Influence of SiGe buffer layer on the optical properties of Ge quantum dots grown on Si substrates by molecular beam epitaxy

Influence of a partially relaxed SiGe buffer layer on the structural and optical properties of Ge quantum dots (QDs) was investigated by atomic force microscopy (AFM) and temperature- and excitation power-dependent photoluminescence (PL) measurements. The Ge QD samples with and without a Si$_{0.75}$Ge$_{0.25}$ buffer layer were grown on a Si (100) substrate by a molecular beam epitaxy (MBE), followed by a Si capping layer. A random, bimodal size distribution and a broad emission were observed for the Ge QDs directly grown on Si substrate without a buffer layer (sample A), while a well-aligned, unimodal size distribution and a narrow emission peak were observed for the Ge QDs grown on the relaxed Si$_{0.75}$Ge$_{0.25}$ buffer layer (sample B). The bimodal and unimodal QD size distributions were clearly reflected in the broadening of the PL spectra of samples A and B, respectively. An increase in PL intensity and a change in PL peak energy with increasing excitation power were another important characteristic in this study. We will discuss the influence of the SiGe buffer layer on strain status, band alignment, and optical properties of Ge QDs.