Optical Properties of Mist CVD Grown α-Ga 2 O 3

We report on the study of optical properties of mist CVD grown α-Ga 2 O 3 with the observation of excitonic absorption in spectral responsivity measurements. 163 nm of Ga 2 O 3 was grown on sapphire using Ga-(acac)3 as the starting solution at a substrate temperature of 450°C. The film was found to be crystalline and of α-phase with an on-axis full width at half maximum (FWHM) of 92 arcsec as confirmed from X-ray diffraction scans. The Eliott-Toyozawa model was used to deduce band gap and excitonic binding energy from the absorption spectrum. The exciton binding energy was extracted to be 90 meV with large Gaussian spread of 0.195 eV. From spectral responsivity (S.R) measurements, a similar value of excitonic binding energy was found. This unusually huge binding energy is attributed to strong interaction between longitudinal optical (LO) phonons and excitons. Further, metal-semiconductor-metal (MSM) photodetectors (PD) with lateral inter-digitated geometry were fabricated on the film. A sharp band edge was observed at 229 nm (~ 5.42 eV) in the spectral response with peak responsivity of ~1 A/W at a bias of 20 V. The UV to visible rejection ratio was found to be ~ 100 while the dark current was measured to be ~ 0.1 nA at a bias voltage of 20 V.