Observation of Phonon Replica Emission in an In-Situ Fe/GaAs Spin LED

GaAs-based spin LEDs were used to study spin injection through a Schottky barrier from a ferromagnetic contact formed by an in-situ technique. Optical measurements were performed in both oblique Hanle and Faraday geometry. In addition to direct recombination at the Gamma-point in the quantum well, emission peaks were seen due to phonon-assisted recombination from the X and L valleys in the semiconductor. The presence of the phonon replica peaks indicates that strong tunneling occurs from the Fe into the X and L minima at the interface. As measured in the oblique Hanle geometry, the satellite peaks display optical polarization, which indicates spin polarized injection into these minima. It is argued that enhanced tunneling into the X and L minima leads to a lower spin polarization in the device due to the transport properties of these valleys. This mechanism then limits the maximum effective spin polarization that can be achieved at a coherent interface.