Vol.6 No.3 2014

Research paper : Development of diamond-based power devices (S. SHIKATA et al.)−150−Synthesiology - English edition Vol.6 No.3 (2013) from 0.8 to 3 m/hr, or over five times, compared to the 0.2 m/hr or less of the conventional plasma density (750 W). To maintain the breakdown voltage of the power device, a thick drift layer is necessary as the active layer, so the drift layer epitaxial growth of 10 m/hr or more is required. In diamond, since the electrical breakdown field is high and only one order less thickness is necessary compared to Si, the epitaxial growth rate obtained in this research is thought to be sufficient for practical application.As described above, the technology for flat-polishing the wafer to obtain arbitrary crystal off angle and off direction was established, and this enabled nano step control as well as epitaxial growth without killer defects. Prior to the researches of devices and crystal epitaxial growth, we were able to develop the technology by returning all the way to the polishing technology. Thus, we pursued the Full Research that involved basic research to application.2) Schottky interface formation to enable high-temperature operationEven the mechanism of the reverse-biased leakage of Schottoky interface was unknown in 2005, and we had to start from basic research. To simplify the process, the investigation at this stage was conducted using the pseudo vertical strucutreTerm 3 shown in Fig. 3.[12] The diamond Schottky barrier diode (SBD) was fabricated and the temperature dependence of the reverse leakage current was analyzed. The leakage current increased with the increase in temperature. For example it increased from 10 A/cm2 (@2 MV/cm) at 23 °C, to 10 mA/cm2 at 120 °C. Such figures for the current density level were several digits less than the leakage current observed for SiC SBD in the same electric field. It is difficult to analyze this leakage current according to the model of decreased barrier induced by electric field that is used generally to understand the reverse-biased leakage in Si SBC and GaAs SBD. It was found that the behavior of current voltage property could be explained mostly by using the TFE modelTerm 4 taking into consideration the tunnel process by the application high electric field.[13][14] It was necessary to increase the barrier height because the device operation limit was reached by the thermal heat emission current, before the operation limitation due to current amplification led by the avalanche breakdown.Term 5 Of course, the operating voltage increases if the barrier height is raised, but this was not a problem assuming the high-temperature operation in this case. Therefore, we attempted the method of introducing the localized level for pinning the Fermi level, by applying surface treatment to the Schottky interface. In considering the dry treatment of the diamond surface, we found a way to maintain the high barrier height by introducing the localized level stably through the UV/O3 treatment.[15] We were also able to observe the reverse field that reached 3.1 MV/cm. Although this localized level has not been identified, we decided to utilize it for engineering purposes. When the Schottky diode was fabricated using this method, the reverse leakage current three order less compared to SiC at high temperature,[16] good forward-biased properties (forward voltage that does not decrease too much at high temperature and low on-resistance due to increased carrier) were observed[17] (Fig. 8).3) Refractory metalThe next explanation is the breakthrough in the search of heat refractory Schottky electrode. At the time, the heat resistant ohmic junction was already developed, and it was known that TiAu materials including TiPtAu and TiMoAu presented extremely high heat resistance.[18] The difficulty was the Schottky junction. The hurdle was high for the simultaneous achievement of Schottky property, low resistance, adhesiveness, simple process (wafer process and wire bond), as well as heat resistance, and the feasibility of the research was unknown. Investigations were done from both aspects of materials that formed carbide by reacting with diamond at high temperature and those that do not form carbides, but the most prospective stable carbide WC had high resistance and sufficiently high heat resistance could not be obtained.[19] Therefore, we shifted the focus to non carbide forming metals with high melting points. Various metals were tested, and Mo was found to have 750 W4000 WKiller defect density (/cm2)Substrate off angle (degree)43210107106105104103102101100 296 K323 K364 K415 K0.95-1.0eV310 K350 K390 K430 K(b) Example of SiC (from Reference [16] T. Hatakeyama et al.)(a) Diamond(device with high barrier height after surface treatment)Current density (A/cm2)Reverse-biased field (MV/cm)Reverse-biased field (MV/cm)10010-810-710-610-510-410-310-210-1SiC SBDDiamond SBD00.511.5Current density (A/cm2)10010-810-710-610-510-410-310-210-100.511.5Fig. 7 Dependence of killer defect, substrate off angle, and plasma densityFig. 8 Reverse leakage current of the Schottky junctio


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