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Sumitomo has developed a novel single-crystal gallium nitride (GaN) substrate that may be used in violet lasers for high-capacity next-generation optical video recording technology.
SEI has started shipping samples and is presently working to mass produce the substrates.
Manufacturers of audio-visual equipment are negotiating to establish a unified standard for a new optical video recording format called "Blu-ray Disc".
Blu-ray Disc enables the recording and writing of data from and to optical discs using a violet laser that has a shorter wavelength than the red laser used in DVD players.
The new substrate has low dislocation (crystal defect) areas arranged in an orderly manner.
Each low dislocation area of new substrate is about 500 microns in diameter.
The density per square centimetre of these low dislocation areas is between 10,000 and 100,000, that is 100,000 times less than the dislocation density of conventional GaN epitaxial layers on sapphire substrates.
These low dislocation areas are arranged in an orderly manner to allow easy device fabrication.
In the epitaxial growth of GaN, sapphire substrates are commonly used and recent research has focused on making GaN-based violet laser devices using sapphire substrates.
As the physical properties of sapphire substrates and GaN are different, sapphire substrates cause dislocation density to become extremely high, which reduces the lifetime of the laser and has led to delays in the practical application of GaN-based violet lasers.
In order to overcome the problem of dislocations, an epitaxial lateral overgrowth (ELO) technique is used in the development of violet lasers.
The ELO technique allows dislocations to be reduced to a small area of a few microns in width.
However, when the ELO technique is used, it is difficult to cleave the crystal and obtain a laser reflective surface.
The development of a high-quality, low-dislocation single-crystal GaN substrate, on which epitaxial growth of GaN can be performed has long been seen as a way to solve this problem.
Sumitomo has extensive experience in the development of GaN substrates and developed the world's first 2in GaN wafer in 2000.
The company has established its own unique GaN substrate fabrication process: the dislocation elimination by epitaxial growth with inverse-pyramidal pits (DEEP) technique reduces dislocations by forming inverse-pyramidal pits on the surface of the crystal.
However, using DEEP alone does not produce low dislocation areas of sufficient size.
Sumitomo has now developed a technique that controls the area where dislocations are concentrated.
This technique enables the formation of larger low-dislocation areas.
Low dislocation areas, each about 500 microns in diameter, are arranged in an orderly manner, allowing easy device fabrication.
For lasers to have sufficient lifetime, the density per square centimetre of these dislocation areas on the substrate must be less than 100,000.
Sumitomo's newly developed low-density GaN substrate has a dislocation density of between 10,000 and 100,000, which is 1/10,000 and 1/100,000 the dislocation density of conventional GaN epitaxial layers on sapphire substrates.
The new substrate also features higher conductivity and offers better cleavages, and so is highly efficient when used in laser devices.
Test developments of a 2in substrate of this type have proved successful and the company has started shipping samples.
After laser device manufacturers evaluate the new GaN substrate, mass-production will start by the time Blu-ray Disc comes to market.
Sumitomo forecasts the production of low-dislocation GaN substrates to reach 300 per month by April 2003.
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