December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
5055
ITO/Ag/ITO multilayer-based transparent conductive electrodes for ultraviolet light-emitting diodes Jae Hoon Lee, Kie Young Woo, Kyeong Heon Kim, Hee-Dong Kim, and Tae Geun Kim* School of Electrical Engineering, Korea University, Seoul 136-701, South Korea *Corresponding author: [email protected] Received September 3, 2013; revised October 17, 2013; accepted October 24, 2013; posted October 25, 2013 (Doc. ID 196923); published November 22, 2013 ITO/Ag/ITO (IAI) multilayer-based transparent conductive electrodes for ultraviolet light-emitting diodes are fabricated by reactive sputtering, optimized by annealing, and characterized with respect to electrical and optical properties. Increasing the annealing temperature from 300°C to 500°C decreased the sheet resistance and increased the transmittance. This may result from an observed improvement in the crystallinity of the IAI multilayer and a reduction in the near-UV absorption coefficient of Ag. We observed the lowest sheet resistance (9.21 Ω∕sq) and the highest optical transmittance (88%) at 380 nm for the IAI multilayer samples annealed in N2 gas at 500°C. © 2013 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (310.4165) Multilayer design; (310.6860) Thin films, optical properties; (310.7005) Transparent conductive coatings. http://dx.doi.org/10.1364/OL.38.005055
GaN-based near-ultraviolet (UV) light-emitting diodes (LEDs) in the wavelength range 380–400 nm have applications in various fields such as analytical instrumentation, water purification, medical treatments, biological agent identification, and white LED displays for general lighting [1–3]. However, unlike blue LEDs, there are few transparent conductive electrodes (TCEs) suitable for near-UV LEDs. Using incompatible TCE materials decreases the external quantum efficiency (EQE) of UV LEDs [4]. In particular, because of the high sheet resistance and low carrier mobility of the p-type thin GaN layer over the whole range of emitted wavelengths, LEDs made from this material exhibit current crowding and low current spreading. Therefore, the p-type electrode must have low contact resistance on a p-GaN layer and high transmittance in the near-UV region [5]. Indium-tin oxide (ITO) is widely used as a TCE material in optoelectronic devices, including flat displays, thin film transistors, and solar cells, because it provides high transparency in the visible region and good electrical conductivity [6–8]. However, ITO suffers from several problems because of absorption in UV region. Several methods have been reported to reduce the light absorption by TCEs in UV LEDs. One method is to use a thin ITO layer, with only a few tens of nanometers [9]. Although this method improves the transmittance of the ITO, it abruptly increases the sheet resistance by reducing the thickness of the thin ITO films. Another approach is to use carbon-based TCEs such as graphene and singlewalled carbon nanotubes (SWNTs) [5,10]. The feasibility of carbon-based TCEs with low sheet resistance and high UV transmittance has been demonstrated [11]. However, before carbon-based TCEs can be widely incorporated into UV LEDs, crucial manufacturing challenges such as ensuring that the carbon nanomaterial film has high quality and is uniform and processed in a manner compatible with the UV LED fabrication must be overcome. Recently, a continuous metal film has also exhibited high performance for the broadband light transparency as TCEs [12,13]. On the other hand, Girtan reported an ITO/Ag/ITO (IAI) multilayer as an alternative TCE [14]. According to this report, the electrical and optical 0146-9592/13/235055-04$15.00/0
properties of IAI-based TCEs can be improved by varying the thickness of the metal layer between the oxide layers. In a well-designed IAI multilayer, the large difference in the refractive index between ITO and Ag suppresses the reflection from the metal layer, which can then become selectively transparent in the visible and UV regions. Nevertheless, IAI-based TCEs in UV LEDs have not yet been fully realized, because the metal tends to agglomerate during the annealing processes that are inevitable in LED fabrication [15]. Therefore, to use IAI-based TCEs in UV LEDs, a method must be found to allow IAI to sustain its properties after annealing. We propose TCEs based on IAI multilayers. Herein, we demonstrate their high conductivity and high transmittance, acceptable for UV LED applications. In addition, we discuss the possible conduction mechanisms in the IAI-based TCEs as a function of the annealing temperature, and the role of Ag in determining the transmission properties of the films. The p-GaN layer of the 380 nm UV LEDs used in this study was grown on c-face sapphire substrates by metal organic chemical vapor deposition (MOCVD). The UV LED structure consisted of a 20-nm-thick GaN low-temperature buffer layer, a 2.6-μm-thick undoped GaN layer, a 4.6-μmthick n-type GaN layer, five AlGaInN/InGaN multiple quantum well (MQW) layers, a 30-nm-thick p-type Al0.2 Ga0.8 N layer, and a 95-nm-thick p-type GaN capping layer. Before the fabrication of the IAI multilayer, the surfaces of p-GaN were ultrasonically degreased with acetone, methanol, deionized (DI) water, and a mixture of sulfuric acid (H2 SO4 ) and hydrogen peroxide (H2 O2 ) for 5 min in each step to remove the surface oxides formed on GaN. After this treatment, a 15-nm-thick bottom ITO film was deposited by RF magnetron sputtering onto the p-GaN surface at 5 mTorr pressure and 100 W. After deposition of the bottom ITO film, the Ag interlayer with a thickness of 7 nm was deposited by RF magnetron sputtering onto the ITO film at 2 mTorr pressure and 100 W. The top ITO film was then deposited using the same conditions as for the bottom ITO film. The resulting IAI multilayer was annealed in a rapid thermal annealing (RTA) system in N2 ambient gas. The sheet resistance of the IAI multilayer was © 2013 Optical Society of America
5056
OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
measured at room temperature using a four-point probe (Advanced Instrument Technology, CMT-SR1000N). Current–voltage (I–V ) characteristics were measured at the interface between IAI-TCEs and p-GaN using a Keithley 4200 measurement system. The surface morphology of the IMI multilayer, as a function of annealing temperature, was analyzed via transmission electron microscopy (TEM). The transmittance of the IAI multilayer was also measured over the wavelength range 280–800 nm using a UV/visible spectrometer (PerkinElmer, Lambda 35). We first investigated the sheet resistance and resistivity of the IAI multilayer as a function of the annealing temperature. Figure 1 shows the effect of postannealing on the sheet resistance and resistivity of the IAI multilayer. It is well established that the resistivity of thin ITO films may vary greatly [16,17]. The thin ITO films in this study exhibited sheet resistance of 130 Ω∕sq and resistivity of 1.08 × 10−2 Ω · cm. For the IAI multilayer, the resistivity decreases from 9.67 × 10−4 to 7.66 × 10−4 Ω · cm after annealing at temperatures of up to 500°C; however, at temperatures over 600°C, the resistivity increases drastically to 2.73 × 10−3 Ω · cm. On the other hand, the sheet resistance decreases from 11.62 to 9.21 Ω∕sq at an annealing temperature of 500°C, which is attributed to the improved crystal quality of ITO films and efficient protection from Ag-oxide formation [18,19]. However, metal agglomeration causes a sudden increase in sheet resistance to 32.77 Ω∕sq at annealing temperatures over 600°C. Figure 2 shows the typical I–V characteristics of IAI-TCE on p-GaN layers as a function of the annealing temperature. The slope of the I–V curve increases with annealing temperature until 500°C, after which it decreases. The as-deposited sample and the samples annealed at 300°C, 400°C, 500°C, 600°C, and 700°C exhibit current values of 3.7 × 10−2 , 4.1 × 10−2 , 4.3 × 10−2 , 4.9 × 10−2 , 3.2 × 10−2 , and 1.6 × 10−2 A at 1 V, respectively. The improvement in conduction observed for the samples annealed at or below 500°C is consistent with the interfacial morphology of the IAI multilayer, as shown by the TEM cross-sectional images (Fig. 3). While the as-deposited sample shows an amorphous phase [Fig. 3(a)], the samples annealed at 500°C show increased crystallinity and columnar grains in both ITO and Ag layers [Fig. 3(b)]. This helps explain the improvement in the electrical properties [20]. However, Fig. 3(c) shows that the Ag films agglomerate when annealed at 700°C. The agglomeration increases the sheet resistance of the IAI multilayer films. To determine the best conditions for fabricating the IAI multilayer as a transparent p-electrode in UV-LEDs, we
Fig. 1. Sheet resistance and resistivity of IAI multilayer as a function of annealing temperature.
Fig. 2. I–V characteristics of the IAI multilayer as a function of annealing temperature.
calculated a figure of merit, ϕTC , from the 380 nm transmittance and the sheet resistance. The ϕTC values are defined by Haacke as follows [21]: φTC
T 10 ; Rsh
(1)
where T is the transmittance and Rsh is the sheet resistance for the films. The figure of merit measured for the as-deposited sample and those annealed at 300°C, 400°C, 500°C, 600°C, and 700°C are 8.4 × 10−3 , 10.0 × 10−3 , 20.0 × 10−3 , 31.7 × 10−3 , 10.0 × 10−3 , and 0.2 × 10−3 Ω−1 , respectively. The samples annealed at 500°C exhibited the highest figure of merit (31.7 × 10−3 Ω−1 ), much higher than that of a typical thin ITO film (2.8 × 10−3 Ω−1 ) [22]. Figure 4 shows the optical transmittance spectra measured for the IAI multilayer on quartz substrates as a function of annealing temperature. The optical transmittance of the as-deposited IAI multilayer is approximately 79% at 380 nm. At 500°C, its transmittance increases from 79% to 88%, an increase of 9% compared to a typical thin ITO film. However, the transmittance may differ depending on the wavelength; this phenomenon may be related
Fig. 3. Cross-sectional TEM image in the interface region of IAI multilayer (a) before and after postannealing at (b) 500°C and (c) 700°C.
Fig. 4. Optical transmittance spectra for IAI multilayer as a function of annealing temperature.
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
to the optical properties of the Ag film. The absorption coefficient of Ag is determined by interband electronic transitions, which are the excitation of electrons from the d-band to the Fermi surface [23]. The absorption coefficient of Ag is high in the longer wavelength near the red part of the visible spectrum; therefore, the transmittance of the IAI multilayer decreases after annealing. On the other hand, in the near-UV region, the transmittance of IAI multilayers increases after annealing, probably owing to the smaller absorption coefficient of Ag in the near-UV region. However, at temperatures over 600°C, the transmittance decreases again because the Ag films are agglomerated to increase the scattering loss. In addition, an increase in transmission is observed with decreasing wavelength in the longer-wavelength regions. It is believed that the transport of free carriers may be weakened at UV wavelengths, reducing the absorption coefficient of Ag [23]. Not shown here, we investigated the thickness dependence of Ag on the electrical and optical properties of the IAI structure to determine the optimal Ag thickness. Before annealing, the transmittance of the ITO 15 nm∕Ag 7 nm∕ITO 15 nm was 79% at 380 nm, whereas those of the ITO 15 nm∕Ag 5 nm∕ ITO 15 nm and ITO 15 nm∕Ag 14 nm∕ITO 15 nm films were ∼67% at 380 nm. This is because each material composing the IAI multilayer has a different refractive index; accordingly, high transmittance is observed at a specific film thickness [24]. On the other hand, after annealing at 600°C, a highest transmittance of 82% was also observed for the ITO 15 nm∕Ag 7 nm∕ ITO 15 nm film at the same wavelength, whereas the ITO 15 nm∕Ag 5 nm∕ITO 15 nm film showed a strong absorption peak at 400 nm due to metal agglomeration. The ITO 15 nm∕Ag 14 nm∕ITO 15 nm film showed lower transmittance than the ITO 15 nm∕ Ag 7 nm∕ITO 15 nm film due to the increased light absorption in Ag layers. On the other hand, the sheet resistance of the ITO 15 nm∕Ag∕ITO 15 nm film decreased from 14.55 to 4.9 Ω∕sq as the Ag thickness increased from 7 to 14 nm, but abruptly increased to 270 Ω∕sq for a 5-nm-thick Ag film due to metal agglomeration, after annealing. Based on these results, we determined the Ag thickness to be 7 nm in this experiment. Figure 5 shows the change in absorption coefficients with respect to the incident beam energies measured for both ITO and multilayers deposited on quartz substrates as a function of postannealing temperature. The relation between the absorption coefficient and
Fig. 5. Plot of absorption coefficient versus photon energy, showing the Burstein–Moss effect in the IAI multilayer as a function of annealing temperature.
5057
energy gap E g can be described with the following formula [25]: αhv ∝ hv − Eg1∕2 ;
(2)
where hν is the photon energy. The optical band gap of ITO is ∼3.78–3.8 eV [26,27]. In Fig. 5, the calculated optical band gap energy of ITO is ∼3.8 eV; however, the band gap energy of the IAI multilayer blue shifts with an increase in the postannealing temperature up to 500°C. This phenomenon can be explained by the Burstein– Moss effect [28]. In other words, the carrier concentration of the multilayers increases with annealing temperature, filling the energy levels in the bottom conduction band [25], thereby shifting the position of the absorption edge to higher energies. In summary, IAI multilayer-based TCEs with high conductivity and good transmittance for UV LED applications were fabricated by magnetron sputtering, and optimized via annealing. The optimal annealing temperature was determined to be 500°C, where the sample exhibited a sheet resistance of 9.21 Ω∕sq and a resistivity of 7.66 × 10−4 Ω⋅cm, while the transmittance was as high as 88% at 380 nm. The multilayer also showed a figure of merit of 31.7 × 10−3 Ω−1 . These low-resistivity and high-transmittance properties were thought to result from the reduction in grain boundary scattering, optical reflection, losses in the ITO films, and the quality of the continuous Ag films during optimization of the annealing process. These IAI-multilayer-based transparent electrodes are expected to increase the EQE of GaN-based UV-A LEDs. This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (No. 2011-0028769). This work was also partially supported by LG Innotek Co., Korea. References 1. C.-H. Chiu, C.-C. Lin, P.-M. Tu, S.-C. Huang, C.-C. Tu, J.-C. Li, Z.-Y. Li, W.-Y. Uen, H.-W. Zan, T.-C. Lu, H.-C. Kuo, S.-C. Wang, and C.-Y. Chang, IEEE J. Quantum Electron. 48, 175 (2012). 2. D. J. Chae, D. Y. Kim, T. G. Kim, Y. M. Sung, and M. D. Kim, Appl. Phys. Lett. 100, 08110 (2012). 3. H. Lu, T. Yu, G. Yuan, X. Chen, Z. Chen, G. Chen, and G. Zhang, Opt. Lett. 37, 17 (2012). 4. J. Mckittrick, M. E. Hannah, A. Piquette, J. K. Han, J. I. Choi, M. Anc, M. Galvez, H. Lugauer, J. B. Talbot, and K. C. Mishra, ECS J. Solid State Sci. Technol. 2, R3119 (2012). 5. T. H. Seo, S. J. Chae, B. K. Kim, G. Shin, Y. H. Lee, and E.-K. Suh, Appl. Phys. Express 5, 115101 (2012). 6. A. Kumar, R. Srivastave, M. N. Kamalasanan, and D. S. Mehta, Opt. Lett. 37, 4 (2012). 7. C. G. Granqvist, Adv. Mater. 15, 1789 (2003). 8. H.-D. Kim, H.-M. An, S. M. Hong, and T. G. Kim, Semicond. Sci. Technol. 27, 125020 (2012). 9. Y. J. Jo, C. H. Hong, and J. S. Kwak, Curr. Appl. Phys. 11, S143 (2011). 10. S.-Y. Jung, K. H. Kim, S.-Y. Jeong, J. Moon, S. Y. Lee, J.-O. Song, Y. T. Byun, and T. Y. Seong, Electrochem. Solid-State Lett. 13, H33 (2010). 11. B.-J. Kim, C. Lee, Y. Jung, K. H. Baik, M. A. Mastro, J. K. Hite, C. R. Eddy, and J. Kim, Appl. Phys. Lett. 99, 143101 (2011). 12. Z. Song, Q. He, S. Xiao, and L. Zhou, Appl. Phys. Lett. 101, 181110 (2012).
5058
OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
13. Z. Liu, G. Liu, H. Zhou, X. Liu, K. Huang, Y. Chen, and G. Fu, Nanotechnology 24, 155203 (2013). 14. M. Girtan, Solar Energy Mater. Solar Cells 100, 153 (2012). 15. K. H. Choi, J. Y. Kim, Y. S. Lee, and H. J. Kim, Thin Solid Films 341, 152 (1999). 16. R. N. Chauhan, C. Singh, R. S. Anand, and J. Kumar, Int. J. Photoenergy 2012, 1 (2012). 17. H. Han, J. W. Mayer, and T. L. Alford, J. Appl. Phys. 100, 083715 (2006). 18. T.-C. Lin, S.-C. Chang, and C.-F. Chiu, Mater. Sci. Eng. B 129, 39 (2006). 19. J.-O. Song and T.-Y. Seong, Appl. Phys. Lett. 85, 6374 (2004). 20. S. Diplas, A. Ulyashin, K. Maknys, A. E. Gunnaes, S. Jorgensen, D. Wright, J. F. Watts, A. Olsen, and T. G. Finstad, Thin Solid Films 515, 8539 (2007).
21. G. Haacke, J. Appl. Phys. 47, 4086 (1976). 22. M. Nisha, S. Anusha, A. Antony, R. Manoj, and M. K. Jayaraj, Appl. Surf. Sci. 252, 1430 (2005). 23. H. Ehrenreich and H. R. Philipp, Phys. Rev. 128, 1622 (1962). 24. Y.-Y. Choi, H.-K. Kim, H.-W. Koo, T.-W. Kim, and S.-N. Lee, J. Vac. Sci. Technol. A 29, 061502 (2011). 25. J. Tauc, R. Grigorovici, and A. Vancu, Phys. Status Solidi 15, 627 (1966). 26. O. Tuna, Y. Selamet, G. Aygun, and L. Ozyuzer, J. Phys. D 43, 055402 (2010). 27. H. Kim, C. M. Gilmore, A. Pique, J. S. Horwitz, H. Murata, Z. H. Kafafi, and D. B. Chrisey, J. Appl. Phys. 86, 11 (1999). 28. N. Balasubramanian and A. Subrahmanyam, J. Phys. D 22, 206 (1989).
5055
ITO/Ag/ITO multilayer-based transparent conductive electrodes for ultraviolet light-emitting diodes Jae Hoon Lee, Kie Young Woo, Kyeong Heon Kim, Hee-Dong Kim, and Tae Geun Kim* School of Electrical Engineering, Korea University, Seoul 136-701, South Korea *Corresponding author: [email protected] Received September 3, 2013; revised October 17, 2013; accepted October 24, 2013; posted October 25, 2013 (Doc. ID 196923); published November 22, 2013 ITO/Ag/ITO (IAI) multilayer-based transparent conductive electrodes for ultraviolet light-emitting diodes are fabricated by reactive sputtering, optimized by annealing, and characterized with respect to electrical and optical properties. Increasing the annealing temperature from 300°C to 500°C decreased the sheet resistance and increased the transmittance. This may result from an observed improvement in the crystallinity of the IAI multilayer and a reduction in the near-UV absorption coefficient of Ag. We observed the lowest sheet resistance (9.21 Ω∕sq) and the highest optical transmittance (88%) at 380 nm for the IAI multilayer samples annealed in N2 gas at 500°C. © 2013 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (310.4165) Multilayer design; (310.6860) Thin films, optical properties; (310.7005) Transparent conductive coatings. http://dx.doi.org/10.1364/OL.38.005055
GaN-based near-ultraviolet (UV) light-emitting diodes (LEDs) in the wavelength range 380–400 nm have applications in various fields such as analytical instrumentation, water purification, medical treatments, biological agent identification, and white LED displays for general lighting [1–3]. However, unlike blue LEDs, there are few transparent conductive electrodes (TCEs) suitable for near-UV LEDs. Using incompatible TCE materials decreases the external quantum efficiency (EQE) of UV LEDs [4]. In particular, because of the high sheet resistance and low carrier mobility of the p-type thin GaN layer over the whole range of emitted wavelengths, LEDs made from this material exhibit current crowding and low current spreading. Therefore, the p-type electrode must have low contact resistance on a p-GaN layer and high transmittance in the near-UV region [5]. Indium-tin oxide (ITO) is widely used as a TCE material in optoelectronic devices, including flat displays, thin film transistors, and solar cells, because it provides high transparency in the visible region and good electrical conductivity [6–8]. However, ITO suffers from several problems because of absorption in UV region. Several methods have been reported to reduce the light absorption by TCEs in UV LEDs. One method is to use a thin ITO layer, with only a few tens of nanometers [9]. Although this method improves the transmittance of the ITO, it abruptly increases the sheet resistance by reducing the thickness of the thin ITO films. Another approach is to use carbon-based TCEs such as graphene and singlewalled carbon nanotubes (SWNTs) [5,10]. The feasibility of carbon-based TCEs with low sheet resistance and high UV transmittance has been demonstrated [11]. However, before carbon-based TCEs can be widely incorporated into UV LEDs, crucial manufacturing challenges such as ensuring that the carbon nanomaterial film has high quality and is uniform and processed in a manner compatible with the UV LED fabrication must be overcome. Recently, a continuous metal film has also exhibited high performance for the broadband light transparency as TCEs [12,13]. On the other hand, Girtan reported an ITO/Ag/ITO (IAI) multilayer as an alternative TCE [14]. According to this report, the electrical and optical 0146-9592/13/235055-04$15.00/0
properties of IAI-based TCEs can be improved by varying the thickness of the metal layer between the oxide layers. In a well-designed IAI multilayer, the large difference in the refractive index between ITO and Ag suppresses the reflection from the metal layer, which can then become selectively transparent in the visible and UV regions. Nevertheless, IAI-based TCEs in UV LEDs have not yet been fully realized, because the metal tends to agglomerate during the annealing processes that are inevitable in LED fabrication [15]. Therefore, to use IAI-based TCEs in UV LEDs, a method must be found to allow IAI to sustain its properties after annealing. We propose TCEs based on IAI multilayers. Herein, we demonstrate their high conductivity and high transmittance, acceptable for UV LED applications. In addition, we discuss the possible conduction mechanisms in the IAI-based TCEs as a function of the annealing temperature, and the role of Ag in determining the transmission properties of the films. The p-GaN layer of the 380 nm UV LEDs used in this study was grown on c-face sapphire substrates by metal organic chemical vapor deposition (MOCVD). The UV LED structure consisted of a 20-nm-thick GaN low-temperature buffer layer, a 2.6-μm-thick undoped GaN layer, a 4.6-μmthick n-type GaN layer, five AlGaInN/InGaN multiple quantum well (MQW) layers, a 30-nm-thick p-type Al0.2 Ga0.8 N layer, and a 95-nm-thick p-type GaN capping layer. Before the fabrication of the IAI multilayer, the surfaces of p-GaN were ultrasonically degreased with acetone, methanol, deionized (DI) water, and a mixture of sulfuric acid (H2 SO4 ) and hydrogen peroxide (H2 O2 ) for 5 min in each step to remove the surface oxides formed on GaN. After this treatment, a 15-nm-thick bottom ITO film was deposited by RF magnetron sputtering onto the p-GaN surface at 5 mTorr pressure and 100 W. After deposition of the bottom ITO film, the Ag interlayer with a thickness of 7 nm was deposited by RF magnetron sputtering onto the ITO film at 2 mTorr pressure and 100 W. The top ITO film was then deposited using the same conditions as for the bottom ITO film. The resulting IAI multilayer was annealed in a rapid thermal annealing (RTA) system in N2 ambient gas. The sheet resistance of the IAI multilayer was © 2013 Optical Society of America
5056
OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
measured at room temperature using a four-point probe (Advanced Instrument Technology, CMT-SR1000N). Current–voltage (I–V ) characteristics were measured at the interface between IAI-TCEs and p-GaN using a Keithley 4200 measurement system. The surface morphology of the IMI multilayer, as a function of annealing temperature, was analyzed via transmission electron microscopy (TEM). The transmittance of the IAI multilayer was also measured over the wavelength range 280–800 nm using a UV/visible spectrometer (PerkinElmer, Lambda 35). We first investigated the sheet resistance and resistivity of the IAI multilayer as a function of the annealing temperature. Figure 1 shows the effect of postannealing on the sheet resistance and resistivity of the IAI multilayer. It is well established that the resistivity of thin ITO films may vary greatly [16,17]. The thin ITO films in this study exhibited sheet resistance of 130 Ω∕sq and resistivity of 1.08 × 10−2 Ω · cm. For the IAI multilayer, the resistivity decreases from 9.67 × 10−4 to 7.66 × 10−4 Ω · cm after annealing at temperatures of up to 500°C; however, at temperatures over 600°C, the resistivity increases drastically to 2.73 × 10−3 Ω · cm. On the other hand, the sheet resistance decreases from 11.62 to 9.21 Ω∕sq at an annealing temperature of 500°C, which is attributed to the improved crystal quality of ITO films and efficient protection from Ag-oxide formation [18,19]. However, metal agglomeration causes a sudden increase in sheet resistance to 32.77 Ω∕sq at annealing temperatures over 600°C. Figure 2 shows the typical I–V characteristics of IAI-TCE on p-GaN layers as a function of the annealing temperature. The slope of the I–V curve increases with annealing temperature until 500°C, after which it decreases. The as-deposited sample and the samples annealed at 300°C, 400°C, 500°C, 600°C, and 700°C exhibit current values of 3.7 × 10−2 , 4.1 × 10−2 , 4.3 × 10−2 , 4.9 × 10−2 , 3.2 × 10−2 , and 1.6 × 10−2 A at 1 V, respectively. The improvement in conduction observed for the samples annealed at or below 500°C is consistent with the interfacial morphology of the IAI multilayer, as shown by the TEM cross-sectional images (Fig. 3). While the as-deposited sample shows an amorphous phase [Fig. 3(a)], the samples annealed at 500°C show increased crystallinity and columnar grains in both ITO and Ag layers [Fig. 3(b)]. This helps explain the improvement in the electrical properties [20]. However, Fig. 3(c) shows that the Ag films agglomerate when annealed at 700°C. The agglomeration increases the sheet resistance of the IAI multilayer films. To determine the best conditions for fabricating the IAI multilayer as a transparent p-electrode in UV-LEDs, we
Fig. 1. Sheet resistance and resistivity of IAI multilayer as a function of annealing temperature.
Fig. 2. I–V characteristics of the IAI multilayer as a function of annealing temperature.
calculated a figure of merit, ϕTC , from the 380 nm transmittance and the sheet resistance. The ϕTC values are defined by Haacke as follows [21]: φTC
T 10 ; Rsh
(1)
where T is the transmittance and Rsh is the sheet resistance for the films. The figure of merit measured for the as-deposited sample and those annealed at 300°C, 400°C, 500°C, 600°C, and 700°C are 8.4 × 10−3 , 10.0 × 10−3 , 20.0 × 10−3 , 31.7 × 10−3 , 10.0 × 10−3 , and 0.2 × 10−3 Ω−1 , respectively. The samples annealed at 500°C exhibited the highest figure of merit (31.7 × 10−3 Ω−1 ), much higher than that of a typical thin ITO film (2.8 × 10−3 Ω−1 ) [22]. Figure 4 shows the optical transmittance spectra measured for the IAI multilayer on quartz substrates as a function of annealing temperature. The optical transmittance of the as-deposited IAI multilayer is approximately 79% at 380 nm. At 500°C, its transmittance increases from 79% to 88%, an increase of 9% compared to a typical thin ITO film. However, the transmittance may differ depending on the wavelength; this phenomenon may be related
Fig. 3. Cross-sectional TEM image in the interface region of IAI multilayer (a) before and after postannealing at (b) 500°C and (c) 700°C.
Fig. 4. Optical transmittance spectra for IAI multilayer as a function of annealing temperature.
December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS
to the optical properties of the Ag film. The absorption coefficient of Ag is determined by interband electronic transitions, which are the excitation of electrons from the d-band to the Fermi surface [23]. The absorption coefficient of Ag is high in the longer wavelength near the red part of the visible spectrum; therefore, the transmittance of the IAI multilayer decreases after annealing. On the other hand, in the near-UV region, the transmittance of IAI multilayers increases after annealing, probably owing to the smaller absorption coefficient of Ag in the near-UV region. However, at temperatures over 600°C, the transmittance decreases again because the Ag films are agglomerated to increase the scattering loss. In addition, an increase in transmission is observed with decreasing wavelength in the longer-wavelength regions. It is believed that the transport of free carriers may be weakened at UV wavelengths, reducing the absorption coefficient of Ag [23]. Not shown here, we investigated the thickness dependence of Ag on the electrical and optical properties of the IAI structure to determine the optimal Ag thickness. Before annealing, the transmittance of the ITO 15 nm∕Ag 7 nm∕ITO 15 nm was 79% at 380 nm, whereas those of the ITO 15 nm∕Ag 5 nm∕ ITO 15 nm and ITO 15 nm∕Ag 14 nm∕ITO 15 nm films were ∼67% at 380 nm. This is because each material composing the IAI multilayer has a different refractive index; accordingly, high transmittance is observed at a specific film thickness [24]. On the other hand, after annealing at 600°C, a highest transmittance of 82% was also observed for the ITO 15 nm∕Ag 7 nm∕ ITO 15 nm film at the same wavelength, whereas the ITO 15 nm∕Ag 5 nm∕ITO 15 nm film showed a strong absorption peak at 400 nm due to metal agglomeration. The ITO 15 nm∕Ag 14 nm∕ITO 15 nm film showed lower transmittance than the ITO 15 nm∕ Ag 7 nm∕ITO 15 nm film due to the increased light absorption in Ag layers. On the other hand, the sheet resistance of the ITO 15 nm∕Ag∕ITO 15 nm film decreased from 14.55 to 4.9 Ω∕sq as the Ag thickness increased from 7 to 14 nm, but abruptly increased to 270 Ω∕sq for a 5-nm-thick Ag film due to metal agglomeration, after annealing. Based on these results, we determined the Ag thickness to be 7 nm in this experiment. Figure 5 shows the change in absorption coefficients with respect to the incident beam energies measured for both ITO and multilayers deposited on quartz substrates as a function of postannealing temperature. The relation between the absorption coefficient and
Fig. 5. Plot of absorption coefficient versus photon energy, showing the Burstein–Moss effect in the IAI multilayer as a function of annealing temperature.
5057
energy gap E g can be described with the following formula [25]: αhv ∝ hv − Eg1∕2 ;
(2)
where hν is the photon energy. The optical band gap of ITO is ∼3.78–3.8 eV [26,27]. In Fig. 5, the calculated optical band gap energy of ITO is ∼3.8 eV; however, the band gap energy of the IAI multilayer blue shifts with an increase in the postannealing temperature up to 500°C. This phenomenon can be explained by the Burstein– Moss effect [28]. In other words, the carrier concentration of the multilayers increases with annealing temperature, filling the energy levels in the bottom conduction band [25], thereby shifting the position of the absorption edge to higher energies. In summary, IAI multilayer-based TCEs with high conductivity and good transmittance for UV LED applications were fabricated by magnetron sputtering, and optimized via annealing. The optimal annealing temperature was determined to be 500°C, where the sample exhibited a sheet resistance of 9.21 Ω∕sq and a resistivity of 7.66 × 10−4 Ω⋅cm, while the transmittance was as high as 88% at 380 nm. The multilayer also showed a figure of merit of 31.7 × 10−3 Ω−1 . These low-resistivity and high-transmittance properties were thought to result from the reduction in grain boundary scattering, optical reflection, losses in the ITO films, and the quality of the continuous Ag films during optimization of the annealing process. These IAI-multilayer-based transparent electrodes are expected to increase the EQE of GaN-based UV-A LEDs. This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (No. 2011-0028769). This work was also partially supported by LG Innotek Co., Korea. References 1. C.-H. Chiu, C.-C. Lin, P.-M. Tu, S.-C. Huang, C.-C. Tu, J.-C. Li, Z.-Y. Li, W.-Y. Uen, H.-W. Zan, T.-C. Lu, H.-C. Kuo, S.-C. Wang, and C.-Y. Chang, IEEE J. Quantum Electron. 48, 175 (2012). 2. D. J. Chae, D. Y. Kim, T. G. Kim, Y. M. Sung, and M. D. Kim, Appl. Phys. Lett. 100, 08110 (2012). 3. H. Lu, T. Yu, G. Yuan, X. Chen, Z. Chen, G. Chen, and G. Zhang, Opt. Lett. 37, 17 (2012). 4. J. Mckittrick, M. E. Hannah, A. Piquette, J. K. Han, J. I. Choi, M. Anc, M. Galvez, H. Lugauer, J. B. Talbot, and K. C. Mishra, ECS J. Solid State Sci. Technol. 2, R3119 (2012). 5. T. H. Seo, S. J. Chae, B. K. Kim, G. Shin, Y. H. Lee, and E.-K. Suh, Appl. Phys. Express 5, 115101 (2012). 6. A. Kumar, R. Srivastave, M. N. Kamalasanan, and D. S. Mehta, Opt. Lett. 37, 4 (2012). 7. C. G. Granqvist, Adv. Mater. 15, 1789 (2003). 8. H.-D. Kim, H.-M. An, S. M. Hong, and T. G. Kim, Semicond. Sci. Technol. 27, 125020 (2012). 9. Y. J. Jo, C. H. Hong, and J. S. Kwak, Curr. Appl. Phys. 11, S143 (2011). 10. S.-Y. Jung, K. H. Kim, S.-Y. Jeong, J. Moon, S. Y. Lee, J.-O. Song, Y. T. Byun, and T. Y. Seong, Electrochem. Solid-State Lett. 13, H33 (2010). 11. B.-J. Kim, C. Lee, Y. Jung, K. H. Baik, M. A. Mastro, J. K. Hite, C. R. Eddy, and J. Kim, Appl. Phys. Lett. 99, 143101 (2011). 12. Z. Song, Q. He, S. Xiao, and L. Zhou, Appl. Phys. Lett. 101, 181110 (2012).
5058
OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013
13. Z. Liu, G. Liu, H. Zhou, X. Liu, K. Huang, Y. Chen, and G. Fu, Nanotechnology 24, 155203 (2013). 14. M. Girtan, Solar Energy Mater. Solar Cells 100, 153 (2012). 15. K. H. Choi, J. Y. Kim, Y. S. Lee, and H. J. Kim, Thin Solid Films 341, 152 (1999). 16. R. N. Chauhan, C. Singh, R. S. Anand, and J. Kumar, Int. J. Photoenergy 2012, 1 (2012). 17. H. Han, J. W. Mayer, and T. L. Alford, J. Appl. Phys. 100, 083715 (2006). 18. T.-C. Lin, S.-C. Chang, and C.-F. Chiu, Mater. Sci. Eng. B 129, 39 (2006). 19. J.-O. Song and T.-Y. Seong, Appl. Phys. Lett. 85, 6374 (2004). 20. S. Diplas, A. Ulyashin, K. Maknys, A. E. Gunnaes, S. Jorgensen, D. Wright, J. F. Watts, A. Olsen, and T. G. Finstad, Thin Solid Films 515, 8539 (2007).
21. G. Haacke, J. Appl. Phys. 47, 4086 (1976). 22. M. Nisha, S. Anusha, A. Antony, R. Manoj, and M. K. Jayaraj, Appl. Surf. Sci. 252, 1430 (2005). 23. H. Ehrenreich and H. R. Philipp, Phys. Rev. 128, 1622 (1962). 24. Y.-Y. Choi, H.-K. Kim, H.-W. Koo, T.-W. Kim, and S.-N. Lee, J. Vac. Sci. Technol. A 29, 061502 (2011). 25. J. Tauc, R. Grigorovici, and A. Vancu, Phys. Status Solidi 15, 627 (1966). 26. O. Tuna, Y. Selamet, G. Aygun, and L. Ozyuzer, J. Phys. D 43, 055402 (2010). 27. H. Kim, C. M. Gilmore, A. Pique, J. S. Horwitz, H. Murata, Z. H. Kafafi, and D. B. Chrisey, J. Appl. Phys. 86, 11 (1999). 28. N. Balasubramanian and A. Subrahmanyam, J. Phys. D 22, 206 (1989).
