Monolithic Broadband InGaN Light-Emitting Diode

Abstract

A monolithic non-phosphor broadband-emission light-emitting diode is demonstrated, comprising a combination of high-density micro-structured and nano-structured InGaN-GaN quantum wells fabricated using a top-down approach. Broadband emission is achieved by taking advantage of low-dimensional-induced strain-relaxation of highly-strained quantum wells, combining light emitted from strain-relaxed nano-tips at wavelengths shorter than the as-grown by as much as 80 nm with longer-wavelength light emitted from the larger non-relaxed microdisks. The localized emission characteristics have been studied by spatially-resolved near-field photoluminescence spectroscopy which enabled both the photoluminescence intensity and spectrum from individual nano-tips to be distinguished from emission at the larger-dimensioned regions. Distinctive blue-green-yellow emission can be observed from the electroluminescent device, whose continuous broadband spectrum is characterized by CIE coordinates of (0.39, 0.47) and color rendering index of 41. Emission can be tuned by adjusting the relative densities of nano-tips and microdisks along the linear color gamut defined by their respective CIE coordinates.temperatures so that their emission spectra are shifted away from the visible to the infrared regions. Fluorescent lamps, on the other hand, are based on mercury gas discharges which emit predominantly at discrete wavelengths in the ultraviolet. White light is be generated through color conversion using color-converters such as phosphors, producing a combined spectrum consisting of a mixture of broad and line emissions. In both cases, the lamps do not emit directly or entirely in the visible region which is useful for illumination, thus compromising on their efficiencies. Such wastage of non-visible spectral components can be avoided by using light-emitting diodes (LEDs) which inherently are monochromatic sources with spectral line-widths in the range of 20 to 50nm.[(1), (2)] However, such widths are insufficiently broadband to cover the entire visible spectral range. Use of phosphors [(3), (4)] in combination with LEDs, analogous to the role of phosphors in fluorescent lamps, is the primary strategy towards achieving broader band emissions. LEDs based on the widebandgap GaN materials are particularly suitable for this purpose due to their abilities to emit at the shorter visible wavelengths with high efficiencies. Together with phosphors such as Ce-doped YAG which typically fluoresce in the yellow color bands, white light can be generated, albeit with spectral incompleteness. Such LED-phosphor solutions have now become commercialized products, but can hardly be regarded ideal white light sources. The large Stokes shifts of nearly 100 nm give rise to significant energy losses negating the efficiencies of the LEDs, not to mention lifetimes and environmental impacts associated with the phosphors.[(5), (6), (7)] The ideal LED-based white light source should thus be monolithic and produce a continuous spectrum across the visible band without co...

Monolithic InGaN Multicolor Light‐Emitting Devices

“…[22][23][24][25][26][27][28][29][30][31] There is also an increase in the internal quantum efficiency (IQE), as the electron-hole wavefunction overlap increases. [32] Moreover, as the size of nanostructures decreases, a larger portion of the nanostructures falls within the region 10 nm from the edge, where the strain relaxation (especially in the quantum wells) is strongest, [31] hence, the degree of strain relaxation in the nanostructure increases giving rise to greater spectral blue shift. [24,31,33] Based on the observation that the spectral blue shift is due to the size-dependent strain relaxation in the quantum wells, [22,[24][25][26][27][28][29][30] it was proposed that nanostructures patterned by nanosphere lithography or other techniques could be used to achieve monolithic multicolor emission.…”

“…A blue-shift range of 61 nm was demonstrated in the smallest nanopillars with a diameter of 100 nm. [32] Using the same strategy, nanostructures with diameters of 800, 100, and 50 nm were patterned by electron-beam lithography on a wafer with 650 nm emission and integrated together to form monolithic red, green, and blue color-tunable LEDs [36][37][38] as shown in Figure 2a. Monolithic RGB emission was demonstrated by connecting the nanostructured RGB channels with metal interconnects as illustrated in Figure 3a.…”

Development of Monolithically Grown Coaxial GaInN/GaN Multiple Quantum Shell Nanowires by MOCVD

“…Recent studies have achieved different emission wavelengths from three-dimensional structures fabricated by local strain engineering [18][19][20]. Using the top-down etching method, the authors of reference [21] fabricated monolithic LEDs combining microstructure and nanostructures of GaInN/GaN, which emit distinctive blue-green-yellow light with a color rendering index (CRI) of 41. However, the fabrication process (even when performed by surface passivation or thermal annealing) can also severely damage the surface and significantly decrease the emission efficiency [22].…”

Monolithic cyan − violet InGaN/GaN LED array

“…Other types of devices are fabricated by the top‐down approach, where a homogeneous QW structure is first grown on typical substrate and then patterned. A broadband monolithic LED obtained by this technique was recently demonstrated by Feng and coworkers . In their work, the broadband emission is obtained in QW structures partially shaped after growth by the means of nano‐ and micromasking (different grades of silica particles dispersion) and ion etching.…”

“…First, a typical substrate is covered with a mask with narrow, stripe-shaped windows. Next, GaN is grown in these windows and, depending on the growth conditions and the window direction, GaN microprisms with different crystallographic facets are obtained, for example (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22), (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), (1-101), (1-100), (0001). Finally, a LED structure is grown on the microprisms and in each microfacet different emission wavelength is obtained [8][9][10][11][12][13][14][15].…”