Optical Properties of Indium-Gallium-Oxide Microcrystalline Alloy Films: From the Visible to the Deep-UV
The tailored optical properties of $(In_xGa_{1-x})_2O_3$ microcrystalline films were studied as a function of composition x via transmission, Urbach energy analysis, and spatial photoluminescence (PL) mapping of the self-trapped hole (STH) emission, with the objective of addressing material characteristics specific to this alloy system. Up to x = 0.46, the optical gap exhibited a redshift of 1 eV from the deep to the near-UV range, while the STH PL was redshifted by 0.5 eV in the visible range. For higher composition, x = 0.63, the transmission spectra indicated the co-existence of two optical gaps attributed to Ga-rich and to In-rich domains, implying that this sample is phase-separated. However, the saturation behavior of the optical gap and that of the STH PL showed that incipient phase separation occurs at a lower composition: x ~ 0.3. This is consistent with the compositional trend found for Urbach energy, implying that phase segregation in the alloys is a major defect even at its incipient stages. Additionally, Urbach analysis of $(In_xGa_{1-x})_2O_3$ was compared to that of $Mg_xZn_{1-x}O$. Both systems were found to have similar compositional dependence: at lower range, Urbach energies exhibited a negligible increase, while at the higher range a significant dependence on the composition was found. The main difference between the two alloy systems is in their Urbach energy: those for $(In_xGa_{1-x})_2O_3$ were significantly larger than those of $Mg_xZn_{1-x}O$. This stems from the strong hole coupling to phonons of $(In_xGa_{1-x})_2O_3$, which provides a dynamic transition additionally to that of defect-type.