Growth mechanisms in atomic layer epitaxy of GaAs

Abstract

We present a study of the different mechanisms governing the growth of GaAs by atomic layer epitaxy (ALE). The gallium precursors such as trimethylgallium (TMGa), triethylgallium (TEGa), and trisneopentylgallium (TNPGa) were characterized and compared during ALE by in situ reflectance difference spectroscopy (RDS). A fundamental difference is observed in the RDS behavior after the alkyl exposure between self-limiting and non-self-limiting precursors. A transient is observed during the purge following alkyl exposure for both self-limiting sources, (TMGa and TNPGa), and is absent with TEGa. This is attributed to the presence of methyl radicals when using self-limiting precursors. Atomic force microscopy measurements of the surface morphology show that a non-self-limiting surface prepared with TEGa has droplets forming on it. In contrast, TMGa and TNPGa show well-defined terraces without droplets. RDS real-time measurements show that Ga incorporating on the c(4×4) surface first reacts with the As in the top layer with a process involving both desorption of As and rearrangement of the remaining top As. We used these measurements to provide a detailed structural model of the process of Ga incorporation, and thus, solve the problem of how an As-rich surface with more than 1 ML coverage can yield 1 ML of material per ALE cycle.