Optical electron spin orientation in Ga‐doped and undoped ZnO films

We have studied the optical orientation and polarization relaxation dynamics of electron spins in Ga‐doped and undoped ZnO films by time‐resolved Kerr rotation (TRKR). In the Ga‐doped film, we have observed a long‐lived spin signal more than 10 ns when the pump‐probe energy is resonant to the neutral donor‐bound excitons (D° X). It is not observed in the undoped film. Meanwhile, when the energy is resonant to free excitons, faster spin relaxation of about 2 ns is observed for itinerant electrons in both the films. The optical spin orientation is always parallel to the c‐axis in the ZnO films, and independent of the pump incidence angle. This contrasts with the observation in crystals with the zincblende structure.