Stress relaxation of AlGaN on nonpolar m-plane GaN substrate

The stress relaxation with increasing thickness of metal-organic vapor phase epitaxy grown Al0.19Ga0.81N on quasi-bulk (10 (1) over bar0) m-plane GaN substrates was investigated by x-ray diffraction. The anisotropic in-plane stress leads to an orthorhombic distortion of the lattice, which requires special mathematical treatment. Extending earlier works, we developed a method to calculate the distortion along [1 (2) over bar 10], [0001], and [10 (1) over bar0] and obtained the lattice parameters, Al content, and strain values. The stress relaxation along the two in-plane directions involves two different mechanisms. First, the stress along [1 (2) over bar 10] relaxes by the onset of misfit dislocations through the {10 (1) over bar0}< 1 (2) over bar 10 > slip system while for thicker layers the stress along [0001] relaxes by crack formation. Comparing the cathodoluminescence emission at room temperature with the expected bandgap showed that both tensile in-plane strains along [1 (2) over bar 10] and [0001] decrease the bandgap.