n+p Junctions in p-type c-Ge based on annealed phosphorous-doped nanocrystalline silicon films

• High quality junctions are created on c-Ge(p) substrates from nc-Si(n) films. • Furnace annealing enhances n + region conductivity and reduces surface recombination. • Fitting experimental J 0p points to dominance of hole diffusion into n + SiGe x alloy. • State-of-the-art photovoltaic efficiency obtained with a proof-of-concept device. Crystalline germanium (c-Ge) substrates epitomize a cost-effective approach in the development of many optoelectronic technologies requiring low bandgap materials such as thermophotovoltaic cells and infrared photodetectors. However, high quality junction formation in these devices is mostly based on epitaxial growth of III-V semiconductors, which jeopardizes the low-cost approach, or silicon heterojunctions requiring complex surface preparations. In this work, we demonstrate an alternative approach for n + /p junctions by furnace annealing (∼700 °C) phosphorous-doped nanocrystalline silicon (nc-Si(n)) films deposited by PECVD. ToF-SIMS measurements reveal silicon and phosphorous diffusion into the c-Ge bulk after the annealing, while Raman spectroscopy indicates the formation of a crystalline SiGe x alloy at the interface. Under optimized annealing conditions, the junction exhibits low hole saturation current densities of 1-2µA/cm 2 and sheet resistances of the n-type region below 50 Ω/sq. We incorporate the junction into a proof-of-concept photovoltaic device whose temperature-dependent J-V measurements indicate a hole current flow with an ideality factor of 1 and an activation energy of 0.79 eV, consistent with recombination in the quasi-neutral region of the diffused SiGe x alloy. The obtained efficiency (6.29 % under AM1.5 g illumination) highlights the potential of this non-epitaxial process for high-quality electron-selective contacts in germanium photovoltaics.