  The Persistent Photoconductivity (PPC) effect was studied in individual tin
oxide (SnO2) nanobelts as a function of temperature, in air, helium, and vacuum
atmospheres, and low temperature Photoluminescence measurements were carried
out to study the optical transitions and to determine of the acceptor/donors
levels and their best representation inside the band gap. Under ultraviolet
(UV) illumination and at temperatures in the range of 200 to 400K we observed a
fast and strong enhancement of the photoconductivity, and the maximum value of
the photocurrent induced increases as the temperature or the oxygen
concentration decreases. By turning off the UV illumination the induced
photocurrent decays with lifetimes up to several hours. The photoconductivity
and the PPC results were explained by adsorption and desorption of molecular
oxygen at the surface of the SnO2 nanobelts. Based on the temperature
dependence of the PPC decay an activation energy of 230 meV was found, which
corresponds to the energy necessary for thermal ionization of free holes from
acceptor levels to the valence band, in agreement with the photoluminescence
results presented. The molecular-oxygen recombination with holes is the origin
of the PPC effect in metal oxide semiconductors, so that, the PPC effect is not
related to the oxygen vacancies, as commonly presented in the literature.
