Devices – Page 4

Modified graphene

Graphene has been touted as the new wonder material for electronics. As we have observed earlier, the excitement is due to the extraordinarily high carrier mobilities, among other interesting properties [1]. Unfortunately, graphene has no band gap, so it must be chemically or physically modified to become a useful semiconductor.

Chemical doping is developing apace, with electron donors (like alkali metals) [3] and hole donors (like bismuth and gold) [2] now experimentally established. Equally interesting is hydrogen “doping” (see references in [1]), realized by reacting graphene with hydrogen. This reaction has generally been carried out globally, but it occurs to me that one might use beam technology to drive localized reactions: proton beam, atomic hydrogen beam, or e-beam induction à la Zeiss MeRiT. All these procedures result in simultaneously splitting the valence and conduction bands and moving the fermi level.

Very recently, Chen’s group at Purdue reports a solid-state reaction under 30 kV e-beam bombardment of graphene on a substrate [4]. This reaction likewise modifies the band structure. (The authors claim that the dose, ~1800 μC / cm^2, is “typical” SEM exposure.)

Physically restructuring graphene also results in changes to its electrical properties, due to edge states (akin to surface states in 3-dimensional materials). Extremely high e-beam doses (~30 C / cm^2 at 200 kV) result in material removal from the beam impact point. Drndić’s group reports [5] creating holes and lines in suspended graphene by this method. The review by Krasheninnikov and Nordlund [6] is a highly valuable resource in regard to such physical restructuring of materials. The shorter review [7], also by Krasheninnikov, focuses on carbon materials.

References:

[1] Berashevich, Chakraborty, “Graphene and graphane: New stars of nanoscale electronics”, arXiv:1003.0044.
[2] Gierz, et al., “Atomic Hole Doping of Graphene”, arXiv:0808.0621.
[3] Ohta, et al., “Controlling the Electronic Structure of Bilayer Graphene”, Science 313, 951 (2006).
[4] Childres, et al., “Effect of electron-beam irradiation on graphene field effect devices”, arXiv:1008.4561.
[5] Fischbein, Drndić, “Electron Beam Nanosculpting of Suspended Graphene Sheets”, arXiv:0808.2974.
[6] Krasheninnikov, Nordlund, “Ion and electron irradiation-induced effects in nanostructured materials”, J Appl Phys 107, 071301 (2010).
[7] Krasheninnikov, Banhart, “Engineering of nanostructured carbon materials with electron or ion beams”, Nature Materials, 6 (2007) 723-33.

Diverse ramifications of geometric phase

Geometric phase (or Pancharatnam-Berry phase) has many surprising effects. What is this variously named “phase”? To quote Wikipedia, “The Berry phase occurs when [two] parameters are changed simultaneously but very slowly (adiabatically), and eventually brought back to the initial configuration.” [1] A light-hearted and easily understood introduction can be found in section 1 of ref. [2]. In what follows, I give a few examples of geometric phase effects that are relevant to nano-technology. (A lengthy review of the Berry phase in electronics can be found in [3].)

A group at TU Denmark have shown in [4] that the Berry phase effects associated with electrical current flowing through a conductive molecular bridge may induce mechanical vibration sufficiently strong to rupture the bridge. This phenomenon is unrelated to electromigration, Joule heating, or other well-known effects. It is purely a result of the quantum mechanical phase of the electric waves.

In an interesting experiment [5], Kohmura-san and colleagues at RIKEN demonstrate millimeter distance translation of X-rays by bent silicon crystals. The crystal was bent only 80 nm or so to achieve a beam displacement of 1.5 mm. See [6] for a brief summary.

The Hasman group at the Technion have constructed optical structures showing technologically applicable geometric phase behavior. In [7] and [8] they display nano-lithographically defined gratings and apertures which can act as optical switches, among other things.

Try the little demonstration in [2]. It will serve as a reminder to keep your eyes open to the geometric phase.

References:

[1] Wikipedia article, http://en.wikipedia.org/wiki/Berry’s_phase
[2] Robert W. Batterman, “Falling Cats, Parallel Parking, and Polarized Light”, http://philsci-archive.pitt.edu/archive/00000794/00/falling-cats.pdf
[3] Di Xiao, et al., “Berry Phase Effects on Electronic Properties”, Rev. Mod. Phys. 82 (2010) 1959-2007, http://rmp.aps.org/abstract/RMP/v82/i3/p1959_1
[4] Jing-Tao Lu, et al., “Blowing the Fuse: Berry’s Phase and Runaway Vibrations in Molecular Conductors”, Nano Letters 10 (2010) 1657-63, http://pubs.acs.org/doi/abs/10.1021/nl904233u
[5] Yoshiki Kohmura, et al., “Berry-Phase Translation of X Rays by a Deformed Crystal”, Phys. Rev. Lett. 104 (2010) 244801, http://prl.aps.org/abstract/PRL/v104/i24/e244801
[6] Adams, “Geometric phase kicks x rays down a new path”, http://physics.aps.org/articles/v3/50
[7] Erez Hasman, et al., “Polarization beam-splitters and optical switches based on space-variant computer-generated subwavelength quasi-periodic structures”, Optics Communications 209 (2002) 45-54, DOI: 10.1016/S0030-4018(02)01598-5, http://www.sciencedirect.com/science/article/B6TVF-4645DRF-1/2/6f0b7ca5cf17b751e48bbf1cf1e77d03
[8] Yuri Gorodetski, “Observation of Optical Spin Symmetry Breaking in Nanoapertures”, Nano Lett., Vol. 9, No. 8, 2009, http://pubs.acs.org/doi/abs/10.1021/nl901437d

Nano-device challenges

As electronic structures become smaller, it appears that the challenges become larger. (Remember the predictions that optical lithography was obsolescent? We have heard this for some 30 years or so.) Our attention usually focuses on transistor gate lengths, oxide thicknesses, etc. The cases of nano-dot junctions and nano-wires may be rather different.

Firstly, it is hard to know what it is we make, when we create a particle that is only a few atoms wide. As this reference explains [1], standard methods like X-ray or neutron scattering work well for bulk, crystalline materials but not for clusters or for amorphous solids. However, by introducing plausible constraints and by utilizing multiple sources of experimental and theoretical information, one may obtain an average or representative structure. But doing so is by no means a trivial undertaking. I think that similar results obtain for the case of nano-wires as well.

Secondly, our understanding of devices must evolve at the nano-scale. The instantaneous electron temperature of a nano-device plays a huge role in the device noise. However, the electron temperature in a nano-device often differs greatly from the lattice temperature. The paper here [2] argues that at the transition between single-electron conduction and mixed single- and multiple-electron conduction, the signal/noise ratio becomes exceptionally small, far more so than one would expect by considering the lattice temperature alone.

In conventional devices, we tend to see digital transistors switched as rapidly as possible in order to maximize speed and to avoid the “unknown” state. But in a nano-scale device, this is exactly the wrong thing to do! This paper [3] shows that nano-transistors must be switched adiabatically in order to avoid driving the device into high-frequency oscillation. We have yet to learn how slowly the switching must be in order to be considered adiabatic.

Circuit designers and manufacturers have worked around electromigration failure, largely by controlling uniformity, roughness, and material composition, and by giving every conductor some extra size margin. If we think of scaling our wires down to a few nanometers, other challenges arise.

When the radius of a silicon nano-wire is somewhere around 10nm or smaller, we expect significant divergences from bulk electronic behavior, according to this review [4]. For example, the scattering radius of an impurity (be it defect or dopant) may be as large as the radius of the nano-wire. This results in very low longitudinal conductivity, since electrons completely backscatter from the impurity.

Toshiba seems to have made a significant advance by creating a nano-wire transistor channel with dimensions around 14nm X 20nm [5]. This is a bit larger than our discussion above, but not much. Review [4] also contains a discussion of why particular crystal orientations within the nano-wire are much preferred over other orientations. Toshiba apparently chose to run their channel along the <100> direction.

We should also expect, as a result of the high surface to volume ratio of a nano-wire, that the mechanical properties also differ from those of the corresponding bulk materials. This experimental study of gold wires [6] warns us of the difficulties that lie ahead for semiconductor devices design and manufacturing.

[1] http://arxiv.org/abs/0912.1971
(Synopsis) http://physics.aps.org/articles/v3/25
[2] http://arxiv.org/abs/0912.2832
(Synopsis) http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.104.196805
[3] http://arxiv.org/abs/0911.3870
(Synopsis) http://physics.aps.org/articles/v3/47
[4] http://arxiv.org/abs/0910.2553
[5] http://www.toshiba.co.jp/about/press/2010_06/pr1501.htm
[6] http://arxiv.org/abs/0910.4139