III-VI semiconductors and oxides: Electronic structure, surface morphology, and transition metal doping of gallium selenide, indium selenide, and gallium oxide.

摘要

The effects of vacancies on the properties of certain III2VI 3 semiconductor compounds are studied with the goal of learning new, interesting physics while laying the groundwork for using these materials in next generation electronic devices. These III-VI materials exhibit intrinsic nanoscale voids or vacancies that order in different ways and impact the electronic structure, dopant incorporation and defect formation.;Ga2Se3 and gamma-In2Se3 are tetrahedrally bonded III-VI semiconductors with 1/3 of the cation sites vacant. Lattice matching allows excellent quality growth of Ga2Se 3 on silicon by molecular beam epitaxy. Comparison of our experimental map of the electronic band structure with theory shows proper theoretical treatment of the vacancies is essential to generate the band structure. We use scanning tunneling microscopy and X-ray absorption to study the Mn-doping of Ga2Se3, an intriguing candidate dilute magnetic semiconductor (DMS) system. Thin uniformly doped films of Mn-doped Ga2Se 3 were grown at low concentrations, but thicker or more concentrated films exhibit islands of MnSe. This may limit the applicability of Mn-doped Ga2Se3 as a DMS.;gamma-In2Se3 has a large lattice mismatch (∼7%) with silicon, which suggests that thick laminar films of In2Se 3 on silicon should not be possible. However, we find that laminar films can be grown up to at least 6 bilayer thickness, and show that this is due to an unusual Se-first interface with the silicon substrate.;beta-Ga2O3 is an optically transparent, III-VI semiconductor that generally exhibits n-type conductivity. We study the electronic structure, magnetic structure, surface termination, and surface morphology of pure, Mn- and Cr-doped beta-Ga2O3 single crystals. Cr3+ and mixed valence Mn2+/3+ occupy the octahedral sites in the structure. Mn incorporation degrades the crystal quality, while Cr does not. We use density functional theory to compute the activation energy of au oxygen vacancy defect, and find, contrary to the commonly held model, that it is too large for oxygen vacancies to contribute substantially to the conductivity in the n-type material. Using hard x-ray photoemission, we show that the bands are bent upward at the surface of air-cleaved beta-Ga 2O3 crystals, lowering the local activation energy.