Studies of ambient dependent electrical behavior of nanocrystalline SnO_(2) thin films using impedance spectroscopy

摘要

Impedance spectroscopy has been used to investigate the properties of sol-gel derived ultrafine grained tin oxide thin films subjected to various ambients and temperatures. The impedance measurements have been carried out in the frequency range 250 kHz-10 mHz and Cole-Cole plots drawn. Equivalent circuit 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 changes in the values of resistive and capacitive components in the equivalent 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 adsorbed ions towards the electrode sample contact region under the influence of 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 water molecules from the surface. The films thereafter exhibit semiconducting behavior till 473 K. A reversal in behavior has been observed during further 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 temperatures 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 region. 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 impedance changes in presence of ethanol but negligible changes in presence of other reducing gases like H_(2) and liquefied petroleum gas.