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Applied Optics

APPLICATIONS-CENTERED RESEARCH IN OPTICS

  • Editor: James C. Wyant
  • Vol. 46, Iss. 23 — Aug. 10, 2007
  • pp: 5974–5978
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Direct evaluation of reflector effects on radiant flux from InGaN-based light-emitting diodes

Hisashi Masui, Natalie N. Fellows, Hitoshi Sato, Hirokuni Asamizu, Shuji Nakamura, and Steven P. DenBaars  »View Author Affiliations


Applied Optics, Vol. 46, Issue 23, pp. 5974-5978 (2007)
http://dx.doi.org/10.1364/AO.46.005974


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Abstract

A metal layer formed on the backside of InGaN∕sapphire-based light-emitting diodes deteriorates the inherent optical power output. An experimental approach of a suspended die is employed to study the effects of such metal layers via a direct comparison in radiant flux from a discrete die with and without a reflector. A sphere package that employs no reflector is proposed and fabricated. Light extraction of the sphere design is discussed; a light source in the sphere package would not have to be either an ideal point or placed at the center of the sphere, due to a finite critical angle at the sphere∕air interface.

© 2007 Optical Society of America

1. Introduction

Emergence of InGaN light-emitting diodes (LEDs) not only led to completion of solid-state light source in the entire visible spectral range, but also paved a way to solid-state general lighting by the invention of simple dichromatic white LEDs [1

1. K. Bando, K. Sakano, Y. Noguchi, and Y. Shimizu, “Development of high-bright and pure-white LED lamps,” J. Light Visual Environ. 22, 2–5 (1998). [CrossRef]

]. The LED has an advantage over conventional lighting technology (such as fluorescent lamps) in ultimate energy efficiency, and for this purpose internal quantum efficiency (IQE) as well as light extraction efficiency is of recent interest [2

2. S. J. Lee, “Study of photon extraction efficiency in InGaN light-emitting diodes depending on chip structures and chip-mount schemes,” Opt. Eng. 45, 014601–14 (2006).

]. Light extraction techniques developed to date include flip-chips [3

3. Y. C. Shen, 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, 2221–2223 (2003). [CrossRef]

], transparent substrates [4

4. F. A. Kish, F. M. Steranka, D. C. DeFevere, D. A. Vanderwater, K. G. Park, C. P. Kuo, T. D. Osentowski, M. J. Peanasky, J. G. Yu, R. M. Fletcher, D. A. Steigerwald, and M. G. Craford, “Very high-efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1x)0.5In0.5P∕GaP light-emitting diodes,” Appl. Phys. Lett. 64, 2839–2841 (1994). [CrossRef]

], chip shaping [5

5. R. Krames, M. Ochiai-Holcomb, G. E. Höfler, C. Carter-Coman, E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J.-W. Huang, S. A. Stockman, F. A. Kish, and M. G. Craford, “High-power truncated-inverted-pyramid (AlxGa1x)0.5In0.5P∕GaP light-emitting diodes exhibiting >50% external quantum efficiency,” Appl. Phys. Lett. 75, 2365–2367 (1999). [CrossRef]

, 6

6. U. Strauss, H.-J. Lugauer, A. Weimar, J. Baur, G. Brüderl, D. Eisert, F. Kühn, U. Zehnder, and V. Härle, “Progress of InGaN light emitting diodes on SiC,” Phys. Status Solidi C 0, 276–279 (2002). [CrossRef]

], surface roughening [7

7. T. Fujii, A. David, Y. Gao, M. Iza, S. P. DenBaars, E. L. Hu, C. Weisbuch, and S. Nakamura, “Cone-shaped surface GaN-based light-emitting diodes,” Phys. Status Solidi C 2, 2836–2840 (2005). [CrossRef]

], photonic crystals [8

8. A. David, T. Fujii, B. Moran, S. Nakamura, S. P. DenBaars, and C. Weisbuch, “Photonic crystal laser lift-off GaN light-emitting diodes,” Appl. Phys. Lett. 88, 133514-3 (2006). [CrossRef]

], and patterned substrates [9

9. M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, and T. Mukai, “InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode,” Jpn. J. Appl. Phys. 41, L1431–L1433 (2002). [CrossRef]

], in addition to conventional resin packages. Many of the LED packages use metal reflectors, either intentionally (e.g., evaporated metal) or unintentionally (e.g., die-bonding glue), since many substrates are transparent to the LED emission [10

10. J. K. Kim, J.-Q. Xi, H. Luo, and E. Fred Schubert, “Enhanced light-extraction in GaInN near-ultraviolet light-emitting diode with A1-based omnidirectional reflector having NiZn∕Ag microcontacts,” Appl. Phys. Lett. 89, 141123-3 (2006). [CrossRef]

].

We were interested in the effects of such reflectors commonly placed on the backside of LED dies. In the present study, an approach was taken that used a suspended mounting of LED dies onto electrical metal leads (called suspended LEDs) to evaluate the effect of die backcoating (i.e., reflector) directly. It was found that power reduction was severe by application of backcoating. A clear sphere package was proposed and fabricated to eliminate backcoating. This sphere package demonstrated the highest extraction efficiency among the packages tested, despite the fact that the sphere shape had not been widely adopted, likely due to the belief in the necessity of a pointlike source [11

11. E. F. Schubert, Light-Emitting Diodes (Cambridge University Press, 2003), p. 120.

]. A performed calculation indicated that the sphere package is tolerable for light sources that have a finite extent directly owing to a finite escape-cone angle at the sphere surface to the free space. Relative comparison of extraction efficiency among the packages was made by the suspended LED approach.

2. Experimental Details

2A. Ingredients

LED dies were prepared as follows. LED structures were grown on sapphire wafers by the metallorganic chemical vapor deposition. They had a multiple quantum-well (MQW) stack consisting of InGaN∕GaN as a light-emitting layer. Emission wavelength was in the blue range (440460  nm). A common mesa structure was applied to make electrical contacts. The LED active area was a 300  μm×300  μm square, where a common Ni∕Au semitransparent layer was deposited on top as a current-spreading layer. The sapphire wafers were then lapped (residual lapping scratches were about a few μm wide) down to 100   μm thick and cut into dies (450   μm×550   μm). Other ingredients were commercially obtained. Headers were silver-finished TO-56 (5.6 mm in diameter). The silver-loaded epoxy (referred as Ag paste hereafter) was Kyocera Chemical CT220HK. Silicone resin was GE Silicone RVT615, whose nominal refractive index was 1.406 and heat conductivity was 0.2W   m1K1. The two-part epoxy resin was also a commercial product designed for lamp–LED packages.

2B. Sample-Fabrication Procedure

All dies were fabricated in the suspended LED form (Fig. 1) first, and optical output power was measured in an integrating sphere. A suspended LED consisted of an LED die suspended in the air by two gold wires that were attached to two posts, which served as electrical leads. In this suspended form, the total optical output power (radiant flux) can be measured with little reflection or absorption (i.e., effects of leads and wires are ignored). Some of those dies were then subjected to a backcoating to determine the effect of the backcoating as a reflector in output power. Two kinds of backcoatings were considered to be of common interest and prepared independently: Ag-paste painting and silver evaporation. The former was prepared by painting the backside of the suspended LED dies with a proper amount of Ag paste and then curing the Ag paste in an oven. The Ag-paste painting was later extended to sidewalls of the die to mimic the climb-up of Ag paste during the die-bonding process in industrial LED fabrication. The latter was prepared in an electron-beam evaporator, which deposited a 150  nm thick silver layer on the backside of the suspended LED dies.

Other dies from the suspended LEDs were taken off the posts and were put into one of two common packages. Several dies were made into industrial standard 5   mm lamps with Ag-paste die bonding and two-part epoxy resin. Other dies were mounted onto the headers using Ag-paste die bonding. In these two packages, the Ag-paste backcoating was practically inevitable due to the nature of die bonding. The advantage of this experimental method employing the suspended LEDs is that it is possible to compare directly the effect of packages in optical output power with an identical die used, rather than relying on statistics where a number of LED dies would be required to obtain reliable results.

Two trial packages were fabricated using silicone casting to enhance light extraction [11

11. E. F. Schubert, Light-Emitting Diodes (Cambridge University Press, 2003), p. 120.

]: a sphere package around the suspended LED and a cone package on the header-mounted LED. The sphere package was of special interest because it enabled the elimination of a backcoating. This sphere package had 2.5   cm of diameter. The cone package had 2.5   cm of diameter at bottom and an apex angle of 86°, slightly smaller than twice of the critical angle between the silicone and air. The LED die on the header sat at the bottom of the cone optic. Two brass molds with polished inner surface were used for casting. This is a standard and industrial compatible technique.

2C. Measurement

Optical output power was measured with a spectrometer (Instrument Systems MAS40) with 7.5   cm and 50   cm integrating spheres under DC operation at ambient room temperature. The dark-noise power readout of the spectrometer (less than 1   μW for a 7.5   cm sphere and less than 0.1   mW for a 50   cm one) has been subtracted from measured power numbers, and resulting power numbers are presented in the present paper. The conversion factor was defined as a ratio of output power at a constant current between two forms of LED packages. A power supply (Keithley 2400) was used as a current source where the applied voltage was monitored in 0.1   mW resolution, which served as an alternative monitor for changes in temperature during LED operation.

3. Results and Discussion

3A. Backcoating

The effect of backcoating was determined using suspended LEDs at 1   mA. Output power from a suspended LED was stably measured at 1   mA where heat generation had little effect. Die-temperature change was monitored by applied-voltage drift, which was sufficiently less than 0.01   V at 1   mA over more than the first ten seconds of LED operation. Heat generation became severe when a suspended LED was driven at 5   mA, confirmed by both the voltage drift and a roll-off in optical output power over several seconds. It was not possible to measure the output power stably at 20   mA because of excessive heat generation.

It was found that the backcoatings hardly worked as an ideal reflector as summarized in Table 1. The evaporated Ag backcoating reduced the output power by 70%. This reduction may be explained by the fact that the reflected light, which would have escaped if there were no backcoating, suffered from the absorption in the MQW and Ni∕Au semitransparent layer as well as multiple reflection and refraction in the LED structure. The Ag-paste backcoating reduced the output power by 45%. The reduction was less than the case of evaporated Ag because the Ag-paste backcoating scattered the incoming light. As a result, a considerable portion of the reflected light did not have to go through the MQW and the semitransparent layer. This contrast between the two types of backcoating is observed in photographs shown in Fig. 2. Scattered light by Ag-paste backcoating is seen well in Fig. 2(b) by the glowing die, while the die with evaporated Ag appeared dark except for the active area as shown in Fig. 2(a). Output power was further reduced when the sidewalls were coated with Ag paste; only 35% of the original output power was obtained. This shows that the sidewall coating prevented not only guided light by sapphire but also scattered light from the Ag-paste backcoating to escape. These results suggested that light absorption at the active layer and semitransparent p-contact metal was considerable.

3B. Common Packages

Conversion factors for the two packages (i.e., lamp and header) with respect to the suspended form were determined at 1 and 5   mA (Table 2). The conversion factor for header (0.6) agreed reasonably with that of the Ag-paste backcoating (0.55) described in the preceding subsection, because Ag-paste was used for the header mount. In lamps, the output power was increased due to extraction enhancement of encapsulant by overcoming the reduction caused by the Ag-paste backcoating. At 5   mA the conversion factor for both packages became a little higher than at 1   mA due to the heat sinking effect of submounts (die-temperature rise was suppressed). The extraction enhancement from header to lamp can be estimated to roughly be a factor 2 (1.1/0.62 at 1  mA; see Table 2). These results showed these common packages were not extracting the full, inherent radiant flux of LED dies.

3C. Trial Packages

A clear silicone sphere was one possible package that could eliminate backcoating. This trial package enhanced extraction efficiency by a factor of 2.3 (Table 2). A photograph of the sphere LED is shown in Fig. 3. The silicone sphere helped heat sinking due to its higher thermal conductivity than air, which was confirmed by stable operation at 20   mA DC. The maximum output power obtained in the present study was by the sphere package, and it was 2.7   mW at 2   mA and 22.7   mW at 20   mA. External quantum efficiency reached 35%.

A cone package (onto a header) increased output power from the header sample by 50% (at 1   mA, see Table 2). This resulting output power was comparable to that of the common lamp package; the cone optic enhanced the light extraction only as much as the lamp optic did.

3D. Light Extraction of the Sphere Package

The problem is set up as illustrated in Fig. 4 to investigate the large enhancement by the sphere package. There are two concentric spheres. The inner sphere (radius r1) is considered to be a light source. The outer sphere (radius r2) is assumed to be a clear packaging resin, with a refractive index n2. Outside the outer sphere is air. It is essential to realize that there exists a finite critical angle θc inside the sphere so that a light ray incident at an angle smaller than θc passing the surface does not experience the total internal reflection. Here we consider an arbitrary point A on the outer sphere. Inside the point A, θc can be calculated as
θc=sin11n2.
(1)

4. Summary

Effects of silver reflectors on LED optical power output were evaluated directly by fabricating suspended LEDs. We found experimentally and have concluded that backcoating on LED dies would not work as an ideal reflector. Based on this conclusion, a sphere LED package was proposed and fabricated, which demonstrated the maximum extraction efficiency among the packages tested in the present study. The light extraction of the sphere package design was analyzed by taking the critical angle between the packaging resin and air into account. The calculation has shown that the light source in a sphere package does not have to be an ideal point, but can be physically extended.

Appendix A.Separation of the Two Effects in the Suspended Approach

When an LED die is attached to a submount, the heat sinking effect of the submount prevents the die from heating up at high currents and influences IQE, as well as changing the light-extraction efficiency. To separate the effects of heating and light extraction, we introduced the following calculation. The output power P=Ps1 for a suspended LED driven at current I=I1, where a rise in die temperature can be negligible (for example, I1=1  mA) [13

13. C. Winnewisser, J. Schneider, M. Börsch, and H. W. Rotter, “In situ temperature measurements via ruby R lines of sapphire substrate based InGaN light emitting diodes during operation,” J. Appl. Phys. 89, 3091–3094 (2001). [CrossRef]

] is expressed as
Ps1=as1I1,
(3)
where as 1 is a proportionality constant implicitly containing IQE. When this LED is driven at a higher current I2 where heating is not negligible, the power can be expressed as
Ps2=as2I2,
(4)
where as2 is another proportionality constant that is responsible for the change in IQE due to the joule heating as well as other possible phenomena. This LED die is then experimentally transplanted to a submount or a certain type of package. The output power at I=I1 is expressed as
Pp1=ap1kI1,
(5)
where k is an enhancement factor in extraction efficiency. Heat generation is negligible so that the die temperature did not differ from the suspended form at I=I1. Another proportionality constant ap1 is set equal to as1, implying IQE does not change. When this packaged LED is driven at I2, the output power is
Pp2=ap2kI2,
(6)
where ap2 is responsible for the change in power due to the heat generation. The enhancement factor has been assumed to be constant at different currents. From Eqs. (3) and (5), a conversion factor for a particular package is defined as
kp=Pp1/Ps1,
(7)
provided power and current have been measured. The change in the IQE can be expressed from Eqs. (5) and (6) implicitly as
ap2/as2=(Pp2Ps1)/(Pp1Ps2).
(8)

Whenever two dies are taken from an LED wafer and are compared in the suspended form, they are likely to have similar performance. That is,
as2/as1=at2/at1.
(13)
If these numbers significantly deviate from Eq. (13), it can be said that one of the dies may have suffered from low IQE due to leakiness. In this case it is often not as1at1.

The authors acknowledge Andrew Weinberg and Nelson Bednersh at College of Engineering, University of California, Santa Barbara, for fine machining work. The present work was supported by The Solid-State Lighting and Display Center at University of California, Santa Barbara, and the Exploratory Research for Advanced Technology (ERATO) Program of the NICP∕Japan Science and Technology Agency.

Table 1. Effects of Backcoating on Suspended LEDs in Optical Output Powera

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Table 2. Conversion Factors for Different Packagesa

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Fig. 1 Schematic of a suspended LED.
Fig. 2 Suspended LEDs under operation (a) with evaporated Ag backcoating and (b) with Ag-paste backcoating. Note that the entire die is glowing in (b) while only the active area is bright in (a).
Fig. 3 Sphere LED under operation.
Fig. 4 Cross section of concentric spheres around the center O with radii r1 and r2. The point A is on the outer sphere where the tangent at point B on the inner sphere intersects. The critical angle θc is determined by the outer sphere material and free space (air).
1.

K. Bando, K. Sakano, Y. Noguchi, and Y. Shimizu, “Development of high-bright and pure-white LED lamps,” J. Light Visual Environ. 22, 2–5 (1998). [CrossRef]

2.

S. J. Lee, “Study of photon extraction efficiency in InGaN light-emitting diodes depending on chip structures and chip-mount schemes,” Opt. Eng. 45, 014601–14 (2006).

3.

Y. C. Shen, 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, 2221–2223 (2003). [CrossRef]

4.

F. A. Kish, F. M. Steranka, D. C. DeFevere, D. A. Vanderwater, K. G. Park, C. P. Kuo, T. D. Osentowski, M. J. Peanasky, J. G. Yu, R. M. Fletcher, D. A. Steigerwald, and M. G. Craford, “Very high-efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1x)0.5In0.5P∕GaP light-emitting diodes,” Appl. Phys. Lett. 64, 2839–2841 (1994). [CrossRef]

5.

R. Krames, M. Ochiai-Holcomb, G. E. Höfler, C. Carter-Coman, E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J.-W. Huang, S. A. Stockman, F. A. Kish, and M. G. Craford, “High-power truncated-inverted-pyramid (AlxGa1x)0.5In0.5P∕GaP light-emitting diodes exhibiting >50% external quantum efficiency,” Appl. Phys. Lett. 75, 2365–2367 (1999). [CrossRef]

6.

U. Strauss, H.-J. Lugauer, A. Weimar, J. Baur, G. Brüderl, D. Eisert, F. Kühn, U. Zehnder, and V. Härle, “Progress of InGaN light emitting diodes on SiC,” Phys. Status Solidi C 0, 276–279 (2002). [CrossRef]

7.

T. Fujii, A. David, Y. Gao, M. Iza, S. P. DenBaars, E. L. Hu, C. Weisbuch, and S. Nakamura, “Cone-shaped surface GaN-based light-emitting diodes,” Phys. Status Solidi C 2, 2836–2840 (2005). [CrossRef]

8.

A. David, T. Fujii, B. Moran, S. Nakamura, S. P. DenBaars, and C. Weisbuch, “Photonic crystal laser lift-off GaN light-emitting diodes,” Appl. Phys. Lett. 88, 133514-3 (2006). [CrossRef]

9.

M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, and T. Mukai, “InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode,” Jpn. J. Appl. Phys. 41, L1431–L1433 (2002). [CrossRef]

10.

J. K. Kim, J.-Q. Xi, H. Luo, and E. Fred Schubert, “Enhanced light-extraction in GaInN near-ultraviolet light-emitting diode with A1-based omnidirectional reflector having NiZn∕Ag microcontacts,” Appl. Phys. Lett. 89, 141123-3 (2006). [CrossRef]

11.

E. F. Schubert, Light-Emitting Diodes (Cambridge University Press, 2003), p. 120.

12.

Ref. [11], p. 90.

13.

C. Winnewisser, J. Schneider, M. Börsch, and H. W. Rotter, “In situ temperature measurements via ruby R lines of sapphire substrate based InGaN light emitting diodes during operation,” J. Appl. Phys. 89, 3091–3094 (2001). [CrossRef]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Optical Devices

History
Original Manuscript: April 10, 2007
Manuscript Accepted: May 30, 2007
Published: August 9, 2007

Citation
Hisashi Masui, Natalie N. Fellows, Hitoshi Sato, Hirokuni Asamizu, Shuji Nakamura, and Steven P. DenBaars, "Direct evaluation of reflector effects on radiant flux from InGaN-based light-emitting diodes," Appl. Opt. 46, 5974-5978 (2007)
http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-46-23-5974


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References

  1. K. Bando, K. Sakano, Y. Noguchi, and Y. Shimizu, "Development of high-bright and pure-white LED lamps," J. Light Visual Environ. 22, 2-5 (1998). [CrossRef]
  2. S. J. Lee, "Study of photon extraction efficiency in InGaN light-emitting diodes depending on chip structures and chip-mount schemes," Opt. Eng. 45, 014601-14 (2006).
  3. Y. C. Shen, 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, 2221-2223 (2003). [CrossRef]
  4. F. A. Kish, F. M. Steranka, D. C. DeFevere, D. A. Vanderwater, K. G. Park, C. P. Kuo, T. D. Osentowski, M. J. Peanasky, J. G. Yu, R. M. Fletcher, D. A. Steigerwald, and M. G. Craford, "Very high-efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes," Appl. Phys. Lett. 64, 2839-2841 (1994). [CrossRef]
  5. R. Krames, M. Ochiai-Holcomb, G. E. Höfler, C. Carter-Coman, E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J.-W. Huang, S. A. Stockman, F. A. Kish, and M. G. Craford, "High-power truncated-inverted-pyramid (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency," Appl. Phys. Lett. 75, 2365-2367 (1999). [CrossRef]
  6. U. Strauss, H.-J. Lugauer, A. Weimar, J. Baur, G. Brüderl, D. Eisert, F. Kühn, U. Zehnder, and V. Härle, "Progress of InGaN light emitting diodes on SiC," Phys. Status Solidi C 0, 276-279 (2002). [CrossRef]
  7. T. Fujii, A. David, Y. Gao, M. Iza, S. P. DenBaars, E. L. Hu, C. Weisbuch, and S. Nakamura, "Cone-shaped surface GaN-based light-emitting diodes," Phys. Status Solidi C 2, 2836-2840 (2005). [CrossRef]
  8. A. David, T. Fujii, B. Moran, S. Nakamura, S. P. DenBaars, and C. Weisbuch, "Photonic crystal laser lift-off GaN light-emitting diodes," Appl. Phys. Lett. 88, 133514-3 (2006). [CrossRef]
  9. M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, and T. Mukai, "InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode," Jpn. J. Appl. Phys. 41, L1431-L1433 (2002). [CrossRef]
  10. J. K. Kim, J.-Q. Xi, H. Luo, and E. Fred Schubert, "Enhanced light-extraction in GaInN near-ultraviolet light-emitting diode with A1-based omnidirectional reflector having NiZn/Ag microcontacts," Appl. Phys. Lett. 89, 141123-3 (2006). [CrossRef]
  11. E. F. Schubert, Light-Emitting Diodes (Cambridge University Press, 2003), p. 120.
  12. Ref. , p. 90.
  13. C. Winnewisser, J. Schneider, M. Börsch, and H. W. Rotter, "In situ temperature measurements via ruby R lines of sapphire substrate based InGaN light emitting diodes during operation," J. Appl. Phys. 89, 3091-3094 (2001). [CrossRef]

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