The use of high-power LED devices is expanding rapidly. These systems are particularly effective in industrial lighting applications, where their high efficiency translates into reduced heat generation. As a result, thermal management becomes simpler and more cost-effective, making them a preferred choice for many manufacturers.
Given the current state of the LED industry, many chip manufacturers are focusing on improving device efficiency to gain a competitive edge in the high-power market. Even small improvements in efficiency—just a few percentage points—can lead to significant advantages, including higher profit margins. A large portion of this benefit comes from the production of high-end LED chips, which are in high demand due to their superior performance.
Another advantage of producing high-efficiency LEDs is their potential success in specialized fields such as automotive headlights. However, these devices have not performed as well in general lighting, where medium and low-efficiency LEDs dominate. Many manufacturers in this segment are struggling due to intense competition and lower profit margins.
**Photon Extraction**
One of the key challenges in creating efficient LEDs is extracting as many photons as possible from the device. This is crucial because the refractive index of gallium nitride (GaN) is much higher than that of air, leading to total internal reflection and trapping most of the light within the chip.
To address this issue, a common approach is to use patterned sapphire substrates. The textured surface scatters photons in various directions, increasing the chances of light escaping. This process typically involves etching patterns into the sapphire using photoresist masks and plasma techniques. Over time, this creates a dome-shaped structure that enhances light extraction.
However, while some research groups have explored using high-refractive-index contrast cavity structures to further improve photon extraction, no commercial production methods have been widely adopted yet.
A notable example is the work by Euijoon Yoon and his team at Seoul National University. They developed nanoscale cavities using hollow silica nanospheres, significantly enhancing light extraction. Additionally, these cavities help reduce compressive stress in the surrounding GaN layer, allowing for thinner sapphire wafers and lower manufacturing costs.
Despite these advancements, there are still limitations. The low density and random placement of the cavities limit the overall improvement in light output. To make this technology viable for mass production, further refinements are needed to ensure consistent and reliable cavity formation.
**From Lab to Factory**
South Korea’s Hexa Solution has developed an innovative cavity sapphire substrate technology that addresses these challenges. Their method was validated through feasibility studies conducted by two Korean research institutes. Laboratory-scale tests showed that LEDs using cavity sapphire substrates outperformed those made with patterned sapphire.
The fabrication process involves forming a dome-shaped photoresist structure, followed by atomic layer deposition of an amorphous alumina layer. After heat treatment in an oxidizing atmosphere, the alumina crystallizes into sapphire, creating a stable and scalable production method. This eliminates the need for additional steps to expose the sapphire seed layer, simplifying the GaN growth process.
One of the key benefits of this design is its optical properties. The cavities create strong diffraction effects, resulting in vivid interference colors under different lighting conditions. Transmission experiments also show that cavity sapphires offer higher transmittance across a wide wavelength range compared to patterned sapphires.
Simulations confirm that the high-refractive-index cavities interact strongly with incoming light, altering its direction and improving light extraction. According to Huygens’ principle, each cavity generates secondary waves that enhance overall light output, reducing losses due to total internal reflection.
**Performance Verification**
To validate the performance of cavity sapphire LEDs, researchers produced devices on both cavity and patterned sapphire substrates simultaneously. Both types of LEDs were processed to create large-area, lateral-type blue-emitting InGaN/GaN chips.
Significant differences were observed between the two substrates. Cavity sapphire featured hemispherical structures with specific dimensions, while patterned sapphire had conical shapes. Testing showed that LEDs on cavity sapphire produced 40% higher optical power at 468 nm compared to those on patterned sapphire. They also emitted at dominant wavelengths of 456 nm and 462 nm, showing consistent performance improvements.
An important advantage of cavity sapphire is that it maintains compatibility with standard MOCVD processes, requiring only minor adjustments. This makes it easier to integrate into existing production lines without major overhauls.
Compared to patterned sapphire, cavity sapphire is also more cost-effective. Patterned sapphire requires expensive plasma etching, while cavity sapphire can be mass-produced using a more stable and affordable atomic layer deposition process.
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