Solid State Phenomena Vol. 362

Abstract: The chemical vapor deposition (CVD) growth of boron carbide (B_xC) layers on 4H-SiC, 4°off substrates was studied. Depending on the polarity of the substrate, different results were obtained. On Si face, the direct CVD growth at 1600°C under a mixture of BCl₃+C₃H₈ systematically led to polycrystalline B_xC films, whatever the C/B ratio in the gas phase. On the C face, heteroepitaxial growth was obtained for C/B ratios = 12 or higher with a step bunched morphology. If a boridation step (10 min at 1200°C under BCl₃ flow) was used before the CVD growth, then heteroepitaxy was successful on both substrate polarities. To explain these results, a mechanism is proposed which involves the nature of the chemical bonds at the early stage of nucleation. It is suggested that a full B coverage of the SiC surface should favor the nucleation of the B-rich (0001) plane of B_xC, promoting thus the heteroepitaxial growth along this direction.

Abstract: In this paper, we report on the growth of highly uniform MoS₂ films, mostly consisting of monolayers, on SiC surfaces with different doping levels (n- SiC epitaxy, ~10¹⁶ cm^-3, and n⁺ SiC substrate, ~10¹⁹ cm^-3) by sulfurization of a pre-deposited ultra-thin MoO_x films. MoS₂ layers are lowly strained (~0.12% tensile strain) and highly p-type doped (<N_h>≈4×10¹⁹ cm⁻³), due to MoO₃ residues still present after the sulfurization process. Nanoscale resolution I-V analyses by conductive atomic force microscopy (C-AFM) show a strongly rectifying behavior for MoS₂ junction with n^- SiC, whereas the p⁺ MoS₂/n⁺ SiC junction exhibits an enhanced reverse current and a negative differential behavior under forward bias. This latter observation, indicating the occurrence of band-to-band-tunneling from the occupied states of n⁺ SiC conduction band to the empty states of p⁺ MoS₂ valence band, is a confirmation of the very sharp hetero-interface between the two materials. These results pave the way to the fabrication of ultra-fast switching Esaki diodes on 4H-SiC.

Abstract: In order to increase productivity and reduce the cost of wafers, we have developed a high temperature chemical vapor deposition (HTCVD) method that can realize the high-speed growth of 4H-SiC crystals. Tokuda et al. reported an interesting study in which the carrier lifetime of a substrate grown by HTCVD (HTCVD substrate) was considerably shorter than that of the substrate grown by physical vapor transport (PVT); moreover, bipolar degradation was highly suppressed when the HTCVD substrate was applied to PiN diodes [1]. Herein, we demonstrate that the short carrier lifetime of the HTCVD substrate is mainly attributable to the carbon vacancy (V_C) and that V_C particularly diffuses from the HTCVD substrate to the epitaxial layer.

Abstract: A detailed knowledge of the growth front geometry during physical vapor transport (PVT) growth of SiC single crystals is beneficial to achieve high quality n+ SiC substrates for power device applications. In this report we show how mapping of resistivity in SiC wafers can shed light on local growth conditions, which are very difficult-to-study in situ. We consider both thermodynamic quantities (absolute temperature T and partial pressure p_N₂) and geometric characteristics of the growth surface relevant to growth kinetic parameters, namely atomic terrace width and atomic step velocity. Specifically, we show how an elevation map of the growth surface can be reconstructed from a spatially-resolved measurement of resistivity in a SiC wafer by integrating the spatial derivative of elevation with respect to the basal plane, which is assumed to be related to local resistivity through dependence of non-equilibrium nitrogen incorporation on the atomic step velocity.

Abstract: This work discusses the use of gallium nitride (GaN)-based solid-state devices for high-power, high-frequency, and high-temperature technology. The article presents the results of an investigation into the Al fraction of AlGaN as a function of GaN cap growth time through µ-Raman and µ-Photoluminescence (µ-PL) spectroscopy under λ=325 and 266 nm laser source. The data exhibit that the detected Al fraction decreases as the GaN cap layer size increases, consistently with the surface quantum well effect in the layer stack. The study confirms that the GaN cap layer is acting as a potential well and enables the design of a non-destructive and quantitative assessment of the grown thickness of the GaN cap layer through UV laser spectroscopy. The interpretation of the data also rules out the possibility of thermal migration of Al in the adjacent GaN layers during MOCVD growth.

Abstract: The layer structure of 3C-SiC stacked on 4H-SiC is implemented by simultaneous lateral epitaxy (SLE). The SLE, involving spontaneous nucleation of 3C-SiC(111) on the 4H-SiC(0001) surface followed by step-controlled epitaxy, facilitates the creation of a single-domain 3C-SiC layer with an epitaxial relationship to the underlying 4H-SiC, establishing a coherent (111)//(0001) interface aligned in the basal plane. An extremely low state density at an interface between thermally grown SiO₂ and SLE-grown 3C-SiC layer is revealed by local deep level transient spectroscopy (local-DLTS) based on scanning nonlinear dielectric microscopy (SNDM).

Abstract: The enlargement of 4H-SiC seed size from 150 mm (6 inch) to 200 mm (8 inch) is currently underway and 8 inches SiC substrate is now facing the market to switch the actual 6-inch technology to 8-inch technology. The aim of this work is to evaluate the influence on the epitaxial layer (using an epi growth campaign made of 21 consecutive runs) using substrates coming from different vendors (3 different suppliers adopted). The same epitaxial process and same reactor were adopted to grow all the samples. After the growth campaign, a difference of thickness uniformity between the three substrate suppliers was observed while no difference of doping uniformity was detected.

Abstract: A new method for reducing the cost and the fabrication time of source material required for SiC crystal growth has been proposed through a heat treatment of recycled powder bulk in this study. The actual crystal growth with using a conventional powder and a recycled powder bulk source has been performed under identical growth condition and then systematically compared in terms of the crystal quality. With applying the recycled powder bulk for SiC crystal growth, similar growth results were obtained as a result grown by conventional high-purity powder source. In terms of crystal defects, slight improvement was observed when high purity recycled powder bulk source was applied.

Abstract: A modified process condition has been proposed for the growth of high-quality 6-inch 4H-SiC single crystal. Temperature gradient (dT[°C] = T_bottom-T_upper) was controlled by changing coil position in order to investigate the effect of the temperature gradient on the SiC crystal quality. SiC ingot surface and etch pit density (EPD) of etched SiC wafer were investigated according to different dT conditions at the initial stage of SiC crystal growth. The surface of SiC crystal ingots grown with different dT for 10h were observed by OM and etched SiC wafers were prepared from SiC crystal ingots after main growth step for 100h. Different dT conditions in the initial growth stage resulted in dramatically different surface images and the crystal quality evaluated by EPD.