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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 5 — Mar. 8, 2004
  • pp: 736–741
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Origin of power fluctuations in GaN resonant-cavity light-emitting diodes

Brendan Roycroft, Mahbub Akhter, Pleun Maaskant, Brian Corbett, Alan Shaw, Louise Bradley, Phillipe de Mierry, and Marie-Antoinette Poisson  »View Author Affiliations


Optics Express, Vol. 12, Issue 5, pp. 736-741 (2004)
http://dx.doi.org/10.1364/OPEX.12.000736


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Abstract

Resonant-cavity light-emitting diodes (RCLEDs) with multiple InGaN/GaN quantum wells have been grown on sapphire substrates. The emission was through the substrate, and the top contact consisted of a highly reflecting Pd/Ag metallization. The peak emission wavelength was measured to be 490 nm. Under constant current biasing, the intensity was observed to fluctuate irregularly accompanied by correlated variations in the voltage. To investigate this further, emission from the RCLED was focused through a GaAs wafer onto a Vidicon camera. This gave a series of infrared, near-field images, spectrally integrated over a wavelength range from 870 nm to 1.9 µm. Flashes from point sources on the RCLED surface were observed, indicating that short-lived, highly localized “hot spots” were being formed that generated pulses of thermal radiation. It is proposed that this phenomenon results from the migration of metal into nanopipes present in this material. The filled pipes form short circuits that subsequently fuse and are detected by bursts of infrared radiation that are recorded in real time.

© 2004 Optical Society of America

1. Introduction

GaN LEDs are finding uses in displays, general lighting, data communications, high-power transistors, data storage, and many other applications [1

1. “Wide-bandgap materials,” Cover Story, Compound Semiconductor8(6), 41–55 (2002).

,2

2. M. Shinoda, K. Saito, T. Kondo, T. Ishimoto, and A. Nakaoki, “High density near field readout over 50 GB capacity using a solid immersion lens with high refractive index,” in Proceedings of the International Symposium on Optical Memory and Optical Data Storage (IEEE Laser and Electro-Optics Society, New Jersey, 2002), pp. 284–286. [CrossRef]

,3

3. R. Behtash, H. Tobler, M. Neuburger, A. Schurr, H. Leier, Y. Cordier, F. Semond, F. Natali, and J. Massies, “AlGaN/GaN HEMTs on Si(111) with 6.6 W/mm output power density,” Electron. Lett. 39, 626–627 (2003). [CrossRef]

]. However, growth of the GaN material is still far from perfect, because the material generally contains a large number of threading dislocations [4

4. R.F. Davis, A.M. Roskowski, E.A. Preble, J.S. Speck, B. Heying, J.A. Freitas Jr., E.R. Glaser, and W.E. Carlos, “Gallium nitride materials—progress, status, and potential roadblocks,” Proc. IEEE 90, 993–1005 (2002). [CrossRef]

,5

5. X.A. Cao, E.B. Stokes, P. M. Sandvik, S. F. LeBoeuf, J. Kretchmer, and D. Walker, “Diffusion and tunneling currents in GaN/InGaN multiple quantum well light-emitting diodes,” Electron. Device Lett. 23, 535–537 (2002). [CrossRef]

]. Although the devices can work fairly well in the presence of such defects [6

6. T. Mukai, “Recent progress in group-III nitride light-emitting diodes,” Sel. Top. Quantum Electron. 8, 264–270 (2002). [CrossRef]

], the dislocations tend to compromise reliability [7

7. G. Koley, H. Kim, L. F. Eastman, and M.G. Spencer, “Electrical bias stress related degradation of AlGaN/GaN HEMTs,” Electron. Lett. 39, 1217–1218 (2003). [CrossRef]

]. In lasers and LEDs, metal migration through dislocations has been attributed as a major failure mechanism [8

8. H. Kim, H. Yang, C. Huh, S.-W. Kim, S.-J. Park, and H. Hwang, “Electromigration-induced failure of GaN multi-quantum well light emitting diode,” Electron. Lett. 36, 908–910 (2000). [CrossRef]

,9

9. D.L. Barton, M. Osinski, P. Perlin, P. G. Eliseev, and J. Lee, “Degradation of single-quantum well InGaN green light emitting diodes under high electrical stress,” in Proceedings of the 36th Reliability Physics Symposium (IEEE, New Jersey, 1998), pp. 119–123.

]. Various techniques are currently being applied in an effort to reduce the number of defects. These include epitaxial overgrowth [10

10. R. F. Davis, O. H. Nam, M. D. Bremser, and T. Zheleva, “Lateral epitaxial overgrowth of and defect reduction in GaN thin films,” in Proceedings of the Lasers and Electro-Optics Society Annual Meeting (IEEE Laser and Electro-Optics Society, New Jersey, 1998), Vol. 1, pp. 360–361.

], bulk crystal growth [11

11. M. Leszczynski, “Optoelectronic devices based on (AlGaIn)N structures on GaN crystals,” in Proceedings of the 3rd International Conference on Novel Applications of Wide Bandgap Layers (IEEE, New Jersey, 2001), pp. 65–66.

], and the inclusion of layers grown at low temperature [12

12. P. R. Tavernier, E. V. Etzkorn, Y. Wang, and D. R. Clarke, “Two-step growth of high quality GaN by hydride vapor-phase epitaxy,” Appl. Phys. Lett. 77, 1804–1806 (2000). [CrossRef]

]. In this paper, we observe failure in resonant-cavity light-emitting diodes (RCLEDs) that we attribute to metal migration along dislocations. This process forms short circuits that subsequently overheat in response to the high current density. This may not result in immediate device failure, but it produces output power fluctuations. After many fluctuations, the devices tend to fail because of a residual resistance left after the short circuits have fused. Here, in contrast to the studies mentioned previously, we observe the failure in real time by monitoring the diode at infrared wavelengths.

2. Sample structure

Fig. 1. Schematic of sample structure.

Two structures were investigated, both based on the generic design shown in Fig. 1 and grown by MOVPE. Sample A had a 4-µm GaN buffer layer on the sapphire substrate, 17 Al0.1Ga0.9N/GaN λ/4 mirror pairs, and a 3λ cavity layer containing three InGaN/GaN quantum wells consisting of 3-nm-thick wells and 10-nm-thick barriers. The peak wavelength of emission of the quantum wells was at 490 nm. The top p-doped GaN layer was coated with Pd/Ag/Ni/Au (3/100/30/300 nm thick) to create a high-reflectivity p contact. The p contacts were not alloyed after deposition, because non-alloyed contacts preserve their high reflectivity and ensure that the metals do not diffuse into the GaN. The p contact was defined by wet etches of KI:I2 (4:1) diluted with 80 parts deionized water, followed by HCl:HNO3 (3:1) diluted with four parts deionized water. A mesa was defined by reactive ion etching (RIE) in a PlasmaTherm 790 series etcher. SiCl4 gas was used as the etchant, at a pressure of 20 mTorr and 225 V dc for 50 min. This etched the top 450 nm of GaN to expose the intracavity n layer, which was then covered in Ti/Al/Pt/Au (200/50/30/300 nm thick) to form the n contact. The device diameter was 150 µm.

Sample B had a similar structure but was grown in a different reactor. It had 10 mirror pairs and 10 quantum wells with 1-nm-thick wells and 8.5-nm-thick barriers. Processing and metallization were the same as for sample A.

The sapphire substrate was thinned, and the devices separated by dicing. They were then packaged at Infineon in a thin shrink small outline package (TSSOP) for handling.

3. Results

When some of the RCLEDs were biased, it was noticed that the output was unstable; that is, the light output flickered in intensity, or would suddenly go to zero. Occasionally, the output power would suddenly increase again to a former level. These effects were captured during a light-current-voltage measurement, as illustrated in a typical result shown in Fig. 2.

Fig. 2. LI curve for circular contact RCLED showing unstable light output.

These curves are different every time they are measured; that is, the light or voltage may drop or increase at any arbitrary moment. However, a clear correlation is seen between light output and voltage. When the light drops, the voltage also drops; when the light increases, the voltage also increases. This suggests a short-circuiting effect where short circuits are first created and then destroyed in a fuse-like process.

Fig. 3. Near-field image of sample B (a), before and (b), after degradation.

Fig. 4. Setup for imaging the thermal signatures of defect-related short circuits on circular contact RCLEDs. Because of the GaAs wafer, the green emission is blocked and the only wavelengths reaching the camera were infrared, between 870 nm and 1.9 µm.
Fig. 5. Device A has transient hot spots that flash for less than 1 s. Dashed circles have been drawn in to represent the outline of the device [432 kB]. The movie shows that flashes start after ~24 s.

The camera was programmed to take images every 0.25 s. Three of these images are shown in Fig. 5, and a typical movie sequence of such images has been published with this article. Usually the image is entirely dark, because the normal visible emission has been blocked. However, at irregular intervals flashes are indeed observed, as indicated by arrows in the figure. These pulses have a duration of just less than 1 s, determined primarily by the size of the heat source and the thermal conductivity of the host material. This indicates that localized short circuiting and fusing are taking place in the material, which results from the contact metal migrating down tubular threading dislocations that are common in this material [see Fig. 6(a)]. After the fusing, the failed point may go open circuit, and so the current passes through the undamaged part of the device, and the device goes back to its high-emission state. Alternatively, there may be a residual resistive short circuit down the sidewalls of the resulting hole in the material. This acts as a parallel resistance, and the light output is not recovered. As the device accumulates more of these, the output drops to unusable levels. Material damage is revealed when the contact metals are removed, as is evidenced by the pitting near the p-metal edge in Fig. 6(b). Damage is highest near the edge of the p contact, as this has highest current density because of current-crowding effects. The large-scale damage evident in Fig. 6(b) also suggests that not all fusing events are captured in Fig. 5, but only the few hottest events. To capture more events, a more sensitive detector is needed.

Fig. 6. (a) Typical 1×1 µm AFM image of GaN surface before degradation, showing open core dislocation density on the order of 3×109 cm-2 and curved growth ledges. (b) SEM image after degradation and metal removal, showing large-scale surface pitting near the edge of the p metallization.

4. Conclusions

A novel infrared measurement technique has been used to study failure in GaN LEDs. Bottom-emitting InGaN/GaN multiple-quantum-well RCLEDs have been manufactured on sapphire substrates. Instabilities were observed, characterized by irregular but correlated variations in the light output and voltage. During measurement in the infrared, bright flashes were also detected at irregular intervals. We have proposed that this is due to metal migration down threading dislocations that form fuses on the nanoscale. The initial short circuit induces a high current density such that the subsequent temperature increases are sufficient to melt the metal. This may result in an open-circuit or a residual resistance. In the case of the former, the device recovers its normal operation, whereas in the latter case light output and voltage are irrecoverably reduced.

Acknowledgments

This research was funded under the EU-IST project AGETHA.

References

1.

“Wide-bandgap materials,” Cover Story, Compound Semiconductor8(6), 41–55 (2002).

2.

M. Shinoda, K. Saito, T. Kondo, T. Ishimoto, and A. Nakaoki, “High density near field readout over 50 GB capacity using a solid immersion lens with high refractive index,” in Proceedings of the International Symposium on Optical Memory and Optical Data Storage (IEEE Laser and Electro-Optics Society, New Jersey, 2002), pp. 284–286. [CrossRef]

3.

R. Behtash, H. Tobler, M. Neuburger, A. Schurr, H. Leier, Y. Cordier, F. Semond, F. Natali, and J. Massies, “AlGaN/GaN HEMTs on Si(111) with 6.6 W/mm output power density,” Electron. Lett. 39, 626–627 (2003). [CrossRef]

4.

R.F. Davis, A.M. Roskowski, E.A. Preble, J.S. Speck, B. Heying, J.A. Freitas Jr., E.R. Glaser, and W.E. Carlos, “Gallium nitride materials—progress, status, and potential roadblocks,” Proc. IEEE 90, 993–1005 (2002). [CrossRef]

5.

X.A. Cao, E.B. Stokes, P. M. Sandvik, S. F. LeBoeuf, J. Kretchmer, and D. Walker, “Diffusion and tunneling currents in GaN/InGaN multiple quantum well light-emitting diodes,” Electron. Device Lett. 23, 535–537 (2002). [CrossRef]

6.

T. Mukai, “Recent progress in group-III nitride light-emitting diodes,” Sel. Top. Quantum Electron. 8, 264–270 (2002). [CrossRef]

7.

G. Koley, H. Kim, L. F. Eastman, and M.G. Spencer, “Electrical bias stress related degradation of AlGaN/GaN HEMTs,” Electron. Lett. 39, 1217–1218 (2003). [CrossRef]

8.

H. Kim, H. Yang, C. Huh, S.-W. Kim, S.-J. Park, and H. Hwang, “Electromigration-induced failure of GaN multi-quantum well light emitting diode,” Electron. Lett. 36, 908–910 (2000). [CrossRef]

9.

D.L. Barton, M. Osinski, P. Perlin, P. G. Eliseev, and J. Lee, “Degradation of single-quantum well InGaN green light emitting diodes under high electrical stress,” in Proceedings of the 36th Reliability Physics Symposium (IEEE, New Jersey, 1998), pp. 119–123.

10.

R. F. Davis, O. H. Nam, M. D. Bremser, and T. Zheleva, “Lateral epitaxial overgrowth of and defect reduction in GaN thin films,” in Proceedings of the Lasers and Electro-Optics Society Annual Meeting (IEEE Laser and Electro-Optics Society, New Jersey, 1998), Vol. 1, pp. 360–361.

11.

M. Leszczynski, “Optoelectronic devices based on (AlGaIn)N structures on GaN crystals,” in Proceedings of the 3rd International Conference on Novel Applications of Wide Bandgap Layers (IEEE, New Jersey, 2001), pp. 65–66.

12.

P. R. Tavernier, E. V. Etzkorn, Y. Wang, and D. R. Clarke, “Two-step growth of high quality GaN by hydride vapor-phase epitaxy,” Appl. Phys. Lett. 77, 1804–1806 (2000). [CrossRef]

OCIS Codes
(160.3380) Materials : Laser materials
(160.6000) Materials : Semiconductor materials
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Research Papers

History
Original Manuscript: November 19, 2003
Revised Manuscript: February 4, 2004
Published: March 8, 2004

Citation
Brendan Roycroft, Mahbub Akhter, Pleun Maaskant, Brian Corbett, Alan Shaw, Louise Bradley, Phillipe de Mierry, and Marie-Antoinette Poisson, "Origin of power fluctuations in GaN resonant-cavity light-emitting diodes," Opt. Express 12, 736-741 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-5-736


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References

  1. �??Wide-bandgap materials,�?? Cover Story, Compound Semiconductor 8(6), 41-55 (2002).
  2. M. Shinoda, K. Saito, T. Kondo, T. Ishimoto, and A. Nakaoki, �??High density near field readout over 50 GB capacity using a solid immersion lens with high refractive index,�?? in Proceedings of the International Symposium on Optical Memory and Optical Data Storage (IEEE Laser and Electro-Optics Society, New Jersey, 2002), pp. 284-286. [CrossRef]
  3. R. Behtash, H. Tobler, M. Neuburger, A. Schurr, H. Leier, Y. Cordier, F. Semond, F. Natali, and J. Massies, �??AlGaN/GaN HEMTs on Si(111) with 6.6 W/mm output power density,�?? Electron. Lett. 39, 626- 627 (2003). [CrossRef]
  4. R.F. Davis, A.M. Roskowski, E.A. Preble, J.S. Speck, B. Heying, J.A. Freitas Jr., E.R. Glaser, and W.E. Carlos, �??Gallium nitride materials�??progress, status, and potential roadblocks,�?? Proc. IEEE 90, 993-1005 (2002). [CrossRef]
  5. X.A. Cao, E.B. Stokes, P. M. Sandvik, S. F. LeBoeuf, J. Kretchmer, and D. Walker, �??Diffusion and tunneling currents in GaN/InGaN multiple quantum well light-emitting diodes,�?? Electron. Device Lett. 23, 535-537 (2002). [CrossRef]
  6. T. Mukai, �??Recent progress in group-III nitride light-emitting diodes,�?? Sel. Top. Quantum Electron. 8, 264- 270 (2002). [CrossRef]
  7. G. Koley, H. Kim, L. F. Eastman, and M.G. Spencer, �??Electrical bias stress related degradation of AlGaN/GaN HEMTs,�?? Electron. Lett. 39, 1217-1218 (2003). [CrossRef]
  8. H. Kim, H. Yang, C. Huh, S.-W. Kim, S.-J. Park, and H. Hwang, �??Electromigration-induced failure of GaN multi-quantum well light emitting diode,�?? Electron. Lett. 36, 908-910 (2000). [CrossRef]
  9. D.L. Barton, M. Osinski, P. Perlin, P. G. Eliseev, and J. Lee, �??Degradation of single-quantum well InGaN green light emitting diodes under high electrical stress,�?? in Proceedings of the 36th Reliability Physics Symposium (IEEE, New Jersey, 1998), pp. 119-123.
  10. R. F. Davis, O. H. Nam, M. D. Bremser, and T. Zheleva, �??Lateral epitaxial overgrowth of and defect reduction in GaN thin films,�?? in Proceedings of the Lasers and Electro-Optics Society Annual Meeting (IEEE Laser and Electro-Optics Society, New Jersey, 1998), Vol. 1, pp. 360-361.
  11. M. Leszczynski, �??Optoelectronic devices based on (AlGaIn)N structures on GaN crystals,�?? in Proceedings of the 3rd International Conference on Novel Applications of Wide Bandgap Layers (IEEE, New Jersey, 2001), pp. 65-66.
  12. P. R. Tavernier, E. V. Etzkorn, Y. Wang, and D. R. Clarke, �??Two-step growth of high quality GaN by hydride vapor-phase epitaxy,�?? Appl. Phys. Lett. 77, 1804-1806 (2000). [CrossRef]

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