The influence of defects on response speed of high gain two-beam photogating in a-Si:H PIN structures

The dynamic characteristics of two-beam photogating in reverse biased a-Si:H pin devices have been investigated. The photogated current switch-on rise time scales approximately inversely with probe beam flux, suggesting the supply and subsequent capture of holes generated in the i-layer by the probe beam is the rate-limiting step. This interpretation leads to an estimated i-layer trap density of 4×1015 cm−3. The switch-off decay is complex, but contains an exponential component with a time constant of 2 s independent of probe beam flux. A mechanism involving emission and subsequent extraction or recombination of holes trapped in a narrow energy range of states some 0.7 eV deep is proposed to account for this. These findings are compared with independent data from dark current, transient photocurrent and constant photocurrent measurements.