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Photoluminescence properties of Sm-doped ZnO grown by sputtering-assisted metalorganic chemical vapor deposition
Journal of Non-Crystalline Solids 358 (2012) 2443–2445
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Photoluminescence properties of Sm-doped ZnO grown by sputtering-assisted metalorganic chemical vapor deposition Takahiro Tsuji ⁎, Yoshikazu Terai, Muhammad hakim bin Kamarudin, Masatoshi Kawabata, Yasufumi Fujiwara Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2–1 Yamadaoka, Suita, Osaka 565–0871, Japan
a r t i c l e
i n f o
Article history: Received 31 August 2011 Received in revised form 19 December 2011 Available online 23 January 2012 Keywords: Rare-earth doped semiconductors; MOCVD; ZnO; Photoluminescence
a b s t r a c t Sm-doped ZnO (ZnO:Sm) thin films with c-axis oriented wurzite structure were grown by sputteringassisted metalorganic chemical vapor deposition (SA-MOCVD). In the photoluminescence (PL) measurements of annealed ZnO:Sm, sharp emission lines from intra-4f transitions in Sm3+ ions were observed at room temperature under the excitation energy above the bandgap energy of ZnO (indirect excitation). In the dependence of PL intensity at 77 K on Sm concentration, the Sm 3+ PL intensity was the largest at Sm concentration of 0.4%. In time-resolved PL measurements, the lifetime of Sm3+ PL became short at higher Sm concentration than 0.4%. These results revealed that a quenching of the Sm3+ PL from ZnO:Sm was induced at higher Sm concentration than 0.4%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth (RE)-doped semiconductor is one of the promising materials for the active layer in electroluminescent (EL) devices [1,2]. In REdoped semiconductors, it is well known that an efficient excitation of RE ions is accomplished by the energy transfer from host semiconductors to doped RE ions. For example, Eu-doped GaN (GaN:Eu) shows an efficient red-emission lines originating from intra-4f shell transitions of 5 D0–7FJ (J= 0–4) at room temperature (RT) [3]. As a new candidate of RE-doped semiconductor, ZnO has a potential for the active layer in visual light range due to the large bandgap of ~3.37 eV. For the development of ZnO-based EL devices, it is necessary to investigate what kinds of RE ions act as efficient luminescent centers in ZnO. In our previous report [4], we have demonstrated red-emission lines from Eu 3+ ions at RT in Eu-doped ZnO (ZnO:Eu) under the excitaion energy above the bandgap of ZnO (indirect excitation). In the ZnO:Eu, the energy transfer from ZnO to Eu3+ ions was confirmed by the photoluminescence excitation spectra (PLE) monitored at 615 nm (5D0–7F2) [5]. The result has opened new research area of RE-doped ZnO. In this study, we have investigated a Sm-doping into ZnO as another candidate of luminescent centers in ZnO. In RE-doped ZnS, it has been reported that Sm 3+ ions act as effective luminescent centers in the red region, but Eu3+ does not show the red-emission lines [6]. The result indicates a possibility that Sm 3+ in ZnO shows a strong photoluminescence (PL) in comparison with Eu3+ in ZnO. The reports of Sm-doped ZnO (ZnO:Sm) have been limited to polycrystalline [7,8] ⁎ Corresponding author. E-mail addresses: [email protected] (T. Tsuji), [email protected] (Y. Terai). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.099
and nanostructures [9], but there is no report on the growth of ZnO: Sm thin films with c-axis oriented wurzite structure. The growth of high-quality ZnO:Sm thin films is necessary to investigate the intrinsic PL properties of Sm3+. In addition, the dependence of Sm 3+ PL properties on Sm concentration has not been investigated. At high Sm concentration, the Sm3+ PL will be quenched by nonradiative recombination processes induced by Sm-doping [10]. It is necessary for the development of EL devices to investigate the suitable doping concentration of Sm 3+ ions. Therefore, we tried the growth of ZnO:Sm thin films with different Sm concentrations by sputtering-assisted metalorganic chemical vapor deposition (SA-MOCVD). In the ZnO:Sm films, the Smconcentration dependence of PL properties under indirect excitation has been investigated by PL and time-resolved PL (TR-PL) measurements. 2. Experimental ZnO:Sm films with a thickness of 650–800 nm were grown on cplane sapphire substrates by SA-MOCVD [11]. Diethylzinc (DEZn) of 0.3 sccm and O2 of 10.0 sccm were introduced on the substrate with Ar carrier gas of 5.0 sccm, resulting in the ZnO growth by the thermal reaction of DEZn and O2. During the growth of ZnO, Sm atoms were doped by RF magnetron sputtering of a Sm2O3 sintered target, which was fixed at the top of the substrate. In the growth technique, the Sm concentration could be controlled by RF power and distance (L) between the target and the substrate. In this study, the Sm2O3 target was sputtered at RF power of 20–60 W, L = 100 mm during the ZnO growth at substrate temperature of 550 °C. The Sm concentration ([Sm]) in the ZnO:Sm was obtained to be [Sm]= 0.2–3.8% in energy dispersive X-ray spectroscopy (EDX) measurements. After the growth, the films were annealed at 600 °C for 30 min in O2 ambient.
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T. Tsuji et al. / Journal of Non-Crystalline Solids 358 (2012) 2443–2445
The crystal structure of ZnO:Sm film was characterized by 2θ–θ X-ray diffraction (XRD) measurement using Cu Kα radiation. PL measurements were carried out at RT and 77 K. The films were excited by the 266 nm line of a Nd:YAG laser (1 ns pulse width, 11 kHz repetition) for the indirect excitation, and the PL was detected by an electronically cooled CCD camera with a 0.3 m monochromator. A tunable dye laser operating at 420 nm (1 ns pulse width, 10 Hz repetition) was used for the PL under ex6 6 4 citation wavelength (λext.) resonant to the H5/2–( P, P)5/2 (~420 nm) 3+ transition in Sm ions (direct excitation). In TR-PL measurements, a pulsed N2 laser with a wavelength of 337.1 nm (1 ns pulse width, 10 Hz repetition) was used as the excitation sources of the indirect excitation. The pulse energy was tuned to 10 μJ. The TR-PL spectra were detected by a gated ICCD camera with a 0.15 m monochromator in the time range of 0–3000 μs. 3. Results Fig. 1 shows the XRD patterns of ZnO:Sm films. The films show the high intensity of ZnO(0002) and ZnO(0004) peaks without other peaks peak, such as Sm2O3. In the pole figure measurement of ZnO 1011 two types of in-plane rotational domains of ZnO 1210 || sapphire || sapphire 1120 were observed in the ZnO: 1100 and ZnO 1010 Sm films. This corresponds to typical growth mode of undoped ZnO on c-plane sapphire substrate [12]. These results show that high-quality caxis oriented ZnO:Sm was successfully grown on the sapphire substrates. As Sm concentration increases, the lattice constant in c-axis obtained by the peak position of ZnO(0004) shifted from 5.207 Å in undoped ZnO to 5.219 Å in ZnO:Sm with [Sm] =3.8%. The expansion of lattice constant indicates that Sm ions with the large ionic radius replace the Zn site in ZnO. In addition, the full width at half maximum (FWHM) of ZnO(0004) peak becomes large at high Sm concentration, indicating a degradation of crystalline orientation by the Sm-doping. The local arrangement of Sm ions in the ZnO:Sm is not clear, but following a possibility is suggested. When the Sm ions replace the Zn site, the charge state of the Sm ions is trivalent. The Sm3+ acts as donor in the ZnO:Sm. In fact, the resistivity of Sm-doped ZnO (2.0 Ω cm) was three orders of magnitude lower than that of undoped ZnO (2000 Ω cm). PL spectra under indirect excitation in ZnO:Sm of [Sm] = 0.4% are shown in Fig. 2. The as-grown ZnO:Sm shows a band-edge emission of ZnO at 370 nm and a broad yellow emission from defect levels
Fig. 2. PL spectra of as-grown and annealed ZnO:Sm of 0.4% under indirect excitation (λext. = 266 nm) at RT. The inset shows detailed Sm3+ PL spectra without the defectrelated PL.
[13,14]. In the annealed ZnO:Sm, sharp emission lines superimposed on the defect-related PL were observed at RT. The inset of Fig. 2 shows the detailed PL spectrum in which the defect-related PL is sub4 6 tracted. The sharp lines at 550–700 nm are identified to G5/2– HJ (J = 5/2, 7/2, 9/2) transition in Sm 3+ ions [15,16]. The result reveals that Sm 3+ in the ZnO thin film acts as luminescent centers in the red region after the thermal annealing. The existence of several PL 4 6 lines from the G5/2– HJ transitions indicates that the local structure 3+ around Sm is inhomogeneous. In the annealed ZnO:Sm, the defectrelated emission became stronger than the as-grown one. When the ZnO is annealed in O2 ambient at high temperature of 600 °C, interstitial oxygen is easily introduced [13]. Therefore, the enhancement of defectrelated emission is due to the increase of interstitial oxygen in the ZnO: Sm. In the PL measurement under direct excitation (λext. = 420 nm), obvious Sm 3+ PL was not observed even at 77 K. The result shows that the excitation efficiency of Sm 3 + by the direct excitation is much low. Therefore, it is suggested that the Sm 3+ luminescent centers are effectively excited by the energy transfer from ZnO to Sm3+ (indirect excitation). In the annealed ZnO:Sm films, the integrated PL intensities of the band-edge and the Sm3+ emission lines under indirect excitation were obtained at 77 K. Fig. 3 shows the Sm-concentration dependence of the band-edge PL and the Sm 3+ PL intensity. The band-edge PL intensity decreases with increase of Sm concentration. On the other hand, the PL intensity of Sm3+ is the largest at [Sm] = 0.4%. The PL decay profiles of Sm3+ PL intensity in the annealed ZnO:Sm are shown in Fig. 4. At the initial decay within 25 μs, the defect-related PL is dominant in the temporal change of PL spectra (not shown). Therefore, we assume the PL after 4 6 25 μs as G5/2– HJ (J= 5/2, 7/2, 9/2) transition of Sm 3+ ions. The life3+ times of Sm PL were obtained by a least-squares fit using exponential decay curves over 25 μs. The fitting results are shown by solid lines in Fig. 4. From the fitting, we obtained three lifetimes of τfast, τmiddle and τslow in the films. The obtained lifetimes of Sm3+ PL are shown in Fig. 5 as a function of Sm concentration. As seen in the figure, the lifetimes become short at higher Sm concentration than [Sm]= 0.4%. The Sm-concentration dependence of lifetimes shows an increase of nonradiative recombination processes at high Sm concentration. 4. Discussion
Fig. 1. XRD patterns of ZnO:Sm films with Sm concentration of 0.2, 0.9, and 3.8%.
The PL properties of Sm 3+ in ZnO have been reported in polycrystalline pellets [7,8], and microstructure [9]. In these reports, Sm 3+ PLs were observed under the direct excitation or an impact excitation by hot electrons, and no energy transfer from ZnO to Sm 3+ ions was
T. Tsuji et al. / Journal of Non-Crystalline Solids 358 (2012) 2443–2445
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Fig. 5. Sm concentration dependence of the lifetimes of Sm3+ PL at 77 K.
Fig. 3. Sm concentration dependence of band-edge and Sm3+ PL intensities in ZnO:Sm at 77 K.
ions was pointed out in the ZnO:Sm films. In the Sm-concentration dependence of PL properties, the quenching of Sm3+ PL intensity was found to be induced at higher Sm concentration than 0.4%. Acknowledgements
observed. However, there is a possibility that these Sm 3+-PL properties depend on extrinsic effects because these ZnO:Sm polycrystalline samples include many grain boundaries and defects which act as nonradiative centers. In the ZnO:Sm thin films in this study, the Sm3+ PL was found to be observed under the indirect excitation. The result shows the contribution of the energy transfer to Sm 3+ PL. The ZnO: Sm thin films are high-quality c-axis oriented thin films. As a result, it is considered that the intrinsic PL properties of Sm 3+ are observed in the ZnO:Sm. In Sm-concentration dependence of PL intensity and lifetime of Sm3+, the quenching of PL intensity was induced at Sm concentration more than [Sm]= 0.4%. In the case of ZnO:Eu, the PL quenching was occurred at 0.1%. These results suggest that more luminescent centers can be formed in ZnO:Sm than ZnO:Eu. 5. Conclusion Sm-doped ZnO (ZnO:Sm) thin films with c-axis oriented wurzite structure were grown by a sputtering-assisted metalorganic chemical vapor deposition technique. The annealed ZnO:Sm films showed redemission lines from intra-4f transition of Sm3+ ions under indirect excitation at RT. The contribution of the energy transfer from ZnO to Sm3+
Fig. 4. Decay profiles of Sm3+ PL intensity in annealed ZnO:Sm. PL intensities are normalized at t = 0 and the curves are shifted vertically for clarification. Solid lines show the fitting results.
This work was supported in part by a Grant-in-Aid for Creative Scientific Research No. 19GS1209 from the Japan Society for the Promotion of Science, and by Priority Assistance for the Formation of Worldwide Renowned Centers of Research — The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] P.D. Rack, P.H. Holloway, Mater. Sci. Eng., R 21 (1998) 171–219. [2] M. Leskelä, J. Alloys Compd. 275–277 (1998) 702–708. [3] A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, Y. Fujiwara, Appl. Phys. Express 2 (2009) 071004. [4] Y. Terai, K. Yoshida, M.H. Kamarudin, Y. Fujiwara, Phys. Status Solidi C 8 (2011) 519–521. [5] Y. Terai, K. Yamaoka, K. Yoshida, T. Tsuji, Y. Fujiwara, Physica E 42 (2010) 2834–2836. [6] Y. Abiko, N. Nakayama, K. Akimoto, T. Yao, Phys. Status Solidi B 229 (2002) 339–342. [7] J.-C. Ronfard-Haret, D. Kouyate, J. Kossanyi, Solid State Commun 79 (1991) 85–88. [8] T. Ohtake, S. Hijii, N. Sonoyama, T. Sakata, Appl. Surf. Sci. 253 (2006) 1753–1757. [9] X. Zeng, J. Yuan, L. Zhang, J. Phys. Chem. C 112 (2008) 3503–3508. [10] G.N. van den Hoven, E. Snoeks, A. Polman, C. van Dam, J.W.M. van Uffelen, M.K. Smit, J. Appl. Phys. 79 (1996) 1258–1266. [11] K. Yamaoka, Y. Terai, T. Yamaguchi, Y. Fujiwara, Phys. Status Solidi C 5 (2008) 3125–3127. [12] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [13] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287–2294. [14] J. Petersen, C. Brimont, M. Gallart, O. Crégut, G. Schmerber, P. Gilliot, B. Hönerlage, C. Ulhaq-Bouillet, J.L. Rehspringer, C. Leuvrey, S. Colis, A. Slaoui, A. Dinia, Microelectron. J. 40 (2009) 239–241. [15] C.K. Jayasankar, E. Rukmini, Opt. Mater. 8 (1997) 193–205. [16] A.R. Zanatta, C.T.M. Ribeiro, U. Jahn, Appl. Phys. Lett. 79 (2001) 488–490.
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Photoluminescence properties of Sm-doped ZnO grown by sputtering-assisted metalorganic chemical vapor deposition Takahiro Tsuji ⁎, Yoshikazu Terai, Muhammad hakim bin Kamarudin, Masatoshi Kawabata, Yasufumi Fujiwara Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2–1 Yamadaoka, Suita, Osaka 565–0871, Japan
a r t i c l e
i n f o
Article history: Received 31 August 2011 Received in revised form 19 December 2011 Available online 23 January 2012 Keywords: Rare-earth doped semiconductors; MOCVD; ZnO; Photoluminescence
a b s t r a c t Sm-doped ZnO (ZnO:Sm) thin films with c-axis oriented wurzite structure were grown by sputteringassisted metalorganic chemical vapor deposition (SA-MOCVD). In the photoluminescence (PL) measurements of annealed ZnO:Sm, sharp emission lines from intra-4f transitions in Sm3+ ions were observed at room temperature under the excitation energy above the bandgap energy of ZnO (indirect excitation). In the dependence of PL intensity at 77 K on Sm concentration, the Sm 3+ PL intensity was the largest at Sm concentration of 0.4%. In time-resolved PL measurements, the lifetime of Sm3+ PL became short at higher Sm concentration than 0.4%. These results revealed that a quenching of the Sm3+ PL from ZnO:Sm was induced at higher Sm concentration than 0.4%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth (RE)-doped semiconductor is one of the promising materials for the active layer in electroluminescent (EL) devices [1,2]. In REdoped semiconductors, it is well known that an efficient excitation of RE ions is accomplished by the energy transfer from host semiconductors to doped RE ions. For example, Eu-doped GaN (GaN:Eu) shows an efficient red-emission lines originating from intra-4f shell transitions of 5 D0–7FJ (J= 0–4) at room temperature (RT) [3]. As a new candidate of RE-doped semiconductor, ZnO has a potential for the active layer in visual light range due to the large bandgap of ~3.37 eV. For the development of ZnO-based EL devices, it is necessary to investigate what kinds of RE ions act as efficient luminescent centers in ZnO. In our previous report [4], we have demonstrated red-emission lines from Eu 3+ ions at RT in Eu-doped ZnO (ZnO:Eu) under the excitaion energy above the bandgap of ZnO (indirect excitation). In the ZnO:Eu, the energy transfer from ZnO to Eu3+ ions was confirmed by the photoluminescence excitation spectra (PLE) monitored at 615 nm (5D0–7F2) [5]. The result has opened new research area of RE-doped ZnO. In this study, we have investigated a Sm-doping into ZnO as another candidate of luminescent centers in ZnO. In RE-doped ZnS, it has been reported that Sm 3+ ions act as effective luminescent centers in the red region, but Eu3+ does not show the red-emission lines [6]. The result indicates a possibility that Sm 3+ in ZnO shows a strong photoluminescence (PL) in comparison with Eu3+ in ZnO. The reports of Sm-doped ZnO (ZnO:Sm) have been limited to polycrystalline [7,8] ⁎ Corresponding author. E-mail addresses: [email protected] (T. Tsuji), [email protected] (Y. Terai). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.099
and nanostructures [9], but there is no report on the growth of ZnO: Sm thin films with c-axis oriented wurzite structure. The growth of high-quality ZnO:Sm thin films is necessary to investigate the intrinsic PL properties of Sm3+. In addition, the dependence of Sm 3+ PL properties on Sm concentration has not been investigated. At high Sm concentration, the Sm3+ PL will be quenched by nonradiative recombination processes induced by Sm-doping [10]. It is necessary for the development of EL devices to investigate the suitable doping concentration of Sm 3+ ions. Therefore, we tried the growth of ZnO:Sm thin films with different Sm concentrations by sputtering-assisted metalorganic chemical vapor deposition (SA-MOCVD). In the ZnO:Sm films, the Smconcentration dependence of PL properties under indirect excitation has been investigated by PL and time-resolved PL (TR-PL) measurements. 2. Experimental ZnO:Sm films with a thickness of 650–800 nm were grown on cplane sapphire substrates by SA-MOCVD [11]. Diethylzinc (DEZn) of 0.3 sccm and O2 of 10.0 sccm were introduced on the substrate with Ar carrier gas of 5.0 sccm, resulting in the ZnO growth by the thermal reaction of DEZn and O2. During the growth of ZnO, Sm atoms were doped by RF magnetron sputtering of a Sm2O3 sintered target, which was fixed at the top of the substrate. In the growth technique, the Sm concentration could be controlled by RF power and distance (L) between the target and the substrate. In this study, the Sm2O3 target was sputtered at RF power of 20–60 W, L = 100 mm during the ZnO growth at substrate temperature of 550 °C. The Sm concentration ([Sm]) in the ZnO:Sm was obtained to be [Sm]= 0.2–3.8% in energy dispersive X-ray spectroscopy (EDX) measurements. After the growth, the films were annealed at 600 °C for 30 min in O2 ambient.
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T. Tsuji et al. / Journal of Non-Crystalline Solids 358 (2012) 2443–2445
The crystal structure of ZnO:Sm film was characterized by 2θ–θ X-ray diffraction (XRD) measurement using Cu Kα radiation. PL measurements were carried out at RT and 77 K. The films were excited by the 266 nm line of a Nd:YAG laser (1 ns pulse width, 11 kHz repetition) for the indirect excitation, and the PL was detected by an electronically cooled CCD camera with a 0.3 m monochromator. A tunable dye laser operating at 420 nm (1 ns pulse width, 10 Hz repetition) was used for the PL under ex6 6 4 citation wavelength (λext.) resonant to the H5/2–( P, P)5/2 (~420 nm) 3+ transition in Sm ions (direct excitation). In TR-PL measurements, a pulsed N2 laser with a wavelength of 337.1 nm (1 ns pulse width, 10 Hz repetition) was used as the excitation sources of the indirect excitation. The pulse energy was tuned to 10 μJ. The TR-PL spectra were detected by a gated ICCD camera with a 0.15 m monochromator in the time range of 0–3000 μs. 3. Results Fig. 1 shows the XRD patterns of ZnO:Sm films. The films show the high intensity of ZnO(0002) and ZnO(0004) peaks without other peaks peak, such as Sm2O3. In the pole figure measurement of ZnO 1011 two types of in-plane rotational domains of ZnO 1210 || sapphire || sapphire 1120 were observed in the ZnO: 1100 and ZnO 1010 Sm films. This corresponds to typical growth mode of undoped ZnO on c-plane sapphire substrate [12]. These results show that high-quality caxis oriented ZnO:Sm was successfully grown on the sapphire substrates. As Sm concentration increases, the lattice constant in c-axis obtained by the peak position of ZnO(0004) shifted from 5.207 Å in undoped ZnO to 5.219 Å in ZnO:Sm with [Sm] =3.8%. The expansion of lattice constant indicates that Sm ions with the large ionic radius replace the Zn site in ZnO. In addition, the full width at half maximum (FWHM) of ZnO(0004) peak becomes large at high Sm concentration, indicating a degradation of crystalline orientation by the Sm-doping. The local arrangement of Sm ions in the ZnO:Sm is not clear, but following a possibility is suggested. When the Sm ions replace the Zn site, the charge state of the Sm ions is trivalent. The Sm3+ acts as donor in the ZnO:Sm. In fact, the resistivity of Sm-doped ZnO (2.0 Ω cm) was three orders of magnitude lower than that of undoped ZnO (2000 Ω cm). PL spectra under indirect excitation in ZnO:Sm of [Sm] = 0.4% are shown in Fig. 2. The as-grown ZnO:Sm shows a band-edge emission of ZnO at 370 nm and a broad yellow emission from defect levels
Fig. 2. PL spectra of as-grown and annealed ZnO:Sm of 0.4% under indirect excitation (λext. = 266 nm) at RT. The inset shows detailed Sm3+ PL spectra without the defectrelated PL.
[13,14]. In the annealed ZnO:Sm, sharp emission lines superimposed on the defect-related PL were observed at RT. The inset of Fig. 2 shows the detailed PL spectrum in which the defect-related PL is sub4 6 tracted. The sharp lines at 550–700 nm are identified to G5/2– HJ (J = 5/2, 7/2, 9/2) transition in Sm 3+ ions [15,16]. The result reveals that Sm 3+ in the ZnO thin film acts as luminescent centers in the red region after the thermal annealing. The existence of several PL 4 6 lines from the G5/2– HJ transitions indicates that the local structure 3+ around Sm is inhomogeneous. In the annealed ZnO:Sm, the defectrelated emission became stronger than the as-grown one. When the ZnO is annealed in O2 ambient at high temperature of 600 °C, interstitial oxygen is easily introduced [13]. Therefore, the enhancement of defectrelated emission is due to the increase of interstitial oxygen in the ZnO: Sm. In the PL measurement under direct excitation (λext. = 420 nm), obvious Sm 3+ PL was not observed even at 77 K. The result shows that the excitation efficiency of Sm 3 + by the direct excitation is much low. Therefore, it is suggested that the Sm 3+ luminescent centers are effectively excited by the energy transfer from ZnO to Sm3+ (indirect excitation). In the annealed ZnO:Sm films, the integrated PL intensities of the band-edge and the Sm3+ emission lines under indirect excitation were obtained at 77 K. Fig. 3 shows the Sm-concentration dependence of the band-edge PL and the Sm 3+ PL intensity. The band-edge PL intensity decreases with increase of Sm concentration. On the other hand, the PL intensity of Sm3+ is the largest at [Sm] = 0.4%. The PL decay profiles of Sm3+ PL intensity in the annealed ZnO:Sm are shown in Fig. 4. At the initial decay within 25 μs, the defect-related PL is dominant in the temporal change of PL spectra (not shown). Therefore, we assume the PL after 4 6 25 μs as G5/2– HJ (J= 5/2, 7/2, 9/2) transition of Sm 3+ ions. The life3+ times of Sm PL were obtained by a least-squares fit using exponential decay curves over 25 μs. The fitting results are shown by solid lines in Fig. 4. From the fitting, we obtained three lifetimes of τfast, τmiddle and τslow in the films. The obtained lifetimes of Sm3+ PL are shown in Fig. 5 as a function of Sm concentration. As seen in the figure, the lifetimes become short at higher Sm concentration than [Sm]= 0.4%. The Sm-concentration dependence of lifetimes shows an increase of nonradiative recombination processes at high Sm concentration. 4. Discussion
Fig. 1. XRD patterns of ZnO:Sm films with Sm concentration of 0.2, 0.9, and 3.8%.
The PL properties of Sm 3+ in ZnO have been reported in polycrystalline pellets [7,8], and microstructure [9]. In these reports, Sm 3+ PLs were observed under the direct excitation or an impact excitation by hot electrons, and no energy transfer from ZnO to Sm 3+ ions was
T. Tsuji et al. / Journal of Non-Crystalline Solids 358 (2012) 2443–2445
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Fig. 5. Sm concentration dependence of the lifetimes of Sm3+ PL at 77 K.
Fig. 3. Sm concentration dependence of band-edge and Sm3+ PL intensities in ZnO:Sm at 77 K.
ions was pointed out in the ZnO:Sm films. In the Sm-concentration dependence of PL properties, the quenching of Sm3+ PL intensity was found to be induced at higher Sm concentration than 0.4%. Acknowledgements
observed. However, there is a possibility that these Sm 3+-PL properties depend on extrinsic effects because these ZnO:Sm polycrystalline samples include many grain boundaries and defects which act as nonradiative centers. In the ZnO:Sm thin films in this study, the Sm3+ PL was found to be observed under the indirect excitation. The result shows the contribution of the energy transfer to Sm 3+ PL. The ZnO: Sm thin films are high-quality c-axis oriented thin films. As a result, it is considered that the intrinsic PL properties of Sm 3+ are observed in the ZnO:Sm. In Sm-concentration dependence of PL intensity and lifetime of Sm3+, the quenching of PL intensity was induced at Sm concentration more than [Sm]= 0.4%. In the case of ZnO:Eu, the PL quenching was occurred at 0.1%. These results suggest that more luminescent centers can be formed in ZnO:Sm than ZnO:Eu. 5. Conclusion Sm-doped ZnO (ZnO:Sm) thin films with c-axis oriented wurzite structure were grown by a sputtering-assisted metalorganic chemical vapor deposition technique. The annealed ZnO:Sm films showed redemission lines from intra-4f transition of Sm3+ ions under indirect excitation at RT. The contribution of the energy transfer from ZnO to Sm3+
Fig. 4. Decay profiles of Sm3+ PL intensity in annealed ZnO:Sm. PL intensities are normalized at t = 0 and the curves are shifted vertically for clarification. Solid lines show the fitting results.
This work was supported in part by a Grant-in-Aid for Creative Scientific Research No. 19GS1209 from the Japan Society for the Promotion of Science, and by Priority Assistance for the Formation of Worldwide Renowned Centers of Research — The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] P.D. Rack, P.H. Holloway, Mater. Sci. Eng., R 21 (1998) 171–219. [2] M. Leskelä, J. Alloys Compd. 275–277 (1998) 702–708. [3] A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, Y. Fujiwara, Appl. Phys. Express 2 (2009) 071004. [4] Y. Terai, K. Yoshida, M.H. Kamarudin, Y. Fujiwara, Phys. Status Solidi C 8 (2011) 519–521. [5] Y. Terai, K. Yamaoka, K. Yoshida, T. Tsuji, Y. Fujiwara, Physica E 42 (2010) 2834–2836. [6] Y. Abiko, N. Nakayama, K. Akimoto, T. Yao, Phys. Status Solidi B 229 (2002) 339–342. [7] J.-C. Ronfard-Haret, D. Kouyate, J. Kossanyi, Solid State Commun 79 (1991) 85–88. [8] T. Ohtake, S. Hijii, N. Sonoyama, T. Sakata, Appl. Surf. Sci. 253 (2006) 1753–1757. [9] X. Zeng, J. Yuan, L. Zhang, J. Phys. Chem. C 112 (2008) 3503–3508. [10] G.N. van den Hoven, E. Snoeks, A. Polman, C. van Dam, J.W.M. van Uffelen, M.K. Smit, J. Appl. Phys. 79 (1996) 1258–1266. [11] K. Yamaoka, Y. Terai, T. Yamaguchi, Y. Fujiwara, Phys. Status Solidi C 5 (2008) 3125–3127. [12] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [13] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287–2294. [14] J. Petersen, C. Brimont, M. Gallart, O. Crégut, G. Schmerber, P. Gilliot, B. Hönerlage, C. Ulhaq-Bouillet, J.L. Rehspringer, C. Leuvrey, S. Colis, A. Slaoui, A. Dinia, Microelectron. J. 40 (2009) 239–241. [15] C.K. Jayasankar, E. Rukmini, Opt. Mater. 8 (1997) 193–205. [16] A.R. Zanatta, C.T.M. Ribeiro, U. Jahn, Appl. Phys. Lett. 79 (2001) 488–490.