1.6K Operation of Diamond FETs with Superconducting Diamond Sources and Drains Targeting JoFET or SCFET Operation

Superconducting currents are induced in the semiconductor channels of Josephson field-effect transistors (JoFETs) or Superconductor FETs (SCFETs) by the proximity effect from the superconductor. The carrier depletion or accumulation induced by the applied gate electric field control the critical currents of them. For example, JoFETs or SCFETs are realized by using Al contacts and Ge channels [1]. They can be applied for ultra-high speed and ultra-low power consumption operation of transistors. Furthermore, JoFETs can be used to call the Qubits of quantum computers in cryogenic environments. Boron-doped diamond exhibits superconductivity, with its superconducting transition temperature <i>T</i>_c of 10 K at its boron concentration of 1×10²² cm^-3 [2] and a few types of Josephson junctions and superconducting quantum interference devices were realized [3][4]. Also, diamond is known as a wide-gap semiconductor. Therefore, it is possible to realize hybrid devices of superconductors and semiconductors with the same material.<br/>We fabricated diamond FETs composed of 2DHG channels induced by hydrogen termination and ALD-Al₂O₃, and Superconducting diamond contacts which were homoepitaxially grown by microwave plasma chemical vapor deposition. The channel length <i>L</i>_sd were miniaturized down to 100 nm. The gate width <i>W</i>_g was 100 µm. The insulator Al₂O₃ was deposited by atomic layer deposition method with 10 nm at 573K. The Al gate electrode was deposited by Electron Beam vapor deposition with 100 nm. We evaluated low temperature operations of 2DHG diamond FETs down to 1.6 K with four terminal methods by applying a current instead of a voltage to examine the possibility of realization of JoFETs or SCFETs.<br/>The <i>I</i>-<i>V</i> characteristics of the FET with <i>L</i>_sd = 100 nm and <i>W</i>_g = 100 µm were obtained at 1.6 K, 8 K and 12 K. We applied current <i>I</i>_ds = 10 µA, 100 µA respectively and applied gate voltage <i>V</i>_gs from -3.5 V to 1.0 V in 0.5 V increments. The drain current <i>I</i>_ds were modulated by applied gate voltage clearly. It is the first demonstration of diamond FET operated below 2K. The on-resistance <i>R</i>_on was 7.9 Ω･mm at 1.6 K at <i>V</i>_gs = -3.5 V. This value is about one-fourth of that at 12 K. That’s because 1.6 K is well below the superconducting diamond<i> T</i>_c (~10 K) and the resistance of the superconducting diamond source and drain was disappeared. Also, it is possible that the proximity effect from superconducting diamond reduced the resistance of the semiconductor channel. Furthermore, we obtained <i>R</i>-<i>T</i> characteristics from 1.6 K to 12 K by applying bias current of 10 µA at <i>V</i>_gs= 0 V, -3.5 V respectively. When the gate voltage was not applied, the resistance drops around 10 K (~<i>T</i>_c) but increase remarkably as the temperature goes down from 10 K. This is because, the barrier of valence band maximum between boron-doped diamond and 2DHG diamond becomes prominent due to the lower temperature (1.6K). However, the increase of the resistance from 10 K down to 1.6 K was reduced by applying gate voltage since the carrier accumulation due to the applied gate voltage cause band bending and holes tunnel through the barrier. In this work, the superconducting current was not observed because the channel mobility at low temperature is decreased by ion scattering.<br/>The 2DHG diamond FET with <i>L</i>_sd = 100 nm was operated in cryogenic environments down to 1.6 K. This result suggests that JoFET or SCFET operation would be possible by high mobility 2DHG diamond channel with further miniaturized structure.<br/>[1] F. Vigneau, S. Franceschi, <i>et al</i>. Nano Lett. 19 (2019) 1023-1027<br/>[2] T. Kageura, HK <i>et</i> <i>al</i>., Diamond and Related Materials 90 (2018) 181-187<br/>[3] T. Kageura, HK <i>et al</i>. Sci. Rep. 9 (2019) 15214<br/>[4] A. Morishita, HK <i>et al</i>., Carbon 181 (2021) 379-388