Pump-probe measurements of carrier capture times in quantum well structures

The captures of electrons and holes into semiconductor quantum well structures have received considerable attention in both fundamental and device-oriented research. In quantum well lasers, the capture efficiency is expected to influence both the quantum efficiency and dynamic performance. Two multiple quantum well (MQW) samples (waveguide structures) were used in our experiments. The first one consists of 55 quantum wells. The well and barrier thicknesses are 7 nm and 12 nm, respectively. Because of the large barrier thickness, we called it an uncoupled QW sample. The second one has 110 quantum wells. The well and barrier thicknesses are 3.5 nm and 5.5 nm, respectively. It is actually a superlattice and we called it a coupled QW sample. In our experiments, we estimate the carrier capture times with subpicosecond pump-probe measurements. By fitting the experimental data to the numerical results based on a set of rate equations, we obtain a capture time of about 15 ps in the uncoupled QW structure and 8 ps in the coupled QW well structure. Actually, the slower decay of barrier carriers in the uncoupled samples, confirms the slower rise of subband carriers.