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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 15 — Jul. 18, 2011
  • pp: 14662–14670
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Implementation of an indium-tin-oxide (ITO) direct-Ohmic contact structure on a GaN-based light emitting diode

Yi-Jung Liu, Chien-Chang Huang, Tai-You Chen, Chi-Shiang Hsu, Jian-Kai Liou, Tsung-Yuan Tsai, and Wen-Chau Liu  »View Author Affiliations


Optics Express, Vol. 19, Issue 15, pp. 14662-14670 (2011)
http://dx.doi.org/10.1364/OE.19.014662


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Abstract

A GaN-based light-emitting diode (LED) with a direct-Ohmic contact structure, formed by an indium-tin-oxide (ITO) transparent film and Au thermal-diffused and removed layer, is studied. By depositing an Au metallic film on the Mg-doped GaN layer followed by thermal annealing and removed processes, an ITO direct-Ohmic contact at p-GaN/ITO interface is formed. An enhanced light output power of 18.0% is also found at this condition. This is mainly attributed to the larger and more uniform light-emission area resulted from the improved current spreading capability by the use of an ITO direct-Ohmic contact structure.

© 2011 OSA

1. Introduction

InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with lateral geometries grown by metal-organic chemical vapor deposition (MOCVD) are very attractive and give variable applications covering visible emission from yellow-green to blue spectra. Recently, due to the significant improvement on epitaxial technique of nitride-based material system, the perfect crystalline quality of InGaN/GaN-based LEDs has become possible. However, the current-crowding phenomenon [1

1. X. Guo and E. F. Schubert, “Current crowding in GaN/InGaN light emitting diodes on insulating substrates,” J. Appl. Phys. 90(8), 4191 (2001). [CrossRef]

] still limits their light output efficiency. This drawback is known to be mainly originated from the high-resistive p-GaN contact layer [2

2. L. Zhou, W. Lanford, A. T. Ping, I. Adesida, J. W. Yang, and A. Khan, “Low resistance Ti/Pt/Au ohmic contacts to p-type GaN,” Appl. Phys. Lett. 76(23), 3451 (2000). [CrossRef]

], due to the high activation energy of Mg dopants and low acceptors (holes) mobility. A higher operation voltage of a GaN-based LED is inevitable because of the Schottky-like characteristic of the p-GaN contact layer, and the induced joule-heating effect underneath n- and p- metal pads deteriorates optical, electrical properties, and the reliability of LEDs [1

1. X. Guo and E. F. Schubert, “Current crowding in GaN/InGaN light emitting diodes on insulating substrates,” J. Appl. Phys. 90(8), 4191 (2001). [CrossRef]

].

Up to date, the indium tin oxide (ITO) layer has been reported to enhance the current spreading performance of GaN-based LEDs [3

3. Y. J. Liu, C. H. Yen, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “On a GaN-based light-emitting diode with a p-GaN/i-InGaN superlattice structure,” IEEE Electron Device Lett. 30(11), 1149–1151 (2009). [CrossRef]

]. However, the poor Ohmic nature between the ITO current-spreading layer and high-resistive p-GaN contact layer [3

3. Y. J. Liu, C. H. Yen, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “On a GaN-based light-emitting diode with a p-GaN/i-InGaN superlattice structure,” IEEE Electron Device Lett. 30(11), 1149–1151 (2009). [CrossRef]

, 4

4. K. M. Chang, J. Y. Chu, and C. C. Cheng, “Highly reliable GaN-based light-emitting diodes formed by p–In0.1Ga0.9N–ITO structure,” IEEE Photon. Technol. Lett. 16, 1807 (2004). [CrossRef]

] seems still remain a miserable problem. Recently, many approaches have been raised trying to enhance this Ohmic contact property. For example, one could apply an InGaN/GaN superlattice (SL) structure above (increases hole concentration for contact) [5

5. C. S. Chang, S. J. Chang, Y. K. Su, C. H. Kuo, W. C. Lai, Y. C. Lin, Y. P. Hsu, S. C. Shei, J. M. Tsai, H. M. Lo, J. C. Ke, and J. K. Sheu, “High brightness InGaN green LEDs with an ITO on n ++ -SPS upper contact,” IEEE Trans. Electron. Dev. 50(11), 2208–2212 (2003). [CrossRef]

]/underlying (modulate current and release strains) [3

3. Y. J. Liu, C. H. Yen, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “On a GaN-based light-emitting diode with a p-GaN/i-InGaN superlattice structure,” IEEE Electron Device Lett. 30(11), 1149–1151 (2009). [CrossRef]

] the p-GaN contact layer, or an InGaN capping layer on the p-GaN layer [6

6. T. Gessmann, Y. L. Li, E. L. Waldron, J. W. Graff, and J. K. Sheu, “Ohmic contacts to p-type GaN mediated by polarization fields in thin InxGa1-xN capping layers,” Appl. Phys. Lett. 80(6), 986 (2002). [CrossRef]

] to lower down the contact resistivity. However, these methods cause a relatively complicated growth process and high cost of LEDs. Without complicating epitaxial growth conditions, a lower contact resistivity was achieved by an In2O3/ITO structure [7

7. Y. J. Liu, C. H. Yen, C. H. Hsu, K. H. Yu, L. Y. Chen, T. H. Tsai, and W. C. Liu, “Impact of An indium oxide/indium–tin oxide mixed structure for GaN-based light-emitting diodes,” Opt. Rev. 16(6), 575–577 (2009). [CrossRef]

] studied by our group. Nevertheless, the p-GaN Ohmic contact property needs to be further improved.

2. Experimental

transmission line model (s-TLM). The light output power was measured by a Si photopic detector integrated with a current source. Dominant wavelengths are around 457 nm for both devices at 20 mA, determined by electroluminescence spectra.

3. Results and discussion

Figure 4
Fig. 4 p-GaN/ITO contact resistances for studied devices A and B. In inset, the corresponding schematic diagrams during contact measurements are also shown.
depicts p-GaN/ITO/p-pad Ohmic contact properties (derived from I-V curves in Fig. 5
Fig. 5 p-GaN/ITO I-V characteristics for studied devices A and B.
) of devices A and B from −5V to 5V. These characteristics are measured directly from two p-contacts on the same mesa regions respectively, as referred in the schematic diagram of the device B in the inset of Fig. 4(where the location of AuinGaout layer is also drawn). Due to the lack of an AuinGaout layer which significantly improves Ohmic contact between p-GaN and ITO layers, the device A still exhibits poor voltage-dependent contact resistances ranging from 44.7 to 67.1 KΩ. On the other hand, Ohmic-like contact resistances from 11.8 to 12.2 KΩ are found for the device B. The corresponding specific contact resistances are evaluated to be 6.60×10−3 and 2.30×10−3 for devices A and B respectively, based on s-TLM. In other words, with a help of an AuinGaout layer on the p-GaN contact layer, the higher holes concentration and improved ITO direct-Ohmic contact property have become possible.

Figure 6
Fig. 6 Forward-biased I-V curves and detailed I-V relations (in inset) of the studied devices A and B.
shows forward-biased I-V curves of devices A and B. The detailed I-V relations are also depicted in the inset. At 20 mA operation current, the devices A and B exhibit turn-on voltages (Vf) of 3.25 and 3.07 V, respectively. The 0.18 V reduction in Vf for the device B than the device A could be explained by the improved ITO direct-Ohmic contact property which relieves parasitic contact resistance [3

3. Y. J. Liu, C. H. Yen, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “On a GaN-based light-emitting diode with a p-GaN/i-InGaN superlattice structure,” IEEE Electron Device Lett. 30(11), 1149–1151 (2009). [CrossRef]

, 14

14. J. S. Jang, “High output power GaN-based light-emitting diodes using an electrically reverse-connected p-Schottky diode and p-InGaN–GaN superlattice,” Appl. Phys. Lett. 93(8), 081118 (2008). [CrossRef]

] between p-GaN and ITO layers. Basically this is ascribed to the higher hole concentration resulted from increased density of gallium vacancies on the p-GaN contact layer, as mentioned above. In addition, since the current injected from p-pad is effectively spread through the ITO current spreading layer due to the good Ohmic contact behavior, a lower dynamic resistance could also be expected. The corresponding dynamic resistances for both devices are shown in Fig. 7
Fig. 7 Dynamic resistance as a function of current for studied devices A and B.
. Clearly the dynamic resistances of the device B are always lower than those of the device A from 10 to 50 mA current operation region, suggesting that the p-GaN/ITO/p-pad contact resistance plays a significant role in device performance. At 20 mA, the device A exhibits dynamic resistance of 25.6 Ω, while device B only 18.0 Ω. The nearly 30% reduction in dynamic resistance is very important to shrinkage joule heating effect and increase power efficiency of LEDs.

Figure 8
Fig. 8 Light output power as a function of current for studied devices A and B.
demonstrates light output power as a function of injection current. At 20, 100, and 150 mA, light output power of 10.0 (11.8), 30.5 (37.4), and 38 (47.5) mW for the device A (B) are obtained, and the corresponding output power improvement are 18.0, 22.6, and 25.0%, respectively. It is clear that the improvement is increased with increasing current injection. We reasonably conclude that this mainly originates from the enhanced internal quantum efficiency of the device B, since both devices were designed as the same geometrical structure which confirms tiny variation in light extraction efficiency. In other words, the LED with an ITO direct-Ohmic contact film to p-GaN obtains a more linear and stable light output power increase with increasing current, due to the reduction of current crowding effect nearby the p-metal. It infers that the device B shows an improved current-spreading performance which results in a larger and more uniform light-emission area. On the other hand, the conventional device A easily suffers from current crowding phenomena, i.e., the injection current is mainly confined around the p-metal contact region. This certainly gives the increase of joule heating which results in rapid saturation of light output power [15

15. Y. J. Liu, C. H. Yen, K. H. Yu, P. L. Lin, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “Characteristics of an AlGaInP-based light emitting diode with an indium-tin-oxide (ITO) direct Ohmic contact structure,” IEEE J. Quantum Electron. 46(2), 246–252 (2010). [CrossRef]

].

Figure 9
Fig. 9 Dominant wavelength as a function of current for studied devices A and B.
shows the emission dominant wavelength as a function of current. The wavelength shift for the device B is around 0.9 nm from 40 to 100 mA. This value is better than that of 1.7 nm for the device A. It could be explained by the fact that the studied p-GaN/AuinGaout/ITO structure exhibits a more uniformity of current spreading, giving rise to a relieved joule-heating effect from LED hetero-junctions. Accordingly, it lowers the current-crowding effect and LED junction temperature which results in a smaller wavelength variation. This means that the proposed structure could significantly improve the performance of GaN LEDs.

Figure 10
Fig. 10 Brightness variation as a function of aging time for studied devices A and B.
shows light output power life time behaviors for both studied devices A and B, respectively. During testing conditions, both devices are always biased at 20 mA operation current. There are only around 1.5 – 2% output power degradations after 260 hrs aging time for both devices, indicating that the Au thermal-diffused and removed process for the device B would not severely influence reliability of light behavior.

Figure 11
Fig. 11 Light output images of the studied devices A and B.
illustrates the output light images of devices A and B under 20 mA dc current. The output light intensity and uniformity on the device surface are clearly displayed referring to the color bar. From these photographs, a more uniform light emission could be observed for the device B, indicating its superior current spreading ability. Nevertheless, the injected current is always crowded nearby the p-metal pad for the device A. This result agrees well with the discussion in Fig. 8.

4. Conclusion

In conclusion, an InGaN/GaN based MQW LED with an ITO direct-Ohmic contact to the p-GaN layer is achieved by means of Au thin film thermal-annealing and followed removed processes. An Au in- and gallium out-diffused (AuinGaout) layer is the primary origin to cause the performance improvement. Experimentally, the AuinGaout layer helps improving holes concentration, resistivity, and sheet resistance of the p-GaN contact layer from 3.15×1017, 1.53, and 5.1×104 to 1.83×1018 cm−3, 0.26 Ω-cm, and 8.6×103 Ω/□. At 20 mA operation current, the studied device (device B) exhibits lower dynamic resistance of 18.0 Ω and forward voltage of 3.07 V, as compared with a conventional device (device A) without the AuinGaout layer (25.6 Ω and 3.25 V). An enhanced light output power of 18.0% is also found at this condition. This is mainly attributed to the larger and more uniform light-emission area resulted from the reduced current-crowding effect by means of an ITO direct-Ohmic contact to the p-GaN layer.

Acknowledgments

This work was supported in part by the National Science Council of Taiwan, R.O.C., under Contract NSC 97-2221-E-006-237-MY3 and Chi Mei Lighting Technology Company.

References and links

1.

X. Guo and E. F. Schubert, “Current crowding in GaN/InGaN light emitting diodes on insulating substrates,” J. Appl. Phys. 90(8), 4191 (2001). [CrossRef]

2.

L. Zhou, W. Lanford, A. T. Ping, I. Adesida, J. W. Yang, and A. Khan, “Low resistance Ti/Pt/Au ohmic contacts to p-type GaN,” Appl. Phys. Lett. 76(23), 3451 (2000). [CrossRef]

3.

Y. J. Liu, C. H. Yen, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “On a GaN-based light-emitting diode with a p-GaN/i-InGaN superlattice structure,” IEEE Electron Device Lett. 30(11), 1149–1151 (2009). [CrossRef]

4.

K. M. Chang, J. Y. Chu, and C. C. Cheng, “Highly reliable GaN-based light-emitting diodes formed by p–In0.1Ga0.9N–ITO structure,” IEEE Photon. Technol. Lett. 16, 1807 (2004). [CrossRef]

5.

C. S. Chang, S. J. Chang, Y. K. Su, C. H. Kuo, W. C. Lai, Y. C. Lin, Y. P. Hsu, S. C. Shei, J. M. Tsai, H. M. Lo, J. C. Ke, and J. K. Sheu, “High brightness InGaN green LEDs with an ITO on n ++ -SPS upper contact,” IEEE Trans. Electron. Dev. 50(11), 2208–2212 (2003). [CrossRef]

6.

T. Gessmann, Y. L. Li, E. L. Waldron, J. W. Graff, and J. K. Sheu, “Ohmic contacts to p-type GaN mediated by polarization fields in thin InxGa1-xN capping layers,” Appl. Phys. Lett. 80(6), 986 (2002). [CrossRef]

7.

Y. J. Liu, C. H. Yen, C. H. Hsu, K. H. Yu, L. Y. Chen, T. H. Tsai, and W. C. Liu, “Impact of An indium oxide/indium–tin oxide mixed structure for GaN-based light-emitting diodes,” Opt. Rev. 16(6), 575–577 (2009). [CrossRef]

8.

S. K. Julita, G. Szymon, L. S. Elzbieta, P. Ryszard, N. Grzegorz, L. Michał, P. Piotr, T. Ewa, K. Jan, and K. Stanisław, “Ni–Au contacts to p-type GaN – Structure and properties,” Solid-State Electron. 54(7), 701–709 (2010). [CrossRef]

9.

J. L. Lee, M. Weber, J. K. Kim, J. W. Lee, Y. J. Park, T. Kim, and K. Lynn, “Ohmic contact formation mechanism of nonalloyed Pd contacts to p-type GaN observed by positron annihilation spectroscopy,” Appl. Phys. Lett. 74(16), 2289 (1999). [CrossRef]

10.

Y. Koide, T. Maeda, T. Kawakami, S. Fujita, T. Uemura, N. Shibata, and M. Murakami, “Effects of annealing in an oxygen ambient on electrical properties of ohmic contacts to p-type GaN,” J. Electron. Mater. 28(3), 341–346 (1999). [CrossRef]

11.

S. Nakahara and E. Kinsbron, “Room temperature interdiffusion study of Au/Ga thin film couples,” Thin Solid Films 113(1), 15–26 (1984). [CrossRef]

12.

M. Puselj and J. Schubert, “Kristallstruktur von Au2Ga,” J. Less-Common Met. 38(1), 83–90 (1974). [CrossRef]

13.

I. Waki, H. Fujioka, M. Oshima, H. Miki, and A. Fukizawa, “Low-temperature activation of Mg-doped GaN using Ni films,” Appl. Phys. Lett. 78(19), 2899 (2001). [CrossRef]

14.

J. S. Jang, “High output power GaN-based light-emitting diodes using an electrically reverse-connected p-Schottky diode and p-InGaN–GaN superlattice,” Appl. Phys. Lett. 93(8), 081118 (2008). [CrossRef]

15.

Y. J. Liu, C. H. Yen, K. H. Yu, P. L. Lin, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “Characteristics of an AlGaInP-based light emitting diode with an indium-tin-oxide (ITO) direct Ohmic contact structure,” IEEE J. Quantum Electron. 46(2), 246–252 (2010). [CrossRef]

OCIS Codes
(160.0160) Materials : Materials
(230.0230) Optical devices : Optical devices
(250.0250) Optoelectronics : Optoelectronics
(260.0260) Physical optics : Physical optics

ToC Category:
Optical Devices

History
Original Manuscript: February 22, 2011
Revised Manuscript: April 2, 2011
Manuscript Accepted: April 2, 2011
Published: July 15, 2011

Citation
Yi-Jung Liu, Chien-Chang Huang, Tai-You Chen, Chi-Shiang Hsu, Jian-Kai Liou, Tsung-Yuan Tsai, and Wen-Chau Liu, "Implementation of an indium-tin-oxide (ITO) direct-Ohmic contact structure on a GaN-based light emitting diode," Opt. Express 19, 14662-14670 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-15-14662


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References

  1. X. Guo and E. F. Schubert, “Current crowding in GaN/InGaN light emitting diodes on insulating substrates,” J. Appl. Phys. 90(8), 4191 (2001). [CrossRef]
  2. L. Zhou, W. Lanford, A. T. Ping, I. Adesida, J. W. Yang, and A. Khan, “Low resistance Ti/Pt/Au ohmic contacts to p-type GaN,” Appl. Phys. Lett. 76(23), 3451 (2000). [CrossRef]
  3. Y. J. Liu, C. H. Yen, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “On a GaN-based light-emitting diode with a p-GaN/i-InGaN superlattice structure,” IEEE Electron Device Lett. 30(11), 1149–1151 (2009). [CrossRef]
  4. K. M. Chang, J. Y. Chu, and C. C. Cheng, “Highly reliable GaN-based light-emitting diodes formed by p–In0.1Ga0.9N–ITO structure,” IEEE Photon. Technol. Lett. 16, 1807 (2004). [CrossRef]
  5. C. S. Chang, S. J. Chang, Y. K. Su, C. H. Kuo, W. C. Lai, Y. C. Lin, Y. P. Hsu, S. C. Shei, J. M. Tsai, H. M. Lo, J. C. Ke, and J. K. Sheu, “High brightness InGaN green LEDs with an ITO on n ++ -SPS upper contact,” IEEE Trans. Electron. Dev. 50(11), 2208–2212 (2003). [CrossRef]
  6. T. Gessmann, Y. L. Li, E. L. Waldron, J. W. Graff, and J. K. Sheu, “Ohmic contacts to p-type GaN mediated by polarization fields in thin InxGa1-xN capping layers,” Appl. Phys. Lett. 80(6), 986 (2002). [CrossRef]
  7. Y. J. Liu, C. H. Yen, C. H. Hsu, K. H. Yu, L. Y. Chen, T. H. Tsai, and W. C. Liu, “Impact of An indium oxide/indium–tin oxide mixed structure for GaN-based light-emitting diodes,” Opt. Rev. 16(6), 575–577 (2009). [CrossRef]
  8. S. K. Julita, G. Szymon, L. S. Elzbieta, P. Ryszard, N. Grzegorz, L. Michał, P. Piotr, T. Ewa, K. Jan, and K. Stanisław, “Ni–Au contacts to p-type GaN – Structure and properties,” Solid-State Electron. 54(7), 701–709 (2010). [CrossRef]
  9. J. L. Lee, M. Weber, J. K. Kim, J. W. Lee, Y. J. Park, T. Kim, and K. Lynn, “Ohmic contact formation mechanism of nonalloyed Pd contacts to p-type GaN observed by positron annihilation spectroscopy,” Appl. Phys. Lett. 74(16), 2289 (1999). [CrossRef]
  10. Y. Koide, T. Maeda, T. Kawakami, S. Fujita, T. Uemura, N. Shibata, and M. Murakami, “Effects of annealing in an oxygen ambient on electrical properties of ohmic contacts to p-type GaN,” J. Electron. Mater. 28(3), 341–346 (1999). [CrossRef]
  11. S. Nakahara and E. Kinsbron, “Room temperature interdiffusion study of Au/Ga thin film couples,” Thin Solid Films 113(1), 15–26 (1984). [CrossRef]
  12. M. Puselj and J. Schubert, “Kristallstruktur von Au2Ga,” J. Less-Common Met. 38(1), 83–90 (1974). [CrossRef]
  13. I. Waki, H. Fujioka, M. Oshima, H. Miki, and A. Fukizawa, “Low-temperature activation of Mg-doped GaN using Ni films,” Appl. Phys. Lett. 78(19), 2899 (2001). [CrossRef]
  14. J. S. Jang, “High output power GaN-based light-emitting diodes using an electrically reverse-connected p-Schottky diode and p-InGaN–GaN superlattice,” Appl. Phys. Lett. 93(8), 081118 (2008). [CrossRef]
  15. Y. J. Liu, C. H. Yen, K. H. Yu, P. L. Lin, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “Characteristics of an AlGaInP-based light emitting diode with an indium-tin-oxide (ITO) direct Ohmic contact structure,” IEEE J. Quantum Electron. 46(2), 246–252 (2010). [CrossRef]

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