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Defects that exhibit some sort of lattice relaxation usually present an energy barrier for electron capture, and the possibility of developing the phenomenon known as persistent photoconductivity (PPC). In this effect, carriers induced in a metastable way remain in a conductive state forever, if the temperature is low enough to avoid the thermally excited retrapping of carriers by large lattice relaxation defects. Although this hypothesis is usually accepted to explain the origin of PPC, there are other principles such as the separation of excited carriers by random local-potential fluctuations. In this paper, we list many sorts of materials that exhibit the PPC effect and the attempts of several researchers to model the transient decay of PPC, seeking parameters in order to understand and model the electrical behavior of these materials. Besides, the PPC effect can be seen as the resulting electrical conduction obtained after removing the electromagnetic irradiation, and it is appealing for applications in a wide range of optoelectronic devices. A personal approach to modeling PPC decay is presented and applied to single crystalline semiconductors, nanocrystalline materials and heterostructures.

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are used for investigating heterostructure samples of GaAs/SnO 2. XANES data are used for analyzing the local organization around Eu in the heterostructure formed by GaAs and Eu-doped SnO 2. The differences between the XANES data for these samples and data obtained for Eu-doped SnO 2 thin films, deposited on glass substrate, are assumed as responsible for the differences in the photoluminescence (PL) spectra concerning the Eu 3+ emission, since films deposited on glass substrate do not present Eu 3+ PL transitions until the annealing temperature is rather high. Eu 3+ emission is explored using two different excitation sources: 350 nm from a Kr + laser (above SnO 2 energy bandgap) and 488 nm from an Ar + laser (below SnO 2 bandgap energy). The existence of more organized regions around the Eu 3+ site observed for the heterostructure surface may be associated with the Eu 3+ luminescent emission. The main and secondary features in the XANES show that there are differences in the average local Eu environment for the SnO 2 :Eu isolated thin films and heterostructures, being more organized in the latter. Electrical characterization evidences that the portion of the resistivity reduction that corresponds to photo-ionized intrabandgap states is responsible for the persistent photoconductivity phenomenon in the heterostructures.

Deposition of an SnO 2 thin film was carried out by sol-gel-dip-coating and doped with Ce 3+ or Eu 3+ , and a GaAs layer was deposited by resistive evaporation or sputtering. This investigation combines the emission properties of these rare-earth ions with the unique transport properties generated by the heterostructure assembly. Illumination with light with energy above the GaAs bandgap and below the SnO 2 bandgap drastically increases the GaAs/SnO 2 heterostructure conductance, which becomes practically temperature-independent. This was associated with the presence of interface conduction, possibly a two-dimensional electron gas at the GaAs/SnO 2 interface. This feature takes place only for the sample where the GaAs bottom layer is deposited via sputtering. Irradiation with energies above the SnO 2 bandgap only excites the top oxide layer. The heterostructure assembly GaAs/SnO 2 :Eu leads to emission from Eu 3+ , unlike SnO 2 deposition directly on a glass substrate, where the Eu 3+ transitions are absent. Eu emission comes along a broad band, located at a higher energy compared to Eu 3+ transitions, which are blue-shifted as the thermal annealing temperature increases. Luminescence from Ce 3+ ions in the heterostructure can be detected, but the ions overlap with emission from the matrix, and a cleaning procedure helps to identify Ce 3+ transitions.

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