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Ultra- short pulse laser ablation of Al70Cu20Fe10 alloy: Nanoparticles generation and thin films deposition
Thin Solid Films 517 (2009) 1880–1886
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Ultra-short pulse laser ablation of Al70Cu20Fe10 alloy: Nanoparticles generation and thin films deposition R. Teghil a,⁎, L. D'Alessio a, A. De Bonis a, D. Ferro b, A. Galasso a, G. Lanza a, A. Santagata c, P. Villani c, D.J. Sordelet d a
Dipartimento di Chimica, Università della Basilicata, via N. Sauro 85, 85100 Potenza — Italy CNR — ISMNS, Sez. Roma 1, P.le A. Moro 5, 00185 Roma — Italy CNR — IMIP, Unità di Potenza, via S. Loja, 85050 Tito Scalo (PZ) — Italy d Materials and Engineering Physics Program, Ames Laboratory, Iowa State University, Ames, IA — USA b c
a r t i c l e
i n f o
Article history: Received 28 August 2007 Received in revised form 16 September 2008 Accepted 25 September 2008 Available online 8 October 2008 Keywords: Laser ablation Metal alloys Deposition process Ultrashort pulse deposition Plume characteristics
a b s t r a c t In this paper the deposition of thin films obtained from femtosecond laser ablation of an Al70Cu20Fe10 alloy is presented. In the plasma produced by ablation, a characteristic feature is the presence of hot nanoparticles that become evident several microseconds after the laser shot. The cooling mechanisms of these particles have been analysed together with the evolution of their composition. The results, compared with those previously obtained for Al65Cu23Fe12 quasicrystal, reveal a clear relation between the final composition of the particles and the high-temperature equilibrium vapor pressures of the different elements, suggesting a direct emission from the target rather than a gas phase formation. Analysis of the elemental composition through the cross-section of the as-deposited films helps to illustrate the role of nanoparticles in the film growth. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Laser ablation has always attracted a large interest either from the point of view of fundamental research or for applications in the field of thin films prepared using the Pulsed Laser Deposition (PLD) technique [1–3]. With the PLD method ultra-short pulses (time duration of 1 ps or less) offer numerous advantages compared to longer pulses. In fact, the laser–target interaction mechanism is different in these two cases; ablation by ultra-short pulses leads to small thermal effects, little mechanical damage or modifications to the target material [4,5] and, in some cases, to a stoichiometric transfer of the target composition to the deposited films. The last characteristic is particularly relevant because, on the one hand, conventional PLD performed with nanosecond pulsed lasers frequently shows some difficulties in retaining the target composition [6]. On the other hand, the phenomena involved in laser ablation performed by ultra-short pulse lasers are still matters of discussion and detailed models are available only for simple systems, such as metals [7,8]. Experimental results on the plasma produced by ultra-short laser pulses have shown the presence of both a primary plume that develops and evolves in the first microsecond after the laser pulse, and a secondary one, which originates after the first microsecond, that is detectable up to several tenths of a microsecond [9–12]. The primary plume is composed of atoms, neu⁎ Corresponding author. Tel.: +39 0971202225; fax: +39 0971202325. E-mail address: [email protected] (R. Teghil). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.095
tral and ionised, molecules, electrons and clusters, while the secondary plume consists of nanoparticles or large clusters [13–17]. In this paper we studied the primary and secondary plumes produced by femtosecond ablation of an Al70Cu20Fe10 metallic alloy, and, in particular, the characteristics of the particles forming the secondary plume. In fact, these nanoparticles, whose origin is still a matter of debate, play a fundamental role in thin films deposition. Moreover, the study of their morphology and composition may help understand the origin of these nanoparticles. We consider the particles as molten droplets ejected directly from the target, and this hypothesis is consistent with our experimental results. The Al70Cu20Fe10 composition, the so called ω phase in the phase diagram of Al–Cu–Fe system [18], was chosen due to the possibility to know or to estimate its thermochemical properties. Moreover, it is a very interesting material, in particular being an approximant of Al65Cu23Fe12 icosahedral quasicrystal [19], closely related to it but with different thermophysical and thermochemical characteristics [20]. From this point of view, a comparative analysis could be useful to clarify the importance of the characteristics of the solid target during the ablation–deposition process. So, we will compare the results on Al70Cu20Fe10 with those already obtained for Al65Cu23Fe12 [21]. 2. Experimental details The ablation and deposition experiments were performed by using the experimental apparatus already described [13]. It consists of a
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35 mm quartz Nikkor lenses was used for acquiring gated images of the overall plasma plume emission lateral view (200–800 nm) induced by single laser pulse. The spatial resolution achieved by this system was 25 μm. The acquisitions were performed by accumulating up to 20 shots, thereby avoiding previously irradiated surfaces. Both ICCD fast imaging and time-resolved spectra were accomplished by delaying the data acquisition of the plasma emission with respect to the laser pulse. Analysis of the deposits' morphology was performed using a 1450 LEO VP Scanning Electron Microscope (SEM), which has a resolution of 4 nm. The films' stoichiometry was determined by Energy Dispersive X-Ray Spectroscopy (EDX) using an INCA 300apparatus, which has boron as its first detectable element. A ZEISS EMIOC transmission electron microscope (TEM,) was used for a morphological characterization of the first deposition steps, and the film crystallinity was studied by X-Ray Diffraction (XRD) using a Siemens D 5000 instrument equipped with Cu Kα1 radiation. 3. Results and discussion 3.1. Plasma analysis
Fig. 1. Normalised time evolution plots of the emission of (a) Al (396.09 nm) and (b) Cu (324.70 nm), collected at 2 mm (●) and 4 mm (■) from the target. The laser fluence was 2.0 J cm− 2. The lines which connect the experimental data were added to facilitate understanding of the graphic.
stainless steel vacuum chamber, evacuated to a pressure of 1.5·10− 4 Pa, equipped with a 1.2 rpm rotating target holder. The distance between the target and the substrate was kept at 2.5 cm and all the experiments were carried out at room temperature. A Light Conversion frequency doubled Nd:glass laser (527 nm emission wavelength, 250 fs pulse duration, 10 Hz repetition rate) was used for the ablation and deposition experiments. In our experiments the laser fluence was varied between 0.2 and 2.0 J cm− 2 and the laser beam was incident at an angle of 45° on the target surface. The laser spot area on the target was 0.1 mm2. The ablation targets were Al70Cu20Fe10 crystalline alloys, supplied by Materials Preparation Center, Ames Laboratory US-DOE, Ames, IA, USA, while the deposition substrates were (111) oriented silicon from Cerac. In the samples prepared for transmission electron microscopy, the deposition substrates were carbon–formvar films from Agar Scientific. The optical emission spectra were detected by means of a Princeton ICCD device (1024 × 1024 pixels). The width of the entrance spectrograph slit and the grating employed were 80 μm and either 1200 or 150 grooves/mm. Thus, the spectral widths obtained were about 15 nm and 150 nm with resolutions of 0.15 nm and 0.5 nm, respectively. The gated system had best time resolution of 2 ns and each acquisition was integrated with 50 laser shots in order to increase the signal-to-noise ratio. Varying the position of the optical elements by a micrometric translation stage, was possible to obtain space-resolved emission spectra at different distances from the ablated target surface. The same ICCD system equipped with a 105/
As already seen for other systems [12], the gaseous phase produced by femtosecond laser ablation of our Al70Cu20Fe10 target consists of two distinct emissions of material. In the primary plume that develops and ends in the first microsecond from the laser pulse, optical emission spectroscopy (OES) indicates the presence of Al, Cu, Fe, Al+ and Cu+ species. The absence of the double ionised species, often found in nanosecond ablation [22], is probably due to the lack of interaction between the expanding plasma and the incoming fs laser radiation. The mean velocities of the different species obtained by time-resolved OES are similar (vAl = 1.6 ± 0.1 × 106 cm s− 1, vCu = 1.0 ± 0.1 × 106 cm s− 1) and in good agreement with those obtained using a quasicrystalline Al65Cu23Fe12 target [13]. Due to the low intensity of the signals, it is impossible to calculate the velocity distribution for iron and for the ionised species. In Fig. 1 the temporal evolution of the emissions of aluminum and copper is reported.
Fig. 2. Overall emission from AlCuFe plume recorded: (a) 200 ns and (b) 500 ns after the laser shot. The gate was 100 ns in both cases. The laser fluence was 2.0 J cm− 2. The light intensity is shown in terms of 20% density contours.
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Fig. 3. Overall emission from AlCuFe plume recorded 30 μs after the laser shot, with a gate of 1 μs. The light intensity is shown in terms of 20% density contours. The laser fluence was 2.0 J cm− 2.
The angular distribution of the material ejected from the target during ablation can be represented by the equation I(θ) = I0cosnθ, where I(θ) is the flux intensity along a direction forming an angle θ with the normal to the target surface, I0 is the intensity corresponding to θ = 0 and n is a parameter related to the anisotropy of the angular distribution. There are other models describing the angular distribution [5] but, for the aim of this work, the simplest one can be considered. The values of n have been calculated by the equation n¼
2ln2 −1 lnð1 þ x2 Þ
where x = b/a is the ratio between the major axis b and minor axis a of the elliptical plume [23].
With regard to the primary plume, ICCD imaging data show a value of 10 for the cosine exponent n and a front velocity of 2.0 ± 0.4 × 106 cm s− 1 (Fig. 2). The ICCD images are reported as intensity contour plots at different time delays after the laser shot, showing the time evolution of the plume. The contours on the left side of the images correspond to the position of the hot target. Another emission develops after the first microsecond from the laser shot and it lasts up to 50 μs (secondary plume). The OES of the secondary plume produced shows typical continuous black-body-like emission spectra, indicating the presence of particles or large clusters, while its ICCD imaging evidences a higher directionality (n = 20) and a lower front velocity (3.9 ± 0.4 × 104 cm s− 1) than the values obtained for the primary plume. In Fig. 3 the intensity contour plots obtained from an ICCD image recorded 30 μs after the laser shot and showing the angular distribution of the secondary plume are reported. Also in this case the target is on the left side of the image. 3.2. Thin film analysis The SEM images of the films deposited on silicon substrates (Fig. 4a) show that the films, which appear to be amorphous in structure as evidenced by XRD, are formed by the coalescence of a large number of droplets of nanometric size. This result confirms the importance of nanoparticles in the films deposited by ultra-short pulse PLD, as already evidenced for the Al–Cu–Fe quasicrystal system [13]. Using TEM bright field images (Fig. 4b), a survey of the first steps of the film growth has been performed, and, once more, the role played by the nanoparticles particles seems to be fundamental. As shown in Fig. 5, the particles size distribution obtained from TEM images shows a lognormal distribution with a mean particles diameter of 24 nm. This value does not change with the deposition time, at least in the first steps of the film growth that we can study by TEM. 3.3. Ablation–deposition mechanism Even if previous studies on fs ablation tended to exclude the melting of the target [24,25], there is now growing agreement about the occurrence of melted material on the target as a consequence of the interaction between an ultra-short laser pulse and the target surface [26–30] and about the presence of liquid nanoparticles in the fs plume [31]. Some of the possible models that allow the ejection of both a gaseous phase and melted particles include phase explosion
Fig. 4. Microphotograph of the surface of an Al70Cu20Fe10 films deposited with a laser fluence of 2.0 J cm− 2 for (a) 60 min (SEM image) and (b) 20 min (TEM image).
Fig. 5. a) Size distribution of Al70Cu20Fe10 particles (for a total of 304) obtained by analysis of the film reported in Fig. 4b. b) Diameter-probability plot for the same size distribution. The solid line fit represents lognormal size distribution. The value at 50% probability gives the mean diameter of the particles.
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Fig. 6. Temperature–time curves for the cooling of Al70Cu20Fe10 particles with a diameter of 20 nm. The dotted line indicates the radiative cooling and the solid line refers to the vaporization cooling and to the total cooling due to both effects. The points represent the experimental temperatures of the secondary plume expanding front.
[32], explosive melt removal [33] and hydrodynamic models [28]. Our results do not allow identifying the mechanism producing the target melting and the ejection of the particles, which, may be due to the occurrence of several mechanisms taking place simultaneously in different parts of the target [29]. Thus, in this current work we only aim to define the mechanism that allows the transfer of the target stoichiometry to the film. For this purpose we have compared the temperature variation of the front of the secondary plume during its expansion with calculations on the particles cooling rate. With this regard, the following cooling rate equation for melted particles has been considered: " # dT 6 Mv RT 1=2 ΔHv ¼− K T 5 −TR5 þ p dt ρCp a RT 2πMv M
ð1Þ
where ρ is the density of the particle, Cp is its thermal capacity and a is the particle diameter, K is constant with a value of 24.88·(2πk5B/c4h4), whereas kB and h are the Boltzmann and Planck constants and c the light velocity; T is the initial temperature of the particle and TR the room temperature, p is the vapour pressure, Mv is the vapour molecular weight, ΔHv is the vaporization enthalpy and M is the molecular weight of the particle. In this equation the former term within the brackets considers the cooling rate of a spherical particle due to radiation emission, whereas the latter term expresses the vaporization cooling process. The term referring to radiation emission differs from the current expressions which use the Stefan–Boltzmann equation [34]. In fact, in this case we have integrated the Wien law over λ considering also a dependence of the emissivity ɛ from the wavelength, as will be shown later. All our experiments have been performed in vacuum, hence it has been neglected the term referring to the cooling due to gas environment. Moreover, the thermionic emission term has been neglected for its poor contribution during cooling rates of similar systems [35]. As a consequence of mass loss due to vaporization, the particle diameter, a, changes with time as follows: da Mv RT 1=2 ¼ −2p dt ρRT 2πMv
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value for ΔHv (304.5 kJ mol− 1) has been estimated by considering as an ideal solution the Al–Cu–Fe melt. These thermochemical data refer to bulk material or to particles with micrometric size so their application to nanoparticles could be a source of errors. Many physical quantities, related to the thermal stability of materials, are predicted to vary with decreasing particle size [38], but, unfortunately, there are no available data about our system in the case of particles with nanometric size. Anyway, a possible decrease of the melting temperature is not particularly significant for our alloy, while more important could be the increase of the vapour pressure, even if this effect cannot be easily estimated. In fact, the variation of the vapour pressure with the particle radius strongly depends on the different systems [39]. The results obtained integrating the coupled Eqs. (1) and (2), for a particle having a diameter of 20 nm are reported in Fig. 6, where the dotted line indicates the radiative cooling, while the solid line refers to the total cooling, which in the temperature and time ranges we have considered, coincides with the vaporization cooling. In fact, radiative cooling becomes important only for a longer time scale, corresponding to relatively low temperatures (b2500 K). These results, indicating that the most important contribution to cooling is the vaporization process, and this is in good agreement with similar calculations reported for Fe/C and W nanoparticles [35]. Conversely, different evaluations about cooling of silicon nanoparticles has been also reported [40]. The reasons for the disagreement with our results could lie in the diverse experimental conditions that lead to different values for the initial temperature and velocity of the particles. In Fig. 6 the experimental temperatures of the secondary plume front obtained by emission spectroscopy are reported. The temperature data have been obtained by fitting the whole UV–Vis detected range of secondary plume emissions by the Wien equation in the form Iλ ~e λ14 e−hc=λkB T , which takes into account that the ICCD detection system measures photons. From Mie theory, the emissivity ɛ of a particle is given by the relation [41] e¼
2 4πNm a m −1 Im λ m2 þ 2
ð3Þ
where λ is the wavelength of the emitted light, Nm is the refractive index of the medium, m = Np/Nm where Np is the refractive index of the particle and a is the particle diameter. Only when (m2 − 1)/(m2 + 2) is considered a weak function of λ can the emissivity be written as ε∝λ− 1, which is the relation commonly used for particles. Since, as reported in literature, this assumption may be problematic for metallic particles [34,40], we have calculated the dependence of ɛ from λ for our system by using Eq. (3). Starting from reference values [42] of
ð2Þ
In order to determine the temporal dependence of the particles' temperature, the coupled Eqs. (1) and (2) have been integrated by the Euler method. For melting of the Al70Cu20Fe10 alloy, we have used ρ = 4.0 × 103 kg m− 1 [36], while as an estimation of Cp the value of 1.1 kJ kg− 1 K− 1 obtained for molten Al75Cu15V10 [37] has been used. A mean
Fig. 7. Logarithmic plot of ɛ vs. λ for aluminum. The slope of the fitting dashed line gives the value for the λ exponent.
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the Al, Cu and Fe complex index of refraction, we calculated the dependence of ɛ from λ given by Eq. (3), and plotted log ɛ vs. log λ, using an algebra software application (Mathematica® version 5, Wolfram). A straight line was readily observed (Fig. 7) indicating an inverse power law dependence of ɛ from λ. The evaluation of the negative slopes by linear regression gave the following values for the λ exponent: Al = 2.2, Cu = 2.0, Fe = 2.0. The obtained exponent values for the single metals can be combined to estimate a value for the alloy. A reasonable choice is to consider the overall emissivity as the product of the single emissivities each raised to a power equal to the mole fraction of the corresponding element. This leads to a combination rule giving an overall value of the λ exponent, x, as a mean of the single exponents, weighted with respect to the atomic content: x ¼ 2:2 0:7 þ 2:0 0:2 þ 2:0 0:1 ¼ 2:1 The results, showing a λ− 2.1 dependence of the particles' emissivity from the wavelength, are in good agreement with the results reported in literature for iron particles [35]. The best fittings of the experimental data (an example is reported in Fig. 8) give a dependence of the emissivity from λ− 2, in perfect accordance with the calculated value. The temperatures reported in Fig. 6 refer to the front of the secondary plume, all except the first one. In fact, the first point, which is representative for the temperature of the secondary plume development initial step, corresponds to a distance of 0 mm from the target, a delay of 200 ns after the laser pulse, with a 500 ns gate. The other points have been taken at 0, 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 mm, with temporal delays of 1, 2, 3, 4, 5, 6, 7 μs, respectively, while the gate of 1 μs was used in all cases. From Fig. 6 the very good agreement between the experimental points and the calculated cooling curve may be noted. If we consider the melted particles as the main components of the films, the retention of the target stoichiometry could be related to the possibility of differential evaporation of the elements from the particles during their flight. The material removal due to evaporation during such a process plays a relevant role only for temperatures beyond 2000 K, as shown in Fig. 9, obtained by integrating Eq. (2). This figure reports the variation of the dimension of a spherical particle due to vaporization as a function of the temperature during a flight time of 100 ns. As a consequence, we have to consider the relative vapour pressures of the different species only for temperatures above 2000 K. For the Al70Cu20Fe10 system, the differences of the vapour pressure among the three elements are quite large at room temperature but they diminish over 2000 K [21,43]. In general, from literature data referring to Al–Pd–Mn quasicrystal [44] and Fe–Ni and Co–Pt alloys [45] it is evident that if the difference of the vapour pressure is larger than one order of magnitude,
Fig. 8. Fitting of an emission spectrum of the secondary plume, giving T = 1860 ± 10 K.
Fig. 9. Variation of the dimension of a spherical particle, due to vaporization, as a function of the temperature, during a flight of 100 ns.
then the melted particles, even if high temperatures occur, cannot retain the target composition and consequent detectable depletion of the most volatile elements can be expected in the deposited films. Of course, this
Fig. 10. Angular distribution of the deposited material in (a) an Al70Cu20Fe10 film and (b) an Al70Pd20Mn10 film. In both cases the fits are represented as the combination of two different components. The composition of the films, in the different zones (a, b, c, d) is reported in Table 1(A) and (B). The deposition time was 1 h for both systems, with a laser fluence of 2.0 J cm− 2.
R. Teghil et al. / Thin Solid Films 517 (2009) 1880–1886 Table 1 Elemental composition of the cross-section of Al70Cu20Fe10 and Al70Pd20Mn10 films in different zones identified in Fig. 10(a) and (b) A Film nominal composition a b c d
Al
Cu
70.3 69.8 70.4 76.0 81.9
19.8 20.3 19.9 16.1 12.5
Fe
Al
Pd
Mn
68.3 65.3 66.7 68.0 79.2
25.2 28.3 26.8 23.1 6.4
6.5 6.4 6.5 8.9 14.4
9.9 9.9 9.7 7.9 5.6
B Film nominal composition a b c d
The films have been deposited at a laser fluence of 2.0 J/cm2 with a deposition time of 1 h.
does not mean that the gaseous phase does not play any role in the deposition process. If we consider the composition with the film cross section (Fig.10a), we can notice that along the borders of the film there is a percentage of Al, the most volatile element at low temperature, higher than the stoichiometric value (Table 1A) [43]. Fig. 10a, obtained from SEM and EDX analyses on a film cross-section, shows that the film angular distribution could be considered the result of the overlapping of two different angular distributions, corresponding quite well to those found for the primary and secondary plumes. In this hypothesis, the droplets, forming the main part of the film, retain the target stoichiometry while the gas phase does not. This hypothesis, which has already been verified for Al65Cu23Fe12 [21], is also supported by the results on another quasicrystal (Al70Pd20Mn10). It is important to remark that the differences among the vapor pressures of the elements forming the quasicrystal are, in this case, larger than two orders of magnitude [43]. The composition of the Al70Pd20Mn10 film generally reflects that of the particles, with a loss of the most volatile elements, while, on the contrary, the border shows the predominance of the most volatile elements. Thus, the final conclusion is that the target stoichiometry cannot be preserved. The cross section of an Al70Pd20Mn10 film is reported in Fig.10b, while the composition of the different zone is shown in Table 1B. An interesting comparison can be carried out between our data and those obtained for plasma spraying of Al–Cu–Fe quasicrystals, evidencing, for the latter, a very marked loss of Al in the melted powders during plasma spraying process of particles finer than 25 μm. This behavior, typical of aluminum-based quasicrystals but not of aluminum-based metal alloys, such as Al–Ni, has been related to the different thermal conductivity of the two types of materials [46]. In plasma spraying the dependence of aluminum loss from the particle dimension and thermal conductivity is clearly connected to the progressive melting of the surface and the absence of such effects in our system confirms that a very rapid non equilibrium transition from solid to melt takes place on the target as a consequence of the interaction with the ultra-short laser pulse. 4. Conclusions In conclusion the stoichiometric composition of Al70Cu20Fe10 films deposited by ultra-short PLD can be most likely understood by considering droplets, which retain the stoichiometry of the starting target, are the main components of the films. The similarity of the results obtained for the Al70Cu20Fe10 crystalline alloy to those obtained for the Al65Cu23Fe12 quasicrystal illustrate that the particles retain no memory about the structure of the ablation target. This seems to confirm our hypothesis about the formation of the droplets from molten material on the target surface, supporting our choice to consider the
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nanoparticles as molten droplets. Of course, on the target surface, we are not in the presence of a conventional equilibrium melting. In such conditions, the most important process that could influence the film composition is the vaporization from the melted particles during their flight towards the substrate. Therefore, the composition of the deposited films depends mainly from differential evaporation of the elements forming the melted particles, taking into account that a substantial loss of material takes place only for temperature higher than 2500 K. Consequently, the differential evaporation is directly related to the different elements vapour pressures at high temperature. Extrapolating available temperature–pressure data to higher temperatures reveals that at temperatures over 2500 K the vapor pressures of the elements tend to become very similar only if the differences among them, at temperatures between 2500 and 2000 K do not exceed one order of magnitude. The parts of the film far from its centre show a different composition, with a higher content of the most volatile elements. Probably, in these peripheral regions a large contribution is due to gas phase condensation from the primary plume, while the particles play a minor role. It should be highlighted that this hypothesis refers only to metallic alloys and to quasicrystals, which, in the melted phase, behave like metals. In other systems, for example covalent compounds, the possible complexity of the equilibrium vaporization could give different results. References [1] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, WileyInterscience, 1994. [2] J.C. Miller, R.F. Haglund (Eds.), Laser Ablation and Desorption, Academic Press, 1998. [3] R. Eamon (Ed.), Pulsed Laser Deposition of Thin Films: Applications-led Growth of Functional Materials, Wiley-Interscience, 2007. [4] K. Furusawa, K. Takahashi, H. Kumagai, K. Midorikawa, M. Obara, Appl. Phys., A Mater. Sci. Process. 69 (1999) S359. [5] F. Korte, S. Nolte, B. Chichkov, T. Bauer, G. Kamlage, T. Wagner, C. Fallnich, H. Welling, Appl. Phys., A Mater. Sci. Process. 69 (1999) S7. [6] A. Santagata, V. Marotta, S. Orlando, R. Teghil, M. Zaccagnino, A. Giardini, Appl. Surf. Sci. 208/209 (2003) 101. [7] S.I. Anisimov, B.L. Kapeliovich, T.L. Perel'man, Sov. Phys. JEPT 39 (1974) 376. [8] S.I. Anisimov, B. Rethfeld, Proc. SPIE 3093 (1997) 192. [9] D. Moreau, O. Albert, R. Benzerga, C. Boulmer-Leborgne, E. Millon, J. Perrière, J. Etchepare, Thin Solid Films 453/454 (2004) 340. [10] D. Scuderi, D. Moreau, O. Albert, P.P. Pronko, J. Etchepare, Appl. Surf. Sci. 248 (2005) 309. [11] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, Phys. Rev. B 67 (2003) 224503. [12] R. Teghil, L. D'Alessio, A. De Bonis, A. Galasso, P. Villani, A. Santagata, Thin Solid Films 515 (2006) 1411. [13] R. Teghil, L. D'Alessio, A. Santagata, M. Zaccagnino, D. Ferro, D.J. Sordelet, Appl. Surf. Sci. 210 (2003) 307. [14] D. Scuderi, R. Benzenga, O. Albert, B. Reynier, J. Etchepare, Appl. Surf. Sci. 252 (2006) 4360. [15] S. Amoruso, G. Ausanio, R. Bruzzese, L. Lanotte, P. Scardi, M. Vitello, X. Wang, J. Phys. Cond. Matter 18 (2006) L49. [16] S. Eliezer, N. Eliaz, E. Grossman, D. Fisher, I. Gouzman, S. Pecker, Y. Horovitz, M. Fraenkel, S. Maman, Y. Lereah, Phys. Rev. B 69 (2004) 144119. [17] F. Garrelie, C. Donnet, A.S. Loir, N. Benchick, Proc. SPIE 6261 (2006) 62610L–1. [18] D. Gratias, Y. Calvayrac, J. Devaud-Rzepsky, F. Faudot, M. Harmelin, A. Quivy, P.A. Bancel, J. Non-Cryst. Solids 153/154 (1993) 482. [19] M. Quiquandon, A. Quivy, J. Devaud, F. Faudot, S. Lefebvre, M. Bessiere, Y. Clavayrac, J. Phys. Cond. Matter 8 (1996) 2487. [20] P.N. Alboni, A.L. Pope, T.M. Tritt, A.R. Ross, C. Jenk, D.J. Sordelet, Mater. Res. Soc. Symp. Proc. 691 (2002) 233. [21] R. Teghil, A. De Bonis, A. Galasso, A. Santagata, P. Villani, D.J. Sordelet, Chem. Phys. Lett. 438 (2007) 85. [22] R. Teghil, L. D'Alessio, M.A. Simone, M. Zaccagnino, D. Ferro, D.J. Sordelet, Appl. Surf. Sci. 168 (2000) 267. [23] A. Santagata, V. Marotta, L. D'Alessio, R. Teghil, D. Ferro, G. de Maria, Appl. Surf. Sci. 109/110 (1997) 376. [24] D. Ashkenasi, A. Rosenfeld, H. Varel, M. Wähmer, E.E.B. Campbell, Appl. Surf. Sci. 120 (1997) 65. [25] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. Tunnermann, Appl. Phys. A 63 (1996) 109. [26] K. Solokowsky-Tinten, J. Bialkowsky, A. Cavalleri, D. vov der Linde, A. Oparin, J. Meyerter-Vehn, S.I. Anisimov, Phys. Rev. Lett. 81 (1998) 224. [27] Wm.T. Ashurst, B.L. Holian, Phys. Rev., E 59 (1999) 6742. [28] T.E. Glover, J. Opt. Soc. Am. B 20 (2003) 125.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Ultra-short pulse laser ablation of Al70Cu20Fe10 alloy: Nanoparticles generation and thin films deposition R. Teghil a,⁎, L. D'Alessio a, A. De Bonis a, D. Ferro b, A. Galasso a, G. Lanza a, A. Santagata c, P. Villani c, D.J. Sordelet d a
Dipartimento di Chimica, Università della Basilicata, via N. Sauro 85, 85100 Potenza — Italy CNR — ISMNS, Sez. Roma 1, P.le A. Moro 5, 00185 Roma — Italy CNR — IMIP, Unità di Potenza, via S. Loja, 85050 Tito Scalo (PZ) — Italy d Materials and Engineering Physics Program, Ames Laboratory, Iowa State University, Ames, IA — USA b c
a r t i c l e
i n f o
Article history: Received 28 August 2007 Received in revised form 16 September 2008 Accepted 25 September 2008 Available online 8 October 2008 Keywords: Laser ablation Metal alloys Deposition process Ultrashort pulse deposition Plume characteristics
a b s t r a c t In this paper the deposition of thin films obtained from femtosecond laser ablation of an Al70Cu20Fe10 alloy is presented. In the plasma produced by ablation, a characteristic feature is the presence of hot nanoparticles that become evident several microseconds after the laser shot. The cooling mechanisms of these particles have been analysed together with the evolution of their composition. The results, compared with those previously obtained for Al65Cu23Fe12 quasicrystal, reveal a clear relation between the final composition of the particles and the high-temperature equilibrium vapor pressures of the different elements, suggesting a direct emission from the target rather than a gas phase formation. Analysis of the elemental composition through the cross-section of the as-deposited films helps to illustrate the role of nanoparticles in the film growth. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Laser ablation has always attracted a large interest either from the point of view of fundamental research or for applications in the field of thin films prepared using the Pulsed Laser Deposition (PLD) technique [1–3]. With the PLD method ultra-short pulses (time duration of 1 ps or less) offer numerous advantages compared to longer pulses. In fact, the laser–target interaction mechanism is different in these two cases; ablation by ultra-short pulses leads to small thermal effects, little mechanical damage or modifications to the target material [4,5] and, in some cases, to a stoichiometric transfer of the target composition to the deposited films. The last characteristic is particularly relevant because, on the one hand, conventional PLD performed with nanosecond pulsed lasers frequently shows some difficulties in retaining the target composition [6]. On the other hand, the phenomena involved in laser ablation performed by ultra-short pulse lasers are still matters of discussion and detailed models are available only for simple systems, such as metals [7,8]. Experimental results on the plasma produced by ultra-short laser pulses have shown the presence of both a primary plume that develops and evolves in the first microsecond after the laser pulse, and a secondary one, which originates after the first microsecond, that is detectable up to several tenths of a microsecond [9–12]. The primary plume is composed of atoms, neu⁎ Corresponding author. Tel.: +39 0971202225; fax: +39 0971202325. E-mail address: [email protected] (R. Teghil). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.095
tral and ionised, molecules, electrons and clusters, while the secondary plume consists of nanoparticles or large clusters [13–17]. In this paper we studied the primary and secondary plumes produced by femtosecond ablation of an Al70Cu20Fe10 metallic alloy, and, in particular, the characteristics of the particles forming the secondary plume. In fact, these nanoparticles, whose origin is still a matter of debate, play a fundamental role in thin films deposition. Moreover, the study of their morphology and composition may help understand the origin of these nanoparticles. We consider the particles as molten droplets ejected directly from the target, and this hypothesis is consistent with our experimental results. The Al70Cu20Fe10 composition, the so called ω phase in the phase diagram of Al–Cu–Fe system [18], was chosen due to the possibility to know or to estimate its thermochemical properties. Moreover, it is a very interesting material, in particular being an approximant of Al65Cu23Fe12 icosahedral quasicrystal [19], closely related to it but with different thermophysical and thermochemical characteristics [20]. From this point of view, a comparative analysis could be useful to clarify the importance of the characteristics of the solid target during the ablation–deposition process. So, we will compare the results on Al70Cu20Fe10 with those already obtained for Al65Cu23Fe12 [21]. 2. Experimental details The ablation and deposition experiments were performed by using the experimental apparatus already described [13]. It consists of a
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35 mm quartz Nikkor lenses was used for acquiring gated images of the overall plasma plume emission lateral view (200–800 nm) induced by single laser pulse. The spatial resolution achieved by this system was 25 μm. The acquisitions were performed by accumulating up to 20 shots, thereby avoiding previously irradiated surfaces. Both ICCD fast imaging and time-resolved spectra were accomplished by delaying the data acquisition of the plasma emission with respect to the laser pulse. Analysis of the deposits' morphology was performed using a 1450 LEO VP Scanning Electron Microscope (SEM), which has a resolution of 4 nm. The films' stoichiometry was determined by Energy Dispersive X-Ray Spectroscopy (EDX) using an INCA 300apparatus, which has boron as its first detectable element. A ZEISS EMIOC transmission electron microscope (TEM,) was used for a morphological characterization of the first deposition steps, and the film crystallinity was studied by X-Ray Diffraction (XRD) using a Siemens D 5000 instrument equipped with Cu Kα1 radiation. 3. Results and discussion 3.1. Plasma analysis
Fig. 1. Normalised time evolution plots of the emission of (a) Al (396.09 nm) and (b) Cu (324.70 nm), collected at 2 mm (●) and 4 mm (■) from the target. The laser fluence was 2.0 J cm− 2. The lines which connect the experimental data were added to facilitate understanding of the graphic.
stainless steel vacuum chamber, evacuated to a pressure of 1.5·10− 4 Pa, equipped with a 1.2 rpm rotating target holder. The distance between the target and the substrate was kept at 2.5 cm and all the experiments were carried out at room temperature. A Light Conversion frequency doubled Nd:glass laser (527 nm emission wavelength, 250 fs pulse duration, 10 Hz repetition rate) was used for the ablation and deposition experiments. In our experiments the laser fluence was varied between 0.2 and 2.0 J cm− 2 and the laser beam was incident at an angle of 45° on the target surface. The laser spot area on the target was 0.1 mm2. The ablation targets were Al70Cu20Fe10 crystalline alloys, supplied by Materials Preparation Center, Ames Laboratory US-DOE, Ames, IA, USA, while the deposition substrates were (111) oriented silicon from Cerac. In the samples prepared for transmission electron microscopy, the deposition substrates were carbon–formvar films from Agar Scientific. The optical emission spectra were detected by means of a Princeton ICCD device (1024 × 1024 pixels). The width of the entrance spectrograph slit and the grating employed were 80 μm and either 1200 or 150 grooves/mm. Thus, the spectral widths obtained were about 15 nm and 150 nm with resolutions of 0.15 nm and 0.5 nm, respectively. The gated system had best time resolution of 2 ns and each acquisition was integrated with 50 laser shots in order to increase the signal-to-noise ratio. Varying the position of the optical elements by a micrometric translation stage, was possible to obtain space-resolved emission spectra at different distances from the ablated target surface. The same ICCD system equipped with a 105/
As already seen for other systems [12], the gaseous phase produced by femtosecond laser ablation of our Al70Cu20Fe10 target consists of two distinct emissions of material. In the primary plume that develops and ends in the first microsecond from the laser pulse, optical emission spectroscopy (OES) indicates the presence of Al, Cu, Fe, Al+ and Cu+ species. The absence of the double ionised species, often found in nanosecond ablation [22], is probably due to the lack of interaction between the expanding plasma and the incoming fs laser radiation. The mean velocities of the different species obtained by time-resolved OES are similar (vAl = 1.6 ± 0.1 × 106 cm s− 1, vCu = 1.0 ± 0.1 × 106 cm s− 1) and in good agreement with those obtained using a quasicrystalline Al65Cu23Fe12 target [13]. Due to the low intensity of the signals, it is impossible to calculate the velocity distribution for iron and for the ionised species. In Fig. 1 the temporal evolution of the emissions of aluminum and copper is reported.
Fig. 2. Overall emission from AlCuFe plume recorded: (a) 200 ns and (b) 500 ns after the laser shot. The gate was 100 ns in both cases. The laser fluence was 2.0 J cm− 2. The light intensity is shown in terms of 20% density contours.
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Fig. 3. Overall emission from AlCuFe plume recorded 30 μs after the laser shot, with a gate of 1 μs. The light intensity is shown in terms of 20% density contours. The laser fluence was 2.0 J cm− 2.
The angular distribution of the material ejected from the target during ablation can be represented by the equation I(θ) = I0cosnθ, where I(θ) is the flux intensity along a direction forming an angle θ with the normal to the target surface, I0 is the intensity corresponding to θ = 0 and n is a parameter related to the anisotropy of the angular distribution. There are other models describing the angular distribution [5] but, for the aim of this work, the simplest one can be considered. The values of n have been calculated by the equation n¼
2ln2 −1 lnð1 þ x2 Þ
where x = b/a is the ratio between the major axis b and minor axis a of the elliptical plume [23].
With regard to the primary plume, ICCD imaging data show a value of 10 for the cosine exponent n and a front velocity of 2.0 ± 0.4 × 106 cm s− 1 (Fig. 2). The ICCD images are reported as intensity contour plots at different time delays after the laser shot, showing the time evolution of the plume. The contours on the left side of the images correspond to the position of the hot target. Another emission develops after the first microsecond from the laser shot and it lasts up to 50 μs (secondary plume). The OES of the secondary plume produced shows typical continuous black-body-like emission spectra, indicating the presence of particles or large clusters, while its ICCD imaging evidences a higher directionality (n = 20) and a lower front velocity (3.9 ± 0.4 × 104 cm s− 1) than the values obtained for the primary plume. In Fig. 3 the intensity contour plots obtained from an ICCD image recorded 30 μs after the laser shot and showing the angular distribution of the secondary plume are reported. Also in this case the target is on the left side of the image. 3.2. Thin film analysis The SEM images of the films deposited on silicon substrates (Fig. 4a) show that the films, which appear to be amorphous in structure as evidenced by XRD, are formed by the coalescence of a large number of droplets of nanometric size. This result confirms the importance of nanoparticles in the films deposited by ultra-short pulse PLD, as already evidenced for the Al–Cu–Fe quasicrystal system [13]. Using TEM bright field images (Fig. 4b), a survey of the first steps of the film growth has been performed, and, once more, the role played by the nanoparticles particles seems to be fundamental. As shown in Fig. 5, the particles size distribution obtained from TEM images shows a lognormal distribution with a mean particles diameter of 24 nm. This value does not change with the deposition time, at least in the first steps of the film growth that we can study by TEM. 3.3. Ablation–deposition mechanism Even if previous studies on fs ablation tended to exclude the melting of the target [24,25], there is now growing agreement about the occurrence of melted material on the target as a consequence of the interaction between an ultra-short laser pulse and the target surface [26–30] and about the presence of liquid nanoparticles in the fs plume [31]. Some of the possible models that allow the ejection of both a gaseous phase and melted particles include phase explosion
Fig. 4. Microphotograph of the surface of an Al70Cu20Fe10 films deposited with a laser fluence of 2.0 J cm− 2 for (a) 60 min (SEM image) and (b) 20 min (TEM image).
Fig. 5. a) Size distribution of Al70Cu20Fe10 particles (for a total of 304) obtained by analysis of the film reported in Fig. 4b. b) Diameter-probability plot for the same size distribution. The solid line fit represents lognormal size distribution. The value at 50% probability gives the mean diameter of the particles.
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Fig. 6. Temperature–time curves for the cooling of Al70Cu20Fe10 particles with a diameter of 20 nm. The dotted line indicates the radiative cooling and the solid line refers to the vaporization cooling and to the total cooling due to both effects. The points represent the experimental temperatures of the secondary plume expanding front.
[32], explosive melt removal [33] and hydrodynamic models [28]. Our results do not allow identifying the mechanism producing the target melting and the ejection of the particles, which, may be due to the occurrence of several mechanisms taking place simultaneously in different parts of the target [29]. Thus, in this current work we only aim to define the mechanism that allows the transfer of the target stoichiometry to the film. For this purpose we have compared the temperature variation of the front of the secondary plume during its expansion with calculations on the particles cooling rate. With this regard, the following cooling rate equation for melted particles has been considered: " # dT 6 Mv RT 1=2 ΔHv ¼− K T 5 −TR5 þ p dt ρCp a RT 2πMv M
ð1Þ
where ρ is the density of the particle, Cp is its thermal capacity and a is the particle diameter, K is constant with a value of 24.88·(2πk5B/c4h4), whereas kB and h are the Boltzmann and Planck constants and c the light velocity; T is the initial temperature of the particle and TR the room temperature, p is the vapour pressure, Mv is the vapour molecular weight, ΔHv is the vaporization enthalpy and M is the molecular weight of the particle. In this equation the former term within the brackets considers the cooling rate of a spherical particle due to radiation emission, whereas the latter term expresses the vaporization cooling process. The term referring to radiation emission differs from the current expressions which use the Stefan–Boltzmann equation [34]. In fact, in this case we have integrated the Wien law over λ considering also a dependence of the emissivity ɛ from the wavelength, as will be shown later. All our experiments have been performed in vacuum, hence it has been neglected the term referring to the cooling due to gas environment. Moreover, the thermionic emission term has been neglected for its poor contribution during cooling rates of similar systems [35]. As a consequence of mass loss due to vaporization, the particle diameter, a, changes with time as follows: da Mv RT 1=2 ¼ −2p dt ρRT 2πMv
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value for ΔHv (304.5 kJ mol− 1) has been estimated by considering as an ideal solution the Al–Cu–Fe melt. These thermochemical data refer to bulk material or to particles with micrometric size so their application to nanoparticles could be a source of errors. Many physical quantities, related to the thermal stability of materials, are predicted to vary with decreasing particle size [38], but, unfortunately, there are no available data about our system in the case of particles with nanometric size. Anyway, a possible decrease of the melting temperature is not particularly significant for our alloy, while more important could be the increase of the vapour pressure, even if this effect cannot be easily estimated. In fact, the variation of the vapour pressure with the particle radius strongly depends on the different systems [39]. The results obtained integrating the coupled Eqs. (1) and (2), for a particle having a diameter of 20 nm are reported in Fig. 6, where the dotted line indicates the radiative cooling, while the solid line refers to the total cooling, which in the temperature and time ranges we have considered, coincides with the vaporization cooling. In fact, radiative cooling becomes important only for a longer time scale, corresponding to relatively low temperatures (b2500 K). These results, indicating that the most important contribution to cooling is the vaporization process, and this is in good agreement with similar calculations reported for Fe/C and W nanoparticles [35]. Conversely, different evaluations about cooling of silicon nanoparticles has been also reported [40]. The reasons for the disagreement with our results could lie in the diverse experimental conditions that lead to different values for the initial temperature and velocity of the particles. In Fig. 6 the experimental temperatures of the secondary plume front obtained by emission spectroscopy are reported. The temperature data have been obtained by fitting the whole UV–Vis detected range of secondary plume emissions by the Wien equation in the form Iλ ~e λ14 e−hc=λkB T , which takes into account that the ICCD detection system measures photons. From Mie theory, the emissivity ɛ of a particle is given by the relation [41] e¼
2 4πNm a m −1 Im λ m2 þ 2
ð3Þ
where λ is the wavelength of the emitted light, Nm is the refractive index of the medium, m = Np/Nm where Np is the refractive index of the particle and a is the particle diameter. Only when (m2 − 1)/(m2 + 2) is considered a weak function of λ can the emissivity be written as ε∝λ− 1, which is the relation commonly used for particles. Since, as reported in literature, this assumption may be problematic for metallic particles [34,40], we have calculated the dependence of ɛ from λ for our system by using Eq. (3). Starting from reference values [42] of
ð2Þ
In order to determine the temporal dependence of the particles' temperature, the coupled Eqs. (1) and (2) have been integrated by the Euler method. For melting of the Al70Cu20Fe10 alloy, we have used ρ = 4.0 × 103 kg m− 1 [36], while as an estimation of Cp the value of 1.1 kJ kg− 1 K− 1 obtained for molten Al75Cu15V10 [37] has been used. A mean
Fig. 7. Logarithmic plot of ɛ vs. λ for aluminum. The slope of the fitting dashed line gives the value for the λ exponent.
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the Al, Cu and Fe complex index of refraction, we calculated the dependence of ɛ from λ given by Eq. (3), and plotted log ɛ vs. log λ, using an algebra software application (Mathematica® version 5, Wolfram). A straight line was readily observed (Fig. 7) indicating an inverse power law dependence of ɛ from λ. The evaluation of the negative slopes by linear regression gave the following values for the λ exponent: Al = 2.2, Cu = 2.0, Fe = 2.0. The obtained exponent values for the single metals can be combined to estimate a value for the alloy. A reasonable choice is to consider the overall emissivity as the product of the single emissivities each raised to a power equal to the mole fraction of the corresponding element. This leads to a combination rule giving an overall value of the λ exponent, x, as a mean of the single exponents, weighted with respect to the atomic content: x ¼ 2:2 0:7 þ 2:0 0:2 þ 2:0 0:1 ¼ 2:1 The results, showing a λ− 2.1 dependence of the particles' emissivity from the wavelength, are in good agreement with the results reported in literature for iron particles [35]. The best fittings of the experimental data (an example is reported in Fig. 8) give a dependence of the emissivity from λ− 2, in perfect accordance with the calculated value. The temperatures reported in Fig. 6 refer to the front of the secondary plume, all except the first one. In fact, the first point, which is representative for the temperature of the secondary plume development initial step, corresponds to a distance of 0 mm from the target, a delay of 200 ns after the laser pulse, with a 500 ns gate. The other points have been taken at 0, 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 mm, with temporal delays of 1, 2, 3, 4, 5, 6, 7 μs, respectively, while the gate of 1 μs was used in all cases. From Fig. 6 the very good agreement between the experimental points and the calculated cooling curve may be noted. If we consider the melted particles as the main components of the films, the retention of the target stoichiometry could be related to the possibility of differential evaporation of the elements from the particles during their flight. The material removal due to evaporation during such a process plays a relevant role only for temperatures beyond 2000 K, as shown in Fig. 9, obtained by integrating Eq. (2). This figure reports the variation of the dimension of a spherical particle due to vaporization as a function of the temperature during a flight time of 100 ns. As a consequence, we have to consider the relative vapour pressures of the different species only for temperatures above 2000 K. For the Al70Cu20Fe10 system, the differences of the vapour pressure among the three elements are quite large at room temperature but they diminish over 2000 K [21,43]. In general, from literature data referring to Al–Pd–Mn quasicrystal [44] and Fe–Ni and Co–Pt alloys [45] it is evident that if the difference of the vapour pressure is larger than one order of magnitude,
Fig. 8. Fitting of an emission spectrum of the secondary plume, giving T = 1860 ± 10 K.
Fig. 9. Variation of the dimension of a spherical particle, due to vaporization, as a function of the temperature, during a flight of 100 ns.
then the melted particles, even if high temperatures occur, cannot retain the target composition and consequent detectable depletion of the most volatile elements can be expected in the deposited films. Of course, this
Fig. 10. Angular distribution of the deposited material in (a) an Al70Cu20Fe10 film and (b) an Al70Pd20Mn10 film. In both cases the fits are represented as the combination of two different components. The composition of the films, in the different zones (a, b, c, d) is reported in Table 1(A) and (B). The deposition time was 1 h for both systems, with a laser fluence of 2.0 J cm− 2.
R. Teghil et al. / Thin Solid Films 517 (2009) 1880–1886 Table 1 Elemental composition of the cross-section of Al70Cu20Fe10 and Al70Pd20Mn10 films in different zones identified in Fig. 10(a) and (b) A Film nominal composition a b c d
Al
Cu
70.3 69.8 70.4 76.0 81.9
19.8 20.3 19.9 16.1 12.5
Fe
Al
Pd
Mn
68.3 65.3 66.7 68.0 79.2
25.2 28.3 26.8 23.1 6.4
6.5 6.4 6.5 8.9 14.4
9.9 9.9 9.7 7.9 5.6
B Film nominal composition a b c d
The films have been deposited at a laser fluence of 2.0 J/cm2 with a deposition time of 1 h.
does not mean that the gaseous phase does not play any role in the deposition process. If we consider the composition with the film cross section (Fig.10a), we can notice that along the borders of the film there is a percentage of Al, the most volatile element at low temperature, higher than the stoichiometric value (Table 1A) [43]. Fig. 10a, obtained from SEM and EDX analyses on a film cross-section, shows that the film angular distribution could be considered the result of the overlapping of two different angular distributions, corresponding quite well to those found for the primary and secondary plumes. In this hypothesis, the droplets, forming the main part of the film, retain the target stoichiometry while the gas phase does not. This hypothesis, which has already been verified for Al65Cu23Fe12 [21], is also supported by the results on another quasicrystal (Al70Pd20Mn10). It is important to remark that the differences among the vapor pressures of the elements forming the quasicrystal are, in this case, larger than two orders of magnitude [43]. The composition of the Al70Pd20Mn10 film generally reflects that of the particles, with a loss of the most volatile elements, while, on the contrary, the border shows the predominance of the most volatile elements. Thus, the final conclusion is that the target stoichiometry cannot be preserved. The cross section of an Al70Pd20Mn10 film is reported in Fig.10b, while the composition of the different zone is shown in Table 1B. An interesting comparison can be carried out between our data and those obtained for plasma spraying of Al–Cu–Fe quasicrystals, evidencing, for the latter, a very marked loss of Al in the melted powders during plasma spraying process of particles finer than 25 μm. This behavior, typical of aluminum-based quasicrystals but not of aluminum-based metal alloys, such as Al–Ni, has been related to the different thermal conductivity of the two types of materials [46]. In plasma spraying the dependence of aluminum loss from the particle dimension and thermal conductivity is clearly connected to the progressive melting of the surface and the absence of such effects in our system confirms that a very rapid non equilibrium transition from solid to melt takes place on the target as a consequence of the interaction with the ultra-short laser pulse. 4. Conclusions In conclusion the stoichiometric composition of Al70Cu20Fe10 films deposited by ultra-short PLD can be most likely understood by considering droplets, which retain the stoichiometry of the starting target, are the main components of the films. The similarity of the results obtained for the Al70Cu20Fe10 crystalline alloy to those obtained for the Al65Cu23Fe12 quasicrystal illustrate that the particles retain no memory about the structure of the ablation target. This seems to confirm our hypothesis about the formation of the droplets from molten material on the target surface, supporting our choice to consider the
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nanoparticles as molten droplets. Of course, on the target surface, we are not in the presence of a conventional equilibrium melting. In such conditions, the most important process that could influence the film composition is the vaporization from the melted particles during their flight towards the substrate. Therefore, the composition of the deposited films depends mainly from differential evaporation of the elements forming the melted particles, taking into account that a substantial loss of material takes place only for temperature higher than 2500 K. Consequently, the differential evaporation is directly related to the different elements vapour pressures at high temperature. Extrapolating available temperature–pressure data to higher temperatures reveals that at temperatures over 2500 K the vapor pressures of the elements tend to become very similar only if the differences among them, at temperatures between 2500 and 2000 K do not exceed one order of magnitude. The parts of the film far from its centre show a different composition, with a higher content of the most volatile elements. Probably, in these peripheral regions a large contribution is due to gas phase condensation from the primary plume, while the particles play a minor role. It should be highlighted that this hypothesis refers only to metallic alloys and to quasicrystals, which, in the melted phase, behave like metals. In other systems, for example covalent compounds, the possible complexity of the equilibrium vaporization could give different results. References [1] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, WileyInterscience, 1994. [2] J.C. Miller, R.F. Haglund (Eds.), Laser Ablation and Desorption, Academic Press, 1998. [3] R. Eamon (Ed.), Pulsed Laser Deposition of Thin Films: Applications-led Growth of Functional Materials, Wiley-Interscience, 2007. [4] K. Furusawa, K. Takahashi, H. Kumagai, K. Midorikawa, M. Obara, Appl. Phys., A Mater. Sci. Process. 69 (1999) S359. [5] F. Korte, S. Nolte, B. Chichkov, T. Bauer, G. Kamlage, T. Wagner, C. Fallnich, H. Welling, Appl. Phys., A Mater. Sci. Process. 69 (1999) S7. [6] A. Santagata, V. Marotta, S. Orlando, R. Teghil, M. Zaccagnino, A. Giardini, Appl. Surf. Sci. 208/209 (2003) 101. [7] S.I. Anisimov, B.L. Kapeliovich, T.L. Perel'man, Sov. Phys. JEPT 39 (1974) 376. [8] S.I. Anisimov, B. Rethfeld, Proc. SPIE 3093 (1997) 192. [9] D. Moreau, O. Albert, R. Benzerga, C. Boulmer-Leborgne, E. Millon, J. Perrière, J. Etchepare, Thin Solid Films 453/454 (2004) 340. [10] D. Scuderi, D. Moreau, O. Albert, P.P. Pronko, J. Etchepare, Appl. Surf. Sci. 248 (2005) 309. [11] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, Phys. Rev. B 67 (2003) 224503. [12] R. Teghil, L. D'Alessio, A. De Bonis, A. Galasso, P. Villani, A. Santagata, Thin Solid Films 515 (2006) 1411. [13] R. Teghil, L. D'Alessio, A. Santagata, M. Zaccagnino, D. Ferro, D.J. Sordelet, Appl. Surf. Sci. 210 (2003) 307. [14] D. Scuderi, R. Benzenga, O. Albert, B. Reynier, J. Etchepare, Appl. Surf. Sci. 252 (2006) 4360. [15] S. Amoruso, G. Ausanio, R. Bruzzese, L. Lanotte, P. Scardi, M. Vitello, X. Wang, J. Phys. Cond. Matter 18 (2006) L49. [16] S. Eliezer, N. Eliaz, E. Grossman, D. Fisher, I. Gouzman, S. Pecker, Y. Horovitz, M. Fraenkel, S. Maman, Y. Lereah, Phys. Rev. B 69 (2004) 144119. [17] F. Garrelie, C. Donnet, A.S. Loir, N. Benchick, Proc. SPIE 6261 (2006) 62610L–1. [18] D. Gratias, Y. Calvayrac, J. Devaud-Rzepsky, F. Faudot, M. Harmelin, A. Quivy, P.A. Bancel, J. Non-Cryst. Solids 153/154 (1993) 482. [19] M. Quiquandon, A. Quivy, J. Devaud, F. Faudot, S. Lefebvre, M. Bessiere, Y. Clavayrac, J. Phys. Cond. Matter 8 (1996) 2487. [20] P.N. Alboni, A.L. Pope, T.M. Tritt, A.R. Ross, C. Jenk, D.J. Sordelet, Mater. Res. Soc. Symp. Proc. 691 (2002) 233. [21] R. Teghil, A. De Bonis, A. Galasso, A. Santagata, P. Villani, D.J. Sordelet, Chem. Phys. Lett. 438 (2007) 85. [22] R. Teghil, L. D'Alessio, M.A. Simone, M. Zaccagnino, D. Ferro, D.J. Sordelet, Appl. Surf. Sci. 168 (2000) 267. [23] A. Santagata, V. Marotta, L. D'Alessio, R. Teghil, D. Ferro, G. de Maria, Appl. Surf. Sci. 109/110 (1997) 376. [24] D. Ashkenasi, A. Rosenfeld, H. Varel, M. Wähmer, E.E.B. Campbell, Appl. Surf. Sci. 120 (1997) 65. [25] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. Tunnermann, Appl. Phys. A 63 (1996) 109. [26] K. Solokowsky-Tinten, J. Bialkowsky, A. Cavalleri, D. vov der Linde, A. Oparin, J. Meyerter-Vehn, S.I. Anisimov, Phys. Rev. Lett. 81 (1998) 224. [27] Wm.T. Ashurst, B.L. Holian, Phys. Rev., E 59 (1999) 6742. [28] T.E. Glover, J. Opt. Soc. Am. B 20 (2003) 125.
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