Optical characterization of Ge quantum dots grown by using rapid thermal chemical vapor deposition

The optical properties of multi-stacked Ge quantum dots (QDs) grown on Si (100) by using rapid thermal chemical-vapor deposition have been studied by using photoluminescence (PL), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM). The HRTEM images demonstrate vertically ordered Ge QDs and full contrast between the Si spacer and the Ge QDs. With increasing growth temperature, the size and the height of the dots increase, but the dot density decreases, as confirmed by using AFM. The Ge QDs had a bimodal shape distribution with domes and pyramids coexisting at specific growth conditions. The two major emission bands observed in the PL spectra are attributed to non-phonon (NP) and transverse-optical (TO) phonon replica of the Ge QDs and they are blue-shifted with decreasing thickness of the Si spacer. The interdiffusion or intermixing effects of Ge QDs have been studied by annealing the samples at various temperatures. As the excitation power increases, the NP energy of the Ge QDs is shifted to higher energy, and a plot of PL intensity vs excitation power shows a sublinear behavior with powers of 0.68 and 0.75 for the NP and the TO PL peaks, respectively. The experimental results are discussed with reference to possible emission mechanisms of Ge QDs/Si.