Photonic crystal technology to enhance the efficiency of LED vendors actively involved

Light emitting diodes (LED) can improve the efficiency of photonic crystals by imprint lithography. In order to improve light extraction and beam shape, many LED manufacturers are developing photonic crystals LED. The challenge is that the wafer is not very clean and smooth, and is easy to surface with a few microns in size.

Recently, a series of reports on the surface, the light-emitting diode (LED) can be used in photonic crystals, LED can be used to narrow the market from the backlight applications to large-scale high brightness LED applications.

Unilite announced that it will use the photonic crystal technology developed by its subsidiary Luxaltek in high brightness lighting products. Lumileds developed a thin layer (700 nm) photonic crystal LED, the extraction rate of up to 73%. Luminus Devices earlier using photonic crystal, and Industries and Nichia Chemical in February signed a cross licensing agreement CREE Research announced at the end of last year, won the 107 lm/W photonic crystal LED, become the new performance marking device. Luxaltek acquired the intellectual property rights of Mesophotonics, and last year to Sweden's Obducat ordered $thirteen million worth of imprint equipment.

In the field of LED, ten years ago, there have been studies on photonic crystals, which can improve the light extraction and beam spot shape of LED. The semiconductor in LED is a flat plate material with high refractive index, which can be used as a very efficient waveguide. In order to extract light from the LED waveguide, researchers have made a lot of efforts, including the surface roughness and the increase of the reflective layer.

Typical waveguide photonic crystals can capture light emitted from the waveguide.

An alternative is the use of photonic crystals. The typical application of photonic crystals is the output dipole. The idea is that if the size of sub array waveguide has a long through hole right, then there is no light through the waveguide, all the light emitted by only perpendicular to the direction of the screen. In terms of the "language" of photonic crystals, there is a band gap in the waveguide".

Unfortunately, in LED, the quantum wells that produce photons cannot be provided with through holes, and such vias may interfere with the passage of electrons arriving at the quantum wells. The result is that the industry is unable to use the "typical" photonic crystals. From the Massachusetts Institute of Technology (MIT), Cree, Devices, Luminus of group Lumileds, Mesophotonics, Nanonex and other universities have released about how to use the photonic crystal in the actual LED device in the. The trend of thinner LED makes the photonic crystals in LED can be used as an approximation of classical photonic crystals, and is more attractive. Luminus Devices Inc. is the first company to sell photonic crystal LED, with the goal of projection display market.

At the right time?

LED has done a good job in efficiency, the next goal is to reduce costs. If there is a high maintenance cost, like traffic lights, LED now has a cost advantage. The biggest obstacle is the heat sink. Although the power of LED is low, it still needs to be much lower than the working temperature of the traditional light source, so the LED device must adopt the heat sink material.

If the photonic crystal can be achieved by a higher extraction efficiency, or pointing to the function - to increase the cost of a little - to reduce power consumption, then the LED will eventually achieve large-scale popularization. The value of a LED wafer is worth hundreds of dollars, so the lithography process with a few tens of billions of dollars is acceptable, and it can be easily achieved by imprint lithography. Whether the influence of photonic crystals is the biggest factor in weakening the efficiency of LED, the choice of the problem is in the hands of LED designers. Once confirmed, the big LED companies will invest a lot of money and manpower in this area.

Who will benefit?

Given the fact that the patents of photonic crystal LED are in the hands of several major companies, the first answer is likely to be the lawyers involved in these legal disputes. Second, patent holders and agents will benefit. Nichia, Unilite, Cree and Lumileds are the mainstay of the LED market, if the use of high brightness LED market in the high-end products in the use of photonic crystals, they will undoubtedly be the biggest winner.

Other winners also include the entire supply chain to achieve imprinting photonic crystal technology on LED. These technologies include photolithography and etching. According to today's standards, the requirements of lithography is not harsh - generally lambda /4 or 200 nm. The challenge is that the wafer is not flat, there will be several micron sized surface projections, and the wafer is not very clean. The warpage of several tens of microns is the result of the thermal expansion coefficient of the substrate material, such as silicon carbide or sapphire and epitaxial growth of semiconductor materials, such as gallium nitride, its growth temperature is higher than 900 C. These two layers of material, in fact, like a double sheet metal, will form a similar potato chip warpage structure. Thermal stress is also hindered by the use of larger size wafers. The surface projection is a by-product of epitaxial growth, and if the lattice of the substrate and the semiconductor material is not fully matched, a protrusion is formed. Finally, the LED also requires a 50 m contact pad, and the pad is usually made in the Fab with a poor cleanliness.

Most of the research on photonic crystals is done by electron beam lithography and early manual lithography system. Much of the team's research, especially on the manual lithography tool, has used Transfer Devices Inc. (TDI, California Santa Clara) production of the soluble mask, the company has also developed a molecular transfer lithography