The ultraviolet–visible absorption spectra are given for 10 nm diameter colloidal particles of 52 of the metallic elements, calculated from the optical constants of the metals by means of Mie theory. For most of the elements the spectra cover the range 200–900 nm. Well resolved absorption bands are observed for colloidal Sc, Ti, V, Y, Cd, Eu, Yb, Hg and Th as well as for colloids of Cu, Ag, Au and the s-block metals. However, for the majority of the colloidal metallic elements in this size range there is only a continuous absorption in the visible range, rising to broad and poorly resolved absorption bands in the ultraviolet near 200 nm. The difference in the way that the spectra of colloidal particles of different metals change when the particle shape is varied from spherical to spheroidal is investigated systematically in the dipole approximation. This is achieved by means of contour plots of the absorbance cross-section for the particles vs. the real and imaginary parts of the dielectric function for the metals, and this method of investigation is extended also to hollow spherical particles. The results suggest that Ca, Sr, Ba, Eu, Yb, Th and possibly Sc, Ti, V and Y may merit experimental investigation as new metals for exhibiting surface-enhanced Raman scattering.

Ultraviolet–visible absorption spectra of the colloidal metallic elements

Silicon carbide (SiC), a material long known with potential for high-temperature, high-power, high-frequency, and radiation hardened applications, has emerged as the most mature of the wide-bandgap (2.0 eV ≲ Eg ≲ 7.0 eV) semiconductors since the release of commercial 6HSiC bulk substrates in 1991 and 4HSiC substrates in 1994. Following a brief introduction to SiC material properties, the status of SiC in terms of bulk crystal growth, unit device fabrication processes, device performance, circuits and sensors is discussed. Emphasis is placed upon demonstrated high-temperature applications, such as power transistors and rectifiers, turbine engine combustion monitoring, temperature sensors, analog and digital circuitry, flame detectors, and accelerometers. While individual device performances have been impressive (e.g. 4HSiC MESFETs with fmax of 42 GHz and over 2.8 W mm−1 power density; 4HSiC static induction transistors with 225 W power output at 600 MHz, 47% power added efficiency (PAE), and 200 V forward blocking voltage), material defects in SiC, in particular micropipe defects, remain the primary impediment to wide-spread application in commercial markets. Micropipe defect densities have been reduced from near the 1000 cm−2 order of magnitude in 1992 to 3.5 cm−2 at the research level in 1995.

Status of silicon carbide (SiC) as a wide-bandgap semiconductor for high-temperature applications: A review

This chapter provides an overview of quasiparticle calculations in solids, and, in particular, the GW approximation (GWA). A successful approximation for the determination of excited states of solids is based on the quasiparticle concept and the Green function method. The Coulomb repulsion between electrons leads to a depletion of negative charge around a given electron, and the ensemble of this electron and its surrounding positive screening charge forms a quasiparticle. The mathematical description of quasiparticles is based on the single-particle Green function (G), whose exact determination requires complete knowledge of the quasiparticle self-energy ∑. The self-energy ∑ is a non-Hermitian, energy-dependent, nonlocal operator that describes exchange and correlation effects beyond the Hartree approximation. A determination of the self-energy can only be approximate, and a working scheme for the quantitative calculation of excitation energies in metals, semiconductors, and insulators is the so-called dynamically screened interaction or the GWA. The chapter also discusses (1) the physics and extensions of the GWA; (2) numerical aspects of GWA calculations; (3) applications of the GWA to semiconductors, insulators and metals; and (4) the relevance of GWA calculations to optical response. Finally, the chapter presents parallel algorithms both for reciprocal and real-space/imaginary-time GWA calculations and several alternative methods to determine excited states of solids within density functional theory.

Quasiparticle calculations in solids

The important material parameters for 6H silicon carbide (6H-SiC) are extracted from the literature and implemented into the 2-D device simulation programs PISCES and BREAKDOWN and into the 1-D program OSSI Simulations of 6H-SiC p-n junctions show the possibility to operate corresponding devices at temperatures up to 1000 K thanks to their low reverse current densities. Comparison of a 6H-SiC 1200 V p-n/sup -/-n/sup +/ diode with a corresponding silicon (Si) diode shows the higher switching performance of the 6H-SiC diode, while the forward power loss is somewhat higher than in Si due to the higher built-in voltage of the 6H-SiC p-n junction. This disadvantage can be avoided by a 6H-SiC Schottky diode. The on-resistances of Si, 3C-SiC, and 6H-SiC vertical power MOSFET's are compared by analytical calculations. At room temperature, such SiC MOSFET's can operate up to blocking capabilities of 5000 V with an on-resistance below 0.1 /spl Omega/cm/sup 2/, while Si MOSFET's are limited to below 500 V. This is checked by calculating the characteristics of a 6H-SiC 1200 V MOSFET with PISCES. In the voltage region below 200 V, Si is superior due to its higher mobility and lower threshold voltage. Electric fields in the order of 4/spl times/10/sup 6/ V/cm occur in the gate oxide of the mentioned 6H-SiC MOSFET as well as in a field plate oxide used to passivate its planar junction. To investigate the high frequency performance of SiC devices, a heterobipolartransistor with a 6H-SiC emitter is considered. Base and collector are assumed to be out of 3C-SiC. Frequencies up to 10 GHz with a very high output power are obtained on the basis of analytical considerations. >

SiC devices: physics and numerical simulation

A full-potential band structure calculation, within the local density approximation to the density functional theory, has been performed for the polytypes 3C, 2H, 4H, and 6H of SiC. The calculated effective electron masses are found to be in very good agreement with experimental values. The electron-optical phonon coupling has been estimated and the polaron masses are calculated to be 3%–13% larger than the corresponding bare masses. The effective electron masses of the second lowest conduction band minima are also presented and the calculated energy difference between the two lowest minima in 4H–SiC is only 0.12 eV. The lowest conduction band in 6H–SiC is found to be very flat and to have a double-well-like minimum along the ML line. The top of the valence bands has been parametrized according to the k⋅p approximation, whereupon the effective hole masses have been determined. The spin-orbit interaction was found to have a strong influence on the value of the effective hole masses. Furthermore, total and partial densities of states are presented.

W.J. Choyke

Papers

Wavelength modulated absorption in SiC

Donor Exciton Satellites in Cubic Silicon Carbide: Multiple Bound Excitons Revisited

Observation of an Electron-Hole Liquid in Cubic SiC

A comparison of the optical properties of single crystal and liquid mercury in the energy range 0.5 to 8.25 eV

The electron-hole-liquid in 15R-SiC