Density-functional theory studies on microscopic processes of gaas growth

Abstract

Results for the elementary processes of MBE growth of GaAs on the frequently used GaAs(001) substrate are reviewed. We propose a bottom-up approach, where a growth model is constructed from the results of density functional theory (DFT) calculations. The implications of such a model can be tested against the information from STM images. First, the stable surface reconstructions are reviewed. Under the most commonly used conditions for MBE growth, the arsenic-rich β2 (2 × 4) reconstruction, which contains As dimers as basic building blocks, is the most stable. Next, the adsorption and diffusion of Ga atoms and As molecules on this surface is described. The DFT calculations support the picture that adsorbed Ga atoms are quite stable against re-evaporation. Thus, their mobility determines the homogeneity of the growing layer. Incorporation of Ga atoms proceeds by splitting the As dimers. We propose a model where growth proceeds in two stages: filling of trenches in the β2 (2 × 4) reconstruction, followed by nucleation of islands on the surface regions where the trenches are filled. We demonstrate how clusters of incorporated Ga atoms act as nuclei for the process of trench filling. Concerning island formation, the role of step formation energies and attachment probabilities of mobile adatoms at steps is discussed. Knowledge of these is crucial for an understanding of island shapes. Ongoing research is aiming at understanding of the microscopic mechanisms giving rise to the transition between the step-flow mode and the island-nucleation mode of growth.