  We investigate the optoelectronic properties of single indium arsenide
nanowires, which are grown vertically on p-doped silicon substrates. We apply a
scanning photocurrent microscopy to study the optoelectronic properties of the
single heterojunctions. The measured photocurrent characteristics are
consistent with an excess charge carrier transport through mid-gap trap states,
which form at the Si/InAs heterojunctions. Namely, the trap states add an
additional transport path across a heterojunction, and the charge of the
defects changes the band bending at the junction. The bending gives rise to a
photovoltaic effect at a small bias voltage. In addition, we observe a
photoconductance effect within the InAs nanowires at large biases.
