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We computationally investigate the tetragonal and orthorhombic Cu2CdGe(SxSe1−x)4 (0 ≤ x ≤ 1) alloy systems and study their mechanical and optoelectronic properties for photovoltaic applications using first principles density functional theory and beyond methods. Formation energies are all below −5.5 eV/f.u., decreasing with x. Energy vs strain calculations estimate the bulk moduli to be between 55 and 69 GPa, the shear moduli to be between 22 and 28 GPa, and Young's moduli to be between 59 and 74 GPa, all monotonically increasing with x. Optoelectronic properties are computed with the hybrid HSE06 functional. Bandgaps between 1.2 and 1.9 eV (increasing with x), low carrier masses in the (001) direction, and favorable optical absorption and reflectivity indicate that the alloys could be used as an absorber layer in solar cells. For a given value of x, the tetragonal system has higher elastic constants, a lower bandgap, and lower effective masses for both electrons and holes compared to the orthorhombic system. Further, the absorption coefficient near the peak of the standard AM 1.5 solar spectrum is calculated to be 5 μ m − 1 larger for the tetragonal system than for the orthorhombic system. These considerations lead to the conclusion that the tetragonal system shows promise as an absorber material for both single-junction and tandem solar cells. [ABSTRACT FROM AUTHOR]

We report on the concentration, chemical bonding, and etching behavior of N at the SiC(0001)/SiO2 interface using photoemission, ion scattering, and computational modeling. For standard NO processing of a SiC MOSFET, a sub-monolayer of nitrogen is found in a thin inter-layer between the substrate and the gate oxide (SiO2). Photoemission shows one main nitrogen related core-level peak with two broad, higher energy satellites. Comparison to theory indicates that the main peak is assigned to nitrogen bound with three silicon neighbors, with second nearest neighbors including carbon, nitrogen, and oxygen atoms. Surprisingly, N remains at the surface after the oxide was completely etched by a buffered HF solution. This is in striking contrast to the behavior of Si(100) undergoing the same etching process. We conclude that N is bound directly to the substrate SiC, or incorporated within the first layers of SiC, as opposed to bonding within the oxide network. These observations provide insights into the chemistry and function of N as an interface passivating additive in SiC MOSFETs. [ABSTRACT FROM AUTHOR]

Degradation mechanisms limiting the electrical reliability of GaN high-electron-mobility transistors (HEMTs) are generally attributed to defect generation by hot-electrons but specific mechanisms for such processes have not been identified. Here we give a model for the generation of active defects by the release of hydrogen atoms that passivate pre-exisiting defects. We report first-principles density-functional calculations of several candidate point defects and their interaction with hydrogen in GaN, under different growth conditions. Candidate precursor point defects in device quality GaN are identified by correlating previously observed trap levels with calculated optical levels. We propose dehydrogenation of point defects as a generic physical mechanism for defect generation in HEMTs under hot-electron stress when the degradation is not spontaneously reversible. Dehydrogenation of point defects explains (1) observed hot electron stress transconductance degradation, (2) increase in yellow luminescence, and opposite threshold voltage shifts in devices where the material was grown under nitrogen- and ammonia-rich conditions. [ABSTRACT FROM AUTHOR]

Oxidation of SiC with the incorporation of Na in the gate oxide was recently found to lead to significantly enhanced electron mobilities in the SiC inversion layer but the underlying mechanism has remained elusive. Here, we report a combination of density functional first-principles calculations and experiments. The new findings demonstrate that neutral Na is essentially a spectator impurity that occupies near interfacial interstitial sites and does not interact with the interface or with interfacial defects. Na ions, however, introduce an effective mass hydrogenic impurity band at the edge of the SiC conduction band that can account for the observed effects. [ABSTRACT FROM AUTHOR]

The application of SiC in electronic devices is currently hindered by low carrier mobility at the SiC/SiO2 interfaces. Recently, it was reported that 4H-SiC/SiO2 interfaces might have a transition layer on the SiC substrate side with C/Si ratio as high as 1.2, suggesting that carbon is injected into the SiC substrate during oxidation or other processing steps. We report finite-temperature quantum molecular dynamics simulations that explore the behavior of excess carbon in SiC. For SiC with 20% excess carbon, we find that, over short time (∼24 ps), carbon atoms bond to each other and form various complexes, while the silicon lattice is largely unperturbed. These results, however, suggest that at macroscopic times scale, C segregation is likely to occur; therefore a transition layer with 20% extra carbon would not be stable. For a dilute distribution of excess carbon, we explore the pairing of carbon interstitials and show that the formation of dicarbon interstitial cluster is kinetically very favorable, which suggests that isolated carbon clusters may exist inside SiC substrate. [ABSTRACT FROM AUTHOR]

We investigate theoretically coherent electron tunneling through three-dimensional microscopic Si[100]/SiO[sub 2]/Si[100] model junctions with oxide thicknesses between 0.4 and 4.6 nm. The transmission probabilities of these structures were calculated using a semiempirical tight-binding scattering method. Our calculations provide a basis for the microscopic understanding of the observed independence of tunneling transmission on the orientation of the bulk silicon and on the nature of inelastic defect-assisted tunneling. We document significant differences between transmission coefficients obtained with the present scheme and with the popular effective-mass-based approaches. The energy dependence of the effective tunneling mass in bulk silicon dioxide is predicted. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Density functional electronic structure calculations are employed to examine hydrogen for a variety of configurations in silicon. A novel complex is found for hydrogen in amorphous silicon. The complex involves the breaking of a weak silicon bond to form two Si-H bonds with both hydrogens in between the original silicon atoms. This complex provides a microscopic model for new metastable complexes observed in amorphous silicon. Mechanisms for hydrogen-related metastability are discussed for amorphous and polycrystalline silicon.