THERMODYNAMIC ANALYSIS OF GAAS GROWTH BY MOLECULAR-BEAM EPITAXY AT THE SURFACE-STRUCTURE TRANSITION FROM 3X1 TO 4X2

The molecular beam epitaxy (MBE) growth of GaAs layers on a single crystal is studied in relation with the stability domain of the GaAs compound in the Ga-As binary system. The growth parameters, i.e. the Ga and As impinging atomic flows, are compared to the necessary flows as calculated by thermodynamics. In order to take into account in the real growth situation, which is not strictly at equilibrium, the flow balance at the surface of the crystal between the impinging flows and the growth and evaporated flows is written for quat equilibrium growth conditions, including condensation and evaporation coefficients that split the ''so-called'' sticking coefficient in parts related to the condensation or the evaporation phenomenon for each gaseous species. A comparison between the quasi-equilibrium simulation of the growth and the experiment is made with the assumption that the surface structure transition from gallium-stabilized to arsenic-stabilized surface corresponds to the growth of a GaAs crystal at its solidus boundary rich in gallium. The surface structure transitions are observed by reflection high energy electron diffraction (RHEED) and the impinging atomic flows are carefully calibrated and also controlled by RHEED oscillations as observed after gallium or arsenic excess as deposited on the surface. The results show that the growth is effectively performed close to equilibrium conditions as evidenced by the values of the condensation and evaporation coefficients. The evaporation coefficient of gallium is 0.4, showing that this component is the supersaturated one at the surface, and this value agrees with theoretical predictions for an evaporation process (during growth) controlled by the surface diffusion process of monoatomic species between steps.