R.D. Hoare

Gas phase monitoring of reactions under InP MOVPE growth conditions for the decomposition of tertiarybutyl phosphine and related precursors

Abstract Ex-situ Fourier transform infrared spectroscopy has been used to study the decomposition of tertiarybutylphosphine (TBP), trimethylindium (TMIn) and mixtures of TBP and TMIn under MOCVD conditions using dihydrogen as a carrier gas. IR bands due to TBP, TMIn, phosphine, isobutene, isobutane, ethene and methane have been monitored as a function of susceptor temperature. The decomposition of TBP alone in dihydrogen is observed to commence at temperatures above 773 K and is accompanied by the formation of isobutene, phosphine and isobutane. The pyrolysis of TBP is observed to be complete at temperatures in excess of 973 K. For TMIn in dihydrogen, the only observable product at temperatures greater than 573 K is methane. For TBP in the presence of TMIn a room temperature reaction is observed, the only detachable product of which is methane. The implication is that TMIn reacts in some way with TBP, possibly forming an “adduct” or polymer; however, decomposition products from TBP are not observed until temperatures are in excess of 573 K, while decomposition is observed to be complete at temperatures of 873 K. Once again isobutene, isobutane and phospine formation accompanies the TBP decomposition at 573K. At temperatures in excess of 900 K, both methane and ethene were observed during both of these experiments and are assumed to arise via the decomposition of isobutene. The deposited product on the reactor wall was found to be InP and phosphorus (rhombohedral) by X-ray diffraction. Some mechanistic steps for these reactions are proposed. In addition, preliminary data for the decomposition of cyclohexylphosphine are presented.

Optical second harmonic generation studies of the nature of the GaAs(100) surface in air

Abstract We present in this paper a study of the non-linear optical response of the GaAs/air interface as measured by second harmonic generation. By careful choice of incidence and analysis electric vectors for the radiation the second harmonic response as a function of crystallographic orientation has been characterized for a GaAs(100) surface. The exact origin of the response remains unclear but comments on the surface specificity of the technique are made and the observed anisotropy in the data is explained in terms of the step density of the crystal surface.

A RHEED and reflectance anisotropy study of the MBE growth of GaAs, AlAs and InAs on GaAs(001)

Abstract MBE growth of GaAs, AlAs and InAs on GaAs(001) has been monitored in real time, using simultaneous reflection high-energy electron diffraction (RHEED) and reflection anisotropy (RA) measurements, the RA measurements all being made with a fixed wavelength laser source operating at 647 nm. For the growth of GaAs and AlAs on GaAs(100), RA oscillations which occurred during the bilayer growth are explained in terms of the variation in the anisotropic distribution of surface “dimer” bonds during the growth process. RA data obtained during the growth of InAs/GaAs heterostructures are presented and discussed in relation to the influence of surface roughening upon both the RA and RHEED measurements.

The Pyrolysis of Precursors for Gaas Mocvd Studied by In-situ and Ex-situ Fourier Transform Infared Spectroscopy

Abstract The decomposition of arsine at atmospheric pressure as studied by in-situ and ex-situ Fourier transform infrared spectroscopy (FTIR) is presented, comparing the techniques. Ex-situ spectroscopic and spectrometric evidence for the formation of methylarsine and dimethylarsine during the reaction of trimethylgallium and arsine, and in-situ spectroscopic evidence for the formation of diethylarsine during the reaction of triethylgallium and arsine is discussed. The observation of alkylarsines strongly implies a stepwise free radical reaction mechanism, however whether these processes are heterogeneous or homogeneous remains uncler.

Do gas phase adducts form during metalorganic vapour phase epitaxial growth of gallium arsenide

The new primary arsine hex-5-enylarsine (hexAsH 2 ) has been synthesized and decomposed in the liquid and gas phases in the presence and absence of trimethylgallium. From the gas phase decomposition by itself in vacuo the main product is hex-1-ene, although smaller amounts of cyclic C 6 products and of conjugated hexadienes are observed. Under growth conditions (1 atm of H 2 > 350°C) the hex-1-ene formed fragments completely because of reactions with intact arsine or other decomposition products. In the liquid phase in the presence of Me 3 Ga, hexAsH 2 first gives the adduct [Me 3 Ga . AshexH 2 ] but this sequentially loses two moles of methane before giving GaAs, methylenecyclopentane and methane, showing that methylenecyclopentane will necessarily be a product if adduct formation occurs. In the gas phase in vacuo the major Ca product from Me 3 Ga and hexAsH 2 is hex-1-ene although there is a small increase in the proportion of methylenecyclopentane and cyclohexene, suggesting that two pathways, one involving adducts (minor) and the other not, operate. In hydrogen, the major Ca products are methylenecyclopentane and cyclohexane confirming that adducts can be important, but, since hex-1-ene fragments under these conditions it is not possible to quantify the extent of adduct formation. These results are analysed in terms of the calculated equilibrium constant for adduct formation together with the residence time in the cold zone of the reactor and the probability of bimolecular reactions occurring. It is concluded that comparative studies under a variety of different conditions are essential for a full understanding of the processes occurring during growth

Optical monitoring of deposition and decomposition processes in MOCVD and MBE using reflectance anisotropy

Abstract A series of experiments have been performed in order to assess the ability of reflection anisotropy (RA) to follow the MBE growth of GaAs and AlAs on (001) GaAs, combined RA/RHEED measurements reveal a layer-by-layer growth mechanism as indicated by periodic RA oscillations. For the growth of the lattice mismatched GaAs/InAs heterostructure large RA transients are observed which are found to reach their maximum during the growth of the first monolayer and before the onset of 3D growth. RA studies have also been carried out during the MOCVD decomposition of triethylgallium (TEGa) on (001) GaAs at several temperatures ranging from 300 to 425°C. Results obtained suggest that TEGa begins to fully decompose at 350°C which is found to correlate with the formation of TEGa gas phase decomposition products. Results are also presented which may indicate a change in surface reconstruction under MOCVD conditions on moving from a Ga-rich to an As-rich surface state.

Optical second harmonic generation studies of the nature of the oxide-covered and clean c(4×4) and (2×4) reconstructed GaAs(001) surfaces

We report the application of the “epioptic” technique of optical second harmonic generation (SHG) to the study of surface chemical processes occurring at the technologically important (001) surfaces of the non-centrosymmetric compound semiconductor GaAs. SHG has been used to monitor the thermal removal of surface contamination from an GaAs(001) substrate mounted in a conventional molecular beam epitaxy (MBE) chamber. Rotational anisotropy plots of the reflected SH signal recorded at near-normal incidence indicate that the oxide layer is not fully disordered and possesses some structural anisotropy, i.e. some preferred net surface orientation. In addition the rotational anisotropy plots were observed to change in both intensity and apparent phase between the oxide-covered surface and the clean reconstructed surfaces. Following heating to 850 K under the background As2 flux present in the chamber, the SH signal from the clean, reconstructed surface was approximately 5 × weaker than that observed for the contaminated surface and was subsequently found to be insensitive to the reconstruction change between the (2 × 4) and c(4 × 4) surfaces, in agreement with theoretical predictions for surfaces having 2mm and 4mm symmetry, respectively. These data are discussed in terms of the difficulties encountered when attempting to observe a surface SH response from a non-centrosymmetric substrate. The sensitivity of the technique towards the thin oxide overlayer suggests that for overlayers which are likely to possess net oriented in-plane dipoles due to the structural mismatch with the substrate, an SH response may be observed in preference to a bulk response when a normal incidence geometry is employed.

Probing surface chemical processes during epitaxial semiconductor crystal growth at near-atmospheric pressures using photon-based techniques

It is demonstrated here that for the study of epitaxial semiconductor crystal growth from vapour-phase precursors, photon-based methods may be applied under true ‘in situ’ conditions and can, where appropriate, reveal a great deal regarding the nature of the growing surface. While several techniques exist which may probe the gaseous phase, the study of the surface phase under conditions of high pressures (up to 1000 mbar) and high temperatures (up to 1000 K) is far from trivial. This paper sets out to emphasise recent developments in technique which permit the study of the surface phase during epitaxial crystal growth. Data are presented for several semiconductor/substrate systems obtained using the linear technique of reflectance anisotropy and the non-linear technique of optical second-harmonic generation (SHG). In particular, the technique of reflectance anisotropy is shown to be a very powerful probe of surface stoichiometry, real-time monitoring of layer-by-layer growth processes, adsorption and desorption kinetics and also the detection of surface reconstructions that occur under high vacuum and near-atmospheric pressure conditions. For the SHG technique it is shown that the application of this method to centroasymmetric substrates such as GaAs is not trivial, yet under certain conditions surface SHG responses can be observed which are directly related to the structure of the surface layers. While the data presented are for special epitaxial semiconductor systems, throughout the paper emphasis is placed upon the applicability of the experimental approach towards the study of crystal growth systems in general.

Reflectance anisotropy from (001) GaAs surfaces during pseudo-ALE growth of GaAs

Abstract We demonstrate that the technique of reflectance anisotropy (RA) has now evolved to the level of sophistication required to be of use in direct MOVPE process control. Comparison with data obtained under high vacuum GaAs MBE growth and vacuum chemical epitaxy growth conditions permits the interpretation of RA changes that occur under higher pressure growth conditions. Inflections in RA responses can indicate changes in surface reconstruction that occur again both under high vacuum conditions and at higher pressures. Under pseudo-ALE growth of GaAs on (001) GaAs from triethylgallium (TEGa) and arsine in a conventional hot-wall MOVPE reactor maintained at a constant pressure of 200 mbar, the onset of Ga incorporation via TEGa decomposition has been monitored as a function of substrate temperature. For this reaction, where the rate-determining step may occur either homogeneously in the gas phase or heterogeneously on the substrate surface, changes in RA intensity have been observed at temperatures as low as 350°C. Under these conditions, while the surface changes resulting from TEGa decomposition occur quite slowly, the subsequent surface recovery upon exposure to arsine gas in the second half of the growth cycle occurs very rapidly, the data suggesting that the surface initially relaxes beyond the initial As-stabilised surface towards a very As-rich surface in response to an oversaturation of the surface with Ga atoms during the first half of the growth cycle. This surface then slowly loses As to return to the stable initial condition. In order to observe the arsine recovery kinetics in more detail, the flow rates were reduced. Under these conditions, while rapid RA variations were again observed, inflections in the RA responses became increasingly prominent, which are discussed in terms of possible surface reconstructions that may occur during recovery back to the initial surface condition.

The use of hex-5-enylarsine as a chemically designed precursor to probe the mechanisms of the metalorganic vapour phase epitaxy growth of gallium arsenide; consequences for reactor design

Abstract Ex-situ Fourier transform infrared spectroscopy has been employed to study the mechanisms of the metalorganic vapour phase epitaxy growth of GaAs, using hex-5-enylarsine and trimethylgallium as precursors. Hex-5-enylarsine was synthesized for the purpose of distinguishing between reductive elimination, free radical and β-hydrogen elimination reactions since all three pathways are theoretically available for its decomposition. However, the reaction under MOVPE conditions is not as simple as envisaged in that fragmentation of the hex-5-enyl alkene chain competes effectively with C-As bond cleavage as a decomposition pathway. Thus for hex-5-enylarsine in dihydrogen decomposition is observed to commence at temperatures of ca. 500°C yielding as yet unassigned alkylarsines, methane and some ethene as products. At higher temperatures ethane, propane are formed while at very high temperatures (> 700°C) there is evidence for the formation of ethyne and 1,3-butadiene. In the presence of trimethylgallium (TMGa), an involatile solid forms at room temperature via the elimination of methane, strongly supporting the proposal that gas-phase adduct formation occurs as the primary process. At higher temperatures, ca. 350°C, this solid begins to decompose yielding the alkylarsines noted above. In addition, methylarsine and dimethylarsine are formed, together with methylene cyclopentane and cyclohexane. The appearance of these latter products appears to correlate with the onset of decomposition of TMGa and is attributed to a mechanism involving the formation of a radical intermediate which may then undergo 1,5 or 1,6 cyclization.