Theoretical calculations have shown that the degrees of spin polarization in proposed nonmagnetic semiconductor resonant tunneling spin filters could be increased through exploitation of bulk inversion asymmetry (BIA). These enhancements would be effected through suitable orientation of spin collectors (or spin- polarization- inducing lateral electric fields), as described below.

Spin filters — more precisely, sources of spin-polarized electron currents — have been sought for research on, and development of, the emerging technological discipline of spintronics (spin-transport electronics). Proposed nonmagnetic semiconductor electron-spin filters were described in a prior NASA Tech Briefs article: "Electron-Spin Filters Based on the Rashba Effect" (NPO- 30635), Vol. 28, No. 10 (October 2004), page 58. To recapitulate: The proposed spin filters were to be based on the Rashba effect, which is an energy splitting of what would otherwise be degenerate quantum states, caused by a spin-orbit interaction in conjunction with a structural-inversion asymmetry (SIA) in the presence of interfacial electric fields in a semiconductor heterostructure. The magnitude of the energy split is proportional to the electron wave number. In a spin filter, the spin-polarized currents produced by the Rashba effect would be extracted by quantum-mechanical resonant tunneling.

Current Spin Polarization was computed as a function of collection angle for different values of the ratio between the BIA and SIA coefficients (αBIA/αSIA), under the simplifying assumption of perfect sub-band filtering.

The origin of the enhancement now proposed lies in recognition that not only the SIA but also bulk inversion asymmetry (BIA) contributes to the spin-dependent energy splitting. The conceptual device structure on which the proposal is based is a spin-filtering resonant tunneling heterostructure, grown along the [001] direction (the z axis), that includes an asymmetric quantum well. The physics of this structure was represented by a simplified two-band, spin-dependent Hamiltonian model. This model was chosen because although it is only approximate, its simplicity facilitates understanding of how BIA could be utilized to enhance spin filtering.

The theoretical calculations were performed using this model. It was found that when only SIA is taken into account, the theoretical upper limit of current spin polarization for a Rashba-effect resonant tunneling spin filter with a one-sided spin collector can be expected to be 2/π (about 63.7 percent), independent of the direction of the collector. When BIA was taken into account along with SIA, it was found that current spin polarization could be changed from the SIA-only value by varying the collection angle: in particular, the greatest and least polarization values were found to occur in the [110] and [11 - 0] directions, representing collection angles of +45° and -45°, respectively.

This work was done by David Ting and Xavier Cartoixa of Caltech for NASA's Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..