Deterministic Coupling of SiGe Quantum Dots to Photonic Crystal Structures

Silicon photonics has advanced to a major research field since it is believed that metallic wiring on a silicon chip could limit the increase in processing speed. Replacing on-chip interconnects by optical solutions is a great challenge because silicon is an indirect bandgap semiconductor. A possible route towards an efficient silicon-based light source is the spatial cofinement of carriers in SiGe heterostructures. The presented work combines the unique ability of photonic crystal structures to systematically manipulate the photonic surroundings of emitters, and the ordered growth of SiGe quantum dots (QDs) to deterministically enhance their light emission at specific wavelengths. We used a three-layer electron beam lithography (EBL) procedure to precisely align SiGe quantum dots with respect to photonic crystal modes. For this purpose, a pit-pattern was etched into a silicon-on-insulator (SOI) substrate. During molecular beam epitaxy (MBE), SiGe quantum dots nucleate in the pre-defined pits. In addition to controlling their position also the homogeneity in size and chemical composition in ordered quantum dots is better defined than in self-assembled QDs grown by the Stranski-Krastanow growth mode. In the last lithography step the photonic crystal structures are aligned to the quantum dots via pre-defined markers such that after a final etching step just a controlled number of QDs remains at pre-defined positions in the PhC. Micro-photoluminescence studies showed significantly enhanced spontaneous emission even for a single quantum dot placed inside a photonic crystal cavity. Thereby, we found that, as predicted by the Purcell effect, the position of the emitter with respect to the electric field maxima of the cavity modes strongly influences its spontaneous emission enhancement.