Characterisation of solution processed zinc oxide quasi-superlattice materials for thin film applications

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Buckley, Darragh
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University College Cork
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Metal oxide thin films are an important area of research due to the broad range of applications in modern optoelectronics. Solution processing allows for novel properties of oxide materials to be investigated through low-cost and scalable techniques in an effort to challenge the current vacuum-based state of the art in metal oxide semiconductor deposition. This work presents the solution deposition of ZnO thin films in the form of single layers and multilayer quasi-superlattices (QSLs). Solution processed growth of single and multi-layer ZnO and Al:ZnO thin films through an iterative spin coating deposition and heating protocol was employed to enable a greater understanding of the resulting crystalline growth of films with predictable thicknesses. Using a wide range of spectroscopic and optoelectrical measurements, the structural, morphological, optical, electronic and crystallographic properties were investigated. The impact of the number of layer depositions on defect composition, surface roughness, emission spectrum and optical band gap were assessed. The angle-resolved reflectance of high crystalline quality, c-axis oriented ZnO and AZO single and periodic multilayered spin coated channels materials are presented. An accurate method for the non-destructive determination of film thickness was derived using UV/vis/NIR reflectance data, agreeing very well with measured thickness of 1 to 20 layered thin films from transmission electron microscopy. Optoelectronic properties of solution processed thin films were investigated via green laser illumination at various power densities during I-V measurements. Repeatable and consistent photovoltaic behavior is exhibited of Al-ZnO thin films from sub-band illumination at 532 nm, leading to unusually high photocurrents of > 3 mA. Experimental data and theoretical analysis corroborate to suggest the formation of a donor level within the ZnO band gap caused by interstitial doping of Al.
Thin film , Solution processed , Zinc oxide , Optoelectronics
Buckley, D. 2020. Characterisation of solution processed zinc oxide quasi-superlattice materials for thin film applications. PhD Thesis, University College Cork.