Detection of intra-band gap defects states in spin-coated sol-gel SnOx nanolayers by photoelectron spectroscopies
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The presence of occupied intra-band gap states in oxygen-deficient tin dioxide (SnO x ; 1 < x < 2) is crucial for efficient manufacturing of multipurpose electronic devices based on transparent conducting oxides. Former experimental determination of these states was conducted for well-defined, usually thick tin oxides obtained under highly controlled vacuum conditions. In this work, we present precise specification of gap defects states for ultra-thin SnO x layers prepared by sol-gel synthesis followed with spin-coat deposition. Post-deposition drying and annealing processing changed layers' surface morphology and bulk crystalline structure as monitored by scanning electron microscopy, atomic force microscopy and x-ray diffraction. An x-ray photoemission spectroscopy (XPS) analysis of chemical composition revealed the presence of both Sn2+ and Sn4+ species in layers with and without post-drying annealing step. A stronger contribution of SnO was found for dried SnO x . In the valence band region, XPS studies revealed pronounced O 2p and hybridised Sn 5p/5s–O 2p states as well as deep, overlapping with the O 2p, band gap states resulting from Sn 5s orbitals. These states—attributed to defect states—indicated enhanced presence of Sn2+ cations, and were assigned to 'bridging' oxygen vacancies. Complementary photoemission yield spectroscopy (PYS) studies of the SnO x band gap region revealed an increased effective density of occupied electronic states below the Fermi level E F for annealed layers. The consequence was a work function reduction by 0.15 eV after the annealing process. PYS results allowed a precise detection of SnO x shallow band gap states close to E F. These states were attributed to surface oxygen vacancies, which was confirmed by computer modelling. Finally, the annealed layers exhibited higher calculated charge carrier concentration, hence the increased n-type character.