Vol.6 No.3 2014

Research paper : Development of diamond-based power devices (S. SHIKATA et al.)−151−Synthesiology - English edition Vol.6 No.3 (2013) excellent properties in various items. The development was continued with Mo as the prime candidate. However, when studying the deterioration in high temperature using various devices, it was found that although excellent property was observed in the Schottky junction that was formed in the no-defect area of the epitaxial layer, the reverse leakage current increased depending on the annealing time in the defective epi layer region. This is shown in Fig. 9. The defective area of epi layer had lost the sp3 bond state, and the carbide was formed as MoC1-x. Since the reverse leakage current increased in the epi defect region with increased high temperature time, it could not be used practically. Amidst such situation, a young post-doctorate researcher suggested Ru that was successfully used in some previous research in which he was involved, and a test was done by borrowing a sputtering equipment from a different section. As a result, it was found that the above five properties, from heat resistance to ease of processing, were simultaneously satisfied using this metal. In the accelerated deterioration test, there were no changes over 1500 hours at 400 °C as shown in Figure 10, regardless of the presence or absence of defects.[20] It was estimated that ultra high thermal resistance of over 300 thousand hours at 250 °C should be obtained when the activated energy of deterioration by surface graphitization was assumed to be l eV. The search for such heat resistant Schottky metal was far from the originally planned R&D. However, by conducting the high-temperature deterioration test at an early stage, we were able to overcome the issue early so it would not be a major problem after the development had progressed for some time. It is also the reality of R&D that progress occurs on a whim as in the Ru suggestion. I mentioned this incidence because I feel that it is very important to maintain some degree of freedom in conducting the R&D.4 Technological syntheses that were verifiedThe developments of breakthroughs were explained, among the several research process of using diamond as power devices. For the verification for diamond advantage, various properties were demonstrated using the pseudo vertical structure as shown in Fig. 3. After the development of the process, device, and implementation technology, the operating properties were investigated using the vertical structure, and the following overall superiority of diamond was demonstrated. Although the details will be abbreviated due to space limitations, the properties can be divided in to the following categories.(1) Electrical breakdown field: verified 3.5 MV/cm with Schottky junction, way surpassing SiC[21](2) Elimination of killer defects by improving the epitaxial growth of the drift layer (described in this paper)(3) Achievement of low leakage current by surface treatment technology and high B (described in this paper)(4) Achievement of ultra high thermal resistant Schottky junction (described in this paper)(5) Development of vertical device process[22](6) Development of field termination structure[23]-[25](7) Verification of high current density at high temperature (5 KA/cm2 @250 °C using small pseudo vertical device)[26]Additional tests other than mentioned above included observations of property unique to diamonds, such as observing that no hotspots would be formed in diamond through temperature mapping of the device in operation.[27]The diamond diode that could achieve both high temperature operation at 250 °C and high current density was developed, and this opened the possibility for a power device that does not require cooling, as well as with low loss at high temperature and high breakdown voltage. This is a concept where the device that reaches high temperature through self-heating does not have to be cooled by a large cooling module using energy, but instead, heat is utilized as is.A prototype of a vertical structure diode was fabricated using 1500 h250 h0 hBias (V)Current density (A/cm2)10310-110-510-310-710-91011050-5-10(a)Maintained for 1500 hr at 400 °C1500 h250 h0 h750 hBias (V)Current density (A/cm2)Bias (V)Current density (A/cm2)10310-110-510-310-710-91011050-5-10(a)(b)100 h250 h10310-110-510-310-710-91011050-5-10(a) Region without epi defect No change in property after 1500 hr at 400 °C(b) Region with epi defect Increase in reverse leakage currentFig. 9 Property of Mo Schottky junction maintained at high temperature(From Reference [20] K. Idea et al.)Fig. 10 Property of Ru Schottky junction maintained at high temperature(From Reference [20] K. Idea et al.)


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