Coulomb correlation effects in zinc monochalcogenides

Electronic structure and band characteristics for zinc monochalcogenides with zinc-blende- and wurtzite-type structures are studied by first-principles density-functional-theory calculations with different approximations. It is shown that the local-density approximation underestimates the band gap and energy splitting between the states at the top of the valence band, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field-splitting energy. Regardless of the structure type considered, the spin-orbit-coupling energy is found to be overestimated for ZnO and underestimated for ZnS with wurtzite-type structure, and more or less correct for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at the top of the valence band is found to be anomalous for ZnO in both zinc-blende- and wurtzite-type structure, but is normal for the other zinc monochalcogenides considered. It is shown that the Zn-3d electrons and their interference with the O-2p electrons are responsible for the anomalous order. The typical errors in the calculated band gaps and related parameters for ZnO originate from strong Coulomb correlations, which are found to be highly significant for this compound. The LDA+U approach is by and large found to correct the strong correlation of the Zn-3d electrons, and thus to improve the agreement with the experimentally established location of the Zn-3d levels compared with that derived from pure LDA calculations

Theoretical study of As overlayers on InP(110) surface: optical properties

The reflectance anisotropy of the As/InP(110) surface is calculated by using an ab initio plane-wave pseudopotential method. We analyze different models of As coverage, ranging from non-reacted epitaxial layers to exchange-reacted geometries. Comparison with experimental data confirms that the annealed, highly ordered surface phase can be described by an InAs monolayer on the InP substrate (exchange reacted model), whereas the reflectance anisotropy of the as-grown, poorly ordered As/InP surface probably is dominated by disorder effects. (C) 1998 Elsevier Science B.V. All rights reserved

Ab initio calculation of the reflectance anisotropy of surfaces: The triangle method

We modify the analytic linear tetrahedron method to calculate the optical properties of surfaces. As st test case, the reflectance anisotropy of InP(110) is calculated within the density-functional theory in the local-density approximation. The convergence with respect to the number of k points and the choice of the triangles is extensively discussed. The resulting spectra are interpreted and compared with experimental results

Heavy-Ion Microbeam Studies of Single-Event Leakage Current Mechanism in SiC VD-MOSFETs

Heavy-ion microbeams are employed for probing the radiation-sensitive regions in commercial SiC VD-MOSFETs with micrometer accuracy. By scanning the beam spot over the die, a spatial periodicity was observed in the leakage current degradation, reflecting the striped structure of the power MOSFET investigated. Two different mechanisms were observed for degradation. At low drain bias (gate and source grounded) only the gate-oxide (at JFET or neck region) is contributing in the ion-induced leakage current. For exposures at drain-source bias voltages higher than a specific threshold, additional higher drain leakage current is observed in the p-n junction region. This provides useful insights for the understanding of basic phenomena of Single Event Effects in SiC power devices.peerReviewe

Interfacial studies of Al2O3 deposited on 4H-SiC(0001)

Al2O3 films deposited on 4H-SiC(0001) by atomic layer deposition (ALD) were characterized by x-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM). The effect of medium and high temperature (873, 1273 K) annealing on samples with oxide thicknesses of 5-8nm and 100-120nm was studied. XPS indicated presence of a thin (~1nm) SiOx layer on the as-grown samples which increased to ~3 nm after annealing above crystallization temperature (1273 K) in Ar atmosphere. Upon annealing the stoichiometry of the interfacial oxide approaches that of SiO2. HRTEM showed that the thickness of the interfacial oxide formed after annealing at 1273 K was not uniform. No significant increase in the thickness of the interfacial oxide, was observed after annealing at 873 K in a N2 (90%) / H2 (10%) atmosphere

Interfacial studies of Al2O3 deposited on 4H-SiC(0001)

Al2O3 films deposited on 4H-SiC(0001) by atomic layer deposition (ALD) were characterized by x-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM). The effect of medium and high temperature (873, 1273 K) annealing on samples with oxide thicknesses of 5-8nm and 100-120nm was studied. XPS indicated presence of a thin (~1nm) SiOx layer on the as-grown samples which increased to ~3 nm after annealing above crystallization temperature (1273 K) in Ar atmosphere. Upon annealing the stoichiometry of the interfacial oxide approaches that of SiO2. HRTEM showed that the thickness of the interfacial oxide formed after annealing at 1273 K was not uniform. No significant increase in the thickness of the interfacial oxide, was observed after annealing at 873 K in a N2 (90%) / H2 (10%) atmosphere

Heavy-Ion Microbeam Studies of Single-Event Leakage Current Mechanism in SiC VD-MOSFETs

Heavy-ion microbeams are employed for probing the radiation-sensitive regions in commercial silicon carbide (SiC) vertical double-diffused power (VD)-MOSFETs with micrometer accuracy. By scanning the beam spot over the die, a spatial periodicity was observed in the leakage current degradation, reflecting the striped structure of the power MOSFET investigated. Two different mechanisms were observed for degradation. At low drain bias (gate and source grounded), only the gate-oxide (at the JFET or neck region) is contributing in the ion-induced leakage current. For exposures at drain–source bias voltages higher than a specific threshold, additional higher drain leakage current is observed in the p-n junction region. This provides useful insights into the understanding of basic phenomena of single-event effects in SiC power devices.ISSN:0018-9499ISSN:1558-157