Interface conduction and photo-induced electrical transport in the heterojunction formed by GaAs and Ce³⁺-doped SnO₂

Electrical and optical properties of heterojunction composed of GaAs and SnO₂ are presented. SnO₂ thin film was deposited by sol-gel-dip-coating and doped with Ce³⁺ whereas the GaAs layer was deposited by resistive evaporation or sputtering. The purpose of this investigation is to combine the blue emission properties of the rare-earth with the unique transport properties generated by the heterojunction assembly. We have found that illumination with light of energy above the GaAs bandgap and below the SnO₂ bandgap decrease drastically the GaAs/SnO₂ heterojunction resistance. Under this condition, the sample exhibits an unusual behavior: the conductivity is practically temperature independent. This behavior was related with the presence of interface conduction, which could be associated to a two-dimensional electron gas at the GaAs/SnO₂ interface. This feature takes places only for the sample where the GaAs bottom layer is deposited by sputtering, which presents a smoother surface as inferred by AFM images. The deposition sequence of the layers is fundamental to provide sample excitation which strongly contributes to the sample conductivity. Energies above the SnO₂ bandgap only excite the top oxide layer. When the GaAs is the top layer it acts as a shield, and only effects from the ions located close to the interface SnO₂/GaAs are observed. Luminescence from the Ce³⁺ ion can be detected, but overlap with emission from the matrix. Results suggest that a more organized GaAs bottom layer may contribute for a more efficient emission and also for signal separation.