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Optical properties of Tb implantation into ZnO
Surface & Coatings Technology 201 (2007) 8534 – 8538 www.elsevier.com/locate/surfcoat
Optical properties of Tb implantation into ZnO A. Çetin a , R. Kibar a , M. Ayvacıklı b , Y. Tuncer b , Ch. Buchal c , P.D. Townsend d , T. Karali e , S. Selvi a , N. Can b,⁎ b
a Ege University, Science Faculty, Department of Physics, 35100 Bornova-İzmir, Turkey Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, 45140 Muradiye, Manisa, Turkey c Forschungszentrum Juelich, ISG1-IT and CNI, D-52425 Juelich, Germany d Science and Technology, University of Sussex, BN1 9QH Brighton, UK e Ege University, Institute of Nuclear Sciences, 35100 Bornova, Izmir, Turkey
Available online 12 March 2007
Abstract ZnO [0001] single crystals were implanted at room temperature with 400 keV Tb+ ions at fluences in the range of 1 × 1016–2 × 1017 ions/cm2. Zinc oxide was chosen because of its potential for photonic applications as a semiconductor with high radiation resistance. After implantation and post-irradiation annealing, optical absorption was measured in a UV–VIS–NIR range and radioluminescence spectra were recorded at room temperature. Emission signals were generated by the Tb+ ion implants and intrinsic emission of the ZnO matrix were observed. The implant signal intensities were comparable with the host radioluminescence, even though the implants modify the surface of the crystal. It is suggested that the presence of Tb at high concentration generates stresses which influence the bulk material and also potentially forms precipitates or nanoparticles in the near surface region. Overall ion implantation of ZnO results in strongly modified luminescence. © 2007 Published by Elsevier B.V. PACS: 78.60.-h; 78.60.-hk; 68.55.-ln Keywords: Radioluminescence; Nanoparticles; ZnO; Ion implantation
1. Introduction There are ongoing developments of applications of new luminescent materials [1]. Many studies have focussed on the alternatives of radioluminescence, thermoluminescence and cathodoluminescence of rare earth (RE) and transition metal (TM) ions as activators of luminescence. Whilst many of the RE and TM ions may be introduced into different hosts, such as oxides, sulfides or semiconductors there are inherent problems of thermodynamic equilibrium if the dopants are at high concentration. High doping levels may result in formation of extensive defect aggregates, precipitation of different phases or impurity exclusion into embedded nanoparticles. Nanoparticle formation can reduce the luminescence efficiency of RE ions in oxide hosts [2] or, as in studies of manganese-doped material provide doped nanocrystals, exhibit specific new properties [3,4]. More recently it has been shown that ZnO is an unexpectedly interesting host for dispersed Cu (possibly in the form of nanoparticles), not least ⁎ Corresponding author. Fax: +90 236 2412158. E-mail address: [email protected] (N. Can). 0257-8972/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.02.066
because the optical features may be compatible with the semiconductor aspects of ZnO [5,6]. RE ions are potentially better candidates for luminescent centres than manganese and copper because of their special 4f intrashell transitions which can result in intense line spectra. Implantation not only places the RE ions at a specific depth beneath the surface but also introduces intense intrinsic lattice damage and/or amorphisation. In order to remove these unwanted effects it is a normal practice to use high temperature annealing to remove lattice damage but this may enable the RE ions to cluster in the form of nanoparticle precipitates [2,7]. For luminescence or laser applications one can circumvent this loss of signal by pulsed laser anneals [8]. There are thus several competing factors which make predictions of the luminescence behaviour of RE implanted material problematic. RE line emission is typical of isolated ions but such features often broaden into wide emission bands if the RE ions interact, as in a nanoparticle. Equally the RE ions may stabilise intrinsic defect sites which could also give broad emission features. To justify that there are nanoparticles one requires detailed transmission electron microscopy. Unfortunately this was not available for the present study but it will be shown that RE aggregation is a realistic outcome of the implants. For the ZnO
A. Çetin et al. / Surface & Coatings Technology 201 (2007) 8534–8538
work Tb ions have been used as they could provide optical luminescence devices in the blue and green spectral regions. There is already some other literature for these ions [9–12]. In this study, we have prepared Tb+ implanted ZnO nanocrystals and studied their optical and radioluminescence properties. 2. Experimental details The ZnO samples were implanted with Tb+ ions at room temperature at an energy of 400 keV to doses of 1 × 1016 to 2 × 1017 ions/cm2. Since the fluence was high the true surface temperature is increased during the implantation. Large doses of 1 × 1016 or 1 × 1017 ions/cm2 were chosen in the expectation that these concentrations exceed the thermodynamic solubility of Tb in ZnO and so after annealing they will precipitate into nanoparticles in the ZnO close to the crystal surface (i.e. the ion range is less than 100 nm). Some diffusion will occur both during the implants at a high dose rate and as the result of subsequent annealing as indicated in more detailed studies of Ag implants in glass and silica where these factors significantly changed the final size and depth distribution of the metal nanoparticles [13]. The ZnO samples were cut to 10 × 5 × 0.4 mm3 by MATECK in Germany. The implants were made with a high ion beam current density and the Tb ions were implanted into the (0001) oxygenface of single crystals. The ion implantation was performed nominally at room temperature in a vacuum of 10− 6 mbar on an EATON 3204 implanter in Juelich, Germany. Optical absorption spectra of ZnO and Tb-implanted ZnO samples were measured using a Perkin-Elmer spectrophotometer (Lambda 950). For radioluminescence (RL) measurements, X-ray irradiation is supplied by a Machlett OEG-50A tube. The system was run at 30 kV and 15 mA to provide intense RL signals. Light arriving at the detection system is dispersed by a Jobin Yvon spectrometer (Triax 552) and recorded by a CCD detector mounted at the exit of the spectrometer. The detector was cooled using liquid nitrogen during the experiment. The ZnO sample which was implanted with 1 × 1016 ions/cm2 Tb ions was annealed in 1 h steps from 700 to 1000 °C in air using a tube furnace and then quenched to room temperature. Rapid cooling of course influences the precipitation conditions and to some extent reduces the growth of nanoparticles. It must also be noted that the ZnO host may also undergo changes as the result of the heat treatments since there are suggestions that the structure is sensitive to distortions. The RL and optical absorption measurements were also taken following annealing treatments. The morphological features of the surface of the ZnO (0001) and Tb-implanted ZnO after annealing at 1000 °C were investigated by an Atomic Force Microscopy (AFM) (Shimadzu SPM-9500).
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un-implanted and low implant dose samples the broad bands thus arise from the host ZnO. The figure does not reveal the line spectra characteristic of isolated Tb ions but the main bands fall in regions where characteristic Tb3+ emissions can occur. These are described by the transitions from 5D3 and 5D4 to 7Fj ( j = 3–6), for which the most intense line is located at 545 nm from 5D4 to 7F5 transition. A small peak observed at about 390 nm for the ZnO sample decreased after Tb implantation, whilst the main peak at 545 nm increased significantly. Fig. 1 shows that since the lower energy features exist in the un-implanted material and increase after implantation (albeit with some wavelength shifts) one must accept that, as there was no Tb initially in the ZnO that these emission bands are basically intrinsic emission bands from the host lattice. The implant intensely damages the surface and introduces a shift of the band edge and so attenuates the bulk signal for higher energy emission (i.e. N 3 eV). The initial role of Tb is thus to introduce and increase the intrinsic defect density which results in the more intense emission but reduces the band-edge signals. This is an extremely surprising result since RL originates from a 0.4 mm thick sample yet the implant zone is one thousand times smaller. Therefore, either the implanted zone is immensely more efficient in generating luminescence, or the modified surface layer has become highly stressed and this has modified the entire sample. Such a stress option is feasible since there are wavelength displacements of the peak signals between the implanted and unimplanted ZnO. A surface modification which triggers a relaxation throughout the bulk is not inconceivable for ZnO and a closely related effect has been observed in luminescence studies of ion implanted SrTiO3 [14–16]. In order to analyse the component emission bands, the signals were transformed from the recorded wavelength data of I (λ)dλ versus λ into energy plots of I(E)dE versus E. An analysis into a set of overlapping Gaussian emission bands was then attempted, Fig. 2. Whilst the analysis clearly showed the presence of numerous features, there is still an uncertainty associated with the true number of components features, since, as will be shown, the spectra included differently resolved components after different annealing temperatures. In the present case, the first assumption is that the higher temperatures have resulted in more isolated Tb ions and their presence increases the luminescence intensity from Tb related
3. Results and discussion Radioluminescence spectra of ZnO: Tb3+ (1 × 1016 ions/cm2) are dominated by a broad band ranging from 1.77 eV to 2.28 eV and a small shoulder at 1.91 eV superimposed on the broad band as shown in Fig. 1. Note that in the RL spectra one has recorded a mixture of signals from throughout the bulk of the crystal as well as features generated in the very shallow implanted surface layer. In
Fig. 1. Comparison of radioluminescence spectra of pure and Tb-implanted ZnO in the range of 1.24 to 4 eV at room temperature.
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A. Çetin et al. / Surface & Coatings Technology 201 (2007) 8534–8538
Fig. 2. Radioluminescence spectra of pure ZnO and Tb-implanted ZnO recorded in the range of 1.24 to 3.5 eV at room temperature for different doses. Attempts to deconvolute the spectra with Gaussian band analyses are shown.
emission bands. However, there is always a doubt in such situations as to whether the RE ion itself is at the origin of the luminescence, or whether the presence of the ion stabilises an intrinsic type defect which is therefore more intense in the doped material. And in view of the preceding suggesting that surface stresses modify the bulk RL signals one must also consider if the stresses relate to the Tb distributions as ions or nanoparticle precipitates. In the present ZnO case, the peak positions and the number of components seem to change with annealing, so changes in the distribution of associated Tb ions and consequent stresses may dominate. The RL spectra of as-grown and Tb-implanted ZnO single crystals following annealing treatments are shown in Fig. 3. In the un-implanted sample there is a weak peak at 390 nm, which can be attributed to intrinsic excited exciton decay. After Tb implantation at a dose of 1 × 1016 ions/cm2, this peak decreased, and after annealing treatments from 900–1000 °C in steps of 50 °C for 1 h in air it almost disappeared. Whilst the peak at the 545 nm increases and shifts after implantation, the peak at the 740 nm increases rapidly after annealing at 900 °C.
Fig. 3. Radioluminescence data of as-grown and Tb-implanted ZnO single crystal following annealing treatments. The annealing treatments were done in steps of 50 °C from 900 to 1000 °C for 1 h in air. Implantation dose is 1 × 1016 ions/cm2.
A. Çetin et al. / Surface & Coatings Technology 201 (2007) 8534–8538
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Fig. 4. RL response of Tb-implanted ZnO as a function of implantation dose for the peak at 550 nm.
Fig. 6. Room temperature optical absorption spectra/μm of Tb-implanted ZnO at 2 × 1017 ions/cm2 after annealing in steps of 50° from 700 to 1000 °C.
Band-edge spectra of the ZnO single crystals shown in Fig. 4 reveal that the intensity of band-edge emission sequentially decreased with increasing Tb dose in the vertical direction. We repeated the experiment using Cu ions, and found that the bandedge intensity depended on the implanted ion mass [17]. These results indicate that the radioluminescence of ZnO is very sensitive to the ion irradiation, but not only where ions introduce defects near the ZnO surface. With increasing Tb3+ concentrations, the 400 nm emission corresponding to the transition from 5 D3 to 7F6 are remarkably quenched, which may be due to a cross-relaxation process between intrinsic defect transitions and those of Tb. The cross-relaxation process can be induced by the resonance between the excited states and the ground states of two Tb3+ ions described in the following equation [18]
Fig. 5 shows the absorption spectrum of the ZnO and ZnO: Tb prepared with the different doses varied from 1 × 1016 to 2 × 1017 ions/cm2 at room temperature. The sharp decrease was
Tb3þ ð5 D3 Þ þ Tb3þ ð7 F6 Þ→Tb3þ ð5 D4 Þ þ Tb3þ ð7 F0 Þ: From the equation, it can be proposed that green to red emissions at 545 nm and 650 nm correspond to transitions from 5D4 to 7F5 and 7F3 multiplet states of Tb3+ are involved. One assumes that any Tb3+ line emission is weak relative to the intense RL generated by the bulk of the ZnO. A future search for the line emission could be considered using cathodoluminescence to minimise bulk effects.
Fig. 5. Optical absorption spectra of ZnO and Tb-implanted ZnO for different doses at room temperature.
Fig. 7. a. AFM image of a ZnO (0001) polished surface. b. AFM image of Tbimplanted ZnO (1 × 1016 ions/cm2) surface after annealing at 1000 °C for 1 h.
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obtained at the 400 nm. This point is attributed to the band gap energy of ZnO. The absorption spectrum of Tb-implanted ZnO at a dose of 2 × 1017 ions/cm2 following annealing treatments from 700– 1000 °C in step of 50 °C for 1 h in air is shown in Fig. 6. Tbimplanted ZnO clearly does not show high transparency after 400 nm. From Fig. 6 it is possible to say that the annealing temperature can strongly influence optical absorption data and ZnO:Tb shows absorption in the visible range as the Tb content increases. The suggestion of surface distortions resulting from the implant will not only introduce stresses into the bulk but can also modify the surface topology. The implantation induced modification of the surface roughness was studied with AFM. AFM images from as-grown, Fig. 7a, and Tb-implanted sample, Fig. 7b were compared. The morphology of the surface changed dramatically. Implantation treatment caused an increment of the surface roughness. Such distortions are consistent with the proposal of long range stress effects. 4. Conclusion Tb ion implantation into ZnO amorphises the surface layer and generates a stress field which extends into the bulk. This alters the radioluminescence behaviour. Such bulk signal modifications are an unexpected, but not unprecedented, result. References [1] S. Kuboto, H. Hara, H. Yamane, M. Shimade, J. Electrochem. Soc. 149 (2002) H68.
[2] A. Polman, D.C. Jacobson, D.J. Eaglesham, R.C. Kistler, J.M. Poate, J Appl. Phys. 70 (1991) 3778. [3] N. Murase, R. Jagannathan, Y. Kanematsu, M. Watanabe, A. Kurite, K.H. Irata, T. Yazawa, T. Kushida, J. Phys. Chem., B 103 (1999) 754. [4] A.A. Bol, A. Meijerink, Phys. Rev., B 58 (1998) R15997. [5] A.L. Stepanov, R.I. Khaibulin, N. Can, R.A. Ganeev, A.I. Ryasnyansky, C. Buchal, S. Uysal, Tech. Phys. Lett. 30 (2004) 846. [6] T. Karali, N. Can, L. Valberg, A.L. Stepanov, P.D. Townsend, Ch. Buchal, R.A. Ganeev, A.I. Ryasnyansky, H.G. Belik, M.L. Jessett, C. Ong, Physica. B+C 363 (2005) 88. [7] P.D. Townsend, P.J. Chandler, L. Zhang, Optical Effects of Ion Implantation, Cambridge University Press, Cambridge, 1994. [8] N. Can, P.D. Townsend, D.E. Hole, H.V. Snelling, J.M. Ballesteros, C.N. Afonso, J. Appl. Phys. 78 (1995) 6737. [9] H. Nakagawa, K. Ebisu, M. Zang, M. Kitaura, J. Lumin. 102–103 (2003) 590. [10] H.J. Lozykowski, W.M. Jadwisienczak, Appl. Phys. Lett. 76 (2000) 861–863. [11] P.M. Guo, F. Zhao, G.B. Li, F.H. Liao, S.J. Tian, X.P. Jing, J. Lumin. 105 (2003) 6. [12] A. Deshkovskaya, Ch. Buchal, V. Komar, I. Skornyakov, Surf. Coat. Technol. 158–159 (2002) 513. [13] A.L. Stepanov, V.A. Zhikharev, D.E. Hole, P.D. Townsend, I.B. Khaibullin, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 166–167 (2000) 26. [14] B. Yang, P.D. Townsend, R. Fromknecht, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 217 (2004) 60. [15] B. Yang, P.D. Townsend, R. Fromknecht, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 226 (2004) 549. [16] B. Yang, P.D. Townsend, R. Fromknecht, J. Phys. Condens. Matter. 16 (2004) 8377. [17] A. Çetin, R. Kibar, M. Ayvacıklı, N. Can, Ch. Buchal, P.D. Townsend, T. Karali, S. Selvi, 17th International Conference on Ion Beam Analysis June 26 – July, , 2005 (Sevilla) Spain. [18] F.S. Kao, T.M. Chen, J. Lumin. 96 (2002) 261.
Optical properties of Tb implantation into ZnO A. Çetin a , R. Kibar a , M. Ayvacıklı b , Y. Tuncer b , Ch. Buchal c , P.D. Townsend d , T. Karali e , S. Selvi a , N. Can b,⁎ b
a Ege University, Science Faculty, Department of Physics, 35100 Bornova-İzmir, Turkey Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, 45140 Muradiye, Manisa, Turkey c Forschungszentrum Juelich, ISG1-IT and CNI, D-52425 Juelich, Germany d Science and Technology, University of Sussex, BN1 9QH Brighton, UK e Ege University, Institute of Nuclear Sciences, 35100 Bornova, Izmir, Turkey
Available online 12 March 2007
Abstract ZnO [0001] single crystals were implanted at room temperature with 400 keV Tb+ ions at fluences in the range of 1 × 1016–2 × 1017 ions/cm2. Zinc oxide was chosen because of its potential for photonic applications as a semiconductor with high radiation resistance. After implantation and post-irradiation annealing, optical absorption was measured in a UV–VIS–NIR range and radioluminescence spectra were recorded at room temperature. Emission signals were generated by the Tb+ ion implants and intrinsic emission of the ZnO matrix were observed. The implant signal intensities were comparable with the host radioluminescence, even though the implants modify the surface of the crystal. It is suggested that the presence of Tb at high concentration generates stresses which influence the bulk material and also potentially forms precipitates or nanoparticles in the near surface region. Overall ion implantation of ZnO results in strongly modified luminescence. © 2007 Published by Elsevier B.V. PACS: 78.60.-h; 78.60.-hk; 68.55.-ln Keywords: Radioluminescence; Nanoparticles; ZnO; Ion implantation
1. Introduction There are ongoing developments of applications of new luminescent materials [1]. Many studies have focussed on the alternatives of radioluminescence, thermoluminescence and cathodoluminescence of rare earth (RE) and transition metal (TM) ions as activators of luminescence. Whilst many of the RE and TM ions may be introduced into different hosts, such as oxides, sulfides or semiconductors there are inherent problems of thermodynamic equilibrium if the dopants are at high concentration. High doping levels may result in formation of extensive defect aggregates, precipitation of different phases or impurity exclusion into embedded nanoparticles. Nanoparticle formation can reduce the luminescence efficiency of RE ions in oxide hosts [2] or, as in studies of manganese-doped material provide doped nanocrystals, exhibit specific new properties [3,4]. More recently it has been shown that ZnO is an unexpectedly interesting host for dispersed Cu (possibly in the form of nanoparticles), not least ⁎ Corresponding author. Fax: +90 236 2412158. E-mail address: [email protected] (N. Can). 0257-8972/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.02.066
because the optical features may be compatible with the semiconductor aspects of ZnO [5,6]. RE ions are potentially better candidates for luminescent centres than manganese and copper because of their special 4f intrashell transitions which can result in intense line spectra. Implantation not only places the RE ions at a specific depth beneath the surface but also introduces intense intrinsic lattice damage and/or amorphisation. In order to remove these unwanted effects it is a normal practice to use high temperature annealing to remove lattice damage but this may enable the RE ions to cluster in the form of nanoparticle precipitates [2,7]. For luminescence or laser applications one can circumvent this loss of signal by pulsed laser anneals [8]. There are thus several competing factors which make predictions of the luminescence behaviour of RE implanted material problematic. RE line emission is typical of isolated ions but such features often broaden into wide emission bands if the RE ions interact, as in a nanoparticle. Equally the RE ions may stabilise intrinsic defect sites which could also give broad emission features. To justify that there are nanoparticles one requires detailed transmission electron microscopy. Unfortunately this was not available for the present study but it will be shown that RE aggregation is a realistic outcome of the implants. For the ZnO
A. Çetin et al. / Surface & Coatings Technology 201 (2007) 8534–8538
work Tb ions have been used as they could provide optical luminescence devices in the blue and green spectral regions. There is already some other literature for these ions [9–12]. In this study, we have prepared Tb+ implanted ZnO nanocrystals and studied their optical and radioluminescence properties. 2. Experimental details The ZnO samples were implanted with Tb+ ions at room temperature at an energy of 400 keV to doses of 1 × 1016 to 2 × 1017 ions/cm2. Since the fluence was high the true surface temperature is increased during the implantation. Large doses of 1 × 1016 or 1 × 1017 ions/cm2 were chosen in the expectation that these concentrations exceed the thermodynamic solubility of Tb in ZnO and so after annealing they will precipitate into nanoparticles in the ZnO close to the crystal surface (i.e. the ion range is less than 100 nm). Some diffusion will occur both during the implants at a high dose rate and as the result of subsequent annealing as indicated in more detailed studies of Ag implants in glass and silica where these factors significantly changed the final size and depth distribution of the metal nanoparticles [13]. The ZnO samples were cut to 10 × 5 × 0.4 mm3 by MATECK in Germany. The implants were made with a high ion beam current density and the Tb ions were implanted into the (0001) oxygenface of single crystals. The ion implantation was performed nominally at room temperature in a vacuum of 10− 6 mbar on an EATON 3204 implanter in Juelich, Germany. Optical absorption spectra of ZnO and Tb-implanted ZnO samples were measured using a Perkin-Elmer spectrophotometer (Lambda 950). For radioluminescence (RL) measurements, X-ray irradiation is supplied by a Machlett OEG-50A tube. The system was run at 30 kV and 15 mA to provide intense RL signals. Light arriving at the detection system is dispersed by a Jobin Yvon spectrometer (Triax 552) and recorded by a CCD detector mounted at the exit of the spectrometer. The detector was cooled using liquid nitrogen during the experiment. The ZnO sample which was implanted with 1 × 1016 ions/cm2 Tb ions was annealed in 1 h steps from 700 to 1000 °C in air using a tube furnace and then quenched to room temperature. Rapid cooling of course influences the precipitation conditions and to some extent reduces the growth of nanoparticles. It must also be noted that the ZnO host may also undergo changes as the result of the heat treatments since there are suggestions that the structure is sensitive to distortions. The RL and optical absorption measurements were also taken following annealing treatments. The morphological features of the surface of the ZnO (0001) and Tb-implanted ZnO after annealing at 1000 °C were investigated by an Atomic Force Microscopy (AFM) (Shimadzu SPM-9500).
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un-implanted and low implant dose samples the broad bands thus arise from the host ZnO. The figure does not reveal the line spectra characteristic of isolated Tb ions but the main bands fall in regions where characteristic Tb3+ emissions can occur. These are described by the transitions from 5D3 and 5D4 to 7Fj ( j = 3–6), for which the most intense line is located at 545 nm from 5D4 to 7F5 transition. A small peak observed at about 390 nm for the ZnO sample decreased after Tb implantation, whilst the main peak at 545 nm increased significantly. Fig. 1 shows that since the lower energy features exist in the un-implanted material and increase after implantation (albeit with some wavelength shifts) one must accept that, as there was no Tb initially in the ZnO that these emission bands are basically intrinsic emission bands from the host lattice. The implant intensely damages the surface and introduces a shift of the band edge and so attenuates the bulk signal for higher energy emission (i.e. N 3 eV). The initial role of Tb is thus to introduce and increase the intrinsic defect density which results in the more intense emission but reduces the band-edge signals. This is an extremely surprising result since RL originates from a 0.4 mm thick sample yet the implant zone is one thousand times smaller. Therefore, either the implanted zone is immensely more efficient in generating luminescence, or the modified surface layer has become highly stressed and this has modified the entire sample. Such a stress option is feasible since there are wavelength displacements of the peak signals between the implanted and unimplanted ZnO. A surface modification which triggers a relaxation throughout the bulk is not inconceivable for ZnO and a closely related effect has been observed in luminescence studies of ion implanted SrTiO3 [14–16]. In order to analyse the component emission bands, the signals were transformed from the recorded wavelength data of I (λ)dλ versus λ into energy plots of I(E)dE versus E. An analysis into a set of overlapping Gaussian emission bands was then attempted, Fig. 2. Whilst the analysis clearly showed the presence of numerous features, there is still an uncertainty associated with the true number of components features, since, as will be shown, the spectra included differently resolved components after different annealing temperatures. In the present case, the first assumption is that the higher temperatures have resulted in more isolated Tb ions and their presence increases the luminescence intensity from Tb related
3. Results and discussion Radioluminescence spectra of ZnO: Tb3+ (1 × 1016 ions/cm2) are dominated by a broad band ranging from 1.77 eV to 2.28 eV and a small shoulder at 1.91 eV superimposed on the broad band as shown in Fig. 1. Note that in the RL spectra one has recorded a mixture of signals from throughout the bulk of the crystal as well as features generated in the very shallow implanted surface layer. In
Fig. 1. Comparison of radioluminescence spectra of pure and Tb-implanted ZnO in the range of 1.24 to 4 eV at room temperature.
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A. Çetin et al. / Surface & Coatings Technology 201 (2007) 8534–8538
Fig. 2. Radioluminescence spectra of pure ZnO and Tb-implanted ZnO recorded in the range of 1.24 to 3.5 eV at room temperature for different doses. Attempts to deconvolute the spectra with Gaussian band analyses are shown.
emission bands. However, there is always a doubt in such situations as to whether the RE ion itself is at the origin of the luminescence, or whether the presence of the ion stabilises an intrinsic type defect which is therefore more intense in the doped material. And in view of the preceding suggesting that surface stresses modify the bulk RL signals one must also consider if the stresses relate to the Tb distributions as ions or nanoparticle precipitates. In the present ZnO case, the peak positions and the number of components seem to change with annealing, so changes in the distribution of associated Tb ions and consequent stresses may dominate. The RL spectra of as-grown and Tb-implanted ZnO single crystals following annealing treatments are shown in Fig. 3. In the un-implanted sample there is a weak peak at 390 nm, which can be attributed to intrinsic excited exciton decay. After Tb implantation at a dose of 1 × 1016 ions/cm2, this peak decreased, and after annealing treatments from 900–1000 °C in steps of 50 °C for 1 h in air it almost disappeared. Whilst the peak at the 545 nm increases and shifts after implantation, the peak at the 740 nm increases rapidly after annealing at 900 °C.
Fig. 3. Radioluminescence data of as-grown and Tb-implanted ZnO single crystal following annealing treatments. The annealing treatments were done in steps of 50 °C from 900 to 1000 °C for 1 h in air. Implantation dose is 1 × 1016 ions/cm2.
A. Çetin et al. / Surface & Coatings Technology 201 (2007) 8534–8538
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Fig. 4. RL response of Tb-implanted ZnO as a function of implantation dose for the peak at 550 nm.
Fig. 6. Room temperature optical absorption spectra/μm of Tb-implanted ZnO at 2 × 1017 ions/cm2 after annealing in steps of 50° from 700 to 1000 °C.
Band-edge spectra of the ZnO single crystals shown in Fig. 4 reveal that the intensity of band-edge emission sequentially decreased with increasing Tb dose in the vertical direction. We repeated the experiment using Cu ions, and found that the bandedge intensity depended on the implanted ion mass [17]. These results indicate that the radioluminescence of ZnO is very sensitive to the ion irradiation, but not only where ions introduce defects near the ZnO surface. With increasing Tb3+ concentrations, the 400 nm emission corresponding to the transition from 5 D3 to 7F6 are remarkably quenched, which may be due to a cross-relaxation process between intrinsic defect transitions and those of Tb. The cross-relaxation process can be induced by the resonance between the excited states and the ground states of two Tb3+ ions described in the following equation [18]
Fig. 5 shows the absorption spectrum of the ZnO and ZnO: Tb prepared with the different doses varied from 1 × 1016 to 2 × 1017 ions/cm2 at room temperature. The sharp decrease was
Tb3þ ð5 D3 Þ þ Tb3þ ð7 F6 Þ→Tb3þ ð5 D4 Þ þ Tb3þ ð7 F0 Þ: From the equation, it can be proposed that green to red emissions at 545 nm and 650 nm correspond to transitions from 5D4 to 7F5 and 7F3 multiplet states of Tb3+ are involved. One assumes that any Tb3+ line emission is weak relative to the intense RL generated by the bulk of the ZnO. A future search for the line emission could be considered using cathodoluminescence to minimise bulk effects.
Fig. 5. Optical absorption spectra of ZnO and Tb-implanted ZnO for different doses at room temperature.
Fig. 7. a. AFM image of a ZnO (0001) polished surface. b. AFM image of Tbimplanted ZnO (1 × 1016 ions/cm2) surface after annealing at 1000 °C for 1 h.
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A. Çetin et al. / Surface & Coatings Technology 201 (2007) 8534–8538
obtained at the 400 nm. This point is attributed to the band gap energy of ZnO. The absorption spectrum of Tb-implanted ZnO at a dose of 2 × 1017 ions/cm2 following annealing treatments from 700– 1000 °C in step of 50 °C for 1 h in air is shown in Fig. 6. Tbimplanted ZnO clearly does not show high transparency after 400 nm. From Fig. 6 it is possible to say that the annealing temperature can strongly influence optical absorption data and ZnO:Tb shows absorption in the visible range as the Tb content increases. The suggestion of surface distortions resulting from the implant will not only introduce stresses into the bulk but can also modify the surface topology. The implantation induced modification of the surface roughness was studied with AFM. AFM images from as-grown, Fig. 7a, and Tb-implanted sample, Fig. 7b were compared. The morphology of the surface changed dramatically. Implantation treatment caused an increment of the surface roughness. Such distortions are consistent with the proposal of long range stress effects. 4. Conclusion Tb ion implantation into ZnO amorphises the surface layer and generates a stress field which extends into the bulk. This alters the radioluminescence behaviour. Such bulk signal modifications are an unexpected, but not unprecedented, result. References [1] S. Kuboto, H. Hara, H. Yamane, M. Shimade, J. Electrochem. Soc. 149 (2002) H68.
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