Characterization and modeling of a-Si:H-based n-i-p photodiodes with protocrystalline silicon p-layers

This article reports on blue-enhanced a-Si:H-based n-i-p photodiodes with protocrystalline silicon (pc-Si:H) p-layers by plasma-enhanced chemical vapor deposition. Microstructure of p-layers was studied by Raman spectroscopy. An optical model has been developed to analyze the device performance. The Transfer Matrix Method was applied to simulate light propagation in the glass/Cr/n-i-p/ZnO:Al stack, while modified Tauc-Lorentz model including the Urbach tail was used to obtain the dispersive optical constants for each semiconductor layer. Two photodiodes with a-Si:H and pc-Si:H p-layers were fabricated, characterized, and modeled for comparison. The deduced optical gaps are 1.8 and 2.33 eV for p-type a-Si:H and pc-Si:H, respectively. Observed band gap widening is the quantum confinement effect due to Si nanocrystals in pc-Si:H. Modeling reveals that incorporation of pc-Si:H leads to a significant reduction of absorption and reflection losses. The optimized heterojunction photodiode exhibits a quantum efficiency up to 92% at 510 nm wavelength. The achieved quantum efficiency of 66% at 400 nm wavelength almost triples that for the homojunction photodiode.