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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 17 — Aug. 13, 2012
  • pp: 19194–19199

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Highly reliable Ag/Zn/Ag ohmic reflector for high-power GaN-based vertical light-emitting diode

Woong-Sun Yum, Joon-Woo Jeon, Jun-Suk Sung, and Tae-Yeon Seong  »View Author Affiliations


Optics Express, Vol. 20, Issue 17, pp. 19194-19199 (2012)
http://dx.doi.org/10.1364/OE.20.019194


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Abstract

We report the improved performance of InGaN/GaN-based light-emitting diodes (LEDs) through Ag reflectors combined with a Zn middle layer. It is shown that the Zn middle layer (5 nm thick) suppresses the agglomeration of Ag reflectors by forming ZnO and dissolving into Ag. The Ag/Zn/Ag contacts show a specific contact resistance of 6.2 × 10−5 Ωcm2 and reflectance of ~83% at a wavelength of 440 nm when annealed at 500 °C, which are much better than those of Ag only contacts. Blue LEDs fabricated with the 500 °C-annealed Ag/Zn/Ag reflectors show a forward voltage of 2.98 V at an injection current of 20 mA, which is lower than that (3.02 V) of LEDs with the annealed Ag only contacts. LEDs with the 500 °C-annealed Ag/Zn/Ag contacts exhibit 34% higher output power (at 20 mA) than LEDs with the annealed Ag only contacts.

© 2012 OSA

1. Introduction

For solid-state lighting application, the fabrication of high-power and high-efficiency GaN-based light-emitting diodes (LEDs) is essential. In this respect, vertical-injection GaN-based LEDs (VLEDs) have been widely investigated [1

W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romano, and N. M. Johnson, “Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off,” Appl. Phys. Lett. 75(10), 1360–1362 (1999). [CrossRef]

–6

H. W. Jang, S. W. Ryu, H. K. Yu, S. Lee, and J. L. Lee, “The role of reflective p-contacts in the enhancement of light extraction in nanotextured vertical InGaN light-emitting diodes,” Nanotechnology 21(2), 025203 (2010). [CrossRef] [PubMed]

]. Compared to conventional lateral-geometry LEDs, VLEDs mounted on conducting supporters have several advantages, e.g. better current injection, excellent heat dissipation, and enhanced reliability with respect to electrostatic discharge [1

W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romano, and N. M. Johnson, “Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off,” Appl. Phys. Lett. 75(10), 1360–1362 (1999). [CrossRef]

–6

H. W. Jang, S. W. Ryu, H. K. Yu, S. Lee, and J. L. Lee, “The role of reflective p-contacts in the enhancement of light extraction in nanotextured vertical InGaN light-emitting diodes,” Nanotechnology 21(2), 025203 (2010). [CrossRef] [PubMed]

]. VLEDs demand low-resistance ohmic contacts to both N-polar n-GaN and Ga-polar p-GaN. In addition, p-type contacts require high reflectance in order to maximize the extraction efficiency. Note that silver (Ag) is the most commonly used reflector because it exhibits reasonable ohmic with p-GaN [7

H. W. Jang and J.-L. Lee, “Mechanism for ohmic contact formation of Ni/Ag contacts on p-type GaN,” Appl. Phys. Lett. 85(24), 5920–5922 (2004). [CrossRef]

–11

S.-Y. Jung, Y.-H. Kim, Y. S. Kong, and T.-Y. Seong, “Improved electrical and thermal properties of Ag contacts for GaN-based flip-chip light-emitting diodes by using a NiZn alloy capping layer,” Superlattices Microstruct. 46(4), 578–584 (2009). [CrossRef]

]. However, Ag only contacts suffer from thermal degradation (e.g. agglomeration) when annealed at temperatures above 300 °C in air [7

H. W. Jang and J.-L. Lee, “Mechanism for ohmic contact formation of Ni/Ag contacts on p-type GaN,” Appl. Phys. Lett. 85(24), 5920–5922 (2004). [CrossRef]

–11

S.-Y. Jung, Y.-H. Kim, Y. S. Kong, and T.-Y. Seong, “Improved electrical and thermal properties of Ag contacts for GaN-based flip-chip light-emitting diodes by using a NiZn alloy capping layer,” Superlattices Microstruct. 46(4), 578–584 (2009). [CrossRef]

]. Thus, to improve the thermal instability, various approaches, such as the introduction of interlayers, capping layers, or Ag alloys, have been suggested until now. It was found that the use of interlayers (such as oxidized NiO/Au [8

D. L. Hibbard, S. P. Jung, C. Wang, D. Ullery, Y. S. Zhao, H. P. Lee, W. So, and H. Liu, “Low resistance high reflectance contacts to p-GaN using oxidized Ni/Au and Al or Ag,” Appl. Phys. Lett. 83(2), 311–313 (2003). [CrossRef]

], a ZnNi solid solution [9

J.-O. Song, D.-S. Leem, J. S. Kwak, O. H. Nam, Y. Park, and T.-Y. Seong, “Low-resistance and highly-reflective Zn–Ni solid solution/Ag ohmic contacts for flip-chip light-emitting diodes,” Appl. Phys. Lett. 83(24), 4990–4992 (2003). [CrossRef]

], and Cu-doped indium oxide [10

J.-O. Song, J.-S. Kwak, and T.-Y. Seong, “Cu-doped indium oxide/Ag ohmic contacts for high-power flip-chip light-emitting diodes,” Appl. Phys. Lett. 86(6), 062103 (2005). [CrossRef]

] was effective in improving both the reflectance and the contact resistivity of Ag contacts. For instance, Hibbard et al. [8

D. L. Hibbard, S. P. Jung, C. Wang, D. Ullery, Y. S. Zhao, H. P. Lee, W. So, and H. Liu, “Low resistance high reflectance contacts to p-GaN using oxidized Ni/Au and Al or Ag,” Appl. Phys. Lett. 83(2), 311–313 (2003). [CrossRef]

], investigating the use of NiO/Au interlayers on the performance of GaN-based LEDs, showed that back surface light emission is ~70% higher than that did LEDs with the Ni/Au contact. Song et al. reported that the introduction of a transparent Cu-doped In2O3 (CIO) layer was fairly effective in improving the electrical properties of Ag contacts. It was shown that LEDs fabricated with the CIO/Ag contacts gave a forward voltage of ~3.0 V at 20 mA, which is much better than that (3.36 V) of LEDs made with Ag only contacts. Furthermore, the thermal stability was shown to be significantly improved when capping layers, such as NiZn [11

S.-Y. Jung, Y.-H. Kim, Y. S. Kong, and T.-Y. Seong, “Improved electrical and thermal properties of Ag contacts for GaN-based flip-chip light-emitting diodes by using a NiZn alloy capping layer,” Superlattices Microstruct. 46(4), 578–584 (2009). [CrossRef]

], La films [12

I.-C. Chen, Y.-D. Chen, C.-C. Hsieh, C.-H. Kuo, and L.-C. Chang, “Highly Reflective Ag/La Bilayer Ohmic Contacts to p-Type GaN,” J. Electrochem. Soc. 158(3), H285–H288 (2011). [CrossRef]

], and Ru [13

J. H. Son, G. H. Jung, and J.-L. Lee, “Highly reflective Ag-Cu alloy-based ohmic contact on p-type GaN using Ru overlayer,” Opt. Lett. 33(24), 2907–2909 (2008). [CrossRef] [PubMed]

], were used. For example, Chen et al. [12

I.-C. Chen, Y.-D. Chen, C.-C. Hsieh, C.-H. Kuo, and L.-C. Chang, “Highly Reflective Ag/La Bilayer Ohmic Contacts to p-Type GaN,” J. Electrochem. Soc. 158(3), H285–H288 (2011). [CrossRef]

], investigating the electrical and optical properties of Ag contact with a La capping layer contacts, reported that the Ag/La bilayer contacts showed a reflectance of 91% at 460 nm and a contact resistivity of 1.6 × 10−4 Ωcm2 when annealed at 450 °C for 1 min. The smooth surface morphology of the annealed bilayer contacts was attributed to the formation of disordered La2O3. Furthermore, Son et al. [13

J. H. Son, G. H. Jung, and J.-L. Lee, “Highly reflective Ag-Cu alloy-based ohmic contact on p-type GaN using Ru overlayer,” Opt. Lett. 33(24), 2907–2909 (2008). [CrossRef] [PubMed]

], investigating the effect of a Ru overlayer on the electrical and optical properties of Ag-Cu alloy contact to p-GaN, reported that the AgCu alloy/Ru contacts produced low contact resistivity as well as high reflectance when annealed at 400 °C in air.

In this work, we introduced a 5-nm-thick Zn layer in between the two Ag layers. The Zn middle layer was employed to serve two functions: first a capping layer for the underlying Ag layer; second, an interlayer for the superjacent Ag layer. We investigated the effect of the Zn middle layer on the electrical and thermal properties of Ag reflectors to p-GaN and consequently on the performance of InGaN/GaN-based LEDs.

2. Experimental

Metalorganic chemical vapor deposition was used to grow blue (440 nm) InGaN/GaN multiple quantum-well (MQW) LED structures on (0001) sapphire substrates. The LED structures consist of a 0.15-μm-thick p-type GaN:Mg (na = 5 × 1017 cm−3) layer, a 20-nm-thick AlGaN electron blocking layer, a 0.1-μm-thick active layer, and a 2.0-μm-thick spreading layer, a 4.0-μm-thick n-type GaN:Si (nd = 5 × 1018 cm−3) layer, and a 2.0-μm-thick undoped GaN layer on the sapphire substrate. Prior to the lithography, the GaN samples were treated with a buffered oxide etch solution for 1 min and rinsed in de-ionized (DI) water. Circular transfer length method (CTLM) patterns were defined by the standard photolithographic technique for measuring specific contact resistance. The outer radius of CTLM patterns was fixed to be 200 μm and the gap spacing between outer and inner radii was varied from 5 to 40 μm. Prior to metal deposition, all samples were treated with a diluted HCl (HCl: DI water = 1: 1) solution for 1 min, rinsed in DI water, and blown dry in a N2 stream. After that, Ag/Zn/Ag (100 nm/5 nm/100 nm) films were deposited on LED samples by electron beam deposition under a base pressure of 6 × 10−7 Torr. For comparison, Ag films (200 nm) were also prepared. Some of the samples were annealed at 500 °C for 1 min in air. For LED samples, Cr/Ni/Au (25 nm/25 nm/50 nm) layers were deposited as an n-type ohmic electrode. Current-voltage (I-V) measurement was carried out by a high-current source-measuring unit (Keithley 238). X-ray photoelectron spectroscopy (XPS, Sigma Probe model) was performed using an Al Kα X-ray source (1486.6eV) in an UHV system in order to characterize the surface characteristics and to understand ohmic mechanisms. In addition, LED chips (500 × 250 μm2 in size) were fabricated and their optical outputs were examined by a Newport dual channel powermeter.

3. Results and discussion

Figure 1 shows the typical I-V characteristics of Ag only (200 nm) and Ag(100 nm)/Zn(5 nm)/Ag(100 nm) contacts before and after annealing at temperatures of 300 and 500 °C in air. The two schemes exhibit similar temperature dependence of the electrical behavior. In other words, both the as-deposited contacts show non-ohmic behavior. Their electrical properties become improved upon annealing, although the 300 °C-annealed Ag/Zn/Ag contacts still show non-linear I-V behavior. Both of the schemes reveal good ohmic behavior when annealed at 500 °C. Measurement showed that the 500 °C-annealed Ag only and Ag/Zn/Ag contacts give specific contact resistances of 5.4 × 10−4 and 6.2 × 10−5 Ωcm2, respectively.

Fig. 1 Typical I-V characteristics of Ag only (200 nm) and Ag(100 nm)/Zn(5 nm)/Ag(100 nm) contacts before and after annealing at temperatures of 300 and 500 °C. Reflectance of Ag only and Ag/Zn/Ag contacts before and after annealing at 300 and 500 °C is shown in the inset.

The reflectance of the Ag only (200 nm) and Ag(100 nm)/Zn(5 nm)/Ag(100 nm) contacts before and after annealing at 300 and 500 °C is show in the inset of Fig. 1. For the Ag only samples, their reflectance is significantly degraded upon annealing, which is related to the presence of voids formed at the Ag/GaN interface due to agglomeration (as will be shown later). However, the reflectance of the Ag/Zn/Ag samples is not seriously degraded after annealing, although the 500 °C-annealed sample shows somewhat lower reflectance in the range 400 – 530 nm than the 300 °C-annealed sample. The Ag only and Ag/Zn/Ag contacts reveal reflectances of 61% and 83% at 440 nm, respectively, after annealing at 500 °C.

Figure 2 exhibits the typical I-V characteristics of blue InGaN/GaN MQW LEDs fabricated with the Ag only and Ag/Zn/Ag contacts before and after annealing at 500 °C. The LEDs with the 500 °C-annealed Ag/Zn/Ag contacts show a forward-bias voltage of 2.98 V at an injection current of 20 mA. On the other hand, the LEDs with the annealed Ag only contacts give a higher forward-bias voltage of 3.02 V at 20 mA. The series resistance of the LEDs with the annealed Ag/Zn/Ag contacts was 5 Ω, whereas the LEDs with the annealed Ag only contacts exhibited a higher series resistance of 6.9 Ω.

Fig. 2 Typical I-V characteristics of blue InGaN/GaN MQW LEDs fabricated with Ag only and Ag/Zn/Ag contacts before and after annealing at 500 °C. Light output-current characteristics of LEDs fabricated with Ag only and Ag/Zn/Ag contacts are shown in the inset.

The light output-current (L-I) characteristics of the LEDs fabricated with the Ag only and Ag/Zn/Ag contacts as a function of the forward drive current are shown in the inset of Fig. 2. The LEDs fabricated with the 500 °C-annealed Ag/Zn/Ag contacts exhibit 18% and 25% higher light output power (at 20 mA) than the LEDs with the Ag only contacts annealed at 300 and 500 °C, respectively. This is consistent with the emission images (at 0.2 mA) of the LEDs fabricated with the Ag only and Ag/Zn/Ag reflectors (Fig. 3 ). A comparison shows that unlike the 500 °C-annealed Ag only contacts (Fig. 3(a)), the 500 °C-annealed Ag/Zn/Ag contact serves as a superior reflector, preventing the leakage of the emitted light (Fig. 3(b)). The leakage is due to the voids formed by agglomeration, as clearly demonstrated by their surface morphologies (the insets in Fig. 3). Unlike the Ag only sample (the inset in Fig. 3(a)), the Ag/Zn/Ag sample reveals a smooth surface without voids (the inset in Fig. 3(b)).

Fig. 3 Emission images (at 0.2 mA) obtained from LEDs fabricated with (a) Ag only and (b) Ag/Zn/Ag reflectors annealed at 500 °C. Their SEM images are shown in the insets (top right). The scale bar in the inset denotes 5 μm.

To analyze the chemical bonding states of Ga, XPS examination was performed on the Ag/Zn/Ag samples. During the XPS examination, the sample surface was Ar + ion-sputtered, and Zn, Ag, and Ga photoelectron signals were carefully monitored. The Ga 2p core levels were finally collected when only the Ga photoelectron peak (i.e., from Ga-N bonding) was detected. Figure 4 shows the XPS Ga 2p core level spectra obtained from the Ag/Zn/Ag contacts on p-GaN before and after annealing at 500 °C. XPS core level peak fittings were carried out with a Shirley-type background and Lorentzian–Doniac–Sunsic curves convoluted with a Gaussian profile. The Ga 2p core levels are composed of components from the Ga-N and Ga-O bonds. It is noted that the Ga 2p core level for the 500 °C-annealed sample shifts toward the lower binding-energy side by 0.25 eV compared to that of the as-deposited sample. This indicates that annealing causes the surface Fermi level to shift toward the valence band edge [14

J.-O. Song, J. S. Kwak, Y. Park, and T.-Y. Seong, “Ohmic and degradation mechanisms of Ag contacts on p-type GaN,” Appl. Phys. Lett. 86(6), 062104 (2005). [CrossRef]

], resulting in a reduction in the band-bending of p-GaN, namely, a lowering of the Schottky barrier height (SBH). The Ga 2p peak shift is indicative of a change of the band-bending since the N 1s core level spectra exhibit a similar shift behavior [15

K. M. Tracy, P. J. Hartlieb, S. Einfeldt, R. F. Davis, E. H. Hurt, and R. J. Nemanich, “Electrical and chemical characterization of the Schottky barrier formed between clean n-GaN(0001) surfaces and Pt, Au, and Ag,” J. Appl. Phys. 94(6), 3939–3948 (2003). [CrossRef]

]. All the samples are shown to contain a small amount of oxygen at the interface region.

Fig. 4 XPS Ga 2p core level spectra obtained from Ag/Zn/Ag contacts on p-GaN (a) before and (b) after annealing at 500 °C.

XPS examination was also carried out to characterize interfacial reactions between the Ag/Zn/Ag layer and GaN. Figure 5 shows the XPS depth profiles obtained from the Ag/Zn/Ag contacts on p-GaN before and after annealing at 500 °C. For the as-deposited sample (Fig. 5(a)), a Zn layer is in between the top and bottom Ag layers. Upon annealing at 500 °C (Fig. 5(b)), most of Zn atoms were out-diffused toward the sample surface region, although a small amount of Zn atoms dissolved into the Ag layer (as indicated by the arrows) [16

T. B. Massalski, H. Okamoto, P. R. Subramanian, and L. Kacprzak, Binary Alloy Phase Diagrams, 2nd ed. (ASM International, Materials Park, Oh, 117, 1990).

]. It is noted that oxygen was introduced into the surface region during annealing, being indicative of the formation of ZnO. A comparison shows that for the 500 °C-annealed sample, some amount of Ga were out-diffused into the Ag layer to form an Ag-Ga solid solution [14

J.-O. Song, J. S. Kwak, Y. Park, and T.-Y. Seong, “Ohmic and degradation mechanisms of Ag contacts on p-type GaN,” Appl. Phys. Lett. 86(6), 062104 (2005). [CrossRef]

], which results in the generation of Ga vacancies at the surface region.

Fig. 5 XPS depth profiles obtained from Ag/Zn/Ag contacts on p-GaN (a) before and (b) after annealing at 500 °C.

The electrical and optical properties of the Ag/Zn/Ag contacts were significantly improved upon annealing at 500 °C for 1 min in air. The annealing-induced improvement can be explained in terms of reduction in the effective SBH due to the shift of the surface Fermi level toward the valence-band edge (Fig. 4) and the improved thermal stability (Fig. 3). The surface Fermi level shift is associated with the formation of a Ag-Ga solid solution, generating acceptor-like Ga vacancies near the GaN surface region and so increase in the carrier concentration at the surface region [14

J.-O. Song, J. S. Kwak, Y. Park, and T.-Y. Seong, “Ohmic and degradation mechanisms of Ag contacts on p-type GaN,” Appl. Phys. Lett. 86(6), 062104 (2005). [CrossRef]

,17

V. M. Bermudez, D. D. Koleske, and A. E. Wickenden, “The dependence of the structure and electronic properties of wurtzite GaN surfaces on the method of preparation,” Appl. Surf. Sci. 126(1–2), 69–82 (1998). [CrossRef]

]. It was shown that the use of the Zn middle layer prevented the formation of voids. This increases the contact areas between the Ag/Zn/Ag reflector and GaN as compared to the Ag only contact/GaN. It was reported that an Ag only layer is agglomerated as a result of surface diffusion to reduce the total free energy and the bulk diffusion of Ag atoms by oxygen–vacancy interaction [18

J. H. Son, G. H. Jung, and J.-L. Lee, “Enhancement of light reflectance and thermal stability in Ag–Cu alloy contacts on p-type GaN,” Appl. Phys. Lett. 93(1), 012102 (2008). [CrossRef]

,19

J.-P. Crocombette, H. de Monestrol, and F. Willaime, “Oxygen and vacancies in silver: A density-functional study in the local density and generalized gradient approximations,” Phys. Rev. B 66(2), 024114 (2002). [CrossRef]

]. Thus, agglomeration could be reduced by suppressing the formation of the oxygen–vacancy cluster [20

Y. H. Song, J. H. Son, G. H. Jung, and J.-L. Lee, “Effects of Mg additive on inhibition of Ag agglomeration in Ag-based ohmic contacts on p-GaN,” Electrochem. Solid-State Lett. 13(6), H173–H175 (2010). [CrossRef]

]. The ZnO layer formed on the sample surface (Fig. 5) would serve as a diffusion barrier to oxygen atoms during annealing. In addition, the presence of Zn atoms within Ag (Fig. 5(b)) might also contribute to the prevention of agglomeration of Ag layer, although this remains to be confirmed. The fact that the LEDs with the Ag/Zn/Ag contacts showed higher output power than the ones with the Ag only contacts (Fig. 2) is consistent with the combined results of the electrical and optical properties. It is worth mentioning that the output power of LEDs is more dominantly dependent on the optical properties than the electrical properties.

4. Summary and conclusion

We investigated the effect of the 5-nm-thick Zn middle layer on the thermal and electrical properties of Ag-based contacts. It was shown that the use of the Zn middle layer significantly improved the thermal stability of the Ag contacts. LEDs fabricated with the 500 °C-annealed Ag/Zn/Ag reflectors produced 25% higher output power (at 20 mA) than did LED with the 500 °C-annealed Ag only contacts. These results imply that the use of the Zn middle layer could serve as a potentially important processing tool for the fabrication of high-power InGaN/GaN-based vertical LEDs.

Acknowledgments

This work was supported by the World Class University program through the National Research Foundation of Korea funded by MEST (R33-2008-000-10025-0) and the Industrial Technology Development Program funded by the Ministry of Knowledge Economy (MKE), Korea.

References and links

1.

W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romano, and N. M. Johnson, “Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off,” Appl. Phys. Lett. 75(10), 1360–1362 (1999). [CrossRef]

2.

S. Y. Lee, K. K. Choi, H.-H. Jeong, H. S. Choi, T.-H. Oh, J.-O. Song, and T.-Y. Seong, “Wafer-level fabrication of GaN-based vertical light-emitting diodes using a multi-functional bonding material system,” Semicond. Sci. Technol. 24(9), 092001 (2009). [CrossRef]

3.

M. K. Kelly, O. Ambacher, B. Dahlheimer, G. Groos, R. Dimitrov, H. Angerer, and M. Stutzmann, “Optical patterning of GaN films,” Appl. Phys. Lett. 69(12), 1749–1751 (1996). [CrossRef]

4.

J.-T. Chu, C.-C. Kao, H.-W. Huang, W.-D. Liang, C.-F. Chu, T.-C. Lu, H.-C. Kuo, and S.-C. Wang, “Effects of Different n-Electrode Patterns on Optical Characteristics of Large-Area p-Side-Down InGaN Light-Emitting Diodes Fabricated by Laser Lift-Off,” Jpn. J. Appl. Phys. 44(11), 7910–7912 (2005). [CrossRef]

5.

O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steigerwald, M. O. Holcomb, P. S. Martin, and M. R. Krames, “High performance thin-film flip-chip InGaN–GaN light-emitting diodes,” Appl. Phys. Lett. 89(7), 071109 (2006). [CrossRef]

6.

H. W. Jang, S. W. Ryu, H. K. Yu, S. Lee, and J. L. Lee, “The role of reflective p-contacts in the enhancement of light extraction in nanotextured vertical InGaN light-emitting diodes,” Nanotechnology 21(2), 025203 (2010). [CrossRef] [PubMed]

7.

H. W. Jang and J.-L. Lee, “Mechanism for ohmic contact formation of Ni/Ag contacts on p-type GaN,” Appl. Phys. Lett. 85(24), 5920–5922 (2004). [CrossRef]

8.

D. L. Hibbard, S. P. Jung, C. Wang, D. Ullery, Y. S. Zhao, H. P. Lee, W. So, and H. Liu, “Low resistance high reflectance contacts to p-GaN using oxidized Ni/Au and Al or Ag,” Appl. Phys. Lett. 83(2), 311–313 (2003). [CrossRef]

9.

J.-O. Song, D.-S. Leem, J. S. Kwak, O. H. Nam, Y. Park, and T.-Y. Seong, “Low-resistance and highly-reflective Zn–Ni solid solution/Ag ohmic contacts for flip-chip light-emitting diodes,” Appl. Phys. Lett. 83(24), 4990–4992 (2003). [CrossRef]

10.

J.-O. Song, J.-S. Kwak, and T.-Y. Seong, “Cu-doped indium oxide/Ag ohmic contacts for high-power flip-chip light-emitting diodes,” Appl. Phys. Lett. 86(6), 062103 (2005). [CrossRef]

11.

S.-Y. Jung, Y.-H. Kim, Y. S. Kong, and T.-Y. Seong, “Improved electrical and thermal properties of Ag contacts for GaN-based flip-chip light-emitting diodes by using a NiZn alloy capping layer,” Superlattices Microstruct. 46(4), 578–584 (2009). [CrossRef]

12.

I.-C. Chen, Y.-D. Chen, C.-C. Hsieh, C.-H. Kuo, and L.-C. Chang, “Highly Reflective Ag/La Bilayer Ohmic Contacts to p-Type GaN,” J. Electrochem. Soc. 158(3), H285–H288 (2011). [CrossRef]

13.

J. H. Son, G. H. Jung, and J.-L. Lee, “Highly reflective Ag-Cu alloy-based ohmic contact on p-type GaN using Ru overlayer,” Opt. Lett. 33(24), 2907–2909 (2008). [CrossRef] [PubMed]

14.

J.-O. Song, J. S. Kwak, Y. Park, and T.-Y. Seong, “Ohmic and degradation mechanisms of Ag contacts on p-type GaN,” Appl. Phys. Lett. 86(6), 062104 (2005). [CrossRef]

15.

K. M. Tracy, P. J. Hartlieb, S. Einfeldt, R. F. Davis, E. H. Hurt, and R. J. Nemanich, “Electrical and chemical characterization of the Schottky barrier formed between clean n-GaN(0001) surfaces and Pt, Au, and Ag,” J. Appl. Phys. 94(6), 3939–3948 (2003). [CrossRef]

16.

T. B. Massalski, H. Okamoto, P. R. Subramanian, and L. Kacprzak, Binary Alloy Phase Diagrams, 2nd ed. (ASM International, Materials Park, Oh, 117, 1990).

17.

V. M. Bermudez, D. D. Koleske, and A. E. Wickenden, “The dependence of the structure and electronic properties of wurtzite GaN surfaces on the method of preparation,” Appl. Surf. Sci. 126(1–2), 69–82 (1998). [CrossRef]

18.

J. H. Son, G. H. Jung, and J.-L. Lee, “Enhancement of light reflectance and thermal stability in Ag–Cu alloy contacts on p-type GaN,” Appl. Phys. Lett. 93(1), 012102 (2008). [CrossRef]

19.

J.-P. Crocombette, H. de Monestrol, and F. Willaime, “Oxygen and vacancies in silver: A density-functional study in the local density and generalized gradient approximations,” Phys. Rev. B 66(2), 024114 (2002). [CrossRef]

20.

Y. H. Song, J. H. Son, G. H. Jung, and J.-L. Lee, “Effects of Mg additive on inhibition of Ag agglomeration in Ag-based ohmic contacts on p-GaN,” Electrochem. Solid-State Lett. 13(6), H173–H175 (2010). [CrossRef]

OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(230.4040) Optical devices : Mirrors

ToC Category:
Optical Devices

History
Original Manuscript: May 9, 2012
Revised Manuscript: July 11, 2012
Manuscript Accepted: July 16, 2012
Published: August 6, 2012

Citation
Woong-Sun Yum, Joon-Woo Jeon, Jun-Suk Sung, and Tae-Yeon Seong, "Highly reliable Ag/Zn/Ag ohmic reflector for high-power GaN-based vertical light-emitting diode," Opt. Express 20, 19194-19199 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-17-19194


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References

  1. W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romano, and N. M. Johnson, “Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off,” Appl. Phys. Lett.75(10), 1360–1362 (1999). [CrossRef]
  2. S. Y. Lee, K. K. Choi, H.-H. Jeong, H. S. Choi, T.-H. Oh, J.-O. Song, and T.-Y. Seong, “Wafer-level fabrication of GaN-based vertical light-emitting diodes using a multi-functional bonding material system,” Semicond. Sci. Technol.24(9), 092001 (2009). [CrossRef]
  3. M. K. Kelly, O. Ambacher, B. Dahlheimer, G. Groos, R. Dimitrov, H. Angerer, and M. Stutzmann, “Optical patterning of GaN films,” Appl. Phys. Lett.69(12), 1749–1751 (1996). [CrossRef]
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