Electrical Transport in doped ZnO Nanoparticles synthesized in the Gasphase
S. Hartner, M. Ali, M. Winterer, H. Wiggers
University of Duisburg-Essen, DE
Keywords: electrical, transport, impedance spectroscopy, ZnO, doped, aluminium
Abstract:Zinc oxide (ZnO) is a group II-VI wide and direct band gap semiconductor and has shown promising results as an inexpensive alternative for transparent conductive materials which are used for flat panel displays, varistors and sensors. Recently, ZnO doped with aluminum gets more attention for being an alternative material for the established but much more expensive Indium Tin Oxide (ITO) due to the fact, that it has comparable electrical and optical properties. The electrical properties of ZnO powders consisting of nanoparticles in the size regime between 6 to 16 nm were investigated by means of impedance spectroscopy. Aluminum doped as well as undoped materials were produced by gas phase synthesis. The crystallite size decreases with rising doping concentration of aluminum, which can be explained by the aluminum dopant that constrains the ZnO particles from sintering. The as-prepared powders were pressed into pellets with a diameter of 5mm and an average thickness of 0.1mm by applying a pressure of 1.02GPa for a period of 30min. The electrical properties of mechanically compacted pellets prepared from the nanosized zinc oxide (ZnO) powders were investigated using impedance spectroscopy (IS). The impedance of the samples was measured in hydrogen and in synthetic air between 323K to 673K. The measurements in hydrogen as well as in synthetic air show both two different transport processes depending on temperature and doping level. The temperature dependent conductivity in hydrogen atmosphere shows a PTC (positive temperature coefficient) behavior for lower dopant concentration and switches to NTC (negative temperature coefficient) towards higher dopant concentration. In synthetic air, the conductivity increases for doping concentrations up to 7.74% of aluminum and it collapses for higher doping levels. This behavior can be explained by generation of free charge carriers due to the incorporation of hydrogen and doping with aluminum, respectively. At higher temperatures and at high doping concentration, scattering processes at grain boundaries as well as lattice defects increasingly affect the charge carrier transport processes. The differing properties in hydrogen and under ambient conditions are reversible.