Hitachi Cable has decided to manufacture and sell samples of two-inch diameter, low defect density, single-crystal gallium nitride (GaN) substrates for blue-violet lasers used in next-generation Blu-ray Discs and other optical discs.
Nine major electronics companies in Japan, the United States, and Europe have established standards for Blu-ray Discs, and GaN-type blue-violet lasers have gained a foothold as the primary means of reading from, and writing to, such discs. There is strong demand for two-inch diameter, low defect density, freestanding single-crystal substrates, which can be used to increase production yields and improve the reliability of laser devices. In the past, freestanding GaN substrates were produced by growing a single GaN crystal on a sapphire substrate that was removed after the crystal had fully grown. However, when this method is used to produce substrates with diameters of two inches or more, it is difficult to remove sapphire base simply and completely, because damage tends to occur when it is removed from the sapphire base.

Using the cutting-edge, freestanding GaN substrate method, known as the Void Assistance Separation (VAS) method, Hitachi Cable has developed new technologies for producing two-inch diameter, freestanding GaN substrates. With the VAS method, a nitride titanium film with meshwork is inserted between the sapphire substrate and the GaN growth layer. As large numbers of microscopic voids are formed in the microstructure of the nitride titanium layer, the GaN crystal can be removed from a large surface area without being damaged. With this method, Hitachi Cable has succeeded in producing two-inch diameter GaN substrates and will be able to make even larger substrates in the future.
One of the drawbacks of growing GaN on sapphire substrates is the difference in lattice constant that occurs between the sapphire and the GaN layer, resulting in a substantial number of dislocations. With the VAS method, the insertion of a nitride titanium film minimizes differences in lattice constant, thereby limiting the dislocation of elements in the GaN crystal. Hitachi Cable has already achieved a low defect density of 10⁶cm^-2 throughout the two-inch diameter substrate and is aiming to lower the defect density even more, to 10⁵cm^-2. Using this method, GaN crystals grown on sapphires have 1/1000 to 1/10,000 the dislocation density of those grown using conventional methods.
Moreover, with conventional methods, surface uniformity can be guaranteed only by polishing the GaN substrate, a process that tends to damage the substrate surface. By removing the need for substrate polishing, the VAS method facilitates both the processing and epitaxial growth stages of laser diode production.
Hitachi Cable plans to produce and sell samples of these two-inch diameter GaN substrates in the spring of 2003. Hitachi Cable intends to build a mass production line that will produce 300 substrate units per month during 2004 at the Company's Takasago Works, thereby contributing significantly to the development of blue-violet lasers for use in optical discs.