Nanocolumn evolution and band-gap engineering in Bi-doped ZnO thin films: experimental investigation and DFT + U insights- S. I. Shulyma,
- N. A. Kurgan,
- V. L. Karbivskyy,
- O. V. Ivanov,
- V. O. Moskaliuk,
- N. K. Shvachko,
- V. V. Zaika &
- S. S. Smolyak
Abstract
The influence of Bi doping on the structural, optical, and electronic properties of ZnO thin films prepared by radio frequency magnetron sputtering was investigated experimentally and confirmed by density functional theory calculations. It was established that Bi doping preserves the wurtzite crystal structure of ZnO thin films, but increases the lattice parameters. It was shown that both undoped and Bi-doped ZnO films are formed from nanocolumns oriented mainly along the [002] direction, perpendicular to the substrate surface. Low Bi concentrations (up to 1.8 at%) do not modify the average nanocolumn thickness (＞ 18 nm), whereas higher Bi concentrations (above 5.2 at%) reduce it to 5 nm. At the same time, Bi doping induces a non-monotonic change in the band gap width: a decrease from 3.20 eV (undoped ZnO films) to 3.09 eV (films with 1.8 at% Bi) results from the expansion of the lattice parameter 扼, while a further increase of Bi concentration (above 5.2 at%) increases the band gap width to 3.18 eV due to the enhanced local electrostatic field, the Moss每Bernstein effect, and quantum confinement associated with a thinner average nanocolumn thickness. It was established that the presence of Bi impurities transforms the direct band gap of ZnO into an indirect one. The transformation mechanism of nanocolumn structure at different Bi concentrations is proposed, in which Bi substitution in ZnO lattice and the formation of the denser 汛-Bi2O3 phase generate significant in-plane stress and hinder the growth of larger nanocolumns at high Bi concentrations.
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