Strained layer (111)B GaAs/InGaAs single quantum well lasers and the dependence of their characteristics upon indium composition

We have studied a series of (111) oriented GaAs–InGaAs single quantum well, broad area lasers with active regions containing a range of indium concentrations, x, in order to understand the advantages and limitations of pseudomorphic strain. For x⩽0.3, both an increased emission wavelength and reduced threshold current were observed with increasing x. The predominant cause of the wavelength increase is the reduction in bulk InGaAs band gap. The reduction in threshold current is attributed mainly to the reduced in-plane density of states caused by the strain induced lifting of the heavy and light hole degeneracy at the valence band edge. For x>0.3, we see a marked deterioration in laser performance. However, we believe that this deterioration is not directly associated with strain relaxation at layer thicknesses beyond the critical value. Rather, imperfections in the AlGaAs cladding layers appear to seed the formation of dislocations within the strained regions. Within the limitation of strain relaxation, we observed monomode continuous wave emission at room temperature at wavelengths up to 1.072 μm and with threshold current densities as low as 74 A/cm2. The differential gain of 1.45×10−15 cm2 is around four times higher than measured on unstrained GaAs/AlGaAs single quantum well lasers. Like the reduction in threshold current density, this relatively high value is attributed to the strain induced reduction in the in-plane, heavy hole effective mass