Relaxation of thermal strain in GaN epitaxial layers grown on sapphire

We have investigated GaN layers of various thicknesses grown on (0001) c-face sapphire by metalorganic vapor phase epitaxy (MOVPE) with an intermediate 10 nm AlN nucleation layer, in order to study the thermal strain in these layers. Using spatially resolved cathodoluminescence (CL) spectroscopy at low temperatures we investigated the relaxation of this stress at the cleaved edges of the samples as can be observed in an energy shift of the luminescence peak. The shift is compared with a theoretical model for the thermal stress in a rectangular plate clamped along one edge. It was found that the sapphire is also strained, but in a tensile way. An effective deformation potential of 12 eV, as found by Amano et al. [6] was confirmed by direct measurement of the energy shift and supplementing X-ray diffractometry. The difference in thermal expansion coefficients between GaN and sapphire perpendicular to the c-face for temperatures of 6-300 K was estimated to 1.2 × 10−6 K−1. Furthermore the freeze-in temperatures for thermal induced dislocation enhancement is estimated.