Simultaneous Spin and Charge Transport in Gated Si Devices

Jing Li and Ian Appelbaum Center for Nanophysics and Advanced Materials and Department of Physics, University of Maryland, College

Park, Maryland 20742, USA

Phone: (301)405-0890, fax: (301)405-3779, email: appelbaum@physics.umd.edu

Recent advances in the development of techniques for electrical injection and detection of

spin-polarized electrons in silicon have aroused intensive research on exploiting devices and circuits

that utilize the spin degree of freedom [1-3] as well as electron charge in this dominant material of the

semiconductor integrated circuits industry.

In this work, lateral spin-transport devices employing ballistic hot-electron injection and

detection methods are used to study temperature-dependent spin and charge transport controlled by an

electrostatic back gate using native oxide (Si02) insulator. Spin transport is studied by the "Larmor

clock" technique [4], where the ns-scale spin transit-time distribution is recovered from quasi-static

precession measurements in a perpendicular magnetic field with potentially sub-ns resolution [see Fig.

1]. The empirical spin transit-time distribution can be directly derived via Fourier transform and later

used to calculate spin mobility through Jis = L2 I(VAC'r) ,where r is the mean of the spin transit-time

distribution, L is the length of lateral transport channel, and VAC is the longitudinal accelerating

voltage across the channel from injector to detector. Simultaneous charge transport characteristics are

investigated by Hall effect, where ballistic injection of electrons into an otherwise undoped silicon

transport channel allows direct measurement of charge mobility (in obvious contrast to ohmic

injection, which directly measures only carrier density). From the transverse voltage across the

channel, charge mobility is derived through Jic = VHLI(WBJ.VAC)' where W is the width of

transport channel, BJ. is the perpendicular magnetic field, and VH is the transverse voltage. The

inset to Fig. 2 shows the linear relationship between Hall voltage and applied field, and the main panel

of Fig. 2 shows that the Hall mobility is indeed independent of the current as expected for

ballistically-injected currents greater than approximately 2f.lA (again, in direct contrast to ohmic

injection, where the Hall voltage is directly proportional to current).

As in Fig. 3, it is found that both charge and spin mobility decrease as temperature rises,

consistent with the role of phonon scattering in both processes. Spin mobility is lower than charge

mobility under the same gate biases, but charge mobility decreases more rapidly than spin mobility

when electrons are attracted to the Si/Si02 interface as shown in Fig. 4. The results indicate that

enhanced momentum scattering near the Si/Si02 interface increases spin relaxation rate, consistent

with the Elliott-Yafet spin relaxation mechanism.

[I] I. Appelbaum et aI., Nature, vol. 447, p. 295 (2007)

[2] B. Huang et aI., Phys. Rev. Lett., vol. 99, p. 177209 (2007)

[2] H.-J. Jang et aI., Phys. Rev. Lett., vol. 103, p. 117202 (2009)

[4] B. Huang et aI., Phys. Rev. B, vol. 82, p. 241202(R) (2010)

978-1-61284-244-8/11/$26.00 ©2011 IEEE 159

.� ..0 co

..0

e 0.. l:J Q) .!::! Cii E o

z

VG= --2V -- OV-- 3V

Fig. I: Transit-time distribution for

spin current under accelerating bias

I6V and gate bias IV at lOOK. Inset I

(upper right): symmetrized spin

precession data, from which the

distribution was calculated using

Fourier transform. Inset 2 (bottom

left): plan view of a wire-bonded

silicon lateral spin-transport device.

--1V--4V Y¢J'!.

Fig. 2: Longitudinal current

dependence of Hall mobility

under different gate bias (O-4V)

at 90K. Inset: Magnetic field

dependence of Hall voltage,

from which Hall mobility was

calculated, under I V gate bias

at different injector bias

(l.O-1.3V). o

"':00 2000 :? 5 1500

� 1000

80

1 2 3 4 5 Longitudinal Current [I-lA)

90 100 110 120 Temperature [K]

Fig. 3: Comparison between spin and

charge mobility temperature dependence

under gate bias IV.

978-1-61284-244-8/11/$26.00 ©2011 IEEE

2000 '00

:? 51500

:0 � 1000

'Charge T=100K

sl �� - �\ ��-

..... !""""

-4 � 0 2 4 6 Gate Bias [V]

8

Fig. 4: Comparison between spin and

charge mobility gate dependence at lOOK.

160