Spin injection into semiconductors: The role of the Fe/A_xGa _1-xAs interface

The influence of the growth and post-growth annealing temperatures of Fe/Al_xGa_1-xAs-based spin light-emitting diodes (LEDs) on the spin injection efficiency is discussed. The extent of interfacial reactions during molecular beam epitaxial growth of Fe on GaAs was determined from in-situ x-ray photoelectron spectroscopy studies. The Fe/GaAs interface results in ≤ 3 monolayers of reaction for Fe grown at -15°C. Intermediate growth temperatures (95°C) lead to ∼5 monolayers of interfacial reactions, and high growth temperatures of 175°C lead to a ∼9 monolayer thick reacted layer. Polarized neutron reflectivity was used to determine the interfacial magnetic properties of epitaxial Fe_0.5Co_0.5/GaAs heterostructures grown under identical conditions. No interfacial magnetic dead layer is detected at the interface for Fe_0.5Co_0.5 films grown at -15°C, a ∼6 Å thick nonmagnetic layer formed at the interface for 95°C growth and a ∼5 Å thick magnetic interfacial reacted layer formed for growth at 175°C. Spin injection from Fe contacts into spin LEDs decreases sharply when reactions result in a nonmagnetic interfacial layer. Significant spin injection signals are obtained from Fe contacts grown between -5°C and 175°C, although the higher Fe growth temperatures resulted in a change in the sign of the spin polarization. Post-growth annealing of the spin LEDs is found to increase spin injection efficiency for low Fe growth temperatures and to a sign reversal of the spin polarization for high growth temperature (175°C). An effective Schottky barrier height increase indicates that post growth annealing modifies the Fe/Al_xGa_1-xAs interface.