Quantum confinement effects in GeSn/SiGeSn heterostructure lasers

The development of a light source on Si, which can be integrated in photonic circuits together with CMOS electronics, is an outstanding goal in the field of Silicon photonics. This could e.g. help to overcome bandwidth limitations and losses of copper interconnects as the number of high-speed transistors on a chip increases. Here, we discuss direct bandgap group IV materials, GeSn/SiGeSn heterostructures and resulting quantum confinement effects for laser implementation. After material characterization, optical properties, including lasing, are probed via photoluminescence spectrometry. The quantum confinement effect in GeSn wells of different thicknesses is investigated. Theoretical calculations show strong quantum confinement to be undesirable past a certain level, as the very different effective masses of r and L electrons lead to a decrease of the L-to Γ-valley energy difference. A main limiting factor for lasing devices turns out to be the defective region at the interface to the Ge substrate due to the high lattice mismatch to GeSn. The use of buffer technology and subsequent pseudomorphic growth of multi-quantum-wells structures offers confinement of carriers in the active material, far from the misfit dislocations region. Performance is strongly boosted, as a reduction of lasing thresholds from 300 kW/cm2 for bulk devices to below 45 kW/cm2 in multi-quantum-well lasers is observed at low temperatures, with the reduction in threshold far outpacing the reduction in active gain material volume.