Stress relaxation of AlGaN on nonpolar m-plane GaN substrate

The stress relaxation with increasing thickness of metal-organic vapor phase epitaxy grown Al 0.19Ga 0.81N on quasi-bulk (10 1 ¯0) 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 ¯10], [0001], and [10 1 ¯0] 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 ¯10] relaxes by the onset of misfit dislocations through the { 10 1 ¯ 0 } ⟨ 1 2 ¯ 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 ¯10] and [0001] decrease the bandgap.