Annealing

In plain words

Heating a material for a while, well below its melting point, so that its atoms can move a little. It drives off water and dirt, removes some of the plastic left behind when a layer is , and lets defects heal or a film settle into a more ordered crystal. For it is a double-edged tool: a gentle anneal cleans a device, but too much heat can rotate stacked layers, oxidise crystals or pull atoms out of the surface.

As the site uses it

Control SrTiO₃ termination by etching and annealing, and count LaAlO₃ unit cells from RHEED oscillations.

LaAlO₃/SrTiO₃ · Oxide interface electron gases

Quote the oxygen partial pressure and the annealing history with every mobility.

SnO · Tin monoxide

Going deeper

An anneal is a ramp, a hold and a cool-down. Up to a few hundred degrees it cleans and settles a device; beyond that it starts to undo the work, and a chip that already carries its wiring sets a ceiling of about 400 °C.

Cleaning after transfer

Every transfer leaves something behind: the polymer that carried the layer, solvent, and water and hydrocarbons from the air. Heating to 200–400 °C in vacuum or in a mixture of argon and hydrogen drives off the water and decomposes much of the polymer, which made it a standard step after transfer from the early graphene work on. It does not remove everything: some residue chars into carbon that stays, and the cleaner sheet settles closer to its – on silicon oxide, graphene annealed this way picks up strong from the oxide below.

In stacks of crystals the heat helps in another way. Contamination trapped between the layers gathers into bubbles, leaving clean interface between them, and annealing lets small bubbles move and merge, which opens up larger clean areas.

Healing and harming

Heated in sulfur or selenium vapour, a can fill some of the it was grown with; heated in vacuum, it can lose more of them. Oxides behave the same way with oxygen, which is why oxide films are annealed in oxygen to set how many they keep. A short anneal after the metal goes on also lowers , by improving the contact between metal and channel.

The risks grow with temperature. Twisted stacks relax towards aligned stacking, so twisted devices are kept cool; air-sensitive crystals such as black phosphorus oxidise faster; and a chip that already carries its wiring tolerates only about 400 °C, which limits what can be done once 2D layers are added to finished circuits.

For specialists

Thermal treatment below the melting or decomposition temperature – in vacuum, inert gas, forming gas (Ar/H₂) or a or oxygen atmosphere – used to desorb adsorbates, decompose polymer residues after transfer, lower contact resistance, heal vacancies and recrystallise grown films. Post-transfer anneals typically run at 200–400 °C; relaxation, oxidation, chalcogen loss and the back-end set the upper limit.

Where this comes from

Atomic structure of graphene on SiO₂ Ishigami et al. · Nano Letters 7, 1643 (2007)
Toward intrinsic graphene surfaces: a systematic study on thermal annealing and wet-chemical treatment of SiO₂-supported graphene devices Cheng et al. · Nano Letters 11, 767 (2011)
Graphene annealing: how clean can it be? Lin et al. · Nano Letters 12, 414 (2012)

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

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

Electron hole

Usually just called a hole: the empty place an electron leaves behind in an otherwise full band. It behaves like a particle of its own with a positive charge. When a neighbouring electron steps into the gap, the gap moves the other way – like the empty space in a queue of cars, which travels backwards as each car edges forward. Many materials conduct mainly with holes, and chips need both kinds of carrier.

An unoccupied state in an otherwise filled band, usually near the valence-band maximum, treated as a quasiparticle with positive charge and a positive effective mass set by the band curvature. Holes carry the current in p-type semiconductors and give a positive Hall coefficient. In 2D semiconductors good p-type transport is harder to obtain than n-type, largely because metal contacts pin the Fermi level nearer the conduction band.

Full explanation

Point defect

A flaw in a crystal that involves a single atomic site: an atom missing (a vacancy), an atom of the wrong kind in a site (a substitution, or an antisite when the crystal’s own two kinds of atom swap places), or an extra atom squeezed in between (an interstitial). Every real crystal has some. In a sheet a few atoms thick each one sits at the surface, so a handful of them can change how the material conducts, glows or reacts far more than in a thick crystal.

A defect confined to one or a few lattice sites – vacancy, interstitial, substitutional impurity, antisite or adatom – or a small complex of them. What it does is set by the levels it introduces: shallow levels dope, deep levels trap carriers and act as non-radiative recombination centres, and an isolated deep level in a wide gap can emit single photons. Formation energies and charge states depend on the chemical conditions of growth and on the Fermi level. In 2D materials every defect is a surface defect, less screened and more exposed to the environment, and densities of 10¹² to 10¹³ per cm² are typical of TMDC monolayers.

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