OSA's Digital Library

Energy Express

Energy Express

  • Editor: Bernard Kippelen
  • Vol. 18, Iss. S4 — Nov. 8, 2010
  • pp: A562–A567
« Show journal navigation

InGaN light-emitting diodes with oblique sidewall facets formed by selective growth on SiO2 patterned GaN film

Jinn-Kong Sheu, Kuo-Hua Chang, Shang-Ju Tu, Ming-Lun Lee, Chih-Ciao Yang, Che-Kang Hsu, and Wei-Chih Lai  »View Author Affiliations


Optics Express, Vol. 18, Issue S4, pp. A562-A567 (2010)
http://dx.doi.org/10.1364/OE.18.00A562


View Full Text Article

Acrobat PDF (921 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

In this study, GaN-based light-emitting diodes (LEDs) with naturally formed oblique sidewall facets (OSFs) were fabricated through a selective regrowth process. The SiO2 mask layer was patterned on a heavily doped n-GaN template layer rather than on a sapphire substrate. As a result, the periphery of the LED included several OSFs around the regrown GaN mesa. While processing the device, dry etching was unnecessary for exposing the n-GaN underlying layer in order to form the n-type Ohmic contacts. This could be attributed to the fact that the n-GaN template layer with an electron concentration of around 8 × 1018/cm3 was exposed after the removal of the SiO2 mask layer. With an injection current of 20 mA, GaN-based LEDs with OSFs exhibited a 21% enhancement in light output compared with those that have vertical sidewall facets. The enhancement is attributed to the fact that photons extracted from OSFs can reduce internal absorption loss.

© 2010 OSA

1. Introduction

2. Experiments

Samples used in this study were grown on c-face (0001) 2-inch sapphire substrates in a vertical metal-organic vapor-phase epitaxy(MOVPE) reactor. Before the growth of LED structures, n-type GaN epitaxial layers, including a 1 μm thick undoped GaN layer and a 2 μm thick Si-doped n-GaN layer, were grown on sapphire substrates as templates for the subsequent regrowth process. The carrier concentration of the n-GaN template layer was around 8 × 1018/cm3. The SiO2 layer was deposited on the n-GaN template layer and then selectively etched using hydroflouric solution to form a series of patterns on the templates. Afterwards, templates with and without the patterned SiO2 layer were respectively loaded into the MOVPE reactor to regrow the InGaN/GaN-based layers. These included a 1.7 μm thick Si-doped n-GaN layer grown at 1,000 °C, a 10-pair In0.3Ga0.7N/GaN MQW structure grown at 750 °C, a 0.05 μm thick Mg-doped p-Al0.15Ga0.85N electron blocking layer grown at 1,000 °C, and a 0.1 μm thick Mg-doped p-GaN top contact layer also grown at 1,000 °C. Finally, a heavily Si-doped short-period superlattice (SPS) structure was grown on the p-GaN contact layer [8

8. J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, Y. K. Su, S. J. Chang, and G. C. Chi, “Low- operation voltage of InGaN/GaN light-emitting diodes with Si-doped In0. 3Ga0. 7N/GaN short-period superlattice tunneling contact layer,” IEEE Electron Device Lett. 22(10), 460–462 (2001). [CrossRef]

]. For the InGaN/GaN MQW active region, each pair consisted of a 3 nm thick In0.27Ga0.73N well layer and a 17 nm thick GaN barrier layer.

Figures 1(a)
Fig. 1 (a) Schematic structure of LED-I and photon paths in LED-I; (b) detailed epitaxial layer structure; (c) typical tilted-angle-view SEM image of LED-I; (d) typical SEM image of LED-I after the formation of Cr/Au electrodes; (e) enlarged tilted-angle-view SEM image taken on the regrown area of LED-I after the removal of SiO2 mask layer; and (f) cross-section view SEM image of LED-I.
and 1(b) show the schematic diagram of the device structure (labeled LED-I) and the detailed epitaxial layer structure, respectively. For comparative analysis, LEDs without OSFs were also prepared and labeled as LED-II. For the device process of LED-II, wafers were partially dry etched until the n-GaN layer was exposed. The dry etching procedure was not necessary for LED-I because its n-GaN layer was exposed only if the SiO2 mask layer was removed using the simple wet etching method (HF solution). Current-voltage (I-V) measurements were performed at room temperature using an HP4156C semiconductor parameter analyzer.

3. Results and discussions

Figure 1(b) shows the typical tilted-view SEM image of LEDs that have been selectively grown on an n-GaN template (LED-I). According to the energy dispersive X-ray spectroscopy analysis (data not shown here), only gallium could be observed on the surface of the SiO2 mask layer. This indicates that GaN-based layers cannot be deposited on the SiO2 layer. The method demonstrates excellent growth selectivity for the GaN film. Figure 1(c) shows a typical SEM image of LED-I after the formation of Cr/Au electrodes. Figures 1(d) and 1(e) show the enlarged tilted-angle and cross-sectional views of the SEM images, respectively. These were taken from the regrown area of LED-I after the removal of the SiO2 mask layer. The periphery of LED-I includes several oblique facets in the mesa. The facets contain {101¯1¯} planes [9

9. H. G. Kim, T. V. Cuong, H. Jeong, S. H. Woo, O. H. Cha, E.-K. Suh, C.-H. Hong, H. K. Cho, B. H. Kong, and M. S. Jeong, “Spatial distribution of crown shaped light emission from a periodic inverted polygonal deflector embedded in an InGaN/GaN light emitting diode,” Appl. Phys. Lett. 92(6), 061118 (2008). [CrossRef]

,10

10. Y. Kato, S. Kitamura, K. Hiramatsu, and N. Sawaki, “Selective growth of wurtzite GaN and AlxGa1−xN on GaN/sapphire substrates by metalorganic vapor phase epitaxy,” J. Cryst. Growth 144(3-4), 133–140 (1994). [CrossRef]

]. The measured angles of regrown GaN sidewall facets were approximately 62 degrees inclined to the c-plane surface. These oblique facets with respect to the plane of (0001) were initiated on the edges of the SiO2 and n-GaN layers during the regrowth process. In addition, the formation of oblique facets can be attributed to the higher growth rate in the vertical direction [0001] compared with the inclined lateral facets [9

9. H. G. Kim, T. V. Cuong, H. Jeong, S. H. Woo, O. H. Cha, E.-K. Suh, C.-H. Hong, H. K. Cho, B. H. Kong, and M. S. Jeong, “Spatial distribution of crown shaped light emission from a periodic inverted polygonal deflector embedded in an InGaN/GaN light emitting diode,” Appl. Phys. Lett. 92(6), 061118 (2008). [CrossRef]

,10

10. Y. Kato, S. Kitamura, K. Hiramatsu, and N. Sawaki, “Selective growth of wurtzite GaN and AlxGa1−xN on GaN/sapphire substrates by metalorganic vapor phase epitaxy,” J. Cryst. Growth 144(3-4), 133–140 (1994). [CrossRef]

]. According to the aforementioned results, one can anticipate that LEDs with these types of OSFs will have higher LEE than LED-II with its vertical sidewall facets. To evaluate the effect of naturally formed OSFs on the LEE of LEDs, light outputs of the bare-chip LEDs were measured using an integral sphere. The device fabrication processes began with the deposition of ITO on top of the regrown mesa area to form the transparent contact layer [4

4. J. K. Sheu, C. M. Tsai, M. L. Lee, S. C. Shei, and W. C. Lai, “InGaN light-emitting diodes with naturally formed truncated micropyramids on top surface,” Appl. Phys. Lett. 88(11), 113505 (2006). [CrossRef]

,7

7. C. C. Kao, H. C. Kuo, H. W. Huang, J. T. Chu, Y. C. Peng, Y. L. Hsieh, C. Y. Luo, S. C. Wang, C. C. Yu, and C. F. Lin, “Light-output Enhancement in a Nitride Based Light-emitting Diode with 22° Undercut Side Walls,” IEEE Photon. Technol. Lett. 17(1), 19–21 (2005). [CrossRef]

]. The Cr/Au bilayer metals were then deposited on the masked n+-GaN template area and the ITO layer to form the anode electrode and the n-type Ohmic contact (i.e., cathode), respectively [11

11. M. L. Lee, J. K. Sheu, and C. C. Hu, “Nonalloyed Cr/ Au-based Ohmic contacts to n-GaN,” Appl. Phys. Lett. 91(18), 182106 (2007). [CrossRef]

] [Figs. 1(a) and 1(c)]. The dimension of the LED chips used in this study was 340 × 340 μm2.

Figure 2
Fig. 2 Typical light output-current (L-I) characteristics of LED-I and LED-II with bare-chip form; the inset shows the typical beam patterns taken from LED-I and LED-II. These LEDs were all bonded on the TO 66.
shows the respective typical light output-current (L-I) characteristics of LED-I and LED-II in bare-chip form. All LEDs have an emission wavelength of around 460 nm when a DC of 20 mA is applied. The total light output of the devices were measured using an integrating sphere to allow light, which was emitted in all directions from the LEDs, to be collected. The results in Fig. 2 show that by utilizing the selective growth process to form the OSFs, the light output in LED-I significantly improved by 21% in terms of magnitude compared with that in LED-II. The enhancement of light output in LED-I can be attributed to the reduction of the re-absorption probability of the photons by the active layers. This assumption can be indirectly supported by the measurements of the beam patterns of the studied LEDs. As shown in the inset of Fig. 2, the typical beam patterns of LED-I and LED-II were determined at a DC driving current of 20 mA. The figure clearly shows that LED-I displays a wider beam pattern due to the fact that the percentage of photons extracted from the sidewall facets of LED-I is higher than that of LED-II, resulting in wider beam patterns observed in the former. For a conventional GaN-based LED, the majority of light emitted from an isotropic source incident on the GaN/air interfaces, including the four sidewall facets and top surface, can undergo total internal reflection or Fresnel reflection, passing small amounts of light through the GaN/air interfaces into the air. The escape probability of photons emitted from a textured semiconductor surface is higher than that from a smooth semiconductor surface [1

1. E. F. Schubert, Light-emitting diodes, pp.150–160 (Second Edition, Cambridge University Press, Cambridge, U.K., 2006.)

]. In principle, the critical angle (θc) of blue light impinging at the top GaN/air interface is around 23.5 degrees. Therefore, a LED-II growing on sapphire with a specular surface and four vertical sidewall facets can result in waveguide modes that trap most photons inside the LED when the incident angle of photons is less than the θc. This allows only a few photons to out-couple outside the LED from the sidewall facets if they are not completely absorbed by the material. However, for LED-I, the oblique facets can play the role of a guided light deflector and facilitate multiple chances for photons to escape from the LED sidewall facets. This allows more photons to be extracted from the OSFs, resulting in a wider beam pattern. This means that the average path length of photons in LED-I is shorter than that in LED-II before the photons escaped from the LEDs. Therefore, the enhancement of light output in LED-I can be attributed to the reduction of the internal absorption losses.

Considering the electrical properties of LED-I, Si and/or O atoms may be able to diffuse in the n-GaN template layer and result in a heavily doped GaN surface layer during the regrowth process at temperatures of 750–1,000 °C [12

12. C. F. Lin, H. C. Cheng, G. C. Chi, C. J. Bu, and M. S. Feng, “Improved contact performance of GaN film using Si diffusion,” Appl. Phys. Lett. 76(14), 1878 (2000). [CrossRef]

,13

13. J. K. Sheu and G. C. Chi, “The doping process and dopant characteristics of GaN,” J. Phys. 14, R657 (2002).

]. In this study, the non-alloyed Cr/Au metal contacts on the dry-etched surface and masked template n+-GaN layers all display linear I-V characteristics and exhibit specific contact resistances (ρc) of approximately 3 × 10−4 and 1 × 10−4 Ω cm2, respectively. Figure 3
Fig. 3 I-V and dynamic resistance characteristics of the LED-I and LED-II.
shows the I-V characteristics of LEDs with oblique (LED-I) and vertical (LED-II) sidewall facets. The forward voltages (Vf) measured at 20 mA are approximately 3.10 and 3.12 V for LED-I and LED-II, respectively. The series resistances (Rs) determined through the extraction of dynamic resistance (Fig. 3) are almost the same in both LED-I and LED-II. This result can be attributed to the fact that the ρc value of the Cr/Au contacts on the etched GaN surface is comparable with that deposited on the SiO2 masked n-GaN template layer.

4. Conclusions

In summary, this research has demonstrated the electrical and optical properties of GaN-based LEDs with naturally formed oblique facets by applying a selective regrowth method. GaN-based LEDs with oblique facets exhibited a 21% enhancement in light output when a DC of 20 mA was applied. The enhancement can be attributed to the reduction of the photon extraction path length and internal absorption losses. Although the ρc value of the Cr/Au contacts deposited on the SiO2 masked n-GaN template layer was slightly lower than that of the Cr/Au contacts on the etched GaN surface, the typical series resistance and the 20 mA-driven forward voltage of LED-I were almost the same as those of LED-II. Thus, the selective regrowth process via the SiO2 layer masked on the n-GaN template layer is not expected to degrade the electrical properties of LED-I.

Acknowledgments

This work was partly supported by Bureau of Energy, Ministry of Economic Affairs under contract No. 98-D0204-6. The authors would also like to acknowledge the National Science Council for the financial support of the research Grant Nos. NSC 97-2221-E-006-242-MY3 and 98-2221-E-218-005-MY3.

References and links

1.

E. F. Schubert, Light-emitting diodes, pp.150–160 (Second Edition, Cambridge University Press, Cambridge, U.K., 2006.)

2.

X. A. Cao, S. J. Pearton, A. P. Zhang, G. T. Dang, F. Ren, R. J. Shul, L. Zhang, R. Hickman, and J. M. Van Hove, “Electrical effects of plasma damage in p-GaN,” Appl. Phys. Lett. 75(17), 2569 (1999). [CrossRef]

3.

C. M. Tsai, J. K. Sheu, P. T. Wang, W. C. Lai, S. C. Shei, S. J. Chang, C. H. Kuo, C. W. Kuo, and Y. K. Su, “High efficiency and improved ESD characteristics of GaN-based LEDs with naturally textured surface grown by MOCVD,” IEEE Photon. Technol. Lett. 18(11), 1213–1215 (2006) (and references therein). [CrossRef]

4.

J. K. Sheu, C. M. Tsai, M. L. Lee, S. C. Shei, and W. C. Lai, “InGaN light-emitting diodes with naturally formed truncated micropyramids on top surface,” Appl. Phys. Lett. 88(11), 113505 (2006). [CrossRef]

5.

C. S. Chang, S. J. Chang, Y. K. Su, C. T. Lee, Y. C. Lin, W. C. Lai, S. C. Shei, J. C. Ke, and H. M. Lo, “Nitride-based LEDs with textured side walls,” IEEE Photon. Technol. Lett. 16(3), 750–752 (2004). [CrossRef]

6.

J.-S. Lee, J. Lee, S. Kim, and H. Jeon, “GaN-based light-emitting diode structure with monolithically integrated sidewall deflectors for enhanced surface emission,” IEEE Photon. Technol. Lett. 18(15), 1588–1590 (2006). [CrossRef]

7.

C. C. Kao, H. C. Kuo, H. W. Huang, J. T. Chu, Y. C. Peng, Y. L. Hsieh, C. Y. Luo, S. C. Wang, C. C. Yu, and C. F. Lin, “Light-output Enhancement in a Nitride Based Light-emitting Diode with 22° Undercut Side Walls,” IEEE Photon. Technol. Lett. 17(1), 19–21 (2005). [CrossRef]

8.

J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, Y. K. Su, S. J. Chang, and G. C. Chi, “Low- operation voltage of InGaN/GaN light-emitting diodes with Si-doped In0. 3Ga0. 7N/GaN short-period superlattice tunneling contact layer,” IEEE Electron Device Lett. 22(10), 460–462 (2001). [CrossRef]

9.

H. G. Kim, T. V. Cuong, H. Jeong, S. H. Woo, O. H. Cha, E.-K. Suh, C.-H. Hong, H. K. Cho, B. H. Kong, and M. S. Jeong, “Spatial distribution of crown shaped light emission from a periodic inverted polygonal deflector embedded in an InGaN/GaN light emitting diode,” Appl. Phys. Lett. 92(6), 061118 (2008). [CrossRef]

10.

Y. Kato, S. Kitamura, K. Hiramatsu, and N. Sawaki, “Selective growth of wurtzite GaN and AlxGa1−xN on GaN/sapphire substrates by metalorganic vapor phase epitaxy,” J. Cryst. Growth 144(3-4), 133–140 (1994). [CrossRef]

11.

M. L. Lee, J. K. Sheu, and C. C. Hu, “Nonalloyed Cr/ Au-based Ohmic contacts to n-GaN,” Appl. Phys. Lett. 91(18), 182106 (2007). [CrossRef]

12.

C. F. Lin, H. C. Cheng, G. C. Chi, C. J. Bu, and M. S. Feng, “Improved contact performance of GaN film using Si diffusion,” Appl. Phys. Lett. 76(14), 1878 (2000). [CrossRef]

13.

J. K. Sheu and G. C. Chi, “The doping process and dopant characteristics of GaN,” J. Phys. 14, R657 (2002).

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 31, 2010
Revised Manuscript: September 29, 2010
Manuscript Accepted: September 30, 2010
Published: October 11, 2010

Citation
Jinn-Kong Sheu, Kuo-Hua Chang, Shang-Ju Tu, Ming-Lun Lee, Chih-Ciao Yang, Che-Kang Hsu, and Wei-Chih Lai, "InGaN light-emitting diodes with oblique sidewall facets formed by selective growth on SiO2 patterned GaN film," Opt. Express 18, A562-A567 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-S4-A562


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. E. F. Schubert, Light-emitting diodes, pp.150–160 (Second Edition, Cambridge University Press, Cambridge, U.K., 2006.)
  2. X. A. Cao, S. J. Pearton, A. P. Zhang, G. T. Dang, F. Ren, R. J. Shul, L. Zhang, R. Hickman, and J. M. Van Hove, “Electrical effects of plasma damage in p-GaN,” Appl. Phys. Lett. 75(17), 2569 (1999). [CrossRef]
  3. C. M. Tsai, J. K. Sheu, P. T. Wang, W. C. Lai, S. C. Shei, S. J. Chang, C. H. Kuo, C. W. Kuo, and Y. K. Su, ““High efficiency and improved ESD characteristics of GaN-based LEDs with naturally textured surface grown by MOCVD,” IEEE Photon. Technol. Lett. 18(11), 1213–1215 (2006) (and references therein). [CrossRef]
  4. J. K. Sheu, C. M. Tsai, M. L. Lee, S. C. Shei, and W. C. Lai, “InGaN light-emitting diodes with naturally formed truncated micropyramids on top surface,” Appl. Phys. Lett. 88(11), 113505 (2006). [CrossRef]
  5. C. S. Chang, S. J. Chang, Y. K. Su, C. T. Lee, Y. C. Lin, W. C. Lai, S. C. Shei, J. C. Ke, and H. M. Lo, “Nitride-based LEDs with textured side walls,” IEEE Photon. Technol. Lett. 16(3), 750–752 (2004). [CrossRef]
  6. J.-S. Lee, J. Lee, S. Kim, and H. Jeon, “GaN-based light-emitting diode structure with monolithically integrated sidewall deflectors for enhanced surface emission,” IEEE Photon. Technol. Lett. 18(15), 1588–1590 (2006). [CrossRef]
  7. C. C. Kao, H. C. Kuo, H. W. Huang, J. T. Chu, Y. C. Peng, Y. L. Hsieh, C. Y. Luo, S. C. Wang, C. C. Yu, and C. F. Lin, “Light-output Enhancement in a Nitride Based Light-emitting Diode with 22° Undercut Side Walls,” IEEE Photon. Technol. Lett. 17(1), 19–21 (2005). [CrossRef]
  8. J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, Y. K. Su, S. J. Chang, and G. C. Chi, “Low- operation voltage of InGaN/GaN light-emitting diodes with Si-doped In0. 3Ga0. 7N/GaN short-period superlattice tunneling contact layer,” IEEE Electron Device Lett. 22(10), 460–462 (2001). [CrossRef]
  9. H. G. Kim, T. V. Cuong, H. Jeong, S. H. Woo, O. H. Cha, E.-K. Suh, C.-H. Hong, H. K. Cho, B. H. Kong, and M. S. Jeong, “Spatial distribution of crown shaped light emission from a periodic inverted polygonal deflector embedded in an InGaN/GaN light emitting diode,” Appl. Phys. Lett. 92(6), 061118 (2008). [CrossRef]
  10. Y. Kato, S. Kitamura, K. Hiramatsu, and N. Sawaki, “Selective growth of wurtzite GaN and AlxGa1−xN on GaN/sapphire substrates by metalorganic vapor phase epitaxy,” J. Cryst. Growth 144(3-4), 133–140 (1994). [CrossRef]
  11. M. L. Lee, J. K. Sheu, and C. C. Hu, “Nonalloyed Cr/ Au-based Ohmic contacts to n-GaN,” Appl. Phys. Lett. 91(18), 182106 (2007). [CrossRef]
  12. C. F. Lin, H. C. Cheng, G. C. Chi, C. J. Bu, and M. S. Feng, “Improved contact performance of GaN film using Si diffusion,” Appl. Phys. Lett. 76(14), 1878 (2000). [CrossRef]
  13. J. K. Sheu and G. C. Chi, “The doping process and dopant characteristics of GaN,” J. Phys. 14, R657 (2002).

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
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited