Low temperature thermally stimulated current characterization of nanoporous TiO₂ films

Abstract

Nanoporous films of TiO 2 have been obtained by deposition on an alumina substrate of a commercial colloid and Au contacts have been put on surface for in-plane conductivity measurements. Samples have been characterized by means of a series of sequential thermally stimulated currents scans, from nearly liquid He temperature up to 200 K, in order to get information on: conduction mechanisms, distribution of intragap density of states and recombination times of free an localized carriers.A model for hopping conductivity has been considered to properly take into account of localized states filling; it has been used to fit the experimental data, without a-priori assumptions on the nature of the conduction mechanisms. It is found that hopping conductivity prevails over free-carrier, in quasi-equilibrium regime, in the investigated temperature range.

Electrical and Optical Characterization of CsPbCl3 Films around the High-Temperature Phase Transitions

“…To improve this simplistic model, we may take into consideration that lattice disorder usually adds a tail of allowed energy states within the gap below the conduction/valence band edge E c . Modeling the intragap DoS as an exponential tail, putting at E = 0 the border between extended and localized states ( E c = 0) [ 24 ]: with E L tailing factor, then is the total density of localized states within the band gap. We may write the decay constant in the form , where K is the Boltzmann constant and is a characteristic temperature.…”

Thermally Stimulated Currents in Nanocrystalline Titania

“…In our model we avoid any a priori assumption about the conduction mechanism, and this is essential because, as we will show, in general both the hopping and free carrier, i.e., the multitrapping, contributions must be taken into account for the electrical conductivity [ 13 ]: the possible prevalence of the one or the other depends on the temperature and the Fermi level position. Developed from reference models given in [ 13 , 14 , 15 ] and first applied in [ 16 ] to get an insight about the prevalence of hopping against multitrapping conductivity in nanocrystalline titania below 200 K, in this paper the model has been further extended to determine main parameters of the band tail, as energy decay constant, defect and recombination centres concentrations, hopping frequency factors. Moreover, our results, extended up to 630 K, show that hopping conductivity can be neglected over 200 K, where conductivity appears to be dominated by discrete energy levels in the forbidden gap, probably related to water, OH groups and oxygen vacancies, which role in transport properties is nowadays still a matter of intense debate [ 17 , 18 ].…”