PNNL

Abstract

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 the hexagonal (h) or a-quartz type GeO2 with nano-textured surface morphology on sapphire substrates using a hybrid synthesis strategy that comprises pulsed laser deposition (PLD) and post-deposition thermal annealing. We performed a comprehensive study to investigate the effect of 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. Thermally activated growth process showed increased average crystallite size, which was varied in the range of 20-130 (±2) nm, while the surface roughness followed a similar trend. The spectral transmittance and band gap also increased with 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 ultra wide band gap semiconductors in cutting-edge technological applications, the results of the current work can be beneficial to realize high structural and optical quality a-quartz structured GeO2 films.