Dilute Nitride Lasers and Spectrally Engineered Semiconductor Laser Resonators

Doktorsavhandling, 2011

The first part of this thesis deals with long wavelength (1.2-1.3 um) InGaAs(N) multiple quantum-well (QW) lasers grown on GaAs, with the aim of understanding and improving their threshold and temperature characteristics. The epitaxial material is grown by molecular beam epitaxy (MBE). By optimizing MBE growth conditions we have obtained record low values for the threshold current density of 107 and 133 A/cm^2/QW for triple QW 1.2 um InGaAs and 1.3 um GaInNAs lasers, respectively. A thorough investigation of the temperature dependence of the threshold current for ridge waveguide GaInNAs double QW lasers is presented. The good temperature stability of GaInNAs lasers is usually attributed to a large conduction band offset as well as strong defect recombination. This work, however, reveals that their good temperature stability also to a large extent arises from a significant and only weakly temperature dependent lateral diffusion current, which is not an effect intrinsic to GaInNAs but rather related to the geometry of the laser resonator. The second part explores a concept used to engineer the spectral properties of a semiconductor Fabry-Perot (FP) laser resonator. A wavelength dependent resonator loss is obtained by introducing perturbations of the effective mode index at key positions along the length of the FP resonator. In a spectrally engineered FP resonator (SE-FPR) this is used to lower the resonator loss for selected longitudinal modes which thereby require less gain for lasing. Previous treatments of SE-FPRs generally relied on an approximation valid for a weakly perturbed resonator. This work extends the treatment to also include strongly perturbed SE-FPRs. The design and fabrication of SE-FPRs supporting either one or two selected modes are investigated. For a strongly perturbed SE-FPR a very large reduction of the resonator loss can be obtained for the selected modes, with the main feedback still provided by the end facets. Fabrication tolerances are, however, strict; in particular the positioning of the perturbations with respect to the end facets is critical.