Epitaxial Graphene Growth on Silicon Carbide Graphene

An epitaxial graphene growth on SiC substrates was achieved using hot filament chemical vapor deposition (HFCVD). Raman, SEM, AFM torsion resonance mode measurements, and EDS analyses have all been conducted on the produced graphene.

HF cleaning of substrate surfaces allows for the production of high-quality few-layer and multilayer graphene layers, and their structures remain virtually indistinguishable from uncleaned substrates for extended growth times.

Characteristics

SiC graphene, formed by exfoliating monolayers of carbon from a wide-bandgap semiconductor substrate, shares many of the properties found in freestanding monolayer graphene. As such, SiC graphene can be utilized in many different applications; such as transistors for modern electronics; solar cells and energy storage; superconductors which offer unparalleled speed and efficiency can all utilize silicon carbide graphene as part of their construction.

Graphene grown on broad-bandgap substrates was studied using various techniques, including low-energy electron microscopy (LEEM), angle-resolved photoelectron spectroscopy (ARPES), Raman spectroscopy, atomic force microscopy in different modes and Hall measurements. Researchers discovered that growth conditions had an immense impact on its morphology and electronic structure – such as when grown on an n-type substrate leading to formation of an unoccupied valley with hole quantum wells (VB).

Researchers found that graphene growing on the C surface of 6H-SiC formed a gapless Dirac point similar to freestanding monolayer graphene due to an interface carbon monolayer or buffer layer which covalently bound with Si atoms on its substrate surface.

Researchers observed a multilayer graphene structure with two stable crystallographic configurations, known as ABA and ABC stacking sequences. For the former, each corner of a carbon hexagon in layer A lies directly beneath a silicon triangular defect in layer B; with ABC stacking each corner lies beneath non-equivalent corners of silicon triangular defects instead.

Researchers conducted experiments to explore how defects affect the morphology and electronic structure of multilayer graphene by employing a controlled sublimation method to grow this material. This technique allowed them to maintain a finite background pressure on the substrate surface while still producing graphene with proper symmetry and energy levels, producing more structurally uniform multilayer graphene than that grown using unconfined ultrahigh vacuum techniques.

Aplicații

Silicon carbide graphene offers an exciting path towards developing novel wafer-scale photonic and electronic devices that are easily integrated with existing silicon electronics. In order to harness its full potential, epitaxial graphene growth on SiC is an integral step. To do so successfully, however, a method must exist that produces high-quality graphene with uniform coverage on substrate while controlling deposition material amounts and defects effectively – recently, Ni/Cu catalyst has proven capable of doing just this, producing epitaxial graphene growth on SiC with great density control while causing no significant substrate damage or delaminacy.

Silicon carbide graphene typically takes the form of a bilayer composed of carbon and silicon atoms. Its basic unit cell is hexagon-shaped with an area of 3.08 A; orientation can either follow an ABA or rhombohedral stacking order; the latter structure leads to hybridization between its valence and conduction bands, providing potential band gap semiconductor performance.

Growing graphene on silicon carbide has been accomplished using several methods, but the most popular approach is hot filament chemical vapor deposition. This technique employs a hot filament to heat SiC substrates and generate a methane vapor phase which reacts with its surface to deposit carbon atoms, creating terraced environments suitable for graphene growth. To increase productivity further, an Ar overpressure can be added during deposition to avoid si sublimation during deposition and suppress any subsequent si sublimation that might arise during deposition – leading to enhanced yield.

Epitaxial graphene on SiC can be examined through scanning electron microscopy (SEM), torsion-resonant conductive atomic force microscopy (TRCAFM), and Raman spectroscopy. Raman mapping enables researchers to map 2D/G intensity ratio, a measure of its number of layers; few-layer samples and multilayer ones can also be identified via Raman mapping; SEM images showing both few-layer and multilayer graphene are displayed below at 140,000x magnification in Figures 4a&b respectively.

Synthesis

SiC graphene holds tremendous promise as an outstanding material due to its wide bandgap, high thermal conductivity and other properties. Synthesizing high-quality graphene has proven challenging, however. There have been various approaches proposed to synthesize high-quality graphene on an SiC substrate including hydrogen etching, nucleation and CVD with an external carbon source; among these approaches CVD appears especially promising due to not relying on sublimation of Si onto its surface and providing greater control of growth processes than others.

CVD involves dehydrogenating methane with an argon plasma to generate carbon species that deposit carbon atoms onto SiC substrate, creating uniform flat terraces that eventually form graphene films. Furthermore, using an external carbon source helps avoid sublimation defects while also guaranteeing superior structural integrity of graphene produced.

Characterizing graphene grown on SiC can be done using various techniques, including Raman, SEM, AFM, EDS and XPS. Raman is an extremely useful technique, providing information about both layers present as well as any defects present within them; furthermore it can reveal structural information as well as reveal any atomic bonding with its substrate.

Note that the growth kinetics of epitaxial graphene on SiC depends on both substrate polytype and polarity. When exposed to humidity, a first C-rich layer called a buffer layer forms on its surface and includes strong bonds between Si atoms of the substrate (inset in Figure 15) and C atoms within this buffer layer, as seen here (inset). Subsequently monolayer graphene forms before being removed leaving behind another monolayer graphene layer characterized by stronger van der Waals interactions than its predecessor.

Tromp et al. have developed a technique to accelerate graphene growth at high quality epitaxial thickness by controlling sublimation temperatures using a confined cavity that maintains a finite Si background pressure while silicate evaporates from substrate surfaces, significantly improving quality compared with non-confined CVD processes.

Properties

Silicon carbide graphene shares many properties similar to pure carbon, including excellent thermal and high temperature mechanical properties, excellent electrical conductivity and outstanding tribological performance. When added to SiC, adding graphene improves these qualities even further and makes this material suitable for applications requiring good tribological and electronic characteristics, such as micro and nanoelectromechanical systems (MEMS and NEMS), high performance semiconductors or batteries.

Graphene on silicon carbide can be produced by epitaxially growing it on its surface. Once grown, this epitaxial graphene can then be transferred onto a clean, transparent Si substrate. A joint team from Georgia Tech and Tianjin International Center for Nanoparticles and Nanosystems recently demonstrated that epitaxial graphene grown on a silicon carbide foundation exhibits semiconducting behavior; its structure allows scientists to tune its transport properties.

Controlling surface reconstruction during the annealing process is made possible by creating an external background pressure using disilane gas, then equilibrating the reconstructed layer into a pressure-temperature phase diagram. This allowed Si atoms to reflect away from their substrates without sublimation occurring, enabling decoupling from Si and growth of true monolayer graphene.

Graphene can be studied using low-energy electron microscopy (LEEM), angle-resolved photoelectron spectroscopy (ARPES), Raman spectroscopy, and atomic force microscopy – each measuring its properties differently – these measurements have revealed that its crystalline quality on silicon carbide plays a key role in its transport properties.

Furthermore, the team investigated how structural properties influenced electronic properties of epitaxial multilayer graphene on SiC. Their results demonstrated that bilayer graphene on SiC is highly stable and exhibits metallic behavior due to Bernal AB stacking where carbon atoms in layer B sit directly above centers of carbon hexagons in layer A; in comparison with conventional AA stacking where all carbon atoms lie above each other.