Temperature Dependence of doped ZnO Nanowire Photoconductance

ZnO nanowires doped with impurity atoms have been fabricated by pulsed laser-assisted chemical vapor deposition method. The as-synthesized nanowires are constructed as a four-probe field-effect transistor with a global back gate. The temperature dependant transport properties based on nanowire field effect transistor are investigated. The ionized impurity scattering is predominant at low temperatures, which has a temperature dependence of $T^{1.5}$. While at high temperatures, electron-acoustic phonon scattering dominates, yielding a temperature dependence of$T^{-1.5}$ The conductance is dramatically affected by light illuminations - HeNe Laser (632 nm) and UV (254 nm). A smooth transition can be observed under laser illumination, $i.e.$ the resistance reaches a minimum at certain temperature, then rises steadily with continued increase of temperature. This stems from the drastic reduction of electron mobility due to the enhanced electron-phonon interaction. In contrast, two reproducible jumps under UV irradiation are observed for all doped nanowires. This originates from the existence of deep defect levels, which locates more than 2 eV below the conduction band edge. They serve as charge traps for electron excitation from the valence band.