Direct Synthesis of Layer-Tunable and Transfer-Free Graphene on
Device-Compatible Substrates Using Ion Implantation
Keywords: Ion
implantation, layer-tunable and transfer-free graphene, dual-metal smart
Janus substrate, growth mechanism, device applications
Direct synthesis of layer-tunable and transfer-free graphene on
technologically important substrates is highly valued for various
electronics and device applications. State of the art in the field is
currently a two-step process: a high-quality graphene layer synthesis on
metal substrate through chemical vapor deposition (CVD) followed by
delicate layer-transfer onto device-relevant substrates. Here, we report
a novel synthesis approach combining ion implantation for a precise
graphene layer control and dual-metal smart Janus substrate for a
diffusion-limiting graphene formation, to directly synthesize large
area, high quality, and layer-tunable graphene films on arbitrary
substrates without the post-synthesis layer transfer process. Carbon (C)
ion implantation was performed on Cu-Ni film deposited on a variety of
device-relevant substrates. A well-controlled number of layers of
graphene, primarily monolayer and bilayer, is precisely controlled by
the equivalent fluence of the implanted C-atoms (1 monolayer
~ 4×1015C-atoms/cm2). Upon thermal annealing to promote Cu-Ni
alloying, the pre-implanted C-atoms in the Ni layer are pushed towards
the Ni/substrate interface by the top Cu layer due to the poor
C-solubility in Cu. As a result, the expelled C-atoms precipitate into
graphene structure at the interface facilitated by the Cu-like alloy
catalysis. After removing the alloyed Cu-like surface layer, the
layer-tunable graphene on the desired substrate is directly realized.
The layer-selectivity, high quality, and uniformity of the graphene
films are not only confirmed with detailed characterizations using a
suite of surface analysis techniques, but more importantly are
successfully demonstrated by the excellent properties and performance of
several devices directly fabricated from these graphene films. Molecular
dynamics (MD) simulations using the
reactive force-field
(ReaxFF) were performed to
elucidate the graphene formation mechanisms in this novel synthesis
approach. With the wide use of ion implantation technology in the
microelectronics industry, this novel graphene synthesis approach with
precise layer-tunability and transfer-free processing has a promise to
advance efficient graphene-device manufacturing and expedite their
versatile applications in many fields.
1. Introduction
Graphene has attracted widespread attention in several areas due to its
distinct two-dimensional (2-D) hexagonal lattice structure and
extraordinary physical properties.[1-5] To achieve
the potential of graphene in integrated circuits for important device
applications, large-area uniform graphene with a layer-tunable character
must be readily and reliably synthesized first since many
physical/chemical features of graphene are associated with its
thickness.[6-8] However, accurate control of the
layer number of graphene is still a significant challenge. Among
multiple synthesis routes, chemical vapor deposition (CVD) on metallic
substrates (both C-soluble and C-insoluble) has become the leading
choice for the large-scale production of large-area
graphene.[9-11] Due to the non-equilibrium
precipitation process, however, it is very challenging in theory to
adjust the thickness of graphene during the CVD when utilizing C-soluble
metallic substrates such as Ni, Co, Pd, etc.[12]For C-insoluble metallic substrates such as Cu, Ag, Pt
etc,[13-14] the ability to control the thickness
of graphene using the CVD is also yet satisfactory, as the self-limiting
surface mechanism leads to an inability to produce graphene beyond the
monolayer thickness. To grow more than single-layer or able to control
the number of layers in graphene synthesis, researchers have combined
the virtues of C-soluble and C-insoluble materials such as Ni-Cu binary
alloys[15-16] and bilayered substrate called smart
Janus substrate such as Ni-Cu[17-19] during the
CVD. However, the ability to control the layer number of graphene films
is still unsatisfactory since both carbon absorption and precipitation
processes are thermally driven and occur simultaneously during the CVD.
Ion implantation has been routinely used in the microelectronics
industry as well as materials R&D labs due to its advantages in precise
control of dopant species, location, and concentration, in large and
uniform area processing ability, as a low-temperature process compared
with diffusion, and in overcoming solubility limits of desired chemical
species from thermodynamically based synthesis approaches such as CVD.
Naturally, C-ion implantation was explored early on to synthesize
layer-tunable graphene on metallic substrates both soluble and insoluble
to C. For C-soluble substrates such as Ni[20-21]it was found that the thickness of graphene on the substrate is
nonuniform and the correlation between the C-implanted fluence and layer
number of graphene is not strictly followed, though the average graphene
thickness agrees roughly with the implanted C-ion fluence (e.g.
4×1015 ions/cm2 ~ 1
monolayer). For C-insoluble substrates such as
Cu[22] it was found that the thickness of graphene
does not vary according to the implanted C-ion fluence and instead the
bilayer graphene was always formed with reasonable uniformity
independent of substantial variation in the C-ion fluence. These initial
results by ion implantation in single elemental metal substrates were
largely disappointing but not totally unexpected when considering the
stochastic nature of ion-solid interactions during ion implantation,
i.e. the number of C-ions implanted in specific location is governed by
Poissonian statistics (i.e. not deterministic). Furthermore, the complex
C absorption and precipitation processes during the post-implant
annealing pose additional uncertainty in precise layer control in
graphene formation by the ion implantation approach.
To decouple thermally driven C absorption and C precipitation on the
Ni-Cu smart Janus substrate during the CVD, our team first proposed and
applied ion implantation processing onto the Ni-Cu smart Janus substrate
and successfully demonstrated layer-tunable graphene synthesis on
metallic substrates using ion implantation and thermal injection
approach.[5,23] The C-ions were implanted in the
Ni sublayer of the Ni-Cu bilayer substrate. During the post-implantation
thermal annealing, the implanted C atoms in the Ni sublayer were
expelled towards the surface by the underneath Cu out-diffusion when
forming a Cu-like alloy. Besides the advantage of the layer number
strictly controlled by the implantation fluence, the C precipitation in
this approach is achieved under steady temperature, which benefits the
defect healing of graphene and leads to the formation of graphene layers
with excellent crystallinity and uniformity.
Despite these important advances in synthesizing layer-tunable graphite
by the CVD and ion implantation through smart Janus substrate, the
required transfer of graphene layer off these metal or alloy substrates
onto technologically more relevant substrates such as Si and SiO2 still
poses a challenge for final device
applications.[24-25] During the transfer
procedure, loss of substrate material and the introduction of defects,
wrinkles, cracks, and contaminations are unavoidable, resulting in a
significant decline in the performance of graphene-based nanoscale
electronic devices.[26-27,29] To avoid these
issues during layer transfer, research efforts on promoting
transfer-free approaches for the direct synthesis of layer-tunable
graphene on device-bound substrates become increasingly important and
urgent.
In this work, we use a reverse-order Janus substrate (Cu-coated
Ni on an arbitrary substrate) instead of the Ni-coated Cu in our
previous ion implantation work[23] to achieve both
layer-tunability and transfer-free characteristics. During the
post-implantation thermal annealing, the top Cu layer behaves like a “C
diffusion barrier” to gradually inward diffuse into the bottom Ni layer
(i.e., Cu atoms “top-down diffusion” and Ni “bottom-up diffusion”).
The poor solubility of C in Cu (<0.001
at.%)[30] facilitates the implanted C ions
(initially located in the Ni layer) to be ejected towards the Cu/Ni
interfacial front and finally converted into graphene on the substrate
promoted through the catalysis impact of the Cu-Ni
alloy.[31-33] Removing the Cu-Ni alloy layer
leaves behind the as-synthesized graphene on the substrate that was
initially used to deposit the Ni film. We have fabricated three ordinary
devices using as-synthesized graphene films on Si, SiO2,
and glass substrates to demonstrate the graphene film quality of our
layer-tunable and transfer-free synthesis approach and the excellent
performance characteristics of these low-cost manufacturing devices:
field-effect transistors, heating devices, and near-infrared
photodetectors. Considering that ion implantation is already widely used
in the microelectronics industry and entirely compatible with current
CMOS technology, we believe that growing layer-controllable and
transfer-free graphene on an arbitrary substrate using this novel
approach can expedite and expand graphene-based device applications.
2. Results and Discussions