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KrF pulsed laser deposition of La5Ca9Cu24O41 thin films on various substrates
Applied Surface Science 255 (2009) 5236–5239
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
KrF pulsed laser deposition of La5Ca9Cu24O41 thin films on various substrates M. Pervolaraki a,*, G.I. Athanasopoulos a, R. Saint-Martin b, A. Revcolevschi b, J. Giapintzakis a a b
Department of Mechanical and Manufacturing Engineering, University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537, Nicosia 1678, Cyprus Laboratoire de Physico-Chimie de L’Etat Solide, UMR 8182, Universite´ Paris-Sud, 91405 Orsay Cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history:
Recent studies on single crystals of cuprate oxides containing spin chains and ladders have reported large anisotropic magnon-mediated thermal conductivity. A potential use of thin films of such materials could be in the thermal management of electronic devices for the guiding of unwanted heat to a heat sink. In this article, the pulsed laser deposition and characterization of La5Ca9Cu24O41 thin films on SrLaAlO4, SrTiO3, MgO, and Si substrates are reported for the first time. The films were grown using a pulsed UV laser (KrF, 248 nm) and various substrate temperatures up to 650 8C. The XRD spectra revealed successful target-film stoichiometric transfer and high texturing of the thin films with (0 k 0) preferred orientation. ß 2008 Elsevier B.V. All rights reserved.
Available online 18 October 2008 Keywords: La5Ca9Cu24O41 Thin films Pulsed laser deposition
1. Introduction Compounds with low-dimensional spin structures have recently attracted considerable attention due to their novel physical properties [1]. Single crystals of such compounds, a few centimeters long, have been grown by Ammerahl and Revcolevschi [2], and their anisotropic thermal conductivity has been investigated [3]. The high non-phonon thermal conductivity of such S = 1/2 cuprate compounds is attributed to the spin-excitation contribution. In the La5Ca9Cu24O41 ladder compound the magnonmediated thermal conductivity is an order of magnitude larger along c-axis in comparison to the in-plane value [1,3]. Thermal management in electronic devices, semiconductor and microchip industry remains a major problem. The generated unwanted heat results in a decrease of the performance and limits the lifetime of the devices. A potential solution to this problem could come from using a thin film of high and anisotropic thermal conductivity for guiding the generated heat to a heat sink. Novel quasi-one dimensional quantum magnet oxides like La5Ca9Cu24O41 are potential candidates since the heat in these oxides is transported predominately in one dimension. To achieve heteroepitaxy, i.e., single crystal thin film growth on a single crystal substrate of a different material, it is vital to match closely the lattice parameters of the substrate to those of the thin film
* Corresponding author. Fax: +357 22892254. E-mail addresses: [email protected] (M. Pervolaraki), [email protected] (J. Giapintzakis). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.09.093
material. To this end, we chose as substrate materials with good lattice parameter matching to the La5Ca9Cu24O41. Pulsed Laser Deposition (PLD) has been successfully used previously for the growth of complex oxides such as the half-metallic oxides like La0.7Sr0.3MnO3, CrO2, Fe3O4 and Sr2FeMoO6 for spintronics applications [4]. We chose PLD for the deposition of La5Ca9Cu24O41 films on various substrates due to the versatility of high oxygen ambient pressure and high success in stoichiometric transfer. To the best of our knowledge this is the first time that La5Ca9Cu24O41 thin films were grown on any substrate using the PLD technique. 2. Experimental Ceramic samples of La5Ca9Cu24O41 were prepared from stoichiometric quantities of high-purity (>99.99%) La2O3, CuO, and CaCO3 powders by conventional solid-state reaction. The powders were mixed in an agate mortar and pressed into pellets 25 mm in diameter and 5 mm thick, using an uniaxial press at pressures ranging from 10 103 to 30 103 kg/cm2. The pellets were annealed in three different steps at 900 8C, 950 8C, and 970 8C. The first two annealing steps lasted 24 h each and the final one 72 h. Between each annealing step, the pellets were re-ground and pressed again, as described above. The composition of the pellets, which were to be used as targets, was checked by XRD, X’pert Pro MPD diffractometer, PANalytical, in the 108 < 2u < 808 range and by Energy Dispersive X-ray spectrometry (EDX), Cambridge 260. Thin La5Ca9Cu24O41 films were deposited using a KrF laser, COMPexPro 201, with pulse duration of 25 ns at a pulse repetition rate of 10 Hz. The laser beam was incident at an angle of 458 with
M. Pervolaraki et al. / Applied Surface Science 255 (2009) 5236–5239
respect to the target surface. The target was rotating during ablation and the distance between the target and the substrate was kept at 4 cm. A thermocouple was utilized for monitoring the temperature in close contact to the mask holder of the substrate. A rotary and a turbomolecular pump evacuated the chamber to a base pressure of 10 3 Pa. The oxygen (99.999%) ambient was around 20 Pa and a fluence of 2 J cm 2 was used in most of the depositions. The substrate temperature was varied from 466 8C to 650 8C. La5Ca9Cu24O41 polycrystalline pellets of density higher than 90% of the theoretical density were used as target materials. We used as deposition substrates single crystals of SrLaAlO4 (1 0 0), SrTiO3 (0 0 1), MgO (1 0 0) and Si (1 0 0). All substrates were ultrasonically cleaned in a sequence of methanol, isopropanol and distilled water, followed by blow drying with pure N2. The thickness of the films was determined by the means of a Xi100 non-contact optical profilometer and found to vary from 37 nm to 450 nm depending on the substrate material and the number of deposition pulses. A SHIMADZU, XRD-6000, X-ray diffractometer was used to confirm the phase purity and to investigate the lattice orientation of the grown films. u–2u scans were performed for all substrates and films. Finally, Scanning Electron Microscopy (SEM) was employed for the topological characterization of the thin films.
5237
Fig. 2. XRD spectra of the SrLaAlO4 substrate (black line) and the La5Ca9Cu24O41– SrLaAlO4 thin film (red line), respectively. The film was deposited at a substrate temperature of 590 8C and laser fluence of 2 J cm 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).
3. Results and discussion An intrinsic problem of the PLD technique is the generation and deposition of particulates on the surface of the film increasing its roughness. The deposition of particulates can be partially eliminated by utilizing low fluences and dense targets. The fluence of 2 J cm 2 was found to be appropriate since it is low enough to reduce the number of particulates deposited at the surface of the film and also sufficient to ensure ablation of the target. Creation and deposition of micron-scale particles can be seen in Fig. 1. In this SEM micrograph a thin La5Ca9Cu24O41 film on a SrLaAlO4 substrate is depicted. The film was deposited at 590 8C and the XRD spectrum shown in Fig. 2 confirms that it is single phase. The La5Ca9Cu24O41–SrLaAlO4 film exhibits mainly (0 k 0) preferred
orientation although the (1 3 7)/(3 3 1) peak, the most intense one for polycrystalline La5Ca9Cu24O41 samples, remains present. Highly textured La5Ca9Cu24O41 films were fabricated on STO substrates; a characteristic example is shown in Fig. 3. The XRD patterns shown in Fig. 4, indicate that the grown film is b-axis oriented (red line) and the only peak that does not correspond to this orientation is the STO peak, see reference spectrum (black line). Using similar substrate temperature of 627 8C a film of the investigated material was deposited on MgO single crystal substrate, shown in Fig. 5. Several peaks expected for La5Ca9Cu24O41 are visible in the La5Ca9Cu24O41–MgO spectrum (red line) such as the (0 4 0) and the (0 8 0) peaks, which indicate weak texturing along (0 k 0). The high intensity (1 3 7)/(3 3 1) peak,
Fig. 1. Scanning electron micrograph of the La5Ca9Cu24O41 film, 435 nm thick, on SrLaAlO4 substrate.
Fig. 3. SEM photo of a 450-nm thick La5Ca9Cu24O41 film on a STO single crystal.
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M. Pervolaraki et al. / Applied Surface Science 255 (2009) 5236–5239
Fig. 4. Reference XRD spectrum of the STO substrate (black line) and the La5Ca9Cu24O41–STO thin film (red line), respectively. The high texturing of the film is evident with clear preferred orientation (0 k 0), (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).
which indicates that partly the material is polycrystalline, and the low intensity (0 0 14) peak, which indicates that higher temperatures are most likely necessary to obtain highly textured films along c-axis, are visible. The lowest amount of particulates deposited on the films was observed for those grown on Si substrates (Fig. 6). It was found that the growth of La5Ca9Cu24O41 thin films on Si takes longer to initiate, almost double the time in comparison to the deposition on perovskites. This film nucleation delay can be attributed to the bonding mismatch between the covalent bonding of silicon and the ionic bonding of the La5Ca9Cu24O41 oxide and the low fluence of 1.1 J cm 2 used in this deposition. Target-film stoichiometric transfer of single phase La5Ca9Cu24O41 material was confirmed in all films grown at substrate temperatures above 500 8C. At the deposition temperature of 466 8C and laser fluence of 1.1 J cm 2 no La5Ca9Cu24O41 peaks could be seen in the XRD spectrum (Fig. 7). However, when both the substrate temperature and the fluence
Fig. 6. Scanning electron micrograph of La5Ca9Cu24O41 thin film on Si substrate. The 37 nm thick La5Ca9Cu24O41–Si film is smooth with only few particulates on the surface.
Fig. 7. XRD spectra of La5Ca9Cu24O41 films grown on Si substrates at 466 8C (black line) and 650 8C (red line), respectively. The highly textured film along b-axis was grown at the highest substrate temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).
were increased to 650 8C and 2 J cm 2, respectively, highly textured films were produced on Si single crystals with well defined (0 4 0), (0 6 0) and (0 8 0) peaks. Preliminary EDX experiments have also indicated that the stoichiometry of the grown thin films is the same as that of the target. 4. Conclusion
Fig. 5. XRD Spectra of the MgO and the La5Ca9Cu24O41–MgO film deposited at 627 8C on the crystal. Four La5Ca9Cu24O41 angles are present along with the (1 3 7)/(3 3 1) having the highest intensity.
Preliminary results on the deposition of the complex La5Ca9Cu24O41 compound using conventional PLD are reported for the first time. All films deposited at temperatures higher than 500 8C on various substrates (SrLaAlO4, STO, MgO and Si) possessed the same stoichiometry as the La5Ca9Cu24O41 target material. Highly textured films were grown on SrLaAlO4, STO and Si crystals with
M. Pervolaraki et al. / Applied Surface Science 255 (2009) 5236–5239
(0 k 0) preferred orientation at substrate temperatures up to 650 8C. The La5Ca9Cu24O41–Si film crystallinity improved at higher substrate temperature and fluence of 2 J cm 2. Further studies are in progress for the investigation of the effect of higher temperature on the degree of crystallinity and the orientation of the La5Ca9Cu24O41 films. Acknowledgements This work was supported by the European Commission through the FET open-STREP NOVMAG, Project Reference 032980. The
5239
authors at the University of Cyprus wish to thank Mr. K. Roushias and Mr. G. Vessiaris for the technical assistance.
References [1] C. Hess, Eur. Phys. J. Special Topics 151 (2007) 73–83. [2] U. Ammerahl, A. Revcolevschi, J. Crystal Growth 197 (1999) 825–832. [3] C. Hess, C. Baumann, U. Ammerahl, B. Buchner, F. Heidrich-Meisner, W. Brenig, A. Revcolevschi, Phys. Rev. B 64 (2001), 184305. [4] A.M. Haghiri-Gosnet, T. Arnal, R. Soulimane, M. Koubaa, J.P. Renald, Phys. Stat. Sol. (a) 201 (2004), No. 7.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
KrF pulsed laser deposition of La5Ca9Cu24O41 thin films on various substrates M. Pervolaraki a,*, G.I. Athanasopoulos a, R. Saint-Martin b, A. Revcolevschi b, J. Giapintzakis a a b
Department of Mechanical and Manufacturing Engineering, University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537, Nicosia 1678, Cyprus Laboratoire de Physico-Chimie de L’Etat Solide, UMR 8182, Universite´ Paris-Sud, 91405 Orsay Cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history:
Recent studies on single crystals of cuprate oxides containing spin chains and ladders have reported large anisotropic magnon-mediated thermal conductivity. A potential use of thin films of such materials could be in the thermal management of electronic devices for the guiding of unwanted heat to a heat sink. In this article, the pulsed laser deposition and characterization of La5Ca9Cu24O41 thin films on SrLaAlO4, SrTiO3, MgO, and Si substrates are reported for the first time. The films were grown using a pulsed UV laser (KrF, 248 nm) and various substrate temperatures up to 650 8C. The XRD spectra revealed successful target-film stoichiometric transfer and high texturing of the thin films with (0 k 0) preferred orientation. ß 2008 Elsevier B.V. All rights reserved.
Available online 18 October 2008 Keywords: La5Ca9Cu24O41 Thin films Pulsed laser deposition
1. Introduction Compounds with low-dimensional spin structures have recently attracted considerable attention due to their novel physical properties [1]. Single crystals of such compounds, a few centimeters long, have been grown by Ammerahl and Revcolevschi [2], and their anisotropic thermal conductivity has been investigated [3]. The high non-phonon thermal conductivity of such S = 1/2 cuprate compounds is attributed to the spin-excitation contribution. In the La5Ca9Cu24O41 ladder compound the magnonmediated thermal conductivity is an order of magnitude larger along c-axis in comparison to the in-plane value [1,3]. Thermal management in electronic devices, semiconductor and microchip industry remains a major problem. The generated unwanted heat results in a decrease of the performance and limits the lifetime of the devices. A potential solution to this problem could come from using a thin film of high and anisotropic thermal conductivity for guiding the generated heat to a heat sink. Novel quasi-one dimensional quantum magnet oxides like La5Ca9Cu24O41 are potential candidates since the heat in these oxides is transported predominately in one dimension. To achieve heteroepitaxy, i.e., single crystal thin film growth on a single crystal substrate of a different material, it is vital to match closely the lattice parameters of the substrate to those of the thin film
* Corresponding author. Fax: +357 22892254. E-mail addresses: [email protected] (M. Pervolaraki), [email protected] (J. Giapintzakis). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.09.093
material. To this end, we chose as substrate materials with good lattice parameter matching to the La5Ca9Cu24O41. Pulsed Laser Deposition (PLD) has been successfully used previously for the growth of complex oxides such as the half-metallic oxides like La0.7Sr0.3MnO3, CrO2, Fe3O4 and Sr2FeMoO6 for spintronics applications [4]. We chose PLD for the deposition of La5Ca9Cu24O41 films on various substrates due to the versatility of high oxygen ambient pressure and high success in stoichiometric transfer. To the best of our knowledge this is the first time that La5Ca9Cu24O41 thin films were grown on any substrate using the PLD technique. 2. Experimental Ceramic samples of La5Ca9Cu24O41 were prepared from stoichiometric quantities of high-purity (>99.99%) La2O3, CuO, and CaCO3 powders by conventional solid-state reaction. The powders were mixed in an agate mortar and pressed into pellets 25 mm in diameter and 5 mm thick, using an uniaxial press at pressures ranging from 10 103 to 30 103 kg/cm2. The pellets were annealed in three different steps at 900 8C, 950 8C, and 970 8C. The first two annealing steps lasted 24 h each and the final one 72 h. Between each annealing step, the pellets were re-ground and pressed again, as described above. The composition of the pellets, which were to be used as targets, was checked by XRD, X’pert Pro MPD diffractometer, PANalytical, in the 108 < 2u < 808 range and by Energy Dispersive X-ray spectrometry (EDX), Cambridge 260. Thin La5Ca9Cu24O41 films were deposited using a KrF laser, COMPexPro 201, with pulse duration of 25 ns at a pulse repetition rate of 10 Hz. The laser beam was incident at an angle of 458 with
M. Pervolaraki et al. / Applied Surface Science 255 (2009) 5236–5239
respect to the target surface. The target was rotating during ablation and the distance between the target and the substrate was kept at 4 cm. A thermocouple was utilized for monitoring the temperature in close contact to the mask holder of the substrate. A rotary and a turbomolecular pump evacuated the chamber to a base pressure of 10 3 Pa. The oxygen (99.999%) ambient was around 20 Pa and a fluence of 2 J cm 2 was used in most of the depositions. The substrate temperature was varied from 466 8C to 650 8C. La5Ca9Cu24O41 polycrystalline pellets of density higher than 90% of the theoretical density were used as target materials. We used as deposition substrates single crystals of SrLaAlO4 (1 0 0), SrTiO3 (0 0 1), MgO (1 0 0) and Si (1 0 0). All substrates were ultrasonically cleaned in a sequence of methanol, isopropanol and distilled water, followed by blow drying with pure N2. The thickness of the films was determined by the means of a Xi100 non-contact optical profilometer and found to vary from 37 nm to 450 nm depending on the substrate material and the number of deposition pulses. A SHIMADZU, XRD-6000, X-ray diffractometer was used to confirm the phase purity and to investigate the lattice orientation of the grown films. u–2u scans were performed for all substrates and films. Finally, Scanning Electron Microscopy (SEM) was employed for the topological characterization of the thin films.
5237
Fig. 2. XRD spectra of the SrLaAlO4 substrate (black line) and the La5Ca9Cu24O41– SrLaAlO4 thin film (red line), respectively. The film was deposited at a substrate temperature of 590 8C and laser fluence of 2 J cm 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).
3. Results and discussion An intrinsic problem of the PLD technique is the generation and deposition of particulates on the surface of the film increasing its roughness. The deposition of particulates can be partially eliminated by utilizing low fluences and dense targets. The fluence of 2 J cm 2 was found to be appropriate since it is low enough to reduce the number of particulates deposited at the surface of the film and also sufficient to ensure ablation of the target. Creation and deposition of micron-scale particles can be seen in Fig. 1. In this SEM micrograph a thin La5Ca9Cu24O41 film on a SrLaAlO4 substrate is depicted. The film was deposited at 590 8C and the XRD spectrum shown in Fig. 2 confirms that it is single phase. The La5Ca9Cu24O41–SrLaAlO4 film exhibits mainly (0 k 0) preferred
orientation although the (1 3 7)/(3 3 1) peak, the most intense one for polycrystalline La5Ca9Cu24O41 samples, remains present. Highly textured La5Ca9Cu24O41 films were fabricated on STO substrates; a characteristic example is shown in Fig. 3. The XRD patterns shown in Fig. 4, indicate that the grown film is b-axis oriented (red line) and the only peak that does not correspond to this orientation is the STO peak, see reference spectrum (black line). Using similar substrate temperature of 627 8C a film of the investigated material was deposited on MgO single crystal substrate, shown in Fig. 5. Several peaks expected for La5Ca9Cu24O41 are visible in the La5Ca9Cu24O41–MgO spectrum (red line) such as the (0 4 0) and the (0 8 0) peaks, which indicate weak texturing along (0 k 0). The high intensity (1 3 7)/(3 3 1) peak,
Fig. 1. Scanning electron micrograph of the La5Ca9Cu24O41 film, 435 nm thick, on SrLaAlO4 substrate.
Fig. 3. SEM photo of a 450-nm thick La5Ca9Cu24O41 film on a STO single crystal.
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M. Pervolaraki et al. / Applied Surface Science 255 (2009) 5236–5239
Fig. 4. Reference XRD spectrum of the STO substrate (black line) and the La5Ca9Cu24O41–STO thin film (red line), respectively. The high texturing of the film is evident with clear preferred orientation (0 k 0), (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).
which indicates that partly the material is polycrystalline, and the low intensity (0 0 14) peak, which indicates that higher temperatures are most likely necessary to obtain highly textured films along c-axis, are visible. The lowest amount of particulates deposited on the films was observed for those grown on Si substrates (Fig. 6). It was found that the growth of La5Ca9Cu24O41 thin films on Si takes longer to initiate, almost double the time in comparison to the deposition on perovskites. This film nucleation delay can be attributed to the bonding mismatch between the covalent bonding of silicon and the ionic bonding of the La5Ca9Cu24O41 oxide and the low fluence of 1.1 J cm 2 used in this deposition. Target-film stoichiometric transfer of single phase La5Ca9Cu24O41 material was confirmed in all films grown at substrate temperatures above 500 8C. At the deposition temperature of 466 8C and laser fluence of 1.1 J cm 2 no La5Ca9Cu24O41 peaks could be seen in the XRD spectrum (Fig. 7). However, when both the substrate temperature and the fluence
Fig. 6. Scanning electron micrograph of La5Ca9Cu24O41 thin film on Si substrate. The 37 nm thick La5Ca9Cu24O41–Si film is smooth with only few particulates on the surface.
Fig. 7. XRD spectra of La5Ca9Cu24O41 films grown on Si substrates at 466 8C (black line) and 650 8C (red line), respectively. The highly textured film along b-axis was grown at the highest substrate temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).
were increased to 650 8C and 2 J cm 2, respectively, highly textured films were produced on Si single crystals with well defined (0 4 0), (0 6 0) and (0 8 0) peaks. Preliminary EDX experiments have also indicated that the stoichiometry of the grown thin films is the same as that of the target. 4. Conclusion
Fig. 5. XRD Spectra of the MgO and the La5Ca9Cu24O41–MgO film deposited at 627 8C on the crystal. Four La5Ca9Cu24O41 angles are present along with the (1 3 7)/(3 3 1) having the highest intensity.
Preliminary results on the deposition of the complex La5Ca9Cu24O41 compound using conventional PLD are reported for the first time. All films deposited at temperatures higher than 500 8C on various substrates (SrLaAlO4, STO, MgO and Si) possessed the same stoichiometry as the La5Ca9Cu24O41 target material. Highly textured films were grown on SrLaAlO4, STO and Si crystals with
M. Pervolaraki et al. / Applied Surface Science 255 (2009) 5236–5239
(0 k 0) preferred orientation at substrate temperatures up to 650 8C. The La5Ca9Cu24O41–Si film crystallinity improved at higher substrate temperature and fluence of 2 J cm 2. Further studies are in progress for the investigation of the effect of higher temperature on the degree of crystallinity and the orientation of the La5Ca9Cu24O41 films. Acknowledgements This work was supported by the European Commission through the FET open-STREP NOVMAG, Project Reference 032980. The
5239
authors at the University of Cyprus wish to thank Mr. K. Roushias and Mr. G. Vessiaris for the technical assistance.
References [1] C. Hess, Eur. Phys. J. Special Topics 151 (2007) 73–83. [2] U. Ammerahl, A. Revcolevschi, J. Crystal Growth 197 (1999) 825–832. [3] C. Hess, C. Baumann, U. Ammerahl, B. Buchner, F. Heidrich-Meisner, W. Brenig, A. Revcolevschi, Phys. Rev. B 64 (2001), 184305. [4] A.M. Haghiri-Gosnet, T. Arnal, R. Soulimane, M. Koubaa, J.P. Renald, Phys. Stat. Sol. (a) 201 (2004), No. 7.