Design and fabrication of multiple quantum well structures for long wavelength laser diodes

Abstract: InGaAsP multiple quantum well (MQW) structures emitting at1300 nm have been designed, fabricated with metal organicvapour phase epitaxy (MOVPE) and evaluated by x-ray diffraction(XRD), photoluminescence (PL) and by laser characterisation. Inaddition the structures were subject to scanning probemicroscopy (STM/AFM) and direct carrier transport measurements.The design goal was to fabricate MQWs with a large number ofperiods, more than a typical 4-5, which is essential forhigh-modulation-speed lasers and vertical cavity surfaceemitting lasers (VCSELs). The demand for temperature-stableproperties has commonly resulted in MQW designs with highbarrier bandgaps for achieving a high electron confinement.This work has shown by laser-simulations that the consequenceof high barriers is a slow hole-transport, which accumulatesthe holes and consequently also the electrons due toCoulomb-attraction in the wells closest to the p-side. Themajor effect of the non-uniform carrier distribution is highnon-radiative carrier losses, degrading the laser performance.The simulations show that by reducing the barrier heights, amore uniform carrier distribution can be achieved andconsequently reduced non-radiative losses. Directhole-transport measurements over a MQW showed a cleardependence of the hole-transfer time over the MQW on thehole-confinement energy. The device performance of lasers wasimproved considerably in terms of lower threshold currents,higher optical power outputs and higher temperature-stabilitywhen optimised barrier bandgaps were used, even if theelectron-confinement is reduced. Another conclusion from thisstudy was that for MQW-structures emitting at 1300 nm, aconstant fraction of Ga in both wells and barriers results inexcellent materials characteristics compared to constant-As orInAsP (wells)-InGaAsP (barriers) alternatives.Keywords:InGaAsP, multiple quantum well, materialsfabrication, semiconductor laser, carrier transport,characteristic temperature, 1300 nm.