Structure and optical properties of Ge/Si superlattice grown at Si substrate by MBE at different temperatures

Multilayer structures in silicon containing submonolayers of Ge in the matrix were grown by MBE on Si substrates at different temperatures. An additional peak at 1.068 eV was observed in photoluminescence spectra (PL) from the samples grown at T=600, 650 and 700 °C, whereas it was absent in specimens grown at 750 °C. Electron microscopy structure investigations showed a high density of spherical, coherent Ge inclusions in samples grown at 650 °C, whereas they were not generated at higher growth temperature, such as at 750 °C. This fact indicates that the observed PL peak is unambiguously related to the Ge inclusions. High temperature growth at 750 °C results in formation of modulated structure where the composition sinusoidally changes with periodicities between 1.2 and 0.44 nm along 113 and 112 directions, respectively. This results in a distortion of the cubic symmetry and in a relaxation of the misfit stress causing changes in the electronic properties.