Multi-color nanowire LEDs on a single chip

We report on the monolithic integration of RGB InGaN dot-in-a-wire LEDs on a single chip. The correlated color temperature can be continuously varied in the range of 1900K to 6800K, while maintaining excellent color rendering index capability (CRI>90). Moreover, submicron scale RGB pixels were demonstrated. Driven by the need for smaller size, reduced power consumption, and enhanced efficiency and functionality, future ultrahigh resolution display technologies require the development of submicron scale, high efficiency, multi-color light sources monolithically integrated on a single chip. In this context, we have investigated the molecular beam epitaxial growth, fabrication and characterization of InGaN nanowire RGB LEDs monolithically integrated on a single chip, including Si and sapphire substrate. We have achieved controllable and tunable full-color light generation from InGaN LEDs [1,2]. The correlated color temperature can be continuously varied in the range of 1900 K to 6800 K, while maintaining excellent color rendering index capability (CRI > 90). Schematically shown in Fig. 1(a), the epitaxy of InGaN nanowires takes place on a GaN buffer layer on sapphire or Si substrate. A thin (~ 10 nm) Ti layer is employed as the growth mask [1-5]. Opening sizes of various shapes are created on the Ti mask, shown in Fig. 1(b). Under selective area epitaxy conditions, Ga adatoms are only nucleated in the opening apertures, and no epitaxy takes place on the Ti mask. As such, the size and morphology of GaN nanowires are precisely determined by the opening apertures in the Ti mask. Figure 1(c) shows the scanning electron microscopy (SEM) image of hexagonal InGaN nanowires grown on GaN template on sapphire substrate. The incorporation of AlGaN barriers in the active region, instead of GaN barriers, leads to the formation of an AlGaN shell surrounding the InGaN quantum dot active region, due to the smaller Al adatom migration length compared to Ga and In adatoms. The resulting core-shell like nanostructures, schematically shown in Fig. 2(a), can effectively suppress nonradiative surface recombination. The photoluminescence emission was measured using a 405 nm laser at room temperature. Shown in Fig. 2(b), it is seen that the photoluminescence intensity is enhanced by nearly a factor of eight, compared to InGaN/GaN nanostructures without the formation of AlGaN shell. We have further studied the optical emission properties of single InGaN dot-in-nanowire structures. Shown in Fig. 3(a), the optical emission exhibits a consistent redshift with decreasing nanowire diameter under otherwise identical epitaxy conditions [1]. The emission wavelengths can be continuously varied from 465 nm to 640 nm by decreasing the nanowire diameter from 2 Pm to 150 nm. The unique size-dependent optical emission is directly related to the diameter dependent incorporation of In and Ga adatoms at the nanowire growth front. We have further demonstrated full-color (red, green, blue – RGB) single nanowire LED pixels monolithically integrated on the same chip, including sapphire and Si substrates [1,2]. Each full-color LED pixel consists of three multi-color subpixels, which can be separately biased and exhibit emission in the blue, green/yellow, and orange/red spectral range. Consequently, light mixing at the chip level can be readily achieved. More importantly, by varying the injection current to each LED subpixel, the spectral power density and the CCT can be controllably tuned. The output spectra of a representative triple-color LED pixel consisting of blue, green and orange/red subpixels are shown in Fig. 3(b). It is seen that by adding green and orange/red light components, the CCT can be varied from ~6500 to 3800 K. For the operation at CCT of 3826 K, the CRI can be as high as 90.7.