Background:
In order to perform measurements of physical quantities, record and compare them in a consistent way, systems of units and standards have been historically developed and agreed upon. At first, the reference standards were based on artefacts or prototypes, but over the last few decades the need for increasingly high accuracy and stability has determined a shift towards standards based on physical phenomena and true constants of nature. Nanoscale systems based on the principles of quantum mechanics are today acknowledged as the most stable and reliable metrological tools, as they can be strongly intertwined with fundamental constants. Exquisitely quantum-mechanical phenomena such as the Josephson Effect and the Quantum Hall Effect have paved the way toward new and more reliable reference standards for the units of voltage and resistance, respectively. Presently, a major research challenge is to find a realization of the electric current in terms of elementary charge, which is the charge of an electron. This will lead to a quantum-based standard for the unit of current, i.e. the ampere.
At present, vast research efforts exploit the discreteness of single electrons to transfer a controlled number of elementary charges and, thus, achieve highly accurate currents. Electronic nanodevices that allow single-electron clocking are known as charge pumps and are routinely used as stable sources of small currents. The most successful generation of these devices makes use of electrostatically-defined quantum dots, which are small regions of semiconductor material where individual electrons can be trapped and released at will. The main identified challenges of this approach are the deterministic initialization of the number of electrons in the dot, and the suppression of pumping errors as the operational speed rises to increase the level of the generated current. Both instances are ultimately related to the electron confinement in the quantum dot. In practice, it has been shown that the tighter the charge confinement within the dot, the better the pump’s performance can be.
Objectives:
The aim of this project is to develop novel nanotechnology to address the issue of confinement in single-charge pumps and, therefore, advance the state-of-the-art of highly accurate current generation. The main aim is to realise ambipolar silicon-based pumps with industrially-compatible metal-oxide-semiconductor technology. Tunable confinement will be attained by means of purposely designed gate electrodes, as well as by implementing controlled transfers of single electrons and single holes at will. By comparing the device performance in different operation modes, detailed information on the effect of confinement will be gathered. Furthermore, conventional transport-based measurement techniques will be validated via a nascent remote sensing technique called radiofrequency reflectometry.
Conclusions:
The project has been successful in producing novel nanotechnology and improve the state-of-the-art of single-charge electronics. In particular, a factor of 3 improvement in the accuracy of current generation has been demonstrated via silicon single-electron pumps. Furthermore, the state-of-the-art of charge detection sensitivity with gate-based reflectometry techniques has been improved by a factor of about 30.
