Influence of the laser wavelength on the microstructure of laser ablated Ba0.5Sr0.5TiO3 films

Abstract

Barium strontium titanate (Ba0.5Sr0.5TiO3 or BST) films were deposited on the (1 1̄ 02)-oriented sapphire substrates using a Nd:YAG laser operating in its second and third harmonics (532 and 355 nm, respectively) and a KrF excimer laser (248 nm). In the same growth conditions, the overall films have the cubic perovskite structure and show randomly oriented polycrystalline BST for the films deposited at laser wavelengths of 532 and 355 nm. The films deposited with the radiation of 248 nm wavelength are preferentially oriented along the [111] direction. Scanning electron micrographs reveal that the film surface is covered by particles whose size and density are laser wavelength dependent. The better film morphology (smaller and fewer particulates) is obtained with a laser wavelength of 248 nm. BST absorbance spectrum measured in the region covering the laser wavelengths used (UV-visible) shows strong variation, with high absorption coefficient in the UV. The influence of the processing variables such as the laser characteristics, and target physical properties during the ablation deposition is discussed. In the high peak power density regime (∼108 W/cm2), we have found that the laser wavelength, through the target absorption coefficient, affects strongly the particulate size and density. Therefore, short laser wavelength is proven as an alternative way to solve the large particulate issue in the case of BST thin films.