Effects of low-level boron doping on the photocurrent of amorphous silicon Schottky photodiodes

Abstract

We have measured the photocurrent-electric field (Jph-F) characteristics of slightly boron (B) doped (0≤[B2H6]/[SiH4]≤10 ppm) amorphous silicon (a-Si:H) Schottky photodiodes with a configuration of Cr/a-Si:H/ITO. The measurements were performed with different bias directions and two light wavelengths (555 and 660 nm), and mobility-lifetime (μτ) products were deduced by two methods. One is by fitting the experimental plots to the theoretical curves developed by Crandall [Semiconductors and Semimetals (Academic, Orlando, FL, 1984), Vol. 21, Pt. B, p. 245.]. The other is a new one we have proposed where the transition electric field Ftr, at which Jph changes from space-charge-limited current to the theoretical curve given by Crandall, is applied to the relationship μτFtr=L, where L is the thickness of a-Si:H. It was found that the former method is applicable only where a-Si:H is homogeneously illuminated and no space charge is formed. On the other hand, the latter method is effective where a-Si:H is inhomogeneously illuminated and a space charge is formed in the carrier transit region. The μhτh deduced by the latter method increased from 2×10−9 to 3×10−8 cm2/V when the B-doping ratio was increased from 0 to 3 ppm, but remained nearly constant with further doping. In contrast, μeτe monotonically decreased from 2×10−8 to 2×10−9 cm2/V with increased doping. The (μτ)fit deduced by the former method coincides with the μhτh at doping ratios higher than 3 ppm, but did not at lower doping ratios probably because of a space-charge formation by deep hole trapping and incomplete homogeneous illumination. These changes in μτ are discussed microscopically in terms of the charge state of the dangling bond state.