Spin relaxation in InGaN/Ga(Mn)N quantum wells

Spin relaxation processes in InGaN/GaN and InGaN/GaMnN multi-quantum wells are studied by transient magneto-optical spectroscopy. Nearly no photoluminescence (PL) polarization was observed immediately after pulsed laser excitation (t=0), regardless of the polarization of the excitation light. Afterwards PL gradually becomes $\sigma ^+$ polarized in an applied magnetic field. This polarization build-up is shown to correspond to an additional decay process (50 ps) of the $\sigma ^-$ PL component. With the aid of the exciton Hamiltonian and rate equations, we show that fast spin relaxation ($<$20 ps) is partly responsible for the vanishing optical polarization at t=0. The fast spin relaxation is attributed to carrier spin relaxation at high K-vectors dominated by the D'yakonov-Perel' (DP) mechanism. When the excitons are at rest (K=0), the DP spin relaxation is suppressed leading to a slower spin relaxation (50 ps).