Structure of SiC-Quantum Wells Studied by TEM and CBED
Summary and Conclusion SiC hetero-polytypic structures have been investigated by conventional and high resolution TEM and the lattice parameters of a thin cubic stripe have been determined by CBED. For the case of the stepped substrate, nearly free of defects, about 2nm thick cubic stripes are formed within the hexagonal layer. For the case of the growth on on-axis substrates, it was demonstrated that 6H/3C as well as 4H/3C stacks are formed on 6H and 4H-SiC substrates, respectively, By CBED the lattice parameters of the cubic stripe were determined to a rhom =0.4360nm a rhom = (89.78±0.02)° demonstrating the rhombohedral distortion of the 3C stripes. It might be suggested that the result is valid also for the 2nm thick cubic stripes in the multi-quantum well structures grown on the stepped SiC substrate, although such layers are generally to thin for the lattice parameter determination by CBED. Acknowledgements The authors are indebted to Dr. A. Chuvilin for the generous scientific support during his stay as guest scientist at Jena University, thank Ch. Zaubitzer for sample preparation and acknowledge the financial support of the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 196.
Artificially layered heteropolytypic structures based on SiC polytypes: molecular beam epitaxy, characterization and properties
Abstract In recent years, new types of semiconductor heterostructures consisting of only one material in different crystal structures, such as wurtzite/zinc-blende heterostructures (heteropolytypic structures) are under discussion. Such heterostructures maintain a completely defect-free, lattice matched, and coherent interface and effects due to different chemical constituents can be avoided. In this field, silicon carbide (SiC) is the most promising candidate because SiC crystallizes in more than two different stable structures. The preparation of heteropolytypic structures by only a change of the crystal structure during the growth is a great challenge and is realized only under well-defined conditions of molecular beam epitaxy (MBE). In this paper an overview is given of the results and conclusions of recent material research on the MBE growth, characterization and properties of SiC heteropolytypic structures and related materials.
Coated-wire electrodes containing polymer immobilized ionophores blended with poly(vinyl chloride).
Polymers containing covalently attached 18-crown-6 or 2.2.2 cryptand units were incorporated into plasticized PVC membranes and the composite membranes were examined as potassium ion sensor elements. Ionophores were linked to carboxy-PVC and to poly(acrylic acid) via amide linkages to an alkyl spacer unit. Coated-wire electrodes (CWEs) from the immobilized ionophores gave acceptable responses, but conventional ion-selective membrane electrodes (ISEs) prepared by solvent casting were inactive. Dip-cast membranes did give active ISEs. Potassium electrode performance was independent of the loading of the ionophore within the acrylate support polymer, but depended upon the spacer length. Ion selectivity varied with the ionophore loading within the support polymer. Selectivity is a composite of the ionophore selectivity and ion-exchange interactions with the acrylate backbone, giving selectivities akin to carboxylate substituted crown ethers, notably enhanced monovalent/divalent ion discrimination relative to the ionophore in solution. Polymer immobilization extended the lifetime of active electrodes.