Single and multiple doping effects of silicon–boron and fluorine on ZnO thin films deposited with sol–gel spin coating technique

Undoped, Si and B single doped, Si–B co-doped and Si–B–F triple doped ZnO thin films were grown by sol–gel spin coating method. Effects of these doping elements on microstructural, morphological and optical properties were investigated. X-ray diffraction studies showed that all films had hexagonal wurtzite structure. Although Si doping increased crystallinity of ZnO, single boron doping, Si–B co-doping and Si–B–F triple doping gave rise to a decreasing in crystallinity. Scanning electron microscope micrographs indicated that the grain size and morphological characters of ZnO depended on doping element type and their concentrations. The micrographs also demonstrated that Si doping deteriorated grain size and their distribution on film surface of ZnO structure. Although single B, doubly Si–B and triple Si–B–F doping at low contents improved grain distribution and film morphology, when their content increased, film morphology started to deteriorate. Optical band gap value of undoped film increased with Si doping irrespective of Si doping content. Although single B, doubly Si–B and triple Si–B–F doping at low contents caused an increase in optical band gap of undoped ZnO, more doping content brought about a decrease in optical band gap. From the optical parameters such as absorption coefficient, the Urbach energy values of the films were calculated by using ln α vs. photon energy graphs. In generally, Urbach energies of the films changed reversely with the band gap energies of the films.