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a-Plane GaN light emitting diodes on self-assembled Ni nano-islands

a-Plane GaN light emitting diodes on self-assembled Ni nano-islands
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  Lester Eastman Conference on High Performance Devices (LEC), 2012, pp.1-2, 7-9 Aug. 2012,  a -Plane GaN Light Emitting Diodes on Self-Assembled Ni Nano-Islands Xiaoli Wang, Wenting Hou, Liang Zhao, Shi You, Theeradetch Detchprohm and Christian Wetzel Future Chips Constellation and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A.  Nitride-based light emitting diodes (LEDs) have received great attention with emission wavelength from ultraviolet to green. However, conventional c -plane GaN LEDs suffer from polarization-related electric fields, which result in poor carrier recombination efficiency and red-shift in the emission wavelength. In order to reduce the undesirable polarization effects, non-polar plane GaN LEDs have been proposed and reported. But the non-polar films also suffer from poor crystalline quality and rough surface morphology, especially for hetero-epitaxy. Several research groups have reported high performance non-polar GaN-based LEDs grown on  bulk GaN substrates[1]. However, high price, small size, and non-uniformity in the substrates also make them difficult for mass production. On the other hand, nano-patterned a -plane GaN on sapphire substrate has been proposed as a solution. It is thought of as a method to achieve low cost, large area, and high uniformity a -GaN on sapphire. In this paper, self-assembled Ni nano-islands[2] are applied to form a -plane GaN nanorod template. We also grew blue and green[3] nano-patterned a -plane GaN LEDs on those nanorod templates. a -Plane GaN templates were first grown on r  -plane sapphire substrates. Then a -plane GaN nanorods were fabricated as follows. First, a SiO 2  film was deposited on the GaN surface by plasma-enhanced chemical vapor deposition (PECVD), which is followed by electron-beam evaporation of a Ni metal film. Then, the samples are rapid thermal annealed (RTA) to form self-assembled nano-sized Ni islands. A reactive ion etch (RIE) was used to transfer the nano-sized patterned defined by the Ni islands into the SiO 2  film. The Ni/SiO 2  pillars then serve as a mask to etch the GaN layer to form GaN nanorods by inductively coupled  plasma-reactive ion etching (ICP-RIE). Fig. 1 shows a scanning electron microscope (SEM) image of the etched sample. The density of the GaN nanorods is on the order of ~1×10 9  cm 2  with a diameter range of 250 to 400 nm and a height range of 850 to 950 nm. The Ni was removed by nitric acid to expose the SiO 2 /GaN nanorods, and the samples were re-loaded into the reactor for a -GaN re-growth. Fig. 2 shows a cross-sectional SEM image of a coalesced a -plane GaN film on a nanorod template. We find two kinds of voids of different sizes at the interface between nanorods and re-grown a -GaN. It is likely that the bigger voids stem from the nanorods, and the smaller ones are related to the SiO 2  mask where the a -GaN did coalesce. High-resolution X-ray diffraction (XRD) measurements were performed to evaluate the crystalline quality of the a - plane GaN samples. We define the azimuth angle as 0° when the projection of the incident beam is parallel to the m -axis, while 90° corresponds to the c -axis. The FWHM values of the [11-20] XRD ω  scans as a function of azimuth angle are shown in Fig. 3. Comparing to the traditional a -GaN grown on r  -plane sapphire, our re-grown a -GaN on the nanorod template eliminates the anisotropy of m - and c - axes of a -GaN, and improves the crystalline quality of a -GaN. Blue and green a -plane GaN LED structures were grown on the nanorod template to demonstrate the advantage of the re-grown a -plane GaN. For comparison, a -plane GaN LEDs with the same structures on low-dislocation-density bulk a -GaN substrate were also prepared. The Fig. 4 shows the LED  partial light output power on nanorod template and bulk a -GaN with different peak wavelengths at a current density of 12.7 A/cm 2 . It is shown that the partial light output powers of  both types of LEDs are almost identical, although with somewhat different peak wavelengths. So apparently, our approach of nanorod formation on a -plane GaN on r  -plane sapphire leads to light output powers that compare well with those on bulk a -plane GaN. In conclusion, we have demonstrated the improvement of the crystalline quality of re-grown a -GaN on GaN nanorod templates and employed these templates for re-grown blue and green a -plane GaN LED structures. The partial light output power of nano-patterned a -plane GaN LED and low-dislocation-density bulk a -plane GaN LED are almost the same for similar wavelengths at a current density of 12.7 A/cm 2 .  Fig. 1 Cross-section SEM image of a-plane GaN nanorod template  Lester Eastman Conference on High Performance Devices (LEC), 2012, pp.1-2, 7-9 Aug. 2012,  Fig. 2 Cross-section SEM image of coalesced re-grown a- plane GaN film 0501001500.  a  -plane GaN on r  -plane sapphire substrate re-growth a  -plane GaN on GaN nanorods template c   axis m   axis    F   W   H   M   (   d  e  g   ) Phi (deg)   Fig. 3 The FWHM of the [11-20] XRD ω  scans as a function of azimuth angle   4004505005500.11current density 12.7 A/cm2   Nano-patterned LEDs with a  -GaN nanorod Bulk LEDs on bulk a  -GaN substrate     P  a  r   t   i  a   l   L   i  g   h   t   O  u   t  p  u   t   P  o  w  e  r   (  m   W   ) Peak Wavelength (nm)    Fig. 4 The partial light output power versus the peak wavelength  A CKNOWLEDGMENT  This work was supported by a United States of America DOE/NETL Solid-State Lighting Contract of Directed Research under DE-FC26-06NT42860. This work was also supported by the National Science Foundation (NSF) Smart Lighting Engineering Research Center (No. EEC-0812056). R  EFERENCES   [1]   Theeradetch Detchprohm, Mingwei Zhu, Yufeng Li, Yong Xia, Christian Wetzel, Edward A. Preble, Lianghong Liu, Tanya Paskova, and Drew Hanser, “Green light emitting diodes on a -plane GaN bulk substrates,”Appl. Phys. Lett., vol. 92, p. 241109, 2008. [2]   K. Xing, Y. Gong, J. Bai, and T. Wang, “InGaN/GaN quantum well structures with greatly enhanced performance on a -plane GaN grown using self-organized nano-masks,” Appl. Phys. Lett., vol. 99, p.181907, 2011. [3]   Yufeng Li, Shi You, Mingwei Zhu, Liang Zhao, Wenting Hou, T. Detchprohm, Y. Taniguchi, N. Tamura, S. Tanaka, and C. Wetzel, “Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire,” Appl. Phys. Lett., vol. 98, p. 151102, 2011.
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