A color synthesis based on InGaN / GaN quantum wells ͑QWs͒ grown on GaN microfacets formed by regrowth on SiO 2 mask stripes is demonstrated. The microfacet structure is composed of ͑0001͒, ͕1122͖, and ͕1120͖ planes, and the InGaN well thickness and composition are spatially inhomogeneous due to the diffusion of the adatoms among the facets. These properties allow microfacet QWs, which, for example, emit yellow from the ͑0001͒ facet and blue from the ͕1122͖ facet, to be fabricated, of which the luminescence appears white due to the additive color mixing. Using a mask pattern that consists of regions with and without stripes, the emissions from the microfacet QWs and from planar QWs are synthesized to produce the desired apparent output colors.

Monolithic polychromatic light-emitting diodes (LEDs) based on micro-structured InGaN/GaN quantum wells are demonstrated. The microstructure is created through regrowth on SiO2 mask stripes along the [1100] direction and consists of (0001) and {1122} facets. The LEDs exhibit polychromatic emission, including white, due to the additive color mixture of facet-dependent emission colors. Altering the growth conditions and mask geometry easily controls the apparent emission color. Furthermore, simulations predict high light extraction efficiencies due to their three-dimensional structures. Those observations suggest that the proposed phosphor-free LEDs may lead to highly efficient solid-state lighting in which the color spectra of light sources are synthesized to satisfy specific requirements for illuminations.

Altering the mask geometry controls the apparent emission color from InGaN / GaN quantum wells ͑QWs͒ grown on GaN microfacets formed by regrowth on SiO 2 mask stripes over a wide spectral range, including white. The mask stripes are along the ͗1100͘ direction and the microfacet structure is composed of the ͑0001͒ and ͕1122͖ planes. With a large occupancy of the mask opening within a period, both facets simultaneously appear and emit different colors. For example, the ͕1122͖ facet QWs emit blue and the ͑0001͒ facet QWs emit green. On the other hand, with a small occupancy of the mask opening, the ͕1122͖ facets become dominant and a greenish-blue light is emitted. To synthesize these spectra, the mask patterns are designed so that two different microfacet structures are included within a period. Hence, the macroscopically observed emission color, which depends on the pattern design, can change from green to purple through white due to the additive color mixture.

The growth mechanisms during metalorganic vapor phase epitaxy of InGaN quantum wells (QWs) on three-dimensional GaN microfacet structures composed of (0001) and {11-22} planes were investigated. The cross section of the microfacet structures formed by regrowth was changed from triangle to trapezoid, depending on the mask geometries for the regrowth. The emission peak wavelength of the (0001) microfacet QWs was always longer than that of the {11-22} microfacet QWs, and the difference is smaller than in planar (0001) and {11-22} QWs. To clarify this observation, the local thicknesses of constituent InN and GaN on the GaN microfacets were estimated. Using a formula derived from the diffusion equation, the distribution of the local thickness is fitted to estimate the apparent diffusion lengths, λ. For Ga, λ was fitted to be 5.2 µm on (0001) and 3.1 µm on {11-22}, while for In, λ was 2.7 µm on (0001) and 1.6 µm on {11-22}. This suggests that the degree of migration of adatoms depends on both facets and atoms, which may cause the facet-dependent In compositions and lead to multiwavelength emission from microfacet InGaN QWs.

Semipolar {nn¯01} InGaN/GaN quantum wells (QWs) (n = 1−3) are fabricated on top of GaN microstructures, which consist of semipolar {11¯01} facets. Semipolar planes are obtained via regrowth of three-dimensional structures on (0001) GaN templates under controlled growth conditions. Compared to QWs on {11¯01} facets, {nn¯01} ridge QWs show an intense emission at ∼440 nm. Time resolved photoluminescence reveals that the radiative lifetime of excitons in {nn¯01} InGaN ridge QWs at 13 K is 310 ps, which is comparable to that in {11¯01} QWs. The estimated internal quantum efficiency at room temperature is as high as 57%.