January 26, 2022

Where are the difficulties of blue LEDs?

Nowadays, LED has been widely used in daily life. Under the policy of promoting energy-saving lighting in China, there are countless manufacturers of LEDs. Such signs make people take it for granted that such a small lamp does not have much scientific content compared to computers and smartphones. I used to think so. But in fact, what our country's overwhelming PV companies do is only the packaging work for LEDs. This is very similar to the production of computers. Enterprises of large and small size just assemble the various parts, and the enterprise of Niu X will design the computer shape by themselves. In the LED industry, all the optoelectronic enterprises, the level of technology almost does not reach the core part of the self-produced LED, that is, the extent of the chip of the light-emitting diode.

Simply put, semiconductors tend to emit red and yellow light, making it difficult to emit blue light.

To illustrate the difficulty in manufacturing blue LEDs, we must start with the principle of LED illumination. The light-emitting area of ​​a light-emitting diode (LED) is a pn junction called an acTIve region. After the electric field is applied to both ends, the holes in the p region and the electrons in the n region move toward the center and eventually recombine in this region. Of course, not all electron-hole pairs will emit photons when they are combined, and the compound that can radiate photons is called radioradiation (radiaTIve recombinaTIon). This process can also be thought of as the transition of electrons from the conduction band to the valence band and radiating a photon, as shown below.

Where are the difficulties of blue LEDs?

Therefore, the color of the radiated light, or the energy of the radiated photons, is completely determined by the band gap. People's demand has led to the rapid development of semiconductor technology, and now it is possible to prepare large single crystal silicon, which is a fairly perfect crystal with few defects. Unfortunately, the first generation of semiconductor silicon is an indirect bandgap semiconductor with low luminous efficiency. For electroluminescent elements, direct bandgap semiconductors are commonly used, and the development process is as follows.

Where are the difficulties of blue LEDs?

However, for direct bandgap semiconductors, how to obtain a perfect crystal has always been a technical problem. II-VI semiconductor compounds are extremely easy to form structural defects and lack the value of commercial applications, so more attention is paid to III-V semiconductor compounds. Prior to 1975, second-generation semiconductor arsenides and phosphides have achieved bright luminescence in the red-yellow region. As can be seen from the figure below, the band gap between GaP and GaAs is small, and the photons radiated are in the red-yellow band. In order to achieve short-wave radiation, it is necessary to increase the content of the phosphorus component, but this causes a significant decrease in luminous efficiency. Where are the difficulties of blue LEDs?

With the development of technology, the advantages of the third generation of semiconductor nitrides have gradually emerged. The vertical axis in the figure is the energy gap width, and the visible light range is about 1.5eV-3eV. As can be seen from the figure, the band gap of InN is 1.9eV, corresponding to the red region; and the band gap of GaN is 3.4eV, corresponding to ultraviolet Light zone. The entire visible light region can be covered by the ratio adjustment of the In and Ga groups. But such a beautiful future was shattered by a cruel reality—the crystal quality of nitrides could not be guaranteed. Due to the different lattice constants of GaN and InN, under the high indium composition, lattice mismatch leads to the generation of a large number of defects, which seriously affects the luminous efficiency of the device. It is mentioned that GaAsP has a low luminous efficiency in short-wavelength, which is caused by physical principles; here is the reason for production technology.

The blue LED chip belongs to the GaN material system, and its main problems are: 1. Yellow-green light band defect (Green-YellowGap) As can be seen from the following figure, two kinds of LEDs of InGaN and AlGaInP are in the visible light region. The end has a high external quantum efficiency (ie, electro-optic conversion efficiency), but the efficiency in the yellow-green region is significantly reduced. The reason is already explained in the previous section.

Where are the difficulties of blue LEDs?

2. Efficiency Droop LEDs have high luminous efficiency under low current injection. However, when the injection current is increased to the extent that it is available, the luminous efficiency of the high power LED will produce a large attenuation of more than 70%. This is not caused by a simple chip fever. The reason is not conclusive. There are two main explanations: Auger recombination and carrier leakage. Therefore, I can only say that when a large lighting company claims that its R&D team "completely solves the problem of luminous efficiency degradation by using semiconductor cooling technology" on the official homepage.

This year's Physical Nobel Prize was awarded to the inventor of the Blue LED. I was very impressed when I saw this news. I have read a lot of Nakamura's literature. The research team led by him has developed pn-junction blue LEDs in 1991 and two-time heterojunction LEDs two years later. To realize the commercialization of LEDs, they have overcome the following difficulties. Class: 1. Growth of GaN crystal, reducing defect density; 2. Design of active region structure to improve luminous efficiency; 3. Fabrication of electrodes to form an ohmic contact between the metal electrode and the semiconductor, thereby enabling The semiconductor chip can be connected to the circuit. Today, many new structures have emerged in the design of LED active areas. The most widely used is the Multiquantum Well (MQ). China's research group has not yet produced a commercially available LED chip. Although energy-saving lighting has been supported by national policies, the development prospects of this industry are not optimistic. Even if China's large and small lighting companies can grow LED chips by themselves, the depreciation of MOCVD equipment for growth is a huge expense, and most of the profits flow into the manufacturers of foreign production equipment. Some of the top ten companies in China can only survive on state subsidies. The status quo of the LED industry is embarrassing, which is why I gave up research in this field.

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