Spin-FET 1D Devices and Micro Hall-Magnetometry
René Eiselt, Christian Uhrich, Guido
Todays field-effect transistors (FET) used in microelectronics are solely based
on the transport of charges. These devices which are functioning as switches
belong to the most important elements in chip technology and determine the speed
as well as the size of processors and integrated circuits (IC).
To further improve the efficiency of the transisitors one promising approach
consists of making use of the spin of the electrons as information carrier .
For that purpose, two ferromagnetic or possibly halfmagnetic electrodes serve
as injector and detector for "spin-polarized transport" (Fig. 1). Within the
semiconductor the spin orientation of its two-dimensional electron system (2DES)
can be adjusted via an electric field with a gate electrode. The spin orientation
of the electrons with respect to the magnetization direction of the detector
decides whether the spin transitor shows a high or low resistance.
Fig. 1: Spin FET proposed by Datta und Das .
The blue arrows show the magnetization direction of the electrodes, the black
arrows the spin polarization of the electrons.
For the realization of a spin transitor some difficulties must be overcome.
One of the main problems consists in the injection of spin-polarized electrons
into the 2DES of the semiconductor. Due to scattering effects most of the spin
information will be lost.
To minimize these effects we are testing ferromagnetic and halfmagnetic materials
in cobination with different structuring methods on InAs heterostructures. Promising
materials for the electrodes are e.g. iron  and Heusler-alloys  with spin
polarizations up to 100%. For the deposition both thermal evaporation and sputter
processes are used.
A further problem in the realization of the spin FET is the control of the electrons'
spin-orientation in the semiconductor. The "rotation" of the spin in the electric
field caused by the Rashba effect  is efficient for a longitudinal movement
through the channel. Electrons which are moving obliquely through the semiconductor
decrease the switching effect. For that reason, it is of great importance that
the semiconducting channel is very narrow , i.e., quasi one-dimensional. Transport
measurements on quantum wires which have been patterned with etching processes
have shown that our InAs heterostrucutres are suitable for 1D-channels (Fig.
2). As things develop in the fabrication of a spin transistor additional split
gates and side gates are planned.
FIG. 2: (a) AFM-measurement of a wet-chemically
etched 1D-channel. The vertical axis is stretched in comparison to the lateral
one. (b) Conductance in dependence of the gate voltage Vg. Conductance
steps are observable in units of 2e2 /h
which are typical for 1D-channels.
Besides transport properties of the spin-polarized electrons in hybrid structures
a comprehensive understanding of the micro-magnetic properties is of great importance.
On the one hand our interest is focused on the domain structure of the electrodes
which are investigated in external magnetic fields with MFM-measurements at
room temperature. On the other hand it is crucial for spin transport to gain
knowledge of the magnetic stray field in the area of the semiconducter channel.
To measure the magnetic stray field of the electrodes we use micro Hall-magnetometry
. This technique shows significant advantages in comparison to other methods.
Examples are the high magnetic field sensitivity in large fields up to 1 Tesla
as well as the applicability in a wide range of temperatures from liquid helium
to room temperature.
The Hall crosses are wet-chemically etched with a Hall bar width in the order
of the electrodes themselves (Fig. 3). In a subsequent step we deposit both
single electrodes and double structures with a thickness of 30 nm in the sensor
area. The magnetic stray field caused by the electrodes generates a Hall effect
in the 2DES. From the Hall voltage conclusions can be drawn on the magnetic
properties of the ferromagnetic structures. Furthermore, we investigate the
switching processes within the domain structure of the electrodes in external
magnetic fields. Comparison of the results of single electrode measurements
with those of the double structure gives information on their interaction properties.
FIG. 3: (a) AFM-measurement of an iron double electrode.
(b) Side view of a nickel structure with an electrode separation of d
= 25 nm. (c) MFM-measurements of a nickel double electrode upon a Hall cross
of width w = 1.2 µm.
 S. Datta and B.
Das, Appl. Phys. Lett. 56, 665 (1990)
 D. Grundler, Phys. Rev. B
63 , R161307 (2001)
 F. Heusler, Verh. Dtsch. Ges.
5 , 219 (1903)
 E. I. Rashba and Yu A. Bychkov,
J. Phys. C 17 , 6039 (1984)
 A. K. Geim et al., Appl. Phys.
Lett. 71 , 2379 (1997)