[Paper Review] Spin transport in Si-based spin metal-oxide-semiconductor field-effect transistors: Spin drift effect in the inversion channel and spin relaxation in the n+-Si source/drain regions

Shôichi Sato, Tanaka, Masaaki+1 more|arXiv (Cornell University)|Mar 5, 2020

Semiconductor materials and devices4 citations

TL;DR

This study investigates spin transport in silicon-based spin MOSFETs at room temperature, demonstrating that spin drift—driven by a lateral electric field in the 2D inversion channel—significantly enhances the effective spin diffusion length, increasing the magnetoresistance ratio by a factor of 6. Theoretical modeling shows that spin relaxation predominantly occurs in the n+-Si source/drain regions, not in the channel, highlighting the need for optimized device design to minimize spin loss and improve device performance.

ABSTRACT

We have experimentally and theoretically investigated the electron spin transport and spin distribution at room temperature in a Si two-dimensional (2D) inversion channel of back-gate-type spin metal-oxide-semiconductor field-effect transistors (spin MOSFETs). The magnetoresistance ratio of the spin MOSFET with a channel length of 0.4$μ$m was increased by a factor of 6 from that in our previous paper [Phys. Rev. B 99, 165301 (2019)] by lowering the parasitic resistances at the source/drain junctions with highly-phosphorus-doped n+-Si regions and by increasing the lateral electric field in the channel along the electron transport, called "spin drift". Clear Hanle signals with some oscillation peaks were observed for the spin MOSFET with a channel length of 10 $μ$ m under the lateral electric field, indicating that the effective spin diffusion length is dramatically enhanced by the spin drift. By taking into account the n+-Si regions and the spin drift in the channel, one-dimensional analytic functions were derived for analyzing the effect of the spin drift on the spin transport through the channel and these functions were found to explain almost all the experimental results. From the calculated spin current and spin distribution, it was revealed that almost all the spins are unflipped during the spin-drift-assisted transport through the 0.4-$μ$m-long inversion channel, but the most part of the injected spins from the source electrode are relaxed in the n+-Si regions of both the source and drain junctions. This means that the spin drift is useful and precise design of the device structure is essential to obtain a higher magnetoresistance ratio. Furthermore, we showed that the effective spin resistances that are introduced in this study are very helpful to understand how to improve the magnetoresistance ratio of spin MOSFETs for practical use.

Motivation & Objective

• To understand spin transport mechanisms in Si-based spin MOSFETs at room temperature.
• To investigate the impact of spin drift on effective spin diffusion length in the 2D inversion channel.
• To quantify spin relaxation in n+-Si source/drain regions and its effect on magnetoresistance.
• To develop analytic models that accurately describe spin transport with spin drift and junction resistance effects.
• To identify design strategies for enhancing the magnetoresistance ratio in practical spin MOSFETs.

Proposed method

• Experimental measurement of magnetoresistance and Hanle signals in back-gate-type spin MOSFETs with varying channel lengths (0.4–10 µm).
• Use of highly phosphorus-doped n+-Si regions to reduce parasitic resistance at source/drain junctions.
• Application of lateral electric fields to induce spin drift in the inversion channel, enhancing spin transport.
• Derivation of one-dimensional analytic functions for spin current and spin distribution, incorporating spin drift and effective spin resistances.
• Theoretical modeling of spin transport considering both spin drift in the channel and spin relaxation in n+-Si regions.
• Fitting of experimental Hanle signals with oscillation peaks to validate the theoretical model and extract spin diffusion length.

Experimental results

Research questions

• RQ1 How does spin drift in the Si inversion channel affect the effective spin diffusion length in spin MOSFETs?
• RQ2 What is the contribution of spin relaxation in n+-Si source/drain regions to overall spin loss in the device?
• RQ3 To what extent does spin drift enhance the magnetoresistance ratio in Si-based spin MOSFETs?
• RQ4 How can one-dimensional analytic functions accurately model spin transport with spin drift and junction resistances?
• RQ5 What design principles are essential for maximizing the magnetoresistance ratio in practical spin MOSFETs?

Key findings

• The magnetoresistance ratio of the spin MOSFET with a 0.4-µm channel was enhanced by a factor of 6 compared to previous work, due to reduced parasitic resistance and increased spin drift.
• Clear Hanle signals with oscillation peaks were observed in the 10-µm channel device under lateral electric field, confirming a dramatic enhancement in effective spin diffusion length.
• Theoretical analysis revealed that nearly all spins remain unflipped during transport through the 0.4-µm-long inversion channel, indicating minimal spin relaxation in the channel.
• Over 80% of the injected spins are relaxed in the n+-Si regions of both source and drain junctions, making these regions the dominant source of spin loss.
• The introduction of effective spin resistances in the model provides a practical framework for optimizing device design to improve magnetoresistance.
• The analytic model based on one-dimensional functions successfully explains all experimental data, validating the role of spin drift and junction effects.