Structural studies of InGaAsP/InP-based lasers : By Thomas Kallstenius

Sammanfattning: Reliable buried-heterostructure (BH) InGaAsP/InP-based laser diodes (LDs) is essentialto the operation of fiber optics communication systems. In this thesis, the structural,electrical and optical properties of such LDs have been studied. The aim has been toidentify materials-related mechanisms causing gradual emission degradation. The resultswill enable improvements in the long-term stability through modifications in laser designand manufacturing processes.Degradation during accelerated aging was studied in bulk as well as multiquantum-well(MQW) lasers having p/n or semi-insulating current-blocking layers. Some MQW lasershad distributed-feedback gratings.In bulk lasers, degradation is shown to be related to dark-defects (DDs) found in electro-and photoluminescence images of the active region. A new preparation method allowedthe DDs to be studied by plan-view transmission electron microscopy (TEM). The DDsare found to be due to dislocation loops. A model for the influence of loops and pointdefects on the threshold current is presented. Differential carrier lifetime measurements and analysis of the current-voltage (I-V) characteristic show observed changes inthreshold current to be due to non-radiative recombination instigated by a generation ofpoint defects. The exponential behavior of the reverse I-V characteristic is found to be due to field-assisted thermal emission from new donor-acceptor pairs.In MQW lasers, degradation is shown to stem from dark-area defects (DADS) beingformed instead of DDs. The DADS consist of [O11]-dislocations along the active region.The degradation is attributed to strain and/or confinement of point defects inside theMQW structure. A model for the time and current dependence of the degradation in thepresence of a finite source of defects explains the experimental results.The defect formation is due to indiffusion of Zn and occasional etch damage of thesidewalls of the active region. The diffusion of Zn and Fe during processing was studiedby means of scanning capacitance microscopy and a recently developed method usingcross-sectional atomic force microscopy in combination with selective etching. Thelatter method, combined with cross-sectional TEM, shows the presence contaminationat several interfaces due to inevitable atmospheric exposure during processing.