Micro-light-emitting diodes (μ-LEDs) are regarded as the cornerstone of next-generation display technology to meet the personalised demands of advanced applications, such as mobile phones, wearable watches, virtual/augmented reality, micro-projectors and ultrahigh-definition TVs. However, as the LED chip size shrinks to below 20 μm, conventional phosphor colour conversion cannot present sufficient luminance and yield to support high-resolution displays due to the low absorption cross-section. The emergence of quantum dot (QD) materials is expected to fill this gap due to their remarkable photoluminescence, narrow bandwidth emission, colour tuneability, high quantum yield and nanoscale size, providing a powerful full-colour solution for μ-LED displays. Here, we comprehensively review the latest progress concerning the implementation of μ-LEDs and QDs in display technology, including μ-LED design and fabrication, large-scale μ-LED transfer and QD full-colour strategy. Outlooks on QD stability, patterning and deposition and challenges of μ-LED displays are also provided. Finally, we discuss the advanced applications of QD-based μ-LED displays, showing the bright future of this technology. Micrometre-sized light-emitting diodes (LEDs) based on quantum dots (QDs) will propel the next generation of display technologies, a review by leading researchers shows. Conventional LED designs, with phosphor coatings that convert light to different colours, are difficult to make smaller than 20 micrometres. Jr-Hau He at City University of Hong Kong and co-workers explain how this problem can be tackled using QDs, tiny particles whose optical properties can be tuned by varying their size, providing brighter and more precise colours. Ultra-high-resolution displays based on phospholuminescent QD-LEDs are now being released to the market thanks to finely-controlled methods for synthesising QDs and depositing them onto films. Further research should focus on the best ways to stabilise and protect QD films within LEDs, and to continue developing electroluminescent QD-LEDs, which could potentially outperform their phospholuminescent cousins.

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Micro-light-emitting diodes with quantum dots in display technology

A proposed network of atomic clocks—using non-local entangled states—could achieve unprecedented stability and accuracy in time-keeping, as well as being secure against internal or external attack.

http://absimage.aps.org/image/DAMOP14/MWS_DAMOP14-2014-000545.pdf

A quantum network of clocks

III-Nitride nanowire optoelectronics

Quantum emitters in two-dimensional materials are promising candidates for studies of light–matter interaction and next generation, integrated on-chip quantum nanophotonics. However, the realization of integrated nanophotonic systems requires the coupling of emitters to optical cavities and resonators. In this work, we demonstrate hybrid systems in which quantum emitters in 2D hexagonal boron nitride (hBN) are deterministically coupled to high-quality plasmonic nanocavity arrays. The plasmonic nanoparticle arrays offer a high-quality, low-loss cavity in the same spectral range as the quantum emitters in hBN. The coupled emitters exhibit enhanced emission rates and reduced fluorescence lifetimes, consistent with Purcell enhancement in the weak coupling regime. Our results provide the foundation for a versatile approach for achieving scalable, integrated hybrid systems based on low-loss plasmonic nanoparticle arrays and 2D materials.

Deterministic Coupling of Quantum Emitters in 2D Materials to Plasmonic Nanocavity Arrays

The large losses of plasmonic nanocavities, orders of magnitude beyond those of photonic dielectric cavities, places them, perhaps surprisingly, as exceptional enhancers of single emitter light–matter interactions. The ultraconfined, sub-diffraction-limited mode volumes of plasmonic systems offer huge coupling strengths (in the 1–100 meV range) to single quantum emitters. Such strengths far outshine the coupling strengths of dielectric microcavities, which nonetheless easily achieve single emitter “strong coupling” due to the low loss rates of dielectric cavities. In fact, it is the much higher loss rate of plasmonic cavities that make them desirable for applications requiring bright, fast-emitting photon sources. Here we provide a simple method to reformulate lifetime measurements of single emitters in terms of coupling strengths to allow a useful comparison of the literature of plasmonic cavities with that of cavity-QED, typically more closely associated with dielectric cavities. Using this approach, we...

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Plasmonic Cavity Coupling

We examine the effect of carrier localization due to random alloy fluctuations on the radiative and Auger recombination rates in InGaN quantum wells as a function of alloy composition, crystal orientation, carrier density, and temperature. Our results show that alloy fluctuations reduce individual transition matrix elements by the separate localization of electrons and holes, but this effect is overcompensated by the additional transitions enabled by translational symmetry breaking and the resulting lack of momentum conservation. Hence, we find that localization increases both radiative and Auger recombination rates, but that Auger recombination rates increase by one order of magnitude more than radiative rates. Furthermore, we demonstrate that localization has an overall detrimental effect on the efficiency-droop and green-gap problems of InGaN light-emitting diodes.

Impact of carrier localization on recombination in InGaN quantum wells and the efficiency of nitride light-emitting diodes: Insights from theory and numerical simulations

Monolithic integration of individually addressable light-emitting diode (LED) color pixels is reported. The integration is enabled by local strain engineering. The use of a nanostructured active region comprising one or more nanopillars allows color tuning across the visible spectrum. In the current work, integration of amber, green, and blue pixels is demonstrated. The nanopillar LEDs exhibit an electrical performance comparable to that of a conventional thin-film LED fabricated on the same wafer. The proposed platform uses only standard epitaxy and a similar process flow as a conventional LED. It is also shown that the emission intensity can be linearly tuned without shifting the color coordinate of individual pixels.

Monolithic integration of individually addressable light-emitting diode color pixels

Single photon emission was observed from site-controlled InGaN/GaN quantum dots. The single-photon nature of the emission was verified by the second-order correlation function up to 90 K, the highest temperature to date for site-controlled quantum dots. Micro-photoluminescence study on individual quantum dots showed linearly polarized single exciton emission with a lifetime of a few nanoseconds. The dimensions of these quantum dots were well controlled to the precision of state-of-the-art fabrication technologies, as reflected in the uniformity of their optical properties. The yield of optically active quantum dots was greater than 90%, among which 13%–25% exhibited single photon emission at 10 K.

Single photon emission from site-controlled InGaN/GaN quantum dots

Monolithically integrating multi-color pixels from a standard InGaN quantum well active region was demonstrated with a wavelength tuning range of 178 nm. Nanopillar structures were fabricated to enable the wavelength tuning. Strain induced wavelength shift was investigated both experimentally and theoretically. A simple one-dimensional strain relaxation model was shown to accurately predict the wavelength shift as a function of the nanopillar diameter. The strain relaxation was found to depend on the indium composition in the quantum well. No noticeable increase of the defect density was observed after the strain relaxation process.

Strain-induced red-green-blue wavelength tuning in InGaN quantum wells

We show over 100-fold enhancement of the exciton oscillator strength as the diameter of an InGaN nanodisk in a GaN nanopillar is reduced from a few micrometers to less than 40 nm, corresponding to the quantum dot limit. The enhancement results from significant strain relaxation in nanodisks less than 100 nm in diameter. Meanwhile, the radiative decay rate is only improved by 10 folds due to strong reduction of the local density of photon states in small nanodisks. Further increase in the radiative decay rate can be achieved by engineering the local density of photon states, such as adding a dielectric coating.

Chu-Hsiang Teng

Papers

Impact of carrier localization on recombination in InGaN quantum wells and the efficiency of nitride light-emitting diodes: Insights from theory and numerical simulations

Monolithic integration of individually addressable light-emitting diode color pixels

Single photon emission from site-controlled InGaN/GaN quantum dots

Strain-induced red-green-blue wavelength tuning in InGaN quantum wells

How much better are InGaN/GaN nanodisks than quantum wells - oscillator strength enhancement and changes in optical properties