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Author: SHEN Guang Beijing Optoelectronic Technology Laboratory Director
　
The semiconductor solid-state lighting adopts a light emitting diode (LED), which not only saves energy, reduces pollution, but also has the advantages of small size, long life, flexible control and the like. At present, the main reason for limiting the improvement of LED performance is that the light extraction efficiency is not high, resulting in low brightness and serious heat generation, which seriously affects the popularity of semiconductor lighting with LED chips as the core. In this paper, the problem of improving the light extraction efficiency in LED devices is introduced. Several common lightes such as transfer substrate structure, current distribution and current expansion structure, chip shape geometric structure and surface microstructure are commonly used in technical research and industrial production. Extraction efficiency improvement technology, analysis of the theoretical principles and application status of these technologies, and pointed out that LED as a new generation of green lighting, there is still huge development potential.
Over the years, with the continuous development of semiconductor lighting, LED has received more and more attention for its high electro-optical conversion efficiency and environmental protection advantages. LED chip, the core component of semiconductor lighting products, has developed rapidly in research and production technology, and the brightness and reliability of the chip have been continuously improved. In the development and production of LED chips, the improvement of quantum efficiency outside the device has always been the core content. Therefore, the improvement of light extraction efficiency is very important.
The light extraction efficiency of the LED is a ratio indicating the photons generated by the electron-hole recombination of the photons available to be used outside the device and the active region of the epitaxial wafer. In conventional LED devices, light extraction efficiency is usually less than 10% due to factors such as substrate absorption, electrode blocking, and total reflection of the light-emitting surface. Most of the photons are limited to the inside of the device and cannot be emitted into heat. Bad factors affecting device reliability, especially in high-power LED devices [1].
In order to improve the light extraction efficiency, the photons generated in the device body are more emitted to the outside of the body, and the internal thermal characteristics of the device are improved. After years of research and practice, various methods for improving the light extraction efficiency have been proposed. The following are some of the more common and effective methods for improving light extraction efficiency.
Flip-flop structure
In order to reduce the absorption of photons by the substrate, a flip-chip structure of the transfer substrate is employed. For GaAs-based AlGaInP red-yellow LEDs, the flip-chip structure is based on Wafer Bonding technology, which bonds the substrate with better conductivity and thermal conductivity to the P-plane of the epitaxial wafer, and then removes the absorbed light and has better thermal conductivity. A poor GaAs substrate transmits light from the N-face of the device. Under normal circumstances, an omnidirectional mirror is formed on the bonding interface, so that photons that are incident on the side of the opaque substrate can be reflected to the light exiting surface, so that the structure greatly improves the light extraction efficiency of the device, and the flip-chip structure The schematic is shown in Figure 1 [2].
For GaN-based blue and green LEDs grown on sapphire epitaxial substrates, the substrate material (such as silicon, copper, etc.) with better thermal conductivity and conductivity is used to remove the sapphire non-conductive substrate. Good, and avoid the current congestion problem in the double-sided electrode device, and greatly save the chip area, which is of great significance in the application of high-power devices.
At present, all scientific research units and manufacturers are aware of the advantages of flip-chip products. The high-power LED chips sold on the market with a size of about 40 mil, in addition to the blue LEDs on the SiC substrate, are mostly chips using Wafer Bonding technology, which have obvious advantages in terms of light intensity and power compared to the positive-loading chips.
However, due to the necessity of bonding technology for flip chip, the yield is not high in mass production, and the capacity of the bonding equipment and the process are limited. The capacity cannot be compared with the ordinary packaged chip, so the flip chip LED chip The production cost is high. However, due to the huge development potential of the chip structure itself, with the in-depth research and development of bonding technology, the yield and capacity problems will be improved, and flip chip will become the mainstream of the high-power LED chip market.
Current distribution and current spreading structure
In the LED device of the upper and lower electrode structures, current is injected from the electrode, and the current of the active region of the light is concentrated under the upper electrode. Since the LED device is thin in the longitudinal direction, the light can only be emitted from the upper surface, and the metal electrode is opaque, so that most of the light emitted by the active region is blocked by the upper electrode and cannot be transmitted. Therefore, in the design of the device, it is desirable to change the direction of current flow in the device, distribute the current as much as possible around the electrode, and then inject the active region to emit light, so that the emitted light can be extracted and the injection current can be fully utilized.
In order to achieve the purpose of improving current transmission, it is necessary to form a current spreading layer under the LED electrode in order to disperse the current outside the electrode. This requires a layer of highly conductive and transparent material on the surface of the epitaxial wafer. Generally, the upper surface of the LED chip is P-type, and it is difficult to achieve high conductivity in the growth process to achieve high conductivity. Therefore, a good current spreading layer is required to achieve a good current spreading effect. Another method is to grow a highly doped N-type semiconductor to form a tunnel junction with the upper surface of the P-type, and use the high conductivity of the N-type semiconductor for current spreading, but this method has not received good results. The improved technology is to use a transparent conductive indium tin oxide film as the current spreading layer, and the electron beam evaporation method is applied on the upper surface of the chip, and a good current spreading effect is obtained, and the excessive voltage drop is not brought. Become a commonly used technology in current expansion.
A more ideal way to improve current transport is to create a current blocking layer under the electrodes that blocks the current so that current does not pass directly under the electrodes. This increases the current distribution in the light exit region, reduces the "shadow" of the electrode, and allows the photons to be better extracted from the device. There are various methods for implementing this structure. One is to introduce a heterojunction barrier or PN junction by a secondary epitaxial process or a selective diffusion and ion implantation process on the current blocking region selected on the epitaxial structure, but due to the process Complex, high cost, have not been promoted.
Chip shape geometric structure
In order to improve the illuminance of the effective current, in-depth research on the electrode and chip geometry, typically a transparent substrate and an inverted pyramid structure, so that the photons generated in the active region form a five-sided light-emitting structure and The photon propagation direction can be changed to form multiple reflections, and the light extraction efficiency of the conventional dressing structure LED is greatly improved compared with the front light output [3].
Surface microstructure
The photons generated by the active area of ​​the LED chip are emitted from the surface of the chip. Since the refractive index of the material of the exit surface of the device is relatively large (for example, the refractive index of GaP is 3.32 and the GaN is 2.5), total reflection occurs at the exit surface, resulting in only part of the surface. The angle of light can be emitted from the device, and other angles of light are reflected back into the chip and cannot be extracted. This is also an important reason for the low efficiency of LED chip light extraction.
To solve this problem, it is necessary to process the LED light-emitting surface. There are generally several practices: antireflection coating technology, surface roughening technology, photonic crystal technology [4-5].
The anti-reflection film technology is to plate a light-emitting surface of the LED as a transparent conductive film with a refractive index between the epitaxial surface material and the air, and increase the exit angle by adjusting the refractive index of the film to make the light transmission of a larger angle. Come out and reduce total reflection. Taking a red LED with a surface of GaP as an example, the surface may be coated with a layer of SiOxN1-x, and the refractive index of the SiOxN1-x is optimized to maximize the exit angle of the light, thereby improving the light extraction efficiency. However, the method of antireflection film is for a small power chip packaged with a transparent resin or a silica gel, since the thickness of the resin or the silica gel is much larger than the thickness of the antireflection film, so that the antireflection effect is "masked". Experiments show that although the light extraction efficiency of the unencapsulated chip with antireflection film is much higher than that of the anti-antireflection film chip, the difference after packaging is not large.
The surface roughening technique is to artificially pattern the surface of the smooth and flat device so that the light-emitting surface is no longer a plane, and the exit angle of the light is no longer strictly obeying the law of refraction, so that the emitted light is diffusely reflected on the light-emitting surface. Thus, the light emitted from the active area is emitted with a greater probability, and the light extraction efficiency is improved. The general practice is to use the etching method to make many hillocks on the light-emitting surface of the device, control the density and shape of the hillock, and improve the light-emitting efficiency by 50%-70%. However, in actual production, it is difficult to roughen the special shape of the surface, and it is difficult to promote it. The process of mass production needs to be further solved.
In structures with periodic changes in refractive index, photons exhibit wave properties similar to those in a lattice, a structure known as photonic crystals. The introduction of defects in the photonic crystal can be achieved in one-dimensional, two-dimensional, three-dimensional form, and localized states are generated in the bandgap, meaning that normal spontaneous radiation can sustain only one desired mode and suppress other modes. A DBR-DBR resonant cavity LED is the concept of a one-dimensional photonic crystal, containing a defect, a cavity of a certain wavelength length.
There are also reports on the application of two-dimensional photonic crystals to LEDs. Various techniques for preparing two-dimensional photonic crystal lattices, such as lithographic etching, electrochemistry, selective oxidation, and the like, have been reported. Since the photonic crystal limits the guided mode, the theoretical light extraction efficiency can reach more than 90%. However, the development of photonic crystal LEDs is still in the theoretical verification and laboratory stage, and is still not mature. However, the attractive prospect of LED light extraction efficiency close to 1 in the future still attracts many research institutions to explore this.
in conclusion
In summary, although the LED device still has the problem of low external quantum efficiency, a variety of light extraction efficiency improvement technologies have been applied to production, and good results have been achieved, and other immature optimization processes are further In the study. On the basis of the production of LED chip products, there is still a lot of room for development. With the development of research and production technology, the new technology will gradually improve the light extraction efficiency of LED chips, and more and more fully reflect the advantages of energy saving and environmental protection of semiconductor lighting.
references:
[1] Fang Zhilie, Semiconductor Lighting Technology, Electronics Industry Press, 2009;
[2] Chih-Hung Yen, Yi-Jung Liu, Nan-Yi Huang, Kuo-Hui Yu, Tzu-Pin Chen, Li-Yang Chen, Tsung-Han Tsai, Chong-Yi Lee, and Wen-Chau Liu; New AlGaInP Multiple-Quantum-Well Light Emitting Diode with a Thin Carbon-Doped GaP Contact Layer Structure; IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.20, No.23, P1923-1925, 2008.
[3]DA VANDERWATER, I.-H. TAN, GE HO ̈ FLER, DC DEFEVERE, AND FA KISH, High-Brightness AlGaInP Light Emitting Diodes, PROCEEDINGS OF THE IEEE, VOL. 85, NO. 11, NOVEMBER 1997, p1752 -1764, invited paper.
[4] DH Kim, CO Cho, YG Roh, H. Jeon, YS Park, and J. Cho et al., "Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns," Appl. Phys. Lett., vol. 87, pp. 203508?C1?C203508?C3, 2005.
[5] Da Xiaoli, 2007 PhD thesis, pp. 97-152
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