Hello,
I have measured the electrical resistivity of TiO2 thin films prepared on glass substrates by sol-gel and spin coating. The amorphous films dried at low temperature give ohmic behaviour and good resistivity (about 105 - 106 ohm cm).
After thermal annealing at different temperatures the values increase (even of orders of magnitude) and some films treated at 400 °C (crystallized as anatase) show no ohmic behaviour nor measurable resistivity.
I would expect an inverse trend, with denser and crystallized coatings more conductive than the low T dried ones.
I have observed a dependence of resistivity on thickness (and I know that films become thinner with annealing), but comparing films with the same thickness (e.g. about 50 nm) the amorphous ones still appear more conductive than the annealed ones.
What could be the explanation?
Resistivity was measured in a longitudinal geometry, with 2 parallel silver contacts drawn on the surface.
Hi Claudio Imparato
The advice is to look for the surface ligands of the TiO2 film before and after annealing through FTIR test. Possibly, these ligands, at low temperature are mostly OH, enhance the hydrophilic tendency and then the conductivity. The broadband gap for the anatase phase, 3.2, may reduce the conductivity too. For preparing more conductive TiO2 films, it is better to work at low temperature, amorphous coating.
Greeting
Dear Claudio Imparato
You did not mention about your grain size! What about your film surface? Generally, we know that resistivity decline with external heat energy (i.e annealing). Conversely, increase resistivity trend appear with higher number of grain boundary in film surface, signify poor crystallinity. Consequently, electron can not move from one grain to another grain, resulting resistivity increases.
Hope, following attached paper may help you:
Article Influence of Tio2 thin film annealing temperature on electri...
Thank you all for your answers and useful suggestions.
The annealing is performed in air, and the resulting grain size is about 10 nm or smaller (evaluated by Scherrer's formula), probably the crystallinity degree is not very high.
Indeed, I see more surface OH groups in the amorphous films, and some of them also contain organic ligands (hybrid films), which may play a role in conductivity, along with their generally higher defectivity with respect to the annealed ones.
The Majibul Babu response it's true when we thing about varistor, for instance. When we increase the annealing time, consequentely, the grain boundering reduce ( barrier potential falls) and the condutivity became higher. However, when oxidation number, in oxide material decreases, the condutity increase, obviosly, the are more free electrons. The oxigens vacancies also contribute to increase the number of free carriers. For instance, films made by sputerring method are deposited at atmosphere of 8% of oxigen, more than that de density of electron carrier decrease. This condition of deposition are very commum in deposition of ITO.
You performed a complex experiment. Resistivity depends on many different factors. But, what about the oxidation state of Ti, it is well known that there are Ti3+ in titania ceramics, which transforms to Ti4+ at heating. In this way resistivity can be chaged drammatically...
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