Spectroscopic Characterization of the Electronic Structure, Chemical Bonding, and Band Gap in Thermally Annealed Polycrystalline Ga2O3 Thin Films

Abstract

Gallium oxide (Ga2O3) thin films were deposited onto Si(100) substrates at 500°C by sputtering the Ga2O3 ceramic target. The effect of thermal annealing in the temperature range of 500–900°C on the crystal structure, chemical bonding, electronic structure, and bandgap of polycrystalline Ga2O3 films was evaluated. Thermal annealing induced improvement in the structural quality and packing density of the Ga2O3 films as evident from X-ray diffraction and Raman spectroscopic analyses. X-ray photoelectron spectroscopic (XPS) analyses indicate the binding energies (BE) of the Ga 2p doublet i.e., the Ga 2p3/2 and Ga 2p1/2 peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga3+) in the as-deposited films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE∼531 eV, which is also characteristic of Ga-O bonds in the Ga2O3 phase. No significant changes were seen in the electronic structure, especially in terms of chemical valence of Ga ions and Ga-O bonds, as function of thermal annealing in the entire temperature range of 500–900°C. However, the presence of carbonyl functional groups become evident in XPS for samples under thermal annealing. Raman analysis also reveals an obvious blueshift for the high-frequency stretching and bending of the GaO4 tetrahedra, which structurally form the β-Ga2O3, with confinement. The bandgap determined from spectrophotometry measurements varies in the range of 4.94–4.78 eV for a variation in annealing temperature in the range of 500–900°C. A correlation between annealing temperature, electronic structure, chemical bonding and bandgap in Ga2O3 films is established.