Effect of Surface Passivation on Doped Tin (IV) Oxide Thin Films for Gas Sensing Applications

In this work, SnO2 thin films were deposited on a glass substrate by spray pyrolysis technique using aqueous solution of tin (IV) chloride (SnCl4.5H2O) dissolved in ethanol. Different types of thin films such as SnO2, F: SnO2, Pd: SnO2 and Pd-F: SnO2 were deposited at a substrate temperature of 450 0 C. Post deposition treatment: annealing and passivation were performed in a tube furnace at 450 0 C. The samples, both as prepared and treated, were thereafter characterized by optical and electrical methods. The optical transmittance was found to be >80% for all undoped and doped SnO2 thin films while their reflectance was < 22%. The band gap energy was calculated using proprietary ScoutTM 98 software. The calculated band gap energy for as prepared undoped SnO2 was found to be 4.1135 eV, which decreased to 4.0594 eV after annealing and increased to 4.1248 eV after passivation. Similarly, the band gap energy for doped SnO2 thin films were calculated. 6.9 at% Pd doping on SnO2 gave the lowest band gap energy value of 3.7332 eV, while the highest calculated band gap energy for Pd:SnO2 was 4.1135 eV at 2.7 at% Pd doping. The calculated band gap energy after doping with fluorine was 3.8014 eV for 16.14 at% F doping being the lowest calculated bandgap energy while 4.1224 eV for 22.74 at% F doping being the highest. Lastly, 2.7 at% Pd:SnO2 was doped with fluorine to get Pd-F:SnO2 which gave a calculated bandgap energy of 4.0253 eV for 8.72 at% F and 4.2289 eV for 19.28 at% F doping, as the lowest and highest calculated band gap energies respectively. Optimum doping of SnO2 with Palladium caused a widening effect on the bandgap energy which is attributed to well known Burstein Moss effect. Optimization on F dopant in SnO2 caused a narrowing effect on the bandgap energy which is attributed to incorporation of Fluorine ions in the crystal lattice of SnO2 which gives rise to donor levels in SnO2 bandgap region causing the conduction band to lengthen causing a reduction in the calculated bandgap value. The electrical resistivity was done by measuring the sheet resistance of the SnO2, Pd:SnO2, F:SnO2 and Pd-F:SnO2 thin films. The as prepared SnO2 this film had a sheet resistivity of 0.5992 Ωcm, which decreased to 0.5358 Ωcm upon annealing and then increased to 0.5746 Ωcm when the thin films were passivated. For undoped SnO2 thin films, the behaviour was similar as observed in calculated band gap energies, that is: at 2.7at% Pd doping and 6.9 at% Pd doping gave the lowest and the highest sheet resistivity of 0.0271 Ωcm and 3.4362 Ωcm, respectively. For F:SnO2 thin films, 0.00075 Ωcm and 0.4599 Ωcm sheet resistivity were observed as the lowest and highest after 16.41 at% F and 22.74 at% F doping, respectively. Lastly, at 16.04 at% F and 19.28 at% F doping of Pd-F:SnO2 gave 0.000164 Ωcm and 0.0464 Ωcm, being the lowest and highest sheet resistivity respectively. Optimum doping of SnO2 with Palladium and Fluorine decreases the sheet resistivity of the resultant thin films. This is due to substitutional incorporation of Pd ions and F ions in the crystal lattice of SnO2 instead of Oxygen vacancies which increases carrier density. The Pd-F:SnO2 gas sensor device was tested for CO2 gas sensitivity using a laboratory assembled gas sensing unit. The performance of the gas sensor device was observed and it was found that the as prepared device was more sensitive to CO2 gas than those subjected to annealing and passivation. The decrease in the sensitivity of the annealed Pd-F: SnO2 gas sensor is attributed to decrease in grain boundary potential as a result of grain growth. This causes a decrement in adsorption properties of CO and O species by the annealed Pd-F: SnO2 thin film. The sensitivity of passivated Pd-F: SnO2 gas sensor was found to be the lowest. The low sensitivity is due to the effects of nitration and decrement in grain boundary potential resulting from grain growth, nevertheless, the sensitivity of the passivated Pd-F: SnO2 thin film was found to be within the range for gas sensing applications.