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 outer cladding layers. The carriers move towards equilibrium by emitting optical phonons. In narrow quantum wells, as compared to the 1-2 ps required in bulk semiconductors, this phonon emission process can be considerably slowed down due to the 2-D density of states and the nature of the electron-optical phonon interaction. This process has been studied theoretically using a Monte Carlo program which allows us to see the carrier distribution as a function of time. Typical times for carrier relaxation are 10-15 ps for a 50 /spl Aring/ GaAs well with Al/sub 0.30/Ga/sub 0.70/As barriers and /spl sim/5 pS for a 200 /spl Aring/ well. These calculations have been complemented by time-resolved photoluminescence measurements on SCH structures where the relaxation time from a 3D distribution into In/sub 0.20/Ga/sub 0.80/As/GaAs wells is measured at T=200 K. Carrier relaxation times of 50, 41, 22, and 17 ps are obtained for wells of sizes 30, 40, 50, and 100 /spl Aring/, respectively. The results show clearly that the use of narrow quantum wells in low threshold lasers will pose a serious limitation to the efficiency and small-signal modulation bandwidth of these devices.<
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