Investigation of a p-CuO/n-ZnO thin film heterojunction for H₂gas-sensor applications

Abstract

A p-CuO/n-ZnO thin film heterojunction is fabricated on a glass substrate by the sol–gel technique. The crystallinity of the junction materials and microstructure of the top p-layer are examined by an x-ray diffractometer (XRD) and scanning electron microscope (SEM). The current–voltage (I–V) characteristics of the p–n heterojunction and its temperature dependence have been investigated in air and H2 ambient. Although the junction possesses linear I–V characteristics from room temperature (RT) to 150 °C in air, at higher temperatures (200 °C to 300 °C), it shows nonlinear rectifying behaviour. The forward current is greatly increased with increasing temperature while the reverse current is increased slightly resulting in a IF/IR ratio as high as 485. The ideality factor (n) is 4.88 at a temperature of 300 °C. The forward current is highly increased by the introduction of H2 gas at 300 °C. However, a simultaneous increase in the reverse current makes the IF/IR ratio 8.4. It is observed that H2 sensitivity of the heterojunction is increased with the increase in temperature as well as the thickness of CuO film. A sensitivity value as high as 266.5 is observed at 300 °C when biased at 3 V in the presence of approximately 3000 ppm of H2.