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High-efficiency vertical AlGaInP light-emitting diodes with conductive omni-directional reflectors
Current Applied Physics 11 (2011) S385eS387
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
High-efficiency vertical AlGaInP light-emitting diodes with conductive omni-directional reflectors Jae Won Seo a, Hwa Sub Oh a, b, Joon Seop Kwak a, * a b
Department of Printed Electronics Engineering (WCU), Sunchon National University, Jeonnam 540-742, Republic of Korea LED Device Team, Korea Photonics Technology Institute, Gwangju 500-460, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2010 Received in revised form 5 October 2010 Accepted 31 January 2011 Available online 26 February 2011
In order to increase extraction efficiency of AlGaInP-based vertical red light-emitting diodes (LEDs), AlGaInP LEDs employing a conductive omni-directional reflector (ODR) using transparent conducting oxide (TCO)/Ag is presented. The ODR consisted of p-type GaP, TCO layer (ITO, IZO, or AZO), and an Ag layer. The transmittance of the ITO, IZO, and AZO films were found 96%, 95%, and 91% after annealing at 350 C at the wavelength of 620 nm, respectively. Forward voltages of the LEDs with TCO/Ag were slightly higher than that of the LED with Ag due to the high resistivity of the TCO. However, the brightness of the AlGaInP LEDs with ODR using ITO, IZO, and AZO measured at 20 mA increased significantly by 42%, 38%, and 29% compared with that of the LED with Ag reflector. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: AlGaInP Omni-directional reflector Vertical red LED TCO
1. Introduction AlGaInP-based materials with a visible spectrum from red to yellow-green have a direct band gap that ranges 1.8e2.3 eV [1,2]. Recently, AlGaInP-based LEDs have proven to be the most reliable choice in many applications, including interior automotive lighting, traffic lights, full-color displays or display signs and billboards [3,4]. Current development efforts of AlGaInP-based LEDs are focused on topics such as high brightness, efficiency, reliability and low cost fabrication. In order to enhance LEDs performance, the GaAs lightabsorbing substrate significantly limits the light extraction of AlGaInP-based LEDs. The problem can be minimized by growing a distributed Bragg reflector (DBR) between the standard LED epitaxial layers and the absorbing substrate [5,6]. Another approach is to replace the GaAs substrate with a Si conductive substrate using a mirror metal via wafer bonding [7,8]. The wafer bonding technology used a mirror metal is more effective than the DBR LEDs to reflect of light from active region because the DBR only reflects light of near-normal incidence [9]. Recently, Kim et al. showed that the RuO2/SiO2/Ag ODR has higher than Ni/Au and even Ag reflectors [10], and Xi et al. reported high reflectivity of ODR employing nanoporous SiO2 (n ¼ 1.23) [11]. The dielectric layer of this ODR was perforated by an array of
* Corresponding author. Tel.: þ82 61 750 3559. E-mail address: [email protected] (J.S. Kwak). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.01.040
metallic micro-contacts to enable electrical conductivity for use in LEDs. On the other hand TCO films are highly degenerate wide band gap semiconductors with high electrical conductivity and optical transparency [12e15]. In this study, a conductive ODR consisting of GaP, Ag, and an intermediate Indium-tin oxide(ITO), Indium-zinc oxide(IZO), or Aluminum-zinc oxide (AZO) low-index layer is incorporated into the AlGaInP vertical LEDs. We reported on the electrical and optical properties of AlGaInP vertical LEDs having the TCO/Ag reflectors and the Ag reflector. 2. Experiments The LEDs employed in this work were grown on temporary nþ-GaAs substrates by metalorganic chemical vapor deposition (MOCVD). The structure consisted of a GaInP etching stop layer grown on an n-GaAs buffer layer, a thick Si-doped n-AlGaInP cladding layer, an undoped active layer with 20 period AlGaInPeGaInP multiple-quantum-wells (MQWs), a Mg-doped n-AlGaInP layer, and a thick pþ-GaP window layer. Fig. 1(a) and (b) show cross-sectional schematic diagrams of the AlGaInP with GaPeTCOeAg ODR structure and Ag reflector. Then, the micro-dot AuBe metal contacts were deposited on p-GaP as p-ohmic contact by e-beam evaporation. The TCO films (ITO, IZO, or AZO films) were deposited on the p-GaP/ micro-dot AuBe contacts by RF magnetron sputtering system, as shown in Fig. 1(b). For measurement of transmittance, the ITO, IZO, and AZO films were also deposited on the glass substrates. The thickness of the TCO films and Ag film was controlled to 40 nm and
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J.W. Seo et al. / Current Applied Physics 11 (2011) S385eS387
Fig. 1. Schematic diagrams of wafer bonding structure with (a) Ag reflector (b) TCO/Ag reflector.
400 nm, respectively. In order to compare reflection of GaP/TCO/Ag ODR structure and Ag reflector, 400 nm-thick Ag metal layers were also deposited on the GaP layer in the other LED sample. The reflectivity of the Ag film was 90% at 620 nm. PteIneAu metal contacts were deposited by e-beam evaporation on top of the Ag reflector. These LED samples were bonded to Si substrate with commercially available metal eutectic bonding process. After wafer bonding, the ntype GaAs substrate and GaInP etching stop layer were removed by chemical etching solution. Then, Ni, Ge, Au metal contacts were deposited on n-GaAs contact layer. Ti, Au metal contacts were deposited on n-AlGaInP as a bonding pad metal and back side metal of p-Si substrate. In order to measure electrical and optical properties of LED samples, these samples were diced into 350 350 ㎛2 chips. 3. Results and discussion
100
100
90
80
80
60
70
AZO IZO ITO
60
Current (mA)
Transmittance (%)
The optical transmission spectra in regions between 350 nm and 800 nm wavelength for ITO, IZO, and AZO films are shown in Fig. 2. The ITO, IZO, and AZO films, which were deposited on glass substrates, were annealed at 350 C, since the annealing temperature
for forming the n-electrode (Ni/Ge/Au contacts on n-GaAs) was 350 C. In visible light range, the most of transmittance of TCO films was highly shown over 80%. ITO, IZO, and AZO films gave transmission of 96%, 95%, and 91% at the wavelength of 620 nm, respectively. Fig. 3 shows typical currentevoltage characteristics of the LEDs fabricated with Ag reflector and of the LEDs fabricated with GaP/TCO/Ag ODR. The vertical AlGaInP LED with Ag reflector shows forward bias voltage of 2.05 V at 20 mA. However, the vertical AlGaInP LEDs with ODR structure using ITO, IZO, and AZO show forward bias voltages of 2.1, 2.13, and 2.2 at 20 mA, respectively, which are slightly higher than that of the AlGaInP LED with Ag reflector. These results can be attributed to the relatively high sheet resistance of the ITO, IZO, and AZO films. The sheet resistances of the 40 nm-thick ITO, IZO, and AZO films were measured as 104, 105, and 503 U/sq, respectively. Fig. 4 exhibits the light output-current characteristics of the AlGaInP LEDs fabricated with Ag reflector and ODR structure using TCO. The output power of the LEDs fabricated with ODR structure was much higher than that of the LED with Ag reflector. In addition,
40
GaP/AZO/Ag GaP/IZO/Ag GaP/ITO/Ag GaP/Ag
20
o
Annealing temperature 350 C 50
400
500
600
700
800
Wavelength (nm) Fig. 2. Optical transmission of TCO films after annealing process of 350 C.
0
0
1
2
3
4
5
Voltage (V) Fig. 3. Forward IeV characteristics of Ag reflector LED and TCO/Ag reflector LEDs.
J.W. Seo et al. / Current Applied Physics 11 (2011) S385eS387
S387
scale and ideality factor of the AlGaInP LEDs fabricated with the Ag reflector and the ODR structure using TCO are shown in Fig. 5. However, the leakage current reduction in low bias was not observed in Fig. 5. In addition, the AlGaInP LEDs with Ag reflector and ODR did not induce significant changes in ideality factors. These results imply that the enhancement in light output of the AlGaInP LEDs with ODR cannot be attributed to the degradation of the LEDs with Ag reflectors by annealing process. Another possible explanation for the increased light output of the LEDs fabricated with ODR is the higher reflectivity of ODR structures. Kim et al. reported that GaInN LEDs with GaN/low-n ITO/Ag ODR show a 31.6% higher light extraction efficiency than LEDs with Ag reflector [17]. In addition, Hsu et al. reported that a 1-mm2 AlGaInP LED employing a ODR using ITO layer showed an external quantum efficiency of 31.8% at 100 mA [18]. Accordingly, the enhancement in light output of the LEDs with ODR can be attributed to the use of a highly reflective ODR and highly transmission of TCO layer. Fig. 4. Light output of the Ag reflector LED and the TCO/Ag reflector LEDs. The inset shows brightness of same LEDs at operating current of 20 mA.
the brightness of same LEDs at operating current of 20 mA is shown in the inset of Fig. 4. The brightness at 20 mA for the ODR LED using ITO is 151 mcd, comparable to that of the ODR LED using IZO, 147 mcd. This is due to the fact that the transmittance of ITO is higher than that of IZO. The LED with GaP/AZO/Ag ODR having the lowest transmittance of ODR LEDs gives 137 mcd. On the other hand, the brightness of LED with the Ag reflector is as low as 106 mcd. The brightness of the LEDs with ODR structure using ITO, IZO, and AZO increased by 42%, 38%, and 29% than that of the AlGaInP LED with Ag reflector, respectively. The light output intensity of the LEDs with ODR corresponded with result of the transmission of TCO. We expected that the enhancement in light output of the LEDs with ODR could be related to damage of Ag reflector and higher reflectivity of ODR. One possible explanation for the increased light output of the AlGaInP LEDs fabricated with ODR is degradation of the Ag reflector LED by deep level defect during the annealing process in order to improve electrical property. Meneghesso et al. reported that the degradation of optical power is accompanied with both increase of generation-recombination current at low forward bias and increase of device ideality factor [16]. To examine the influence of degradation of Ag reflector, IeV characteristics plotted on logelog
1
GaP/AZO/Ag GaP/IZO/Ag GaP/ITO/Ag GaP/Ag
Current (mA)
0.01 1E-4 1E-6 1E-8 1E-10 0
1
2
3
Voltage (V) Fig. 5. Forward IeV characteristics plotted on logelog scale of Ag reflector LED and TCO/Ag reflector LEDs. The inset shows ideality factors of same LEDs.
4. Conclusion In summary, we have investigated electrical and optical properties of the vertical AlGaInP LEDs with Ag reflector and GaP/TCO/Ag ODR structure. ITO, IZO, and AZO films showed transmittance of 97%, 96%, and 91% at the wavelength of 620 nm, respectively. Because of highly resistivity of TCO films, AlGaInP LEDs with ODR structure using TCO films produced slightly higher forward voltages than AlGaInP LED with Ag reflector. However, the AlGaInP LEDs with ODR structure using ITO, IZO, and AZO showed 42, 38, 29% higher brightness at 20 mA than the AlGaInP LED with Ag reflector, respectively. Acknowledgment This work was supported by Mid-career Researcher Program through NRF grant funded by the "MEST (No. 2009-0080313), and was also supported by the WCU program at Sunchon National University, and this work was supported in part by Research Foundation of Engineering College, Sunchon National University. References [1] H. Sugawara, M. Ishikawa, G. Hatakoshi, Appl. Phys. Lett. 58 (1991) 1010. [2] H.C. Wang, Y.K. Su, C.L. Lin, W.B. Chen, S.M. Chen, W.L. Li, Jpn. J. Appl. Phys. 43 (2004) 1934. [3] Y.J. Lee, H.C. Tseng, H.C. Kuo, S.C. Wang, C.W. Chang, T.C. Hsu, Y.L. Yang, M.H. Hsieh, M.J. Jou, B.J. Lee, IEEE Photonics Technol. Lett. 17 (2005) 1041. [4] R. Windisch, C. Rooman, S. Meinlschmidt, P. Kiesel, D. Zipperer, G.H. Dohler, B. Drtta, M. Kuijk, G. Borghs, P. Heremans, Appl. Phys. Lett. 79 (2001) 2315. [5] S.W. Chiou, C.P. Lee, C.K. Huang, C.W. Chen, J. Appl. Phys. 87 (2000) 2052. [6] H.S. Oh, J.H. Joo, J.H. Lee, J.H. Baek, J.W. Seo, J.S. Kwak, Jpn. J. Appl. Phys. 47 (2008) 6214. [7] K. Streubel, N. Linder, R. Werth, A. Jaeger, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 321. [8] S. Illek, U. Jacob, A. Ploessl, P. Strauss, K. Streubel, W. Wegleiter, R. Wirth, Compd. Semicond. 8 (2002) 39. [9] T. Gessmann, E.F. Schubert, J. Appl. Phys. 58 (2004) 1010. [10] J.K. Kim, T. Gessmann, H. Luo, E.F. Schubert, Appl. Phys. Lett. 84 (2004) 4508. [11] J.Q. Xi, M. Ojha, W.J. Cho, J.L. Plawsky, W.N. Gill, T. Gessmann, E.F. Schubert, Opt. Lett. 30 (2005) 1518. [12] T. Minami, H. Sonohra, T. Kakumu, S. Takata, Thin Solid Films 270 (1995) 37. [13] P. Lippens, A. Segers, J. Haemers, R. De Gryse, Thin Solid Films 317 (1998) 405. [14] G. Hu, B. Kumar, H. Gong, E.F. Chor, P. Wu, Appl. Phys. Lett. 88 (2006) 101901. [15] W.J. Lee, Y.K. Fang, J.J. Ho, C.Y. Chen, L.H. Chiou, S.J. Wang, F. Dai, T. Hsieh, R.Y. Tsai, D. Huang, F.C. Ho, Solid State Electron. 46 (2002) 477. [16] G. Meneghesso, S. Levada, R. Pierobon, F. Rampazzo, E. Zanoni, A. Cavallini, A. Castaldini, G. Scamarcio, S. Du, I. Eliashevich, IEDM Technical Digest (2002) 103 IEEE, San Francisco. [17] J.K. Kim, Th. Gessmann, E.F. Schubert, J.Q. Xi, H. Luo, J. Cho, C. Sone, Y. Park, Appl. Phys. Lett. 88 (2006) 013501. [18] S.C. Hsu, D.S. Wuu, C.Y. Lee, J.Y. Su, R.H. Horng, IEEE Photonics Technol. Lett. 19 (2007) 492.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
High-efficiency vertical AlGaInP light-emitting diodes with conductive omni-directional reflectors Jae Won Seo a, Hwa Sub Oh a, b, Joon Seop Kwak a, * a b
Department of Printed Electronics Engineering (WCU), Sunchon National University, Jeonnam 540-742, Republic of Korea LED Device Team, Korea Photonics Technology Institute, Gwangju 500-460, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2010 Received in revised form 5 October 2010 Accepted 31 January 2011 Available online 26 February 2011
In order to increase extraction efficiency of AlGaInP-based vertical red light-emitting diodes (LEDs), AlGaInP LEDs employing a conductive omni-directional reflector (ODR) using transparent conducting oxide (TCO)/Ag is presented. The ODR consisted of p-type GaP, TCO layer (ITO, IZO, or AZO), and an Ag layer. The transmittance of the ITO, IZO, and AZO films were found 96%, 95%, and 91% after annealing at 350 C at the wavelength of 620 nm, respectively. Forward voltages of the LEDs with TCO/Ag were slightly higher than that of the LED with Ag due to the high resistivity of the TCO. However, the brightness of the AlGaInP LEDs with ODR using ITO, IZO, and AZO measured at 20 mA increased significantly by 42%, 38%, and 29% compared with that of the LED with Ag reflector. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: AlGaInP Omni-directional reflector Vertical red LED TCO
1. Introduction AlGaInP-based materials with a visible spectrum from red to yellow-green have a direct band gap that ranges 1.8e2.3 eV [1,2]. Recently, AlGaInP-based LEDs have proven to be the most reliable choice in many applications, including interior automotive lighting, traffic lights, full-color displays or display signs and billboards [3,4]. Current development efforts of AlGaInP-based LEDs are focused on topics such as high brightness, efficiency, reliability and low cost fabrication. In order to enhance LEDs performance, the GaAs lightabsorbing substrate significantly limits the light extraction of AlGaInP-based LEDs. The problem can be minimized by growing a distributed Bragg reflector (DBR) between the standard LED epitaxial layers and the absorbing substrate [5,6]. Another approach is to replace the GaAs substrate with a Si conductive substrate using a mirror metal via wafer bonding [7,8]. The wafer bonding technology used a mirror metal is more effective than the DBR LEDs to reflect of light from active region because the DBR only reflects light of near-normal incidence [9]. Recently, Kim et al. showed that the RuO2/SiO2/Ag ODR has higher than Ni/Au and even Ag reflectors [10], and Xi et al. reported high reflectivity of ODR employing nanoporous SiO2 (n ¼ 1.23) [11]. The dielectric layer of this ODR was perforated by an array of
* Corresponding author. Tel.: þ82 61 750 3559. E-mail address: [email protected] (J.S. Kwak). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.01.040
metallic micro-contacts to enable electrical conductivity for use in LEDs. On the other hand TCO films are highly degenerate wide band gap semiconductors with high electrical conductivity and optical transparency [12e15]. In this study, a conductive ODR consisting of GaP, Ag, and an intermediate Indium-tin oxide(ITO), Indium-zinc oxide(IZO), or Aluminum-zinc oxide (AZO) low-index layer is incorporated into the AlGaInP vertical LEDs. We reported on the electrical and optical properties of AlGaInP vertical LEDs having the TCO/Ag reflectors and the Ag reflector. 2. Experiments The LEDs employed in this work were grown on temporary nþ-GaAs substrates by metalorganic chemical vapor deposition (MOCVD). The structure consisted of a GaInP etching stop layer grown on an n-GaAs buffer layer, a thick Si-doped n-AlGaInP cladding layer, an undoped active layer with 20 period AlGaInPeGaInP multiple-quantum-wells (MQWs), a Mg-doped n-AlGaInP layer, and a thick pþ-GaP window layer. Fig. 1(a) and (b) show cross-sectional schematic diagrams of the AlGaInP with GaPeTCOeAg ODR structure and Ag reflector. Then, the micro-dot AuBe metal contacts were deposited on p-GaP as p-ohmic contact by e-beam evaporation. The TCO films (ITO, IZO, or AZO films) were deposited on the p-GaP/ micro-dot AuBe contacts by RF magnetron sputtering system, as shown in Fig. 1(b). For measurement of transmittance, the ITO, IZO, and AZO films were also deposited on the glass substrates. The thickness of the TCO films and Ag film was controlled to 40 nm and
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J.W. Seo et al. / Current Applied Physics 11 (2011) S385eS387
Fig. 1. Schematic diagrams of wafer bonding structure with (a) Ag reflector (b) TCO/Ag reflector.
400 nm, respectively. In order to compare reflection of GaP/TCO/Ag ODR structure and Ag reflector, 400 nm-thick Ag metal layers were also deposited on the GaP layer in the other LED sample. The reflectivity of the Ag film was 90% at 620 nm. PteIneAu metal contacts were deposited by e-beam evaporation on top of the Ag reflector. These LED samples were bonded to Si substrate with commercially available metal eutectic bonding process. After wafer bonding, the ntype GaAs substrate and GaInP etching stop layer were removed by chemical etching solution. Then, Ni, Ge, Au metal contacts were deposited on n-GaAs contact layer. Ti, Au metal contacts were deposited on n-AlGaInP as a bonding pad metal and back side metal of p-Si substrate. In order to measure electrical and optical properties of LED samples, these samples were diced into 350 350 ㎛2 chips. 3. Results and discussion
100
100
90
80
80
60
70
AZO IZO ITO
60
Current (mA)
Transmittance (%)
The optical transmission spectra in regions between 350 nm and 800 nm wavelength for ITO, IZO, and AZO films are shown in Fig. 2. The ITO, IZO, and AZO films, which were deposited on glass substrates, were annealed at 350 C, since the annealing temperature
for forming the n-electrode (Ni/Ge/Au contacts on n-GaAs) was 350 C. In visible light range, the most of transmittance of TCO films was highly shown over 80%. ITO, IZO, and AZO films gave transmission of 96%, 95%, and 91% at the wavelength of 620 nm, respectively. Fig. 3 shows typical currentevoltage characteristics of the LEDs fabricated with Ag reflector and of the LEDs fabricated with GaP/TCO/Ag ODR. The vertical AlGaInP LED with Ag reflector shows forward bias voltage of 2.05 V at 20 mA. However, the vertical AlGaInP LEDs with ODR structure using ITO, IZO, and AZO show forward bias voltages of 2.1, 2.13, and 2.2 at 20 mA, respectively, which are slightly higher than that of the AlGaInP LED with Ag reflector. These results can be attributed to the relatively high sheet resistance of the ITO, IZO, and AZO films. The sheet resistances of the 40 nm-thick ITO, IZO, and AZO films were measured as 104, 105, and 503 U/sq, respectively. Fig. 4 exhibits the light output-current characteristics of the AlGaInP LEDs fabricated with Ag reflector and ODR structure using TCO. The output power of the LEDs fabricated with ODR structure was much higher than that of the LED with Ag reflector. In addition,
40
GaP/AZO/Ag GaP/IZO/Ag GaP/ITO/Ag GaP/Ag
20
o
Annealing temperature 350 C 50
400
500
600
700
800
Wavelength (nm) Fig. 2. Optical transmission of TCO films after annealing process of 350 C.
0
0
1
2
3
4
5
Voltage (V) Fig. 3. Forward IeV characteristics of Ag reflector LED and TCO/Ag reflector LEDs.
J.W. Seo et al. / Current Applied Physics 11 (2011) S385eS387
S387
scale and ideality factor of the AlGaInP LEDs fabricated with the Ag reflector and the ODR structure using TCO are shown in Fig. 5. However, the leakage current reduction in low bias was not observed in Fig. 5. In addition, the AlGaInP LEDs with Ag reflector and ODR did not induce significant changes in ideality factors. These results imply that the enhancement in light output of the AlGaInP LEDs with ODR cannot be attributed to the degradation of the LEDs with Ag reflectors by annealing process. Another possible explanation for the increased light output of the LEDs fabricated with ODR is the higher reflectivity of ODR structures. Kim et al. reported that GaInN LEDs with GaN/low-n ITO/Ag ODR show a 31.6% higher light extraction efficiency than LEDs with Ag reflector [17]. In addition, Hsu et al. reported that a 1-mm2 AlGaInP LED employing a ODR using ITO layer showed an external quantum efficiency of 31.8% at 100 mA [18]. Accordingly, the enhancement in light output of the LEDs with ODR can be attributed to the use of a highly reflective ODR and highly transmission of TCO layer. Fig. 4. Light output of the Ag reflector LED and the TCO/Ag reflector LEDs. The inset shows brightness of same LEDs at operating current of 20 mA.
the brightness of same LEDs at operating current of 20 mA is shown in the inset of Fig. 4. The brightness at 20 mA for the ODR LED using ITO is 151 mcd, comparable to that of the ODR LED using IZO, 147 mcd. This is due to the fact that the transmittance of ITO is higher than that of IZO. The LED with GaP/AZO/Ag ODR having the lowest transmittance of ODR LEDs gives 137 mcd. On the other hand, the brightness of LED with the Ag reflector is as low as 106 mcd. The brightness of the LEDs with ODR structure using ITO, IZO, and AZO increased by 42%, 38%, and 29% than that of the AlGaInP LED with Ag reflector, respectively. The light output intensity of the LEDs with ODR corresponded with result of the transmission of TCO. We expected that the enhancement in light output of the LEDs with ODR could be related to damage of Ag reflector and higher reflectivity of ODR. One possible explanation for the increased light output of the AlGaInP LEDs fabricated with ODR is degradation of the Ag reflector LED by deep level defect during the annealing process in order to improve electrical property. Meneghesso et al. reported that the degradation of optical power is accompanied with both increase of generation-recombination current at low forward bias and increase of device ideality factor [16]. To examine the influence of degradation of Ag reflector, IeV characteristics plotted on logelog
1
GaP/AZO/Ag GaP/IZO/Ag GaP/ITO/Ag GaP/Ag
Current (mA)
0.01 1E-4 1E-6 1E-8 1E-10 0
1
2
3
Voltage (V) Fig. 5. Forward IeV characteristics plotted on logelog scale of Ag reflector LED and TCO/Ag reflector LEDs. The inset shows ideality factors of same LEDs.
4. Conclusion In summary, we have investigated electrical and optical properties of the vertical AlGaInP LEDs with Ag reflector and GaP/TCO/Ag ODR structure. ITO, IZO, and AZO films showed transmittance of 97%, 96%, and 91% at the wavelength of 620 nm, respectively. Because of highly resistivity of TCO films, AlGaInP LEDs with ODR structure using TCO films produced slightly higher forward voltages than AlGaInP LED with Ag reflector. However, the AlGaInP LEDs with ODR structure using ITO, IZO, and AZO showed 42, 38, 29% higher brightness at 20 mA than the AlGaInP LED with Ag reflector, respectively. Acknowledgment This work was supported by Mid-career Researcher Program through NRF grant funded by the "MEST (No. 2009-0080313), and was also supported by the WCU program at Sunchon National University, and this work was supported in part by Research Foundation of Engineering College, Sunchon National University. References [1] H. Sugawara, M. Ishikawa, G. Hatakoshi, Appl. Phys. Lett. 58 (1991) 1010. [2] H.C. Wang, Y.K. Su, C.L. Lin, W.B. Chen, S.M. Chen, W.L. Li, Jpn. J. Appl. Phys. 43 (2004) 1934. [3] Y.J. Lee, H.C. Tseng, H.C. Kuo, S.C. Wang, C.W. Chang, T.C. Hsu, Y.L. Yang, M.H. Hsieh, M.J. Jou, B.J. Lee, IEEE Photonics Technol. Lett. 17 (2005) 1041. [4] R. Windisch, C. Rooman, S. Meinlschmidt, P. Kiesel, D. Zipperer, G.H. Dohler, B. Drtta, M. Kuijk, G. Borghs, P. Heremans, Appl. Phys. Lett. 79 (2001) 2315. [5] S.W. Chiou, C.P. Lee, C.K. Huang, C.W. Chen, J. Appl. Phys. 87 (2000) 2052. [6] H.S. Oh, J.H. Joo, J.H. Lee, J.H. Baek, J.W. Seo, J.S. Kwak, Jpn. J. Appl. Phys. 47 (2008) 6214. [7] K. Streubel, N. Linder, R. Werth, A. Jaeger, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 321. [8] S. Illek, U. Jacob, A. Ploessl, P. Strauss, K. Streubel, W. Wegleiter, R. Wirth, Compd. Semicond. 8 (2002) 39. [9] T. Gessmann, E.F. Schubert, J. Appl. Phys. 58 (2004) 1010. [10] J.K. Kim, T. Gessmann, H. Luo, E.F. Schubert, Appl. Phys. Lett. 84 (2004) 4508. [11] J.Q. Xi, M. Ojha, W.J. Cho, J.L. Plawsky, W.N. Gill, T. Gessmann, E.F. Schubert, Opt. Lett. 30 (2005) 1518. [12] T. Minami, H. Sonohra, T. Kakumu, S. Takata, Thin Solid Films 270 (1995) 37. [13] P. Lippens, A. Segers, J. Haemers, R. De Gryse, Thin Solid Films 317 (1998) 405. [14] G. Hu, B. Kumar, H. Gong, E.F. Chor, P. Wu, Appl. Phys. Lett. 88 (2006) 101901. [15] W.J. Lee, Y.K. Fang, J.J. Ho, C.Y. Chen, L.H. Chiou, S.J. Wang, F. Dai, T. Hsieh, R.Y. Tsai, D. Huang, F.C. Ho, Solid State Electron. 46 (2002) 477. [16] G. Meneghesso, S. Levada, R. Pierobon, F. Rampazzo, E. Zanoni, A. Cavallini, A. Castaldini, G. Scamarcio, S. Du, I. Eliashevich, IEDM Technical Digest (2002) 103 IEEE, San Francisco. [17] J.K. Kim, Th. Gessmann, E.F. Schubert, J.Q. Xi, H. Luo, J. Cho, C. Sone, Y. Park, Appl. Phys. Lett. 88 (2006) 013501. [18] S.C. Hsu, D.S. Wuu, C.Y. Lee, J.Y. Su, R.H. Horng, IEEE Photonics Technol. Lett. 19 (2007) 492.