CRYOFREE

Application noteOn the way to a graphene spin field effect transistor
How Prof. Barbaros Özyilmaz is using a TeslatronTMPT from Oxford Instruments at theDepartment of Physics, NanoCore and Graphene Research Centre, National University of Singapore (NUS)

Prof. Barbaros ÖzyilmazFigure 1a. TeslatronTMPT Cryofree® system
at Barbaros Özyilmaz lab

Introduction
This application note describes some experiments at low temperature in a magnetic
environment, used for the development of a graphene-based spin field effect transistor.
The measurements have been carried out using Oxford Instruments’ TeslatronPTCryofree® superconducting magnet system, with two specially designed measurement
probes. The main experimental work has been published in Reference [1].

BackgroundFigure 1b. Rotating probewith chip holder

Graphene, being only one atom thick, is sensitive to the properties of the substrate on
which it is placed. For example, graphene is not itself a superconductor, but inherits suchproperties when deposited on top of a superconducting material, due to the proximity
effect. When graphene is deposited on hexagonal boron nitride, the energy spectrum ofgraphene is modified and additional Dirac points are formed. Therefore, new phenomena
could emerge when graphene interacts with a substrate. Such flexibility in material
and thus device design is central to new applications which can be enabled by twodimensional (2D) materials.
Intrinsic spin-orbit coupling (SOC) in graphene is weak, making graphene a promisingmaterial for spintronics, where the long spin mean-free-path of charge carriers is essential
for practical purposes. However, weak SOC also means poor control of the spin. This
limitation has been overcome by using the proximity effect when graphene is deposited
on a tungsten disulphide substrate, which has strong SOC (reference [1]). By means ofgraphene with strong and weak SOC, one would expect that room temperature operationof graphene-based spin field effect transistor is getting closer.
In the study of non-local magneto-resistance in graphene deposited on top of tungsten
disulphide (WS2), high magnetic field with two orientations with respect to the graphene’s
plane is used. These measurements are essentially to distinguish between orbital andspin effects. A rotational probe (Figure 1b) is used because it is desirable to perform all
experiments without exposing the sample to ambient conditions.

Figure 1c. Hall bars
(Graphene is not visible)

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Application noteMethods
The heterostructure is fabricated by sequential transfer of WS2 andgraphene on top of an oxidised silicon substrate. Graphene is patterned
into a Hall bar by electron beam lithography. Then, Cr/Au Ohmic contacts
to graphene are formed as shown in Figure 1c.
Standard four-terminal measurements of the resistance are performed
using a lock-in amplifier (Stanford Research Systems SR830). The lowfrequency oscillator output is connected to the current contact number
1 in series with a large resistance (10 MOhm), as shown in the circuit
diagram in the inset of Fig. 2a on the next page. The current contact
number 6 is grounded. The voltage drop across the sample is measured(numbers 3 and 5 on the diagram) by two probes connected to thedifferential input of the lock-in amplifier. The gate voltage is applied usinga DC source (Keithley 6430). For non-local measurements a constantcurrent flows from contact 2 to contact 3 and the voltage is measured by
the pair of contacts 5 and 4. The current contacts used for measurements
are indicated by black colour and the potential probes are shown by red
numbers in the insets of Figure 2a.

The front and back view of
the chip holder attached to
the rotating probes

Chip holder

Special measurement probes and cryomagnetic system
Oxford Instruments’ 12 T, 50 mm bore VTI TeslatronPT system is used forthis experiment to control temperature and to sweep magnetic field. Twomeasurement probes were specially designed for the graphene experiments
at NUS.
The first probe is a two position rod (0 and 90 degrees) fixed at roomtemperature. The second is a manual 180 degree rotator. One of the probes may
be rotated parallel to the field, via a rotator provided at the NW50 flange at thetop of the VTI, which enables all experiments to be performed without exposing
the sample to ambient conditions.
The key advantages of using two these special measurement probes are:
The sample is in a vacuum can that is pumped to 1 x10-5 Torr and heated to420 K outside the VTI. This enables the sample to be outgassed, or annealed,and protects it from coming into contact with air or helium gas
The sample is on a chip carrier with multiple electrical connections
When loaded into the VTI, the sample will achieve a base temperature of lessthan1.6 K, even with the sample in vacuum, and a maximum temperature of300 K
The sample can be at different angles to the field
Both probes are fitted with 48 wires for use, 44 connected to the chip and four
left free. 42 SWG PTFE coated constantan wire is used. In addition, four 42 SWG
constantan wires and two 36 SWG copper wires are fitted for the CernoxTM
sensor and the Watlow Firerod heater fitted to the rotator body.

Specially designed
measurement probes

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Application note

Experimental results

Figure 2a shows the conductivity of graphene as a function of gate voltage measured at 1.5 K.
Hole mobility is estimated to be 18,000 cm2V-1s-1 whereas for the electrons it is ~25,000 cm2V-1s-1.
Saturation of conductivity is observed above the threshold voltage of 15 V. This is caused by thesaturation of the electron concentration, as can be seen in Figure 2b, where the quantum Hall effectin graphene is modified by the presence of underlying WS2.
The most remarkable result is measured by non-local spin transport at room temperature as shownin Figure 2c. This is a common feature for the non-local resistance near zero gate voltage, whichis due to an Ohmic leakage contribution. However, the largest value of the non-local resistance isobserved in the saturated regime above 15 V. To clarify the origin of this unusual signal, non-local
resistance was measured in parallel to the graphene plane magnetic field. An apparent signal, which
depends on the in-plane magnetic field, is measured at 30 V (Figure 2d). This indicates a strong SOC
in graphene induced by the substrate. Ab initio fully relativistic density functional theory calculations
show that this effect is caused by the sulphuric vacancies in WS2.

Figure 2a. Conductivity of graphene.
Figure 2b. Resistance of graphene in high magnetic field.
Figure 2c. Non-local resistance measured at room temperature.
Figure 2d. Non-local resistance as a function of in-plane magnetic field for two gate voltages.

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Application note

Conclusion and outlook

In summary, The Barbaros Özyilmaz lab has demonstrated substrate-induced
enhancement of SOC in graphene. At the same time the mobility of charge
carriers in graphene has not been decreased, which paves the way to creationof a graphene spin field effect transistor that operates at room temperature.
Reference:[1] A. Avsar, J. Y. Tan, T. Taychatanapat, J. Balakrishnan, G. K. W. Koon, Y. Yeo, J. Lahiri, A.
Carvalho, A. S. Rodin, E. C. T. O’Farrell, G. Eda, A. H. Castro Neto & B. Özyilmaz Nature Commun.
5, 4875 (2014).

About the TeslatronPT and special probes
The TeslatronPT is a Cryofree superconducting magnet system,
providing top loading access to a sample in a variable magnetic field / lowtemperature environment. These systems provide magnetic fields up to18 T, with their integrated variable temperature inserts providing sample
temperatures from 1.5 K to 300 K. These capabilities enable measurements
to be carried out for innovative experiments in the field of nanotechnology
and nano-structures, such as the one explained in this application note.
The two special probes described here, including one with automatedperpendicular rotation, have been developed by Oxford Instruments inconsultation with the Barbaros Özyilmaz group specifically for graphene and2D materials research, and are generally available.

TeslatronPT
Cryofree system

About the Barbaros Özyilmaz lab at NUS
The main lab is a Cryogen free low temperature high magnetic fieldmeasurement facility consisting of six individual set ups. These range from VTI
systems with magnetic fields up to 16 T, to top-loading dilution refrigeratorswith vector fields and base temperatures down to 20 mK. The Graphene
Research Centre is dedicated to device physics, based on 2D crystals andtheir heterostructures. This lab works in diverse research areas ranging fromfundamental research including spintronics and chemical reactivity studies
to applied research, such as in the fields of flexible electronics, nanoengines,
transparent conducting electrodes and energy storage and conversion.

Contact us at:
Oxford Instruments NanoSciencenanoscience@oxinst.com
www.oxford-instruments.com

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May 2016, Issue 4