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 bea m 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 evo lution of the surface, the RHEED intensity is computed based on kinematical theory of electron diffraction with an As-As interplanar distance of 2.48 Angstrom, for the physisorbed As layer and a (100) Ga-As crystalline interp lanar distance of 1.41 Angstrom. The model results show ROs at low and high temperatures but not in the intermediate range of 300-450 degrees 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 depen ds upon that of the underlying crystalline surface. Thus a temporal variati on of the step density of the crystalline GaAs surface results in step dens ity variation of the PA layer which, in turn, yields ROs. Since the height of the PA layer is uniformly 2.48 Angstrom in this case, the RHEED beam see s a step height; equal to the GaAs interplanar distance of 1.41 Angstrom, a nd the specular intensity of the RHEED beam will respond to the temporal va riations in the underlying GaAs surface, yielding ROs if the growth is laye r-by-layer. At high temperatures the crystalline GaAs is exposed to the RHE ED beam due to evaporation of PA layer and the ROs appear due to periodic s tep density oscillations with a step height of 1.41 Angstrom which is the G a-As crystalline interplanar distance. At intermediate temperatures, the pa rtial coverage of the surface by the PA layer and crystalline GaAs, coupled with very different interplanar distances in these layers, results in a co mplete destructive interference of the RHEED intensity. The RO dependence o n the As BEP is also presented and discussed.