Cation segregation observed in an (In,Ga) 2O3 material thin film library beyond the miscibility limit of the bixbyite structure

Structural, morphological, and optical properties of (${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{)}_{2}{\mathrm{O}}_{3}$ thin films are reported as a function of the cation composition. A material library with $0.1\ensuremath{\le}x\ensuremath{\le}0.64$ was fabricated by discrete combinatorial synthesis on $r$-plane sapphire substrates using pulsed laser deposition. The samples crystallize in the cubic bixbyite phase for $x\ensuremath{\le}0.35$. The lattice constant and absorption edge energy systematically decrease and increase, respectively, with increasing Ga content up to $x=0.2$. For higher Ga admixtures, both saturate. In addition, a significant change in surface morphology occurs at $x\ensuremath{\sim}0.2$. Transmission electron microscopy examinations of selected samples show a homogeneous incorporation of ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ into cubic ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ for $x=0.11$, while a segregation of Ga-rich and In-rich regions can be seen for $x=0.22$ and $x=0.35$. In the sample with $x=0.35$, the Ga-rich regions exhibit a preferred orientation with an angle of ${45}^{\ensuremath{\circ}}$--${55}^{\ensuremath{\circ}}$ with respect to the substrate normal, which has been shown to result from a correspondingly faceted In-rich bixbyite layer at the substrate--thin film interface.