ZnSe nucleation on the GaAs(001):Se-(2×1) surface observed by scanning tunneling microscopy

Abstract

We have used scanning tunneling microscopy (STM) to study the initial stages of the growth of ZnSe by molecular-beam epitaxy on the GaAs(001):Se-(2×1) surface. The GaAs(001):Se-(2×1) surface is formed by depositing Se onto the GaAs(001):Se-(2×4) surface and annealing at above 520 °C. It is a highly ordered (2×1) array of Se dimers, and so is structurally similar to the Se-terminated ZnSe(001)-(2×1) surface, and might be expected to provide a good starting surface for two-dimensional ZnSe growth. However, we find that ZnSe grows by the formation of three-dimensional islands on the GaAs(001):Se-(2×1) surface. Islands grow several layers high while much of the Se-terminated GaAs surface remains uncovered. We have compared our STM images to a simple statistical model of growth and conclude that the sticking coefficient of ZnSe on the GaAs(001):Se-(2×1) surface is about 1/5 of that on a continuous ZnSe film, whereas the sticking coefficient on top of the small ZnSe islands is close to that on a continuous ZnSe film. We can understand the low reactivity of the Se dimers on GaAs compared to Se dimers on ZnSe by considering electron counting. On the ZnSe surface, Se dimers are neutral, whereas on the GaAs surface Se dimers have a net positive charge which results in a lowering of the dimer energy, producing a relatively inert surface. We conclude that in order to grow structurally high quality ZnSe films on GaAs, which requires two-dimensional growth, formation of a Se-(2×1) structure on the GaAs(001) surface must be avoided.