The cavity type sapphire substrate LED technology a not into the general lighting market can be broken

The use of high power LED market is expanding rate. These devices are used in industrial lighting applications, will bring high efficiency. It is because of the higher efficiency, lower heat dissipation, finally, thermal management is simple and cheap.

In view of the LED industry, so many chip manufacturers are working to improve the efficiency of the equipment, and achieved more success in the power industry and greater profit margins. Even if these equipment manufacturers only higher than their peers more than a few percent of the efficiency, but it will bring them a considerable advantage, they can have a lot of room for profit, of which a large part of the production from high-end LED chip.

Another advantage is the high efficiency of production equipment, if they have reliability, then they can be successful in the headlight challenge. However, they did not succeed in places but in general lighting. In general lighting, widely used in low LED, many manufacturers of these devices suffer days.

Photon extraction

The key to a high LED requirements is one of the most of the photons generated from them. This is a very important point, because the refractive index of GaN is much higher than the refractive index of air, resulting in most of the photons due to total internal reflection and trapped in the internal device.

The commonly used method to overcome this problem is to produce a device on patterned sapphire. The corrugated surface can be scattered in many directions on the photon, increase the light from the possibility of escaping the chip.

These substrates are formed by using a flat piece of sapphire, using photoresist mask and inductively coupled plasma etching pattern inside. Through this process, the mask gradually eroded, appear dome shaped surface. If you want a cone, can be formed by adjusting the etching conditions.

When light propagates through the formation of the patterned sapphire substrates of LED, because of the difference of refractive index, light reflection and diffraction. The propagation of light has the biggest change at the interface, the refractive index of the biggest differences, which appear in the boundary between GaN and air. Therefore, many research groups to high refractive index contrast cavity structure into LED. This structure is successful, can increase the light extraction, but so far, not on the use of production level, in the LED industry into the cavity technology report.

A research team has made significant contribution to the cavity LED which is Euijoon Yoon and colleagues at Seoul National University in South Korea team. The nano cavity they spent several years with the formation of hollow silica nanospheres made of LED, to increase light extraction by introducing cavity in LED. In addition, because the cavity around the GaN in compressive stress reduction, they reduce the warpage. This means that the Sapphire Wafers thinner can be used for LED manufacturing, reducing the cost of production.

However, there is still room for improvement. Due to the randomness of the cavities in the lower density and position of the light extraction increase is not high. To solve these shortcomings, in order to create a better and more reliable cavity forming process for LED production.

From the laboratory to the factory

Here is South Korea's Hexa Solution company is how to solve these problems, they are the core technology of sapphire substrate cavity. Two research institutions in South Korea conducted a feasibility study, a laboratory scale production showed that the combination of the LED sapphire substrate cavity design patterned sapphire substrate is better than LED.

They produce process of cavity of the sapphire substrate is very robust, and is scalable (see Figure 1). The process from the photoresist pattern, the reflow process first cylindrical photoresist made into the shape of the dome. Then, in all exposed on the surface temperature of 120 DEG 80nm thick amorphous alumina layer by atomic layer deposition. Alumina partially covered sapphire, partially covered by photoresist.

Figure 1. for the formation of manufacturing process of cavity type substrate including amorphous alumina are fully crystallized, make it a sapphire. This greatly simplifies the subsequent growth of GaN.

The next step is to conduct heat treatment in oxidizing atmosphere. This can be through the porous alumina layer of oxygen diffusion, oxidation by-products will spread outward, the result is a dome shaped cavity.

At the same time, amorphous alumina becomes fully crystallized, from sapphire contact area, completed in the photoresist dome top. One of the benefits of the crystallization method is an amorphous alumina phase is sapphire. Therefore, due to alumina during heat treatment crystallization into sapphire, so do not need additional processing steps to expose the sapphire seed layer and start GaN growth (see Figure 2).

Figure 2. scanning electron microscope shows that the hexagonal dome embedded cavity formed after GaN growth.

The great advantage of the cavity type substrate is their unique optical properties. They have a strong interference, leading to the naked eye can see a series of color. Compared with the patterned sapphire, due to the strong diffraction in the cavity, incandescent and fluorescent lighting will produce interference colors more vivid (see Figure 3).

Figure 3. compared with patterned sapphire, sapphire in cavity type diffraction stronger, more bright colors in the light.

In addition, the transmission experiments show that the transmittance of sapphire cavity in a wide wavelength range is higher than the patterned sapphire.

The finite difference time domain simulations show that the high refractive index contrast cavity and plane wave coming very strong interaction. The results showed that the cavity in changing the direction of light propagation is effective (see Figure 4).

Figure 4. the finite difference time domain simulations show that in cavity type Sapphire