Investigation of a p-CuO/n-ZnO thin film heterojunction for H2 gas-sensor applications

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 H-2 ambient. Although the junction possesses linear I-V characteristics from room temperature (RT) to 150 degrees C in air, at higher temperatures (200 degrees C to 300 degrees 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 I-F/I-R ratio as high as 485. The ideality factor (n) is 4.88 at a temperature of 300 degrees C. The forward current is highly increased by the introduction of H-2 gas at 300 degrees C. However, a simultaneous increase in the reverse current makes the I-F/I-R 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 degrees C when biased at 3 V in the presence of approximately 3000 ppm of H-2.