| GaN material is direct band gap and wide band gap semiconductor material with excellent optical, electrical and thermal conduction properties. It is suitable to fabricate short-wavelength optoelectronic devices. Since the first commercial GaN/InGaN blue LED came on the market in1993, GaN-based light-emitting devices and their applications have been global research hotspot. In recent years, through the phosphor coating, GaN blue LEDs have achieved white light emission, and has been used in the field of solid-state lighting. Electricity for lighting accounts for20%of the power consumption of the whole society. In order to achieve the green lighting requirements, high bright GaN LED with higher photoelectric conversion efficiency is demanded.Internal quantum efficiency and chip light extraction efficiency are the main factors affecting the photoelectric conversion efficiency of LED. With the development of epitaxy technology, internal quantum efficiency of GaN/InGaN LED has exceeded80%. However, GaN has a high refractive index about2.3. Theoretically, when the ambient is air, due to total internal reflection between the GaN and the air, only4%of the lights in LED could come out, which severely reduces the light extraction efficiency of the GaN/InGan LED chip.GaN devices are mostly prepared by epitaxy technology, such as homoepitaxy on GaN substrate, or heteroepitaxy on sapphire, SiC, ZnO, and Si substrate. As the bulk GaN crystal is difficult to grow, GaN substrate is very expensive and only has been used in the field of scientific research. Sapphire substrate has been used by the vast majority of scientists and LED manufacturers, due to its low price and mature GaN epitaxial technology on it. With the small lattice mismatch and high thermal conductivity, SiC substrate also takes part in LED manufacture, although the price of SiC substrate is relative high. By the virtue of SiC crystal growth technology’s advantages, Cree Company is the only supplier for providing large number GaN LED chips on SiC substrate in the market. The manufacture technology of GaN LED on the SiC substrate is also monopolized by Cree Company.GaN, SiC and sapphire are transparent substrate materials, which is conducive to the LED light extraction. Based on the transparent substrate, chip shaping technology and laser lift off technology could be used to improve the light extraction efficiency of LEDs. In this dissertation, we investigated different chip technology to improve the light extraction of GaN/InGaN LED. The content of this study and the significant results are as follows:In chapter2, we investigated the light extraction efficiency of LED with truncated inverted pyramid (TIP) geometric structure.First, the influence of the properties of the substrate, including refractive index and absorption coefficient, as well as side wall inclination angle and chip size on the light extraction efficiency of LED chip is studied by optical simulation method. Simulation results show that TIP shaping is an effective method to improve the light extraction efficiency of GaN LED chip with transparent substrate, especially for the chip with high refractive index. The material with a refractive index of2.3is most suitable to be the substrate of TIP GaN LED. In reality, GaN (n=2.4) and SiC (n=2.4) is the closest to the optimal value. The result of the calculation is that for a LED chip with dimension of400μm*400μm*100μm, TIP structure can enhance the light extraction efficiency of the chip by100%, when the absorption index of the substrate is set to be0.The influence of the side wall angle of the chip on the light extraction efficiency is studied. For sapphire substrate, the optimal side wall angle is25°. For SiC substrate, in the range of0-60°, there exist a maxima value at35°, and a maximum value at60°. The influence of the absorb index of the substrate is also been studied. The light extraction efficiency decreases exponentially with the increase of the absorb index. For the TIP shaped chip, the light extraction efficiency deceases faster than that for regular rectangular shaped LED chip.The simulation results indicate that as the chip size increases, the light extraction efficiency enhanced by TIP shaping decreases rapidly. The effect of TIP shaping is very small for millimeter-size chips. Solution for large-size transparent substrate LED chip substrate is divided it into multiple connected TIP structure.Referring simulation results, the GaN LED chips on SiC substrate with different sizes and different structures were actually prepared. The prepared10mil×23mil size TIP structure GaN LED chips on SiC substrate have a package power of22mW, and its photoelectric conversion efficiency is36%. The light extraction efficiency of TIP shaped GaN LED chips on SiC substrate with different size was measured. The measured results coincide with simulation results. The rationality and usability of the optical simulation method is also proved.In chapter3, we studied the key technology to fabricate vertical GaN LED by laser lift off sapphire substrate method, including p-GaN ohmic-reflector technology, bonding technology, N-face n-GaN ohmic contact technology and roughing technology. The sapphire substrate was lifted off in high quality with optimized threshold laser power.The influence of capping layer on the electrical and optical properties of Ni/Ag alloy on p-GaN was investigated. Under the conditions of the oxygen atmosphere, Nano-thick Pt and Ni capping layer could effectively reduce the surface energy of the Ag film, and suppressed silver agglomeration. Meanwhile, sufficient oxygen could penetrate through the metal film and make Ni/Ag and p-GaN to form good ohmic contact. Nano-thick Ni capping layer shows better performance than Pt layer. Thick composite capping layer could maintain the good reflectance of the Ni/Ag mirror. However, because oxygen penetration is very difficult when annealing and it is not conducive to the form of Ga vacancy, Ni/Ag shows poor electrical contact property.Ohmic contact on N-face n-GaN is studied. It is found that metal could form good ohmic contact on flat surface of N-face n-GaN. However, it is hard to form ohmic contact on KOH solution roughed N-face n-GaN. From XPS test, the mechanism is attributed to a large amount of acceptor-type Ga vacancy defects, which reduces the n-type carrier concentration at the surface of the N-face n-GaN and increases the Schottky barrier height of metal/n-GaN. Surface roughening of N-face n-GaN by ICP dry etching technique was achieved. The influence of the gas flow ratio of Cl2and BC13on the surface morphology of N-face n-GaN was studied. High density of GaN pillars appear after ICP etching. The density of the pillar decreases and the size of the pillar increases gradually as the proportion of the Cl2gas increases. The density of the GaN pillar is about1×109cm-2, which is close to the dislocation density of GaN film. The mechanism of dry roughing is described. Dry roughening technology could be used as an alternative method for wet roughing, and be used in chip process which is not compatible with wet roughening technology.The process route of the vertical GaN LED was optimized. Laser scribing technique was used to achieve the isolation of the GaN epi-layer. The GaN epi-layer was divided into two parts, mesa area and reserved area. Vertical GaN LED was prepared by mesa-selective laser lift off. A yield of vertical GaN chip as high as80%was obtained. The process route is simple, efficient, and can be applied to industrial production.We investigated the aging failure mechanism of vertical structure GaN LED. It is found that short-term failure of vertical GaN LED is caused by silver migration. We inhibit silver migration by encasing the Ag layer with a Pt barrier layer. The improved vertical chip shows good reliability. The drop of the light output power of the vertical GaN chip is less than5%after4000hours’accelerated aging.In addition, the defects in LED related material, such as in SiC substrate and GaN epi-layer, has an important impact on the LED luminous efficiency. In chaper4, we studied the low angle grain boundary in SiC and yellow luminescence of GaN. The microstructure of low angle grain boundary in SiC is revealed by theoretical simulation and polarizing microscope observation. The low angle grain boundary is formed by symmetrical arranged pure edge dislocations, which have3<a<1120> Burgers vectors. N-face n-GaN was obtained by laser lift off method, and its photoluminescence spectrum was studied. The n-face n-GaN was etched with KOH solution. The intensity of yellow luminescence of N-face n-GaN was enhanced with KOH etching time. The influencing factors such as dislocation density, surface roughness and tested sample position were excluded. XPS measurement show that, after KOH etching, Ga2p and N1s core-levels of the N-face sample move to low banding energy direction, and the fitted Ga/N atom ratio decreases. It is concluded that Ga vacancies is generated at the surface of KOH treated N-face n-GaN. Thus, it is proved that yellow luminescence of GaN was related to Ga vacancy defect. |
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