Optical Properties Tuning of SnO2 Films by Metal Incorporation (Pt,Pd): Correlation with Microstructure Change

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In this work, we report on the effect of noble metal doping (namely Pd or Pt) on the optical properties of SnO2 thin films. The optical constants ($n$ and $k$) of the films, as a function of noble metal nature and content, were obtained using variable angle spectroscopic ellipsometry in the ultraviolet–visible–near infrared (UV–vis–NIR) regions. Ellipsometry analysis showed that we can tune the optical constants of SnO2 films by changing Pt or Pd doping concentration. In particular, their refractive index increases from 1.6 to ${\sim}2$ while varying Pt content from 3 to 12 at. %. The origin of this optical behaviour was correlated to the microstructure change induced by metal doping. X-ray diffraction (XRD) was used to investigate the effect of doping on SnO2 lattice parameter, on crystallite size and on film preferential orientation. Atomic force microscopy (AFM) was used to estimate the surface roughness of the films. A metal concentration of ${\sim}3$ at. % (for both Pt and Pd), which is known to yield the highest SnO2 gas sensing response, was found to correspond to the highest contraction of the lattice parameter of the films. Finally, the energy band gap of undoped SnO2 thin films (estimated to 4 eV) was found to shift to lower value while increasing doping concentrations.
2009-07-25