Effects of a compositionally graded buffer layer on stress evolution during GaN and Al_xGa_1-xN MOCVD on SiC substrates

In situ substrate curvature measurements were used to monitor stress evolution during metal organic chemical vapor deposition (MOCVD) of Al_xGa_1-xN epilayers (x=0, 0.5, 0.7) on 6H-SiC (0 0 0 1) substrates that were grown using compositionally graded AlN to Al_xGa_1-xN buffer layers. The results were compared to identical growths carried out using thin (∼80 nm) AlN buffer layers in order to assess the impact of the graded layer on the mean film stress and structural properties of Al_xGa_1-xN. Compositionally graded (AlN-GaN) layers effectively increased the mean compressive stress in GaN epilayers grown on SiC compared to samples grown on AlN buffer layers, with the mean stress in the GaN layer increasing with buffer layer thickness. Increasing buffer thickness correlated with a decrease in the threading dislocation density measured by plan-view-TEM. In contrast, the mean stress in high Al-fraction Al_xGa_1-xN epilayers (x=0.5 and 0.7) was not significantly altered by the use of AlN to Al_xGa_1-xN graded buffer layers and in some cases, the graded layer leads to a decrease in mean compressive stress in the films. The differences in stress evolution are explained in terms of the available compressive strain energy, which is suggested to be responsible for dislocation bending observed in the GaN graded layer samples.