Arsenic desorption processes from GaAs surfaces with various crystallographic orientations were studied in the MOVPE reactor under H
2 atmosphere (13.3 kPa) by the surface photo-absorption (SPA) method. We found two desorption processes (fast and slow processes) on the GaAs (001) surface. The real part of the refractive index of the surface layer which is removed during the fast As desorption process, is different from that of the GaAs substrate. Both the real and the imaginary parts of the refractive index of the layer which is removed during the slow As desorption process, are different from those of the GaAs substrate. We observed single As desorption processes on the GaAs (111) A and (111) B surfaces. We also studied SPA signals while AsH
3 and TMG were being alternately supplied to those surfaces. We observed the ALE process on the (001) surface around 480°C. Growth rates were less than one monolayer per cycle on (111) A within the entire temperature range of our experiments (from 420°C to 550°C).
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