Multi-color broadband visible light source via GaN hexagonal annular structure Young-Ho Ko 1[+], Jie Song 2, Benjamin Leung 2, Jung Han 2 and Yong-Hoon Cho 1* 1 Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701 Korea [+] The first author is currently affiliated with Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon, Korea. 2 Department of Electrical Engineering, Yale University, New Haven, Connecticut 06520, USA The InGaN/GaN MQWs on the GaN hexagonal annular structure provided the multi-color emission with broadband spectra. To discriminate the InGaN emission from the defect-related yellow luminescence of GaN, we performed a photoluminescence excitation (PLE) experiment. Figure S1. shows a PL spectrum with the excitation wavelength of 325 nm and PLE spectra were obtained with detection wavelength of 379 nm, 403 nm and 545 nm. The emission around 379 nm was expected to donor-accept pair emission of GaN because the absorption edge is located in the energy of GaN. However the emission around 403 nm and 545 nm were considered as the InGaN emission from semi-polar and polar facet, respectively because the absorption edges were located at the energy of each InGaN layers (black arrow). Therefore we conclude that the multi-color emission was originated from the InGaN on the multi-facets (i.e., not from the defect-related yellow luminescence of GaN). The semi-polar and polar facets of the annular structures had the different emission wavelength. The origin of different wavelength for each facet was considered as the In-composition, well thickness and strain-induced internal electric field. To characterize the effect of strain, we carried out the cathodoluminescence (CL) measurement at 80 K. Figure S2a. shows. the top-view scanning electron microscope (SEM) image with the lines for CL scanning. The line scanning map of CL spectra were displayed in the Figure S2b.(from 1u to 1d for {10 11}), the Figure S2c.(from 2d to 2u for {11 22}) and the Figure S2d.(from 3i to 3o for (0001)). The left images of line scanning map were for the peak position of GaN and the right images for the peak position of InGaN. For the semi-polar facets of {10 11} and {11 22}, the GaN peak shows blue-shift as the height goes upward, meaning that the strain in the inside sidewalls of annular structure varied with the height (more compressive strain for higher region). Although the strain was varied with the height, the InGaN emission
wavelength was not changed. The InGaN on the {10 11} has longer wavelength emission than the InGaN on the {11 22}, even though the peak positions of GaN were almost same for two semi-polar facets at the same height. Therefore, the origin of different wavelength for each facet was not due to the strain-induced internal electric field. As a result, we concluded that the {10 11} facet had higher efficiency of In-incorporation than the {11 22} facet from the analysis of transmission electron microscope. For the (0001), the emission of InGaN was much longer wavelength than the semi-polar facets. So, the InGaN on the hexagonal annular structure emitted the multi-color emission. To verify the result for higher efficiency of In-corporation for {10 11}, we fabricated the triangular stripe structures grown on the line patterned masks. The {10 11} of the horizontal stripes and the {11 22} of the vertical stripes were grown at the same time with the same growth condition and the same opening area of line pattern (2 µm opening and 10 µm period). Therefore we can consider that the horizontal and vertical stripe structure had almost the same growth rate, resulting in similar thickness of InGaN MQWs. The CL spectra of the vertical and horizontal stripes were shown in Figure S3. The {10 11} of the horizontal stripes had longer wavelength of 461 nm than the {11 22} of the vertical stripes of 414 nm. Consequently, the {10 11} had higher efficiency of In-incorporation and the hexagonal annular structure provided the broadband emission with high internal quantum efficiency of semi-polar facets.
Wavelength (nm) 600550 500 450 400 350 300 Temp. : 10 K PL Intensity (a. u.) 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 Photon energy (ev) Figure S1. The PL and PLE spectra of the hexagonal annular structure measured at 10 K. The PL spectrum was obtained with excitation wavelength of 325 nm. The detection wavelength for the PLE spectra were 379 nm (violet line), 403 nm (blue line) and 545 nm (green line).
Figure S2. The line scanning maps of CL peak positions of GaN and InGaN measured at 80 K. a, The SEM image of a hexagonal annular structure and the indication of scanning directions for CL. The CL spectra map of line scanning along the b, from 1u to 1d, c, from 2d to 2u and d, from 3i to 3o.
Figure S3. The CL spectra of vertical and horizontal stripes structures. a, The CL spectra measured at 300 K for the vertical stripe structure of {10 11} and b, for the horizontal stripes of {11 22}. The inset shows the top-view SEM images of each triangular stripe structures. The red spots indicate the excitation region for CL spectrum.