Atomic layer deposition TiO2–Al2O3 stack: An improved gate dielectric on Ga-polar GaN metal oxide semiconductor capacitors

Abstract

This research focuses on the benefits and properties of TiO2–Al2O3 nanostack thin films deposited on Ga2O3/GaN by plasma-assisted atomic layer deposition (PA–ALD) for gate dielectric development. This combination of materials achieved a high dielectric constant, a low leakage current, and a low interface trap density. Correlations were sought between the films' structure, composition, and electrical properties. The gate dielectrics were approximately 15 nm thick and contained 5.1 nm TiO2, 7.1 nm Al2O3, and 2 nm Ga2O3 as determined by spectroscopic ellipsometry. The interface carbon concentration, as measured by x-ray photoelectron spectroscopy depth profile, was negligible for GaN pretreated by thermal oxidation in O2 for 30 min at 850 °C. The RMS roughness slightly increased after thermal oxidation and remained the same after ALD of the nanostack, as determined by atomic force microscopy. The dielectric constant of TiO2–Al2O3 on Ga2O3/GaN was increased to 12.5 compared to that of pure Al2O3 (8–9) on GaN. In addition, the nanostack's capacitance–voltage (C-V) hysteresis was small, with a total trap density of 8.74 × 1011 cm−2. The gate leakage current density (J = 2.81 × 10−8 A/cm2) was low at +1 V gate bias. These results demonstrate the promising potential of PA–ALD deposited TiO2/Al2O3 for serving as the gate dielectric on Ga2O3/GaN based MOS devices.