PhD Defense: "Electrical Spin Injection and Detection in Ferromagnet/Semiconductor Heterostructures"

Qi Hu

June 20th (Monday), 3:00pm
Elings Hall, Rm 1601

A comprehensive picture of all-electrical spin injection and detection in ferromagnet/semiconductor Schottky tunnel barrier heterostructures will be 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 ferromagnetic metal contact. However, recent experiments have revealed that spin accumulation in the semiconductor depends strongly on bias conditions at the ferromagnet/semiconductor Schottky interface, which indicates that the Schottky barrier profile plays an important role in understanding the spin-dependent tunneling mechanism.

I will show by electron charge transport and tunneling spectroscopy experiments that an accumulation region forms adjacent to the Schottky barrier in the semiconductor and electrons in this accumulation region are confined in discrete 2-dimensional energy subbands. I will further demonstrate by spin transport experiment that electrical spin injection and detection are significantly influenced by the existence of this accumulation region and the confinement of electrons therein. First, the accumulation region establishes an electrochemical potential equilibrium of spins between the ferromagnetic metal and the semiconductor in the absence of an applied bias voltage, which is a prerequisite for electrical spin detection. Second, the spin species extracted from confined electrons in the accumulation region is opposite to that from free electrons in the bulk semiconductor in Fe/GaAs sytems, which explains the puzzling sign reversal of spin accumulation in the bias dependence experiment. Third, the accumulation region functions as the source of significant spin currents flowing into the bulk of the semiconductor under small bias voltages. A quantitative rate equation analysis based on this picture predicts that the optimal thickness of the accumulation region is in the range of 20 to 25 nm for Si dopings of 5e18 cm^-3 in GaAs, in reasonable agreement with the experiment.

In addition to the details of the Schottky barrier profile near the interface, both the sign and magnitude of the spin currents are sensitive to very small changes in the ferromagnet/semicondcutor interfacial electronic structure, as will be demonstrated in annealing experiments. The knowledge attained in Schottky tunnel barrier heterostructures provides valuable insight into the design of future semiconductor-based spintronic devices.

Hosted by: Professor Christopher Palmstrøm

Hosted by: Professor Christopher Palmstrøm