ICYS Annual Report 2023

24/50

ICYS Annual Report 2023Semiconductor devices are indispensable in our daily lives. We use various types of semiconductor devices in many places. Recently, the importance of semiconductor devices called power devices has been increasing. Power devices are semiconductor devices used for power control. However, power devices inevitably incur power losses during power conversion. From the viewpoints of realizing a decarbonized society and effective use of natural resources, it is necessary to reduce the power loss as much as possible. One approach is to fabricate power devices using semiconductor materials that are suitable for reducing power loss. From this perspective, diamond is a promising semiconductor material. This is because diamond has excellent semiconductor properties suitable for power devices, such as wide bandgap, high breakdown field, high thermal conductivity, and high mobility. The realization of high-performance power devices that take advantage of diamond's superior semiconductor properties is expected to significantly reduce power losses during power conversion.In this study, I am working on the development of diamond field-effect transistors (FETs). Specifically, I aim to realize diamond FETs with high mobility. Higher mobility leads to lower switching and conduction losses. Previously, we have fabricated FETs with reduced surface acceptor density by using hexagonal boron nitride (h-BN) as a gate insulator and a fabrication process that does not expose the hydrogen-terminated diamond surface to air. We successfully achieved a high mobility of 680 cm2V-1s-1 in the FETs. [1-4] In FY2023, I aim to achieve even higher mobility.To achieve higher mobility, I thought that the surface quality of diamond should be improved. In our previous work, FETs were fabricated on polished diamond surfaces. Roughness on such polished surfaces can induce the formation of surface states. Charges trapped in the surface states can reduce the mobility due to Coulomb scattering. In addition, scattering due to surface roughness also reduces the mobility. In this study, to reduce surface roughness, I formed a mesa structure on a diamond substrate by oxygen plasma etching and grew diamond by chemical vapor deposition (CVD) with a low methane concentration. Relatively flat diamond films were grown on the mesa. FETs were fabricated on the diamond films using h-BN as the gate insulator. The source/drain electrodes are placed parallel to the step of diamond. I used a modified technique for the lamination of h-BN.Figure 1 shows the temperature dependence of the mobility normalized to the mobility at 300 K. The red circles indicate the FETs fabricated in this study, and the blue and green circles indicate the FETs fabricated in our previous study. The FETs shown in red circles were fabricated on CVD-grown diamond, Research Digest 1. Outline of Research2. Research ActivitiesYosuke SASAMAFig. 1. The temperature dependence of (a) Hall mobility and (b) Hall mobility normalized by the Hall mobility at 300 K.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).while the FETs shown in blue and green circles were fabricated on a polished diamond surface. The FETs shown in red and blue circles were fabricated with air-free process, while the FETs with green circles were fabricated on air-exposed hydrogen-terminated diamond surfaces. The mobility was evaluated by Hall effect measurements. As shown in Figure 1, the mobility of the FETs fabricated in this study increased with decreasing temperature to lower temperatures than our previous FETs. This indicates that the Coulomb scattering and surface roughness scattering are suppressed compared to the previous FETs. Thus, the high quality of the diamond/h-BN interface suppresses carrier scattering and improves the mobility of the FETs.22High-Performance Diamond Electronic Devices