Materials Science Forum Vols. 778-780

Abstract: The optical properties of isotope-pure ²⁸Si¹²C, natural SiC and enriched with ¹³C isotope samples of the 4H polytype are studied by means of Raman and photoluminescence spectroscopies. The phonon energies of the Raman active phonons at the Γ point and the phonons at the M point of the Brillouin zone are experimentally determined. The excitonic bandgaps of the samples are accurately derived using tunable laser excitation and the phonon energies obtained from the photoluminescence spectra. Qualitative comparison with previously reported results on isotope-manipulated Si is presented.

Abstract: We have investigated Raman spectra of p-type 4H-SiC crystals with hole concentration ranging from 3×10¹⁶ to 1×10²¹ cm^-3. For folded transverse acoustic (FTA) doublet modes, Fano interference profiles were analyzed, and the frequency shift due to Fano interference was obtained as a function of hole concentration. We demonstrated that the shift of the FTA doublet modes is a quantitative measure for evaluating of hole concentration of p-type 4H-SiC. Spectral features of transverse optical (TO) and longitudinal optical phonon-plasmon coupled (LOPC) modes were also studied for various carrier concentrations. The results show that the full width at half maximum of the LOPC and relative intensity of TO and the LOPC can be used as other calibration measures for hole concentration.

Abstract: Crystallographic structures of 8H-and 10H-SiC crystals were analyzed by the Raman spectroscopy and diffraction methods. Two and four transverse-optical modes for 8H-and 10H-SiC were observed, and their values were different from those of 4H-and 6H-SiC. Crystallinity for the wide and narrow areas of these crystals was analyzed by the Laue diffraction and the transmission electron microscope, respectively. Based on these results, the stacking sequences of these polytype were discussed.

Abstract: For the characterization of n-channel 4H-SiC MOSFETs, current-voltage and Hall-effect measurements were carried out at room temperature. To interpret the Hall-effect measurements, the Hall factor for the electron transport in the channel of SiC MOSFETs was evaluated, for the first time. The method of the Hall factor calculation is based on the interdependence with mobility components via the respective scattering relaxation times. The results of the calculation reveal a strong dependence of the Hall factor on the gate voltage. Depending on the gate voltage applied, the values of the Hall factor vary between 1.3 and 1.5. Sheet carrier density and drift mobility values derived from the Hall-effect measurements using our new gate-voltage-dependent Hall factor show very good agreement with simulations performed with Sentaurus Device of Synopsys.

Abstract: In this study, we present results on electrical characterization of bipolar pn-diodes to investigate the temperature and electrical field dependent behavior of ambipolar mobility in n-doped 4H-SiC. Therefore, static current-voltage measurements to calculate the specific differential resistance and dynamical reverse recovery measurements to determine the mean carrier concentration were carried out for different temperatures and forward current densities. The specific differential resistance of the drift layer decreased from 10 mΩcm² at 80 Acm^-2 to 6.6 mΩcm² at 180 Acm^-2, whereas the mean carrier concentration only increased from 4^.10¹⁵ cm^-3 to 8^.10¹⁵ cm^-3, indicating a decreasing ambipolar mobility. The calculated reduction of the ambipolar mobility from 800 cm²V^-1s^-1 to 650 cm²V^-1s^-1 in dependence on the current density has to be attributed to an increasing electric field from 150 Vcm^-1 to 250 Vcm^-1 and increasing carrier scattering due to higher carrier concentrations. For example, at a constant conduction current density of 160 Acm^-2, the ambipolar mobility decreases from 710 cm²V^-1s^-1 at 300 K to 650 cm²V^-1s^-1 at 450 K.

Abstract: THz imaging was performed in 2 s intervals with 1 mm resolution on a 3 in., 0.42 mm thick, as-cut n-type Silicon Carbide wafer. Carrier density, relaxation time, mobility, and resistivity obtained from imaging results are 0.91 × 10¹⁸ cm^-3, 4.36 × 10^-14 s, 218 cm²V^-1s^-1, and 3.14 × 10^-2 Ωcm, respectively. Compared with the standard values provided by the manufacturers, the results suggest that THz imaging has reliable precision and accuracy.

Abstract: Well addressable and controllable point defects in device friendly semiconductors are desired for quantum computational and quantum informational processes. Recently, such defect, an ultra-bright single photon emitter, the antisite carbon carbon vacancy pair, was experimentally investigated in 4H-SiC. In our theoretical work, based on ab initio calculation and group theory analysis, we provide a deeper understanding of the features of the electronic structures and the luminescence process of this defect.

Abstract: We calculated the hyperfine structure and the zero-field splitting parameters of divacancies in 3C, 4H and 6H SiC in the ground state and in the excited state for 4H SiC within the framework of density functional theory. Besides that our calculations provide identification of the defect in different polytypes, we can find some carbon atoms next to the divacancy that of the spin polarizations are similar in the ground and excited states. This coherent nuclear spin polarization phenomenon can be the base to utilize ¹³C spins as quantum memory.

Abstract: The photolytic hydrogen generation using sunlight attracts attention as a next generation energy technology. A key of this technology is a selection of materials for the photolysis and SiC is one of the candidate materials for this application. The conversion efficiency from the solar to the hydrogen energy would be affected by the carrier lifetime in SiC. Therefore, in this study, we measured carrier lifetimes in SiC and compared them with photocurrents in electrolytes that is directly correlated to the conversion efficiency.

Abstract: We demonstrate a simple method for direct observation of the stacking orientation on 4H/6H-SiC {0001} surfaces by low-voltage SEM. The difference in the direction of the stacking orientation is observed as SEM contrast. By utilizing this technique, the bond configuration at {1-10n} steps can be determined by the SEM contrast.