Papers by Author: Tetsuo Hatakeyama

Abstract: SiO2/4H-SiC interfaces are examined by high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and spatially resolved electron energy-loss spectroscopy (EELS). HRTEM and HAADF-STEM images of SiO2/4H-SiC interfaces reveal that abrupt interfaces are formed irrespective of the fabrication conditions. Transition regions around the interfaces reported by Zheleva et al. were not observed. Using EELS, profiles of the C/Si and O/Si ratios across an interface were measured. Our measurements did not reveal a C-rich region on the SiC side of the interface, which was reported by Zheleva et al.

Abstract: Previous simulation works and experiments on the loss of 4H-SiC floating junction Schottky barrier diodes (Super-SBDs) show that the loss is related to the doping concentration in the drift region and the pattern of the floating layer. The effect of the doping concentration for lowering the loss is characterized the breakdown voltage (Vbd) and the on-state resistances (RonS) of the Super-SBDs based on Baliga’s figure of Merit (BFOM). Experimental devices with two doping concentrations in the drift region are fabricated to investigate the static characteristics: Vbd and RonS. The Vbd of the Super-SBDs is close to the simulation result, near 3000 V. However the tendency of the Vbd by the doping concentration is not similar to the simulation result. And the RonS are about 3.22 mcm2 which is higher than that of simulation result. The doping concentration optimized in this study does not show significant lowering loss and the design of the floating layer in the termination region affect the low-loss static characteristics of the Super-SBD. In addition, adopting PiN structure with floating layer (Super-PiN) affects the low-loss dynamic characteristics, optimizing the doping concentration in the drift region. We conclude that the fabricated Super-SBDs with the floating layer in the termination region, the drift region with a doping concentration of 1.01016 cm-3 and mesa-shaped termination structure, have excellent Vbd of 2990 V which is almost same as that of simulation result and RonS of 3.22 mcm2.

Abstract: The oxide reliability of metal-oxide-semiconductor (MOS) capacitors on 4H-SiC(000-1) carbon face was investigated. The gate oxide was fabricated by using N2O nitridation. The effective conduction band offset (Ec) of MOS structure fabricated by N2O nitridation was increased to 2.2 eV compared with Ec = 1.7 eV for pyrogenic oxidation sample of. Furthermore, significant improvements in the oxide reliability were observed by time-dependent dielectric breakdown (TDDB) measurement. It is suggested that the N2O nitridation as a method of gate oxide fabrication satisfies oxide reliability on 4H-SiC(000-1) carbon face MOSFETs.

Abstract: The calculation for 4H-SiC floating junction Schottky barrier diodes (Super-SBDs) was carried out by device simulation and the optimized device structure was fabricated. The best characteristics of the Super-SBDs were breakdown voltage of 2700V and the specific on-resistance of 2.57m*cm2. The world record of Bariga’s Figure of Merit (BFOM) for SiC-SBD expressed by 4Vbd 2/Ron was improved to 11,354MW/cm2.

Abstract: Scaling theory is applied in the design of power devices. The scaling law for power devices is presented. A new figure of merit (HFOM) is derived as an invariant of scale transformation, which is a function of avalanche breakdown field and regarded as a measure of the performance of a power device. The optimization of a SiC Schottky barrier diode with the floating junction structure (Super-SBD) has been performed using the HFOM as a measure of the performance. The performance of the optimized Super-SBD surpasses the performance limit of 4H-SiC devices with the conventional structure.

Abstract: 4H-SiC floating junction Schottky barrier diodes (Super-SBDs) were fabricated. It was found that their properties are closest to the theoretical limitation, defined by the relationship between specific on-state resistance and breakdown voltage of 4H SiC-unipolar devices. They have a p-type floating layer designed as line-and-spacing. The specific on-state resistances of Super-SBDs with a few micrometers of spacing width were found to be nearly equal to those of conventional SBDs without p-type floating layer. The breakdown voltages of Super-SBDs were higher than those of conventional SBDs. Accordingly the properties of Super-SBDs have improved the trade-off between specific on-state resistance and breakdown voltage, and the highest value to date for Baliga’s Figure of Merit (BFOM) has been obtained.

Abstract: In this study, evaluation of residual stress in nano-TiO2 film on ITO glass is carried out. The films with thickness less than 30 nm are prepared by the dual-arcs magnetron sputtering with gas pressure 10 and 20 Pa. The surface microstructure and grain morphology of the nano-TiO2 films are observed by the atomic force microscopy (AFM). In order to accurately evaluate the residual stress in the film, the Young’s modulus of the film is determined by the nanoindentation with three point bending method at first, then the internal residual stress in the film is measured by high energy X-ray diffraction with the synchrotron radiation facility Spring-8. The measured residual stresses of nano-TiO2 films prepared with gas pressure 10 and 20 Pa are -11.6 and -9.1 GPa, respectively. It is shown that the residual stress of TiO2 films decrease with the increasing of gas pressure.

Abstract: This paper describes process and device simulation results of SiC floating junction Schottky barrier diodes (Super-SBDs). Two-dimensional process simulation of a SiC device is implemented using the customized ISE’s process simulator “DIOS”. The simulation results reproduce the experimentally observed buried floating junction structure of a SiC Super-SBD. The device simulation method using the anisotropic impact ionization coefficients is formulated. The effect of anisotropic avalanche breakdown field on termination structures of SiC SBDs is examined. Finally, by the device simulation we have shown that the trade-off between the on-state resistance and the breakdown voltage of the super-SBD that contains two drift layers exceeds that of the conventional SBD.

Abstract: A 4H-SiC 600 V class Deep-Implanted gate Vertical JFET (DI-VJFET) is examined. The DI-VJFET exhibited a specific on-resistance of 13 mΩcm2, drain current of 5 A, and a blocking-voltage of 600 V. In this paper, the very high temperature dependence (R.T.~ 400 oC) of the I-V characteristics is measured and the dominant factor of the on-resistance and the blocking-voltage is discussed. Moreover, the switching waveform of SiC DI-VJFET with SiC SBD is measured by using a half bridge, double-pulse circuit with inductive load at R.T. and 200 oC. The turn-off time is 300 ns at an inductance of 4 mH and an external gate resistance of 100 Ω.

Abstract: This paper reports on the degradation of inversion channel mobility of SiC MOSFET caused by the increase of channel doping. SiC MOSFETs were fabricated on three wafers, the doping concentrations of the epitaxial layer of which were 16 10 2× cm-3 (sample A), 17 10 2× cm-3 (sample B) and 17 10 4× cm-3 (sample C). The field effect mobility sharply decreases as the doping concentration increases. Hall mobility measurements have been done to investigate the degradation of the mobility due to doping. The measurement of sample A shows that, as a consequence of the decrease of the free carrier density due to MOS interface traps, the Hall mobility is as much as a factor of ten higher than the field effect mobility. In contrast, in regard to the measurement of sample B and sample C, we encountered unstable Hall voltage and could not obtain reproducible results. This implies that such high-density traps are generated that a channel disappears in the higher-doping samples.