Normally-Off Operation in (001) Vertical Diamond MOSFET Using Oxidized Si Termination Diamond Channel

The development of p-FETs corresponding to n-FETs is necessary to realize compact, high-power high-speed switching complementary inverters. To date, n-FETs using wide bandgap semiconductor materials such as GaN, SiC, and Ga₂O₃ have been studied extensively, but there are few reports on p-FETs. We have reported vertical diamond MOSFETs using 2-Dimensional Hole Gas(2DHG) which is induced independently of crystal orientation by hydrogen terminated diamond surface(C-H) and high-temperature ALD-Al₂O₃[1-4]. However, all conventional vertical diamond MOSFETs use C-H surface as channel and exhibit normally-on operation (+16 V ~ +30 V) with positively large threshold voltage, and normally-off operation has not been achieved[1-4]. Normally-off operation is essential for industrial power electronic application, and we have already demonstrated normally-off operation in lateral diamond FETs using oxidized Si terminated (C-Si-O) diamond as channel[5-7] which can be applied to vertical devices. In this study, we have fabricated (001) vertical diamond MOSFET with C-Si-O diamond channel and achieved the first normally-off operation(-10.2 V) in vertical diamond FETs.<br/>Fabrication process is as follows. A total of 1.5 μm of nitrogen-doped layer was deposited on the (001) p⁺ diamond substrate as current blocking layer by microwave plasma chemical vapor deposition(MPCVD), and trench with depth(<i>W</i>_T) of 3 μm was formed by inductively coupled plasma reactive ion etching(ICP-RIE). After trench formation, a 200 nm regrown undoped layer was deposited by MPCVD to induce 2DHG. Then, SiO₂ was deposited 300 nm as a mask by tetraethyl orthosilicate CVD, and the p++ layer([B]: 1×10²¹ cm^-3) was deposited by MPCVD after etching 300 nm of SiO2 and 40 nm of the Regrown layer in the p++ layer growth area by ICP-RIE. At this time, C-Si-O diamond channel was formed by SiO₂ and diamond reaction in reductive and high temperature atmosphere(~1223 K). After SiO₂ mask removal by HF and ICP-RIE, device isolation was performed by oxygen plasma. Then, the source electrode(Ti/Pt/Au) was formed, and 200 nm of Al₂O₃ was deposited as the gate insulator by high-temperature ALD method(450°C) using H₂O as the oxidant. Finally, drain electrode(Ti/Au) was formed on the backside of the substrate and gate electrode(Al) on the surface of the substrate to complete the device.<br/>The lengths of source-source(<i>L</i>_SS), source-drain(<i>L</i>_SD), effective channel(<i>L</i>_CH), gate-drain(<i>L</i>_GD), trench width(<i>W</i>_T), and gate width(<i>W</i>_G) were 18 μm, 9.2 μm, 8.8 μm, 0.4 μm, 6 μm, and 25 μm, respectively. The maximum drain current density(<i>I</i>_D,max) by the gate width was 8.05 mA/mm. The gate leakage current was less than 10^-9 A at 300 K and on-off ratio was 10⁷. The threshold voltage(<i>V</i>_th) was -10.2 V, achieving normally-off operation. For device with different dimensions (<i>L</i>_SS: 8 μm, <i>L</i>_SD: 9.2 μm, <i>L</i>_CH: 8.8 μm, <i>W</i>_T: 4 μm), <i>I</i>_D,max was 102 mA/mm, <i>R</i>_on was 258 Ω mm, and <i>V</i>_th was -1.9 V, confirming normally-off operation. Compared to the threshold voltage of the conventional vertical C-H diamond MOSFET, the threshold voltage was shifted in the negative direction by more than 15 V. This large negative shift of the threshold voltage can be attributed to decrease upward band bending between diamond and Al₂O₃ due to increase the electron affinity of the C-Si-O interface (-0.25 eV)[8] compared to that of the C-H interface (-1.3 eV)[9].<br/><br/>[1] N. Oi, H. K. et al., Scientific reports, 8(1), 1-10(2018).<br/>[2] J. Tsunoda, H. K. et al., Carbon 176, 349-357(2021).<br/>[3] J. Tsunoda, H. K. et al., IEEE TED, 68(7), 3490-3496(2021).<br/>[4] J. Tsunoda, H. K. et al., IEEE EDL, 43(1), 88-91(2021).<br/>[5] W. Fei, H. K. et al., APL, 116(21), 212103(2020).<br/>[6] Y. Fu, H. K. et al., IEEE TED, 69(5), 2236-2242(2022).<br/>[7] X. Zhu, H. K. et al., Appl. Sur. Sci., 593, 153368(2022).<br/>[8] A. Schenk, C. I. Pakes et al., J. Phys., Conds. Matt, 29, 025003(2017).<br/>[9] H. K. et al., Sci. Rep, 7, 42368(2017).