Reducing the Threshold Voltage Instability of β-Ga2O3 Nanobelt Field-Effect Transistors Using Ozone Treatment

Abstract

β-Ga2O3 is a promising semiconducting material for next-generation power devices owing to its wide bandgap (4.8 eV), high breakdown electric field (8 MV/cm), and good thermal stability. β-Ga2O3 has a monoclinic crystal structure with weak covalent bonding along the direction of (100), so bulk β-Ga2O3 can be exfoliated into ultra-thin flakes, which can be integrated with van der Waals materials. An example of the application is metal-insulator-semiconductor field-effect transistors (MISFETs) using h-BN as a gate dielectric. H-BN has an atomically flat surface without dangling bond, minimizing interfacial states between the channel and insulator layer. Moreover, the high thermal conductivity of h-BN (30 W/m·K along the c-plane) makes heat management easier (cf. SiO2: 1.4 W/m·K). Therefore, β-Ga2O3/h-BN MISFETs are expected to enhance device performance with a synergetic effect of both materials.However, there are remaining reliability issues about the gate-bias stress instability (BSI), the temporary or permanent degradation of electrical properties from gate bias stress (ex. threshold voltage, field-effect mobility, on/off ratio). β-Ga2O3 FETs have been developed for high voltage operation in harsh environments, so that suppression of BSI is required. BSI is mainly attributed to charge trapping in the channel, channel/dielectric interface, or dielectric layer under gate-bias stress. In this work, we tried to reduce bias-stress instability of β-Ga2O3/h-BN MISFETs by UV-O3 treatment. Numerous reports demonstrated the annealing of oxygen-related defects by UV-O3 treatment in oxide-based materials. Therefore, UV-O3 treatment was expected to also work on β-Ga2O3.First, β-Ga2O3 (200-300 nm) and h-BN (40-50 nm) flakes were exfoliated from each bulk crystal with adhesive tape. H-BN flake was dry-transferred on SiO2/p++Si substrate, and subsequently β-Ga2O3 flake was stacked on h-BN. The source, drain, and gate electrodes (Ti/Au 50 nm/100 nm) were defined by e-beam lithography, followed by e-beam evaporation and lift-off processes. Bias-stress characteristics were systematically measured by Measure-Stress-Measure (MSM) method—repeating the sequence of 1) IDS-VGS, 2) VGS stress, 3) IDS-VGS sweep. β-Ga2O3/h-BN MISFET was UV-O3 treated using a commercial UV-O3 cleaning system (PSD-UV, Novascan Technologies, Ames, IA, USA). By following bias-stress characteristics measurement, we could observed reduced shifts of electrical properties regarding as annealing effect on oxygen-related defect by UV-O3 treatment. This work grants facile way to achieve β-Ga2O3 FETs stable to stress and open up new opportunities for the practical implementation of β-Ga2O3 power device. The details of methods and results will be presented at the conference.