Growth of highly transparent oxide layers by pulsed laser deposition: reduction of droplet density

Thin layers of ZnO and CeO2 have been grown on Corning glass and Si substrates by pulsed laser deposition (PLD) method. Under optimised conditions, films exhibiting absorption coefficients in the visible region as low as 4×102 cm−1 and 3×103 cm−1 for ZnO and CeO2, respectively, have been deposited at a substrate temperature of only 350°C. When using the PLD technique, the grown layers very often exhibit some micrometer sized droplets. Although the surface density of these can be dramatically reduced, there is still much effort being directed at completely eliminating their presence, which could clearly restrict the applications. There is still much controversy concerning the mechanisms of droplet formation, whether they are caused by a sub-surface super-heating effect or by explosive boiling. Careful scanning electron microscopy investigations of various target surfaces after the laser ablation process and numerical simulations of the temperature distribution inside these targets during the action of the laser pulse tend to support the sub-surface super-heating effect as one of the main causes of droplet emission.