The efforts to develop high-performance next-generation displays is at an advanced stage, with the focus being to address the limitations of the conventional organic LEDs (OLEDs) and liquid crystal displays (LCDs). In particular, InGaN-based micro-LEDs (µLEDs) have attracted significant research attention owing to their fast response, high contrast, long lifetime and a wider color gamut than their counterparts. With potential application in numerous fields, market reports have suggested the need to reduce the lateral dimension of the µLED mesa, probably below 10 µm, to meet the cost targets needed for large-scale production and commercialization. Additionally, it is desirable to operate µLEDs at a higher operational current density for the enhanced brightness. In applications that require both brightness and higher communication speeds (for example, visible light communication (VLC) included AR glasses), gadgets that contain superior characteristic µLEDs may stand out of the crowd.
To date µLEDs only employ high dislocation density GaN templates (generally polar c-plane substrates) due to cost and size availability. The limitations of InGaN LEDs include low external quantum efficiency (EQE) attributed to a decrease in the lateral mesa dimensions due to etching effects. Plasma etching used in defining µLEDs mesas found to be crucial in creating leakage paths from the sidewalls. This is detrimental to the overall device functionality. To this end, several techniques have been devised to mitigate the sidewall damages. Passivating the sidewalls via a combination of atomic layer deposition method and chemical treatment has proved effective. Moreover, polar c-plane substrate µLEDs have been prone to large polarization effects called quantum confined stark effects (QCSE), which require large operational currents to mitigate. However, at large operational currents polar c-plane device efficiency decreases due to droop phenomenon. Although this problem can be solved by using non-polar and semipolar substrates, they are expensive and impractical to commercialize.
Recently, a robust approach combining epitaxial lateral overgrown (ELO) and cryogenic treatment was found to be effective for fabricating semi- and non-polar devices by lifting them off from their native substrates [Srinivas Gandrothula et al 2020 Appl. Phys. Express 13 041003]. This new workflow has been applied to demonstrate edge emitting laser [Opt. Express 27, 24717-24723 (2019)], GaN n-DBR mirror for VCSEL [Srinivas Gandrothula et al 2021 Appl. Phys. Express 14 031002]. Authors believe the same technology can be used to transfer semi/non-polar GaN crystallographic orientation high quality µLEDs onto displays panels. To appeal this new advantage to the GaN community, the same team of researchers from the University of California, Santa Barbara: Dr. Srinivas Gandrothula, Mr. Takeshi Kamikawa, Dr. Pavel Shapturenka, Mr. Ryan Anderson, Dr. Matthew Wong, Dr. Haojun Zhang, Professor James Speck, Professor Shuji Nakamura and Professor Steven Denbaars investigated the electrical and optical characteristics of µLEDs grown on lower defect density epitaxial layers. µLEDs of mesa sizes and were specifically grown on native semipolar (2021) substrates and on ELO wings of the substrate. Their light-voltage-current characteristics were systematically investigated and compared with µLEDs fabricated on planar substrates, which have relatively more dislocations. In this work authors reported simple sidewall passivation scheme. The work is currently published in the journal, Applied Physics Letter.
The research team showed that the ELO µLEDs exhibited a reduction in the leakage current under both forward and reverse bias voltages. The leakage current was reported to be lower than 10-10 A, which was significantly lower than that of planar µLEDs subjected to similar sidewall passivation and fabrication conditions. Consequently, the electrical characterization results showed that the mesa sidewall of the ELO µLEDs was less damaged during plasma etching. This was attributed to lower material defect density of ELO µLEDs. Furthermore, ELO µLEDs also exhibited superior optical performance. Notably, the choice of the semipolar ) was attributed to its ability to reduce QCSE effects and attain wider ELO wings.
In summary, Dr. Srinivas Gandrothula and colleagues successfully fabricated µLEDs on ELO wing of native semipolar (2021) GaN substrate and compared its characteristics with those fabricated on planar substrates. Under similar passivation and fabrication procedures, ELO µLEDs exhibited superior light-current-voltage characteristics and performance than planar µLEDs due to the low defect density of the crystalline layers of the ELO wings. In a statement to Advances in Engineering, the authors explained that a combination of the liftoff methods and ELO µLEDs would provide a simple and robust route for fabricating high-performance µLEDs displays.
Gandrothula, S., Kamikawa, T., Shapturenka, P., Anderson, R., Wong, M., Zhang, H., Speck, J., Nakamura, S., & Denbaars, S. (2021). Optical and electrical characterizations of micro-LEDs grown on lower defect density epitaxial layers. Applied Physics Letters, 119(14), 142103.