Summary and Outlook
Summary and outlook of the doctoral thesis of Tim David Germann showing key achievements and possibilities for future work.
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Summary and Outlook
Summary MOVPE of long wavelength I nGa As/Ga As QDs emitting around 1.3 µm was systematically studied. Growth processes for three completely different laser concepts based on QWs, QDs, and SMLs were developed and devices were realized. In the course of this work about 2500 different MOVPE samples were grown, measured, and characterized. Among these were: • Multiple QD lasers with AlGa As and I nGa P designs that exhibited record low thresholds and emission wavelengths up to 1.3 µm. • First MOVPE grown SML and QD-based VECSELs, emitting at 950, 1040 and 1210 nm were fabricated. High-power, high-brilliance output and excellent temperature stability were demonstrated. • Realization of the novel EOM VCSEL concept based on a QW double-cavity design unveiled an ultra-high intrinsic bandwidth potential surpassing direct modulated VCSEL. For the systematic study of QD growth and QD stacking processes for long wavelength applications, a standardized PL test structure design with constant excitation volume was adopted. Multiple computer-based tools were implemented to deal with the substantial number of samples. Based on an existing I nGa As/Ga As QD growth process, the influence of individual growth parameters on the QD luminescence characteristic was studied in detail, and the QD growth process was developed further. A series of PL test samples revealed the important role of the V/III ratio during overgrowth of QDs. A strong influence of the V/III ratio on QD properties was determined. Reduced V/III ratios improved the integration of QDs into the Ga As matrix crystal, enabling a reduced FWHM, longer emission wavelength, and a significantly improved QD stacking ability. The improved QD properties are attributed to a reduction of point defects in the proximity of the QD layer. In consequence, the vacancy-driven indium segregation was suppressed. Effects of the V/III ratio on QD properties were found not only for QD deposition and the capping process itself, but also for all layers deposited subseT. D. Germann, Design and Realization of Novel GaAs Based Laser Concepts, Springer Theses, DOI: 10.1007/978-3-642-34079-6_8, © Springer-Verlag Berlin Heidelberg 2012
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quent to a QD layer. This finding is explained by a group three vacancy rich growth surface for high arsenic partial pressures, leading to enhanced indium segregation out of the QDs. Through these improvements of the QD growth process, the successful complete suppression of QD emission blue shift upon overgrowth and annealing was achieved, even for long wavelength QDs at 1.3 µm [1]. Edge-emitting lasers based on this advanced QD growth process were fabricated, achieving ultra-low threshold values of only 66 A/cm2 at 1.25 µm [2, 3]. Additional to the conventional AlGa As-based design, a completely aluminum-free design based on lattice matched I nGa P cladding was developed and realized, achieving an extremely low transparency-current density of only 35 A/cm2 at the same wavelength. Complete suppression of any uninten
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