Stability of surface reconstructions on hexagonal GaN grown by molecular beam epitaxy

Control of the reversible transformation between the (2 x 2) surface reconstruction and the unreconstructed (1 x 1) surface of hexagonal-phase GaN during growth by molecular beam epitaxy is examined with respect to substrate temperature, gallium flux, nitrogen flow rate, nitrogen plasma fell power and system pressure. The transition is considered to occur at a particular surface stoichiometry which depends on the net arrival rate of reactant species at the growth surface. The (2 x 2) surface reconstruction as observed by RHEED is stable under a nitrogen environment, whereas the disappearance of the half-order reconstruction occurs during Ga-rich growth yielding a (1 x 1) RHEED pattern and eventual accumulation of metallic Ga on the growth surface. Based on these observations, a model describing the flux balance at the transition stoichiometry is developed. It is shown to be useful for estimating the concentration of active nitrogen species available, determining the interdependence of growth parameters, and optimizing growth conditions.