Tail-states induced semiconductor-toconductor like transition under sub-bandgap light excitation in the zinc-tin-oxide photothinfilm transistors

We report on a giant persistent photoconductivity (PPC) induced semiconductor-to-conductor like transition in zinc-tin-oxide (ZTO) photo-thinfilm transistors (TFT). The active ZTO channel layer was prepared by remote-plasma reactive sputtering and possesses an amorphous structure. Under subbandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~10 -4 A (a photo-to-dark current ratio of ~10 7) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits gigantic PPC with long-lasting recovery time, which leads the ZTO compound to undergo a semiconductor-to-conductor like transition. In the present case, the conductivity changes over six orders of magnitude, from ~10-7 to 0.92 {\Omega} -1cm-1. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep-states and tail-states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-gap tail-states of the ZTO in the giant PPC, while deep-states contribute to mild PPC.