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ITO multilayers as a current spreading layer to enhance the light output of ultraviolet light-emitting diodes
Journal of Alloys and Compounds 776 (2019) 960e964
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optimized ITO/Ag/ITO multilayers as a current spreading layer to enhance the light output of ultraviolet light-emitting diodes Sei Young Lee a, Young Soo Park b, Tae-Yeon Seong a, b, * a b
Department of Nanophotonics, Korea University, Seoul 02841, South Korea Department of Materials Science and Engineering, Korea University, Seoul 02841, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2018 Received in revised form 4 October 2018 Accepted 27 October 2018 Available online 30 October 2018
ITO/Ag/ITO multilayers were optimized at different annealing conditions and were employed as a current spreading layer (CSL) for 365 nm UVLEDs. The ITO (40 nm) layer gave a transmittance of 83.9% at 365 nm, while the ITO/Ag/ITO (13 nm/14 nm/23 nm) multilayers annealed at 600 C in N2 ambient had 92.3%. The ITO/Ag/ITO multilayer annealed at 600 C in N2 ambient yielded a sheet resistance of 3.8 U/sq., while the ITO layer had 186.8 U/sq. The ITO/Ag/ITO multilayers annealed at 600 C had a Haacke's figure of merit (FOM) of 106.1 103 U1, which was far larger than that of the ITO layer (1.5 103 U1). The annealed multilayers revealed optical bandgaps of 3.74e3.94 eV. The high-angle annular dark-field images showed that unlike the N2-annealed samples, the Ag layer of the air-annealed sample experienced agglomeration. Ultraviolet light-emitting diodes (UVLEDs) fabricated with the 600 C-annealed ITO/Ag/ITO CSL displayed 68.5% higher light output at 100 mA than the reference ITO UVLED. Unlike the UVLED with reference ITO, the UVLEDs with the annealed multilayers exhibited uniform light emission across the chip area. The higher light output was attributed to the combined effects of the high transmittance, and better current injection and current spreading of the annealed multilayers. © 2018 Elsevier B.V. All rights reserved.
Keywords: ITO/Ag/ITO multilayer Transmittance Sheet resistance Current spreading Ultraviolet light-emitting diodes
1. Introduction III-nitride-based ultraviolet light-emitting diodes (UVLEDs) have attracted a great deal of interest because of their applications in sterilization, disinfection, water and air purification, biochemistry, and solid-state lighting [1e4]. These applications have stimulated the technological development of UVLEDs. Compared with visible LEDs, however, UVLEDs exhibit still relatively low external quantum efficiencies (EQEs) [1,2]. One of the approaches of increasing EQEs is to enhance light extraction efficiency (LEE) and current injection efficiency (CIE) [1,2,4]. To accomplish these goals, the development of a highly transparent and conductive current spreading layer (CSL), which leads to reduction in the current crowding and photon absorption at p-electrodes of lateral UVLEDs, is crucial [5,6]. Sn-doped indium oxide (ITO) is commercially used as a transparent conducting oxide (TCO) because of its excellent electrical and optical properties [7,8]. However, there is a trade-off between the transmittance and sheet resistance of ITO. In other
* Corresponding author. Department of Materials Science and Engineering, Korea University, Seoul 02841, South Korea. E-mail address: [email protected] (T.-Y. Seong). https://doi.org/10.1016/j.jallcom.2018.10.368 0925-8388/© 2018 Elsevier B.V. All rights reserved.
words, thin ITO (<80 nm) has a high transmittance but gives relatively high resistivity [9]. Thus, to solve the issue, thin metal films sandwiched between two conducting oxide TCO films, e.g., ITO/Ag/ ITO, have been widely studied [10e14]. For example, Guillen and Herrero [10], characterizing the optical and electrical properties of sputtering-deposited ITO/Ag/ITO films as functions of Ag and ITO layer thicknesses, reported that the ITO/Ag/ITO (10 nm/20e40 nm/ 10 nm) films gave sheet resistance of about 6 U/sq. and the ITO/Ag/ ITO (30 nm/10 nm/30 nm) films have a transmittance of ~90% at ~600 nm. Lee et al. [11], investigating the opto-electrical properties of sputtered ITO/Ag/ITO layers which were annealed in N2 gas, showed that the sheet resistance and the transmittance depended on temperatures varying from 300 C to 500 C. The films gave a sheet resistance of 9.21 U/sq. and a transmittance of 88% at 380 nm when annealed at 500 C. In addition, Ren et al. [12], investigating the optical and electrical properties of sputtering-deposited ITO/ Ag/ITO films, reported a transmittance of 94.25% at 550 nm, figure of merit (FOM) of 80.9 103 U1 for ITO (48 nm)/Ag (15 nm)/ITO (42 nm) films, and surface roughnesses of 0.65e1.26 nm. Furthermore, these ITO/Ag/ITO films were applied as a current spreading layer of LEDs [15,16]. For instance, Hong et al. [15] investigated the effect of Ag layer thickness and electron beam irradiation on the
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electrical and optical properties of sputtered ITO/Ag/ITO films and reported that the optical band gap of the ITO/Ag/ITO multilayer was 4.35 eV and gave a transparency of ~80% at 375 nm. UVLEDs (375 nm) fabricated with the ITO/Ag/ITO p-type electrode with electron beam irradiation yielded 19% higher output power than those without beam irradiation, which was attributed to the low absorption loss in the p-type electrode. Lee et al. [16] also investigated the performance of ITO/Ag-based multilayer electrodes and reported that the ITO/Ag/ITO (5 nm/6 nm/20 nm) layer had a transmittance of 82% at 385 nm and a sheet resistance of 20 U. UVLEDs (385 nm) with ITO/Ag/ITO electrode showed a forward voltage of 3.55 V at 20 mA somewhat higher than that with ITO electrode. To our knowledge, ITO/Ag/ITO films have not been hitherto applied to UVLEDs (below 365 nm). Thus, in this study, we optimized the optical and electrical properties of ITO/Ag/ITO layers at different annealing conditions and then applied the optimal ITO/ Ag/ITO films (13 nm/14 nm/23 nm) as a CSL for 365 nm UVLEDs. The performance of UVLEDs fabricated with the multilayer CSLs was characterized and compared with those of reference LEDs with 40 nm-thick ITO CSL. 2. Experimental procedure UVLEDs were grown on 6-inch sapphire substrates by metalorganic chemical vapor deposition. The LED structures comprised a 20 nm-thick ex-situ AlN NL deposited by physical vapor deposition, a 2 mm-thick undoped GaN layer, a 2 mm-thick n-Al0.07Ga0.93N layer (1 1019 cm3), an AlGaN/InGaN multiple quantum well (MQW), an 80 nm-thick Mg-doped Al0.3Ga0.7N electron blocking layer (2 1020 cm3), and a p-GaN layer (1 1019 cm3) that was in-situ activated at 700 C for 5 min in a N2 stream within the MOCVD chamber. The samples were ultrasonically degreased with trichloroethylene, acetone, and methanol for 5 min per cleaning agent and then rinsed with deionized water. After cleaning the samples using a buffered oxide etch solution for 20 min, the samples were blown dry by nitrogen gas. ITO/Ag/ITO films were deposited on sapphire substrates by an electron beam (e-beam) evaporator and a radio frequency (RF) magnetron sputter. ITO target (99.999% purity) and Ag target (99.99% purity) were used at room temperature under a base pressure of ~1 Torr. For reference sample, an ITO (40 nm-thick) CSL was deposited on p-GaN by ebeam in order to avoid sputtering-induced plasma damage, followed by annealing at 600 C in air for 5 min. For the ITO/Ag/ITO multilayers, Ag (14 nm) and ITO (23 nm) layers were sequentially deposited on the e-beam-evaporated ITO by RF sputtering. These multilayer CSLs were annealed at 300 C and 600 C for 1 min in N2 ambient, and 600 C in air. For UVLEDs, Cr/Ni/Au (10 nm/200 nm/ 50 nm) layers were deposited as an n-type electrode and a p-type pad by e-beam for wire bonding. Schematic of UVLEDs fabricated with ITO and ITO/Ag/ITO CSLs are shown in Fig. 1. Hall measurements by the van der Pauw technique were conducted to estimate the carrier concentrations of the multilayers with a magnetic field of 0.55 T (HMS 3000, Ecopia). The sheet resistances were examined with the four-point-probe technique. Transmittance was estimated using a UV/Visible spectrometer (UV-100, Shimadzu). Scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) examination was made to investigate the interfacial structures of the ITO/Ag/ITO layers annealed at different conditions. To characterize the performance of UVLEDs (chip size: 300 600 mm2), current-voltage (IeV) characteristics were examined with an Agilent B1505A system. 3. Results and discussion In our previous study [17], the scattering matrix method was
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used to achieve a near-unity transmittance dielectric/Ag/ITO electrode for GaN-based LEDs. The transmittance of a dielectric layer/a thin metal layer (Ag, Au, or Al)/an ITO layer was investigated as a function of the thickness and the optical constant of each constituent layer. Calculations showed that the transmittance of a dielectric/metal/ITO multilayer film was maximized with an about 10-nm-thick Ag layer. It was found that the dielectric/Ag/ITO structure had a transmittance of 0.97 by optimizing the thickness of the dielectric layer and the ITO layer. Thus, on the basis of the design rules, ITO/Ag/ITO layers were designed and optimized. Differently from our expectation [17], however, the ITO/Ag/ITO (13 nm/14 nm/23 nm) layer had higher transmittance than the ITO/Ag/ ITO (23 nm/14 nm/23 nm) layer. This can be ascribed to the fact that e-beam-evaporated ITO has the density and dielectric constant different from those of sputtered ITO. Transmittances of the ITO/Ag/ ITO (13 nm/14 nm/23 nm) multilayers were measured as a function of annealing conditions, Fig. 2. The inset reveals a HAADF image of the ITO/Ag/ITO multilayer. Unlike a 40 nm-thick ITO layer, the transmittance of the multilayers is found to reach a maximum at ~350 ~400 nm and gradually decrease with increasing wavelength. This feature was explained by the destructive or constructive interference of reflected partial waves at the ITO/Ag interfaces [17]. Measurement showed that the ITO layer had a transmittance of 83.9% at 365 nm, while the multilayer annealed at 600 C in N2 ambient gave 92.3%. The carrier concentration and mobility of the ITO/Ag/ITO multilayers were also measured. The ITO/Ag/ITO multilayers exhibit higher carrier concentration, but lower mobility than reference ITO (40 nm), Table 1. Furthermore, the annealed multilayers show lower resistivity and sheet resistance than ITO. For example, the multilayers annealed at 600 C in N2 ambient yield a sheet resistance of 3.8 U/sq., while the ITO layer has 186.8 U/sq. As listed in Table 1, figure of merit (FOM), 4TC, of the ITO and ITO/Ag/ 10 ITO was estimated using Haacke's equation [18], 4TC ¼ Tav /Rs, where Rs is the sheet resistance and Tav is the average UV transmittance (360e400 nm), Table 1. Tav is defined as, Tav ¼ ! T(l)V(l)dl/!V(l)dl, where T(l) is the transmittance and V(l) is the photopic luminous efficiency function defining the standard observer for photometry [19,20]. The multilayer annealed at 600 C in N2 ambient shows the highest FOM of 106.1 103 U1, while the reference ITO displays much lower FOM of 1.5 103 U1. Fig. 3 displays absorption coefficients of the reference ITO and ITO/Ag/ITO multilayers annealed at different conditions. The absorption coefficient as a function of energy gap, Eg, can be given with the relation [21], a(hn) f (hn e Eg)½, where hn is the photon energy. The optical band gap of the reference ITO was measured to be 3.8 eV, which is in good agreement of previous findings [22]. The optical bandgaps of the annealed multilayers were estimated to be 3.74e3.94 eV. In particular, the multilayer annealed at 600 C in N2 ambient had higher bandgap (3.94 eV) than the ITO, which is consistent with the transmittances of the multilayers, Fig. 2. As shown in Table 1, the ITO/Ag/ITO multilayers annealed at 600 C in N2 ambient show higher transmittance and mobility than the multilayers annealed at 600 C in air. The STEM HAADF images and elemental mapping results (Fig. 4) reveal that unlike the N2annealed multilayer [Fig. 4(b)], the Ag layer of the air-annealed sample became roughened with a hillock (and voids), as marked by the arrows, Fig. 4(c), resulting in a rough Ag/ITO interface. This is consistent with the fact that a Ag layer becomes agglomerated by surface diffusion and through the bulk diffusion of Ag atoms by oxygen-vacancy interaction [23] when annealed at temperatures higher than 300 C. Thus, the lower transmittance of the airannealed sample could be explained by photon scattering at the rough Ag/ITO interface. Furthermore, mobility characteristic is known to be described by scattering mechanisms, e.g., phonon, grain-boundary, surface, interface, and ionized-impurity scattering
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Fig. 1. Schematic of UVLEDs fabricated with (a) ITO and (b) ITO/Ag/ITO CSLs.
Fig. 2. Transmittances of reference ITO and ITO/Ag/ITO (13 nm/14 nm/23 nm) multilayers annealed at different conditions. The inset reveals a HAADF image of the asdeposited ITO/Ag/ITO multilayer.
Fig. 3. Absorption coefficients of reference ITO and ITO/Ag/ITO multilayers annealed at different conditions.
[14]. With the different annealing conditions, this indicates that the interface scattering may be responsible for the lower mobility of the air-annealed sample. To verify the current spreading characteristic of the ITO/Ag/ITO multilayers, 365 nm UVLEDs were fabricated with reference ITO and ITO/Ag/ITO multilayer CSLs. Fig. 5 exhibits the IeV characteristics of UVLEDs with different CSLs. It is shown that UVLEDs with the reference ITO, ITO/Ag/ITO annealed at 300 C in N2 ambient, and ITO/Ag/ITO annealed at 600 C in N2 ambient give forward-bias voltages of 3.59, 3.50, and 3.51 V at 20 mA and series resistances of 9.24, 7.55 and 7.64 U, respectively. The light output-current (LI) characteristics of UVLEDs exhibit that the annealed multilayer samples produce higher light output than the reference ITO UVLED, Fig. 6. For example, the UVLED with the 600 C-annealed ITO/Ag/
ITO CSL shows 68.5% higher light output at 100 mA than the reference ITO UVLED. The higher light output can be attributed to the combined effects of the higher transmittance and lower sheet resistance of the annealed ITO/Ag/ITO multilayer, namely, the improved current injection, extraction, and spreading. This is consistent with the fact that the annealed ITO/Ag/ITO multilayer has higher transmittance (92.3% at 365 nm) than reference ITO (83.9%) and lower sheet resistance (3.8 U/sq.) than ITO (186.8 U/ sq.). To examine the current spreading and light extraction behaviour of UVLEDs fabricated with different CSLs, the emission images of the UVLEDs were taken at 50 mA. Fig. 7 reveals the plan-view emission images from the UVLEDs with reference ITO, ITO/Ag/ ITO annealed at 300 C in N2 ambient, and ITO/Ag/ITO annealed at 600 C in N2 ambient. The MATLAB program was used to reduce
Table 1 Summary of the optical and electrical properties of ITO/Ag/ITO multilayers annealed at different conditions.
Ref. ITO (40 nm) ITO/Ag/ITO (as-dep) ITO/Ag/ITO (300 C, N2) ITO/Ag/ITO (600 C, N2) ITO/Ag/ITO (600 C, air)
Average transmittance (%)
Carrier concentration (cm3)
Mobility (cm2/V s)
Resistivity (U cm)
Sheet resistance (U/sq.)
FOM (U1)
88.1 85.3 88.1 91.3 81.3
2.63 1020 2.28 1022 2.35 1022 2.39 1022 2.10 1022
31.8 9.2 13.3 15.0 10.6
7.47 104 2.42 105 1.97 105 1.89 105 2.81 105
186.8 4.8 3.9 3.8 5.6
1.5 103 42.6 103 72.5 103 106.1 103 22.6 103
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Fig. 4. STEM HAADF images and elemental mapping results obtained from ITO/Ag/ITO multilayers annealed at (a) room temperature, (b) 600 C in N2 ambient and (c) 600 C in air.
Fig. 5. IeV characteristics of UVLEDs fabricated with reference ITO and ITO/Ag/ITO multilayers annealed at room temperature, 300 C, and 600 C in N2 ambient.
the original emission images to 250 different colours. Fig. 7(a) shows an image from the reference UVLED, whose photoemission is inhomogeneous and localized around the p-pad owing to current crowding. On the other hand, the UVLEDs with ITO/Ag/ITO multilayers annealed at 300 C and 600 C (Fig. 7(b) and (c),
Fig. 6. LI characteristics of UVLEDs with reference ITO and ITO/Ag/ITO multilayers annealed at room temperature, 300 C, and 600 C in N2 ambient.
respectively) display uniform light emission across the chip area due to their lower sheet resistances (3.8e3.9 U/sq.). A comparison of the light output powers and emission images shows that the ITO/Ag/ITO multilayer annealed at 600 C in N2 ambient can serve as an excellent CSL for the fabrication of high-performance UVLEDs.
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Fig. 7. Plan-view emission images of UVLEDs with (a) reference ITO, (b) ITO/Ag/ITO annealed at 300 C in N2 ambient, and (c) ITO/Ag/ITO annealed at 600 C in N2 ambient.
4. Conclusion ITO/Ag/ITO multilayers were optimized at different annealing conditions and were employed as a CSL for 365 nm UVLEDs. The ITO/Ag/ITO (13 nm/14 nm/23 nm) multilayers annealed at 600 C in N2 ambient exhibited the highest transmittance at 365 nm and the lowest sheet resistance. The ITO/Ag/ITO multilayer annealed at 600 C in N2 ambient had the highest optical energy gap of 3.94 eV. The ITO/Ag/ITO multilayers annealed at 600 C showed much larger Haacke's FOM than the reference ITO layer. The STEM HAADF images displayed that annealing in air caused agglomeration of the Ag layer. UVLED fabricated with the ITO/Ag/ITO annealed at 600 C in N2 ambient yielded 68.5% higher light output at 100 mA than the reference ITO UVLED. These results suggest that the ITO/Ag/ITO annealed at 600 C in N2 ambient could serve as a promising CSL for high-performance UVLEDs. Acknowledgments This work was supported by the Global Research Laboratory (GRL) program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT (NRF2017K1A1A2013160). References [1] A. Khan, K. Balakrishnan, T. Katona, Nat. Photon. 2 (2) (2008) 77e84.
[2] M. Kneissl, T. Kolbe, C. Chua, V. Kueller, N. Lobo, J. Stellmach, A. Knauer, H. Rodriguez, S. Einfeldt, Z. YangN, M. JohnsonM. Weyers, Semicond. Sci. Technol. 26 (1) (2011), 014036. [3] H. Jeong, R. Salas-Montiel, G. Lerondel, M.S. Jeong, J. Alloys Compd. 708 (2017) 612e618. [4] J.-S. Park, J.-H. Kim, D.-H. Kim, D. Kang, T.-Y. Seong, J. Alloys Compd. 703 (2017) 198e203. [5] J.-O. Song, J.S. Kwak, Y. Park, T.-Y. Seong, Appl. Phys. Lett. 86 (2005), 213505. [6] D.-S. Leem, T. Lee, T.-Y. Seong, Solid State Electron. 51 (2007) 793. [7] A. Kumar, C. Zhou, ACS Nano 4 (2010) 11e14. [8] M. Oh, H.J. Jeong, M.S. Jeong, K.-S. Ahn, K.-K. Kim, J.-H. Ryou, H. Kim, J. Alloys Compd. 724 (2017) 813e819. [9] Y.Z. Yoo, J.-Y. Na, Y.S. Choi, Y.J. Lim, J.-H. Kim, J.-H. Kim, S.-K. Kim, T.-Y. Seong, Small 14 (2018), 1800056. [10] C. Guillen, J. Herrero, Optic Commun. 282 (2009) 574e578. [11] K.H. Choi, J.Y. Kim, Y.S. Lee, H.J. Kim, Thin Solid Films 341 (1999) 152e155. [12] N. Ren, J. Zhu, S. Ban, AIP Adv. 7 (2017), 055009. [13] J.H. Lee, K.Y. Woo, K.H. Kim, H.-D. Kim, T.G. Kim, Opt. Lett. 38 (2013) 5055e5058. [14] A. Indluru, T.L. Alford, J. Appl. Phys. 105 (2009), 123528. [15] C.H. Hong, Y.J. Jo, H.A. Kim, M.J. Park, J.S. Kwak, J. Nanosci. Nanotechnol. 12 (2012) 4163e4167. [16] B.R. Lee, T.H. Lee, K.-R. Son, T.G. Kim, J. Alloys Compd. 741 (2018) 21e27. [17] H.-K. Lee, J.-Y. Na, Y.-J. Moon, T.-Y. Seong, S.-K. Kim, Curr. Appl. Phys. 15 (2015) 833e838. [18] G. Haacke, J. Appl. Phys. 47 (1976) 4086e4089. [19] W.G. Driscoll, W. Vaughan, Handbook of Optics, McGraw-Hill, Ohio, USA, 1978. [20] S. Yu, W. Zhang, L. Li, D. Xu, H. Dong, Y. Jin, Thin Solid Films 552 (2014) 150e154. [21] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627e638. [22] O. Tuna, Y. Selamet, G. Aygun, L. Ozyuzer, J. Phys. D 43 (2010), 055402. [23] J.-S. Sung, J. Han, D.-Y. Noh, T.-Y. Seong, Scripta Mater. 80 (2014) 5e8.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optimized ITO/Ag/ITO multilayers as a current spreading layer to enhance the light output of ultraviolet light-emitting diodes Sei Young Lee a, Young Soo Park b, Tae-Yeon Seong a, b, * a b
Department of Nanophotonics, Korea University, Seoul 02841, South Korea Department of Materials Science and Engineering, Korea University, Seoul 02841, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2018 Received in revised form 4 October 2018 Accepted 27 October 2018 Available online 30 October 2018
ITO/Ag/ITO multilayers were optimized at different annealing conditions and were employed as a current spreading layer (CSL) for 365 nm UVLEDs. The ITO (40 nm) layer gave a transmittance of 83.9% at 365 nm, while the ITO/Ag/ITO (13 nm/14 nm/23 nm) multilayers annealed at 600 C in N2 ambient had 92.3%. The ITO/Ag/ITO multilayer annealed at 600 C in N2 ambient yielded a sheet resistance of 3.8 U/sq., while the ITO layer had 186.8 U/sq. The ITO/Ag/ITO multilayers annealed at 600 C had a Haacke's figure of merit (FOM) of 106.1 103 U1, which was far larger than that of the ITO layer (1.5 103 U1). The annealed multilayers revealed optical bandgaps of 3.74e3.94 eV. The high-angle annular dark-field images showed that unlike the N2-annealed samples, the Ag layer of the air-annealed sample experienced agglomeration. Ultraviolet light-emitting diodes (UVLEDs) fabricated with the 600 C-annealed ITO/Ag/ITO CSL displayed 68.5% higher light output at 100 mA than the reference ITO UVLED. Unlike the UVLED with reference ITO, the UVLEDs with the annealed multilayers exhibited uniform light emission across the chip area. The higher light output was attributed to the combined effects of the high transmittance, and better current injection and current spreading of the annealed multilayers. © 2018 Elsevier B.V. All rights reserved.
Keywords: ITO/Ag/ITO multilayer Transmittance Sheet resistance Current spreading Ultraviolet light-emitting diodes
1. Introduction III-nitride-based ultraviolet light-emitting diodes (UVLEDs) have attracted a great deal of interest because of their applications in sterilization, disinfection, water and air purification, biochemistry, and solid-state lighting [1e4]. These applications have stimulated the technological development of UVLEDs. Compared with visible LEDs, however, UVLEDs exhibit still relatively low external quantum efficiencies (EQEs) [1,2]. One of the approaches of increasing EQEs is to enhance light extraction efficiency (LEE) and current injection efficiency (CIE) [1,2,4]. To accomplish these goals, the development of a highly transparent and conductive current spreading layer (CSL), which leads to reduction in the current crowding and photon absorption at p-electrodes of lateral UVLEDs, is crucial [5,6]. Sn-doped indium oxide (ITO) is commercially used as a transparent conducting oxide (TCO) because of its excellent electrical and optical properties [7,8]. However, there is a trade-off between the transmittance and sheet resistance of ITO. In other
* Corresponding author. Department of Materials Science and Engineering, Korea University, Seoul 02841, South Korea. E-mail address: [email protected] (T.-Y. Seong). https://doi.org/10.1016/j.jallcom.2018.10.368 0925-8388/© 2018 Elsevier B.V. All rights reserved.
words, thin ITO (<80 nm) has a high transmittance but gives relatively high resistivity [9]. Thus, to solve the issue, thin metal films sandwiched between two conducting oxide TCO films, e.g., ITO/Ag/ ITO, have been widely studied [10e14]. For example, Guillen and Herrero [10], characterizing the optical and electrical properties of sputtering-deposited ITO/Ag/ITO films as functions of Ag and ITO layer thicknesses, reported that the ITO/Ag/ITO (10 nm/20e40 nm/ 10 nm) films gave sheet resistance of about 6 U/sq. and the ITO/Ag/ ITO (30 nm/10 nm/30 nm) films have a transmittance of ~90% at ~600 nm. Lee et al. [11], investigating the opto-electrical properties of sputtered ITO/Ag/ITO layers which were annealed in N2 gas, showed that the sheet resistance and the transmittance depended on temperatures varying from 300 C to 500 C. The films gave a sheet resistance of 9.21 U/sq. and a transmittance of 88% at 380 nm when annealed at 500 C. In addition, Ren et al. [12], investigating the optical and electrical properties of sputtering-deposited ITO/ Ag/ITO films, reported a transmittance of 94.25% at 550 nm, figure of merit (FOM) of 80.9 103 U1 for ITO (48 nm)/Ag (15 nm)/ITO (42 nm) films, and surface roughnesses of 0.65e1.26 nm. Furthermore, these ITO/Ag/ITO films were applied as a current spreading layer of LEDs [15,16]. For instance, Hong et al. [15] investigated the effect of Ag layer thickness and electron beam irradiation on the
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electrical and optical properties of sputtered ITO/Ag/ITO films and reported that the optical band gap of the ITO/Ag/ITO multilayer was 4.35 eV and gave a transparency of ~80% at 375 nm. UVLEDs (375 nm) fabricated with the ITO/Ag/ITO p-type electrode with electron beam irradiation yielded 19% higher output power than those without beam irradiation, which was attributed to the low absorption loss in the p-type electrode. Lee et al. [16] also investigated the performance of ITO/Ag-based multilayer electrodes and reported that the ITO/Ag/ITO (5 nm/6 nm/20 nm) layer had a transmittance of 82% at 385 nm and a sheet resistance of 20 U. UVLEDs (385 nm) with ITO/Ag/ITO electrode showed a forward voltage of 3.55 V at 20 mA somewhat higher than that with ITO electrode. To our knowledge, ITO/Ag/ITO films have not been hitherto applied to UVLEDs (below 365 nm). Thus, in this study, we optimized the optical and electrical properties of ITO/Ag/ITO layers at different annealing conditions and then applied the optimal ITO/ Ag/ITO films (13 nm/14 nm/23 nm) as a CSL for 365 nm UVLEDs. The performance of UVLEDs fabricated with the multilayer CSLs was characterized and compared with those of reference LEDs with 40 nm-thick ITO CSL. 2. Experimental procedure UVLEDs were grown on 6-inch sapphire substrates by metalorganic chemical vapor deposition. The LED structures comprised a 20 nm-thick ex-situ AlN NL deposited by physical vapor deposition, a 2 mm-thick undoped GaN layer, a 2 mm-thick n-Al0.07Ga0.93N layer (1 1019 cm3), an AlGaN/InGaN multiple quantum well (MQW), an 80 nm-thick Mg-doped Al0.3Ga0.7N electron blocking layer (2 1020 cm3), and a p-GaN layer (1 1019 cm3) that was in-situ activated at 700 C for 5 min in a N2 stream within the MOCVD chamber. The samples were ultrasonically degreased with trichloroethylene, acetone, and methanol for 5 min per cleaning agent and then rinsed with deionized water. After cleaning the samples using a buffered oxide etch solution for 20 min, the samples were blown dry by nitrogen gas. ITO/Ag/ITO films were deposited on sapphire substrates by an electron beam (e-beam) evaporator and a radio frequency (RF) magnetron sputter. ITO target (99.999% purity) and Ag target (99.99% purity) were used at room temperature under a base pressure of ~1 Torr. For reference sample, an ITO (40 nm-thick) CSL was deposited on p-GaN by ebeam in order to avoid sputtering-induced plasma damage, followed by annealing at 600 C in air for 5 min. For the ITO/Ag/ITO multilayers, Ag (14 nm) and ITO (23 nm) layers were sequentially deposited on the e-beam-evaporated ITO by RF sputtering. These multilayer CSLs were annealed at 300 C and 600 C for 1 min in N2 ambient, and 600 C in air. For UVLEDs, Cr/Ni/Au (10 nm/200 nm/ 50 nm) layers were deposited as an n-type electrode and a p-type pad by e-beam for wire bonding. Schematic of UVLEDs fabricated with ITO and ITO/Ag/ITO CSLs are shown in Fig. 1. Hall measurements by the van der Pauw technique were conducted to estimate the carrier concentrations of the multilayers with a magnetic field of 0.55 T (HMS 3000, Ecopia). The sheet resistances were examined with the four-point-probe technique. Transmittance was estimated using a UV/Visible spectrometer (UV-100, Shimadzu). Scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) examination was made to investigate the interfacial structures of the ITO/Ag/ITO layers annealed at different conditions. To characterize the performance of UVLEDs (chip size: 300 600 mm2), current-voltage (IeV) characteristics were examined with an Agilent B1505A system. 3. Results and discussion In our previous study [17], the scattering matrix method was
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used to achieve a near-unity transmittance dielectric/Ag/ITO electrode for GaN-based LEDs. The transmittance of a dielectric layer/a thin metal layer (Ag, Au, or Al)/an ITO layer was investigated as a function of the thickness and the optical constant of each constituent layer. Calculations showed that the transmittance of a dielectric/metal/ITO multilayer film was maximized with an about 10-nm-thick Ag layer. It was found that the dielectric/Ag/ITO structure had a transmittance of 0.97 by optimizing the thickness of the dielectric layer and the ITO layer. Thus, on the basis of the design rules, ITO/Ag/ITO layers were designed and optimized. Differently from our expectation [17], however, the ITO/Ag/ITO (13 nm/14 nm/23 nm) layer had higher transmittance than the ITO/Ag/ ITO (23 nm/14 nm/23 nm) layer. This can be ascribed to the fact that e-beam-evaporated ITO has the density and dielectric constant different from those of sputtered ITO. Transmittances of the ITO/Ag/ ITO (13 nm/14 nm/23 nm) multilayers were measured as a function of annealing conditions, Fig. 2. The inset reveals a HAADF image of the ITO/Ag/ITO multilayer. Unlike a 40 nm-thick ITO layer, the transmittance of the multilayers is found to reach a maximum at ~350 ~400 nm and gradually decrease with increasing wavelength. This feature was explained by the destructive or constructive interference of reflected partial waves at the ITO/Ag interfaces [17]. Measurement showed that the ITO layer had a transmittance of 83.9% at 365 nm, while the multilayer annealed at 600 C in N2 ambient gave 92.3%. The carrier concentration and mobility of the ITO/Ag/ITO multilayers were also measured. The ITO/Ag/ITO multilayers exhibit higher carrier concentration, but lower mobility than reference ITO (40 nm), Table 1. Furthermore, the annealed multilayers show lower resistivity and sheet resistance than ITO. For example, the multilayers annealed at 600 C in N2 ambient yield a sheet resistance of 3.8 U/sq., while the ITO layer has 186.8 U/sq. As listed in Table 1, figure of merit (FOM), 4TC, of the ITO and ITO/Ag/ 10 ITO was estimated using Haacke's equation [18], 4TC ¼ Tav /Rs, where Rs is the sheet resistance and Tav is the average UV transmittance (360e400 nm), Table 1. Tav is defined as, Tav ¼ ! T(l)V(l)dl/!V(l)dl, where T(l) is the transmittance and V(l) is the photopic luminous efficiency function defining the standard observer for photometry [19,20]. The multilayer annealed at 600 C in N2 ambient shows the highest FOM of 106.1 103 U1, while the reference ITO displays much lower FOM of 1.5 103 U1. Fig. 3 displays absorption coefficients of the reference ITO and ITO/Ag/ITO multilayers annealed at different conditions. The absorption coefficient as a function of energy gap, Eg, can be given with the relation [21], a(hn) f (hn e Eg)½, where hn is the photon energy. The optical band gap of the reference ITO was measured to be 3.8 eV, which is in good agreement of previous findings [22]. The optical bandgaps of the annealed multilayers were estimated to be 3.74e3.94 eV. In particular, the multilayer annealed at 600 C in N2 ambient had higher bandgap (3.94 eV) than the ITO, which is consistent with the transmittances of the multilayers, Fig. 2. As shown in Table 1, the ITO/Ag/ITO multilayers annealed at 600 C in N2 ambient show higher transmittance and mobility than the multilayers annealed at 600 C in air. The STEM HAADF images and elemental mapping results (Fig. 4) reveal that unlike the N2annealed multilayer [Fig. 4(b)], the Ag layer of the air-annealed sample became roughened with a hillock (and voids), as marked by the arrows, Fig. 4(c), resulting in a rough Ag/ITO interface. This is consistent with the fact that a Ag layer becomes agglomerated by surface diffusion and through the bulk diffusion of Ag atoms by oxygen-vacancy interaction [23] when annealed at temperatures higher than 300 C. Thus, the lower transmittance of the airannealed sample could be explained by photon scattering at the rough Ag/ITO interface. Furthermore, mobility characteristic is known to be described by scattering mechanisms, e.g., phonon, grain-boundary, surface, interface, and ionized-impurity scattering
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Fig. 1. Schematic of UVLEDs fabricated with (a) ITO and (b) ITO/Ag/ITO CSLs.
Fig. 2. Transmittances of reference ITO and ITO/Ag/ITO (13 nm/14 nm/23 nm) multilayers annealed at different conditions. The inset reveals a HAADF image of the asdeposited ITO/Ag/ITO multilayer.
Fig. 3. Absorption coefficients of reference ITO and ITO/Ag/ITO multilayers annealed at different conditions.
[14]. With the different annealing conditions, this indicates that the interface scattering may be responsible for the lower mobility of the air-annealed sample. To verify the current spreading characteristic of the ITO/Ag/ITO multilayers, 365 nm UVLEDs were fabricated with reference ITO and ITO/Ag/ITO multilayer CSLs. Fig. 5 exhibits the IeV characteristics of UVLEDs with different CSLs. It is shown that UVLEDs with the reference ITO, ITO/Ag/ITO annealed at 300 C in N2 ambient, and ITO/Ag/ITO annealed at 600 C in N2 ambient give forward-bias voltages of 3.59, 3.50, and 3.51 V at 20 mA and series resistances of 9.24, 7.55 and 7.64 U, respectively. The light output-current (LI) characteristics of UVLEDs exhibit that the annealed multilayer samples produce higher light output than the reference ITO UVLED, Fig. 6. For example, the UVLED with the 600 C-annealed ITO/Ag/
ITO CSL shows 68.5% higher light output at 100 mA than the reference ITO UVLED. The higher light output can be attributed to the combined effects of the higher transmittance and lower sheet resistance of the annealed ITO/Ag/ITO multilayer, namely, the improved current injection, extraction, and spreading. This is consistent with the fact that the annealed ITO/Ag/ITO multilayer has higher transmittance (92.3% at 365 nm) than reference ITO (83.9%) and lower sheet resistance (3.8 U/sq.) than ITO (186.8 U/ sq.). To examine the current spreading and light extraction behaviour of UVLEDs fabricated with different CSLs, the emission images of the UVLEDs were taken at 50 mA. Fig. 7 reveals the plan-view emission images from the UVLEDs with reference ITO, ITO/Ag/ ITO annealed at 300 C in N2 ambient, and ITO/Ag/ITO annealed at 600 C in N2 ambient. The MATLAB program was used to reduce
Table 1 Summary of the optical and electrical properties of ITO/Ag/ITO multilayers annealed at different conditions.
Ref. ITO (40 nm) ITO/Ag/ITO (as-dep) ITO/Ag/ITO (300 C, N2) ITO/Ag/ITO (600 C, N2) ITO/Ag/ITO (600 C, air)
Average transmittance (%)
Carrier concentration (cm3)
Mobility (cm2/V s)
Resistivity (U cm)
Sheet resistance (U/sq.)
FOM (U1)
88.1 85.3 88.1 91.3 81.3
2.63 1020 2.28 1022 2.35 1022 2.39 1022 2.10 1022
31.8 9.2 13.3 15.0 10.6
7.47 104 2.42 105 1.97 105 1.89 105 2.81 105
186.8 4.8 3.9 3.8 5.6
1.5 103 42.6 103 72.5 103 106.1 103 22.6 103
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Fig. 4. STEM HAADF images and elemental mapping results obtained from ITO/Ag/ITO multilayers annealed at (a) room temperature, (b) 600 C in N2 ambient and (c) 600 C in air.
Fig. 5. IeV characteristics of UVLEDs fabricated with reference ITO and ITO/Ag/ITO multilayers annealed at room temperature, 300 C, and 600 C in N2 ambient.
the original emission images to 250 different colours. Fig. 7(a) shows an image from the reference UVLED, whose photoemission is inhomogeneous and localized around the p-pad owing to current crowding. On the other hand, the UVLEDs with ITO/Ag/ITO multilayers annealed at 300 C and 600 C (Fig. 7(b) and (c),
Fig. 6. LI characteristics of UVLEDs with reference ITO and ITO/Ag/ITO multilayers annealed at room temperature, 300 C, and 600 C in N2 ambient.
respectively) display uniform light emission across the chip area due to their lower sheet resistances (3.8e3.9 U/sq.). A comparison of the light output powers and emission images shows that the ITO/Ag/ITO multilayer annealed at 600 C in N2 ambient can serve as an excellent CSL for the fabrication of high-performance UVLEDs.
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Fig. 7. Plan-view emission images of UVLEDs with (a) reference ITO, (b) ITO/Ag/ITO annealed at 300 C in N2 ambient, and (c) ITO/Ag/ITO annealed at 600 C in N2 ambient.
4. Conclusion ITO/Ag/ITO multilayers were optimized at different annealing conditions and were employed as a CSL for 365 nm UVLEDs. The ITO/Ag/ITO (13 nm/14 nm/23 nm) multilayers annealed at 600 C in N2 ambient exhibited the highest transmittance at 365 nm and the lowest sheet resistance. The ITO/Ag/ITO multilayer annealed at 600 C in N2 ambient had the highest optical energy gap of 3.94 eV. The ITO/Ag/ITO multilayers annealed at 600 C showed much larger Haacke's FOM than the reference ITO layer. The STEM HAADF images displayed that annealing in air caused agglomeration of the Ag layer. UVLED fabricated with the ITO/Ag/ITO annealed at 600 C in N2 ambient yielded 68.5% higher light output at 100 mA than the reference ITO UVLED. These results suggest that the ITO/Ag/ITO annealed at 600 C in N2 ambient could serve as a promising CSL for high-performance UVLEDs. Acknowledgments This work was supported by the Global Research Laboratory (GRL) program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT (NRF2017K1A1A2013160). References [1] A. Khan, K. Balakrishnan, T. Katona, Nat. Photon. 2 (2) (2008) 77e84.
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