Strain determination in heteroepitaxial GaN

Residual strains have been evaluated in a variety of GaN layers grown on sapphire or 6H-SiC from wafer curvature at 293 K, which avoids needing to know the unstrained lattice parameters or energy gap of GaN in advance. Estimated strains at 1.7 K are correlated with the energy of the $A$ free exciton to determine its strain dependence. We find that strain-free GaN has an $A$ exciton energy of $\text{3.468±0.002\hspace{0.17em}}\mathrm{eV}$ at 1.7 K, and 293 K lattice parameters $\text{a=3.1912\hspace{0.17em}Å}$ and $\text{c=5.1836\hspace{0.17em}Å.}$ These values imply that GaN on SiC is frequently under net biaxial compressive stress due to residual lattice mismatch stress, and that several hundred μm thick GaN layers on sapphire and homoepitaxial layers grown on bulk platelets grown at high pressure are both under about ${\text{1×10}}^{\text{−3}}$ in-plane compressive strain. These conclusions conflict with most previous assumptions.