REFLECTANCE ANISOTROPY FROM (001) GAAS-SURFACES DURING PSEUDO-ALE GROWTH OF GAAS

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-degrees-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.