Mingyu Jo

Position controlled nanowires for infrared single photon emission

We report the experimental demonstration of single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires is fabricated by epitaxial growth on an electron-beam patterned substrate. Photoluminescence spectra taken on single quantum dots show narrow emission lines. Superconducting single photon detectors, which have a higher sensitivity than avalanche photodiodes in the infrared, enable us to measure auto and cross correlations. Clear antibunching is observed [g(2)(0) = 0.12] and we show a biexciton–exciton cascade, which can be used to create entangled photon pairs.

Fabrication and single-electron-transfer operation of a triple-dot single-electron transistor

A triple-dot single-electron transistor was fabricated on silicon-on-insulator wafer using pattern-dependent oxidation. A specially designed one-dimensional silicon wire having small constrictions at both ends was converted to a triple-dot single-electron transistor by means of pattern-dependent oxidation. The fabrication of the center dot involved quantum size effects and stress-induced band gap reduction, whereas that of the two side dots involved thickness modulation because of the complex edge structure of two-dimensional silicon. Single-electron turnstile operation was confirmed at 8 K when a 100-mV, 1-MHz square wave was applied. Monte Carlo simulations indicated that such a device with inhomogeneous tunnel and gate capacitances can exhibit single-electron transfer.

Fabrication and evaluation of series-triple quantum dots by thermal oxidation of silicon nanowire

Series-connected triple quantum dots were fabricated by a simple two-step oxidation technique using the pattern-dependent oxidation of a silicon nanowire and an additional oxidation of the nanowire through the gap of the fine gates attached to the nanowire. The characteristics of multi-dot single-electron devices are obtained. The formation of each quantum dot beneath an attached gate is confirmed by analyzing the electrical characteristics and by evaluating the gate capacitances between all pairings of gates and quantum dots. Because the gate electrode is automatically attached to each dot, the device structure benefits from scalability. This technique promises integrability of multiple quantum dots with individual control gates.

Excitonic spin-state preservation mediated by optical-phonon resonant excitation in a single quantum dot

(Received ## March 2008; published ## ## 2008) High degree of preservation of spin states during energy relaxation processes mediated by optical phonons is demonstrated in a single quantum dot. Optical-phonon resonance and relevant suppression of spin relaxation are clearly identified as dip structures in photoluminescence excitation spectra probed by the positive trion emission. The absence of continuum states makes this observation possible under the cross-circularly polarized detection with respect to a circularly polarized pumping. Consequently, distinguishably high degree of circular polarization up to ~0.85 is achieved without applying external magnetic field at the optical-phonon resonance. Rate equation analysis reveals that the spin-flip probability during energy relaxation is restricted to less than 7.5%. It is also indicated that the spin flip time of the positive trion ground state is extended by more than 3 times compared with that of neutral exciton ground state. This corresponds to the spin flip time longer than 11 ns for the positive trion ground state. The influence of nuclear polarization to the present measurements is also discussed.

Full Adder Operation Based on Si Nanodot Array Device with Multiple Inputs and Outputs

We demonstrate a highly functional Si-nanodot-array device that has three input gates and two output terminals. The device was fabricated on an SOI wafer using Si MOS processes. We confirmed that a single device can operate as both a half adder and a full adder when we carefully select the operation voltages.

Differential resistance oscillations with microwave irradiation in a superconductor-semiconductor junction

Transport properties of a superconductor-semiconductor junction are measured. Differential resistance as a function of bias voltage of the junction showed oscillation with a microwave irradiation of specific frequencies (1.715 GHz), which indicate the modulation of Andreev reflection probability by microwave irradiation.

Capacitance evaluation of compact silicon triple quantum dots by simultaneous gate voltage sweeping

We propose a simple method to evaluate the triple quantum dots (TQDs) coupled in series with the compact device structure. Compact coupled quantum dots, each with an attached control gate, offer promising applications as quantum computing and single-electron transfer devices. However, device reduction required in practical applications creates a capacitive cross-talk between a control gate and its neighboring dots making it difficult to determine the charge transition boundaries in each dot. To properly evaluate the electron-transfer characteristics of TQDs, a method is proposed whereby the three gate voltages are simultaneously swept. We studied the charge stability diagram of the compact TQDs using Monte Carlo simulations, and confirmed the effectiveness of the method. Compact Si-TQDs were actually fabricated by the use of pattern-dependent oxidation and additional oxidation method for this study. The method was then applied to the stability diagrams obtained from the devices. The nine measurements of t...

Evaluation of serially coupled triple quantum dots with a compact device structure by a simultaneous voltage-sweeping method

Series coupled triple quantum dots (TQDs) have been studied as an important element of quantum computers. It is needed to fabricate QD array connected in series with a compact structure to scale up the quantum bits. To develop quantum bits based on the TQDs, evaluation of the charge stability diagram is needed. However, since the center gate usually couples to all dots in the compact TQDs, it is difficult to achieve the charge stability diagram of TQDs by simply scanning the gate voltages. Here, we propose a simple method to achieve the stability diagram of TQDs. First, we check the stability diagram of the compact TQDs by the use of Monte Carlo simulations and succeed in developing a method to achieve the stability diagram by simultaneously sweeping of three gate voltages in the simulation. Then the method is applied to a silicon TQDs device fabricated by pattern-dependent oxidation. As the result, the formation of TQDs together with its stability diagram is confirmed.

Evaluation of the origin of excited states appeared in small Si single-electron transistors

Single electron transistors (SETs) have attracted attention not only to the transistor function but also their interesting characteristics. We reported that the negative conductance appeared in Coulomb diamond characteristics of silicon based SETs made by pattern dependent oxidation. This is attributable to the excited states in the quantum dot. However, it is necessary to consider the effect of electrostatic energy level in electron reservoir such as source and drain electrodes because the portion of the electrodes near the quantum dot is similar with the dot in size. In this study, we fabricated two SETs with a different structure, and investigated the influence of the energy level in small source and drain electrodes on the negative conductance.

Analog memory characteristics of 1T1R MoOx resistive random access memory

Multilevel resistive random access memory (ReRAM) is known as a key device for neural networks and reconfigurable computing. Control of the resistance is important to utilize ReRAM as an analog-resistance memory. In this study, we confirm that MoOx ReRAM shows analog memory characteristics by the control of compliance current. In addition, we propose a verification method to achieve target resistances.