Materials Science Forum Vols. 527-529

Abstract: The atomic structure of the 4H-SiC(11 2 0) surface including possible phase transformations via Si deposition and annealing has been investigated using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The sample is initially prepared by hydrogen etching before loading into the ultra-high vacuum system. The sample is then out-gassed to remove oxygen from the surface. To explore the existence of ordered surface phases, Si is deposited on the sample at 850°C for 15 minutes followed by a series of sequential annealing steps. Throughout this process, the surface is monitored by LEED, AES and XPS. LEED shows that the surface continuously maintains a (1×1) periodicity. Yet, two unique and distinguishable (1×1) phases can be identified. The changes between these phases are clearly demonstrated by the LEED spot intensities. Simultaneously, the Auger and XPS data show a decrease in Si intensity.

Abstract: The morphology and atomic structure of 4H-SiC(1102) and 4H-SiC(1102) surfaces, i.e. the surfaces found in the triangular channels of porous 4H-SiC, have been investigated using AFM, LEED and AES. After hydrogen etching the surfaces show steps parallel and perpendicular to the caxis, yet drastically different morphologies for the two isomorphic orientations. Both surfaces immediately display a sharp LEED pattern. Together with the presence of oxygen in the AES spectra this indicates the development of an ordered oxide. Both surfaces show an oxygen free, well ordered surface after Si deposition and annealing.

Abstract: The effects of argon and nitrogen bombardment of 3C-SiC surfaces at acceleration voltages below 2 keV were studied by stylus profilometry, reflectometry, reflection high energy electron diffraction and Auger electron spectroscopy (AES). The erosion rate of the SiC surface was determined. It was found that the sputtering rate for argon was three times higher compared to nitrogen. AES measurements revealed argon and nitrogen incorporation at a depth of a few nanometers as well as stoichiometric changes at the same depth scale independent of the acceleration voltage. In the case of the interaction of nitrogen ions with the 3C-SiC surface the formation of a SiCNalloy was detected.

Abstract: Preliminary results for the phonon dispersion curves of hexagonal 4H-SiC from experimental inelastic neutron (INS) and X-ray scattering (IXS) are reported and contrasted with those of cubic 3C-SiC and silicon. The experimental frequencies and scattering intensities are in excellent agreement with those from first-principles calculations using density-functional methods. The relative merits of the two experimental techniques and aspects of the density functional perturbation theory and the so-called frozen phonon method for the determination of the basic phonon properties are briefly outlined.

Abstract: We have performed a combined investigation of experiment and theory on the infrared reflectance from cubic SiC grown on Si by chemical vapor deposition. A damping behavior of the interference fringes away from the reststrahlen band and a dip or notch within the “flat top” are observed from some samples while they does not occur in high quality 3C-SiC/Si samples. The former is interpreted due to an interfacial transition layer existed between SiC-Si and a rough surface, while the latter can be demonstrated by a three-component effective medium model.

Abstract: The coefficient of thermal expansion (CTE) of SiC single crystals is important, in particular, for both designing device assembly and controlling stress distributions in heteroepitaxial thin film structures grown onto SiC substrates. We have performed very precise measurements of the CTEs for SiC single crystals comprising of single 4H polytype PVT-grown in NIPPON Steel Corporation for a temperature range from 123 K to 473 K using a laser interferometry method. This method allows us to directly measure the temperature dependent variation in thermal expansion of the crystal volume with much higher accuracy, and enables us to straightforwardly obtain practical information of CTE data. Furthermore in order to discuss the CTE behavior for a wider temperature range the CTEs at higher temperatures up to 1573 K have been also measured using dilatometer method. The CTE obtained for a nitrogen-doped 4H-SiC single crystal increases continuously from 0.8 ppm/K to 3.1 ppm/K for temperatures of 273 K and 423 K respectively, and further increases to 5.4 ppm/K at 1273 K. We conclude from our data that the CTE variations are likely to be almost independent of the crystal axis directions of SiC from 123 K up to 1573 K.

Abstract: In this paper we combine the use of photo-thermal techniques, Thermal lens (TL) spectrometry, Photoacoustic and heat capacity measurements to determine the optical path dependence with temperature (ds/dT) of a polycrystalline 3C-SiC sample. Results obtained for the polycrystalline sample with the TL technique show that ds/dT is negative at room temperature. This means that the thermal lens formed in 3C-SiC acts as a divergent lens when light impinges the sample. Our measurements, demonstrate that photo-thermal techniques can be used to obtain thermal parameters in circumstances where other techniques cannot be used, for example, in harsh environments.

Abstract: The work deals with the highly important problem of the qualitative temperature dependence of avalanche breakdown voltage in p-n junctions based on 4H-SiC. As it has been shown before, the temperature coefficient of avalanche breakdown voltage (TCABV) is negative in seven SiC polytypes, including 4Н-SiC. This effect has been explained by the Wannier-Stark localization (WSL). It is worth noting that the plane of the investigated p-n junctions coincided with the basal plane (0001). However the current SiC device technology prefers 4H-SiC p-n junction formation on a plane that has 8о disorientation from (0001). This may result in a weakening of the WSL and, correspondingly, in a positive TCABV. This problem has been elucidated in the present paper. The photocurrent of 4H-SiC p-n junctions in a strong electric field has been scrutinized, that has allowed to discover a negative differential conductivity region and it has testified to the WSL process and negative TCABV.