SOME INSIGHT INTO THE NATURE OF THE SURFACE CHEMICAL PROCESSES INVOLVED IN THE MOVPE GROWTH OF GaAs FROM ARSINE AND TRIMETHYL- OR TRIETHYL-GALLIUM

Abstract

A now established method of studying reaction pathways in GaAs growth is via the use of surface science and related techniques. This paper sets out to review progress made to date via the use of such techniques in terms of the two primary aims of any mechanistic investigation, the identification of reaction intermediates and the measurement of kinetic processes. A comparison is made between trimethylgallium (TMGa) and triethylgallium (TEGa) in terms of their behaviour upon adsorption at GaAs (100) surfaces. Both metallorganics adsorb readily at 300 K in a manner which depends only slightly upon the As-coverage of the substrate surface, indicating that there can be only small barriers to adsorption although clear differences in the subsequent chemistry are observed for As-rich and Garich GaAs (100) surfaces, with more rapid uptake at the As-rich surface and the spontaneous loss of one alkyl unit (i.e. C1 for TMGa and C2 for TEGa). For TMGa, some evidence of multilayer formation at 300 K is presented. Triethylgallium adsorption and thermal decomposition on GaAs (100) surfaces under high vacuum conditions results in clean, intramolecular surface chemical pathways involving intermediates in which the ethyl groups remain intact. For trimethylgallium, the evidence suggests that adsorption leaves the methyl groups intact yet subsequent thermal decomposition does not occur cleanly in the absence of a source of surface hydrogen, although methyl radical recombination is clearly identified as a possible alternative route to the loss of surface carbon. Reflection IR spectroscopy has been used for the first time to study TMGa adsorption on GaAs (100) at 300 K and has produced vibrational data that are shown to agree with EELS data for the adsorption of TMGa on Si surfaces, also implying that the methyl groups remain intact. As a source of surface hydrogen, arsine adsorption and reaction is shown to lead to a surface sub-hydride phase represented as AsHn, which is shown to be consistent with the concept of stabilisation of GaAs (100) surfaces at high temperatures via an arsine (and not arsenic) overpressure. Finally, recent data obtained using the non-linear optical surface diagnostic technique of second harmonic generation are reported for the interaction of trimethylgallium with GaAs (100) surfaces at 300 K, which are discussed in the light of the available surface science data.