Observation of discrete dopant potential and its application to Si single-electron devices

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Abstract

Single-electron devices are attractive because of their ultimate capabilities such as single-electron transfer, single-electron memory, single-photon detection and high sensitivity to elemental amount of charge. We studied single-electron transport in doped nanoscale-channel field-effect transistors in which the channel potential is modulated by ionized dopants. These devices work as arrays of quantum dots with dimensions below present lithography limits. We demonstrate the ability of dopant-induced quantum dot arrays to mediate the transfer of individual electrons one at a time (single-electron transfer). We also monitored the actual dopant distribution and observed single dopant potentials using low temperature Kelvin probe force microscopy.

Introduction

Single-electron devices (SEDs) are attractive candidates for future electronics due to their ultimate capabilities of controlling elementary charge transport and due to their ultrahigh sensitivity [1]. Applications such as single-electron transfer [2], [3], [4], single-electron memories [5], single-photon detection [6], and detection of single dopant ionization [7] have been already proposed and demonstrated in devices containing quantum dots (QDs). Reducing the size of the QDs to nanometer scale is expected to improve the capabilities of SEDs from the viewpoints of operation temperature and speed. However, this reduction is strongly relying on progress in nanolithography techniques.

Different from the above-mentioned approach, we have recently investigated the properties of doped-nanowire field-effect transistors (FETs) in which QD arrays are formed by the superposition of individual dopant potentials. We have demonstrated the capabilities of these structures to realize single-electron transfer operation, i.e., to mediate the time-correlated transfer of single electrons under the application of gate pulses [8]. We have also found by extensive simulations that single-electron transfer ability is enhanced for three-QD systems, particularly when the central dot is larger [9]. These results provide important insights for future design optimization of single-electron transfer devices utilizing dopant potentials. Monitoring the discrete dopant distribution at nanoscale is an important component of such designs. We have utilized a technique based on low temperature Kelvin probe force microscopy to detect individual dopant potential in nanoscale devices even under operating conditions [10]. In this paper, we are describing these recent findings focusing on the importance of the interplay between single electrons and individual dopant atoms.

Section snippets

Dopant-induced quantum dot arrays

Dopants have been an essential component of electronic devices playing the role of extrinsic charge providers for transport. They are also used to create the p–n junctions necessary inside the FETs — the basic building block of the electronic industry. However, as FET channel dimensions are brought down to only few tens of nanometers, the number of dopants in the channel is strongly reduced and their discrete distribution starts to play a significant role in device operation [11]. On the other

Single-electron transfer in doped-nanowire FETs

Single-electron transfer [2], [3], i.e., shuttling of only one electron between source and drain electrodes every cycle of ac gate voltages, is one of the key issues for future electronics. This would open the path to utilizing individual electrons for conveying one bit of information in logic or memory circuits. Another application of interest is developing a current metrological standard which is currently not a stand-alone one [17]. Realizing this ordered function based on the

Direct observation of individual dopant potentials by low temperature KFM

Monitoring the dopant distribution in nanoscale channels is of paramount importance considering the fact that at these small dimensions the discreteness of the dopants plays a significant role on determining the device parameters. Several methods are available for shallow dopant profiling or carrier concentration evaluation, such as scanning tunneling microscopy (STM) [20] or scanning capacitance microscopy (SCM) [21]. These methods have, however, some important drawbacks: STM can only provide

Summary

At present Si nanodevices are evolving toward single-atom sizes. For single-electron applications, an attractive direction of research concerns transport properties of individual dopant potentials. This may lead to a new era for electronic devices, completely different from classical devices that utilize statistical and continuous nature of matter due to the huge number of dopants involved in the operation. For future Si nanodevices, discreteness of both transferred and fixed charges will play

Acknowledgements

This work was partially supported by MEXT KAKENHI (16106006 and 18063010). The authors wish to thank Dr. Y. Ono from NTT Basic Research Laboratories, Prof. H. Inokawa and Assoc. Prof. H. Ikeda for their valuable contributions.

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