Each crystal has its own lattice constant, which is essentially the distance between atoms that constitute the crystal. We use a sapphire substrate, which has a lattice mismatch of 13.5 percent between itself and GaN. A GaAs substrate is used for II-VI epitaxial growth. The lattice mismatch between the GaAs substrate and II-VI materials is almost zero percent. So they're essentially lattice matched. For epitaxial growth, the ideal is that the lattice mismatch between the substrate and epitaxial materials is zero percent.
Defects cause degradation and a lower power output.
For GaN growth, we didn't have a lattice-matched substrate. It means that the crystal quality of GaN growth on sapphire was terrible. The structure is shown in Figures 2 and 3. Total thickness is about 6 microns. The important thing for me at that time was that no one else was working on it.