Self-Healing of Defect-Mediated Disorder in ZnO Thin Films Grown by Atomic Layer Deposition

Electronics with semiconductors rely strongly on defect concentrations and on the properties of these defects. Here we study ZnO thin films which were grown by atomic layer deposition. An interesting mechanism of build-up and of self-healing of Zn interstitial defects as a function of layer thickness d was found, based on measurements of photoabsorption (PA), photoluminescence (PL) and x-ray diffraction as a function of d. The concentration of Zn interstitial defects increases up to d = 19 nm, coupled with a corresponding increase of the Urbach energy, E_u, in PA. At this layer thickness, the growth mode changes from the formation of a homogeneous layer to a layer of nano-crystals, where the nano-crystals grow in size with d. Surprisingly, the Zn interstitial concentration decreases spontaneously once the layer thickness exceeds d = 38 nm. We explain this behavior by a reduction of diffusion barriers for Zn interstitials as a function of average ZnO particle size leading to spontaneous diffusion to the particle surface and subsequent oxidation therein. At the same time, the concentration of oxygen vacancies, mostly located at the particle surface, is greatly reduced with increasing film thickness. The study is of importance in designing opto- and nano-electronic devices by means of appropriate selection of ZnO film thickness, for targeted quality, property and further practical applications.