Selective Growth Of Graphene Research Articles

Study of selective graphene growth on non-catalytic hetero-substrates

To incorporate graphene layers into a complementary metal oxide semiconductor (CMOS), various growth techniques are needed, such as direct and selective growth at a low temperature. The selective growth on a heterogeneous substrate is the key technology for the realistic synthesis of various structures, including graphene. In this study, we addressed the origin of selective growth of graphene on a semiconductor over the dielectric materials. Based on ab initio simulations, we found that the adsorption energy of CH4 on SiO2 is much higher than that on Si and Ge. The growth differences of graphene on Ge and SiO2 were also experimentally confirmed. While a continuous graphene layer was formed on the Ge surface at the initial growth stage that became thicker at increasing growth times, an incubation time was required for the nucleation of graphene on SiO2. The analysis of the interface between graphene and SiO2 indicated that the initially incorporated carbons could not contribute to the graphene growth because they reduced SiO2 by forming CO as a by-product. Finally, 3.0 nm-thick graphene layers were selectively grown on a SiO2-patterned Ge/Si(0 0 1) substrate. The selective growth of graphene on patterned semiconductor substrates will help broaden CMOS applications.

Selective growth of uniform single-layer graphene on Cu foil and fabrication of damage-free field effect transistor combining with direct transfer

During selective growth of graphene by using silicon dioxide (SiO2) patterns on Cu foil (SOCF), multilayer graphene was grown on SOCF under the same conditions that are used to synthesize single-layer graphene (SLG) on blank Cu foil. The authors demonstrated that oxygen (O2) species that can be released from the SiO2 film did not affect the layer increase and that the SiO2 film of SOCF reduced the area of the exposed Cu surface and thereby increased the relative concentration of hydrogen (H2) to the Cu surface and initially grown graphene; as a result, extra graphene layers grew on SOCF. By adjusting the H2 supply and SiO2 coverage, uniformly-grown SLG patterns were obtained on SOCF. A damage-free graphene field effect transistor (GFET) was fabricated using selectively-grown SLG and direct transfer using parylene-C. The field effect mobility of the GFET was 7538.81 cm2/(V s), which is quite high compared to those of chemical vapor deposition based GFETs on flexible substrates that have been reported.

Selective LPCVD growth of graphene on patterned copper and its growth mechanism

Copper-catalyzed graphene low-pressure chemical-vapor deposition (LPCVD) growth has been regarded as a viable solution towards its integration to CMOS technology, and the wafer-bonding method provides a reliable alternative for transferring the selective graphene grown on a patterned metal film for IC manufacturing. In this paper, selective LPCVD graphene growth using patterned copper dots has been studied. The Raman spectra of grown films have demonstrated large dependence on the growth conditions. To explain the results, the growth mechanisms based on surface adsorption and copper-vapor-assisted growth are investigated by the comparison between the blanket copper films with/without the additional copper source. The copper vapor density is found to be critical for high-quality graphene growth. In addition, the copper-vapor-assisted growth is also evidenced by the carbon deposition on the SiO2 substrate of the patterned-copper-dot sample and chamber wall during graphene growth. This growth mechanism explains the correlation between the growth condition and Raman spectrum for films on copper dots. The study on the copper-catalyzed selective graphene growth on the hard substrate paves the way for the synthesis and integration of the 2D material in VLSI.

Selective growth of graphene in layer-by-layer via chemical vapor deposition.

Selective and precise control of the layer number of graphene remains a critical issue for the practical applications of graphene. First, it is highly challenging to grow a continuous and uniform few-layer graphene since once the monolayer graphene fully covers a copper (Cu) surface, the growth of the second layer stops, resulting in mostly nonhomogeneous films. Second, from the selective adlayer growth point of view, there is no clear pathway for achieving this. We have developed the selective growth of a graphene adlayer in layer-by-layer via chemical vapor deposition (CVD) which makes it possible to stack graphene on a specific position. The key idea is to deposit a thin Cu layer (∼40 nm thick) on pre-grown monolayer graphene and to apply additional growth. The thin Cu atop the graphene/Cu substrate acts as a catalyst to decompose methane (CH4) gas during the additional growth. The adlayer is grown selectively on the pre-grown graphene, and the thin Cu is removed through evaporation during CVD, eventually forming large-area and uniform double layer graphene. With this technology, highly uniform graphene films with precise thicknesses of 1 to 5 layers and graphene check patterns with 1 to 3 layers were successfully demonstrated. This method provides precise LBL growth for a uniform graphene film and a technique for the design of new graphene devices.

Catalytic decomposition of SiO2 by Fe and the effect of Cu on the behavior of released Si species

Si supply mechanism in the spontaneous formation of SiO2 pattern on a Cu foil, which was previously reported to enable the selective growth of graphene in no direct contact with organic materials, was investigated. Upon annealing at 1000 °C and atmospheric pressure, SiO2 in the Fe-coated SiO2/Al2O3 template covered by a Cu foil was found to be severely decomposed by Fe droplets. No signature of reaction between SiO2 and Fe droplets suggested that Si species, which contributed to the spontaneous formation of SiO2 pattern on a Cu foil, originated from SiO2 catalytically decomposed by Fe droplets. In the absence of a Cu foil covering an Fe-coated SiO2/Al2O3 template, the catalytic activity of Fe droplets was significantly suppressed because of the encapsulation of Fe droplets by a 2-nm-thin-Fe3O4 layer. Furthermore, a part of the Fe film on SiO2/Al2O3 without a Cu foil turned into nanodroplets and assisted the formation of SiO2 nanowires. These results reveal that a Cu foil made an intriguing role in preventing Fe from being encapsulated by Fe3O4, so the catalytic role of Fe droplets was not terminated until SiO2 was fully decomposed. Our experimental results provided insights into better understanding of these intriguing behaviors of Fe and Cu on SiO2 at high temperature and at atmospheric pressure.

Graphene Growth on Pre-patterned Copper Film with Nickel as a Buffer Layer

Selective graphene growth has been simultaneously achieved on oxidized silicon substrate with three kinds of pre-patterned rectangular metal films, i.e., Cu/Ni double layer, and Ni and Cu single layer film, by atmospheric chemical vapor deposition at 1020°C. The top graphene maintains the micron-scale patterning of the metal film underneath. It was found that single layer graphene growth is more favorable on the Cu/Ni double layer film than on either single layer. The morphology and structure study of the pre-patterned metal substrates before and after graphene growth indicated that Ni functions as a buffer layer to significantly weaken the lattice mismatch between the copper and silicon substrate, resulting in a smoother and larger grain-sized Cu surface. It is also suggested that Ni diffuses to the Cu surface and participates in the graphene growth during the chemical vapor deposition (CVD) process. Defect-free single layer graphene growth can be obtained when the ratio of Cu/Ni is appropriate with respect to their thickness and the feature size of rectangular patterning.

Lattice Selective Growth of Graphene on Sapphire Substrate

In this work, we report a systematic study of CVD synthesizing graphene on different crystal faces of sapphire (c-Plane, m-Plane, and r-Plane). Nickel films are deposited on sapphire substrates with c-Plane, m-Plane, and r-Plane to catalyzing the growth of graphene by CVD (Chemical Vapor Deposition) method. It is only on c-Plane sapphire substrates that graphene can be found after growth and etching off nickel. In the case of m-Plane and r-Plane sapphire substrates, graphene forms only on the top surface of nickel films, but none at the interface between nickel and sapphire. Oxygen plasma treatment is introduced to certify that graphene on c-Plane sapphire does not originate from the top surface of nickel film. Selective formation of graphene is attributed to lattice structures of sapphire’s different faces. Moreover, influences of nickel’s thickness and growth time are studied by a series of control experiments.

Selective area growth of Bernal bilayer epitaxial graphene on 4H-SiC (0001) substrate by electron-beam irradiation

We report selective area growth of large area homogeneous Bernal stacked bilayer epitaxial graphene (BLEG) on 4H-SiC (0001) substrate by electron-beam irradiation. Sublimation of Si occurs by energetic electron irradiations on SiC surface via breaking of Si–C bonds in the localized region, which allows the selective growth of graphene. Raman measurements ensure the formation of homogeneous BLEG with weak compressive strain of −0.08%. The carrier mobility of large area BLEG is ∼5100 cm2 V−1 s−1 with a sheet carrier density of 2.2 × 1013 cm−2. Current-voltage measurements reveal that BLEG on 4H-SiC forms a Schottky junction with an operation at mA level. Our study reveals that the barrier height at the Schottky junction is low (∼0.58 eV) due to the Fermi-level pinning above the Dirac point.

Spontaneous pattern transfer and selective growth of graphene on a Cu foil

When a 25-μm-thick Cu foil placed on top of a selectively FeCl3-coated quartz substrate was heated inside a furnace at 1000°C in a H2/Ar atmosphere, a FeCl3 mask pattern on a quartz was spontaneously transferred to the front side of the Cu foil. Due to this spontaneous pattern transfer, a subsequent heating process in H2/Ar/CH4 atmosphere led to selective growth of graphene on the front side of the Cu foil. Surface analysis revealed that a spontaneously transferred mask on a Cu foil was made of SiO2, and Si in SiO2 was found to be from the quartz substrate. Computational simulation of surface diffusion of Si on a Cu foil was consistent with experimentally observed microstructures of spontaneously transferred pattern boundaries, which suggests that oxidation of diffused Si atoms, which originated from the FeCl3-coated region of a quartz, on the Cu surface was the crucial mechanism of the spontaneous pattern transfer.

Graphene synthesis by C implantation into Cu foils

Cu foils of 2×2cm2 have been implanted with 70keV C− ions to nominal fluences of (2–10)×1015cm−2 at room temperature (RT) and subsequently annealed at 900–1100°C for 15min, before being cooled to RT to form graphene layers on the Cu surfaces. Analyses with Raman spectroscopy and atomic force microscopy demonstrate that a continuous film of bi-layer graphene (BG) is produced for implant fluences as low as 2×1015cm−2, much less than the carbon content of the BG films. This suggests that the implanted carbon facilitates the nucleation and growth of graphene, with additional carbon supplied by the Cu substrate (0.515ppm carbon content). No graphene was observed on unimplanted Cu foils subjected to the same thermal treatment. This implantation method provides a novel technique for the selective growth of graphene on Cu surfaces.

Graphene nanoribbons on vicinal SiC surfaces by molecular beam epitaxy

We present a method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consists of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1$\overline{1}$00], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high-density GNRs, enable us to observe the electronic structure at $K$ points by angle-resolved photoemission spectroscopy, showing a clear band-gap opening of at least 0.14 eV.

Selective Graphene Formation on Copper Twin Crystals

Selective graphene growth on copper twin crystals by chemical vapor deposition has been achieved. Graphene ribbons can be formed only on narrow twin crystal regions with a (001) or high-index surface sandwiched between Cu crystals having (111) surfaces by tuning the growth conditions, especially by controlling the partial pressure of CH(4) in Ar/H(2) carrier gas. At a relatively low CH(4) pressure, graphene nucleation at steps on Cu (111) surfaces is suppressed, and graphene is preferentially nucleated and formed on twin crystal regions. Graphene ribbons as narrow as ~100 nm have been obtained in experiments. The preferential graphene nucleation and formation seem to be caused primarily by a difference in surface-dependent adsorption energies of reactants, which has been estimated by first principles calculations. Concentrations of reactants on a Cu surface have also been analyzed by solving a diffusion equation that qualitatively explains our experimental observations of the preferential graphene nucleation. Our findings may lead to self-organizing formation of graphene nanoribbons without reliance on top-down approaches in the future.

Drawing graphene nanoribbons on SiC by ion implantation

We describe a straightforward technique for selective graphene growth and nanoribbon production onto 4H- and 6H-SiC. The technique presented is as easy as ion implanting regions where graphene layers are desired followed by annealing to 100 °C below the graphitization temperature (TG) of SiC. We find that ion implantation of SiC lowers the TG, allowing selective graphene growth at temperatures below the TG of pristine SiC and above TG of implanted SiC. This results in an approach for patterning device structures ranging from a couple tens of nanometers to microns in size without using conventional lithography and chemical processing.

Synthesis of graphene ribbons using selective chemical vapor deposition

A new method for implementing graphene ribbons using selective graphene growth on metal-sidewall by chemical vapor deposition has been proposed. In this method, Ni catalyst is pre-patterned before chemical vapor deposition, and graphene film is selectively grown on the sidewall of the nickel for graphene ribbons. The graphene ribbons were confirmed by TEM image and Raman spectroscopy, and the fabricated graphene ribbon transistors showed well gate-modulated output characteristics. We believe this sidewall-graphene could be useful for applications such as graphene sensors which require high surface area of graphene.

Synthesis of monolithic graphene–graphite integrated electronics

Encoding electronic functionality into nanoscale elements during chemical synthesis has been extensively explored over the past decade as the key to developing integrated nanosystems with functions defined by synthesis. Graphene has been recently explored as a two-dimensional nanoscale material, and has demonstrated simple device functions based on conventional top-down fabrication. However, the synthetic approach to encoding electronic functionality and thus enabling an entire integrated graphene electronics in a chemical synthesis had not previously been demonstrated. Here we report an unconventional approach for the synthesis of monolithically integrated electronic devices based on graphene and graphite. Spatial patterning of heterogeneous metal catalysts permits the selective growth of graphene and graphite, with a controlled number of graphene layers. Graphene transistor arrays with graphitic electrodes and interconnects were formed from the synthesis. These functional, all-carbon structures were transferable onto a variety of substrates. The integrated transistor arrays were used to demonstrate real-time, multiplexed chemical sensing and more significantly, multiple carbon layers of the graphene-graphite device components were vertically assembled to form a three-dimensional flexible structure which served as a top-gate transistor array. These results represent substantial progress towards encoding electronic functionality through chemical synthesis and suggest the future promise of one-step integration of graphene-graphite based electronics.

Selective graphene growth from DLC thin film patterned by focused-ion-beam chemical vapor deposition

The authors were able to grow a few-layer graphene film on a SiO2 substrate directly and selectively by using focused-ion-beam chemical vapor deposition (FIB-CVD) and a Ga evaporation process. FIB-CVD can control the thickness of the diamondlike carbon (DLC) film and potentially control the number of graphene layers, and Ga evaporation causes the direct transformation of patterned DLC into graphene. DLC was graphitized by annealing with a Ga film at 1000 °C and 1100 °C. Increasing the thickness of the Ga film resulted in a graphitized pattern that had a strong Raman 2D band. The formation of graphene above and around a dimple on the SiO2 layer indicates that Ga dissolved the DLC and graphene precipitated not between the liquid Ga and the SiO2 substrate but, rather, on the small droplets of Ga. This behavior is different than that reported in previous studies of graphitization at the interface between a carbon layer and liquid Ga.