| Abstract |
A comparative study was carried out on the growth of ZnO thin films by UV pulsedlaser deposition (PLD) using 35 nanosecond and 450 femtosecond pulses at 248 nm from an excimer laser system. Hot-pressed, high purity ZnO was employed as a target. The thin films were grown on Si(100) wafer or quartz plate substrates. 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 irradiation. However, those produced with the ultra-short 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 timeresolved 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 Zn* were observed in the case of nanosecond pulses while additional lines attributable to Zn+* were recorded in femtosecond ablation. The electronically excited species Zn* and Zn+*, are considered to arise as a result of recombination of electrons with nascent ablation species, carrying an extra positive charge that is, Zn+ and Zn+2 respectively. By monitoring the evolution of the emission of Zn* and Zn+* it was calculated that the mean propagation velocity for the Zn+ along the surface normal is ~10 kms-1 (kinetic energy of few tens of eV) in both nanosecond and femtosecond ablation plume. The propagation velocity of Zn+2, evident only in the femtosecond ablation plume is of the order of 33 kms-1 (kinetic energy of few hundredths of eV). Such high energy species in the ablation plume are likely to be responsible for the inferior quality of the ZnO films produced by femtosecond PLD as their impact on the film surface could lead to non-uniform crystallite growth.
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Ανάπτυξη επιστρώσεων ZnO με τη μέθοδο φωτοαποδόμησης με παλμικό λέιζερ.Φασματοσκοπική μελέτη της δυναμικήςτου πλάσματος
