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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 23 — Nov. 7, 2011
  • pp: 23036–23041
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Enhanced electroluminescence of a-plane InGaN light emitting diodes grown on oxide-patterned r-plane sapphire substrates

Sung-Min Hwang, Hooyoung Song, Yong Gon Seo, Ji-Su Son, Jihoon Kim, and Kwang Hyeon Baik  »View Author Affiliations


Optics Express, Vol. 19, Issue 23, pp. 23036-23041 (2011)
http://dx.doi.org/10.1364/OE.19.023036


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Abstract

We report on the new fabrication method of a-plane InGaN light emitting diodes (LEDs) using the epitaxy on patterned insulator on sapphire substrate (EPISS). Cathodoluminescence spectrum of the fully coalesced a-plane GaN template showed that band edge emission intensity of the wing region was four times higher than that of the window region. Threading dislocations and basal stacking faults densities in wing region were ~1×107 cm−2 and ~5☓104 cm−1, respectively. Blue-emitting (443.4 nm) a-plane InGaN LED employing EPISS showed the optical power of 3.1 mW and the EL FWHM of 25.2 nm at the injection current of 20 mA.

© 2011 OSA

1. Introduction

Nonpolar III-nitrides have attracted a lot of interest due to the potential for eliminating undesirable field effects and the possibility of producing devices with higher efficiencies than conventional c-plane III-nitrides [1

1. F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024–R10027 (1997). [CrossRef]

3

3. S. Pimputkar, J. Speck, S. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photonics 3(4), 180–182 (2009). [CrossRef]

]. Since there are large anisotropic lattice mismatch and thermal expansion coefficients in the parallel and perpendicular to the c-axis [4

4. U. T. Schwarz and M. Kneissl, “Nitride emitters go nonpolar,” Phys. Status Solidi (RRL) 1(3), A44–A46 (2007). [CrossRef]

9

9. K. H. Baik, Y. G. Seo, S.-K. Hong, S. Lee, J. Kim, J.-S. Son, and S.-M. Hwang, “Effects of basal stacking faults on electrical anisotropy of nonpolar a-plane (11-20) GaN light-emitting diodes on sapphire substrate,” IEEE Photon. Technol. Lett. 22(9), 595–597 (2010). [CrossRef]

], nonpolar GaN films grown on hetero-substrates generally contain a high density of threading dislocations (TDs) and basal stacking faults (BSFs) which significantly deteriorate the device performances. In order to reduce these microstructural defects, several research groups have focused on the epitaxially laterally overgrown (ELOG) technique and the development of nonpolar bulk GaN, but these still have critical drawbacks for commercial applications because of the cost disadvantage [10

10. A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura, and U. K. Mishra, “Nonpolar InGaN/GaN emitters on reduced-defect lateral epitaxially overgrown a-plane GaN with drive-current-independent electroluminescence emission peak,” Appl. Phys. Lett. 85(22), 5143–5145 (2004). [CrossRef]

,11

11. H. Yamada, K. Iso, M. Saito, H. Hirasawa, N. Fellows, H. Masui, K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Comparison of InGaN/GaN light emitting diodes grown on m-plane and a-plane bulk GaN substrates,” Phys. Status Solidi (RRL) 2(2), 89–91 (2008). [CrossRef]

]. If nonpolar GaN-based light emitting diodes (LEDs) are to be used more widely, cost-effective growth technique insuring high structural quality is required. Especially, nonpolar growth on r-plane sapphire substrates is very attractive for scale-up of wafer size fabrication because the growth directions of sapphire ingot are generally along the a-axis or the r-axis. Therefore the fabrication of large-size r-plane sapphire wafer from the r-axis ingot is much easier when compared with processing c-plane wafer from the a-axis ingot. In addition, direct growth methods with the minimization of extended defects on sapphire substrates would be required for future industrial production. We previously reported milliwatt-class a-plane InGaN/GaN LEDs on r-plane sapphire substrates by using several defect reduction techniques such as multi-buffer layers and SiNx interlayers [12

12. Y. G. Seo, K. H. Baik, K.-M. Song, S. Lee, H. Yoon, J.-H. Park, K. Oh, and S.-M. Hwang, “Milliwatt-class non-polar a-plane InGaN/GaN light-emitting diodes grown directly on r-plane sapphire substrates,” Curr. Appl. Phys. 10(6), 1407–1410 (2010). [CrossRef]

]. However, the output power was still limited when compared with that of c-plane InGaN/GaN LEDs. The growth issues still remain challenging. And in order to overcome the limitations, additional defect reduction and surface smoothening techniques should be investigated. In this study, we introduce new defect-reduction technique using oxide-patterned sapphire substrates for high quality nonpolar GaN films without any regrowth technique, and report on the enhanced electroluminescence of a-plane InGaN/GaN LEDs employing this technique.

2. Experimental details

Before the GaN growth, the 150nm thick SiO2 was deposited on r-plane sapphire substrates by using plasma-enhanced chemical vapor deposition. Hexagonal patterns were transferred onto SiO2 with conventional photolithography and wet etching process. Patterns were designed to 8 μm width and 2 μm spacing. And then, 6 μm thick a-plane GaN template and 2 μm InGaN/GaN LED structure was grown on the oxide-patterned substrate by using metal organic chemical vapor deposition. Our new fabrication method, the epitaxy on patterned insulator on sapphire substrate (EPISS), is different from conventional c-plane GaN growth in that high-temperature NL is used. While low-temperature NL yields polycrystalline GaN on SiO2 that reduces the crystalline quality, high-temperature NL does not grow on SiO2. EPISS does not need any regrowth process, which has to be done after the patterning process of SiO2 on GaN film in the conventional ELOG process. The a-plane GaN template consists of 4 layers and the growth procedures in detail are as follows. At first, 100 nm thick nucleation layer (NL) was grown in the high temperature of 1050 °C. And then, for 3-dimensional growth, 700 nm thick GaN layer was grown with the conditions of 0.5 nm/s growth rate and 8000 sccm NH3 flow at the growth temperature of 1100 °C. In the third layer, 2-dimensional growth was performed, in which the lateral growth is faster than vertical growth in order to reduce dislocation density and surface roughness. In situ SiNx layer was inserted to improve the crystalline quality between the second and the third layer. Finally, to enhance the coalescence process, we elevated the growth temperature and diminished the reactor pressure and NH3 flow. On the a-plane GaN template, we grew a 1.5-μm Si-doped n-type GaN layer with the doping concentration of ~2 × 1018 cm−3, followed by the active region of single quantum well with 10 nm thick undoped GaN barrier and 9 nm thick InGaN well, followed by a 150 nm thick p-type GaN: Mg layer with the activated hole concentration of ~2 × 1018 cm−3. The optimization of quantum well structures was carried out by changing thickness and number of quantum wells, and we found that 9 nm single quantum well is most effective even compared with 3 nm thickness and 5 periods quantum wells used in conventional c-plane LEDs. This result corresponds to the polarization-free characteristics in nonpolar heterostructures. LED chips with an area of 300 × 600 μm2 were formed through conventional photolithography, followed by inductively coupled plasma dry etching techniques. Ti/Al/Ni/Au and ITO were used as n-type and p-type GaN contacts, respectively.

3. Results and discussion

Coalescence and smoothness of film surface was investigated by using scanning electron microscopy (SEM). Figure 1
Fig. 1 SEM image for the surface of as-grown a-plane GaN template using EPISS, and optical micrograph image of oxide patterns on r-plane sapphire substrate shown in inset.
shows SEM image of as-grown a-plane GaN template grown on the oxide-patterned sapphire substrate. The hexagonal shape and array of oxide patterns on r-plane sapphire substrate are shown in the optical micrograph image of the inset. A fully coalesced a-plane GaN template shows very smooth surface morphology without any observable pits which induce deteriorative effects on device performance. When the growth condition is not optimized, surface pits typically appear on a-plane GaN films [13

13. Q. Sun, B. H. Kong, C. D. Yerino, T.-S. Ko, B. Leung, H. K. Cho, and J. Han, “Morphological and microstructural evolution in the two-step growth of nonpolar a-plane GaN on r-plane sapphire,” J. Appl. Phys. 106(12), 123519 (2009). [CrossRef]

,14

14. M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, “Threading dislocation reduction via laterally overgrown nonpolar (11-20) a-plane GaN,” Appl. Phys. Lett. 81, 1201–1203 (2002). [CrossRef]

]. Since different growth rates of Ga- and N-polar wings together with wing tilt create a major obstacle for achieving a smooth surface, SEM result indicates that the growth conditions of multi-buffer layers used for a-plane GaN template were very appropriate for the growth optimization of EPISS technique.

To verify the enhancement of luminescence properties with improved crystal quality, cathodoluminescence (CL) image and spectrum were measured at room temperature. Figure 2
Fig. 2 The cross-sectional CL image of as-grown a-plane GaN template on oxide-patterned sapphire substrates.
shows cross-sectional composite CL (green) and SEM (red) images for a-plane GaN films on oxide-patterned sapphire substrates. Bright area in the top layer of the GaN template is attributed to the n-type carrier doping. The spectrally integrated CL intensities are consistently low in the window regions where vertical growth takes place, whereas in the laterally overgrown wing regions the integrated CL intensities are significantly higher. Thus, we conclude that the nonradiative recombination processes are reduced in the wing regions. Also since the threading dislocations in GaN layers generally act as nonradiative recombination centers, the analysis of extended defects in wing and window regions will be further studied in transmission electron microscopy (TEM) results.

Figure 3
Fig. 3 CL plan-view image and spectra (inset) for the surface of as-grown a-plane GaN template on oxide-patterned sapphire substrates.
shows a plan-view CL image of the a-plane GaN template grown on the oxide-patterned sapphire substrate. Similar to the cross-sectional CL results, higher CL emission intensity was clearly observed on oxide-patterns. The CL image contrast reveals the planar array of oxide patterns, and high TDs densities between patterns. In the inset of Fig. 3, CL spectrum observed from bright spot (point 1) and dark spot (point 2) shows that the band edge emission intensity of the wing region is four times higher than that of the window region. These CL results clearly indicate that the enhanced a-plane GaN crystal quality on oxide patterns improves luminescence properties.

In Fig. 4(a)
Fig. 4 (a) The cross-sectional TEM image viewed along m-direction with g = 11-20, and (b) the plan-view TEM image (g = 1-100) of a-plane GaN template for wing and window (inset) regions.
, the cross-sectional TEM image with two beam condition (g = 11-20) shows the microstructure evolution of a-plane GaN template grown on an oxide-patterned sapphire substrate. Using g•b extinction rules, when g = 11-20, partial dislocations (b = 1/5<20-23>), several types of threading dislocations, and prismatic stacking faults (R = 1/2 [10

10. A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura, and U. K. Mishra, “Nonpolar InGaN/GaN emitters on reduced-defect lateral epitaxially overgrown a-plane GaN with drive-current-independent electroluminescence emission peak,” Appl. Phys. Lett. 85(22), 5143–5145 (2004). [CrossRef]

,11

11. H. Yamada, K. Iso, M. Saito, H. Hirasawa, N. Fellows, H. Masui, K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Comparison of InGaN/GaN light emitting diodes grown on m-plane and a-plane bulk GaN substrates,” Phys. Status Solidi (RRL) 2(2), 89–91 (2008). [CrossRef]

]) could be in contrast. The dislocation density of GaN layer in wing region significantly reduced as can be expected in CL results. Dislocation densities of wing region and window region were measured to be ~1 × 107 cm−2 and ~3 × 109 cm−2, respectively. Figure 4 (b) shows plan-view TEM images using g = 1-100 of wing and window (inset) regions for a-plane GaN template. Whereas the density of I1-type BSFs in window region was ~2 × 106 cm−1, the density of BSFs in wing region was decreased down to ~5 × 104 cm−1. It is quite interesting to see that the length of BSFs became longer than that in window region of a-plane GaN films, presumably due to reduced density of partial dislocations terminating I1-type BPSF. Also in general, dislocation density is reduced by two orders of magnitude in the wing region of conventional ELOG a-plane GaN films [15

15. C. F. Johnston, M. J. Kappers, M. A. Moram, J. L. Hollander, and C. J. Humphreys, “Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire,” J. Cryst. Growth 311(12), 3295–3299 (2009). [CrossRef]

]. Therefore the capability of defect reduction of EPISS is similar to the ELOG technique. However area proportion of wing region in EPISS is about 75%, which is higher than that in ELOG of about 50% [15

15. C. F. Johnston, M. J. Kappers, M. A. Moram, J. L. Hollander, and C. J. Humphreys, “Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire,” J. Cryst. Growth 311(12), 3295–3299 (2009). [CrossRef]

]. In addition to CL results, TEM results also indicated that the reduction of extended defects density in a-plane GaN template could be achieved by using the EPISS technique. Therefore we suggest that the enhanced crystal quality of a-plane GaN template employing the EPISS technique could contributed to the improvement of nonpolar LEDs performance. Low refractive index materials such as SiO2 patterns and voids near the substrate shown in Fig. 4(a) could partially increase the enhancement of extraction efficiency.

Figure 5
Fig. 5 L-I-V curve as a function of the injection current for nonpolar a-plane InGaN LEDs. The left inset shows EL image of a-plane InGaN LEDs with oxide-patterned sapphire substrates at 20 mA, and the right inset shows EL emission spectrum of a-plane InGaN LED.
shows outstanding optical output powers for the a-plane InGaN LED using EPISS technique, as a function of the injection current measured on wafer-level, where we collected lights from the surface as well as sapphire backside. The optical output power of the a-plane LED was 3.1 mW at the current of 20 mA and a forward voltage of 3.67 V, and 12.6 mW at 100 mA and at 5.37 V. Compared to a-plane LED only using the multi-step buffer layer [12

12. Y. G. Seo, K. H. Baik, K.-M. Song, S. Lee, H. Yoon, J.-H. Park, K. Oh, and S.-M. Hwang, “Milliwatt-class non-polar a-plane InGaN/GaN light-emitting diodes grown directly on r-plane sapphire substrates,” Curr. Appl. Phys. 10(6), 1407–1410 (2010). [CrossRef]

], the optical power of the LED using EPISS is enhanced by 250% at 20 mA and 286% at 100 mA. This result indicates that the EPISS is highly efficient to improve the a-plane LED performance. Corresponding to TEM results, the improvement of crystal quality using EPISS was attributed to the optical power enhancement. In the same measurement system, the output power of the commercialized c-plane InGaN blue LEDs with patterned sapphire substrate (PSS) was 20 mW at 20 mA, and the output power of the LEDs without PSS was 9 mW at 20 mA. In our newly-developed EPISS process, the light extraction efficiency is not much higher than that of InGaN blue LEDs with PSS. We believe that better performance will be obtained by improving the extraction efficiency and solving the lateral current spreading issue which are present in nonpolar and semipolar III-N LEDs on foreign substrates [9

9. K. H. Baik, Y. G. Seo, S.-K. Hong, S. Lee, J. Kim, J.-S. Son, and S.-M. Hwang, “Effects of basal stacking faults on electrical anisotropy of nonpolar a-plane (11-20) GaN light-emitting diodes on sapphire substrate,” IEEE Photon. Technol. Lett. 22(9), 595–597 (2010). [CrossRef]

]. The left inset in Fig. 5 is an EL image of a-plane InGaN LEDs at 20 mA, showing the light scatterings on the SiO2 patterns. The right inset shows the EL spectra at 20 mA with the full-width at half maximum (FWHM) of 25.2 nm and the peak wavelength of 443.3 nm. The FWHM of EL spectrum was significantly reduced by using the SiO2 patterned substrates when compared with the FWHM of ~30 nm of a-plane GaN LEDs with flat sapphire substrates.

4. Conclusion

In this study, we report on the new fabrication method named EPISS for the high quality of a-plane GaN layers grown on r-plane sapphire substrates. Fully coalescence and smooth surface of the a-plane GaN template on the oxide-patterned sapphire substrate were confirmed by the SEM image. Cross-sectional and plan-view CL images showed that luminescence intensity in the wing region of a-plane GaN template was greatly improved compared to that in the window region due to the reduction of TDs in the wing region. Band edge emission intensity of the wing region was four times higher than that of the window region. Reduction of extended defect densities and improved crystal quality were also confirmed by using various TEM analyses. Compared to defect densities in window region, TDs and BSFs densities in wing region decreased from ~3×109 cm−2 and ~2☓106 cm−1 to ~1×107 cm−2 and ~5☓104 cm−1, respectively. Fabricated blue-emitting (443.4 nm) a-plane InGaN LED employing EPISS showed the optical power of 3.1 mW and the EL FWHM of 25.2 nm at the injection current of 20 mA. Since ELOG need thicker GaN films more than 10 μm and take additional fabrication process steps and the cost, our EPISS technique can become the promising solution for the development of commercially available nonpolar LEDs in the near future.

Acknowledgements

This work was supported by the Ministry of Knowledge Economy at Korea Electronics Technology Institute by the IT R&D program (Project No. K1002099)

References and links

1.

F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024–R10027 (1997). [CrossRef]

2.

P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature 406(6798), 865–868 (2000). [CrossRef] [PubMed]

3.

S. Pimputkar, J. Speck, S. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photonics 3(4), 180–182 (2009). [CrossRef]

4.

U. T. Schwarz and M. Kneissl, “Nitride emitters go nonpolar,” Phys. Status Solidi (RRL) 1(3), A44–A46 (2007). [CrossRef]

5.

C. Chen, V. Adivarahan, J. Yang, M. Shatalov, E. Kuokstis, and M. A. Khan, “Ultraviolet light emitting diodes using non-polar a-plane GaN-AlGaN multiple quantum wells,” Jpn. J. Appl. Phys. 42, L1039–L1040 (2003). [CrossRef]

6.

A. Chitnis, C. Chen, V. Adivarahan, M. Shatalov, E. Kuokstis, V. Mandavilli, J. Yang, and M. A. Khan, “Visible light-emitting diodes using a-plane GaN-InGaN multiple quantum wells over r-plane sapphire,” Appl. Phys. Lett. 84(18), 3663–3665 (2004). [CrossRef]

7.

M. Araki, N. Mochimizo, K. Hoshino, and K. Tadatomo, “Effect of misorientation angle of r-plane sapphire substrate on a-plane GaN grown by metalorganic vapor phase epitaxy,” Jpn. J. Appl. Phys. 47(1), 119–123 (2008). [CrossRef]

8.

S. Hwang, Y. G. Seo, K. H. Baik, I. Cho, J. H. Baek, S. Jung, T. G. Kim, and M. Cho, “Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate,” Appl. Phys. Lett. 95(7), 071101 (2009). [CrossRef]

9.

K. H. Baik, Y. G. Seo, S.-K. Hong, S. Lee, J. Kim, J.-S. Son, and S.-M. Hwang, “Effects of basal stacking faults on electrical anisotropy of nonpolar a-plane (11-20) GaN light-emitting diodes on sapphire substrate,” IEEE Photon. Technol. Lett. 22(9), 595–597 (2010). [CrossRef]

10.

A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura, and U. K. Mishra, “Nonpolar InGaN/GaN emitters on reduced-defect lateral epitaxially overgrown a-plane GaN with drive-current-independent electroluminescence emission peak,” Appl. Phys. Lett. 85(22), 5143–5145 (2004). [CrossRef]

11.

H. Yamada, K. Iso, M. Saito, H. Hirasawa, N. Fellows, H. Masui, K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Comparison of InGaN/GaN light emitting diodes grown on m-plane and a-plane bulk GaN substrates,” Phys. Status Solidi (RRL) 2(2), 89–91 (2008). [CrossRef]

12.

Y. G. Seo, K. H. Baik, K.-M. Song, S. Lee, H. Yoon, J.-H. Park, K. Oh, and S.-M. Hwang, “Milliwatt-class non-polar a-plane InGaN/GaN light-emitting diodes grown directly on r-plane sapphire substrates,” Curr. Appl. Phys. 10(6), 1407–1410 (2010). [CrossRef]

13.

Q. Sun, B. H. Kong, C. D. Yerino, T.-S. Ko, B. Leung, H. K. Cho, and J. Han, “Morphological and microstructural evolution in the two-step growth of nonpolar a-plane GaN on r-plane sapphire,” J. Appl. Phys. 106(12), 123519 (2009). [CrossRef]

14.

M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, “Threading dislocation reduction via laterally overgrown nonpolar (11-20) a-plane GaN,” Appl. Phys. Lett. 81, 1201–1203 (2002). [CrossRef]

15.

C. F. Johnston, M. J. Kappers, M. A. Moram, J. L. Hollander, and C. J. Humphreys, “Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire,” J. Cryst. Growth 311(12), 3295–3299 (2009). [CrossRef]

OCIS Codes
(230.0230) Optical devices : Optical devices
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Optical Devices

History
Original Manuscript: August 2, 2011
Revised Manuscript: October 12, 2011
Manuscript Accepted: October 16, 2011
Published: October 28, 2011

Citation
Sung-Min Hwang, Hooyoung Song, Yong Gon Seo, Ji-Su Son, Jihoon Kim, and Kwang Hyeon Baik, "Enhanced electroluminescence of a-plane InGaN light emitting diodes grown on oxide-patterned r-plane sapphire substrates," Opt. Express 19, 23036-23041 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-23-23036


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References

  1. F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B56(16), R10024–R10027 (1997). [CrossRef]
  2. P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature406(6798), 865–868 (2000). [CrossRef] [PubMed]
  3. S. Pimputkar, J. Speck, S. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photonics3(4), 180–182 (2009). [CrossRef]
  4. U. T. Schwarz and M. Kneissl, “Nitride emitters go nonpolar,” Phys. Status Solidi (RRL)1(3), A44–A46 (2007). [CrossRef]
  5. C. Chen, V. Adivarahan, J. Yang, M. Shatalov, E. Kuokstis, and M. A. Khan, “Ultraviolet light emitting diodes using non-polar a-plane GaN-AlGaN multiple quantum wells,” Jpn. J. Appl. Phys.42, L1039–L1040 (2003). [CrossRef]
  6. A. Chitnis, C. Chen, V. Adivarahan, M. Shatalov, E. Kuokstis, V. Mandavilli, J. Yang, and M. A. Khan, “Visible light-emitting diodes using a-plane GaN-InGaN multiple quantum wells over r-plane sapphire,” Appl. Phys. Lett.84(18), 3663–3665 (2004). [CrossRef]
  7. M. Araki, N. Mochimizo, K. Hoshino, and K. Tadatomo, “Effect of misorientation angle of r-plane sapphire substrate on a-plane GaN grown by metalorganic vapor phase epitaxy,” Jpn. J. Appl. Phys.47(1), 119–123 (2008). [CrossRef]
  8. S. Hwang, Y. G. Seo, K. H. Baik, I. Cho, J. H. Baek, S. Jung, T. G. Kim, and M. Cho, “Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate,” Appl. Phys. Lett.95(7), 071101 (2009). [CrossRef]
  9. K. H. Baik, Y. G. Seo, S.-K. Hong, S. Lee, J. Kim, J.-S. Son, and S.-M. Hwang, “Effects of basal stacking faults on electrical anisotropy of nonpolar a-plane (11-20) GaN light-emitting diodes on sapphire substrate,” IEEE Photon. Technol. Lett.22(9), 595–597 (2010). [CrossRef]
  10. A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura, and U. K. Mishra, “Nonpolar InGaN/GaN emitters on reduced-defect lateral epitaxially overgrown a-plane GaN with drive-current-independent electroluminescence emission peak,” Appl. Phys. Lett.85(22), 5143–5145 (2004). [CrossRef]
  11. H. Yamada, K. Iso, M. Saito, H. Hirasawa, N. Fellows, H. Masui, K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Comparison of InGaN/GaN light emitting diodes grown on m-plane and a-plane bulk GaN substrates,” Phys. Status Solidi (RRL)2(2), 89–91 (2008). [CrossRef]
  12. Y. G. Seo, K. H. Baik, K.-M. Song, S. Lee, H. Yoon, J.-H. Park, K. Oh, and S.-M. Hwang, “Milliwatt-class non-polar a-plane InGaN/GaN light-emitting diodes grown directly on r-plane sapphire substrates,” Curr. Appl. Phys.10(6), 1407–1410 (2010). [CrossRef]
  13. Q. Sun, B. H. Kong, C. D. Yerino, T.-S. Ko, B. Leung, H. K. Cho, and J. Han, “Morphological and microstructural evolution in the two-step growth of nonpolar a-plane GaN on r-plane sapphire,” J. Appl. Phys.106(12), 123519 (2009). [CrossRef]
  14. M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, “Threading dislocation reduction via laterally overgrown nonpolar (11-20) a-plane GaN,” Appl. Phys. Lett.81, 1201–1203 (2002). [CrossRef]
  15. C. F. Johnston, M. J. Kappers, M. A. Moram, J. L. Hollander, and C. J. Humphreys, “Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire,” J. Cryst. Growth311(12), 3295–3299 (2009). [CrossRef]

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