Semiconductor electron-spin filters of a proposed type would be based on the Rashba effect, which is described briefly below. Electron-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-based electronics). There have been a number of successful demonstrations of injection of spin-polarized electrons from diluted magnetic semiconductors and from ferromagnetic metals into nonmagnetic semiconductors. In contrast, a device according to the proposal would be made from nonmagnetic semiconductor materials and would function without an applied magnetic field.

The Rashba effect, named after one of its discoverers, 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 in the presence of interfacial electric fields in a semiconductor heterostructure.

ImageThe magnitude of the energy split is proportional to the electron wave number. The present proposal evolved from recent theoretical studies that suggested the possibility of devices in which electron energy states would be split by the Rashba effect and spin-polarized currents would be extracted by resonant quantum-mechanical tunneling. Accordingly, a device according to the proposal would be denoted an asymmetric resonant interband tunneling diode [a-RITD]. An a-RITD could be implemented in a variety of forms, the form favored in the proposal being a double-barrier heterostructure containing an asymmetric quantum well.

It is envisioned that a-RITDs would be designed and fabricated in the InAs/GaSb/AlSb material system for several reasons: Heterostructures in this material system are strong candidates for pronounced Rashba spin splitting because InAs and GaSb exhibit large spin-orbit interactions and because both InAs and GaSb would be available for the construction of highly asymmetric quantum wells. This material system affords a variety of energy-band alignments that can be exploited to obtain resonant tunneling and other desired effects. The no-common-atom InAs/GaSb and InAs/AlSb interfaces would present opportunities for engineering interface potentials for optimizing Rashba spin splitting.

More specifically, a device of this type would comprise an asymmetric composite InAs-GaSb well, sandwiched between AlSb barriers and InAs electrodes. Unpolarized electrons from the conduction band of an InAs emitter electrode would tunnel rapidly through one AlSb barrier and through an asymmetric InAs-GaSb quantum well, where Rashba spin splitting would occur; they would then tunnel through the other AlSb barrier into the conduction band of an InAs collector electrode. With appropriately chosen thicknesses of layers, this device could be made to operate in either of two regimes (see figure):

  • Under low bias, in a resonant-interband-tunneling regime, in which electrons would traverse valence subband states in GaSb or
  • Under moderate bias, in an intrabandresonant-tunneling regime, in which electrons would traverse conduction subband states in InAs. Computational simulations have led to an expectation that the interband regime would yield better performance.

This work was done by David Z.-Y. Ting, Xavier Cartoixà, and Thomas C. McGill of Caltech; Jeong S. Moon, David H. Chow, and Joel N. Schulman of HRL Laboratories, LLC; and Darryl L. Smith of Los Alamos National Laboratory for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
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Refer to NPO-30635, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Electron-Spin Filters Based on the Rasba Effect

(reference NPO-30635) is currently available for download from the TSP library.

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This article first appeared in the October, 2004 issue of NASA Tech Briefs Magazine.

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