Influence of physisorbed arsenic on RHEED intensity oscillations during low-temperature GaAs molecular beam epitaxy

In a recent work, RHEED specular spot intensity oscillations were obtained during low-temperature molecular beam epitaxy (MBE) growth of GaAs when near stoichiometric V/III flux ratios (less than or equal to 2:1) were used. The cause of these oscillations has not been fully explained. In this work, we have developed a stochastic model of growth which correctly describes the RHEED intensity dynamics over a wide range of growth conditions. A critical and novel improvement to the stochastic model is the inclusion of a physisorbed state. The experimental RHEED responses for various growth conditions were matched by taking into account the build-up of a physisorbed As layer and its effect on the specular spot RHEED intensity. The dynamics of the physisorbed As layer were introduced into the stochastic model by including the thermally activated processes of chemisorption and evaporation of the As physisorbed state. Model results indicate that for typical low-T GaAs growth temperatures (200 degrees C), the steady-state coverage of physisorbed As ranged from 0.72 at a V/III flux ratio of 2.1 to 0.24 at a flux ratio of 1:1. Increased scattering of the RHEED beam due to the higher physisorbed As coverage at 2:1 leads to a factor of 5 decrease in the steady-state amplitude of the RHEED oscillations compared to the 1:1 case. This is in excellent agreement with the experimental results of Ibbetson et al. The RHEED intensity oscillates under these conditions due to layer-by-layer growth and a periodic variation in the coverage of physisorbed As, even though the surface migration rates are small. A factor in maintaining this growth mode is that arsenic stays in the physisorbed state with lifetimes in the range of 10(-3) to 10(-5) s and incorporates only when an appropriate configuration of Ga atoms forms on the surface. The temperature dependence of the evaporation and chemisorption time constants of physisorbed As yield activation energies of 0.24 eV acid 0.39 eV, respectively, which are in excellent agreement with the experimental data.