Conductivity and Structure of Sputtered ZnO:Al on Flat and Textured Substrates for Thin-Film Solar Cells

Aluminum-doped zinc oxide (ZnO:Al) is a prominent representative of the material class denoted as transparent conductive oxides (TCO). TCOs feature electrical conductivity while being transparent in the visible range. These unique properties constitute the wide application of TCOs in opto-electronic devices. This work targets the application of TCOs for thin-film silicon and chalcopyrite-based solar cells. Generally, TCOs are deposited onto at substrates. However, TCO growth on textured, light scattering substrates for thin-film silicon solar cells and on the rough chalcopyrite absorber also call for the optimization of TCO deposition on textured substrates. Therefore, the deposition of sputtered ZnO:Al on at as well as on textured substrates is elaborated. The focus is the understanding and optimization of electrical conductivity accompanied by a detailed investigation of the material's structural properties. On at substrates, I propose a conductivity model that comprises three scattering mechanisms, namely ionized-impurity, electron-phonon, and grain boundary scattering. The prominent feature of the model is the analytical description of grain boundary scattering by feld emission, i.e. quantum mechanical tunneling of electrons through potential barriers at grain boundaries. For this purpose, a theory of Stratton(R. Stratton, $\textit{Theory of Field Emission from Semiconductors}$, Phys. Rev. $\textbf{125}$ (1962), 67 - 82) is adapted to double Schottky barriers at grain boundaries. The conductivity model is applied to a wide range of literature data to show its applicability and explanatory power. After establishing the basic understanding of ZnO:Al conductivity, two optimization routes are presented. The first route allows for a reduction of deposition temperature by 100 $^{\circ}$C without deteriorating conductivity, transparency, and etching morphology by means of a seed layer concept. Seed and subsequently grown bulk layers were deposited from ZnO:Al$_{2}$O$_{3}$ targets with 2 wt% and 1 wt% Al$_{2}$O$_{3}$, respectively. I investigated the effect of bulk and seed layer deposition temperature as well as seed layer thickness on electrical, optical, and structural properties of ZnO:Al films. The positive effect of the highly doped seed layer was attributed to the beneficial role of the dopant aluminum that induces a surfactant effect. Furthermore, the seed layer induced increase of tensile stress is explained on the basis of the grain boundary relaxation model. Finally, temperature-dependent conductivity measurements, optical fits, and etching characteristics revealed that seed [...