Xuezhen Jia1,Kento Narita1,Kosuke Ota1,2,Akira Takahashi1,Ryosuke Yamamoto1,Atsushi Hiraiwa1,Tatsuya Fujishima2,Hiroshi Kawarada1,2,3
School of Fundamental Science & Engineering, Waseda University1,Power Diamond Systems, Inc.2,The Kagami Memorial Laboratory for Materials Science and Technology, Waseda University3
Xuezhen Jia1,Kento Narita1,Kosuke Ota1,2,Akira Takahashi1,Ryosuke Yamamoto1,Atsushi Hiraiwa1,Tatsuya Fujishima2,Hiroshi Kawarada1,2,3
School of Fundamental Science & Engineering, Waseda University1,Power Diamond Systems, Inc.2,The Kagami Memorial Laboratory for Materials Science and Technology, Waseda University3
The hydrogen-terminated (C-H) diamond surface has been used in diamond MOSFETs for its p-type high conductivity[1][2]. So far, we have reported diamond MOSFETs using 2-dimensional hole gas (2DHG), which is induced by the surface dipole formed by hydrogen-carbon bonds. The negatively charged adsorbates are also considered to have an important role to obtain high surface conductivity[3]. In the fabrication of diamond MOSFETs, even if C-H surface is preserved, we noticed that the surface conductivity decreases after several device processing sequences, because of different ambient and high temperature processing condition. In this work, we developed the recovering method of conductivity by irradiating extra negative ions in air on the hydrogen-carbon (C-H) surface to enhance the current density of the C-H diamond surface. The low conductivity of C-H diamond substrate was improved successfully. This result indicates the necessity of negatively charged adsorbates on the C-H diamond surface. It also makes the fabrication of C-H diamond FET more reliable by intentionally introducing additional negative ion formed by corona discharge to the C-H diamond surface.<br/>In this study, the surface of (001) undoped diamond was hydrogen-terminated. Ti/Pt/Au (30nm/20nm/100nm) was evaporated as source/drain electrodes to define the channel length. Before introducing additional negative ion, the current density was 8 mA/mm at V<sub>DS </sub>= 30 V, with L<sub>ch </sub>= 5 μm, W<sub>ch </sub>= 70 μm, which is more than 1 order of magnitude below the current density of C-H diamond channel reported [4]. To increase the conductivity of C-H surface, we introduced a large amount of negative ion by corona discharge near the substrate surface to provide additional negative charge on the hydrogen-carbon surface for 30 min[5]. The change of current density over time was measured. With additional negative ion in the air, the current density increased rapidly from 9 mA/mm to 164 mA/mm in 30 min and saturated. Sheet resistance decreased from 656 kΩ/sq to 38kΩ/sq. The current density maintained the maximum for more than 3 days. All the experiment and measurement were performed in regular air.<br/>The increase of current density indicated the amount of negative charge on the C-H surface has a great influence on the density of polarization-induced 2DHG. The disorption of negatively charged adsorbates in the device processing probably leads to a low density of 2DHG,which results in low conductivity of hydrogen-terminated surface. It takes more than several hours for negatively charged adsorbates to cover the surface. This low conductivity issue can be simply improved by introducing additional negative ion to the C-H surface. This process only takes about 30 minutes in air without using toxic gas such as NO<sub>2</sub>. Introducing additional negatively charged adsorbates in the device fabrication process is recommended to secure the higher density of polarization-induced 2DHG, higher p-type conductivity and a higher yield of C-H diamond p-FET.<br/><br/>[1] H. Kawarada, Makoto Aoki, Masahiro Ito, Appl. Phys. Lett. 65(12): 1563–1565. (1994)<br/>[2] H. Kawarada, Surface Science Reports. 26(7), 205-259 (1996)<br/>[3] K. Hirama, H. Kawarada, H. Umezawa, et al., Appl. Phys. Lett., 92(11), 112107-1～112107-3 (2008)<br/>[4] K. Kudara, H. Kawarada, et al., Carbon,188, pp.220-228. (2022)<br/>[5] D.K.Schroder,<i> Semiconductor Material and Device Characterization</i>, 3rd ed. John Wiley & Sons Inc., 2005, pp.438-439.