Effect of Ar and O₂ Atmospheres on the Fundamental Properties of the Plasma Produced by Laser Ablation of Lithium Niobate

Abstract

The in uence of gas atmospheres (at 1 torr) of different natures, both reactive (O 2 ) and inert (Ar), on the spatial evolution of the electron temperature (T e ) and electron density (N e ) of the plasma generated by laser ablation of a LiNbO 3 target is evaluated by optical emission spectroscopy techniques. It is found that the behavior of N e in the plasma produced in vacuum, argon, and oxygen atmospheres exhibit quite different trends as a function of the distance from the target to the substrate, but the behavior of T e is nearly constant. The spatial constancy of T e in all the atmospheres studied is probably related not only to the low laser irradiance used (0.06 GW cm 22 ), but also to a possible collisional decoupling of the heavy species in the central part of the expanding plasma plume where a low pressure region is formed. The fact that N e is lower in Ar than in O 2 indicates the presen ce of fewer positive ions in the plasma generated in Ar. The impact of this result on the better crystallinity observed in the LiNbO 3 lm s grown in Ar is also discussed.Index Headings: Pulsed laser ablation; Lithium niobate; Argon and oxygen atmospheres; Plasma diagnosis. INT RODUCTIO NPulsed laser deposition (PLD) has become a very promising tool for both advanced micromachining and thin lm deposition. Among the parameters involved in the PLD processes, the laser uence together with the background gas pressure (and nature) and the distance at which the substrate is located are relevant variables inuencing the lm growth process. M oreover, the ablation of solid targets produces, in most cases, a lum inous plasma form ed by neutral and ionized species whose expansion dynamics have been widely studied in order to understand the physical mechanisms controlling the thin lm deposition process and properties. 1 Nevertheless, a very limited number of works have focused on the study of the fundamental plasma parameters, such as the electron temperature and electron density, or the nature of the dominant plasma excitation and ionization processes in different spatial and temporal regions. 2-6 Although they are essential to evaluating the energy and m aterial transport in the plasma, as well as to understanding the plasm a reactivity in connection with its effectiveness in the processes of thin-lm growth and composition, such studies have only been perform ed in ablation of m etals, 2,3 graphite, 4 and YBa 2 Cu 3 O 7 . 5 Lithium niobate (LiNbO 3 ) is a well-known m aterial with excellent nonlinear properties of interest for m any applications that normally require the production of high quality thin lms.7 Although PLD has proven to be one of the m ost successful techniques in growing complex oxide materials, 8 the lms deposited by ablation of LiNbO 3 in vacuum or low pressure (,1 torr) atmospheres are often found to be Li de cient. 9,10 However, recent results indicate that when the growth is carried out at a pressure of about 1 torr, independent of the nature of the ambient atmosphere used (either inert or reactive), the LiN...

Spectroscopic characterization of laser-induced tin plasma

“…It was noted that the three-body recombination rate coefficient strongly depended on the nature of the ambient gas used. 29 Gordillo-Vazquez et al 29 estimated the ionization rate coefficient and three-body recombination rate coefficient for laser-produced lithium niobate plasma and found these values to vary with distance for different ambient gases. Compared to the expansion into a vacuum, the interaction of the plume with an ambient gas is a far more complex gas dynamic process that involves deceleration, attenuation, thermalization of the ablated species, confinement, formation of shock waves, etc.…”

Investigation of local thermodynamic equilibrium of laser induced Al2O3–TiC plasma in argon by spatially resolved optical emission spectroscopy

“…In addition, this increase of T ion with increasing distance, as shown in Fig. 2, could be explained by a spatial increase of the ionization rate coefficient, 25 such a spatial increase of T ion disappears at high pressure. By increasing the argon pressure, T ion approaches T exc which means that LTE becomes more valid.…”

Influence of Ar and O2 atmospheres on the Li atom concentration in the plasma produced by laser ablation of LiNbO3

“…However, for plasmas produced by lasers with a pulse duration of several nanoseconds, as in our case, it happens to be that the relaxation time of electrons 15 ͑the thermalization time͒ is much shorter than any characteristic time scale of the laser-produced plasma. The above considerations also apply to the plasma produced after laser ablation of LiNbO 3 in Ar and O 2 atmospheres at 1 Torr since the measured N e and Te are of the same order (ϳ10 16 cm Ϫ3 and ϳ0.6 -0.9 eV, respectively 14 ͒ as those obtained in vacuum. Author to whom correspondence should be addressed; electronic mail: vazquez@io.cfmac.csic.es Consequently, the time-dependent rate equations for calculating the concentration of the electrons in the transient plasmas of interest here can be solved by assuming that electrons are in steady state.…”

“…2 ϩ ,e Ϫ ). Therefore, since we have measurements of the spatial evolution of N e and T e in our laser-produced parent plasma, 14 we can introduce them in the collisional-radiative model for atomic oxygen proposed by Soon and Kunc 22 to estimate the ground and excited state concentrations of oxygen atoms in our parent plasma. According to this model, the populations of the three lower energy levels of an oxygen atom almost always follow a Boltzmann distribution with N O(1) Х0.90ϫN O where N O is the total amount ͑ground and excited͒ of oxygen atoms in the plasma ͑see Fig.…”

“…The present model is a generalization of the one reported earlier, 10 hereafter referred to as article I, for laser-generated lithium-like plasmas in vacuum which was based on a diagnostic method originally proposed by Suckewer 11 and Kunc. 12 The comparison between the concentration of Li atoms predicted by the model under vacuum conditions and in Ar and O 2 atmospheres at 1 Torr will provide: ͑i͒ quantitative information on the spatial evolution of ground and excited Li atom concentrations in vacuum, and in Ar and O 2 atmospheres at 1 Torr by simply introducing the previously reported N e and T e values 13,14 into the model equations, ͑ii͒ qualitative agreement with the spatial evolution of available experimental emission intensities from excited Li atoms in Ar and O 2 environments at 1 Torr, and ͑iii͒ quantitative information on the relative loss and gain electron-impact rate coefficients contributing to, respectively, population and depopulation of the different energy levels of Li atoms present in the laser-produced plasmas of interest here.…”