THRESHOLD CURRENT ANALYSIS OF COMPRESSIVE STRAIN (0-1.8-PERCENT) IN LOW-THRESHOLD, LONG-WAVELENGTH QUANTUM-WELL LASERS

A comprehensive study of the effect of compressive strain on the thres hold current performance of long-wavelength (1.5 mum) quantum well (QW ) lasers is presented. Model predictions of threshold currents in such devices identify QW thickness as a parameter that must be considered in optimizing laser performance when Auger currents are present. A min imum in threshold current density is thus calculated when QW thickness es are maintained between 70 and 100 angstrom, presently achieved in s trained QW's using InGaAsP, and thereby isolating strain effects from any thickness and energy dependences. Experimental comparisons between strained and unstrained devices thus reveal strain-induced reductions in internal transparency current density per QW from 66 to 40 A/cm2, an increase in peak differential modal gain from 0.12 to 0.23 cm/A, an d evidence for the elimination of intervalence band absorption as comp ressive strain increases from 0 to 1.8%. However, most of these improv ements arise in the first approximately 1% of compressive strain. In o rder to fabricate low-threshold 1.5 mum buried hetero-structure BH dev ices in InP using the strained QW active regions, an optimized design is derived which shows that threshold current is at its lowest when th e stripe width is approximately 0.6-0.7 mum. Example uncoated BH laser s exhibit room temperature pulsed threshold currents of 5.9 mA in stru ctures without current blocking layers, and 4.1 mA in structures with current blocking layers.