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

Abstract

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 × 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 Ω 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.