In this paper, we present a comprehensive, correlative study of the structural, transport, optical and thermoelectric properties of high-quality VO₂ thin films across its metal-insulator phase transition. Detailed x-ray diffraction study shows that it's textured polycrystalline along [010]_M1, with in-plane lattice orienting along three equivalent crystallographic directions. Across the metal-insulator transition, the conductivity increases by more than 3 orders of magnitude with a value of 3.8 × 10³ S/cm in the metallic phase. This increase is almost entirely accounted for by a change in electron density, while the electron mobility changes only slightly between the two phases, yet shows strong domain boundary scattering when the two phases coexist. Electron effective mass was determined to be ∼65m₀ in the insulating phase. From the optical and infrared reflection spectra in the metallic phase, we obtained the plasma edge of VO₂, from which the electron effective mass was determined to be ∼23m₀. The bandgap of VO₂ was determined from optical absorption to be 0.70 ± 0.05 eV at room temperature and rapidly shrinks before the phase transition occurs. In the temperature range where metallic and insulating phases coexist, the Seebeck coefficient was found to be significantly lower than that predicted by a linear combination of volumetric contributions from the insulating and metallic domains, indicating abnormal thermoelectric effect at the metal/insulator domain walls in such two-dimensional domain structure.

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.

Engineering oxide interfaces with defined electronic band structures is of vital importance for designing all-oxide devices with controllable multifunctionality and improved performance. Here, we report the band alignment, band bending, and transport mechanism in the NiO/ $\beta $ -Ga ₂ O ₃ p-n heterojunction (HJ) which exhibits high performances with a rectification ratio over 10 ¹¹ , a turn-on voltage of 1.87 V and specific ON-resistance of 10.2 $\text{m}\Omega \cdot $ cm ² . A type-II band alignment is identified at NiO/ $\beta $ -Ga ₂ O ₃ HJs with a valence band offset of 3.60 eV and a conduction band offset of 2.68 eV, respectively, determined from the depth-profiled X-ray photoelectron spectroscopic analysis. Besides band edge discontinuity, an additional built-in potential of 0.78 V is observed at the interface due to the charge transfer across the p-n-junction. In comparison, the NiO/ $\beta $ -Ga ₂ O ₃ p-n HJ has lower leakage current and higher breakdown voltage than that of the Ni/Ga ₂ O ₃ Schottky barrier diode. Capacitance–frequency analysis indicates the presence of interfacial states, and interface recombination is the dominant transport mechanism. The type-II NiO/Ga ₂ O ₃ HJ provides favorable energetics for facile separation and transportation of photogenerated electrons and holes, which is important for all-oxide devices that require bipolar operation and power devices with higher conversion efficiencies.

A high-speed c-plane InGaN/GaN blue micro-scale light-emitting diodes (Micro-LEDs) with a pixel diameter of $60~\mu \text{m}$ , and a high −3 dB modulation bandwidth of 1.53 GHz at the current density of 5996 A/cm ² were demonstrated. The ultra-thin quantum well (QW) structure with InGaN layer of 1 nm was designed, and optimized variable temperature growth processes were adopted to increase In incorporation and localization effect in the quantum wells, which greatly reduce the polarization field and increase the overlap of electron-hole wave functions. The differential carrier lifetime was effectively shortened, increasing the modulation bandwidth of the Micro-LED. And a data rate of 5.27 Gbps with a bit error rate (BER) of $3.1\times 10^{-{3}}$ was demonstrated with orthogonal frequency-division multiplexing (OFDM) signal for a 1 m free space real-time visible light communication (VLC) based on such high-speed micro-LED. Promising results in this work may support a great development of high-speed VLC applications.