Electronic Transport in Tin(IV) Oxide Nanocrystalline Films: Two-Medium Transport with Three-Dimensional Variable-Range Hopping Mechanism for the Ultrasmall Nanocrystallite Size Regime

Homogeneous, nanocrystalline films of tin(IV) oxide with controllable crystalline grains in the ultrasmall size range of 4-12 nm have been prepared by using a simple method of spin-coating followed by annealing in oxygen at different postannealing temperatures (T-anneal). These nanocrystalline films all exhibit a high optical transparency of 90-100% in the visible region with a band gap of 3.71 +/- 0.05-3.87 +/- 0.05 eV compared to 3.6 eV for bulk SnO2, indicating a high carrier density for all the TO films. The films obtained with T-anneal >= 350 degrees C, marking the onset of crystallization, are found to be conductive. The ac resistivity is measured as a function of temperature between 50 and 280 K for all the conductive films, and two distinct behaviors are observed between 50 and 90 K (LT) and 120-280 K (HT). The presence of two different media, i.e., the crystalline grains and the charge-depletion layer, can explain the observed resistivity behavior. The excellent fit of a parallel resistor model to the resistivity data for samples obtained with T-annel = 350-700 degrees C further validates the presence of the two media, revealing energy barrier heights of 48.0 +/- 0.4-60.5 +/- 0.4 meV for transport across the grain boundaries. The resistivity behavior in each medium is best described by the three-dimensional variable-range hopping (3D-VRH) model, given its excellent fit to the experimental data. On the basis of the resistivity results as analyzed within this model, we conclude that increasing T-anneal leads to a reduction in the carrier density as defect density decreases. The 3D-VRH fits to the resistivity in the LT region further reveal that above the onset of exponential growth at T-anneal = 500 degrees C, a remarkable improvement in the charge transport occurs likely due to the observed enhanced crystallinity. Postannealing at different temperatures, therefore, has a direct effect on the extent of crystallization in the amorphous matrix and the size of the resulting nanocrystallites, both of which affect the defect density and transport channels, and can therefore be used to provide fine control on the resistivity of the nanocrystalline SnO2 film.