Dependences of the Transport Scattering Time and Quantum Lifetime on the Two-Dimensional Electron Gas Density in Modulation-Doped Single GaAs Quantum Wells with AlAs/GaAs Short-Period Superlattice Barriers

The dependences of the transport scattering time τ_t, quantum lifetime τ_q, and their ratio τ_t/τ_q on the density n_e of the electron gas in modulation-doped single GaAs quantum wells with AlAs/GaAs short-period super-lattice barriers are investigated. The experimental dependences are explained in terms of electron scattering by remote ionized donors with an effective two-dimensional concentration n_R^* and background impurities with a three-dimensional concentration n_B. An expression for n_R^*(n_e) is obtained including the contribution of X-valley electrons localized in AlAs layers to the suppression of scattering by the random potential of remote donors. It is shown that the experimentally observed abrupt increase in τ_t and τ_q with an increase in n_e above a certain critical value n_ec is related to a decrease in n_R^*. It is established that the drop in τ_t/τ_q observed for electron densities n_e > n_ec occurs because scattering by the random potential of background impurities in this two-dimensional system with a decrease in n_R^* limits an increase in τ_t more considerably than an increase in τ_q.