Gento Yamahata

Gigahertz single-trap electron pumps in silicon

Manipulation of single electrons is the key to developing ultimate electronics such as single-electron-based information processors and electrical standards in metrology. Especially, high-frequency and high-accuracy single-electron pumps are essential to realize practical current standards. While electrically defined quantum dots are widely used to build single-electron pumps, a localized state in semiconductors is also a potential candidate for accurate pumps because it can have a large activation energy for the captured electron. However, the transfer mechanism of such localized-state-mediated single-electron pumps for high-accuracy operation at a high frequency has not been well examined. Here we demonstrate a single-electron pump using a single-trap level with an activation energy of a few ten millielectron volts in Si nanotransistors. By means of gate control of capture and emission rates, the pump operates at a frequency of 3 GHz with an accuracy of better than 10−3 at 17 K, indicating that an electric field at the trap level lowers the capture and emission time to less than 25 ps.

Control of Inter-Dot Electrostatic Coupling by a Side Gate in a Silicon Double Quantum Dot Operating at 4.5 K

We report on electron transport measurements of a lithographically-defined silicon double quantum dot (DQD) coupled in series with a top gate and side gates. The structure of the top gate coupled uniformly to the DQD is suitable for realizing a few-electron regime. The obtained small DQD enables us to observe a clear honeycomb-like charge stability diagram at a temperature of 4.5 K. The validity of the DQD structure is confirmed by theoretical calculations. Furthermore, we demonstrate successful modulation of the inter-dot electrostatic coupling by the side gate. Externally tunable coupling is essential for practical implementation of spin-based quantum information devices.

Control of electrostatic coupling observed for silicon double quantum dot structures

We study the electrostatic coupling in the silicon double quantum dot (DQD) structure as a key building block for a chargebased quantum computer and a quantum cellular automaton (QCA). We realize the three interdot coupling regimes of the DQD structure only by optimizing the DQD design and the thermal oxidation condition. We then demonstrate that the electrostatic coupling between DQDs can be modulated by tuning the negative voltage of the side gate electrode. Note that the interdot coupling was largely modulated with a small decrease in the gate voltage from 0 to � 100 mV because our structure initially has the DQD geometry. Furthermore, the device fabrication is compatible with the conventional silicon complementary metal–oxide–semiconductor (CMOS) process. This structure is suitable for the future integration of CMOS devices. In addition, we show the derivation of the DQDs’ capacitances, including the gate cross capacitances, as a function of the spacing between the two adjacent charge triple points. By using these capacitances, the electron transport properties of the DQD structure are simulated, and the modulation of the electrostatic coupling is successfully simulated as the change of the total capacitance in DQDs. [DOI: 10.1143/JJAP.47.4820]

Position-Controllable Ge Nanowires Growth on Patterned Au Catalyst Substrate

A well position-controllable single Ge nanowire array was grown on patterned Au catalysts substrate by low-pressure chemical vapor deposition. Both transmission electron microscope and X-ray diffraction results indicate that Ge nanowires are single crystalline with diamond structure. By optimizing the electron-beam lithography process, Au patterns with a diameter of 10 nm were prepared by lift-off method. The growth of Ge nanowires can be precisely controlled by adjusting the location of catalysts, which may offer the possibility of in situ fabrication of nanowire devices.

Gigahertz single-electron pumping in silicon with an accuracy better than 9.2 parts in 107

High-speed and high-accuracy pumping of a single electron is crucial for realizing an accurate current source, which is a promising candidate for a quantum current standard. Here, using a high-accuracy measurement system traceable to primary standards, we evaluate the accuracy of a Si tunable-barrier single-electron pump driven by a single sinusoidal signal. The pump operates at frequencies up to 6.5 GHz, producing a current of more than 1 nA. At 1 GHz, the current plateau with a level of about 160 pA is found to be accurate to better than 0.92 ppm (parts per million), which is a record value for 1-GHz operation. At 2 GHz, the current plateau offset from 1ef (∼320 pA) by 20 ppm is observed. The current quantization accuracy is improved by applying a magnetic field of 14 T, and we observe a current level of 1ef with an accuracy of a few ppm. The presented gigahertz single-electron pumping with a high accuracy is an important step towards a metrological current standard.

Gigahertz single-hole transfer in Si tunable-barrier pumps

We report high-speed single-hole (SH) transfer using Si tunable-barrier pumps comprising p-type metal-oxide-semiconductor field-effect transistors. A clear SH-transfer-current plateau with the current level of about 160 pA was observed when a clock signal having a frequency of 1 GHz was applied to one of the gates. Temperature dependence measurements of the transfer current reveal that the transfer probability is dominated by non-equilibrium SH escape by thermal hopping from the electrically formed charge island. The lower bound of the relative error rate for the 1-GHz transfer is about 10−3 at a temperature of about 17 K. In addition, we investigate the frequency dependence of the transfer, where we discuss possible sources causing the change in the error rate. These results pave the way for accurate manipulation of SHs and its application to metrological current standards.

Realization of Lithographically‐Defined Silicon Quantum Dots without Unintentional Localized Potentials

We have fabricated lithographically‐defined Si quantum dots (QDs) within a metal‐oxide‐semiconductor field‐effect transistor (MOSFET) structure. In this architecture, the top gate is used to tune the carrier density whereas side gates control the potentials of the QDs and tunneling barriers. These lithographically‐defined and electrically‐tunable Si QDs were successfully realized without unintentional localized potentials.

Direct observation of subband structures in (110) PMOSFETs under high magnetic field: Impact of energy split between bands and effective masses on hole mobility

The band structures and carrier transport in (110) pFETs are thoroughly studied over a wide temperature range under high magnetic fields. In (110) pFETs, the degenerate hole bands in bulk Si are separated into the higher energy band (H band) and the lower energy band (L band). The energy difference between these bands is experimentally evaluated. The effective masses of each band are directly obtained from the Shubnikov-de Haas (SdH) oscillation analysis. It is demonstrated that mobility in the higher energy band is worse than that in the lower energy band, resulting in sharp mobility drop at higher surface carrier concentrations (Ns) and a clear hump in Id-Vg characteristics at low temperatures of less than 20 K. In order to further enhance mobility in (110) pFETs, the increase in the energy split between H and L bands is important.

High-accuracy current generation in the nanoampere regime from a silicon single-trap electron pump

A gigahertz single-electron (SE) pump with a semiconductor charge island is promising for a future quantum current standard. However, high-accuracy current in the nanoampere regime is still difficult to achieve because the performance of SE pumps tends to degrade significantly at frequencies exceeding 1 GHz. Here, we demonstrate robust SE pumping via a single-trap level in silicon up to 7.4 GHz, at which the pumping current exceeds 1 nA. An accuracy test with an uncertainty of about one part per million (ppm) reveals that the pumping current deviates from the ideal value by only about 20 ppm at the flattest part of the current plateau. This value is two orders of magnitude better than the best one reported in the nanoampere regime. In addition, the pumping accuracy is almost unchanged up to 7.4 GHz, probably due to strong electron confinement in the trap. These results indicate that trap-mediated SE pumping is promising for achieving the practical operation of the quantum current standard.

Enhanced tunnel conductance due to QCA cotunneling processes observed for silicon serial triple quantum dots

We study single-electron tunneling characteristics of silicon serial triple quantum dots which consist of lithographically-defined double quantum dots interconnected with a naturally-formed and smaller quantum dot. By controlling the single-electron tunneling through the triple quantum dots electrostatically using multiple side gates, the charge stability diagrams are characterized experimentally and theoretically. Several charge quadruple points are observed where sequential tunneling throughout the triple quantum dots is enabled. In addition, enhancement of tunnel conductance is observed along the two-hold degeneracy boundaries across which two electrons exhibit quantum cellular automata (QCA) cotunneling processes.