Experimental realization of a silicon spin field-effect transistor
aa r X i v : . [ c ond - m a t . m t r l - s c i ] M a y The following article has been submitted to Applied Physics Letters
Experimental realization of a silicon spin field-effect transistor
Biqin Huang ∗ Electrical and Computer Engineering Department,University of Delaware, Newark, Delaware, 19716
Douwe J. Monsma
Cambridge NanoTech Inc., Cambridge MA 02139
Ian Appelbaum
Electrical and Computer Engineering Department,University of Delaware, Newark, Delaware, 19716
A longitudinal electric field is used to control the transit time (through an undoped silicon verticalchannel) of spin-polarized electrons precessing in a perpendicular magnetic field. Since an appliedvoltage determines the final spin direction at the spin detector and hence the output collector current,this comprises a spin field-effect transistor. An improved hot-electron spin injector providing ≈ ≈
38% electron current spin polarization after transportthrough 10 µ m undoped single-crystal silicon, is used for maximum current modulation. The spin field effect transistor (spinFET) proposed byDatta and Das[1] has stimulated much research in spinprecession-controlled electronic semiconductor devices[2,3]. Because silicon (Si) has a very long intrinsic elec-tron spin lifetime[4, 5] and is the cornerstone of modernsemiconductor microelectronics, it could be the materi-als basis of a future semiconductor spintronics paradigmutilizing these types of devices. However, spintronicstechniques which worked so well for other semiconduc-tors, most notably GaAs[6, 7, 8, 9, 10], are ineffective
Si n-Si
Spin TransportI C2 I C1 V C1 V E Al Al O Sin-Si 10 m m FIG. 1: Schematic illustration of the Si spin field-effect deviceused in this work (left), and associated conduction band dia-gram (right). The vertical structure (top to bottom) is 40nmAl/Al O /5nm Al/5nm Co Fe /5nm Cu/10 µm undopedSi/4nm Ni Fe /4nm Cu/n-Si. Hot electrons are injectedby an emitter voltage ( V E ) from Al ballistically through theAl/Co Fe /Cu anode base and into the conduction band ofthe 10 µm -thick undoped Si drift layer forming injected cur-rent I C . Detection on the other side is with spin-dependentballistic hot electron transport through the Ni Fe thin film.Our spin-transport signal is the ballistic current transportedinto the conduction band of the n-Si collector ( I C ). ∗ [email protected] with silicon for both bandstructure and materials growthreasons.[5]To solve this problem, we have recently demonstratedspin transport in silicon using hot-electron transportthrough ferromagnetic (FM) metal thin films for all-electrical spin-polarized injection and detection.[11] Be-cause the device design includes rectifying Schottky bar-riers on either side of the Si transport layer, an appliedaccelerating voltage induces little spurious current, al-lowing transit-time control of final spin direction at thespin detector during precession in a perpendicular mag-netic field. Two of us have recently proposed to use thiseffect as the basis of a transit-time spinFET.[12]To demonstrate the transit-time spinFET, the outputcollector current magnetocurrent change must be largerthan any magnetically-independent current rise inducedby accelerating voltage increase. However, successful op-eration of previously demonstrated devices in this pro-posed mode is prevented by the low magnetocurrent sig-nal of only ≈ -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5681012 Perpendicular Magnetic Field [kOe] C o ll ec t o r C u rre n t [ p A ]
286 Oe -300 -200 -100 0 100 200 30068101214 C o ll ec t o r C u rre n t [ p A ] In-Plane Magnetic Field [Oe] ab -1.5 -1.0 -0.5 0.0 0.5 1.0 1.51012141618202224 Perpendicular Magnetic Field [kOe] C o ll ec t o r C u rre n t [ p A ]
978 Oe c -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5681012 Perpendicular Magnetic Field [kOe] C o ll ec t o r C u rre n t [ p A ]
286 Oe -300 -200 -100 0 100 200 30068101214 C o ll ec t o r C u rre n t [ p A ] In-Plane Magnetic Field [Oe] ab -1.5 -1.0 -0.5 0.0 0.5 1.0 1.51012141618202224 Perpendicular Magnetic Field [kOe] C o ll ec t o r C u rre n t [ p A ]
978 Oe c FIG. 2: (a) In-plane spin-valve effect for the device with emit-ter tunnel junction bias V E =-1.6V and V C =0V at 85K,showing ≈ V E =-1.6V and accelerating voltage V C =0V. (c) Sameas in (b), but with V C =3V. Al/Al O /5nm Al/5nm Co Fe /5nm Cu. Unpolar-ized electrons tunneling from the normal metal Al acrossthe Al O oxide barrier are subsequently spin polarizedby the hot-electron ballistic spin filtering effect (spin-dependent scattering) through the Co Fe layer beforeconduction-band injection over the Cu/Si Schottky bar-rier.After vertical transport through the 10 µ m-thickundoped single-crystal silicon device layer, the spin-polarized electrons are ejected from the Si conductionband into the detector FM thin film (Ni Fe ) abovethe Fermi energy. The ballistic component of this hot-electron current is collected by the second Schottky bar-rier with a n-Si substrate, forming the collector cur-rent and spin-transport signal ( I C ). By manipulat-ing the relative orientation of the injector and detectorFM layer magnetizations with an in-plane external mag-netic field, I C can be changed correspondingly. Thisin-plane spin-valve hysteresis at constant emitter bias V E =-1.6V is shown in Fig. 2(a). The magnetocurrent C o ll ec t o r C u rre n t [ p A ] Accelerating Voltage [V]
Accelerating Voltage [V] C o ll ec t o r C u rre n t [ p A ] Accelerating Voltage [V] N o r m a li ze d C u rre n t [ % ] N o r m a li ze d C u rre n t [ % ] Accelerating Voltage [V] C o ll ec t o r C u rre n t [ p A ] Accelerating Voltage [V] N o r m a li ze d C u rre n t [ % ] Accelerating Voltage [V] abc def C o ll ec t o r C u rre n t [ p A ] Accelerating Voltage [V]
Accelerating Voltage [V] C o ll ec t o r C u rre n t [ p A ] Accelerating Voltage [V] N o r m a li ze d C u rre n t [ % ] N o r m a li ze d C u rre n t [ % ] Accelerating Voltage [V] C o ll ec t o r C u rre n t [ p A ] Accelerating Voltage [V] N o r m a li ze d C u rre n t [ % ] Accelerating Voltage [V] abc def