Step-density variations and reflection high-energy electron-diffraction intensity oscillations during epitaxial growth on vicinal GaAs(001)

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Abstract
Systematic measurements and simulations are reported for the transition from growth by nucleation and accretion of two-dimensional islands to step advancement on misoriented GaAs(001) surfaces during epitaxial growth. The growth conditions have been chosen in order to satisfy as much as possible the underlying assumptions of a solid-on-solid model of epitaxial growth, namely, that the adatom mobility is isotropic (by using a surface misoriented toward the [010] direction), the effect of the As is not rate determining (by using a As/Ga ratio of 2.5), and that the presence of any surface reconstruction can be subsumed in effective migration parameters, i.e., that no explicit account of surface reconstruction is required if the reconstruction does not change (by maintaining the 2×4 reconstruction). The diffraction conditions for reflection high-energy electron-diffraction (RHEED) measurements were chosen to eliminate as much as possible the contribution of well-known incoherent features to the specular intensity on GaAs(001). Using these growth and diffraction conditions, the parameters of a solid-on-solid model have been optimized by performing extensive simulations to quantitatively reproduce the measured misorientation-angle dependence and the Ga flux dependence of the growth-mode transition temperature. Since at the chosen diffraction conditions, the kinematic diffraction is insensitive to surface morphologies, we have modeled the growth-induced loss of intensity from the specular beam as being due to the steps on the surface, with each step acting as an individual source of scattering. Direct comparisons between the time-dependent density of surface steps of the simulated surfaces and the RHEED specular intensity profiles measured during growth reveal several qualitative and quantitative similarities. The most striking of these is that the two quantities show the same relative change of magnitude with time and temperature for a given misorientation and Ga flux. The implications of these comparisons are discussed for the growth dynamics of GaAs(001), for the scattering processes in RHEED, and the morphological sensitivity of RHEED for these diffraction conditions.