Vol.6 No.3 2014
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Research paper : Development of diamond-based power devices (S. SHIKATA et al.)−149−Synthesiology - English edition Vol.6 No.3 (2013) conventional polishing technology. We decided to take this to the basics, and started from the design and fabrication of the polishing device. The two points of the polishing device development were as follows.(1) An x-ray Laue goniometer was mounted on the polishing head to measure the off angle and off direction by x-ray analysis, to enable polishing in any arbitrary direction.(2) A weight was placed on the high-rigidity arm for weight adjustment, and the lap was designed with a low vibration structure.By developing the polishing plate and polishing process as well as the polishing device, we were able to achieve step formation after applying ultra flat processing (arithmetical mean roughness of Ra < 1 nm) on substrates with various off angles and off directions. Using such formations, we investigated the epitaxial layer growth. The epitaxial layer was formed using the CH4 and H2 gases with trimethylboron (TMB) as B dopant gas, using the 2.45 GHz microwave chemical vapor deposition (CVD) that is employed normally. While the details will be abbreviated, it was found that in the microwave CVD growth using low density plasma, the abnormal particle defects did not decrease even by changing the off angles and off directions, step flow did not occur well, and there were some variations depending on plasma density. Therefore, we remodeled the CVD equipment to use high-powered plasma. The dependence of the off direction was studied by increasing the power of microwave from 0.75 kW to 4 kW.As a result, it was found that a giant growth hillock tended to form between <110> and <100>. There were no such hillocks at directions <110> and <100>, nor were there abnormal particles, and an extremely flat surface could be obtained at 2 degrees or more, without dependence on the off angle.[11] Particularly for direction <110>, it was easily estimated that the step flow growth was readily formed since the carbon atoms on the surface formed the dimer row. Figure 7 shows the dependence of the defect formed by the epitaxial growth and the off angles. The situation obtained when the plasma density was changed is also shown. Hence, we succeeded in reducing the killer defect of 105 cm-2 to almost zero.[11] The flatness Ra obtained by measuring the epitaxial film by AFM was 1.1 nm. When the mobility of the holes in the diamond was measured by hole effect measurement, it was high at 1540 cm2/Vs, and it was found to be a high quality film. The rate of epitaxial growth in this session (4 kW) was high Elimination of epitaxial layer defect⑥Electrical field relaxationPhase 1: Concept verification Subject of this paper (1 A class trial)⑤Process development⑧High-speed switchingPhase 2: Development of practical device Future effort (100 A class verification)Ultra low-defect epi②Elimination of killer defect③Surface treatment①Electrical breakdown fieldBreakthroughBreakthroughBreakthroughBreakthrough④Refractory SchottkyElemental verification: pseudo vertical structure deviceUltra heat resistance⑦High current densityHigh-temperature operationHigh-temperature operationHigh breakdown voltageSuperiority verification: small vertical deviceHigh-current operation100 A class vertical structure diodeTransistor developmentHigh-temperature packaging technologygyHigh-temperature durable packagingMIS structure(a)Pseudo vertical structure used in initial experiment(b)Vertical structurep-p-p+p+OhmicOhmicInsulated substrateSchottkySchottky20 um (Killer defect)0 0.2 0.4 0.6 0.8 1 0 4 8 12 Yield=A*exp (-defect × surface area)Defect density=1 × 105/cm2Device surface area (cm-2)Device yield (%)Fig. 3 Synthesiology tree diagram for the superiority verification of the diamond power device(Numerals are the order of researches conducted)Fig. 4 Structural diagram of the deviceFig. 6 Relationship between the defect in device and its yieldFig. 5 Killer defect present in the epi layer of diamond

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