CARRIER CAPTURE AND RELAXATION IN NARROW QUANTUM-WELLS

In separate confined heterostructure (SCH) lasers, injected electrons and holes thermalize into a quantum well after diffusion through the o uter cladding layers. The carriers move towards equilibrium by emittin g optical phonons. In narrow quantum wells, as compared to the 1-2 ps required in bulk semi-conductors, this phonon emission process can be considerably slowed down due to the 2-D density of states and the natu re of the electron-optical phonon interaction. This process has been s tudied theoretically using a Monte Carlo program which allows us to se e the carrier distribution as a function of time. Typical times for ca rrier relaxation are 10-15 ps for a 50 angstrom GaAs well with Al0.30G a0.70As barriers and approximately 5 ps for a 200 angstrom well. These calculations have been complemented by time-resolved photoluminescenc e measurements on SCH structures where the relaxation time from a 3D d istribution into In0.20Ga0.80As/GaAs wells is measured at T = 200 K. C arrier relaxation times of 50, 41, 22, and 17 ps are obtained for well s of sizes 30, 40, 50, and 100 angstrom, respectively. The results sho w clearly that the use of narrow quantum wells in low threshold lasers will pose a serious limitation to the efficiency and small-signal mod ulation bandwidth of these devices.