Band Gap and Phase Stability in (Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}})_{2}$O$_{3}$ Alloy Films

Gallium oxide is a transparent semiconductor (E$_{\mathrm{g}}=$ 4.8 eV) that exhibits n-type conductivity; it has been proposed for a variety of uses ranging from ``solar-blind'' conductive coatings to chemical sensing. An intriguing possibility is development of transparent, high power transistors based on carrier accumulation at an epitaxial Ga$_{2}$O$_{3}$--(Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}})_{2}$O$_{3}$ alloy interface. Using pulsed laser deposition, composition-spread (Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}})_{2}$O$_{3}$ thin films were fabricated on sapphire and silicon substrates, with $x$ varying smoothly across the surface. Position-dependent X-ray diffraction revealed [-201]-oriented Ga$_{2}$O$_{3}$ on c-plane sapphire, and unoriented Ga$_{2}$O$_{3}$ on silicon with native oxide. Alloy (Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}})_{2}$O$_{3}$ films on sapphire remain in the $\beta $-Ga$_{2}$O$_{3}$ phase for $x$ \textless 0.30 then relax to the $\alpha $-Al$_{2}$O$_{3}$ phase, whereas films on silicon remain in the $\beta $-Ga$_{2}$O$_{3}$ phase for $x $\textless 0.35 and then relax into the cubic $\gamma $-Al$_{2}$O$_{3}$ phase. Photoemission spectroscopy shows core and valence levels shifting to higher binding energy and decreasing work function, while spectroscopic ellipsometry reveals the absorption edge moving to higher photon energy, consistent with a widening band gap.