Plasma shielding of a XeCl-laser-irradiated YBCO target

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1 Appl. Phys. A 71, (2) / Digital Object Identifier (DOI) 1.17/s Applied Physics A Materials Science & Processing Plasma shielding of a XeCl-laser-irradiated YBCO target T. Efthimiopoulos 1,, D. Dogas 2, I. Palli 3, C. Gravalidis 1,M.Campbell 2 1 Laser Physics Lab., Physics Dept. University of Crete, Heraklion, Crete 713, Greece 2 Physical Sciences Dept., Glasgow Caledonian University, G4 BA Glasgow, UK 3 Mat. Science Dept., National Technical University of Athens, Athens, Greece Received: 7 February 2/Accepted: 2 April 2/Published online: 2 August 2 Springer-Verlag 2 Abstract. Plasma shielding during the laser ablation process of YBCO high-t c superconductor is demonstrated by observing the transmissivity of a probe beam. A plasma electron density of cm 3 at the end of the laser pulse is estimated. PACS: 52.4; 81.6 Pulsed laser ablation has been established as a very useful method for the deposition of thin films of high-criticaltemperature superconductors [1, 2]. Because of the technological importance of the ablation process, many efforts have been made for the understanding of the hydrodynamics of the laser-induced plasma [2 6], since many parameters such as the shape and the velocity of the plasma front, play an important role regarding the deposition processes. One of the main outcomes of these efforts was the establishment of an adiabatic expansion model [2, 3] which best fits to the hydrodynamical characteristics of the expanding plasma. According to this model, the hydrodynamical characteristics of the plasma (such as forward peaking) are strongly dependent on the flux of the evaporated particles ejected from the target surface and therefore on the laser fluence used for the ablation of the ceramic superconducting targets [2 5]. In addition, as was shown by Venkatesan et al. [6], the stoichiometry of the plasma plume and of the produced thin films depends on the laser fluence. Other observations [2, 7 1] show that the laser fluence affects the crystal structure and the surface morphology of the films. Also, the fluence is connected to the presence of film surface nanoscale particles which are believed to be a result of the formation of clusters during the recombination processes within the plasma. The work that have been done so far yields an insight into the dynamics and the stoichiometry of the plasma plume during the ablation of high-t c superconductors. However, the Corresponding author. (Fax: +3-81/3942, efthim@physics.uoc.gr) observations mainly concern the later stages of the plasma expansion, several hundreds of ns after the initial impact of the laser onto the target surface and the initialization of the plasma. Harilal et al. [11] recently reported on the electron density and temperature of the plasma during pulsed laser ablation of the YBCO ceramic superconductor. They have measured the Stark broadening of a specific spectral line to derive the electron density and used spectral lines of successive ionization states to calculate the temperature. The plasma electron density and temperature have been measured at least 3 ns after the incidence of the laser pulse (which typically lasts about 25 ns in the case of an excimer laser), since for smaller delays, the emission lines could not be observed because of the dominant Bremsstralung radiation of the plasma. The observation of the intensity of a probing laser pulse which passes through the plasma during the ablation process, provides information with respect to the transmissivity of the plasma and we can calculate the electron density and temperature at the early stages of the plasma generation. The influence of the strong emission of continuous radiation from the plasma during the transmissivity measurements can be minimized with the use of a spectrometer. In our present work, plasma shielding is observed, during the XeCl excimer laser irradiation of bulk YBCO, by measuring the profile of a XeCl laser pulse through the plasma. The observed drop of the plasma transmissivity and the truncation of the temporal profiles of the laser pulses for laser energy density above 1.4J/cm 2, indicate that plasma shielding is initialized slightly above the ablation threshold of 1.J/cm 2. As a result, the laser intensity that reaches the target surface is a fraction of the total intensity of the laser pulse. Additionally, by using the transmissivity data obtained from our experiments, we were able to estimate the electron density of the plasma at the end of each laser pulse. To the best of our knowledge this is the first time that observation of plasma shielding during the XeCl excimer laser ablation of YBCO is reported and the electron concentration of the plasma is estimated.

2 326 1 Experimental details In our experiments, the plasma was generated in air by irradiation of a high-t c superconducting YBCO pellet (having a mass density equal to 93% of the theoretical value), with a Lumonics PM884 XeCl excimer laser (λ = 38 nm, τ = 3 ns). Figure 1a shows the experimental setup for the measurement of the delay of the onset of the luminous emission of the plasma. A beam splitter divides the laser beam into two components. The beam with the lowest intensity (about 4% of the intensity of the initial beam) is delivered to a photodiode. The distance between the beam splitter and the laser-sensing photodiode and that between the beam splitter and the target were made equal in order to determine the zero point on the time axis. The laser pulse of the main beam was focused onto the sample surface with a quartz lens. The plasma emission was focused on a second photodiode with an achromatic lens which was not transparent to the 38-nm photons, in order to block the laser light scattered by the plasma. A 1-mm-diameter circular aperture was placed right in front of the target as a mask. The photodiode signals were terminated to the 5 Ω impedance inputs of a HP 2-Gsample/s digital oscilloscope connected to a PC for further processing of the data. Special care was taken to avoid saturation of the photodiodes, by using neutral density filters. Figure 1b shows the experimental arrangement for the measurements of the profiles of the laser pulses. The main laser beam was split into three components by two quartz beam splitters. The highest power component was focused on the YBCO surface with a quartz lens. One of the two lower intensity components was used as a probe directed through the plasma, while the other one was directed to the triggering photodiode. The probing laser beam reached the YBCO surface simultaneously with the ablating laser beam. Assuming an average velocity of the particles of the plasma front expanding in air of the order of cm/s [11] and a laser pulse duration of 3 ns we find that the distance of the plasma front from the target surface is of the order of 1 µm. Thus, at the end of the laser pulse, the dimensions of the plasma would be of the order of 1 µm which limits the probing laser beam to be of the same spatial dimensions. In our experiment this was achieved by passing the probing laser beam through two pinholes of 1-mm and.5-mm diameter without focusing it. The dimensions of the probe were further reduced by using a knife edge placed above the YBCO target surface before the ablation region. The probing excimer laser beam was focused on the entrance slit of a 45-cm-focal-length spectrometer and a 12-line/mm grating, located several meters away in order to avoid the scattered light. A PMT coupled to the output slit of the monochromator was connected to the sampling oscilloscope. The laser energy flux on the target surface was controlled with a variable attenuator. The relative position of the focusing lens to the target surface was adjusted in order to maintain the same spot area of 2 mm 2. 2 Results Laser attenuator (a) (b) Laser attenuator Apperture Aperture PD splitter Plasma plume YBCO target splitter Mirror Trigger PD splitter Plasma plume Pinhole Scope Mirror lens YBCO target PC PMT prism Monochromator lens Fig. 1. a Experimental arrangement for the measurement of the delay of the onset of the plasma luminus emission. b Experimental arrangement for the measurement of the plasma transmissivity and of the trasmitted laser temporal profiles Figure 2a c presents the emission of the plasma which is created during the ablation process for three different energy flux values. It can be seen that the emission starts earlier in time for increased energy flux. It has been shown in another publication of ours [22], using an acoustic technique, that the melting with subsequent vaporization threshold for YBCO is.4j/cm 2 which coincides with the minimum energy flux to detect any fluorescence from the plasma. The same vaporization threshold value was also found by Singh and Narayan [3] and by Dyer et al. [12] and it is indicative of the thermal process of the excimer laser decomposition of YBCO at low energy flux. The plot of the plasma onset time with respect to the arrival of the laser pulse on the sample, as a function of the laser energy flux, is shown in Fig. 2d. It can be seen that a fast drop of the onset occurs up to the ablation threshold value of 1.J/cm 2 found by the acoustic wave method [17, 22]. Above the ablation threshold the onset approaches a value of 5 ns. Plotted on a logarithmic scale, the onset consists of a double exponential, one below the ablation threshold and one above, indicating the different mechanism of direct solid-togas phase transformation of the material. The acoustic wave amplitude, detected at the back of the sample shows a linear dependence against the laser fluence with a change of slope at.4j/cm 2 (the melting point) and also a sharp change of the wave amplitude at threshold due to the momentum transfer by the ablated mass directly from the solid phase into the gas phase [22]. Also, the delay of arrival of the acoustic wave to the detector shows a sharp re-

3 327 Intensity (au) F=1.1 J/cm (a) F=1.4 J/cm (b) Intensity (au) F=2.5 J/cm (c) Fig. 3a c. Transmitted excimer laser pulses for several energy flux values of the ablating laser pulse. There is no absorption of the probe beam for the energy flux of 1.1J/cm 2 (a). Truncation of the laser pulses is observable for energy flux of 1.4J/cm 2 (b) and over (c) 3 Plasma onset (ns) (d) Fig. 2. a c Laser pulses and plasma plume luminus emission profiles at.4j/cm 2,.7J/cm 2 and 1.6J/cm 2. d Delay of the luminous emission of the plasma plume versus the laser energy density. The delay is measured between the onset of the laser pulse and the onset of the plasma emission pulse Transmissivity (I/Io) duction at threshold, proportional to the depth of the crater created, which can be used to estimate the ablation rate if the acoustic wave velocity is known. These effects suggest that below threshold the mechanism of normal solid to liquid to gas evaporation takes place, whereas above threshold the process involves direct solid to gas phase conversion. Absorption of the laser radiation by the plasma takes place above threshold as shown in Fig. 3 where a and a probe intensity profile are recorded for three energy flux values above threshold. Similar truncated probe pulses were reported by Mao and Russo [13] for a glass sample. Song and Xu [14] also reported recently on the ablation of metals and showed similar results with respect to the onset of the plasma emission and the pulse truncation. Figure 4a shows the transmissivity of the plasma as a function of the laser energy flux, and Fig. 4b shows the temporal width (at FWHM) of the probe pulse. Both plots show a sharp change at the flux of approximately 1.4J/cm 2. Pulse truncation and plasma transmissivity reduction occur at the same energy flux threshold which is close to and above the ablation threshold value of 1.J/cm 2 found by the acoustic wave method [22]. Temporal width (ns) Fig. 4a,b. Transmissivity of the plasma for the XeCl excimer laser as a function of the energy flux of the ablating laser beam (a). Temporal width (FWHM) of the probing laser pulse as a function of laser energy flux (b) 3 Discussion So far from the experimental measurements and observations we have the following facts:

4 328 (a) the emission of the plasma starts during the laser pulse and its onset moves to earlier times for higher energy flux values, (b) the probe pulse truncation starts during the ablating laser pulse when the energy flux is close to 1.4J/cm 2, (c) the plasma emission onset is at.4j/cm 2, (d) the acoustic wave amplitude as a function of flux measurements indicates a melting threshold of.4j/cm 2, (e) the ablation threshold energy flux measured by the acoustic wave method is 1.J/cm 2. These observations indicate that the vapor phase can be produced either by melting with subsequent evaporation or by a direct solid-to-vapor phase transformation. This is supported by our acoustic wave method measurements and the results of other researchers such as Song and Xu [14] and Dyer et al. [12]. The vapor produced during the laser pulse, if the flux is above the vaporization threshold of.4j/cm 2, will in turn absorb photons from the incoming laser pulse and partly ionize. The number of electrons in the plasma can increase mainly either by direct photoinization of the neutral atoms or by the inverse Bremsstralung process where the free electrons colliding with ions can absorb photons in a free-to-free transition. These energetic electrons collide with neutrals and ions subsequently to produce more electrons and ions. Song and Xu [14] calculated the cross section for the two processes for Ni samples and found that the photoionization process is the dominant mechanism for excimer-laser-induced plasma at low flux, by several orders of magnitude which is our case as well. The more dense the plasma, the more absorption of the laser pulse flux occurs. It is of interest to be able to estimate the temperature of the electrons and ions of the plasma and their density. There are several methods to measure the basic parameters T e, n e of the plasma such as Thomson scattering, emission spectroscopy, Langmuir probe interferometry and transmission of laser-generated surface harmonics described by Pfalzner and Gibbon [23] and the s cited. Harilal et al. [11] measured the electron density of the plasma of YBCO in vacuum, ablated by a Nd:YAG (1.6 µm) laser, using the Stark-broadened profile method for the BaI line at nm. For the case of local thermodynamic equilibrium, which is their case, and for the adiabatic expansion case (1/z dependence of the density, where z is the distance from the target) they measured an electron density of cm 3 3 ns after the plasma onset. The electron temperature was determined from the relative intensities of neutral and ionized barium spectral lines and they found an electron temperature of 2.36 ev for the same time delay of 3 ns. In our case we used an XeCl excimer laser of.38 µm wavelegth and the absorption coefficient of YBCO is very different for the Nd:YAG and XeCl wavelengths. Additionally we have done the present experiments in air at 1 Atm. Consequently their experimental conditions are quite different from ours and we cannot make direct comparisons. However, since we do our measurements during the incidence of the laser pulse with the plasma expanding in air, we expect to have a much higher electron density by a few orders of magnitude. From the transmissivity measurement of Fig. 4a we can estimate the electron density n e. The plasma is in local thermodynamic equilibrium since there are many collisions during the heating time of the plasma which is of the order of 3 ns, the FWHM of the laser pulse. When the plasma frequency equals the laser frequency the transmission drops to zero and then the formula for a weakly coupled nondegenerate plasma is valid: ne ω laser = ω P = e. ε m e From this relation and since I = Transmissivity = 4n I (1 + n) 2, with n the index of refraction of the plasma [16] given by ( ) n = 1 ωp 2, ω we can estimate ne. The plot of n e as laser a function of the laser energy flux is shown in Fig. 5. For high laser energy flux, the electron density saturates because above the critical density, the plasma totally reflects the probe laser beam. According to Harilal et al. [11], the electron density shows a t 2 dependence on time. Therefore, the maximum electron density of cm 3 that the above authors observed at 3 ns after the laser pulse, will be scaled to a value of cm 3 at 3 ns, which is the end of our laser pulse duration. Additionally, the 1/z scaling due to the adiabatic expansion, taking into account that the plasma length in vacuum is an order of magnitude higher than the plasma length in air, will increase the electron density by a factor of 1. Harilal et al. [11] performed their experiment using a Nd:YAG laser (1.6 µm) whereas our laser was a XeCl (.38 µm). Consequently, the maximum electron density in our case will be higher by the ratio of the frequencies squared, which is approximately 1. Therefore, the expected electron density is of the order of cm 3. This value is of the same order with the value of , observed in our case, considering the uncertainties involved. With respect to the electron temperature, measuring the line intensities of two successive energy states of BaI (553.5 nm and nm), we estimated a temperature of 5.5eV at t = 3 ns after the onset of the plasma in air. The corresponding value found by Harilal et al. [11] for Electron density (cm -3 ) 1.2x x x x x x x x Fig. 5. Electron density of the laser induced plasma at the end of the laser pulse as a function of the laser energy flux

5 329 plasma expansion in vacuum is 2.36 ev, for the same time delay of 3 ns, 3 mm above the target surface. The cooling effect of the plasma due to the adiabatic expansion in the vacuum case, partly justifies the higher temperature we observed. It is of interest to know the kind of plasma produced by the laser ablation of YBCO. The coupling coefficient of the plasma, which is the ratio of the potential energy of the ions to their kinetic energy [23] is given by Γ = e 2 /r i k B T, with 1/n i = (4π/3)r 3 i, n i the number density of ions, T the ion temperature (T i T e )andn i n e. For the density and temperature measured by Harilal et al. [11] and ourselves, Γ<1 and consequently the plasma is weakly coupled. In this case, a Maxwellian distribution of the electron energy is valid. With respect to the degeneracy θ of the plasma, we have θ = k B T/E F which is the ratio of the kinetic energy to the Fermi energy E F = (3π 2 n e ) 2/3 h 2 /2m e. In our case we have θ 2 and the plasma produced is not degenerate [24]. 4 Conclusions Measurements of the transmissivity of the laser-induced plasma have been carried out in the case of XeCl-excimerlaser irradiation of YBCO. Plasma shielding was observed during the laser pulse. The transmissivity of the plasma drops considerably for laser energy flux above 1.4J/cm 2, whereas for the same energy flux region, the electron density of the plasma saturates to a critical value corresponding to total reflectance of the laser photons by the plasma. These observations provide a useful insight into the ablation process of YBCO, especially for the deposition of thin films, where the structure and surface morphology of the produced thin films depend on the fraction of the laser pulse energy that reaches the target surface and on the spatial and compositional characteristics of the plasma plume. An electron temperature of the order of 5. ev and an electron density of the order of 1 22 cm 3 were estimated. Transmitted laser temporal profiles start to become truncated at energy flux of 1.4J/cm 2. Mao et al. [19] developed a model of thermal heating and inverse Bremsstrahlung to explain the plasma shielding effect in metals. They found that the pulse truncation energy flux was almost independent of the target material [2], which is not true for YBCO. Acknowledgements. We would like to thank Dr C. Andreouli, CERECO S.A., for providing the YBCO targets. This work was done at the Materials- Laser Lab., Technical Educational Institute (TEI) of Crete. We acknowledge the support of Dr C. Savakis, Dr L. Naoumidis and Mr Y. Bertachas of the above Institute. References 1. X.D. Wu, S.R. Foltyn, R.C. Dye, A.R. Garcia, N.S. Nogar, E. Muenchausen: Thin Solid Films 218, 31 (1992) 2. D.B. Chrisey, G.K. Hubler (Eds.): Pulsed Laser Deposition of Thin Films (Wiley, New York 1994) 3. R.K. Singh, J. Narayan: Phys. Rev. B 41, 8843 (199) 4. D.B. Geohegan: Appl. Phys. Lett. 6, 2732 (1992) 5. P.E. Dyer, A. Issa, P.H. Key: Appl. Phys. Lett. 57, 186 (199) 6. T. Venkatesan, X.D. Wu, A. Inam, I.B. Wachtman: Appl. Phys. Lett. 52, 1193 (1988) 7. R. Ramesh, A. Inam, T. Sands, C.T. Rogers: Mater. Sci. Eng. B 14, 188 (1992) 8. K. Bierleutgeb, S. Proyer: Appl. Surf. Sci. 19/11, 331 (1997) 9. V. Boffa, T. Petrisor, L. Ciontea, U. Gambardella, S. Barbanera: Physica C 276, 218 (1997) 1. S. Gaponov, J. Gavrilov, M. Jelinek, E. Kluenkov, L. Mazo: Supercond. Sci. Technol. 5, 645 (1992) 11. S.S. Harilal, C.V. Bindu, V.P.N. Nampoori, C.P.G. Vallabhan: Appl. Phys. B 66, 633 (1998) 12. P.E. Dyer, S.R. Farrar, P.H. Key: Appl. Surf. Sci. 54, 255 (1992) 13. X. Mao, R.E. Russo: Appl. Phys. A 64, 1 (1997) 14. K.H. Song, X. Xu: Appl. Phys. A 65, 477 (1997) 15. J.J. Chang, B.E. Warner: Appl. Phys. Lett. 69, 473 (1996) 16. H. Hora: Nonlinear Plasma Dynamics at Laser Irradiation (Springer, Berlin, Heidelberg 1979) 17. T. Efthimiopoulos, E. Kritsotakis, H. Kiagias, C. Savvakis, Y. Bertachas: J. Phys. D: Appl. Phys. 31, 2648 (1998) 18. S. Amoruso, M. Armenante, V. Berardi, R. Bruzzese, N. Spinelli: Appl. Phys. A 65, 265 (1997) 19. X.L. Mao, W.T. Chan, M. Caetano, M.A. Shannon, R.E. Russo: Appl. Surf. Sci. 96-8, 126 (1995) 2. M.A. Shannon, X.L. Mao, A. Fernadez, W.T. Chan, R.E. Russo: Analyt. Chem. 67, 4522 (1995) 21. A. Miotello, R. Kelly: Appl. Phys. Lett. 67, 3535 (1995) 22. D. Dogas, T. Efthimiopoulos, C. Andreouli, M. Campbell: submitted to Appl. Phys. A, unpublished 23. S. Pfalzner, P. Gibbon: Phys. Rev. E 57, 4698 (1998) 24. M.A. Berkovsky, Y.K. Kurilenkov, H.M. Milchberg: Phys. Lett. A 168, 416 (1992)

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