SUPERCONDUCTING JOSEPHSON JUNCTIONS

Abstract: Superconducting Josephson junctions having a layer of niobium nitride on a substrate, an epitaxial layer of a pseudo-binary compound on the layer of niobium nitride, where the pseudo-binary compound has the composi­ tion 3 atomic per cent MgO - 97 atomic per cent CaO, to 97 atomic per cent MgO % 3 atomic per cent CaO, and an epitaxial layer of niobium nitride on the layer of pseudo-­ binary compound. Also a method of making a Josephson junction by depositing a layer of niobium nitride onto a suitable substrate, depositing an epitaxial layer of a pseudo-binary compound onto the layer of niobium nitride, where the pseudo-binary compound has a composition of 3 atomic per cent MgO - 97 atomic per cent CaO, to 97 atomic per cent MgO - 3 atomic per cent CaO, and depositing an epitaxial layer of niobium nitride onto the layer said pseudo-binary compound.

Description: This invention relates to superconducting Josephson junctions. To increase the speed with which an integrated circuit can process information, it is necessary to reduce the distance that the electrical signals have to travel. While this can be accomplished by reducing the size of the circuit components, it can also be accomplished by reducing the electrical resistance in the circuit, which reduces power dissipation and permits closer packing of electrical components. In a superconducting circuit, the electrical resistance is reduced to zero by cooling the circuit below the temperature at which its components become superconducting. While commercial superconducting electronics with medium or large scale integration have not yet been built, some research has been done on designing the components. The state of the art design for a Josephson tunnel junction for superconducting electronics is a trilayer, on a substrate, formed of a layer of superconducting niobium nitride (NbN), an insulating layer of magnesium oxide (MgO), and a second layer of niobium nitride on top. Reference is made to the following prior art documents: Appl. Phys. Lett., vol. 46, no. 11, 1st June 1985, pp. 1098-1099, AIP, New York, US; A. Shoji et al. and IEE Transactions on Magnetics, vol. MAG-23, no. 2, March 1987, pp. 1467-1471, IEEE, New York, US; A. Shoji et al. While the said start of the art design structure is operable, it is not optimal because the lattice spacing of magnesium oxide does not match the lattice spacing of niobium nitride, although they do have the same structure and crystal orientation. Niobium nitride has a B1 (sodium chloride) structure with a lattice constant, (a₀) of 4.38 angstroms; magnesium oxide and calcium oxide have the same B1 structure, but magnesium oxide has a lattice constant of 4.21 angstroms. Because of this mismatch, the initial layer of the top niobium nitride will be partly disordered, the superconducting energy gap reduced, and the operating temperature of the junction reduced. Accordingly, the present invention resides in a superconducting Josephson junction comprising (A) a layer of NbN on a substrate; (B) a solid solution of an epitaxial layer of a pseudo-binary compound on said layer of NbN, said solid solution lying between 3 atomic % MgO and 97 atomic % CaO, and 97 atomic % MgO and 3 atomic % CaO; and (C) an epitaxial layer of NbN on said layer of pseudo-binary compound. The invention also includes a method of making a superconducting Josephson junction characterized by (A) depositing a layer of NbN onto a suitable substrate; (B) depositing an epitaxial layer of a pseudo-binary compound onto said layer of NbN, where said compound comprises from 3 atomic % MgO and 97 atomic % CaO, to 97 atomic % MgO and 3 atomic % CaO; and (C) depositing an epitaxial layer of NbN onto said layer of said compound. It has been discovered that a superconducting Josephson junction can be formed from a pseudo-binary solid solution of magnesium and calcium oxides sandwiched between two niobium nitride layers. Calcium oxide has the same B1 structure as MgO but a lattice constant of 4.84 angstroms. By selecting the proportion of magnesium oxide to calcium oxide in the pseudo-binary layer, we are able to match the lattice spacing of the niobium nitride so that both the pseudo-binary layer and the top layer of niobium nitride are epitaxial with the bottom layer of niobium nitride. The discovery that a pseudo-binary layer of magnesium oxide-calcium oxide can be formed at room temperature is quite remarkable because the equilibrium phase diagram for the MgO-CaO system indicates that at room temperature solid solutions containing between 2% MgO-98% CaO and 98% MgO-2% CaO do not exist. That is, below 1600°C a mixture of Mg 1-x Ca x O where 0.02 < x < 0.98 will crystallize as separate domains of practically pure MgO and practically pure CaO, rather than as a solid solution of Mg 1-x Ca x O. Since MgO-CaO solid solutions have the same structure and orientation as NbN, and epitaxial layers can be grown having the same lattice spacing, the Josephson junctions of this invention have a more uniform oxide layer than do the Josephson junctions formed with MgO. (Theoretical and experimental background is found in J. W. Mathews et al., "Effect of Coherency Strain and Misfit Dislocations on the Mode of Growth of Thin Films," Thin Solid Films 26 pp. 129-134 (1975).) Also, the superior oxide layer of this invention results in a better quality niobium nitride top layer on the oxide layer. Until now, it was very difficult to get a good quality niobium nitride top layer, especially the bottom of the niobium nitride top layer. However, because the oxide layer of this invention has the same lattice spacing as the niobium nitride layer, we are able to obtain a very good quality niobium nitride top layer. As a result, the superconducting transition temperature of the niobium nitride layer is higher. In order that the invention can be more clearly understood, a convenient embodiment thereof will now be described, by way of example, with reference to Figs. 2A to 2D of the accompanying drawings which are side views, in ection, illustrating one method of constructing a Josephson junction. The equilibrium phase diagram shown in Figure 1 was taken from an article by R. C. Doman et al. in the Journal of the American Ceramics Society , July, 1983, titled "Phase Equilibria in the System CaO-MgO," which is herein incorporated by reference. The ordinate gives the temperature in degrees centigrade and the abscissa gives the atomic percent of calcium oxide in the MgO-CaO system. The phase diagram shows that below a temperature of 2370°C, separate solid solutions (SS) of magnesium oxide and calcium oxide form, and that below 1600°C no mixed solid solutions of magnesium oxide and calcium oxide exist where the amount of calcium oxide is greater than 2% or less than 98%. In Figure 2A a substrate 1 has deposited thereon a base electrode 2 of niobium nitride, a barrier layer 3 of the pseudo-binary compound, and a counterelectrode 4 also of niobium nitride. In Figure 2B portions of the counterelectrode barrier layer and base electrode have been etched away. In Figure 2C an insulating layer, preferably of silicon dioxide, has been applied over portions of the structure. In Figure 2D a contact layer 6, also of niobium nitride, has been deposited. Any substrate material that can provide mechanical support for the junction and that helps to promote growth of niobium nitride can be used in this invention. The preferred materials are sapphire (α-Al₂O₃) or magnesium oxide (MgO) as these materials closely match the lattice spacing of niobium nitride. The substrate may be of any size and thickness but is typically about 6 millimeters square to about 50.8 mm (2 inches) diameter and about 0.508 mm (20 mils) thick. The substrate may have any orientation, but we have found that the (100) orientation of a magnesium oxide substrate produces smooth single-crystal layers. Also, we have found that the (111) niobium nitride single-crystal films grown on a sapphire substrate had the best junction properties. The (111) niobium nitride orientation can be obtained if the sapphire orientation is (11 2 0) or (0001). Sapphire is preferred to magnesium oxide as a substrate because it is more available and easier to use. In constructing the superconducting Josephson junction according to this invention, the first step is to deposit a layer of niobium nitride on the substrate. This can be accomplished, for example, by chemical vapor deposition or sputtering, processes that are well known in the art. In chemical vapor deposition, a niobium-containing gas is passed over a hot surface causing a reaction to occur that forms niobium nitride, which deposits upon the substrate. Either radio frequency (RF) sputtering or direct current (DC) sputtering can also be used, but reactive DC sputtering is preferred as it produces a higher superconducting transition temperature. Reactive DC sputtering is performed by sputtering niobium in a gas containing nitrogen and argon (and no oxygen), where the niobium reacts with the nitrogen to form niobium nitride at the substrate. (The conditions for depositing a single crystal niobium nitride superconductor by DC magnetron sputtering are reported in the literature. See, for example, V. L. Noskov et al., Soviet Physics Crystallography, 25(4), 504 (1980).) The niobium nitride layers may be either polycrystalline or single crystal, but single crystal layers are preferred as they are easier to test. The niobium nitride layer may be from 40 nm to 500 nm (400 angstroms to 5000 angstroms) thick, and is preferably from 150 nm to 250 nm (1500 to 2500 angstroms) thick. If the niobium nitride layer is too thick, it becomes difficult to cover the steps that are formed when the layers are etched; and if the niobium nitride layer is too thin, the speed of the circuit decreases. The thickness of the layer is determined by the deposition time. In the next step of the process of this invention, a layer of the pseudo-binary compound is deposited upon the layer of niobium nitride; the pseudo-binary compound grows on the layer of niobium nitride by single crystal epitaxy and has the same structure as the layer of niobium nitride. The composition of the pseudo-binary compound can vary from 3 atomic percent MgO and 97 atomic percent CaO to 97 atomic percent MgO and 3 atomic percent CaO. A preferred range is from 22 to 35 atomic percent CaO (and the remainder, MgO) because we have determined by calculation that the optimal composition is 0.73 atomic percent MgO-0.27 atomic percent CaO, and this range gives a lattice spacing mismatch of 1% or less. The layer of pseudo-binary compound can be deposited, for example, by RF sputtering or by evaporation. Evaporation is accomplished by heating an appropriate mixture of MgO-CaO above the melting point of MgO (2800°C). It should be noted that the composition of the layer of pseudo-binary compound may not be exactly the same as the composition of the material which was used to form the layer due to differences in vapor pressure between the magnesium oxide and calcium oxide as they are being evaporated. However, once the differences in composition have been determined for the particular apparatus and deposition parameters being used, the composition of the starting material can be adjusted to obtain the desired composition of the deposited layer. The thickness of the insulating layer of pseudo-binary compound is preferably about 0.5 nm to about 2.5 nm (5 to about 25 angstroms) as thinner layers may result in superconducting shorts, and thicker layers have greater resistance which reduces the circuit speed. In the next step of the process of this invention, the second niobium nitride layer is deposited upon the layer of pseudo-binary compound. The deposition process and parameters for that layer can be the same as for the first layer of niobium nitride. Once the trilayer on the substrate has been deposited, various processes can be used, as is well known in the art, to form circuit patterns and attach connections. These processes include etching, the application of photoresists, the development of photoresist to form circuit patterns, and the deposition of contact layers. The Josephson junctions of this invention, in addition to being used in superconducting computers, can also be used in other types of apparatus such as radar signal processing apparatus. The invention will now be illustrated with reference to the following Example: EXAMPLE A polished single-crystal sapphire plate 6.35 mm (1/4 inch) by 6.35 mm (1/4 inch) by 0.508 mm (20 mils) thick having a (11 2 0) orientation was placed in a DC magnetron and was heated to 700°C. Using a gas mixture of 0.5333 Pa (4 mtorr) argon, 0.10666 Pa (0.8 mtorr) nitrogen, and 6.6661 mPa (0.05 mtorr) methane, niobium nitride was sputtered onto the substrate a rate of 4 nm (40 angstroms) per minute. The resulting film was about 100 nm (1,000 angstroms) thick and had a (111) crystal orientation. Maintaining the temperature at 700°C, a barrier layer was deposited on the niobium nitride layer from a mixture of 73 mole percent magnesium oxide -- 27 mole percent calcium oxide using RF magnetron sputtering in pure argon at 0.9333 Pa (7 mtorr). The deposition rate was 0.6 nm (6 angstroms) a minute and the barrier film was about 2 nm (20 angstroms) thick. The temperature of the coated substrate was cooled to 150°C and the counter electrode was deposited in the same manner as the base electrode at the same deposition rate. The counter electrode was about 50 nm (500 angstroms) thick and had a (111) crystal orientation. Using conventional photolithography, a photoresist was spun onto a substrate and was exposed and developed to define a junction area. The exposed area was etched using reactive ion etching down to the base electrode as shown in Figure 2B. A second layer of photoresist was applied, leaving an opening for contact to the top electrode. This photoresist was left in place as an insulator as shown in Figure 2C. Finally, a contact layer of niobium nitride was applied as shown in Figure 2D in the same manner as the other layers of niobium nitride. The completed Josephson junction was tested in a magnetically shielded dewar filled with liquid helium at 4.2°K by passing a slowly increasing current through it while measuring the voltage. When the voltage had increased to about 7 millivolts, the experiment was terminated. Figure 3 is a graph of current versus voltage which gives the results of this experiment. The graph proves that conduction through the Josephson junction was by tunnelling rather than by metal conduction. The graph also shows that the slope of Curve A, the mixed oxide, was highest at a voltage of 4.8 millivolts (the gap voltage). This compares very favorably with the gap voltage of a typical Josephson junction using a magnesium oxide barrier layer, Curve B, which has a gap voltage of 4.5 millivolts. The significantly higher gap voltage indicates that the Josephson junction of this invention will be able to operate at a higher superconducting transition temperature since the gap voltage is directly proportional to the transition temperature: where V g equals the gap voltage, k B equals Boltzmann's constant and, and T c is the superconducting transition temperature.

Claims: A superconducting Josephson junction characterized in that said junction comprises (A) a layer of NbN on a substrate; (B) a solid solution of an epitaxial layer of a pseudo-binary compound on said layer of NbN, said solid solution lying between 3 atomic % MgO and 97 atomic % CaO, and 97 atomic % MgO and 3 atomic % CaO; and (C) an epitaxial layer of NbN on said layer of pseudo-binary compound. A junction according to claim 1, characterized in that the substrate is sapphire having a (1120) or a (0001) orientation. A junction according to claim 1, characterized in that the substrate is MgO having a (100) orientation. A junction according to claim 1, 2 or 3, characterized in that layers (A) and (C) are each from 40 to 500 nm (400 to 5000 Angstroms) thick. A junction according to claim 1, 2, 3 or 4, characterized in that the layers of NbN are both single crystal layers. A junction according to any of claims 1 to 5, characterized in that the substrate consists of single crystal MgO or single crystal sapphire; the layer of NbN is from 40 to 500 nm (400 to 5000 Angstroms) thick on said substrate; the epitaxial layer is of a single crystal pseudo-binary compound from 0.5 to 2.5 nm (5 to 25 Angstroms) thick, having a composition comprising from 78 atomic % MgO and 22 atomic % CaO, to 65 atomic % MgO and 35 atomic % CaO; and the layer of epitaxial NbN is from 40 to 500 nm (400 to 5000 Angstroms) thick. A junction according to claim 6, characterized in that the layers of single crystal NbN are each from 150 to 250 nm (1500 to 2500 Angstroms) thick. A junction according to claim 6 or 7, characterized in that the layers of NbN are both single crystal or polycrystalline layers. A method of making a superconducting Josephson junction characterized by (A) depositing a layer of NbN onto a suitable substrate; (B) depositing an epitaxial layer of a pseudo-binary compound onto said layer of NbN, where said compound comprises from 3 atomic % MgO and 97 atomic % CaO, to 97 atomic % MgO and 3 atomic % CaO; and (C) depositing an epitaxial layer of NbN on said layer of said compound. A method according to claim 9, characterized in that the substrate is sapphire having a (1120) or a (0001) orientation. A method according to claim 10, characterized in that the substrate is MgO having a (100) orientation. A method according to claim 9, 10 or 11, characterized in that the NbN layers are each from 40 to 500 nm (400 to 5000 Angstroms) thick. A method according any of claims 9 to 13, characterized in that the NbN layers are deposited using reactive DC sputtering in an atmosphere of N₂ and Ar. A method according any of claims 9 to 13, characterized in that the pseudo-binary compound is deposited using RF sputtering. A method according any of claims 9 to 14, characterized in that the pseudo-binary compound is deposited using evaporation. A method according any of claims 9 to 15, characterized in that the layer of pseudo-binary compound is from 0.5 to 2.5 nm (5 to 25 Angstroms) thick. A method according to any of claims 9 to 16, characterized in that the layers of NbN are both single crystal layers. A method according to any of claims 9 to 17, characterized in that the junction is etched, a photoresist is applied and developed on said junction to form a circuit pattern, and contact layers are deposited. A superconducting integrated circuit characterized in that said circuit comprises an array of electrically connected junctions according to any of claims to 1 to 8. A superconducting processor characterized in that said processor comprises a multiplicity of electrically connected superconducting integrated circuits according to claim 19.

Inventors: BRAGINSKI, ALEKSANDER IGNACE, GAVALER, JOHN RAYMOND, JANOCKO, MICHAEL ANDREW, TALVACCHIO, JOHN JOSEPH
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1987
Main classification: H01L39
IPC codes: H01L39