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Effect of Tm doping on the properties of electrodeposited ZnO nanorods
Materials Chemistry and Physics 125 (2011) 813–817
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of Tm doping on the properties of electrodeposited ZnO nanorods F. Fang a, A.M.C. Ng a, X.Y. Chen a, A.B. Djuriˇsic´ a,∗, Y.C. Zhong b, K.S. Wong b, P.W.K. Fong c, H.F. Lui c, C. Surya c , W.K. Chan d a
Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong c Dept. of Electronic and Information Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong d Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong b
a r t i c l e
i n f o
Article history: Received 13 December 2009 Received in revised form 16 August 2010 Accepted 22 September 2010 PACS: 81.07.Vb ;81.15.Pq
a b s t r a c t ZnO nanorods were fabricated by electrodeposition from solution with and without thulium precursors and characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, photoluminescence and time-resolved photoluminescence. In spite of the low incorporation of thulium into ZnO nanorods, both the morphology and the optical properties of the nanorods were affected by the presence of thulium. The light emitting diodes with ZnO nanorods with and without Tm have been demonstrated. © 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO nanorods Photoluminescence
1. Introduction ZnO is a material of considerable interest for optoelectronic applications due to its wide band gap and high exciton binding energy [1,2]. Optical properties of ZnO nanostructures have been extensively studied [1,2] and progress in the growth of nanorods and their application in light emitting diodes (LEDs) has been achieved [2]. For device applications of ZnO and control over its optical and electronic properties, doping with different impurities is needed. Among various dopants for ZnO, rare earth ions are of interest for tailoring the optical properties [3]. Doping of ZnO with rare earth ions has been studied for different ZnO morphologies and fabrication methods, as well as for different dopant ions [3–15]. Among different rare earth ions, Tm doping is of interest since Tm3+ emits in spectral ranges which are rarely observed in ZnO (blue and near infrared). Fabrication and optical properties of Tm:ZnO have been studied previously [3–11]. Electroluminescence has been demonstrated from devices containing Tm:ZnO [3,7,12], for example films sandwiched between insulating layers [3]. However, the bias voltages required to observe light emission were high in all cases [3,7,12]. Two small peaks attributed to Tm transitions were superimposed onto a broad ZnO defect emission in case of SiO2 insulating layers, while no Tm lines were observed for LiF [3]. Characteristic Tm emis-
∗ Corresponding author. Tel.: +852 2859 7946; fax: +852 2559 9152. ´ E-mail address: [email protected] (A.B. Djuriˇsic). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.09.051
sion lines were observed in some devices, but at significantly higher voltages exceeding 100 V [7,12]. All the reports on electroluminescence (EL) from Tm:ZnO have been for bulk (thin film or pellet) samples. In this work, we have prepared heterojunction light emitting diodes (LEDs) based on ZnO nanorods (with and without Tm) in an attempt to achieve low turn-on voltage devices. ZnO and Tm:ZnO nanorods have been fabricated by electrodeposition. Electrodeposition represents a simple and inexpensive low temperature aqueous solution-based growth technique with a high growth rate [2]. Low cost, scalability, and high growth rate make this technique of particular interest for low cost LED fabrication. In addition, electrodeposition has been used to fabricate ZnO films and nanostructures doped with different elements [13–16]. 2. Experimental details Indium tin oxide (ITO) on quartz (cleaned by sonication in toluene, acetone, ethanol and deionized water) was used as the substrate for nanorod growth. Electrodeposition was performed in a two electrode setup, with a platinum foil as the anode while the substrate was used as the cathode. The synthesis solution was composed of 70 mg of zinc nitrate hydrate, 30 mg hexamethylenetetramine and 60 mg potassium chloride with 4 mg thulium nitrate pentahydrate as the doping source in 30 ml deionized water. The solution was heated up on a hotplate, and voltage of 2.2 V was applied when the temperature of the solution reached 80 ◦ C. Throughout the 30 min deposition time, the temperature of the solution and the applied voltage were maintained constant.
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F. Fang et al. / Materials Chemistry and Physics 125 (2011) 813–817
The morphology and composition of the nanorods was examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) using JEOL (JSM-7001F) SEM. X-ray diffraction (XRD) patterns were obtained using a Bruker AXS SMART CCD diffractometer. Low-temperature and room temperature photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were carried out using a liquid-helium cryostat and a Hamamatsu model C4334 streak camera coupled to a spectrometer for the TRPL and a fiber spectrometer (USB2000, Ocean Optics Inc.) for PL. Decay curves were obtained from TRPL measurements, with the window details as follows: width of the wavelength was 6 nm, center wavelengths were 371 nm and 378 nm for 8 K and room temperature, respectively, and the time scale of the streak camera was 1 ns. A femtosecond titanium–sapphire oscillator was used as the excitation source. A wavelength of 351 nm (second harmonic of the laser output at 702 nm) was used as the pumping source. The repetition rate and average power were 76 MHz and 0.5 mW, respectively. For LED fabrication, p-GaN layers grown on sapphire using metal organic chemical vapor deposition (680 nm Mg:GaN p-type/500 nm highly resistive Mg:GaN/2.2 m undoped GaN layer/30 nm GaN nucleation layer/sapphire substrate) were used as the substrates. The p-GaN substrates were activated by annealing in nitrogen at 825 ◦ C, and Ni (30 nm) and Au (70 nm) were evaporated using thermal evaporation (AST PEVA 500 EL) as an ohmic contact to p-GaN. For electrodeposition of nanorods on p-GaN, the solution composition and temperature were the same as for the ITO substrate, but in this case deposition was performed at a constant current (10 mA for ∼30 s, followed by 1 mA for 30 min) due to different substrate properties (sheet resistance and surface roughness). To prevent contact between top electrode and p-GaN, after nanorod growth and annealing at 200 ◦ C in oxygen gas flow spin-on-glass (SOG) was spin-coated at 3000 rpm for 40 s, followed by annealing at 200 ◦ C for 1 min [17]. At optimized spin coating conditions, SOG fills up the space between nanorods while only extremely thin coating is present on the top of the nanorods, as verified by SEM and ohmic contact checks. Then, Ag (1 mm radius, 200 nm thickness) was evaporated by thermal evaporation (AST PEVA 500 EL). For device characterization, Keithley 2400 source meter was used for the I–V curve measurement and for providing constant current bias for EL measurements. The EL spectra were recorded using a PDA-512 USB (Control Development Inc.) fiberoptic spectrometer. Luminance was measured using Minolta Luminance Meter LS-100.
3. Results and discussion Fig. 1 shows the SEM images of the ZnO nanorods grown with and without Tm precursor in the solution. It can be observed that after addition of the Tm, the shape of the nanorod changes from the flat top into a sharp pointed top. From EDS results at different points in the samples, Tm incorporation is typically below 2 wt% (or <0.4 at%). In general, achievement of Tm3+ incorporation into ZnO is difficult due to large difference in charge state and ionic radii of Zn2+ and Tm3+ [3]. XRD patterns of the nanorods with and without Tm are shown in Fig. 2. The strongest peaks located at 31.78◦ , 34.4◦ and 36.26◦ correspond to the (1 0 0), (0 0 2), and (1 0 1) directions of ZnO, respectively (JCPDS 36-1451). Besides the three most obvious peaks, other peaks corresponding to the (1 0 2), (1 1 0), (1 0 3), and (1 1 2) directions of ZnO can also be observed, while remaining peaks correspond to the ITO [18]. After Tm doping, peak shift can be observed but no new peaks appear, unlike some of the previous works on Tm:ZnO where peaks corresponding to Tm2 O3 were identified [3]. From the position of the peaks, lattice constants with and without Tm doping can be calculated [19]. For undoped ZnO, the lattice constant a
Fig. 1. SEM images of (a, b) ZnO nanorods and (c, d) Tm:ZnO nanorods.
is 0.325 ± 0.0004 nm and c is 0.521 ± 0.0006 nm. As for Tm doped ZnO, the lattice constant a increases to 0.326 ± 0.0004 nm, and c increases to 0.523 ± 0.0006 nm, which is ascribed to the radius of ˚ Tm3+ being larger than that of Zn2+ (the radius of Tm3+ is 0.87 A, larger than 0.74 A˚ [3]). PL spectra from the nanorods at ∼8 K and room temperature (RT) are shown in Fig. 3. As-grown and samples annealed at 200 ◦ C in oxygen gas flow (to reduce defect emission [20]) were investigated. It can be observed that Tm:ZnO exhibits lower defect emission compared to undoped ZnO, but no new peaks can be observed. In addition to UV emission peak due to excitonic transition near the band edge, ZnO typically exhibits visible emission due to defects [1]. In samples grown in aqueous solutions, yellow-orange defect emission is often observed [20]. This emission is related to presence of adsorbates, such as OH groups in the surface [20], but the exact chemical nature of the defects responsible for this emission is unknown. The peaks in the red spectral region are likely due to the noise in the measurement system used. Therefore, presence of Tm ions in the growth solution resulted in differences in the native defect concentrations, and consequently different optical properties in spite of the absence of Tm emission. Different optical properties have been reported in the literature for Tm:ZnO. For Tm implanted samples, Tm emission in near infrared spectral region (790–815 nm) was observed at 13 K [4] and ∼800 nm at room
Fig. 2. XRD patterns of ZnO and Tm:ZnO. Peaks corresponding to ZnO and ITO substrate are labeled.
F. Fang et al. / Materials Chemistry and Physics 125 (2011) 813–817
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Fig. 5. PL decay curves of as-grown and annealed ZnO and Tm:ZnO at (a) 8 K and (b) RT. Fig. 3. PL spectra of as-grown and annealed ZnO and Tm:ZnO at (a) 8 K and (b) RT.
temperature [11]. On the other hand, blue emission was observed in Tm:ZnO nanoparticles prepared by combustion method [6]. Both ZnO and Tm emission were observed in Tm,Cl:ZnO sintered pellets [11]. However, in this work we observed only the broad ZnO emission, similar to the EL spectra of Tm:ZnO films in the absence of SiO2 layers [3]. Schematic energy level diagram for ZnO (including only relevant defect levels for yellow-red emissions) and Tm3+ ions is shown in Fig. 4 [1,10,21]. While some other reports in the literature found characteristic Tm emission in Tm:ZnO, here we only observed emissions present in ZnO, although the defect emission was affected by introduction of Tm. This is possibly due to unfavorable energy level alignment between Tm and ZnO [22].
To further examine the optical properties, TRPL measurements were performed and the obtained decay curves are shown in Fig. 5. The obtained curves could be fitted to a single-exponential decay in all cases. At 8 K, the obtained time constants for as grown samples were 13.8 ps and 14.4 ps for ZnO and ZnO:Tm samples, and after annealing the obtained values were 19.8 ps and 31.0 ps, respectively. At room temperature, decay times were 12.7 ps and 12.8 ps for as-grown ZnO and Tm:ZnO, while after annealing the time constants increased to 14.6 ps and 17.0 ps, respectively. We can observe that in both cases samples annealed at 200 ◦ C exhibit longer PL decay time compared to as-grown samples. At 8 K, we can observe slower decay for annealed Tm:ZnO samples compared to undoped ZnO, but at room temperature they exhibit similar decay. This difference then likely originates
Fig. 4. Schematic energy level diagrams of (a) ZnO and (b) Tm3+ ion [1,10,21].
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Fig. 8. I–V curves of LEDs with ZnO and Tm:ZnO nanorods.
Fig. 6. SEM images of (a) ZnO and (b) ZnO:Tm nanorods on p-GaN substrates.
from different concentrations of shallow defects in the two samples. Since samples annealed at 200 ◦ C exhibited superior optical properties compared to as-grown samples, LEDs were fabricated with annealed ZnO nanorods. SEM images and PL spectra of ZnO and Tm:ZnO nanorods grown on p-GaN substrates are shown in Figs. 6 and 7, while the obtained I–V curves and the EL spectra of the LEDs are shown in Figs. 8 and 9, respectively. The introduction of Tm in this case as well leads to a similar change in the ZnO nanorod shape as for ITO substrates, namely sharpened top instead of a flat-top nanorod. In addition, undoped ZnO nanorods exhibit higher density on the substrate. In terms of PL emission, UV
Fig. 7. PL spectra of ZnO and Tm:ZnO on p-GaN substrates. PL from bare p-GaN substrate is also shown.
emission and orange emission originating from ZnO are observed in both samples, while in Tm:ZnO sample a small blue peak is also observed. From the peak position and since this peak is weak and broad, it may originate from p-GaN substrate (due to lower nanorod density) rather than from Tm transitions. As for LED performance, we can observe that I–V curves exhibit a backward diode shape curve [23], similar to our previous results [17]. This is likely due to high carrier concentration in solutiongrown ZnO and large energy band offsets at GaN/ZnO interface, which favor tunneling [17]. It can be observed that for the same bias voltage, lower current is observed in Tm:doped samples, which would be expected in the case of a lower shallow donor concentration. Both samples exhibit yellow emission under reverse bias (no emission under forward bias up to 30 V was observed). This emission likely originates from the defect levels of ZnO or
Fig. 9. EL spectra of (a) p-GaN/ZnO and (b) p-GaN/Tm:ZnO LEDs.
F. Fang et al. / Materials Chemistry and Physics 125 (2011) 813–817
interfacial defects. No emission lines corresponding to Tm were observed. Electroluminescence spectra exhibiting characteristic Tm3+ transitions (1 G4 → 3 H6 at ∼485 nm and 3 H4 → 3 H6 at ∼800 nm) have been previously achieved for Tm:ZnO pellets sintered at 1200 ◦ C, but the required bias voltage was higher than 100 V [7]. In this case, however, turn-on voltage is ∼5 V for Tm:ZnO and ∼4 V for undoped ZnO. Tm:ZnO samples exhibit higher emission intensity, and the luminance at 20 mA bias was 12.46 cd/m2 for undoped ZnO and 16.57 cd/m2 for Tm:ZnO. Thus, we have achieved considerable reduction in turn-on voltage of electroluminescent devices based on Tm:ZnO (by one [3] or two [7,12] orders of magnitude). However, no Tm emission lines were observed in the EL spectra. 4. Conclusions ZnO and Tm:ZnO nanorods were fabricated by electrodeposition. The presence of Tm ions in the solution resulted in the change of the morphology and optical properties of the nanorods. Observed differences in the optical properties were attributed to different concentrations in native defects in the rods. P-GaN/n-ZnO nanorod LEDs were fabricated, and higher brightness was achieved for Tm:ZnO compared to undoped ZnO. Acknowledgements Financial support from the Strategic Research Theme, University Development Fund, Small Project Grant, Outstanding Young Researcher Award (administrated by The University of Hong Kong), and Innovation & Technology Fund Grant ITS/129/08 is acknowledged.
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References ´ Y.H. Leung, Small 2 (2006) 944. [1] A.B. Djuriˇsic, ˜ [2] M. Willander, O. Nur, Q.X. Zhao, L.L. Yang, M. Lorenz, B.Q. Cao, J. Zúniga Pérez, C. Czekalla, G. Zimmermann, M. Grundmann, A. Bakin, A. Behrends, M. AlSuleiman, A. El-Shaer, A.C. Mofor, B. Postels, A. Waag, N. Boukos, A. Travlos, H.S. Kwack, J. Guinard, D.L.S. Dang, Nanotechnology 20 (2009) 332001. [3] S.A.M. Lima, M.R. Davolos, C. Legnani, W.G. Quirino, M. Cremona, J. Alloy Compd. 418 (2006) 35. [4] E. Rita, E. Alves, U. Wahl, J.G. Correia, A.J. Neves, M.J. Soares, T. Monteiro, Physica B 340–342 (2003) 235. [5] E. Rita, E. Alves, U. Wahl, J.G. Correia, T. Monteiro, M.J. Soares, A. Neves, M. Peres, Nucl. Instrum. Methods Phys. Res. B 242 (2006) 580. [6] Y.X. Liu, C.F. Xu, Q.B. Yang, J. Appl. Phys. 105 (2009) 084701. [7] J.C. Ronfard-Haret, J. Kossanyi, Chem. Phys. 241 (1999) 339. [8] M. Peres, J. Wang, M.J. Soares, A. Neves, T. Monteiro, E. Rita, U. Wahl, J.G. Correia, E. Alves, Superlattices Microstruct. 36 (2004) 747. [9] T. Monteiro, A.J. Neves, M.J. Soares, M.C. Carmo, M. Peres, E. Alves, E. Rita, Appl. Phys. Lett. 87 (2005) 192108. [10] Y.S. Liu, W.Q. Luo, R.F. Li, H.M. Zhu, X.Y. Chen, Opt. Exp. 17 (2009) 9748. [11] W.M. Jadwisienczak, H.J. Lozykowski, A. Xu, B. Patel, J. Electron. Mater. 31 (2002) 776. [12] S. Bachir, C. Sandouly, J. Kossanyi, J.C. Ronfardt-Haret, J. Phys. Chem. Solids 57 (1996) 1869. [13] T. Pauporté, F. Pellé, B. Viana, P. Aschehoug, J. Phys. Chem. C 111 (2007) 15427. [14] G.-R. Li, X.-H. Lu, C.-Y. Su, Y.-X. Tong, J. Phys. Chem. C 112 (2008) 2928. [15] X.-H. Lu, G.-R. Li, W.-X. Zhao, Y.-X. Tong, Electrochim. Acta 53 (2008) 5180. [16] B. Seipel, A. Nadarajah, B. Wutzke, R. Könenkamp, Mater. Lett. 63 (2009) 763. ´ W.K. Chan, S. Gwo, H.L. Tam, K.W. [17] A.M.C. Ng, Y.Y. Xi, Y.F. Hsu, A.B. Djuriˇsic, Cheah, P.W.K. Fong, H.F. Lui, C. Surya, Nanotechnology 20 (2009) 445201. [18] S.-G. Chen, C.-H. Li, W.-H. Xiong, L.-M. Liu, H. Wang, Mater. Lett. 59 (2005) 1342. [19] H. Li, Y. Zhang, X. Pun, H. Zhang, T. Wang, E. Xie, J. Nanopart. Res. 11 (2009) 917. ´ Y.H. Leung, K.H. Tam, Y.F. Hsu, L. Ding, W.K. Ge, Y.C. Zhong, K.S. [20] A.B. Djuriˇsic, Wong, W.K. Chan, H.L. Tam, K.W. Cheah, W.M. Kwok, D.L. Phillips, Nanotechnology 18 (2007) 095702. [21] U. Hömmerich, E.E. Nyein, D.S. Lee, A.J. Steckl, J.M. Zavada, Appl. Phys. Lett. 83 (2003) 4556. [22] P. Dorenbos, E. van der Kolk, Proc. SPIE 6473 (2007) 647313. [23] K.K Ng, Complete Guide to Semiconductor Devices, John Wiley & Sons Inc., NY, USA, 2002, p. 63.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of Tm doping on the properties of electrodeposited ZnO nanorods F. Fang a, A.M.C. Ng a, X.Y. Chen a, A.B. Djuriˇsic´ a,∗, Y.C. Zhong b, K.S. Wong b, P.W.K. Fong c, H.F. Lui c, C. Surya c , W.K. Chan d a
Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong c Dept. of Electronic and Information Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong d Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong b
a r t i c l e
i n f o
Article history: Received 13 December 2009 Received in revised form 16 August 2010 Accepted 22 September 2010 PACS: 81.07.Vb ;81.15.Pq
a b s t r a c t ZnO nanorods were fabricated by electrodeposition from solution with and without thulium precursors and characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, photoluminescence and time-resolved photoluminescence. In spite of the low incorporation of thulium into ZnO nanorods, both the morphology and the optical properties of the nanorods were affected by the presence of thulium. The light emitting diodes with ZnO nanorods with and without Tm have been demonstrated. © 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO nanorods Photoluminescence
1. Introduction ZnO is a material of considerable interest for optoelectronic applications due to its wide band gap and high exciton binding energy [1,2]. Optical properties of ZnO nanostructures have been extensively studied [1,2] and progress in the growth of nanorods and their application in light emitting diodes (LEDs) has been achieved [2]. For device applications of ZnO and control over its optical and electronic properties, doping with different impurities is needed. Among various dopants for ZnO, rare earth ions are of interest for tailoring the optical properties [3]. Doping of ZnO with rare earth ions has been studied for different ZnO morphologies and fabrication methods, as well as for different dopant ions [3–15]. Among different rare earth ions, Tm doping is of interest since Tm3+ emits in spectral ranges which are rarely observed in ZnO (blue and near infrared). Fabrication and optical properties of Tm:ZnO have been studied previously [3–11]. Electroluminescence has been demonstrated from devices containing Tm:ZnO [3,7,12], for example films sandwiched between insulating layers [3]. However, the bias voltages required to observe light emission were high in all cases [3,7,12]. Two small peaks attributed to Tm transitions were superimposed onto a broad ZnO defect emission in case of SiO2 insulating layers, while no Tm lines were observed for LiF [3]. Characteristic Tm emis-
∗ Corresponding author. Tel.: +852 2859 7946; fax: +852 2559 9152. ´ E-mail address: [email protected] (A.B. Djuriˇsic). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.09.051
sion lines were observed in some devices, but at significantly higher voltages exceeding 100 V [7,12]. All the reports on electroluminescence (EL) from Tm:ZnO have been for bulk (thin film or pellet) samples. In this work, we have prepared heterojunction light emitting diodes (LEDs) based on ZnO nanorods (with and without Tm) in an attempt to achieve low turn-on voltage devices. ZnO and Tm:ZnO nanorods have been fabricated by electrodeposition. Electrodeposition represents a simple and inexpensive low temperature aqueous solution-based growth technique with a high growth rate [2]. Low cost, scalability, and high growth rate make this technique of particular interest for low cost LED fabrication. In addition, electrodeposition has been used to fabricate ZnO films and nanostructures doped with different elements [13–16]. 2. Experimental details Indium tin oxide (ITO) on quartz (cleaned by sonication in toluene, acetone, ethanol and deionized water) was used as the substrate for nanorod growth. Electrodeposition was performed in a two electrode setup, with a platinum foil as the anode while the substrate was used as the cathode. The synthesis solution was composed of 70 mg of zinc nitrate hydrate, 30 mg hexamethylenetetramine and 60 mg potassium chloride with 4 mg thulium nitrate pentahydrate as the doping source in 30 ml deionized water. The solution was heated up on a hotplate, and voltage of 2.2 V was applied when the temperature of the solution reached 80 ◦ C. Throughout the 30 min deposition time, the temperature of the solution and the applied voltage were maintained constant.
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F. Fang et al. / Materials Chemistry and Physics 125 (2011) 813–817
The morphology and composition of the nanorods was examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) using JEOL (JSM-7001F) SEM. X-ray diffraction (XRD) patterns were obtained using a Bruker AXS SMART CCD diffractometer. Low-temperature and room temperature photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were carried out using a liquid-helium cryostat and a Hamamatsu model C4334 streak camera coupled to a spectrometer for the TRPL and a fiber spectrometer (USB2000, Ocean Optics Inc.) for PL. Decay curves were obtained from TRPL measurements, with the window details as follows: width of the wavelength was 6 nm, center wavelengths were 371 nm and 378 nm for 8 K and room temperature, respectively, and the time scale of the streak camera was 1 ns. A femtosecond titanium–sapphire oscillator was used as the excitation source. A wavelength of 351 nm (second harmonic of the laser output at 702 nm) was used as the pumping source. The repetition rate and average power were 76 MHz and 0.5 mW, respectively. For LED fabrication, p-GaN layers grown on sapphire using metal organic chemical vapor deposition (680 nm Mg:GaN p-type/500 nm highly resistive Mg:GaN/2.2 m undoped GaN layer/30 nm GaN nucleation layer/sapphire substrate) were used as the substrates. The p-GaN substrates were activated by annealing in nitrogen at 825 ◦ C, and Ni (30 nm) and Au (70 nm) were evaporated using thermal evaporation (AST PEVA 500 EL) as an ohmic contact to p-GaN. For electrodeposition of nanorods on p-GaN, the solution composition and temperature were the same as for the ITO substrate, but in this case deposition was performed at a constant current (10 mA for ∼30 s, followed by 1 mA for 30 min) due to different substrate properties (sheet resistance and surface roughness). To prevent contact between top electrode and p-GaN, after nanorod growth and annealing at 200 ◦ C in oxygen gas flow spin-on-glass (SOG) was spin-coated at 3000 rpm for 40 s, followed by annealing at 200 ◦ C for 1 min [17]. At optimized spin coating conditions, SOG fills up the space between nanorods while only extremely thin coating is present on the top of the nanorods, as verified by SEM and ohmic contact checks. Then, Ag (1 mm radius, 200 nm thickness) was evaporated by thermal evaporation (AST PEVA 500 EL). For device characterization, Keithley 2400 source meter was used for the I–V curve measurement and for providing constant current bias for EL measurements. The EL spectra were recorded using a PDA-512 USB (Control Development Inc.) fiberoptic spectrometer. Luminance was measured using Minolta Luminance Meter LS-100.
3. Results and discussion Fig. 1 shows the SEM images of the ZnO nanorods grown with and without Tm precursor in the solution. It can be observed that after addition of the Tm, the shape of the nanorod changes from the flat top into a sharp pointed top. From EDS results at different points in the samples, Tm incorporation is typically below 2 wt% (or <0.4 at%). In general, achievement of Tm3+ incorporation into ZnO is difficult due to large difference in charge state and ionic radii of Zn2+ and Tm3+ [3]. XRD patterns of the nanorods with and without Tm are shown in Fig. 2. The strongest peaks located at 31.78◦ , 34.4◦ and 36.26◦ correspond to the (1 0 0), (0 0 2), and (1 0 1) directions of ZnO, respectively (JCPDS 36-1451). Besides the three most obvious peaks, other peaks corresponding to the (1 0 2), (1 1 0), (1 0 3), and (1 1 2) directions of ZnO can also be observed, while remaining peaks correspond to the ITO [18]. After Tm doping, peak shift can be observed but no new peaks appear, unlike some of the previous works on Tm:ZnO where peaks corresponding to Tm2 O3 were identified [3]. From the position of the peaks, lattice constants with and without Tm doping can be calculated [19]. For undoped ZnO, the lattice constant a
Fig. 1. SEM images of (a, b) ZnO nanorods and (c, d) Tm:ZnO nanorods.
is 0.325 ± 0.0004 nm and c is 0.521 ± 0.0006 nm. As for Tm doped ZnO, the lattice constant a increases to 0.326 ± 0.0004 nm, and c increases to 0.523 ± 0.0006 nm, which is ascribed to the radius of ˚ Tm3+ being larger than that of Zn2+ (the radius of Tm3+ is 0.87 A, larger than 0.74 A˚ [3]). PL spectra from the nanorods at ∼8 K and room temperature (RT) are shown in Fig. 3. As-grown and samples annealed at 200 ◦ C in oxygen gas flow (to reduce defect emission [20]) were investigated. It can be observed that Tm:ZnO exhibits lower defect emission compared to undoped ZnO, but no new peaks can be observed. In addition to UV emission peak due to excitonic transition near the band edge, ZnO typically exhibits visible emission due to defects [1]. In samples grown in aqueous solutions, yellow-orange defect emission is often observed [20]. This emission is related to presence of adsorbates, such as OH groups in the surface [20], but the exact chemical nature of the defects responsible for this emission is unknown. The peaks in the red spectral region are likely due to the noise in the measurement system used. Therefore, presence of Tm ions in the growth solution resulted in differences in the native defect concentrations, and consequently different optical properties in spite of the absence of Tm emission. Different optical properties have been reported in the literature for Tm:ZnO. For Tm implanted samples, Tm emission in near infrared spectral region (790–815 nm) was observed at 13 K [4] and ∼800 nm at room
Fig. 2. XRD patterns of ZnO and Tm:ZnO. Peaks corresponding to ZnO and ITO substrate are labeled.
F. Fang et al. / Materials Chemistry and Physics 125 (2011) 813–817
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Fig. 5. PL decay curves of as-grown and annealed ZnO and Tm:ZnO at (a) 8 K and (b) RT. Fig. 3. PL spectra of as-grown and annealed ZnO and Tm:ZnO at (a) 8 K and (b) RT.
temperature [11]. On the other hand, blue emission was observed in Tm:ZnO nanoparticles prepared by combustion method [6]. Both ZnO and Tm emission were observed in Tm,Cl:ZnO sintered pellets [11]. However, in this work we observed only the broad ZnO emission, similar to the EL spectra of Tm:ZnO films in the absence of SiO2 layers [3]. Schematic energy level diagram for ZnO (including only relevant defect levels for yellow-red emissions) and Tm3+ ions is shown in Fig. 4 [1,10,21]. While some other reports in the literature found characteristic Tm emission in Tm:ZnO, here we only observed emissions present in ZnO, although the defect emission was affected by introduction of Tm. This is possibly due to unfavorable energy level alignment between Tm and ZnO [22].
To further examine the optical properties, TRPL measurements were performed and the obtained decay curves are shown in Fig. 5. The obtained curves could be fitted to a single-exponential decay in all cases. At 8 K, the obtained time constants for as grown samples were 13.8 ps and 14.4 ps for ZnO and ZnO:Tm samples, and after annealing the obtained values were 19.8 ps and 31.0 ps, respectively. At room temperature, decay times were 12.7 ps and 12.8 ps for as-grown ZnO and Tm:ZnO, while after annealing the time constants increased to 14.6 ps and 17.0 ps, respectively. We can observe that in both cases samples annealed at 200 ◦ C exhibit longer PL decay time compared to as-grown samples. At 8 K, we can observe slower decay for annealed Tm:ZnO samples compared to undoped ZnO, but at room temperature they exhibit similar decay. This difference then likely originates
Fig. 4. Schematic energy level diagrams of (a) ZnO and (b) Tm3+ ion [1,10,21].
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Fig. 8. I–V curves of LEDs with ZnO and Tm:ZnO nanorods.
Fig. 6. SEM images of (a) ZnO and (b) ZnO:Tm nanorods on p-GaN substrates.
from different concentrations of shallow defects in the two samples. Since samples annealed at 200 ◦ C exhibited superior optical properties compared to as-grown samples, LEDs were fabricated with annealed ZnO nanorods. SEM images and PL spectra of ZnO and Tm:ZnO nanorods grown on p-GaN substrates are shown in Figs. 6 and 7, while the obtained I–V curves and the EL spectra of the LEDs are shown in Figs. 8 and 9, respectively. The introduction of Tm in this case as well leads to a similar change in the ZnO nanorod shape as for ITO substrates, namely sharpened top instead of a flat-top nanorod. In addition, undoped ZnO nanorods exhibit higher density on the substrate. In terms of PL emission, UV
Fig. 7. PL spectra of ZnO and Tm:ZnO on p-GaN substrates. PL from bare p-GaN substrate is also shown.
emission and orange emission originating from ZnO are observed in both samples, while in Tm:ZnO sample a small blue peak is also observed. From the peak position and since this peak is weak and broad, it may originate from p-GaN substrate (due to lower nanorod density) rather than from Tm transitions. As for LED performance, we can observe that I–V curves exhibit a backward diode shape curve [23], similar to our previous results [17]. This is likely due to high carrier concentration in solutiongrown ZnO and large energy band offsets at GaN/ZnO interface, which favor tunneling [17]. It can be observed that for the same bias voltage, lower current is observed in Tm:doped samples, which would be expected in the case of a lower shallow donor concentration. Both samples exhibit yellow emission under reverse bias (no emission under forward bias up to 30 V was observed). This emission likely originates from the defect levels of ZnO or
Fig. 9. EL spectra of (a) p-GaN/ZnO and (b) p-GaN/Tm:ZnO LEDs.
F. Fang et al. / Materials Chemistry and Physics 125 (2011) 813–817
interfacial defects. No emission lines corresponding to Tm were observed. Electroluminescence spectra exhibiting characteristic Tm3+ transitions (1 G4 → 3 H6 at ∼485 nm and 3 H4 → 3 H6 at ∼800 nm) have been previously achieved for Tm:ZnO pellets sintered at 1200 ◦ C, but the required bias voltage was higher than 100 V [7]. In this case, however, turn-on voltage is ∼5 V for Tm:ZnO and ∼4 V for undoped ZnO. Tm:ZnO samples exhibit higher emission intensity, and the luminance at 20 mA bias was 12.46 cd/m2 for undoped ZnO and 16.57 cd/m2 for Tm:ZnO. Thus, we have achieved considerable reduction in turn-on voltage of electroluminescent devices based on Tm:ZnO (by one [3] or two [7,12] orders of magnitude). However, no Tm emission lines were observed in the EL spectra. 4. Conclusions ZnO and Tm:ZnO nanorods were fabricated by electrodeposition. The presence of Tm ions in the solution resulted in the change of the morphology and optical properties of the nanorods. Observed differences in the optical properties were attributed to different concentrations in native defects in the rods. P-GaN/n-ZnO nanorod LEDs were fabricated, and higher brightness was achieved for Tm:ZnO compared to undoped ZnO. Acknowledgements Financial support from the Strategic Research Theme, University Development Fund, Small Project Grant, Outstanding Young Researcher Award (administrated by The University of Hong Kong), and Innovation & Technology Fund Grant ITS/129/08 is acknowledged.
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