There are many substrate materials used to grow GaN films which are requisite for the preparation of blue LEDs. Sapphire and SiC are the most common ones. In recent years, sapphire has become the most important substrate material due to its small lattice mismatch coefficient with GaN and good transparency.
As one important LED sapphire substrate material and compared with sapphire substrate, SiC is of relatively low resistance thus being used to make electrodes. Besides, its lattice constant and thermal expansion coefficient are much closer to those of GaN and it is soluble and of blue light emission characteristics. Nevertheless, it also has its shortcomings. Owing to quite different thermal expansion coefficient from that of GaN which could easily lead to cracks in the epitaxial GaN layer, it is not suitable for mass use and relatively expensive.
LED sapphire substrate is currently the best and most commonly used substrate material. Better match with GaN lattice, the single crystal sapphire substrate have good high temperature stability and mechanical properties, and meet the high temperature resistance of GaN film growth. In addition, it has better light transmittance in the visible light range, more relevant researches, relatively mature production technology, and cheaper price, making it to be the key substrate material for GaN substrates of white, blue, green, and blue-green light.
