Tuning positive and negative photoconductivity in Zn1-xCdxO films

Photoconductivity investigation is performed on Zn1-xCdxO films and a transition from positive to negative photoconductivity is observed at room temperature as Cd concentration x is varied. Low and high Cd-content samples are studied by temperature dependent photoconductivity measurements in the temperature range of 77-300 K. Our results exhibited additional transitions manifested by an anomalous persistent effect on sample with x ∼ 0.25 and a transition from negative to positive photoconductivity for sample with x ∼ 0.75 as temperature changes. The observed negative photoconductivity effect is attributed to trap levels while the anomalous behavior of persistent photoconductivity amplitude is caused by random local potential fluctuations, responsible to spatial separation of electrons and holes at high temperatures. This interpretation is further supported by photoluminescence and electrical resistance measurements.