Comparison of the microstructural characterizations of GaN layers grown on Si (111) and on sapphire

Due to the large differences in the lattice constants and the thermal expansion coefficients between GaN and Si, GaN growth on a Si substrate usually leads initially to high defect densities and cracks. If high-quality GaN films on Si substrate are to be obtained, it is essential to understand the different growth characteristics of GaN layers grown on Si and on sapphire. In this study, the GaN specimens were grown on sapphire and Si (111) substrates with AlGaN and AlN buffer layers, respectively, by metalorganic chemical vapor deposition (MOCVD). Using transmission electron microscopy (TEM) and micro-Raman spectroscope, we carried out a comparative investigation of GaN growth by characterizing lattice coherency, defect density, and residual strain. These analyses revealed that the GaN layers grown on Si have much residual tensile strain and that strain has an effect on the formation of InGaN/GaN multiple quantum wells (MQWs) above the GaN layers.