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Optical spectroscopy diagnostics and thin film deposition of laser ablated rare earth–Ni2B2C plasma plumes
13 February 2002
Chemical Physics Letters 353 (2002) 1–6 www.elsevier.com/locate/cplett
Optical spectroscopy diagnostics and thin film deposition of laser ablated rare earth–Ni2B2C plasma plumes X. Wang a, S. Amoruso a,c, R. Bruzzese a, N. Spinelli a, A. Tortora a, R. Velotta a,*, C. Ferdeghini b, G. Grassano b, W. Ramadan b a
c
INFM and Dipartimento di Scienze Fisiche, Universita di Napoli ‘‘Federico II’’, Via Cintia 26 Ed. G, I-80126 Napoli, Italy b INFM and Dipartimento di Fisica, Via Dodecaneso 33, 16146 Genova, Italy Dipartimento di Ingegneria e Fisica dell’Ambiente, Universit a della Basilicata C.da Macchia Romana, I-85100 Potenza, Italy Received 20 September 2001; in final form 15 November 2001
Abstract We describe optical spectroscopy diagnostics of UV laser ablated rare earth–Ni2 –B2 –C superconducting targets. In the light of this characterization, we have optimized the parameters for high-quality thin film deposition of borocarbide compounds. Our measurements have evidenced relevant differences in the flow velocity as well as in the spatial divergence of different plasma components. This is related to the influence of the different masses of the atoms of the multicomponent target during plume expansion. These results have led us to identify the target-to-substrate distance and substrate temperature as the critical parameters for the deposition of high-quality thin films, as clearly evidenced by residual resistivity ratio measurements. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The superconducting compounds of the series RE–Ni2 B2 C (RE being a rare earth element or Y) have been the subject of intensive studies in recent years because of their peculiar characteristics [1]. In particular, when RE is magnetic, these materials show an interplay between magnetic ordering and superconductivity that manifests itself in surprisingly rich effects depending on the specific RE element used [2]. On the other hand, when RE is nonmagnetic, a variety of phenomenology sug-
*
Corresponding author. E-mail address: [email protected] (R. Velotta).
gesting unconventional d-wave pairing is observed [3]. Moreover, although they present a layered crystalline structure similar to that of the superconducting copper oxides, borocarbides have almost three-dimensional properties, since also the B–C bond, oriented along the c-axis, takes part in the electronic density of states at the Fermi level. This gives rise to nearly isotropic electronic properties, and to a multiband character for the superconducting state [4]. The highest quality samples available are polycrystals or single crystals of a size of some millimeters. On the other hand, the strong interest in producing borocarbide samples in the form of high-quality thin films stems from their usefulness both in the study of particular physical properties,
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 4 8 2 - 8
2
X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
and in applications in various devices (e.g., borocarbide-based Josephson junctions) [5]. Despite the efforts in producing samples in the form of thin films, and whatever the deposition technique, the quality of these samples has shown to be lower than that of bulk samples [6] and, only very recently, the possibility of obtaining epitaxial films was reported [7]. In this Letter we discuss our studies on the deposition of borocarbide thin films by pulsed laser ablation/deposition (PLD). As well known, excimer laser induced PLD is widely used for the deposition of thin solid films, and the technique has been applied successfully to a wide range of solids (metals, semiconductors, insulators and superconductors) [8,9], and, more recently, liquid materials [10,11]. The rapid deployment of laser energy into a thin layer of the target surface within the short excimer laser pulse duration triggers an explosive evaporation. This leads to congruent evaporation that is considered to be one of the main advantages of pulsed laser ablation, especially for multicomponent film growth. The characterization of laser ablated plasmas is typically carried out by using optical emission and absorption spectroscopy [8,11], and mass spectrometry [8,12,13] which is particularly suited for the analysis of the charged component of the plume. We have focused our attention on the optical spectroscopy diagnostics of the ablated plasma plume [8], and on the dependence of the superconducting properties of the deposited films on important macroscopic parameters of the ablation process such as laser fluence and target-to-substrate distance. This has allowed us to determine the main cause of the difficulties characterizing the production of thin films of these superconducting compounds by the PLD technique. More generally, our results can provide useful guidelines for the deposition by PLD of thin films of multicomponent materials with atoms greatly differing in their masses. We have deposited both magnetic and nonmagnetic borocarbide thin films [14,15], and here we discuss the optimization of the deposition parameters only in the case of nonmagnetic compounds, i.e., Y-based and Lu-based samples, in the
light of the spectroscopy analysis of the laser plasma. These two last compounds exhibit very similar physical properties and present very similar, optimized deposition parameters.
2. Experimental In our spectroscopy experiments the plasma was generated by ablation of a YNi2 B2 C sample using a XeF excimer laser ( 20 ns FWHM pulse duration) operating at 5 Hz. The target was mounted on a rotating holder and placed in a vacuum chamber evacuated to a residual pressure of 108 mbar. The laser energy density at the target surface was varied in the range 1–6 J=cm2 , which is typical in PLD of these materials. The bright plasma emission was viewed through a side window at right angles to the plume expansion direction. A slice of the plasma was imaged onto the entrance slit ð100 lmÞ of a 0.25 m monochromator equipped with a 1200 grooves per mm grating blazed at 500 nm, and with a maximum resolution of 0.05 nm. By using appropriate collimating and focusing lenses we obtained a one-to-one correspondence between the sampled area of the plume (with a spatial resolution of 100 lm) and the image. The monochromator was coupled to an intensified charged coupled device (ICCD) camera, with a minimum temporal gate of 5 ns, to record one-dimensional images of the plume along the target normal. The overall spectral resolution of the system was about 0.4 nm. In our experiment the emission spectra were recorded at different times after the end of the laser pulse, and by varying the distance d between the sample target and the spatial point of the plume imaged. We have thus obtained time- and spaceresolved optical spectra of the YNi2 B2 C expanding plume. The spectrum at a fixed distance and delay was obtained by averaging multiple laser shots (typically 100).
3. Results and discussions The results presented here start at a distance of 1 mm from the target surface to be sure that the
X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
contribution from the continuum can be neglected [16]. In this condition, characteristic line emissions from transitions in B, Ni, and Y atoms, as well as in their ions, are seen, whereas no emission from C has been detected, due to the weakness of its spectral lines. Fig. 1 shows the time of flight (TOF) curves of the B I 249.7 nm (doublet), Ni I 305 nm (triplet), and Y I 410.2 nm spectral lines, measured at three different distances (d) from the YNi2 B2 C target irradiated by a laser fluence of 4 J=cm2 . The experimental points were obtained by integrating the area under the corresponding emission lines. Each TOF distribution is normalized to its area. The analysis of other transitions from the plume (e.g., Y I 408 nm) led to TOF distributions very similar to those reported in Fig. 1. The main feature observed in Fig. 1 is the considerable temporal delay accumulated in traveling toward the substrate by most of the heavier Y neutral atoms with respect to the lighter Ni and B atoms. This delay is as large as 200 ns at
Fig. 1. TOF distributions of the B I (249.7 nm), Ni I (305 nm), and Y I (410.2 nm) spectral lines, measured at three different distances from the YNi2 B2 C target irradiated by a laser fluence of 4 J=cm2 .
3
d ¼ 5:5 mm. From the TOF distributions of Fig. 1 we have inferred the velocity of the center of mass of the various species. The results are shown in Fig. 2 where the temporal mean values of the TOF distributions for Y I, Ni I and B I are reported as a function of the distance from the target. The linear dependence of the TOF mean values on the distance confirms that the species in the plume travel with a constant velocity, given by the inverse of the line slope, which is mass dependent. This constant velocity is due to the fact that the expansion of the plume after the end of the laser pulse is adiabatic and an asymptotic velocity is reached after only few tens of ns, as discussed in detail in the theoretical model of Singh and Narayan in the case of laser produced YBCO plumes [17]. Such an asymptotic velocity is also mass dependent because of the isothermal expansion occurring during the laser pulse. In this respect, in the above quoted analytical model it was also estimated that, because of collisions, a dependence M a with a < 0:5 is generally obtained, M being the atom mass. By taking into account only B I and Y I (as discussed below, Ni I lines are not considered due to the overlapping with almost coincident Y II lines), we have measured in our conditions a 0:17. The drift velocity inferred for the different species, and through different lines, from the TOF spectral distributions are summarized in Table 1. We stress the good agreement between the values obtained for a given species through different spectral lines, and, above all, that the drift velocity correspond-
Fig. 2. Center of mass of the TOF distributions for Y I, Ni I and B I as a function of the distance from the target.
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X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
Table 1 Analyzed optical transitions with corresponding wavelength, relative flow velocity v and expansion coefficient p Specie
k (nm)
v (cm/ls)
p
BI Ni I Ni I YI YI Y II
249.7 305 313.4 408.4 410.2 324.2
1:9 0:1 2:2 0:4 2:5 0:2 1:3 0:1 1:3 0:1 2:0 0:1
2:6 0:3 1:9 0:2 1:8 0:1 2:0 0:2 2:0 0:2 1:7 0:2
ing to the heavier Yttrium atoms is about 33% smaller than that of the other sample species, thus leading to temporal delays as high as several hundreds ns (up to 600 ns) in the arrival times on a substrate placed even at a very short distance of 15 mm from target. This in turn can give rise to a lack of stoichiometry in the thin film. It must be pointed out that according to the atomic mass dependence of the expansion velocities in the plume [17], the Ni I TOF distribution is expected to be intermediate between the B I and Y I ones. On the other hand, in Fig. 1 Ni I excited atoms seem to travel even faster than B I. This is caused by the limited spectral resolution of the ICCD camera (0.4 nm) which introduces overlapping of contributions from Y II lines (304.5, 304.7, 305.5 nm) to the 305 nm Ni spectral line. Since ions of a given species have a velocity larger than the corresponding neutrals [8], the spurious contributions of Y II transitions to the Ni I line leads to a partially distorted (faster) Ni I TOF distribution. To further elucidate this point, we have also spectrally studied the temporal dynamics of Y ions (e.g., the strong transition at 324.2 nm). We have indeed found that the corresponding TOF distributions have a steeper leading edge and are narrower than those pertaining to neutrals (see Fig. 1), thus confirming the role of Y II in the anomalous behavior observed for Ni I. We wish to point out that such a spurious effect also influences the emission line at 313 nm of Ni I, whose analysis leads to results very similar to those discussed for the 305 nm Ni I line. We now discuss the optimal deposition parameters, that we have experimentally determined, in the light of the plume spectroscopy character-
ization above presented. Substrate temperature and target-to-substrate distance are the key parameters for the stabilization of the correct phase [15]. The film transition temperature approaches the value for bulk samples for a substrate deposition temperature higher than 860 °C, in agreement with [18,19]. This temperature is unusually high and imposes very strict requirements in the choice of the growth substrate [15]. The necessity of such high temperatures can be explained in terms of the observed difference in the flow velocity of the different atomic species expanding toward the substrate (see Fig. 1). At high substrate temperature the adatoms have a high surface mobility, thus reducing the effects of the different arrival times of the species. The same goal of minimising the consequences of species dependent flow velocities is obviously also obtained by reducing the targetto-substrate distance. To verify this hypothesis, we have deposited samples at different substrate-totarget distance. In Fig. 3 the resistance of four samples deposited at distances decreasing from 4.5 up to1.4 cm, are shown. The sample are deposited at 900 °C and at a laser energy fluence of about 4 J=cm2 [15]. Decreasing the distance up to the shortest one allowed by the deposition geometry of our chamber leads to a progressive increase in the residual resistivity ratio defined as Rð300 KÞ= Rð17 KÞ, R being the thin film resistivity, and Tc values, corresponding to the crossover from a semiconducting to a metallic and then supercon-
Fig. 3. Resistance vs temperature for three samples grown at 900 °C varying the target-to-substrate distance. It is evident an improvement of the properties decreasing the distance, from 4.5 up to 1.4 cm.
X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
ducting behavior; this is correlated to a complete lack of the 1221 phase, to a stressed, slightly nonstoichiometric phase, and then to stoichiometric, not stressed phase, respectively, as evidenced by X-ray measurements. A second interesting aspect of our plume analyses stems from the fact that the densities of species ablated from a solid target have a dependence on the distance d from the target of the kind d p , p being a parameter ranging from 1 (linear expansion) to 3 (spherical expansion), as reported in the model of Singh and Narayan [17]. In general, the value of p depends on M, as well as on the target composition (see, e.g., the theoretical study of Saenger [20]). In the case of YNi2 B2 C, by analyzing the line intensity vs distance we have found a value of p ranging from 2.5 for B I to 1.6 for Y (see Table 1). A mass dependent spatial distribution has been already reported for YNi2 B2 C [21,22], and has also been recently found on Si and Ge and on Li and Nb, during laser ablation of SiGe and LiNbO3 targets in vacuum, respectively [23,24]. These results clearly show that because of the great complexity characterizing the hydrodynamic expansion of a laser ablated plasma, also the spatial divergences of different atomic species explosively evaporated from the surface of a multicomponent target, and expanding into vacuum, can have considerably different values. In turn, this peculiar feature of a laser plume can play a key role in the stoichiometric homogeneity of large size, thin films of multicomponent materials produced by PLD. This is in good agreement with the spatial distribution of the superconducting properties of the films; in fact, we have deposited a film on a large MgO substrate ð15 15 mm2 Þ in the optimal deposition conditions and we have found that the sample shows anular, concentric regions observable by eye. Only the central part of the film, with a diameter of about 7 mm, corresponding to complete on-axis deposition, turns out to be superconducting, i.e., with the right stoichiometry, the external rings being not superconducting at all. We wish to point out that the analysis we have described for the specific laser fluence of 4 J=cm2 has also been carried out for 2.5 and 6 J=cm2 , namely, the typical laser fluence interval where
5
PLD is carried out. We have only found a very slight dependence of the atomic drift velocities on fluence, in agreement to the model described in [17], whereas the change in the p exponent was not appreciable. This indicates that in the investigated range, laser fluence does not seem to be a very critical parameter. In fact, but for very low energy densities, where the deposited samples are not superconducting, the increase in energy fluence – starting from about 1.5–2 up to 10 J=cm2 – does not bear any significant advantage in the phase formation, while strongly deteriorating the surface morphology because of the very high density of particulate coming from the target. In conclusion, we have performed optical emission spectroscopy studies of laser ablated plasma plumes of borocarbide compounds. Our measurements have evidenced relevant differences in the flow velocity of different ablated species, with a corresponding arrival time delay on substrate. This can mainly be related to the influence of the different masses of the atoms of the multicomponent target during plume expansion. These results have led us to obtain the optimal experimental conditions for PLD of these samples, thus identifying the target-to-substrate distance, and substrate temperature as the critical parameters. This comparison among the spectroscopic data and deposition parameters provides some useful and more general suggestions for the deposition of borocarbides and other complex-phase thin films by PLD. References [1] P.C. Canfield, P.L. Gammel, D.J. Bishop, Phys. Today 51 (1998) 40. [2] J.W. Lynn, S. Skanthakumar, Q. Huang, S.K. Sinha, Z. Hossain, L.C. Gupta, R. Nagarajan, C. Godart, Phys. Rev. B 55 (1997) 6584. [3] V. Metlushko, U. Loshelev, I. Aranson, G.W. Crabtree, P.C. Canfield, Phys. Rev. Lett. 79 (1997) 1738. [4] S.L. Drechsler, S.V. Shulga, K.H. Muller, G. Fuchs, J. Freudenberger, G. Behr, H. Eschrig, L. Schulz, M.S. Golden, H. von Lips, J. Fink, V.N. Narozhnyi, H. Rosner, P. Zahn, A. Gladun, D. Lipp, A. Kreyssig, M. Loewenhaupt, K. Koepernik, K. Winzer, K. Krug, Physica C 318 (1999) 117. [5] G. Grassano, M.R. Cimberle, C. Ferdeghini, M. Iavarone, R. Di Capua, R. Vaglio, F. Canepa, Physica C 341 (2000) 757.
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[6] C. Ferdeghini, M.R. Cimberle, G. Grassano, D. Marre, M. Putti, F. Canepa, Int. J. Mod. Phys. B 13 (1999) 1049. [7] G. Grassano, F. Canepa, D. Marre, M. Putti, W. Ramadan, A.S. Siri, C. Ferdeghini, in: K.-H. M€ uller, V Narozhnyi (Eds.), Rare Earth Transition Metal Borocarbides (Nitrides): Superconducting, Magnetic and Normal State Properties, Nato Sciences Series, Kluwer Academic Publishers, Dordrecht, 2001, p. 369. [8] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, J. Phys. B 32 (1999) R131. [9] R.E. Russo, D.B. Geohegan, R.F. Haglund, K. Murakami (Eds.), Laser Ablation, Elsevier, Amsterdam, 1998. [10] D.A. Allwood, P.E. Dyer, J. Gozalo, H.V. Snelling, M. Hird, Chem. Phys. Lett. 301 (1999) 91. [11] K. Hatanaka, T. Itoh, T. Asahi, N. Ichinose, S. Kawanishi, T. Sasuga, H. Fukumura, H. Masuhara, Chem. Phys. Lett. 300 (1999) 727. [12] T. Kimura, T. Nakamura, K. Ishikawa, F. Kokai, Y. Koha, Chem. Phys. Lett. 340 (2001) 296. [13] Y. Tang, Q. Qin, Chem. Phys. Lett. 343 (2001) 452.
[14] C. Ferdeghini, F. Canepa, M.R. Cimberle, G. Grassano, S. Gariglio, D. Marre, F. Napoli, M. Putti, Intermetallics 7 (1999) 1389. [15] G. Grassano, D. Marre, I. Pallecchi, F. Ricci, A.S. Siri, C. Ferdeghini, Supercond. Sci. Technol. 14 (2001) 117. [16] J. Gonzalo, F. Vega, C.N. Afonso, J. Appl. Phys. 77 (1995) 6588. [17] R.K. Singh, J. Narayan, Phys. Rev. B 41 (1990) 8843. [18] J.M. Eyhorn, D.G. Naugle, T.O. Callaway, Physica C 755 (2000) 341. [19] K. H€ase, B. Holzapfel, L. Schulz, Physica C 288 (1997) 28. [20] K. Saenger, in: D. Chrisey, G. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [21] X. Wang, S. Amoruso, M. Armenante, R. Bruzzese, N. Spinelli, R. Velotta, Appl. Surf. Sci. 168 (2000) 100. [22] S. Amoruso, M. Armenante, R. Bruzzese, N. Spinelli, R. Velotta, X. Wang, Appl. Phys. A 71 (2000) 215. [23] C.B. Arnold, M.J. Aziz, Appl. Phys. A 69 (1999) S23. [24] A. Perea, J. Gonzalo, C.N. Afonso, C. Vivien, Ch. Leborgne, Appl. Phys. A 69 (1999) S141.
Chemical Physics Letters 353 (2002) 1–6 www.elsevier.com/locate/cplett
Optical spectroscopy diagnostics and thin film deposition of laser ablated rare earth–Ni2B2C plasma plumes X. Wang a, S. Amoruso a,c, R. Bruzzese a, N. Spinelli a, A. Tortora a, R. Velotta a,*, C. Ferdeghini b, G. Grassano b, W. Ramadan b a
c
INFM and Dipartimento di Scienze Fisiche, Universita di Napoli ‘‘Federico II’’, Via Cintia 26 Ed. G, I-80126 Napoli, Italy b INFM and Dipartimento di Fisica, Via Dodecaneso 33, 16146 Genova, Italy Dipartimento di Ingegneria e Fisica dell’Ambiente, Universit a della Basilicata C.da Macchia Romana, I-85100 Potenza, Italy Received 20 September 2001; in final form 15 November 2001
Abstract We describe optical spectroscopy diagnostics of UV laser ablated rare earth–Ni2 –B2 –C superconducting targets. In the light of this characterization, we have optimized the parameters for high-quality thin film deposition of borocarbide compounds. Our measurements have evidenced relevant differences in the flow velocity as well as in the spatial divergence of different plasma components. This is related to the influence of the different masses of the atoms of the multicomponent target during plume expansion. These results have led us to identify the target-to-substrate distance and substrate temperature as the critical parameters for the deposition of high-quality thin films, as clearly evidenced by residual resistivity ratio measurements. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The superconducting compounds of the series RE–Ni2 B2 C (RE being a rare earth element or Y) have been the subject of intensive studies in recent years because of their peculiar characteristics [1]. In particular, when RE is magnetic, these materials show an interplay between magnetic ordering and superconductivity that manifests itself in surprisingly rich effects depending on the specific RE element used [2]. On the other hand, when RE is nonmagnetic, a variety of phenomenology sug-
*
Corresponding author. E-mail address: [email protected] (R. Velotta).
gesting unconventional d-wave pairing is observed [3]. Moreover, although they present a layered crystalline structure similar to that of the superconducting copper oxides, borocarbides have almost three-dimensional properties, since also the B–C bond, oriented along the c-axis, takes part in the electronic density of states at the Fermi level. This gives rise to nearly isotropic electronic properties, and to a multiband character for the superconducting state [4]. The highest quality samples available are polycrystals or single crystals of a size of some millimeters. On the other hand, the strong interest in producing borocarbide samples in the form of high-quality thin films stems from their usefulness both in the study of particular physical properties,
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 4 8 2 - 8
2
X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
and in applications in various devices (e.g., borocarbide-based Josephson junctions) [5]. Despite the efforts in producing samples in the form of thin films, and whatever the deposition technique, the quality of these samples has shown to be lower than that of bulk samples [6] and, only very recently, the possibility of obtaining epitaxial films was reported [7]. In this Letter we discuss our studies on the deposition of borocarbide thin films by pulsed laser ablation/deposition (PLD). As well known, excimer laser induced PLD is widely used for the deposition of thin solid films, and the technique has been applied successfully to a wide range of solids (metals, semiconductors, insulators and superconductors) [8,9], and, more recently, liquid materials [10,11]. The rapid deployment of laser energy into a thin layer of the target surface within the short excimer laser pulse duration triggers an explosive evaporation. This leads to congruent evaporation that is considered to be one of the main advantages of pulsed laser ablation, especially for multicomponent film growth. The characterization of laser ablated plasmas is typically carried out by using optical emission and absorption spectroscopy [8,11], and mass spectrometry [8,12,13] which is particularly suited for the analysis of the charged component of the plume. We have focused our attention on the optical spectroscopy diagnostics of the ablated plasma plume [8], and on the dependence of the superconducting properties of the deposited films on important macroscopic parameters of the ablation process such as laser fluence and target-to-substrate distance. This has allowed us to determine the main cause of the difficulties characterizing the production of thin films of these superconducting compounds by the PLD technique. More generally, our results can provide useful guidelines for the deposition by PLD of thin films of multicomponent materials with atoms greatly differing in their masses. We have deposited both magnetic and nonmagnetic borocarbide thin films [14,15], and here we discuss the optimization of the deposition parameters only in the case of nonmagnetic compounds, i.e., Y-based and Lu-based samples, in the
light of the spectroscopy analysis of the laser plasma. These two last compounds exhibit very similar physical properties and present very similar, optimized deposition parameters.
2. Experimental In our spectroscopy experiments the plasma was generated by ablation of a YNi2 B2 C sample using a XeF excimer laser ( 20 ns FWHM pulse duration) operating at 5 Hz. The target was mounted on a rotating holder and placed in a vacuum chamber evacuated to a residual pressure of 108 mbar. The laser energy density at the target surface was varied in the range 1–6 J=cm2 , which is typical in PLD of these materials. The bright plasma emission was viewed through a side window at right angles to the plume expansion direction. A slice of the plasma was imaged onto the entrance slit ð100 lmÞ of a 0.25 m monochromator equipped with a 1200 grooves per mm grating blazed at 500 nm, and with a maximum resolution of 0.05 nm. By using appropriate collimating and focusing lenses we obtained a one-to-one correspondence between the sampled area of the plume (with a spatial resolution of 100 lm) and the image. The monochromator was coupled to an intensified charged coupled device (ICCD) camera, with a minimum temporal gate of 5 ns, to record one-dimensional images of the plume along the target normal. The overall spectral resolution of the system was about 0.4 nm. In our experiment the emission spectra were recorded at different times after the end of the laser pulse, and by varying the distance d between the sample target and the spatial point of the plume imaged. We have thus obtained time- and spaceresolved optical spectra of the YNi2 B2 C expanding plume. The spectrum at a fixed distance and delay was obtained by averaging multiple laser shots (typically 100).
3. Results and discussions The results presented here start at a distance of 1 mm from the target surface to be sure that the
X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
contribution from the continuum can be neglected [16]. In this condition, characteristic line emissions from transitions in B, Ni, and Y atoms, as well as in their ions, are seen, whereas no emission from C has been detected, due to the weakness of its spectral lines. Fig. 1 shows the time of flight (TOF) curves of the B I 249.7 nm (doublet), Ni I 305 nm (triplet), and Y I 410.2 nm spectral lines, measured at three different distances (d) from the YNi2 B2 C target irradiated by a laser fluence of 4 J=cm2 . The experimental points were obtained by integrating the area under the corresponding emission lines. Each TOF distribution is normalized to its area. The analysis of other transitions from the plume (e.g., Y I 408 nm) led to TOF distributions very similar to those reported in Fig. 1. The main feature observed in Fig. 1 is the considerable temporal delay accumulated in traveling toward the substrate by most of the heavier Y neutral atoms with respect to the lighter Ni and B atoms. This delay is as large as 200 ns at
Fig. 1. TOF distributions of the B I (249.7 nm), Ni I (305 nm), and Y I (410.2 nm) spectral lines, measured at three different distances from the YNi2 B2 C target irradiated by a laser fluence of 4 J=cm2 .
3
d ¼ 5:5 mm. From the TOF distributions of Fig. 1 we have inferred the velocity of the center of mass of the various species. The results are shown in Fig. 2 where the temporal mean values of the TOF distributions for Y I, Ni I and B I are reported as a function of the distance from the target. The linear dependence of the TOF mean values on the distance confirms that the species in the plume travel with a constant velocity, given by the inverse of the line slope, which is mass dependent. This constant velocity is due to the fact that the expansion of the plume after the end of the laser pulse is adiabatic and an asymptotic velocity is reached after only few tens of ns, as discussed in detail in the theoretical model of Singh and Narayan in the case of laser produced YBCO plumes [17]. Such an asymptotic velocity is also mass dependent because of the isothermal expansion occurring during the laser pulse. In this respect, in the above quoted analytical model it was also estimated that, because of collisions, a dependence M a with a < 0:5 is generally obtained, M being the atom mass. By taking into account only B I and Y I (as discussed below, Ni I lines are not considered due to the overlapping with almost coincident Y II lines), we have measured in our conditions a 0:17. The drift velocity inferred for the different species, and through different lines, from the TOF spectral distributions are summarized in Table 1. We stress the good agreement between the values obtained for a given species through different spectral lines, and, above all, that the drift velocity correspond-
Fig. 2. Center of mass of the TOF distributions for Y I, Ni I and B I as a function of the distance from the target.
4
X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
Table 1 Analyzed optical transitions with corresponding wavelength, relative flow velocity v and expansion coefficient p Specie
k (nm)
v (cm/ls)
p
BI Ni I Ni I YI YI Y II
249.7 305 313.4 408.4 410.2 324.2
1:9 0:1 2:2 0:4 2:5 0:2 1:3 0:1 1:3 0:1 2:0 0:1
2:6 0:3 1:9 0:2 1:8 0:1 2:0 0:2 2:0 0:2 1:7 0:2
ing to the heavier Yttrium atoms is about 33% smaller than that of the other sample species, thus leading to temporal delays as high as several hundreds ns (up to 600 ns) in the arrival times on a substrate placed even at a very short distance of 15 mm from target. This in turn can give rise to a lack of stoichiometry in the thin film. It must be pointed out that according to the atomic mass dependence of the expansion velocities in the plume [17], the Ni I TOF distribution is expected to be intermediate between the B I and Y I ones. On the other hand, in Fig. 1 Ni I excited atoms seem to travel even faster than B I. This is caused by the limited spectral resolution of the ICCD camera (0.4 nm) which introduces overlapping of contributions from Y II lines (304.5, 304.7, 305.5 nm) to the 305 nm Ni spectral line. Since ions of a given species have a velocity larger than the corresponding neutrals [8], the spurious contributions of Y II transitions to the Ni I line leads to a partially distorted (faster) Ni I TOF distribution. To further elucidate this point, we have also spectrally studied the temporal dynamics of Y ions (e.g., the strong transition at 324.2 nm). We have indeed found that the corresponding TOF distributions have a steeper leading edge and are narrower than those pertaining to neutrals (see Fig. 1), thus confirming the role of Y II in the anomalous behavior observed for Ni I. We wish to point out that such a spurious effect also influences the emission line at 313 nm of Ni I, whose analysis leads to results very similar to those discussed for the 305 nm Ni I line. We now discuss the optimal deposition parameters, that we have experimentally determined, in the light of the plume spectroscopy character-
ization above presented. Substrate temperature and target-to-substrate distance are the key parameters for the stabilization of the correct phase [15]. The film transition temperature approaches the value for bulk samples for a substrate deposition temperature higher than 860 °C, in agreement with [18,19]. This temperature is unusually high and imposes very strict requirements in the choice of the growth substrate [15]. The necessity of such high temperatures can be explained in terms of the observed difference in the flow velocity of the different atomic species expanding toward the substrate (see Fig. 1). At high substrate temperature the adatoms have a high surface mobility, thus reducing the effects of the different arrival times of the species. The same goal of minimising the consequences of species dependent flow velocities is obviously also obtained by reducing the targetto-substrate distance. To verify this hypothesis, we have deposited samples at different substrate-totarget distance. In Fig. 3 the resistance of four samples deposited at distances decreasing from 4.5 up to1.4 cm, are shown. The sample are deposited at 900 °C and at a laser energy fluence of about 4 J=cm2 [15]. Decreasing the distance up to the shortest one allowed by the deposition geometry of our chamber leads to a progressive increase in the residual resistivity ratio defined as Rð300 KÞ= Rð17 KÞ, R being the thin film resistivity, and Tc values, corresponding to the crossover from a semiconducting to a metallic and then supercon-
Fig. 3. Resistance vs temperature for three samples grown at 900 °C varying the target-to-substrate distance. It is evident an improvement of the properties decreasing the distance, from 4.5 up to 1.4 cm.
X. Wang et al. / Chemical Physics Letters 353 (2002) 1–6
ducting behavior; this is correlated to a complete lack of the 1221 phase, to a stressed, slightly nonstoichiometric phase, and then to stoichiometric, not stressed phase, respectively, as evidenced by X-ray measurements. A second interesting aspect of our plume analyses stems from the fact that the densities of species ablated from a solid target have a dependence on the distance d from the target of the kind d p , p being a parameter ranging from 1 (linear expansion) to 3 (spherical expansion), as reported in the model of Singh and Narayan [17]. In general, the value of p depends on M, as well as on the target composition (see, e.g., the theoretical study of Saenger [20]). In the case of YNi2 B2 C, by analyzing the line intensity vs distance we have found a value of p ranging from 2.5 for B I to 1.6 for Y (see Table 1). A mass dependent spatial distribution has been already reported for YNi2 B2 C [21,22], and has also been recently found on Si and Ge and on Li and Nb, during laser ablation of SiGe and LiNbO3 targets in vacuum, respectively [23,24]. These results clearly show that because of the great complexity characterizing the hydrodynamic expansion of a laser ablated plasma, also the spatial divergences of different atomic species explosively evaporated from the surface of a multicomponent target, and expanding into vacuum, can have considerably different values. In turn, this peculiar feature of a laser plume can play a key role in the stoichiometric homogeneity of large size, thin films of multicomponent materials produced by PLD. This is in good agreement with the spatial distribution of the superconducting properties of the films; in fact, we have deposited a film on a large MgO substrate ð15 15 mm2 Þ in the optimal deposition conditions and we have found that the sample shows anular, concentric regions observable by eye. Only the central part of the film, with a diameter of about 7 mm, corresponding to complete on-axis deposition, turns out to be superconducting, i.e., with the right stoichiometry, the external rings being not superconducting at all. We wish to point out that the analysis we have described for the specific laser fluence of 4 J=cm2 has also been carried out for 2.5 and 6 J=cm2 , namely, the typical laser fluence interval where
5
PLD is carried out. We have only found a very slight dependence of the atomic drift velocities on fluence, in agreement to the model described in [17], whereas the change in the p exponent was not appreciable. This indicates that in the investigated range, laser fluence does not seem to be a very critical parameter. In fact, but for very low energy densities, where the deposited samples are not superconducting, the increase in energy fluence – starting from about 1.5–2 up to 10 J=cm2 – does not bear any significant advantage in the phase formation, while strongly deteriorating the surface morphology because of the very high density of particulate coming from the target. In conclusion, we have performed optical emission spectroscopy studies of laser ablated plasma plumes of borocarbide compounds. Our measurements have evidenced relevant differences in the flow velocity of different ablated species, with a corresponding arrival time delay on substrate. This can mainly be related to the influence of the different masses of the atoms of the multicomponent target during plume expansion. These results have led us to obtain the optimal experimental conditions for PLD of these samples, thus identifying the target-to-substrate distance, and substrate temperature as the critical parameters. This comparison among the spectroscopic data and deposition parameters provides some useful and more general suggestions for the deposition of borocarbides and other complex-phase thin films by PLD. References [1] P.C. Canfield, P.L. Gammel, D.J. Bishop, Phys. Today 51 (1998) 40. [2] J.W. Lynn, S. Skanthakumar, Q. Huang, S.K. Sinha, Z. Hossain, L.C. Gupta, R. Nagarajan, C. Godart, Phys. Rev. B 55 (1997) 6584. [3] V. Metlushko, U. Loshelev, I. Aranson, G.W. Crabtree, P.C. Canfield, Phys. Rev. Lett. 79 (1997) 1738. [4] S.L. Drechsler, S.V. Shulga, K.H. Muller, G. Fuchs, J. Freudenberger, G. Behr, H. Eschrig, L. Schulz, M.S. Golden, H. von Lips, J. Fink, V.N. Narozhnyi, H. Rosner, P. Zahn, A. Gladun, D. Lipp, A. Kreyssig, M. Loewenhaupt, K. Koepernik, K. Winzer, K. Krug, Physica C 318 (1999) 117. [5] G. Grassano, M.R. Cimberle, C. Ferdeghini, M. Iavarone, R. Di Capua, R. Vaglio, F. Canepa, Physica C 341 (2000) 757.
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