thermal decomposition of TMGa on GaAs(100). The effect of UV photons on both surface TMGa and MMGa has also been investigated, but no photochemistry was observed at wavelengths longer than 230 nm.

Low pressure and low temperature gallium arsenide homoepitaxy employing in-situ generated arsine

Removed from the deposition region, an upstream hydrogen microwave plasma etches a surface of solid arsenic located downstream to generate arsenic hydrides. The latter are used with trimethylgallium (TMGa) to achieve low temperature (400–490° C) and low pressure (750 mTorr) homoepitaxial GaAs films. No active or afterglow plasma exists in the growth region. The homoepitaxial growth activation energy of 62 kcal/mole is consistent with the heterogeneous decomposition of TMGa in the absence of arsine. Precursor V-III ratios as low as 0.25 are used to achieve homoepitaxial films, but with high levels of carbon impurities (1019 to mid 1020 cm−3). Carbon incorporation increases at low V-III ratios (0.25 to 0.5) for increasing temperatures with an activation energy of 23 kcal/mole. As the V-III ratios are increased above 1.0, the carbon incorporation activation energy decreases slightly to 15 kcal/mole.

Selective growth of GaAs wire structures by electron beam induced metalorganic chemical vapor deposition

We succeeded in the fabrication of GaAs wires by an electron beam induced selective growth technique, for the first time. In situ irradiation of the electron beam, with simultaneous supply of tri-methyl-gallium (TMG) and cracked AsH3, formed a GaAs quasiquantum wire structure as narrow as 300 nm selectively. Auger analysis and dependence of the growth on source materials and types of substrate suggest that the selective growth results from the decomposition of TMG by the electron beam irradiation.

SSIMS identification of surface intermediates in the thermal decomposition of TMGa on Si(100)

The surface chemistry of trimethylgallium (TMGa), dosed on Si(100) at 115 K, has been studied using SSIMS, TPD, and AES. The results are compared with CD 3I to clarify the role played by methyl migration from Ga to Si and to identify subsequent decomposition channels. For TMGa, the TPD products are multilayer parent (150 K), hydrogen (800 K), and atomic gallium (925 K). SIMS of TMGa suggests Ga-C bond cleavage and attachment of the methyl group to Si starting as low as 115 K and going to completion at 420 K. When TMGa is coadsorbed with CD 3I, observation of SiCH 3 + and GaCH 3CD 3 + in SSIMS confirms the facile cleavage of Ga-C bonds and the formation of Si-C bonds. In agreement with previous studies, we find that carbon contamination of the silicon substrate is intrinsic to TMGa decomposition.

MOVPE growth of GaAs using metallic arsenic and trimethylgallium

GaAs homoepitaxial layers were grown using trimethylgallium (TMG) and metallic arsenic as a source material for metalorganic vapor phase epitaxy (MOVPE). p-Type GaAs layer was obtained with a growth rate of 3–8nm/s. The full width at half-maximum (FWHM) of double-crystal X-ray diffraction was found to be comparable to that of bulk GaAs (0.015°) when TMG concentration is less than 3.3×10 -4 mol/ m 3. Further increase of TMG concentration resulted in the increase of FWHM. Carrier concentration of (-4)×10 16 cm -3 and Hall mobility of 203–348 cm 2/V.s were obtained at a substrate temperature of 650°C, V/III ratio of 29 and total pressure of 1.3×10 4 Pa. Methyl radical (CH 3), methane-d 0 (CH 4) and methane-d 1 (CH 3D) were found to be formed by mass spectroscopy using D 2 as replacement of H 2. The formation of CH 3D is explained by the reaction of CH 3 with D 2. CH 4 is produced directly by the decomposition of TMG and/or produced by the reaction of CH 3 with CH 3.

Adsorption and decomposition of trimethylgallium on Pt(111)

The adsorption and decomposition of trimethylgallium (TMG) on Pt(111) has been studied by high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and Auger electron spectroscopy (AES). Partial decomposition (Ga-C bond cleavage) occurs during adsorption on Pt(111) at 120 K. The products are identified as CH 3 and Ga(CH 3) 2, both bound to Pt. Upon heating, multilayer TMG desorbs at 155 K and further Ga-C cleavage within the chemisorbed TMG increases the surface methyl group concentration. Carbon-hydrogen bond breaking occurs just below 300 K and supplies H atoms for the hydrogenation of methyl groups. The methane formed desorbs promptly. Hydrogen atoms also recombine to form H 2. Other than the parent, these are the only C- and H-containing species found in desorption. There is some evidence for Ga=C double bonds above 300 K. Gallium desorbs between 1000 and 1100 K, whereas residual carbon is not removed below 1200 K.

The adsorption and decomposition of trimethylgallium and triethylgallium at GaAs(100) surfaces

This paper outlines the results of an investigation into the adsorption of trimethylgallium (THGa) on GaAs(100) surfaces at 300 K and its subsequent thermal chemistry carried out using Auger electron spectroscopy, temperature programmed desorption methods and reflection-absorption IR spectroscopy (RAIRS). The data are compared to previously obtained data for triethylgallium adsorption at these surfaces and discussed in terms of reactions that lead to carbon deposition and the relative reactivities of As-rich and Ga-rich surfaces. Very preliminary RAIR data for TMGa adsorption at the Ga-rich surface reveal that some surface reaction does indeed occur at 300 K but that at least some of the methyl groups appear to remain intact. Thermal decomposition of the species formed by TMGa adsorption is found to be highly sensitive to experimental conditions and generally results in high levels of residual carbon, in marked contrast to the behaviour of TEGa.

Kinetics of thermal decomposition of triethylgallium, trimethylgallium, and trimethylindium adsorbed on GaAs(100)

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. The relative coverage of carbon was monitored by x-ray photoelectron spectroscopy (XPS) as the substrate temperature was slowly increased (0.6–3.2 °C/min). Products were detected at faster heating rates (0.7–6 °C/s) with a differentially pumped quadrupole mass spectrometer. The substrate temperature was measured by infrared laser interferometric thermometry. The kinetic analysis also makes use of XPS and mass spectrometric data on laser-induced, rapid thermal decomposition (heating rates of ∼1011 °C/s ). TEGa dissociatively chemisorbs on GaAs(100) at room temperature. Heating the substrate from room temperature to ∼500 °C results in desorption of a Ga–alkyl at low temperature, ascribed mostly to diethylgallium (DEGa) and possibly some TEGa. At higher temperature, C2H4 and C2H5 desorb in parallel after most of the Ga–alkyl has desorbed. The hydrocarbon desorption is described well by simple first order kinetics with an activation energy, Eact=32±4 kcal/mol, and a pre-exponential A factor of 2.5×1010±1.5 s−1. Ga–alkyl desorption is more complicated; the Arrhenius parameters for assumed first order desorption exhibit strong coverage dependences. A fit to all the data was obtained for A=5×108 s−1 and Eact=18 kcal/mol at saturated coverage, with a large decrease in Eact (or increase in A) with decreasing coverage. TMGa decomposes to yield a Ga–alkyl desorption product (either dimethylgallium, TMGa, or a mixture of the two) at low temperature, and CH3 at higher temperature. CH3 desorption has a first order activation energy of 43±2 kcal/mol for an assumed A factor of 1×1013 s−1. For the Ga–alkyl, A=108 s−1 and Eact=19 kcal/mol, with a coverage dependence similar to DEGa desorption from TEGa decomposition. TMIn undergoes a methyl exchange reaction on GaAs(100). Upon heating above room temperature, a Ga–alkyl desorbs first, followed by desorption of CH3 at higher temperature. The Ga–alkyl has with the same cracking pattern as observed for TMGa decomposition. No In–alkyls desorb, and In desorption does not occur until all carbon-containing species desorb. CH3 starts to desorb at lower temperature than for TMGa decomposition. Assuming an A factor of 1×1013 s−1, CH3 desorption over the observed wide temperature range indicates a range of activation energies from 33–43 kcal/mol. Ga–alkyl desorption is similar to that observed during TMGa decomposition. At saturated coverage, A=108 and Eact=17 kcal/mol. However, the coverage dependence is not as strong as for TMGa, so that Ga–alkyl desorption peaks at lower temperature for TMIn. Decomposition mechanisms for these group-III metal alkyls are discussed, along with implications for growth of III–V compound semiconductor films from these precursors by chemical vapor deposition and molecular beam techniques.

Reactions of trimethylgallium multilayers on silicon (100)

Trimethylgallium (TMGa) is an important source gas for metal-organic chemical vapor deposition (MOCVD) and metal-organic molecular beam epitaxy (MOMBE). In a previous paper Lee et al. (Surf. Sci. 216 (1989) 173) found that at coverages below 2 × 10 14 molecules/cm 2, TMGa decomposes via an intramolecular transfer hydrogen process to yield methane, carbon, gallium, and hydrogen. Here we consider the mechanism of TMGa decomposition on top of this first monolayer of TMGa using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). It is found that at TMGa exposures > 1 × 10 16 molecules/cm 2, TMGa transiently populates a weakly bound state. The TMGa desorbs from this weakly bound state if the sample is held in vacuum for several minutes at 300 K. However, the weakly bound TMGa will react if the sample is flashed immediately after dosing. The weakly bound TMGa decomposes at 400–600 K to yield dimethylgallium (DMGa (abs)). methane, and CH x groups. The DMGa reacts upon further heating to yield monomethylgallium (MMGa (abs)), methane and additional CH x groups. The MMGa decomposes above 700 K, to yield methyl radicals and gallium. Traces of ethylene are also seen. These results verify that carbon deposition is an intrinsic part of TMGa decomposition on Si(100). In contrast, MMGa can decompose cleanly. Hence, MMGa may be a promising intermediate for MOMBE.