Single-electron sensing for silicon-based charge pumps

Abstract

There is currently considerable international effort aimed at utilising single electron tunnelling effects to develop a highly accurate electrical current standard that is dependent only on the electron charge (e) and a highly accurate frequency (f) standard, by transferring one electron at a time at a frequency in the GHz range. However, the accuracy requirements remain challenging due to measurement limitations. In this thesis, we focus on a silicon (Si) based single-electron pump with an in-situ Al-SET (aluminium single-electron transistor) charge sensor for error detection, which avoids many limitations present in direct current measurements. We aim to design and fabricate these integrated devices for high-fidelity charge sensing. Firstly, we designed and compared device structures using modelling and simulation. The simulation methods were verified by comparing simulations with experimental results on earlier generation pumps. This enabled identification of a new design, incorporating an Al-SET sensor with up to forty times greater sensitivity than that of previous pumps with Si-SET sensors. Next, we fabricated stand-alone Al-SETs of the selected design. We found that when using a fabrication process previously used in our laboratory, only 30% of the batches had complete and clear metal structures. By adding an O2 plasma ash and modifying the evaporation process, 90% of the batches were then free from this problem. These Al-SETs showed good superconducting operation when tested at millikelvin temperatures. The measured conductance of 9.2 nA/e is excellent for charge sensing purposes, and almost an order of magnitude better than the Si-SET sensor in the old design. Finally, we fabricated three batches of integrated devices, incorporating both a Si-MOS pump and Al-SET sensors. In these initial trials, we used 15nm-thick gates, and demonstrated successful functioning of such thin metal gates. The low yield of the Al-SETs meant that we could not demonstrate a device with full pump and sensor functionality. However, the work done in this thesis provides a good foundation for the project. Further investigations will be required to increase the yield of Al-SETs to achieve full device functionality and demonstrations of high-accuracy error counting.