Defects and dopants in zinc oxide: a study of the optoelectronic properties of thin films prepared by spray pyrolysis

Abstract

Interest in transparent conducting oxides (TCOs) has intensified over the past decade, driven by the requirement to find a suitable replacement to indium tin oxide (ITO). Of the many possible candidates identified, zinc oxide (ZnO) was selected owing to its favourable optical properties, abundance, low toxicity and chemical stability. This thesis is directed towards finding low cost routes to producing transparent conducting ZnO thin films that could be utilised in a range of current and emerging optoelectronic devices. The spray pyrolysis technique is demonstrated as a highly appropriate low cost, large area deposition route to produce ZnO thin films. The necessity for, and the construction of, an automated rig for the deposition process is detailed. Excellent reproducibility is demonstrated compared with manual deposition. The structural, electrical and optical properties of native and doped ZnO are examined with the overall aim to optimise performance for TCO applications. The novel, AC Hall effect technique is employed to study the factors controlling charge carrier concentration and charge mobility across the a wide doping range, including in previously unexplored regions such as in the undoped and low doping regimes. Investigations into the optimal crystal structure for the greatest charge carrier concentration and charge mobility are made and links between the degree of (002) texturing and charge mobility suggested. Further studies into the position of Al, Ga and In dopants in the crystalline lattice show that the ionic radius plays a key role in the solubility of the dopant in the lattice. Ingress and egress of the zinc vacancy (VZn), and emergence of the hybrid dopant substitution/zinc vacancy (XZnVZn where X = Al, Ga, In) defects are suggested as likely candidate to explain observed electronic behaviour. Finally, the effect of annealing ZnO films is studied, and the necessity for Zn-rich films for TCO applications determined. Annealing in oxygen poor environments is shown to greatly reduce resistivity of films, with VZn defects suggested to be the main contributor to this effect. The stability of electronic improvements following annealing is presented; no observable degradation in performance is seen subsequent to 180 days storage in ambient conditions.