REFLECTION-ABSORPTION IR SPECTROSCOPY AS AN IN-SITU PROBE OF THE SURFACE CHEMISTRY OF SEMICONDUCTOR GROWTH INTERMEDIATES : THE ADSORPTION OF TRIMETHYLGALLIUM AT GaAs (100) SURFACES AT 300 K

FTIR spectroscopy has been used in reflection-absorption mode to monitor the adsorption of trimethylgallium (TMGa) at GaAs (100) surfaces at 300 K. Intense, positive going transmittance bands are observed, which confirm the theoretical prediction that the presence of an adsorbate layer at a semiconducting substrate which itself does not absorb the IR radiation, will result in a net increase in absolute IR reflectivity. Three bands are observed in the CH deformation region, at 1050 cm-l, 1174 cm-1and 1420 cm-1. The latter two bands are shown to be consistent with the formation of a surface species in which the methyl groups are intact, although lack of data in the Ga-C region prevents an unambiguous assignment. These band positions together with their spectral intensities are shown to be consistent with the formation of a physisorbed TMGa species. However, the band at 1050 cm-1is not consistent with this assignment. Some alternative modes of adsorption are discussed in the light of data for TMGa adsorption at Si surfaces. The data presented here are the first external reflection IR data available for possible III-V growth surface intermediates at GaAs (100) surfaces and indicate the potential of the reflection-absorption method for the detection of adsorbed species under real MOVPE and related growth conditions.