Yixin Xiao

The continued drive toward device miniaturization and monolithic integration has created an urgent need for nanoscale light sources with high efficiency and controllable carrier dynamics. Here, we report a gate-all-around (GAA) nanoscale light-emitting diode (nano-LED), in which a conformal gate electrode surrounds both the n-GaN region and the InGaN/GaN multiple-quantum-wells (MQWs) active region. By simply electrically modulating the surface potential, the GAA structure suppresses surface carrier recombination and confines radiative recombination to the undamaged central bulk region, overcoming a primary efficiency bottleneck in submicron III-nitride emitters. Moreover, the proposed GAA nano-LED exhibits transistor-like current modulation, with positive gate bias enhancing carrier injection and negative bias suppressing it, enabling full on-off switching of electroluminescence. Importantly, negative gate bias reduces surface recombination and non-radiative leakage, resulting in a nearly fourfold enhancement of peak quantum yield. Hence, this work demonstrates, for the first time to the best of our knowledge, a vertically integrated GAA field effect transistor-LED platform at the nanoscale and establishes a promising route for achieving high-efficiency nanowire LEDs and large-scale three-dimensional optoelectronic integration.

To date, the performance of AlGaN-based mid and deep ultraviolet (UV) light-emitting diodes (LEDs) is limited by the presence of a high density of defects, quantum-confined Stark effect, inefficient doping, and optical polarization transition from transverse electric to transverse magnetic mode in high Al-content AlGaN quantum wells. In this study, we address some of these fundamental challenges by utilizing monolayer (ML) GaN active regions in AlGaN-based UV LEDs. We show, both theoretically and experimentally, that charge carrier recombination occurs predominantly via the excitonic transition in ML GaN active regions, wherein the exciton binding energy exceeds 0.12 eV. We further demonstrate that double ML UV LEDs emitting at 300 nm exhibit an external quantum efficiency and a wall-plug efficiency of 5.6% and 3.5%, respectively, with excellent spectral stability.

InGaN-based micro-light-emitting diodes (micro-LEDs) are emerging to revolutionize the display and lighting technologies, particularly for their excellent robustness, high brightness, high efficiency and small pixel size. Despite the success of blue LEDs, long-wavelength emission, particularly the red emission, has been a challenge for InGaN-based micro-LEDs. Overcoming the low quantum efficiency, color instability, and broad emission in the red wavelength regime are among the most urgent and critical problems that inhibit the commercial implementation of micro-LED technology. In this work, we utilize a nanowire photonic crystal (PhC) structure to reform the radiation behavior of red-emitting InGaN micro-LEDs. Through detailed optimization on the PhC design and device fabrication, we demonstrate red-emitting micro-LEDs with a peak wavelength at 617 nm and a full-width-at-half-maximum (FWHM) of 5 nm, which is about one order of magnitude narrower than previous reported values and is paramount for achieving high color purity. The chromaticity property is highly stable with varying injection currents due to the coupling of emission to photonic band edge mode. A high external quantum efficiency of over 10% was measured from micro-LEDs with a size of 1 µm ² . This work provides a vital strategy for high-performance red-emitting micro-LEDs and a potential pathway for full-color micro-LED technology by using all III-nitride semiconductors.

Semiconductor quantum dots (QD) promise unique electronic, optical, and chemical properties, which can be exquisitely tuned by controlling the composition, size, and morphology. Semiconductor QDs have been synthesized primarily via two approaches, namely, epitaxial growth and wet-chemical synthesis. However, the properties of epitaxial QDs (eQDs) are susceptible to wetting layer formation and substrate dislocations, while colloidal QDs (cQDs) face fluorescence intermittency issues. Here, we report on the synthesis of a class of QDs that can overcome the fundamental limitations of eQDs and cQDs. By exploiting the sp ² bonding of layered hexagonal boron nitride (hBN), we show that GaN QDs can be epitaxially grown through a weak van der Waals (vdW) interaction without two-dimensional wetting layer formation. The photoluminescence intensity of GaN van der Waals quantum dots (vQDs) is more than six times stronger than that of conventional GaN eQDs and no optical blinking was observed from vQDs. We show that the interadatom bond strength is about one order of magnitude stronger compared with that between the adatoms and the hBN substrate. This work shows that vQDs have unique properties that are difficult to achieve using existing QDs synthesis methods and thus can potentially enable new classes of high-performance optoelectronic and quantum devices.

III-nitride-based micro-light-emitting diodes (micro-LEDs) are currently under rapid development for next-generation high-resolution and high-brightness displays and augmented/virtual reality (AR/VR) technologies. However, it remains elusive to achieve red-emitting III-nitride micro-LEDs with a microscale size, high efficiency, and high spectral stability, posing significant impediments to the development of full-color micro-LEDs. In this work, through detailed strain engineering and control of charge carrier transport, we achieved pure red emission (≥620 nm) micro-LEDs over 2 orders of magnitude of injection current variation. We show both theoretically and experimentally that the combination of a short-period InGaN/GaN superlattice and a thick n-type GaN interlayer can not only relieve the quantum-confined Stark effect in the active region but also suppress parasitic emission from the superlattice. The optimized deep red micro-LEDs with a device lateral dimension of ∼1 μm feature a maximal external quantum efficiency of over 3% emitting at ∼660 nm.