High-Speed 1.55 µm Quantum Dot Lasers with Electronically Coupled Quantum Well - Dot Active Regions

The expansion of long-distance data communication at a wavelength of 1.55 µm calls for larger data transfer rates. Consequently, an improvement of emission and modulation properties of lasers emitting in this wavelength region is of high interest. The utilization of so-called tunnel-injection (TI) structures in InP-based quantum dot (QD) lasers is promising to fulfill this task. The aim of this thesis was the realization and analysis of TI structures in the lattice matched InP material system and incorporation into a high-speed laser design. These consist of an InGaAs quantum well (QW), an InAlGaAs barrier as well as InAs quantum dots or a stack of multiple QD structures. The samples for this thesis were prepared using molecular beam epitaxy. The optimization of the QD growth yielded a low-temperature small full width at half maximum (FWHM) photoluminescence (PL) linewidth below 30 meV and an emission wavelength of 1.55 µm. The challenging fabrication of the QWs (needed for the TI structures), grown by shuttering technique, resulted in lattice matched InGaAs QWs with a specific transition energy. To optimize the energy band structure of the lasers, the strong influence of the QW (energy band alignment) and the barrier (coupling strength) thicknesses on the emission properties of the aforementioned TI structures was investigated. Interestingly, a shift from QW to QD dominated emission or vice versa was observed at room temperature in dependence of these conditions. An optimum thickness of 3 nm and 1.8 nm was discovered for the QW and barrier, respectively, for the highest achievable ratio between the integrated intensities of QD and QW related emission. Moreover, a narrowing of the FWHM PL linewidth was observed for the TI structures in comparison to QD reference structures at 10 K, even though the emission was originating from the QDs. Growth related structural and morphological changes were excluded, which makes a narrowing by the selection of the emitting dots through the tunneling process plausible. The TI structures were implemented into a high-speed QD laser design and compared to standard QD lasers. By improving the energy band alignment, an increase in modal gain of 11 cm-1 to 17 cm-1 per dot layer was demonstrated. In addition, small signal modulation measurements revealed a maximum modulation bandwidth of 14.9 GHz for the QD laser, while the best performing TI QD laser showed a bandwidth of 8.6 GHz. The relative difference was not as big for large signal modulation measurements, since the highest achievable data rates were 28 Gb/s and 23 Gb/s for the QD and TI QD lasers, respectively. However, an incorporation of p-type doping on the one hand increased the modal gain to up to 23 cm-1 per dot layer. On the other hand, the threshold current densities raised by up to 40 %. A strong increase in small signal modulation bandwidth of the TI QD laser by 23 % was the result. Furthermore, overall a deterioration of the temperature stability was detected for the doped laser structures. In order to further improve the performance of QD and TI QD lasers, the application of post growth rapid thermal annealing (RTA) was investigated. By applying different RTA temperatures to both laser types, an enormous improvement of the evaluated static para-meters was observed for the QD laser. However, the emission properties of the TI QD laser did change unpredictably. Different strengths of the emission shifts of QW and QDs and the resulting misalignment of the energy band structure working against the improving material quality for higher RTA temperatures could be the reason.