Practical application of laser-Induced plasma expansion models for thin film deposition

This chapter presents a study that combines several plasma expansion models andplasma characterization techniques to answer the following question, can the optimumconditions for the growth of good quality ZnO thin films be predicted from models or atleast bounded between some limits? Using ion signals, acquired with a Langmuir probe, it is corroborated that theAnisimov model describes successfully the expansion of laser produced plasma invacuum. Furthermore, plasma properties such as initial energy and specific heats ratioobtained from it can be used as input for the initial conditions of the plasma expansion ina background gas.For plasma expansion in a background gas, firstly, a full characterization of theexpansion is performed using intensified charged coupled device (ICCD) photographyand ion signals. The commonly used shock wave model (Sedov-Tylor model) is appliedand it is shown that it describes the general features of the expansion, i.e. front expansion~ t^2/5; yet it failed in a full description of the plasma properties. However a differentapproach of another shock wave model, the Freiwald-Axford model, shows to be moreappropriate. The proposed approach describes the plasma contact front instead of thecommonly described shock front. This allowed using the plasma specific heats ratio asobtained from Anisimov model instead of approximated values of the same physicalmagnitude but for the gas background. A detailed comparison between the two modelsand the correctness of the plasma expansion velocities, positions and pressurescorroborated that such an approach is correct. It is shown that the use of ion signals as extracted using the Langmuir probe is not correct for describing the plasma front. On theother hand the use of ICCD photography is correct. It is corroborated that in abackground gas the plasma reaches a maximum in its expansion which is well predictedby the Predtechensky model. Beyond this distance the plasma enters in a diffusive regimewhich is well described by a 3-D diffusion model.The results obtained from the thin film depositions are presented. Structure, surfacesmoothness and optical properties of the ZnO thin films are studied and shown to presentgood quality. It was shown that oxygen incorporation into the growing thin film occurs,preferentially, during the material condensation on the substrate.Finally, it is presented a discussion that answers the question posed above.