The simultaneous growth of monocrystalline and polycrystalline silicon films with controlled parameters

The influence of the temperature of deposition of nucleation layers on the structure of polycrystalline silicon films which grow locally during the epitaxial growth of monocrystalline films was investigated.The structure of polycrystalline films grown in the high temperature chloride process has been found to depend on the deposition mode of the nucleation layer. For temperature changes in the range 800–860°C, the grain sizes in polycrystalline silicon increase by more than one order of magnitude.It was shown that heat treatment of films of small grain size in an atmosphere of oxygen leads to a decrease in their resistance owing to recrystallization of the grains, whereas during the annealing of films of large grain size the oxidation of intergrain boundaries is the dominant process resulting in an increase in the resistance.The possibility of using polycrystalline films fabricated under specific technological conditions for the isolation of integrated circuit elements was demonstrated.     

Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition

The strong interest in graphene has motivated the scalable production of high-quality graphene and graphene devices. As the large-scale graphene films synthesized so far are typically polycrystalline, it is important to characterize and control grain boundaries, generally believed to degrade graphene quality. Here we study single-crystal graphene grains synthesized by ambient chemical vapour deposition on polycrystalline Cu, and show how individual boundaries between coalescing grains affect graphene's electronic properties. The graphene grains show no definite epitaxial relationship with the Cu substrate, and can cross Cu grain boundaries. The edges of these grains are found to be predominantly parallel to zigzag directions. We show that grain boundaries give a significant Raman 'D' peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. Finally, we demonstrate an approach using pre-patterned growth seeds to control graphene nucleation, opening a route towards scalable fabrication of single-crystal graphene devices without grain boundaries.     

Control of thickness uniformity and grain size in graphene films for transparent conductive electrodes

Large-scale and transferable graphene films grown on metal substrates by chemical vapor deposition (CVD) still hold great promise for future nanotechnology. To realize the promise, one of the key issues is to further improve the quality of graphene, e.g., uniform thickness, large grain size, and low defects. Here we grow graphene films on Cu foils by CVD at ambient pressure, and study the graphene nucleation and growth processes under different concentrations of carbon precursor. On the basis of the results, we develop a two-step ambient pressure CVD process to synthesize continuous single-layer graphene films with large grain size (up to hundreds of square micrometers). Scanning electron microscopy and Raman spectroscopy characterizations confirm the film thickness and uniformity. The transferred graphene films on cover glass slips show high electrical conductivity and high optical transmittance that make them suitable as transparent conductive electrodes. The growth mechanism of CVD graphene on Cu is also discussed, and a growth model has been proposed. Our results provide important guidance toward the synthesis of high quality uniform graphene films, and could offer a great driving force for graphene based applications.     

Synthesis of hexagonal graphene on polycrystalline Cu foil from solid camphor by atmospheric pressure chemical vapor deposition

Understanding of graphene nucleation and growth on a metal substrate in chemical vapor deposition (CVD) process is critical to obtain high-quality single crystal graphene. Here, we report synthesis of individual hexagonal graphene and their large cluster on Cu foil using solid camphor as a carbon precursor in the atmospheric pressure CVD (AP-CVD) process. Optical and scanning electron microscopy studies show formation of hexagonal graphene crystals across the grain, grain boundaries and twin boundaries of polycrystalline Cu foil. Electron backscattered diffraction analysis is carried out before and after the growth to identify Cu grain orientation correlating with the graphene formation. The influence of growth conditions and Cu grain structure is explored on individual hexagonal graphene formation in the camphor-based AP-CVD process.     

Hard magnetic two-phase Co-Cu thin films

The magnetic properties of sputtered films of 25 Co-75 Cu and 50 Co-50 Cu before and after annealing were investigated. In the as-sputtered state the films exhibit the structure of a metastable fcc solid solution. Annealing at 500 to 700°C causes decomposition into two phases, Cu and fcc 89 Co-11 Cu. The decomposition supposedly occurs by heterogeneous nucleation at the grain boundaries, and growth by grain boundary diffusion. The 89 Co-11 Cu phase exists in the form of small particles with magnetic single domain behavior. The films have coercivities up to 280 Oe. Squareness ratios between 0.7 and 0.9 were found. No strain sensitivity of the magnetic properties could be detected. This material is regarded to be suitable for magnetic recording.     

Investigation of interfacial reactions in thin film couples of aluminum and copper by measurement of low temperature contact resistance

Interfacial reactions in bimetallic thin film couples of aluminum and copper were investigated by measurement of contact resistance between 4.2 and 300 K. Results for as-deposited couples indicate that electrical properties, such as resistance and temperature coefficient of resistance, correspond closely to those computed from a simple model involving current flow parallel to the interface. These properties change appreciably after step annealing at elevated temperatures. Results in this case indicate that interfacial reactions commence by nucleation and growth of Al₂Cu along grain boundaries at 145°C, whereas significant solid solution alloying precedes nucleation and growth of intermetallic compounds at 250°C. Homogenization of these couples proceeds, sequentially, by (1) rapid diffusion of aluminum along grain boundaries in copper, (2) rapid diffusion of copper along grain boundaries in aluminum and (3) slower diffusion of each species into grain interiors of the other until a uniform composition, controlled by relative thickness of the constituent films, is achieved. This interpretation of the evolution of contact resistance is entirely consistent with complementary microstructural characterization by optical and transmission electron microscopy and chemical characterization by Auger electron spectroscopy.     

Effects of polycrystalline cu substrate on graphene growth by chemical vapor deposition

Chemical vapor deposition of graphene on Cu often employs polycrystalline Cu substrates with diverse facets, grain boundaries (GBs), annealing twins, and rough sites. Using scanning electron microscopy (SEM), electron-backscatter diffraction (EBSD), and Raman spectroscopy on graphene and Cu, we find that Cu substrate crystallography affects graphene growth more than facet roughness. We determine that (111) containing facets produce pristine monolayer graphene with higher growth rate than (100) containing facets, especially Cu(100). The number of graphene defects and nucleation sites appears Cu facet invariant at growth temperatures above 900 °C. Engineering Cu to have (111) surfaces will cause monolayer, uniform graphene growth.     

Epitaxial Growth of 6 in. Single‐Crystalline Graphene on a Cu/Ni (111) Film at 750 °C via Chemical Vapor Deposition

The future electronic application of graphene highly relies on the production of large‐area high‐quality single‐crystal graphene. However, the growth of single‐crystal graphene on different substrates via either single nucleation or seamless stitching is carried out at a temperature of 1000 °C or higher. The usage of this high temperature generates a variety of problems, including complexity of operation, higher contamination, metal evaporation, and wrinkles owing to the mismatch of thermal expansion coefficients between the substrate and graphene. Here, a new approach for the fabrication of ultraflat single‐crystal graphene using Cu/Ni (111)/sapphire wafers at lower temperature is reported. It is found that the temperature of epitaxial growth of graphene using Cu/Ni (111) can be reduced to 750 °C, much lower than that of earlier reports on catalytic surfaces. Devices made of graphene grown at 750 °C have a carrier mobility up to ≈9700 cm² V^?1 s^?1 at room temperature. This work shines light on a way toward a much lower temperature growth of high‐quality graphene in single crystallinity, which could benefit future electronic applications.     

Effect of copper foil annealing process on large graphene domain growth by solid source-based chemical vapor deposition

Controlling the metal catalyst surface structure is a critical factor to achieve growth of large graphene domains. In this prospect, we explored the annealing process to create an oxide layer and subsequent recrystallization of Cu foil for growth of large graphene domain by the atmospheric pressure chemical vapor deposition (AP-CVD) technique. We revealed the transformation of Cu surface crystallographic structures in every step of annealing process by electron back-scattered diffraction analysis. Initially, electroless polished Cu foils are annealed in Ar and then in H₂ atmosphere to obtain a smoother surface with reduced graphene nucleation sites. The transformation of Cu grain structures at various annealing steps was confirmed, where the gas atmosphere and annealing duration have significant influence. Graphene domains with the size more than 560 µm are obtained on the processed Cu surface using polystyrene as solid precursor. It is obtained that the oxidation and recrystallization process of Cu foil surface significantly influence the nucleation density, which enable growth of larger graphene domain in the developed CVD process.     

Low‐Temperature and Rapid Growth of Large Single‐Crystalline Graphene with Ethane

Future applications of graphene rely highly on the production of large‐area high‐quality graphene, especially large single‐crystalline graphene, due to the reduction of defects caused by grain boundaries. However, current large single‐crystalline graphene growing methodologies are suffering from low growth rate and as a result, industrial graphene production is always confronted by high energy consumption, which is primarily caused by high growth temperature and long growth time. Herein, a new growth condition achieved via ethane being the carbon feedstock to achieve low‐temperature yet rapid growth of large single‐crystalline graphene is reported. Ethane condition gives a growth rate about four times faster than methane, achieving about 420 µm min^?1 for the growth of sub‐centimeter graphene single crystals at temperature about 1000 °C. In addition, the temperature threshold to obtain graphene using ethane can be reduced to 750 °C, lower than the general growth temperature threshold (about 1000 °C) with methane on copper foil. Meanwhile ethane always keeps higher graphene growth rate than methane under the same growth temperature. This study demonstrates that ethane is indeed a potential carbon source for efficient growth of large single‐crystalline graphene, thus paves the way for graphene in high‐end electronical and optoelectronical applications.     

Preparation of bilayer graphene utilizing CuO as nucleation sites by CVD method

Graphene is an attractive 2D material for optoelectronics applications. However, due to the spontaneous nucleation characteristics of graphene growth on the metal substrates using chemical vapor deposition method, the polycrystalline graphene exhibited many crystal defects, leading to poor crystal quality. Properly controlling the density of nucleation sites is an important and necessary mean to increase the quality of graphene material. In this work, a new method to synthesize high-quality graphene on Cu substrate was reported by utilizing the CuO nanoparticles as nucleation sites. It was found that when annealing the copper substrate at 300 °C for 30 min with Ar:O₂ flow ratio of 64:1, the copper substrate showed the lowest roughness and the density of CuO nucleation sites after hydrogen etching (H₂ 21 sccm at 1035 °C). Bilayer graphene with diagonal length of ~?3 µm was successfully prepared centering on the CuO nucleation sites. This work supplied a new clue for high quality and monocrystalline graphene preparation.     

High‐Quality Monolayer Graphene Synthesis on Pd Foils via the Suppression of Multilayer Growth at Grain Boundaries

The segregation of carbon from metals in which carbon is highly soluble, such as Ni (≈1.1 atom% at 1000 °C), is a typical method for graphene growth; this method differs from the surface‐catalyzed growth of graphene that occurs on other metals such as Cu (<0.04 atom%). It has not been established whether strictly monolayer graphene could be synthesized through the traditional chemical vapor deposition route on metals where carbon is highly soluble, such as Pd (≈3.5 atom%). In this work, this issue is investigated by suppressing the grain boundary segregation using a pretreatment comprising the annealing of the Pd foils; this method was motivated by the fact that the typical thick growths at the grain boundaries revealed that the grain boundary functions as the main segregation channel in polycrystalline metals. To evaluate the high crystallinity of the as‐grown graphene, detailed atomic‐scale characterization with scanning tunneling microscopy is performed.     

Influence of copper morphology in forming nucleation seeds for graphene growth

We report that highly crystalline graphene can be obtained from well-controlled surface morphology of the copper substrate. Flat copper surface was prepared by using a chemical mechanical polishing method. At early growth stage, the density of graphene nucleation seeds from polished Cu film was much lower and the domain sizes of graphene flakes were larger than those from unpolished Cu film. At later growth stage, these domains were stitched together to form monolayer graphene, where the orientation of each domain crystal was unexpectedly not much different from each other. We also found that grain boundaries and intentionally formed scratched area play an important role for nucleation seeds. Although the best monolayer graphene was grown from polished Cu with a low sheet resistance of 260 Ω/sq, a small portion of multilayers were also formed near the impurity particles or locally protruded parts.     

Rapid growth of angle-confined large-domain graphene bicrystals

In the chemical vapor deposition growth of large-area graphene polycrystalline thin films,the coalescence of randomly oriented graphene domains results in a high density of uncertain grain boundaries (GBs).The structures and properties of various GBs are highly dependent on the misorientation angles between the graphene domains,which can significantly affect the performance of the graphene films and impede their industrial applications.Graphene bicrystals with a specific type of GB can be synthesized via the controllable growth of graphene domains with a predefined lattice orientation.Although the bicrystal has been widely investigated for traditional bulk materials,no successful synthesis strategy has been presented for growing two-dimensional graphene bicrystals.In this study, we demonstrate a simple approach for growing well-aligned large-domain graphene bicrystals with a confined tilt angle of 30° on a facilely recrystallized single-crystal Cu (100) substrate.Control of the density of the GBs with a misorientation angle of 30° was realized via the controllable rapid growth of subcentimeter graphene domains with the assistance of a cooperative catalytic surface-passivation treatment.The large-area production of graphene bicrystals consisting of the sole specific GBs with a tunable density provides a new material platform for fundamental studies and practical applications.     

Controlled Growth of Single-Crystal Graphene Films

Grain boundaries produced during material synthesis affect both the intrinsic properties of materials and their potential for high-end applications. This effect is commonly observed in graphene film grown using chemical vapor deposition and therefore caused intense interest in controlled growth of grain-boundary-free graphene single crystals in the past ten years. The main methods for enlarging graphene domain size and reducing graphene grain boundary density are classified into single-seed and multiseed approaches, wherein reduction of nucleation density and alignment of nucleation orientation are respectively realized in the nucleation stage. On this basis, detailed synthesis strategies, corresponding mechanisms, and key parameters in the representative methods of these two approaches are separately reviewed, with the aim of providing comprehensive knowledge and a snapshot of the latest status of controlled growth of single-crystal graphene films. Finally, perspectives on opportunities and challenges in synthesizing large-area single-crystal graphene films are discussed.     

Void growth in thin silver films

A high density of small voids was observed in thin films of silver on examination with a high resolution transmission electron microscope. Void growth was studied in these films by annealing them in a vacuum as well as in the electron microscope using a specimen heating stage. Heating of the films to 700°C led to thermal grooving at the grain boundaries and finally to grain separation. It appears that the void growth occurs essentially as a result of annihilation of excess vacancies trapped in the film. The observed phenomena of thermal grooving and grain separation can be explained in terms of surface diffusion of silver atoms.     

Towards Wafer‐Scale Monocrystalline Graphene Growth and Characterization

Since its discovery in 2004, graphene has boosted numerous fundamental sciences and technological applications due to its massless Dirac particle‐like linear band dispersion, that causes unprecedented physical properties. Among the various methods for synthesizing graphene, chemical vapor deposition is the most suitable approach for scalable production on a wafer scale, which is a critical step for practical applications. Graphene grain boundaries (GGBs), consisting of nonhexagonal carbon rings and therefore modulating the properties of graphene films, are inevitably formed via the merging of adjacent graphene domains with different orientations. Large‐area monocrystalline graphene synthesis without forming GGBs has been challenging, let alone observing such boundaries. Here, an up‐to‐date review is presented of how to grow wafer‐scale monocrystalline graphene without GGBs. One approach is to make single domain sizes as large as possible by reducing or passivating the number of nucleation sites. Another approach is to align graphene domains in identical orientations, and then merge them atomically. The recently developed methods for observing graphene orientation and GGBs both at the atomic and macro‐scales are also presented. Finally, perspectives for future research in graphene growth are discussed.     

Effect of doping on properties of Zno:Cu and Zno:Ag thin films

ZnO:Cu and ZnS thin films were grown by metal-organic chemical vapour deposition (MOCVD) under atmospheric pressure onto glass substrates. The ZnO:Ag films were fabricated from ZnS films by non-vacuum method that consists of simultaneous oxidation and Ag-doping by the close spaced evaporation (CSE) of silver at the temperature of 500–600 °C. Photo-assisted rapid thermal annealing (PARTA) at ambient air during 10–30 s at the temperature of 700–800 °C was used for the ZnO:Cu films. The samples were studied by X-ray diffraction technique (XRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements. The grain size of ZnO:Cu films increased with an increase of Cu concentration. PL spectra of as-deposited ZnO:Cu films depended on Cu concentration and contained the bands typical for the copper. After PARTA at high temperature the emission maximum shifted towards the short-wave region. During the fabrication of ZnO:Ag films the grain growth process was strongly affected by the Ag loading level. The grain size increased with an increase of Ag concentration and ZnO:Ag films with surface roughness of 8 nm were obtained. Observed 385 nm PL peak for these samples can be attributed to the exciton–exciton emission that proves the high quality of the obtained ZnO:Ag films.     

Direct formation of wafer scale graphene thin layers on insulating substrates by chemical vapor deposition

Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.