Mechanisms of the laser plume expansion during the ablation of LiMn2O4

Abstract

Films of LiMn 2 O 4 prepared by pulsed laser ablation deposition are typically lithium deficient when grown at background pressures ranging between 10 −4 and 20 Pa. The deficiency of light atomic species such as lithium in LiMn 2 O 4 thin films occurs as a result of the different behavior of the species during plume expansion and the deposition of the film at elevated deposition temperatures. The plasma plume evolution in vacuum and 20 Pa oxygen pressure are studied using two spectroscopic techniques: emission spectroscopy and plume imaging. Higher velocities and a wider spatial distribution of lithium atoms are observed in vacuum when compared with manganese and oxygen species. Plume species are slowed down due to collisions with gas molecules when ablating LiMn 2 O 4 in an oxygen atmosphere. As a result, a strong deceleration of the plasma front occurs, and the effect is more pronounced for the light species, e.g., Li. Comparing the spatial manganese and lithium distribution within the plasma, the lithium species are again scattered into a wider angular range. These similar spatial distributions of Li atoms detected in the pressure range between 10 −4 and 20 Pa suggest that it is not possible to achieve a congruent lithium transfer to a growing film if the target contains heavier elements besides lithium. The general implications for the ablation of materials containing a combination of light and heavy elements are that as-grown films are inherently deficient with respect to the content of the light elements. If the mass ratio between the light and the heavy elements is not too different, is should be possible to find a set of deposition parameters which will result in the growth of a film with the desired composition.

Nanosecond and femtosecond ablation of La0.6Ca0.4CoO3: a comparison between plume dynamics and composition of the films

“…During expansion in a background gas, the plume undergoes multiple scattering with the oxygen molecules, resulting in a spatial splitting into species which travel without collisions (fast) and species undergoing collisions (slow). This effect has been reported in the literature [21], and is observed for Table 4. In ns ablation the plume front position follows a τ 2/5 time dependence, in agreement with the shock wave (SKW) propagation model in gases [25].…”

“…7c, d). A non-uniform distribution of elements in the plasma, as reported for Ba in BaTiO 3 [33] and Li in LiMn 2 O 4 [21] is also not likely because no pronounced differences in the distribution of the elements is detected. The small differences in the kinetic energies of the ablated species are probably not being enough to explain the non stoichiometric transfer in fs ablation.…”

“…from Ca(II) to Ca(I) and from La(II) to La(I). The difference in velocities between ions and neutrals has been previously reported in the literature [21] and has been explained by the ambipolar diffusion mechanism [22]. This mechanism suggests that in early times of the plume expansion, the lighter particles (electrons) will diffuse much faster than the heavier particles (ions and neutrals), leading to the formation of an electron rich and electron depleted layer in the plume.…”

Synthesis and electronic properties of Ruddlesden-Popper strontium iridate epitaxial thin films stabilized by control of growth kinetics

“…It has been generally recognized that during the PLD growth of multicomponent oxide thin films, the background gas pressure can induce significant variations of the angular distribution of different species in the plume, which in turn results in off-stoichiometry of films with respect to the chemical composition of a target. [51][52][53][54][55] More specifically, in nearly vacuum, the plume is primarily composed of elemental atomic/ionic species. Compared to heavier ions, lighter species tend to propagate with higher velocity and hence a narrower angular distribution towards the substrate normal.…”

Dynamics of the plumes produced by ultrafast laser ablation of metals

“…A similar effect has been observed in ns laser ablation of LiNbO 3 where the Li had a wider distribution than Nb. 32 Fitting the angular distributions to Eq. ͑6͒, values of k zy Ϸ 4.4 for the aspect ratio of the main Ni ion feature and k zy Ϸ 2.3 for the contamination component were found.…”