Room-temperature deposition of nanocrystalline PbWO_4 thin films by pulsed laser ablation

摘要

Pulsed laser ablation (PLA) was applied to synthesize nanocrystalline PbWO_4 thin films onto glass substrates. The effects of Ar background gas pressure on phase evolution, microstructures and optical characteristics of PbWO_4 thin films were investigated in detail. The PLA processes were carried out at room temperature without substrate heating or post-annealing treatment. XRD and HR-TEM results revealed that the PbWO_4 thin films are composed of nanocrystalline and amorphous phases. Moreover, the films contained a high density of lattice defects such as twin boundaries and edge dislocations. The crystallite size and crystallinity increased, which were associated with a change in surface morphology as the Ar pressure increased. Reduced tungsten states W~(5+) or W~(4+) induced by oxygen vacancies were observed at 10 Pa and the atomic concentration of all constituent element was almost stoichiometric, especially the [Pb]/[W] ratio, which was nearly unity above 50 Pa. The optical energy band-gap was 3.03 eV at 50 Pa and increased to 3.35 eV at 100 Pa, which are narrower than the reported value (4.20 eV). This optical band-gap narrowing could be attributed to localized band-tail states and new energy levels induced by the amorphous structure and inherent lattice defects.