Micro-light-emitting diode (microLED) display technology has drawn a lot of attention in display industry recently owing to the superior optical and electrical properties. Compared to conventional display technologies, microLED display demonstrates higher efficiencies, longer lifetime, much higher brightness, ultra-high pixel densities, faster response time and larger color gamut. In this work, the microLED display production technology, including high efficiency microLED chip process and novel inspection technologies are discussed. At the same time, microLED display for various applications are demonstrated. Finally, challenges to achieve commercialization of microLED display are also analyzed.

III-Nitride based light-emitting diodes based on InGaN active layers formed on GaN, are fine for the blue and green emitting LEDs. However, the large lattice mis-match between red-emitting active layers and the GaN substrate still limits the efficiencies to very low values, typically <5%. We propose to use seeding techniques originally developed for nanowire growth, to seed the formation of ternary InGaN pyramids which later are converted to thin c-facet platelets of InGaN. I will in this presentation show that such relaxed, and dislocation-free, InGaN platelets with In-composition about 20%, have the potential as ideal templates for red-emitting microLEDs. Of special significance is the fact that our technology provides all three RGB microLED sources with one and the same InGaN technology, and with the same efficiency and with the maintained efficiency down to pixel sizes of just 1-2 µm in diameter.

This conference presentation was prepared for the Light-Emitting Devices, Materials, and Applications XXVII conference at SPIE OPTO, 2023.

Semiconductor heterostructures utilizing the III-N material aluminium nitride (AlN) have attracted significant interest for optoelectronic device applications in the ultraviolet (UV) spectral range. We will discussion the impact of alloy fluctuations on the electronic and optical properties of (Al,Ga)N and (Al,In)N materials by means of atomistic theoretical studies. Moreover, we present a theoretical analysis of the impact of random alloy fluctuations on the electronic and optical properties of c-plane (Al,Ga)N/AlN quantum wells. Finally, we will give an outlook for potential ways to tailor the electronic and optical properties of UV light emitters by utilizing for instance boron containing III-N alloys.

In this talk, I will discuss results from atomistic calculations that uncover the mechanisms that limit the efficiency of UV light emitters and predict excitonic heterostructures with improved efficiency. In particular, I will show how Fermi-level control during the growth of p-type AlGaN contacts can promote acceptor incorporation and decrease charge compensation, increasing the electrical conductivity. I will also discuss the detrimental role of statistically random alloy disorder in AlGaN to the efficiency of carrier transport and recombination. Alternatively, we propose atomically thin GaN/AlN superlattices and quantum wells as a mechanism to improve the carrier mobility and to form stable excitons at room temperature that increase the radiative recombination rate. I will also discuss how the integration of hexagonal BN with nitride heterostructures is a promising material to realize efficient excitonic UV light emitters despite its indirect band gap.

MicroLEDs are making rapid progress as the next generation display technology due to high brightness, higher power efficiency and better contrast as compared to existing LCD and OLED based display technologies. MicroLEDs are being introduced into large displays and smart watches and are expected to penetrate other display segments including those for AR/VR applications within the next five years. The MOCVD technology needed for GaN based microLEDs must deliver on the industry’s stringent performance requirements of material quality, uniformity and low defectivity while reducing costs via high throughput, yield, and low operating expenses. A large operating process space (in terms of pressure, temperature, and growth rates) is needed for optimization of device stacks, with excellent boundary layer control over the full process range. In this talk, we discuss the latest status of Gallium Nitride based epitaxy for microLEDs using single wafer deposition technology.

Epitaxy is a way of growing single-crystalline thin film devices, initially driven by forming chemical bonds between wafer and epi-layer. Both chemical and physical vapor-phase epitaxy have contributed to manufacturing quantum-functional devices with composition-modulated ultrathin heterostructures. However, the strongly bonded epi-layer/wafer makes it difficult to fabricate highly free-form and multi-functionally integrated high-density devices unless the wafer is easily peeled off. In this talk, we introduce the unconventional epitaxy as a platform for fabricating deformable and densely stackable devices, and share the results of our recent works on van der Waals and remote epitaxy of gallium nitride microrod and thin films for deformable lighting devices and vertical lighting-pixel-stacks. We also discuss the opportunities and challenges of the novel epitaxy for future electronics and display technology.

In recent years, reports on highly efficient UV LEDs and UV laser diodes have been published one after another. Considering the application field of UV semiconductor light-emitting devices, it is essential to achieve high output, i.e., high current density operation, and it is important to establish a fabrication process for vertical devices to realize this. A 1cm square wafer with deep-UV LEDs stacked on a sapphire substrate was successfully separated from the substrate to fabricate vertical LEDs. In this study, an Al0.68Ga0.32N underling layer was formed on an AlN template with periodic pillars, and a process that enables reproducible substrate detachment was successfully developed. The fabricated vertical LEDs successfully exhibit remarkable luminescence characteristics (peak wavelength: 298 nm) up to a current density of ~43 kA cm^-2 at room temperature and pulse driving. Applications to high-power ultraviolet region LEDs and laser diodes are expected.

In this talk, I will discuss using AlGaN nanowires for surface-emitting semiconductor deep UV LEDs at short wavelengths in vertical geometry. I will first show that by using high Al content AlGaN nanowires, devices emitting down to 207 nm can be obtained. The incorporation of polarization engineered layers further improves the device electrical performance significantly, whereas replacing conventional metal top electrode with graphene improves the device optical performance. I will also show in this talk that, by using nanowire template assisted AlN buffer layers, vertical semiconductor deep UV LEDs emitting down to 247 nm can be achieved.

AlGaN-based ultra-violet light emitting diodes (UV LEDs) are promising for a range of applications, including water purification, air disinfection and medical sensing. However, widespread adoption of UV LEDs is limited by the poor device efficiency. This has been attributed to the strong internal light absorption and poor electrical injection efficiency for the conventional UV LED structures, which typically use an absorbing p-GaN layer for p-type contact. Recent development of ultra-wide banggap AlGaN tunnel junctions enabled a novel UV LED design with the absence of the absorbing p-GaN contact layer. In this presentation, we will discuss recent progress of the AlGaN tunnel junctions and the development of tunnel junction-based UV LEDs, and discuss the challenges and future perspectives for the realization of high power, high efficiency UV LEDs.

The effects of visible light on our health and well-being are widely known and have led to the emergence of circadian-friendly or ‘human-centric’ lighting. The effects of far-red or near-infrared (NIR) light on our bodies are less known (in the lighting community). These effects, called ‘photobiomodulation’ are mostly employed in specialty or therapeutic devices for wound healing or pain relief. However, a double-blind and placebo-controlled clinical study now shows that photobiomodulation is not only a local effect: generally healthy people that are exposed to NIR light benefit from improved health and well-being. Unfortunately, indoors we are deprived of NIR light as LED-based lamps only emit in the visible spectrum, and most windows block all wavelengths beyond the visible. Consequently, there is a need for NIR light sources that provide people indoors with the right dose of NIR light in an energy-efficient way to improve their health and well-being.

High Al content (Al,G)aN is the most promising material system to produce deep UV light-emitting diodes and lasers and has prospects to realize high power electronics. Growth of these layers on native AlN substrates with low dislocation densities holds the promise to realize devices with high external quantum efficiency. Recent results, however, show despite high structural perfection large hexagonal hillocks, with a lateral extent of several µm form, depending on the film thickness. In this paper we combine AFM, defect selective etching, CL, TEM and nano-beam scanning X-ray diffraction to experimentally disentangle the mutual influence of surface steps, dislocations, strain, and composition on the forming surface morphology. To rationalize our experimental findings, we develop a theoretical model that accounts for the adatom kinetics of Al and Ga on the growth surface in the presence of a-type dislocations.

Organic and inorganic halide perovskites have demonstrated rapid progress of optoelectronic figures-of-merit, strongly supporting their application in electroluminescent devices, photovoltaics as well as X-ray and optical detectors. However, the lack of a mature fabrication technology is a major challenge for their commercialization. Chemical vapor deposition (CVD) is an appealing choice due to its flexibility/scalability, high purity and superior process control. In this work, we present a prototype perovskite hot-wall CVD reactor, including details of powder-based sublimation sources, reactor design as well as selected results from deposited perovskite compounds, first optoelectronic devices and remaining challenges.

Incorporation of tunnel junctions (TJs) to device structure enabled vertical integration of multicolor light emitting diodes (LEDs) and laser diodes (LDs). The TJs allows to control the current path in distributed-feedback LDs and micro-LEDs. It opens possibility to design new architecture devices like “inverted” LEDs or LDs with TJs located below active region. These devices have the sequence of p and n type layers similar to structures grown on hypothetical p-type (0001) GaN substrate, which is beneficial for high carrier injection efficiency, and enables operation at cryogenic temperatures. Finally, we also discuss the properties of bi-directional LEDs and wavelength-tunable LEDs.

Two-dimensional (2D) materials are gaining great attention due to their extraordinary thickness-dependent properties. 2D Ga-VI semiconductors have bandgaps in the UV region making them candidates for several LED concepts. The Indium-containing counterparts of Ga-chalcogenides moreover have small electron effective masses and high mobilities. Hence, 2D III-chalcogenides are promising materials for next-generation optoelectronic applications. We establish metal organic chemical vapor deposition (MOCVD) to find suitable growth routines for 2D materials. We will summarize our current understanding of the MOCVD growth of III-chalcogenides by systematic variation of the growth parameters and correlate the findings to optoelectronic properties of the layers.

The widespread adoption of solution-processed halide perovskites LEDs demands to surpass the luminous efficacies of conventional LEDs while having full control over the emission properties. In this talk, we report directional and enhanced emission in a green perovskite LED through hybrid photonic-plasmonic modes. We employ advanced optical modelling powered by genetic algorithms to find the best combination of materials and structures compatible with the fabrication of efficient LEDs. The experimental realisation of the optimum designs allows us to show low-temperature processed devices with enhanced emission and fine control over the emission angle. This methodology is versatile and easily integrable in cost-effective LEDs across the whole visible spectrum, opening avenues for their application in displays and light sources where the angular dispersion of light is crucial.

Hexagonal boron nitride (hBN) is a two-dimensional van der Waals material and is composed of boron and nitrogen atoms in a hexagonal lattice. hBN is the wide-bandgap semiconductor with a band of 6.4 eV and shows the efficient band edge cathodoluminescence at 215 nm as well as lasing behavior. Here I will present the efficient DUV electroluminescence (EL) in band edge emission at 215 nm as well as broad 303-333 nm emission peaks from hBN van der Waals heterostructure. We observed that 303-333 nm broad emissions with phonon replica of optical phonon energy of hBN based on the Franck-Condon principle, which are attributed to the electric field induced color centers and its highly localized excitons features. I will also present the tunable DUV light emission around band edge emission of 215 nm as a function of electric field direction and discuss the possible origin of DUV EL from hBN van der Waals heterostructures.

This conference presentation was prepared for the Light-Emitting Devices, Materials, and Applications XXVII conference at SPIE OPTO, 2023.

Hexagonal boron nitride (h-BN), an insulating two-dimensional layered material, has recently attracted a great attention due to its fascinating optical, electrical, and thermal properties, and promising applications across the fields of photonics, quantum optics, and electronics. Here, we exploit the scalable approach to grow h-BN on epitaxial gallium nitride (GaN) substrate by using metal-organic chemical vapor deposition (MOCVD). It was found that at a specific MOCVD growth condition, a very unique h-BN film can be grown on GaN substrates, in which few-layer h-BN film is suspended on GaN nanoneedles. The combination of state-of-the-art microscopic and spectroscopic analyses revealed that the suspended h-BN films exhibit unprecedented DUV photoluminescence spectra. In addition, the h-BN films show unprecedented atomic stacking configuration, the mechanism of which will be discussed with optical and structural characterizations and theoretical calculations.

The recent miniaturization of semiconductor light emitting devices (LEDs), such as mini- and micro-LEDs demands stable and efficient down-converting materials with nano-to-submicron dimensions. YAG:Ce, because of its many beneficial properties, is an interesting candidate for these applications when available at the nanoscale. This talk gives an overview of our technologies on achieving stable, well-defined nano-YAG:Ce phosphors with high energy conversion efficiencies. We discuss different synthesis approaches, after treatments and encapsulation technologies, and how these influence the structural and optical properties, and offers nano-YAG:Ce from sub-micron particles down to sizes in the order of ten to 20 nm.

This conference presentation was prepared for the Light-Emitting Devices, Materials, and Applications XXVII conference at SPIE OPTO, 2023.

Pulsed diode lasers have found widespread applications in many fields of time-resolved fluorescence spectroscopy and microscopy. Often, use of different excitation wavelengths in the same system requires the use of different diode heads, whose beam paths need to be combined for measurements. Here we present a new stand-alone picosecond laser (Prima) with three integrated colors, which can easily be switched in software. We integrate the laser into a time-resolved fluorescence spectrometer (FluoTime 300) and a confocal microscope (MicroTime 100). There, its performance is compared to that of a standard laser diode, especially for the measurements of long luminescence lifetimes in the µs range.

While state-of-the-art RGB microdisplays rely on quantum-dot color-converters (CC), the next generation of very bright and high-resolution AR microdisplays could be based on MQW CC due to their better photostability and higher blue-light absorption. However, their practical implementation is still hindered by their low light extraction efficiency (LEE), their Lambertian-like emission patterns, as well as the challenges related to their integration on on-Silicon blue µLED arrays (transfer and pixelization). In this work, we use photonic-crystals (PhCs) as light extractors and investigate all the challenges related to the design, integration & fabrication as well as testing of novel MQW based CC.

Efficient Light Emitting Diodes (LED) with a wide color gamut for high-resolution RGB display applications is still a challenging objective for state of the art technologies. The current industrial technology compromises the narrow RGB peaks emission with device efficiency. In this work, we demonstrate a CMOS compatible monolithic hybrid approach the fabrication of GaN Blue micro-LED co-integrated with Green or Red top emitting OLED on 8’’ GaN epitaxy on Si, with LED sizes as low as 5µm. This approach fulfills the need of a good color purity, while taking advantage of the best of both technology efficiencies and emission width.