Multicolor Emission from Ultraviolet GaN-based Photonic Quasicrystal Nanopyramid Structure with Semipolar InxGa1-xN/GaN Multiple Quantum Wells

Cheng-Chang Chen (  chencc@itri.org.tw ) Industrial Technology Research Instutude Hsiang-Ting Lin Research Center for Applied Sciences, Academia Sinica, Taipei Shih-Pang Chang Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University Hao-Chung Kuo Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University Hsiao-Wen Hung Energy and Environment Research Laboratories, Industrial Technology Research Institute Kuo-Hsiang Chien Energy and Environment Research Laboratories, Industrial Technology Research Institute Yu-Choung Chang Energy and environment Research Laboratories, Industrial Technology Research Institute Min-Hsiung Shih Academia Sinica

increasing the e ciency of the light source is imperative because of its multitude of potential applications. GaN-based nanorods possess low dislocation, low internal eld, and high light extraction e ciency, which are the factors intrinsic in improving photoluminescence (PL) intensity. [15,16] Various approaches have been employed to increase the light extraction e ciency for III-nitride LEDs, such as rough surfaces, [17][18][19][20] sapphire microlenses, [21] oblique mesa sidewalls, [22] nanopyramids, [23] graded refractive index materials, [24] self-assembled lithography patterning, [25] colloidal-based microlens arrays, [26,27] and photonic crystals. [28][29][30][31] Photonic crystals have been reported in quasicrystal or defective two-dimensional (2D) grating con gurations, and lead to improved light extraction e ciency in LEDs. [32][33][34][35] The photonic crystal structure is periodic with translational symmetry. The periodic structure can exhibit a photonic band gap to inhibit the propagation of guided modes and uses a photonic crystal structure to couple guided modes with radiative modes. [36][37][38][39] Photonic crystal lasers based on the band-edge effect have several advantages, such as high-power emissions, single mode operation, and coherent oscillation. [40][41][42][43] E-beam lithography and laser interference lithography have been used to produce the photonic crystal structure. [44,45] Furthermore, because the emitting units are separated and the emission surfaces face each other, the light can be mixed effectively. Thus, nanorods are considered to have a great potential for improving the luminous e ciency in the green-to-red emission region, and numerous efforts have been adopted. [46,47] However, nanoimprint lithography (NIL) offers high-level resolution, low-cost, and high throughput compared with other forms of lithography including laser interference and e-beam lithography. [48][49][50] In this study, we demonstrated the multiple color emission from a GaN-based 2D photonic quasicrystal (PQC) structure as illustrated in Fig. 1.The PQC structure was fabricated using NIL. [41,43] The total area of the PQC pattern is approximately 4 cm x 4 cm(2-in. sapphire substrate) and possessed 12-fold symmetry, [51,52] with a lattice constant of approximately 750 nm, a diameter of 300 nm and the depth of the nanopillars is approximately 1 μm. The PQC structure formed a complete band gap with the regrowth of 430-nm-tall GaN pyramids and 10-pair semipolar {10-11} In x Ga 1-x N/GaN (3 nm/12 nm) multiple quantum well (MQW) nanostructures, as illustrated in Fig. 1.
Under room temperature pumping operation, the device demonstrates laser action with a low threshold power density and the multiple color emission simultaneously. We had reported the single color laser action from the GaN PQC structure. [41,43] This PQC platform exhibits the advantages in low fabrication costs, and better integration of GaN-based material with multi-color systems. In the future, the multiplecolor GaN-based lasers can be expected with the optimization of regrowth procedure and the high quality photonic crystal cavity. of the quantum-con ned Stark effect on the quantum e ciency of LEDs due to the surface stability and suppression of polarization effects. [53][54][55][56] To study the optical properties of the GaN-based PQC with nanopyramid structure, two GaN PQC samples were prepared: A, In 0.1 Ga 0.9 N/GaN MQWs, and B, In 0.3 Ga 0.7 N/GaN MQWs with regrowth fabrication.
During the regrowth step, the temperature is the key to control the ratio of indium composition. The control temperature of blue In 0.1 Ga 0.9 N is 760~780°C and the control temperature of green In 0.3 Ga 0.7 N is 730~740°C. To demonstrate the optical mode from the photonic quasicrystal structure, samples A and B were optically pumped by a continuous-wave (CW) He-Cd laser at 325 nm with an incident power of approximately 50 mW. The light emission from the device was collected by a 15× objective lens through a multimode ber, and coupled into a spectrometer with charge-coupled device detectors. Figure 4(a) illustrates the measured PL spectra under He-Cd 325 nm CW laser pumping. The spectrum of the black curve is the light emission with a wavelength of 366nm from the GaN-based PQC structure displayed in respectively, as illustrated in Fig. 4(b). Thus, this hybrid platform has several possibilities for multicolor LEDs. It should be note that the peak of the sample B is broader than the one of sample A in Fig. 4(a). The slight broad spectrum from the sample B was attributed to the existence of defects and dislocations generated by the higher indium composition [57-59].
In order to con rm the optical resonant modes were the PQC band-edge modes, the nite-element method (FEM)[60, 61] was used to perform a simulation for the 12-fold symmetry photonic quasicrystal lattices. The calculated transmission spectra of the PQC with incident angles along with 0, 5°, 10°, 15°, 20°, and 25° as indicated in Fig. 5(a) was presented in Fig. 5(b). Due to the symmetry of this PQC lattices, the spectra would repeat for every 30° incident angle. The high transmission value in the spectra (blue color) indicate that the incident signal coupled into the PQC lattice resonant modes which are the band diagram areas. The low transmission (yellow color) regions indicate several photonic band gaps (PBGs) of the PQC structure. The ratio of high-to-low transmission is more than four order which show the PQC lattices take the strong effect to select the propagation modes in the device. The observed lasing actions occur around the band-edges of the PQC bandstructure, which are the boundaries between the hightransmission and low-transmission regimes in the Fig. 5(b). The at dispersion curve near the band-edge implies a low group velocity of light and strong localization, and lead to the lasing actions of the devices.  [43,45]. For the regrown In 0.1 Ga 0.9 N and In 0.3 Ga 0.7 N which coupled to M 2 and M 1 , the emission blue and green light would be boosted. Therefore, leveraging the coupling between the optical modes of PQC structure and In x Ga 1-x N/GaN, e cient multicolor LEDs, LDs could be realized in such hybrid platform. The length of the nanorods in photonic crystal lattices is also important to generate the high quality color enhancement. In this study, in order to achieve high quality color enhancement, the photonic crystal nanorod length was etched to 1000 nm which is more than four times of the effective wavelength. To realize the multicolor emission from a single PQC device in the future, the multiple regrowth procedures should be added in the epitaxial process.
In summary, a 12-fold symmetric GaN PQC nanopillars was fabricated using the NIL technology. High-

Competing interests
The authors declare that they have no competing interests.
Authors' contributions CCC participated in the design of the study and measured the optical properties and drafted the manuscript. HTL calculated transmission spectrum of the 12-fold symmetry photonic quasicrystal lattices by FEM and helped to draft the manuscript. SPC carried out the study of ultraviolet GaN-based photonic quasicrystal nanopyramid structure and drafted the manuscript. HWH, KHC and YCC helped to fabricate the process of nanoimprint lithography, analyzed the optical properties, and helped to draft the manuscript. MHS and HCK conceived of the study, participated in its design and coordination, and helped draft the manuscript. All authors read and approved the nal manuscript.