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

In this study, we demonstrated large-area high-quality multi-color emission from the 12-fold symmetric GaN photonic quasicrystal nanorod device which was fabricated using the nanoimprint lithography technology and multiple quantum wells regrowth procedure. High-efficiency blue and green color emission wavelengths of 460 and 520 nm from the regrown InxGa1−xN/GaN multiple quantum wells were observed under optical pumping conditions. To confirm the strong coupling between the quantum well emissions and the photonic crystal band-edge resonant modes, the finite-element method was applied to perform a simulation of the 12-fold symmetry photonic quasicrystal lattices.


Background
The GaN-based materials with the wide band gap and unique properties had been applied in many optoelectronic systems and devices, including light emitting diodes (LEDs) [1][2][3] and laser diodes (LDs) [4,5]. The GaN-based LEDs have been applied in traffic signals, display backlights [6][7][8], solid-state lighting [9,10], biosensors [11], and optogenetics [12]. One of the challenges for the advanced GaN LEDs is to realize the phosphor-free white LEDs, including multichip white LEDs, monolithic LEDs, and color-conversion white LEDs [13,14]. GaNbased nanorod LED with low dislocation, low internal field, and high light extraction efficiency [15,16] could be a possible solution. Various approaches have been employed to increase the light extraction efficiency for III-nitride LEDs, such as rough surfaces [17][18][19][20], sapphire microlenses [21], oblique mesa structure [22], nanopyramids [23], graded refractive index materials [24], self-assembled lithography patterning [25], colloidalbased microlens arrays [26,27], and photonic crystals [28][29][30][31]. Photonic crystals have been reported in quasicrystal or defective two-dimensional (2D) grating configurations and lead to improved light extraction efficiency 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]. E-beam lithography and laser interference lithography have been used to produce the photonic crystal structure [43,44] light can be mixed effectively. Thus, nanorods are considered to have a great advantage for improving the luminous efficiency in the green-to-red emission region, and numerous efforts have been adopted [45,46]. However, nanoimprint lithography (NIL) offers highlevel resolution, low-cost, and high throughput compared with other forms of lithography including laser interference and e-beam lithography [47][48][49]. 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 [41,42]. The total area of the PQC pattern is approximately 4 cm × 4 cm(2-in. sapphire substrate) and possessed 12-fold symmetry [50,51], 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 com- 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 [41,42]. 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 multiple-color GaN-based lasers can be expected with the optimization of regrowth procedure and the high-quality photonic crystal cavity. Figure 2 illustrates the schematic procedures of the device fabrication. The fabrication procedures included epitaxial growth of a GaN wafer, NIL of PQC patterns, and dry etching. The GaN-based material was grown in a low-pressure metalorganic chemical vapor deposition reactor on a C-plane (0001) sapphire substrate. To prepare a clean surface of the sapphire substrate, the substrate was immersed into a burning solution of sulfuric acid: phosphoric acid = 3:1, then heat the beaker to a constant temperature for 1 h. The substrate was cleaned with DI water under ultrasonic oscillation. A GaN (1-μm thick) was first grown on a 2-inch sapphire substrate at 1160 °C. A 0.4-μm SiO 2 mask and 0.2-μm polymer mask were then deposited. After the polymer film was dry, a patterned mold of a 2-inch PQC structure was placed onto it by applying high pressure (Fig. 2. step 1). The substrate was heated to higher than the polymer's glass transition temperature (T g ). The substrate and the mold were then cooled to room temperature to release the mold. The PQC patterns were defined on the polymer layer (Fig. 2, step 2). The patterns were then transferred into a SiO 2 layer with reactive ion etching (RIE) by using a CHF 3 /O 2 mixture (Figure, step 3). The SiO 2 layer was used as a hard mask. The structure was then etched using inductively coupled plasma RIE with a Cl 2 /Ar mixture. The mask of SiO 2 layer was removed at the end of the etching process (Fig. 2, step 4).

Design and Fabrication of Sample
Before the regrowth process, the sample was passivated with porous SiO 2 at the sidewall of the nanopillars. The pyramid-shaped GaN structures were regrown on top of the GaN nanopillars at 730 °C. The 0.43-μm-high pyramids contained 10-pair In x Ga 1-x N/GaN (3 nm/12 nm) quantum wells, which supported different wavelengths of blue and green color emission, with the ratio of in composition: In x Ga 1−x N/GaN-dependent InN fraction variations. In 0.1 Ga 0.9 N/GaN MQWs and In 0.3 Ga 0.7 N/ GaN MQWs corresponded to 460-and 520-nm emission wavelengths, respectively (Fig. 2, step 5). The etch depth of the nanorods was approximately 1 μm, as illustrated in Fig. 3a. Figure 3b  Step 1 Step 2 S tep 3 Step

Results and Discussion
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 fiber, and coupled into a spectrometer with charge-coupled device detectors. Figure 4a 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 366 nm from the GaN-based PQC structure displayed in Fig. 3a. Both samples A (blue curve) and B (green curve) had a strong emission peak which corresponded to wavelengths of approximately 460 and 520 nm, respectively, resulting from the In x Ga 1-x N/GaN MQWs structure. The spectrum linewidths of the samples A and B were 40 and 60 nm, respectively. Figure 4a also displays photographs of the PQC structure of samples A and B during measurement. The CIE coordinates of PL from samples A and B were (0.19, 0.38) and (0.15, 0.07), respectively, as illustrated in Fig. 4b. 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. 4a. The slight broad spectrum from the sample B was attributed to the existence of defects and dislocations generated by the higher indium composition [56][57][58].
In order to confirm the optical resonant modes were the PQC band-edge modes, the finite-element method (FEM) [59,60] 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 500nm 500nm  Fig. 5a are presented in Fig. 5b. 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) indicates 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 high-transmission and low-transmission regimes in Fig. 5b.  [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, efficient 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.

Conclusions
In summary, a 12-fold symmetric GaN PQC nanopillars was fabricated using the NIL technology. High-efficiency blue and green color emissions from In