Chemical synthesis and electrical characterization of nickel oxide thin films for sensor applications

Abstract

The semiconducting properties and wide bandgaps of some metal oxides have made them useful in sensing applications and printed electronics. Doping of common metal oxides to achieve p-type conductivity and the formation of p-n junctions with them is not feasible. However, nickel oxide (NiO) is known for p-type conductivity due to intrinsic defects. This research is aimed at synthesizing NiO thin films by means of chemical solution methods, and to characterize the films to determine the effects of processing methods and conditions of deposition on the properties of the films with the aim to use the films in light or gas sensors. NiO films were synthesized on glass substrates using chemical bath deposition (CBD), sol-gel spin-coating, and spray pyrolysis. Some conditions of deposition such as the concentration of precursors and the processing temperature of the films were varied and the effects of the variations on the properties of the films were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning probe microscopy (SPM), ultraviolet-visible spectrophotometry, and Raman spectroscopy. Electrical characterization was carried out using a linear four-point probe and current-voltage measurement systems.

The XRD results confirmed that the as-deposited films grown by CBD were hydrated nickel hydroxide. Thermogravimetric analysis (TGA) showed that the transformation of this phase to NiO required annealing at a temperature above 350 degree Celsius. The films were of porous morphology and were made of nanowalls of varying thickness with an average of 77 nm. This shrank to 52 nm after annealing at 450 degrees Celsius.

NiO thin films grown by the sol-gel method were granular, and the crystallinity, grain size and electrical conductivity of the films depended on the temperature at which they were processed. Minimum electrical resistivity of 125 Ohm.cm was measured for the film that was dried at 250 degrees Celsius and annealed at 500 degrees Celsius.

For the films deposited through spray pyrolysis, the sheet resistance decreased with increasing concentration of precursor and decreased with annealing at 500 degree Celsius for one hour.

Generally, the films were polycrystalline, with the most prominent peak in their XRD patterns due to diffraction in the (111) crystallographic plane in the films that were produced by the CBD and spray pyrolysis. However, the (200) peak was most prominent in films that were produced by the sol-gel spin-coating. In all the films, the presence of a Ni-O bond was confirmed by the observation of the Ni-O stretching mode in one-phonon first-order and two-phonon second-order Raman peaks. Spray pyrolysis was found to be the best method for producing transparent and conducting NiO thin films. The current-voltage characteristics of the junction between p-NiO and n-type silicon, showed rectification of two orders of magnitude both in the dark and under illumination. The device was weakly sensitive to solar radiation.