Search for: All records

Rutile germanium oxide (r-GeO2), an ultrawide bandgap semiconductor, is a promising material for next-generation power electronics. Understanding and controlling the structure and morphology of r-GeO2 thin films are crucial for advancing their integration into future devices. In this work, r-GeO2 thin films were grown using RF magnetron sputtering on r plane sapphire substrates. Postdeposition annealing (PDA) was performed in an oxygen ambient atmosphere to crystallize the films. PDA at 950 °C resulted in the formation of needle-like nanostructures, predominantly originating at the edges of the film and growing inward toward the sample center. Sequential annealing at increasing temperatures indicated that these needle-like structures begin forming at temperatures above 925 °C. Next, the effect of the PDA duration on the structure was studied. It was seen that PDA at 950 °C for durations of 1 to 15 min promoted formation of the rutile phase, and extending the PDA duration allowed greater surface coverage of the nanostructures. However, annealing even longer, i.e., for 120 min, resulted in mixed phases of α-quartz and rutile GeO2. These findings demonstrate that controlling the PDA temperature and duration can effectively modulate the morphology of rutile-phase GeO2 thin films. 

In this work, we demonstrate the growth and phase stabilization of ultrawide bandgap polycrystalline rutile germanium dioxide (GeO2) thin films. GeO2 thin films were deposited using RF magnetron sputtering on r-plane sapphire (Al2O3) substrates. As-deposited films were x-ray amorphous. Postdeposition annealing was performed at temperatures between 650 and 950 °C in an oxygen or nitrogen ambient. Annealing at temperatures from 750 to 950 °C resulted in mixed-phase polycrystalline films containing tetragonal (rutile) GeO2, hexagonal (α-quartz) GeO2, and/or cubic (diamond) germanium (Ge). When nitrogen was used as the anneal ambient, mixed GeO2 phases were observed. In contrast, annealing in oxygen promoted stabilization of the r-GeO2 phase. Grazing angle x-ray diffraction showed a preferred orientation of (220) r-GeO2 for all crystallized films. The combination of O2 annealing and O2 flux during growth resulted in r-GeO2 films with highly preferential alignment. Using electron microscopy, we observed an interfacial layer of hexagonal-oriented GeO2 with epitaxial alignment to the (11¯02) Al2O3 substrate, which may help stabilize the top polycrystalline r-GeO2 film.