Spin-transport characteristics in a Si-based spin metal-oxide-semiconductor field-effect transistor (spin MOSFET): Bias dependence of the spin polarization in Si and magnetoresistance in spin-valve signals
Abstract
We have studied the spin transport characteristics of a spin metal-oxide-semiconductor field-effect transistor (spin MOSFET), particularly the bias voltage dependence of the electron spin polarization PS in Si and the magnetoresistance ratio MR in spin-valve signals, to optimize the device performance. The spin MOSFET device has an 8-nm-thick p-Si channel with a back gate (G) and ferromagnetic source / drain (S/D) junctions consisting of Fe/Mg/MgO/SiOx/n+-Si. In addition to transistor characteristics with an on-off ratio of 104, clear spin-valve signals and Hanle spin precession signals were observed at 4 K in a wide range of the source-to-gate VGS and source-to-drain VDS bias voltages. We achieved a high PS of 50% and a high MR of 0.35% as the maximum values in their single-peaked curves plotted as a function of the junction voltage VJ, mainly because the ferromagnetic S/D junction can generate high PS and the spin diffusion length is very long in the Si inversion channel. These PS and MR values are the highest ever reported in spin-MOSFETs. Our spin transport model for our spin MOSFET structure was improved in this study by taking into account the electron distribution and band profile of the n+-Si regions in the ferromagnetic S/D junctions, which enables the accurate estimation of PS. Detailed analyses with various VGS and VJ clarified that PS is determined only by VJ. Our analyses also revealed that the main parameters for determining MR, such as PS and the resistance-area products of the S/D ferromagnetic junctions, have different VJ dependences, leading to the finding that the present device does not exploit the full potential of the ferromagnetic S/D junctions to maximize MR. Based on the results, we discuss the device physics and engineering for further enhancement of MR, with a focus on the electrical and spin-related properties of the ferromagnetic S/D junctions.
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