Structural defects in II-VI epitaxial Layers

Abstract

This study has principally been concerned with the structural assessment of a range of II-VI epitaxial layers using the combined techniques of cross-sectional transmission electron microscopy and microdiffraction. Iodine reactive ion sputtering has been used for the final stage of sample preparation, and this has been shown to produce electron transparent foils with a greatly reduced artefactual defect content, as compared with samples prepared by argon ion milling.- The two conflicting conventions for the polarity determination of CdTe, as proposed by Warekois (1962) and Fewster (1983), have led to considerable confusion in the recent international literature. This issue has been resolved using the microdiffraction technique to identify the best CdTe polar face for the epitaxial growth of (Hg,Cd)Te. This proved to be the Te: surface, and this result is in agreement with the Fewster convention for CdTe crystal polarity. The study of wide band-gap II- VI compounds has been mainly concerned with heterostructures of ZnSe/ZnS grown by MOVPE on (001) oriented GaAs. Observations along orthogonal < 110 > directions demonstrated the strong anisotropy in the defect distribution of these epitaxial layers. Microtwins were found exclusively in the [IĪ0] epi- layer projection lying on advancing A planes. A model has been proposed, based on the differential motion of a and β dislocations in the sphalerite structure, to explain the defect anisotropy. It is considered that the large difference in the ionic radii of Zn(^2+) and S(^2-), and material doping are primarily responsible for this phenomenon. The study of narrow band-gap II-VI compounds has been mainly concerned with the structural assessment of hybrid substrates for the epitaxial growth of (Hg,Cd)Te. Lamella twins lying parallel to the interfacial plane characterised CdTe epitaxial layers grown by MOVPE on and B GaAs, and on B CdTe substrates, while the CdTe/GaAs system exhibited a network of misfit dislocations. MOVPE grown layers of (Hg,Cd)Te on both B CdTe and CdTe/B GaAs hybrid substrates have also been investigated. This study is complemented by an assessment of the anisotropic defect distribution in MBE grown (Cd,Zn)Te/CdTe strained layer superlattices, which have potential application as high quality lattice matched substrates for (Hg,Cd)Te. Finally a provisional study of the intermediate band-gap ZnTe and ZnTe/CdTe multilayer system is presented.