High-Quality Gallium Nitride Crystals

Ricoh has employed the flux method to realize growth of high-quality crystals having dislocation density reduced by three orders of magnitude.

Crystal Growth Using the Flux Method Enhances the Characteristics of Gallium Nitride

Gallium nitride is a semiconductor applied in blue LEDs, white LEDs, and violet laser diodes used for various applications, including LED displays, lighting, and the light source for Blu-ray disc drives. To develop high-performance devices that bring out the maximum potential of gallium nitride, it is necessary to achieve high-quality gallium nitride substrates having low dislocation density. As gallium nitride has a high melting point and decomposes at high temperatures, it is difficult to grow crystals from melt, the process used with silicon. Accordingly, Ricoh engages in research and development to achieve high-quality, large gallium nitride crystals using the flux method.

The flux method is a solution method for growing gallium nitride (GaN) by dissolving nitrogen in a mixed solution of metallic sodium (Na) and gallium (Ga) (Figure 1). The use of this method makes it possible to grow high-quality gallium nitride crystals at the comparatively low temperature of 900°C and atmospheric pressure of 8MPa (Figure 2).
Figure 1: Schematic of crystal growth by the flux method Figure 2: Micrograph of GaN crystals grown by the flux method
Figure 1: Schematic of crystal growth by the flux method Figure 2: Micrograph of GaN crystals grown by the flux method

The Superiority of Crystals Grown by the Flux Method

This method makes it possible to grow crystals with fewer defects than previously. Currently, gallium nitride substrates are produced by thick-film crystal growth using a vapor growth method called the VPE method. However, with this method a defect involving dislocation at a density of about 104 to 106cm-2 (10,000 to 1,000,000 defects per square centimeter) is generated, and this defect greatly deteriorates device performance. On the other hand, the flux method, a liquid-phase growth method, makes possible higher-quality crystals having dislocation density of about 103cm-2. This makes it possible to dramatically increase device performance. Also, the flux method enables growth of platelet crystals having a c-plane (polar plane) and prismatic crystals having an m-plane (non-polar plane) as a facet, which are important for device production, by controlling crystal growth conditions such as temperature and pressure (Figure 3).
(a) Platelet crystal (b) Prismatic crystal
(a) Platelet crystal (b) Prismatic crystal

Figure 3: Micrographs of GaN crystals grown by the flux method

Possibilities Abound for High-Quality Gallium Nitride Crystals

As the band gap energy of gallium nitride is approximately 3.4eV, which corresponds to the blue-ultraviolet optical region, the development of blue-light-emitting devices has advanced. Currently, gallium nitride is used in white LEDs that use yellow phosphor and in violet semiconductor lasers used as the light source for Blu-ray disc drives.

In the coming years, the production of high-quality gallium nitride crystals will lead to progress in the development of higher-efficiency white LEDs for lighting and currently unachievable violet laser diodes. Future availability of RGB semiconductor lasers will open the way to products such as ultra-small projectors and laser televisions, and application to medical equipment that uses ultraviolet radiation will also progress. Furthermore, high-efficiency power transistors and high-frequency transistors will promote energy conservation in the industrial equipment and electronics fields.

In this way, the realization of high-quality gallium nitride crystals will make possible low-power operation that will contribute to the mitigation of global warming.

Page Top