Low temperature MBE of GaAs: A theoretical investigation of RHEED oscillations

Surface dynamics dominate the temporal variation of reflection high energy electron diffraction (RHEED) intensity in the low temperature molecular beam epitaxy (MBE) of (100) gallium arsenide (GaAs). A rate equation model is proposed which includes the presence and dynamics of a physisorbed arsenic (PA) layer riding the growth surface. Using the results of the temporal evolution of the surface, the RHEED intensity is computed based on kinematical theory of electron diffraction with an As-As interplanar distance of 2.48 Å for the physisorbed As layer and a (100) Ga-As crystalline interplanar distance of 1.41 Å. The model results show ROs at low and high temperatures but not in the intermediate range of 300–450°C which is in good agreement with experiments. At low temperatures, the surface is covered by the PA layer whose vertical distribution across the layers depends upon that of the underlying crystalline surface. Thus a temporal variation of the step density of the crystalline GaAs surface results in step density variation of the PA layer which, in turn, yields ROs. Since the height of the PA layer is uniformly 2.48Å in this case, the RHEED beam sees a step height equal to the GaAs interplanar distance of 1.41Å, and the specular intensity of the RHEED beam will respond to the temporal variations in the underlying GaAs surface, yielding ROs if the growth is layer-by-layer. At high temperatures the crystalline GaAs is exposed to the RHEED beam due to evaporation of PA layer and the ROs appear due to periodic step density oscillations with a step height of 1.41Å which is the Ga-As crystalline interplanar distance. At intermediate temperatures, the partial coverage of the surface by the PA layer and crystalline GaAs, coupled with very different interplanar distances in these layers, results in a complete destructive interference of the RHEED intensity. The RO dependence on the As BEP is also presented and discussed