Defect density and diffusion length of holes in nanocrystalline silicon devices

摘要

We report on the measurement of doping density, defect density, and minority carrier diffusion length in nanocrystalline Si p~+nn~+ solar cell devices fabricated using very high frequency (VHF) and electron-cyclotron-resonance (ECR) plasma deposition techniques. Doping and midlevel defect densities were estimated from a measurement of capacitance versus voltage at different frequencies. The as grown layers were always n-type. The doping in as-grown base layers could be reduced by either compensating the material during growth with ppm levels of boron, or increased by adding ppm levels of phosphorus. It was found that there was a distinct correlation between shallow donor density, and deep defect density, situated at 0.35-0.5 eV below the conduction band, even in unintentionally doped materials. Increasing doping increased the midlevel defect density. The diffusion length of holes was measured using combinations of capacitance and quantum efficiency measurements. The nanocrystalline Si p-n junction is shown to be a diffusion controlled, and not a drift-controlled device.