Isothermal capacitance transient spectroscopy study on trap levels in polycrystalline SnO 2 ceramics
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Kug Hyun Song and Soon Ja Park Department of Inorganic Materials Engineering, Seoul National University, Seoul 151-742, Korea (Received 29 July 1992; accepted 6 October 1992)
The unusual sigmoidal behavior between electrical resistivity and inverse temperature in polycrystalline porous SnO 2 was intensively investigated by detecting the localized traps formed at the interparticle through the measurement of Isothermal Capacitance Transient Spectroscopy. A trap of 1.0 eV was evaluated at a higher temperature range of 460-500 °C, and 0.6 eV trap at a lower temperature range 200-280 °C. It seems that the trap of 1.0 eV originates from O2~ and 0.6 eV from O~ defect center. Time constants which were determined at the peak position of ICTS spectra, S(t), were increased with the increase of temperature at the intermediate temperature range, indicative of the competition among various kinds of traps. The active competition among various traps is one of the causes that result in prominent gas sensitivity in polycrystalline porous SnO 2 .
I. INTRODUCTION SnO 2 has been developed extensively as a prominent gas-sensing material and has widened its application in the field of town gas sensor, humidity sensor, toxic gas sensor, etc.1 The pure SnO2 shows the versatility of nonstoichiometry,2"4 according to the applied atmosphere and temperature, and so the sensitivity on gas or humidity is very active. In addition the history of powder preparation and the fired schedule of SnO 2 significantly influence the electrical property, such as the electrical conductivity or sensing property.2 As an n-type semiconductor, the stoichiometric excess of cation is sustained by oxygen vacancy,3'4 and the grain boundary surface characteristics due to the material process essentially modify its conductivity mechanism.5 It is known that a double Schottky barrier is formed5 at the interparticle interface of polycrystalline SnO2 by the preferential adsorption of oxygen or hydroxyl ion at the interface between particles during the firing process. The nature and concentration of surface oxygen species are intimately related with the gas-sensing response of polycrystalline SnO 2 under all conditions in which these sensors are used. On the other hand, porously sintered SnO 2 pellets show a substantial conductivity change when only small concentrations of a combustible gas are present in a large excess of oxygen. A simple defect reaction mechanism of oxygen vacancy formation and oxygen adsorption on the particle surface cannot explain this significant conductivity change. Therefore, it is said that the conductance is controlled by surface 1368 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 6, Jun 1993 Downloaded: 28 Sep 2015
processes that are not in equilibrium with the bulk of SnO 2 particles.5 In order to understand the surface property by adsorbed species and sample history, it is important to investigate the information on the interface traps which are formed at the interparticles, but there are scarce discussions on the interface trap
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