Electron spin and momentum lifetimes in two-dimensional Si accumulation channels: Demonstration of Schottky-barrier spin metal-oxide-semiconductor field-effect transistors at room temperature

We have investigated the electron spin lifetime tau(S) and momentum lifetime tau in a two-dimensional (2D) accumulation channel of Schottky-barrier spin metal-oxide-semiconductor field-effect transistors (spin MOSFETs). The spin MOSFETs examined in this study have Fe/Mg/MgO/Si Schottky-tunnel junctions at the source/drain and a 15-nm-thick nondegenerated Si channel with a phosphorus donor doping concentration N-D of 1 x 10(17) cm(-3). We estimated tau and the electron diffusion coefficient D-e in the 2D accumulation channel from experimental results of a Hall-bar-type MOSFET device and self-consistent calculations using Poisson's and Schrodinger's equations. The spin MOSFETs with various channel lengths L-ch (= 0.3-10 mu m) exhibited transistor characteristics with a high on/off ratio of similar to 10(6) as well as clear spin-valve signals at 295 K. From the spin-valve signals measured with various gate electric fields (2-5 MV/cm), tau(S) and the electron spin diffusion length lambda(S) were estimated. We found that the spin-flip rate per one momentum scattering event tau/tau(S) is similar to 1/14 000, which is almost unchanged by the gate electric field. The proportionality between tau(S) and tau indicates that the Elliott-Yafet mechanism is dominant in the Si 2D electron accumulation channel, and that the spin-flip rate per one phonon scattering event and that per one surface roughness scattering event are the same. Based on the Elliott-Yafet theory, there is a possibility that the spin-orbit coupling in the Si 2D accumulation channel is almost twice as strong as that in bulk Si materials.