Morphology and optical property control of electrodeposited zinc oxide

Abstract

Active-matrix liquid crystal displays (LCDs) can be made brighter and more efficient by using optically transparent thin film transistors (TFTs). Economical pro-duction, small size, and control of defects are all essential factors in introducing transparent semiconductors into TFT device applications. As potential candidates for TFT applications, semiconducting ZnO thin films (300 nm to 2 μm thickness) were economically synthesized on stainless steel substrates using electrodeposition in this thesis work. Scanning Electron Microscopy images show that applied deposition voltage affects ZnO crystallite morphologies and deposition time influences crystallite sizes. Ultraviolet-Visible reflectance measurements show that the ZnO bandgap energy increases with more negative deposition potentials, and varies inversely with film thickness and aging time, ranging from 3.1 eV to 3.4 eV. One of the likely reasons for the bandgap energy dependence with film thickness, applied potential, and aging time is a hydrogen-induced Moss-Burstein effect. Hydrogen is evolved from the aqueous solution during electrodeposition, and it can be incorporated as an electron donor and increase the bandgap energy. Defects also appear to play a role in restricting hydrogen diffusion.