Phase-change material (PCM)

Everyday term

In plain words

A material that can be switched between a and an ordered arrangement of its atoms by a pulse of heat or light, and then stays that way. Because the two states reflect light and carry current very differently, the switch stores a bit – which is how rewritable DVDs and Blu-ray discs hold their data.

Going deeper

A phase-change material is cycled between an amorphous and a crystalline state by heat pulses, and stays in whichever one it was left. The two states differ enormously in both resistance and refractive index, which is what makes the arrangement itself a stored bit.

One composition, two states, kept without power

A short, intense pulse melts a small volume and the heat runs away fast enough that the atoms freeze in place, leaving a glass. A longer, gentler pulse holds the same volume above the crystallisation temperature but below melting, and it orders. Both states are stable at room temperature for years, so the memory is non-volatile, and both transitions take nanoseconds to tens of nanoseconds.

What makes the family useful is the size of the contrast. In the crystalline state these bond in a way that is unusually polarisable – often described as metavalent, sitting between and – which gives a high optical and relatively high conductivity. The amorphous state bonds ordinarily. The result is a resistance ratio of orders of magnitude and a refractive index change large enough to see, from the same atoms rearranged.

From optical discs to memory to photonics

The first commercial use read the : rewritable CDs and DVDs used a focused laser to write and read amorphous marks. Electrical memory followed, with Ge–Sb–Te switched by current pulses in a confined cell, and it reached products: Intel’s Optane storage-class memory, sold from 2017 until it was discontinued in 2022, and embedded memory in STMicroelectronics’ Stellar automotive microcontrollers. The same cells, switched partially, hold intermediate resistances, which is why the family keeps appearing in analogue and computing proposals.

is the newer direction, and it changes the figure of merit. There the number that matters is not a resistance ratio but the refractive index change against the optical loss, because a switch that absorbs light is useless in a waveguide. Ge–Sb–Te absorbs too much in the near infrared, which is why interest moved to layered antimony chalcogenides and selenide-substituted alloys with far lower loss in the telecom bands.

What still limits them

The amorphous state is not a single state: it relaxes slowly toward lower energy, and its resistance drifts upward as it does. For a binary bit that is tolerable; for multi-level storage it is the central problem, since the levels wander into each other over time. Endurance is the second limit – repeated melting segregates the elements and opens voids – and the third is the current needed to melt a volume, which scales down only as the cell does.

The 2D connection runs in two directions. Layered chalcogenides are themselves phase-change materials, and superlattices of GeTe and Sb₂Te₃ have been reported to switch with much lower energy by rearranging atoms between layers rather than melting the whole volume, though how that works is still argued over. In the other direction, thin layered films are the practical route to putting a phase-change switch on top of a photonic waveguide without spoiling it.

For specialists

A compound cycled between amorphous and crystalline states by melt-quench and crystallisation pulses, giving a large non-volatile contrast in and in refractive index. Ge–Sb–Te alloys are the established family for memory; layered antimony chalcogenides extend the same trick to programmable photonics, where the figure of merit is index change against optical loss rather than a resistance ratio.

Where this comes from

Phase-change materials for rewriteable data storage Wuttig and Yamada · Nature Materials 6, 824 (2007) cited by 3,670
Phase-change materials for non-volatile photonic applications Wuttig et al. · Nature Photonics 11, 465 (2017) cited by 1,425

Disorder and localisation

Every real crystal has some disorder: missing or misplaced atoms, impurities, stray charges in the substrate, a random mixture of elements in an alloy. Electrons moving through it scatter off these imperfections, which raises the resistance. If the disorder is strong enough, electron waves become trapped in small regions – localised – and the material stops conducting, even if it would be a metal when perfect. In a sheet one atom thick, disorder in its surroundings matters especially.

Deviations from perfect periodicity – point defects, substitutional or alloy disorder, interface roughness, charged impurities and strain variations in the substrate – that scatter carriers and, beyond a threshold, localise their wavefunctions through interference (Anderson localisation). Scaling theory predicts that non-interacting electrons in two dimensions without spin–orbit coupling are localised by any disorder; its precursor, weak localisation, appears as a logarithmic rise of resistance at low temperature that a magnetic field suppresses, while strong spin–orbit coupling gives weak antilocalisation. In graphene on SiO₂, charged impurities break the sheet near charge neutrality into electron–hole puddles, which hBN encapsulation reduces. Strong disorder leads to hopping conduction, broadens optical lines and smears phase transitions; in frustrated magnets and correlated oxides, structural or cation disorder can mimic exotic states.

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

Chemical bond

The glue that holds atoms together. In a covalent bond two atoms share electrons, as within a sheet of graphene; in an ionic bond one atom hands electrons to another and the resulting charges attract, as in table salt; in a metal the outer electrons are shared by all the atoms at once. Much weaker van der Waals forces hold molecules and the layers of a layered crystal together. A 2D material is a crystal with strong bonds within its layers and only weak ones between them – which is why it can be peeled.

The attractive interactions that hold atoms in molecules and solids, classified by how electrons are shared: covalent bonds, from overlapping orbitals holding shared electron pairs, strong and directional (sp² carbon, Mo–S in TMDCs); ionic bonds, from electron transfer and electrostatic attraction (halides, oxides); metallic bonding, with electrons delocalised in a partly filled band; and the weak, non-directional van der Waals interaction from correlated charge fluctuations, besides hydrogen bonds. Real bonds are mixtures, characterised by electronegativity differences and charge analyses. Layered crystals combine in-plane bonds of several electronvolts with interlayer binding of only about 20 meV per Å² (some 0.3 J/m²), the criterion high-throughput screens use to identify exfoliable compounds.

Full explanation

Metal, semimetal and insulator

The three basic ways a solid treats electricity, set by how its electrons fill the energies open to them. In a metal they fill a band of energies only partly, so the smallest push sets them moving, and it conducts at any temperature. In an insulator they fill a band completely and a wide gap separates it from the next, so nothing can move. A semimetal sits in between: two bands just touch or overlap a little, leaving a small number of mobile electrons and holes. NbSe₂ is a metal, graphene a semimetal, hBN an insulator.

A classification by where the Fermi level falls in the band structure. In a metal it crosses a partly filled band, leaving a Fermi surface, a carrier density of order one per atom (about 10²² cm⁻³) and a resistivity that falls on cooling. In a band insulator it lies in a gap of several electronvolts; a semiconductor is an insulator whose gap is small enough to be doped or gated. A semimetal has a slight overlap between conduction and valence bands (graphite, bismuth, bulk WTe₂) or bands that touch at isolated points (graphene), giving low and nearly equal densities of electrons and holes. Interactions can open gaps that band theory does not predict: Mott insulators from half-filled bands, excitonic insulators from semimetals.

Full explanation

Dielectric screening

The way a material weakens the pull between two charges inside it: its own electrons shift slightly and partly cancel the field between them. A sheet one atom thick has almost nothing around it to do this, so charges in it attract each other far more strongly than in a thick crystal – which is why light makes tightly bound pairs in 2D materials, and why whatever a sheet rests on changes its properties.

The reduction of the Coulomb interaction by the polarisation of bound electrons and ions and, in metals and doped layers, by free carriers, described by a dielectric function ε(q, ω). In a monolayer the field lines between distant charges run through the surroundings, so ε(q) approaches 1 as q → 0 and the screening is non-local and set by the environment. Exciton binding energies reach hundreds of meV, and a change of substrate or encapsulation shifts the quasiparticle gap by 100 meV or more while the optical gap barely moves.

Full explanation

Alloy and solid solution

A crystal in which two similar kinds of atom share the same sites at random – some molybdenum sites taken by tungsten, say, or some sulfur sites by selenium – so that one crystal structure holds a mixture. By choosing the mix, its properties can be set almost anywhere between those of the two pure compounds, much as mixing two paints gives any shade between them. Chemists call it a solid solution; alloy is the everyday word.

A single-phase crystal in which two or more species share a sublattice at random, such as Mo_1−xW_xS₂ or MoS_2(1−x)Se_2x, with lattice constants close to Vegard’s law and band gaps that vary continuously with x, often with a small bowing. Complete miscibility needs similar radii, the same structure type and favourable mixing energetics; otherwise the mixture separates into phases or orders. Random disorder adds alloy scattering and inhomogeneous broadening.

Full explanation

Silicon photonics

Guiding light instead of electricity through tiny channels on a silicon chip, made with the same factories and tools as ordinary computer chips. Light carries data faster and with less loss over distance, so such chips already connect the servers in data centres. Silicon itself is poor at a few jobs – it hardly emits light, cannot detect the infrared light used in fibre networks, and switches light only weakly – and 2D materials laid on top of the light channels are one way to add the missing functions.

Photonic integrated circuits in silicon-on-insulator, in which submicron silicon waveguides, with their high index contrast to the buried oxide, route light at telecom wavelengths through splitters, rings and gratings made with CMOS tools; germanium detectors and silicon carrier-depletion modulators are standard, and lasers are bonded or grown from III–V semiconductors. 2D materials couple to the evanescent field of the waveguide: graphene gives broadband, fast modulators and photodetectors that can be integrated at wafer scale, thin MoTe₂ and black phosphorus emit and detect in the near infrared, and low-loss phase-change Sb₂S₃ and Sb₂Se₃ give non-volatile switches and tuning. The obstacles are wafer-scale transfer or growth, contacts, optical loss and reproducibility to foundry standards.

Full explanation

Resistivity and sheet resistance

Two ways to put a number on how strongly a material resists a current. Resistivity belongs to the material itself, whatever the size of the piece: copper’s is tiny, glass’s enormous. For a sheet so thin that its thickness is hard to pin down, the sheet resistance is used instead: the resistance of a square of it, measured from one edge to the opposite one, which comes out the same for a square of any size. It is quoted in ohms per square.

Resistivity ρ relates electric field to current density, in Ω·m or Ω·cm, so that a bar of length L and cross-section A has resistance ρL/A. For a film of thickness t the sheet resistance is ρ/t, quoted in Ω per square, and a strip L long and W wide has the sheet resistance times L/W. For an atomically thin layer it is the natural quantity, since the thickness is a convention, and it equals 1/neμ for sheet density n and mobility μ. Both are measured with four probes – a Hall bar, a collinear four-point probe or the van der Pauw method for arbitrary shapes – so that the voltage is sensed away from the current contacts and the contact resistance drops out.

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