Red, green, and blue LEDs are III-V compounds such as phosphorus, arsenic, and nitrogen, such as gallium arsenide (GaAs), gallium phosphide (GaP), gallium arsenide (GaAsP), and gallium nitride ( Made of semiconductors such as GaN). The LED lighting technology route includes various aspects such as epitaxy, substrate, package, and white LED.
1. Epitaxial technology
The epitaxial wafer material is the core part of the LED, and the photoelectric parameters such as the wavelength, forward voltage, brightness or luminescence of the LED are basically determined by the epitaxial wafer material. Epitaxial technology and equipment are the key to epitaxial wafer fabrication technology. Metal organic vapor deposition (MOCVD) is the main method for growing thin-layer single crystals of III-V, II-VI and alloys. The dislocation of the epitaxial wafer acts as a non-radiative non-radiative recombination center, which has a very important influence on the photoelectric performance of the device. In the past ten years, the industry has improved the dislocation density by improving the epitaxial growth process. However, the mismatch between the lattice and thermal expansion coefficients of gallium nitride GaN and the substrate of the blue LED for mainstream white light illumination still results in a high dislocation density. It has always been the goal pursued by LED technology to study LED epitaxial technology to minimize defect density and improve crystal quality. Epitaxial structure and epitaxial technology research: 1 Droop effect: After years of development, the epitaxial layer structure and epitaxial technology of LED have been relatively mature, the internal quantum efficiency of LED has reached more than 90%, and the internal quantum efficiency of red LED is even close to 100. %. However, in the research of the high-power LED, it is found that the quantum efficiency drop under high current injection is more significant, which is called the Droop effect. The cause of the Droop effect of GaN-based LEDs tends to be the localization of carriers, leakage or overflow from active regions, and Auger recombination. It has been found that the use of a wider quantum well to reduce the carrier density and optimize the electron blocking layer of the P-type region can alleviate the Droop effect. 2 Quantum Well Active Region The InGaN/GaN quantum well active region is the core of the LED epitaxial material. The key to growing the InGaN quantum well is to control the stress of the quantum well and reduce the effect of the polarization effect. Conventional growth techniques include the growth of InGaN pre-well release stress of low in composition before multiple quantum wells, and act as a carrier "reservoir", and then grow the GaN barrier layer to increase the crystal quality of the barrier layer, and grow lattice matching. The InGaAlN barrier layer or the growth stress complementary InGaN/AlGaN structure or the like. Other specific technologies in the study of epitaxial structure and epitaxial technology are as belows:
1 Substrate stripping technology: This technology was first realized by Hewlett-Packard Company in AlGaInP/GaAsLED. The GaAs substrate absorbs light greatly. After stripping GaAs, AlGaInP is pasted on a transparent GaP substrate, and the luminous efficiency is nearly doubled. The sapphire substrate laser stripping technology is based on the development of GaN homoepitaxial stripping. The substrate is irradiated with ultraviolet laser and the transition layer is melted. In 2003, OSRAM stripped the sapphire with this process, and the light extraction rate was increased to 75%, which is the traditional 3 times and formed a production line.
2 Surface roughening technology: Because the refractive index of the epitaxial material is different from the packaging material, part of the emitted light will be reflected back to the epitaxial layer. The roughening of the epitaxial surface is equivalent to changing the exit angle to avoid the total reflection of the emitted light and improving the light extraction rate. The epitaxial surface is directly processed on the process, which easily damages the epitaxial active layer, and the transparent electrode is more difficult to fabricate. It is feasible to achieve surface roughening by changing the growth conditions of the epitaxial layer.
The microstructure of the 3D photonic crystal can improve the light extraction efficiency. In September 2003, Matsushita Electric Industrial Co., Ltd. produced an LED with a diameter of 1.5 microns and a height of 0.5 microns, and the light extraction rate was increased by 60%.
4 flip chip technology According to the US Lumileds company data, the sapphire substrate LED increases the light output by 1.6 times.
5 omnidirectional reflection film: In addition to the front side of the light, the exit light from the other side is reflected as far as possible back into the epitaxial layer, and the light output rate from the front is finally raised.
2, substrate technology
The commonly used chip substrate technology routes mainly include sapphire substrates, silicon carbide substrates, and silicon substrates which have been commercialized, and gallium nitride and zinc oxide which are under development. The evaluation of the substrate material mainly has the following aspects:
1 The structure of the substrate and the epitaxial film layer are matched. The crystal structure of the two materials is the same or similar, the lattice constant mismatch is small, the crystallization property is good, and the defect density is low;
2 The thermal expansion coefficient matching between the substrate and the epitaxial film layer is too large, which will cause the growth quality of the epitaxial film to decrease. During the working process of the device, the device may be damaged due to heat generation.
3 The chemical stability of the substrate and the epitaxial film layer is matched. The substrate material has good chemical stability, is not easily decomposed and corroded in the temperature and atmosphere of the epitaxial growth, and does not chemically react with the epitaxial film to deteriorate the quality of the epitaxial film;
4 The difficulty of material preparation and the cost of industrialization: The preparation of industrial substrate materials should be simple and the cost should not be very high. The large size of the substrate wafer makes the overall cost relatively low.
At present, substrates for GaN-based LEDs which have been widely used for commercialization have only sapphire and silicon carbide substrates. China's silicon substrate technology has achieved technological breakthroughs and is working hard to develop into large-scale industrial applications. Other substrate materials that can be used for GaN-based LEDs are GaN homogeneous substrates and ZnO substrates that are still some distance away from industrialization.
1 Sapphire (Al2O3 Al2O3) is the earliest application of LED substrate technology, and its large output makes its current relative cost lower. Sapphire substrates have many advantages: First, the production technology of sapphire substrate is mature and the device quality is good. Secondly, sapphire has good stability and can be used in high temperature growth process. Finally, sapphire has high mechanical strength and is easy to handle and clean. The disadvantages of sapphire as a substrate: First, the lattice mismatch and thermal stress mismatch are large, resulting in a large number of defects in the epitaxial layer, and at the same time causing difficulties in subsequent device processing; secondly, sapphire is an insulator and cannot be fabricated. In the vertical structure, only the N-type and P-type electrodes are formed on the upper surface of the epitaxial layer, so that the effective light-emitting area is reduced, and the photolithography and etching processes are increased, so that the material utilization rate is reduced and the cost is increased. Furthermore, in order to avoid the problem of P-type GaN doping, it is common to prepare a metal transparent electrode on P-type GaN to diffuse current to achieve uniform illumination, but the transparent electrode will absorb about 30% of the emitted light, while the chemistry of the GaN-based material has stable performance, high mechanical strength, and better equipment for etching. In addition, sapphire is second only to diamond in hardness, and cutting, thinning and processing, it requires some more expensive equipment, resulting in increased production equipment and costs. Sapphire has poor thermal conductivity (about 25W/(m·K) at 100°C) and needs to be fixed at the bottom of the substrate with a silver paste that is not good in thermal conductivity. These are disadvantageous for high power LED devices that generate a lot of heat.
