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Luminescence of a titanate compound under europium ion implantation
Nuclear Instruments and Methods in Physics Research B 326 (2014) 106–109
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Luminescence of a titanate compound under europium ion implantation O. Plantevin a,⇑, E. Oliviero a, G. Dantelle b, L. Mayer b a b
Centre de Sciences Nucléaires et de Sciences de la Matière CSNSM, Univ Paris-Sud, CNRS/IN2P3, 91405 Orsay Cedex, France Laboratoire de Physique de la Matière Condensée LPMC, Ecole Polytechnique, CNRS, 91128 Palaiseau, France
a r t i c l e
i n f o
Article history: Received 28 June 2013 Received in revised form 8 October 2013 Accepted 10 October 2013 Available online 27 January 2014 Keywords: Ion implantation Rare earth Photoluminescence Oxyde
a b s t r a c t The ability to incorporate europium ions in a near-surface layer of a nonlinear optical material KTiOPO4 by ion implantation is reported here. Europium diffusion as well as surface modification were characterized after annealing using RBS. The photoluminescence of the as-implanted samples indicates that the creation of defects gives rise to green visible emission centered about 550 nm. Annealing up to 1000 °C does not allow the oxidation to the 3+ valence state of the europium ion. However it is shown that annealing up to such high temperature gives rise to an intense near infra-red photoluminescence in the range 800–1100 nm in implanted samples at an optimal fluence of 2 1013 europium ions/cm2. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Ion implantation has been shown a versatile method for modifying surface properties of materials since it allows a precise control of dopant concentration and localization. It opens up the possibility to introduce purposely a new property in a material through the introduction of new elements or through designed strain. In the specific case of oxide materials, ion implantation showed to be of particular interest for the fabrication of optical waveguides and waveguide lasers [1]. Of particular interest is also the possibility to tailor optical emission property of a nonlinear material such as KTiOPO4. Indeed, crystals belonging to the family of potassium titanyl phosphate KTiOPO4 (KTP) have attracted a particular attention due to their excellent nonlinear optical properties, as second harmonic generation (SHG). These crystals show ferroelectric properties with a high Curie temperature (934 °C for KTP), and it is thought that the structural origin of the nonlinear properties is related to the presence of long chains that connects deformed TiO6 octahedra [2]. It was also previously shown that titanate octahedra and their 1D, 2D or 3D framework govern the luminescence properties of the titanates compound family [3,4]. The strong specifity of KTP within the titanate family is the way the titanate octahedra connect in zig-zag chains, sharing oxygen corners. On another hand, rare earth (RE) doped solid inorganic compounds have found considerable interest due to their wide range of applications in optoelectronic devices, such as optical amplifiers, lamp phosphors or micro lasers. Number of different ⇑ Corresponding author. E-mail address: [email protected] (O. Plantevin). 0168-583X/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.058
techniques for the introduction of the dopants are used and ion implantation can represent a physical alternative when a precise control of doping profile is looked for. The ion implantation of europium ions in different wide gap oxides as strontium titanate [5], LiNbO3 [6], sapphire or MgO [7] was studied and the optical transitions within the Eu3+ ions were measured using photoluminescence for the latters. On another hand, erbium ion photoluminescence was also observed at low temperature after implantation and annealing in potassium titanyl phosphate (KTP) and rubidium titanyl phosphate (RTP) [8,9]. We focus on this work on the photoluminescence properties of europium implanted KTP. 2. Experimental Potassium titanyl phosphate KTiOPO4 (KTP) crystals belong at room temperature to the mm2 class of the orthorhombic system with noncentrosymetric space group Pna21. Crystals of the KTP family are the best known nonlinear optical materials for frequency doubling of continuous-wave or pulsed Nd3+ laser radiation. The KTP single crystals were made from flux-growth process by Crystal Laser company. They were cut in 5 5 2 mm3 for ion implantation. No surface treatment was performed. The samples were implanted at CSNSM using the IRMA implantor [10]. An ion beam energy of 150 keV was used with europium ion fluences between 2 1013 ions/cm2 and 2 1014 ions/cm2. The implantations were performed with an ion current density below 3 lA/cm2, at room temperature. For comparison, an implantation was performed with gold ions accelerated at an energy of 500 keV and with a fluence of 1015 ions/cm2. Samples were annealed in air at 900 °C and 1000 °C using both rapid
107
O. Plantevin et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 106–109
3. Results
A. RBS As shown in Fig. 1 and detailed in Fig. 2, the heavier Eu ion is well separated from the Ti, K, P and O contributions of the bulk material. The vertical line in Fig. 2 indicates the energy (in channel units) of the backscattered He ions from Eu atoms situated at the sample surface. The energy shift DE indicated in Fig. 2 is due to the He ions energy loss when traveling from the sample surface to the Eu distribution profile, and allows to extract the depth and
3 -2
2E13 cm
1000 C 4h
-2
2E14 cm 900 C 4h Pristine KTP
20 O
yield (a.u.)
KTP REF -2
2E14 cm 900 C 4h 2000
-2
2E14 cm
DE
900 C RTA
Eu
-2
2E13 cm 1000 C 4h 1500
simulation 900 C 4h simulation 900 C RTA
1000
500
According to the SRIM simulation program [11] (http:// www.srim.org), the mean projected range for the europium ions at 150 keV, Rp, and straggling, dRp, are 53 nm and 15 nm respectively. The maximum Eu concentration for an ion fluence of 2 1014 Eu+ ions/cm2 is 5 1019 cm 3 at a depth of 53 nm, which is rather low, about 0.08 atomic %. On the other side, the total vacancy concentration following the simulation is estimated to have a maximum of 5.6 1020 cm 3, which corresponds to 0.75 atomic %. This value is expected to be much lower due to direct vacancyinterstitial recombination and only about 20% of the defects is generally estimated to survive at room temperature. Nuclear energy loss is directly responsible for defect creation in the irradiated samples. The nuclear energy loss in the near surface region is estimated about 300 eV/Å ion. Gold ions implanted with an energy of 500 keV give approximately the same nuclear energy loss, which allow to reproduce the defect creation in the sample. In that case, the mean projected range is 117 nm.
25x10
2500
yield (a.u.)
thermal annealing (RTA 1 min rise time, annealing for 1 min, 1 min cool time) and classical annealing (1 h rise time, annealing for 4 h, 1 h cool time). Rutherford backscattering (RBS) measurements were performed using a 1.4 MeV He+ ion beam produced at the accelerator ARAMIS in CSNSM, and backscattering was measured under an angle of 165°. Photoluminescence emission was measured at room temperature using the 488 nm line of a Coherent Ar laser and a Triax320 spectrometer from Horiba–Jobin Yvon equipped with a Photomultiplier Hamamatsu R928S for the visible range up to 800 nm and a LN2 cooled InGaAs detector for the infra-red region up to 1600 nm.
0 850
900
950
1000 Channel
1050
1100
1150
Fig. 2. Focus at the Europium peak position on the RBS spectra of a pristine KTP crystal and Eu implanted KTP as described in Fig. 1.
profile of europium ions inside the KTP matrix. From the simulation of the RBS measurements, as represented in Fig. 2, one obtains an in-depth shift of the europium profile after annealing at 900 °C with a maximum respectively at 58 nm and 64 nm for the RTA and classical annealing (4 h). After RTA, the Eu profile shows essentially diffusion towards the sample volume, without any contribution within the first 12 nm, as in the as-implanted sample. Differently, the sample annealed for a long time (4 h) at the same temperature shows Eu diffusion towards the sample surface as can be seen from Fig. 2. No double profile indicating a trapping of the implanted ions in the defect region was observed as reported in the case of KTP implanted with Yb and Er ions and annealed at 800 °C [12]. Also, the sample annealed at 900 °C for 4 h shows a higher first step starting at the Ti position, indicating a titanium surface enrichment, which is consistent with the TiO2 phase formation already discussed in Ref. [13]. Simulation of the RBS profile indicates phosphorus and potassium depletion within the first 50 nm as compared to the reference sample, with respectively 10 atomic % and 9 atomic % instead of 15% and 11% in the reference sample. Also, titanium enrichment can be estimated to be in the order of 14 atomic % at 50 nm depth instead of 10% in the reference sample. In the case of the sample implanted with 2 1013 ions/cm2 and annealed at 1000 °C, the Eu diffusion is much higher, leading to an uniform distribution profile from the surface to few hundreds of nanometer inside the sample volume as can be deduced from the RBS measurement shown in Figs. 1 and 2.
15
B. Photoluminescence P 10
K Ti
5
Eu
0 0
200
400
600
800
1000
1200
Channel Fig. 1. RBS spectra of a pristine KTP crystal and Eu implanted KTP at fluences of 2 1013 ions/cm2 and 2 1014 ions/cm2, after annealing respectively at 1000 °C and 900°C.
We performed optical absorption measurements on pristine as well as implanted with 2 1014 ions/cm2 europium ions, and the energy gap was found at about 380 nm (3.25 eV), consistent with the value of 3.2 eV usually reported. A higher slope of the absorption curve was observed indicating defect level creation within the band gap after implantation. Photoluminescence in the visible range from pristine KTP is shown in Fig. 3 as a continuous line, indicating some mechanisms most probably due to intrinsic defects are giving rise to visible luminescence in the range 510– 650 nm. After europium ion implantation, another luminescence contribution centered around 550 nm is observed as an additional component. Gold ion irradiation at 500 keV and with a fluence of 1015 ions/cm2 is also responsible for a very similar luminescence
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O. Plantevin et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 106–109
Au 1E15 cm
1400
-2
-2
Eu 1E14 cm
-2
Eu 2E13 cm
1200
-2
Eu 1E14 cm RTA 900 C
Intensity (a.u.)
1000
Pristine KTP
800 600 400 200 0 550
600 (nm)
650
700
Fig. 3. Photoluminescence emission spectra of pristine KTiOPO4 (KTP) crystals and KTP crystals implanted with Eu ions at an energy of 150 keV and fluences of 2 1013 ions/cm2 and 1014 ions/cm2, as well as KTP crystals implanted with Au ions at 500 keV and a fluence of 1015 ions/cm2 for comparison. Spectrum of the sample implanted at 1014 ions/cm2 after rapid thermal annealing (RTA) at 900 °C for 1 min is also represented. The excitation source consists in the 488 nm line of an Ar laser at 100 mW power.
Intensity (a.u.)
200
400
(a)
-2
(b)
Eu 2E14 cm annealed 1000 C 4h -2
KTP annealed 1000 C 4h KTP annealed 900 C 4h Pristine KTP
Eu 2E14 cm annealed 900 C 4h
300
-2
Eu 2E14 cm
Intensity (a.u.)
250
signal. This is a conclusive indication that the europium implanted samples give rise to green defect induced luminescence. After rapid thermal annealing (RTA) at 900 °C for 1 min, one observes a reduction of this luminescence, which is consistent with the disparition of irradiation defects that were formed during the ion implantation. One can notice the absence of any narrow lines in the photoluminescence spectrum that would sign the presence of Eu3+ ions. On another hand, the Eu2+ ion luminescence is expected to give rise to a rather broad component due to 4f65d1 to 4f7 transition which is known to be much dependent on host material and can be centered around various wavelengths from blue to red [14]. If present, this contribution is rather weak and difficult to separate from the defect induced luminescence. When comparing the luminescence spectra from the sample implanted at a fluence of 1014 ions/cm2 and annealed at 900 °C (RTA) with the pristine sample, we can observe a broad and weak component centered about 600 nm that may have its origin in the luminescence from Eu2+ ions. We also observed that a photoluminescence contribution is present in the near-IR (NIR) region, as already found [13]. We focus on this contribution in Fig. 4, where the range between 700 nm and 1300 nm was measured with a LN2 cooled InGaAs detector. In some spectra, narrow peaks around 1000 nm corresponding to the x/2 contribution from the incident laser beam at 488 nm and Raman peaks are present, depending on sample orientation. The pristine sample gives a rather weak NIR contribution, which increases after annealing at 1000 °C (4 h) as shown in Fig. 4(a). Ion implantation with a fluence of 2 1014 ions/cm2 does not change
150
100
Pristine KTP 200
100 50
0 800
900
1000 (nm) 2500
1100
1200
800
(c)
900
1000 (nm)
1100
1200
-2
Eu 2E13 cm annealed 1000 C 4h -2
2000
Eu 2E13 cm annealed 900 C 4h
Intensity (a.u.)
-2
Eu 2E13 cm 1500
Pristine KTP
1000
500
0 800
900
1000 (nm)
1100
1200
Fig. 4. Photoluminescence emission spectra in the infra-red region of pristine KTiOPO4 (KTP) crystals before and after annealing at 900 °C and 1000 °C for 4 h (a) and KTP crystals implanted with Eu ions at fluences of 2 1014 ions/cm2, before and after annealing at 900 °C and 1000 °C for 4 h (b), as well as KTP crystals implanted with Eu ions at fluences of 2 1013 ions/cm2, before and after annealing at 900 °C and 1000 °C for 4 h (c). The excitation source consists in the 488 nm line of an Ar laser at 100 mW power.
O. Plantevin et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 106–109
the NIR contribution as compared to the pristine sample, as well as after an annealing at 900 °C. However, after annealing at 1000 °C (4 h), this NIR contribution shows a strong increase by a factor 5 as shown in Fig. 4(b). This emission was ascribed to surface decomposition under annealing at high temperature, leaving a TiO2 enriched near-surface region, as evidenced from RBS measurements. Reduction of this TiO2 phase during the decomposition process of the surface could liberate Ti3+ ions giving rise to NIR luminescence, as for instance in Ti-sapphire lasers. Ion sputtering during implantation roughens the surface and can also modify the surface stoechiometry, which would explain the important increase of the NIR luminescence in the samples first implanted before annealing at high temperature. Even more surprising is the dramatic increase by a factor 50 of the NIR contribution as compared to the pristine sample for the sample implanted with a fluence of 2 1013 ions/cm2 and annealed at 1000 °C (4 h) as shown in Fig. 4(c). In that case an annealing at 900 °C is already responsible for a strong increase of the NIR photoluminescence signal. Implantation at 2 1014 ions/cm2 may reduce the thickness of the surface layer giving rise to the NIR photoluminescence signal because of stronger sputtering from the surface, and could explain the reason why a lower europium implantation fluence may give rise to a stronger optical emission. 4. Conclusion Photoluminescence measurements indicate the presence of optically active irradiation defects in the as-implanted KTP giving a broad luminescence signal centered around 550 nm (E = 2.25 eV) at room temperature. Annealing at 900 °C using both RTA or conventional furnace decreases this luminescence because of defect annealing without any indication of oxidation of the europium ions to the Eu3+ valence state, contrary to other oxide materials. Another broad and intense infra-red luminescence, centered
109
about 950 nm, appears after annealing implanted samples above 900 °C and may be ascribed to surface decomposition. We found that the NIR photoluminescence is stronger for samples implanted at a lower fluence. These results indicate the possibility to couple the nonlinear optical property of the KTP crystal with green visible or near infra-red emission, using ion implantation and annealing. This optical coupling could open new possibilities, for instance in term of biological imaging using functionalized KTP particles. Acknowledgment We thank J. Moeyaert, J. Bourçois and C. Bachelet for the help with the implantation. References [1] P.D. Townsend, P.J. Chandler, L. Zhang, Optical Effects of Ion Implantation, Cambridge Studies in Modern Optics: 13, Cambridge University Press, 1994. [2] N.I. Sorokina, V.I. Voronkova, Cryst. Rep. 52 (1) (2007) 80. [3] B. Bouma, G. Blasse, J. Phys. Chem. Solids 56 (2) (1995) 261. [4] G. Blasse, M. Wiegel, Mat. Chem. Phys. 41 (1995) 257. [5] S.M.M. Ramos, B. Canut, P. Moretti, P. Thevenard, D. Poker, Thin Solid Films 259 (1995) 113. [6] P. Moretti, B. Canut, S.M.M. Ramos, R. Brenier, P. Thevenard, D. Poker, J.B.M. Dacunha, L. Amaral, A. Vasquez, J. Mat. Res. 8 (10) (1993) 2679. [7] E. Alves, C. Marques, N. Franco, L.C. Alves, M. Peres, M.J. Soares, T. Monteiro, Nucl. Instr. Meth. Phys. Res. B 268 (2010) 3137. [8] Th. Opfermann, T. Bachmann, E. Lux, W. Wesch, Nucl. Instr. Meth. Phys. Res. B 127 (128) (1997) 483. [9] A. Kling, M. Rico, C. Zaldo, M. Aguiló, F. Díaz, Nucl. Instr. Meth. Phys. Res. B 218 (2004) 271. [10] J. Chaumont, F. Lalu, M. Salomé, A.-M. Lamoise, H. Bernas, Nucl. Instrum. Meth. Phys. Res. 189 (1981) 193. [11] J. Ziegler, J. Biersack, U. Littmark, The Stopping of Ions in Matter, Pergamon, New York, 1985. [12] K.M. Wang, P.J. Ding, W. Wang, W.A. Lanford, Y. Li, J.-S. Li, Y.-G. Liu, Appl. Phys. Lett. 64 (1994) 3101. [13] K.T. Stevens, N.C. Giles, L.E. Halliburton, Appl. Phys. Lett. 68 (1996) 897. [14] I. Sun Cho, D. Kyun Yim, C. Hyun Kwak, J. Sul An, H. Suk Roh, K. Sun Hong, J. Lumin. 132 (2012) 375.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Luminescence of a titanate compound under europium ion implantation O. Plantevin a,⇑, E. Oliviero a, G. Dantelle b, L. Mayer b a b
Centre de Sciences Nucléaires et de Sciences de la Matière CSNSM, Univ Paris-Sud, CNRS/IN2P3, 91405 Orsay Cedex, France Laboratoire de Physique de la Matière Condensée LPMC, Ecole Polytechnique, CNRS, 91128 Palaiseau, France
a r t i c l e
i n f o
Article history: Received 28 June 2013 Received in revised form 8 October 2013 Accepted 10 October 2013 Available online 27 January 2014 Keywords: Ion implantation Rare earth Photoluminescence Oxyde
a b s t r a c t The ability to incorporate europium ions in a near-surface layer of a nonlinear optical material KTiOPO4 by ion implantation is reported here. Europium diffusion as well as surface modification were characterized after annealing using RBS. The photoluminescence of the as-implanted samples indicates that the creation of defects gives rise to green visible emission centered about 550 nm. Annealing up to 1000 °C does not allow the oxidation to the 3+ valence state of the europium ion. However it is shown that annealing up to such high temperature gives rise to an intense near infra-red photoluminescence in the range 800–1100 nm in implanted samples at an optimal fluence of 2 1013 europium ions/cm2. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Ion implantation has been shown a versatile method for modifying surface properties of materials since it allows a precise control of dopant concentration and localization. It opens up the possibility to introduce purposely a new property in a material through the introduction of new elements or through designed strain. In the specific case of oxide materials, ion implantation showed to be of particular interest for the fabrication of optical waveguides and waveguide lasers [1]. Of particular interest is also the possibility to tailor optical emission property of a nonlinear material such as KTiOPO4. Indeed, crystals belonging to the family of potassium titanyl phosphate KTiOPO4 (KTP) have attracted a particular attention due to their excellent nonlinear optical properties, as second harmonic generation (SHG). These crystals show ferroelectric properties with a high Curie temperature (934 °C for KTP), and it is thought that the structural origin of the nonlinear properties is related to the presence of long chains that connects deformed TiO6 octahedra [2]. It was also previously shown that titanate octahedra and their 1D, 2D or 3D framework govern the luminescence properties of the titanates compound family [3,4]. The strong specifity of KTP within the titanate family is the way the titanate octahedra connect in zig-zag chains, sharing oxygen corners. On another hand, rare earth (RE) doped solid inorganic compounds have found considerable interest due to their wide range of applications in optoelectronic devices, such as optical amplifiers, lamp phosphors or micro lasers. Number of different ⇑ Corresponding author. E-mail address: [email protected] (O. Plantevin). 0168-583X/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.058
techniques for the introduction of the dopants are used and ion implantation can represent a physical alternative when a precise control of doping profile is looked for. The ion implantation of europium ions in different wide gap oxides as strontium titanate [5], LiNbO3 [6], sapphire or MgO [7] was studied and the optical transitions within the Eu3+ ions were measured using photoluminescence for the latters. On another hand, erbium ion photoluminescence was also observed at low temperature after implantation and annealing in potassium titanyl phosphate (KTP) and rubidium titanyl phosphate (RTP) [8,9]. We focus on this work on the photoluminescence properties of europium implanted KTP. 2. Experimental Potassium titanyl phosphate KTiOPO4 (KTP) crystals belong at room temperature to the mm2 class of the orthorhombic system with noncentrosymetric space group Pna21. Crystals of the KTP family are the best known nonlinear optical materials for frequency doubling of continuous-wave or pulsed Nd3+ laser radiation. The KTP single crystals were made from flux-growth process by Crystal Laser company. They were cut in 5 5 2 mm3 for ion implantation. No surface treatment was performed. The samples were implanted at CSNSM using the IRMA implantor [10]. An ion beam energy of 150 keV was used with europium ion fluences between 2 1013 ions/cm2 and 2 1014 ions/cm2. The implantations were performed with an ion current density below 3 lA/cm2, at room temperature. For comparison, an implantation was performed with gold ions accelerated at an energy of 500 keV and with a fluence of 1015 ions/cm2. Samples were annealed in air at 900 °C and 1000 °C using both rapid
107
O. Plantevin et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 106–109
3. Results
A. RBS As shown in Fig. 1 and detailed in Fig. 2, the heavier Eu ion is well separated from the Ti, K, P and O contributions of the bulk material. The vertical line in Fig. 2 indicates the energy (in channel units) of the backscattered He ions from Eu atoms situated at the sample surface. The energy shift DE indicated in Fig. 2 is due to the He ions energy loss when traveling from the sample surface to the Eu distribution profile, and allows to extract the depth and
3 -2
2E13 cm
1000 C 4h
-2
2E14 cm 900 C 4h Pristine KTP
20 O
yield (a.u.)
KTP REF -2
2E14 cm 900 C 4h 2000
-2
2E14 cm
DE
900 C RTA
Eu
-2
2E13 cm 1000 C 4h 1500
simulation 900 C 4h simulation 900 C RTA
1000
500
According to the SRIM simulation program [11] (http:// www.srim.org), the mean projected range for the europium ions at 150 keV, Rp, and straggling, dRp, are 53 nm and 15 nm respectively. The maximum Eu concentration for an ion fluence of 2 1014 Eu+ ions/cm2 is 5 1019 cm 3 at a depth of 53 nm, which is rather low, about 0.08 atomic %. On the other side, the total vacancy concentration following the simulation is estimated to have a maximum of 5.6 1020 cm 3, which corresponds to 0.75 atomic %. This value is expected to be much lower due to direct vacancyinterstitial recombination and only about 20% of the defects is generally estimated to survive at room temperature. Nuclear energy loss is directly responsible for defect creation in the irradiated samples. The nuclear energy loss in the near surface region is estimated about 300 eV/Å ion. Gold ions implanted with an energy of 500 keV give approximately the same nuclear energy loss, which allow to reproduce the defect creation in the sample. In that case, the mean projected range is 117 nm.
25x10
2500
yield (a.u.)
thermal annealing (RTA 1 min rise time, annealing for 1 min, 1 min cool time) and classical annealing (1 h rise time, annealing for 4 h, 1 h cool time). Rutherford backscattering (RBS) measurements were performed using a 1.4 MeV He+ ion beam produced at the accelerator ARAMIS in CSNSM, and backscattering was measured under an angle of 165°. Photoluminescence emission was measured at room temperature using the 488 nm line of a Coherent Ar laser and a Triax320 spectrometer from Horiba–Jobin Yvon equipped with a Photomultiplier Hamamatsu R928S for the visible range up to 800 nm and a LN2 cooled InGaAs detector for the infra-red region up to 1600 nm.
0 850
900
950
1000 Channel
1050
1100
1150
Fig. 2. Focus at the Europium peak position on the RBS spectra of a pristine KTP crystal and Eu implanted KTP as described in Fig. 1.
profile of europium ions inside the KTP matrix. From the simulation of the RBS measurements, as represented in Fig. 2, one obtains an in-depth shift of the europium profile after annealing at 900 °C with a maximum respectively at 58 nm and 64 nm for the RTA and classical annealing (4 h). After RTA, the Eu profile shows essentially diffusion towards the sample volume, without any contribution within the first 12 nm, as in the as-implanted sample. Differently, the sample annealed for a long time (4 h) at the same temperature shows Eu diffusion towards the sample surface as can be seen from Fig. 2. No double profile indicating a trapping of the implanted ions in the defect region was observed as reported in the case of KTP implanted with Yb and Er ions and annealed at 800 °C [12]. Also, the sample annealed at 900 °C for 4 h shows a higher first step starting at the Ti position, indicating a titanium surface enrichment, which is consistent with the TiO2 phase formation already discussed in Ref. [13]. Simulation of the RBS profile indicates phosphorus and potassium depletion within the first 50 nm as compared to the reference sample, with respectively 10 atomic % and 9 atomic % instead of 15% and 11% in the reference sample. Also, titanium enrichment can be estimated to be in the order of 14 atomic % at 50 nm depth instead of 10% in the reference sample. In the case of the sample implanted with 2 1013 ions/cm2 and annealed at 1000 °C, the Eu diffusion is much higher, leading to an uniform distribution profile from the surface to few hundreds of nanometer inside the sample volume as can be deduced from the RBS measurement shown in Figs. 1 and 2.
15
B. Photoluminescence P 10
K Ti
5
Eu
0 0
200
400
600
800
1000
1200
Channel Fig. 1. RBS spectra of a pristine KTP crystal and Eu implanted KTP at fluences of 2 1013 ions/cm2 and 2 1014 ions/cm2, after annealing respectively at 1000 °C and 900°C.
We performed optical absorption measurements on pristine as well as implanted with 2 1014 ions/cm2 europium ions, and the energy gap was found at about 380 nm (3.25 eV), consistent with the value of 3.2 eV usually reported. A higher slope of the absorption curve was observed indicating defect level creation within the band gap after implantation. Photoluminescence in the visible range from pristine KTP is shown in Fig. 3 as a continuous line, indicating some mechanisms most probably due to intrinsic defects are giving rise to visible luminescence in the range 510– 650 nm. After europium ion implantation, another luminescence contribution centered around 550 nm is observed as an additional component. Gold ion irradiation at 500 keV and with a fluence of 1015 ions/cm2 is also responsible for a very similar luminescence
108
O. Plantevin et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 106–109
Au 1E15 cm
1400
-2
-2
Eu 1E14 cm
-2
Eu 2E13 cm
1200
-2
Eu 1E14 cm RTA 900 C
Intensity (a.u.)
1000
Pristine KTP
800 600 400 200 0 550
600 (nm)
650
700
Fig. 3. Photoluminescence emission spectra of pristine KTiOPO4 (KTP) crystals and KTP crystals implanted with Eu ions at an energy of 150 keV and fluences of 2 1013 ions/cm2 and 1014 ions/cm2, as well as KTP crystals implanted with Au ions at 500 keV and a fluence of 1015 ions/cm2 for comparison. Spectrum of the sample implanted at 1014 ions/cm2 after rapid thermal annealing (RTA) at 900 °C for 1 min is also represented. The excitation source consists in the 488 nm line of an Ar laser at 100 mW power.
Intensity (a.u.)
200
400
(a)
-2
(b)
Eu 2E14 cm annealed 1000 C 4h -2
KTP annealed 1000 C 4h KTP annealed 900 C 4h Pristine KTP
Eu 2E14 cm annealed 900 C 4h
300
-2
Eu 2E14 cm
Intensity (a.u.)
250
signal. This is a conclusive indication that the europium implanted samples give rise to green defect induced luminescence. After rapid thermal annealing (RTA) at 900 °C for 1 min, one observes a reduction of this luminescence, which is consistent with the disparition of irradiation defects that were formed during the ion implantation. One can notice the absence of any narrow lines in the photoluminescence spectrum that would sign the presence of Eu3+ ions. On another hand, the Eu2+ ion luminescence is expected to give rise to a rather broad component due to 4f65d1 to 4f7 transition which is known to be much dependent on host material and can be centered around various wavelengths from blue to red [14]. If present, this contribution is rather weak and difficult to separate from the defect induced luminescence. When comparing the luminescence spectra from the sample implanted at a fluence of 1014 ions/cm2 and annealed at 900 °C (RTA) with the pristine sample, we can observe a broad and weak component centered about 600 nm that may have its origin in the luminescence from Eu2+ ions. We also observed that a photoluminescence contribution is present in the near-IR (NIR) region, as already found [13]. We focus on this contribution in Fig. 4, where the range between 700 nm and 1300 nm was measured with a LN2 cooled InGaAs detector. In some spectra, narrow peaks around 1000 nm corresponding to the x/2 contribution from the incident laser beam at 488 nm and Raman peaks are present, depending on sample orientation. The pristine sample gives a rather weak NIR contribution, which increases after annealing at 1000 °C (4 h) as shown in Fig. 4(a). Ion implantation with a fluence of 2 1014 ions/cm2 does not change
150
100
Pristine KTP 200
100 50
0 800
900
1000 (nm) 2500
1100
1200
800
(c)
900
1000 (nm)
1100
1200
-2
Eu 2E13 cm annealed 1000 C 4h -2
2000
Eu 2E13 cm annealed 900 C 4h
Intensity (a.u.)
-2
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1000
500
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Fig. 4. Photoluminescence emission spectra in the infra-red region of pristine KTiOPO4 (KTP) crystals before and after annealing at 900 °C and 1000 °C for 4 h (a) and KTP crystals implanted with Eu ions at fluences of 2 1014 ions/cm2, before and after annealing at 900 °C and 1000 °C for 4 h (b), as well as KTP crystals implanted with Eu ions at fluences of 2 1013 ions/cm2, before and after annealing at 900 °C and 1000 °C for 4 h (c). The excitation source consists in the 488 nm line of an Ar laser at 100 mW power.
O. Plantevin et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 106–109
the NIR contribution as compared to the pristine sample, as well as after an annealing at 900 °C. However, after annealing at 1000 °C (4 h), this NIR contribution shows a strong increase by a factor 5 as shown in Fig. 4(b). This emission was ascribed to surface decomposition under annealing at high temperature, leaving a TiO2 enriched near-surface region, as evidenced from RBS measurements. Reduction of this TiO2 phase during the decomposition process of the surface could liberate Ti3+ ions giving rise to NIR luminescence, as for instance in Ti-sapphire lasers. Ion sputtering during implantation roughens the surface and can also modify the surface stoechiometry, which would explain the important increase of the NIR luminescence in the samples first implanted before annealing at high temperature. Even more surprising is the dramatic increase by a factor 50 of the NIR contribution as compared to the pristine sample for the sample implanted with a fluence of 2 1013 ions/cm2 and annealed at 1000 °C (4 h) as shown in Fig. 4(c). In that case an annealing at 900 °C is already responsible for a strong increase of the NIR photoluminescence signal. Implantation at 2 1014 ions/cm2 may reduce the thickness of the surface layer giving rise to the NIR photoluminescence signal because of stronger sputtering from the surface, and could explain the reason why a lower europium implantation fluence may give rise to a stronger optical emission. 4. Conclusion Photoluminescence measurements indicate the presence of optically active irradiation defects in the as-implanted KTP giving a broad luminescence signal centered around 550 nm (E = 2.25 eV) at room temperature. Annealing at 900 °C using both RTA or conventional furnace decreases this luminescence because of defect annealing without any indication of oxidation of the europium ions to the Eu3+ valence state, contrary to other oxide materials. Another broad and intense infra-red luminescence, centered
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about 950 nm, appears after annealing implanted samples above 900 °C and may be ascribed to surface decomposition. We found that the NIR photoluminescence is stronger for samples implanted at a lower fluence. These results indicate the possibility to couple the nonlinear optical property of the KTP crystal with green visible or near infra-red emission, using ion implantation and annealing. This optical coupling could open new possibilities, for instance in term of biological imaging using functionalized KTP particles. Acknowledgment We thank J. Moeyaert, J. Bourçois and C. Bachelet for the help with the implantation. References [1] P.D. Townsend, P.J. Chandler, L. Zhang, Optical Effects of Ion Implantation, Cambridge Studies in Modern Optics: 13, Cambridge University Press, 1994. [2] N.I. Sorokina, V.I. Voronkova, Cryst. Rep. 52 (1) (2007) 80. [3] B. Bouma, G. Blasse, J. Phys. Chem. Solids 56 (2) (1995) 261. [4] G. Blasse, M. Wiegel, Mat. Chem. Phys. 41 (1995) 257. [5] S.M.M. Ramos, B. Canut, P. Moretti, P. Thevenard, D. Poker, Thin Solid Films 259 (1995) 113. [6] P. Moretti, B. Canut, S.M.M. Ramos, R. Brenier, P. Thevenard, D. Poker, J.B.M. Dacunha, L. Amaral, A. Vasquez, J. Mat. Res. 8 (10) (1993) 2679. [7] E. Alves, C. Marques, N. Franco, L.C. Alves, M. Peres, M.J. Soares, T. Monteiro, Nucl. Instr. Meth. Phys. Res. B 268 (2010) 3137. [8] Th. Opfermann, T. Bachmann, E. Lux, W. Wesch, Nucl. Instr. Meth. Phys. Res. B 127 (128) (1997) 483. [9] A. Kling, M. Rico, C. Zaldo, M. Aguiló, F. Díaz, Nucl. Instr. Meth. Phys. Res. B 218 (2004) 271. [10] J. Chaumont, F. Lalu, M. Salomé, A.-M. Lamoise, H. Bernas, Nucl. Instrum. Meth. Phys. Res. 189 (1981) 193. [11] J. Ziegler, J. Biersack, U. Littmark, The Stopping of Ions in Matter, Pergamon, New York, 1985. [12] K.M. Wang, P.J. Ding, W. Wang, W.A. Lanford, Y. Li, J.-S. Li, Y.-G. Liu, Appl. Phys. Lett. 64 (1994) 3101. [13] K.T. Stevens, N.C. Giles, L.E. Halliburton, Appl. Phys. Lett. 68 (1996) 897. [14] I. Sun Cho, D. Kyun Yim, C. Hyun Kwak, J. Sul An, H. Suk Roh, K. Sun Hong, J. Lumin. 132 (2012) 375.