Micro light-emitting diode (micro-LED) will play an important role in the future generation of smart displays. They are found very attractive in many applications, such as maskless lithography, biosensor, augmented reality (AR)/mixed reality etc, at the same time. A monitor that can fulfill saturated color rendering, high display resolution, and fast response time is highly desirable, and the micro-LED-based technology could be our best chance to meet these requirements. At present, semiconductor-based red, green and blue micro-LED chips and color-conversion enhanced micro-LEDs are the major contenders for full-color high-resolution displays. Both technologies need revolutionary ways to perfect the material qualities, fabricate the device, and assemble the individual parts into a system. In this roadmap, we will highlight the current status and challenges of micro-LED-related issues and discuss the possible advances in science and technology that can stand up to the challenges. The innovation in epitaxy, such as the tunnel junction, the direct epitaxy and nitride-based quantum wells for red and ultraviolet, can provide critical solutions to the micro-LED performance in various aspects. The quantum scale structure, like nanowires or nanorods, can be crucial for the scaling of the devices. Meanwhile, the color conversion method, which uses colloidal quantum dot as the active material, can provide a hassle-free way to assemble a large micro-LED array and emphasis the full-color demonstration via colloidal quantum dot. These quantum dots can be patterned by porous structure, inkjet, or photo-sensitive resin. In addition to the micro-LED devices, the peripheral components or technologies are equally important. Microchip transfer and repair, heterogeneous integration with the electronics, and the novel 2D material cannot be ignored, or the overall display module will be very power-consuming. The AR is one of the potential customers for micro-LED displays, and the user experience so far is limited due to the lack of a truly qualified display. Our analysis showed the micro-LED is on the way to addressing and solving the current problems, such as high loss optical coupling and narrow field of view. All these efforts are channeled to achieve an efficient display with all ideal qualities that meet our most stringent viewing requirements, and we expect it to become an indispensable part of our daily life.

Augmented reality and visual reality (AR & VR) micro-displays require micro light emitting diodes (µLEDs) with an ultra-small dimension (≤ 5 µm), high external quantum efficiency (EQE) and narrow spectral linewidth. Unfortunately, dry etching which is the most crucial step for the fabrication of µLEDs in current approaches introduces severe damages, which seems to become an insurmountable challenge for achieving ultra-small µLEDs with high EQE. Furthermore, it is well-known that µLEDs which require InGaN layers as an emitting region naturally exhibit significantly broad spectral linewidth, which becomes increasingly severe towards long wavelengths such as green. In this paper, we have reported a combination of our selective overgrowth approach developed very recently and epitaxial lattice-matched distributed Bragg reflectors (DBRs) embedded in order to address all these fundamental issues. As a result, our µLEDs with a diameter of 3.6 µm and an inter-pitch of 2 µm exhibit an ultra-high EQE of 9% at ~500 nm. More importantly, the spectral linewidth of our µLEDs has been significantly reduced down to 25 nm, the narrowest value reported so far for III-nitride green µLEDs.

A direct epitaxial approach to achieving ultra-small and ultra-bright InGaN micro light emitting diodes (µLEDs) has been developed, leading to the demonstration of ultra-small, ultra-efficient and ultra-compact green µLEDs with a dimension of 3.6 µm and an inter-pitch of 2 µm. The approach does not involve any dry-etching processes which are exclusively used by any current µLED fabrication approaches. As a result, our approach has entirely eliminated any damage induced during the dry-etching processes. Our green µLED array chips exhibit a record external quantum efficiency (EQE) of 6% at ~515 nm in the green spectral region, although our measurements have been performed on bare chips which do not have any coating, passivation, epoxy or reflector which is generally used for standard LED packaging in order to enhance extraction efficiency. High luminance of >10^7 cd/m^2 has been obtained on the µLED array bare chips. Temperature dependent measurements show that our µLED array structure exhibits an internal quantum efficiency (IQE) of 28%. It is worth highlighting that our epitaxial approach is fully compatible with any existing micro-display fabrication techniques.

The application of deep ultraviolet detection (DUV) in military and civil fields has increasingly attracted the attention of researchers’ attention. Compared with the disadvantages of organic materials, such as complex molecular structure and poor stability, inorganic materials are widely used in the field of DUV detection because of their good stability, controllable growth, and other characteristics. Rapid advances in preparing high-quality ultrawide-bandgap (UWBG) semiconductors have enabled the realization of various high-performance DUV photodetectors with different geometries, which provide an avenue for circumventing numerous disadvantages in traditional detectors. Herein, the development history and types of DUV detectors are briefly introduced. Typical UWBG detection materials and their preparation methods, as well as their research and application status in the field of DUV detection, are emphatically summarized and reviewed, including III-nitride semiconductors, gallium oxide, diamond, etc. Finally, problems pertaining to DUV detection materials, such as the growth of materials, the performance of devices, and their future development, are also discussed.