Effect of off-axis substrate angles on β-Ga2O3 thin films and solar-blind ultraviolet photodetectors grown on sapphire by MOCVD

Abstract

β-Ga2O3 thin films were heteroepitaxially grown on c-plane sapphire substrates with different off-axis angles toward <11‾00> direction by metalorganic chemical vapor deposition (MOCVD), where the crystal quality of the films and the deep-ultraviolet photodetectors (PDs) fabricated on these films were characterized. The effect of sapphire substrates with different off-axis angles on the β-Ga2O3 thin films and the optical performance of β-Ga2O3 PDs are investigated for the first time. (2‾01)-oriented β-Ga2O3 thin films were grown on sapphire substrates with off-axis angles of 0°, 1.5°, 4°, and 6°. The full width at half maxima (FWHM) of rocking curves decreased significantly with the increase of off-axis angles. The β-Ga2O3 thin films grown on the 6° off-axis angle substrate showed a FWHM of 0.55°, indicating excellent crystal quality. Substrate off-axis angles play a critical role in the nucleation process in the growth of β-Ga2O3 thin films. The step edge on the off-axis substrates serves as the preferential incorporation sites for Ga adatoms. The step terrace width is controlled by the off-axis angle and influences the growth modes depending on the relative magnitude of the step terrace width compared with the Ga adatom diffusion length. In addition, β-Ga2O3 photodetectors with a metal-semiconductor-metal (MSM) structure have been fabricated using these β-Ga2O3 thin films. The β-Ga2O3 PDs fabricated on 6° off-axis sapphire substrate showed excellent optical performance with a large photocurrent of 61 μA, an outstanding photo/dark current ratio of 2.3 × 103, and peak responsivity of 74.6 mA/W. These results can provide important guidance on heterogeneous epitaxy of β-Ga2O3 thin films and photodetectors on cost-effective mass-producible sapphire substrates by MOCVD.