TOKYO — Shimei Semiconductor Co. has developed a blue LED grown on a silicon wafer that it plans to make available by next April.

Using silicon wafers as a substrate for GaN epitaxy could drastically lower the cost, simplify LED structure, extend the lifetime and enable the integration of an optical device in CMOS circuits, the company claims.

The prototype LED emits blue light of the 450-nm wavelength and an output power of 10 mW. Shimei said it expects to improve performance through future prototyping. LED lifetime was disclosed.

Currently, sapphire substrates are widely used for GaN ataxia growing. If silicon wafers can be used for the substrate of GaN crystalline deposition, "silicon wafers have a lot of advantages [and] it is easy to make large area LEDs, and drivers and other circuits can be fabricated on the same silicon substrate," said Hirofumi Yamamoto, founder and chief technology officer of Shimei Semiconductor (Kyoto, Japan).

Shimei Semiconductor's blue LED on a silicon wafer

The device has a layered structure consisting of a cathode on the bottom, the silicon substrate, buffer layers, an emitting layer and an anode on top. As the silicon substrate absorbs light generated at the emitting layer, a reflective film is formed on the silicon to improve efficiency.

Many attempts were made to form a GaN buffer layer on a substrate in order to overcome the wide lattice consonant gap, but Yamamoto said the buffer layers are different from conventional buffers. He declined to elaborate.

The company claims a proprietary material composition and a turned material organic chemical vapor phase epitaxial growth (MOVPE) process that enabled GaN epitaxy on silicon. In addition to the blue LED, Shimei said it is working on LEDs with longer wavelengths, including green and even red. It has also started blue laser development.

Shimei said it is readying production lines with a monthly capacity of 3 million units and will start sampling in April 2007. The LEDs will be supplied in bare chip or wafer form.