Although confusion remains over whether Blu-ray or the Advanced Optical disc format offers the better standard for next-generation high-capacity optical disc equipment, this isn't an issue for Sumitomo Electric Industries Ltd. of Tokyo.The company claims that it is the first to offer mass production of low-dislocation single-crystal GaN substrate for violet lasers that should support systems based on either format.Sumitomo is banking so much on optical disc technology to replace conventional DVD players that it expects the substrate production ultimately to become the third element of its compound semiconductor business alongside GaAs and InP. Researcher Kensaku Motoki reports that monthly production capacity is at 200 GaN wafers, with each wafer capable of generating thousands of lasers. This should ramp up to 500 wafers by October and some 5000 wafers per month by 2007. The standard substrate will be 2 in. in size.The problem with conventional DVD players is their inability to record and play back digital terrestrial high-definition programs. Unlike this equipment, which uses GaAs as the light source for reading and writing data to and from optical discs, next-generation optical disc players based on violet lasers will have five times the storage capacity.According to Sumitomo engineers, a semiconductor laser is basically a single-crystal substrate with a single-crystal epitaxial layer grown on it. This emits laser light when an electric current passes through it. Violet semiconductor lasers emit from the GaN epitaxial layer. The problem is that this compound evaporates and decomposes at high temperature, so a single-crystal growth process, such as is used with silicon and GaAs, will not work. Therefore, conventional GaN layers are grown on sapphire substrates, which, Motoki and colleagues report, results in high dislocation density and cleavage that reduce laser output power and lifetime to levels unacceptable for next-generation player technology.Sumitomo resolves this issue by using a vapor phase preparation technique to grow GaN crystal on another material and then removing the substrate to obtain a single crystal. First reported in 2000, the technique called DEEP -- for dislocation elimination by epitaxial growth with inverse-pyramidal pits -- produces a density per square centimeter of dislocation areas that is 10,000 times less than that of conventional GaN epitaxial layers on sapphire substrates.This level of dislocation control reportedly leads to better operating parameters for violet lasers.
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