Materials Science Forum Vol. 1089

Abstract: High-quality, low resistivity n-type (nitrogen-doped) single crystal 4H-SiC wafers are needed to grow high-quality epitaxial SiC layers used for the active blocking layers of high-voltage power devices. The resistance of the substrate constitutes a portion of the device resistance for vertical devices, and therefore the SiC substrate properties must be fully characterized. In this study we report the 4H-SiC substrate electrical properties as a function of temperature measured using van der Pauw structures to measure resistivity from 4-point measurements, and carrier concentration and mobility from Hall effect measurements. We find that the SiC substrate resistivity has a minimum around 425K for typical substrate doping levels, due to a competition between the decreasing mobility and increasing carrier concentration with increasing temperature. The measured energy levels of the N donor (hexagonal / cubic sites) are extracted for a 5.8×10¹⁸ cm^-3 N-doped substrate, and found to be 15 meV and 105 meV, respectively.

Abstract: The reconstruction of 4°off 4H-SiC surfaces was investigated using Si melting at 1550°C in a SiC/Si/SiC sandwich configuration. Despite systematically obtaining a macrostepped morphology over the entire areas in contact with the liquid Si, the steps were found wavy when using as-received 4H-SiC wafers. The regularity and straightness of the steps were significantly improved when the surface reconstruction was performed on processed surfaces: on re-polished surfaces the steps were found to be regular and straight in some cases, while this was constantly observed on as-grown epitaxial layers. After a reconstruction process of 2h, the best regularity of the steps was obtained with an average width of ̴ 3-5 μm. Increasing the processed area from 1.44 to 4 cm² did not affect the results which suggests a good scalability of the process.

Abstract: Hysteresis response of epitaxially grown graphene nanoribbons devices on semi-insulating 4H-SiC in the armchair and zigzag directions is evaluated and studied. The influence of the orientation of fabrication and dimensions of graphene nanoribbons on the hysteresis effect reveals the metallic and semiconducting nature graphene nanoribbons. The hysteresis response of armchair based graphene nanoribbon side gate and top gated devices implies the influence of gate field electric strength and the contribution of surface traps, adsorbents, and initial defects on graphene as the primary sources of hysteresis. Additionally, passivation with AlO_x and top gate modulation decreased the hysteresis and improved the current-voltage characteristics.

Abstract: We discuss the quantification of the secondary electron doping contrast in the scanningelectron microscope on 4H-SiC. It has been observed and studied at length mostly on silicon, but noconclusive theoretical model has been proposed yet. Therefore, we propose a simple physical modelthat allows for a quantification of the doping contrast. It is based on the changes in effective ionizationenergy for different doping concentrations and types. For a better agreement between our model andthe experiment, a locally increased temperature of the electron system or separate quasi Fermi levelsfor electrons and holes have to be assumed. A line profile of the sample under investigation is comparedwith a SRIM simulation of the corresponding implant and shows very good agreement.

Abstract: Photoluminescence (PL) signatures of 4H-SiC bare and epitaxial wafers from a surface inspection tool have been studied. Large variations in PL black or white dot densities were confirmed for comparable crystal quality and growth process conditions. Comparison with KOH etching results confirms that both PL black and white dots are tied to discrete threading dislocations. PL spectra results suggest dislocation decoration by donor-acceptor pairs.

Abstract: Nanostructuring of the surface occurs after annealing at high temperature of 4H-SiC samples. The surface morphology becomes needle-shaped like black silicon. The roughness of the surface also increases due to annealing and a slight etching of nanostructured zones occurs with an accentuated phenomenon at the boundaries. Electroluminescence is obtained by applied forward bias on fabricated PIN diode structures with localized nanostructurated windows in surface. Light intensity seems to be more sensitive to the initial orientation of the substrate and less to the annealing temperature in the 1500°C-1700°C range.

Abstract: In 4H-silicon carbide crystals, basal plane slip is the predominant deformation mechanism. However, prismatic slip is often observed in single crystals grown by the physical vapor transport method as the diameter expands to 6 inches or larger. Thermal modeling has shown that occurrence of prismatic slip is attributed to increased radial thermal gradients. While X-ray topography can be used to characterize the presence and extent of prismatic slip, the feasibility of using the chemical etching method to assess the extent of prismatic slip in an industrial setting is investigated. The distribution of scallop shaped etch pits oriented along the directions that correspond to prismatic dislocations, correlate well with the results of the thermal model that predicts the occurrence of prismatic slip dislocations. This capability of the etch pit method to characterize prismatic slip can be used to manage radial thermal gradients during PVT growth.

Abstract: An increasing interest in the non-contact corona charge-based electrical characterization technique, CnCV, for wide bandgap semiconductors, is justified by the reduction of cost and the reduction of testing feedback time [1]. In addition, the technique expands measurement capabilities. Regarding SiC, recent progress includes expanded dopant concentration range and dopant measurement on fresh epitaxial wafers. The latter is made possible with an ultraviolet wafer pretreatment technique [2]. The novel applications to AlGaN/GaN HEMT on insulating substrates demonstrate the benefits of a noninvasive top side edge contact, TSEC [3], that eliminates the problem of the floating surface potential. This development enables a unique three variable, charge-voltage-capacitance (Q-V-C), characterization of AlGaN/GaN on sapphire and SI-SiC. The quasistatic CnCV measurement, not affected by series resistance, is shown to be suitable for wafer mapping of HEMT parameters. The CnCV version with TSEC can be combined with an eddy current technique enabling non-contact 2DEG mobility measurements vs. electron sheet density.

Abstract: In this work, we investigate the correlation between tensile residual stress and Q-factor of double-clamped beams fabricated on epitaxial 3C-SiC layers grown on both <100> and <111> silicon substrates, using a completely optical measurement setup to measure the Q-factor of the resonators and the residual stress of the layers by means of purposely designed micromachined test structures. From the measurements, a clear correlation appears between the residual stress of the SiC layer and the Q-factor of the resonators, with Q-factor values above half a million for resonators fabricated on <111> substrates, showing residual stress around 1 GPa.