We analyzed the applicability of original Datta-Das proposal for spin-field-effect transistor ͑spin-FET͒ to nonballistic regime based on semiempirical Monte Carlo simulation for spin transport. It is demonstrated that the spin helix state in two-dimensional electron gas system is sufficiently robust against D'yakonov-Perel' spin relaxation to allow an operation of Datta-Das-type spin-FET in the nonballistic transport regime. It is also shown that the spin diffusion length of the spin helix state can be increased with an in-plane electrical field along the ͓110͔ direction. In marked contrast to early proposals for nonballistic spin-FETs, the "on" and "off" states are characterized by a 180°phase difference in the spin precession motions, which is highly advantageous in terms of device flexibility.
The D'yakonov-Perel' spin relaxation process in the ͑001͒ InAs quantum well system is studied based on Monte Carlo ͑MC͒ simulation. The present space-resolved MC analysis demonstrates that the relaxation of spins oriented in any axes is totally suppressed with equal strength of Rashba and Dresselhaus effects, which is in marked contrast with the spin relaxation anisotropy reported previously in time-resolved analyses. Our calculation also shows a substantial contribution of the cubic term of the wave number vector in the Dresselhaus model onto the spatial spin distribution. DOI: 10.1103/PhysRevB.75.241308 PACS number͑s͒: 72.25.Rb, 72.25.Dc, 85.75.Hh Carrier spin transport in a zinc-blende semiconductor involves decay of spin polarization coherence, i.e., a spin relaxation process due to several mechanisms 1-4 and suppression of the spin relaxation process is a prerequisite for realizing spintronics devices such as the spin-field-effect transistor ͑spin-FET͒. 5,6 At room temperature, among several mechanisms, electron spin relaxation due to the D'yakonovPerel' ͑DP͒ mechanism is the most dominant in a quantum well ͑QW͒ system grown on ͑001͒ substrate. In an asymmetric QW system, bulk-inversion asymmetry ͑BIA͒ and structure-inversion asymmetry ͑SIA͒ lead to the Dresselhaus 7 and Rashba 8 spin-orbit terms in the effective Hamiltonian, respectively, and their corresponding effective magnetic fields are involved in the DP process. It has been demonstrated in several theoretical works 9-13 that the interplay between the Dresselhaus and Rashba effects causes a spin relaxation anisotropy so that the spin relaxation rate largely depends on the orientation of spin. The existence of such an anisotropy has been in fact confirmed by means of Hanle effect measurements in a recent work.14 Of particular importance in these theoretical findings is that when the Rashba and Dresselhaus linear-in-k terms have equal strength, the relaxation of spin oriented in one of the ͗110͘ axes, more specifically the ͓110͔ axis within the present discussion, is totally suppressed, while the relaxation time of spins along the other axis is finite. Hence, it has been generally acknowledged that the spin along a different axis from ͓110͔ is not robust against the spin relaxation. The strong suppression of spin relaxation along ͓110͔ is the essential ingredient for nonballistic spin-FET proposed in Ref. 5.The spin relaxation essentially proceeds in both temporal and spatial coordinates. In the theoretical works mentioned above, the spin relaxation anisotropy has been discussed only in a time-resolved analysis with the assumption of a homogeneous spin distribution. Based on a semiclassical Monte Carlo ͑MC͒ approach of spin transport, [15][16][17][18] in the present study, it is demonstrated that the time-resolved analysis can highlight only one aspect of anisotropic spin relaxation phenomena, overlooking an important feature inherent in the spin transport, i.e., spatial coherence of spin polarization. The present space-resolved MC simulation revea...
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