Optical studies of the nature of the GaAs (100) surface during MOVPE growth of GaAs

This paper describes the application of two surface sensitive optical techniques to the study of III-V growth under MOVPE conditions. The first technique, optical second harmonic generation, is shown to be well suited to in-situ MOVPE studies of III-V growth, despite the fact that typical substrates should give rise to intense bulk SHG signals. Model SHG studies of GaAs (100) are described which allow the proposal of a suitable experimental geometry for the application of this technique to a substrate in an MOVPE reactor. Reflectance anisotropy measurements have been made from a GaAs (100) surface in an MOVPE reactor during a series of pump-purge cycles commonly used to remove volatile contaminants from the reactor prior to MOVPE growth. The RA response observed from the substrate at 300 K varies in intensity with both the number of pump-purge cycles and the extent of evacuation reached in each cycle, suggesting that a change in surface structure or composition may be induced via evacuation in an increasingly cleaner reactor environment. RA data recorded during a pre- growth substrate bake in arsine clearly show the onset of the formation of an As-stabilized surface at temperatures around 670 K, where the decomposition of arsine becomes appreciable. Temperature-programmed desorption from the resulting As-stabilized surface reveals an initial loss of As from the surface commencing at low temperatures (470 K) followed by recovery of the As-stabilization at higher temperatures (670 K) due to arsine decomposition. These data are compared with data for As-desorption from GaAs (100) recorded during MBE growth and it is shown that inflection in the RA traces could relate to surface reconstruction changes occurring under these high overpressure conditions.