OSA's Digital Library

Energy Express

Energy Express

  • Editor: Christian Seassal
  • Vol. 20, Iss. S6 — Nov. 5, 2012
  • pp: A1019–A1025
« Show journal navigation

Vertical InGaN light-emitting diode with a retained patterned sapphire layer

Y.C. Yang, Jinn-Kong Sheu, Ming-Lun Lee, C. H. Yen, Wei-Chih Lai, Schang Jing Hon, and Tsun Kai Ko  »View Author Affiliations


Optics Express, Vol. 20, Issue S6, pp. A1019-A1025 (2012)
http://dx.doi.org/10.1364/OE.20.0A1019


View Full Text Article

Acrobat PDF (1136 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We present an efficient vertical InGaN light-emitting diode (LED) in which the proposed vertical LEDs were fabricated with patterned sapphire substrates (PSS) using thinning techniques. After the thinning of sapphire substrate, selective dry etching process was performed on the remainder sapphire layer to expose the n-GaN contact layer instead of removing the sapphire substrate using the laser lift-off technique. These processes feature the LEDs with a sapphire-face-up structure and vertical conduction property. The PSS was adopted as a growth substrate to mitigate the light-guided effect, and thereby increase the light-extraction efficiency. Compared with conventional lateral GaN LEDs grown on PSS, the proposed vertical LEDs exhibit a higher light output power and less power degradation at a high driving current. This could be attributed to the fact that the vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction.

© 2012 OSA

1. Introduction

2. Experiments

After the LLO of sapphire substrate, the CV-LEDs with planar and textured top n-GaN surface were labeled as CV-LED-I and CV-LED-II, respectively. To texture the n-GaN surface of the CV-LEDs, 6 M KOH solution at an elevated temperature of 60 °C for 180 seconds. All the experimental LEDs used in this study were in bare-chip form on TO can without encapulant and had a chip area of 1 mm × 1 mm. Current-voltage (I-V) characteristics of experimental LEDs were measured using the HP-4156C semiconductor parameter analyzer. Light output power-current (L-I) of the LEDs was measured using a calibrated integrating sphere combined with a source meter of Keithly 2400.

3. Results and discussions

Figure 2(a)
Fig. 2 (a) A typical cross-sectional view SEM image of LED-I (b) a focus at the trench (c) enlarged SEM image taken from the bottom surface of trench (d) a top-view image taken from the LED-I by optical microscope.
shows the typical cross-sectional view scanning electron microscopy (SEM) image of LED-I, which was taken to inspect the etching profile of sapphire and the bonding interface between the Si substrate and GaN layer. The typical SEM image indicates that the bonding interface was void-free. Vertical thin-GaN LEDs with a smooth bonding interface facilitates heat dissipation, mechanical support, and current conduction. The presence of voids around the bonding interface reduces the reliability and performance of the LEDs [9

9. H. Y. Kuo, S. J. Wang, P. R. Wang, K. M. Uang, T. M. Chen, S. L. Chen, W. C. Lee, H. K. Hsu, J. C. Chou, and C. H. Wu, “Use of elastic conductive adhesive as the bonding agent for the fabrication of vertical structure GaN-based LEDs on flexible metal substrate,” IEEE Photon. Technol. Lett. 20(7), 523–525 (2008). [CrossRef]

]. On the other hand, periodic convex patterns were observed on the bottom surface of the trench, as shown in Fig. 2(b). To clearly examine the feature, an enlarged SEM image taken from a trench of the etched sapphire substrate on the emitting surface of LED-I is shown in Fig. 2(c). The formation of convex patterns is due to that fact that the conformal etching of PSS took place during the formation of via trenches through the remaining sapphire layer. Figure 2(d) shows a top-view image taken from LED-I by optical microscope.

Figure 3
Fig. 3 Typical L-I and I-V characteristics of the experimental LEDs.
shows the typical L-I characteristics of experimental LEDs. With an injection current of 350 mA, the light output powers were 360, 310, 250 and 309 mW for LED-I, LED-II, CV-LED-I and CV-LED-II, respectively. This corresponded to have external quantum efficiencies of 37.2%, 32.1%, 25.8% and 32.0% for LED-I, LED-II, CV-LED-I and CV-LED-II, respectively. The significant improvement in the output power of LED-I was attributable to the increase in its LEE, compared with those of LED-II and conventionnal vertical LEDs. Considering the LED-II with p-side-up emitting surface, the indium tin oxide (ITO) layer possesses a relatively lower critical angle for light escaping into the air, because the refractive index of ITO is larger than that of sapphire. In addition, the remaining top sapphire layer, with a thickness of 7 μm, was significantly thicker than that of the ITO layer. Therefore, the top sapphire layer served as a window layer to facilitate light extraction. Although the LED-I and LED-II both possess textured GaN/sapphire substrate to reflect and/or redirect light for facilitating light extraction, the nature of thin-GaN LEDs for LED-I would be further beneficial to light extraction [2

2. W. S. Wong, M. Kneissl, P. Mei, D. W. Treat, M. Teepe, and N. M. Johnson, “Continuous-wave InGaN multiple-quantum-well laser diodes on copper substrates,” Appl. Phys. Lett. 78(9), 1198–1200 (2001). [CrossRef]

4

4. D. S. Wuu, S. C. Hsu, S. H. Huang, C. C. Wu, C. E. Lee, and R. H. Horng, “GaN/mirror/Si light-emitting diodes for vertical current injection by laser lift-off and wafer bonding techniques,” Jpn. J. Appl. Phys. 43(8A), 5239–5242 (2004). [CrossRef]

]. That is, downward-propagating light is reflected up by a metal reflector. On the other hand, the light-shadowing effect by opaque electrodes in LED-I is relatively insignificant to that in LED-II, as shown in the insets of Figs. 1(a) and 1(b). On the other hand, CV-LED-II with textured n-GaN emitting surface exhibited a marked enhancement in light output power compared with CV-LED-I. Although the light output power of CV-LED-II could be improved by further texturing the surface of LED, the texture process may cause the increase of reverse leakage current, as shown in Fig. 4(a)
Fig. 4 (a) Reverse I-V characteristics of the experimental LEDs (b) emission peak wavelength against on injection current for the experimental LEDs.
. Considering the CV-LED-II and LED-I, the difference of light output powers could be attributed the fact that they had different emitting surface layers. In addition, the substantial roughness of light scattering interface was also different between the LEDs.

Figure 3 also shows the typical forward I-V characteristics of experimental LEDs. The forward voltages at a driving current of 350 mA (Vf) were 3.28, 3.48, 3.28 and 3.27 V for LED-I, LED-II, CV-LED-I and CV-LED-II, respectively. The slightly higher Vf observed in LED-II can be attributed to its relatively higher series resistance (Rs), compared with LED-I and CV-LEDs. In fact, the Rs extracted from the I-V characteristics were approximately 1.1 and 1.6 Ω for LED-I(or CV-LED) and LED-II, respectively. As shown in Fig. 1(b), the current path between the anode and cathode electrodes in the vertical LEDs(LED-I and CV-LEDs) could be expected to be shorter than those in LED-II. Although the vertical LEDs consist of a greater number of metal/semiconductor interfaces and an additional Si substrate, these factors in the vertical LEDs with adequate bonding and ohmic contact layers contribute limited magnitude in Rs. Therefore, the higher Rs of LED-II were attributed to the relatively larger distance between anode and cathode electrodes. In other words, the low Rs in the vertical LEDs can be attributed to a relatively better current-spreading effect, compared with that of the lateral LEDs. In addition to the superior forward I-V characteristics, typical reverse leakage current (Ir) of the LED-I was comparable with the LED-II, but markedly lower than the CV-LEDs(CV-LED-I and CV-LED-II), as shown in Fig. 4(a).

The relative higher Ir could be attributed to the fact that strain release and laser-induced damage on the GaN-based epitaxial layers led to the degradation of electrical properties of the conventional vertical LEDs. Another benefit of the design scheme of LED-I is that the heat generated in the LED flows directly from the p-n junction to the Si substrate. However, heat extraction through the thermally resistive sapphire substrate is inevitable for conventional GaN/sapphire-based LEDs, i.e., LED-II. The degradation of light output power of LED-II, attributable to junction heating, is expected when the LED-II is operated under a high driving current. This is because of the low thermal conductivity of sapphire. As shown in Fig. 3, LED-I and CV-LEDs can operate up to 1.0 A without significant power degradation. However, LED-II exhibits substantial power degradation when the injection current exceeds 850 mA. Furthermore, the heating effect on the redshift of the emission wavelength was also observed. Figure 4(b) shows the typical curves of emission peak wavelengths against the injection current for the experimental LEDs. At a low injection current, the quantum-confined Stark effect (QCSE), attributable to the polarization field in the QWs, reflects on the blueshift of the emission wavelengths [10

10. T. Mukai, M. Yamada, and S. Nakamura, “Current and temperature dependences of electroluminescence of InGaN-based UV/blue/green light-emitting diodes,” Jpn. J. Appl. Phys. 37(Part 2, No. 11B), L1358–L1361 (1998). [CrossRef]

]. The redshift of the emission wavelength, resulting from band-gap shrinkage at a high injection current, is attributed to the junction heating effect. LED-I and CV-LEDs exhibited low Rs and excellent heat conduction, resulting in low junction heating. As shown in Fig. 4(b), the redshift of the emission wavelength was not observed in LED-I and CV-LEDs, even when the injection current reached 1000 mA. In contrast, LED-II exhibited a significant redshift as the injection current exceeded 200 mA. The redshift occurring in the relatively lower current levels revealed that junction temperature increased rapidly with increasing injection current, and that the heat extraction rate of LED-II through sapphire was lower than the heat generation rate. Thus, the band-gap shrinkage rate, resulting from junction heating versus injection current, was higher than that of the band-gap widening rate (i.e., band-filling effect).

3. Conclusions

GaN/Si-based vertical LEDs emitted from a sapphire surface were demonstrated to have higher light output power and lower forward voltage. Owing to the retained sapphire layer of the vertical LEDs, the use of a patterned sapphire substrate was shown to be able to mitigate the light-guided effect, and thereby increase the light-extraction efficiency. In addition, the proposed vertical LEDs behaved in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction, and exhibited low power degradation and minor red shifts in emission wavelength at high current driving. The proposed vertical GaN LEDs can potentially be applied to high-power LEDs used for future solid-state lighting.

Acknowledgments

This work was supported from the Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC and National Science Council for the financial support under contract Nos. 101-D0204-6, 101-2221-E-218-012-MY3, 101-2221-E-006-171-MY3, 100-2112-M-006-011-MY3 and 100-3113-E-006-015-. The authors would also like to acknowledge the LED Lighting Research Center and the Research Center for Energy technology and Strategy of National Cheng Kung University.

References and links

1.

E. F. Schubert, Light-emitting diode, 2nd ed. (Cambridge University Press, 2006) pp. 150–160.

2.

W. S. Wong, M. Kneissl, P. Mei, D. W. Treat, M. Teepe, and N. M. Johnson, “Continuous-wave InGaN multiple-quantum-well laser diodes on copper substrates,” Appl. Phys. Lett. 78(9), 1198–1200 (2001). [CrossRef]

3.

S. C. Hsu, B. J. Pong, W. H. Li, E. T. Beechem, S. Graham, and C. Y. Liu, “Stress relaxation in GaN by transfer bonding on Si substrates,” Appl. Phys. Lett. 91(25), 251114 (2007).

4.

D. S. Wuu, S. C. Hsu, S. H. Huang, C. C. Wu, C. E. Lee, and R. H. Horng, “GaN/mirror/Si light-emitting diodes for vertical current injection by laser lift-off and wafer bonding techniques,” Jpn. J. Appl. Phys. 43(8A), 5239–5242 (2004). [CrossRef]

5.

Y. C. Sermon Wu, J. H. Cheng, W. C. Peng, and H. Ouyang, “Effects of laser sources on the reverse-bias leakages of laser lift-off GaN-based light-emitting diodes,” Appl. Phys. Lett. 90(25), 251110 (2007).

6.

S.-H. Huang, R.-H. Horng, S.-C. Hsu, T.-Y. Chen, and D.-S. Wuu, “Surface texturing for wafer-bonded vertical-type GaN/mirror/Si light-emitting diodes,” Jpn. J. Appl. Phys. 44(5A), 3028–3031 (2005). [CrossRef]

7.

C. H. Jang, J. K. Sheu, S. J. Chang, M. L. Lee, C. C. Yang, S. J. Tu, F. W. Huang, and C. K. Hsu, “Effect of growth pressure of undoped GaN layer on the ESD characteristics of GaN-based LEDs grown on patterned sapphire,” IEEE Photon. Technol. Lett. 23(14), 968–970 (2011). [CrossRef]

8.

C. M. Tsai, J. K. Sheu, W. C. Lai, M. L. Lee, S. J. Chang, C. S. Chang, T. K. Ko, and C. F. Shen, “GaN-based LEDs output power improved by textured GaN/sapphire interface using in-situ SiH4 treatment process during epitaxial growth,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1275–1280 (2009). [CrossRef]

9.

H. Y. Kuo, S. J. Wang, P. R. Wang, K. M. Uang, T. M. Chen, S. L. Chen, W. C. Lee, H. K. Hsu, J. C. Chou, and C. H. Wu, “Use of elastic conductive adhesive as the bonding agent for the fabrication of vertical structure GaN-based LEDs on flexible metal substrate,” IEEE Photon. Technol. Lett. 20(7), 523–525 (2008). [CrossRef]

10.

T. Mukai, M. Yamada, and S. Nakamura, “Current and temperature dependences of electroluminescence of InGaN-based UV/blue/green light-emitting diodes,” Jpn. J. Appl. Phys. 37(Part 2, No. 11B), L1358–L1361 (1998). [CrossRef]

OCIS Codes
(230.0250) Optical devices : Optoelectronics
(230.3670) Optical devices : Light-emitting diodes
(230.5590) Optical devices : Quantum-well, -wire and -dot devices

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: August 9, 2012
Revised Manuscript: September 21, 2012
Manuscript Accepted: October 17, 2012
Published: November 2, 2012

Citation
Y.C. Yang, Jinn-Kong Sheu, Ming-Lun Lee, C. H. Yen, Wei-Chih Lai, Schang Jing Hon, and Tsun Kai Ko, "Vertical InGaN light-emitting diode with a retained patterned sapphire layer," Opt. Express 20, A1019-A1025 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S6-A1019


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. E. F. Schubert, Light-emitting diode, 2nd ed. (Cambridge University Press, 2006) pp. 150–160.
  2. W. S. Wong, M. Kneissl, P. Mei, D. W. Treat, M. Teepe, and N. M. Johnson, “Continuous-wave InGaN multiple-quantum-well laser diodes on copper substrates,” Appl. Phys. Lett.78(9), 1198–1200 (2001). [CrossRef]
  3. S. C. Hsu, B. J. Pong, W. H. Li, E. T. Beechem, S. Graham, and C. Y. Liu, “Stress relaxation in GaN by transfer bonding on Si substrates,” Appl. Phys. Lett.91(25), 251114 (2007).
  4. D. S. Wuu, S. C. Hsu, S. H. Huang, C. C. Wu, C. E. Lee, and R. H. Horng, “GaN/mirror/Si light-emitting diodes for vertical current injection by laser lift-off and wafer bonding techniques,” Jpn. J. Appl. Phys.43(8A), 5239–5242 (2004). [CrossRef]
  5. Y. C. Sermon Wu, J. H. Cheng, W. C. Peng, and H. Ouyang, “Effects of laser sources on the reverse-bias leakages of laser lift-off GaN-based light-emitting diodes,” Appl. Phys. Lett.90(25), 251110 (2007).
  6. S.-H. Huang, R.-H. Horng, S.-C. Hsu, T.-Y. Chen, and D.-S. Wuu, “Surface texturing for wafer-bonded vertical-type GaN/mirror/Si light-emitting diodes,” Jpn. J. Appl. Phys.44(5A), 3028–3031 (2005). [CrossRef]
  7. C. H. Jang, J. K. Sheu, S. J. Chang, M. L. Lee, C. C. Yang, S. J. Tu, F. W. Huang, and C. K. Hsu, “Effect of growth pressure of undoped GaN layer on the ESD characteristics of GaN-based LEDs grown on patterned sapphire,” IEEE Photon. Technol. Lett.23(14), 968–970 (2011). [CrossRef]
  8. C. M. Tsai, J. K. Sheu, W. C. Lai, M. L. Lee, S. J. Chang, C. S. Chang, T. K. Ko, and C. F. Shen, “GaN-based LEDs output power improved by textured GaN/sapphire interface using in-situ SiH4 treatment process during epitaxial growth,” IEEE J. Sel. Top. Quantum Electron.15(4), 1275–1280 (2009). [CrossRef]
  9. H. Y. Kuo, S. J. Wang, P. R. Wang, K. M. Uang, T. M. Chen, S. L. Chen, W. C. Lee, H. K. Hsu, J. C. Chou, and C. H. Wu, “Use of elastic conductive adhesive as the bonding agent for the fabrication of vertical structure GaN-based LEDs on flexible metal substrate,” IEEE Photon. Technol. Lett.20(7), 523–525 (2008). [CrossRef]
  10. T. Mukai, M. Yamada, and S. Nakamura, “Current and temperature dependences of electroluminescence of InGaN-based UV/blue/green light-emitting diodes,” Jpn. J. Appl. Phys.37(Part 2, No. 11B), L1358–L1361 (1998). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2 Fig. 3
 
Fig. 4
 

« Previous Article

OSA is a member of CrossRef.

CrossCheck Deposited