Investigation of Optical functions, sub-bandgap transitions, and Urbach tail in the absorption spectra of Ga2O3 thin films deposited using mist-CVD

We report on the investigation of optical properties of epitaxial Ga2O3 thin films deposited at different temperatures using mist-Chemical Vapor Deposition on c-plane sapphire. The deposited films were of highly crystalline quality, with measured full width at half maxima (FWHM) of on-axis rocking curve scans as low as 62 arcsec for alpha-Ga2O3 sample grown at 500 degrees C. Pure phase kappa-Ga2O3 was found to get stabilized as temperature increased to 650 degrees C. The optical properties of the as-deposited films were studied using absorbance, reflectance, and transmittance spectra. Various optical functions have been calculated, such as refractive index (n), extinction coefficient (k), real part of dielectric function (epsilon(real)), imaginary part of dielectric function (epsilon(imaginary)) and high-frequency dielectric constant (epsilon(infinity)). Furthermore, absorption spectra were modeled for sub-bandgap transitions and exponential onset near band-edge using Elliott-Toyozawa and Urbach-Martienssen models, respectively. Codi's rule was used to estimate the value of the structural disorder parameter (chi) associated with Urbach's tail. Variation of the extracted parameters, for instance, Urbach's Energy (E-U), phonon energy (h nu(p)), steepness parameter (sigma), electron-phonon interaction strength (Ee-phonon), Codi's structural disorder parameter (chi), bandgap (E-g) and excitonic binding energy (E-exc) were studied against deposition temperature; these parameters were found to correlate with deposition temperature and hence the crystalline quality of the film. In light of the results, it was inferred that the exponential tail is inevitable at finite temperature because of the dominant... phonon mode present at similar to 430 cm(-1) in alpha- or alpha- dominated Ga2O3 phase.