Strain distribution in GaN hexagons measured by Raman spectroscopy

Abstract

Epitaxial growth of GaN layers normally starts in a three-dimensional growth mode at nucleation sites. Growth islands are formed which coalesce when a certain layer thickness is achieved. In our samples these islands show hexagonal structure and in some cases a flat surface plane perpendicular to the (0001) growth direction. We studied these hexagons by spatially resolved Raman spectroscopy. Raman spectroscopy is a powerful tool to determine strain in GaN layers, because the E2 Raman modes are very sensitive to the change of the elastic properties of the material. In all cases we found that the centre of the hexagons is nearly strain free whereas at the edges there is a tensile strain. This tensile strain, however, is not as high as in the surrounding film.