Vol.6 No.3 2014

Research paper : Development of diamond-based power devices (S. SHIKATA et al.)−148−Synthesiology - English edition Vol.6 No.3 (2013) started for the first time in the world on the research of its use in wafers and devices for application in power devices. For wafers, there is a report on part of the developmental process (direct wafer technology where the single crystals are fabricated as if being copied, and the realization of crystals with 12 mm sides).[7] This was followed by the achievement of mosaic crystal,[8] and recently the 20×40 mm2 dimension has been achieved.[9] In this paper, we report Phase 1 of an R&D where the superiority of diamond was verified from the vantage point of devices, taking the example of the Schottky barrier diode (SBD).2 Research scenarioTo realize diamond as a next-generation power semiconductor device, there are, of course, various issues in each phase. The following points must be verified to establish superiority over other materials in Phase 1 (superiority verification).1) High breakdown voltage (verification of property that surpasses other materials)2) Operation in high current (density) (verification that high output can be achieved in high temperature)3) Operation in high temperature (verification of property that enables new concept)4) Verification of high-speed switching operationAmong the above points, 1) and 3) can be verified using the pseudo vertical device (to match the explanation in the figure) where the process can be carried out relatively easily, but for 2) and 4), a vertical structureTerm 3 that enables practical use is necessary.The above points were summarized from the perspective of synthesiology in Fig. 3. These are like the combination of the aufheben and breakthrough types that are basic synthesis methods,[10] and it can be concluded that the accumulation of elemental technologies and the breakthrough that enables that are necessary. Figure 4 supplements the explanation of the pseudo vertical and vertical devices shown in Fig. 3. In this synthesis diagram, the low-defect epitaxial growth in the active layer of device (elimination of killer defect in Phase 1) and the heat resistant Schottky formation for high-temperature operation are considered very difficult issues. As shown in Fig. 5, there are defects present due to abnormal growth of diamond in the epitaxial film. In this example, the defects in the growth hillock appear as holes, and these are “killer defects” that are fatal to device operation. This was determined since we detected what seemed like a superimposition of the ohmic flow-through current when we conducted property assessment after fabricating the diode. By studying the relationship between the device yield and the surface area, it was shown quantitatively that such defects directly affect the device yield as shown in Fig. 6. In the example of this epi film, the defect density reached 105 defect/cm2.3 Example of the elemental technology developmentIn this paper, the items that were breakthroughs for solving the issues of fundamental technology of the devices will be outlined.1) Elimination of killer defectsFirst, pertaining to the low-defect epitaxial film growth in the drift layer that is the active layer of the device, it is widely known that low defect can be obtained by step flow growth in the normal semiconductor material. Since the diamond has bond energy three times the strength of SiC, arbitrary polishing was difficult, creating reproducible steps on the crystal surface was difficult, and experiments could not be carried out easily. Therefore, we conducted the R&D of polishing technology for obtaining a flat surface in arbitrary direction on the diamond crystal and then forming steps. It became clear that this was totally impossible with the 10210010-110-210-310-4Room temperatureDiamondSiC250 ℃Specific on-resistance(Ωcm2)Breakdown voltage(V)10310410510210010-110-210-310-4Specific on-resistance(Ωcm2)Breakdown voltage(V)103104105SiCDiamondFig. 2 Comparison of the relationship of on-resistance and breakdown voltage at room temperature and 250 °C


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