Large-area ultraviolet GaN-based photonic quasicrystal laser with high-efficiency green
color emission of semipolar {10-11} In0.3Ga0.7N/GaN multiple quantum wells
Cheng-Chang Chen, Ching-Hsueh Chiu, Shih-Pang Chang, M. H. Shih, Ming-Yen Kuo, Ji-Kai Huang, Hao-Chung Kuo, Shih-Pu Chen, Li-Ling Lee, and Ming-Shan Jeng
Citation: Applied Physics Letters 102, 011134 (2013); doi: 10.1063/1.4775373 View online: http://dx.doi.org/10.1063/1.4775373
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/1?ver=pdfcov Published by the AIP Publishing
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Large-area ultraviolet GaN-based photonic quasicrystal laser with
high-efficiency green color emission of semipolar {10-11}
In
0.3Ga
0.7N/GaN multiple quantum wells
Cheng-Chang Chen,1Ching-Hsueh Chiu,2Shih-Pang Chang,2M. H. Shih,2,3,a) Ming-Yen Kuo,3Ji-Kai Huang,2Hao-Chung Kuo,2Shih-Pu Chen,1Li-Ling Lee,1 and Ming-Shan Jeng1
1
Green Energy and Environment Research Laboratories, Industrial Technology Research Institute (ITRI), 195, Sec. 4, Chung-Hsin Road, Chutung 310, Taiwan
2
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
3
Research Center for Applied Sciences (RCAS), Academia Sinica 115, Taiwan
(Received 9 October 2012; accepted 21 December 2012; published online 11 January 2013) In this study, a multi-color emission was observed from the large-area GaN-based photonic quasicrystal (PQC) nanopillar laser. The GaN PQC nanostructure was fabricated on an n-GaN layer by using nanoimprint lithographic technology. The regrown InGaN/GaN multiple quantum wells (MQWs) formed a nanopyramid structure on top of the PQC nanopillars. A lasing action was observed at ultraviolet wavelengths with a low threshold power density of 24 mJ/cm2, and a green color emission from InGaN/GaN MQWs was also achieved simultaneously. VC 2013 American
Institute of Physics. [http://dx.doi.org/10.1063/1.4775373]
In recent years, GaN-based material and high-brightness GaN-based light-emitting diodes (LEDs)1,2have attracted sig-nificant attention because of their wide and direct band gap. In particular, significant efforts have been made to improve the quality of GaN-based LEDs because of their uses, such as in laser diodes,3–5 solid-state lighting,6 display technology, traffic signals, color printing, and optical storage.
However, the performance of the light source must been improved, and high extraction efficiencies must be increased. Recent approaches based on surface roughness increase,7,8 colloidal-based microlens arrays,9,10 colloidal-based micro-structures arrays,11,12 and photonic crystals13–16 had been reported in quasi-crystal or defective 2-D grating configura-tions, which in turn leads to improved light extraction effi-ciency in LEDs. The photonic crystal structure has a periodic structure with translational symmetry. The periodic structure can exhibit a photonic band gap (PBG) to inhibit the propa-gation of guided modes17and use a photonic crystal structure to couple guided modes with radiative modes.18–20Photonic crystal lasers based on the band-edge effect21–23 possess many advantages, such as high-power emissions, single-mode operation, and coherence oscillation over large areas. E-beam lithography24 and laser interference lithography25 have been used to fabricate the photonic crystal structure. However, when comparing the two methods to nanoimprint lithography (NIL), NIL is suitable for mass production of LED devices because of its good resolution and higher throughput with low fabrication costs. This study demon-strates the GaN-based 2D photonic quasicrystal (PQC) struc-ture with the regrowth of GaN pyramids and 10-pair semipolar f10-11g InxGa1xN/GaN (3 nm/12nm) multiple
quantum well (MQW) nanostructures, as shown in Fig.1(a). In addition to the use of semi/non-polar QWs,26,27approaches by using various polar QWs with large optical matrix element
designs28–32 had also been reported for suppressing charge separation and improving optical gain in III-nitride QWs. Ex-perimental results show that the device contains lasing action and different color emission simultaneously. This laser struc-ture can be a potential platform to achieve a multi-color emis-sion from a single device, which benefits future lighting and detection applications. Large-area PQC patterns were defined by the NIL,33such as the PQC structures possessing 12-fold symmetry,33,34 and forming a complete band gap. The regrowth of GaN nanopyramids with semipolar f10-11g InxGa1xN/GaN MQW is grown by selective area growth
(SAG).35–37Different wavelength emissions have been stud-ied by changing the ratio of In composition of InxGa1xN/
GaN, or by controlling the growth rate of InxGa1xN/GaN
MQWs on the facets of trapezoid microstructures. The semi-polar f10-11g planes can also reduce the influence of the quantum-confined Stark effect on the quantum efficiency of LEDs because of the surface stability and suppression of polarization effects.38,39
The GaN-based material was grown by a low-pressure metal-organic chemical vapor deposition (MOCVD) system. A 2 -lm-thick GaN layer was first grown on a 2-in. C-plane (0001) sapphire substrate. The GaN contained 1 lm undoped GaN and 1 lm n-type GaN and were grown at 1150C and 1160C, respectively. The photonic crystal patterns were formed using an NIL technique. First, a 400 nm SiO2 layer
and a 200 nm polymer layer were deposited as the masks dur-ing the process (step 1 of Fig.1(b)). A patterned mold of the photonic crystal structure was then placed onto the dried poly-mer film (step 2 of Fig.1(b)). Under high pressure, the sub-strate was heated over the glass transition temperature (Tg) of
the polymer. The substrate and the mold were then cooled to room temperature to release the mold (step 3 of Fig.1(b)).
After defining the photonic crystal patterns on the poly-mer layer, the patterns were transferred into the SiO2layer
by reactive ion etching (RIE) with a CHF3/O2mixture and
a)
into the GaN layer by inductively coupled plasma etching (ICP) with a Cl2/Ar mixture. The mask layers were removed
at the end of the processes. Before the regrowth process, the sample was passivated with porous SiO2at the sidewall. The
pyramid-shaped n-type GaN structures were regrown on top of the GaN nanopillars at 730C. The 450-nm-tall pyramids contain 10-pair In0.3Ga0.7N/GaN (3 nm/12 nm) quantum
wells, which support an emission of approximately 500 nm in wavelength. The fabrication procedure is shown in Fig.
1(b). The total area of the PQC pattern is approximately 4 cm 4 cm with a lattice constant of (a) approximately 460 nm and a diameter of 350 nm. The etch depth of the nanopillars is approximately 1 lm, as shown in Fig.2(a)top view and (b) angle view. Fig. 2(c) shows the SEM image of the PQC structure with a semipolar f10-11g In0.3Ga0.7
N/GaN MQW cross-sectional view. To study the optical properties of the GaN-based PQC laser, two GaN PQC sam-ples (samsam-ples A and B) were prepared. Sample B is a GaN-based PQC structure with an In0.3Ga0.7N/GaN MQW
regrowth procedure, and Sample A has an identical PQC
structure without the regrowth step. The devices were opti-cally pumped by using a frequency-tripled Nd: YVO4 355 nm pulsed laser with a pulse width of 0.5 ns and a repeti-tion rate of 1 kHz. The light emission from the device was collected by a 15 objective lens through a multimode fiber, and coupled into a spectrometer with charge-coupled device (CCD) detectors. Fig.3(a)shows the measured spectra from the PQC pattern of Sample A (blue curve) and Sample B (green curve) above the threshold and photoluminescence (blue-dashed curve) of GaN. The lasing action was observed at 366 nm wavelength because of the distributed feedback of light at the photonic band edge of the PQC structure.22The threshold power density of Sample A is approximately 9.0 W/cm2. This ultralow threshold, which is one of lowest reported thresholds for GaN lasers, indicates the strong enhancements from PQC lattices. Sample B has a lasing-mode approximating a wavelength of 366 nm resulting from the PQC structure. The color emission resulting from InxGa1xN/GaN MQW is located in the green range.
Fig.3(b)shows the light-in light-out (L-L) curve of the GaN PQC laser from sample B. The threshold power density of sample B is approximately 24 W/cm2. The value corre-sponds to a threshold power density of 24 mJ/cm2. The threshold of the PQC structure in sample B is higher than the threshold of the structure in sample A. This difference is mainly attributed to geometrical changes of GaN nanopillars and absorption of top InGaN/GaN MQW. Another strong emission of approximately 500 nm wavelength was observed in the PQC structure in sample B. This green light, which is emitted from the top of InGaN/GaN MQW, is shown in Fig.
4(b). When the pump power is over the threshold of UV las-ing, the intensity of the green emission is clamped. It is a sign of UV lasing in this multi-color GaN PQC structure. The UV lasing action was supported by GaN in the nanopil-lars. While the green luminescence was attributed to the absorption from optical pump lasers as well as UV emission from the nanopillars. To have a clear spectrum for the green emission, samples A and B were optically pumped using a continuous-wave (CW) He-Cd laser at 325 nm with an inci-dent power of 48 mW. The measured setup is the same as the micro-PL setup previously described. Fig.4(a)shows the measured PL spectra under He-Cd 325 nm CW laser pump-ing. When comparing sample B (green curve) with sample A (black curve), a strong emission peak was observed at a wavelength of approximately 500 nm resulting from the In0.3Ga0.7N/GaN MQWs structure. The spectrum linewidth
is approximately 60 nm. The results show possible applica-tions for LEDs. Fig.4(b)shows the photography of the PQC structure of sample B during measurement. The white light region is caused the pumping light source of the He-Cd 325 nm CW laser. The UV lasing and green emission modes are corresponded to the band-edge resonant modes of the GaN PQC structure22and could be manipulated by photonic crystal geometry and regrowth InGaN/GaN materials. This hybrid platform generates many possibilities for multi-color LEDs or multi-color lasers systems.
In summary, a 12-fold symmetric GaN PQC nanopillar structure was fabricated using NIL technology. The ultravio-let (UV) lasing action was observed at an approximate 366 nm wavelength with an ultralow threshold power density
FIG. 1. (a) Schematic structure of the GaN-based PQC structure with the regrowth of semipolar{10-11} GaN pyramids and 10-pair In0.3Ga0.7N/GaN
(3 nm/12 nm) MQW; (b) Illustrations of the fabrication process of nanoim-print technology and the regrowth process.
of 9.0 kW/cm2corresponding to the threshold energy density of 9 mJ/cm2. With the regrowth procedure of the top InxGa1xN/GaN MQWs by SAG, UV lasing from GaN PQC
and high-efficiency green color emissions from InxGa1x
N/GaN MQW were simultaneously achieved. These methods of fabrication demonstrated high potential in low fabrication costs, excellent techniques in fabricating semipolarf10-11g InxGa1xN/GaN LEDs, and better integration of GaN-based
FIG. 3. (a) The measured spectrum of samples A and B above threshold. The lasing wavelength is 366 nm. (b) The light-in light-out (L-L) curve and linewidth narrowing of sample B.
FIG. 4. (a) PL spectra from the PQC structure of sample A (black) and sam-ple B (green) under He-Cd 325 nm CW laser pumping. (b) The photography of the PQC structures on sample B during the experiment, and the white light on the center caused by the pumping light source of the He-Cd laser.
FIG. 2. (a) The top-view and (b) the angle-view SEM images of the PQC structure. (c) The cross-sectional SEM image of the PQC structure after the regrowth procedure.
photonic crystal lasers and multi-color light sources in future applications.
The authors are grateful to National Chiao Tung Univer-sity’s Center for NanoScience and Technology. The study is supported by the Bureau of Energy, Ministry of Economics Affairs of Taiwan, and the National Science Council (NSC) of the Republic of China, Taiwan under Contract No. NSC 99-2112-M-001-003-MY3.
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