Devices

Atomic scale modelling at Synopsys

As devices become smaller, quantum effects come to dominate so completely that one must think in quantum terms from the beginning. Whether one’s concern is device operation (consider tunneling) or device creation (consider EUV exposure of resist), quantum mechanics is essential.

QuantumATK is a multi-scale atomic and electronic modelling package available under license from Synopsys. QuantumATK has its origins in the former QuantumWise, which in turn was based on the Atomistix package. It covers the range of DFT and tight-binding methods for electronic structure calculations, electron transport models with electron-phonon and electron-photon interactions, to force field models for atomic motion.

This post is prompted by the appearance of a technical overview on the arXiv: arXiv:1905.02794. See also, “From QuantumWise to QuantumATK” at Synopsys.

Atomically precise GNR-FET gates

Continuing work first appearing in 2013 [1], the Bokor group at UC Berkeley has demonstrated FETs whose graphene nano-ribbon gates are precisely 9 and 13 atoms wide [2]. While the gate lengths are still defined lithographically, this line of development shows that two of the channel dimensions (width and thickness) can be exactly controlled. Some work remains to control device characteristic dispersion, but one expects that the chemical synthetic approach to gate definition is sound.

References:
[1] Bennett, et al., “Bottom-up graphene nanoribbon field-effect transistors”, Appl. Phys. Lett. 103, 253114 (2013), http://dx.doi.org/10.1063/1.4855116.
[2] Llinas, et al., “Short-Channel Field Effect Transistors with 9-Atom and 13-Atom wide Graphene Nanoribbons”, arXiv:1605.06730.

Single molecule transistor not so simple

In an impressive piece of experimental work, researchers have created transistors utilizing a single phthalocyanine molecule (gate) sitting atop a cleaved InAs surface (drain) and probed by a cryogenic STM tip (source). The data reveals a much larger band gap than can be explained purely electronically. Imaging the phthalocyanine along with its InAs host reveals that the charge state of the phthalocyanine, induced by electron transfer from the tip, causes the phthalocyanine to rotate relative to the InAs lattice. The authors explain that the phthalocyanine is bound to the InAs surface only through van der Waals forces. One might therefore expect similar behavior in molecular scale systems with weak binding.

Ref.: J Martinez-Blanco, et al., “Gating a single-molecule transistor with individual atoms”, Nature Physics 11, 640–644 (2015); reprinted as arXiv:1603.00908.

Moly-disulfide/graphene transistors over wide area

While transition metal dichalcogenides, graphene and related two-dimensional materials have been much researched, not so much has been done toward manufacturing active devices over large areas using these materials. The situation is changing with the posting of a preprint jointly authored by researchers from Berkeley and Cornell in December of last year. After transferring graphene to an SiO₂ substrate, gaps (to become the transistor channels) were opened photolithographically. Next, MoS₂ was grown into the channels by a CVD process. Finally, top gates were patterned over the channels and contacting electrodes over the graphene to complete the field-effect transistors.

The authors convincingly demonstrate that the MoS₂ grows only in the channels and not atop the graphene, postulating that MoS₂ growth occurs inward from the graphene edges. If nothing else, this self-aligned growth eliminates a host of error sources affecting channel length. It also results in a manufacturing flow which procedurally differs from standard silicon-based device production only in the initial transfer of the graphene onto the substrate.

Reference: M Zhao, et al., “Chemical assembly of atomically thin transistors and circuits in a large scale”, arXiv:1512.05016.

Sub-10 femtosecond electron point source

In a cooperative development between the Fritz-Haber Institute (Berlin) and the University of Colorado (Boulder), researchers have demonstrated an electron source whose temporal resolution lies below 10 fs. In their (electron) lens-less system, they couple brief laser pulses to a grating on the side of a metallic cone, generating surface plasmon polaritons which focus the pulse energy to the conical tip, whereupon electron emission occurs from the tip alone.

In addition to various measurements of the emission itself, the authors demonstrate scans over an isolated InP nanowire, in which the P/N junction is clearly visible while the scanning tip is 3 µm distant.

Reference: M Müller, et al., “A nanofocused plasmon-driven sub-10 femtosecond electron point source”, arXiv:1512.07037.