Chemical vapour deposition (CVD)

Also called chemical vapor deposition

Everyday term

In plain words

A way of growing thin films from gases. The ingredients arrive as vapour, react on a hot surface and leave a solid layer behind. It is how large sheets of graphene and other are made for industry.

Going deeper

A typical tube-furnace CVD growth of MoS₂. Carrier gas sweeps sulfur vapour from a gently heated boat downstream to the hot zone, where it meets vapour from MoO₃ and reacts on the substrate. Growth starts from scattered nuclei that become triangular single-crystal domains; given time they merge into a continuous film, with grain boundaries wherever misaligned domains meet.

Building a layer from vapours

Chemical vapour deposition grows a film from gas-phase precursors that react or decompose on a heated . For graphene the precursor is usually methane over copper foil at about 1000 °C: carbon barely dissolves in copper, so growth largely stops once the surface is covered, which makes large uniform practical. For a metal source – an oxide such as MoO₃ or WO₃, or a metal–organic compound – meets vapour, and the reaction forms the layer directly on the substrate, typically at several hundred degrees Celsius.

Nucleation, domains and grain boundaries

A CVD film does not appear everywhere at once. Growth begins at scattered nuclei, each of which becomes a domain – for TMDCs usually a triangle whose edges follow the crystal lattice. Domains grow until they meet, and where two meet with different orientations they leave a . The density of nuclei, set by temperature, precursor supply and substrate, therefore decides the grain size and much of the film’s electrical quality. The route to single crystals is to make every domain point the same way, for example by growing on a crystalline substrate such as sapphire whose surface steps align them, so that merging domains stitch together without boundaries.

What is traded for scale

CVD produces areas no can, and grows directly on substrates suitable for manufacturing. The costs are defect densities higher than in the best exfoliated , uneven thickness where second layers nucleate, contamination from precursors and, often, a that adds wrinkles and residue. Variants address particular problems: lowers growth temperatures and enlarges domains, and metal–organic CVD gives more uniform thickness over whole wafers at the price of carbon contamination. Growth temperature also decides compatibility with finished chips, which tolerate only a limited .

For specialists

Growth of films from gas-phase precursors that react or decompose on a heated substrate. For 2D materials this includes graphene on copper foil, powder-source CVD of TMDCs from metal oxides and chalcogens, and metal-organic CVD (MOCVD) for wafer-scale TMDC films; grain size, nucleation density and residues are the usual quality limits.

Where this comes from

Large-area synthesis of high-quality and uniform graphene films on copper foils Li et al. · Science 324, 1312 (2009) cited by 11,246

Substrate

The base a thin film or flake sits on – often a polished slice of silicon, glass or sapphire. For a crystal one atom thick, the substrate is not just a table: its bumps, stray charges and vibrations reach right into the sheet and change how it conducts and glows. That is why the same material can behave differently on two substrates, and why the flattest, cleanest one – boron nitride – gives the best results.

The material beneath a 2D layer. It acts on the layer through surface roughness, charged impurities, surface optical phonons, dielectric screening, strain and charge transfer, and during growth it sets orientation through epitaxy. Replacing SiO₂ with hexagonal boron nitride, which is atomically flat and nearly free of dangling bonds and charge traps, improved the mobility and charge homogeneity of graphene by close to an order of magnitude.

Full explanation

Chalcogenide, halide and pnictide

Family names for compounds, taken from the element in them that carries the negative charge. Chalcogenides contain sulfur, selenium or tellurium; halides contain fluorine, chlorine, bromine or iodine; pnictides contain phosphorus, arsenic, antimony or bismuth. Most layered crystals belong to one of these families, and the catalogue groups many of them that way.

Compounds named for their most electronegative constituent: chalcogenides of group 16 (S, Se, Te; oxides are conventionally counted apart), halides of group 17 (F, Cl, Br, I) and pnictides of group 15 (P, As, Sb, Bi; nitrides usually keep their own name). Down each group the anion grows larger and less electronegative, so bonding turns more covalent, gaps shrink and spin–orbit coupling grows. Chalcogenides give the MX₂ semiconductors and metals, halides mostly ionic, often magnetic insulators built from edge-sharing octahedra, and pnictides many metals, semimetals and the parents of the iron-based superconductors; mixed-anion compounds such as oxychalcogenides and chalcohalides combine two.

Full explanation

Single crystal

A piece of material in which the atoms are lined up in one unbroken pattern from one side to the other, with every row pointing the same way. Most solids are not like that: they are polycrystalline, made of many small crystals stuck together at random angles, like a pavement of tiles laid every which way. The boundaries between those grains scatter electrons, weaken the material and blur measurements, so physics experiments and the best devices want single crystals – a centimetre-sized block to peel flakes from, or a sheet grown across a whole wafer in one orientation.

A solid whose lattice is continuous and uniformly oriented throughout, without grain boundaries – unlike polycrystalline material, made of misoriented grains, or amorphous solids without long-range order. Bulk single crystals of layered compounds are grown by chemical vapour transport, flux or Bridgman methods and are the usual source of exfoliated flakes; their quality is judged by X-ray rocking-curve widths, the residual resistivity ratio and the absence of twins and stacking faults. Large single-crystal films grow either from one nucleus or from aligned nuclei that merge without boundaries, as for graphene on Cu(111) or Ge(110) and MoS₂ or hBN on vicinal sapphire or copper, where substrate steps fix one orientation and remove the twin boundaries that two antiparallel orientations would form.

Full explanation

Wafer and wafer-scale growth

A wafer is the thin, round slice of ultra-pure silicon – today 300 millimetres across – on which computer chips are made, hundreds at a time. Wafer-scale growth means making a 2D material evenly over a whole wafer rather than as scattered flakes, which is what any factory needs. For the best 2D materials that is still hard: a sheet one atom thick has to have the same thickness, quality and orientation from one edge of a dinner-plate-sized disc to the other.

The substrate format of the semiconductor industry – 200 and 300 mm silicon wafers, with sapphire, SiC and glass for particular processes – and the scale at which a material must be grown, transferred and processed to enter manufacturing. Wafer-scale growth of 2D materials uses CVD and MOCVD of TMDCs, CVD of graphene and hBN, and MBE, with targets of uniform layer number, low defect and grain-boundary density and, increasingly, a single orientation on vicinal sapphire or Cu(111). The obstacles are the growth temperature against the thermal budget of a finished wafer (growth on the back end of line needs to stay below about 400 to 450 °C), the transfer from growth substrate to device wafer, and metrology that confirms thickness and quality across the wafer at production speed.

Full explanation

Layer transfer

Moving an atomically thin layer from where it was made to where it is needed – from the copper foil it grew on to a silicon wafer, or from a flake on tape onto another flake. The layer is too thin to pick up on its own, so it is carried on a polymer film or a soft stamp, put down, and the carrier removed. Each step risks tears, wrinkles, trapped bubbles and a film of leftover polymer, which is why transfer is often the dirtiest step in making a 2D device.

The step that moves a 2D layer from its growth substrate or exfoliation source to a target substrate or stack. Wet transfer coats the layer with a supporting polymer, usually PMMA, detaches it by etching the growth substrate or by electrochemical bubbling, and floats it onto the target before the polymer is dissolved. Dry transfer picks layers up with a viscoelastic PDMS stamp or a polymer film such as polycarbonate, often using the van der Waals adhesion of a top flake, and releases them by heating. The costs are cracks and wrinkles, trapped water, hydrocarbons and polymer residue, strain and doping, and – at wafer scale – yield that compounds with every transfer, which is why growth directly on the target and transfer-free routes are pursued.

Full explanation