Carrier dynamics in a coupled quantum well hetero nipi structure determined from nonlinear transmission

A large number of lifetime measurements were taken to characterize a coupled-quantum-well hetero nipi structure (CQW-H-nipi). The GaAs coupled quantum wells were 48 angstrom each and separated by a 36-angstrom Al_0.30Ga_0.70As barrier. Twelve coupled pairs were separated by 100-angstrom AlGaAs. The entire structure, consisting of three layers of nipi regions for a total of seventy-two coupled pairs, was grown by molecular beam epitaxy on a GaAs substrate. To measure the changes in absorption, a standard pump-probe technique was used. Instantaneous lifetimes were calculated from the slope of the decay and plotted as a function of time. The data show a lifetime that increases with time. Initial lifetimes are approximately 300 μs and increase to several milliseconds. To separate recombination lifetime and effects due to carrier diffusion, different spot sizes were used, and CW measurements of the nonlinear signal were separated. An effective diffusion length of 2 mm (defined as the separation distance where the nonlinear signal falls to 1/e) was measured. Other results show that for a given nonlinear signal (induced current density), the carrier decay rate at long times after the pump pulse depends on the initial pump intensity. The difference is due to the decreased tunneling rate of photocarriers out of the quantum wells in the smaller space-charge fields that accompany higher excitation.