Optimizing the optical properties of fluorine-doped ZnO thin films deposited by sol-gel spin-coating

The optical properties of sol-gel spin-coated ZnO and F-doped ZnO (FZO) thin films were investigated using ultraviolet-visible and temperature-dependent photoluminescence spectroscopies. The FZO films showed on average of > 80% optical transmittance over the entire visible region, and the transmittance spectra and the optical bandgaps were blueshifted owing to the Burstein-Moss effect. The 4 at% FZO film showed the lowest Urbach energy and, thus, the best crystallinity of all the films. Changes in the free exciton, neutral-donor-bound exciton (D⁰X), donor-acceptor pair, and second-order longitudinal optical phonon replica peaks were represented from the temperature-dependent photoluminescence spectra. The strength of the exciton-phonon coupling was investigated using the Bose-Einstein approximation to fit the experimental data, and the thermal activation energies of the as-deposited ZnO and 4 at% FZO thin films were determined using Arrhenius plots for the integrated PL intensities of the D⁰X peak.