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

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 × 10¹¹cm^-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.