Single-electron (SE) transfer is a technique for accurately conveying an SE in synchronization with a clock signal, which is expected to be applied to a current standard and to low-power-consumption information processing. For the current standard, high-speed operation (more than gigahertz) with an error rate below 10-8 is necessary. While SE transfer using an electrically formed quantum dot has been widely studied, SE transfer using a trap level with a large activation energy is expected to have higher accuracy. However, the possibility of high-speed trap-mediated SE transfer is not obvious. Here, we report the achievement of high-speed SE transfer at 3.5 GHz using a trap level in Si .
Figure 1(a) shows a schematic of the device. Double-layer polycrystalline-Si gate electrodes on a Si wire with a width of a few ten nanometers were formed using electron beam lithography. Each lower gate (G1, G2) is used to form a potential barrier in the Si wire. The upper gate is used to modulate the potential between G1 and G2. We selected a device that has a single trap level (most likely an interface trap) under the right edge of G1 and measured SE transfer current via the trap level at 17 K. To perform the SE transfer, we modulate a potential barrier by applying a high-frequency signal (frequency f) to G1, with a fixed negative voltage applied to G2 to create another potential barrier [Fig. 1(b)]. An SE is captured by the trap level from the source when the barrier under G1 is low. After that, when the barrier under G1 is high, the captured SE is emitted to the drain. This results in a transfer current level of ef (e is the elementary charge). Detailed measurements reveal that the capture and emission can become fast by lowering the barrier height under G1 during the capture phase and by applying a strong electric field at the trap level during the emission phase, respectively. This can be achieved when the high-frequency signal has a large amplitude. Under this condition, we achieved high-speed operation at 3.5 GHz [Fig. 1(c)], in which the transfer error rate is below the level that can be measured using a commercial current meter (~ 10-3). Theoretically, we found that 1-GHz operation with an error rate below 10-8 may be possible. In the future, we will evaluate the absolute accuracy  of the trap-mediated SE transfer to realize high-accuracy and high-speed SE transfer toward application to the current standard.
This work was partly supported by the Funding Program for Next Generation World-Leading Researchers of JSPS (GR 103).