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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 19 — Sep. 18, 2006
  • pp: 8654–8660
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Light extraction enhancement from nanoimprinted photonic crystal GaN-based blue lighte-mitting diodes

Hyun Kyong Cho , Junho Jang, Jeong-Hyeon Choi, Jaewan Choi, Jongwook Kim, Jeong Soo Lee, Beomseok Lee, Young Ho Choe, Ki-Dong Lee, Sang Hoon Kim, Kwyro Lee, Sun-Kyung Kim, and Yong-Hee Lee  »View Author Affiliations


Optics Express, Vol. 14, Issue 19, pp. 8654-8660 (2006)
http://dx.doi.org/10.1364/OE.14.008654


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Abstract

The nano-imprint lithography method was employed to incorporate wide-area (375×330µm2) photonic-crystal (PC) patterns onto the top surface of GaN-based LEDs. When the 280-nm-thick p-GaN was partly etched to ~140nm, the maximal extraction-efficiency was observed without deteriorating electrical properties. After epoxy encapsulation, the light output of the PC LED was enhanced by 25% in comparison to the standard LED without pattern, at a standard current of 20mA. By three-dimensional finite-difference time-domain method, we found that the extraction efficiency of the LED tends to be saturated as the etch-depth in the GaN epitaxial-layer becomes larger than the wavelength of the guided modes.

© 2006 Optical Society of America

1. Introduction

GaN-based light-emitting diodes (LEDs) have been employed for various applications, including traffic signals, full-color displays, back light units for liquid-crystal display and so forth. Likewise other conventional semiconductor LEDs, the high refractive index of GaN (n=2.46) prohibits light beyond a critical angle (θc) from being extracted because of the phase-mismatch. For instance, the ratio of the total flux reaching the capping layer filled with epoxy (n=1.4) through the top GaN surface upon the first reflection is merely 0θc(1r(θ)2)sinθdθ=15.4%, where r(θ) is the Fresnel reflection coefficient. This value covers the light going downward initially and then reflecting back at a mirror beneath the sapphire substrate. Certainly, if one wishes to define the extraction efficiency rigorously, one should also consider the horizontal radiation through four side-facets surrounding the LED [1

1. A. David, T. Fujii, R. Sharma, K. Mcgroody, S. Nakamura, S. P. DenBaars, E. L. Hu, and C. Weisbuch, “Photonic-crystal GaN light-emitting diodes with tailored guided mode distribution,” Appl. Phys. Lett. 88, 061124 (2006). [CrossRef]

]. Typically, the amount of photons escaping through the side-facets of GaN layer can be negligible for large-area LEDs, because the active material in the GaN LED has a fairly large extinction-coefficient (k~0.05) [2

2. E. Stefanov, B. S. Shelton, H. S. Venugopalan, T. Zhang, and I. Eliashevich, “Optimizing the external light extraction of nitride LEDs,” in Solid State Lighting II, I. T. Ferguson, N. Narendran, S. P. DenBaars, and Y.-S. Park, eds., Proc. SPIE4776, 223234 (2002). [CrossRef]

]. In comparison, the transmission through the side-facets of the non-absorbing, sapphire-substrate contributes more to the horizontal radiation. In most schemes, the amount of the horizontal radiation is almost constant and it may not be a matter of main concern. Thus, considering the total portion of the extracted light, one can urge that there are still a plenty of rooms of improvement for the light extraction.

There have been two basic schemes toward the high-efficiency of GaN LED. One is to directly manipulate the physical properties (radiation pattern and decay-time) of the light source by the microcavity effect [3

3. Y. C. Chen, J. J. Wierer, M. R. Krames, M. J. Ludowise, M. S. Misra, F. Ahmed, A. Y. Kim, G. O. Mueller, J. C. Bhat, S. A. Stockman, and P. S. Martin, “Optical cavity effects in InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes,” Appl. Phys. Lett. 82, 2221 (2003). [CrossRef]

, 4

4. Y. J. Lee, S. H. Kim, J. Huh, G. H. Kim, Y. H. Lee, S. H. Cho, Y. C. Kim, and Y. R. Do, “A highe-xtraction-efficiency nanopatterned organic light-emitting diode,” Appl. Phys. Lett. 82, 3779 (2003). [CrossRef]

, 5

5. K. Okamoto, I. Niki, A. Shvartster, Y. Narukawa, T. Mukai, and A. Scherer,” Nature 3, 601 (2004). [CrossRef]

]. In this case, a highly reflective metal is placed close to the light-emitting element. The other is to incorporate nanopatterns to convert the guided waves trapped in an epoxy-GaN-sapphire multimode waveguide into external leaky waves [6

6. Y. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, “III-nitride blue and ultraviolet photonic crystal light emitting diodes,” Appl. Phys. Lett. 84, 466 (2004). [CrossRef]

,7

7. L. Chen and A. V. Nurmikko, “Fabrication and performance of efficient blue light emitting III-nitride photonic crystals,” Appl. Phys. Lett. 85, 3663 (2004). [CrossRef]

,8

8. A. David, C. Meier, R. Sharma, F. S. Diana, S. P. DenBaars, E. Hu, S. Nakamura, and C. Weisbuch, “Photonic bands in two-dimensionally patterned multimode GaN waveguides for light extraction,” Appl. Phys. Lett. 87, 101107 (2005). [CrossRef]

]. The former scheme is available only for flip chips or laser lift-off GaN LEDs [9

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

]. Although a number of attempts incorporating the photonic crystal (PC) into various LEDs have been reported, the effect of the etch-depth on the light extraction was not investigated thoroughly. In this study, we focused on the etch-depth for maximum extraction efficiency, employing PC patterns on the top surface of the GaN LED. Computationally, we employed the three-dimensional finite-difference time-domain (3D-FDTD) method. This analysis shows the escape route of the light generated in LEDs into the capping layer for a given time.

From the viewpoint of practical manufacturing, the fabrication of the sub-micron feature has been the nontrivial issue [10

10. D. H. Kim, C. O. Cho, Y. G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q-Han Park, “Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns,” Appl. Phys. Lett. 87, 203508 (2005) [CrossRef]

]. So far, the wavelength-scale patterns have been mostly defined by the electron-beam lithography. Here, we tried nano-imprint lithography (NIL) techniques to define large-area PC patterns (375×330µm2) for the PC GaN-LEDs. For comparison purposes, we prepared GaN LEDs of various PCs with different etch-depths. The relative enhancement in light output was measured by an integration-sphere setup after epoxy encapsulation

2. Fabrication of PC GaN LEDs by Nano-imprint Lithography

The GaN-based LED structures were grown on sapphire substrates by metal organic chemical vapor deposition. The schematic of the PC GaN-LED is shown in Fig. 1(a). On the bottom sapphire substrate, the LED structure consists of a low-temperature GaN buffer layer, a 2µmthick undoped GaN layer, a 3µm-thick Si-doped n-type GaN layer, followed by 0.1µm-thick InGaN/GaN multiple-quantum-well (MQW) layers and a 0.3µm-thick Mg-doped p-type GaN. Prior to defining PC patterns, a 2800Å transparent indium tin oxide (ITO) film was deposited by sputtering process. After standard fabrication steps for normal LED devices (375×330µm2), two-dimensional square-lattice air-hole arrays with a lattice constant of 1200nm are formed by NIL techniques [11

11. The issue on the lattice constant will be described in the other paper.

]. The NIL is essentially a micro-molding process in which the topography of a template defines the patterns on a substrate [12

12. L Jay Guo, “Recent progress in nanoimprint technology and its applications,” J. Phys. D Appl. Phys. 37, R123 (2004) [CrossRef]

]. The resolution of the NIL is limited by that of the template. This process promises important advantages in manufacturing over photolithography and other lithography techniques. Note that the NIL does not require expensive projection optics, advanced illumination sources, or specialized resist materials that are the central to the others. The NIL is especially suitable for large-area processing.

In order to generate patterns on ITO/GaN, a SiO2 thin film layer and a Cr metal layer were used as etch masks. These layers were deposited by sputtering process. Finally, ITO and GaN layers were etched using inductively coupled plasma reactive ion etching with CH4 and BCl3/Cl2, respectively as shown in Fig. 1(b).

3. Computation of extraction-efficiency with various etch-depths

As aforementioned, the total internal reflection (TIR) prevents LED devices from gaining high-efficiency. The TIR is attributed to the mismatch of the in-plane directional momentum of light when θ>θc. Generally, the periodic PC pattern can add (or subtract) the Bloch momentum to photons such that they can penetrate into a lower index medium; otherwise, the trapped photons become extinct through the absorption. Since the absorption losses are unavoidable, it is important to find ways to turn the guided modes into leaky continuum modes before they are lost.

4. Comparison between simulation and experimental result

Computationally, the extraction efficiency increases gradually with the etch-depth until it reaches the MQWs. For comparison purposes, various PC LEDs of different etch-depths were fabricated. The other critical parameters such as lattice constants (a~1200nm) and filling-factors (f~0.20) were fixed. Generally, the large filling factor leads to the high extraction-efficiency. We could observe the same tendency till the filling-factor approached around 0.20. However, when the filling-factor was even larger than this value, electrical properties become deteriorated. We attribute this fact to the increase of the lateral resistance. We also prepared a normal LED without nanopatterns on the same wafer as a reference. Before encapsulation, we examined the performance of the device as is. For the shallow-etched PC LED, the near-field pattern looks uniform over the whole surface except the electrodes [Fig. 3(a)]. On the other hand, for the device etched deeper than the MQWs, almost no light emission is observed indicating the strong non-radiative recombination.

Before measuring current-voltage (IV) characteristic curves and light-output power, all LED devices were mounted p-side-up on the lead frame and molded in epoxy resin. The refractive index of epoxy resin is about 1.4. Figure 3(b) shows the change of the relative enhancement in light output as a function of etch-depth. The light output was measured by a conventional integration sphere to collect the omnidirectional radiation over whole emission wavelength. The center wavelength of the GaN LED is 457nm and its spectral width is 26nm (as a FWHM). The maximum relative enhancement of ~25% (averaged by ten samples) is recorded when the etch depth is 140nm, at a standard current of 20mA. Unexpectedly, the extraction efficiency decreases at the etch depth of 190nm. This decrease is partly attributed to the resistance increase resulting from the reduction of p-GaN volume. Experimentally, the resistance increases from 11Ω for 140nm p-GaN etch-depth to 15Ω for 200nm p-GaN etch-depth. The light output was measured by a conventional integration-sphere setup to collect the omnidirectional radiation. For the same sample, the relative enhancement measured at the normal direction before encapsulation was ~85%. The relative enhancement after encapsulation is smaller because of the reduction of the index contrast and the contribution of the horizontal radiation to the light output. For samples etched deeper than the QW depth, the enhancement becomes rather smaller owing to the nonradiative recombination.

The entire L-I characteristics of the PC LED exhibiting the maximum enhancement are shown in Fig 4(a). Up to 100mA, the PC LED maintains significant improvement in light output, proving its robustness even at such a high current. Finally, we measured I–V characteristics to determine the operation voltage (Vop) in the LED. The forward operation voltage is unchanged and the slope of the PC LED is also similar to that of the conventional LED [Fig. 4(b)]. Therefore, the incorporation of PCs with moderate etch-depth was hardly detrimental to its electrical properties. However, the LED device with PC patterns etched deeper than the QWs did not follow the normal I-V characteristic curve and we could not define a definite Vop.

Conclusion

We fabricated various PC GaN-based LEDs with different etch-depths by NIL techniques and measured the relative enhancement in the light output. The NIL technique was used to generate wide-area, high-throughput PC patterns. According to the 3D-FDTD simulation, the extraction-efficiency of the PC LED increases steadily with the etch-depth. However, the increment is reduced as the etch-depth becomes larger than ~λ/n. In the experiment, the maximum enhancement (~25%) was obtained at the condition that the p-GaN layer was etched to 140nm. As seen from the IV characteristic curve, it is worth pointing out that the enhancement was achieved without sacrificing the electrical properties. We expect that further optimization of PC parameters such as the lattice constant and the filling-factor will promise the brighter GaN LEDs.

Fig. 1. (a). Schematic of a photonic-crystal (PC) GaN-based LED. The PC patterns are employed on the top surface of the GaN LED. (b) Top-view scanning electron microscope (SEM) image of a nanoimprinted PC GaN-LED. The lattice constant (a) is 1200nm and the radius of holes are 0.25a.
Fig. 2. (a). Snap shots of the calculated electric-field distribution of the GaN LED without PC (top) and with PC (bottom). The gray bars at the side edges represent perfect-mirrors with reflectance of 100%. Particularly, the thickness of the sapphire substrate is shrunk down to 1.5µm because of the limited memory size. (b) The vertically-extracted efficiency of the PC GaN-LED with various etch-depths as a function of the propagation-distance. The extraction-efficiency is defined as a ratio of the energy through the top GaN surface and the total generated energy. This simulation is performed by the 3D-FDTD method. (c) The vertical extraction-efficiency as a function of the lattice constant. The extraction efficiency is an integrated value till the light propagates 30µm. The etch-depths are 140nm in all cases.
Fig. 3. (a). A typical near-field image for shallow-etched (p-GaN 140nm-etch) PC GaN-LED. It shows the uniform radiation over the whole surface. (b) The measured relative-enhancement as a function of etch-depth. When the device is etched deeper than the MQWs, almost no light emission is observed.
Fig. 4. (a). The entire L-I characteristics of the normal LED (unfilled circles) and the PC LED for 140nm etch-depth of p-GaN (filled squares) and the PC LED for 200nm etch-depth of p-GaN (filled diamonds). The relative enhancement of PC LED is 25% at a standard current of 20mA. (b) IV characteristics of the normal LED (unfilled circles) and the PC LED (filled squares). For the p-GaN 140-nm-etched device, the electrical properties (Vop, slope) are hardly changed.

References and links

1.

A. David, T. Fujii, R. Sharma, K. Mcgroody, S. Nakamura, S. P. DenBaars, E. L. Hu, and C. Weisbuch, “Photonic-crystal GaN light-emitting diodes with tailored guided mode distribution,” Appl. Phys. Lett. 88, 061124 (2006). [CrossRef]

2.

E. Stefanov, B. S. Shelton, H. S. Venugopalan, T. Zhang, and I. Eliashevich, “Optimizing the external light extraction of nitride LEDs,” in Solid State Lighting II, I. T. Ferguson, N. Narendran, S. P. DenBaars, and Y.-S. Park, eds., Proc. SPIE4776, 223234 (2002). [CrossRef]

3.

Y. C. Chen, J. J. Wierer, M. R. Krames, M. J. Ludowise, M. S. Misra, F. Ahmed, A. Y. Kim, G. O. Mueller, J. C. Bhat, S. A. Stockman, and P. S. Martin, “Optical cavity effects in InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes,” Appl. Phys. Lett. 82, 2221 (2003). [CrossRef]

4.

Y. J. Lee, S. H. Kim, J. Huh, G. H. Kim, Y. H. Lee, S. H. Cho, Y. C. Kim, and Y. R. Do, “A highe-xtraction-efficiency nanopatterned organic light-emitting diode,” Appl. Phys. Lett. 82, 3779 (2003). [CrossRef]

5.

K. Okamoto, I. Niki, A. Shvartster, Y. Narukawa, T. Mukai, and A. Scherer,” Nature 3, 601 (2004). [CrossRef]

6.

Y. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, “III-nitride blue and ultraviolet photonic crystal light emitting diodes,” Appl. Phys. Lett. 84, 466 (2004). [CrossRef]

7.

L. Chen and A. V. Nurmikko, “Fabrication and performance of efficient blue light emitting III-nitride photonic crystals,” Appl. Phys. Lett. 85, 3663 (2004). [CrossRef]

8.

A. David, C. Meier, R. Sharma, F. S. Diana, S. P. DenBaars, E. Hu, S. Nakamura, and C. Weisbuch, “Photonic bands in two-dimensionally patterned multimode GaN waveguides for light extraction,” Appl. Phys. Lett. 87, 101107 (2005). [CrossRef]

9.

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

10.

D. H. Kim, C. O. Cho, Y. G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q-Han Park, “Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns,” Appl. Phys. Lett. 87, 203508 (2005) [CrossRef]

11.

The issue on the lattice constant will be described in the other paper.

12.

L Jay Guo, “Recent progress in nanoimprint technology and its applications,” J. Phys. D Appl. Phys. 37, R123 (2004) [CrossRef]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(220.3740) Optical design and fabrication : Lithography
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Materials

History
Original Manuscript: June 2, 2006
Revised Manuscript: August 2, 2006
Manuscript Accepted: August 29, 2006
Published: September 18, 2006

Citation
Hyun Kyong Cho, Junho Jang, Jeong-Hyeon Choi, Jaewan Choi, Jongwook Kim, Jeong Soo Lee, Beomseok Lee, Young Ho Choe, Ki-Dong Lee, Sang Hoon Kim, Kwyro Lee, Sun-Kyung Kim, and Yong-Hee Lee, "Light extraction enhancement from nano-imprinted photonic crystal GaN-based blue light-emitting diodes," Opt. Express 14, 8654-8660 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-19-8654


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References

  1. A. David, T. Fujii, R. Sharma, K. Mcgroody, S. Nakamura, S. P. DenBaars, E. L. Hu, and C. Weisbuch, "Photonic-crystal GaN light-emitting diodes with tailored guided mode distribution," Appl. Phys. Lett. 88, 061124 (2006). [CrossRef]
  2. E. Stefanov, B. S. Shelton, H. S. Venugopalan, T. Zhang, and I. Eliashevich, "Optimizing the external light extraction of nitride LEDs," in Solid State Lighting II, I. T. Ferguson, N. Narendran, S. P. DenBaars, Y.-S. Park, eds., Proc. SPIE 4776, 223 234 (2002). [CrossRef]
  3. Y. C. Chen, J. J. Wierer, M. R. Krames, M. J. Ludowise, M. S. Misra, F. Ahmed, A. Y. Kim, G. O. Mueller, J. C. Bhat, S. A. Stockman, and P. S. Martin, "Optical cavity effects in InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes," Appl. Phys. Lett. 82, 2221 (2003). [CrossRef]
  4. Y. J. Lee, S. H. Kim, J. Huh, G. H. Kim, Y. H. Lee, S. H. Cho, Y. C. Kim, and Y. R. Do, "A high-extraction-efficiency nanopatterned organic light-emitting diode," Appl. Phys. Lett. 82, 3779 (2003). [CrossRef]
  5. K. Okamoto, I. Niki, A. Shvartster, Y. Narukawa, T. Mukai, and A. Scherer," Nature 3, 601 (2004). [CrossRef]
  6. Y. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, "III-nitride blue and ultraviolet photonic crystal light emitting diodes," Appl. Phys. Lett. 84, 466 (2004). [CrossRef]
  7. L. Chen, and A. V. Nurmikko, "Fabrication and performance of efficient blue light emitting III-nitride photonic crystals," Appl. Phys. Lett. 85, 3663 (2004). [CrossRef]
  8. A. David, C. Meier, R. Sharma, F. S. Diana, S. P. DenBaars, E. Hu, S. Nakamura, and C. Weisbuch, "Photonic bands in two-dimensionally patterned multimode GaN waveguides for light extraction," Appl. Phys. Lett. 87, 101107 (2005). [CrossRef]
  9. W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romando, and N. M. Johnson, "Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off," Appl. Phys. Lett. 75, 1360 (1999) [CrossRef]
  10. D. H. Kim, C. O. Cho, Y. G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q-Han Park, "Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns," Appl. Phys. Lett. 87, 203508 (2005) [CrossRef]
  11. The issue on the lattice constant will be described in the other paper.
  12. L Jay Guo, "Recent progress in nanoimprint technology and its applications," J. Phys. D Appl. Phys. 37, R123 (2004) [CrossRef]

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