Physical Insight Into Multiple Gate-Voltage Dependencies of Off-State Photocurrent in Amorphous InZnO Thin-Film Transistors

The multiple gate-voltage (
${V}_{\text {g}}$
) dependencies of the OFF-state photocurrent (
${I}_{\mathrm {ph-{\scriptscriptstyle OFF}}}$
) with increasing, decreasing, or being constant are observed in amorphous InZnO (a-IZO) thin-film transistors (TFTs) during reverse sweeps of
${V}_{\text {g}}$
from high to low values. This study provides a physical insight into the multiple
${V}_{\text {g}}$
dependencies by considering the joint effects of
${V}_{\text {g}}$
and the dynamic ionization of oxygen vacancy (
${V}_{\text {O}}$
) on the source-to-channel barrier that ultimately determines
${I}_{\mathrm {ph-{\scriptscriptstyle OFF}}}$
. When the photo-induced ionization of
${V}_{\text {O}}$
reaches a steady state during the reverse
${V}_{\text {g}}$
sweep,
${I}_{\mathrm {ph-{\scriptscriptstyle OFF}}}$
is primarily dominated by
${V}_{\text {g}}$
, resulting in a decreasing
${I}_{\mathrm {ph-{\scriptscriptstyle OFF}}}$
with
${V}_{\text {g}}$
. When the ionization of
${V}_{\text {O}}$
is far from reaching a steady state,
${I}_{\mathrm {ph-{\scriptscriptstyle OFF}}}$
is mainly affected by the increasing
${V}_{\text {O}}$
ionization,
${I}_{\mathrm {ph-{\scriptscriptstyle OFF}}}$
leading to an increasing with
${V}_{\text {g}}$
. An intermediate state exists, where the effects of
${V}_{\text {O}}^{{2}+}$
are comparable, resulting in an almost constant
${I}_{\mathrm {ph-{\scriptscriptstyle OFF}}}$
. The proposed mechanism is verified by experimentally measured results.