Fabrication of n-ZnO/p-Si++ Hetero-junction Devices for Hydrogen Detection: Effect of Annealing Temperature

Abstract

In the present study, we report the fabrication of n-ZnO/p-Si++ hetero-junction devices for the detection of hydrogen leakage in ambient air environment. For the fabrication of n-ZnO/p-Si++ hetero-junction devices, high quality ZnO thin films are grown by controlled thermal evaporation technique on the highly doped p-type silicon substrates at 400 oC. The two sets of films deposited at 400 o C are further annealed at 500 and 600 oC to examine the effect of annealing temperature on the structural, morphological, electrical and gas sensing properties of the deposited films. It is revealed from the x-ray diffraction studies that the crystallite size, and the density of the films increase from 22.55 to 24.95 nm, and from 5.65 to 5.68 g/cm3, respectively, on increasing the fabrication temperature from 400 to 600 oC. In contrast to this, the grain boundary specific surface area decreases from 8.79 × 107 to 7.88 × 107 m−1 on changing the fabrication temperature from 400 to 600 oC. The hydrogen gas sensing responses of the fabricated devices have also been recorded at different operating temperatures and different hydrogen concentrations (200 to 1000 ppm) in air ambient. It is found that the gas sensing responses of the fabricated devices increase with increase in operating temperature up to 100 oC and decease beyond this temperature. The gas sensing responses of the devices fabricated at 400, 500 and 600 oC are found to be 97.22, 64.23 and 40.77 % respectively at 1000 ppm of hydrogen. A decrease in gas sensing response with fabrication temperature is attributed to the increase in crystallite size (quantum size effect), density of film (i.e. lower penetration) and decrease in grain boundary specific surface area (i.e. active sites) with annealing temperature. The mechanism of the gas sensing in these devices has also been systematically analyzed under different models.