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Nd:YVO4 crystalline film grown by pulsed laser deposition
Optical Materials 31 (2009) 1331–1333
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nd:YVO4 crystalline film grown by pulsed laser deposition C. Calì a, F. Cornacchia b, A. Di Lieto b,*, F. Marchetti c, M. Tonelli d a
Dipartimento di Ingegneria Elettrica, Elettronica e delle Telecomunicazioni, Università di Palermo, Italy NEST – Scuola Normale Superiore, Pisa, Italy Dipartimento di Chimica – Università di Pisa, Pisa, Italy d NEST – Dipartimento di Fisica, Università di Pisa, Italy b c
a r t i c l e
i n f o
Article history: Available online 28 November 2008 PACS: 81.15.Fg 78.66.w 68.37.d
a b s t r a c t We present the preliminary results obtained in the growth of thin films of Nd-doped YVO4 (YVO) by pulsed laser deposition (PLD) on amorphous substrate. The films were obtained by ablating bulk YVO crystals doped with Nd3+ ions with a Q-switched tripled Nd:YAG laser in a UHV chamber. The samples have been characterized both morphologically (with X-ray diffraction and atomic force microscope measurements) and spectroscopically, by measuring fluorescence spectra and lifetime. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Pulsed laser deposition Rare-earth doped oxides Nanostructures
1. Introduction
2. Experimental apparatus
The rare-earth orthovanadate YVO is a well-known laser host materials from more than 40 years [1]. Laser action has been reported for many rare-earth doped YVO crystals, including Nd, Er, Ho, Tm [2] and more recently Yb [3]. Pulsed laser deposition has been proven to be a simple, versatile, and reliable technique [4] for the fabrication of optical devices [5,6] such as waveguide lasers [7], and recently this technique has been used to produce thin film of both oxides [8] and fluorides doped with rare-earth ions [9]. In the literature a first result regarding the preparation of Nd: YVO thin film on sapphire substrate using the pulsed laser deposition technique is reported in Ref. [10]. In this work, we present the preliminary results obtained in the deposition of Nd3+:YVO4 film on amorphous (quartz) substrate. Different morphological and spectroscopical techniques have been applied to characterize the film quality: the crystallinity of the films has been estimated by X-ray diffraction (XRD) measurements, while the surface topography has been investigated using an atomic force microscope (AFM) in non-contact mode. The spectroscopic measurements include both the fluorescence spectra and the fluorescence lifetime of the 4F3/2 manifold.
The targets used in the presented ablation experiments were single crystals of YVO, doped with 0.25% atomic concentration of Nd3+ ions. The target has been ablated with a Q-switched tripled Nd:YAG laser providing optical pulses of 8 ns duration, repetition rate 20 Hz, and energy up to 150 mJ at a wavelength of 355 nm in a vacuum chamber where oxygen gas can be introduced through an electromechanical valve, if necessary. A mechanical pump followed by a turbomolecular pump has been used to evacuate the chamber. Electronic feedback between electromechanical valve and vacuum meter stabilized the gas pressure inside the chamber (2 102 mbar). The laser beam was sent to the target with an angle of incidence of 25° through a quartz window and focused by a quartz converging lens (f = 0.3 m). The laser fluence (625 J/cm2) can be easily adjusted moving the focusing lens toward or away from the target. Best results have been got with a fluence of 13 J/cm2. The target was situated on the periphery of a revolving holder. The motion of an x y computer-controlled microtranslator assembly placed outside the vacuum chamber was transmitted to the axis of the revolving holder by a vacuum feedthrough. Such unconventional solution assures (a) uniform and full ablation of the target surface and (b) facility to change the target by a manual rotation of the axis of the revolving holder. A controlled high temperature heater allowed that the temperature of the substrate was between 25 °C and 1200 °C. Tested films have been deposited on
* Corresponding author. Tel.: +39 050 2214555; fax: +39 050 2214333. E-mail address: [email protected] (A. Di Lieto). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.032
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C. Calì et al. / Optical Materials 31 (2009) 1331–1333
quartz substrates heated to 800 °C. Position of the substrate from target has been fixed to 8 cm. An optical set-up based on the well knows relation between the thickness of a dielectric layer and the reflectivity has been used to monitor the growing of the film. The apparatus for X-ray diffraction measurements was a Bragg– Brentano diffractometer, using Cu Ka radiation at k = 0.154178 nm and adopting a h 2h scheme. A preliminary survey of the deposition surface has been performed by using a commercial profilometer (Dektak Veeco), with low force sensor option, allowing stylus forces down to a few lg. The vertical measurement range is up to 1 mm, with a repeatability better than a few nm. The apparatus for recording the fluorescence spectra of the samples include a diode laser source, with astigmatism compensation made by a couple of anamorphic prisms and a collimating lens, a focusing system, a 30 cm focal length monochromator to disperse the fluorescence with resolution in the range 0.12–1.6 nm, and a detector connected to a lock-in amplifier. For the fluorescence lifetime measurements of the 4F3/2 manifold, the samples were excited by a pulsed Ti:Al2O3 laser, featuring 30 ns pulse width and 10 Hz of pulse repetition frequency, that can be tuned between 770 and 920 nm. In order to observe a uniformly pumped volume and to reduce the influence of the radiation trapping, the fluorescence was collected from a short portion of the sample; furthermore the laser power incident on the film was reduced as much as possible by means of an attenuator to minimize the non-linear effects. The signal was detected by an experimental apparatus similar to that used for recording the fluorescence spectra, and the signal was sent, by a fast amplifier, to a digital oscilloscope connected to a computer. The response time of the system was about 1 ls.
As pointed out from the film analysis with the atomic force microscope, above the surface some irregularities are present, with a height of the order of tens of nm. The Fig. 1 shows part of the surface, where a few islands are superimposed to the film. Their shape is probably distorted by the sensor, but the density and the height gives an estimation of the film quality. The crystallinity of the films was determined by X-ray diffraction measurements (h 2h scan) of the film, and comparing it to the scan of the original target. In Fig. 2 are shown the scans obtained respectively from the quartz substrate (trace A) and from the film (trace B). The trace C was obtained from fine powder (a few lm) obtained by grinding a piece of undoped YVO crystal. All the scan have been recorded in similar conditions (scan from 15° to 75° with 0.02° step) but the vertical scales have been rearranged. In the trace B are present only diffraction peaks corresponding to [hkl] values with l = 0 or l = 1. All peaks are consistent with those of the pure material, and are compatible with the small thickness of the deposition.
3. Results The thickness of the deposition has been measured with the low-force profilometer. All the scans started from an edge of the substrate, which has been covered by pliers during the film deposition, in order to have a reference for the clean surface, and the length of the path was of the order of a few millimeters. From these data, all the deposition seems quite regular, with a core thickness ranging from 100 to 500 nm on different samples.
Fig. 2. X-ray diffraction spectra (h 2h scans) of a film on amorphous substrate. A (black line): scan of the pure substrate. B (red line): scan of the film. C (blue line): scan of undoped YVO4 powders. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 1. Image of part of the film surface, taken with the atomic force microscope.
C. Calì et al. / Optical Materials 31 (2009) 1331–1333
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As last check of the spectroscopical quality of the film, we have compared the fluorescence lifetime of the film and of the bulk, by recording as a function of time the fluorescence signal decay after a pulsed excitation of the Nd3+ ions into the 4F3/2 manifold. Fig. 4 shows the results obtained for the bulk sample (empty triangles) and for the film (black squares). The different behavior between the film and the bulk can be ascribed to cooperative effects, like the up-conversion energy transfer mechanism, which depends on the square of the excited level population. This effect has been already reported in the literature even for low concentrated samples, because it produces both a shortening of the measured lifetime and a non-exponential decay shape (see Ref. [11] and references therein). The continuous curve superimposed to the data are their best fit with the model described by equation (3) of Ref. [11]. The calculated lifetime are respectively 82.1 ± 0.2 ls for the bulk and 64.1 ± 0.8 ls for the film. Fig. 3. Unpolarized fluorescence spectra in the region near 900 nm, corresponding to the emission from the 4F3/2 to the 4I9/2 manifolds. Black continuous line: unpolarized film fluorescence; red dashed line: Ekc bulk fluorescence; blue dotted line: E \ c bulk fluorescence. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusion This paper describes the realization of a crystalline sub-micrometric YVO film grown by pulsed laser deposition on amorphous substrate. The spectroscopical characterizations indicates that the film contains Nd3+ ions with a doping level similar than that of the originating material, as suggested from the analysis of the fluorescence spectra. The order degree of the film is confirmed by the exponential decay of the excited levels fluorescence, even if a shortening of the lifetime has been observed. The structure of the deposition, as revealed from the morphological analyses, suggests a good degree of order in the film, and confirms the feasibility of monocrystalline rare-earth doped oxide submicrometric layers with promising optical qualities. Experiments are in progress to produce other depositions with the modification of the growth parameters, and to measure all the other optical features. Acknowledgement
Fig. 4. Fluorescence of the Nd3+ ion 4F3/2 manifold recorded from the bulk sample (empty triangles) and from the film (black squares) as a function of time.
The XRD signal was collected from an area comparable to the whole film deposition, and the scan reported in the figure suggest a good crystallinity of the entire film. Fig. 3 shows the comparison between the fluorescence in the region around 900 nm recorded from the film and from a bulk of monocrystalline YVO doped with the same amount of Nd3+ ions used for the target. The film fluorescence was recorded unpolarized, and it corresponds to the black continuous line. The bulk fluorescence was recorded by using a polarizer in order to distinguish between Ekc (red dashed line) and E \ c (blue dotted line) signals. All fluorescence records were obtained in similar condition, by pumping with a diode laser emitting 300 mW and tuned around the absorption peak at 809 nm. The monochromator resolution was settled in order to increase the signal-to-noise ratio for the film, without affecting the spectra linewidths. Nevertheless, even if the signal-to-noise ratio of the film is evidently poorer, the spectra show very similar features, with a strong coincidence of the position of the emitted peaks.
The authors express their gratitude to P. Pingue for the measurements with the profilometer and with the AFM. This research has been co-funded by the Italian MIUR with the PRIN’04 Project No. 2004023130. References [1] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [2] A.A. Kaminskii, Crystals, Springer-Verlag, New York, 1990. [3] V.E. Kisel, A.E. Troshin, N.A. Tolstik, V.G. Shcherbitsky, N.V. Kuleshov, V.N. Matrosov, T.A. Matrosova, M.I. Kupchenko, Opt. Lett. 29 (2004) 2491. [4] B.D. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, J. Wiley and Sons, New York, 1994. [5] Z.G. Dong, F.Y. Wang, Y.X. Fan, P. Lu, S.N. Zhua, P.K. Lim, T.B. Tang, Appl. Phys. Lett. 86 (2005) 151908. [6] A.M. Grishin, E.V. Vanin, S.I. Khartsev, O.V. Tarasenko, P. Johansson, Appl. Phys. Lett. 89 (2006) 021114. [7] Y. Kuzminykh, A. Kahn, G. Huber, Opt. Mater. 28 (2006) 883. [8] S. Bar, H. Scheife, G. Huber, Opt. Mater. 28 (2006) 681. [9] S. Barsanti, F. Cornacchia, A. Di Lieto, A. Toncelli, M. Tonelli, P. Bicchi, Thin Solid Films (2007), doi:10.1016/j.tsf.2007.06.144. [10] M.B. Korzenski, Ph. Lecoeur, B. Mercey, B. Raveau, Chem. Mater. 13 (2001) 1545. [11] L. Palatella, F. Cornacchia, A. Toncelli, M. Tonelli, J. Opt. Soc. Am. B 20 (2003) 1708.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nd:YVO4 crystalline film grown by pulsed laser deposition C. Calì a, F. Cornacchia b, A. Di Lieto b,*, F. Marchetti c, M. Tonelli d a
Dipartimento di Ingegneria Elettrica, Elettronica e delle Telecomunicazioni, Università di Palermo, Italy NEST – Scuola Normale Superiore, Pisa, Italy Dipartimento di Chimica – Università di Pisa, Pisa, Italy d NEST – Dipartimento di Fisica, Università di Pisa, Italy b c
a r t i c l e
i n f o
Article history: Available online 28 November 2008 PACS: 81.15.Fg 78.66.w 68.37.d
a b s t r a c t We present the preliminary results obtained in the growth of thin films of Nd-doped YVO4 (YVO) by pulsed laser deposition (PLD) on amorphous substrate. The films were obtained by ablating bulk YVO crystals doped with Nd3+ ions with a Q-switched tripled Nd:YAG laser in a UHV chamber. The samples have been characterized both morphologically (with X-ray diffraction and atomic force microscope measurements) and spectroscopically, by measuring fluorescence spectra and lifetime. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Pulsed laser deposition Rare-earth doped oxides Nanostructures
1. Introduction
2. Experimental apparatus
The rare-earth orthovanadate YVO is a well-known laser host materials from more than 40 years [1]. Laser action has been reported for many rare-earth doped YVO crystals, including Nd, Er, Ho, Tm [2] and more recently Yb [3]. Pulsed laser deposition has been proven to be a simple, versatile, and reliable technique [4] for the fabrication of optical devices [5,6] such as waveguide lasers [7], and recently this technique has been used to produce thin film of both oxides [8] and fluorides doped with rare-earth ions [9]. In the literature a first result regarding the preparation of Nd: YVO thin film on sapphire substrate using the pulsed laser deposition technique is reported in Ref. [10]. In this work, we present the preliminary results obtained in the deposition of Nd3+:YVO4 film on amorphous (quartz) substrate. Different morphological and spectroscopical techniques have been applied to characterize the film quality: the crystallinity of the films has been estimated by X-ray diffraction (XRD) measurements, while the surface topography has been investigated using an atomic force microscope (AFM) in non-contact mode. The spectroscopic measurements include both the fluorescence spectra and the fluorescence lifetime of the 4F3/2 manifold.
The targets used in the presented ablation experiments were single crystals of YVO, doped with 0.25% atomic concentration of Nd3+ ions. The target has been ablated with a Q-switched tripled Nd:YAG laser providing optical pulses of 8 ns duration, repetition rate 20 Hz, and energy up to 150 mJ at a wavelength of 355 nm in a vacuum chamber where oxygen gas can be introduced through an electromechanical valve, if necessary. A mechanical pump followed by a turbomolecular pump has been used to evacuate the chamber. Electronic feedback between electromechanical valve and vacuum meter stabilized the gas pressure inside the chamber (2 102 mbar). The laser beam was sent to the target with an angle of incidence of 25° through a quartz window and focused by a quartz converging lens (f = 0.3 m). The laser fluence (625 J/cm2) can be easily adjusted moving the focusing lens toward or away from the target. Best results have been got with a fluence of 13 J/cm2. The target was situated on the periphery of a revolving holder. The motion of an x y computer-controlled microtranslator assembly placed outside the vacuum chamber was transmitted to the axis of the revolving holder by a vacuum feedthrough. Such unconventional solution assures (a) uniform and full ablation of the target surface and (b) facility to change the target by a manual rotation of the axis of the revolving holder. A controlled high temperature heater allowed that the temperature of the substrate was between 25 °C and 1200 °C. Tested films have been deposited on
* Corresponding author. Tel.: +39 050 2214555; fax: +39 050 2214333. E-mail address: [email protected] (A. Di Lieto). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.032
1332
C. Calì et al. / Optical Materials 31 (2009) 1331–1333
quartz substrates heated to 800 °C. Position of the substrate from target has been fixed to 8 cm. An optical set-up based on the well knows relation between the thickness of a dielectric layer and the reflectivity has been used to monitor the growing of the film. The apparatus for X-ray diffraction measurements was a Bragg– Brentano diffractometer, using Cu Ka radiation at k = 0.154178 nm and adopting a h 2h scheme. A preliminary survey of the deposition surface has been performed by using a commercial profilometer (Dektak Veeco), with low force sensor option, allowing stylus forces down to a few lg. The vertical measurement range is up to 1 mm, with a repeatability better than a few nm. The apparatus for recording the fluorescence spectra of the samples include a diode laser source, with astigmatism compensation made by a couple of anamorphic prisms and a collimating lens, a focusing system, a 30 cm focal length monochromator to disperse the fluorescence with resolution in the range 0.12–1.6 nm, and a detector connected to a lock-in amplifier. For the fluorescence lifetime measurements of the 4F3/2 manifold, the samples were excited by a pulsed Ti:Al2O3 laser, featuring 30 ns pulse width and 10 Hz of pulse repetition frequency, that can be tuned between 770 and 920 nm. In order to observe a uniformly pumped volume and to reduce the influence of the radiation trapping, the fluorescence was collected from a short portion of the sample; furthermore the laser power incident on the film was reduced as much as possible by means of an attenuator to minimize the non-linear effects. The signal was detected by an experimental apparatus similar to that used for recording the fluorescence spectra, and the signal was sent, by a fast amplifier, to a digital oscilloscope connected to a computer. The response time of the system was about 1 ls.
As pointed out from the film analysis with the atomic force microscope, above the surface some irregularities are present, with a height of the order of tens of nm. The Fig. 1 shows part of the surface, where a few islands are superimposed to the film. Their shape is probably distorted by the sensor, but the density and the height gives an estimation of the film quality. The crystallinity of the films was determined by X-ray diffraction measurements (h 2h scan) of the film, and comparing it to the scan of the original target. In Fig. 2 are shown the scans obtained respectively from the quartz substrate (trace A) and from the film (trace B). The trace C was obtained from fine powder (a few lm) obtained by grinding a piece of undoped YVO crystal. All the scan have been recorded in similar conditions (scan from 15° to 75° with 0.02° step) but the vertical scales have been rearranged. In the trace B are present only diffraction peaks corresponding to [hkl] values with l = 0 or l = 1. All peaks are consistent with those of the pure material, and are compatible with the small thickness of the deposition.
3. Results The thickness of the deposition has been measured with the low-force profilometer. All the scans started from an edge of the substrate, which has been covered by pliers during the film deposition, in order to have a reference for the clean surface, and the length of the path was of the order of a few millimeters. From these data, all the deposition seems quite regular, with a core thickness ranging from 100 to 500 nm on different samples.
Fig. 2. X-ray diffraction spectra (h 2h scans) of a film on amorphous substrate. A (black line): scan of the pure substrate. B (red line): scan of the film. C (blue line): scan of undoped YVO4 powders. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 1. Image of part of the film surface, taken with the atomic force microscope.
C. Calì et al. / Optical Materials 31 (2009) 1331–1333
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As last check of the spectroscopical quality of the film, we have compared the fluorescence lifetime of the film and of the bulk, by recording as a function of time the fluorescence signal decay after a pulsed excitation of the Nd3+ ions into the 4F3/2 manifold. Fig. 4 shows the results obtained for the bulk sample (empty triangles) and for the film (black squares). The different behavior between the film and the bulk can be ascribed to cooperative effects, like the up-conversion energy transfer mechanism, which depends on the square of the excited level population. This effect has been already reported in the literature even for low concentrated samples, because it produces both a shortening of the measured lifetime and a non-exponential decay shape (see Ref. [11] and references therein). The continuous curve superimposed to the data are their best fit with the model described by equation (3) of Ref. [11]. The calculated lifetime are respectively 82.1 ± 0.2 ls for the bulk and 64.1 ± 0.8 ls for the film. Fig. 3. Unpolarized fluorescence spectra in the region near 900 nm, corresponding to the emission from the 4F3/2 to the 4I9/2 manifolds. Black continuous line: unpolarized film fluorescence; red dashed line: Ekc bulk fluorescence; blue dotted line: E \ c bulk fluorescence. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusion This paper describes the realization of a crystalline sub-micrometric YVO film grown by pulsed laser deposition on amorphous substrate. The spectroscopical characterizations indicates that the film contains Nd3+ ions with a doping level similar than that of the originating material, as suggested from the analysis of the fluorescence spectra. The order degree of the film is confirmed by the exponential decay of the excited levels fluorescence, even if a shortening of the lifetime has been observed. The structure of the deposition, as revealed from the morphological analyses, suggests a good degree of order in the film, and confirms the feasibility of monocrystalline rare-earth doped oxide submicrometric layers with promising optical qualities. Experiments are in progress to produce other depositions with the modification of the growth parameters, and to measure all the other optical features. Acknowledgement
Fig. 4. Fluorescence of the Nd3+ ion 4F3/2 manifold recorded from the bulk sample (empty triangles) and from the film (black squares) as a function of time.
The XRD signal was collected from an area comparable to the whole film deposition, and the scan reported in the figure suggest a good crystallinity of the entire film. Fig. 3 shows the comparison between the fluorescence in the region around 900 nm recorded from the film and from a bulk of monocrystalline YVO doped with the same amount of Nd3+ ions used for the target. The film fluorescence was recorded unpolarized, and it corresponds to the black continuous line. The bulk fluorescence was recorded by using a polarizer in order to distinguish between Ekc (red dashed line) and E \ c (blue dotted line) signals. All fluorescence records were obtained in similar condition, by pumping with a diode laser emitting 300 mW and tuned around the absorption peak at 809 nm. The monochromator resolution was settled in order to increase the signal-to-noise ratio for the film, without affecting the spectra linewidths. Nevertheless, even if the signal-to-noise ratio of the film is evidently poorer, the spectra show very similar features, with a strong coincidence of the position of the emitted peaks.
The authors express their gratitude to P. Pingue for the measurements with the profilometer and with the AFM. This research has been co-funded by the Italian MIUR with the PRIN’04 Project No. 2004023130. References [1] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [2] A.A. Kaminskii, Crystals, Springer-Verlag, New York, 1990. [3] V.E. Kisel, A.E. Troshin, N.A. Tolstik, V.G. Shcherbitsky, N.V. Kuleshov, V.N. Matrosov, T.A. Matrosova, M.I. Kupchenko, Opt. Lett. 29 (2004) 2491. [4] B.D. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, J. Wiley and Sons, New York, 1994. [5] Z.G. Dong, F.Y. Wang, Y.X. Fan, P. Lu, S.N. Zhua, P.K. Lim, T.B. Tang, Appl. Phys. Lett. 86 (2005) 151908. [6] A.M. Grishin, E.V. Vanin, S.I. Khartsev, O.V. Tarasenko, P. Johansson, Appl. Phys. Lett. 89 (2006) 021114. [7] Y. Kuzminykh, A. Kahn, G. Huber, Opt. Mater. 28 (2006) 883. [8] S. Bar, H. Scheife, G. Huber, Opt. Mater. 28 (2006) 681. [9] S. Barsanti, F. Cornacchia, A. Di Lieto, A. Toncelli, M. Tonelli, P. Bicchi, Thin Solid Films (2007), doi:10.1016/j.tsf.2007.06.144. [10] M.B. Korzenski, Ph. Lecoeur, B. Mercey, B. Raveau, Chem. Mater. 13 (2001) 1545. [11] L. Palatella, F. Cornacchia, A. Toncelli, M. Tonelli, J. Opt. Soc. Am. B 20 (2003) 1708.