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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 9 — Apr. 26, 2010
  • pp: 9728–9732
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Enhanced light extraction of nonpolar a-plane (11-20) GaN light emitting diodes on sapphire substrates by photo-enhanced chemical wet etching

Younghun Jung, Jihyun Kim, Soohwan Jang, Kwang Hyeon Baik, Yong Gon Seo, and Sung-Min Hwang  »View Author Affiliations


Optics Express, Vol. 18, Issue 9, pp. 9728-9732 (2010)
http://dx.doi.org/10.1364/OE.18.009728


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Abstract

The extraction efficiency of nonpolar a-plane (11-20) GaN LEDs on sapphire substrates has been enhanced by selectively etching the mesa sidewall faces and the n-type GaN surfaces with photoenhanced chemical wet etching. Submicron-sized trigonal prisms having prismatic planes of {1-100} were clearly displayed on the n-type GaN surfaces as well as the sidewall face after 5 min etching at 60°C. The radiation patterns have shown that more light is extracted in all directions and the output powers of surface textured a-plane GaN LEDs have increased by 25% compared with control samples. PEC wet etching produced unique feature of etching morphology on the mesa sidewall faces and the n-type GaN surface.

© 2010 OSA

1. Introduction

There have been intensive efforts to increase the extraction efficiency in GaN LEDs to improve the external quantum efficiency. It is well known that a significant fraction of generated lights are trapped inside the cavity due to total internal reflection. Thus, a variety of extraction methods are currently employed such as highly reflective p-electrode, chip geometry deformation, patterned sapphire substrate, photonic crystals, and surface roughening. Since Minsky et al. had demonstrated photoenhanced chemical (PEC) wet etching on GaN, the surface texturing with hexagonal pyramidal structures on chemically active nitrogen-face GaN is now commonly used in vertical thin-film GaN LEDs due to its simple and damage-free etching process [14

14. M. S. Minsky, M. White, and E. L. Hu, “Room‐temperature photoenhanced wet etching of GaN,” Appl. Phys. Lett. 68(11), 1531–1533 (1996). [CrossRef]

,15

15. T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). [CrossRef]

]. It is obvious that PEC wet etching can be also implemented in nonpolar and semipolar GaN films by selectively etching different crystallographic planes [16

16. Y. Jung, M. Mastro, J. Hite, C. R. Eddy Jr, and J. Kim, “Electrical and structural characterizations of non-polar AlGaN/GaN heterostructures,” Thin Solid Films 518(6), 1747–1750 (2010). [CrossRef]

18

18. A. C. Tamboli, M. C. Schmidt, A. Hirai, S. P. DenBaars, and E. L. Hu, “Photoelectrochemical undercut etching of m-plane GaN for microdisk applications,” J. Electrochem. Soc. 156(10), H767–H771 (2009). [CrossRef]

]. Hardy et al. recently demonstrated a nonpolar m-plane superluminescent diode by forming hexagonal pyramids on –c (000-1) face via PEC etching [19

19. M. T. Hardy, K. M. Kelchner, Y.-D. Lin, P. S. Hsu, K. Fujito, H. Ohta, J. S. Speck, S. Nakamura, and S. P. DenBaars, “m-Plane GaN-Based Blue Superluminescent Diodes Fabricated Using Selective Chemical Wet Etching,” Appl. Phys. Express 2(12), 121004 (2009). [CrossRef]

]. However, there are few reports available on PEC wet etching on heteroepitaxial a-plane GaN LEDs. In this work, we investigated how selective wet etching changes the morphology of a-plane GaN surfaces as well as mesa sidewall faces in nonpolar a-plane GaN LEDs on r-plane sapphire substrates.

2. Experimental details

High crystalline a-plane (11-20) GaN film was directly grown on r-plane (1-102) sapphire substrates by metalorganic chemical vapor deposition, utilizing a two-step growth method with high-temperature GaN buffer. Nonpolar a-plane InGaN/ GaN LEDs were subsequently grown on 4.5 μm thick a-plane GaN films. The doping concentration of the Si-doped n-GaN layer was 3 × 1018 cm−3, and four periods of 5 nm thick InGaN QW layer and 9 nm thick GaN barrier layer were grown for the active region, followed by 150 nm thick Mg-doped p-GaN layer with doping of 1.4 × 1018 cm−3. For a p-electrode, Ni/Au (3 nm/3 nm) was deposited on the p-GaN layer using an e-beam evaporator, followed by annealing in air ambient to form Ohmic contacts. The mesa pattern with dimension of 200 μm × 500 μm was transferred into the n-GaN layer down to 1 μm by inductively coupled plasma etching. Cr/Au (50 nm/400 nm) was deposited as the n-electrode.

A schematic diagram of heteroepitaxial a-plane InGaN/GaN LED structure is presented in Fig. 1(A)
Fig. 1 (A). Schematic diagram of heteroepitaxial a-plane InGaN/GaN LED structure (B). Schematic diagram of GaN planes showing c-axis, c-plane and a-plane. (C) Microscope image of the processed a-plane InGaN/GaN LED.
. Note that our a-plane non-polar InGaN/GaN LED devices are aligned with c-axis [Fig. 1(B) and 1(C)]. Figure 1(C) is a plan-view optical microscope image of LED chip. Fabricated LED devices were then immersed in a potassium hydroxide (KOH) solution for different periods of time. PEC etchings were performed at stirring rate of 300 rpm at 60°C with ultraviolet illuminations. Electrical and optical properties of LEDs were characterized by an on-wafer measurement system.

3. Results and discussion

Plan-view scanning electron microscope (SEM) images of the mesa sidewall face in the + c-axis [0001] direction are shown Fig. 2(A)
Fig. 2 SEM images in the direction of + c-axis [0001] of the mesa sidewall of LED samples (A) before and (B) after 5 min etching.
before and Fig. 2(B) after KOH etching in KOH solution. Figure 2(B) shows a clear evidence of PEC etching on the mesa sidewall in the + c-axis direction. A bundle of trigonal prisms having various widths from hundreds of nanometer to 1 micron were observed. These submicron-sized trigonal prisms with prismatic planes of {1-100} also indicate that the normal plane is a-plane (11-20). It is notable that the p-GaN layer was roughened in this case with relatively low temperature, and the p-GaN layer was intact after PEC etching due to Ni/Au contact layer.

Figure 3(A)
Fig. 3 SEM images of PEC textured LED samples in the directions of (A) + c-axis [0001], (B) + m-direction [1-100], and (C) –c-axis [000-1].
, 3(B), and 3(C) show SEM images of the sidewall faces after 5 min etching in the direction of the + c-axis, the + m-axis [1-100], and the –c-axis [000-1], respectively. The sidewall face in the + m-axis direction turned into rough surface morphology, which was the same case for the sidewall face in the –m-axis direction. Figure 3(C) is the sidewall face in the –c-axis direction, showing different surface morphology with the one in the + c-axis direction. There appeared a specific crystallographic plane as an etch-stop plane at the bottom of the sidewall, which was previously reported to be (−1-12-2) plane [20

20. Y. Gao, M. D. Craven, J. S. Speck, S. P. Den Baars, and E. L. Hu, “Dislocation- and crystallographic-dependent photoelectrochemical wet etching of gallium nitride,” Appl. Phys. Lett. 84(17), 3322–3324 (2004). [CrossRef]

]. Typical cone-shaped morphology can be seen on top of the sidewall in the –c-axis direction. It is important to note that the surface of n-type GaN between mesa sidewalls and n-contact is also textured simultaneously, showing a high density of trigonal prisms with a few hundred nanometer scale aligned along the + c-axis. As can be seen Fig. 2 and 3, PEC wet etching produced unique feature of etching morphology on the mesa sidewall faces and the n-type GaN surface, which is beneficial to increase the light extraction efficiency of a-plane GaN LEDs.

In order to investigate the effects of PEC wet etching, the output powers of etched LED samples were measured at the current injection of 20mA from on-wafer measurements (Fig. 4
Fig. 4 The optical output powers of control and PEC textured LED samples for 5 min, measured on-wafer at the current injection of 20mA.
). The light output powers of 5 min etched LEDs were 25% higher in average than those of control samples. The increase of output power can be attributed to the roughened n-type GaN surface as well as the sidewall faces. As shown in Fig. 5
Fig. 5 The comparison of radiation patterns of control and PEC textured a-plane LEDs, measured in m-axis and c-axis rotations.
, the radiation patterns confirm that more lights are extracted in all directions for the c-axis and m-axis rotations at 20 mA. PEC wet etching produces unique feature of etching morphology on the sidewall faces and the n-type GaN surfaces in a-plane GaN LEDs and significantly improves the extraction efficiency of heteroepitaxial a-plane GaN LEDs.

4. Conclusion

We report on the enhanced light extraction of heteroepitaxial a-plane GaN LEDs by selectively etching the sidewall faces and the n-type GaN surface via PEC wet etching. The light output powers of PEC etched LED samples have increased by 25% at 20mA current injection. The radiation patterns have also shown that lights are extracted at all directions, indicating that PEC wet etching is also quite effective to improve the extraction efficiency of heteroepitaxial a-plane GaN LEDs.

Acknowledgments

The work at Korea University was supported by the Carbon Dioxide Reduction and Sequestration Center, one of the 21st Century Frontier R&D Program funded by the Ministry of Education, Science and Technology of Korea. The work at KETI was supported by the IT R&D program (2009-F-022-01) and the International Joint R&D Program (10030797) by the Ministry of Knowledge Economy. Authors would like to thank Prof. J. I. Shim at Hanyang Univ. for measuring the radiation patterns.

References and links

1.

F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024–R10027 (1997). [CrossRef]

2.

J. Seo Im, H. Kollmer, J. Off, A. Sohmer, F. Scholz, and A. Hangleiter, “Reduction of oscillator strength due to piezoelectric fields in GaN/AlxGa1-xN quantum wells,” Phys. Rev. B 57(16), R9435–R9438 (1998). [CrossRef]

3.

P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature 406(6798), 865–868 (2000). [CrossRef] [PubMed]

4.

T. Detchprohm, M. Zhu, Y. Li, Y. Xia, C. Wetzel, E. A. Preble, L. Liu, T. Paskova, and D. Hanser, “Green light emitting diodes on a-plane GaN bulk substrates,” Appl. Phys. Lett. 92(24), 241109 (2008). [CrossRef]

5.

S.-M. Hwang, Y. G. Seo, K. H. Baik, I.-S. Cho, J. H. Baek, S. Jung, T. G. Kim, and M. Cho, “Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate,” Appl. Phys. Lett. 95(7), 071101 (2009). [CrossRef]

6.

A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. Denbaars, S. Nakamura, and U. K. Mishra, “Demonstration of Nonpolar m-Plane InGaN/GaN Light-Emitting Diodes on Free-Standing m-Plane GaN Substrates,” Jpn. J. Appl. Phys. 44(5), L173–L175 (2005). [CrossRef]

7.

M. Funato, M. Ueda, Y. Kawakami, Y. Narukawa, T. Kosugi, M. Takahashi, and T. Mukai, “Blue, Green, and Amber InGaN/GaN Light-Emitting Diodes on Semipolar {11-22} GaN Bulk Substrates,” Jpn. J. Appl. Phys. 45(26), L659–L662 (2006). [CrossRef]

8.

B. Neubert, T. Wunderer, P. Bruckner, F. Scholz, M. Feneberg, F. Lipski, M. Schirra, and K. Thonke, “Semipolar GaN/GaInN LEDs with more than 1 mW optical output power,” J. Cryst. Growth 298, 706–709 (2007). [CrossRef]

9.

M. Schmidt, K.-C. Kim, H. Sato, N. Fellows, H. Masui, S. Nakamura, S. P. DenBaars, and J. S. Speck, “High Power and High External Efficiency m-Plane InGaN Light Emitting Diodes,” Jpn. J. Appl. Phys. 46(7), L126–L128 (2007). [CrossRef]

10.

K.-C. Kim, M. C. Schmidt, H. Sato, F. Wu, N. Fellows, M. Saito, K. Fujito, J. S. Speck, S. Nakamura, and S. P. DenBaars, “Improved electroluminescence on nonpolar m -plane InGaN/GaN quantum wells LEDs,” Phys. Status Solidi 1, 125–127 (2007) (RRL).

11.

Y. Saito, K. Okuno, S. Boyama, N. Nakada, S. Nitta, Y. Ushida, and N. Shibata, “m-Plane GaInN Light Emitting Diodes Grown on Patterned a-Plane Sapphire Substrates,” Appl. Phys. Express 2, 041001 (2009). [CrossRef]

12.

Y.-D. Lin, A. Chakraborty, S. Brinkley, H. C. Kuo, T. Melo, K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Characterization of blue-green m-plane InGaN light emitting diodes,” Appl. Phys. Lett. 94(26), 261108 (2009). [CrossRef]

13.

H. Masui, S. Nakamura, S. P. DenBaars, and U. K. Mishra, “Nonpolar and Semipolar III-Nitride Light-Emitting Diodes: Achievements and Challenges,” IEEE Trans. Electron. Dev. 57(1), 88–100 (2010). [CrossRef]

14.

M. S. Minsky, M. White, and E. L. Hu, “Room‐temperature photoenhanced wet etching of GaN,” Appl. Phys. Lett. 68(11), 1531–1533 (1996). [CrossRef]

15.

T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). [CrossRef]

16.

Y. Jung, M. Mastro, J. Hite, C. R. Eddy Jr, and J. Kim, “Electrical and structural characterizations of non-polar AlGaN/GaN heterostructures,” Thin Solid Films 518(6), 1747–1750 (2010). [CrossRef]

17.

A. C. Tamboli, A. Hirai, S. Nakamura, S. P. DenBaars, and E. L. Hu, “Photoelectrochemical etching of p-type GaN heterostructures,” Appl. Phys. Lett. 94(15), 151113 (2009). [CrossRef]

18.

A. C. Tamboli, M. C. Schmidt, A. Hirai, S. P. DenBaars, and E. L. Hu, “Photoelectrochemical undercut etching of m-plane GaN for microdisk applications,” J. Electrochem. Soc. 156(10), H767–H771 (2009). [CrossRef]

19.

M. T. Hardy, K. M. Kelchner, Y.-D. Lin, P. S. Hsu, K. Fujito, H. Ohta, J. S. Speck, S. Nakamura, and S. P. DenBaars, “m-Plane GaN-Based Blue Superluminescent Diodes Fabricated Using Selective Chemical Wet Etching,” Appl. Phys. Express 2(12), 121004 (2009). [CrossRef]

20.

Y. Gao, M. D. Craven, J. S. Speck, S. P. Den Baars, and E. L. Hu, “Dislocation- and crystallographic-dependent photoelectrochemical wet etching of gallium nitride,” Appl. Phys. Lett. 84(17), 3322–3324 (2004). [CrossRef]

OCIS Codes
(230.0230) Optical devices : Optical devices
(230.3670) Optical devices : Light-emitting diodes
(230.4000) Optical devices : Microstructure fabrication

ToC Category:
Optical Devices

History
Original Manuscript: April 5, 2010
Revised Manuscript: April 21, 2010
Manuscript Accepted: April 22, 2010
Published: April 23, 2010

Citation
Younghun Jung, Jihyun Kim, Soohwan Jang, Kwang Hyeon Baik, Yong Gon Seo, and Sung-Min Hwang, "Enhanced light extraction of nonpolar a-plane (11-20) GaN light emitting diodes on sapphire substrates by photo-enhanced chemical wet etching," Opt. Express 18, 9728-9732 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-9-9728


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References

  1. F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024–R10027 (1997). [CrossRef]
  2. J. Seo Im, H. Kollmer, J. Off, A. Sohmer, F. Scholz, and A. Hangleiter, “Reduction of oscillator strength due to piezoelectric fields in GaN/AlxGa1-xN quantum wells,” Phys. Rev. B 57(16), R9435–R9438 (1998). [CrossRef]
  3. P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature 406(6798), 865–868 (2000). [CrossRef] [PubMed]
  4. T. Detchprohm, M. Zhu, Y. Li, Y. Xia, C. Wetzel, E. A. Preble, L. Liu, T. Paskova, and D. Hanser, “Green light emitting diodes on a-plane GaN bulk substrates,” Appl. Phys. Lett. 92(24), 241109 (2008). [CrossRef]
  5. S.-M. Hwang, Y. G. Seo, K. H. Baik, I.-S. Cho, J. H. Baek, S. Jung, T. G. Kim, and M. Cho, “Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate,” Appl. Phys. Lett. 95(7), 071101 (2009). [CrossRef]
  6. A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. Denbaars, S. Nakamura, and U. K. Mishra, “Demonstration of Nonpolar m-Plane InGaN/GaN Light-Emitting Diodes on Free-Standing m-Plane GaN Substrates,” Jpn. J. Appl. Phys. 44(5), L173–L175 (2005). [CrossRef]
  7. M. Funato, M. Ueda, Y. Kawakami, Y. Narukawa, T. Kosugi, M. Takahashi, and T. Mukai, “Blue, Green, and Amber InGaN/GaN Light-Emitting Diodes on Semipolar {11-22} GaN Bulk Substrates,” Jpn. J. Appl. Phys. 45(26), L659–L662 (2006). [CrossRef]
  8. B. Neubert, T. Wunderer, P. Bruckner, F. Scholz, M. Feneberg, F. Lipski, M. Schirra, and K. Thonke, “Semipolar GaN/GaInN LEDs with more than 1 mW optical output power,” J. Cryst. Growth 298, 706–709 (2007). [CrossRef]
  9. M. Schmidt, K.-C. Kim, H. Sato, N. Fellows, H. Masui, S. Nakamura, S. P. DenBaars, and J. S. Speck, “High Power and High External Efficiency m-Plane InGaN Light Emitting Diodes,” Jpn. J. Appl. Phys. 46(7), L126–L128 (2007). [CrossRef]
  10. K.-C. Kim, M. C. Schmidt, H. Sato, F. Wu, N. Fellows, M. Saito, K. Fujito, J. S. Speck, S. Nakamura, and S. P. DenBaars, “Improved electroluminescence on nonpolar m -plane InGaN/GaN quantum wells LEDs,” Phys. Status Solidi 1, 125–127 (2007) (RRL).
  11. Y. Saito, K. Okuno, S. Boyama, N. Nakada, S. Nitta, Y. Ushida, and N. Shibata, “m-Plane GaInN Light Emitting Diodes Grown on Patterned a-Plane Sapphire Substrates,” Appl. Phys. Express 2, 041001 (2009). [CrossRef]
  12. Y.-D. Lin, A. Chakraborty, S. Brinkley, H. C. Kuo, T. Melo, K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Characterization of blue-green m-plane InGaN light emitting diodes,” Appl. Phys. Lett. 94(26), 261108 (2009). [CrossRef]
  13. H. Masui, S. Nakamura, S. P. DenBaars, and U. K. Mishra, “Nonpolar and Semipolar III-Nitride Light-Emitting Diodes: Achievements and Challenges,” IEEE Trans. Electron. Dev. 57(1), 88–100 (2010). [CrossRef]
  14. M. S. Minsky, M. White, and E. L. Hu, “Room‐temperature photoenhanced wet etching of GaN,” Appl. Phys. Lett. 68(11), 1531–1533 (1996). [CrossRef]
  15. T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). [CrossRef]
  16. Y. Jung, M. Mastro, J. Hite, C. R. Eddy, and J. Kim, “Electrical and structural characterizations of non-polar AlGaN/GaN heterostructures,” Thin Solid Films 518(6), 1747–1750 (2010). [CrossRef]
  17. A. C. Tamboli, A. Hirai, S. Nakamura, S. P. DenBaars, and E. L. Hu, “Photoelectrochemical etching of p-type GaN heterostructures,” Appl. Phys. Lett. 94(15), 151113 (2009). [CrossRef]
  18. A. C. Tamboli, M. C. Schmidt, A. Hirai, S. P. DenBaars, and E. L. Hu, “Photoelectrochemical undercut etching of m-plane GaN for microdisk applications,” J. Electrochem. Soc. 156(10), H767–H771 (2009). [CrossRef]
  19. M. T. Hardy, K. M. Kelchner, Y.-D. Lin, P. S. Hsu, K. Fujito, H. Ohta, J. S. Speck, S. Nakamura, and S. P. DenBaars, “m-Plane GaN-Based Blue Superluminescent Diodes Fabricated Using Selective Chemical Wet Etching,” Appl. Phys. Express 2(12), 121004 (2009). [CrossRef]
  20. Y. Gao, M. D. Craven, J. S. Speck, S. P. Den Baars, and E. L. Hu, “Dislocation- and crystallographic-dependent photoelectrochemical wet etching of gallium nitride,” Appl. Phys. Lett. 84(17), 3322–3324 (2004). [CrossRef]

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