U.S. patent application number 14/486149 was filed with the patent office on 2016-03-17 for oriented bottom-up growth of armchair graphene nanoribbons on germanium.
The applicant listed for this patent is Wisconsin Alumni Research Foundation. Invention is credited to Michael Scott Arnold, Robert Michael Jacobberger.
Application Number | 20160079357 14/486149 |
Document ID | / |
Family ID | 55450239 |
Filed Date | 2016-03-17 |
United States Patent
Application |
20160079357 |
Kind Code |
A1 |
Arnold; Michael Scott ; et
al. |
March 17, 2016 |
ORIENTED BOTTOM-UP GROWTH OF ARMCHAIR GRAPHENE NANORIBBONS ON
GERMANIUM
Abstract
Graphene nanoribbon arrays, methods of growing graphene
nanoribbon arrays and electronic and photonic devices incorporating
the graphene nanoribbon arrays are provided. The graphene
nanoribbons in the arrays are formed using a scalable, bottom-up,
chemical vapor deposition (CVD) technique in which the (001) facet
of the germanium is used to orient the graphene nanoribbon crystals
along the [110] directions of the germanium.
Inventors: |
Arnold; Michael Scott;
(Middleton, WI) ; Jacobberger; Robert Michael;
(Madison, WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wisconsin Alumni Research Foundation |
Madison |
WI |
US |
|
|
Family ID: |
55450239 |
Appl. No.: |
14/486149 |
Filed: |
September 15, 2014 |
Current U.S.
Class: |
257/29 ;
438/492 |
Current CPC
Class: |
H01L 29/0673 20130101;
H01L 29/045 20130101; H01L 21/02603 20130101; H01L 21/0262
20130101; H01L 21/02658 20130101; H01L 29/1606 20130101; H01L
21/02433 20130101; H01L 21/02381 20130101; H01L 21/02527 20130101;
H01L 21/02609 20130101 |
International
Class: |
H01L 29/06 20060101
H01L029/06; H01L 29/16 20060101 H01L029/16; H01L 21/02 20060101
H01L021/02; H01L 29/04 20060101 H01L029/04 |
Goverment Interests
REFERENCE TO GOVERNMENT RIGHTS
[0001] This invention was made with government support under
DE-SC0006414 awarded by the US Department of Energy. The government
has certain rights in the invention.
Claims
1. A graphene nanoribbon array comprising: a germanium substrate
having a (001) facet; and a plurality of graphene nanoribbons on
the (001) facet of the germanium substrate; wherein the graphene
nanoribbons have aspect ratios of at least 5 and the average aspect
ratio of the graphene nanoribbons in the plurality of graphene
nanoribbons is at least 20; and further wherein the graphene
nanoribbons have the armchair crystallographic direction of
graphene running all the way along their long axes and an armchair
configuration along their edges, and further wherein the long axes
of the graphene nanoribbons are oriented along a [110] direction of
the germanium.
2. The array of claim 1, wherein the plurality of graphene
nanoribbons comprises graphene nanoribbons having edges with an rms
roughness of 1 nm or lower over edge lengths of at least 40 nm.
3. The array of claim 1, wherein the plurality of graphene
nanoribbons comprises graphene nanoribbons having edges with an rms
roughness of 0.5 nm or lower over edge lengths of at least 40
nm.
4. The array of claim 1, wherein the plurality of graphene
nanoribbons comprises graphene nanoribbons having edges with an rms
roughness of 0 nm over edge lengths of at least 10 nm.
5. (canceled)
6. The array of claim 1, wherein the plurality of graphene
nanoribbons comprises a first portion of graphene nanoribbons
having their long axis oriented along a first [110] direction of
the germanium and a second portion of graphene nanoribbons having
their long axis oriented along a second [110] direction of the
germanium.
7. The array of claim 1, wherein the (001) facet of the germanium
comprises a series of (001) faceted terraces separated by steps
that are a multiple of two atomic layers in height along a single
[110] direction, and the graphene nanoribbons are oriented along
the terraces with the armchair crystallographic direction of the
graphene and the long nanoribbon axes oriented along a single [110]
direction of the germanium.
8. The array of claim 7, wherein the plurality of graphene
nanoribbons include graphene nanoribbons having widths of 5 nm or
less.
9. The array of claim 1, wherein the plurality of graphene
nanoribbons include graphene nanoribbons having widths of no
greater than about 10 nm and average lengths of at least 200
nm.
10. A method of growing graphene nanoribbons, the method comprising
growing graphene on the (001) facet of a germanium substrate via
chemical vapor deposition from a mixture of methane gas and
hydrogen gas, wherein the partial pressures of the methane and
hydrogen are selected to result in the growth of a plurality of
graphene nanoribbons having their long axis oriented along the
[110] directions of the germanium substrate and having the armchair
crystallographic direction of graphene running parallel to the long
nanoribbon edges, wherein the graphene nanoribbons are grown with
an average aspect ratio of at least 12.
11. The method of claim 10, wherein the plurality of graphene
nanoribbons includes graphene nanoribbons grown with widths of less
than 5 nm.
12. The method of claim 10, wherein the plurality of graphene
nanoribbons includes graphene nanoribbons grown with widths of no
greater than about 10 nm and average lengths of at least 200
nm.
13. The method of claim 10, wherein the (001) facet of the
germanium is miscut toward the 110 direction, such that a series of
(001) faceted terraces separated by steps that are a multiple of
two atomic layers in height are formed along a single [110]
direction, and the graphene nanoribbons are grown along the
terraces with the armchair crystallographic direction of graphene
and the long nanoribbon axis oriented along a single [110]
direction of the germanium.
14. The method of claim 10, further comprising incorporating the
graphene nanoribbons and the germanium substrate into an electronic
or photonic device.
15. The method of claim 10, further comprising releasing at least a
portion of the graphene nanoribbons from the germanium substrate
and transferring the released graphene nanoribbons to a second
substrate.
16. The method of claim 10, wherein seed particles are disposed on,
or embedded in, the surface of the germanium substrate, and the
seed particles catalyze the growth of, and control the location of,
the graphene nanoribbons on the germanium substrate.
Description
BACKGROUND
[0002] Graphene is a two-dimensional carbon allotrope, the
electronic, optical, and magnetic properties of which can be tuned
by engineering two-dimensional graphene sheets into one-dimensional
structures with confined width, known as graphene nanoribbons. The
properties of graphene nanoribbons are highly dependent on their
width and edge structure.
[0003] It has been proposed that graphene nanoribbons will
outperform conventional materials and lead to next-generation
technologies. Graphene nanoribbons have already shown tremendous
promise for providing enhanced performance in nanoelectronics,
spintronics, optoelectronics, plasmonic waveguiding,
photodetection, solar energy conversion, molecular sensing, and
catalysis. However, the full potential of graphene nanoribbons in
such applications has not been realized.
[0004] A major challenge facing graphene nanoribbon-based devices
is that scalable approaches to create high-quality graphene
nanoribbons with atomically-smooth edges are lacking. Conventional,
top-down techniques in which graphene nanoribbons are etched from
continuous graphene sheets result in structures with rough,
disordered edges that are riddled with defects, which significantly
degrade graphene's exceptional properties. This blunt top-down
etching can be avoided by synthesizing nanoribbons from the
bottom-up. For instance, organic synthesis can yield ribbons with
smooth edges, defined widths, and complex architectures. However,
organic synthesis forms short nanoribbons (typically .about.20 nm
in length) and is not adapted to technologically relevant
substrates, such as insulators or semiconductors, limiting its
potential for commercial development.
SUMMARY
[0005] Graphene nanoribbon arrays, methods of growing graphene
nanoribbon arrays and electronic and photonic devices incorporating
the graphene nanoribbon arrays are provided.
[0006] One embodiment of a graphene nanoribbon array comprises: a
germanium substrate having a (001) facet; and a plurality of
graphene nanoribbons on the (001) facet of the germanium substrate.
The graphene nanoribbons have aspect ratios of at least 5 and the
average aspect ratio of the graphene nanoribbons in the plurality
of nanoribbons is at least 10 and, in some embodiments, at least
20. The graphene nanoribbons comprise two edges that run
substantially parallel along the length of the nanoribbon and they
have the armchair crystallographic direction of graphene running
parallel to the nanoribbon edges. That is--they have carbon-carbon
bonds running parallel to the long nanoribbon axis. The graphene
nanoribbons are characterized in that the long ribbon axis is
oriented along the [110] directions of the germanium surface.
[0007] One embodiment of a method of growing graphene nanoribbons
comprises the steps of: growing graphene on the (001) facet of a
germanium substrate via chemical vapor deposition from a mixture of
methane gas and hydrogen gas, wherein the partial pressures of
methane and hydrogen are selected to result in the growth of a
plurality of graphene nanoribbons having their long axis oriented
along the [110] directions of the germanium substrate and the
armchair crystallographic direction of graphene running parallel to
the long nanoribbon axis, wherein the graphene nanoribbons are
grown with an average aspect ratio of at least 10 and, in some
embodiments, at least 20.
[0008] Some embodiments of the graphene nanoribbon arrays include
graphene nanoribbons having edges with an rms roughness of 1 nm or
lower over edge lengths of at least 40 nm.
[0009] Once grown on the germanium substrate, the graphene
nanoribbons can be released from that substrate and transferred to
an arbitrary second substrate to provide a graphene nanoribbon
array.
[0010] Other principal features and advantages of the invention
will become apparent to those skilled in the art upon review of the
following drawings, the detailed description, and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Illustrative embodiments of the invention will hereafter be
described with reference to the accompanying drawings, wherein like
numerals denote like elements.
[0012] FIG. 1 is a scanning electron microscopy image of an array
of graphene nanoribbons on a Ge (001) surface. The orientation of
the long nanoribbon axis is controlled along two perpendicular
[110] directions of germanium.
[0013] FIG. 2. Table 1. Growth conditions used to grow graphene
nanoribbons, including the growth temperature, (T), growth time
(t), Ar flux (Ar), H.sub.2 flux (H.sub.2), CH.sub.4 flux
(CH.sub.4), number of nanoribbons analyzed (n), average nanoribbon
width (w) and standard deviation (.sigma..sub.w), average
nanoribbon length (l) and standard deviation (m), average
nanoribbon aspect ratio and standard deviation (.sigma..sub.ar),
and growth rate of nanoribbon width (R.sub.w) and length
(R.sub.l).
[0014] FIG. 3. Table 2. Other growth conditions that yielded
graphene nanoribbons, including the growth temperature, (T), growth
time (t), Ar flux (Ar), H.sub.2 flux (H.sub.2), and CH.sub.4 flux
(CH.sub.4).
[0015] FIG. 4 is a plot of graphene nanoribbon width vs. growth
time with constant CH.sub.4 mole fraction (0.0092), H.sub.2 mole
fraction (0.33), total pressure (760 Torr), and growth temperature
(910.degree. C.). Error bars indicate standard deviation.
[0016] FIG. 5 is a plot of graphene nanoribbon length vs. growth
time with constant CH.sub.4 mole fraction (0.0092), H.sub.2 mole
fraction (0.33), total pressure (760 Torr), and growth temperature
(910.degree. C.). Error bars indicate standard deviation.
[0017] FIG. 6. Growth rate of nanoribbon length (R.sub.l) plotted
against methane mole fraction (xCH.sub.4). The growth temperature
is 910.degree. C., Ar flux is 200 sccm, H.sub.2 flux is 100 sccm,
and total pressure is 760 Torr. Error bars indicate standard
deviation.
[0018] FIG. 7. Growth rate of nanoribbon length (R.sub.l) plotted
against hydrogen mole fraction (xH.sub.2). The growth temperature
is 910.degree. C., CH.sub.4 flux is 2.0 sccm, and total pressure is
760 Torr. The Ar flux is adjusted so that the total Ar and H.sub.2
flux is 300 sccm. Error bars indicate standard deviation.
[0019] FIG. 8. Nanoribbon aspect ratio plotted against width for
different xCH.sub.4 in the range from about 0.00662 to about
0.0164. Growth conditions: H.sub.2 mole fraction is 0.33, total
pressure is 760 Torr, temperature is 910.degree. C.
[0020] FIG. 9. Nanoribbon aspect ratio plotted against width for
different xH.sub.2 in the range from about 0.331 to about 0.215.
Growth conditions: CH.sub.4 mole fraction is 0.0066, total pressure
is 760 Torr, temperature is 910.degree. C.
[0021] FIG. 10. Nanoribbon length (R.sub.l) plotted against
temperature (T). The Ar flux is 200 sccm, H.sub.2 flux is 100 sccm,
CH.sub.4 flux is 2.8 sccm, and total pressure is 760 Torr. Error
bars indicate standard deviation.
[0022] FIG. 11. Scanning tunneling microscope (STM) image of a
nanoribbon having atomically smooth edge segments. The nanoribbon
was grown at 910.degree. C. with Ar, H.sub.2, and CH.sub.4 fluxes
of 200, 100, and 2.8 sccm, respectively, for 1.5 hours at 760 Torr.
The image is 20 nm by 40 nm.
[0023] FIG. 12. Schematic illustration of a graphene nanoribbon
with armchair edges perfectly aligned along the Ge[110] direction
(upper nanoribbon) and a graphene nanoribbon with armchair edges
aligned within .+-.about 3.degree. (2.9.degree.) of the Ge[110]
direction.
DETAILED DESCRIPTION
[0024] Graphene nanoribbon arrays, methods of growing graphene
nanoribbon arrays and electronic and photonic devices incorporating
the graphene nanoribbon arrays are provided. The graphene
nanoribbons in the arrays are narrow, elongated strips (or
"ribbons") of monolayer graphene having widths and crystallographic
edge structures that provide the ribbons with electronic
properties, such as electronic bandgaps, that are absent in
continuous two-dimensional films of graphene. The graphene
nanoribbons in the arrays are formed using a scalable, bottom-up,
chemical vapor deposition (CVD) technique to grow oriented graphene
nanoribbons with atomically-smooth edges on the (001) facet of
germanium substrates. CVD is an inexpensive scalable technique,
offering high throughput and compatibility with planar processing.
The graphene nanoribbons are grown directly on a semiconductor
platform, germanium, which can be purchased as a single-crystal
wafer or can be epitaxially grown on silicon wafers and gallium
arsenide wafers. These materials are available in large formats and
are prevalent in the semiconductor industry. During the CVD growth
process, the (001) facet of the germanium is used to orient the
long edges of the graphene nanoribbons as well as the armchair
direction of the graphene lattice within the ribbons along the
[110] directions of the germanium. In addition, the graphene growth
rate is highly anisotropic, resulting in faceted graphene
nanoribbons with high aspect ratios. The nanoribbons are grown with
exclusively or almost exclusively armchair configurations along
their edges. Although there may be a minute portion of zigzag
segments near defects or kinks at the nanoribbon edges.
[0025] The armchair crystallographic direction of graphene, as well
as the long nanoribbon edges, may not run perfectly parallel to the
Ge[110] directions. Therefore, for the purposes of this disclosure
the graphene nanoribbons are considered to be "oriented along a
[110] direction of the germanium" if their armchair direction and
long nanoribbon axis are aligned within .+-.4.degree. of the [110]
direction of the germanium. This is illustrated schematically in
FIG. 11. In that figure, the lighter grey circles represent the top
two layers of germanium in the substrate, while the darker grey
circles, which are only partially visible under the top two layers,
represent germanium atoms in lower layers of the substrate. The
arrow represents the Ge[110] direction. The upper graphene
nanoribbon is perfectly aligned along the Ge[110] direction. In the
lower graphene nanoribbon, the armchair crystallographic direction
of graphene and the long axis of the ribbons deviate by about
3.degree. from perfect alignment.
[0026] Because the graphene nanoribbons are grown, rather than
patterned lithographically from graphene sheets, they are formed
with atomically smooth edges. The degree of edge smoothness can be
characterized by the average root mean square (rms) roughness of
the edges of the nanoribbons in the array. The rms edge roughness
along the length of a nanoribbon can be measured using scanning
tunneling microscopy (STM), as illustrated in the Example. The
average rms edge roughness for the nanoribbons in a nanoribbon
array will vary since longer nanoribbons within the array will tend
to have rougher edges. For the purposes of this disclosure, a
nanoribbon is considered to have an atomically smooth edge if its
edge has an rms roughness of less than 1 nm over a length of at
least 40 nm. Some embodiments of the present nanoribbon growth
methods form nanoribbons having an rms roughness of less than 0.5
nm over an edge length of at least 40 nm. This includes embodiments
of the growth methods that form nanoribbons that have perfectly
atomically smooth edges, that is--having an rms roughness of 0 nm,
over an edge length of at least 10 nm. An STM image of one such
nanoribbon is shown in FIG. 11.
[0027] Short nanoribbons having atomically smooth edges, including
those with perfectly atomically smooth edges, are well-suited for
use as channel materials in FETs having channel lengths of 40 nm or
less, including those having channel lengths as low as 10 nm.
Therefore, short nanoribbons having atomically smooth edges can be
identified within the nanoribbon arrays and selectively
incorporated into an FET. Alternatively, short nanoribbon segments
having atomically smooth edges can be cut from longer nanoribbons
and selectively incorporated into an FET.
[0028] Some aspects of the invention can be attributed, at least in
part, to the inventors' discovery that the germanium facet and
conditions for depositing graphene via CVD from a hydrocarbon
precursor can be selected to provide graphene nanoribbon growth. As
a result, the present methods are distinguishable from methods in
which CVD is used to grow graphene sheets on germanium via island
growth. In order to distinguish the present graphene nanoribbons
from other graphene structures that have been grown via CVD,
graphene nanoribbons are defined, for the purposes of this
disclosure, as graphene structures having aspect ratio of at least
5 and having at least two edges that run substantially parallel
with each other. In addition, the present graphene nanoribbons, as
grown on the germanium substrate, are characterized by the armchair
crystallographic direction of graphene running substantially
parallel to the long nanoribbon axis, which is oriented along the
[110] directions of the germanium surface. The phrase
`substantially parallel` is used in recognition of the fact that
the edges may be parallel on a global scale, but might include edge
portions that deviate slightly from perfectly parallel on an atomic
scale due to edge roughness.
[0029] The graphene nanoribbons are grown on the (001) facet of a
germanium substrate using hydrocarbon precursor molecules, such as
methane. As illustrated in the Example below, the graphene
nanoribbons can be deposited from a mixture of methane gas and
hydrogen gas. By varying the composition of the precursor gas
mixture during growth, the duration of the growth time, and the
growth temperature, the graphene nanoribbon width, length, and
aspect ratio can be controlled. This control over the nanoribbon
structure makes it possible to tune the graphene properties. For
example, graphene undergoes a metallic-to-semiconducting transition
as the nanoribbon width decreases, wherein the induced bandgap is
inversely proportional to the nanoribbon width. Therefore, the
present approach makes it possible to control the width of the
nanoribbons and, therefore, to tailor their electronic structure.
By tuning the precursor composition and growth time, nanoribbons
with widths below the current lithography resolution can be
achieved. For example, nanoribbons with widths below 10 nm and
lengths greater than 200 nm can be grown.
[0030] Key parameters for realizing anisotropic growth are the mole
fractions of the precursor molecules and the carrier molecules used
in the CVD gas mixture, where the mole fractions can be adjusted by
adjusting the partial pressures of the precursor and carrier gases.
However, these parameters are not independent, so the optimal value
for one of the parameters will depend on the others. By way of
illustration only, anisotropic growth of nanoribbons from a mixture
of H.sub.2 and CH.sub.4 can be achieved at certain combinations of
temperatures in the range from about 860 to 935.degree. C., H.sub.2
mole fractions in the range from about 5.0.times.10.sup.-3 to 0.33
and CH.sub.4 mole fractions in the range from about
3.0.times.10.sup.-5 to 2.0.times.10.sup.-2. Guidance for selecting
an appropriate combination of temperatures and mole fractions
(partial pressures) is provided in the Example, below. In general,
nanoribbon growth is favored by a high H.sub.2 mole fraction and
low CH.sub.4 mole fraction, which corresponds to a slow growth
rate. For example, in some embodiments of the growth methods, the
growth conditions are selected to provide growth rates of no
greater than 500 nm/hr. This includes embodiments in which the
growth conditions are selected to provide growth rates of no
greater than 300 nm/hr, further includes embodiments in which the
growth conditions are selected to provide growth rates of no
greater than 100 nm/hr and still further includes embodiments in
which the growth conditions are selected to provide growth rates of
no greater than 35 nm/hr. These growth rates refer to the growth
rate of the fastest growing edge dimension of the graphene crystal
structures.
[0031] The growth time also plays a role in determining the
dimensions of the CVD-grown graphene nanoribbons. Generally, as
growth time is decreased, narrower, shorter nanoribbons are formed.
Therefore, by tuning the duration of the growth time and the ratio
of precursor gas to carrier gas in the gas mixture, nanoribbons
with desired lengths and widths can be selectively grown using
bottom-up CVD growth.
[0032] The optimal conditions for achieving anisotropic graphene
growth may vary somewhat depending upon the laboratory conditions.
For example, in a cleaner environment, the growth rate at a given
set of conditions would be expected to be slower than in a dirtier
environment. Therefore, to achieve the same low growth rate
observed under standard laboratory conditions in a cleaner system,
such as a clean room, a higher CH.sub.4 mole fraction and/or a
lower H.sub.2 mole fraction could be used.
[0033] By way of illustration, in some embodiments of the graphene
nanoribbon arrays, the average aspect ratio of the nanoribbons in
the array is at least 10. This includes graphene nanoribbon arrays
in which the average aspect ratio of the nanoribbons in the array
is at least 20, further includes graphene nanoribbon arrays in
which the average aspect ratio of the nanoribbons in the array is
at least 25 and still further includes graphene nanoribbon arrays
in which the average aspect ratio of the nanoribbons in the array
is at least 30. Included in some embodiments of such arrays are
graphene nanoribbons having aspect ratios of at least 30, at least
40, at least 50, at least 60 and/or at least 70.
[0034] The average width of the graphene nanoribbons in some of the
graphene nanoribbon arrays is no greater than 60 nm. This includes
graphene nanoribbon arrays in which the average width of the
graphene nanoribbons is no greater than 40 nm and further includes
graphene nanoribbon arrays in which the average width of the
graphene nanoribbons is no greater than 20 nm. Included in some
embodiments of such arrays are graphene nanoribbons having widths
of no greater than 10 nm and/or widths of no greater than 5 nm.
[0035] The average length of the graphene nanoribbons in some of
the graphene nanoribbon arrays is at least 100 nm. This includes
graphene nanoribbon arrays in which the average length of the
graphene nanoribbons is at least 200 nm and further includes
graphene nanoribbon arrays in which the average length of the
graphene nanoribbons is at least 500 nm.
[0036] Optionally, nucleation sites can be introduced into or onto
the germanium growth surface in order to control the locations at
which graphene nanoribbon growth originates and the time at which
graphene nanoribbon growth begins. These nucleation sites can take
the form of, for example, roughed areas on the surface, grooves in
the surface or metal and/or metal oxide seed particles on or
embedded in the surface to catalyze nanoribbon growth.
[0037] While the CVD-based graphene nanoribbon growth methods can
be used to grow graphene nanoribbons having their graphene crystal
lattice oriented along the two equivalent [110] directions of the
Ge(001) surface, as shown in FIG. 1, they can also be used to grow
graphene nanoribbons with their graphene lattice selectively
oriented along only one [110] direction of the germanium. Selective
growth along a single [110] direction can be achieved by miscutting
the growth surface of the germanium toward the 110 direction, such
that a series of parallel (001) faceted terraces, separated by
steps that are a multiple of two atomic layers in height, are
formed and the graphene nanoribbons are grown with a parallel
alignment along the terraces. For example, the steps can be two,
four, six, eight, ten, etc., atomic layers in height. The terraces
can be formed using a miscut angle of, for example, about 9.degree.
or higher in order to provide steps that are two atomic layers
high.
[0038] The present methods can produce graphene nanoribbon arrays
having a high density of very narrow, smooth-edged, nanoribbons
over a large area. Arrays of these atomically-smooth nanoribbons
can include thousands of nanoribbons and can be formed over areas
of, for example, at least 1 .mu.m.sup.2. This includes high density
nanoribbon arrays that extend over areas of at least 100
.mu.m.sup.2, at least 1 mm.sup.2, and at least 10 mm.sup.2, the
area of the nanoribbon arrays being limited only by the area of the
surface of the substrate.
[0039] The graphene nanoribbon arrays can be incorporated into a
variety of devices, including electronic and photonic devices, such
as field effect transistors and photodetectors. One embodiment of a
field effect transistor comprising a graphene nanoribbon array
comprises: a source electrode; a drain electrode; a gate electrode;
a conducting channel in electrical contact with the source
electrode and the drain electrode; a gate dielectric disposed over
the conducting channel, but below the gate electrode; and a
germanium substrate having a (001) facet. In the transistor, the
conducting channel comprises a plurality of graphene nanoribbons on
the (001) facet of the germanium substrate, wherein the long
nanoribbon axis is oriented along the [110] directions of the
germanium surface and the nanoribbons have the armchair
crystallographic direction of graphene running parallel to the long
edges of the nanoribbons.
[0040] Alternatively, once the graphene nanoribbon arrays are
formed on the germanium substrates, they can be transferred to
other substrates, such as silicon substrates, polymer (e.g.,
plastic) substrates, dielectric substrates or metal substrates.
Methods for transferring graphene from a germanium substrate to
another substrate are described in Wang et al., Scientific Reports
3, Article number: 2465. The transfer procedure described in the
example that follows illustrates a modified version of the methods
described in Wang et al. The transferred graphene nanoribbons and
the substrate onto which they are transferred can then be
incorporated into a variety of devices.
EXAMPLE
[0041] This example illustrates methods of growing graphene
nanoribbons on the (001) facet of a germanium substrate. By varying
the CVD synthesis conditions, the widths, lengths, and aspect
ratios of the grown nanoribbons were controlled.
[0042] Methods: Before growth, the Ge(001) substrates (Wafer World,
resistivity>40 .OMEGA.-cm, miscut<1.degree.) were cleaned via
sonication in acetone and isopropyl alcohol for 15 minutes followed
by etching in water (18 M.OMEGA.) at 90.degree. C. for 15 minutes.
The Ge(001) substrate were loaded into a horizontal tube furnace
with a quartz tube diameter of 1.25 inches and the system was
evacuated to .about.10.sup.-6 Torr to remove contamination. The
system was then filled to atmospheric pressure with a mixture of Ar
(99.999%) and H.sub.2 (99.999%). The Ge(001) samples were annealed
prior to introducing CH.sub.4 (99.99%) at the desired growth
temperature. All growth occurred at atmospheric pressure. In order
to terminate growth, samples were rapidly cooled in the same
atmosphere used during synthesis by sliding the furnace away from
the growth region.
[0043] In order to transfer the graphene from the Ge(001) to an
arbitrary substrate, PMMA was first spin-coated onto the
graphene/Ge(001). Then, the PMMA/graphene/Ge(001) stack was floated
on a solution of water, hydrofluoric acid, and hydrogen peroxide to
etch the Ge(001). The PMMA/graphene stack was then transferred to
water to clean the graphene. Next, the PMMA/graphene stack was
scooped onto the target substrate. The PMMA was dissolved using
acetone and further removed by annealing in vacuum at 300.degree.
C., leaving the graphene ribbons on an arbitrary substrate.
[0044] After growth, the graphene/Ge(001) samples were imaged with
scanning electron microscopy (Zeiss LEO 1530) to characterize the
graphene morphology. For each nanoribbon synthesis, the width and
length of approximately 250 ribbons was measured from SEM images
using ImageJ.
[0045] Discussion: First, graphene was synthesized via CVD on
Ge(001) at atmospheric pressure using a variety of growth
temperatures (860 to 930.degree. C.), H.sub.2 mole fractions
(5.0.times.10.sup.-3 to 0.33), and CH.sub.4 mole fractions
(3.0.times.10.sup.-5 to 2.0.times.10.sup.-2). Growth was terminated
before full graphene coverage was obtained and the graphene
crystals were characterized. The growth was highly anisotropic,
resulting in narrow graphene nanoribbons with high aspect ratios.
The ribbons preferentially oriented along the [110] directions of
the Ge(001) template, resulting in two ribbon orientations that
were perpendicularly aligned. These two orientations exist with
equal probability. FIG. 1 is a scanning electron microscopy image
of the graphene nanoribbons on the Ge (001) surface. The
orientation of the nanoribbons is controlled along two
perpendicular [110] directions. The graphene nanoribbons shown in
FIG. 1 were grown using a growth temperature of 910.degree. C., a
growth time of two hours and H.sub.2 and CH.sub.4 mole fractions of
0.33 and 0.0092, respectively.
[0046] Graphene nanoribbons nucleated over a wide range of
experimental conditions, which are summarized in Table 1, which is
provided in FIG. 2. However, the nanoribbon dimensions and the
percentage of graphene that nucleated as nanoribbons can vary
greatly depending on the growth parameters. It was found that in
order to obtain high aspect ratio nanoribbons, it was important to
operate in a regime in which the growth rate is especially slow,
for example, on the order of 5 nm/h for growth in the direction of
the nanoribbon width. The one-dimensional nature of growth was
insensitive to the bulk Ge doping concentration
(0<N.sub.Sb<1.5.times.10.sup.18 cm.sup.-3), the Ge surface
termination (OH, H, and Cl) prior to synthesis, and the annealing
time before growth. Graphene nanoribbons were not observed on
Ge(101) or Ge(111) under any growth condition, indicating that
nanoribbon nucleation is unique to the Ge(001) surface.
[0047] FIGS. 4 and 5 are graphs of the growth evolution of the
graphene nanoribbons over time between 1 and 13.75 hours, with a
constant growth temperature (910.degree. C.), Ar flux (200 sccm),
H.sub.2 flux (100 sccm), and CH.sub.4 flux (2.8 sccm). The
nanoribbon width (FIG. 4) and length (FIG. 5) both decrease as the
growth time was reduced. For example, after growth for 13.75 h, the
average nanoribbon width and length were 305.+-.109 nm and
2747.+-.710 nm, respectively, and after growth for 1 h, the average
nanoribbon width and length decreased to 9.8.+-.4.0 nm and
182.+-.42 nm, respectively. Importantly, by using short growth
time, a high yield of ribbons with sub-10 nm width, which is below
the resolution of optical or electron-beam lithography, can be
achieved. The average nanoribbon width can be narrowed by
decreasing the growth time even further; however, characterization
of the size distribution of these nanoribbons was not performed
because the nanoribbon widths were below the lateral resolution of
scanning electron microscopy and atomic force microscopy.
[0048] Next, the effect of CVD conditions on the growth kinetics
and evolution of graphene nanoribbons on Ge(001) was studied.
First, the effect of the precursor composition on the graphene
growth rate was explored. To determine the effect of the CH.sub.4
mole fraction (x.sub.CH4), the CH.sub.4 flux was varied between 2.0
and 5.0 sccm at a constant growth temperature (910.degree. C.), Ar
flux (200 sccm), and H.sub.2 flux (100 sccm). Consequently, the Ar
and H.sub.2 mole fractions remained nearly constant at 0.66 and
0.33, respectively, while x.sub.CH4 was varied between 0.0066 and
0.016. Similarly, to study the effect of the H.sub.2 mole fraction
(x.sub.H2), the H.sub.2 flux was varied between 65 and 100 sccm at
a constant growth temperature (910.degree. C.), and CH.sub.4 flux
(2.0 sccm). The Ar flux was adjusted so that the total Ar and
H.sub.2 flux was 300 sccm. Consequently, x.sub.CH4 remained
constant at 0.0066, while x.sub.H2 was varied between 0.22 and
0.33. The results of the growth rate studies are summarized in
Table 1. Table 2 in FIG. 3 shows other growth conditions that
resulted in nanoribbon formation. By dividing the length and width
of the ribbon by the growth time and by a factor of two (to account
for growth in both directions), the growth rate of the ribbon
length, R.sub.l, and width, R.sub.w, respectively, were calculated.
Consistent with the definition of nanoribbon presented above, the
data in Tables 1 and 2 take into account only graphene structures
having aspect ratios of at least 5, having at least two edges that
run substantially parallel with each other along the [110]
direction of the germanium growth substrate. In each experiment,
graphene nanoribbons made up the majority of the graphene crystal
structures. The non-nanoribbon graphene crystal structures were
island structures, typically having an aspect ratio of about 1 to
about 2 and were not characterized by parallel edges running along
the [110] direction of the germanium.
[0049] FIGS. 6 and 7 show the dependence of R.sub.l on x.sub.CH4
and x.sub.H2. R.sub.l was used to compare the growth rate under
different CVD conditions because it stayed relatively constant with
time. Increasing (decreasing) x.sub.CH4 (x.sub.H2) increased the
growth rate. The R.sub.l could be controlled over an order of
magnitude by changing x.sub.CH4 and x.sub.H2. For example, using
low x.sub.CH4 of 0.0066 and high x.sub.H2 of 0.33, R.sub.l of
39.8.+-.8.1 nm/h was measured and an R.sub.w of 1.42.+-.0.57 nm/h
was achieved. In contrast, increasing x.sub.CH4 to 0.016 or
decreasing x.sub.H2 to 0.22 resulted in an R.sub.l of 285.+-.120
and 205.+-.84 nm/h, respectively. These slow growth rates makes it
possible to finely tune the nanoribbon width and length, and
therefore, the graphene properties, with high precision.
[0050] From these studies with varying precursor compositions, it
was also found that the nanoribbon aspect ratios decreased with
increasing x.sub.CH4 and decreasing x.sub.H2, corresponding to a
fast growth rate, for a given nanoribbon width (FIGS. 8 and 9).
Importantly, some nanoribbons with sub-10 nm width had aspect
ratio>50, which makes the nanoribbons suitable for device
applications in which the graphene nanoribbons need to be
electrically contacted.
[0051] Next, the effect of the growth temperature on R.sub.l was
studied by varying the temperature between 895 and 930.degree. C.,
while maintaining constant Ar flux (200 sccm), H.sub.2 flux (100
sccm), and CH.sub.4 flux (2.8 sccm). FIG. 10 is a graph showing the
dependence of R.sub.l on the growth temperature.
[0052] The word "illustrative" is used herein to mean serving as an
example, instance, or illustration. Any aspect or design described
herein as "illustrative" is not necessarily to be construed as
preferred or advantageous over other aspects or designs. Further,
for the purposes of this disclosure and unless otherwise specified,
"a" or "an" means "one or more".
[0053] The foregoing description of illustrative embodiments of the
invention has been presented for purposes of illustration and of
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and modifications and
variations are possible in light of the above teachings or may be
acquired from practice of the invention. The embodiments were
chosen and described in order to explain the principles of the
invention and as practical applications of the invention to enable
one skilled in the art to utilize the invention in various
embodiments and with various modifications as suited to the
particular use contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto and their
equivalents.
* * * * *