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

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• 作者：Fikry El Akkad ; Tressia A.P. Paulose
• 关键词：Tin oxide ; F ; SnO2 ; FTO ; Defects ; Electrical properties ; Optical properties ; Thin films ; Transparent conducting oxides ; Atomic force microscopy
• 刊名：Applied Surface Science
• 出版年：15 March, 2014
• 年：2014
• 卷：295
• 期：Complete
• 页码：8-17
• 全文大小：1682 K

文摘

FTO films were deposited on borosilicate glass using chemical spray pyrolysis at 450 掳C then subjected to post deposition annealing in air at 500, 550 and 600 掳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 (1 1 0) and (2 0 0) planes and with grain size varying with annealing temperature in the range 20-100 nm. Electron concentration and oxygen vacancy concentration in the range (2.61-7.07) 脳 10²⁰ cm^鈭? and (1.49-2.41) 脳 10²² cm^鈭? 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 卤 0.07 eV, E₂ = 4.39 卤 0.07 eV and E₃ = 4.81 卤 0.08 eV. Taking into account the Moss-Burstein and the Urbach tailing effects, E₂ was identified as being due to a direct optical transition across the gap. The mean value of the width of this gap is determined to be 3.86 卤 0.14 eV. The two other energies E₁ and E₃ are assigned to electronic transitions originating from the lower valence bands and respectively to a defect level at E_c 鈭?.61 卤 0.02 eV 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 F_O at higher annealing temperatures possibly by thermal dissociation.