Optical transitions and point defects in F:SnO2 films: Effect of annealing

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FTO films were deposited on borosilicate glass using chemical spray pyrolysis at 450°C then subjected to post deposition annealing in air at 500, 550 and 600°C. The films are characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical and electrical measurements. They are found to have the Rutile structure with strong orientation along the (110) and (200) planes and with grain size varying with annealing temperature in the range 20–100nm. Electron concentration and oxygen vacancy concentration in the range (2.61–7.07)×1020cm−3 and (1.49–2.41)×1022cm−3 were determined using Hall and XPS measurements respectively. The analysis of optical absorption spectra revealed the presence of three direct optical transitions of energies E1=3.78±0.07eV, E2=4.39±0.07eV and E3=4.81±0.08eV. Taking into account the Moss–Burstein and the Urbach tailing effects, E2 was identified as being due to a direct optical transition across the Γ3v+−Γ1c+ gap. The mean value of the width of this gap is determined to be 3.86±0.14eV. The two other energies E1 and E3 are assigned to electronic transitions originating from the lower valence bands Γ5v− and Γ1v+ respectively to a defect level at Ec −0.61±0.02eV attributed to the second ionization state of the oxygen vacancy. On the other hand, the analysis of the Hall mobility results on the basis of current theories provides evidence that fluorine is at the origin of a double donor which, according to XPS measurements, must contain FSn bonds. This double donor, suggested to be the complex center [F–Sn–F]++, dominates the electrical properties of as-deposited films and creates isolated substitutional fluorine FO at higher annealing temperatures possibly by thermal dissociation.