Superconducting device

Abstract: A pair of superconducting electrodes are so formed as to interpose a semiconductor therebetween, and a control electrode is formed on the semiconductor through an insulator film so as to control the superconductive weak coupling state in the semiconductor between the supercon­ ducting electrodes. The distance between the superconduct­ ing electrodes is determined by the thickness of the super­ conductor interposed between the two electrodes, whereby the interelectrode distance is settled with a high precision to improve the uniformity of the device characteristic. And in an arrangement where two superconducting electrodes are formed on a semiconductor layer and the superconductive weak coupling state between such two electrodes is controlled by a third electrode, the gain is increadable by furnishing a varied impurity distribution in the semiconductor layer.

Description: The present invention relates to a superconducting device. In the conventional switching devices operative at extremely low temperatures, there is known a Josephson element. It has some advantages including a high switching speed and a small power consumption of several microwatts or so. However, this element is basically a two-terminal device, which is therefore not adapted for use in forming a circuit by the prior techniques accumulated in connection with the known semiconductor transistors, and employment of an AC power supply is requisite. In an attempt to solve the problems mentioned above, a superconducting device has been developed by means of combining a superconductor and a semiconductor, as disclosed in Japanese Patent Laid-open No. 57-106186. In this device, a pair of superconducting layers are deposited on a semiconductor substrate with the respective end faces opposed to each other, and a control conductor is disposed in such opposed portions through an insulator layer. In such prior art, a superconducting layer formed on the semiconductor substrate is separated into two superconducting electrodes by photolithography and etching. Subsequently the portion thus separated is thermally oxidized to form an insulator film, and then a desired control electrode is formed by evaporation. For enabling the two superconductors of such device to interact in superconductive weak coupling through a semiconductor at extremely low temperatures, it is necessary to dispose the two superconducting electrodes so close to each other that the distance therebetween becomes about 10 times the coherence length of an electron pair in the superconductor, i.e. less than 0.5 µm (microns) or so. However, in the conventional structure, it is impossible to facilitate formation of a control electrode in a narrow gap of 0.5 microns or less, whereby the dimensional precision cannot be enhanced in manufacture to eventually bring about difficulties in attaining high reliability and uniformity. Another known superconducting device composed of a semiconductor with an electrode to control the characteristics thereof is JOFET (Hybrid J osephson F ield E ffect T ransistors) proposed by T. D. Clark as reported in J. Appl. Phys., vol. 51, pp. 2736 - 2743, 1980. In such JOFET, an electrode of a superconductor is formed on a semiconductor substrate doped to a high concentration, or a dope layer is formed on a high-purity buffer layer and an electrode of a superconductor is formed thereon. In such device, control of its characteristics is executed by applying a voltage to the control electrode and extending the inversion layer from the control electrode side toward the semiconductor side. However, since the impurity concentration is high in the semiconductor layer immediately below the control electrode, the voltage to be applied to the control electrode reaches as high as several hundred millivolts while an output voltage obtainable is approximately equal to or even lower than that. Accordingly, it has been impossible heretofore to use circuits similar to those employed in the prior art of semiconductors. In addition to the above, there are the following references related to superconducting devices. 1) IEEE Trans. Magn., vol. MAG-15, pp. 435 - 438. 1979 2) IEEE Trans. Electron Devices, vol. ED-28, pp. 1394 - 1397, 1981 3) Japanese Patent Laid-open No. 57-176781 It is known from said article entitled "Feasability of Hybrid Josephson Field Effect Transistors" by T.D. Clark et al in the Journal of Applied Physics, Vol. 51 May 1980 (p. 2736 to 2743) to form a superconducting device in which source and drain superconducting electrodes are formed in contact with a semiconductor substrate and a gate (control) electrode is formed on the substrate but separated therefrom by an insulator film. Thus, this document corresponds to the precharacterising part of claim 1. The present invention seeks to provide a structure for superconducting device, preferably one of the transistor type, which is composed of a combination of a semiconductor and a superconductor and is capable of enhancing the manufacturing precision to consequently improve the uniformity and reproducibility of the device characteristics. Therefore, according to a first aspect of the present invention, there is provided a superconducting device comprising a semiconductor substrate, a pair of superconducting electrodes, each of the superconducting electrodes being in contact with the semiconductor, and a control electrode formed on the semiconductor with an insulator film therebetween; wherein: a semiconductor projection, which is a part of the semiconductor, is interposed between the superconducting electrodes and equals the distance between the superconducting electrodes; and the control electrode is formed in an upper or lower portion of the semiconductor projection through the insulator film. According to a second aspect of the present invention, there is provided a superconducting device comprising a semiconductor substrate, a pair of superconducting electrodes, each of the superconducting electrodes being formed on the semiconductor substrate, and a control electrode formed on the semiconductor substrate with an insulating film therebetween: characterised in that: a semiconductor projection, which is a part of the semiconductor substrate, is interposed between the superconducting electrodes and equals the distance between the superconducting electrodes; the control electrode is formed on the top of the projection with the insulator film therebetween; and at the surfaces of the projection and the semiconductor in contact with the superconducting electrodes a diffusion layer is provided, leaving a low impurity concentration region at the top of the projection. The superconducting device of the present invention may thus be suitable for high-density integration. The present invention may also make it possible to provide a superconducting device in which the parasitic capacity is reducible to enable a high speed operation. Furthermore, the present invention may provide a superconducting device which retains a great gain in controlling the coupling state between superconducting electrodes by an applied voltage. It is highly desirable taht the distance between the superconducting electrodes should be less than 10 times the coherence length ξn of an electron pair. The manufacturing precision can be enhanced by determining the interelectrode distance in accordance with the thickness of the semiconductor interposed between the superconducting electrodes, whereby the uniformity of the device characteristics can be improved as well. Determination of the interelectrodes distance in accordance with the semiconductor thickness is achieved by first etching a semiconductor substrate to form a projection thereon and then disposing a pair of super-conducting electrodes on the two sides of such projection. According to this process, the accuracy of the spatial distance between the superconducting electrodes is dependent on the precision of the semiconductor substrate. Therefore a high manufacturing precision is attainable regardless of any other condition such as the hardness of the superconductor material. It is desired that the height of the projection be equivalent to at least the thickness of the superconducting electrode. The electrode for controlling the superconductive weak coupling state is formed in an upper or lower portion of the semiconductor projection through an insulator film. It is a matter of course that the control electrode may be provided in each of upper and lower portions. The superconducting device of such structure furnished with a control electrode particularly in an upper portion of the projection is of a planar type adapted for realizing an arrangement of high-density integration. Reduction of a parasitic capacity is achievable by shaping a channel-defining semiconductor into an insular contour. That is, first an insular semiconductor is formed on one side of the insulator film, then superconducting electrodes are formed in such a manner as to interpose the semiconductor therebetween, and subsequently a control electrode is formed on the other side of the insulator film. The above procedure eliminates the substrate capacity to eventually accomplish a satisfactory superconducting device which is capable of performing a high speed operation. The superconducting device of such structure can be shaped into a planar type with facility so that high-density integration thereof is rendered possible. 2) At least two superconducting electrodes are disposed adjacent each other on a semiconductor layer (or substrate), and at least one control electrode is formed for controlling current flow between the superconducting electrodes to change the interelectrode superconductive weak coupling. Preferably, the distribution of impurities contained in the semiconductor layer is so arranged as to include at least one region of a high impurity concentration above the average and at least one region of a low impurity concentration below the average. Due to such varied concentration distribution of impurities contained in the semiconductor layer that serves as the superconducting electrode, the gain is increasable in controlling the interelectrode coupling state by an applied voltage. Since the semiconductor layer in the conventional superconducting device contains impurities uniformly at a high concentration, attainable extension of a charge storage layer or inversion layer is relatively small in comparison with the voltage applied to the control electrode. However, in the present invention where the impurities are in a varied distribution, the extension of the charge storage layer or inversion layer is caused in a different manner as compared with that induced in a uniform distribution of impurities, so that application of a lower voltage is sufficient to extend the charge storage layer or inversion layer with facility, hence increasing the gain. For example, in a preferred embodiment of the present invention where two superconducting electrodes are disposed on one side of a semiconductor layer while a control electrode is disposed on the other side of the semiconductor layer through an insulator layer, the impurity concentration in a semiconductor region proximate to the control electrode is made to be high while the impurity concentration in other semiconductor regions proximate to the superconducting electrodes is made to be low in such a manner that the concentration distribution is formed in the direction of depth of the semiconductor layer. In such impurity distribution, the charge storage layer is extendable readily from the high concentration layer toward the low concentration layer by the application of a low control voltage. Therefore, utilizing the characteristic phenomenon mentioned above, the superconductive weak coupling state can be changed with facility by a low control voltage to eventually bring about an effect of increasing the gain. As for the inversion layer, its extension is not induced easily until the control voltage exceeds a given value (i.e. until the inversion layer extends to the high impurity concentration layer), but upon rise of the voltage to cause extension of the inversion layer to the low impurity concentration layer, desired extension is induced readily even with a subsequent slight rise of the voltage. Consequently, utilization of such characteristic phenomenon realizes a satisfactory superconducting device of a great gain in which the characteristic thereof remains substantially unchanged until arrival of the control voltage at a certain fixed value out is sharply changed upon subsequent excess of the control voltage over the fixed value. Embodiments of the present invention will now be described in detail, by way of example, wich reference to the accompanying drawings, in wich: Fig. 1 (a) through (d) are sectional views illustrating the steps of producing a first embodiment of the superconducting device according to the present invention; Fig. 2 is a sectional view of a second embodiment of the superconducting device according to the invention; Fig. 3 (a) and (b) are a sectional view and a front view, respectively, of a third embodiment of the superconducting device according to the invention; and Fig. 4 is a sectional view of a fourth embodiment of the superconducting device according to the invention. Embodiment 1 Hereinafter a first embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 (c) is a sectional view of a superconducting transistor structure according to the present invention, wherein a projection 4 is formed in a portion of a semiconductor substrate 1, and a pair of mutually opposed superconducting electrodes 2 and 3 are disposed on the two sides of the projection 4. And a control electrode 5 with insulated lateral surfaces is disposed on the projection 4 through an insulator layer 6. In case the superconducting electrodes 2 and 3 are composed of a semiconductor such as a silicon, a narrow gap of 0.2 µm (microns) is kept therebetween. Superconducting electron pairs are emitted from the superconducting electrodes 2 and 3 onto the semiconductor substrate 1 or the projection 4 thereof and then are advanced to flow in the semiconductor interposed between the superconducting electrodes. Such flow of the superconducting electron pairs is controllable by a voltage applied to the control electrode 5. In this superconducting device, the superconducting electrodes 2 and 3 are coupled to each other by superconductive weak interaction when cooled below the transition temperature of the superconducting electrode material, and the maximum Josephson current Im flowing between the two superconducting electrodes due to such coupling is given by Im = 4πΔ/2eR N where Δ is gap energy of the superconducting electrodes 2 and 3; e is an elementary charge; and R N is a normal-conduction tunnel resistance in the superconductive weak coupling. In order to achieve such superconductive weak coupling, it is desired that the gap between the two superconducting electrodes 2 and 3 be smaller than 300 nanometers and the two electrodes be spatially separated from each other. When a negative or positive voltage is applied to the control electrode 5 with respect to the superconducting electrode 2 or 3, a positive or negative charge is induced on the semiconductor side of the interface between the semiconductor and the insulator film 6, so that the state of a tunnel barrier is altered with such charge to eventually vary the resistance R N to a greater value, hence reducing the maximum Josephson current Im which is permitted to flow between the electrodes 2 and 3 without causing generation of a voltage. The steps of producing this device will now be described below with reference to Fig. 1 (a) through (d). First a substrate 1 composed of monocrystal silicon is oxidized for an hour in a steam-containing oxygen atmosphere at 1200°C to form thereon an insulator layer 6 of SiO₂ having a thickness of 80 nanometers, and then a control electrode 5 of polycrystal silicon is formed to a thickness of 0.3 µm (microns) by electron beam evaporation in a high vacuum receptacle, as shown in Fig. 1 (a). Subsequently, using an electron beam resist as a mask, plasma etching is executed with CF₄ gas to a portion of the substrate 1, as shown in Fig. 1 (b). In the next step, superconducting electrodes 2 and 3 composed of Pb and having a thickness of 300 nanometers are formed by resistance heating evaporation and lift-off. In this step, the superconducting electrodes may be formed in contact with a projection 4 as shown in Fig. 1 (c) or may be formed separately therefrom as shown in Fig. 1 (d). And finally the lateral surface of the control electrode 5 is thermally oxidized to form an insulator layer 7 thereon. A material suitable for such insulator layers 6 and 7 may be selected from SiO, Si₃N₄ and Al₂O₃ in addition to the aforesaid SiO₂. As for the semiconductor, its material may be Si or Ge containing an impurity such as P, B or As at a concentration higher than 10¹⁹ cm⁻³; or GaAs, InAs or InP containing an impurity such as Si, Zn or Ge at a concentration higher than 10¹⁷ cm⁻³. The control electrode 5 may be composed of Ti, W, Mo, Poly-Si, Al, Cu or Pt as well. And a material for the superconducting electrodes 2 and 3 may be Pb or Pb-based alloy (e.g. Pb-Au, Pb-In, Pb-Au-In or Pb-Bi); or Nb or its compound (e.g. NbN, Nb₃Al, Nb₃Ge or Nb₃Sn). In the superconducting transistor thus produced, the projection 4 serving as a channel layer can be easily formed with a high accuracy to have a dimension (approx, 0.5 µm (microns) or less although different depending on the semiconductor material, impurity concentration and other condition) which is about 10 times the coherence length of the superconductor. Due to the high precision attained with respect to the spatial distance between the superconducting electrodes, it becomes possible to minimize the characteristic variation as well as to achieve a high yield rate in manufacture. Furthermore, relative to the principle of operation based on the phenomenon that the superconductive weak coupling state between the two superconducting electrodes 2 and 3 is changed by a voltage applied to the control electrode 5, any malfunction is preventable in this exemplary embodiment where the insulator layer 7 is formed around the control electrode 5 to ensure interelectrode separation. Since the superconducting electrodes 2 and 3 are disposed opposite to each other, current flow therebetween is rendered ready to bring about a higher operation speed in the superconducting transistor. In addition, a self-align process is applicable to eventually reduce the number of masks and that of steps required, hence facilitating the manufacture in view of the process. Embodiment 2 In a second embodiment of the present invention shown in Fig. 2, a control electrode 25 composed of niobium and having a thickness of 200 nanometers and a width of 10 µm (microns) is formed on a substrate 21 of monocrystal silicon which is covered with an oxide film 28 of 0.7 µm (microns) in thickness, and the surface of the control electrode 25 is oxidized to form an insulator film 26 thereon. Subsequently a semiconductor layer 24 composed of polycrystal or amorphous silicon and having a thickness of 300 nanometers is formed by electron beam evaporation in a high vacuum receptacle, and the semiconductor 24 is etched, partially or entirely by the same method as employed for the first embodiment with an electron beam resist used as a mask, to form a projection 29. It is desired that the etching be executed in a depth equivalent to at least the thickness of superconducting electrodes 22 and 23 which are to be formed in the next step. The superconductor layer is composed of niobium and measures 200 nanometers in thickness. In the case of using a fast-etchable material such as niobium or a compound thereof, it is not exactly easy to attain a high dimensional precision because of deformation of the resist material or the like during the etching. However, according to the method mentioned above, the precision of the spatial distance between the two superconducting electrodes is determined regardless of the superconductor material and merely by the manufacturing precision of the semiconductor substrate, hence not dependent on any ther condition such as the hardness of the superconductor material. Consequently, even when the superconductor material is changed from Pb alloy to Nb or its compound, high-precision processing can be performed to ensure remarkable uniformity in the device characteristic, whereby a high yield rate is attainable in manufacture. Embodiment 3 In this embodiment, as shown in Fig. 3 (a), a superconducting device is constituted by first forming electrodes 52 and 53 of a superconductor on either a substrate 51, which becomes an insulator at extremely low temperatures, or an insulator film 58 provided previously on the substrate 51, then on the electrodes 52 and 53 forming a semiconductor layer 54 having a projection 59, further forming an insulator layer 56 thereon, and finally forming a control electrode 55 on the insulator layer. The semiconductor layer, where a superconducting or normal conducting current flows, is electrically isolated from the substrate so that no substrate capacity is existent therein. Accordingly it becomes possible to increase the switching speed in the circuit operation. Since a high resistance material is usable for the substrate, the necessity of providing particular means to separate the components from one another can be eliminated when arraying a multiplicity of superconducting devices on the substrate. A heat treatment is effective for improving the crystalline and electrical properties of the semiconductor layer. In this case, however, there arises a problem that deterioration may be induced in the superconductivity of the superconductor located under the semiconductor layer. As a result of the studies accomplished by the present inventors, it has been confirmed that such deterioration in the characteristic of the superconductor under the semiconductor layer is preventable despite a heat treatment of the semiconductor layer if any of NbN, Nb₃Si, Nb₃Ge, MoN and V₃Si is used as a material for the superconductor. This discovery permits a heat treatment of the semiconductor layer without raising the aforementioned problem, hence enabling monocrystallization of the [100] face principally by, for example, increasing the crystal grain diameter in the semiconductor or recrystallizing it to consequently improve the characteristic of the superconducting device. Monocrystallization of the [100] face is achievable by forming the semiconductor layer astride the superconductor and the insulator film, as shown in Fig. 3 (b). Embodiment 4 A fourth embodiment of the superconducting device according to the present invention has such a feature that, for the purpose of realizing a planar structure and increasing the gain, two impurity introduced regions are formed on a semiconductor substrate to reduce the Schottky barrier, and a low impurity concentration region is formed therebetween in such a manner as to be disposed in a projection of a height which is equivalent to or smaller than the thickness of superconducting electrodes formed on the semiconductor. Fig. 4 is a sectional view showing principal portions of the fourth exemplary superconducting device embodying the present invention. The surface of a silicon substrate 501 containing boron as an impurity at a concentration of 1 x 10¹⁴ cm⁻³ or less is oxidized to form thereon an oxide film 502 of SiO₂ to a thickness of 30 nanometers and, after a photoresist pattern is formed thereon to be used as a mask, the surface of the silicon substrate is processed by plasma etching with CF₄ gas or by ion beam etching with Ar or CF₄ gas, thereby forming a projection 503. It is desired that the height of the projection 503, which corresponds to the amount of the silicon substrate removed by etching, should be equivalent to or smaller than the thickness of superconducting electrodes to be formed later. In this embodiment, such height is selected to be 200 manometers. And a sesirable width of the projection 503 ranges from 0.3 to 0.1 µm (micron). Subsequently, boron is diffused throughout the entire surface of the sample at a surface concentration of 1 x 10¹⁹ cm⁻³ to form a diffusion layer 504. The process of diffusion may be replaced with ion implantation in forming the layer 504 to accomplish the object of this embodiment. An exemplary thickness of the diffusion layer is selected to be 50 nanometers from the surface. Such diffusion layers are formed on the two sides of the projection 503 while a low impurity concentration region is left at the center. In the next step, niobium is evaporated by electron beam heating in a high vacuum receptacle at 1 x 10⁻⁶ Pa or less, whereby a niobium film is deposited to a thickness of about 200 nanometers. For deposition of such niobium film, sputtering is also applicable in place of electron beam evaporation. With a photoresist pattern used as a mask, the niobium film is processed by plasma etching with CF₄ gas to produce superconducting electrodes 505 and 506. The projection 503 and the superconducting electrodes 505 and 506 are existent substantially at the same height to realize the aim of flattening the device to a planar structure. Subsequently an oxide film of SiO₂ is formed by vapor phase growth to a thickness of 50 nanometers and then is processed by plasma etching to form an insulator film 507. And in the final step, a niobium film is formed by vacuum evaporation and is processed by plasma etching to produce a control electrode 508, hence completing the superconducting device of the present invention. In forming the SiO₂ film, the aforesaid vapor phase growth is replaceable with sputtering. An integrated circuit manufactured by the use of such superconducting device is cooled to the temperature (4.2 K) of liquid helium and is placed in operation. Then emission of superconducting electrons into the semiconductor is expedited due to that the impurity concentration in a region corresponding to the channel is as low as 1 x 10¹⁴ cm⁻³ in this superconducting device and also that the diffusion layer is existent in a portion of the semiconductor adjacent to the superconductor And the coherent length in the semiconductcr is changed by a voltage of several millivolts applied to the control electrode, whereby the current flowing between the two superconducting electrodes is variable 10 to 100 times. Furthermore, since the device is planar in shape as mentioned, wiring on the control electrode and lead layers for connection thereof can be arrayed in a multilayer arrangement three-dimensionally to realize high density integration and an enhanced yield rate in manufacture. In this embodiment where the oxide film 502 is previously formed to produce a gate insulator film, the interface level can be lowered to stabilize the device operation. It is a matter of course that silicon used for the semiconductor is replaceable with some other suitable substance such as GaAs, InAs, InP or InSb. Also the same effect is achievable if niobium used for the superconducting electrodes is replaced with Pb alloy; Nb compound such as NbN, Nb₃Si, Nb₃Al or Nb₃Sn; or Mo compound such as MoN. As for the impurity, boron may be replaced with phosphorus, arsenic or antimony. Although it is preferred to employ an impurity of the same conduction type as that of the substrate, an impurity of a different conduction type is also usable to attain the aimed effect of the invention.

Claims: A superconducting device comprising a semiconductor substrate (1,24,54), a pair of superconducting electrodes (2,3,22,23,52,53), each of the superconducting electrodes being in contact with the semiconductor, and a control electrode (5,25,55) formed on the semiconductor with an insulator film (6,26,56) therebetween; characterised in that: a semiconductor projection (4,29,59), which is a part of the semiconductor (1,24,54), is interposed between the superconducting electrodes (2,3,22, 23,52,53) and equals the distance between the superconducting electrodes (2,3,22, 23,52,53); and the control electrode (5,25,55) is formed in an upper or lower portion of the semiconductor projection (4,29,59) through the insulator film (6,26,56). A superconducting device according to claim 1, wherein the distance between the superconducting electrodes (2,3,22,23,52,53) is less than 10 times the coherence length of an electron pair. A superconducting device according to claim 1 or claim 2, wherein the height of the projection (4,29,59) is equal to the thickness of the superconducting electrodes. A superconducting device according to any one of claims 1-3, wherein an impurity contained in the semiconductor projection (4,29,59) is so distributed as to form at least one high impurity concentration region having a concentration above the average value, and at least one low impurity concentration region having a concentration below the average value. A superconducting device comprising a semiconductor substrate (501), a pair of superconducting electrodes (505, 506), each of the superconducting electrodes being formed on the semiconductor substrate (501), and a control electrode (508) formed on the semiconductor substrate with an insulating film (507) therebetween: characterised in that: a semiconductor projection (503), which is a part of the semiconductor substrate (501), is interposed between the superconducting electrodes (505,506) and equals the distance between the superconducting electrodes (505,506); the control electrode (508) is formed on the top of the projection (503) with the insulator film (507) therebetween; at the surfaces of the projection (503) and the semiconductor (501) in contact with the superconducting electrodes (505,506) a diffusion layer (504) is provided, leaving a low impurity concentration region (502) at the top of the projection (503).

Inventors: AOKI MASAAKI, HARADA YUTAKA, HIRANO MIKIO, KAWABE USHIO, MIYAKE MUTSUKO, NISHINO TOSHIKAZU, AOKI, MASAAKI, HARADA, YUTAKA, HIRANO, MIKIO, KAWABE, USHIO, MIYAKE, MUTSUKO, NISHINO, TOSHIKAZU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1985
Main classification: H01L39
IPC codes: H01L21, H01L39