Highly reflective and passivated ohmic contacts in p-Ge by laser processing of aSiCx:H(i)/Al2O3/aSiC films for thermophotovoltaic applications

Crystalline germanium (c-Ge) has historically been regarded as a cost-effective alternative to III-V semiconductors for thermophotovoltaic (TPV) device fabrication. However, Ge-based devices have not yet reported high efficiencies, partially due to the lack of an efficient back-surface reflector that turns back to the heat source out-band (sub-bandgap) thermal radiation. The difficulty of implementing back surface reflectors in Ge TPV cells is related to the simultaneous requirement of good back surface passivation, low electrical resistivity, and high out-band optical reflectivity. In this study, we demonstrate a highly reflective ohmic contact to p-type c -Ge (doping concentration of 2 x 1015 cm-3) made of an aSiCx(1 nm)/Al2O3 (50 nm)/aSiC (45 nm) stack that is laser processed using Nd:YVO4 laser emitting at 355 nm to create punctual p+ contacts (locally diffused Al regions). This stack is finally caped with a thick (1000 nm) Al layer that behaves as a metallic mirror and back electrode. As the laser processed area increases from 0.1 to 3 %, which is the typical range in the final devices, the surface recombination velocity increase from 10.5 to 60.0 cm/s, while the effective contact resistance reduces from 0.462 to 0.036 omega cm2. Moreover, a sub-bandgap reflectance of 90-98 % is achieved. Simulations assuming ideal device configuration indicate that implementing these back contacts could potentially enable TPV cell conversion efficiencies comparable to the reported high-efficiency c-Ge TPV cells operating at similar illumination temperature.

Automated summary

In this study, a highly reflective ohmic contact to p-type c-Ge material is demonstrated, which can improve the efficiency of thermophotovoltaic devices. The experimental results show that this contact can simultaneously meet the requirements of good back surface passivation, low electrical resistivity, and high reflectivity. Moreover, simulations suggest that implementing these back contacts has the potential to achieve conversion efficiencies comparable to high-efficiency c-Ge TPV cells.