Japanese start-up touts GaN-on-silicon LED
Shimei Semiconductor, a start-up company based in Kyoto, Japan, is reported to have developed a blue LED using GaN-on-silicon epitaxy.
According to reports at both Nikkei.net and EE Times, Shimei is planning to begin shipping samples of the technology in April 2007.
Because native nitride substrates aren't yet available in large diameters, blue LEDs are normally fabricated on either SiC or sapphire wafers for volume applications such as cell phone keypad backlights.
Although silicon would provide a cheaper, conducting, non-native substrate and be available in very large diameters, the problem is that it is opaque in the blue spectrum.
To get around this, the Shimei technical team, led by co-founder and CTO Hirofumi Yamamoto, have developed a way to deposit a reflecting layer within the device structure so that all the light is emitted out of the top of the LED.
Unlike sapphire substrates, silicon is conductive, so the bottom electrode of the chip can sit below the silicon and eliminate the need to etch down from the top of the device.
According to the Nikkei.net report, Shimei is readying a production system capable of manufacturing 3 million units per month. It adds that the LEDs can deliver up to 2 cd of 450 nm light when driven at 20 mA.
Shimei s is not the only research team to have worked on silicon as a substrate for blue LEDs. Alois Krost's group at the Otto-von-Guericke University, Magdeburg, Germany, reported a similar device at the 5th European Conference on Silicon Carbide and Related Materials, held in Italy in late 2004 (see related magazine article).
Krost said that although the different thermal expansion coefficients of GaN and silicon could result in cracks throughout the structure, monitoring the strain in the devices as they are grown allows crack-free epitaxy.