Silicon also has a poor lattice constant compared to sapphire. Silicon has a lattice constant of 3.84 compared to GaN’s 3.19A, while sapphire has a slightly closer match of 2.75.
In general, sapphire has the best lattice constant when matched with GaN, followed by silicon carbide (SiC), while silicon wafer trails last. Due to the poor lattice constant match, even if lighting manufacturers could save 10% on silicon wafer material, companies often will need to spend more effort on epitaxy growth. The lower yield rates have actually made GaN-on-Si LEDs a net cost adder.
GaN-on-GaN LEDs are also very expensive to manufacture because of the smaller wafer size. It is estimated GaN wafer prices will remain high in the next 5 years. In contrast, sapphire prices have declined significantly over the last 5 years. In a word, GaN-on-GaN LED or GaN-on-Si is not always cost effective, and it may take several years before GaN or silicon substrate based LEDs can match current LEDs.