X-ray Photoelectron Spectroscopy of Novel Ternary Materials

Abstract

In this thesis the best synthesis conditions and compositional dependency of optoelectronic properties for novel ternary n-type amorphous Zinc Tin oxide (a-ZTO) and p-type nanocrystalline CuxCrO2 Transparent Conducting Oxides (TCOs) are explored.

a-ZTO was synthesised employing physical vapour deposition (magnetron sputtering) and chemical vapour deposition (spray pyrolysis) methods. Comprehensive compositional analysis was performed on magnetron sputtered a-ZTO via X-ray photoelectron spectroscopy (XPS). Two distinct compositions were identified and found to correspond to ZnSn2O5 and ZnSn3O7, structures which are unstable in bulk yet which could exist locally. The role that local bond order, confirmed by Raman spectroscopy, play in the electrical properties of the material is discussed. An in situ sample transfer across magnetron sputtering => XPS/Ultraviolet photoelectron spectroscopy => Scanning Tunneling Microscopy/Spectroscopy (STM/STS) systems allowed for measurement of stoichiometric formula ZnSn2.9O6.7 that is close to calculated unstable polymorph ZnSn3O7. UPS measurements reveal the work function of the material to be 3-3.1 eV. The STM/STS characterisation indicated the bandgap of a-ZTO is approximately 3.3 eV. Annealing at 600? C under ultra high vacuum conditions did not induce long range order crystallinity, but formed local crystalline clusters on the surface. a-ZTO was studied by cost effective, stable and reliable spray pyrolysis technique using various precursors combinations. At optimised deposition conditions films with uniform thickness and the highest conductivity ~300 S/cm reported for non-vacuum, solution processed films are achieved. A systematic study of a-ZTO synthesis utilizing dichlorozinc and stannous octoate precursors in a methanol solution demonstrated that final film stoichiometry and conductivity values are highly dependent on the kinetically limited growth and derived morphology of the film. A new methodology for studying the decomposition of precursors during spray pyrolosis was developed. The methodology utilised Reflectance anisotropy spectroscopy to monitor the optical response of a growing film in real time. In order to ensure that a strong signal a substrate coated with Au plasmonic dimers was used. This method applied to thermal decomposition of Cu(acac)2 and Cr(acac)3 precursors and allowed for identification of the physical origin of the very narrow growth window for p-type CuxCrO2. Beyond this specific material, this technique is applicable to a wide variety of other spray pyrolysis processes. Expected use cases are identifying ideal growth conditions for various ternary oxides or finding alternative precursors to further reduce growth temperatures to minimize the thermal budget of thin film synthesis.