Effect of rapid thermal annealing on the structural and electrical properties of solid ZnO/NiO heterojunctions prepared by a chemical solution process

Transparent pn-heterojunction electrodes composed of n-type zinc oxide (ZnO) nanorod arrays and p-type NiO are fabricated by a chemical solution route combined with rapid thermal annealing (RTA). The effects of RTA treatment (450 °C, 1 min) in reducing atmosphere on the structural properties and alignment of the ZnO nanorods are investigated. SEM images and XRD patterns show that the average diameter and length are increased, and the alignment and crystal quality of the nanorod arrays are significantly improved. The RTA-treated ZnO shows uniform and well-aligned nanorod arrays with a high aspect ratio of ∼12.4. The structural improvements associated with ZnO crystalline and defect reduction are further verified from the enhanced UV to visible emission ratio by a photoluminescence analysis. Moreover, the effect of RTA on the I – V characteristics of the ZnO/NiO heterojunction electrodes is also investigated; the electrical parameters of the junction are determined from the forward-bias I – V characteristics using Cheung's method. The results reveal that the series resistance is significantly reduced from 784.4 to 114.8 Ω, which leads to better rectifying behaviour with rectifying slope increased to ∼6.67 mA V −1 . The RTA-processed ZnO/NiO heterojunctions with excellent optoelectronic properties may find wide applications in sensors and solid-state oxide junction-based devices.