Structural studies of diluted magnetic semiconductor thin films and bulk ferroelectric relaxors using TEM

Structural properties of diluted magnetic semiconductor thin films and lead magnesium niobate ferroelectric relaxors were investigated by means of selected area diffraction, dark/bright field imaging, and high-resolution lattice imaging. In order to establish a correspondence between the structural features and the experimental lattice images, computer image simulation was performed in conjunction with high-resolution transmission electron microscopy.; The structural properties of Zn{dollar}sb{lcub}rm 1-x{rcub}{dollar}Fe{dollar}sb{lcub}rm x{rcub}{dollar}Se and Zn{dollar}sb{lcub}rm 1-x{rcub}{dollar}Co{dollar}sb{lcub}rm x{rcub}{dollar}Se epilayers grown on (001) GaAs or InP substrates using molecular beam epitaxy have been investigated. A low density of misfit dislocations and stacking faults, and no evidence for microtwins in these epilayers were observed, indicating high crystalline quality. Chemical ordering was observed in Zn{dollar}sb{lcub}rm 1-x{rcub}{dollar}Fe{dollar}sb{lcub}rm x{rcub}{dollar}Se (x {dollar}approx{dollar} 0.5). Especial attention was given to this ordered phase because of the possibility of obtaining magnetic ordering associated with chemical ordering.; The structural properties of (ZnSe/FeSe) and (ZnSe/MnSe) superlattices grown on (001) GaAs substrate by molecular beam epitaxy have also been investigated. High-quality strained (ZnSe/FeSe) superlattices were obtained when a ZnSe buffer layer was grown on the substrate prior to the growth of the superlattice. In contrast, nominal (ZnSe/FeSe) superlattices grown directly on the substrate showed evidence for interdiffusion between the constituent layers in the superlattice followed by ordering of Zn and Fe atoms. The misfit strains between the superlattice and the substrate are believed to be responsible for the interdiffusion and ordering. Well defined layers were observed in the (ZnSe/MnSe) superlattices, but with a high density of misfit dislocations and microtwins. The MnSe layers were found to have a zinc-blende structure.; Chemical ordering in lead magnesium niobate solid solutions has also been investigated. Computer image simulation was performed for various defocusing conditions, local Mg/Nb ratios, and long-range order parameters. These studies revealed that long-range order parameter dominates the patterns in these images. It was also shown that the ordered structure with a long-range order parameter of 0.4 {dollar}<{dollar} S {dollar}<{dollar} 0.8 consists of alternating Mg- and Nb-preferred sublattices along the {dollar}langle{dollar}111{dollar}rangle{dollar} directions. A strain modulation observed along the {dollar}langle{dollar}111{dollar}rangle{dollar} directions is believed to be associated with the relaxor properties reported in these materials.