Size Tunable High-Efficiency Micro-LED Array Grown on Sapphire Nano-Membrane Template

Micro-LED displays are emerging displays with a lot of potential advantages over conventional display technologies. They are new displays with high pixel density, high brightness, superior stability, low energy consumption and most importantly, no limitation in size, capable of covering from micro-displays for AR/VR/MR to large area displays for consumer TV. Recent demonstrations by companies and institutes shows that development of micro-LED technology is in progress. However, further research is needed before they are commercialized. The current micro-LED fabrication process contains singluation of LED film into micro-scale using plasma etching. During the etching process, the active layers are inherently damaged, resulting in degradation in quantum efficiency. Another obstacle is the unacceptable loss of LED film known as kerf loss. The mentioned problems become more crucial as the chip size shrinks, affecting the cost of the micro-LED displays. It is highly desired to establish a scheme to fabricate high-efficiency micro-LEDs without a singulation process.<br/>To fabricate micro-LEDs with self-passivated structures, we proposed a unique growth template called sapphire nano-membrane (SNM). The SNMs are 3D bridge structures consisted of 100 nm thick single crystalline Al₂O₃. The fabrication process of the SNM array started with photolithography to make photoresist (PR) pattern on a sapphire substrate. Then, an amorphous Al₂O₃ layer with a thickness of 120 nm was deposited by atomic layer deposition. Second photolithography and wet etching by H3PO4 solution was conducted to make a discrete SNM array. After removal of PR using acetone, the template was annealed at 1100 °C for 2 hours in air to crystallize the amorphous Al₂O₃ into single crystalline Al₂O₃ through solid phase epitaxy.<br/>An array of discrete micro-LEDs surrounded by a major (0001) plane, and (11-20) and (1-101) planes at sidewalls was grown on SNM template using MOCVD. Self-passivated structure of the fabricated micro-LEDs, with MQWs sandwiched between n-GaN at the core and p-GaN at the outer shell, were confirmed with TEM and STEM. The size of micro-LEDs was controlled by adjusting the width and space of the SNM array. By placing several SNMs closely, larger micro-LEDs were obtained via lateral overgrowth and merge of the GaN. Structural and optical properties of the fabricated micro-LEDs were analyzed to confirm the compliant substrate effect of the SNM. Micro-Raman spectroscopy result showed reduced strain on the GaN and consequent reduction of threading dislocations were confirmed with panchromatic CL analysis. Micro-PL result showed improved internal quantum efficiency and spatially resolved CL analysis showed different spectrums emitted from various facets.<br/>After the growth of LED structures, p-contact metal was formed using e-beam evaporation followed by flip-chip bonding to a target substrate with an electrode array. Utilizing ultra-thin profile of the SNM, micro-LED array was massively transferred to the target substrate via mechanical lift-off process. The micro-LEDs showed improved electrical properties with lower leakage current level compared to dry-etched micro-LEDs. Moreover, our devices showed reduced quantum confined stark effect with negligible wavelength shift depending on the current density change. We assume that it is due to relaxed strain which resulted in reduced band bending. The improved properties were observed regardless of the chip size, implying that the size effect problem in micro-LED display could be controlled using the SNM technique. We believe that results presented in this work would provide a significant step towards the commercialization of micro-LED displays.<br/>This work was supported by the BK21Plus SNU Materials Division for Educating Creative Global Leaders (21A20131912052), National Research Foundation of Korea (NRF) grand funded by the Korea government (MSIT) (2021M3D1A2039641)