A simple approach to the fabrication of fluorine-doped zinc oxide thin films by atomic layer deposition at low temperatures and an investigation into the growth mode

Abstract

A simple low-temperature fabrication of fluorine-doped ZnO (ZnO:F) thin films by atomic layer deposition (ALD) was investigated and the growth mode of the films was analyzed. A novel method for fluorine doping into a ZnO matrix was successfully developed that uses ALD with a home-made fluorine source at a low deposition temperature of 140 °C, which is low enough for films to be applied to a plastic substrate. The fluorine doping concentration was controlled from 0 at.% to 1.2 at.% by manipulating the pulse sequence ratio of the oxygen source (deionized water) and the home-made fluorine source. Structural and morphological properties were investigated by X-ray diffraction and field emission scanning electron microscopy. The growth mode of the films was studied by grazing-incidence wide-angle X-ray diffraction. The grain growth orientation was found to change significantly as the fluorine concentration was increased due to the characteristics of fluorine doping in the oxygen sites of ZnO. This phenomenon could be explained by a perturbation and passivation effect resulting from the fluorine doping mechanism, with the fluorine anions filling oxygen-related defect sites in the ZnO lattice. A photoluminescence study confirmed the diminishment of defect sites and showed that, due to the Burstein–Moss effect, the optical transmittance in the visible region increased significantly with an increase in the fluorine doping concentration from 80.87% to 83.02%. The lowest resistivity was 1.876 × 10⁻³ Ω cm for ZnO thin films doped with 1.0 at.% fluorine when the carrier concentration and mobility were 1.375 × 10²⁰ cm⁻³ and 24.20 cm² V⁻¹ s⁻¹, respectively.