A SiGe quantum cascade laser based on light hole transitions

A number of highly promising applications, among them the monolithic integration of silicon-based optoelectronics, has been driving research efforts on the realization of a group-IV laser source. Up to now, the only lasing devices demonstrated in Si, among them a laser based on highly doped, strained Ge on Si, essentially lack the advantages associated with the silicon system by requiring an external pump laser source. One of the most promising concepts for the realization of an electrically pumped Si-based laser source is that of quantum cascade lasers (QCLs), which is successfully implemented in the III-V system. But while infrared electroluminescence (EL) of various wavelengths has been demonstrated for p-type SiGe quantum cascade structures, lasing has yet to be achieved. QC electroluminescence (EL) emitters realized in p-type SiGe, which operate in the mid-infrared spectral region, have commonly been based on transitions between two heavy-hole (HH) states. In contrast, the EL devices operating in the terahertz region, which have been demonstrated so far, exploit transitions between light-hole (LH) and HH states. Despite the successful demonstration of electroluminescence for all of the mentioned devices, in none of the QC structures population inversion could be achieved. This project proposes basing a SiGe QCL on a transition between two LH states, where the targeted lasing energy lies in the terahertz range of the electromagnetic spectrum. The proposal presents the design of a strain-balanced SiGe QC structure, which is to be grown on a Si0.7Ge0.3 virtual substrate. The novel design promises long lifetimes of carriers in the upper state of the lasing transition as well as an ultra-fast depopulation of its lower state by optical phonons, enabled by employing a diagonal lasing transition and by preventing the formation of parasitic relaxation paths via intermediate states. The latter is made feasible by basing the QCL on a LH-LH transition, where further advantages associated with this choice for the lasing transition include a high oscillator strength due to the light confinement mass of the involved states. As the proposed structure further features relaxed growth requirements and should not exhibit any parasitic currents from the upper lasing state, it overcomes the difficulties which ultimately prevented the demonstration of population inversion in SiGe structures so far. Thus the proposed design should enable the successful demonstration of laser operation. The project includes the design, fabrication, experimental characterization and optimization of a LH-based SiGe QCL, as well as its integration in a double-metal terahertz waveguide, where the experience and facilities of the hosting institution, namely the Capasso group at Harvard University, are essential for the successful completion of the proposed work packages and the achievement of the final goal, which is the demonstration of the first electrically pumped silicon-based laser source.

In the course of the project, the first silicon-germanium cascade emitters based on transitions between light-hole states have been demonstrated. The achieved results are a crucial step towards the realization of a practical silicon-based laser source, which is of high interest for compact and robust sensing and detection systems for various applications. Due to the prospect of CMOS compatibility, silicon photonics is highly attractive as a platform for next-generation data transfer solutions and spectroscopy and detection systems. However, up to now the monolithic integration of all components required for such compact optoelectronic systems has been prevented by the lack of an essential building block: That of a convenient silicon-based laser source. The achievements of the presented project are an important step towards realizing such a laser. A novel optically active material based on nanometer-thin layers of silicon-germanium compounds with varying composition has been designed and fabricated, forming a so-called quantum cascade structure. In other material systems, lasers based on the quantum cascade concept are realized with great success. However, all of the quantum cascade lasers demonstrated up to now employ radiative transitions between electron states in the conduction band. This is not possible for silicon-germanium structures, as no sufficiently deep potential wells (quantum wells) for electrons can be formed. However, silicon-germanium quantum cascades can be realized for holes, which suffer from the disadvantage of generally being heavier than electrons and experience different masses depending on the occupied band. The charge carrier mass strongly affects the performance of the optical material, and up to now silicon-germanium quantum cascade structures have been based on disadvantageous heavy-holes only. In contrast, the approach demonstrated in this project uses transitions between light-hole states, which are closer to electrons in their properties and allow more efficient emission of radiation and charge transport. The realized light-hole based structures show a highly promising performance, and will lay the foundation for the first silicon-based quantum cascade laser. Once such a device is demonstrated, it can be integrated with silicon-based optoelectronic circuits. Potential applications of integrated optoelectronic platforms include medical diagnosis (lab-on-achip) and breath analysis, environmental and air pollution control, threat reduction, industrial process control and water purity monitoring.