An Ultrahigh Efficiency Excitonic Micro-LED

Abstract

High efficiency micro-LEDs, with lateral dimensions as small as one micrometer, are desired for next-generation displays, virtual/augmented reality, and ultrahigh-speed optical interconnects. The efficiency of quantum well LEDs, however, is reduced to negligibly small values when scaled to such small dimensions. Here, we show such a fundamental challenge can be overcome by developing nanowire excitonic LEDs. Harnessing the large exciton oscillator strength of quantum-confined nanostructures, we demonstrate a submicron scale green-emitting LED having an external quantum efficiency and wall-plug efficiency of 25.2% and 20.7%, respectively, the highest values reported for any LEDs of this size to our knowledge. We established critical factors for achieving excitonic micro-LEDs, including the epitaxy of nanostructures to achieve strain relaxation, the utilization of semipolar planes to minimize polarization effects, and the formation of nanoscale quantum-confinement to enhance electron–hole wave function overlap. This work provides a viable path to break the efficiency bottleneck of nanoscale optoelectronics.

Impact of Charge Carrier Transfer and Strain Relaxation on Red-Emitting InGaN/GaN Heterostructures

“…A prepatterned N-polar GaN on a sapphire substrate was used for the growth. The detailed patterning process can be found in our previous publications. ,, The epitaxy initiates with a 500 nm long Si-doped GaN nanowire segment, followed by a short-period superlattice (SPSL) consisting of 4 pairs of InGaN/GaN (thicknesses ∼8 nm/8 nm) for the purpose of reducing dislocations and effective strain relaxation in the subsequent high indium composition InGaN active region epitaxy. The active region consists of a 25 nm thick InGaN segment that is enclosed by GaN barriers.…”

“…Figure d shows a representative low-magnification, high-angle annular dark-field (HAADF) image of the nanowire array, wherein each layer is indicated by the atomic-number-sensitive image contrast. It was observed that for the SPSL and the active region, the center region of the InGaN is incorporated on the polar c -plane, while the edge is on the semipolar plane, which can be attributed to the unique partial faceting toward the nanowire sidewalls during the initial GaN elongation. ,− The indium map of the InGaN active region, shown in Figure d, was collected by electron-dispersive spectroscopy (EDS). The estimated indium compositions from the c -plane and semipolar plane are around 33 and 40%, respectively.…”

“…III-nitride visible light emitters with micro- or submicrometer sizes are essential building blocks for next-generation displays and the emerging applications including augmented and virtual reality (AR/VR) systems. − Recently, significant progress has been made in high-efficiency blue and green microlight-emitting diodes (μLEDs) as well as multicolor integration, − while extensive efforts have been devoted in high-performance red-emitting μLEDs with high indium composition InGaN as the active regions. − However, the incorporation of high indium composition causes alloy substitutional disorder in the active region and reduces the exciton binding energy, which enhances the Auger recombination coefficient. Polar nitrides also suffer from quantum-confined Stark effect (QCSE) due to spontaneous and piezoelectric polarizations, which reduces the overlap of electron–hole wave functions and causes color instability as is depicted in Figure a.…”

Recent Progress in Long‐Wavelength InGaN Light‐Emitting Diodes from the Perspective of Epitaxial Structure

“…[ 111 ] Pandey et al reported an ultrahigh efficiency nanowire excitonic LED in the submicron green light spectrum. [ 112 ] By utilizing the large exciton oscillator strength of quantum‐confined nanostructures, with an EQE of 25.2% and a WPE of 20.7%, which are the currently highest values known for the same size, providing an effective way to break the efficiency bottleneck of green LEDs. In addition, the insertion of AlGaN barriers in the QWs of nanowire LEDs enables the screening of the QCSE, which is realized by the transfer of free electrons into the QWs induced by polarization positive bond charges, thus effectively solving the problem of wavelength blueshift.…”

“…[ 113 ] Through optimization of the Mg doping within the nanowires, the plasma damage‐induced deleterious effect can be eliminated, resulting in a peak EQE and WPE of ≈8.3% and ≈4.6% for submicrometer red LEDs with an emission wavelength at ≈650 nm. [ 114 ] Remarkably, as the diameter of nanowires decreases, the lateral diffusion of In adatoms is significantly enhanced, resulting in an increase of In incorporation and longer emitting wavelengths. [ 115 ] This observation is crucial for the development of monolithically integrated full‐color micro‐LEDs, offering a new avenue toward the creation of ultrasmall and ultrahigh resolution displays, as shown in Figure 7b–d.…”