Gallium arsenide (GaAs) is a semiconductor that is necessary to meet with a red laser requirements. It can be produced by methods similar to those developed previously for silicon. Tiles made from this material are the ideal substrate on which, with great precision, atoms can be placed to form the active part of the laser that generates light (from quantum wells) with a thickness of over a dozen layers of atoms. It is important that the substrate does not have defects, called dislocations, and that the distance between atoms making up the ground and those of the quantum wells are the same.
These conditions are relatively easy to produce in case of red lasers. In the case of blue semiconductor lasers the best ground is another semiconductor - gallium nitride (GaN). The process of producing single crystals of GaN is much harder than GaAs. It is similar to the process of manufacturing synthetic diamonds, since both diamonds and GaN are formed at very high pressures and temperatures. Many technical challenges make it difficult to manufacture GaN, one of which is the need to use high-pressure nitrogen gas.
The process of high-pressure crystallization of GaN seemed to be impractical and since the 1960s attempts to replace the surface of the GaN substrates with readily available sapphire have not worked. Mismatch between sapphires structure and Gallium Nitride created a large number of structural defects (dislocation), which prevented the implementation of efficient blue light-generating devices.
In 1992, the Japanese inventor Shuji Nakamura invented the first efficient blue LED, and four years later, the first blue laser. Nakamura used the material deposited on the sapphire substrate, although the number of defects remained high (106-1010/cm2). The presence of defects in the structure of the laser made it difficult to build a high-power laser.
In the early 90s the Institute of High Pressure Physics at the Polish Academy of Sciences in Warsaw (Poland), under the leadership of Dr. Sylwester Porowski was developing technology to create gallium nitride crystals. Those crystals had very high structural quality and the number of defects did not exceed 100/cm2. It was at least 10 000 times less than in the case of the best material deposit on sapphire.
In 1999, Shuji Nakamura tried to use Polish crystals to see how defects in this crystal affected the properties of lasers. Laser built on Polish crystal have proved repeatedly to be better than previously constructed, both in terms of lifetime and efficiency. The lifetime at a power of 30 mW has increased 10-fold (from 300 to 3 000 hours), and the yield more than twice.
A further development of the technology has led to the launch of the first mass production of the device. Today - blue lasers utilize sapphire surface covered with layer of gallium nitride (this technology is used by Japanese company Nichia, which has an agreement with Sony), and blue semiconductor lasers utilize a gallium nitride mono-crystal surface (Polish company TopGaN).
After 10 years in Japan it was possible to master the production of a blue laser with 60 mW of power, making them applicable in reading a dense high-speed stream of data from Blu-ray, BD-R, and BD-RE. Polish technology is cheaper than Japanese but has a smaller share of the market. There is one more Polish high-tech company which creates gallium nitride crystal - Ammono, but this company does not produce blue lasers.
Nakamura's technological success, which created the basis for a new field of blue-laser utilization in the electronics industry, has been honored with the Millennium Technology Prize awarded in 2006 year.
As with conventional CDs and DVDs, Blu-ray plans to provide a wide range of formats including ROM/R/RW. The following formats are part of the Blu-ray Disc specification:
BD-ROM - read-only format for distribution of HD movies, games, software, etc.
BD-R - recordable format for HD video recording and PC data storage.
BD-RE - rewritable format for HD video recording and PC data storage.
There's also plans for a BD/DVD hybrid format, which combines Blu-ray and DVD on the same disc so that it can be played in both Blu-ray players and DVD players.
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