Research Digest 23amorphous insulator. [1-3] Furthermore, I fabricated diamond FETs with a reduced surface acceptor density by using an air-free process in which the hydrogen-terminated surface is not exposed to air, and realized a high mobility of 680 cm2V-1s-1. [4] In this study, I aim to further improve the mobility of diamond FETs.2. Research ActivitiesTo further improve the mobility of diamond FETs, I consider that it needs to reduce the density of surface states. If charges are trapped in the surface states, the mobility can be reduced due to Coulomb scattering caused by the trapped charges. Surface states may be caused by roughness of the diamond surface. Dangling bonds on the diamond surface are hydrogen-terminated by the exposure of hydrogen plasma, but hydrogen termination is likely to be non-uniform on rough surfaces. The non-uniformity of hydrogen termination can contribute to the formation of surface states. I consider that the density of surface states can be reduced by the reduction of surface roughness. If surface roughness can be reduced, mobility can also be enhanced by reducing surface roughness scattering. Surface roughness could be reduced by chemical vapor deposition (CVD) of diamond. Previous studies reported that atomically flat diamond can be grown by CVD with a low methane concentration on mesa structure formed on a diamond substrate [5].In FY2022, I formed mesa structure on a diamond substrate and grew diamond at low methane concentration. The mesa structures on diamond substrates are useful for growing atomically flat diamond, but they may cause disconnection of leads and other problems when fabricating FETs. I constructed a process to fabricate FETs on mesas. I also improved the lamination method of h-BN on diamond to improve the quality of diamond/h-BN interface. Based on these results, I aim to realize the further enhancement of mobility of diamond FETs.Fig. 1. Comparison of semiconductor properties of Si, SiC, GaN, and diamond.One of the promising power device applications is the field effect transistor (FET), which controls the current flowing between the source and drain electrodes by changing the voltage applied between the gate and drain electrodes (gate voltage). The mobility, which is one of the indicators of FET performance, is desired to be high, because it contributes to the improvement of switching speed and the reduction of losses. However, most of the diamond FETs reported so far have low mobility, which is less than one-tenth of the intrinsic mobility of bulk diamond.Previously, I successfully realized a relatively high mobility of about 300 cm2V-1s-1 in diamond FETs by using monocrystalline hexagonal boron nitride (h-BN) as a gate insulator instead of an References1) Y. Sasama, K. Komatsu, S. Moriyama, M. Imura, T. Teraji, K. Watanabe, T. Taniguchi, T. Uchihashi, T. Yamaguchi, APL Mater. 6, 111105 (2018).2) Y. Sasama, K. Komatsu, S. Moriyama, M. Imura, S. Sugiura, T. Terashima, S. Uji, K. Watanabe, T. Taniguchi, T. Uchihashi, T. Yamaguchi, Phys. Rev. Mater. 3, 121601 (2019).3) Y. Sasama, T. Kageura, K. Komatsu, S. Moriyama, J. Inoue, M. Imura, K. Watanabe, T. Taniguchi, T. Uchihashi, T. Yamaguchi, J. Appl. Phys. 127, 185707 (2020).4) Y. Sasama, T. Kageura, M. Imura, K. Watanabe, T. Taniguchi, T. Uchihashi, T. Yamaguchi, Nat. Electron. 5, 37 (2022).5) N. Tokuda, H. Umezawa, S. Ri, M. Ogura, K. Yamabe, H. Okushi, S. Yamasaki, Diamond Relat. Mater. 17, 1051 (2008).High-Performance Diamond Electronic DevicesYosuke SASAMA1. Outline of ResearchElectric power is indispensable in our daily lives. To use electric power in consumer electronics, and so on, power conversion is mostly required. Power conversion is often performed by semiconductor devices called power devices. However, power devices inevitably incur power losses during power conversion. From the view point of realizing a decarbonized society and effective use of natural resources, there is a strong need to reduce power losses. One approach to achieve this is to fabricate power devices using semiconductor materials that are suitable for reducing power losses. One promising material is diamond, since it has excellent semiconductor properties such as wide bandgap, high breakdown electric field, high mobility, and high thermal conductivity (Fig. 1). The realization of high-performance power devices that take advantage of diamond's superior semiconductor properties will greatly reduce power losses during power conversion and is expected to contribute the realization of a decarbonized society.
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