Materials Science Forum Vols. 717-720

Abstract: Optical Beam Induced Current (OBIC) measurements have been performed on 4H-SiC avalanche diodes with very thin and highly doped avalanche region. The light source used in this study is an Ar-laser with a wavelength of 351 nm which results in a mixed carrier injection. From these measurements, impact ionization coefficients for 4H-SiC have been extracted in the electric field range from 3 to 4.8 MV/cm. In combination with ionization coefficients in our previous paper extracted from diodes with lowly doped avalanche region, we propose a set of parameters of impact ionization coefficients for 4H-SiC, applicable to a wide electric field range.

Abstract: Polarization effect characteristically occurs in detectors based on wide-bandgap materials at considerable concentrations of radiation defects. The appearance of an electromotive force in the bulk of a detector is due to the long-term capture of carriers at deep levels related to radiation centers. The kinetics and strength of the polarization field have been determined. The capture can be controlled by varying the detector temperature, with a compromise reached at the "optimal" temperature between the generation current and the position of the deepest of the levels whose contribution to the loss of charge via capture is negligible. It has been found that the depth of a level (related to the energy gap width) is close to 1/3, irrespective of a material. The optimal temperatures are strictly individual for materials.

Abstract: We report on efficient terahertz emission in the region of 1.5-2 THz from high electric field biased 6H-SiC structures with a natural superlattice at liquid helium temperatures. The shape of the emission spectrum, the linear dependence of its maximum on the bias and the characteristic field strength required to achieve the emission allow the emission to be attributed to steady-state Bloch oscillations of electrons in the SiC natural superlattice.

Abstract: In this work we report the enhancement of the 3C-SiC band edge luminescence induced by the SiO2 shell in SiC/SiO2 core/shell nanowires (NWs) system. We demonstrate that the shell enhances the SiC near band edge luminescence and we argue the formation of a type-I quantum well between the SiC core and the SiO2 shell, with the consequent injection of carriers from the larger band-gap shell to the narrower band-gap core.

Abstract: We present, for the first time, simulations of thermoelectric properties of silicon carbide (SiC) nanowires as a function of the wire cross section at high temperature (500K), based on non-equilibrium classical molecular dynamics simulations for the lattice thermal transport and non-equilibrium green's function for the electrical transport. Our calculations show that figure of merit (ZT) was increasing with decreasing cross section area: ZT of SiC nanowire at 2x2 nm2 has maximum value in the range of 0.65 - 0.89 at 500K, which is 7 - 8 times larger than maximum ZT of SiC thin film value (0.125 at 973 K). These results show that SiC may be a promising material for thermoelectric applications operating at high temperature.

Abstract: Recently, polytypism has been observed in nanowires in materials, for which normally only one crystal structure is stable. For example, GaAs, nanowires can have wurtzite or mixed zincblende/wurtzite. Here we provide band structure parameters for wurzite and 4H GaAs and use them for modeling the nanowire electronic states. The band gap, crystal field splitting, and its strain dependence, as well as the effective mass parameters are calculated using the quasiparticle self-consistent GW method. The nanowire electronic states are obtained in the envelope function approximation within a simplified cylindrical model. The crystal field splitting of the wurtzite GaAs valence band is found to be 180 meV while in 4H-GaAs it is less than half 69 meV, suggesting a downward bowing as function of hexagonality. The conduction band minimum at Γ changes symmetry character under strain. We discuss the consequences for nanowires and determine the conditions under which a polarization reversal of photoluminescence can occur from mostly perpendicular to parallel to the wire.

Abstract: In this work, we made an off-axis surface of SiC with Mechano-chemical polishing method to examine the influence that an atomic step gave for electronic properties of devices. Low energy ion scattering spectroscopy was shown Si-face and C-face with SiC structure were polished with monocrystal. We consider that there results above the atomic step on the surface influence to a device properties, especially a leakage current of Schottky diode. C atom at the step and terrace cause leakage event at metal-semiconductor interface of Schottky diode have been clarified by conductive atomic force microscopy.

Abstract: Step-terrace structures were observed at on-axis/4o off 4H-SiC {0001} surfaces after Si-vapor etching which we have been supposed as an original technique to planarize and etch the SiC surfaces by utilizing a TaC crucible in temperature ranged from 1600 to 2200 oC. The structures obtained after the Si-vapor etching obviously indicated temperature dependence. There were two types of step-terrace structures in terms of the step height and the shape of the step edges at on-axis surfaces. Step bunched surfaces consisting of full unit cell height (= 1.0 nm) steps with {1-10n} facets at the step edges were observed at 4H-SiC (0001) in lower temperatures below 2000 oC, while smooth isotropic surfaces with half unit cell height (= 0.5 nm) steps and without any stable facets at the step edges were observed at 4H-SiC (0001) in higher temperatures above 2000 oC and in all temperature conditions (1600 - 2200 oC) at 4H-SiC (000-1). Similar tendency was also confirmed at 4o off 4H-SiC {0001} surfaces. From the comparison with 6H-SiC, macro step bunching (~10 nm height) was revealed to be a unique phenomenon at 4H-SiC (0001) surface in the etching.

Abstract: The morphological instability appeared at step-free 4H-SiC (0001) surfaces was investigated. The step-free surfaces were fabricated at the bottom of inverted-mesa structure by the method combining a laser digging and Si-vapor etching. By repeated Si-vapor etching treatments, randomly created crater and maze structures were cyclically appeared at the step-free surfaces. These structures were distinctly classifiable by their depths from the step-free surfaces. Crater structures have 0.2 - 0.3 nm depth and maze structures have 0.5 nm depth. The morphological evolutions indicate the process of destruction of the step-free (0001) basal plane and generation of steps from step-free surfaces in the Si-vapor etching process.

Abstract: It has been reported that SiC surface can be etched off in a concentrated aqueous solution of hydrogen fluoride with making contacts with Pt plate catalyst, named Catalyst-referred Etching (CARE) [1]. In this report, we present first-principles molecular-dynamic simulations on the initial stage of the etching process. Reaction barrier heights of dissociative absorption reactions of hydrogen fluoride molecule breaking into back-bonds at step edge of 3C-SiC(111) are analyzed.