Spin lifetimes in InGaAs quantum wells*

We have carried out Hanle spin relaxation time measurements in InGaAs quantum well structures as well as in Fe-based spin-LEDs that incorporate InGaAs QWs. In the InGaAs QW structures the spin lifetime T$_{S}$ at T = 5 K is equal to 2 ns while in spin LEDs T$_{S}$ is only 0.17 ns. For the undoped QWs, T$_{S}$ increases from 2ns at 5K to 4.5ns at 15K and then decreases monotonically. On the other hand, T$_{S}$ in the LEDs increases monotonically with temperature. The origin for the difference in the spin lifetimes between undoped QWs and that within a LED structure can be understood by considering the corresponding differences in the band structure between the symmetric quantum well and the spin-LED. In the latter, the Rashba spin-orbit interaction due to the built-in asymmetry brings about a significant enhancement in spin relaxation. In addition, bias conditions in the spin-LED play a crucial role in determining the temperature trend of the spin relaxation. The bias voltage tunes the electron density and accordingly the spin dephasing could be either suppressed or enhanced by electron-electron scattering.