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 (CaGe2) 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 CaGe2 layers with a high figure of merit (FOM) as transparent conducting material. Ab initio electronic band structure calculations and optical modeling confirmed the CaGe2 semimetal nature, which is responsible for a demonstrated high FOM. To test CaGe2 electrodes under actual conditions, a planar Ge photodetector (PD) with a metal–semiconductor–metal structure was fabricated, where the CaGe2/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 CaGe2 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 CaGe2 thin films have great potential in numerous applications promoting the era of advanced Ge optoelectronics.