Alternative Look at Calcium Digermanide CaGe₂: A High-Performing Semimetal Transparent Conducting Material for Ge Optoelectronics

Following a recently manifested guide of how to team up infrared transparency and high electrical conductivity within semimetal materials [ Cui, C. Prog. Mater. Sci. 2023, 136, 101112 ], we evaluate the applicability of the calcium digermanide (CaGe 2 ) thin-film electrodes for advanced Ge-based optical devices. Rigorous growth experiments were conducted to define the optimal annealing treatment and thickness of the Ca–Ge mixture for producing stable CaGe 2 layers with a high figure of merit (FOM) as transparent conducting material. Ab initio electronic band structure calculations and optical modeling confirmed the CaGe 2 semimetal nature, which is responsible for a demonstrated high FOM. To test CaGe 2 electrodes under actual conditions, a planar Ge photodetector (PD) with a metal–semiconductor–metal structure was fabricated, where the CaGe 2 /Ge interface acts as a Schottky barrier. The resulting Ge PD with semimetal electrodes outperformed commercially available Ge devices in terms of both photoresponse magnitude and operated spectral range. Moreover, by using femtosecond-laser projection lithography, a mesh CaGe 2 electrode with a relative broadband transmittance of 90% and a sheet resistance of 20 Ω/sq. was demonstrated, which further enhanced the Ge PD photoresponse. Thus, the obtained results suggest that CaGe 2 thin films have great potential in numerous applications promoting the era of advanced Ge optoelectronics.