Kunikazu Izumi

Epitaxial growth of thick 4H–SiC layers in a vertical radiant-heating reactor

A vertical radiant-heating reactor has been developed for thick silicon carbide (SiC) epitaxial growth, in which the susceptor and substrates are heated by radiation from the hot wall. The benefit of the heating and sample-holding method is demonstrated by improvements in the curvature of crystal bending and FWHM of X-ray ω-rocking curves followed by epitaxial growth. The typical growth rate is 13 16 μm/h at 1530 1550 C at the susceptor top under reduced pressure as low as 50 70 mbar. Low background doping at low 10 13 cm 3 (N d - N a ) was achieved, and some of the 4H SiC epilayers exhibited a high resistivity. We also succeeded in growing a 4H SiC epilayer over 240 μm-thick with minimal surface roughness. Little sigh of impurities was observed by low-temperature photoluminescence (LTPL), and no impurities (Al, B, Ti, V and Cr) exceeding I × 10 14 cm 3 were found by secondary ion mass spectroscopy (SIMS) for a 150 μm-thick 4H SiC epilayer. Thickness and doping uniformity along the gas flow of 5% and 11%, respectively, were obtained for 2-in substrates. Molten KOH etching analysis revealed that some of the micropipes were dissociated into closed core screw dislocations during epitaxial growth. The electrical performance of high-voltage devices was also demonstrated.

Development and testing of prototype models for a high-performance 300 MW self-commutated AC/DC converter

A technical project is under way in Japan to develop a high-performance self-commutated power converter for future HVDC transmission and DC interconnection applications. In the first stage of the project, prototype power converter models for a 300 MW self-commutated converter were developed. The models were subjected to factory testing to verify the technology for series connection of a large number of gate turn-off thyristors (GTOs), a gate power supply from the high voltage main circuit and energy regeneration using snubber circuits. Satisfactory results were obtained.

Structural Transformation of Screw Dislocations via Thick 4H-SiC Epitaxial Growth

In this paper, we studied the structural transformation of screw dislocations through gas-phase 4H-SiC epitaxial growth. We confirmed based on the numbers and features of etch pits on a surface after KOH treatment that some micropipes were closed in the epitaxial layer, and were divided into several elementary screw dislocations. We discuss the magnitude of Burgers vectors of micropipes in 4H-SiC substrates in relation to the number of elementary screw dislocations generated by micropipe closing. A depth analysis further revealed that most micropipe closings took place in the initial stage of epitaxial growth.

INFRARED SPECTROSCOPY OF HYDRIDES ON THE 6H-SIC SURFACE

Hydrides on the 6H-SiC(0001) and (0001) surfaces before and after hydrogen annealing were investigated by infrared attenuated total reflection spectroscopy. The absorption bands of CH2 and CH3 were observed from the 6H-SiC(0001) and (0001) surfaces before hydrogen annealing. After hydrogen annealing, a sharp C–H stretching vibration attributable to a monohydride appeared on the 6H-SiC(0001) surface, in contrast to a sharp Si–H stretching vibration which can be observed on the 6H-SiC(0001) surface.

Infrared attenuated total reflection spectroscopy of 6H–SiC(0001) and (0001̄) surfaces

Fourier-transformed infrared attenuated total reflection (FTIR–ATR) spectroscopy was used to study the adsorbates on 6H–SiC(0001) and (0001) surfaces after chemical treatments and after heat treatments in hydrogen. We obtained clear absorption bands attributable to hydrides and oxides on the SiC surfaces. The ATR spectroscopy revealed polarity dependencies of Si–H and C–H stretch modes between the 6H–SiC(0001) and (0001). The surface Si–H bonds on 6H–SiC(0001) formed by the heat treatment in hydrogen were found to have a quite long life time against air exposure, and the heat treatment in hydrogen also produced a flat (0001) surface with ordered surface C–H bonds. The oxidation characteristics of 6H–SiC(0001) and (0001) surfaces were also studied, and we discussed the roles of surface treatments using H2O2 solution and H2SO4 solution at elevated temperature.

Si–H Bonds on the 6H–SiC(0001) Surface after H2 Annealing

The Si–H bonds on the 6H–SiC(0001) on-axis surface were investigated using Fourier-transformed infrared attenuated total reflection (FTIR-ATR). The clear absorption bands of the Si–H stretching vibrations were observed from the 6H–SiC(0001) on-axis surface after H2 annealing. The configuration of the Si–H bonds on the surface was discussed from the polarized spectra, the chemical characteristics and the electronegativities of the atoms bonded to the Si. The FTIR-ATR spectra suggested that the 6H–SiC(0001) on-axis surface after H2 annealing at 1000° C was primarily terminated by silicon monohydride with high regularity.

Influence of 4H-SiC Growth Conditions on Micropipe Dissociation

In this study, we investigated the influence of 4H–SiC growth conditions on micropipe dissociation. The C/Si ratio of the reactant gases for chemical vapor deposition (CVD) epitaxial growth has a major influence on the probability of micropipe dissociation. A high probability of micropipe dissociation of over 98% was successfully obtained at a low C/Si ratio. We also investigated the surface morphology of 4H–SiC epilayers around dissociated and continuous micropipes grown under different C/Si growth conditions.

Investigation on abnormal phenomena of contacts using disconnecting switch and detachable bus in 300 kV GIS

Using the disconnecting switch and the detachable bus of a gas-insulated switchgear (GIS), rated voltage 300 kV and rated current 2000 A, current and voltage-current tests were conducted in simulation of abnormal contact likely to occur in actual operations, yielding the following results: under the abnormal condition of the arcing contact of the disconnecting switch, continuous arcing occurs with a current above a certain level, resulting in a ground fault. Under the abnormal condition of the main contacts of the disconnecting switch, continuous arcing occurs, resulting in a ground fault if products decomposed by arcing are deposited on the contacts. In conductors of the detachable bus, arcing occurs with current above a certain level if there is a gap between the current-carrying surfaces even when bolts are used. The progress thereafter can be roughly divided into two categories. With narrow gaps between the current-carrying surfaces, the current-carrying surfaces are welded together with the molten metal produced by arcing. With wide gaps between the current-carrying surfaces, continuous arcing occurs. Rapid overheating or arcing at contacts is likely to occur not only when the current is raised by also when sunlight shifts. >

Chemical states of crystalline silicon carbide surfaces

Chemical states of 6H polytype crystalline silicon carbide (6H-SiC) surfaces were investigated using X-ray photoelectron spectroscopy (XPS). Surface contaminants such as C-C, C-H and C-O species were evaluated from C1s photoelectron spectra after treatment under various conditions. Clean SiC surfaces were found to appear after the chemical etching of a thermal oxide ( SiO2) film using a buffered HF solution. Furthermore, to clarify the chemical etching characteristics of SiO2 formed on 6H-SiC substrates, the depth profiling of the SiO2 was also performed using XPS. The C1s peak at a binding energy of about 286.2 eV was detected on the surfaces of the 6H-SiC substrates at the moment when only the SiO2 was completely removed, and the C1s peak was characterized as due to the C-O bonds formed near the SiO2/SiC interface.

Improvement in Electrical Properties of 4H-SiC Epilayers by Micropipe Dissociation

In this study, the effect of micropipe dissociation via 4H-SiC vapor phase epitaxy on the electrical properties of the epilayer was investigated. Ni/4H-SiC Schottky barrier diodes (SBDs) were fabricated on a 21-µm-thick epilayer, and their reverse blocking characteristics were examined. Some micropipes dissociated into closed core screw dislocations during epitaxial growth and some of them propagated into the epilayer. The SBDs that included a propagated micropipe showed poor reverse blocking performance, as reported previously, while the breakdown voltages were below -400 V. On the other hand, the SBDs that included a dissociated micropipe withstood breakdown up to -1000 V, where the leakage current density was 10-2 to 10-6 A/cm2. This demonstrated that a great improvement of the electrical properties could be achieved by the structural transformation of micropipes into closed core screw dislocations.