Materials Science Forum Vols. 527-529

Abstract: We have proposed a novel model of hydrogen etching of SiC based on thermal equilibrium and have confirmed the validity of our model through the analysis of H2 etching experiments. The experimental results obtained showed that the etching rate is expressed by a linear equation with the H2 flow rate, by an exponential function with the reciprocal of the temperature and by a power law with the pressure. These results agree well with the theoretical behaviors derived from our model.

Abstract: In this paper, we attempted to grow semi-insulating SiC epitaxial layer by in-situ iron doping. Homoepitaxial growth of iron-doped 4H-SiC layer was performed by MOCVD using organo-silicon precursor, bis-trimethylsilylmethane (BTMSM, [C7H20Si2]) and metal organic precursor, t-butylferrocene ([C14H17Fe]). Doping-induced crystallinity degradation showed different tendency depending on conducting type of substrate. The crystal quality of epilayer grown on n-type substrate was not degraded significantly despite of the Fe doping but in case of semi-insulating substrate, crystallinity was remarkably degraded as increasing iron contents. For measurement of resistivity of highly resistive iron-doped 4H-SiC epilayer, we used the on-resistance technique which is firstly attempted for measuring resistivity of epilayer. From on-resistance of epilayer measured by I-V, it is shown that the residual donor concentration of epilayer was decreased as increasing partial pressure of t-butylferrocene. The resistivity of iron-doped 4H-SiC epilayer was about 107 Ωcm. From this result, it is concluded that Fe could effectively act as a compensation center in the iron-doped 4H-SiC.

Abstract: 4H-SiC layers have been homoepitaxially grown on 4°off-axis (0001) and (000-1) under various conditions by horizontal hot-wall CVD. We have investigated surface morphology and background doping concentration of the epi-layers on 4°off-axis substrates. Surface morphology grown on the (0001) Si-face showed strong step bunching under C-rich conditions. On the other hand, smooth surface morphology on the (000-1) C-face could be grown in the wide C/Si ratio range at 1600 °C. Site-competition behavior is clearly observed under low-pressure growth conditions on 4°off-axis (000-1) C-face, leading to a lowest doping concentration of 4.4x1014 cm-3.

Abstract: We have investigated the morphology and doping characteristics of 4H-SiC epilayers grown on 13o-22o off-axis (0001) substrates by horizontal hot-wall CVD. Step bunching is not observed on 18o off-axis substrates as well as on other substrates with large off-angles. The rms roughness is a minimum (as small as 0.10 nm) on 18o off-axis (0001). Under C-rich condition (C/Si >1.0), the donor concentration increases by increasing the off-angle, when the off-angle is larger than 15o. This trend in doping characteristic is enhanced in CVD growth at 1450-1500oC. At a high temperature of 1600oC, however, the off-angle dependence of donor concentration is significantly reduced. Epitaxial growth on 4H-SiC(000-1) with large off-angles is also reported.

Abstract: Thick layers have been grown on (1120) and (1 1 00)6H-SiC substrates. The thicknesses are up to 90 #m obtained with growth rate of 180 #m/h. Even in such thick layers the surfaces are smooth and only disturbed by polishing scratches. The surfaces contain steps which facilitate reproduction of the substrate polytype.

Abstract: Propagation and nucleation of basal plane dislocations (BPDs) in 4H-SiC(000-1) and (0001) epitaxy were compared. Synchrotron reflection X-ray topography was performed before and after epitaxial growth to classify the BPDs into those propagated from the substrate into the epilayer and those nucleated in the epilayer. It was revealed that the propagation ratio of BPDs for the (000-1) epitaxy was significantly smaller than that for the (0001) epitaxy. Growing (000-1) epilayers at a high C/Si ratio of 1.2 achieves a further reduction in BPDs to only 3 cm-2 for those propagated from the substrate, and 16 cm-2 for those nucleated in the epilayer. A dramatic increase was also found in the nucleation of BPDs omitting the re-polishing and in-situ H2 etching procedure.

Abstract: The origin of the polytypes of SiC has been investigated from the viewpoint of surface reactions by the density functional theory (DFT) within the Projector Augmented Wave-Generalized Gradient Approximation. Three radicals were considered here as the major species in the crystal growth process: Si, Si2C and SiC2. We supposed that these radicals contribute to the crystal growth directly through the adsorption on the 4H-SiC (000-1) C-face surface. The DFT calculations showed that the Si2C, which relatively has a similar structure with the SiC crystal, had no activation barrier to be adsorbed chemically to the 4H-SiC C-face surface. On the other hand, SiC2 with Si showed an activation barrier of 0.79eV to form the 4H-SiC crystal. In order to investigate the arrangements to decide polytypism in SiC, we compared the adsorption energies between the different sites, which correspond to the 4H-SiC crystal and a disordered arrangement. The activation energies had almost no difference. Our calculations indicated that these radicals do not contribute to the origin of the polytypes of SiC.

Abstract: This work presents results on the growth of thick epitaxial layers on 4° off-oriented 4HSiC in a commercially available hot-wall CVD system. Results on background doping level, homogeneity of thickness and doping, and run-to-run reproducibility will be shown. Defect structures we observed on 4° off-oriented substrates only are discussed. AFM measurements are presented to show the degree of step-bunching. 6.5 kV PiN-diodes with an active device area of 5.7 mm2 were fabricated and electrically characterized. In spite of the surface defects and stepbunching, up to 50% of the devices per wafer fulfilled our strict yield criteria even at 6.5 kV. Up to the onset of avalanche, the devices exhibited extremely low leakage currents. These results turn the cheaper 4° off-oriented substrates into a promising choice for producing higher volumes of highvoltage SiC power devices at reasonable costs.

Abstract: A method was developed in our laboratory to grow low basal plane dislocation (BPD) density and BPD-free SiC epilayers. The key approach is to subject the SiC substrates to defect preferential etching, followed by conventional epitaxial growth. It was found that the creation of BPD etch pits on the substrates can greatly enhance the conversion of BPDs to threading edge dislocations (TEDs) during epitaxy, and thus low BPD density and BPD-free SiC epilayers are obtained. The reason why BPD etch pits can promote the above conversion is discussed. The SiC epilayer growth by this method is very promising in overcoming forward voltage drop degradation of SiC PiN diodes.

Abstract: We report on further observations of homoepitaxially grown 4H silicon carbide (SiC) cantilevers on commercial on-axis mesa patterned substrates. Mesa shapes with hollow interiors were designed to significantly increase the ratio of dislocation-free cantilever area to pregrowth mesa area. Mesas that did not contain axial screw dislocations (SD’s) continued to expand laterally until uncontrolled growth in the trench regions rises up to interfere / merge with the laterally expanding cantilevers. Molten KOH etching revealed high defect density in regions where trench growth merged with the laterally expanding cantilevers. The remaining portions of the cantilevers, except for central coalescence points, remained free of dislocations.