Nanoheteroepitaxy of Ge and SiGe on Si: role of composition and capping on quantum dot photoluminescence

We investigate the nanoheteroepitaxy (NHE) of SiGe and Ge quantum dots (QDs) grown on nanotips (NTs) substrates realized in Si(001) wafers. Due to the lattice strain compliance, enabled by the nanometric size of the tip and the limited dot/substrate interface area, which helps to reduce dot/substrate interdiffusion, the strain and SiGe composition in the QDs could be decoupled. This demonstrates a key advantage of the NHE over the Stranski-Krastanow growth mechanism. Nearly semi-spherical, defect-free, similar to 100 nm wide SiGe QDs with different Ge contents were successfully grown on the NTs with high selectivity and size uniformity. On the dots, thin dielectric capping layers were deposited, improving the optical properties by the passivation of surface states. Intense photoluminescence was measured from all samples investigated with emission energy, intensity, and spectral linewidth dependent on the SiGe composition of the QDs and the different capping layers. Radiative recombination occurs in the QDs, and its energy matches the results of band-structure calculations that consider strain compliance between the QD and the tip. The NTs arrangement and the selective growth of QDs allow to studying the PL emission from only 3-4 QDs, demonstrating a bright emission and the possibility of selective addressing. These findings will support the design of optoelectronic devices based on CMOS-compatible emitters.