  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.
