Pulsed laser deposition (PLD)

In plain words

Growth by firing short, intense laser pulses at a target so that a plume of material blows off it and settles on a nearby . Its main attraction is that a complicated composition transfers to the film almost unchanged.

Going deeper

Pulsed laser deposition ablates a target with short, intense pulses and lets the plume condense on a substrate. Its distinguishing virtue is that a complicated composition arrives almost unchanged; its distinguishing problems are droplets, uniformity and the energy of what lands.

Why the composition survives

A nanosecond pulse focused hard enough removes material faster than the constituent elements can segregate by evaporation. Everything in the illuminated volume leaves together, as a plume of atoms, ions and clusters, and lands together. That is the whole appeal: for a compound with four or five elements, getting the film to match the target is the hard part of every other growth method and comes almost free here.

Two further levers come with it. The chamber can hold a reactive background gas – oxygen, nitrogen – at pressures that would be impossible in a molecular beam system, which both feeds the film and slows the plume. And because growth is far from equilibrium and the substrate can stay relatively cool, phases that are not the thermodynamic ground state can be stabilised.

What it has managed on layered materials

The method is at home with oxides, and it is increasingly used for layered and tellurides where is the difficulty. MoS₂ has been grown this way over large areas and down to a single on sapphire, GaN and SiC, quasi-, with rocking-curve widths of 0.01°, a root-mean-square roughness of 0.27 nm and thickness uniform enough to check by .

The same work shows what still needs attention: those films came out as grown, where exfoliated MoS₂ is usually – a sign that defects or impurities, not design, set the doping. As with every chalcogenide growth method, the chalcogen has to be replenished during growth rather than assumed to survive the trip.

The costs, and what is done about them

Ablation throws off not only atoms but molten droplets, which land as particulates in the film. The plume is also narrow and forward-directed, so the composition and thickness vary across a far more than in methods with a broad, thermalised flux. Both problems have partial fixes – off-axis geometry, mechanical velocity filters, lower fluence, larger target-to-substrate distances, substrate rotation – and each costs deposition rate.

The third problem is specific to surfaces. Species in the plume arrive with energies of tens of electronvolts, well above the threshold for knocking atoms out of a monolayer, so the very first layer can be damaged by the growth of the second. A background gas that thermalises the plume is the usual remedy, at the price of the control that made the technique attractive.

For specialists

Ablation of a target by nanosecond pulses, with the plume condensing on a heated substrate. It transfers multi-element stoichiometry congruently, tolerates reactive background gases and reaches metastable phases, at the cost of droplets, a narrow useful plume and an energetic flux that can damage a van der Waals surface; for chalcogenides the chalcogen still has to be replenished during growth.

Where this comes from

Synthesis of novel thin-film materials by pulsed laser deposition Lowndes et al. · Science 273, 898 (1996) cited by 645
Highly crystalline MoS₂ thin films grown by pulsed laser deposition Serrao et al. · Applied Physics Letters 106, 052101 (2015) cited by 145

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

Stoichiometry

The ratio in which the elements of a compound are combined – the numbers in its formula. MoS₂ should have two sulfur atoms for every molybdenum atom. Real crystals often deviate slightly, with a few atoms missing or a few extra squeezed in, and in 2D materials a deviation of a few percent can change how a crystal conducts, glows or behaves magnetically. That is why careful work measures the composition rather than assuming the formula.

The proportions of the elements in a compound, ideally the integer ratios of its formula. Real crystals are often non-stoichiometric, through vacancies, interstitials, antisites or intercalated atoms whose concentrations depend on the chemical potentials during growth and on the Fermi level, as point-defect thermodynamics describes. Deviations dope the material and can decide its ground state: chalcogen-deficient TMDCs are n-type, self-intercalated titanium makes TiS₂ metallic rather than semiconducting, the alkali content of AV₃Sb₅ and the europium content of EuSn₂As₂ shift their transitions, and melt-grown InSe varies along the boule. Composition is measured by X-ray photoelectron or energy-dispersive spectroscopy, Rutherford backscattering or electron-probe microanalysis, with accuracies of about a percent at best, so small deviations are often inferred from properties instead.

Full explanation

Epitaxy

Growing a crystal on top of another so that its atoms line up with the ones underneath and carry on the pattern of the crystal below. It is how most semiconductor chips, lasers and LEDs are built, and it works cleanly only when the two crystals’ atomic spacings match closely – a restriction that layered 2D materials largely escape.

Growth of a crystalline film whose orientation is fixed by a single-crystal substrate: homoepitaxy on the same material, heteroepitaxy on a different one. A covalently bonded film strains to the substrate’s lattice until, beyond a critical thickness that falls steeply with mismatch, misfit dislocations relax it, so mismatches above a few percent permit only very thin coherent films. In van der Waals epitaxy the film is aligned but not bonded and mismatch is largely irrelevant; in remote epitaxy the substrate’s potential aligns the film through a graphene interlayer.

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

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