Electrical, optical and structural investigation of plasma-enhanced chemical-vapor-deposited amorphous silicon oxynitride films for solar cell applications

Abstract

Hydrogenated amorphous silicon oxynitride (a-SiOxNy:H) films, which are deposited by the plasma decomposition of silane and nitrous oxide at low temperatures (Tdep<300°C), are investigated in order to evaluate the potential of these films for photovoltaic applications. In this work both, intrinsic and doped a-SiOxNy:H films are investigated in terms of their electrical, optical and structural properties using Fourier-transform infra-red (FTIR) and secondary-ion mass-spectroscopy (SIMS), as well as high-resolution transmission electron microscopy (HRTEM), photo-conductance decay (PCD), spectral ellipsometry and temperature-dependent conductivity measurements. The plasma deposition parameters are optimized in terms of effective minority carrier lifetime, dark conductivity and low absorbance (i.e. high optical band gap). The optical band gap of the a-SiOxNy:H films can be widened up to 2.2eV compared to a-Si:H due to the incorporation of oxygen and nitrogen into the amorphous network. Not only the optical band gap but also the passivation quality and the dark conductivity of the films are well correlated with the oxygen and nitrogen concentration, which are monitored by means of SIMS measurements. When applying an a-SiOxNy:H film with an optical band gap of 2.0eV, a very high effective minority carrier lifetime of 2.5ms is measured. In case of doped films, conductivities up to σdark=4.5×10−3S/cm for the n-type doping and σdark=3.9×10−4S/cm for the p-type doping are achieved. Combining the intrinsic and doped a-SiOxNy:H films to heteroemitter stacks on a crystalline Si base, a very high implied open circuit voltage of up to 733mV is demonstrated. FTIR and HRTEM measurements reveal a homogenous distribution of Si–Si and Si–O–Si bonds in the a-SiOxNy:H films.