Novel surface termination for advanced diamond electronics

Abstract

Diamond is an interesting semiconductor for high-power and high-frequency devices due to its ultra-wide bandgap, high carrier mobility, and superior thermal conductivity. However, traditional doping is limited by the deep energy levels of most impurities, which limit carrier activation at room temperature. As a consequence, despite its exceptional intrinsic properties, relatively few reproducible and high-performance diamond field-effect transistors have been reported in recent literature. This motivates the exploration of alternative doping strategies, such as hydrogen-terminated diamond with surface transfer doping. Hydrogen-terminated diamond provides a conductive two-dimensional hole gas (2DHG) with low activation energy and relatively high carrier mobility, making it highly attractive for electronic devices. At the same time, the surface conductivity of H-diamond is highly sensitive to surface chemistry, oxide interfaces, and fabrication processes, posing significant challenges for achieving stable and controllable device operation. In this work, different surface terminations have been explored for negative electron affinity (NEA) and positive electron affinity (PEA), to clarify their respective roles in enabling surface transfer doping or suppressing surface conductivity. Subsequently, the behaviour of different contact metals on H-diamond was examined, with emphasis on their ability to form reliable ohmic contacts. The influence of different oxide layers and deposition methods on H-diamond transfer doping was studied, revealing that thermal ALD HfO₂ can enhance the 2DHG by promoting transfer doping, whereas electron-beam–deposited Al₂O₃ with prior in-situ annealing effectively suppresses surface conductivity without degrading the hydrogen termination. Building on these findings, accumulation-channel hydrogen-terminated diamond MOSFETs were successfully fabricated using an optimised and reproducible process flow. The devices exhibit normally-off, enhancement-mode operation with an Ion/Ioff ratio of 10⁷, achieving drain current densities exceeding 35 mA/mm at room temperature. These results demonstrate a viable pathway towards stable and controllable diamond MOSFETs, addressing key technical barriers that have limited progress in the field. A stable and reproducible Au-based fabrication process was established for hydrogen-terminated diamond devices, providing a robust contact platform for the demonstrated enhancement-mode MOSFETs