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Multi-color broadband visible light source via three-dimensional GaN hexagonal annular microstructures

Broadband visible light emitting, three-dimensional hexagonal annular microstructures with InGaN/GaN multiple quantum wells (MQWs) are fabricated via selective-area epitaxial growth. The single hexagonal annular structure is composed of not only polar facet of (0001) on top surface but also semi-polar facets of {10-11} and {11-22} in inner and outer sidewalls, exhibiting multi-color visible light emission from InGaN/GaN MQWs formed on the different facets. The InGaN MQWs on (0001) facet emits the longer wavelength (green color) due to the larger well thickness and the higher In composition, while those on semi-polar facets of {10-11} and {11-22} had highly-efficient shorter wavelength (violet to blue color) emission caused by smaller well thickness and smaller In composition. By combining the multiple color emission depending on different facets, high efficiency broadband visible light emission could be achieved. The emission color can be changed with excitation power density owing to the built-in electric field on the (0001) facet, which is confirmed by time-resolved luminescence experiments. The hexagonal annular structures can be a critical building block for highly efficient broadband visible light emitting sources, providing a solution to previous problems related to the fabrication issues for phosphor-free white light emitting devices.


Introduction
The group III-nitrides, as representative materials for light-emitting diodes (LEDs), has attracted a wide range of attention in science and technology. [1][2][3] The broadband emission and multi-color emission have been important issue in the solid-state lighting. [4][5][6] The conventional white LEDs generally combined with phosphors had unavoidable energy conversion loss and poor color-rendering index. So, there have been various approaches to obtain broadband emission of InGaN without using phosphors. [7][8][9] The GaN micro-structures such as micro-rods, micro-stripes and micro-pyramids could be considered as important building blocks for realizing broadband and multi-color emission. It was possible to obtain multi-color emission from InGaN/GaN multi-quantum wells (MQWs) on the GaN multifacets of the micro-structures, which were grown by the selective-area epitaxial growth (SAG) technique. [10][11][12][13] T. Wunderer et al. studied three-dimensional structures of GaN providing multi-facets to obtain broadband emission. The InGaN/GaN MQWs on the triangular stripe structures of GaN were demonstrated as electrically-driven LEDs. The specific geometry influenced variation of thickness and In-composition, resulting in broadband emission. [14] To obtain multi-color emission with the triangular stripe structures, there were many approaches by combining different structures of the planar and the triangular structures, which had different wavelength emissions. [15,16] But there were difficulties to fabricate electrodes on the microstructures because of the different height of the planar and the stripe structures. GaN nanorod structures could be applied as the LED of the multi-color emission. Y. Hong et al.
fabricated nano-rod based LED by growing InGaN/GaN MQW on the GaN nano-rod which had multi-color emission from the multi-facets of nano-rod. Because the current path was varied along the height of the nano-rods, the emission color was varied with injection current density. [17] The nano-rod or micro-rod structures not only had the non-uniform current path, but also had the leakage current, which were severe problems for the electrical fabrication due to geometrically difficult fabrication process.
The GaN pyramid structures composed of semi-polar facets have been applied to optical devices. It has been widely studied because the pyramid structure provides not only a reduced strain and dislocation densities but also diminished polarization field, resulting in enhancement of recombination efficiency. [18][19][20][21] Recently, we have studied the optical properties of pyramid structures and its application of electrical demonstration. It provided good optical performance due to high quantum efficiency of quantum dot and semi-polar MQWs in the GaN pyramids. [22] The broadband emission could be realized due to broad distribution of In-composition and well thickness in the pyramids. Therefore, it was possible to obtain white-color emission by combining planar and pyramid structures with optimizing the ratio of two structures. [23] It still had difficulties on the electric fabrication because of nonequivalent height between planar and pyramid structures.
To overcome the aforementioned problems, we proposed the hexagonal annular structure of GaN as a single micro-structure with equivalent height, which can have the multi-facet in both inner and outer sidewalls for broadband and multi-color emission. The hexagonal annular structures have been widely adopted for the investigation of the growth mode because the GaN hexagonal annular structure contains various polar-and semi-polarfacets. [24,25] From the multi-facet of hexagonal annular structure, we expected that the

Selective-Area Epitaxial Growth
A schematic illustration of the hexagonal annular structure is shown in Figure 1a. The hexagonal annular structure of GaN was grown by SAG with a ring-patterned mask on a 2 µm-thick GaN on a c-sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The inside and outside of diameters for ring pattern were 6 µm and 12 µm, respectively. The center-to-center distance of ring pattern was 16 µm. The 10-period InGaN/GaN MQW layers were grown on the GaN hexagonal annular structures. We confirmed that the hexagonal annular structure was grown well with the uniform array and the clear surfaces from top-view scanning electron microscope (SEM) image as shown in

Spatially-Resolved Optical Properties
The cathodoluminescence (CL) of hexagonal annular structure was measured to characterize spatially-resolved optical properties. For the same facet of {1011}, the inside facet had longer wavelength than outside facet, which could be due to the geometric origin of differences of In-incorporation efficiency, well thickness, and built-in electric field. As a result, the InGaN on the hexagonal annular structure provided broadband and multi-color emission in visible wavelength from the mixture of the polar and semi-polar facets.

Structural Characteristics
To confirm the origin of each facet emitting different wavelength emission, we measured the well thickness and In-composition from the transmission electron microscope (TEM). resulting in red shifted emission for {1011}. The peak shift due to the variation of well thickness for our sample was estimated as around 2.5 nm. [26,27] But the experimental result of CL peak shift of 28 nm was much larger than calculated value. To confirm the effect of strain-induced piezoelectric field, we considered the peak position of GaN from the CL measured at 80 K. It was known that the near-band edge emission of GaN is shifted with the strain. [28] We obtained the CL spectra along the height for each facet. Because the CL peak position of GaN for {1011} and {1122} facets were almost same, we expected that the origin of CL peak shift was not due to the effect of bulit-in electric field. (cf. Supporting Information for a detailed discussion). Therefore, we concluded that the origin of different emission of InGaN on {1011} and {1122} facets was mostly difference of In composition.
We carried out the energy dispersive spectrometer (EDS) to measure the In-composition.
Because of the limitation of resolution for detecting a single layer of InGaN, we measureed the relative In contents for the same area of MQWs (dotted-line rectangles in Figure 3).

Multi-color Broadband Visible Light Emission
The photoluminescence (PL) of a hexagonal annular structure was measured with changing the excitation power density at 10 K to characterize the optical properties, as shown in InGaN MQWs on polar facet were measured to be 168 ps and 1.83 µs, respectively. The decay lifetime at 10 K was considered as the radiative lifetime by assuming that 10 K was low enough to disregard the non-radiative processes. The decay lifetime of InGaN MQWs on semi-polar facets had much shorter than InGaN MQWs on polar facet, which was due to the smaller built-in electric field and thiner layers of InGaN on semi-polar facets resulting in higher recombination rate. Consequently, we realized the broadband and multi-color visible emission from the multi-facets of hexagonal annular, which had the advantages of high quantum efficiency of semi-polar facets. It was so meaningful because the hexagonal annular structure can provide the possibility for fabrication of optical devices as a single microstructure avoiding many drawbacks of combined structures for the broadband and white-color emission.

Conclusion
We have successfully obtained the broadband and multi-color emission by growing Therefore, the hexagonal annular structures will provide a solution to previous problems related to the fabrication of single structure emitting broadband and multi-color emission and it will be building blocks for highly efficient broadband visible light emitting devices.

Experimental Section
A hexagonal annular structure of GaN was grown by MOCVD through the SAG technique with a ring patterned mask. A 100 nm thick SiO 2 layer was deposited on a 2 µm thick c-GaN template grown on sapphire substrate. We obtained the ring patterned mask by photo