With funding from the Samsung Future Technology Incubation Center, Professor Sang Hyeon Kim and his team of researchers from the Department of Electrical and Electronic Engineering at Korea Advanced Institute of Science & Technology (KAIST) have made significant progress in addressing the challenges of MicroLED technology.
A little background before we get started on what could be a meaningful addition to the body of research on MicroLEDs. Quantum barriers (QBs) are essential components in the active region of LEDs, functioning as layers that help control how electrons and holes move within the device. Researchers have been working on improving the efficiency of GaN-based LEDs by adjusting the thickness of these QBs, balancing their polarization, and modifying their doping concentration.
In larger LEDs, reducing the thickness of QBs has been found to help with efficiency issues. Thinner QBs allow electrons and holes to interact better and reduce the energy needed for carriers to escape due to the internal electric field in the QBs. However, when working with MicroLEDs, it’s important to keep in mind that changing the thickness of QBs can also lead to other effects, such as the creation of a miniband, changes in the energy barrier height, and differences in the tunneling rate. The tunneling rate refers to how quickly a carrier can move between quantum wells (QWs) and can be estimated using an approximation called the Wentzel–Kramers–Brillouin (WKB) method. This method considers factors like QB thickness and energy barrier height.
In short, the thickness of QBs is crucial for LED performance, especially in the active region. Reducing QB thickness can help with efficiency issues in larger LEDs, but when designing MicroLEDs, it’s important to also consider other factors like miniband formation, energy barrier height changes, and tunneling rates.
Past research into mitigating lateral defects in MicroLEDs focused on post-processing techniques, such as the use of a highly selective lateral etch technique aimed to remove non-radiative defects on the sidewalls of the MicroLEDs. While this approach demonstrated some improvement, it faced limitations due to its implementation after the epitaxial structure growth. In contrast, the new research conducted by KAIST centers on the epitaxial structure itself.
Epitaxy involves stacking gallium nitride crystals, which serve as light-emitting bodies, onto ultrapure silicon or sapphire substrates used for MicroLEDs. The fabrication of MicroLEDs requires forming pixels by cutting the epitaxial structure grown on a wafer into cylindrical or cuboid shapes through an etching process, which incorporates plasma-based processes that generate defects on the pixel sides.
The KAIST researchers found that the movement of electric current to the sides of MicroLEDs depended on the design of the epitaxial structure, which includes QBs. They created a new design that was less affected by defects on the sides, improving the efficiency of smaller MicroLED devices. Moreover, their innovative structure reduced heat generation by about 40% during operation compared to traditional designs, making it a significant step toward commercializing ultra-high-resolution MicroLED displays.
What Do We Think?
KAIST focused on developing epitaxial structure engineering to address the efficiency degradation in MicroLEDs by developing a structure less sensitive to sidewall defects, resolving efficiency issues associated with these devices. In the context of QBs, optimizing the thickness and other parameters of the quantum barriers is crucial for improving the overall efficiency of MicroLEDs.
The KAIST research team’s work on epitaxial structure engineering can be seen as an advancement in this field, as it contributes to a better understanding of the factors that influence the performance of MicroLEDs, such as carrier confinement and efficiency droop. By focusing on the epitaxial structure itself and designing a structure that is less sensitive to sidewall defects, the KAIST research team tackled a different aspect of the efficiency problem compared to past studies, which were primarily concerned with the optimization of QB thickness.
The KAIST research complements the existing body of knowledge on QBs and their influence on LED efficiency. It contributes to the ongoing effort to improve MicroLED performance, making them more suitable for commercialization in ultrahigh-resolution displays and wearable devices.
Reference
Baek, W.J., Park, J., Shim, J. et al. Ultra-low-current driven InGaN blue micro light-emitting diodes for electrically efficient and self-heating relaxed microdisplay. Nat Commun 14, 1386 (2023). https://doi.org/10.1038/s41467-023-36773-w
