Tokyo-based Hitachi Cable Ltd says that it has developed new mass-production technology for gallium nitride (GaN) templates (see Figure 1), in which a high-quality gallium nitride (GaN) single-crystal thin film is grown on a sapphire substrate, and that it will start selling the templates.
Using the template as a base substrate for an epitaxial wafer for white LEDs makes it possible to drastically improve the productivity of white LED epiwafers as well as the LED properties, claims the firm. The product is hence expected to become an effective solution for white LED makers to boost their position in the industry, where there is severe competition.
Demand for white LEDs is growing rapidly, used in recent years in backlight units (BLUs) in liquid-crystal displays (LCDs) as well as ordinary lighting devices due to their energy efficiency and long service life. The structure of a white LED epiwafer consists of a thin active layer and a p-type GaN layer with a total thickness of about 1μm, on top of an n-type GaN layer with a thickness of about 10μm, grown on a sapphire substrate (see Figure 2). In the usual manufacturing process, all these crystal layers are produced using the metal-organic vapor phase epitaxy (MOVPE) method. MOVPE is suitable for growing active layers that require atomic-level control of the film thickness, but it takes a long time to grow a high-quality, thick n-type GaN layer, says Hitachi Cable. White LED epiwafers can therefore be grown only about once or twice a day at the most, thus there is a need for a high-efficiency production method, adds the firm.
Hitachi Cable had previously developed single-crystal free-standing GaN substrates (used to make blue-violet lasers) and also developed unique hydride vapor phase epitaxy (HVPE) growth technology and reactors (for mass producing GaN substrates). Based on this technology, Hitachi Cable has now developed?new high-efficiency production technology and machines for mass producing high-quality GaN templates - consisting of an n-type GaN layer grown on sapphire - as a base substrate for MOVPE growth.
Using a GaN template means that LED makers do not need to grow an n-type GaN buffer layer, roughly halving their required growth time compared with conventional methods, it is reckoned. Hitachi Cable says that its GaN templates are also suitable for high-output LEDs, which require large currents because they allow both low resistance and high crystal formation.
Hitachi Cable lists the main characteristics of the GaN template as follows.
high crystal quality and high surface quality (based on growth technology established in developing free-standing GaN substrates);? a low-resistance n-type GaN buffer, suitable for high-output wafers and bonding-type LEDs (with electrodes on both the front and rear side, made by bonding an LED epiwafer on a highly heat dissipating supporting substrate and removing the sapphire – after which the surface should have a low resistance in order to achieve low contact resistance);? templates on flat-surface sapphire substrates and various types of patterned sapphire substrate (PSS) are available;? wafers with diameters of 2-6 inches in are available (an 8-inch version is planned for development).
Hitachi Cable says that, with the new GaN template now added to its range of GaN substrates and GaN epiwafers, it will strengthen and expand its GaN product group and offer compound semiconductor products catering for a variety of needs.
The firm will exhibit the GaN templates at the International Conference on Compound Semiconductor Manufacturing Technology (CS MANTECH 2013) in New Orleans (13-16 May).
