Design and growth of group IV laser structures

The development of information technology during the last century was substantially pushed forward by silicon technology. The rapid growth of the need for interconnected devices requires the optimization of the energy efficiency of information transfer. Therefore, there is a great endeavor to replace the data transfer by electrons with transfer by photons, with a laser source as the key device. One proposed approach to achieve a laser in so-called Opto-electronic integrated circuits (OEICs), is the monolithic integration of silicon technology compatible group IV alloys. GeSn alloys with their potential to achieve a direct bandgap are here highly interesting. In this work, possibilities to optimize group IV laser materials are investigated. Starting from bulk GeSn, 8-Band k · p band structure calculations show a large achievable gain. In this regard, the influence of strain and composition on band structure is evaluated, demonstrating advantages of low Sn content GeSn under tensile strain. Following the development of III-V laser materials, the opportunities and challenges of quantum structures were investigated. GeSn/SiGeSn multi quantum wells (MQWs) were found to be promising, in this respect, showing strong carrier confinement and material gain. Limitations in the well thickness and n-type doping concentration, due to quantization effects and absorption losses, were studied. Compared to MQWs, substantially lower lasing thresholds can be achieved with quantum dots (QDs). Therefore, GeSn QDs were studied experimentally and theoretically. Using transmission electron microscopy and atom probe tomography the morphology and composition of Sn-rich precipitates obtained by thermally treated thin GeSn layers embedded in Ge were investigated. Based on the promising findings for GeSn/SiGeSn MQWs, band alignment calculations were performed for cone shaped GeSn QDs in an SiGeSn matrix. High energy barriers are found for hydrostatically and biaxially strained direct bandgap QDs. At the end of this thesis, several approaches are discussed which could, additionally, improve the laser performance. First, a GeSn/SiGeSn MQW LED was grown by reduced-pressure chemical vapor deposition, where a grading in doping concentration was integrated into bottom and top contacts. This design could substantially reduce losses of the lasing mode in highly doped regions. A powerful approach to separate the growth of e.g. photodetectors and waveguides is the selective epitaxial growth. Using different substrates, growth temperatures and adding HCl, growth parameters guaranteeing selectivity were evaluated for GeSn. As a last concept, the ternary CGeSn was considered. Using the precursor tetrabromomethane (CBr4), CGeSn layers were grown at different flow rates and growth temperatures. C contents exceeding the solid solubility of C in Ge were found, while still maintaining high crystal quality. Band energy calculations predict a direct bandgap for ternaries grown at low temperatures. For all layers a displacement of Sn atoms from lattice sites and a significant increase in the defect density was measured. CGeSn alloys could represent a new way to tune the band structure of group IV alloys