Technological Breakthroughs in Chip Fabrication, Transfer, and Color Conversion for High-Performance Micro-LED Displays.

The implementation of high-efficiency and high-resolution displays has been the focus of considerable research interest. Recently, micro light-emitting diodes (micro-LEDs), which are inorganic light-emitting diodes of size <100 µm2 , have emerged as a promising display technology owing to their superior features and advantages over other displays like liquid crystal displays and organic light-emitting diodes. Although many companies have introduced micro-LED displays since 2012, obstacles to mass production still exist. Three major challenges, i.e., low quantum efficiency, time-consuming transfer, and complex color conversion, have been overcome with technological breakthroughs to realize cost-effective micro-LED displays. In the review, methods for improving the degraded quantum efficiency of GaN-based micro-LEDs induced by the size effect are examined, including wet chemical treatment, passivation layer adoption, LED structure design, and growing LEDs in self-passivated structures. Novel transfer technologies, including pick-up transfer and self-assembly methods, for developing large-area micro-LED displays with high yield and reliability are discussed in depth. Quantum dots as color conversion materials for high color purity, and deposition methods such as electrohydrodynamic jet printing or contact printing on micro-LEDs are also addressed. This review presents current status and critical challenges of micro-LED technology and promising technical breakthroughs for commercialization of high-performance displays.

Self-Assembled Size-Tunable Microlight-Emitting Diodes Using Multiple Sapphire Nanomembranes.

Microlight-emitting diode (Micro-LED) is the only display production technology capable of meeting the high-performance requirements of future screens. However, it has significant obstacles in commercialization due to etching loss and efficiency reduction caused by the singulation process, in addition to expensive costs and a significant amount of time spent on transfer. Herein, multiple-sapphire nanomembrane (MSNM) technology has been developed that enables the rapid transfer of arrays while producing micro-LEDs without the need for any singulation procedure. Individual micro-LEDs of tens of µm size were formed by the pendeo-epitaxy and coalescence of GaN grown on 2 µm width SNMs spaced with regular intervals. We have successfully fabricated micro-LEDs of different sizes including 20 × 20 µm2, 40 × 40 µm2, and 100 × 100 µm2, utilizing the membrane design. It was confirmed that the 100 × 100 µm2 micro-LED manufactured with MSNM technology not only relieved stress by 80.6% but also reduced threading dislocation density by 58.7% compared to the reference sample. It was proven that micro-LED arrays of varied chip sizes using MSNM were all transferred to the backplane. A vertical structure LED device could be fabricated using a 100 × 100 µm2 micro-LED chip, and it was confirmed to have a low operation voltage. Our work suggests that the development of the MSNM technology is promising for the commercialization of micro-LED technology.

A discrete core-shell-like micro-light-emitting diode array grown on sapphire nano-membranes.

A discrete core-shell-like micro-light-emitting diode (micro-LED) array was grown on a 100 nm-thick sapphire nano-membrane array without harmful plasma etching for chip singulation. Due to proper design for the sapphire nano-membrane array, an array of multi-faceted micro-LEDs with size of 4 µm × 16 µm was grown. Threading dislocation density in the micro-LED formed on sapphire nano-membrane was reduced by 59.6% due to the sapphire nano-membranes, which serve as compliant substrates, compared to GaN formed on a planar substrate. Enhancements in internal quantum efficiency by 44% and 3.3 times higher photoluminescence intensity were also observed from it. Cathodoluminescence emission at 435 nm was measured from c-plane multiple quantum wells (MQWs), whereas negligible emissions were detected from semi-polar sidewall facets. A core-shell-like MQWs were formed on all facets, hopefully lowering concentration of non-radiative surface recombination centers and reducing leakage current paths. This study provides an attractive platform for micro-LEDs by using sapphire nano-membrane.

Enhanced performance of InGaN/GaN multiple-quantum-well light-emitting diodes grown on nanoporous GaN layers.

We demonstrate the high efficiency of InGaN/GaN multiple quantum wells (MQWs) light-emitting diode (LED) grown on the electrochemically etched nanoporous (NP) GaN. The photoluminescence (PL) and Raman spectra show that the LEDs with NP GaN have a strong carrier localization effect resulting from the relaxed strain and reduced defect density in MQWs. Also, the finite-difference time-domain (FDTD) simulation shows that the light extraction efficiency (LEE) is increased by light scattering effect by nanopores. The output power of LED with NP GaN is increased up to 123.1% at 20 mA, compared to that of LED without NP GaN. The outstanding performance of LEDs with NP GaN is attributed to the increased internal quantum efficiency (IQE) by the carrier localization in the indium-rich clusters, low defect density in MQWs, and increased LEE owing to the light scattering in NP GaN.

Laterally graded porous silicon optical filter fabricated by diffusion-limited etch process.

We report a method of producing a lateral gradient in the optical properties of anodically etched porous silicon layers. Lateral gradation details of the porous silicon layer are governed by the etch mask pattern involved. Unlike other methods that rely on uneven hole current distribution, we believe that in our method the diffusion of reactive ions in the etchant plays a key role. As an implementation of the proposed method, we demonstrate a linearly graded optical bandpass filter operating at the lambda=1550 nm range by employing a tapered etch window opening. The resultant optical filter exhibited a approximate 60 nm tuning range with a sharp transmission bandwidth of approximately 3 nm. Computer simulations indicate that an uneven hole current distribution cannot be the reason for the observed gradient along the taper axis, supporting the view that the diffusion-limited etch process plays the key role.