Growth and characterization of electron beam evaporated NiO thin films for room temperature formaldehyde sensing

Herein, we report the formaldehyde gas sensing performance of NiO thin films deposited by the electron beam evaporation technique. NiO thin films were grown over glass substrates by the variation of deposition temper-ature from room temperature (RT) to 500 degrees C. The structure, chemical composition and morphological studies were conducted by XRD, FESEM, EDS, AFM, XPS, RBS and HRTEM techniques. The XRD characterization revealed the formation of NiO in the polycrystalline phase with the face-centered cubic structure without im-purity phases. The surface and morphological characterization confirmed the growth of the NiO thin layer and the homogeneous distribution of Ni and O elements on the surface of the films. The optical transmittance increased with the rise in deposition temperature. The optical band gap was estimated using Tauc relation and it was shown a decreasing trend with the rise in deposition temperature. The electrical characteristics of thin films were carried out using Hall measurements and presented. The developed gas sensors tested at room temperature have displayed good selectivity towards formaldehyde with a low detection limit of 10 ppm. The NiO thin film sensor prepared at a deposition temperature of 400 degrees C has exhibited a short response and recovery time of 25 s and 9 s respectively towards 50 ppm concentration of formaldehyde. The detailed formaldehyde sensing mechanism for p-type NiO thin films was discussed.