Investigation of sensing properties of sol–gel processed 4 at%Sb:SnO2/TiO2 thin films

Abstract

In this work we investigate the gas and photo sensing properties of the antimony doped tin oxide and titanium oxide (4 at%Sb:SnO 2 /TiO 2 ) nanocrystalline thin films deposited by sol-gel dip-coating. Photoconductivity measurements are carried out under solar light spectra irradiation at different powers. These results show a photo sensitivity of the films in a lateral junction due to interfacial defects. Gas sensitivity was studied at different pressures, and higher conductivity is presented at lower pressure compared to oxygen-rich atmosphere. It occurs due to absence of oxygen adsorption on the semiconductors surface. TiO 2 films are also investigated concerning its properties to gas sensing under photo-excitation with InGaN LED light source with wavelength centered in 450 nm. The decay of photo-induced current evaluated under O 2 and vacuum atmospheres shows that the sample illumination may contribute to higher gassensitivity. This measurement allows determining the charge carrier capture energy, that is related to trapping dominated by distinct defects in each atmosphere.

On the photo-induced electrical conduction related to gas sensing of the Sb:SnO2/TiO2 heterostructure

“…4(b) yields the value of (E cap -) = 78 ± 11 meV. The temperature range (240-300 K) for the evaluation of photo-induced decay parameters is in good agreement with previous publications [20,34,35]. For a wider range, other defects may become relevant for the trapping process and compete for electron capture, leading to the overlap of the decay curves, as observed.…”

“…After the abrupt current fall in the first seconds, the current decay becomes slower in all the three atmospheres. This abrupt decaying current rate did not occur in sole TiO 2 film measured under similar conditions as reported in a previous work [35], which implies that the interface between TiO 2 and Sb:SnO 2 of the heterostructure is contributing to the higher gas-sensing on the sample. Under excitation, the gas-adsorption improvement on the TiO 2 surface may occur due to the increase of electrons in the TiO 2 layer coming from excitation and from the SnO 2 layer.…”

“…Firstly, the 4at%Sb:SnO 2 layer was deposited, followed by a calcination at 80 • C for 10 min, and a thermal annealing at 500 • C for 2 h. Then, the TiO 2 was deposited, as the top layer semiconductor, with same calcination and annealing processes. The final thicknesses of these films are approximately 380 nm and 360 nm, for 4at%:SnO 2 and TiO 2 respectively, as reported in a recent work with the same deposition parameters [35]. Aluminum electrodes were deposited paralelly on the TiO 2 surface, using shadow masks, through thermal evaporation at pressure of about 10 −6 torr.…”