Refractive indices and extinction coefficients of p-type doped Germanium wafers for photovoltaic and thermophotovoltaic devices

Our work is primarily driven by the absence of optical parameters for p-type Ge wafers, adaptable to any specific doping value, and, at the same time, able to cover any application from infrared to ultraviolet regime. To address this, we have determined the complex refractive indices of p-type commercial Ge wafers through a wide spectral range (200-25000 nm) employing spectroscopic ellipsometry and transmittance measurements. The doping levels in these wafers vary between 1015 and 1018 cm-3, which are most commonly used in photovoltaic, thermophotovoltaic and optoelectronic applications. Our data fitting procedure resulted in a dielectric function that successfully reproduces not only the critical points associated with interband transitions above the bandgap, but also absorption features below the bandgap connected to intraband processes. Consequently, we obtained the complex refraction indices of these Ge wafers as a function of dopant concentration, and these have been corroborated through experimental reflectance and transmittance measurements. Additionally, we achieved a successful validation of these refractive indices by simulating the external quantum efficiency of Ge single-junction solar cell with two different thickness fabricated on a Ge wafer with a different doping level than those analyzed in this study.