  We report on the study of optical properties of mist CVD grown alpha Gallium
oxide with the observation of excitonic absorption in spectral responsivity
measurements. 163 nm of Gallium oxide was grown on sapphire using Gallium
acetylacetonate as the starting solution at a substrate temperature of 450 deg
C. The film was found to be crystalline and of alpha phase with an on axis full
width at half maximum of 92 arcsec as confirmed from X ray diffraction scans.
The Taucs plot extracted from absorption spectroscopy exhibited two transitions
in the UV regime at 5.3 eV and 5.6 eV, corresponding to excitonic absorption
and direct band to band transition respectively. The binding energy of exciton
was extracted to be 114 meV from spectral responsivity measurements. Further,
metal semiconductor metal photodetectors with lateral inter digitated geometry
were fabricated on the film. A sharp band edge was observed at 230 nm in the
spectral response with peak responsivity of around 1 Amperes per Watt at a bias
of 20 V. The UV to visible rejection ratio was found to be around 100 while the
dark current was measured to be around 0.1 nA.
