U.S. patent application number 15/463229 was filed with the patent office on 2017-07-06 for homoepitaxial tunnel barriers with functionalized graphene-on-graphene and methods of making.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is Adam L. Friedman, Berend T. Jonker, Connie H. Li, Jeremy T. Robinson, Olaf M. T. van 't Erve. Invention is credited to Adam L. Friedman, Berend T. Jonker, Connie H. Li, Jeremy T. Robinson, Olaf M. T. van 't Erve.
Application Number | 20170194468 15/463229 |
Document ID | / |
Family ID | 54322614 |
Filed Date | 2017-07-06 |
United States Patent
Application |
20170194468 |
Kind Code |
A1 |
Friedman; Adam L. ; et
al. |
July 6, 2017 |
Homoepitaxial Tunnel Barriers with Functionalized
Graphene-on-Graphene and Methods of Making
Abstract
This disclosure describes a method of making a tunnel
barrier-based electronic device, in which the tunnel barrier and
transport channel are made of the same material--graphene. A
homoepitaxial tunnel barrier/transport device is created using a
monolayer chemically modified graphene sheet as a tunnel barrier on
another monolayer graphene sheet. This device displays enhanced
spintronic properties over heteroepitaxial devices and is the first
to use graphene as both the tunnel barrier and channel.
Inventors: |
Friedman; Adam L.; (Silver
Spring, MD) ; van 't Erve; Olaf M. T.; (Falls Church,
VA) ; Li; Connie H.; (Alexandria, VA) ;
Robinson; Jeremy T.; (Washington, DC) ; Jonker;
Berend T.; (Waldorf, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Friedman; Adam L.
van 't Erve; Olaf M. T.
Li; Connie H.
Robinson; Jeremy T.
Jonker; Berend T. |
Silver Spring
Falls Church
Alexandria
Washington
Waldorf |
MD
VA
VA
DC
MD |
US
US
US
US
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Washington
DC
|
Family ID: |
54322614 |
Appl. No.: |
15/463229 |
Filed: |
March 20, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14629939 |
Feb 24, 2015 |
9614063 |
|
|
15463229 |
|
|
|
|
61980448 |
Apr 16, 2014 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/4236 20130101;
H01L 21/02293 20130101; H01L 21/3081 20130101; H01L 29/1606
20130101; H01L 29/167 20130101; H01L 21/02115 20130101; H01L
21/02527 20130101; H01L 21/02227 20130101; H01L 29/45 20130101;
H01L 21/042 20130101; H01L 21/043 20130101; H01L 29/66984
20130101 |
International
Class: |
H01L 29/66 20060101
H01L029/66; H01L 21/04 20060101 H01L021/04; H01L 29/45 20060101
H01L029/45; H01L 29/167 20060101 H01L029/167; H01L 21/02 20060101
H01L021/02; H01L 29/16 20060101 H01L029/16 |
Claims
1. A method of making a homoepitaxial tunnel barrier transport
device with functionalized graphene-on-graphene, comprising:
growing a first monolayer graphene film; growing a second monolayer
graphene film; transferring the second monolayer graphene film onto
the first monolayer graphene film; fluorinating the second
monolayer graphene film; and forming the homoepitaxial tunnel
barrier transport device with functionalized
graphene-on-graphene.
2. The method of making a homoepitaxial tunnel barrier transport
device with functionalized graphene-on-graphene of claim 1, wherein
the step of fluorinating the second monolayer graphene film
comprises the step of preventing any edge state conduction.
3. The method of making a homoepitaxial tunnel barrier transport
device with functionalized graphene-on-graphene of claim 1, further
comprising the step of utilizing the tunneling behavior.
4. The method of making a homoepitaxial tunnel barrier transport
device with functionalized graphene-on-graphene of claim 1, further
comprising the step of operating the homoepitaxial tunnel barrier
transport device with functionalized graphene-on-graphene as a spin
valve.
5. A method of making a homoepitaxial tunnel barrier transport
device with functionalized graphene-on-graphene, comprising:
providing a substrate; providing a monolayer graphene film on the
substrate; and providing a chemically modified monolayer graphene
film on the monolayer graphene film.
6. The homoepitaxial tunnel barrier transport device with
functionalized graphene-on-graphene of claim 5, wherein the
chemically modified monolayer graphene film is a fluorinated
monolayer graphene film.
7. The homoepitaxial tunnel barrier transport device with
functionalized graphene-on-graphene of claim 5, wherein the
chemically modified monolayer graphene film is a hydrogenated
monolayer graphene film.
8. The homoepitaxial tunnel barrier transport device with
functionalized graphene-on-graphene of claim 6, wherein the
fluorinated monolayer graphene film is a tunnel barrier.
9. The homoepitaxial tunnel barrier transport device with
functionalized graphene-on-graphene of claim 5, wherein there is no
electrical connection between the monolayer graphene film and the
chemically modified monolayer graphene film.
10. The homoepitaxial tunnel barrier transport device with
functionalized graphene-on-graphene of claim 5, wherein the
tunneling spin efficiency is from about 26% to about 46%.
Description
[0001] This application claims priority to and the benefits of U.S.
Patent Application No. 61/980,448 filed on Apr. 16, 2014, and U.S.
patent application Ser. No. 14/629,939 filed on Feb. 24, 2015, the
entireties of each are herein incorporated by reference.
BACKGROUND
[0002] The quantum phenomenon of tunneling enables novel
charge-based devices with ultra-low power consumption, and is key
to the emerging field of spintronics.
[0003] Tunnel devices typically require mating dissimilar materials
and maintaining monolayer level control of thickness, raising
issues that severely complicate fabrication and compromise
performance. The recent discoveries of intrinsically 2-dimensional
materials such as graphene and h-BN have created new perspectives
on tunnel barriers. Their strong in-plane bonding promotes
self-healing of pinholes and a well-defined layer thickness,
important because the tunnel current depends exponentially upon the
barrier thickness.
[0004] There has been keen interest in utilizing graphene, a
two-dimensional (2D) honeycomb lattice of carbon atoms, as a high
mobility transport channel. Its linear band dispersion, ambipolar
conduction, and remarkable in-plane electronic transport properties
have stimulated development of RF transistors and wafer-scale
fabrication of graphene circuits. Graphene also exhibits
exceptional in-plane spin transport characteristics, including long
spin diffusion lengths due to its low spin-orbit interaction, which
has stimulated ideas for novel spin devices.
[0005] The highest values for spin diffusion lengths and spin
lifetimes have been measured using mechanically exfoliated
graphene, which, although it possesses extraordinary electrical
properties, is not amenable for device scalability, as devices must
be fabricated on individual, randomly placed and sized flakes.
Moreover, spin injection into graphene from a ferromagnetic metal
contact typically requires the use of an oxide tunnel barrier such
as Al.sub.2O.sub.3 or MgO to accommodate the large conductivity
mismatch. These materials do not wet the graphene surface, making
it very difficult to control the thickness and uniformity of the
tunnel barrier.
[0006] In addition, the mobility of graphene is significantly
degraded by coupling to phonons or charged impurities/defects in an
adjacent oxide. Consequently, significant effort has focused on
exploiting other carbon thin films and 2D materials such as h-BN or
MoS.sub.2 as a substrate, gate dielectric, or tunnel barrier for
graphene devices. This improves operating characteristics, but
significantly complicates the fabrication, and often relies upon
sequential mechanical exfoliation to produce a few device
structures.
[0007] Although single layer graphene itself has been shown to
function as a tunnel barrier in a heterostructure, it does not
effectively serve as a tunnel barrier on another layer of graphene
because there is electrical interaction between the two layers,
regardless of the stacking orientation, except in a large magnetic
field.
[0008] One can markedly alter graphene's physical properties with
chemical functionalization by fluorination or hydrogenation.
Fluorinated graphene is an excellent in-plane insulator, and no
electrical communication is observed between adjacent layers of
fluorographene and graphene, allowing for its use as a tunnel
barrier in an all-graphene tunnel-transport homoepitaxial
structure.
[0009] Only two other methods have been devised for making tunnel
barriers on graphene. First, a method of high-energy electron-beam
lithographic decomposition of vaporized carbon can produce
amorphous carbon layers on the surface of the graphene channel and
can act as a tunnel barrier. Although this method produces tunnel
barriers, the high-energy electron beam adds charged impurities to
the substrate, affecting the transport properties of the graphene
channel, and it can induce physical damage to the graphene by
driving off individual carbon atoms from the lattice. A second
alternative method involves the chemical vapor deposition growth of
thin layer hexagonal-BN, which is then transferred to the graphene
transfer. However, this process does not produce exceptional
results and is not homoepitaxial, requiring the growth and transfer
of two completely different materials with vastly different growth
mechanisms and properties. Thus, it is also unsuitable for
industrial scaling.
BRIEF SUMMARY OF THE INVENTION
[0010] This disclosure describes a process to fabricate a
completely new kind of tunnel barrier-based electronic device, in
which the tunnel barrier and transport channel are made of the same
material--graphene. A never-before-seen homoepitaxial tunnel
barrier/transport structure is created using a monolayer chemically
modified graphene sheet as a tunnel barrier on another monolayer
graphene sheet. The new type of device displays enhanced spintronic
properties over heteroepitaxial devices and is the first to use
graphene as both the tunnel barrier and channel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIGS. 1A, 1B, and 1C illustrate images and electrical
characterization before and after device fluorination. FIG. 1A
illustrates an optical image of a device with fluorination only
under the Py contacts. The scale bar is 20 .mu.m. FIG. 1B
illustrates an optical image of a device fluorinated everywhere.
The contrast of the upper layer of graphene changes, becoming
transparent, as it is fluorinated. The scale bar is 20 .mu.m. FIG.
1C illustrates an IV curve of a 2T graphene device before
fluorination. FIG. 1D illustrates an IV curve of the same device
after fluorination, showing that the graphene is now an
insulator.
[0012] FIGS. 2A, 2B, and 2C are demonstrations of tunneling
behavior. FIG. 2A illustrates Dirac curves taken using the
SiO.sub.2/Si substrate as a back gate. One line illustrates between
contacts 2 and 3 (only on the fluorographene). Another line
illustrates between contacts 1 and 4 (only on the graphene
channel). The fluorographene shows no Dirac point, indicating that
it is fully insulating and electrically uncoupled from the graphene
that is underneath it. FIG. 2B illustrates current-voltage curves
for a typical device. Taken between contacts 1 and 2 or 1 and 3
(includes the tunnel barrier), the curves are non-Ohmic. Taken
between contacts 1 and 4 (graphene channel only) the curve is
linear. The inset further highlights this by showing dV/dI vs. V
when the tunnel barrier is included in the circuit. FIG. 2C
illustrates zero bias resistance vs. temperature for the Py
contacts showing a weak temperature dependence (non-metallic
behavior) that is a hallmark of a good tunnel barrier.
[0013] FIGS. 3A, 3B, 3C, and 3D illustrate spin transport data.
FIG. 3A illustrates non-local spin valve (NLSV) measurement taken
at 10 K. The arrows below the curve indicate the direction of the
field sweep. The arrows above the curve indicate the ferromagnetic
contact magnetization directions. A small constant background of
.about.400 Ohms was subtracted from the data. FIG. 3B illustrates
4T Hanle (top and right axes) vs. 2T Hanle and witness sample
(bottom and left axes). All data taken at 10 K. The dotted lines
show fits to the appropriate models. Here, .tau..sub.s for 2T/4T
was 96 ps/205 ps. FIG. 3C illustrates bias dependence of 2T
(triangle) and 4T (square) Hanle signal amplitude. FIG. 3D
illustrates bias dependence of the NLSV plateau .DELTA.R.sub.NL and
the spin polarization efficiency, showing evidence of spin-filtered
tunneling.
[0014] FIG. 4 illustrates gate voltage dependence of the 4T spin
lifetime at 10 K. The bias current was -10 .mu.A.
DETAILED DESCRIPTION
[0015] This disclosure describes a process to fabricate a
completely new kind of tunnel barrier-based electronic device, in
which the tunnel barrier and transport channel are made of the same
material, graphene. A never-before-seen homoepitaxial tunnel
barrier/transport structure is created using a monolayer chemically
modified graphene sheet as a tunnel barrier on another monolayer
graphene sheet. The new type of device displays enhanced spintronic
properties over heteroepitaxial devices and is the first to use
graphene as both the tunnel barrier and channel.
[0016] The quantum phenomenon of tunneling enables novel
charge-based devices with ultra-low power consumption, and is key
to the emerging field of spintronics. Tunnel devices typically
require mating dissimilar materials and maintaining monolayer level
control of thickness, raising issues that severely complicate
fabrication and compromise performance. The recent discoveries of
intrinsically 2-dimensional materials such as graphene and h-BN
have created new perspectives on tunnel barriers. Their strong
in-plane bonding promotes self-healing of pinholes and a
well-defined layer thickness, important because the tunnel current
depends exponentially upon the barrier thickness.
[0017] There has been keen interest in utilizing graphene, a
two-dimensional (2D) honeycomb lattice of carbon atoms, as a high
mobility transport channel. Its linear band dispersion, ambipolar
conduction, and remarkable in-plane electronic transport properties
have stimulated development of RF transistors and wafer-scale
fabrication of graphene circuits. Graphene also exhibits
exceptional in-plane spin transport characteristics, including long
spin diffusion lengths due to its low spin-orbit interaction, which
has stimulated ideas for novel spin devices. The highest values for
spin diffusion lengths and spin lifetimes have been measured using
mechanically exfoliated graphene, which, although it possesses
extraordinary electrical properties, is not amenable for device
scalability, as devices must be fabricated on individual, randomly
placed and sized flakes. Moreover, spin injection into graphene
from a ferromagnetic metal contact typically requires the use of an
oxide tunnel barrier such as Al.sub.2O.sub.3 or MgO to accommodate
the large conductivity mismatch. These materials do not wet the
graphene surface, making it very difficult to control the thickness
and uniformity of the tunnel barrier. In addition, the mobility of
graphene is significantly degraded by coupling to phonons or
charged impurities/defects in an adjacent oxide. Consequently,
significant effort has focused on exploiting other carbon thin
films and 2D materials such as h-BN or MoS.sub.2 as a substrate,
gate dielectric, or tunnel barrier for graphene devices. This
improves operating characteristics, but significantly complicates
the fabrication, and often relies upon sequential mechanical
exfoliation to produce a few device structures.
[0018] Although single layer graphene itself has been shown to
function as a tunnel barrier in a heterostructure, it does not
effectively serve as a tunnel barrier on another layer of graphene
because there is electrical interaction between the two layers,
regardless of the stacking orientation, except in a large magnetic
field. One can markedly alter graphene's physical properties with
chemical functionalization by fluorination or hydrogenation.
Fluorinated graphene is an excellent in-plane insulator, and no
electrical communication is observed between adjacent layers of
fluorographene and graphene, suggesting its use as a tunnel barrier
in an all-graphene tunnel-transport homoepitaxial structure.
[0019] Here, described is a method of fabrication and demonstration
of the operation of the world's first homoepitaxial
graphene-on-graphene tunnel barrier/transport structure.
[0020] Demonstrated is the increased performance of our new
structure by fabricating spintronic devices where a monolayer of
chemically functionalized graphene acts as a tunnel barrier on a
monolayer of non-functionalized graphene, and demonstrate
electrical spin injection, lateral transport, and detection by
4-terminal non-local spin valve and Hanle effect measurements. We
find the highest spin efficiency values yet measured for graphene,
and present evidence for the theoretically predicted enhancement of
tunnel spin polarization.
Example 1
Formation of the Homoepitaxial Graphene Tunnel Barrier/Transport
Channel Device
[0021] Graphene was grown by chemical vapor deposition (CVD) via
decomposition of methane in small Cu foil enclosures. This method
produces monolayer graphene films with grain sizes on the order of
hundreds of microns containing minimal defects. After growth, the
graphene is removed from the Cu growth substrate by etching the
copper in acid solution, and then it is mechanically transferred
onto a SiO.sub.2/Si substrate.
[0022] Care is taken to eliminate the exposure of graphene to
standard optical photoresists, which can leave significant residues
on graphene. Instead, a PMMA-based process is used, which produces
fewer residues. The first layer of graphene is spin-coated with a
thin layer of PMMA followed by Shipley S1818 photoresist. Using
photolithography, a mesa pattern is defined in the photoresist and
02 plasma is used to etch through the PMMA and unwanted graphene.
The sample is rinsed in acetone and isopropyl alcohol (IPA) to
remove the etch mask. Ohmic reference contacts and bond pads are
then defined using a MMA/PMMA mask with features defined using
high-current (.about.7 nA) electron-beam lithography writing. Ti/Au
is deposited using electron beam deposition and lift-off in
acetone.
[0023] A second layer of graphene that will act as the tunnel
barrier is then deposited on top of these devices using the same
methods as above. A second mesa etch, similar to the first, is
performed. Electron-beam lithography using a MMA/PMMA resist is
then used to define trenches for deposition of ferromagnetic
contacts by electron beam deposition and lift-off. The graphene in
these trenches is fluorinated by placing the sample in XeF.sub.2
gas until the resistance of a concurrently fluorinated 2-terminal
graphene device reaches approximately 50 G.OMEGA., indicating that
the upper graphene layer is fully insulating. This is shown in FIG.
1. NiFe/Au is then deposited by electron beam evaporation and
lift-off is performed in acetone.
[0024] Just prior to placing the devices in a cryostat for
measurement, a final fluorination is performed to fluorinate the
remaining upper layer of graphene to prevent any edge state
conduction. Again, IV characteristics of a concurrently fluorinated
2-terminal graphene devices are measured and the observation of
high resistance ensures that the fluorination process has
succeeded. After this fluorination, the upper layer of graphene
(now fluorographene) changes optical contrast, becoming visibly
almost transparent. This contrast change is further evidence that
the fluorination was successful.
Example 2
Demonstration of Tunneling Behavior
[0025] FIG. 1B shows an image of the device structure. It consists
of two ferromagnetic permalloy/fluorinated graphene tunnel contacts
(contacts 2 and 3) placed between two Au/Ti contacts (1 and 4). The
Au/Ti contacts show Ohmic behavior, as expected. The Conductance
vs. Back Gate Voltage, measured between the two Ohmic Au/Ti
contacts (FIG. 2A), shows the Dirac point of the bottom graphene
channel at .about.80V, indicating a high electron concentration.
The transistor characteristics measured between the two permalloy
(Py, Ni.sub.80Fe.sub.20) contacts that only contact the top
fluorinated graphene film (FIG. 2A), shows no modulation or Dirac
point. This confirms that the graphene layers are indeed not
communicating electrically, as expected after fluorination. The
conductance of the device between these two Py contacts is orders
of magnitude less than the conductance of the fluorinated graphene
film, indicating that all of the electrical transport measured is
due to tunneling through the fluorinated graphene and into the
underlying graphene transport channel.
[0026] FIG. 2B shows IV curves taken between the Py and the Ohmic
Au/Ti contacts. These curves exhibit markedly non-Ohmic behavior,
further emphasized in the inset of FIG. 2B with a graph of the
differential conductance vs. voltage, and provide additional
support that the fluorinated graphene is acting as a tunnel
barrier. The temperature dependence of the zero bias resistance
(FIG. 2C) is weak and insulator-like in character, changing by a
factor less than 1.7 for both Py contacts. Non-Ohmic IV curves and
a weakly temperature dependent zero bias resistance has been shown
to be firm confirmation of tunneling behavior in the contacts.
Example 3
Operation of the Device as a Spin Valve
[0027] In non-local spin valve (NLSV) measurements, a bias current
is applied between one of the FM contacts and the nearest Ohmic
reference contact, and a spin-polarized charge current is injected
from the FM across the fluorinated graphene tunnel barrier and into
the graphene transport channel. Spin simultaneously diffuses in all
directions, creating a pure spin current on one side, and the
corresponding spin accumulation results in a spin-splitting of the
chemical potential. This is manifested as a voltage on the second
FM contact, which is outside of the charge current path and
referred to as the non-local detector. An in-plane magnetic field
is used to control the relative orientation of the magnetizations
of the FM injector and detector contacts. When the magnetizations
are parallel, the voltage measured will be smaller than when they
are antiparallel. Sweeping the magnetic field causes the contact
magnetizations to reverse in-plane at their respective coercive
fields and produce a measurable voltage peak.
[0028] In order to observe this effect, we fabricate the Py
contacts with two different widths (0.5 .mu.m and 3 .mu.m) to
exploit magnetic shape anisotropy so that the coercivities of the
ferromagnetic contacts are different. This NLSV behavior is clearly
observed in FIG. 3A, where distinct steps in the non-local
resistance (the measured voltage divided by the bias current)
appear at the coercive fields of the wide and narrow FM contacts,
producing plateaus of higher resistance when the FM contact
magnetizations are antiparallel. This demonstrates successful spin
injection and detection at the FM/fluorinated graphene tunnel
contacts, and lateral spin transport in the graphene channel.
[0029] The spin lifetime corresponding to this pure spin current is
quantitatively determined using the Hanle effect, in which a
magnetic field B.sub.z applied along the surface normal causes the
spins in the graphene transport channel to precess at the Larmor
frequency, .omega..sub.L=g.mu..sub.BB.sub.z/h, and dephase. Here g
is the Lande g-factor (g.about.2 for graphene), .mu..sub.B is the
Bohr magneton, and h is Planck's constant. As the magnetic field
increases, the net spin polarization and corresponding spin voltage
decreases to zero with a characteristic pseudo-Lorentzian line
shape. FIG. 3B shows Hanle spin precession curves for both
non-local and local contact geometries for a typical device used in
this study in comparison to a witness sample device where the top
graphene layer was not fluorinated. We note that no NLSV signal or
Hanle effect is apparent in the witness sample, demonstrating that
the fluorinated graphene tunnel barrier is necessary to achieve
spin injection.
[0030] We measure Hanle spin precession in two different electrical
configurations. The spin lifetime of the pure spin current is
measured in the NLSV or 4T configuration, where the
full-width-half-max of the measured change in voltage is directly
proportional to the steady-state spin polarization at the detector,
given by
S ( x 1 , x 2 , B z ) = S 0 .intg. 0 .infin. 1 4 Dt e - ( x 2 - x 1
- v d t ) 2 / 4 Dt cos ( .omega. L t ) e - t / .tau. s dt ( 1 )
##EQU00001##
where spin is injected into the graphene at x.sub.1 and t=0 and
detected at x.sub.2. So is the spin injection rate, D is the
electron diffusion constant, .nu..sub.d is the electron drift
velocity (=0 for diffusive transport), and .tau..sub.s is the spin
lifetime. Secondly, the spin current can be injected and the spin
voltage detected with same Py contact in a 2-terminal (2T)
configuration. Here, we measure the spin accumulation and lifetime
directly under the Py contact, and the voltage
.DELTA.V.sub.2T(B.sub.z) decreases with B.sub.z with a Lorentzian
line shape given by
.DELTA.V.sub.2T(B.sub.z)=.DELTA.V.sub.2T(0)/[1+(.omega..sub.Lt.sub.s).sup-
.2]. In this way, fits to the Hanle curves allow us to extract the
spin lifetime (for the 2T and 4T case) and the spin diffusion
constant (for the 4T case).
[0031] In FIG. 3B we see a strong Hanle signal from the 4T
non-local measurement and the 2T measurement. The Hanle signal
persists up to .about.200 K. Average 4T spin lifetimes were
.about.200 ps and average 2T spin lifetimes were .about.100 ps. The
spin diffusion length is given by L.sub.SD=(Dt.sub.s).sup.1/2 where
D is the diffusion constant. We find an average L.sub.SD.about.1.5
.mu.m, based on t.sub.s-200 ps and D.about.0.01224 m.sup.2/s. The
observation of both the non-local Hanle effect and the NLSV
provides strong evidence that the fluorinated graphene tunnel
barrier indeed enables efficient spin injection, transport, and
detection in the graphene channel. Based on the magnitude of the
NLSV signal (FIG. 3A) and the calculated spin diffusion length from
the 4T Hanle measurements, we can determine the tunneling spin
polarization, P, of the Py/fluorinated graphene contact using the
formula:
.DELTA. R NL = P 2 L SD W.sigma. exp ( - L / L SD ) ( 2 )
##EQU00002##
where .sigma. is the measured conductivity of
1.29.times.10.sup.-4.OMEGA..sup.-1 for the device shown in FIG. 3A,
L is the center to center contact spacing of 5.75 .mu.m, L.sub.SD
is the spin diffusion length of 1.5 .mu.m, W is the width of the
graphene channel of 5 .mu.m, and .DELTA.R.sub.NL.about.3.3.OMEGA.
is the magnitude of the NLSV plateau for a bias current of -10
.mu.A. From this, we find P.about.26%. FIG. 3D summarizes the bias
dependence .DELTA.R.sub.NL and P. Both increase monotonically with
decreasing bias, typical of graphene NLSV devices. We measure
values of P up to .about.45% at low bias, which is at the upper
limit of what can be expected for an intrinsic spin polarization of
Ni.sub.80Fe.sub.20, which is 32%-48%. This value is also larger
than the highest values measured to date (P=26-30%) in graphene
NLSV devices with alumina or MgO tunnel barriers. This indicates
that spin-filtering occurs at the Py/fluorinated graphene
interface, consistent with theoretical predictions for spin
transport across Ni/graphene lattice-matched interfaces, and
provides further evidence for the efficacy of the fluorinated
graphene tunnel barrier.
[0032] As a final demonstration of the effectiveness of our
homoepitaxial structure, we show gate modulation of the spin
lifetime in FIG. 4. While most early graphene spin experiments
showed spin lifetimes that are constant in gate voltage, other work
shows that spin lifetimes are affected by changes in carrier
density. Studies that observe gate voltage dependence have in
common high contact resistance in the oxide tunnel barrier
contacts, indicative of pinhole-free tunnel barriers that prevent
back diffusion into the FM contact and subsequent fast spin
relaxation. The discrepancy between these measurements is thus
likely related to differences in the quality of the tunnel barrier
contacts. In all of these cases, it would be difficult to measure
the intrinsic properties of the graphene itself. The single-atom
thick fluorinated graphene tunnel barrier offers an elegant
solution. Our experiments show a clear gate voltage dependent spin
signal that follows the Dirac curve, just as predicted by
theory.
[0033] Our structure demonstrates the first homoepitaxial tunnel
barrier/transport system in which the tunnel barrier and transport
channel are comprised of the same material, graphene. In previous
art, the tunnel barrier and transport channel are very different
materials, and such devices require mating dissimilar materials,
raising issues of heteroepitaxy, layer uniformity, interface
stability and electronic defect states that severely complicate
fabrication and compromise performance.
[0034] Our new approach obviates these issues. Our approach does
not rely upon a second material "wetting" the graphene surface to
obtain a uniform and complete tunnel barrier. Graphene, by
definition, is uniform in thickness down to a single atom, has very
few defects, does not easily form vacancies, and does not intermix
readily with other materials--these are key characteristics for a
tunnel barrier, in which the tunnel current depends exponentially
on the barrier thickness.
[0035] Our approach provides a simple and effective way to form a
tunnel barrier on graphene. The functionalized graphene tunnel
barrier does not affect the adjacent transport channel because it
is comprised of the same material, contrary to evaporated
dielectric or oxidized metal tunnel barriers, which can
structurally damage the graphene or add impurity dopants. This is
readily indicated by our high spin polarization values, spin
relaxation lengths on par with the highest quality graphene
devices, and our ability to control the spin relaxation time with
the application of an electrostatic back gate.
[0036] Our complete tunnel barrier/transport channel structure also
provides for the thinnest of this type of structure ever made,
allowing it to be used in applications where space is a premium.
Furthermore, due to the thinness of the tunnel barrier, and the
advantage that it allows for true electron tunneling, our structure
has lower impedance and less loss than other previously made
designs, allowing its use in ultra low-power electronics
architectures.
[0037] A majority of previous tunnel barrier devices using graphene
as the conductive transport channel rely on deposited oxides or
post deposition oxidized metals, usually both consisting of
Al.sub.2O.sub.3 or MgO. The deposition is performed with three
types of methods: 1) evaporative methods with either thermal or
electron beam evaporation to deposit an oxide. The evaporated oxide
or metal tends to ball up on the surface, causing cracks and
pinholes that limit tunneling. 2) Sputter evaporation of oxide or
metal. It has been shown that the graphene transport channel can be
irreversibly damaged. 3) Atomic layer deposition of oxides.
Successful deposition usually requires a chemical pretreatment of
the graphene film, which adds dopants that affect the transport
properties. Moreover, oxide tunnel barriers are known to be very
difficult to form on graphene since they exhibit de-wetting in the
absence of prior chemical treatment of the graphene, and attempts
to mitigate this to create a good surface for oxide growth may
induce scatterers and defects.
[0038] Many modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that the claimed invention may be practiced otherwise
than as specifically described. Any reference to claim elements in
the singular, e.g., using the articles "a," "an," "the," or "said"
is not construed as limiting the element to the singular.
* * * * *