High optical transparency, electrical conductivity and NO2 gas response of spray pyrolytic fluorine doped ZnO thin films

Abstract

Fluorine doped zinc oxide (FZO) thin films with doping concentration in the range 0-20 at.% were prepared by following a simple thin film deposition process like spray pyrolysis. Optimum electrical properties [Conductivity 22.12 S.cm(-1), Carrier concentration 1.637 x 10(17) cm(-3), Carrier mobility 8.446 x 10(2) cm(2)/Vs] with high optical transparency (> 80% at 550 nm) was obtained for 15 at.% F-doping concentration, measured at room temperature (similar to 27 degrees C). The sample (15 at.% FZO thin film) demonstrated superior NO2 gas sensing ability compared to undoped ZnO. The gas response was found to be similar to 10 times better to NO2 gas (40 ppm in air) with lowering of operating temperature from 200 degrees C to 150 degrees C. It also showed better selectivity towards NO2 gas than C3H6O, NH3, CO2, LPG and SO2. Formation of larger crystallites and orientation along (002) direction was favoured as found from XRD analysis. XPS analysis revealed increase of oxygen-defect site binding energy by similar to 0.76 eV, while decrease of Zn 2p(3/2) and Zn 2p(1/2) binding energies by similar to 0.16 eV. This indicated the change in chemical environment around zinc and oxygen ions, as expected owing to difference of oxidation state and electronegativity between oxygen and fluorine. The optical bandgap of ZnO increased from 3.11 eV (for undoped case) with F-doping and found optimum (3.20 eV) at 15 at.% doping level. FESEM micrographs depicted merger of grains, having a positive impact on electrical conductivity. Water contact angle measurement indicated better hydrophobic nature of the FZO thin film. These results proved effectiveness of FZO thin films as NO2 gas sensor and also indicate the possibility of preparation of a low-cost TCO material by spray pyrolysis technique.