Connected to the LED lighting industry, and compete in the global market (Part 1), (Middle)
There are more and more high-brightness LED packages used in lighting applications, some of which integrate multiple microchips in a single package, and some use only one large chip. In large chips, the light path from generation to emission is long, and the attenuation therein reduces the luminous efficiency. This problem has recently been solved by modifying the chip structure to coat a GaN (gallium nitride) layer on the substrate.
OSRAM Opto Semiconductors has developed ThinGaN technology that uses a laser to strip a sapphire substrate from a GaN-based chip and attach it to a germanium wafer. ThinGaN LEDs have a high light utilization rate, and 97% of the light can be emitted from the surface of the chip.
The company also uses chip-level conversion (CLC) technology to directly apply phosphors to the chip's emitting surface (see Figure 7), which emits blue and yellow light from the same surface. Good results are obtained when used with a lens.
The idea of ​​nano-etching chip surfaces was proposed a few years ago, and some LED chip manufacturers are already adopting this technology. SCIVAX Vice President Okuda Delu explained why it can improve luminous efficiency at low cost: “Traditional technology is difficult to handle large-area wafers, but with nano-printing technology, the cost of processing a single LED wafer can be reduced to only A few hundred yen."
First, the resin is spin-coated on the surface of the p-type GaN layer of the LED wafer, and then the silicon mold with the concave-convex pattern is pressed onto the resin, and the GaN is stripped from the p-type GaN by ion etching to display the concave-convex pattern. . The depth and diameter of the microvoids in the pattern is approximately 200 nm and needs to be optimized to match specific characteristics (such as wavelength of light and chip composition).
The simulation results show that: This technology can increase the brightness of the LED chip by 20% to 30%. Further, the process can also be applied to a sapphire substrate before the formation of the light-emitting layer, thereby suppressing reflection of the interface between the sapphire and the buffer film. The company noted that this process can also reduce lattice defects during GaN crystal growth.
Other companies are developing m-plane GaN substrate LEDs, but they have encountered the problems of low productivity and high cost. Mitsubishi Chemical Corporation uses a relatively low-cost liquid phase growth technology. According to the company's Chuan Mingzhen, by 2015, its manufacturing costs are expected to drop to the level comparable to blue LED chips.
Mitsubishi Chemical currently uses a traditional LED package that includes a blue LED and a yellow phosphor (see Figure 9). The company will use its phosphor-specific technology to combine red and green phosphors with blue LEDs to create LED packages with high color rendering. At present, such packages have low luminous efficiency, and the company hopes to achieve a luminous efficiency of 100 lm/W in 2010 by improving phosphors.
The m-plane GaN substrate is also ideal for future LEDs. In order to improve luminous efficiency and provide good color rendering, Mitsubishi Chemical engineers are trying to combine near-ultraviolet LEDs with red, green and blue phosphors. The company plans to launch samples at the end of fiscal 2009 and begin mass production in 2011.
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