Spatially resolved photocurrent measurements on a-C:H/c-Si and a-C:H/a-Si:H heterojunction photodiodes

Abstract

Abstract Heterojunction photodiodes were prepared by r.f. plasma-enhanced chemical vapour deposition of a-C:H films on single crystalline p-type silicon substrates ( N A = 10 14 cm −3 , orientation 〈111〉) and a-Si:H substrates. We studied the quantum efficiency of devices working in photovoltaic and reverse-biased mode in the wavelength region between 350 and 1200 nm. Post-deposition treatment of a-C:H films with hydrogen at high pressures (up to 300 bar) and elevated temperatures (up to 650 K) changed the bulk properties of the a-C:H films significantly. Photoconductivity was observed within the films after the hydrogen processing. The responsivity of the photoconducting a-C:H films, however, is too small to account for the observed quantum efficiencies (~ 20–30%) of the devices. Multiplication of charge carriers is indicated by quantum efficiencies exceeding 100%, which are observed for reverse-biased devices after the hydrogen treatment. According to C-V measurements the formation of the heterojunction leads to an inversion layer on the silicon surface which is essential for properly working devices. In our paper we propose a model for the operation of photodiodes which is supported by optical beam-induced current measurements.