Impurity-Driven Two-Dimensional Spin Relaxation Induced by Intervalley Spin-Flip Scattering in Silicon

Abstract

Through the theoretical study of electron spin lifetime in the two-dimensional electron gas (2DEG) confined near the surface of doped Si, we highlight a dominant spin relaxation mechanism induced by the impurity central-cell potential near an interface via intervalley electron scattering. At low temperatures and with modest doping, this Yafet spin flip mechanism can become more important than the D'yakonov-Perel' spin relaxation arising from the structural Rashba or Dresselhaus spinorbit coupling field. As the leading-order impurity-induced spin flip happens only between two non-opposite valleys in Si, 2DEG systems in Si MOSFETs or SiGe heterostructures are a natural platform to test and utilize this spin relaxation mechanism due to the valley splitting near the interface and the tunability by electrical gating or applied stress. Our proposed new spin relaxation mechanism may explain a part of the spin relaxation contribution to Si-based 2DEG systems, and should have spintronic applications in Si-based devices.

Electron Spin Transport in a Metal-Oxide-Semiconductor Si Two-Dimensional Inversion Channel: Effect of Hydrogen Annealing on Spin-Scattering Mechanism and Spin Lifetime

“…Our finding leads to a simple guideline that S at room temperature is enhanced by the enhancement of µ. This conclusion is also supported by our result that even the highest µ value of 3065 cm 2 V −1 s −1 at 4 K is not high enough for the appearance of the DP mechanism [33]. Hence, it is important to note that the established Si technology for the enhancement of µ in ordinary MOSFETs is also useful for spin MOSFETs, as we demonstrated that the SiO2/Si interface with the lower NDEF achieved by annealing leads to the higher S value at a certain T than that with the higher NDEF.…”

“…Under such circumstances, the electron momentum scattering occurs through the intra-valley scatterings owing to the fixed defect states in the SiO2 layer [40,41]. On the other hand, although a theory on the impurity-dominated electron spin-flip scattering in Si 2D inversion channels was reported [33], it cannot be applied to our analysis because it assumes that dopant atoms are ionized and the electron spin flip occurs through the inter-valley f process between the 2-fold and 4-fold subbands even at low temperatures. Besides, as pointed out in ref.…”

“…Besides, as pointed out in ref. [33], the spin flip due to the Coulomb scattering through fixed defect states near a SiO2/Si interface has not been clarified. To qualitatively analyze the Coulomb scattering, the strength of SOC was estimated using the theory in ref.…”

“…From the difference in the threshold gate voltage Vth between the theoretical and experimental values in the annealed device, the fixed defect states density NDEF was estimated to be 2.3×10 11 cm −2 [19] that corresponds to the volume density nDEF = 1  10 17 cm −3 , according to the same conversion procedure from the sheet to volume densities in ref. [33]. Using this nDEF value and S = 9.3 ns, the theory in ref.…”

“…The EY mechanism is supported by the increased S with increase in μ by the reduction in the fixed defect state density due to annealing. This is because the anti-correlation between S and μ should be obtained when the D'yakonov-Perel' (DP) process is dominant [33,34]. It should be noted that the /S value (~1/25000) in the p-Si channel with lightly boron doping (NA = 1×10 15 cm −3 ) is smaller than that (~1/14000) in the phosphorous-doped 2D accumulation channel (ND = 1×10 17 cm −3 ) when T = 295 K [8].…”