Carrier distribution, spontaneous emission and gain engineering in lasers with nonidentical quantum wells

Novel quantum-well lasers with a 100-Å GaAs quantum-well (QW) (λ~840 nm) and a higher energy 40-Å AlGaInAs QW (λ~810 nm) in a graded Al_xGa_1-xAs separate confinement heterostructure (SCH) with the bandgap increasing from the p- to the n-side are characterized. A sizeable variation in the QW carrier densities can be achieved as a function of well composition and placement due to the built-in electric field that forces carriers toward the p-side of the SCH and the different densities of states and carrier capture rates of the QW´s. Transversely emitted spontaneous emission (SE) is measured through a windowed contact to determine the relative contribution from each QW to the total SE. Information from these measurements is incorporated into a detailed device model to determine the carrier density and evaluate the gain characteristics in a new way. Since the nonidentical wells emit different photon energies, it is shown for the first time that the carrier density can be determined at a specific location within the SCH of a semiconductor laser. By changing the placement of dissimilar QW´s and the grading of the SCH, it is found that the gain spectrum can be substantially engineered