Detailed explanation of the principle and process of blue LED photonic crystal technology

In order to avoid Nichia Chemical's patented blue LED  plus phosphor technology, various industry players have invested in other LED  technologies that can emit white light. Currently, the most anticipated technology is to use UV LED to achieve the purpose of white light. However, UV LED still has two difficulties that are difficult to overcome: light leakage and low brightness . In addition to continuing to work hard to solve related problems, they have to seek other materials or technologies to achieve LED technology that emits white light.

In 1987, two scholars of different nationalities and living in different places, Eli Yablonovitch and Sajeev John, discovered in theory at almost the same time that the propagation state of electromagnetic waves in periodic dielectrics has a band structure, and that materials with photon energy bands are achieved by periodically changing two or more materials with different refractive indices (or dielectric constants) . So today, nearly 20 years after the discovery of photonic crystals , their applications in various fields have been quite impressive, and they have always been a technology that has attracted much attention from researchers.

At present, the technology of using two-dimensional photonic crystals to achieve white light LED  has seen breakthrough developments, making the future Photonic Crystal LED the focus of attention and the hope of breaking away from Nichia Chemical's patents.

1. Photonic crystal properties and structure

Photonic crystals appear in periodic structures with different wavelengths, and can be developed into one-dimensional, two-dimensional and three-dimensional photonic crystals. Among these structures, the most famous should be the three-dimensional photonic crystal structure. However, the manufacturing and commercialization of three-dimensional photonic crystals are very difficult in terms of today's technology. The reason is that the main research field is still retained in the two-dimensional photonic crystals. Therefore, the photonic crystal LEDs that various companies in the LED field are competing to develop today are also two-dimensional photonic crystals.

The general material structure is a fixed structure, so the material itself will have a certain refractive index. The influence of wave number (Wave Number) and frequency on the refractive index of general materials, the horizontal axis is the wave number (Wave Number) of the material, the vertical axis is the frequency, and the oblique line represents the refractive index. The refractive index grows in a very proportional manner, which means that no matter what the wave number and wavelength are, its refractive index is constant. So what kind of structure is the photonic crystal, and then explain it from another angle. The characteristic of photonic crystal is periodic structure, which also produces multiple reflections. The frequency curve of the wave number vector number and the frequency ratio of light formed by the photonic crystal is not so simple, and the curve has become very complicated. This curve will change with the multi-directionality of light, that is, anisotropy, and with its polarization, it can be used to design different products. Photonic crystal has a very famous characteristic, I believe everyone knows that it has an optical energy gap. In this area of ​​the optical energy gap, light does not exist. The slope of the curve here is the same as that of Figure 1A, which is the opposite of the refractive index. As long as it is at this point, the slope is equal to zero. Therefore, beyond this point, the speed of light will not reach zero. Therefore, it can be said that photonic crystals can also control the speed of light.

To put it simply, the purpose of using photonic crystals can be summarized in one sentence: to use periodic structures to artificially control the optical properties.

2. Differences between photonic crystals and solid-state light- emitting devices

Photonic crystals have three optical properties that can be controlled artificially to achieve different purposes. The first property is that if the optical energy gap is used, light can be blocked from passing through. This property can be used to lock light in a very small area. Currently, the industry is using this property to gather light in an area and make an integrated circuit .

Another characteristic is that photonic crystals are anisotropic. The light from photonic crystals will scatter in many directions. The reason is that photonic crystals can be transparent or opaque depending on the polarization angle of the light (it can be transparent at certain angles, but not at other angles).

The third characteristic is that the curve of photonic crystal is very complex and varied. Because the curve of photonic crystal changes very quickly and irregularly, as long as the wavelength changes slightly, you can see that the angle of light entering the photonic crystal will deviate greatly. In terms of advantages, the area of ​​photonic crystal is one thousandth smaller than that of traditional integrated circuits, so, relatively speaking, the integration of circuits has increased by 1,000 times compared to the past. Another advantage is that the refractive multiple can reach 1,000 times the previous one.

In addition, polarization can also be used to change the properties of light, and the square polarization can be concentrated into one thousandth of the previous volume. In simple terms, what are the benefits and characteristics of photonic crystals? High concentration, small size, and low cost. 3. Using photonic crystals to make LEDs

In addition, photonic crystals have other characteristics. Using its characteristics, photonic crystal LEDs can be made. Generally, they can be divided into two types, one is LED and the other is laser diode. LD laser diodes can be divided into photonic crystal DFB laser diodes (Photonic crystal DFB LD) and photonic crystal de fec t LD. Photonic crystal DFB laser diodes are a structure that everyone is more familiar with. Its laser value can be controlled in a very low area for emission. For such a structure, there must be an optical energy gap area. Because of this, it is more difficult to commercialize such a structure.

Relatively speaking, it is relatively simple to make LED using the structure of photonic crystal. The part about photonic crystal that is often confused is that it is thought to be using DFB laser, so some people think whether it is using a specific period or wavelength? In fact, the answer is wrong. The reason is that the incident (Incident) and diffracted (Diffracted) light of DFB laser and photonic crystal LD ​​are limited. However, the incident light angle and diffracted light angle of photonic crystal are not limited. Therefore, it is not using a specific period or wavelength to enhance efficiency. This feature is very important for LED.

4. Photonic crystal blue LED

White LEDs made from blue LEDs emit blue light, but each blue light will  be converted into yellow light according to the YAG phosphor part. Using blue and yellow light, LEDs can produce white light. White LEDs are used in white light lamps and LCD backlights. This type of white LED is called solid white lighting. This type of light has three characteristics: small size, energy saving, and long life, but there is a big problem to overcome: compared to fluorescent lamps, the luminous is relatively poor. In order to solve this problem, photonic crystals can be used to solve this problem.

In order to overcome the problem of low luminous efficiency of blue LED, photonic crystals can be placed in blue LEDs to improve luminous efficiency. The blue photonic crystal LED produced in this way has the characteristic of long cycle. There are several very important technologies to improve luminous efficiency. The production of traditional LED is very simple, but the problem is that the luminous efficiency is relatively poor, because the traditional blue LED surface is totally reflected, and the light coming out of the active layer will be totally reflected by the surface. Such light cannot be emitted outside the LED. In response to this problem, CREE  made some improvements in the production process. In the Deformed Chip, it can be seen that there is a slope next to the active layer. Using such a slope structure, the luminous efficiency can be improved. In order to improve the efficiency, we designed a two-dimensional accumulation surface. Using such a structure, the luminous efficiency of the surface can be improved. Therefore, we use the semiconductor Planar technology, which is a very precise technology to control this structure.

Penetration uses the active layer of the second element to let light pass through. This structure can make the luminous efficiency as high as 80%, but there is also a problem that needs to be overcome, that is, the internal quantum efficiency will be reduced. In order to let the light pass through the active layer, the internal quantum efficiency will be reduced because of the purpose of passing through the active layer. Resonant Cavity is to load a resonator on the photonic crystal LED. This design is called a resonator LED. We configure photonic crystals around the LED. Using this design, the LED efficiency can be increased by 60%. Although the Surface Grating design method we developed using Planar technology mentioned above is good, there are some problems in the injection of current.

Compared with Surface Grating, although Resonant Cavity is easier to inject current, Resonant Cavity itself also has problems. That is, resonant LED is more difficult to manufacture. The difficulty in manufacturing means that the cost will increase. For LED, everyone hopes to mass produce it at a low cost, which has caused a development bottleneck. The two designs of Penetration and Resonant Cavity are just adding a two-dimensional design on the LED. Such a design can be used on the existing LED.

5. Operation principle of photonic crystal blue LED

In the existing LED structure, we can see that its total reflection has a relatively small critical angle, mainly because the surface reflects all the light. In contrast, the LED designed with photonic crystal blue LED can correct the angle of light due to diffraction. The corrected light can be smaller than the critical angle and can enter the critical angle and be projected outside, improving the problem of all the light from the LED being reflected in the past. We can radiate the light emitted from the active layer of the LED 360 degrees, but previous LEDs were only limited by the critical angle and could only emit light within the critical angle range. Only light within the critical angle can be emitted. We know that the area within the critical angle range only accounts for 4% of the entire range, so the light of the photonic crystal is relatively wide, and there is more area to reflect the light. This principle is used to improve the luminous efficiency. 6. Key points of photonic crystal design

There are some key points in the design of photonic crystals. One indicator is the period. The period is related to the diffraction distance. If the period is smaller, the diffraction distance will be larger. Even after correction, there is still no way to emit light to the outside. Conversely, if the period becomes larger, the diffraction distance will be smaller. Because of this relationship, the light can be moved to the outside, so it is necessary to find the most suitable period in the design.

Another key point is height. Height is closely related to diffraction efficiency. In fact, not all light is affected by diffraction. Light affected by diffraction is related to the diffraction rate. Therefore, these two important indicators need to be calculated when developing photonic crystal LEDs. The most appropriate G value must be calculated in the design, so the G value must be obtained through very precise calculations.

In design, how to calculate how much light is needed on the LED surface can be done using the FDTD calculation method, which is commonly used in photonic crystals. Non-photonic crystal LEDs are LEDs with relatively flat surfaces. After non-photonic crystal LEDs generate light, the part of the light source that contacts the air will be reflected by the surface.

The design of photonic crystal LED can make light not be affected by reflection and reflect light to the outside. The height part is also distributed in a curve. When it reaches a certain height, the efficiency is the highest. It can be seen that the period with the highest luminous efficiency is at 1.5 microns, and the highest luminous efficiency is 0.25 microns. It can be seen that in this area, there is a very long period and a very short height. This shows that the production of photonic crystals is very simple. Just find the most suitable period of 1.5 microns, which is longer than the wavelength of light. However, it is often said that existing LEDs must at least overcome such conditions. However, from the design here, it can be seen that even if this period is very long, high efficiency can still be achieved. Therefore, this photonic crystal design is called a long-period photonic crystal.

Therefore, the designed photonic crystal LED cycle is relatively long. In addition, there is another feature, that is, a whole surface of the photonic crystal is coated with a thin film, which is a transparent electrode. Through this thin film design, light can be emitted from the entire surface .

The effect of transparent electrode on the produced photonic crystal LED is explained. It can be seen that whether or not the transparent electrode is applied does not have a great impact on the luminous efficiency. Based on this result, we can safely cover the photonic crystal with a layer of transparent electrode. Using sapphire as the substrate, and then through MOCVD , EB and RIE ETCHING processes, a two-dimensional photonic crystal LED is produced.

According to us, we are currently using the EB method, but in the future when we officially mass-produce or commercialize it, we will use another method with lower cost, and we will also do dry etching to form a transparent electrode and electrode plate. In theory, the result after calculation should be 3 times higher, but after this experiment, the result is only 50% higher. The reason for the analysis may be that the value used in the manufacturing process of the photonic crystal formation is not the most appropriate value. So we believe that as long as this process is changed, the luminous efficiency should reach 3 times the calculated value.

In addition, another possibility is that there is a small flaw in the manufacturing process, that is, there is a small crack in the chip, and the appearance of this crack will also affect the luminous efficiency of the entire LED.

8. Large-area luminescence can be achieved through transparent electrodes

We are the first to introduce photonic crystals into blue LEDs, and it was very successful. The luminous efficiency reached 1.5 times. I believe that through such continuous research, the industry has shown that the commercialization of solid white light lighting should be just around the corner. This technology can definitely be used and mass-produced. Another point is that the unique design of photonic crystals makes long-period structures possible. Because such long-period structures make the application of GaN photonic crystals easier to achieve. In addition, after actual production, we have also confirmed one thing, that the surface of the photonic crystal is covered with a whole surface of transparent electrodes. Such a unique design makes large-area luminescence possible.