Synthesis and characterization of GeNx films with different nitrogen ratios: An experimental and DFT analysis

In this study, GeN_x thin films with varying nitrogen ratios (30 %, 50 %, and 70 %) were deposited on sapphire and silicon substrates at room temperature using the RF magnetron sputtering technique. Optical properties of the films deposited on sapphire were characterized using UV–VIS–NIR spectrophotometry, which provided insights into their transmittance and energy band gap values. For films grown on silicon, spectroscopic ellipsometry was employed to determine their refractive indices. The XRD patterns of the films grown on sapphire indicated the presence of the (220) crystallographic plane of GeN_x, which became pronounced in the sample produced with 50 % nitrogen content. Notably, the energy band gap of the films varied between 3.40 eV and 4.10 eV, exhibiting a dependence on the nitrogen ratio as it increased from 30 % to 70 %. Similarly, a change in the refractive index values was observed with increasing nitrogen ratio. It was observed that the film with a 30 % nitrogen ratio had the highest refractive index. The mode at ∼303 cm⁻¹ for both films grown on Si and sapphire is well-known TO (c-Ge) vibration. To further support and validate the experimental findings, Density Functional Theory (DFT) calculations were performed to correlate the experimentally determined energy band gaps. It was observed that the experimentally found energy band gap values, especially the 30 % nitrogen film, were very close to γ-Ge₃N₄ from theoretical calculations. This computational approach enhanced the reliability and consistency of the results.