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YBa2Cu3Ox tunnel junctions
Physica C 235-240 (1994)3343 3344 North-Holland
PHYS[gA
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New Insulating Material for YBa2Cu3Ox/Insulator/YBa2Cu3Ox Tunnel Junctions Yasuo Tazoh, Masashi Mukaida, and Shintaro Miyazawa NTT LSI Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-01,
Japan
BaSnO3 is proposed as a new insulating material for YBCO-SIS tunnel junctions from the viewpoint of the superconducting behavior of a layer-structured YBCO. YBCO and BaSnO 3 thin films were grown in-situ by a reactive co-evaporation in oxygen radicals. XRD, AFM, and RHEED observations confirm that (001) oriented YBCO and (100) oriented BaSnO3 thin films grow epitaxially on each other. Cross-sectional TEM observations reveal that about 4 nm thick BaSnO3 thin films perfectly cover the lower YBCO thin film surface steps with a height of one unit cell length of YBCO. Tc of the lower and upper YBCO thin films is higher than 80 K. 1. INTRODUCTION While many attempts have been made to fabricate SIS tunnel junctions made of high-Tc oxide superconductors,the Josephson tunnel characteristics accompanied with a superconducting gap structure have still not been observed. Some experimental results suggest the existence of pin holes through the insulator layer between the lower and upper superconducting electrodes and the degradation of superconductivity at the I/S and Sfl interfaces. It is therefore important to select a suitable insulating material that provides good surface coverage (wettability) of the lower superconductor and maintains superconductivity at both interfaces. This paper reports a new material for the insulating layer of YBCO-SIS tunnel junctions from the viewpoint of the superconducting behavior of a layer-structured YBCO.
Spectroscopy[3]. These results indicate that superconductivity is caused by carrier injection from the charge reservoir layers of BaO-CuO-BaO into the CuO2 plane layers. Ba ions in CuO chain layers therefore seems to play an important role in the appearance of superconductivity. These features led us to the following guiding principle for selecting insulating materials: the materials should contain Ba ions to provide a charge reservoir layer of BaO-CuO-BaO at the interfaces and thus maintain superconductivity at the interfaces, and their crystal structures should be similar to that of YBCO to ensure good surface coverage and wettability to YBCO thin films[4]. The insulating material that we selected from the viewpoint of crystal chemistry, was BaSnO3. This has a cubic perovskite structure and its lattice constant is 0.4116 nm. The lattice mismatch between YBCO and BaSnO3 is 6.7 %.
2. GUIDING PRINCIPLE FOR SELECTING OF INSULATING MATERIALS Terashima et al. investigated the superconducting transition of a 1-unit cell length thick (1-UCT) YBCO thin film sandwiched between PrBa2Cu3Ox and various cap materials (PrOx, La2CuO4, CaO, CuO, SrO, BaO, and PrBa2Cu3Ox)[1]. They found that 1-UCT YBCO thin films covered with BaO and PrBa2Cu3Ox showed superconducting transitions around 20 and 30 K, respectively, but 1-UCT YBCO thin films covered with the other cap materials were nonsuperconducting. They concluded from these experimental results and TEM analysis that the surface termination of YBCO thin film is a CuOchain layer. This was also supported by other analyses, including Z-contrast Transmission Electron Microscopy[2] and Ion Scattering
3. EXPERIMENTAL The epitaxial properties of YBCO on BaSnO3 and BaSnO 3 on YBCO were investigated. Both thin films were grown in-situ using reactive coevaporation in a radical oxygen atmosphere. Details of the deposition system and procedure have already been described elsewhere[5]. The substrate used was (110) NdGaO3 single crystal. The ultralow growth rate (about 0.0065 nm/sec.) played an important role in producing atomically flat YBCO thin film[5]. Figure 1 shows a typical RHEED pattern of BaSnO3 thin film grown in-situ on a YBCO thin film/NdGaO3 substrate. This RHEED pattern was taken when the BaSnO3 thin film was about 6 nm. Oscillations in RHEED intensity were usually observed during the growth of BaSnO3 thinner than about 10 nm. The lattice parameter estimated from
0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)02237-2
3344
)i T~tzoh et al., tJhv.~ica (" 235 240 (1994) 3343 3344
Figure 1. Typical RHEED pattern of BaSnO 3 thin film grown in-situ on YBCO thin film/NdGaO 3 substrate.
rlnl ¸ Figure 2. Cross-sectional TEM image for a YBCO/BaSnO3/YBCO trilayers. the distance between streaks was about 0.41 nm, which coincides with the lattice parameter of BaSnO 3. These results suggest that BaSnO 3 ultrathin films grew epitaxially and quasi-twodimensionally on YBCO, despite the relatively large lattice mismatch between YBCO and BaSnO 3. The strong, sharp streak patterns suggest that the film had a smooth surface with good crystallinity. In fact, AFM observations indicate that the surface roughness was about as smooth as that of the bottom YBCO thin film, i.e., almost atomically flat. Moreover, XRD analysis of YBCO/BaSnO3/YBCO trilayers confirms that (001) oriented YBCO and (100) oriented BaSnO 3 thin films with atomically smooth surfaces grew epitaxially on each other.
Figure 2 shows a cross-sectional TEM image lor a YBCO/BaSnO3/YBCO trilayer. The thicknesses of the lower and upper YBCO and the BaSnO3 thin films were about 10 ran, 100 nm, and 4 nm, respectively. Ultra-thin BaSnO3 film (about 4 nm thick) was grown on the lower YBCO thin film uniformly and with good wettability, and it perfectly covered the YBCO surface steps with a height of one unit cell length (-1.17 nm) of YBCO. The relative dielectric constant er of BaSnO3 was evaluated by measuring the capacitance of a capacitor consisting of a BaSnO 3 pellet sandwiched between Cu electrodes using an impedance meter. The estimated value was 22.0 at 77 K and 600 kHz. Tc of the trilayer were measured using a usual four-probe method, where T c was defined as the temperature showing zero resistance. T c of the upper YBCO had no dependence of BaSnO3 fihn thickness and was around 88 K. T c of the lower YBCO ranged from 80 to 88 K when the BaSnO 3 thickness was less than about 4 nm. However, a reduction in T c to 55 K was observed when the B a S n O 3 thickness was about 20 nm. This is however not a problem because insulating tunneling BaSnO3 layer is usually less than about 4 nm. We are now making process techniques for evaluating the electrical characteristics of SIS tunnel junctions. 4. C O N C L U S I O N A new insulating material BaSnO 3 of YBCOSIS tunnel junctions was proposed from the viewpoint of the superconducting behavior of a layer-structured YBCO. The (001) oriented YBCO and (100) oriented BaSnO3 thin films grew epitaxially on each other and BaSnO 3 thin fihns with a thickness of even 4 nm perfectly covered YBCO thin film surface steps with a height of one unit cell length of YBCO. BaSnO 3 is therefore a promising insulating material for SIS tunnel junctions because the degradation of Tc of the lower and upper YBCO was also small.
REFERENCES [1] T. Terashima et al., Proc. of ISS'92, Advances in Superconductivity -V (1993) 827. [2] S. J. Pennycook, Annu. Rev. Mater. Sci, vol. 22 (1992) 171. [3] T. Nakamura et al.,, Proc. of ISS'92, Advances in Superconductivity -V (1993) 833. [4] Y. Tazoh et al., Proc. of EUCAS'93, Applied Superconductivity (1994) 1119. [5] Y. Tazoh et al.,, Appl. Phys. Left., vol.62, no.4 (1993) 408.
PHYS[gA
(~
New Insulating Material for YBa2Cu3Ox/Insulator/YBa2Cu3Ox Tunnel Junctions Yasuo Tazoh, Masashi Mukaida, and Shintaro Miyazawa NTT LSI Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-01,
Japan
BaSnO3 is proposed as a new insulating material for YBCO-SIS tunnel junctions from the viewpoint of the superconducting behavior of a layer-structured YBCO. YBCO and BaSnO 3 thin films were grown in-situ by a reactive co-evaporation in oxygen radicals. XRD, AFM, and RHEED observations confirm that (001) oriented YBCO and (100) oriented BaSnO3 thin films grow epitaxially on each other. Cross-sectional TEM observations reveal that about 4 nm thick BaSnO3 thin films perfectly cover the lower YBCO thin film surface steps with a height of one unit cell length of YBCO. Tc of the lower and upper YBCO thin films is higher than 80 K. 1. INTRODUCTION While many attempts have been made to fabricate SIS tunnel junctions made of high-Tc oxide superconductors,the Josephson tunnel characteristics accompanied with a superconducting gap structure have still not been observed. Some experimental results suggest the existence of pin holes through the insulator layer between the lower and upper superconducting electrodes and the degradation of superconductivity at the I/S and Sfl interfaces. It is therefore important to select a suitable insulating material that provides good surface coverage (wettability) of the lower superconductor and maintains superconductivity at both interfaces. This paper reports a new material for the insulating layer of YBCO-SIS tunnel junctions from the viewpoint of the superconducting behavior of a layer-structured YBCO.
Spectroscopy[3]. These results indicate that superconductivity is caused by carrier injection from the charge reservoir layers of BaO-CuO-BaO into the CuO2 plane layers. Ba ions in CuO chain layers therefore seems to play an important role in the appearance of superconductivity. These features led us to the following guiding principle for selecting insulating materials: the materials should contain Ba ions to provide a charge reservoir layer of BaO-CuO-BaO at the interfaces and thus maintain superconductivity at the interfaces, and their crystal structures should be similar to that of YBCO to ensure good surface coverage and wettability to YBCO thin films[4]. The insulating material that we selected from the viewpoint of crystal chemistry, was BaSnO3. This has a cubic perovskite structure and its lattice constant is 0.4116 nm. The lattice mismatch between YBCO and BaSnO3 is 6.7 %.
2. GUIDING PRINCIPLE FOR SELECTING OF INSULATING MATERIALS Terashima et al. investigated the superconducting transition of a 1-unit cell length thick (1-UCT) YBCO thin film sandwiched between PrBa2Cu3Ox and various cap materials (PrOx, La2CuO4, CaO, CuO, SrO, BaO, and PrBa2Cu3Ox)[1]. They found that 1-UCT YBCO thin films covered with BaO and PrBa2Cu3Ox showed superconducting transitions around 20 and 30 K, respectively, but 1-UCT YBCO thin films covered with the other cap materials were nonsuperconducting. They concluded from these experimental results and TEM analysis that the surface termination of YBCO thin film is a CuOchain layer. This was also supported by other analyses, including Z-contrast Transmission Electron Microscopy[2] and Ion Scattering
3. EXPERIMENTAL The epitaxial properties of YBCO on BaSnO3 and BaSnO 3 on YBCO were investigated. Both thin films were grown in-situ using reactive coevaporation in a radical oxygen atmosphere. Details of the deposition system and procedure have already been described elsewhere[5]. The substrate used was (110) NdGaO3 single crystal. The ultralow growth rate (about 0.0065 nm/sec.) played an important role in producing atomically flat YBCO thin film[5]. Figure 1 shows a typical RHEED pattern of BaSnO3 thin film grown in-situ on a YBCO thin film/NdGaO3 substrate. This RHEED pattern was taken when the BaSnO3 thin film was about 6 nm. Oscillations in RHEED intensity were usually observed during the growth of BaSnO3 thinner than about 10 nm. The lattice parameter estimated from
0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)02237-2
3344
)i T~tzoh et al., tJhv.~ica (" 235 240 (1994) 3343 3344
Figure 1. Typical RHEED pattern of BaSnO 3 thin film grown in-situ on YBCO thin film/NdGaO 3 substrate.
rlnl ¸ Figure 2. Cross-sectional TEM image for a YBCO/BaSnO3/YBCO trilayers. the distance between streaks was about 0.41 nm, which coincides with the lattice parameter of BaSnO 3. These results suggest that BaSnO 3 ultrathin films grew epitaxially and quasi-twodimensionally on YBCO, despite the relatively large lattice mismatch between YBCO and BaSnO 3. The strong, sharp streak patterns suggest that the film had a smooth surface with good crystallinity. In fact, AFM observations indicate that the surface roughness was about as smooth as that of the bottom YBCO thin film, i.e., almost atomically flat. Moreover, XRD analysis of YBCO/BaSnO3/YBCO trilayers confirms that (001) oriented YBCO and (100) oriented BaSnO 3 thin films with atomically smooth surfaces grew epitaxially on each other.
Figure 2 shows a cross-sectional TEM image lor a YBCO/BaSnO3/YBCO trilayer. The thicknesses of the lower and upper YBCO and the BaSnO3 thin films were about 10 ran, 100 nm, and 4 nm, respectively. Ultra-thin BaSnO3 film (about 4 nm thick) was grown on the lower YBCO thin film uniformly and with good wettability, and it perfectly covered the YBCO surface steps with a height of one unit cell length (-1.17 nm) of YBCO. The relative dielectric constant er of BaSnO3 was evaluated by measuring the capacitance of a capacitor consisting of a BaSnO 3 pellet sandwiched between Cu electrodes using an impedance meter. The estimated value was 22.0 at 77 K and 600 kHz. Tc of the trilayer were measured using a usual four-probe method, where T c was defined as the temperature showing zero resistance. T c of the upper YBCO had no dependence of BaSnO3 fihn thickness and was around 88 K. T c of the lower YBCO ranged from 80 to 88 K when the BaSnO 3 thickness was less than about 4 nm. However, a reduction in T c to 55 K was observed when the B a S n O 3 thickness was about 20 nm. This is however not a problem because insulating tunneling BaSnO3 layer is usually less than about 4 nm. We are now making process techniques for evaluating the electrical characteristics of SIS tunnel junctions. 4. C O N C L U S I O N A new insulating material BaSnO 3 of YBCOSIS tunnel junctions was proposed from the viewpoint of the superconducting behavior of a layer-structured YBCO. The (001) oriented YBCO and (100) oriented BaSnO3 thin films grew epitaxially on each other and BaSnO 3 thin fihns with a thickness of even 4 nm perfectly covered YBCO thin film surface steps with a height of one unit cell length of YBCO. BaSnO 3 is therefore a promising insulating material for SIS tunnel junctions because the degradation of Tc of the lower and upper YBCO was also small.
REFERENCES [1] T. Terashima et al., Proc. of ISS'92, Advances in Superconductivity -V (1993) 827. [2] S. J. Pennycook, Annu. Rev. Mater. Sci, vol. 22 (1992) 171. [3] T. Nakamura et al.,, Proc. of ISS'92, Advances in Superconductivity -V (1993) 833. [4] Y. Tazoh et al., Proc. of EUCAS'93, Applied Superconductivity (1994) 1119. [5] Y. Tazoh et al.,, Appl. Phys. Left., vol.62, no.4 (1993) 408.