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Silicon based and light extraction technology for high cost performance lighting

时间:2017-04-06 10:40:28    来源:

Conventional gallium nitride (GaN) LED devices are usually based on sapphire or silicon carbide (SiC), because the lattice matching of these two materials is better than that of GaN, and the substrate size is usually 2 "or 4"". The industry has been committed to developing GaN by supplying more abundant silicon wafers (6 or larger) because silicon substrates can significantly reduce costs and can be manufactured on automated IC production lines. It is reasonably estimated that this substrate saves 80% of the cost compared to conventional techniques.
However, the problem with the silicon substrate is that it does not match the mechanical and thermal aspects of the GaN seriously, which results in severe warpage of the wafer forming the LED element and deterioration of the crystal material quality. Now, University of Cambridge CamGan (acquired by Plessey in 2012) silicon based GaN technology has solved this mismatch problem, and has been successfully applied to its wafer processing plant in Plymouth, uk. As a result, the industry's first low-cost, entry-level commercial silicon based GaN LED is now on the market. Primary products are mainly directed light and key lighting market, the light efficiency of 30-40lm/W, three this year, the fourth quarter will launch 70 lm/W products, supply to more general lighting market.
Figure 1: vertical LED production flow chart.
GaN, on, Si, Growth: silicon based gallium nitride growth
Mirror layer added: additional mirrored layers
Wafer: using wafers
Flip, bonded, wafer: flip chip
Substrate removal: substrate removal
Metallisation, and, surface, texturing: spray metal layer and surface texture
The production of LED on silicon substrates requires some process steps to overcome the light absorption problems inherent in the silicon material in the architecture and to produce efficient components. In the wafer fabrication process (shown in Figure 1), a vertical LED element is designed on the GaN architecture (based on the 6 silicon wafer through MOCVD growth). Immediately following the deposition and adhesion of a high reflective contact (usually 95% reflectivity), the metal layer is then fabricated to paste the wafer onto the replacement substrate.
Then the welding line in the casting welding layer, the conductive and heat fusible Jin Xiceng (heavy melting point temperature is about 280 degrees Celsius) together with other metal layer, as the weld metal and carrier between components or substitute. After welding line is finished, the parent wafer is removed, and the seed layer for epitaxial growth of GaN layer is exposed. Flip the wafer for the next LED component patterning process. The metal coating is patterned on the wafer and placed over the barrier to minimize the amount of light coverage. Most of the current is delivered by top metal (usually 2m). Finally, light extraction patterning was performed and etched onto the GaN layer (exposed behind the solder line) to remove parent wafers. The final step is especially critical for remote phosphor applications, as it enables the control of blue light LED emission patterns.
Since the reflection index of GaN semiconductor is very high (the reflection index of 445nm blue is about 2.45), only a small amount of light escapes into the free space. According to Snell's law, the narrow optical escape cone is about 25 degrees. If we assume that the light distribution within the semiconductor has favorable space, and the mirror reflection index greater than 90%, then only 8% of the total light can escape from the top surface of the semiconductor, the other is limited to the total internal reflection inside, and eventually absorbed component materials.
In order to improve the light extraction, using a semiconductor coupled to a large dome lens (the radius ratio of semiconductor light emitting zone size is 1.5 times larger) the simple design. Ideally, quaquaversal reflectance index (n~2.45) lens should be made with the GaN approximation of the material, which makes more than 90% of the light to escape free space.
But in fact, there is no reflection and GaN index, and cost-effective, can be made into a dome shaped lens material, so LED manufacturers usually to use reflective index for easy access to about 1.5 of the epoxy or silicon materials. However, adding reflection index for the Dome Lens 1.5, only the light extraction rate reached 12%. In order to overcome the weak light extraction performance caused by total internal reflection, it is necessary to optimize the optical path of light to increase the possibility of appearing in the escape cone.
Most of the traditional extraction methods of high efficient cost-effective technology is determined based on surface roughness. This surface technology is critical because it limits the LED component to the final angle of light. This is very suitable for remote fluorescent powder applications, especially the blue light LED light-emitting pattern control. Other patterned reflectors are used to scatter light, which can be further improved by light extraction. In essence, similar microarchitecture, such as fireflies, the sawtooth of the inner structure of fireflies, can enhance the intensity of illumination.
Most of the LED light emission in a spatial pattern, and the light intensity changes with the angle cosine value changes, showing the distribution of the standard number. When these standard LED are used in an array to form an illumination distribution panel, the propagation of light results in some unusual patterns of illumination that are not in the desired range (as shown in Figure 2), which we call "hot spots."". In Figure 2, the brightness of the LED has remained low to help illustrate the problem.
Figure 2: an example of an existing lighting distribution panel.
For the extraction of light and the formation of the spatial pattern is more uniform, showing more beautiful effect for consumers, LED light should not be a Lambeau distribution, but a distribution of bats. In this way, light can reach a wider side area, thereby maximizing pumping efficiency of the phosphor, and reducing losses by improved blue light conversion.

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