Theory of spin-polarized transport in photoexcited semiconductor/ferromagnet tunnel junctions

Abstract

We present a theory for spin-polarized transport in tunnel junctions consisting of a ferromagnet and a semiconductor, in which spin-polarized carriers are created by optical orientation. The model includes, for both spin orientations, the current due to tunneling between the ferromagnet and the semiconductor surface as well as the photoinduced and the thermionic emission currents through the semiconductor subsurface region. Tunneling is described in terms of a spin-dependent tunnel conductance, taking account of the magnetic structure of the ferromagnet. We consider spin depolarization of photoexcited electrons in the semiconductor bulk material and in surface states that have a spin-dependent occupation. The total tunnel current is evaluated as well as current modulations due to modulated spin polarization of photoelectrons ͑CPM signal͒ or modulated optical intensity. The calculations show that the CPM signal is proportional to the tunnel conductance polarization and is relatively insensitive to spin depolarization of photoelectrons during their transport to the surface. A severe signal reduction can, however, result from spin relaxation in semiconductor surface states. In addition, it is demonstrated that a crucial role is played by the operating regime of the junction, i.e., photoamperic or photovoltaic, where the selection is determined mainly by the choice of applied bias voltage. We find that the photovoltaic mode is favored, as it yields the highest contribution from spin-polarized tunneling, combined with the smallest sensitivity for unwanted light intensity modulations. ͓S0163-1829͑98͒03207-X͔

Photoassisted tunneling from free-standing GaAs thin films into metallic surfaces

“…20 For a gold (non-magnetic) surface, the interpretation of the results is relatively simple as the density of empty states depends only weakly on energy. 21 The quantitative agreement between the model and the experimental data demonstrates that, unlike earlier work, 13 the tunnel photocurrent originates from the conduction band. For a large tunnel distance, the observed exponential relationship of the tunnel photocurrent is caused by the bias dependent tunnel barrier height.…”

“…12 Jansen et al proposed that electrons tunnel from midgap surface states thereby obtaining good agreement with experimental results for small values of the bias applied to the metal. 13,14 These studies considered an energy-independent density of surface states and neglected surface recombination as well as the bias-dependent tunnel barrier height. Injection of free carriers across a semiconductorliquid interface has also considered, 15 with tunnelling from the conduction or valence band accounted for.…”

“…13 Analysis of the results has allowed us to eliminate the effect of possible distance inhomogeneities so that, after correction, the metal-semiconductor interface can be considered as planar, in the sense of a parallel plate capacitor.…”

Ballistic spin filtering across the ferromagnetic-semiconductor interface

“…In the spin filtering process across FM/SC interface, the spin polarized ensemble excited by right or left circularly polarized light on the semiconductor side had to first relax down to the band edge, then transit through the tunneling barrier into the FM side [17]. In the previous work, a physical model for the spin transport across Fe/Al 2 O 3 /n-GaAs tunneling structure under optical spin orientation was established [18].…”

Use of voltage pulses to detect spin-polarized tunneling

“…͓DOI: 10.1063/1.1556958͔ Spin-polarized scanning tunneling microscopy ͑SP-STM͒ and spectroscopy ͑SP-STS͒ are the most powerful tools for studying surface magnetism at an atomic scale. [1][2][3][4][5][6][7] To detect SP tunneling, W tips coated with ultrathin magnetic films, 1-3 tips made of soft magnetic materials whose magnetization can be easily switched, 4 and optically pumped GaAs tips [5][6][7] have been used. Although the use of tips covered with ultrathin magnetic layers has great advantages compared to the use of bulk magnetic tips with respect to stray field effects 4 and to optically pumped GaAs tips with respect to simplicity, 5-7 it still needs a lot of efforts to prepare these tips for SP tunneling experiments.…”