The natural n-type conduction of tin dioxide (SnO 2 ) may be compensated by trivalent rare-earth doping. In this work, SnO 2 thin films doped with Eu 3+ have been deposited by the sol-gel dip-coating (SGDC) process, topped by a GaAs layer deposited by the resistive evaporation technique. The goal is the combination of a very efficient rare-earth emitting matrix with a high-mobility semiconductor. The x-ray diffraction pattern of SnO 2 :Eu/GaAs heterojunctions showed simultaneously the crystallographic plane characteristics of GaAs as well as cassiterite SnO 2 structure. The electric resistance of the heterojunction device is much lower than the resistance of the SnO 2 :2 at.%Eu and GaAs films considered separately. Micrographs obtained by scanning electron microscopy (SEM) of the cross-section showed that the interface is clearly identified, exhibiting good adherence and uniformity. A possible explanation for the low resistivity of the SnO 2 :2 at.%Eu/GaAs heterojunction is the formation of small channels with two-dimensional electron gas (2DEG) behavior.

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Resumo. Espectroscopia Raman mostra a evolução dos modos de vibração intra-grãos de SnO 2 com o aumento da temperatura de tratamento térmico. Palavras-chave: dióxido de estanho, fi lmes fi nos, cério, terras-raras, transporte elétrico. Abstract Incorporation of

Incorporação de Ce3+ ou Ce4+ em filmes finos de SnO2 depositados via sol-gel-dip-coating aumenta drasticamente a resistividade elétrica. No primeiro caso, temos comportamento aceitador do dopante, levando a matriz à alta compensação de carga. Por outro lado, para Ce4+, verifica-se aumento na largura da região de depleção do contorno de grão, resultando em maior espalhamento de elétrons. Medidas de caracterização elétrica sob pressão ambiente levam à barreiras de potencial mais altas do que as medidas sob vácuo, devido a adsorção de oxigênio na superfície das partículas. A presença de Ce3+ aumenta a transmitância no infravermelho, o que significa menor quantidade de elétrons livres. Dados de XANES confirmam que o tratamento térmico a 550 ºC dos filmes, ainda que promova oxidação parcial para Ce4+, preserva uma quantidade significativa (em torno de 60%) no estado Ce3+. Espectroscopia Raman mostra a evolução dos modos de vibração intra-grãos de SnO2 com o aumento da temperatura de tratamento térmico.

Abstract. Optical excitation of Ce 3+ -doped SnO 2 thin films, obtained by the sol-gel-dip-coating technique, is carried out and the effects on electrical transport are evaluated. Samples are doped with 0.1at% of Ce, just above the saturation limit. The excitation is done with an intensity-controlled halogen-tungsten lamp through an interference filter, yielding an excitation wavelength of 513nm, 9 nm wide (width at half intensity peak). Irradiation at low temperature (25K) yields a conductivity increase much lower than above bandgap light. Such a behavior assures the ionization of intra-bandgap defect levels, since the filter does not allow excitation of electron-hole pairs, what would happen only in the UV range (below about 350nm). The decay of intra-bandgap excited levels in the range 250-320 K is recorded, leading to a temperature dependent behavior related to a thermally excited capture cross section for the dominating defect level.