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 i ntermediate 10 mm AIN nucleation laver, In order to study the thermal strain in these lavers. 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 clam ped along one edge. It was found that the sapphire is also strained, b ut in a tensile way. An effective deformation potential of 12 eV, as f ound by Amano et al. [6] was confirmed by direct measurement of the en ergy shift and supplementing X-ray diffractometry. The difference in t hermal expansion coefficients between GaN and sapphire perpendicular t o the c-face for temperatures of 6-300 K was estimated to 1.2 x 10(-6) K-1. Furthermore the freeze-in temperatures for thermal induced dislo cation enhancement is estimated. (C) 1997 Elsevier Science S.A.