Synthesis of fluorine doped zinc oxide by reactive magnetron sputtering

ZnO and fluorine doped ZnO (FZO) thin films were prepared by d.c. reactive magnetron sputtering using a zinc target in an Ar/O2(/F2) mixture. In a first attempt ZnO films were synthesized in order to optimize the matrix properties in terms of crystalline properties and transparency. The parameters studied were the d.c. power (Pdc), the total pressure (PTot) and the O2 content in the discharge (%O2). The highest grain size of ∼25 nm is obtained for Pdc = 70 W, PTot = 30 mtorr and %O2 = 7.5%. F2 was then introduced in the discharge. The influence of the presence of fluorine on the crystallographic, chemical, electrical and optical properties of the deposited films were evaluated. Our X-ray photoelectron spectroscopy and X-ray diffraction (XRD) data suggest that only a certain part of the measured fluorine atoms substitute for oxygen atoms in the ZnO structure. The rest of fluorine could be adsorbed as F2 on the grain boundaries or located in interstices of the ZnO structure. XRD data reveal a decrease in the crystallite size with an increase in the fluorine content. Above a fluorine concentration of ∼2% the FZO films become amorphous. The electrical properties have been investigated by Hall effect measurements. The optimal synthesis conditions (∼2% of fluorine in the film) were a charge carrier density of ∼1020 cm−3, an electrical resistivity of 10−2 Ω cm, and a charge mobility of 4 cm2 V s−1. Finally, all deposited FZO films had >80% transmission in the visible range.