Electromagnetismo Y Sus Aplicaciones

JOURNAL OF APPLIED PHYSICS 103, 063718 2008

Drift-diffusion crossover and the intrinsic spin diffusion lengths in semiconductors
M. Idrish Miah1,2,a
1

Nanoscale Science and Technology Centre and School of Biomolecular and Physical Sciences, Griffith University, Nathan, Brisbane Queensland 4111, Australia 2 Department of Physics, University of Chittagong, Chittagong, Chittagong 4331,Bangladesh

Received 4 December 2007; accepted 17 January 2008; published online 27 March 2008 We study the propagation of electron spin density polarization and spin currents in n-doped semiconductors within the two-component drift-diffusion model in an applied electric field E . The drift and diffusion contributions to the spin currents are examined, which shows how the spin current could beenhanced. We find that there is a crossover field Ex , where the drift and diffusion contribute equally to the spin current in the downstream direction. This suggests a possible way to identify whether the process for a given E would be in the drift or diffusion regime. We derive the expression for Ex and show that the intrinsic spin diffusion length in a semiconductor can be calculated directly from Ex.The results will be useful in obtaining transport properties of the carriers’ spin in semiconductors. This investigation, however, highlights the need for further experiments to be conducted to measure Ex in semiconductors. © 2008 American Institute of Physics. DOI: 10.1063/1.2898408 There has recently been an increasing interest in the emerging field of spin-sensitive electronics or spintronics,where the electron spin degree of freedom is exploited and new devices, such as spin field effect transistors SFETs or spin storage/memory devices, are proposed.1–3 The first metallic spintronic components were passive devices, namely, magnetic multilayers sandwiched structures consisting of alternating ferromagnetic and nonmagnetic metal layers whose electric conductance depends strongly on theexternal magnetic field. Depending on the relative orientation of the magnetizations in the magnetic layers, the device conductance changes from large parallel magnetizations to small antiparallel magnetizations . These devices are already implemented as giant magnetoresistance read heads and nonvolatile magnetic random access memory.2,3 The proposal of utilizing electron spin for quantum informationprocessing is a natural extension of spintronics since the electron provides an ideal condition for a quantum bit. However, the first active semiconductor spintronic device was suggested by Datta and Das in 1990,4 where they proposed an electronic analogue of an electro-optical modulator, that was later termed “SFET,” in a two-dimensional electron gas contacted with two ferromagnetic electrodes: oneas a source for the injection of spin-polarized electrons and the other as an analyzer for electron spin polarization. The Datta–Das SFET device relies on the basic concept of modulating the transistor’s source-to-drain current by varying the Rashba interaction in the channel with a gate voltage. The necessary requirements for such an active device are the efficient injection of spinpolarizedelectrons into a semiconductor, transporting them reliably over reasonable distances, i.e. without spin flipping or spin relaxation and then detecting them since information is carried by the carrier spin . Much effort has thus been
a

dedicated1 in understanding these important issues. The injection and detection of spin-polarized electrons or spin currents in semiconductors have been eitheroptically achieved or by electrical methods with varying degrees of success.1,5 In this paper, we focus our attention on the spin transport under applied electric field E within a semiconductor, leaving aside the problems of spin injection and detection. We study the propagation of electron spin density polarization and spin currents in n-doped semiconductors within the driftdiffusion model. We find that...