Spin relaxation in InAs QDs*

We have studied the circular polarization of the electroluminescence emitted from Fe spin-LEDs which incorporate a single layer of InAs QDs at the center of the device as function of magnetic field applied along the growth axis. The circular polarization below 50 K shows a dramatic drop around 5 tesla, indicating the presence of an efficient spin relaxation mechanism that is tuned by the externally applied magnetic field. Calculations indicate that such a mechanism exists at $B$ = 5 tesla if the QDs are populated by 3 electron-hole pairs. The spin relaxation mechanism consists of a two step process. In the first step we have spin relaxation via spin-orbit interaction; in the second step we have energy relaxation via phonon emission. It is assumed that the QDs are anisotropic. This model was put to the test by studying a spin-LED which has a lower QD density and in which we can control the number of electron-hole pairs occupying each QD by changing the bias voltage. The predicted resonance was observed in this experiment verifying the proposed theoretical model.