| Toshiba Develops Blue-Purple Semiconductor Laser Technology
11 September, 1996
Researchers at institutions throughout the world are conducting intense studies of short wavelength semiconductor lasers. Interest arises from the central role the lasers have to play in realizing even higher-density record and read levels for DVD discs. In addition to boosting memory for computer data, greatly increased disc capacity will allow reproduction of high resolution moving pictures and promote development of richer, highly featured software.
For example, the high-definition motion pictures will require a DVD disc with a capacity of 15 gigabytes (GB) per side -- while today's standard DVD disc contains 4.7GB. Current DVD technology uses a red semiconductor laser with a wavelength of 650nm, but a wavelength between 400nm and 430nm will be necessary to achieve a 15GB capacity in a single sided disc. Toshiba's new development achieves a wavelength of 417nm, well within the required target.
Toshiba's researchers used a gallium-nitride (GaN) compound for the semiconductor laser, in the belief that the III-V compound material is more suitable for producing a stable, short wavelength laser beam than other materials, including such II-VI compounds as zinc selenide materials.
Two breakthroughs allowed realization of the blue-purple laser emission. First was success in fabricating very thin layers of GaN compound crystals in a multi-quantum-well structure. Such thin layers are an essential requirement, as the fineness of the layer determines how easily a laser beam can be emitted. The means to achieving this was utilization of Toshiba's originally-developed metalorganic chemical vapor deposition (MOCVD) technology.
The second breakthrough came in a new technology for cleaving the GaN crystal grown on the C-face sapphire substrate and assuring a smooth surface. Cleaving GaN crystals on a sapphire substrate usually produces a rough surface unsuitable for laser emission, so dry etching has been used instead. However, cleaving is less expensive than dry etching and requires fewer manufacturing processes. Toshiba's new cleavage technology will be more practical when blue-purple semiconductor lasers are commercialized in the future.
Toshiba will continue to refine the new technologies to realize a blue-purple semiconductor laser suitable for practical application, and which achieves continuous-wave operation and with better characteristics.
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