Annealing induced bandgap engineering and interfacial oxide formation in nanostructured TiO 2 -Ge thin film heterojunctions for photovoltaic applications

Nanostructured thin films of germanium (Ge) and titanium dioxide (TiO 2 ) were successfully deposited onto silicon (Si) substrates using a combination of direct current (DC) and radio frequency (RF) magnetron sputtering techniques. Thickness measurements conducted via Rutherford Backscattering Spectrometry (RBS) revealed film thicknesses of approximately 484 nm for Ge and 5 nm for TiO 2 slightly below the intended thickness of 500 nm and 10 nm, respectively. The films were subsequently subjected to controlled thermal annealing in ambient air using a muffle furnace at temperatures ranging from 100 °C to 600 °C for 2 hours. Significant thermally induced modifications were observed, particularly at elevated annealing temperatures of 500 °C and 600 °C. These included visible changes in film coloration from a metallic silver sheen to darker hues, alongside structural alterations and reduction in film uniformity. RBS analysis further confirmed the oxidation of Ge and the formation of interfacial mixed oxides such as GeO 2 and GeTiO 2 , resulting from diffusion and chemical interaction at the TiO 2 -Ge interface. Optical characterization using UV–Vis–NIR spectroscopy revealed a pronounced shift in the optical band gap. Initially measured at 1.45 eV, the optical band gap decreased to 1.25 eV after annealing at 400 °C, attributed to the formation of nanocrystalline Ge domains and quantum confinement effects. At 500 °C, the band gap remained nearly unchanged (1.23 eV) due to the competing effects of crystallization and the onset of oxidation with initial GeO 2 formation, while a significant increase to 1.60 eV at 600 °C was associated with the progressive formation of GeO 2 and GeTiO 2 nanostructures within the multilayered system. These findings demonstrate that the TiO 2 -Ge thin film system exhibits temperature dependent tunability in both structure and optical response. The observed interfacial engineering and band gap modulation position these films as promising candidates for next-generation optoelectronic and photovoltaic applications.