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Vertical LED with Diamond-Like Carbon (DLC) Interface for High-Power Illumination
Keywords: Vertical LED, Diamond-Like Carbon, CTE
Most blue light LED chips are made by growing GaN epitaxy on a sapphire substrate. Because sapphire is an insulator, the two electrodes on a conventional LED die must lie on the same side. As a result, electrical current travels in a curved path between the electrodes. The consequence of this type of design is a hot spot in the curvature and unused quantum wells on far side of P-N junction. This horizontally current path limits LED longevity due to the hot spot and and decreases efficiency due to the un-optimal utilization of the junction. To improve upon existing technology, several companies are developing vertical stacked LED designs by coating P-type GaN with a reflector (e.g. Ag) that is soldered (e.g. via Au-Sn) to an electrode as the substrate. The electrode can be made from a semiconductor material (e.g. Si by CREE and Epistar) or a low CTE (Coefficient of Thermal Expansion) alloy (e.g. CuW by Formosa Epitaxy). Alternatively, metal (e.g. Ni-Cu) may also be electroplated directly onto the reflector (e.g. SemiLED). These techniques have their limitations. Soldering may not permeate completely throughout the thin gap between wafer and substrate, so poorly bonded dies are always present. On the other hand, electroplating metal only forms a mechanical contact with the GaN wafer so the interface is vulnerable to delamination. Due to the much larger CTE (over 12 ppm/C) of metal relative to GaN (about 6 ppm/C), microcracks induced at the interface can cause a LED's brightness to decay rapidly (e.g. 20% for 6000 hours). If an LED die is driven by a larger current (e.g. 1A), hot spots along interface may develop so lifetime is greatly shortened. In this report, we introduce a methodology to producing a high-powered LED with a thin-film DLC (diamond like carbon) interface that can effectively bridge the semiconductor GaN and metallic substrate. DLC can not only moderate the thermal mismatch, but also to enhance the heat spreading since DLC has a thermal conductivity (475 W/mC) that is significantly higher than even copper. In addition, the metal substrate of the LED can optionally be replaced by a diamond-metal (Ni or Cu) composite that further minimizes the CTE mismatch and boost heat spreading efficacy. Such a DLC LED design can sustain a high drive current so that reduced number of enhanced dies can be used in place of conventional chips for small form-factor general illumination applications.
Michael Sung, CEO
SinoDiamond LED
Sunnyvale, CA
USA


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