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Lithium niobate films on periodic poled lithium niobate substrates prepared by liquid phase epitaxy
Journal of Crystal Growth 237–239 (2002) 596–601
Lithium niobate films on periodic poled lithium niobate substrates prepared by liquid phase epitaxy ! D. Callejo, V. Bermudez, M.D. Serrano*, E. Die! guez ! Departamento of F!ısica de Materiales, Facultad de Ciencias, Universidad Autonoma de Madrid, Madrid 28049, Spain
Abstract Lithium niobate thin films were grown by liquid phase epitaxy technique on periodic poled lithium niobate substrates doped with erbium. The composition and structure of the films were characterised by wave dispersion X-ray analysis and high resolution X-ray diffraction, respectively. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Lm; 77.84.Dy Keywords: A1. High resolution X-ray diffraction; A3. Liquid phase epitaxy; B1. Niobates
1. Introduction Second harmonic generation (SHG) by quasiphase-matching (QPM) in LiNbO3 (LN) via periodic modulation of the non-linear coefficient, provides an attractive route for the realisation of blue and green light sources with several mW of output power [1,2]. Electric-field poling has been used extensively for periodic domain reversal in ferroelectric materials, and hence modulation of the non-linear coefficient, but it has the limitation of the size of the samples and the use of laser dopant ions into the crystals [3,4]. However, periodic poled lithium niobate (PPLN) prepared by off-centred Czochralski (Cz) technique, has the possibility to introduce laser ions in the periodic structure, and to use bigger cross-size samples without poling. *Corresponding author. Tel.: +34-91-397-4784; fax: +3491-397-8579. E-mail address: [email protected] (M.D. Serrano).
In the integrate-optic field, it is necessary to work with waveguide geometry, in order to incorporate all the components in an optical circuit [5,6]. In this way, PPLN samples have to be used as waveguide, in order to connect the samples with the conventional optoelectronic devices. There are so many techniques to prepare waveguides over LN, and also over PPLN crystals, but each one has its own drawback [7–9]. Among them, the liquid phase epitaxy (LPE) technique is based on the epitaxial growth of LN over a crystalline substrate, which can be either LN or other material with suitable properties. The film could be doped during growth with optically active ions, like rear earths (RE), opening up the possibility of creating a laser-waveguide. The refractive index profile between the film and substrate is stepped allowing to obtain good results in terms of light propagation [10]. Moreover, it is possible to control the film depth to solve the problems associated with waveguides obtained by diffusion techniques [11].
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 9 6 7 - 4
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
The aim of this work is to study the LPE process over doped PPLN substrates in order to use this technique to growth crystalline films to prepare QPM-waveguides devices.
2. Experimental procedure LN thin films were grown on erbium (Er)-doped PPLN substrates. The substrates were cut from an Er:PPLN bulk crystal grown by the off-centred Cz technique in the a direction and were optically polished in order to obtain a flat surface [12]. The ferroelectric structure of the substrates was verified by chemical etching. The set-up used for the film preparation was a Czochralski modified equipment [13], consisted of a resistance furnace, 5 cm in diameter, with two thermocouples located at the furnace wall and at the bottom of the crucible, respectively. The mass variation during the growth process was recorded using a computer connected with a balance supporting the Pt crucible. A Pt square foil of 10 10 mm2 was used as a substrate holder, which can be moved vertically with a speed of several mm/h, allowing a very precise control of contact between the substrate and the melt. The flux used was 55 mol% of Li2CO3, 40 mol% of V2O5 and 5 mol% of Nb2O5, which reduces the growth temperature to 8501C [10]. The flux was homogenised at 9501C during 10 or 40 h depending on the experiment. After the homogenisation process was completed, the temperature was lowered at a rate of 201C/h to 501C above the growth temperature and then at a rate of 101C/h down to the growth temperature. Larger cooling rates produce spontaneous nucleation due to supercooling. The growth temperature in this experiments ranged from 7301C to 8901C. The substrate was placed vertically in the holder for the contact with the melt. When the film growth was completed, the substrate was withdrawn from the melt and cooled down to room temperature at a rate of 401C/h. The flux remaining attached to the sample was removed with water. The crystalline quality of the film was studied by high resolution X-ray diffraction (HRXRD) in a Siemens D5000HR diffractometer using the
597
(0 0.1 2) reflection of the Z face with a resolution of 0.00021. The film composition was analysed by wave dispersion X-ray (WDX) analysis in a Hewlett Packard JXA-8900 M microscope, also used as scanning electron microscope (SEM). The profile of the surface films was studied by a mechanical profilometer.
3. Results and discussion We have performed several LPE process in order to achieve the optimal growth conditions, either in temperature and time of the epitaxial process. The growth temperature was ranged between 7301C and 8901C. For the highest temperatures there were not epitaxial growth, whereas for the lowest temperatures, the velocity of the process is too fast, the surface exhibits islands and the quality of the surface is not good enough to use the samples as QPM-waveguide devices. Fig. 1 shows the Rocking Curves (HRXRD) spectra of two samples prepared at two different temperatures but with the same growth duration. The first curve (a) corresponds to a sample grown at 7701C, during 10 min and presents a shoulder at the right part of the substrate peak. Taking into account the results, we can estimate that the thickness of the film is around 1.5 m [14]. The signal due to the film is weak and it is not possible to determine the FWHM. The second curve (b) shows the HRXRD of a sample grown at 8401C during the same period of time. The intensity of the shoulder in this curve is lower than the previous one, and it is difficult to discriminate it from the substrate peak. This means that the last conditions give films thinner than the first one, and we can obtain thickness smaller than 1 m [14]. After the LPE process on a PPLN substrate the surface of the samples presents a periodic structure, which reproduce the periodic ferroelectric structure of the substrate. It is possible to explain this periodic structure due to the different growth velocities between the positive and the negative ferroelectric domain, the epitaxial process is faster on the positive domain than on the negative one [15]. Fig. 2 shows both a SEM image and the
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
598
80000
(00 12)
Substrate peak
70000
Intensity (a.u.)
60000 50000 40000 30000
Film signal
20000 10000 0 -10000 41.35
41.40
41.45
41.50
41.55
2 θ (degree)
(a) 160000 140000
(0 0 1 2 )
Substrate peak
In te n s ity (a .u .)
120000 100000 80000 60000 40000
Film signal
20000 0 -2 0 0 0 0 4 1 .3 5
4 1 .4 0
(b)
4 1 .4 5
4 1 .5 0
2 θ (degree)
Fig. 1. Rocking curve spectra of LPE samples grown at (a) 7701C and (b) 8401C during 10 min.
profilometer curve of the film surface prepared at 8401C during 10 min, and for this sample it is possible to observe that the difference in height between the film grown on a positive and a negative domain, is less than 0.3 m. This fact opens up the possibility to use this kind of superficial structure as ridge waveguides, very useful to achieve higher efficiencies in the QPM process [6]. The ferroelectric analysis of the surface after the LPE process shows that the whole film presents a negative face, which means that the positive domain in these processes have suffered a domain inversion. In the negative domain there are not domain inversion and the ferroelectric direction of the substrate is maintained in the film. After the structural characterization, the samples were studied by WDX, in order to obtain the composition of the films. Fig. 3 shows a WDX
scanning of the film prepared at 8401C during 10 min. It is possible to observe the profile of the films after the LPE process, and the periodic variation of the Nb and Er signal by WDX analysis. The Nb variation through the surface of the film is due to the Nb variation in the PPLN substrate. In a typical PPLN crystal, the Nb composition varies between the positive and negative domain, the concentration of Nb in a negative domain is higher than in a positive one [16]. Besides, the composition of the films prepared by LPE technique depends on the stoichiometry of the substrate, where a higher content of Li means more Li in the film [13]. This fact explains the Nb behaviour of our films. On the other hand, the Er signal varies through the periodic structure due to the different thickness of the film. The WDX technique shows the composition up to 2 m in
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
Fig. 2. (a) Profile and (b) SEM picture of the film surface prepared at 8401C during 10 min.
Er Nb
12000
In te n s ity ( a .u .)
10000
8000
6000
4000
2000
Surface structure
0 0
10
20
30
40
50
Width (microns)
Fig. 3. WDX analysis of Nb and Er of the film prepared at 8401C during 10 min.
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600
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
Fig. 4. SEM image of a film prepared at 8401C during 40 min.
depth of the samples, and these films are thinner than 2 m, so the Er of the substrate is also observed. This fact explains the presence of an Er signal in our WDX spectrum, and its variation through the surface. The Er signal is smaller in the positive domain, where the film thickness is bigger than on the negative one, due to a smaller penetration in the substrate. For longer LPE processes, we have also observed growth of LN on the walls of the periodic structure. Fig. 4 shows a SEM picture of the film surface of a sample prepared at 8401C during 40 min. Due to this lateral growth, the regions of faster growth velocity corresponding to positive domains of PPLN substrate are brocaded. This fact smoothes over the surface of the films, opening up the possibility to prepare waveguides with a flat surface.
4. Conclusions We have determined the better possible growth conditions for obtaining high quality LN films over Er:PPLN substrates by LPE technique. It is possible to prepare samples for ridge waveguides using their superficial structure. Further investiga-
tions to study the waveguide properties of these samples are necessary in order to optimise the quality of the film and to study the effect of longer growth periods to obtain films with a flat surface to use as usual waveguides.
Acknowledgements One of the authors (D.C.) acknowledges the ! y Ciencia for a fellowship Ministerio de Educacion in the frame of the ESP98-1340 project fellowship. ! V.B. acknowledges the Comunidad Autonoma de Madrid for a postdoctoral fellowship.
References [1] J. Amin, V. Pruneri, J. Webjorn, P. Russell, D.C. Hanna, J.S. Wilkinson, Opt. Commun. 135 (1997) 41. [2] C. Balslev, L. Torner, Opt. Lett. 24 (1) (1999) 7. [3] M. Fujimura, Y.K. Shin, H. Nishihara, Jpn. J. Appl. Phys. 39 (2000) L288. [4] M. Fujimura, T. Kodama, T. Suhara, H. Nishihara, IEEE Phot. Tech. Lett. 12 (2000) 1041. [5] I.E. Barry, G.W. Ross, P.G.R. Smith, Appl. Phys. Lett. 74 (10) (1999) 1487. [6] R. Nevado, E. Cantelar, G. Lifante, F. Cusso, Jpn. J. Appl. Phys. 39 (2000) L488.
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601 [7] I. Cristiani, C. Liberale, V. Degiorgio, G. Tartarini, P. Bassi, Opt. Commun. 187 (2001) 263. [8] R.J. Holmes, D.M. Smyth, J. Appl. Phys. 55 (1984) 3531. [9] J. Ichikawa, S. Uda, K. Shimamura, T. Fukuda, Appl. Phys. Lett. 76 (2000) 1498. [10] H. Tamada, A. Yamada, M. Sayito, J. Appl. Phys. 70 (1991) 2536. [11] T. Kawaguchi, H. Kitayama, M. Imaeda, T. Fukuda, J. Crystal Growth 178 (1997) 524. [12] V. Bermudez, J. Capmany, J. Garcia-Sole, E. Dieguez, Appl. Phys. Lett. 73 (5) (1998) 593.
601
[13] V. Bermudez, M.D. Serrano, E. Dieguez, J. Crystal Growth 198/199 (1999) 526. ! [14] D. Callejo, S. Manotas, M.D. Serrano, V. Bermudez, F. Agull!o-Rueda, E. Di!eguez, J. Crystal Growth 226 (2001) 488. [15] A.A. Ballman, H. Brown, J. Crystal Growth 29 (1975) 289. [16] V. Bermudez, D. Callejo, F. Caccavale, F. Segato, F. Agullo-Rueda, E. Dieguez, Solid State Commun. 114 (2000) 555.
Lithium niobate films on periodic poled lithium niobate substrates prepared by liquid phase epitaxy ! D. Callejo, V. Bermudez, M.D. Serrano*, E. Die! guez ! Departamento of F!ısica de Materiales, Facultad de Ciencias, Universidad Autonoma de Madrid, Madrid 28049, Spain
Abstract Lithium niobate thin films were grown by liquid phase epitaxy technique on periodic poled lithium niobate substrates doped with erbium. The composition and structure of the films were characterised by wave dispersion X-ray analysis and high resolution X-ray diffraction, respectively. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Lm; 77.84.Dy Keywords: A1. High resolution X-ray diffraction; A3. Liquid phase epitaxy; B1. Niobates
1. Introduction Second harmonic generation (SHG) by quasiphase-matching (QPM) in LiNbO3 (LN) via periodic modulation of the non-linear coefficient, provides an attractive route for the realisation of blue and green light sources with several mW of output power [1,2]. Electric-field poling has been used extensively for periodic domain reversal in ferroelectric materials, and hence modulation of the non-linear coefficient, but it has the limitation of the size of the samples and the use of laser dopant ions into the crystals [3,4]. However, periodic poled lithium niobate (PPLN) prepared by off-centred Czochralski (Cz) technique, has the possibility to introduce laser ions in the periodic structure, and to use bigger cross-size samples without poling. *Corresponding author. Tel.: +34-91-397-4784; fax: +3491-397-8579. E-mail address: [email protected] (M.D. Serrano).
In the integrate-optic field, it is necessary to work with waveguide geometry, in order to incorporate all the components in an optical circuit [5,6]. In this way, PPLN samples have to be used as waveguide, in order to connect the samples with the conventional optoelectronic devices. There are so many techniques to prepare waveguides over LN, and also over PPLN crystals, but each one has its own drawback [7–9]. Among them, the liquid phase epitaxy (LPE) technique is based on the epitaxial growth of LN over a crystalline substrate, which can be either LN or other material with suitable properties. The film could be doped during growth with optically active ions, like rear earths (RE), opening up the possibility of creating a laser-waveguide. The refractive index profile between the film and substrate is stepped allowing to obtain good results in terms of light propagation [10]. Moreover, it is possible to control the film depth to solve the problems associated with waveguides obtained by diffusion techniques [11].
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 9 6 7 - 4
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
The aim of this work is to study the LPE process over doped PPLN substrates in order to use this technique to growth crystalline films to prepare QPM-waveguides devices.
2. Experimental procedure LN thin films were grown on erbium (Er)-doped PPLN substrates. The substrates were cut from an Er:PPLN bulk crystal grown by the off-centred Cz technique in the a direction and were optically polished in order to obtain a flat surface [12]. The ferroelectric structure of the substrates was verified by chemical etching. The set-up used for the film preparation was a Czochralski modified equipment [13], consisted of a resistance furnace, 5 cm in diameter, with two thermocouples located at the furnace wall and at the bottom of the crucible, respectively. The mass variation during the growth process was recorded using a computer connected with a balance supporting the Pt crucible. A Pt square foil of 10 10 mm2 was used as a substrate holder, which can be moved vertically with a speed of several mm/h, allowing a very precise control of contact between the substrate and the melt. The flux used was 55 mol% of Li2CO3, 40 mol% of V2O5 and 5 mol% of Nb2O5, which reduces the growth temperature to 8501C [10]. The flux was homogenised at 9501C during 10 or 40 h depending on the experiment. After the homogenisation process was completed, the temperature was lowered at a rate of 201C/h to 501C above the growth temperature and then at a rate of 101C/h down to the growth temperature. Larger cooling rates produce spontaneous nucleation due to supercooling. The growth temperature in this experiments ranged from 7301C to 8901C. The substrate was placed vertically in the holder for the contact with the melt. When the film growth was completed, the substrate was withdrawn from the melt and cooled down to room temperature at a rate of 401C/h. The flux remaining attached to the sample was removed with water. The crystalline quality of the film was studied by high resolution X-ray diffraction (HRXRD) in a Siemens D5000HR diffractometer using the
597
(0 0.1 2) reflection of the Z face with a resolution of 0.00021. The film composition was analysed by wave dispersion X-ray (WDX) analysis in a Hewlett Packard JXA-8900 M microscope, also used as scanning electron microscope (SEM). The profile of the surface films was studied by a mechanical profilometer.
3. Results and discussion We have performed several LPE process in order to achieve the optimal growth conditions, either in temperature and time of the epitaxial process. The growth temperature was ranged between 7301C and 8901C. For the highest temperatures there were not epitaxial growth, whereas for the lowest temperatures, the velocity of the process is too fast, the surface exhibits islands and the quality of the surface is not good enough to use the samples as QPM-waveguide devices. Fig. 1 shows the Rocking Curves (HRXRD) spectra of two samples prepared at two different temperatures but with the same growth duration. The first curve (a) corresponds to a sample grown at 7701C, during 10 min and presents a shoulder at the right part of the substrate peak. Taking into account the results, we can estimate that the thickness of the film is around 1.5 m [14]. The signal due to the film is weak and it is not possible to determine the FWHM. The second curve (b) shows the HRXRD of a sample grown at 8401C during the same period of time. The intensity of the shoulder in this curve is lower than the previous one, and it is difficult to discriminate it from the substrate peak. This means that the last conditions give films thinner than the first one, and we can obtain thickness smaller than 1 m [14]. After the LPE process on a PPLN substrate the surface of the samples presents a periodic structure, which reproduce the periodic ferroelectric structure of the substrate. It is possible to explain this periodic structure due to the different growth velocities between the positive and the negative ferroelectric domain, the epitaxial process is faster on the positive domain than on the negative one [15]. Fig. 2 shows both a SEM image and the
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
598
80000
(00 12)
Substrate peak
70000
Intensity (a.u.)
60000 50000 40000 30000
Film signal
20000 10000 0 -10000 41.35
41.40
41.45
41.50
41.55
2 θ (degree)
(a) 160000 140000
(0 0 1 2 )
Substrate peak
In te n s ity (a .u .)
120000 100000 80000 60000 40000
Film signal
20000 0 -2 0 0 0 0 4 1 .3 5
4 1 .4 0
(b)
4 1 .4 5
4 1 .5 0
2 θ (degree)
Fig. 1. Rocking curve spectra of LPE samples grown at (a) 7701C and (b) 8401C during 10 min.
profilometer curve of the film surface prepared at 8401C during 10 min, and for this sample it is possible to observe that the difference in height between the film grown on a positive and a negative domain, is less than 0.3 m. This fact opens up the possibility to use this kind of superficial structure as ridge waveguides, very useful to achieve higher efficiencies in the QPM process [6]. The ferroelectric analysis of the surface after the LPE process shows that the whole film presents a negative face, which means that the positive domain in these processes have suffered a domain inversion. In the negative domain there are not domain inversion and the ferroelectric direction of the substrate is maintained in the film. After the structural characterization, the samples were studied by WDX, in order to obtain the composition of the films. Fig. 3 shows a WDX
scanning of the film prepared at 8401C during 10 min. It is possible to observe the profile of the films after the LPE process, and the periodic variation of the Nb and Er signal by WDX analysis. The Nb variation through the surface of the film is due to the Nb variation in the PPLN substrate. In a typical PPLN crystal, the Nb composition varies between the positive and negative domain, the concentration of Nb in a negative domain is higher than in a positive one [16]. Besides, the composition of the films prepared by LPE technique depends on the stoichiometry of the substrate, where a higher content of Li means more Li in the film [13]. This fact explains the Nb behaviour of our films. On the other hand, the Er signal varies through the periodic structure due to the different thickness of the film. The WDX technique shows the composition up to 2 m in
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
Fig. 2. (a) Profile and (b) SEM picture of the film surface prepared at 8401C during 10 min.
Er Nb
12000
In te n s ity ( a .u .)
10000
8000
6000
4000
2000
Surface structure
0 0
10
20
30
40
50
Width (microns)
Fig. 3. WDX analysis of Nb and Er of the film prepared at 8401C during 10 min.
599
600
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601
Fig. 4. SEM image of a film prepared at 8401C during 40 min.
depth of the samples, and these films are thinner than 2 m, so the Er of the substrate is also observed. This fact explains the presence of an Er signal in our WDX spectrum, and its variation through the surface. The Er signal is smaller in the positive domain, where the film thickness is bigger than on the negative one, due to a smaller penetration in the substrate. For longer LPE processes, we have also observed growth of LN on the walls of the periodic structure. Fig. 4 shows a SEM picture of the film surface of a sample prepared at 8401C during 40 min. Due to this lateral growth, the regions of faster growth velocity corresponding to positive domains of PPLN substrate are brocaded. This fact smoothes over the surface of the films, opening up the possibility to prepare waveguides with a flat surface.
4. Conclusions We have determined the better possible growth conditions for obtaining high quality LN films over Er:PPLN substrates by LPE technique. It is possible to prepare samples for ridge waveguides using their superficial structure. Further investiga-
tions to study the waveguide properties of these samples are necessary in order to optimise the quality of the film and to study the effect of longer growth periods to obtain films with a flat surface to use as usual waveguides.
Acknowledgements One of the authors (D.C.) acknowledges the ! y Ciencia for a fellowship Ministerio de Educacion in the frame of the ESP98-1340 project fellowship. ! V.B. acknowledges the Comunidad Autonoma de Madrid for a postdoctoral fellowship.
References [1] J. Amin, V. Pruneri, J. Webjorn, P. Russell, D.C. Hanna, J.S. Wilkinson, Opt. Commun. 135 (1997) 41. [2] C. Balslev, L. Torner, Opt. Lett. 24 (1) (1999) 7. [3] M. Fujimura, Y.K. Shin, H. Nishihara, Jpn. J. Appl. Phys. 39 (2000) L288. [4] M. Fujimura, T. Kodama, T. Suhara, H. Nishihara, IEEE Phot. Tech. Lett. 12 (2000) 1041. [5] I.E. Barry, G.W. Ross, P.G.R. Smith, Appl. Phys. Lett. 74 (10) (1999) 1487. [6] R. Nevado, E. Cantelar, G. Lifante, F. Cusso, Jpn. J. Appl. Phys. 39 (2000) L488.
D. Callejo et al. / Journal of Crystal Growth 237–239 (2002) 596–601 [7] I. Cristiani, C. Liberale, V. Degiorgio, G. Tartarini, P. Bassi, Opt. Commun. 187 (2001) 263. [8] R.J. Holmes, D.M. Smyth, J. Appl. Phys. 55 (1984) 3531. [9] J. Ichikawa, S. Uda, K. Shimamura, T. Fukuda, Appl. Phys. Lett. 76 (2000) 1498. [10] H. Tamada, A. Yamada, M. Sayito, J. Appl. Phys. 70 (1991) 2536. [11] T. Kawaguchi, H. Kitayama, M. Imaeda, T. Fukuda, J. Crystal Growth 178 (1997) 524. [12] V. Bermudez, J. Capmany, J. Garcia-Sole, E. Dieguez, Appl. Phys. Lett. 73 (5) (1998) 593.
601
[13] V. Bermudez, M.D. Serrano, E. Dieguez, J. Crystal Growth 198/199 (1999) 526. ! [14] D. Callejo, S. Manotas, M.D. Serrano, V. Bermudez, F. Agull!o-Rueda, E. Di!eguez, J. Crystal Growth 226 (2001) 488. [15] A.A. Ballman, H. Brown, J. Crystal Growth 29 (1975) 289. [16] V. Bermudez, D. Callejo, F. Caccavale, F. Segato, F. Agullo-Rueda, E. Dieguez, Solid State Commun. 114 (2000) 555.