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