Manipulation of Elementary Charge in a Silicon Charge-Coupled Device
Akira Fujiwara and Yasuo Takahashi
Device Physics Laboratory
Single-electron (SE) devices have been attracting much attention in light of the underlying physics and the application to future integrated circuits. Though the manipulation of single electrons becomes possible in the so-called SE pump consisting of coupled conductive islands, the fabrication is no easy because it needs the integration of multiple tunnel capacitors. The SE manipulation has not yet been performed in Si devices. Recently we have developed a novel-type SE device oriented for large-scale integration ; it is the SE charge-coupled device (CCD), in which a single hole was stored and transferred at 25K. For sensing the hole, we demonstrate a new scheme based on the electron-hole system in Si nanostructures.
Figure 1(a) shows a top-view SEM image of the device. It is a closely packed array of two Si-wire MOSFETs with fine poly-Si gates. The Si wire is T-shaped so that each end is connected to three n-type electrodes, enabling us to detect the electron currents through each MOSFET separately. Figure 1(b) and (c) describe how the single hole is stored and transferred between the two Si channels, and is sensed by the electron currents. The hole is generated by illumination. Its storage and transfer are done simply by applying the negative voltages to the poly-Si gates and controlling them like the CCD. For the sensing, the hole and the electrons are kept apart by the large electric field across the Si wire, and therefore do not recombine soon. Since the wire diameter is as small as 15-20 nm, the electron current is highly sensitive to the number of holes (nh), thus realizing the elementary-charge sensitivity.
The results for the single-hole manipulation are shown in Fig. 2. The sensing and the transfer of the single hole were repeated a few times. The sensing currents alternately have a high level. This means that the single hole was successfully transferred back and forth between the two Si-wire MOSFETs.
 A. Fujiwara and Y. Takahashi, Nature 410 (2001) 560.
Fig. 1. (a) Top-view SEM image of the device.
(b) Transfer and sensing of the single hole.
(c) Cross-sections of the Si wire.
Fig. 2. Manipulation of a single hole between the two MOSFETs.
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