Photoresponse characteristics of n-type tetrahedral amorphous carbon/p-type Si heterojunction diodes

Abstract

The photovoltaic behavior and spectral response of n-type (nitrogen-doped) tetrahedral amorphous carbon (ta-C)/p-type crystalline silicon heterojunction photodiodes are reported. Abrupt step junction type characteristics are observed with ta-C films ranging in thickness from 40 to 160 nm. The photovoltage increases and the prominent peak in responsivity shifts from 800 nm to longer wavelengths of 1000 nm as the doping in the ta-C films is increased, indicative of a widening of the depletion region in the Si. Use of the responsivity vs wavelength data with the Donnelly and Milnes model for an abrupt heterojunction is successful in predicting the depletion width in the Si and the doping level in the higher doped ta-C. The latter value coupled with the x-ray photoemission spectroscopy derived N concentration in the ta-C films enables an estimation of the doping efficiency. Secondary ion mass spectroscopy studies also confirm the abrupt nature of the junctions.