II/VI metastable sulphides : fundamental studies of MBE growth, characterization and nanostructure formation

Abstract

This thesis presents a fundamental study of II-VI metastable sulfides. Details of the growth by Molecular Beam Epitaxy (MBE) and characterization of CrS,ZnS,MgS, and ZnMgSSe structures on GaP and GaAs substrates are presented. Multilayer structures of ZnS/CrS and ZnMgS/CrS were grown on GaP substrates and characterized using X-ray interference. Asymmetric X-ray spectra indicated the presence of mosaic structure in the layers. A discussion on solutions to the disordered surface is presented. Superlattice structures were designed to contain 100Å of CrS. These samples were sent to the Polish academy of science for further analysis of magnetic properties. The surface morphology of zinc blende MgS layers grown on ZnSe buffer layers has been investigated by AFM. Nanowire formation is studied as a function of flux ratio and MgS layer thickness. Typical wire dimensions were found to be 0.6μm (length), 900Å (width), and 20Å (height). The method of wire formation is consistent with an anisotropic relaxation of the MgS layer by mismatch dislocations leading to a surface instability by the Asaro-Tiller-Grinfield mechanism. RHEED patterns from zincblende MgS (100) surfaces have been observed during growth, annealing and both during and after irradiation of the surface with an Mg flux. The results are compatible with the c(2x2) surface being formed from a ZnMgS alloy which converts to a pure MgS 4x1 reconstruction on Mg irradiation. Samples containing ZnMgSSe alloy were grown and analyzed by X-ray Interference. The alloy composition was found to be Zn0.20Mg0.80S0.64Se0.36. Samples with this composition are shown to be single phase using transmission electron microscopy. Structures with Zn0.20Mg0.80S0.64Se0.36 barriers were grown with ZnSe quantum wells and CdSe quantum dots, and showed good quantum confinement. The incorporation of Zn0.20Mg0.80S0.64Se0.36 in an epitaxial lift off technology has also been demonstrated.