Sprayed NiO-Doped p-Type Transparent ZnO Thin Films Suitable for Gas-Sensing Devices

Abstract

The spray pneumatic method has been successfully employed for the preparation of polycrystalline NiO-doped ZnO thin films. The effect of NiO content (0, 1, 2, 3, and 6 at %) is studied on structural, optical, and electrical properties of NiO-doped ZnO thin films. The thin films were successfully deposited on a glass substrate at 450°C using the organic solar heater. XRD patterns of NiO-doped ZnO thin films indicate that the obtained ZnO thin films are polycrystalline with (100), (002), and (101) highest peaks of ZnO phase. However, α-Ni(OH)₂ and β-Ni(OH)₂ were observed at 6 and 3 at % NiO, respectively. The crystal structure was improved for doped thin films, the crystallite size decreased by increasing the NiO content up to 6 at % NiO. All thin films have a high optical transmission in the visible region of about 85%. The optical band gap energy decreased from 3.26 eV for 0% to 3.34 eV for 1 at %, and further decreased to 3.27 eV for 6 at % NiO. The thin film deposited with 3 at % NiO has the lowest value of Urbach energy of 0.091 eV. The electrical conductivity of the NiO-doped ZnO films increased greatly from 0.016 (Ω cm)^–1 for 0% NiO to 0.042 (Ω cm)^–1 for 3 at % NiO. It can be noted that the deposited film after 3 at % NiO is a p-type semiconductor.