In the manufacture of optical devices such as laser diodes (LD) and light emitting diodes (LED) based upon gallium nitride (GaN) thin films, it is of utmost importance to minimize defects in the GaN film. For a GaN-based LD to have a long life, i.e. several thousand hours of continuous operation, it is essential to limit the defect density in the GaN film to no more than 10 million per square centimeter. In order to achieve this low defect density, very complex schemes of growing, the GaN film epitaxially on a sapphire substrate have been developed. Such growth schemes are very time-consuming, however, and have thus far prevented the commercialization of laser diodes based on GaN. For instance, the overall scheme for producing low defect density in GaN films is to first grow an aluminum nitride (AIN) film on a silicon carbide (SiC) substrate and then deposit a GaN film on top of the AIN film. Such an AIN film is generally known as a buffer layer. The GaN/AIN/SiC system has reasonably close matching in lattice parameters. However, sole reliance on lattice matching is insufficient for the production of defect-free films. GaN films grown on AIN buffer layers on SiC substrates still exhibit a defect density similar to those grown on sapphire substrates, about 1010 dislocations per square centimeter. The reason for the high defect density in the GaN film is largely due to dislocations propagating from the AIN/SiC substrate/buffer-layer interface, which in turn, originate from the inherent defects and imperfections existing on the SiC substrate surface.Researchers at Arizona State University have developed a new method of achieving extremely low defect densities in heteroepitaxial GaN films. In fact, examination of interfaces between SiC substrates and AIN buffer layers using high-resolution cross-sectional transmission electron microscopy (TEM) reveals very sharp and very clean surfaces. A near-perfect epitaxial relationship is observed between the AIN buffer layer and the SiC substrate with very few interfacial defects. Although the nearly defect-free interface is obtained with the SEE method of deposition, low defect densities at the interface can be also be achieved with other standard methods of deposition such as metallorganic chemical vapor deposition (MOCVD), hydride vapor phase deposition (HVPE), and molecular beam epitaxy (MBE).
|Original language||English (US)|
|Publication status||Published - Jan 1 1900|