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To: John Finley who wrote (104)	12/7/1999 10:46:00 PM
From: kinkblot	Respond to of 565

I'm wondering about GaN substrates because of Nakamura. He's going that route, after a fashion. Since he's the guy who found the holy grail, the enabling technology, whatever he does is worth paying attention to. ttec.org This is something called Int'l Tech Research Institute, Tech Transfer Division. ? Judging by the sponsors, our tax dollars are once again at work. <g> see: Site Report ---> Nichia Chemical (6/98) In the section GaN Blue Laser Diodes, they give a slightly different description of ELOG: A key development in obtaining reliable CW laser performance has been defect reduction by using an epitaxial lateral overgrowth GaN (ELOG) substrate. In this technique a silicon dioxide mask is propagated by block dislocation, and a "defect free" film is achieved in the laterally overgrown region. After 100 microns of growth, a fully coalesced GaN thin film is achieved, and a proprietary process removes the sapphire substrate. see: Panel Presentation ---> Status of Japan and U.S. GaN Materials Tech (12/98) About 10 slides in is a cartoon of the overgrowth technique, showing that defects are blocked by a silicon dioxide layer. This is a lot like what Kopin does (used to do) with their CLEFT and ISE techniques. I thought they might be using a selective etching technique to release the 100 micron layer. I'm not so sure the EETimes article has it right about the polishing; maybe they just polish after cleaving? The slide "Size of GaN Technology Effort in Japan" shows what we're up against. Some of the other Presentations are also worth looking at, though they're about a year old now. WT

To: John Finley who wrote (104)	12/8/1999 9:26:00 AM
From: kinkblot	Read Replies (1) \| Respond to of 565

J. Mater. Res., Vol. 14, No. 7, July 1999, pages 2716-2731 This is a paper on the topic of current interest: InGaN/GaN/AlGaN-based laser diodes grown on epitaxially laterally overgrown GaN, by Shiji Nakamura. He does do it by polishing. In his words: After obtaining the 20-micron-thick ELOG substrate, the GaN growth was continued up to a thickness of 100 microns. After the 100-micron thick GaN growth, the sapphire substrate was removed by polishing in order to obtain a pure GaN substrate with a thickness of approximately 80 microns. For comparison, he fabricates laser diodes on both 80 micron thick pure GaN substrates and ELOG on sapphire substrates of the same total thickness (70 microns sapphire + 10 microns ELOG). The only cleaving he does is within the GaN crystal to create a mirror facet for laser operation. This helps him achieve single mode emission without any interference effects, and output power at current of 200 mA of 90 mW, the highest value reported (at publication) for a III-V nitride-based laser diode under RT-CW operation. Lifetime is increased by about a factor of 4 for the devices built on the pure GaN substrate versus ELOG on sapphire, he believes due to the lower thermal resistivity of the substrate resulting from the higher thermal conductivity of GaN. He concludes his paper: "The ultimate substrate for GaN growth is GaN." WT