Interplay of spin dynamics of trions and two-dimensional electron gas in a n-doped CdTe single quantum well

We used the photoinduced Faraday rotation technique to study the low-temperature spin relaxation of excitons, trions, and two-dimensional electron gas in n-doped single CdTe/CdMgTe quantum wells with different concentrations of electrons. A pulsed (ps) optical excitation tuned to an exciton line allows the simple observation of the exciton spin relaxation (18–36 ps), whereas an excitation resonant with a trion state demonstrates a biexponential spin relaxation. The shorter characteristic time $(40\mathrm{}\pm 10\mathrm{ps})$ then belongs to the photocreated hole spin relaxation, and the longer one $(180\mathrm{}\pm 20\mathrm{ps})$ is attributed to the spin relaxation of the electron gas, initially polarized through the creation of trions by using circularly polarized light. This interpretation is confirmed by the oscillations shown by the photoinduced Faraday rotation in a transverse magnetic field, which provide the Landé factor of the free electron in a CdTe quantum well $(|g_{e\perp }|=1.342\mathrm{}\pm 0.001).$ Finally, the observed interplay of trions and electron gas polarization is quantitatively explained with a system of four rate equations involving the spin populations of trions and those of the electron gas.