Spectral Behavior of Bias-Dependent Photocurrent and Photoluminescence in Sputtered ZnO Layers

The bias-dependent behavior of the photocurrent (PC) and photoluminescence (PL) of sputtered ZnO layers has been investigated. Based on PC spectroscopy results, the PC intensity of the observed free exciton increased strongly up to electric field of 60 V/cm, after which its rate of increase slightly reduced due to disturbance of field-assisted dissociation of radical ion pairs, which leads to photocarrier generation. Thus, the energy of excitonic PC peaks showed a tendency to red-shift with increasing electric field, being attributed to the induced Stark effect. Therefore, it is concluded that the strong interaction between free excitons and photogenerated PC carriers leads to displacement or widening of the spectrum. In the PL measurements, near-band-edge (NBE) and violet emissions were observed. With increasing electric field, two PL emissions were progressively quenched. The combined PL/PC results reveal that the PL ions associated with the NBE and violet emissions readily interact with the PC carriers of photogenerated electrons and holes. This behavior reduces the recombination ratio and the lifetime of PL ions. So, the PL intensity quenching originates from a decrease in the number of carriers participating in recombination. Consequently, we find that the quenching mechanism of the NBE and violet emissions is strongly related to low external electric field.