Effect of doping concentration and temperature on the morphology, crystallinity and electrical conductivity of Al:ZnO nanostructured films grown from aqueous solution

Abstract

Al-doped ZnO nanostructures were grown on glass substrate by hydrothermal method, in the temperature range of 70–95°C, from aqueous solutions of zinc nitrate he333xahydrate and aluminum chloride hexahydrate. Based on scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy and electrical measurement data, the effect of 2–6wt.% Al doping on the morphology, microstructure and electrical properties was investigated. The morphology and crystalline structure are strongly dependent on the Al-doping and growth temperature. When Al doping increases from zero to 2 and 4wt.%, the morphology changes from nanowires with higher optical phonon confinement, to combined morphologies including nanowires, nanoblades and transparent nanosheets. Lowering the temperature of the growth solution and/or increasing the concentration of aluminum dopant above 4wt.%, 1D (nanowires) and 2D (nanoblades and nanosheets) nanostructures collapse into an amorphous structure with totally changed morphology. Micro-Raman spectra and X-ray diffraction patterns confirmed the wurtzite structure with preferential c-axis growth direction of samples containing up to 4wt.% aluminum. The average crystallite size decreases continuously from 23.8 to 21.1nm with increasing the Al concentration from 0 to 4wt.%. The electrical conductivity of the obtained nanostructures significantly depends on the aluminum doping concentration. 2wt.% Al-doped ZnO nanostructured layer grown at 95°C showed the best electrical conductivity of 140.9Ω−1cm−1. The ambivalent role of aluminum, donor or acceptor, depending on doping concentration was highlighted.