Detection of intra-band gap defects states in spin-coated sol-gel SnOx nanolayers by photoelectron spectroscopies

The presence of occupied intra-band gap states in oxygen-deficient tin dioxide (SnOx; 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 SnOx 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 SnOx. 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 SnOx 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 SnOx 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.