The use of high-power LED technology is rapidly expanding. In industrial lighting applications, these devices offer remarkable efficiency. The higher the efficiency, the less heat is generated, making thermal management simpler and more cost-effective in the long run.
Given the current state of the LED industry, many chip manufacturers are focused on improving device efficiency to gain a competitive edge in the high-power market. Even small improvements in efficiency can lead to significant profit margins, especially when producing high-end LED chips. This has become a key strategy for companies aiming to capture a larger share of the growing market.
Another benefit of high-efficiency LED production is its potential for success in automotive lighting. However, in general lighting, where medium- and low-efficiency LEDs dominate, many manufacturers struggle. This highlights the importance of continuous innovation in LED technology.
**Extracting Photons**
One of the main challenges in creating efficient LEDs is extracting as many photons as possible from the device. This is crucial because gallium nitride (GaN), the material commonly used in LEDs, has a much higher refractive index than air. As a result, most photons are trapped inside the device due to total internal reflection.
A common solution to this problem is using patterned sapphire substrates. These substrates have a textured surface that scatters light in multiple directions, increasing the chances of photons escaping the chip. The process involves etching patterns into a planar sapphire using a photoresist mask and inductively coupled plasma. Over time, the mask erodes, forming a dome-shaped surface. Adjusting the etching conditions allows for different shapes, such as conical structures.
When light travels through an LED on a patterned sapphire substrate, it reflects and diffracts at the interface between GaN and air, where the refractive index change is greatest. Many research groups have explored incorporating high-refractive-index contrast cavity structures into LEDs. While these structures improve light extraction, they have not yet been widely adopted in mass production.
One notable group is led by Euijoon Yoon at Seoul National University. They developed nanoscale cavities using hollow silica nanospheres, which significantly improved light extraction. Additionally, the reduced compressive stress around the cavities allowed for thinner sapphire wafers, lowering production costs. However, challenges remain, including the low density and random placement of the cavities, which limit their effectiveness.
**From the Laboratory to the Factory**
Hexa Solution from South Korea has made significant progress in addressing these issues. Their core technology involves a cavity sapphire substrate, which was tested through feasibility studies by two Korean research institutes. Lab-scale results showed that LEDs with cavity designs outperformed those with patterned sapphire substrates.
Their manufacturing process is both robust and scalable. It begins with a photoresist pattern that is reflowed into a dome shape. An 80 nm amorphous alumina layer is then deposited via atomic layer deposition at 120°C. The alumina partially covers the sapphire and photoresist. A subsequent heat treatment in an oxidizing atmosphere causes oxygen to diffuse through the porous alumina, forming a dome-shaped cavity. During this process, the amorphous alumina crystallizes into sapphire, eliminating the need for additional steps to expose the seed layer for GaN growth.
The unique optical properties of cavity substrates create strong light disturbances, resulting in vivid interference colors under various lighting conditions. Transmission experiments also show that cavity sapphire has higher transmittance over a wide wavelength range compared to patterned sapphire.
Simulations reveal that the high-refractive-index cavities strongly interact with incoming light, changing its direction and improving light extraction. According to Huygens’ principle, each air cavity generates a secondary wave, enhancing overall light output.
**Performance Verification**
To validate the performance of cavity sapphire LEDs, researchers produced devices on both cavity and patterned sapphire substrates simultaneously. The resulting LEDs were 1075 μm × 750 μm in size and mounted on the same package. Measurements using an integrating sphere showed that cavity sapphire LEDs produced significantly higher optical power.
At 240 mA, cavity sapphire LEDs emitted at 468 nm with 40% higher power than patterned sapphire ones. They also outperformed conventional LEDs at other wavelengths, though the difference was smaller.
A major advantage of cavity technology is that it uses standard MOCVD processes, requiring minimal changes to existing production lines. Compared to patterned sapphire, cavity sapphire is cheaper to produce, thanks to the stable and cost-effective atomic layer deposition and heat treatment process.
In summary, cavity sapphire technology represents a promising advancement in LED manufacturing, offering better performance, lower costs, and greater scalability. As the industry continues to evolve, such innovations will play a key role in shaping the future of solid-state lighting.
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