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

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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.