Influence of oxygen pressure on the expansion dynamics of Ba-hexaferrite ablation plumes and on the properties of deposited thin films

Using the pulsed laser deposition (PLD) technique to prepare Ba, Sr-hexaferrite films from stoichiometric targets, it has been found that the degree of perpendicular anisotropy of the films is sensitive to, among other parameters, the background oxygen ambient. In an effort to better understand the ablated material transport under Ba-hexaferrite film growth conditions, an emission spectroscopy study was initiated. Temporally integrated spectra were collected as a function of distance above the target surface and as a function of oxygen pressure (PO2). Only emission lines from Ba, Fe neutrals and singly ionised Ba species were observed. However, emission features from oxide molecules were not identified even in the presence of oxygen. The plasma appeared highly ionised close to the target and, during its propagation towards the substrate, expanded to a low density weakly ionised vapour, mostly populated with Ba and Fe neutrals. High local equilibrated gas temperatures (of the order of 0.8 eV) in the deposition region are thought to facilitate film crystallisation and oriented growth. The emission signal was found to depend on oxygen pressure and the distance from the target. Generally, it decreased with distance and increased with PO2. It was deduced that the optical emission is excited by electron impact excitation. Moreover, both temporal profiles of the constituent line intensities and time-resolved images of the overall optical plasma emission consistently demonstrated that the oxygen pressure confines the plasma, slows its expansion and enhances the emission particularly at its expanding front. The change observed in the quality of Ba ferrite films with oxygen pressure is discussed in terms of the behaviour of the expanding plume with increasing PO2.