Growth of ZnO thin films by ultraviolet pulsed-laser ablation: Study of plume dynamics

Abstract

A comparative study is presented on the growth of ZnO thin films by ultraviolet pulsed-laser deposition using nanosecond and femtosecond pulses at 248nm. Different experimental parameters were examined including substrate temperature, oxygen ambient pressure, and laser energy density at the target. At optimum conditions the films grown exhibited high optical transmittance and a preferential orientation along the c axis, both with nanosecond and femtosecond irradiations. However, those produced with the ultrashort laser pulses are composed of smaller dimension crystallites, with higher angular distribution on the substrate and higher roughness, suggesting different film growth mechanism. This was investigated by studying the plume dynamics of the ejected material accompanying laser ablation of the ZnO targets by means of spatially and time-resolved optical emission spectroscopy in order to obtain information on the nature of the ejected species and the distribution of their propagation velocities. Employing irradiation conditions, typically used in the deposition experiments, emission lines assigned to electronically excited neutral zinc atoms (Zn*) were observed both in the case of nanosecond and femtosecond pulses, while the latter additionally gave rise to emission attributable to zinc ions (Zn+*). The mean propagation velocities measured suggest the presence of highly energetic Zn cations in the case of femtosecond ablation, which are likely to be responsible for defect formation on the films, disturbing the smooth growth of crystallites that takes place under nanosecond irradiation, and giving rise to films composed of smaller crystallites with higher mosaicity and roughness.