Ab-initio simulations of the Al/ZnO interface

This thesis is part of a broad effort aimed at gaining a better description of the active elements in functionalized nanostructures for energy applications, and a deeper understanding of charge and energy transfer mechanisms in these complex systems. In particular this work focused on Aluminum doped ZnO, recently proposed for interesting photovoltaic applications, and on the Al/ZnO interface since Metal/ZnO contacts are central to all ZnO electronic devices. Beside the fundamental interest in the challenging topics related to material science and nano structures, the control of the interactions at these interfaces offers a unique opportunity to unravel the interplay between structures and functionalities of increasing complexity and technological relevance. Our approach is based on ab initio Density Functional Theory (DFT) simulations to characterize microscopic properties and processes that constitute the fundamental building blocks of ideal structures and interfaces. Firstly we studied the effects of aluminum doping on the electronic and optical properties of ZnO, via DFT simulations. We discussed the bandstructure and absorption properties of Al:ZnO as a function of the dopant concentration, and compared with recent experimental data. Our results support the formation of a transparent conductive oxide compound up to an incorporation of Al of about 3% in substitutional Zn sites. We propose an explanation to the observed degradation of conductivity in terms of interstitial defects expected to occur at high doping concentrations, beyond the Al solubility limit. Finally, we addressed the problem of the Al/ZnO interface, the main target of the PhD project. For the first time, the Al/ZnO Schottky barrier was calculated on a reduced mismatch system, taking into account the macroscopic dipole field effects adapting well-established methods to the present case. Our results highlight the Ohmic character of contact, clearly showing the strong influence of the interface microscopic details on the barrier height.