Microcrystalline silicon p-i-n photodetectors for telecommunications and photovoltaic applications

Very-high-frequency plasma enhanced chemical vapor deposition was used to fabricate microcrystalline silicon p-i-n photodetectors. Optical simulation of reflectance measurements was used to monitor the crystalline fraction, which determines the quantum efficiency in the near infrared range, relevant for telecommunication applications. To improve the i/p interface quality, a variable silane concentration was used in the early stages of the i-layer deposition. It is observed that a 50% large-grain fraction in the initial (200 nm) region of the i-layer results in a microcrystalline material well beyond the transition region between the amorphous and microcrystalline phase. The large-grain concentration in such initial layer is correlated with the open circuit voltage of the final p-i-n device. We suggest that the total elimination of the amorphous component in the heterophase transition region would result in excessive crystallization of the i-layer in the p-i-n device, which is to be technologically hindered by an on-time modulation of the deposition parameters.