Ultra-small micro-LEDs are essential for next-generation display technology. However, micro-LEDs below 5 μm have been seldom reported. In this work, we demonstrate InGaN-based blue and green micro-LEDs from 1 to 20 μm by using laser direct writing lithography. The 1-μm blue micro-LEDs show a peak external quantum efficiency of 13.02%, which is 9.57% for green ones. By characterizing the size-dependent external quantum efficiency and simply assuming that this variety is dominantly determined by the dry-etching induced dead zone, we deduce that the dead zone sizes of carrier injection at the edge of chips are 0.18 and 0.15 μm in blue and green ones, respectively. A time-resolved photoluminescence measurement also shows that carrier lifetime reduction at the edge of blue ones is more serious than that of green ones, reflecting the easier carrier lateral diffusion in the former than the latter. These results exhibit the ability of laser direct writing lithography on micro-LED fabrication and also provide a reference for predicting the limit of their chip size scaling-down.

Efficiency droop in GaN-based light emitting diodes (LEDs) under high injection current density perplexes the development of high-power solid-state lighting. Although the relevant study has lasted for about 10 years, its mechanism is still not thoroughly clear, and consequently its solution is also unsatisfactory up to now. Some emerging applications, e.g., high-speed visible light communication, requiring LED working under extremely high current density, makes the influence of efficiency droop become more serious. This paper reviews the experimental measurements on LED to explain the origins of droop in recent years, especially some new results reported after 2013. Particularly, the carrier lifetime of LED is analyzed intensively and its effects on LED droop behaviors are uncovered. Finally, possible solutions to overcome LED droop are discussed.

The ultra-small size InGaN red micro-LED has attracted a lot of research interest for AR micro-display applications. However, it still faces the challenge of maintaining the emission wavelength and efficiency that meet the needs of micro-display in small size and high current density. Here, we demonstrate 1–20 μm InGaN red micro-LEDs based on freestanding GaN substrates, showing enhanced indium incorporation compared with those grown on traditional sapphire substrates. For the 1 μm devices, an external quantum efficiency (on-wafer) of 0.86% and a wavelength of 613.6 nm are achieved at a current density of 50 A/cm2. In addition, the emission uniformity of 1 μm array grown on GaN substrates is significantly superior to that grown on sapphire substrates. These results indicate that freestanding GaN substrates are benefits for improving performance of red InGaN micro-LEDs.

The application of high-speed micro-scale light-emitting diode (micro-LED) technology in short-distance data transmission breaks through the limitations of traditional electrical connections. This is particularly crucial as the demand for large-capacity and high-speed communications becomes more urgent, with a high-speed micro-LED light source being a key component. In this work, the alternating growth interruption method is used to grow a 3-layer InGaN nanostructure with smaller size and higher density. This approach increases local carrier concentration and reduces differential carrier lifetime. Based on the 3-layer improved InGaN nanostructure, a blue micro-LED is designed and fabricated, achieving the output optical power exceeding 0.5 mW while the electro-electric (E-E) −3 dB bandwidth exceeding 2.6 GHz. Utilizing this high-speed micro-LED as the light source, three non-return-to-zero on-off keying (NRZ-OOK) real-time communication systems are demonstrated: single-channel transmission via free-space and quartz optical fiber, and 4-channel transmission via plastic imaging fiber, achieving data rates of 3.07, 2.60, and 12.00 Gbps, respectively. Additionally, the 4-channel system achieves bit-power loading discrete multi-tone (DMT) transmission of 15.05 Gbps.