Carrier capture time and its effect on the efficiency of quantum-well lasers

Carrier capture time of the quantum well, which is an important parameter in the laser operation, was estimated for separate-confinement-heterostructure single-quantum-well (SCH-SQW) lasers by measuring the spontaneous emission from the optical confinement layers, which increases with current even above the laser threshold due to finite capture time. By fitting theoretical analysis to the measurement, hole capture time was found to be the dominant factor for the spontaneous emission increase, and was estimated to be 0.2-0.3 ps for GaInAs/GaInAsP/InP step- and graded-refractive-index-(GRIN-) SCH-SQW lasers, independent of the optical confinement structures. The same measurement was done for multiquantum-well lasers, and it was found that transport across the barrier was also responsible for the spontaneous emission increase and inhomogeneous injection into each well. The effect of the hole capture time and the transport time on the threshold current and the quantum efficiency was analyzed for high-power operation, considering the absorption loss by the carriers in the optical confinement layers. GRIN-SCH structure is shown to keep high differential efficiency in high-power operation in comparison with step-SCH structures, because the carrier density in the confinement layer is small and increases very little above threshold