α‑Quartz Phase Stabilization, Surface Texturing, and Tunable Optical Properties of Nanocrystalline GeO₂ Films Made by Pulsed-Laser Deposition: Implications for Optical and Optoelectronic Applications

Germanium oxide (GeO₂) has great potential in multifunctional devices and next-generation power electronics due to its high thermal conductivity and ambipolar doping capability. However, the complexity of synthesizing the desirable polymorph with a controlled phase, surface/interface quality, microstructure, and functional properties is the main barrier to GeO₂ utilization in advanced applications. In this regard, we present a method to realize hexagonal (h) or α-quartz type GeO₂ with a nanotextured surface morphology on sapphire substrates using a hybrid synthesis strategy that comprises pulsed laser deposition (PLD) and postdeposition thermal annealing. We performed a comprehensive study to investigate the effect of the annealing temperature, which was varied in a wide range (600–1100 °C), on the crystal structure, phase, surface morphology, chemical stoichiometry, defect states, and optical properties of PLD-grown GeO₂ films. As-deposited GeO₂ films at 500 °C were amorphous. Upon annealing, the GeO₂ films induced an amorphous-to-crystalline phase transformation; GeO₂ films annealed at higher annealing temperatures (≥900 °C) stabilized in the hexagonal phase and demonstrated excellent crystal quality and chemical stability. The thermally activated growth process showed increased average crystallite size, which was varied in the range 20–130 (±2) nm, whereas the surface roughness followed a similar trend. The spectral transmittance and band gap also increased with an increasing annealing temperature. The resulting h-GeO₂ films, particularly those obtained at annealing temperatures in the 900–1100 °C range, had a higher band gap of 6.2–6.3 eV and displayed excellent optical transmittance in the visible region. Moreover, the absence of extended valence band maxima and reduced optical defect density support the quality improvement upon annealing. When considering phase-pure bulk and nanostructured GeO₂ as a possible candidate for ultrawide band gap semiconductors in cutting-edge technological applications, the results of the current work can be beneficial to realize high structural and optical quality α-quartz structured GeO₂ films.