The growth of epitaxial graphene on SiC and its metal intercalation: a review

High-quality epitaxial graphene (EG) on SiC is crucial to high-performance electronic devices due to the good compatibility with Si-based semiconductor technology. Metal intercalation has been considered as a basic technology to modify EG on SiC. In the past ten years, there have been extensive research activities on the structural evolution during EG fabrication, characterization of the atomic structure and electronic states of EG, optimization of the fabrication process, as well as modification of EG by metal intercalation. In this perspective, the developments and breakthroughs in recent years are summarized and future expectations are discussed. A good understanding of the growth mechanism of EG and subsequent metal intercalation effects is fundamentally important.


Introduction
Two-dimensional (2D) materials are composed of a few nanometers thick atoms, which have attracted extensive attention due to the unique physical properties and flexibility in preparing non-traditional heterojunctions [1].This review is devoted to graphene, which is the first ground-breaking 2D material recognized in 2004 [2].The discovery of graphene spurs extensive research activities, stimulating a myriad of applications of 2D materials [3,4].The first discoverers of Andre Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Geim and Konstantin Novoselov, physicists at University of Manchester, won the Nobel Prize in Physics in 2010.Different production methods for graphene have been reported in scientific literatures.Mechanical exfoliation used to prepare graphene sheets typically has low production efficiency [5].Oxidation and reduction of graphite can be used to fabricate large-scale graphene sheets but with unwanted defects and impurities [6].Chemical vapor deposition (CVD) can produce large-scale and high-quality graphene sheets on metals, but the complicated transferring technique is still challenge [7,8].Thermal decomposition of SiC is one of the most promising approaches to produce high-quality epitaxial graphene (EG) directly on insulating substrates [9][10][11].During thermal decomposition of SiC at high temperature, Si-C bonds are broken and Si atoms are sublimated from SiC substrate, while the remaining C atoms nucleate and form as graphene structure on SiC [12].According to the stoichiometric theory, the number of carbon atoms decomposed from three Si-C bilayers is approximately equal to that of one graphene layer [12].Because of the insulating SiC substrate, no transfer process of EG layer is needed for device fabrication [13][14][15].Accordingly, the EG/SiC structure is compatible with Si-based semiconductor techniques and the method has been applied to prepare high-performance electronics [16].So that, EG on SiC is a kind of hot 2D material pursued and explored by scientists, and many new technologies are emerged for the preparation and modification [17,18].
However, there are strong coupling effects due to the disordered buffer layer between SiC substrate and graphene layer [19].The coupling effects can cause strong charge transfer and electron scattering, resulting in severely disruption of π electron and deterioration of graphene carrier mobility [20].As a result, the electrical properties and practical applications of EG are greatly limited and hindered.Therefore, how to prepare and modify EG on SiC is one of the important topics.Metal intercalation is a common method to effectively regulate the properties of graphene [21,22].Metal atoms can insert into graphene layer through surface defect during annealing and form as a sandwich structure of graphene/metal/substrate [23].Meanwhile, the formed metallic layer at interface can decouple graphene with near-free property, and the graphene layer can also protect the underlying metallic layer [22].The confined 2D metallic layer is extremely thin, which is highly customizable through van der Waals force [24,25].It provides an effective approach to design and control the laminated material's interface [26].Therefore, metal intercalation raises the research of the fabrication and application of EG on SiC, which arouses the enthusiasm of researchers and shows great applications of this new 2D materials in various fields.
In this review, recent developments and breakthroughs pertaining to EG/SiC are summarized.The growth mechanism, structural characterization, optimization of the fabrication process, and metal intercalation of EG are discussed.It should give indication on the understanding of new physics and modification phenomena of graphene as well as device.

Growth mechanism of EG on SiC
SiC substrates are varied by poly-type (3C, 4H, 6H, 15R etc), doping (n-type, p-type, semi-insulating) and miscut (on-axis, off-axis, or other facets).There are two surfaces for on-axis SiC substrates, Si terminated SiC(0001) and C terminated SiC(000-1) are mainly considered in this review.Both of the two SiC substrates can be decomposed under certain conditions to produce EG [9-11, 27, 28].The morphology of EG on SiC can be optimized by inert gas or special techniques, such as Ar atmosphere [9], confinement controlled sublimation (CCS) [10] and face to face decomposition [29].Due to the particularity and randomness of decomposition process, SiC substrates with different terminating surface involve different surface reconstruction, buffer layer structure and graphene layer thickness, which ultimately leads to certain differences in the morphology and properties of EG [28].EG growth mechanisms on Si-and C-terminated SiC substrate through thermal decomposition have been well investigated by experimental observations and proved by theoretical simulations [30][31][32][33][34][35].Although the tracing of the individual particles is difficult due to the complexity of SiC binary component system, it is still highly desirable to probe the microscopic nucleation behavior of Si and C atoms during the early stage of thermal decomposition of SiC [36].
The growth mechanism of EG layer on Si-and Cterminated SiC is different [37].Meanwhile, the growth temperature has also significant influence on the quality of graphene layer, and very high temperatures may destroy the SiC substrate [38].The growth of EG layer on Si-terminated SiC follows the mechanism of graphene layer grown on Cu substrate, where the surface carbon epitaxially diffuses to the edge of pre-formed graphene-like structure [30].Thus, monolayer EG can be easily produced on Si-terminated SiC substrate.The structural transition from buffer layer to graphene lattice has been probed by both theoretical calculation and experimental observation [39][40][41].The Si terminated SiC surface undergoes a series of reconstructions with increasing temperatures, resulting in the formation of single or multilayer EG layer.However, due to high temperature cracking, there are many pits or holes on graphene surface, which is not conducive to the formation of continuous and uniform EG layer [12].The nucleation mechanism in early growth stage can be simulated by atomic model and the atomic structure of graphene on SiC can be characterized by scanning tunneling microscopy/spectroscopy (STM/S) [30,31,36,41].EG layer on SiC exhibits many great physical properties, including high quality and mobility [42], quasi-free standing feature and wafer-scale fabrication [43].Moreover, there is no necessary transfer process for electronic device fabrication on EG layer because of the insulating SiC substrate [44,45].So that, EG layer on SiC is much more convenient for practical application, especially compared to CVD and mechanically exfoliated graphene.EG layer on SiC can be used directly to produce high sensitivity gas sensor or photo detector without transferring [46].P. Mallet et al have provided a quick understanding on the role of pseudospin in quasiparticle interferences in monolayer and bilayer graphene on SiC(0001) through high resolution STM [47].Device with high thermal transport can be fabricated based on the heterogeneous interface of SiC and graphene layer [48].Graphene nano-ribbon can be assembled on SiC, which also has special application for electronic device [49].
As for C-terminated SiC, the segregation of carbon atoms is much easy.The growth of EG layer on C-terminated SiC is governed by carbon segregation from the surface and bulk of SiC substrate, which follows the mechanism of graphene layer grown on Ni substrate [30].Usually, multi-layer EG with randomly rotating angle can be synthesized on C-terminated SiC substrate.Meanwhile, there is no buffer layer structure, making the interlayer coupling between adjacent EG layers is relatively weak [50,51].Thus, EG layer on C-terminated surface can maintain the electronic properties of single-layer near-free graphene [52][53][54].However, disordered stacking can also result in a large number of grain boundaries, making it difficult to control the number of layers and thus reducing the graphene quality.Hass et al have demonstrated the rotational stacking faults feature on C-terminated SiC during graphene growth [55].Different from Si-terminated SiC, there is no buffer layer formed on C-terminated SiC.That is why multilayer graphene on 4H-SiC(000-1) behaves like a single sheet of graphene.Because of the thickness of SiC lose due to Si sublimation cannot completely be countered by the thickness of resulted graphene, so plenty of graphene covered basins (GCBs) are formed [34].The GCBs are most likely nucleated at the threading dislocations, which exhibit as ridges observed in continuous graphene film.The GCBs expand through erosion of the surrounding SiC substrate walls, eventually coalescing into continuous films during subsequent annealing.Ingemar Persson et al have investigated the morphology and atomic structure of few layers of graphene on C-terminated 6H-SiC(000-1), of which the nature of azimuthal disorder can be identified by STM [54].The superstructures of Moiré patterns are due to the misorientation angle between consecutive layers.And, the stacking faults are expected to lead free standing feature of single graphene even for multilayer samples, indicating the apparent electronic decoupling of graphene layers on SiC [54].As measured by Keskin et al through friction force microscope, the friction coefficient of graphene on the Si-terminated face of SiC is about two times bigger than the one grown on its C-terminated face [56].The difference in friction coefficients cannot be related to the roughness of graphene layers due to the alternating periodicity of charge distribution on graphene layer.The electro-catalytic property of C-terminated SiC is highly suppressed, which more likely leads to high energy efficiency [57].Moreover, EG layer on the C-faced SiC exhibits stronger Raman scattering signal because of the natural twisting and disordering graphene layers [58].

Evolution and atomic structure of SiC buffer layer
The buffer layer between SiC substrate and graphene layer takes part in the first-stage graphitization process during thermal decomposition of SiC, which affects the quality, electron mobility and thermal conductivity of EG [59][60][61][62][63].Because of the intrinsic structure and electronic properties, the interfacial buffer layer has great influence on EG layer [41].Even the stacking order of SiC substrate can induce doping variation in EG layer [64].So that, buffer layer is often studied only as the intermediate to the synthesis of SiC supported graphene [41].Usually, graphitization of SiC involves three surface reconstructions at elevated temperature: [12,36].The surface of (6 √ 3 × 6 √ 3)R30 o reconstruction is carbon enriched and serves as the interfacial buffer layer for growth of EG.Moiré patterns with 6 × 6 periodicity are often observed due to the lattice mismatch between graphene and SiC substrate.Since graphene becomes transparent under a bias of 0.1 eV, the atomically resolved structure of the buffer layer is quite evident even when EG is covered.As depicted in figure 1, the atomic configuration evolution of the buffer layer can be monitored by STM by changing the bias voltage [36].The buffer layer normally retains the long-range order of 6 × 6 reconstruction despite the disordered shortrange atomic arrangements during thermal decomposition and serves as the template for subsequent nucleation and growth of graphene.Hence, the buffer layer can be built up and the intrinsic structural and electronic properties of the buffer layer on SiC have been studied by DFT calculation to corroborate experimental observation and provide insights in to the formation mechanism of EG [41].

Large-scale fabrication of EG
Large-scale or wafer-size graphene is always important goal and urgently needed for industrial applications.Thermal decomposition of SiC is a non-equilibrium process, which can produce a discontinuous surface on graphene with undesirable pits and edges [33].As a result, the morphology of EG on SiC is usually irregular after SiC decomposition [40].Emtsev et al have observed that large-size EG domains can be formed on Si-terminated SiC if thermal decomposition is performed in an argon atmosphere and the electronic mobility in EG reaches ∼2000 cm 2 •V −1 •s −1 at 27 K [9].de Heer et al have developed the process of CCS and produced large-scale EG [10].Continuous single-layer graphene can also be produced on SiC by annealing using a metal flux [65].In the process, the sublimation rate of Si is reduced by the metal atmosphere and the metal weakens C-C bonds and promotes graphitization.Actually, thermal decomposition of SiC is directly related to the annealing rate and diffusion of Si/C atoms, which determine the step structure and defect density.Rapid thermal annealing has been proposed and there have been studies on the optimization of the thermal decomposition process to fabricate large-scale and high-quality EG [66].Prof. Zhao and his team from Peking University realized the fabrication of EG based on the evaporation of Si atoms through injecting carbon ions into SiC substrate by ion implantation [67].This method reduces the fabrication temperature and accurately controls the carbon ion dose, which enables the large-scale preparation of highquality graphene directly on SiC.Moreover, graphene nanobands can be grown on the surface of SiC by the decomposition process assisted by foreign atoms of C or Ar atmosphere [49], which exhibit excellent physical properties and can be applied to graphene patterning.
There are strong coupling effects due to the disordered buffer layer between SiC substrate and graphene layer [27].Buffer layer is considered as the zero graphene layer, which is covalently bonded to SiC substrate [27].The covalent bonding between buffer layer and EG layer occurs only at the edges, which is missing in the areal parts [27].The coupling effects can cause strong charge transfer and electron scattering [40], resulting in severely disruption of π electron and denotation of graphene carrier mobility [63].As a result, the electrical properties and practical applications of EG are greatly limited and hindered [20].Therefore, how to modify and decouple EG on SiC is also one of the important topics for large scale EG fabrication.Flash annealing at a high temperature can be employed to fabricate near-free-standing graphene on a large scale but a rough morphology is frequently produced regardless of the stacking configuration, polar faces, and surface reconstruction of SiC [38].As depicted in figure 2, the morphology as well as decoupling characteristic of EG layer can be controlled by flash annealing under different atmosphere and temperature [38].

Mechanism and features of metal intercalation
Metal intercalation is a promising method to modify and decouple EG on SiC for electronics applications [21,[69][70][71].Many elements, including Li [72], Ge [73], Pt [74], Pb [75] and rare-earth metals [76] have been adopted for intercalation.Pd [77] and Cu [78] can penetrate the SiC buffer layer to dope the graphene layer.Moire patterns are commonly produced as a result of metal intercalation and the lattice mismatch between intercalated metal and graphene [79][80][81], which usually show interesting and unique properties such as robust superlubricity and superconductivity [82][83][84].Thus, metal intercalation is an effective technique to optimize the morphology and properties of EG on SiC, propelling the performance and functionalization of graphene and electronic devices [22,85,86].Normally, single or multi-element metal atoms can be used to intercalate graphene.Figure 3 depicts the mechanism of indium intercalation [68], which can passivate buffer layer and decouple graphene layer without damaging the structure of graphene.For example, the intercalation of alkali metal atoms and magnesium metal atoms can solve the bonding problem between graphene and SiC substrate, optimizing the SiC buffer layer to improve the performance of graphene [87].Because of the intercalation of Mg atom, the monolayer graphene can transfer into decoupled bilayer graphene, exhibiting special electronic band structure in the intercalated region [87].The intercalation of Ta atoms not only forms n-type doped graphene, but also enables the graphene decoupled from SiC substrate [88].
Metal intercalation can introduce strong correlation effects into graphene, which has been widely studied [89].Philipp Rosenzweig, a scientist from Max Planck Institute, has reported that the epitaxially grown buffer layer graphene on SiC(0001) can be decoupled from SiC substrate and strongly n doped up to its van Hove singularity via ytterbium intercalation [90].It opens up a direct way of tuning graphene to a desired doping level in the vicinity of the van Hove singularity by controlling the annealing temperature [90].Large area of intercalated 2D metal can also be achieved.For example, large scale of intercalated Ag atoms can make the top layer of graphene with n-type doping characteristics [85].The intercalated Ag monolayer exhibits as semiconducting, which is different from bulk silver.There is very similar work on Au intercalation, in which large scale of 2D gold layer can be synthesized and stabilized between SiC substrate and monolayer graphene [91].Semiconductor to metal transition in 2D-Au can be induced by tuning the amount of intercalated gold atoms [91].
In addition, metal intercalation can improve graphene properties.Some special metals, such as low melting point metal Ga, can be easily intercalated into graphene with liquid state under atmospheric conditions at room temperature, facilitating the fabrication of large-area metal-intercalated graphene [92].As proved by x-ray technique, the intercalation of metal Li can effectively eliminate the surface folds of graphene, independently of the intercalation structure and density.A small amount of Li atom intercalation can decouple the entire graphene layer, promoting the fabrication of large area of intercalated graphene with ultra-high conductivity [93].The intercalation of multi-element or compound is also the trending.Russian scientist Pronin realized the intercalation of compound through silicification on the substrate of Co intercalated graphene [86].The intercalation of such compounds can passivate SiC substrate and effectively prevent the transfer of electrons from buffer layer to graphene layer, decoupling the upper graphene with quasi-free property.First principles calculation shows that large scale silicide intercalation between SiC substrate and buffer layer is feasible, which is much more stable even under different temperature and pressure [94].Dynamically, van der Waals forces, ridge structure and atomic defects play major roles in regulating the intercalation of metal atoms [21].Therefore, metal intercalation can propel the largescale fabrication of EG on SiC [92,95].The sandwich structure of graphene/metal/substrate exhibits a broad application prospect in the field of next-generation high-performance electronic devices.

Atomic structure of the intercalated metal atoms
Theoretically, atomic thin lamellar structures are unstable and exist only in high vacuum.However, the special interface between SiC substrate and graphene provides an effective framework to protect the two-dimensional intercalated metal atoms [96].Thus, metal intercalation between EG/SiC can promote the formation of 2D materials that do not exist in nature [21].The interface between the intercalated metal layer and adjacent 2D layers provides confined space for chemical reactions consequently spurring a new research area of 'chemistry under 2D cover' [22].The confined 2D metallic layer can be stable even at atmosphere, which give rise to many unique physical phenomena.It provides an excellent platform for the exploration of moiré pattern, charge spin, superlubrication, Raman enhancement and other interesting physics [22].The special confined space is also considered as 2D nano-container, which has a very important application in the fields of electrochemistry and energy [21,22].Thus, it is indicated that the confined 2D metallic layer underneath graphene is still a hot research topic for scientists.Based on metal intercalation, Prof. Joshua A. Robinson and his team from University of Pennsylvania reported wafer scale fabrication of 2D metal at graphene/SiC interface [97].Prof. Walt A. de Heer from Georgia Institute of Technology summarized that the sandwich structure of SiC/metal/graphene has promising application because of the stable metallic layer confined and protected by graphene [98].As proved by previous studies, the arrangements of Au and Ag atoms inserted into graphene layers are different from the bulk crystal, causing the confined 2D metal behave as semi-conductive feature [85].
The atomic and electronic structures of interface between graphene layer and SiC substrate play an important role for the superconductivity of graphene [100].As shown in figure 4, the atomic arrangement of the intercalated Pb atoms underneath graphene layer can be different, resulting in mottled or striated moiré patterns as detected by STM [75,99].As shown in figure 5, the intercalated In atoms underneath graphene exhibits different thickness [68,101].The various structures of ordered interfacial Sn atoms can be identified by the means of high-resolution spot profile analyzing low energy electron diffraction [102].The asymmetric alternating arrangement of metal atoms breaks the symmetry of 2D metal's potential energy, changing the band structure of top layer graphene with certain band gap [101].Therefore, the structure of 2D metallic layer is key to determine the properties of graphene.The accurate measurement and theoretical verification of the structure of the intercalated 2D metal is of great significance.It provides an important reference for the design and regulation of material interfaces, promoting the functionalization and devicalization of 2D material of graphene.

Summary and future developments
The aim of the paper is to overview the aspects of graphene on SiC, including the growth mechanism, structural and electronic properties, metal intercalation and applications.Thermal decomposition is a promising method to fabricate high-quality EG on SiC on a large scale for electronics applications and recent research activities have advanced our understanding on EG on SiC substrate.However, future researching works are required in order to elucidate the growth mechanisms and interactions between graphene and SiC substrate.Produce industrial grade EG on SiC and find a facile way to apply EG/SiC in various fields are responsible for the novel properties.
Scientists have been focused on the preparation mechanism and application of near-free EG on SiC.The surface morphology of graphene/SiC can be well controlled through appropriate heating rate [103].It is reported that the most preferable heating rate of ∼250 • C min −1 can result in the most homogeneous monolayer graphene on SiC substrate [103].The group of H. W. Schumacher reported the preparation of uniform EG with large-area and near-free properties through gas shielding atmosphere, which is important for the formation of SiC buffer layer and the growth of graphene [43].The kind of quasi-free-standing graphene on SiC shows widely potential applications.It is reported that visible light detector with enhanced performance can be made with quasi-free graphene on SiC [104].In addition, the decoupled graphene on SiC substrate can transform the immune response into electrical output [15], which is much suitable for the rapid detection of novel coronavirus (SARS-COV-2) and shows potential application in the field of bioscience [14].The technology has ultra-high sensitivity, excellent signal-to-noise ratio and ultra-fast response speed, providing a new portable diagnostic method for virus detection [105].It is indicated that EG on SiC has a wide range of applications in high-performance electronic devices, biosensors and other fields [15,104,105].
Recent research activities have advanced the understanding on the intercalation mechanism, the characteristics of intercalated graphene and the structure of intercalated metal confined underneath graphene layer [69,70,106,107].It opens the door for the development of 2D materials in biomedicine.The era of 2D materials will come in future, and high quality preparation and functionalization of 2D materials will be key important to promote its basic research and industrial application [108,109].However, the complexity and diversity of EG/SiC surface seriously affects the metal intercalation, whether Si terminated SiC with buffer layer structure or C terminated SiC without buffer layer [28].These deficiencies largely hinder the in-depth development of metal intercalation on EG on SiC.Therefore, future researching works are required in order to elucidate the metal intercalation on different types of EG/SiC substrates.Producing large-scale intercalated EG on SiC and finding a facile way to apply the intercalated and decoupling EG in various fields are responsible for the novel properties.

Figure 1 .
Figure 1.Atomic configuration evolution of buffer layer with increasing temperature observed by STM: (a) 1100 • C, (b) 1200 • C, (c) 1250 • C, and (d) 1300 • C. The characteristic unit cells are marked.Panels (e)-(h) are the corresponding line scans showing the heights along the green lines in (a)-(d), respectively.Reprinted from [36], with the permission of AIP Publishing.

Figure 2 .
Figure 2. Morphology and Raman results of EG controlled by flash annealing under different atmosphere and temperature.(a) and (b) STM images show EG morphology produced by thermal annealing in UHV and Ar atmosphere, respectively.(c) AFM image shows EG morphology produced by HT flash annealing in UHV.(d)-(f) The topological profiles acquired from the green lines marked in (a)-(c).(g)-(i) The corresponding Raman spectra.Reprinted from [38], Copyright (2017), with permission from Elsevier.

Figure 4 .
Figure 4. Pb-intercalated graphene (PbG) exhibits mottled and striated moiré patterns, which are corresponding to Pb(111) and Pb(110) arrangements underneath graphene layer.(a)-(c) Atomically resolved structure, showing the consist of three PbG regions, including mottled moiré patterns, striated moiré patterns and irregular regions.The insets in (a) and (b) show the DFT calculation results acquired from the models of G/Pb(111) and G/Pb(110) at low energy level, reflecting the STM image under a small bias voltage.(d) The electronic properties measured on the three different regions through STS.Reprinted from [99], Copyright (2021), with permission from Elsevier.

Figure 5 .
Figure 5. Atomic structures and corresponding 2D-FFTs of one layer In-intercalated graphene (O-InG) and two layer In-intercalated graphene (T-InG).(a) Morphology of O-InG and T-InG.(b) and (c) Atomic structure of O-InG observed under different sample bias voltages.(d)-(f) 2D-FFT obtained from (a)-(c), respectively.(g) and (h) Atomic structure of T-InG magnified from the red square in (a).(i) FFT obtained from (g) and (h).Reprinted from [68], Copyright (2021), with permission from Elsevier.