Detection of a Spin Accumulation in Nondegenerate Semiconductors

Abstract

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...

Electrical injection and detection of spin-polarized electrons in silicon through an Fe3Si/Si Schottky tunnel barrier

“…[11] Using such contacts (without the n + -Si layer), we can not detect spin-polarized carriers electrically due to additional resistance originating from a wide width of the depletion re- gion (∼ 500 nm). [2,14] In contrast, we identify that |I rev. | for Diode A (red) shows almost no variation with decreasing temperature, as shown in Fig.…”

“…For other samples without the n + -Si layer, such hysteretic features could not be obtained because of the limitation of the current flow. [2,14] The hysteretic nonlocal signals (∆V /I) shown in Figs. 2(c) and 2(d) indisputably exhibit the experimental demonstrations of electrical spin injection and detection in Si-based devices via a Schottky tunnel barrier.…”

“…For spin injection from Fe 3 Si directly into Si, the I − V bias characteristic in the reverse bias regime (V bias < 0) is important. [2,13,14] We note that the reverse-bias |I| (|I rev. |) for Diode A is extremely large more than three orders of magnitude compared to that for Diode B.…”

Spin dynamics in semiconductors

“…Spin related phenomena in silicon have also attracted much attention recently because of the long spin lifetime in silicon and the compatibility to the current industrial semiconductor technologies [149,150,279,280,282,290,291,295,776,781,[1045][1046][1047][1048][1049][1050][1051][1052][1053][1054][1055][1056][1057]. A major difficulty for studying spin related properties in silicon is spin generation and detection.…”

Sign of tunnel spin polarization of low-work-function Gd/Co nanolayers in a magnetic tunnel junction

“…[9][10][11] In particular, the work function of the ferromagnetic electrode is a crucial parameter that allows for the necessary suppression of the Schottky barrier and the depletion region in the semiconductor.…”