Electrical Spin Injection and Detection in Ferromagnet-Semiconductor Heterostructures

A comprehensive picture of all-electrical spin injection and detection in ferromagnet/semiconductor Schottky tunnel barrier heterostructures is described. It is widely assumed that the spin current flowing in the bulk of a semiconductor is directly related to the spin polarization of the tunneling current flowing from the ferromagnetic contact. However, recent experiments have revealed that spin accumulation in the semiconductor depends strongly on interfacial bias conditions, which indicates that the Schottky barrier profile plays an important role in understanding the tunneling mechanism. My experiments demonstrate that an accumulation region forms adjacent to the depletion region of the Schottky barrier in the semiconductor, and electrons in this accumulation region are confined in discrete 2-dimensional energy subbands. The existence of this accumulation region and the confinement of electrons provide a quantitative explanation for the experimentally observed charge and spin transport in our devices. First, the accumulation region supports a significant tunneling current at small bias voltages. Second, the accumulation region establishes a well-defined electrochemical potential reservoir which functions as the source of spin currents flowing into the bulk of the semiconductor. This flow of spin currents even at zero bias in the detection contact is essential to achieve electrical detection of spins. Third, the spin species extracted from confined electrons in the accumulation region is opposite to that from free electrons in the bulk semiconductor, which explains the puzzling sign reversal of spin accumulation in the bias dependence experiment. Lastly, a rate equation analysis based on this picture shows that the optimal thickness of the highly doped GaAs layer at the interface is in the range of 20 to 25 nm for Si dopings of 5 x 1018 cm-3, in reasonable agreement with the experiment. In addition to the details of the Schottky barrier profile, both the sign and magnitude of the spin currents are sensitive to very small changes in ferromagnet/semicondcutor interfacial electronic structure, as will be demonstrated in annealing experiments. The knowledge attained in Schottky tunnel barrier heterostructures will be applied to the design of future spintronic devices based on semiconductors with large spin-orbit coupling.