High-speed Single-hole Transfer in a Si Tunable-barrier Pump

Gento Yamahata, Takeshi Karasawa, and Akira Fujiwara
Physical Science Laboratory

　A single-electron (SE) pump can transfer SEs in synchronization with a clock signal. It is expected to be used for low-power-consumption devices, current standards, and single-photon sources. In particular, high-accuracy and high-speed operation is necessary for the current standard, but there is so far no device that has a performance suitable for practical use (gigahertz operation with an error rate of less than 10^-8). A tunable-barrier pump is a promising device, with which we can achieve gigahertz SE pumping [1]. One factor determining its accuracy is the effective mass of charge carriers. We expect an accuracy improvement by using a hole (electric charge e), which has a heavy effective mass. However, there is no report on high-speed single-hole (SH) transfer. Here, we report the achievement of high-speed SH transfer using a Si tunable-barrier pump [2].
　Figure 1(a) shows a schematic of the device. It has a double-layer gate structure on a Si wire. S and D are heavily doped to form p-type leads. We apply negative voltage V_UG to the upper gate (UG) to generate holes in the Si wire and apply positive voltages to the two lower gates (G1, G2) to form hole potential barriers. As a result, there is an SH island in the Si wire between G1 and G2. In addition, we apply a high-frequency signal with frequency f to G1 to transfer SHs from S to D [Fig. 1(b)]. When the barrier under G1 is low, holes are loaded to the island. Since the island potential rises when the barrier is raised because of a capacitive coupling, the loaded holes escape to S. However, when the rise rate of the barrier is much larger than the escape rate, holes are captured by the island at a non-equilibrium state. The captured holes are ejected to D. Since our device has a very small island, SH addition energy E_add is much larger than the energy of thermal fluctuations. In this case, a rate of SH escape from the island with two holes can be much larger than that from the island with one hole, leading to capture of an SH in the island. When the number of transferred holes is n, the current is nef. The n can be tuned by changing the island potential by applying V_UG.
　Figure 1(c) shows a measurement result for the current at 1 GHz (red circles). We observe a current plateau with decreasing V_UG. A fit using a theoretical model of the non-equilibrium hole capture agrees well with the data (blue curve), indicating non-equilibrium hole transfer. From the fit, the transfer error rate is estimated to be about 10^-3 at 17 K. A theory predicts that a transfer accuracy of 10^-8 can be achieved at 9 K. These results pave the way for accurate manipulation of SHs and its application to metrological standards.