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 threshold 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 minimum in threshold current density is thus calculated when QW thicknesses are maintained between 70 and 100 angstrom, presently achieved in strained 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, and evidence for the elimination of intervalence band absorption as compressive strain increases from 0 to 1.8%. However, most of these improvements arise in the first approximately 1% of compressive strain. In order to fabricate low-threshold 1.5 mum buried hetero-structure BH devices in InP using the strained QW active regions, an optimized design is derived which shows that threshold current is at its lowest when the stripe width is approximately 0.6-0.7 mum. Example uncoated BH lasers exhibit room temperature pulsed threshold currents of 5.9 mA in structures without current blocking layers, and 4.1 mA in structures with current blocking layers.