Optical and structural properties of Si-doped ZnO thin films

Transparent thin films of Si-doped ZnO were prepared by the sol-gel spin coating method. The optical and structural properties of the films were investigated to see the effects of Si dopant concentration for three different heat treatments: 250, 350, and 550 °C. Doping is most significant at the highest annealing temperature, where the structure of the films is crystalline at low Si values, and deteriorates as Si levels are increased. Optically, the result is increased transmittance and decreased reflectance with doping for the near-UV and visible ranges. The large changes in the near-UV region (20-50% increase in transmittance relative to pure ZnO) are due to weaker excitonic absorption as crystallinity is destroyed by doping. The films at 250 and 350 ° C were amorphous, but the 350 ° C samples showed optical trends qualitatively similar to 550 ° C (though less pronounced). Band gaps were shifted slightly upwards with doping, though at higher annealing temperatures the values remained within 2% of the 3.3 eV pure ZnO result at all Si levels. AFM measurements of RMS surface roughness varied in a small range from ≲ 1 to 7 nm, with the roughest samples being pure ZnO at 250 °C, and low Si dopings at 550 °C. The optical characteristics for all the films, regardless of heat treatment or doping, were successfully modeled using a Tauc-Lorentz-based dielectric function, consisting of two oscillator terms describing near-band-gap absorption, and an exponential Urbach tail in the sub-gap regime.