At present, the quality of ultra-high brightness white or blue LED depends on the material quality of GaN, which is closely related to the surface processing quality of the sapphire substrate. The lattice constant mismatch rate between the sapphire (single crystal Al2O3) C-plane and the Ⅲ-Ⅴ and Ⅱ-Ⅵ group deposited films is small, and it also meets the requirements of high temperature resistance in the GaN epitaxial process, making sapphire wafers a key material for making white, blue or green LED substrates.
Generally speaking, the following factors must be considered when evaluating LED substrate materials:
1. The matching degree of the structure of the substrate and the epitaxial film must be considered. The crystal structures of the epitaxial material and the substrate material are the same or similar, lattice constant mismatch is small with good crystallization performance, and low defect density.
2. The matching degree of the thermal expansion coefficient of the substrate and the epitaxial film must be considered. It’s very important for the thermal expansion coefficient to be matched, because the large difference in thermal expansion coefficient between the epitaxial film and the substrate material may not only reduce the quality of the epitaxial film, but also cause damage to the device due to heating during working process.
3. Matching degree of the chemical stability of the substrate and the epitaxial film must be considered. The substrate material must have good chemical stability, so that it is not easy to decompose and corrode in the temperature and atmosphere of the epitaxial growth, and its quality cannot be reduced due to the chemical reaction with the epitaxial film.
4. How difficult the materials preparation is and how high the cost is must be considered. Taking into account the needs of industrial development, the preparation of substrate materials requires simple preparation, the cost should not be high, and the substrate is generally not less than 2 inches in size.
Sapphire crystal is particularly suitable as an LED substrate because it meets the requirements of LED substrate materials in all aspects with many excellent characteristics such as high hardness (Mohs 9), high melting point (2045°C), good light permeability, good thermal stability, and stable chemical properties.