Umklapp electron-electron scattering in so-called moiré superlattices made from aligned layers of 2D materials could degrade the high-temperature mobility of charge carriers. That’s the conclusion of a new study by researchers in the UK, US and Japan, who say the effect would limit the potential applications of these technologically important materials in high-mobility devices operating a room temperature. The problem could be overcome, however, by misaligning, or twisting, the 2D layers with respect to each other.
Umklapp electron-electron (Uee) scattering is the process that gives pure metals an electrical resistance, explain the research team, which was led by Vladimir Fal’ko and Andre Geim of the National Graphene Institute at the University of Manchester in the UK. It is the only intrinsic mechanism that allows electrons to transfer momentum to the crystal lattice, but it is difficult to measure in an experiment because it is often masked by other dissipation phenomena.
Fal’ko and colleagues have now shown that Uee scattering dominates the transport properties of superlattices that are made by placing layers of graphene on top of hexagonal boron nitride (hBN). Uee processes in these heterostructures lead to giant excess resistivity that rapidly grows as the period of the superlattice increases, causing their room-temperature mobility to plummet by more than an order of magnitude when compared to ordinary, non-superlattice graphene devices.
‘Twistronics’ tunes 2D material properties
The researchers engineered their superlattices by aligning graphene with an hBN substrate. This process produces a moiré pattern as a result of the small lattice mismatch (of 1.8%) between the two materials. The moiré lattice produces a potential with a period of around 15 nm when the two materials are perfectly aligned, and this periodic potential dramatically alters the material’s electronic properties – transforming it from a “zero-gap” semiconductor to one that does have a band gap.
In a zero-gap semiconductor, the electron valence and conduction bands just touch each other at the so-called Dirac point, while conventional semiconductors have an energy gap between the bands. At the Dirac point, the relationship between the energy and momentum of the electrons can be described by the Dirac equation and resembles that of a photon, with the electrons moving at a very high, relativistic, speed.
One of the most important things to happen in a moiré superlattice is the creation of a mini Brillouin zone around the Dirac point. The size of this zone depends on the misalignment angle between the graphene and hBN and the resulting moiré period. Since the Brillouin zone is small compared to that in normal metals, Uee scattering is the dominant effect in graphene/hBN superlattices, say the researchers, who obtained their result by measuring the resistivity of different superlattice devices and a “reference” device in which the graphene and hBN were intentionally misaligned by more than 15°. This device had a moiré period of less than 3 nm.
“Uee scattering in long-period moiré superlattices degrades the intrinsic high-temperature mobility of graphene’s charge carriers,” they report in Nature Physics 10.1038/s41567-018-0278-6. “This limits the potential applications of epitaxially-grown graphene/hBN heterostructures, which are inherently aligned, for room-temperature high-mobility devices.”
The researchers advise that the 2D crystals that form the heterostructure should be misaligned, or twisted, to achieve high carrier mobility at room temperature.
