Single-electron counting statistics and its circuit application in nanoscale field-effect transistors at room temperature

Thermally driven single-electron stochastic resonance

Stochastic resonance (SR) in a single-electron system is expected to allow information to be correctly carried and processed by single electrons in the presence of thermal fluctuations. Here, we comprehensively study thermally driven single-electron SR. The response of the system to a weak voltage signal is formulated by considering the single-electron tunneling rate, instead of the Kramers' rate generally used in conventional SR models. The model indicates that the response of the system is maximized at finite temperature and that the peak position is determined by the charging energy. This model quantitatively reproduces the results of a single-electron device simulator. Single-electron SR is also demonstrated using a GaAs-based single-electron system that integrates a quantum dot and a high-sensitivity charge detector. The developed model will contribute to our understanding of single-electron SR and will facilitate accurate prediction, design, and control of single-electron systems.

On the performance of a photosystem II reaction centre-based photocell

The photosystem II reaction centre is the photosynthetic complex responsible for oxygen production on Earth. Its water splitting function is particularly favoured by the formation of a stable charge separated state via a pathway that starts at an accessory chlorophyll. Here we envision a photovoltaic device that places one of these complexes between electrodes and investigate how the mean current and its fluctuations depend on the microscopic interactions underlying charge separation in the pathway considered. Our results indicate that coupling to well resolved vibrational modes does not necessarily offer an advantage in terms of power output but can lead to photo-currents with suppressed noise levels characterizing a multi-step ordered transport process. Besides giving insight into the suitability of these complexes for molecular-scale photovoltaics, our work suggests a new possible biological function for the vibrational environment of photosynthetic reaction centres, namely, to reduce the intrinsic current noise for regulatory processes.

A Probabilistic Finite State Logic Machine Realized Experimentally on a Single Dopant Atom

Exploiting the potential of nanoscale devices for logic processing requires the implementation of computing functionalities departing from the conventional switching paradigm. We report on the design and the experimental realization of a probabilistic finite state machine in a single phosphorus donor atom placed in a silicon matrix electrically addressed and probed by scanning tunneling spectroscopy (STS). The single atom logic unit simulates the flow of visitors in a maze whose topology is determined by the dynamics of the electronic transport through the states of the dopant. By considering the simplest case of a unique charge state for which three electronic states can be resolved, we demonstrate an efficient solution of the following problem: in a maze of four connected rooms, what is the optimal combination of door opening rates in order to maximize the time that visitors spend in one specific chamber? The implementation takes advantage of the stochastic nature of electron tunneling, while the output remains the macroscopic current whose reading can be realized with standard techniques and does not require single electron sensitivity.

Single-electron thermal noise

We report the observation of thermal noise in the motion of single electrons in an ultimately small dynamic random access memory (DRAM). The nanometer-scale transistors that compose the DRAM resolve the thermal noise in single-electron motion. A complete set of fundamental tests conducted on this single-electron thermal noise shows that the noise perfectly follows all the aspects predicted by statistical mechanics, which include the occupation probability, the law of equipartition, a detailed balance, and the law of kT/C. In addition, the counting statistics on the directional motion (i.e., the current) of the single-electron thermal noise indicate that the individual electron motion follows the Poisson process, as it does in shot noise.

Observation of single electron transport via multiple quantum states of a silicon quantum dot at room temperature

Single electron transport through multiple quantum levels is realized in a Si quantum-dot device at room-temperature conditions. The energy spacing of more than triple the omnipresent thermal energy is obtained from an extremely small ellipsoidal Si quantum dot, and high charge stability is attained through a construction of the gate-all-around structure. These properties may move us a step closer to practical applications of quantum devices at elevated temperatures. An in-depth analysis on the transport behavior and quantum structure is presented.

Donor-based single electron pumps with tunable donor binding energy

We report on single electron pumping via a tunable number of individual donors. We use a device that essentially consists of a silicon nanowire with local arsenic implantation between a set of fine gates. A temperature-dependent characterization of the pumped current allows us to extract the ionization energy of a single arsenic donor. We observe the ionization energy to be tunable by the gate electric field over a large range of energies.

Device variability and circuit redundancy in signal processing based on nanoswitches

Signal processing based on molecular switches whose conductance can be tuned by an external stimulus between two (on and off) states has been proposed recently (Cervera et al 2008 J. Appl. Phys. 104 084317). The basic building block is a metal nanoparticle linked to two electrodes by an organic ligand and a nanoswitch. The net charge delivered by this nanostructure exhibits a sharp resonance when the alternating potential applied between the electrodes has the same frequency as the periodic variation between the on and off conductance states induced on the nanoswitch. This resonance can be used to process an external signal by selectively extracting the weight of the different harmonics. However, because of the fabrication process at the nanoscale, the nanostructures will show a significant variability in the physical characteristics. By using a phenomenological model that includes this variability, the stochastic nature of electron transference, and the thermal noise, we demonstrate that reliable signal processing can still be achieved by adapting the number of nanoswitches per bit of information (circuit redundancy) to the nanostructure tolerance (device variability). Extensive kinetic Monte Carlo simulations show that a moderate level of redundancy can compensate for significant nanostructure variability. This result gives support to the concept of ensembles of redundant switches as reliable components for signal processing at the nanoscale.