Introduction to MicroLED Technology
In the ideal scenario, each manufacturing step in compound semiconductor device production adds functionality without compromising performance. For example, adding an anti-reflective coating to multi-junction solar cells can improve efficiency without limiting their lifespan. Similarly, integrating mirrors into lasers can enhance their reflectivity without introducing light-quenching defects.
In general, the process of fabricating devices from epitaxial wafers follows this formula: a series of positive steps leading to high-performance chips. However, there is an exception: MicroLED. When creating these smaller emitters, their size is typically determined by etching, which introduces defects, including suspended bonds that lower efficiency. Technologies like plasma-enhanced CVD and atomic layer deposition can help recover device performance through passivation. However, wouldn’t it be better to avoid damaging these tiny devices in the first place?
Polar Light Technologies’ Approach
Polar Light Technologies, a Swedish startup spun off from the research team of Per Olof Holtz at Linköping University, is aggressively pursuing this highly promising approach. This company has been accelerating its efforts this year, aiming to expand the spectral range of its devices, collaborate with other firms, and secure additional funding to transition from R&D to production.
The company’s core technology revolves around pyramid-shaped LEDs, which are formed by growing GaN structures on patterned SiC substrates using MOCVD.
Target Market and Technology Development
According to Ivan Martinovic, the company’s Chief Product Officer and co-founder, the initial target market for these new emitters is the light sources for augmented reality (AR) headsets. Martinovic believes that the technology is a competitive candidate for creating monochrome panels for companies like Jade Bird Display, where optical systems can form full-color images by outputting red, green, and blue light sources.
Looking ahead, Martinovic envisions MicroLED—including the MicroLEDs developed by Polar Light Technologies—penetrating the automotive industry due to its robustness. In the smartwatch sector, MicroLED might also capture market share from OLED, as both technologies are expected to achieve several days of battery life.
For the first target market—augmented reality headsets—the pixel size of displays should approach two microns. “Sub-pixels should be even smaller, and with the gap included, it would be sub-micron level,” Martinovic notes.
Cost Efficiency and Performance
Beyond improving fidelity for users, MicroLEDs small enough to meet these requirements can also offer cost benefits.
Martinovic further explains: “Whether it’s a single integrated microdisplay on silicon or large-scale transfer for big panels, you want extremely small LEDs. Even for large panels, you can’t use 5 or 10-micron MicroLEDs, as it would waste too much epitaxial material.”
However, it’s important to note that producing such small, high-performance traditional MicroLEDs is virtually impossible. Martinovic points out that “damage caused by etching quickly gets out of control and is nearly impossible to manage.”
While pyramid-shaped MicroLEDs can perform well at such small sizes, they do face a cost disadvantage in terms of substrate because of their use of SiC, compared to more traditional LEDs. Still, Martinovic argues that in the face of etching challenges, this additional cost is negligible, especially as the SiC substrates continue to grow in size while maintaining a stable price. “So, in this case, you could say the price is actually dropping,” he says.
The Advantages of Pyramid-shaped MicroLEDs
The pyramid-shaped MicroLEDs from Polar Light Technologies offer numerous advantages, including improved light extraction efficiency and enhanced luminescent characteristics that optimize the use of light.
So far, the company’s technology demonstrations have focused on devices emitting blue and green light, with red light capabilities still in development.
Challenges in Red Light Emission
Achieving red light emission from GaN-based LEDs is no easy task. Success requires increasing the indium content in the quantum wells, but this leads to significant strain, which introduces non-radiative defects. Switching to a wider quantum well alleviates this strain, but at the cost of reduced efficiency, as the overlap of electrons and holes decreases, and under the influence of the internal electric field, electrons and holes are pulled toward the edges of the quantum well.
These issues are reduced in pyramid-shaped LEDs. Thanks to their geometry, pyramid-shaped LEDs are semi-polar, which helps reduce the intensity of the internal electric fields that hinder radiative recombination.
Company Growth and Future Prospects
Polar Light Technologies has been refining its pyramid-shaped MicroLEDs for years. Founded in 2014, the company initially focused on research and patent filings. In 2021, it began receiving funding from several local venture capital firms.
Ivan Martinovic is currently engaged in a “medium-scale” fundraising round, expected to close by the end of this year. He explains: “We’ve built a solid network of Swedish and Nordic venture capitalists. We’ve chosen to focus on these investors because we want to maintain this network.”
“By the end of next year, we’ll be ready to secure larger-scale funding—perhaps for Polar Light Technologies’ Series A round,” he adds.
To date, the company has completed all internal process steps, including manufacturing silicon chips to drive MicroLEDs.
Martinovic shares, “We are using a cold bonding method to hybridize GaN LEDs with silicon CMOS.” He explains that the slight differences in thermal expansion coefficients can cause misalignment, which affects spacing on the display. This makes bonding methods involving heat unfeasible.
Collaborations and Productization
Martinovic and his team are actively seeking collaborations. As he notes, this internal approach is unlikely to continue: “We are more open to collaborations than ever. It’s part of the productization process. We will not go it alone.”
This collaborative push is a significant driver, as greater cooperation will be key to the success of MicroLEDs in many display applications, where miniature emitters that don’t require etching will shine.
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