Unlocking the THz Emission Potential of Sputtered Germanium Thin Films Through Structural and Optoelectronic Properties Optimization

ABSTRACT We report on a systematic optimization of RF‐magnetron‐sputtered, n‐doped germanium thin films for enhanced terahertz (THz) emission under femtosecond laser excitation. By jointly investigating the effects of deposition temperature (25–650°C) and film thickness (280–1330 nm), we have been able to establish direct correlations between microstructure, optoelectronic properties, and THz emission efficiency. Ge thin films deposited at 500°C with an optimal thickness of 570 nm exhibit a highly (111)‐oriented structure, enlarged crystallite size (∼40 nm), low electrical resistivity (∼0.04 Ω·cm), and a narrowed effective direct bandgap of ∼0.77 eV. Under 1035 nm excitation, these optimized films generate a ∼35‐fold increase in total THz irradiance compared to room‐temperature‐deposited Ge. The enhanced THz emission is shown to result from the combined effects of strong (111) texture, maximized optical penetration depth, and increased carrier concentration and mobility. Laser power‐ and azimuthal angle‐dependent measurements further reveal that THz generation is governed by transient photocurrents driven by surface depletion fields and the photo‐Dember effect mechanisms, with an additional contribution from electric‐field‐induced optical rectification. The optimized Ge (111) thin films developed here are compatible with integration into potential on‐chip THz sources and devices.