Field-Effect-Driven Synthetic Rashba Spin–Orbit Coupling in n-Si Metal-Oxide Semiconductor

Recent extensive studies in condensed matter physics have revealed a wide variety of physical phenomena such as spin manipulation without magnetic field [1], interconversion between spin and charge currents [2], and giant spin splitting at an interface [3]. The typical material with strong spin–orbit coupling is a single heavy element such as Pt, Ta, and Bi. Breaking structural inversion symmetry is also one of the pivotal approaches to generate the spin–orbit coupling, called Rashba-type. For instance, structural inversion symmetry breaking at the Bi-Ag interface induces giant spin splitting [3]. Because of this background, materials with inversion symmetry and light elements have been outside the scope of spin–orbit coupling physics. Indeed, because Si has a light atomic number and inversion symmetric crystal structure, Si has been believed to be an unsuitable material for study on spin–orbit coupling. Here, we focus on Si metal-oxide semiconductor (MOS). Application of gate voltage in Si MOS modulates carrier density at the Si/oxide interface, and an interplay of a gate-electric field and carrier accumulation at the Si/oxide interface can synthetically induce Rashba-type spin–orbit coupling. In this study, we observe spin lifetime anisotropy of propagating spins in n-Si induced by the formation of an emergent effective magnetic field due to the Rashba-type spin–orbit coupling, when a gate voltage is applied to the n-Si MOS.<br/>Spin-polarization in n-Si is electrically generated, transported, and detected in a non-local spin transport device, which is comprised of ferromagnetic electrodes and non-magnetic reference electrodes on an n-Si channel. The gate voltage is applied via the backside silicon dioxide of the silicon-on-insulator substrate from 0 V to 100 V. To investigate an anisotropy of spin lifetime in n-Si, spin precession measurement is exploited at 300 K, applying an oblique magnetic field. In this measurement, non-local spin signal (<i>V</i>_NL4T) under an applied magnetic field is measured as a function of the tilt angle (<i>β</i>) of the magnetic field. In a system with an isotropic spin lifetime, a linear relationship between the magnitude of <i>V</i>_NL4T(<i>β</i>)/<i>V</i>_NL4T(0) and cos²<i>β</i> is predicted. To examine this approach quantitatively, experimental angular dependence is fitted by the theoretical equation, which is derived from the spin diffusion equation with anisotropic spin lifetime [4]. From the fitting, anisotropy ratio, which is quantified by a ratio of an out-of-plane spin lifetime to an in-plane spin lifetime, is extracted. We carry out a measurement of the angular dependence of <i>V</i>_NL4T in each gate voltage. Anisotropy ratios are 0.99 ± 0.02 and 0.75 ± 0.02 in gate voltages of 10 V and 100 V, respectively. Our results indicate that synthetic Rashba-type spin–orbit coupling is indeed induced in n-Si MOS by a field-effect when a strong gate voltage is applied [5]. A more detailed discussion will be given in the presentation.<br/>[1] J. Nitta <i>et al</i>., Physical Review Letters <b>78</b>, 1335 (1997).<br/>[2] S. O. Valenzuela and M. Tinkham, Nature <b>442</b>, 176 (2006).<br/>[3] C. R. Ast <i>et al</i>., Physical Review Letters <b>98</b>, 186807 (2007).<br/>[4] B. Raes <i>et al</i>., Nature Communications <b>7</b>, 11444 (2016).<br/>[5] S. Lee, M. Shiraishi <i>et al</i>., Nature Materials <b>20</b>, 1228 (2021).