Si Single-electron Transistors Research Articles

Reduced Electron Temperature in Silicon Multi-Quantum-Dot Single-Electron Tunneling Devices.

The high-performance room-temperature-operating Si single-electron transistors (SETs) were devised in the form of the multiple quantum-dot (MQD) multiple tunnel junction (MTJ) system. The key device architecture of the Si MQD MTJ system was self-formed along the volumetrically undulated [110] Si nanowire that was fabricated by isotropic wet etching and subsequent oxidation of the e-beam-lithographically patterned [110] Si nanowire. The strong subband modulation in the volumetrically undulated [110] Si nanowire could create both the large quantum level spacings and the high tunnel barriers in the Si MQD MTJ system. Such a device scheme can not only decrease the cotunneling effect, but also reduce the effective electron temperature. These eventually led to the energetic stability for both the Coulomb blockade and the negative differential conductance characteristics at room temperature. The results suggest that the present device scheme (i.e., [110] Si MQD MTJ) holds great promise for the room-temperature demonstration of the high-performance Si SETs.

(Invited) Characteristics of Si Single-Electron Transistor under Illumination

Current of single-electron transistors (SETs) flows by means of one-by-one electron transfer. In a SET, the number of charges in the SET dot is determined by the gate voltage and capacitance between the gate and the dot. If the SETs are made of a semiconductor, the dot accumulates electrons and holes according to the positive and negative gate voltages, respectively. However, since the source and drain electrodes of SETs are made of n-type semiconductor, holes cannot be injected to the dot. A single-hole transistor (SHT) usually needs a p-type semiconductor for source and drain [1]. Here, we measured the SHT characteristics by the use of SET structure device under illumination. Holes excited in the semiconductor is thought to flow through the SET island according to the negative gate voltages. We made silicon SETs, which have n-type source and drain electrodes on thin SOI (silicon on insulator) wafer, by the use of pattern dependent oxidation (PADOX) method [2]. Clear current oscillations and Coulomb diamonds as a function of positive gate voltage were achieved. When the gate voltage was scanned to negative direction, almost no current was observed in the dark or under week photo-excitation. However, current oscillations were observed under the strong photo- excitation of 330 μW/cm2 from a halogen-lamp, though the peak current was limited by the photo-excited currents. Coulomb diamonds were also detected at negative gate voltage region. Photo-excited carriers in Si quantum dots also show another interesting features such as single-electron CCD [3] where electrons and holes exist in the same dot at the same time. In a SET, when a few holes generated by photo-excitation are trapped in the one side of the SET island under the vertical electric field, the number of trapped holes is detected by the electron current flowing through the other side of the island [4]. In the few electron regime, not only the number of net charges but also the each number of electrons and holes plays an important role to decide the electrical characteristics of SETs. However, these characteristics appeared only when the electrons and holes are divided by strong electric field. In this report, we show the characteristics of photo-excited SETs without strong vertical electric fields. When the gate voltage (V g) of a SET was scanned from low to high, drain current (I d) shows interesting features depending on the photo-excited conditions. Figure 1 shows I d-V g characteristics under three different conditions. In any conditions, Coulomb oscillations were observed. The characteristics measured in the dark are fundamental ones, in which the first peak corresponds to the first electron transferred to the SET island. When the SET was continuously irradiated, a new current peak emerges at the lower V g region, which is thought to be an effect of a positive charge (a hole) generated by photo excitation. The facts that the new current peak suddenly dropped and recovered again can be understood that the generated hole recombined with electron flowing through the SET island and another hole was generated again. In the one-shot excitation condition, the SET was illuminated only once at the starting V g, but scan was performed in the dark. Although the new peak was detected first, the current characteristics return to those of in the dark after the sudden current drop. These phenomena were thought to be caused by a photo-excited hole trapped in SiO2 near the SET island. Acknowledgement This work is supported by JSPS KAKENHI (15H01706, 16H0433906 and 16K18073) and by the Cooperative Research Project of the Research Center for Biomedical Engineering with Research Institute of Electronics, Shizuoka University.

(Invited) Characteristics of Si Single-Electron Transistor under Illumination

Photo-excited carriers in Si quantum islands and their surrounded area exhibit interesting features due to electron-hole generation. We show Si single- hole transistor (SHT) characteristics in a Si single-electron transistor (SET) comprising n-type source and drain electrodes under illumination. Holes excited in the Si are expected to flow through the SET island by applying negative gate voltages. Interesting phenomenon is the co-existence of electrons and holes around the island. In a SET, when holes generated by photo-excitation are trapped in the one side (either top or bottom side) of the SET island under the vertical electric field, the SET characteristics are changed depending on the number of trapped holes because effective total number of electrons can be negative. Here, we show the characteristics of photo-excited SETs without strong vertical electric fields.

(Invited) Evaluation of Coupled Triple Quantum Dots with Compact Device Structure

Semiconductor multiple-quantum-dot devices have been attracted as a single-electron pump and quantum bits (qubits) [1, 2]. In particular, silicon-based coupled-quantum-dot has been paid attentions for the applications as qubits in solid state quantum computers due to their long coherent time of electron spins [3]. In the previous reports, these devices have been demonstrated by the use of two-dimensional electron gas (2DEG) and multiple control gates, where many gates were used to form tunnel barriers. This device concept makes it difficult to integrate quantum dots (QDs) due to the interference of many gates. For the applications in quantum computing, development of coupled QDs with a smaller number of control gates is considered. We have already succeeded in the fabrication of serially coupled Si triple-quantum-dots (TQD) by pattern-dependent oxidation (PADOX) method [4] and additional oxidation of the silicon nanowire on a SOI wafer with three control gates [5, 6]. The formation of a Si TQD, where each QD is formed just under each control gate, was confirmed experimentally. The equivalent circuit of the fabricated TQD is shown in Fig. 1 (a). The device has nine gate capacitances and two inter-dot coupling capacitances. It is crucial to know these capacitances for controlling the potential and the number of electrons of each dot. It is well known that the nine gate capacitances can be evaluated from the stability diagram that consists of boundary of the charge transition in the QDs. If the distance between the three gates is relatively long enough to ignore the influence of non-adjacent QD, it is easy to get the stability diagrams of charges. However, when the TQD become compact in size, the outer QD cannot be ignored because the center gate strongly couples to all three QDs, i.e., C 23 or C 21 is comparable to C 22. The stability diagram of such TQD usually shows very complicated behavior because three boundaries of the charge transition lines of individual QDs are superimposed. Fig. 1(b) shows the stability diagram which is the contour plot of current of TQD at 10 K simulated as a function of the two gate voltages V 1 and V 2 by the use of Monte Carlo method. Green dotted lines in the figures are the boundaries of charge transition of QD3, which are difficult to determine from the diagram. In addition, the coupling capacitance between the QDs also makes the stability diagrams complicate. Figs. 1(c) and (d) show stability diagrams of the TQD when the coupling capacitances, C 2 and C 3, become larger. As shown in the figures, the current peaks (white regions) are split as coupling capacitance C 2 and C 3 increase.In this study, we propose a simple analyzing method to evaluate the TQD by defining the contributions from each QD in stability diagrams. In order to divide the complicated stability diagram of TQD into the diagram of each QD, we employ the simultaneous scanning of three gate voltages with different sweeping ratios [7]. The effectiveness of the method is also confirmed even when the inter-dot coupling capacitance is large as shown in Fig. 1(c) and (d). The analysis using this method was applied to a Si TQD device fabricated on SOI wafer and successfully achieved the nine gate capacitances of the TQD. Acknowledgement This work was partly supported by JSPS KAKENHI (nos. 25420279, 26630141, 15H01706, 16H04339 and 16K18073). References H. Pothier, et al.: “Single electron pump fabricated with ultrasmall normal tunnel junction,” Physica B, 169, 573 1991. W. G. van der Wiel, et al.: “Electron transport through double quantum dots,” Rev. Mod. Phys, 75, 1 2002. B. M. Maune, et al.: “Coherent singlet-triplet oscillations in a silicon-based double quantum dot,” Nature, 481, 344 2012. Y. Takahashi, et al.: “Fabrication technique for Si single-electron transistor operating at room temperature,” Electron. Lett. 31, 136 1995. T. Uchida, et al.: “Coupling capacitance between double quantum dots tunable by the number of electrons in Si quantum dots,” J. Appl. Phys. 117, 084316 2015. T. Uchida, et al.: “Fabrication and evaluation of series-triple quantum dots by thermal oxidation of silicon nanowire,” AIP Advances. 5, 117144 2015. T. Uchida, et al.: “Capacitance evaluation of compact silicon triple quantum dots by simultaneous gate voltage sweeping,” J. Appl. Phys. 120, 234502 2016. Figure 1

Nano-Scale Silicon Quantum Dot-Based Single-Electron Transistors and Their Application to Design of Analog-to-Digital Convertors at Room Temperature

Room-temperature analog-to-digital converters (ADCs) based on nanoscale silicon (Si) quantum dot (QD)-based single-electron transistors (SETs) can be very attractive for high-speed processors embedded in future generation nanosystems. This paper focuses on the design and modeling of advanced single-electron converters suited for operation at room temperature. In contrast to conventional SETs with metallic QD, the use of sub-10-nm Si QD results in stable operation at room temperature, as the observable Coulomb blockade regime covers effectively the higher temperature range. Si QD-based SETs are also fully compatible with advanced CMOS technology and they can be manufactured using routine nanofabrication steps. At first, we present the principles of operation of Si SETs used for room-temperature operation. Possible flash-type ADC architectures are then investigated and the design considerations of possible Coulomb oscillation regimes are addressed. A modified design procedure is then introduced for [Formula: see text]-bit SET-based ADCs, and validated through simulation of a 3-bit ADC with a sampling frequency of 5 GS/s. The ADC core is comprised from a capacitive signal divider followed by three periodic symmetric functions (PSFs). Simulation results demonstrate the stability of output signals at the room-temperature range.

Multiple logic functions from extended blockade region in a silicon quantum-dot transistor

We demonstrate multiple logic-functions at room temperature on a unit device of the Si single electron transistor (SET). Owing to the formation of the multi-dot system, the device exhibits the enhanced Coulomb blockade characteristics (e.g., large peak-to-valley current ratio ∼200) that can improve the reliability of the SET-based logic circuits. The SET displays a unique feature useful for the logic applications; namely, the Coulomb oscillation peaks are systematically shifted by changing either of only the gate or the drain voltage. This enables the SET to act as a multi-functional one-transistor logic gate with AND, OR, NAND, and XOR functions.

High-Speed Operation of Si Single-Electron Transistor

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(Invited) High-Speed Operation of Si Single-Electron Transistor

Single-electron transistors (SETs) are expected as extremely low power devices. One of the critical issues for their application is that the operation speed is thought to be limited to be very slow due to their high tunnel resistance. We investigated the high-frequency limit of Si SET operation. The SETs were fabricated by the use of pattern-dependent oxidation method, and achieved the SETs with low tunnel resistance closed to quantum limit. And then, we employed a special rectifying effect, which occurs due to the asymmetry of Coulomb diamond, when alternating current voltage was applied to the drain terminal. By the use of the effect, rectifying characteristics of Si SETs were evaluated and revealed that there is no cutoff frequency in the rectifying effect set by the capacitance and tunneling resistance. These results indicate that the rectifying effect of AC voltage is usable beyond the THz regime.

Biomolecule detection based on Si single-electron transistors for practical use

Experimental and theoretical analyses demonstrated that ultra-sensitive biomolecule detection can be achieved using a Si single-electron transistor (SET). A multi-island channel structure was used to enable room-temperature operation. Coulomb oscillation increases transconductance without increasing channel width, which increases detection sensitivity to a charged target. A biotin-modified SET biosensor was used to detect streptavidin at a dilute concentration. In addition, an antibody-functionalized SET biosensor was used for immunodetection of prostate-specific antigen, demonstrating its suitability for practical use. The feasibility of ultra-sensitive detection of biomolecules for practical use by using a SET biosensor was clearly proven through this systematic study.

Biomolecule detection based on Si single-electron transistors for highly sensitive integrated sensors on a single chip

Biomolecule detection was achieved using a Si single-electron transistor (SET) for highly-sensitive detection. A multiple-island channel-structure was used for the SET to enable room-temperature operation and to increase sensitivity. Coulomb oscillation shifted against the gate voltage due to biotin-streptavidin binding. Coulomb oscillation has a possibility to increase transconductance (gm), and a higher gm leads to greater sensitivity to a charged target. Since a Si structure is important for integrating label-free-biomolecule and/or ion sensors into large-scale-integrated circuits, a Si SET with multiple islands should enable the integration of a sensor system on a single chip for multiplexed detections and simultaneous diagnoses.

Fluctuation of average position of electrons in Coulomb island in Si single-electron transistor

Average position of electrons along thickness direction in a Coulomb island in an n-channel Si single-electron transistor is estimated by analyzing the back-gate voltage dependence of peak voltage (defined as the gate voltage giving a drain current peak) as a function of peak number. It is found that the accuracy of estimated average position is better than 0.5nm and that the average position fluctuates as the peak number increases.

Highly sensitive ion detection using Si single-electron transistors

Si single-electron transistors (SETs) were used for highly sensitive ion detection. A multiple-island channel structure was adapted in the SET for room-temperature operation. Clear Coulomb oscillation and diamonds were observed at room temperature. Using the Coulomb oscillation, clear pH responses of drain current (Id)-gate voltage (Vg) characteristics were obtained despite the existence of Id noise. Because Coulomb oscillations have a possibility to increase the slope of Id over Vg near the half-maximum current of the peaks, high resolving power of ion, and/or biomolecule concentration can be expected. A Si-structure will make it possible to integrate the sensors on a single chip.

Si single electron transistor fabricated by chemical mechanical polishing

The authors report the results of a novel single electron transistor (SET) fabrication technique that combines the unique advantages of chemical mechanical polishing and the versatility and scalability of silicon processing. A thin (∼15 nm) line of degenerately doped silicon on insulator is embedded in a planarized plasma enhanced chemical vapor deposition oxide through nanoencapsulation. A pit in this line is formed by a highly selective silicon dry etch, and the tunnel oxide is grown on the sidewalls of the pit using rapid thermal oxidation. Degenerately doped amorphous silicon is deposited into the pit to form the SET island, the overburden of silicon is removed, and the structure is thinned down by chemical mechanical polishing. The SET’s charging diagram indicates a charging energy of around 20 meV, credited to its small tunnel junction size as well as possible “puddle” formation from the dopant distribution within the island and the leads. The observed anomalies, such as missing and split diamonds in the charging plots and random telegraph signals, suggest that the donor sites play an important role in the device operation. Several fabricated SETs show a similar Coulomb blockade at 4 K, demonstrating reasonable yield for the process.

Microwave-assisted transport via localized states in degenerately doped Si single electron transistors

Resonant microwave-assisted and dc transport are investigated in degenerately doped silicon single electron transistors. A model based on hopping via localized impurity states is developed and first used to explain both the dc temperature dependence and the ac response. In particular, the non-monotonic power dependence of the resonant current under irradiation is proved to be consistent with spatial Rabi oscillations between these localized states.

Detection of a single-charge defect in a metal-oxide-semiconductor structure using vertically coupled Al and Si single-electron transistors

An Al-AlO_x-Al single-electron transistor (SET) acting as the gate of a narrow (~ 100 nm) metal-oxide-semiconductor field-effect transistor (MOSFET) can induce a vertically aligned Si SET at the Si/SiO_2 interface near the MOSFET channel conductance threshold. By using such a vertically coupled Al and Si SET system, we have detected a single-charge defect which is tunnel-coupled to the Si SET. By solving a simple electrostatic model, the fractions of each coupling capacitance associated with the defect are extracted. The results reveal that the defect is not a large puddle or metal island, but its size is rather small, corresponding to a sphere with a radius less than 1 nm. The small size of the defect suggests it is most likely a single-charge trap at the Si/SiO_2 interface. Based on the ratios of the coupling capacitances, the interface trap is estimated to be about 20 nm away from the Si SET.

Fabrication of double-dot single-electron transistor in silicon nanowire

We propose a simple method for fabricating Si single-electron transistors (SET) with coupled dots by means of a pattern-dependent-oxidation (PADOX) method. The PADOX method is known to convert a small one-dimensional Si wire formed on a silicon-on-insulator (SOI) substrate into a SET automatically. We fabricated a double-dot Si SET when we oxidized specially designed Si nanowires formed on SOI substrates. We analyzed the measured electrical characteristics by fitting the measurement and simulation results and confirmed the double-dot formation and the position of the two dots in the Si wire.

Lack of charge offset drift is a robust property of Si single electron transistors

One of the challenges that single-electron transistors (SETs) face before they can be considered technologically useful is the charge offset drift. Recently, two different types of Si SETs were shown to have a drift of only 0.01e (the fundamental charge) over several days. Those devices came from one fabrication source. Here, we present the results for Si SETs fabricated by our group (a different source) demonstrating their operation as SETs. We confirm that the charge offset drift is less than 0.01e, demonstrating the lack of charge offset drift is generic to Si devices and not dependent on the fabrication source.

Coulomb blockade in a Si channel gated by an Al single-electron transistor

We incorporate an Al–AlOx–Al single-electron transistor as the gate of a narrow (∼100nm) metal-oxide-semiconductor field-effect transistor (MOSFET). Near the MOSFET channel conductance threshold, we observe oscillations in the conductance associated with Coulomb blockade in the channel, revealing the formation of a Si single-electron transistor. Abrupt steps present in sweeps of the Al transistor conductance versus gate voltage are correlated with single-electron charging events in the Si transistor, and vice versa. Analysis of these correlations using a simple electrostatic model demonstrates that the two single-electron transistor islands are closely aligned, with an interisland capacitance approximately equal to 1∕3 of the total capacitance of the Si transistor island, indicating that the Si transistor is strongly coupled to the Al transistor.

Electrical Characteristics of Si Single-Electron Transistor Based on Multiple Islands

We investigated the electrical characteristics specifically associated with cotunneling and quantum effects of single-electron transistors (SETs) with three geometrically defined islands. Clear Coulomb oscillation and negative differential conductance were observed. The drain current for the peaks and valleys of the Coulomb oscillation was measured as a function of temperature and drain voltage. Consequently, it was found that the valley current was well described by the inelastic cotunneling theory at temperatures above 40 K in the low-drain-voltage region; on the other hand, elastic cotunneling dominantly contributed to the valley current in the low-temperature and low-drain-voltage region. However, for a high gate voltage, the SET behaved as a single-island device. This is probably because the three islands were electrically enlarged and merged into a single island owing to the high applied gate voltage.

Cyclotron localization in a sub-10-nm silicon quantum dot single electron transistor

The authors have fabricated and measured a lateral Si single electron transistor consisting of a succession of a big island and small quantum dots. The big island gives rise to a small period Coulomb oscillation riding on the large irregular oscillation arising from the small quantum dots. The peaks of the latter shift in the presence of a magnetic field, which is analyzed in the context of field-induced Landau level shift with a soft-wall confinement potential. Furthermore, the current peak was suppressed for fields beyond a threshold value. An explanation based on cyclotron localization at noninteracting Landau levels is presented.