Early stages of heteroepitaxial growth of ZnSe on GaAs by MOVPE, MOASE and MBE

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We have used cross-sectional transmission electron microscopy (XTEM) X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED) to study the early stages of heteroepitaxial growth of ZnSe on GaAs grown by metal-organic vapor phase epitaxy (MOVPE), metal-organic alternating source epitaxy (MOASE) and molecular beam epitaxy (MBE). In MOVPE growth of ZnSe on GaAs, we have observed 3D islands in the early stages of growth. We observe no difference when using nominally (100) oriented substrates and substrates tilted 2° toward [011]. Changing the carrier gas from conventionally used H 2 to Ar affects the initial sticking of deposited atoms to the substrate, but does not seem to affect the growth mode. In contrast, XTEM observations show that the islands that form in the early stages of MOASE growth are anisotropic; that is, islands are elongated along the [011] direction. In MBE, we find that the growth mode is 3D in the very early stages of growth, with no strong sign of anisotropy in the shape of the islands as seen in XTEM observations for both (NH 4) 2S and Se pretreated substrates. However, we do find that the sticking coefficient of the deposited atoms during the early stages of film formation are affected by the substrate pretreatment process. Among the substrate pretreatment processes studied to date, ex situ ammonium sulfide pretreatment is associated with a lower initial sticking coefficient compared to pretreatments which involve an in situ anneal with a Se flux, with or without an ex situ (NH 4) 2S pretreatment.