OPTIMISATION OF 670nm STRATh4ED QUANTUM WELL LASER DIODES

There is increasing interest in the use of visible emitting GaXlnl.XP/(Al5Ga)5)O52InO4SP quantum well lasers for optical interconnections using polymer waveguides and this calls for the optimisation of device structure for operation at a specific wavelength and usually at an elevated temperature. We concentrate on the mechanisms by which compressive strain modifies the threshold current in a regime where well composition (x) (strain) and quantum well width are adjusted to maintain a transition wavelength of 670nm. In our model we assume a parabolic band structure, which is a reasonable approximation in this case since strain enhanced splitting of the valence bands is large. and we include the effects of monolayer fluctuations in well width and carrier-carrier scattering ( where we calculate an energy and carrier density dependent lifetime). Using our model we examine the relative merits of various well composition (x) / well width combinations. We predict a minimum threshold current as a function of decreasing well width (increasing compressive strain), as in the case of 633nm lasers. The decrease in threshold current with decreasing well width is due both to a decrease in the number of sub-bands in the quantum well and to an increase in the splitting of the valence bands due to increasing strain. The increase in threshold current for ver thin wells is due to increasing broadening of the gain by well width fluctuations and an increasing contribution from thermally activated leakage current. We predict an optimum threshold current density for a two well, 68A well width and 41% gallium composition device for operation at 400K.