Surface science studies of MOVPE processes: a review of progress

Abstract

The increasing use of lower total pressures in GaAs MOVPE growth necessarily results in an increased dependency of the properties of the epilayer upon surface chemistry. This paper describes some of the advances in our understanding of the possible surface chemical pathways involved in MOVPE growth that have resulted from surface science studies. It is demonstrated that although such experiments are often carried out under idealised static conditions, they reveal details of the underlying fundamental chemical pathways likely to be operating under real growth conditions. Data are presented which describe the interactions of three species employed in the growth of GaAs-H 2, AsH 3 and (C 2H 5) 3Ga on GaAs (100) substrates. It is demonstrated that at H 2 pressures of 0.5 mbar and temperatures as low as 700 K, clean GaAs (100) surfaces may be generated. This treatment depletes As from the topmost layers of the substrate, most likely via AsH 3 formation, however As levels may be restored via the heterogeneous decomposition of AsH 3. The extent of As deposition under these conditions is consistent with a low AsH 3 effective sticking coefficient. TEG is shown to adsorb reactively on an As-stabilised GaAs (100) surface at 300 K yielding a species which has the stoichiometry 1Ga:4C±0.5, with the implication that a C 2 unit is liberated via reactive adsorption at 300 K. The resulting surface species is shown to generate both ethene and ethene in the ratio 2:1 in a sequential decomposition pathway at higher temperatures which leaves little residual carbon. This ratio is compared with that expected for a completely intramolecular decomposition pathway. Coverage estimates made from AES data suggest that TEG saturates at a fractional coverage of 0.3±0.1 monolayers with respect to substrate surface atom density. It is demonstrated that these estimates are in reasonable agreement with the saturation coverages predicted for certain species on the basis of Van der Waals radii. It is further demonstrated that the surface science approach can facilitate development of novel methods capable of operating under real growth conditions. This is illustrated via results for the dynamic adsorption of (C 2H 5) 3Ga on GaAs (100) at 300 K obtained using the technique of optical second harmonic generation. Analysis of these data using a simple model generates a value for the saturation coverage of TEG at 300 K which is in good agreement with that estimated from the AES data and also suggest that the system adopts non-Langmuirian adsorption kinetics together with dissociative molecule-surface interactions. By analogy with metal/adsorbate systems, the sticking coefficient for TEG on GaAs (100) at 300 K would appear to increase with increasing exposure. It is suggested that although the fit of the experimental SHG data to a Langmuir isotherm is quite poor, the availability of As surface sites plays a major role in the adsorption kinetics. The surface science/SHG data are compared to results obtained elsewhere and the role of this fundamental surface chemistry is discussed in the light of commonly accepted MOVPE technology.