JP4865822B2 - Single-electron element impedance measuring apparatus and single-electron element impedance measuring method - Google Patents

Description

  The present invention relates to a charge detector in the vicinity of a single electron transistor, wherein the impedance of the single electron transistor, that is, the resistance component and the capacitance component, accompanying the charge transfer through a tunnel barrier composed of a resistance component and a capacitance component constituting the single electron device. The present invention relates to a device for detecting a phase on an element as a signal.

  As a conventional charge detector operating at high speed, a high-frequency single-electron transistor configured as shown in FIG. 5 is known. In this high-frequency single-electron transistor, two tunnel barriers 103 and 105 are formed in a microconductive region 101 (hereinafter referred to as microisland 101) in which the charging energy of a single electron is equal to or higher than the thermal energy at the operating temperature. Electrons can be taken in and out via Usually, such a small island 101 is obtained by finely processing a semiconductor or metal, or by forming a tunnel junction between a small conductive molecule or fine particle and a semiconductor or metal.

  In a typical high-frequency single-electron transistor circuit, a small island 101 has an electrode 104 via a tunnel barrier 105 and an electrode 102 via a tunnel barrier 103 joined. The electrode 104 is grounded, and the electrode 102 is grounded via an artificially or parasitically formed capacitor 106. A high-frequency signal 108 is input to the electrode 102 via the inductor 107. Hereinafter, the electrode 102 will be referred to as the input / output terminal 102.

  Here, when a high frequency having a resonance frequency determined by the capacitor 106 and the inductor 107 is input to the input / output terminal 102, a reflected signal appears at the input / output terminal 102. At this time, the impedance of the single electron transistor can be determined at high speed and with high sensitivity by measuring the ratio of the reflected signal to the high frequency input signal, that is, the reflection coefficient. Here, since the impedance of the single-electron transistor is determined by the charge state of the micro island 101, it can be used as a high-speed and highly sensitive charge detector. Non-patent document 1 is known as such a technique.

R. Schoelkopf, "The radio-frequency single-electron transistor: a fast and ultrasensitive electrometer" Science, 280, pp.1238-1242

  However, the reflection coefficient measured in such a conventional high-frequency single-electron transistor is, in principle, 1 when the single-electron transistor has a high resistance (Coulomb blockade state), and the single-electron transistor has a low resistance (10 kilohms). Even in the case of (above), the circuit constant normally used is slightly less than 1. Therefore, in order to obtain the impedance, there is a problem that it is necessary to measure the slight difference between the reflection coefficients and a large accuracy (number of digits) is required.

  Further, the conventional high-frequency single-electron transistor operates only at a resonance frequency (usually a megahertz band) determined by a capacitor and an inductor in the circuit, and has a problem that the frequency variable width is extremely narrow. Therefore, there is a problem that only a limited charge state determined by the resonance frequency can be detected. Further, when the resonance frequency is changed, it is necessary to reconfigure a circuit incorporating circuit components such as a capacitor and an inductor, and there is a problem that complexity is required.

  The present invention has been made in view of such circumstances, and its object is to reduce the number of circuit components for measurement and widen the frequency band to be input, and to obtain the impedance of a single-electron transistor with high sensitivity. An object of the present invention is to provide a single-electron element impedance measuring apparatus capable of

The present invention has been made to solve the above-described problems. The invention according to claim 1 is a micro-conducting region in which the charging energy of single electrons is equal to or higher than the thermal energy at the operating temperature. A single-electron transistor having a small island and an input electrode that is an electrode joined to the small island through a tunnel barrier is electrically insulated from the vicinity of the single-electron transistor and the conduction path of the single-electron transistor. A point junction that is electrostatically coupled to the small island, and one of the electrodes joined to the point junction and the input electrode receives a phase variable high-frequency voltage signal having a wideband frequency as an input signal. The single-electron transition associated with temporal charge transfer between the micro-island and the input electrode, which is independently input and induced by the input signal The impedance of the capacitor, a single electron element impedance measuring apparatus characterized by having a charge detection meter for phase detection by the output signal outputted from the other electrode to be bonded to said point bonding.

  The invention according to claim 2 is the single-electron element impedance measuring apparatus according to claim 1, wherein the single-electron transistor has a plurality of the micro-islands coupled quantum mechanically.

  The invention described in claim 3 is characterized in that the micro island is one of a GaAs compound semiconductor, silicon, a carbon-based material, and a single molecule. This is a single-electron element impedance measuring device.

  According to a fourth aspect of the present invention, the single-electron according to any one of the first to third aspects, wherein the phase variable high-frequency voltage signal having a wideband frequency is a sine wave signal or a pulse signal. It is an element impedance measuring device.

According to the fifth aspect of the present invention, a micro island that is a micro conductive region in which the charging energy of a single electron is equal to or higher than the thermal energy at the operating temperature is joined to the micro island through a tunnel barrier. A single-electron transistor having an input electrode that is an electrode, and another conductive path in the vicinity of the single-electron transistor and electrically insulated from the conductive path of the single-electron transistor, and electrostatically and point bonding, which is coupled to the a single-electron element impedance measuring methods used in the single-electron element impedance measuring device having a charge detection meter for detecting the impedance of the single electron transistor, said charge detection meter, A phase-variable high-frequency voltage signal having a broadband frequency is input to the one electrode joined to the point junction and the input electrode. Independently inputted Te, other electrode of the impedance of the single-electron transistor with time charge transfer between the micro-island and the input electrode induced by the input signal, is joined to said point bonding A method of measuring impedance of a single electron element, characterized in that phase detection is performed by an output signal output from the first electronic element.

According to the present invention, a phase variable high-frequency voltage signal having a wideband frequency is independently input as an input signal to one electrode joined to a point junction that is electrostatically coupled to the microisland and to an input electrode joined to the microisland. The input charge detector outputs the impedance variation of the single-electron transistor associated with the temporal charge transfer between the micro-island and the input electrode induced by the input signal from the other electrode joined to the point junction. The phase is detected by the output signal to be detected.
As a result, the present invention has the effect of reducing the number of circuit components for measurement and widening the input frequency band to obtain the impedance of the single electron transistor with high sensitivity.

It is a block diagram which shows the structure of the single-electron element impedance measuring apparatus by 1st Embodiment of this invention. It is a graph which shows the gate voltage Vg dependence of the time average current < _IQPC > when the frequency of a phase variable high frequency voltage signal is 1 kHz. 6 is a graph showing a change in current peak (dip) with respect to a phase difference θ of a phase variable high-frequency voltage signal input to an electrode 20 and an electrode 21. It is a block diagram which shows the structure of the single-electron element impedance measuring apparatus by 2nd Embodiment of this invention. It is a block diagram which shows the structure of the conventional high frequency single electron transistor.

<First Embodiment>
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic block diagram showing the configuration of a single-electron element impedance measuring apparatus according to the first embodiment of the present invention.

  The single-electron transistor 30 is joined to a micro-island 1 that is a micro-conductive region in which the charge energy of a single electron is approximately equal to or higher than the thermal energy at the operating temperature, and the micro-island 1 is joined via a tunnel barrier 3. An electrode 2 (input electrode) and an electrode 4 joined to the small island 1 via a tunnel barrier 5 are provided.

  The micro island 1 is formed by GaAs compound semiconductor, silicon, carbon-based material (carbon nanotube, graphene, fullerene), single molecule that is fullerene or organic molecule, or by microfabrication of metal, or minute conductive molecule or fine particle. And a semiconductor or a metal.

  Electrons can be taken in and out of the small island 1 one by one through the tunnel barrier 3 or the tunnel barrier 5. For example, in this embodiment, electrons can be taken in and out from the electrode 2 to the micro island 1 through the tunnel barrier 3 one by one. For example, in this embodiment, the electrode 4 is grounded. Similarly to the tunnel barrier 3, electrons can be taken in and out from the electrode 4 to the micro island 1 through the tunnel barrier 5.

  An electrode 2 connected to such a small island 1 through a tunnel barrier 3 is an input terminal, and a phase variable high-frequency voltage signal 20 is input to this electrode 2. The phase variable high frequency voltage signal 20 is, for example, a sine wave signal. The phase variable high-frequency voltage signal 20 and a phase variable high-frequency voltage signal 21 to be described later are variable in phase.

  The point junction 7 is electrostatically coupled to the micro island 1 via the capacitor 6. Further, although the conduction path constituting the point junction 7 and the conduction path constituting the single-electron transistor 30 having the micro island 1 are close to each other (the distance between them is about 100 nanometers), another conduction path is used. And is electrically insulated. Here, since the conductance of the point junction 7 is modulated by electrons entering and exiting the micro island 1 from the electrode 2, the point junction 7 functions as a charge detector for the single electron transistor 30.

  In the present embodiment, phase variable high-frequency voltage signals 20 and 21 having a broadband frequency are independent of the electrode 2 to which the electrode 2 of the single electron transistor 30 and the charge detector 10 are joined via the electrode 9 and the point junction 7. Phase detection on the element.

  That is, phase variable high-frequency voltage signals 20 and 21 having a broadband frequency are independently input as input signals to one electrode 8 joined to the point junction 7 and the electrode 2 that is the input terminal of the single-electron transistor 30. Then, the charge detector 10 detects the impedance variation of the single-electron transistor 30 accompanying the temporal charge transfer between the micro island 1 and the electrode 2 induced by the input signal, and joins the other to the point junction 7. Phase detection is performed by the output signal output from the electrode 9. The other end of the charge detector 10 is connected to one end of a DC power source 11. The other end of the DC power source 11 is grounded.

For example, when the phase variable high-frequency voltage signal 20 having the frequency fop is input to the micro island 1 through the electrode 2, the potential of the micro island 1 is modulated, and the single electron transistor configured by the micro island 1, the electrode 2, and the electrode 4. The impedance ZQD (fop) of 30 changes. The frequency fop point bonding 7 via the electrode 8, by entering the phase variable high-frequency voltage signal 21 of the phase difference theta, time average current detected by the charge detecting meter 10 connected to the electrodes 9 <IQPC> is It is proportional to Z QD (fop) e ^iθ . Therefore, by modulating the phase difference θ, the resistance component and reactance component of the impedance ZQD (fop) can be detected independently.

For example, when the time average current <I _QPC > is measured while changing the gate voltage Vg for adjusting the height of the tunnel barrier 3, a current peak (or dip) with a background component is observed. FIG. 2 is a graph showing the dependence of the time average current <I _QPC > on the gate voltage Vg when the frequency of the phase variable high-frequency voltage signal is 1 kHz. The background component is caused by electrostatic coupling between the electrode 2 and the electrode 8.

  As shown in FIG. 2, in the boundary region where the number of electrons in the micro island 1 changes, the conductance of the charge detector 10 accompanying the increase in the impedance of the single-electron transistor 30 composed of the micro island 1, the electrode 2 and the electrode 4 is shown. A current peak is observed due to the increase in. Here, the peak or dip of the signal of the charge detector 10 composed of the point junction 7 is determined by the positional relationship between the micro island 1 and the point junction 7.

  FIG. 3 is a graph showing changes in the current peak (dip) in the signal detected by the charge detector 10 with respect to the phase difference θ of the phase variable high-frequency voltage signal input to the electrode 2 and the electrode 8. From this graph, when the frequency is low, the peak intensity is maximized when the phase difference is zero, and when the phase difference is 90 degrees, the peak intensity is 0. Thus, it can be seen that phase detection is performed. . As the frequency increases, the peak intensity decreases and the phase shifts (see the arrow in FIG. 3). This is because the decrease in the peak intensity approaches the time for the frequency of the input voltage to pass through the tunnel barrier 3, so that charge transfer is less likely to occur. The phase shift also ensures that the charge detector 10 detects the impedance of the single electron transistor 30.

  In the single-electron element impedance measuring apparatus according to the first embodiment described above, one electrode 8 joined to the point junction 7 electrostatically coupled to the micro island 1 and the input electrode joined to the micro island 1 2, phase variable high-frequency voltage signals 20 and 21 having a wideband frequency are independently input as input signals. Then, the charge detector 10 joins the impedance variation of the single-electron transistor due to the temporal charge transfer between the micro island 1 and the input electrode 2 induced by the input signal to the other junction that joins the point junction 7. Detection is performed by phase detection using the output signal output from the electrode 9.

  Thereby, in the single-electron element impedance measuring apparatus according to the first embodiment, since there is no need to measure the reflection coefficient with the input electrode 2 as in the prior art, a slight difference in reflection coefficient is obtained in order to obtain impedance. There is no need to measure. Further, since it is not necessary to operate at a resonance frequency (usually a megahertz band) determined by a capacitor and an inductor in the circuit as in the prior art, the frequency variable width can be widened. In addition, since it is not necessary to operate at the resonance frequency as in the prior art, it is not necessary to reconfigure a circuit incorporating circuit components such as a capacitor and an inductor which are necessary when changing the resonance frequency.

  That is, the single-electron element impedance measuring apparatus according to the first embodiment can measure the impedance of a single-electron transistor in a wide band from about several hertz to several tens of gigahertz without requiring circuit components such as capacitors and inductors. Is determined by the charge state in the microisland, so that it is possible to detect the charge state in a wide band.

  Therefore, the single-electron element impedance measuring apparatus according to the first embodiment has an effect that the impedance of the single-electron transistor can be obtained with high sensitivity by reducing the circuit components for measurement and widening the input frequency band. Play.

<Second Embodiment>
Next, the configuration of the single-electron element impedance measuring apparatus according to the second embodiment will be described with reference to FIG. The single-electron element impedance measuring apparatus according to the second embodiment has a single-electron transistor 30 coupled in series with a plurality of quantum-mechanically coupled single-electron transistors 30 as compared to the single-electron element impedance measurement apparatus according to the first embodiment. It has a small island 1.

  4, components corresponding to those in FIG. 1 are denoted by the same reference numerals, description thereof is omitted, and only components different from those in FIG. 1 are described. The single electron transistor 30A in FIG. 4 corresponds to the single electron transistor 30 in FIG. Here, a case where the single electron transistor 30A has two micro islands 1 that are quantum-mechanically coupled, that is, the micro island 1A and the micro island 1B will be described.

  The single electron transistor 30A has a plurality of small islands connected in series instead of the small island 1 of FIG. For example, the single-electron transistor 30A has a micro island 1A and a micro island 1B instead of the micro island 1 in FIG. The small island 1A and the small island 1B are connected in series via the tunnel barrier 3A.

  Also in the second embodiment, similarly to the case of the first embodiment described with reference to FIG. 1, electrons can be taken in and out one by one from the electrode 2 to the small island 1B through the tunnel barrier 3B. Is possible. It is also possible to put electrons in and out from the electrode 4 to the small island 1A through the tunnel barrier 5 one by one. Furthermore, it is also possible to transfer electrons one by one from the small island 1A to the small island 1B via the tunnel barrier 3A, or vice versa, from the small island 1B to the small island 1A via the tunnel barrier 3A.

  A point junction 7 is associated with each of the plurality of small islands. For example, the point junction 7A is electrostatically coupled to the micro island 1A via the capacitor 6A, and the point junction 7B is electrostatically coupled to the micro island 1B via the capacitor 6B. Further, the point junction 7A and the point junction 7B are connected in series like the small island 1A and the small island 1B.

  In the single-electron element impedance measuring apparatus according to the second embodiment, as in the single-electron element impedance measuring apparatus according to the first embodiment, a plurality of small islands 1 (1A and 1B) connected in series and static One electrode 8 joined to a plurality of point junctions 7 (7A and 7B) that are electrically coupled, and a micro island 1B of a plurality of micro islands 1 (1A and 1B) connected in series Broadband frequency phase variable high-frequency voltage signals 20 and 21 are independently input as input signals to the input electrode 2 to be joined. Then, the charge detector 10 joins the impedance variation of the single-electron transistor due to the temporal charge transfer between the micro island 1B and the input electrode 2 induced by the input signal to the other junction that joins the point junction 7A. Detection is performed by phase detection using the output signal output from the electrode 9.

  As a result, the single-electron element impedance measuring apparatus according to the second embodiment reduces the number of circuit components for measurement and widens the input frequency band in the same manner as the single-electron element impedance measurement apparatus according to the first embodiment. The impedance of the single electron transistor can be obtained with high sensitivity.

According to the first or second embodiment described above, the following effects are obtained.
-Impedance (resistance component and reactance component) of a single electron transistor can be obtained.
The electron tunneling time can be measured by measuring the change in impedance of the single electron transistor accompanying the charge transfer through the tunnel barrier 3 and the tunnel barrier 5.
-Since the relaxation time of electron spin in a micro island is several hundred microseconds, a single spin state can be detected by the technique according to the present embodiment.

Moreover, according to 2nd Embodiment mentioned above, there exists the following effect.
-If two micro islands coupled quantum mechanically are used, the impedance accompanying the charge transfer of the barrier between dots can be measured.
-In a system in which a plurality of small islands are coupled in series, the charge state of the plurality of small islands can be detected.
-It is possible to distinguish and detect an anti-bonding orbital state and a bonding orbital state, which are quantum mechanical states formed by electric charges, in two micro-islands that are quantum-mechanically coupled. In order to distinguish both states, a measurement faster than the phonon relaxation time (several gigahertz) is required, and can be detected by this technique.

In addition, as a semiconductor material which comprises a micro island, a GaAs compound semiconductor, silicon, a carbon-type material (carbon nanotube, graphene, fullerene), a single molecule (fullerene or an organic molecule), etc. are mentioned.
The same effect can be obtained even if the sine wave signal in the phase variable high frequency voltage signal is replaced with a pulse signal.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes design and the like within a scope not departing from the gist of the present invention.

  1, 1A, 1B, 101 ... micro islands, 2, 4, 8, 9, 102, 104 ... electrodes, 3, 3A, 3B, 5, 103, 105 ... tunnel barriers, 6, 6A, 6B, 106 ... capacitors, 7, 7A, 7B: Point junction, 10: Charge detector, 11: DC power supply, 20, 21: Phase variable high frequency voltage signal, 107: Inductor, 108: High frequency signal

Claims (5)

A single island having a small island that is a small conduction region in which the charging energy of a single electron is equal to or higher than the thermal energy at the operating temperature, and an input electrode that is an electrode joined to the small island through a tunnel barrier. An electronic transistor;
A point junction in the vicinity of the single-electron transistor and in another conductive path that is electrically insulated from the conductive path of the single-electron transistor, and electrostatically coupled to the microisland;
A phase variable high-frequency voltage signal having a wide-band frequency is independently input as an input signal to one of the electrodes and the input electrode that are joined to the point junction, and between the micro-island and the input electrode induced by the input signal. the impedance of the single-electron transistor with time charge transfer, a charge detection meter for phase detection by the output signal outputted from the other electrode to be bonded to said point bonding,
A single-electron element impedance measuring apparatus comprising: The single electron transistor is
Having a plurality of said microislands coupled quantum mechanically,
The single-electron element impedance measuring apparatus according to claim 1. The microislands are
One of GaAs compound semiconductor, silicon, carbon-based material, single molecule,
The single-electron element impedance measuring apparatus according to claim 1 or 2, characterized by the above-mentioned. The broadband variable phase variable high-frequency voltage signal is
The single-electron element impedance measuring apparatus according to any one of claims 1 to 3, wherein the single-electron element impedance measuring apparatus is a sine wave signal or a pulse signal. A single island having a small island that is a small conduction region in which the charging energy of a single electron is equal to or higher than the thermal energy at the operating temperature, and an input electrode that is an electrode joined to the small island through a tunnel barrier. A point junction that is in a separate conduction path in the vicinity of the single-electron transistor and electrically isolated from the conduction path of the single-electron transistor, and is electrostatically coupled to the microisland; wherein a single-electron element impedance measuring methods used in the single-electron element impedance measuring device having a charge detection meter for detecting the impedance of the single-electron transistor,
The charge detector is
A phase variable high-frequency voltage signal having a wide-band frequency is independently input as an input signal to one of the electrodes and the input electrode that are joined to the point junction, and between the micro-island and the input electrode induced by the input signal. the impedance of the single-electron transistor with time charge transfer, and detecting the phase detection by the output signal outputted from the other electrode to be bonded to said point bonding,
A single-electron element impedance measuring method.

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