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Electrical detection of a spin accumulation in a nondegenerate semiconductor using a tunnel barrier and ferromagnetic contact is shown to be fundamentally affected by the energy barrier associated with the depletion region. This prevents the ferromagnet from probing the spin accumulation directly, strongly suppresses the magnetoresistance in current or potentiometric detection, and introduces nonmonotonic variation of spin signals with voltage and temperature. Having no analogue in metallic systems, we identify energy mismatch as an obstacle for spin detection, necessitating control of the energy landscape of spin-tunnel contacts to semiconductors. DOI: 10.1103/PhysRevLett.99.246604 PACS numbers: 72.25.Hg, 72.25.Dc, 73.40.Gk, 85.75.ÿd The control and usage of the spin degree of freedom in semiconductor structures lie at the heart of spintronics [1][2][3][4]. Research is fueled by prospects of novel electronic devices (spin transistors and the like) with improved performance or novel functionality and the development of a spin-based quantum computer. Thereby, a wealth of intriguing spin-related phenomena have been discovered in semiconductors [1][2][3], including carrier-mediated ferromagnetism, long spin-coherence times, control of spin polarization by electric field, strain, or currents, and the spin-Hall effect. Crucial to many spintronic devices is the ability to (i) inject spin-polarized carriers into the semiconductor, (ii) transport them with or without spin manipulation in the semiconductor, and (iii) detect the spins with a second magnetic contact [4 -6].Spin injection and detection has been successful in nonmagnetic metals [7,8], metallic carbon nanotubes [9], and graphene [10]. For semiconductors, progress has been more difficult. A fundamental obstacle identified for spin injection is the conductivity mismatch [11] between a low resistivity ferromagnetic metal (FM) and a semiconductor (SC), causing the injected current to lack significant spin polarization as it is controlled by the large, spinindependent resistance of the SC. This issue was shown to be solved by introducing an extra spin-dependent interface resistance, for instance, a tunnel barrier, between FM and SC [12,13]. Despite this solution, significant magnetoresistance in a two-terminal FM/SC/FM device with diffusive transport is yet to be observed. Spin transport in a fully electrical device has been observed in 4-terminal nonlocal geometry (for GaAs [14] ) where the resistance of the detection contact is not an issue, or using hot-electron transport (using undoped Si [15] ) where the conductivity mismatch does not play a role.Theoretical models [4,11,16 -19] for spin transport (and conductivity mismatch) in such SC structures are based on diffusive transport and/or tunneling transport across the FM/SC interface. While this is appropriate for metal systems, it is questionable to describe the SC as a nonmagnetic metal with low conductivity and flat energy bands as this neglects the specific energy landscape associated with semiconduct...

Utilizing Co/Al2O3/Co magnetic tunnel junctions (MTJs) with Co electrodes of different crystalline phases, a clear relationship between electrode structure and junction transport properties is presented. For junctions with one fcc(111) textured and one polycrystalline (poly-phase and poly-directional) Co electrode, a strong asymmetry is observed in the magnetotransport properties, while when both electrodes are polycrystalline the magnetotransport is essentially symmetric. These observations are successfully explained within a model based on ballistic tunneling between the calculated band structures (DOS) of fcc-Co and hcp-Co.