Studies of ambient dependent electrical behavior of nanocrystalline SnO2 thin films using impedance spectroscopy

Impedance spectroscopy has been used to investigate the properties of sol-g el derived ultrafine grained tin oxide thin films subjected to various ambi ents and temperatures. The impedance measurements have been carried out in the frequency range 250 kHz-10 mHz and Cole-Cole plots drawn. Equivalent ci rcuit models have been suggested and fitting of the experimentally obtained data done on the basis of the approach of "universal dielectric behavior" introduced by Jonscher. The films have shown strong interaction with water vapor in the atmosphere at room temperature (300 K) as reflected from the c hanges in the values of resistive and capacitive components in the equivale nt circuits at different humidity levels. The conduction mechanism appears to be due to the transfer of protons through the physisorbed layer of water molecules. A low frequency spur has been observed in the impedance diagram at medium and high humidity levels and attributed to the migration of adso rbed ions towards the electrode sample contact region under the influence o f the electric field. These ions accumulate in the electrode sample contact region, giving rise to a non-Debye capacitance. The spur in the impedance diagram disappears on heating the films to 373 K due to the desorption of w ater molecules from the surface. The films thereafter exhibit semiconductin g behavior till 473 K. A reversal in behavior has been observed during furt her increase of temperature due to the conversion of O-2(-) ions to highly active O- ions which abstract electrons from the grain bulk. At very high t emperatures in excess of 573 K, the spur in the impedance diagram reappears , but this time due to accumulation of the adsorbed O- ions, thereby giving rise to another non-Debye capacitance in the electrode sample contact regi on. The spur disappears in the presence of ethanol due to removal of oxygen ions by ethanol molecules. The films on float glass have exhibited large i mpedance changes in presence of ethanol but negligible changes in presence of other reducing gases like H-2 and liquefied petroleum gas. (C) 2000 Amer ican Institute of Physics. [S0021-8979(00)04510-2].