Evandro A. de Morais

Electro-optical properties of Er-doped SnO2 thin films

Abstract Photoconductivity of SnO 2 sol-gel films is excited, at low temperature, by using a 266 nm line—fourth harmonic—of a Nd:YAG laser. This line has above bandgap energy and promotes generation of electron-hole pairs, which recombines with oxygen adsorbed at grain boundary. The conductivity increases up to 40 times. After removing the illumination on an undoped SnO 2 film, the conductivity remains unchanged, as long as the temperature is kept constant. Adsorbed oxygen ions recombine with photogenerated holes and are continuously evacuated from the system, leaving a net concentration of free electrons into the material, responsible for the increase in the conductivity. For Er doped SnO 2 , the excitation of conductivity by the laser line has similar behavior, however after removing illumination, the conductivity decreases with exponential-like decay.

Sb doping effects and oxygen adsorption in SnO2 thin films deposited via sol-gel

Transparent electrically conducting antimony-doped SnO2 thin films have been prepared by sol-gel dip-coating process from colloidal aqueous suspension. The effect of doping content on the structural, optical and electrical properties is analyzed. Results from infrared optical transmission and reflection have shown that the higher the Sb concentration the lower the transmission intensity and the higher the reflection signal. Absorption intensity increases as well. Results of X-ray reflectometry and electron microscopy have shown that the density of films fired at 400 °C after each dip is higher than that of multi-dipped films prepared with a single annealing. Both the electrical characteristics in the dark and the increase in conductivity as function of illumination through different filters, at 190 K, evidence that the transport properties of these films are dominated by the presence of defects, including the trapping at grain boundary due to excess of oxygen.

Er rare-earth ion incorporation in sol-gel SnO2

Er-doped SnO2 thin films and xerogels are obtained by sol-gel technique. In order to understand the Er³+ rare-earth ion activity in SnO2 matrix, a characterization of Er incorporation is done through emission and excitation spectra of xerogels, besides thin film electrical characterization. Effects to grain dimensions are also analyzed based on X-ray diffraction data showing the particle growth with annealing temperature and inhibition of this growth by Er doping. Electrical characterization results suggest that Er³+ has an acceptor-like character in SnO2, and that codoping with Yb³+ allows an energy transfer process Yb³+ ® Er³+.

Visible emission from Er-doped SnO2 thin films deposited by sol-gel

Emission from Er-doped SnO2 thin film deposited via sol-gel by the dip coating technique is obtained in the range 500-700 nm with peak at 530 nm (green). Electron-hole generation in the tin dioxide matrix is used to promote the rare-earth ion excitation. Evaluation of crystallite dimensions through X-ray diffraction results leads to nanoscopic size, what could play a relevant role in the emission spectra. The electron-hole mechanism is also responsible for the excitation of the transition in the 1540 nm range in powders obtained from the same precursor solution of films. The thin film matrix presents a very useful shape for technological application, since it allows integration in optical devices and the application of electric fields to operate electroluminescent devices.

Analysis of Er3+ incorporation in SnO2 by optical investigation

Er3+ emission in the wide bandgap matrix SnO2 is observed either through a direct Er ion excitation process as well as by an indirect process, through energy transfer in samples codoped with Yb3+ ions. Electron-hole generation in the tin dioxide matrix is also used to promote rare-earth ion excitation. Photoluminescence spectra as function of temperature indicate a slight decrease in the emission intensity with temperature increase, yielding low activation energy, about 3.8meV, since the emission even at room temperature is rather considerable.

Optical excitation of charge carriers from intra‐bandgap states in Ce‐doped SnO2 thin films

Optical excitation of Ce3+‐doped SnO2 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 513 nm, 9 nm wide (width at half intensity peak). Irradiation at low temperature (25 K) 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 350 nm). 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.