Electrical properties of aluminum-doped zinc oxide (AZO) nanoparticles synthesized bychemical vapor synthesis

Abstract

Aluminum-doped zinc oxide nanoparticles have been prepared by chemical vapor synthesis,which facilitates the incorporation of a higher percentage of dopant atoms, farabove the thermodynamic solubility limit of aluminum. The electrical propertiesof aluminum-doped and undoped zinc oxide nanoparticles were investigated byimpedance spectroscopy. The impedance is measured under hydrogen and syntheticair between 323 and 673 K. The measurements under hydrogen as well as undersynthetic air show transport properties depending on temperature and doping level.Under hydrogen atmosphere, a decreasing conductivity with increasing dopantcontent is observed, which can be explained by enhanced scattering processesdue to an increasing disorder in the nanocrystalline material. The temperaturecoefficient for the doped samples switches from positive temperature coefficientbehavior to negative temperature coefficient behavior with increasing dopantconcentration. In the presence of synthetic air, the conductivity firstly increases withincreasing dopant content by six orders of magnitude. The origin of the increasingconductivity is the generation of free charge carriers upon dopant incorporation. Itreaches its maximum at a concentration of 7.7% of aluminum, and drops forhigher doping levels. In all cases, the conductivity under hydrogen is higher thanunder synthetic air and can be changed reversibly by changing the atmosphere.