Towards monomaterial p-n junctions: Single-step fabrication of tin oxide films and their non-destructive characterisation by angle-dependent X-ray photoelectron spectroscopy

Abstract

The application of a non-destructive method for characterization of electronic structure of an ultra-thin SnO1<x<2 layer synthesized by spin coating on Si wafers was demonstrated. Utilizing angle dependent XPS, we quantified stoichiometry changes inside the SnO1<x<2 layers of thickness comparable with the electron attenuation length. The O/Sn concentration varied from 1.25 near the SnOx surface to 1.10 near the substrate/overlayer interface. Deviations from ideal stoichiometry are caused by defects, and defect levels affect the band structure of the SnOx layers. By investigation of the valence band region, followed by main core level position tracking, changes of electronic parameters like energy levels shift were identified. The results indicated a downward energy levels shift by 0.45 eV in SnOx layers at the SiO2/SnOx interface. In combination with the detected upward energy levels shift in the substrate's electronic structure, these results suggest a negative charge displacement across the SiO2 layer. As a consequence, there is a significant carrier concentration gradient in the layer, from a nearly insulating oxide at the SnOx surface to a semiconducting one at the bottom of the SnOx film. The results showed that the application of a simple and cost-effective method allows tuning the materials' properties towards the one-step fabrication of materials with ambipolar doping.