WO1999014811A1 - Superconducting element for a micro-electronic circuit - Google Patents
Superconducting element for a micro-electronic circuit Download PDFInfo
- Publication number
- WO1999014811A1 WO1999014811A1 PCT/NL1998/000526 NL9800526W WO9914811A1 WO 1999014811 A1 WO1999014811 A1 WO 1999014811A1 NL 9800526 W NL9800526 W NL 9800526W WO 9914811 A1 WO9914811 A1 WO 9914811A1
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- WIPO (PCT)
- Prior art keywords
- layer
- superconducting
- compounds
- semiconductor
- control means
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N60/00—Superconducting devices
- H10N60/10—Junction-based devices
- H10N60/128—Junction-based devices having three or more electrodes, e.g. transistor-like structures
Abstract
Superconducting element for a micro-electronic circuit, comprisisng a substrate with a first layer of a superconducting first material provided with a first terminal for an electric current, a second layer of a superconducting second material provided with a second terminal for an electric current, and a third layer of a third material, which third layer forms a Josephson contact between the first and the second layer, wherein control means are provided for controlling the occupancy of the energy states in the third layer.
Description
SUPERCONDUCTING ELEMENT FOR A MICRO-ELECTRONIC CIRCUIT
The invention relates to a superconducting element for a micro-electronic circuit, comprising a substrate with a first layer of a superconducting first material provided with a first terminal for an electric current, a second layer of a superconducting second material provided with a second terminal for an electric current, and a third layer of a third material , which third layer forms a Josephson contact between the first and the second layer.
Such an element is known from the American patent no. 4334158. The known superconducting element comprises in combination a tunnel junction assembled from two electrodes separated by a tunnel barrier, one electrode of which is a superconductor and the tunnel junction has a threshold power density wherein an excessive number of quasiparticles exists which causes the superconducting gap of said superconducting electrode to disappear, in addition to means for injecting power in the order of said threshold power density into said superconducting electrode.
The known element forms a switching device having three terminals ("three terminal device"), practical application of which however has a number of drawbacks .
The extent to which the supercurrent in the known element can be controlled is highly dependent on the superconducting materials used in the element, as a result of which the oper- ation of the known element is greatly dependent on these applied superconducting materials.
Another practical drawback of the known element is that manufacture thereof comprises a series of technically time- consuming production steps (for instance the manufacture of thin oxide tunnel junctions) and for this reason can only be realized with a limited number of superconducting materials.
Another drawback of the known element is that the switching rate thereof is intrinsically bounded by the time requi-
red to suppress the superconductivity therein.
A further drawback of the known element is the hysteresis in the current-voltage characteristics, as a result of which so-called latching occurs when this element is used. The object of the invention is to provide a superconducting element for a micro-electronic circuit in which the su- percurrent is controllable independently of the superconducting materials used in the element.
Another object is to realize the manufacture of such an element in a technically relatively simple manner with a diversity of superconducting materials, including for instance ceramic superconductors with high Tc .
A further object of the invention is to provide a superconducting element for a micro-electronic circuit which can operate at a relatively low voltage level, so that the power requirement and the dissipation level of the element are correspondingly low, so that the number of elements which can be integrated per unit of surface area on a substrate is relatively high. A further object is to provide an element which is free of latching, so that an element which is switched in a circuit by a control signal from a determined first state to a determined second state returns to this first state after the control signal is switched off. An object is further to provide an element which is suitable for amplifying voltage respectively power.
Other further objects are to provide an element with a good input/output insulation, i.e. an element of which the output state does not influence the signal required at the input, which is suitable to be used for frequencies in the THz range (i.e. in the order of 1012 Hz), which is scalable to dimensions in the submicron range and which is suitable for being integrated with superconducting strip lines for the purpose of rapid communication on a chip. These objects are achieved, and additional advantages obtained, with an element of the type stated in the preamble in which according to the invention control means are provid-
ed for controlling the occupancy of the energy states in the third layer.
Other than in an element according to the prior art, in which the critical value of the supercurrent Ic through the element is influenced by influencing the concentration ns of charge carriers, resulting in influencing of the energy gap in the superconducting first or second layer, in an element according to the invention this critical value is influenced by influencing the occupancy of the energy states in the normally conductive third layer.
In a Josephson junction a supercurrent Is flows across a layer of non-superconducting material with a maximum value Ic which is determined by the difference in the macroscopic phases (respectively Φ1 and Φ2) of two superconductors cou- pled on either side of this non-superconducting layer. This is summarized mathematically as follows:
Iβ = ICF(Φ1-Φ2) (1)
wherein F is a periodic function of the phase difference, in the lowest order the sine function, so that:
Iβ = I0sin(Φ1-Φ2) (2)
The invention follows from the observation that the value of Ic in equation (1) is a function of Δ, the energy gap of the superconducting material which influences the energy levels En of the material forming the Josephson contact (for the sake of simplicity it is here assumed that the first and the second superconducting material are identical) . The maximum supercurrent which can flow is dependent on Δ, on the material properties of the Josephson contact and on the occupancy f (En) of the states En, as follows:
Ic = KF(ns, Δ, f(En), fixed properties of the chosen material) (3)
wherein K is a numeric constant and n the concentration of
the charge carriers. By controlling the occupation f (En) of the states En it is thus possible to control the maximum supercurrent .
For an element in thermal equilibrium it can be assumed that the occupation f (En) of the states En satisfies the Fer- mi-Dirac distribution function
AE > (4)
wherein EF is the Fermi energy, kB the Boltzmann constant and Te the electron temperature. In equilibrium Te is equal to the ambient temperature or equal to the temperature Tph of the phonon lattice.
The maximum current through an element in thermal equi- librium is preferably controlled according to the invention by controlling the occupancy of the energy states in the third layer subject to the electron temperature in this third layer.
In an element according to the invention in which the occupancy of the energy states in the third layer is controllable when the element is not in thermal equilibrium, the control means comprise for instance a structure with two tunnel junctions separated by an intermediate layer, which intermediate layer extends to the third layer of the element . The structure with two tunnel junctions separated by an intermediate layer is for instance a per se known SINIS structure, wherein the letters S, I and N respectively designate a superconducting metal, an insulator and a normal metal.
In an embodiment of an element according to the invention the third material is a metal.
A Josephson contact consisting of a metal offers the advantage that it can be relatively thick (for instance a factor of 100 times thicker than a Josephson contact of an insulating material having a comparable electrical resis- tance) .
A degenerately doped semiconductor is a semiconductor of which the conduction band has fallen below the Fermi level as
a result of the doping.
In yet another embodiment of an element according to the invention the third material forms a constituent of a semiconductor heterostructure, of which the electron gas realized therein can be coupled to the superconducting first and second materials .
The semiconductor in an element according to the invention is selected for instance from the group of materials comprising silicon and compounds of the type III-V such as for instance indium arsenide (InAs) and indium gallium arsenide (InGaAs) .
This embodiment provides the possibility of combining semiconductor and superconductor technologies .
In a embodiment of an element according to the invention the distance between the first and the second layer is for instance smaller than about 10~6 m (1 μm) .
In an embodiment the control means for controlling the occupancy of the energy states in the third layer comprise an electrical conductor, via which energy can be supplied to the third layer to adjust the temperature thereof.
The electrical conductor extends for instance from a third terminal along a determined distance L of the third layer, separated from this third layer by a fourth layer of a fourth material , to a fourth terminal . In an embodiment the length L and the fourth material, the diffusion constant of which is represented by D, are selected such that the value of (L/2)2/D is less than 2 x 10"11 sec .
An element with such a value of (L/2)2/D (the transport time) can operate at frequencies higher than 50 GHz.
The value of (L/2)2/D is preferably less than 2 x 10~12 sec, so that the element can operate at frequencies higher than 500 GHz.
More preferably the transport time is less than 10"12 sec, so that the element can operate at frequencies higher than 1 THz.
An element with a transport time of less than 10~12 sec is
for instance realized in a structure in which the third layer comprises a ballistic semiconductor in a heterostructure, the transport time of which is determined by the value L/vF (wherein vF represents the Fermi speed in the heterostructure) . In this embodiment the fourth material is preferably identical to the third material, and the fourth terminal coincides with the first or second terminal, so that a relatively simple element with three terminals ("three terminal device") is provided. In an element according to the invention the first and the second material are for instance selected from the group comprising superconducting elements, for instance niobium (Nb) and lead (Pb) , superconducting compounds of the type Bl, for instance niobium nitride (NbN) , superconducting compounds of the type A15, for instance niobium tin (Nb3Sn) and ceramic superconductors. An electrical conductor in an element according to the invention comprises for instance a strip of a metal selected from the metals gold (Au) , silver (Ag) and copper (Cu) . The invention will be elucidated in the following on the basis of an embodiment, with reference to the drawings. In the drawings fig. 1 shows a reproduction of an SEM ("scanning electron micrograph") image of a first embodiment of an element ac- cording to the invention, fig. 2 shows a reproduction of an SEM image of a second embodiment of an element according to the invention, fig. 3 shows a graph of the maximum supercurrent Ic through the element of fig. 1 as a function of the tempera- ture of the Josephson contact, fig. 4 shows the current-voltage characteristic of the element of fig. 1 for different values of a control current flowing wholly outside the Josephson contact, and fig. 5 shows the current-voltage characteristic of the element of fig. 2 for different values of a control current flowing through the Josephson contact .
Corresponding components in the figures are designated
with the same reference numerals.
Fig. 1 shows a reproduction of an SEM image of a part of an element 1 consisting of a substrate 2 of an oxidized silicon wafer on which a pattern of gold is applied in per se known manner using a sputter or vapour-deposition technique. The gold pattern comprises two U-shapes in mirror relation to one another, each with parallel legs 3, 4 respectively 3', 41 and a connecting leg 5 respectively 5', which U-shapes are mutually connected by a bridge part 6. From the longitudinal edges of bridge part 6 extend two layers of niobium 7, 8 mutually separated by bridge part 6 at a distance of about 0.2 μm. Together with the bridge part 6 of gold, the superconducting niobium layers 7, 8 form a Josephson junction.
Fig. 2 shows a reproduction of an SEM image of a part of an element 20 which differs from element 1 of fig. 1 in the use of metal plate-like conductors 9, 9' on either side of bridge part 6 , the length L of which corresponds with the distance between the the arrowheads 11-11'. It has been found that the configuration of the element 20 results in a markedly shorter transport time than the configuration of the element 1.
Fig. 3 shows the maximum supercurrent Ic through the series connection of layers 7, 6 and 8 of element 1 of fig. 1 as a function of the temperature of the Josephson contact 6. The figure clearly demonstrates the effect that the maximum supercurrent in an element according to the invention is greatly dependent on said temperature, and can therefore be influenced by means of this temperature.
Fig. 4 shows the result of an experiment in which the effect demonstrated in fig. 3 is utilized by an indirect local heating of the Josephson contact 6 by means of a control current which flows successively through a first leg 3, connecting leg 5 and second leg 4 of one of the U-shapes. The control current in this experiment thus flows only outside the Josephson contact 6. The figure shows current-voltage characteristics a, b, c, d and e of the element of fig. 1, (measured through and across the superconducting layers 7 and
8 at a temperature of 4.2 K) for a value of the control current of respectively 0 μA, 90 μA, 180 μA, 270 μA and 500 μA.
Fig. 5 shows the result of an experiment in which the effect demonstrated in fig. 3 is utilized by a direct local heating of the Josephson contact 6 in the element 20 of fig. 2 by means of a control current which flows successively through a first plate-like conductor 9, the bridge part 6 between the two plate-like conductors 9, 9' and the second plate-like conductor 9'. The control current in this experiment therefore flows through the Josephson contact 6. The figure shows current-voltage characteristics a, b, c, d, e and f of element 20, (measured through and across the superconducting layers 7 and 8 at a temperature of 1.7 K) for a value of the control current of respectively 0 μA, 90 μA, 150 μA, 210 μA, 320 μA and 400 μA. Similar current-voltage characteristics are obtained in a configuration wherein the control current is transmitted through one of the plate-like conductors 9, 9' via the bridge part, i.e. the Josephson contact 6, to one of the two superconducting parts 7, 8 of the element, in which configuration the element functions as switching element with three terminals ("three terminal device" ) .
An element according to the invention is applicable as signal amplifier, which can be understood as follows with reference to fig. 2. The element 20 applied as signal amplifier is set to a fixed direct current through the junction (i.e. through layers 7 and 8 in fig. 2) above the critical current, and to a fixed direct current through the control line (i.e. the points 11 and 11' in fig. 2) . As a result a voltage is created across the junction (between 7 and 8) and across the control line (between 11 and 11 ' ) . A small voltage δVιn applied over the control line (input of the amplifier) results in a change of the control current with a quantity δVin/R, wherein R is the resistance of the control line (in the embodiment of fig. 2 this resistance amounts to 2.8 Ω. This change results in a change in the critical current of
the junction, and consequently in a change δVout in the voltage over the outer ends (the output voltage of the amplifier) . The element has a voltage amplification if the inequality δVout/δVin > 1 is satisfied. It has been found that with the element according to fig. 2 this inequality is satisfied at a temperature of 1.7 K.
Although the described embodiments relate to lateral structures, i.e. structures in which the different layers are deposited adjacently of each other on the substrate, the invention is in no way limited to such lateral structures but within the scope of the appended claims also extends to for instance structures in which the layers are wholly or partially stacked on top of each other.
Claims
1. Superconducting element for a micro-electronic circuit, comprising a substrate with a first layer of a superconducting first material provided with a first terminal for an electric current, a second layer of a superconducting second material provided with a second terminal for an electric current, and a third layer of a third material, which third layer forms a Josephson contact between the first and the second layer, characterized in that control means are provided for controlling the occupancy of the energy states in the third layer.
2. Element as claimed in claim 1, characterized in that the control means are provided to control the occupancy subject to the electron temperature in the third layer.
3. Element as claimed in claim 1, characterized in that the control means comprise a structure with two tunnel junctions separated by an intermediate layer, which intermediate layer extends to the third layer of the element.
4. Element as claimed in any of the claims 1-3, charac- terized in that the third material is a metal.
5. Element as claimed in any of the claims 1-3, characterized in that the third material is a degenerately doped semiconductor .
6. Element as claimed in any of the claims 1-3, charac- terized in that the third material forms a constituent of a semiconductor heterostructure, of which the electron gas realized therein can be coupled to the superconducting first and second materials .
7. Element as claimed in either of the claims 5-6, char- acterized in that the semiconductor is selected from the group of materials comprising silicon and compounds of the type III-V.
8. Element as claimed in claim 7, characterized in that the semiconductor is selected from a group of compounds of the type III-V which comprises indium arsenide (InAs) and indium gallium arsenide (InGaAs) .
9. Element as claimed in any of the foregoing claims , characterized in that the distance between the first and the second layer is smaller than about 10~6 m (1 ╬╝m) .
10. Element as claimed in any of the foregoing claims, characterized in that the control means comprise an electrical conductor.
11. Element as claimed in claim 10, characterized in that the electrical conductor extends from a third terminal along a determined distance L of the third layer, separated from this third layer by a fourth layer of a fourth material.
12. Element as claimed in claim 11, characterized in that the length L and the fourth material, the diffusion constant of which is represented by D, are selected such that the value of (L/2)2/D is less than 2 x 10"11 sec.
13. Element as claimed in claim 12, characterized in that the value of (L/2)2/D is less than 2 x 10"12 sec.
14. Element as claimed in claim 11, wherein the third layer comprises a ballistic semiconductor in a heterostructure, characterized in that the fourth material, the length L and the third material, of which the Fermi speed in the heterostructure is represented by vF, are selected such that the value of L/vF is less than 10"12 sec.
15. Element as claimed in any of the claims 11-14, characterized in that the fourth material is identical to the third material .
16. Element as claimed in any of the foregoing claims, characterized in that the first and the second material are selected from the group comprising superconducting elements, superconducting compounds of the type Bl, superconducting compounds of the type A15 and ceramic superconductors .
17. Element as claimed in claim 16, characterized in that the group of superconducting elements comprises niobium (Nb) and lead (Pb) .
18. Element as claimed in claim 16, characterized in that the superconducting compounds of the type Bl comprise niobium nitride (NbN) .
19. Element as claimed in claim 16, characterized in that the superconducting compounds of the type A15 comprise niobium tin (Nb3Sn) .
20. Element as claimed in any of the claims 10-19, characterized in that the electrical conductor comprises a strip of a metal selected from the metals gold (Au) , silver (Ag) and copper (Cu) .
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AU90974/98A AU9097498A (en) | 1997-09-17 | 1998-09-11 | Superconducting element for a micro-electronic circuit |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL1007048A NL1007048C2 (en) | 1997-09-17 | 1997-09-17 | Superconducting element for a micro-electronic circuit. |
| NL1007048 | 1997-09-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| WO1999014811A1 true WO1999014811A1 (en) | 1999-03-25 |
Family
ID=19765686
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/NL1998/000526 WO1999014811A1 (en) | 1997-09-17 | 1998-09-11 | Superconducting element for a micro-electronic circuit |
Country Status (3)
| Country | Link |
|---|---|
| AU (1) | AU9097498A (en) |
| NL (1) | NL1007048C2 (en) |
| WO (1) | WO1999014811A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102008036993A1 (en) | 2007-08-08 | 2010-02-18 | Universität Tübingen | Geometric Π-Josephson contact |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4589001A (en) * | 1980-07-09 | 1986-05-13 | Agency Of Industrial Science & Technology | Quasiparticle injection control type superconducting device |
| JPH01290271A (en) * | 1988-05-18 | 1989-11-22 | Seiko Epson Corp | Josephson transistor |
-
1997
- 1997-09-17 NL NL1007048A patent/NL1007048C2/en not_active IP Right Cessation
-
1998
- 1998-09-11 AU AU90974/98A patent/AU9097498A/en not_active Abandoned
- 1998-09-11 WO PCT/NL1998/000526 patent/WO1999014811A1/en active Application Filing
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4589001A (en) * | 1980-07-09 | 1986-05-13 | Agency Of Industrial Science & Technology | Quasiparticle injection control type superconducting device |
| JPH01290271A (en) * | 1988-05-18 | 1989-11-22 | Seiko Epson Corp | Josephson transistor |
Non-Patent Citations (3)
| Title |
|---|
| J.CUOMO ET AL.: "FABRICATION OF SUPERCONDUCTIVE TRANSISTORS", IBMTDB, vol. 27, no. 2, July 1984 (1984-07-01), pages 1260 - 1267, XP002065047 * |
| PATENT ABSTRACTS OF JAPAN vol. 014, no. 073 (E - 0887) 9 February 1990 (1990-02-09) * |
| TOSHIHIKO MAEMOTO ET AL: "SUPERCONDUCTING WEAK LINK DEVICES USING LEAD ALLOY-INAS/ALGASB HETEROSTRUCTURES", PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON INDIUM PHOSPHIDE AND RELATED MATERIALS, HOKKAIDO, MAY 9 - 13, 1995, no. CONF. 7, 9 May 1995 (1995-05-09), INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, pages 377 - 380, XP000630654 * |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102008036993A1 (en) | 2007-08-08 | 2010-02-18 | Universität Tübingen | Geometric Π-Josephson contact |
| DE102008036993B4 (en) * | 2007-08-08 | 2011-12-29 | Universität Tübingen | Geometric Π-Josephson contact |
Also Published As
| Publication number | Publication date |
|---|---|
| AU9097498A (en) | 1999-04-05 |
| NL1007048C2 (en) | 1999-03-18 |
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