Superconducting device having an extremely thin superconducting channel formed of oxide superconductor material

Abstract: A superconducting device comprising a substrate having a principal surface, a non-superconducting oxide layer having a similar crystal structure to that of the oxide superconductor, a first and a second superconducting regions formed of c -axis oriented oxide superconductor thin films on the non-superconducting oxide layer separated from each other and gently inclining to each other, a third superconducting region formed of an extremely thin c -axis oriented oxide superconductor thin film between the first and the second superconducting regions, which is continuous to the first and the second superconducting regions.

Description: Background of the Invention Field of the invention The present invention relates to a superconducting device, and more specifically to a superconducting device having an extremely thin superconducting channel formed of oxide superconductor material. Description of related art Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature. Josephson device is one of well-known superconducting devices. However, since Josephson device is a two-terminal device, a logic gate which utilizes Josephson devices becomes complicated configuration. Therefore, three-terminal superconducting devices are more practical. Typical three-terminal superconducting devices include two types of super-FET (field effect transistor). The first type of the super-FET includes a semiconductor channel, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on both side of the semiconductor channel. A portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulating layer on the portion of the recessed or undercut rear surface of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode. A superconducting current flows through the semiconductor layer (channel) between the superconductor source electrode and the superconductor drain electrode due to the superconducting proximity effect, and is controlled by an applied gate voltage. This type of the super-FET operates at a higher speed with a low power consumption. The second type of the super-FET includes a channel of a superconductor formed between a source electrode and a drain electrode, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel. Both of the super-FETs mentioned above are voltage controlled devices which are capable of isolating output signal from input one and of having a well defined gain. However, since the first type of the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be positioned within a distance of a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, a distance between the superconductor source electrode and the superconductor drain electrode has to be made less than about a few ten nanometers, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material. However, it is very difficult to conduct a fine processing such as a fine pattern etching, so as to satisfy the very short separation distance mentioned above. On the other hand, the super-FET having the superconducting channel has a large current capability, and the fine processing which is required to product the first type of the super-FET is not needed to product this type of super-FET. In order to obtain a complete ON/OFF operation, both of the superconducting channel and the gate insulating layer should have an extremely thin thickness. For example, the superconducting channel formed of an oxide superconductor material should have a thickness of less than five nanometers and the gate insulating layer should have a thickness more than ten nanometers which is sufficient to prevent a tunnel current. In the super-FET, since the extremely thin superconducting channel is connected to the relatively thick superconducting source region and the superconducting drain region at their lower portions, the superconducting current flows substantially horizontally through the superconducting channel and substantially vertically in the superconducting source region and the superconducting drain region. Since the oxide superconductor has the largest critical current density J c in the direction perpendicular to c -axes of its crystal lattices, the superconducting channel is preferably formed of a c -axis oriented oxide superconductor thin film and the superconducting source region and the superconducting drain region are preferably formed of a -axis oriented oxide superconductor thin films. In a prior art, in order to manufacture the super-FET which has the superconducting channel of c -axis oriented oxide superconductor thin film and the superconducting source region and the superconducting drain region of a -axis oriented oxide superconductor thin films, a c -axis oriented oxide superconductor thin film is formed at first and the c -axis oriented oxide superconductor thin film is etched and removed excluding a portion which will be the superconducting channel. Then, an a -axis oriented oxide superconductor thin film is deposited so as to form the superconducting source region and the superconducting drain region. In another prior art, at first an a -axis oriented oxide superconductor thin film is deposited and etched so as to form the superconducting source region and the superconducting drain region, and then a c -axis oriented oxide superconductor thin film is deposited so as to form the superconducting channel. The European Patent Application N° EP-A-0 475 838 describes a superconducting device comprising a substrate coated with a non-superconducting oxide layer. In order to manufacture such a device, a first and a second superconducting regions are formed of c-axis oriented oxide superconducting thin films, deposited directly on the non-superconducting oxide layer, and are separated from each other by a third oxide superconducting region of an extremely thin thickness. On the other hand, the European Patent Application N° EP-A-0 477 103 describes a superconducting device, in which a first oxide superconductor thin film having a very thin thickness is formed on a principal surface of a substrate, and a stacked structure of a gate insulator and a gate electrode is formed on a portion of this first thin film. Then, a first and second superconducting regions having a relatively thick thickness are formed at opposite sides of the gate electrode, and are electrically isolated from the gate electrode. However, in the prior art, the oxide superconductor thin film is degraded during the etching so that the superconducting characteristics is affected. In addition, the etched surface of the oxide superconductor thin film is roughened, therefore, if another oxide superconductor thin film is formed so as to contact the rough surface, an undesirable Josephson junction or a resistance is generated at the interface. By this, the super-FET manufactured by the above conventional process does not have an enough performance. Summary of the Invention Accordingly, it is an object of the present invention to provide a superconducting device having a superconducting region constituted of an extremely thin oxide superconductor film, which have overcome the above mentioned defects of the conventional ones. The above and other objects of the present invention are achieved in accordance with the present invention by a superconducting device comprising: a substrate having a principal surface; a superconducting source region and a superconducting drain region formed of an oxide superconductor thin film electrically isolated from each other disposed on a non-superconducting oxide layer on the principal surface of the substrate, the non-superconducting oxide layer having a crystal structure similar to that of the oxide superconductor thin film; an extremely thin superconducting channel formed or a c -axis oriented oxide superconductor thin film which electrically connects the superconducting source region and the superconducting drain region, so that superconducting current can flow through the superconducting channel between the superconducting source region and the superconducting drain region ; and a gate electrode on a gate insulator on the superconducting channel for controlling the superconducting current flowing through the superconducting channel , wherein the superconducting source region and the superconducting drain region are formed of a c -axis orientated oxide superconductor thin film, and the superconducting source region the superconducting drain region have upper surfaces gently inclined to the superconducting channel by a tilt angle of less than 40°. In the superconducting device in accordance with the present invention, upper surfaces of the first and second superconducting regions gently inclines to the third superconducting region of an extremely thin oxide superconductor thin film. Therefore, superconducting current flows into or flows from the third superconducting region efficiently so that the current capability of the superconducting device can be improved. In the first embodiment, the third superconducting region forms a superconducting channel, so that superconducting current can flow between the first and second superconducting region through the third superconducting region. In this case, the superconducting deyice further includes a gate electrode formed on the third superconducting region, so that the superconducting device constitutes a super-FET, and the superconducting current flowing between the first and second superconducting region through the third superconducting region is controlled by a voltage applied to the gate electrode. In the superconducting device in accordance with the present invention, the non-superconducting oxide layer preferably has a similar crystal structure to that of a c -axis oriented oxide superconductor thin film. In this case, the superconducting channel of a c -axis oriented oxide superconductor thin film can be easily formed. Preferably, the above non-superconducting oxide layers is formed of a Pr 1 Ba 2 Cu 3 O 7-ε oxide. A c -axis oriented Pr 1 Ba 2 Cu 3 O 7-ε thin film has almost the same crystal lattice structure as that of a c -axis oriented oxide superconductor thin film. It compensates an oxide superconductor thin film for its crystalline incompleteness at the bottom surface. Therefore, a c -axis oriented oxide superconductor thin film of high crystallinity can be easily formed on the c -axis oriented Pr 1 Ba 2 Cu 3 O 7-ε thin film. In addition, the effect of diffusion of the constituent elements of Pr 1 Ba 2 Cu 3 O 7-ε into the oxide superconductor thin film is negligible and it also prevents the diffusion from substrate. Thus, the oxide superconductor thin film deposited on the Pr 1 Ba 2 Cu 3 O 7-ε thin film has a high quality. In a preferred embodiment, the oxide superconductor is formed of high-T c (high critical temperature) oxide superconductor, particularly, formed of a high-T c copper-oxide type compound oxide superconductor for example a Y-Ba-Cu-O compound oxide superconductor material, a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a TI-Ba-Ca-Cu-O compound oxide superconductor material. In addition, the substrate can be formed of an insulating substrate, preferably an oxide single crystalline substrate such as MgO, SrTiO 3 , CdNdAlO 4 , etc. These substrate materials are very effective in forming or growing a crystalline film having a high degree of crystalline orientation. However, the superconducting device can be formed on a semiconductor substrate if an appropriate buffer layer is deposited thereon. For example, the buffer layer on the semiconductor substrate can be formed of a double-layer coating formed of a MgAlO 4 layer and a BaTiO 3 layer if silicon is used as a substrate. According to another aspect of the present invention, there is provided a superconducting device comprising: a substrate having a principal surface; a non-superconducting oxide layer on the principal surface of the substrate ; two superconducting regions formed of an oxide superconductor thin film on the non-superconducting oxide layer electrically isolated from each other, the non-superconducting oxide layer having a crystal structure similar to that of the oxide superconductor thin film ; and an extremely thin superconducting region formed of an oxide superconductor thin film which electrically connects the two superconducting regions and forms a weak link of Josephson junction, wherein the two superconducting regions are formed of one c -axis oriented oxide superconductor thin film which has a gently concave upper surface with a tilt angle of less than 40° and an insulating region at its center portion including an impurity concentration sufficient to inhibit superconductivity and the extremely thin superconducting region is formed of a c -axis oriented oxide superconductor thin film on the insulating region. In one preferred embodiment, the ions which are implanted so as to form the insulating region are selected from Ga ions, Al ions, In ions, Si ions, Ba ions and Cs ions. Brief Description of the Drawings Figures 1A to 1F are diagrammatic sectional views for illustrating a first embodiment of the process in accordance with the present invention for manufacturing the super-FET; Figures 2A to 2C are diagrammatic sectional views for illustrating featured steps of a second embodiment of the process in accordance with the present invention for manufacturing the super-FET; Description of the Preferred embodiments Embodiment 1 Referring to Figures 1A to 1F, the process in accordance with the present invention for manufacturing the super-FET will be described. As shown in Figure 1A, a MgO (100) single crystalline substrate 5 having a substantially planar principal surface is prepared. As shown in Figure 1B, an oxide layer 20 having a thickness of 100 nanometers composed of a c -axis oriented Pr 1 Ba 2 Cu 3 O 7-ε thin film is deposited on the principal surface of the substrate 5 and a c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 having a thickness of about 300 nanometers is deposited on the oxide layer 20, by for example a sputtering, an MBE (molecular beam epitaxy), a vacuum evaporation, a CVD, etc. A condition of forming the c -axis oriented Pr 1 Ba 2 Cu 3 O 7-ε thin film and the c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 by off-axis sputtering is as follows: Pr 1 Ba 2 Cu 3 O 7-ε thin film Temperature of the substrate 750°C Sputtering Gas Ar: 90% O 2 : 10% Pressure 10 Pa Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film Temperature of the substrate 700 °C Sputtering Gas Ar: 90% O 2 : 10% Pressure 10 Pa Then, as shown in Figure 1C, a center portion of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 is processed by He ion-beam accelerated by an energy of 3 to 50 keV so as to form a concave portion 14 which is concave gently. The tilt angle of the concave portion 14 is less than 40° and its length is about 100 nanometers. Thereafter, Ga ions are implanted into a bottom portion of the concave portion 14 by an energy of 50 to 150 keV so as to form an insulating region 50, as shown in Figure 1D. In this connection, Al ions, In ions, Si ions, Ba ions and Cs ions can be also used instead of Ga ions. The Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 is divided into a superconducting source region 2 and a superconducting drain region 3 by the insulating region 50. Then, the substrate 5 is heated to a temperature of 350 to 400 °C under a pressure lower than 1.333 x 10 -7 Pa (1 × 10 -9 Torr) so as to clean the surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1. This heat-treatment is not necessary, if the surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 is clean enough. Thereafter, as shown in Figure 1E, a c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 having a thickness on the order of about 5 nanometers is deposited on the surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 by an MBE (molecular beam epitaxy). A condition of forming the c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 by an MBE is as follows: Molecular beam source Y: 1250°C Ba: 600°C Cu: 1040°C O 2 or O 3 atmosphere Pressure 1.333 x 10 -3 Pa (1 × 10 -5 Torr) Temperature of the substrate 700°C Since the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 is formed on the gently curved surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1, it becomes an uniform c -axis oriented oxide superconductor thin film. A portion of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 on the insulating region 50 becomes a superconducting channel. Finally, as shown in Figure 1F, a gate insulating layer 7 is formed of Si 3 N 4 , MgO or SrTiO 3 on the superconducting channel 10 and a gate electrode 4 is formed of Au on the gate insulating layer 7. Metal electrodes may be formed on the superconducting source region 2 and the superconducting drain region 3, if necessary. With this, the super-FET in accordance with the present invention is completed. As explained above, the superconducting channel, the superconducting source region and the superconducting drain region of the above mentioned super-FET manufactured in accordance with the embodiment of the method of the present invention are formed of c -axis oriented oxide superconductor thin films. Therefore, the super-FET has no undesirable resistance nor undesirable Josephson junction between the superconducting channel and the superconducting source region and between the superconducting channel and the superconducting drain region. In addition, since the superconducting source region and the superconducting drain region gently inclines to the superconducting channel, superconducting current efficiently flows into and flows from the superconducting channel. By this, the current capability of the super-FET can be improved. In the above method, if the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 is deposited to have a grain boundary so as to form a weak link of the Josephson junction on the insulating region 50, a Josephson junction device is manufactured. In this case, the superconducting source region and the superconducting drain region are two superconducting electrodes. Almost all the above mentioned features of the super-FET can apply to the Josephson junction device. Embodiment 2 Referring to Figures 2A to 2C, a second embodiment of the process for manufacturing the superconducting device will be described. In this second embodiment, the same processings as those shown in Figures 1A to 1B are performed. Then, as shown in Figure 2A, the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 is processed by He ion-beam accelerated by an energy of 3 to 50 keV so that the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 is divided into a superconducting source region 2 and a superconducting drain region 3 which have inclined surfaces gently inclined to each other. The tilt angle of the inclined surfaces is less than 40°. The oxide layer 20 of Pr 1 Ba 2 Cu 3 O 7-ε is exposed between the superconducting source region 2 and the superconducting drain region 3. Then, the substrate 5 is heated to a temperature of 350 to 400 °C under a pressure lower than 1.333 x 10 -7 Pa (1 × 10 -9 Torr) so as to clean the surfaces of the superconducting source region 2 and the superconducting drain region 3 and the exposed surface of the oxide layer 20. This heat-treatment is not necessary, if the surfaces of the superconducting source region 2 and the superconducting drain region 3 and the exposed surface of the oxide layer 20 are clean enough. Thereafter, as shown in Figure 2B, a c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 having a thickness on the order of about 5 nanometers is deposited on the surfaces of the superconducting source region 2 and the superconducting drain region 3 and the exposed surface of the oxide layer 20 by an MBE (molecular beam epitaxy). A condition of forming the c -axis oriented Y 1 BA 2 Cu 3 O 7-δ oxide superconductor thin film 11 by an MBE is the same as that of Embodiment 1. Since the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 is formed on the gently curved surfaces of the superconducting source region 2 and the superconducting drain region 3 and the exposed surface of the oxide layer 20, it becomes an uniform c -axis oriented oxide superconductor thin film. A portion of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 on the exposed surface of the oxide layer 20 becomes a superconducting channel 10. Finally, as shown in Figure 2C, a gate insulating layer 7 is formed of Si 3 N 4 , MgO or SrTiO 3 on the superconducting channel 10 and a gate electrode 4 is formed of Au on the gate insulating layer 7. Metal electrodes may be formed on the superconducting source region 2 and the superconducting drain region 3, if necessary. With this, the super-FET in accordance with the present invention is completed. As explained above, the superconducting channel, the superconducting source region and the superconducting drain region of the above mentioned super-FET manufactured in accordance with the embodiment of the method of the present invention are formed of c -axis oriented oxide superconductor thin films. Therefore, the super-FET has no undesirable resistance nor undesirable Josephson junction between the superconducting channel and the superconducting source region and between the superconducting channel and the superconducting drain region. In addition, since the superconducting source region and the superconducting drain region gently inclines to the superconducting channel, superconducting current efficiently flows into and flows from the superconducting channel. By this, the current capability of the super-FET can be improved. In the above method, if the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 is deposited to have a grain boundary so as to form a weak link of the Josephson junction on the exposed surface of the oxide layer 20, a Josephson junction device is manufactured. In this case, the superconducting source region and the superconducting drain region are two superconducting electrodes. Almost all the above mentioned features of the super-FET can apply to the Josephson junction device.

Claims: A superconducting device, comprising: a substrate (5) having a principal surface; a superconducting source region (2) and a superconducting drain region (3) formed of an oxide superconductor thin film electrically isolated from each other disposed on a non-superconducting oxide layer (20) on the principal surface of the substrate (5), the non-superconducting oxide layer having a crystal structure similar to that of the oxide superconductor thin film ; an extremely thin superconducting channel (10) formed of a c -axis oriented oxide superconductor thin film which electrically connects the superconducting source region (2) and the superconducting drain region (3), so that superconducting current can flow through the superconducting channel (10) between the superconducting source region (2) and the superconducting drain region (3); and a gate electrode (4) on a gate insulator (7) on the superconducting channel (10) for controlling the superconducting current flowing through the superconducting channel (10), wherein the superconducting source region (2) and the superconducting drain region (3) are formed of a c -axis orientated oxide superconductor thin film, and the superconducting source region (2) and the superconducting drain region (3) have upper surfaces gently inclined to the superconducting channel by a tilt angle of less than 40°. A superconducting device as claimed in Claim 1, characterized in that the superconducting source region (2) and the superconducting drain region (3) are formed of one c -axis oriented oxide superconductor thin film (1) which has a gently concave upper surface (14) and an insulating region (50) at its center portion including an impurity concentration sufficient to inhibit superconductivity. A superconducting device as claimed in Claim 1, characterized in that the superconducting channel (10) is formed on the non-superconducting oxide layer (20). A superconducting device, comprising: a substrate (5) having a principal surface; a non-superconducting oxide layer (20) on the principal surface of the substrate (5); two superconducting regions (2, 3) formed of an oxide superconductor thin film on the non-superconducting oxide layer (20) electrically isolated from each other, the non-superconducting oxide layer (20) having a crystal structure similar to that of the oxide superconductor thin film ; and an extremely thin superconducting region (10) formed of an oxide superconductor thin film which electrically connects the two superconducting regions (2, 3) and forms a weak link of Josephson junction, wherein the two superconducting regions (2, 3) are formed of one c -axis oriented oxide superconductor thin film (1) which has a gently concave upper surface (14) with a tilt angle of less than 40° and an insulating region (50) at its center portion including an impurity concentration sufficient to inhibit superconductivity and the extremely thin superconducting region (10) is formed of a c -axis oriented oxide superconductor thin film on the insulating region (50). A superconducting device as claimed in anyone of Claims 1 or 4, characterized in that the non-superconducting oxide layer (20) has a similar crystal structure to that of a c -axis oriented oxide superconductor thin film. A superconducting device as claimed in claim 5, characterized in that the non-superconducting oxide layer (20) is formed of a Pr 1 Ba 2 Cu 3 O 7-ε thin film. A superconducting device as claimed in anyone of Claims 1 to 5, characterized in that the oxide superconductor is a high-T c (high critical temperature) oxide superconductor. A superconducting device as claimed in Claim 7, characterized in that the oxide superconductor is a high-T c copper-oxide type compound oxide superconductor. A superconducting device as claimed in Claim 8, characterized in that the oxide superconductor is an oxide superconductor material selected from the group consisting of a Y-Ba-Cu-O compound oxide superconductor material, a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material. A superconducting device as claimed in anyone of Claims 1 to 9, characterized in that the substrate is formed of a material selected from the group consisting of a MgO (100) substrate, a SrTiO 3 (100) substrate and a CdNdAlO 4 (001) substrate, and a semiconductor substrate. A superconducting device as claimed in Claim 10, characterized in that the substrate is formed of a silicon substrate and a principal surface of the silicon substrate is coated with an insulating material layer which is formed of a MgAl 2 O 4 layer and a BaTiO 3 layer.

Inventors: IIYAMA MICHITOMO, INADA HIROSHI, NAKAMURA TAKAO, IIYAMA, MICHITOMO, INADA, HIROSHI, NAKAMURA, TAKAO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: H01L39
IPC codes: H01L39