Controlled Phase Stabilization Enabled Tunable Optical Properties of Nanocrystalline GeO₂ Films

Germanium oxide (GeO2) exhibits two polymorphs, namely, α-quartz and rutile, which belongs to the ultrawide band gap (UWBG) semiconductor group. With exceptional thermal conductivity and ambipolar capability by doping, GeO2 has tremendous prospects for next-generation semiconductor electronics and other multifunctional device applications. However, the primary bottleneck for GeO2 utilization in advanced applications is the complexity in selectively synthesizing the desired polymorph with a controlled phase and properties. Free formation energies of these polymorphs are relatively close to each other and prone to stabilize in various metastable states or a complex mixture of respective phases. In this context, we present an approach to successfully demonstrate nanotextured hexagonal (h) and tetragonal (t) phases of GeO2 on Si and quartz substrates through a hybrid synthesis route, which involves film deposition by magnetron sputtering followed by postdeposition thermal oxidation. The resulting GeO2 thin films exhibit exceptional optical behavior with a high band gap at 6.21 eV for the α-quartz phase and 5.29 eV for the rutile phase. Precise control over the deposition and annealing conditions under oxygen atmosphere results in the formation of amorphous (a) and crystalline phases of GeO2 films. The evolution of h-GeO2 and t-GeO2 phases is further analyzed using a variety of analytical methods selectively determining the crystal structure, surface morphology, and optical properties. The findings of the current study can be further endorsed while fabricating phase-pure bulk and nanostructured GeO2 on numerous platforms as a potential candidate for UWBG semiconductors for advanced technological applications.