Hot Carrier Relaxation in Quantum Well and Bulk GaAs at High Carrier Densities: Femtoseconds to Nanoseconds

An investigation of hot carrier relaxation in GaAs/A1_xGa_1-xAs multiple quantum wells and bulk GaAs in the high carrier density limit is presented. Two techniques have been employed: luminescence upconversion with < 80 fs temporal resolution has been used to cover the range from 100 fs to 100 ps, and time-correlated single-photon counting to cover the range from 100 ps to 2 ns. Electron temperatures as a function of time were determined from the slope of the high energy tail of the time-resolved photoluminescence spectra. Our results show that hot electron cooling rates in the quantum wells begin to become significantly slower than that in the bulk when the photogenerated carrier density is above a critical value of ∼ 2 x 10¹⁸ cm^-3; the difference in cooling rates increases rapidly with increasing carrier density. The time constant characterizing the power loss of hot carriers is also determined and discussed. A comparison is made with previous publications to resolve the confusion concerning the difference in cooling rates between quasi-2D and 3D systems.