Using self-consistent band-structure methods, we analyze the remarkable anomalies (& 50%) in the energy-band gaps of the ternary IB-IIIA-VL42 chalcopyrite semiconductors (e.g. , CuGaS2) relative to their binary zinc-blende analogs IIB-VIA (e.g. , ZnS), in terms of a chemical factor AEg"' and a structural factor EEg. We show that AE~"' is controlled by a p-d hybridization effect AEg and by a cation electronegativity effect AEg, whereas the structural contribution to the anomaly is controlled by the existence of bond alternation (R&c&R~c) in the ternary system, manifested by nonideal anion displacements u-4&0. All contributions are calculated self-consistently from band-structure theory, and are in good agreement with experiment. We further show how the nonideal anion displacement and the cubic lattice constants of all ternary chalcopyrites can be obtained from elemental coordinates (atomic radii) without using ternary-compound experimental data. This establishes a relationship between the electronic anomalies and the atomic sizes in these systems. difference AEg =Eg Eg (band-gap anomaly) with respect to the binary analogs. E~"' values are compiled from Refs. 8 and 15-22, and correspond to room temperature, except as noted. The binary band gaps Eg ' are from Ref. 23 taken at the corresponding temperatures. Uncertain values are denoted by an asterisk.

The electronic structure of six Cu-based ternary chalcopyrite semiconductors is calculated selfconsistently for the first time within the density-functional formalism. The chemical trends in the band structures, electronic charge densities, density of states, and chemical bonding are analyzed.

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Copper thiocyanate (CuSCN) is a candidate as a transparent solid p-type conductor for optoelectronic and photovoltaic applications, such as solar cells. We calculate the band structure, bonding characteristics, and basic native defect configurations of hexagonal β-CuSCN. β-CuSCN is predicted to be an indirect-gap semiconductor with an unusual orbital character: although the highest valence bands have the expected character of Cu 3d levels hybridized with S 3p states, the conduction band minimum (at the K point of the hexagonal Brillouin zone) has mostly cyanide antibonding character. This quasi-molecular character results in some unusual properties, including that the electron effective masses are comparable to or even larger than the hole effective masses. Calculated results match well with the valence band spectrum of thin film CuSCN, although optical absorption measurements do not conclusively confirm the predicted indirect nature of the lowest transitions. The dominant p-type character of this material is explained in terms of copper vacancies; CN unit vacancies, which are also expected to be acceptors, are proposed as a mechanism to increase p-type conduction.

In zinc-blende semiconductors, the nonpolar ͑110͒ surface is more stable than all polar surfaces because the formation of the latter requires the creation of charge-neutralizing but energetically costly surface reconstruction. Our first-principles calculations on CuInSe 2 reveal this in the double-zinc-blende ͑chalcopyrite͒ structure, the defect-induced reconstructions make the ͑112͒-cation plus (1 1 2)-anion polar facets lower in energy than the nonpolar ͑110͒ plane, despite the resulting increased surface area. We show that this spontaneous facetting results from the remarkable stability of surface defects ͑Cu vacancy, Cu-on-In antisite͒ in chalcopyrites, and explains the hitherto puzzling formation of polar microfacets when one attempts to grow epitaxialloy a nonpolar chalcopyrite surface.

Chemical states and electrical properties of a high-k metal oxide/silicon interface with oxygen-gettering titaniummetal-overlayer Appl.We present theoretical and experimental results regarding the thermodynamic stability of the high-k dielectrics ZrO 2 and HfO 2 in contact with Si and SiO 2 . The HfO 2 /Si interface is found to be stable with respect to formation of silicides whereas the ZrO 2 /Si interface is not. The metal-oxide/SiO 2 interface is marginally unstable with respect to formation of silicates. Cross-sectional transmission electron micrographs expose formation of nodules, identified as silicides, across the polycrystalline silicon/ZrO 2 /Si interfaces but not for the interfaces with HfO 2 . For both ZrO 2 and HfO 2 , the x-ray photoemission spectra illustrate formation of silicate-like compounds in the MO 2 /SiO 2 interface.

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