Electrical Generation of Spin Polarization in Strained III-V Semiconductors.

Abstract

For the advancement of spin-based electronics applications, as well as the advancement of semiconductor technology in general, an understanding of spin-related phenomena in semiconductors is of crucial importance. This work focuses on two effects, namely the manipulation of electron spins via spin-orbit coupling, and the generation of spin polarization using an all electrical means known as current-induced spin polarization. Optical measurements via Faraday/Kerr rotation are systematically conducted on strained n-type InGaAs epilayers. The anisotropic spin-orbit splitting is mapped for several samples taken from the same wafer, and is represented as a spin-orbit effective magnetic field. Measurements of electrically generated spin polarization are performed on the same sample locations. In accordance with previous predictions, spins are dynamically polarized along the direction of the spin-orbit field. However, contrary to previous predictions, the steady-state spin polarization is deviated from the spin-orbit field direction. This is characterized quantitatively in this work presenting a new model based on the anisotropic spin relaxation rate. Furthermore, the magnitude of current-induced spin polarization is not proportional to the spin-orbit splitting, but rather the two obey a negative differential relationship. That is, the crystal direction having the weakest spin-orbit splitting exhibits the strongest current-induced spin polarization. This is characterized phenomenologically by introducing a term that allows for spin-dependent scattering accompanied by a spin flip. This is the first work to establish a relationship between current-induced spin polarization and an anisotropic spin-orbit splitting. Our model agrees with previous measurements in the field. Furthermore, the crystal directions corresponding to a spin polarization and spin-orbit field maxima are nearly orthogonal. We point out that this phenomenon has the potential to be extremely useful for the advancement of spintronics applications, as it allows for independent spin polarization and manipulation by application of orthogonal electric fields.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108822/1/bmnorman_1.pd