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Right-click to download interview with Vyacheslavs Kashcheyevs (9.7 MB MP3)

“Nanoelectronics has arrived,” says Vyacheslavs Kashcheyevs, whose research at the University of Latvia focuses on quantum transport. He has a strong argument. Tech-savvy consumers checking the specifications of a potential smartphone or PC purchase are looking at devices where the basic unit of the circuit measures just tens of nanometres. At a talk at Euronanoforum 2015 he described how in that conference room alone there were billions of nanoparticles under precise electronic control. So what would it mean to scale that from nanoparticle to atom?

A lot of existing nanoelectronics uses nanotransistors based on quantum dots, also referred to as ‘artificial atoms’ because of the way electron behaviour confined on these semiconductor islands resembles atomic orbitals. But as Kashcheyevs tells, electronics is full of bona fide atoms used as dopants and it is one of the aims of the SiAM project (Silicon at the Atomic and Molecular scale) to exploit the atomic nature of dopants thoughout microelectronics using ICT devices and circuits. “So the challenge is to have single atom transistors operated in a well defined and controlled way.”

Single electrons in mainstream technology

The potential to manipulate single atoms and electrons has caught the imaginations of several research groups working on novel computing paradigms. Back in the 1990s, Supriyo Bandyopadhyay, Biswajit Das and Albert Miller at the University of Notre-Dame in France described how to inscribe and manipulate ‘bits’ of logic information in the spin of single electrons. Among the advantages of this computing architecture, they list speed, information density, robustness and power efficiencies. Other groups have also studied how control of single electrons may benefit quantum computing.

Yet alongside some of the more exotic next-generation computing technologies attracting attention, mainstream electronics also has a fundamental interest in scaling electronics down to single charges. Kashcheyevs describes this as “the final frontier” that industrial electronics companies are fighting for to keep expanding Moore’s Law. Given industry’s role in driving these developments, it is no surprise that the approaches adopted to tackle the challenge of fabricating these systems differ little from standard CMOS approaches, although there are some tricks to successfully scaling down to this degree.

Simply reducing the size of quantum dots to existing fabrication limits runs into difficulties as the quantum confinement becomes increasingly sensitive to impurities and roughness in the quantum dot. Enrico Prati from Laboratorio MDM in Italy, Marc Sanquer from the Commissariat á l’Énergie Atomique Grenoble and Université Joseph Fourier in France and colleagues at Universität Tübingen in Germany, Delft University of Technology in the Netherlands and the University of New South Wales in Australia use a doping gradient to elicit single-electron characteristics.

The European collaboration used electron-beam lithography, etching and thermal treatments to produce a silicon nanowire in silicon oxide, and then an oxide gate on three sides. They then introduce spacers of silicon nitride with modulated arsenic doping at the source and drain gates so that these gates do not overlap with the transistor. The researchers can then home in on the characteristics of single dopants from the modulated doping profile.

Studies of the conductance for different voltages applied to the device demonstrated quantized electronic transport that was further validated by simulations. As Prati and Sanquer conclude in their report of the work, “The fabrication of single electron transistors in CMOS technology provides the possible ground for integration between traditional microelectronics and quantum circuit oriented nanoelectronics.”

Kashcheyevs also highlights the potential for using these types of structures for investigating quantum effects. “Atoms in the highest occupied band may be close to the conduction band of the surrounding semiconductor material, and that makes their wave functions quite extended to tens and hundreds of Bohr radii in size,” he explains. “That allows us to bridge the order of magnitude between the atomic scale and the technologically controllable nanoworld.”

Thermodynamics of shrinking systems

The quantum statistics of systems change when the number of particles is very few, and can be described by the theoretical model put forward by Enrico Prati analysed in 2009. The divergence from the familiar thermodynamical characteristics of many body systems raises some interesting questions, such as – what actually happens to energy lost as heat? Kashcheyevs also tentatively posits the idea of evaporative cooling in hot single-electron systems, emphasising the debate as to what that might mean. The states of matter that single-electron systems or condensates might resemble are also an area of keen interest for researchers.

“In the old days of statistical physics and thermodynamics, those were hypothetical questions, and any conclusions to be tested would have to be inevitably macroscopic,” says Kashcheyevs. “Now being able to control those particles on an individual level allows you to test these predictions.”

On a practical level, thermal effects may complicate some of the measurements that aim to use single-electron transport experiments to quantify the electron charge with unprecedented precision for a future SI system of units. Driving electrons around nanoelectronic systems requires relatively strong signals, and this in turn introduces additional energy into the system so that care is needed to distinguish thermally induced electron escape from charge traps from purely quantum-limited transport.

In their recent review, Bernd Kaestner at the Physikalisch-Technische Bundesanstalt in Germany and Kashcheyevs describe how semiconductor quantum dots with tunable tunnelling barriers can yield robust and accurate quantized charge pumps. Traditionally, an excessive signal amplitude would be expected to compromise any quantized response in a nanoelectronic system. However, the work that Kaestner and Kashcheyevs overview shows how to improve the quantum characteristics in the system’s statistical dynamics by introducing a level of tunability in adiabatic single-electron pumping.

“It is those connections to stochastic thermodynamics and non-equilibrium thermodynamics that we are eager to explore again,” explains Kashcheyevs. “And what is great is one group may work on something and the results may find unexpected application in a new area.”

The breadth of impact of a particular advance in scientific research is a widely accepted signature greatness, and with the broad range of disciplines touching on single-electron technologies, it is easy to understand the excitement these developments generate. In the words of Stephen Hawking, “Science is beautiful when it makes simple explanations of phenomena or connections between different observations.”

Further details on non-adiabatic quantized charge pumping with tunable-barrier quantum dots are reviewed in Reports on Progress in Physics.