Plasma-Assisted Chemical Vapor Synthesis of Aluminum-Doped Zinc Oxide Nanopowder and Synthesis of AZO Films for Optoelectronic Applications

Abstract

Transparent conducting oxide aluminum-doped zinc oxide (AZO) nanoparticles were synthesized by a plasma-assisted chemical vapor synthesis route using zinc nitrate and aluminum nitrate as the precursors. The injected precursors vaporized in the plasma flame, followed by vapor-phase reaction and subsequent quenching of the vaporized precursors, producing nanosized AZO powder. The amount of aluminum nitrate was varied to obtain samples with 2 at.%, 4 at.% and 8 at.% Al, designated as AZO1, AZO2 and AZO3, respectively. The XRD patterns of the AZO1 and AZO2 nanoparticles indicated the presence of a wurtzite structure without any alumina peaks except in the AZO3 sample, and scanning electron microscopy micrographs revealed spherical particles. The magnetization measurements revealed a ferromagnetic behavior at room temperature in the AZO1 sample, and the ferromagnetic order is decreased in the high field region with an increase in the Al doping amount. AZO thin films were deposited on glass substrates by spin-coating a dispersion of nanoparticles. All the AZO films had a hexagonal wurtzite structure and exhibited a c-axis preferred orientation perpendicular to the substrate. The Hall effect measurements yielded a minimum resistivity of 9.9 × 10−4 Ω cm for the AZO2 film and optical transmission of 80% for both the AZO1 and AZO2 films. However, with 8 at.% Al in the AZO3 film, deterioration in crystallinity, electrical and optical properties were observed. Post-annealing of the AZO1 film in H2 atmosphere caused a significant decrease in resistivity from 1.2 × 10−3 Ω cm to 8.7 × 10−4 Ω cm. The optical band gap energies of the AZO films were determined from the transmission spectra. The blue shift in the band gap from 3.2 eV to 3.28 eV, observed with an increase in Al doping, was interpreted by the Burstein–Moss effect. The photoluminescence spectra of the AZO films revealed a UV near-band edge emission and a green emission peak.