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Growth and properties of high Tc films in Y1Ba2Cu3O7−x perovskite by molecular beam epitaxy
Journal of Crystal Growth 111 (1991) 965—972 North-Holland
965
Growth and properties of high 7, films in Y1Ba2Cu3O7_~perovskite by molecular beam epitaxy J. Kwo AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA
Thin film synthesis of the high temperature superconductors in Y1Ba2Cu3O7_~perovskite has been successfully carried out by the advanced molecular beam epitaxy technique. The development of an efficient activated oxygen source led to in-Situ growth of high quality films of Y1 Ba 2Cu307_ and Dy1Ba2Cu 307_ without high temperature annealing. Excellent superconducting properties have been achieved: the T~,~ normal state resistivity p, and critical current density J~equal to the best records reported to date for Y1Ba2Cu307_~films. This paper reviews in-situ MBE growth, structural and superconducting characteristics of the films. The results of tunneling in planar junctions of Y1Ba2Cu307_~/nativebarrier/Pb are also presented in the context of quasiparticle tunneling and Josephson behavior.
1. Introduction Recent discovery of high temperature superconductivity in the layered perovskite oxides has generated much interest in synthesizing high ~ superconducting films for both fundamental and device research. Many experimental studies suggest that the occurrence of superconductivity is directly connected with itsbeam layered, intergrowth structure. The molecular epitaxy (MBE) technique is best known for its capabilities of low temperature and layer-by-layer growth, as has been demonstrated successfully for semiconductor materials. It is therefore anticipated that the MBE method is ideally suited for the thin film synthesis of high 7~oxide superconductors. Furthermore, it provides opportunities of fabricating artificial oxide heterostructures or metastable phases tailored on atomic scale. Studies of these synthetic, layered oxides may shed light on the theoretical mechanism responsiblc for high T~superconductivity, which, in turn, offer promise of producing even higher 7~materials. The paper reviews our recent successful application of the molecular beam epitaxy method to produce high T~Y1Ba 2Cu 307_ and Dy1Ba 2Cu3 07_ thin films on a number of substrates includ0022-0248/91/$03.50 © 1991
—
ing MgO, SrTiO3, and LaAIO3. The growth techniques are described, along with the design and operation of an activated oxygen source. Based on in-situ observations by RHEED, a model is proposed for the growth mode of the perovskites. Typical superconducting properties are the following: 7~(R= 0) = 92 K, i.~7~(inductive) = 0.4 K, ~(iOO2.K)Furthermore, = 55 jt~2 cm, tunneling and J~(77 K) = 4.5 10~ studies onX thin A/cm film junctions consisting of Y 1Ba 2Cu 307_ s/native barrier/ Pb indicated that the in-situ film surface exhibits quasiparticle tunneling characteristics similar to etched bulk single crystal. Josephson behavior was observed for the first time in this type of planar junctions.
2. MBE growth aided with an activated oxygen source Two basic requirements are demanded for the MBE growth of high 7~perovskites; namely, precisc compositional control, and an efficient activated oxygen source for oxidation [1—3].In ordinary MBE growth process, the first requirement is met with relative ease. However, for our present research it remains as a challenging task
Elsevier Science Publishers BY. (North-Holland)
966
J. Kwo
/
Growth and properties of high 7~films in YjBa
due to a large chemical unit cell containing multiple cation species. Discussions on this subject will be given later in the section. The necessity of activated oxygen species follows from the oxidation characteristics intrinsic of the YiBa2Cu3O7_~ compound. The thermodynamic phase diagram of oxygen partial pressure versus temperature determined for Y1Ba 2Cu 307_ indicates that for a given oxidation state there exists at a given temperature a minimal oxygen partial pressure below which that phase becomes unstable [1,2]. For instance, at a growth temperature of 650°C, the minimal oxygen pressure of forming Y1Ba 2Cu 306 is 6.0 x 10 ~ Torr. However, the molecular flow regime generic to MBE requires that the mean free path be greater than typical source to substrate distance (— 20—40 cm), and such condition implies an operating pressure less than 1 x i0-~ Torr. The abundance of activated oxygen species enables oxidations of the cation elements at an oxygen partial pressure of several orders lower, while still sustaining the long free path necessary for MBE. The in-situ growth of oxides in our work was achieved by reacting metal flux with neutral oxygen radicals (primarily atomic oxygen and excited molecular oxygen) at the substrates held at an elevated temperature [3—6].The design of our activated oxygen source is based on the concept of a down-stream plasma excited by microwave discharges contained in a fast flow reactor. Due to a distance 20 inch long between the discharge and the substrates, it is essential to achieve a high flow rate exceeding 10 SCCM to minimize wall recombinations. The initial design using a simple straight tubing with a small hole near the end experienced a loss of activated species by as much as 60% in the flow path due to a low flow rate [3,4]. The latest design shown in fig. 1 has succeeded to retain the activated oxygens by as high as 87% of those initially produced near the discharge [5,6]. Furthermore, the usage of a quartz ring containing multiple holes allows uniform depositions over a 1.0 inch diameter area. The pressure near the discharge is — 2 x 10_I Torr, and the oxygen pressure near the substrates is low 10 ~ Torr. The overall chamber pressure during growth is maintamed at low 10~~ Torr. The activated oxygen
2Cup7_ perovskite by MBE
I
HEATER
[~~J CRYOPUMP
/11
// I /~ j~\ / /
°2
e-
\H~/ y
Ba
CRYOPUMP
\J7” Cu
Fig. 1. The design of an activated oxygen source using a fast flow tube reactor and the internal system configuration.
flux near the substrates is estimated to be about 2 x 1016 species/s cm~. Metal cations are evaporated from individual thermal sources using a combination of effusion cells and e-beam evaporators. The coevaporation method is used, and no shuttered growth is employed thus far. The alkaline earths like Ba are evaporated from effusion cells at 600 °C. The flux detection and control of e-beam evaporation are made by Inficon Sentinel monitors based on electron impact emission spectroscopy. While this method proved to be very successful for pure metal depositions [7], the interference of the metal cation signal (especially for Y) with a large oxygen signal from the background pressure makes the rate control of Y quite difficult. The deposition of Cu is less affected because its light emission occurs at a different wavelength. An alternative solution is to use effusion cells to evaporate the rare earths as well as Cu. Recently, Dy1Ba 2Cu 307_ thin films were produced by evaporating Dy from an effusion cell held at — 1150°C [8]. The Dy flux was kept constant within 1% by regulating the cell temperature, and varied very little with the 02 pressure. The precise compositional control is necessary for producing a highly ordered superconducting structure free of impurity phases. A number of provisions were also made in the construction of the sample manipulator to be compatible with a highly oxidizing environment. Refractory materials like molybdenum and tantalum were avoided because they are subject to oxidation easily, and their oxides are highly volatile at 650 °C.The substrate block material is now replaced by nickel due to its less volatile oxide —
J. Kwo
/ Growth and properties of high
than Mo, and a better thermal conductor than other oxidation resistant materials like stainless steel [9].The heater is made of 0.020 inch rhenium wire wound into a flower pattern. The substrate temperature is measured by an infrared pyrometer. Consistent readings between the pyrometer and the thermal-couple methods have been obtamed once the Ni block surface became oxidized after multiple depositions. Good thermal contact between the substrates and the sample block is crucial for growing the perovskites due to poor thermal conductivity of the substrate materials. Presently Ag pastes are used to provide the thermal contact. The substrate temperature at which the best films were produced occurred in a range of 650—700 °C. Following the deposition, the films were cooled to 200°Cin an oxygen pressure 4 times higher than during growth, while keeping the plasma running continuously,
3. Structural properties Reflection high energy electron diffraction (RHEED) was used during growth to optimize the deposition conditions [3]. Sharp streaky pattern accompanied with Kikuchi arcs were generally seen. Typical diffraction patterns along azimuthal [100] and [110] axes are shown in figs. 2a and 2b. This observation, in general, implies that the growth takes place in a layer by layer mode,
T~films in 1’~Ba
2Cu3O7 _ ~ perouskite by MBE
967
producing a highly ordered, atomically smooth film surface. Furthermore, the alignments of the in-plane axes of Y1Ba2Cu 307 films with those of the underlying substrates of MgO(100) and SrTiO3(100) are evidence for epitaxial growth. The overall crystal structure was examined by X-ray diffraction [3]. Under present deposition conditions, growth on epitaxial substrates of (100) orientation including MgO, SrTiO3, and LaA1O3 produces c-axis oriented Y3Ba2Cu307_~ films. There is little evidence for a-axis oriented grains. Typical rocking curve of the (005) Bragg reflection is less than 0.3°. The detailed growth mechanism of the oxides still remains an interesting subject of study. Recently, strong oscillations of the specular RHEED intensity during growth of several perovskites including Y1Ba 2Cu 307 were observed by Terashima et al using the reactive evaporation method [10]. Furthermore, one period of oscillation corresponds to the height of one minimal unit cell which satisfies charge neutrality. The result is shown in fig. 3. In contrast to the layer-by-layer growth mode commonly known for metals or semiconductors, the growth of the perovskites is described by the unit-by-unit fashion. Presumably, it is related to the strong character of the ionic bond of oxides. This observation has an important implication that the oxide growth occurs by the formation of a 2D nuclei which is the minimum unit satisfying electrical neutrality. Moreover, it also suggests that within each growth unit the
Fig. 2. RHEED patterns along azimuthal (a) [100J and (b) [110] axes of a Y1Ba 2Cu 307
_
_
film 1000
A thick grown on
MgO(100).
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J. Kwo
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Growth and properties of high T~films in 1’~Ba
2Cu3O7 —
perovskite by MBE
(a)
start
C
-o
~
TIME (Arb. Unit)
Ba
Cu
,.fs1~ 02
Cu
(b)
Ba
j Cu02
Cu 02
~
~Ju1.7A
Fig. 3. (a) RHEED specular intensity as a function of the deposition time and (b) a proposed model for the Y1Ba2Cu3O7_~film growth, after Terashima et al. [10].
stacking sequence starts and ends with specific cation-oxide layers. 4. Superconducting properties
07
MBE grown Y1Ba2Cu3O7_~and Dy1Ba2Cu3 films showed excellent superconducting _
properties including 2~(R= 0), J~,and normal state resitivity p. Depositions on a variety of substrates including MgO(100), SrTiO3(100), and LaA1O3(100) have produced comparably good results, although the best superconducting transport properties were usually found for SrTiO3(100) of closest lattice match (— 1.0%) with Y1Ba2Cu307_~ [8]. In comparison, the very large lattice mismatch
J. Kwo
/
200
Growth and properties of high T~films in
I
‘ I
I
A
1000
I
FILM
60
15 0
(c) (Dl (a)
0 90 92 94 96
~
>-
-
(a) SrTiO (100) (b) LaAIO (100) Cc) MgO (100)
0 I
0
I
I I
100
200
I
300
1(K) Fig. 4. Resistivity versus temperature for Dy1Ba2Cu307_~ films 1000 A thick grown on (a) SrTiO3(100), (b) LaA1O3(100), and (c) MgO(100). The superconducting transitions in details are plotted in the inset.
9%) between Y1Ba 2Cu307 ~ and MgO conduces 7~,onset by — 2 K, and a higher 100 to K) abylower10%. p( The temperature dependence of the normal state resistivity for typical Dy1Ba2Cu3O7....~films 1000 A thick is shown in fig. 4 for three different substrates. The p versus T curves show a linear temperature dependence above T~,a common feature for current conductions along the Cu—Ox planes in the cuprates. The ratios of p(300 K)/p(100 K) in all three cases are 3.0, and the p versus T curves extrapolate to zero resistivity at T = 0. The detailed superconducting transition is plotted in the inset for each substrate. Notable difference in the shape of the transition was observed. Specifically, the 7(R = 0) is 92.3, 91, and 90.5 K for films grown on SrTiO3, LaA1O3, and MgO, respectively. The 10%—90% transition widths are 1.2 K for SrTiO3, and LaA1O3, and is only 0.5 K for MgO. The value of normal state resistivity p(lOO K) has been commonly used in the high T~field as a figure of merit to assess the materials quality. Note that p(l00 K) of the film grown on SrTiO3 is as low as 55 ~sQcm, which equals to the best value reported for bulk single crystals at present. (—
_
—
—
969
Screening measurements on Dy1Ba2Cu307_~ films showed a sharp superconducting transitions at 90 K, and a width less than 0.4 K [11]. The
mean field BCSthat behavior T~of 90 K.only Itwas also concluded such witha behavior is true for
/~~(b) a)
100
50
2Cu3O7_ ~ perovskite by MBE
penetration surement followed depth derived a temperature from thedependence screening meaof a
///..
40 20
________
88
//,/
}‘~ Ba
good film qualities in the absence of cracks or inhomogeneities [11]. The critical current densities, .i~,at 77 K in zero field were measured by the transport method. The typical 2 for values SrTiOare 4.5 x 106 and 2.0 x 106 A/cm 3(100) and MgO(100), respectively. Our J~results equal to the highest record reported by other groups using different growth techniques including reactive evaporation, pulsed laser deposition, and inverted cylindrical magnetron sputtering [12—14]. Thinner films generally show slightly poorer superconducting transport properties due to the lattice clamping effect and an increasing tendency of forming a tetragonal structure near the interface. For instance, reducing the film thickness to 500 A has led to an increase in the resitivity by about = 0) on decreases 88.5 K,10%. for The films1~(R grown SrTiO to 90.6 and 3 and MgO, respectively. Continuing reduction of the film thickness to below 100 A gives rise to substantially broadened transitions, especially for films grown on MgO. For a 90 A Y1Ba2Cu3O7_5 film on MgO, the zero resistivity is not reached until 70 K [6]. .
5. Tunneling studies of planar junctions The high t perovskites exhibit dramatic variations in conductivities with minor variations in the cation compositions and oxygen stoichiometry. This unusual property has offered new exciting possibilities of fabricating novel electronic devices in configurations of S/I/S or S/N/S oxide heterostructures. In comparison, such freedom in materials choices for devices was not available for traditional Josephson technology using low 7, Nb based superconductors. However, the progress toward fabricating all high 1 tunneling devices has been hampered due to the following difficulties. First, the coherence lengths of the high 7~super-
J. Kwo / Growth and properties of high 7 films in YjBafJu
970
3O7 —
I
I
I I
~
-
-
I
-
100K,
.,_,,,,,,,,—‘---..
-
70K C
1
30K V V
8K
I
150
I
I
I
100
50 0 -50 -100 -150 VOLTAGE (mV) Fig. 5. Quasipartscle tunneling data of dynamic resistance versus bias for a Y1Ba2Cu307~film/native barrier/Pb Junc-
perovskite byMBE
faces and to obtain meaningful tunneling results, in the first phase of study we focused the efforts on a simpler junction configuration consisting of a Y1Ba2Cu3O7_~thin film, native barrier, and a low 7~superconductor, Pb [15]. Low leakage tunnel barriers were formed by native insulating oxides of the in-situ grown Y1Ba2Cu3O7_~film surface exposed to room air or annealed at 450°Cfor 10 mm in 02. At the moment, the exact nature of the barrier is not clear, and still awaits further surface chemical analysis. However, empirically, the junction resistance appears to correlate with the film resistivity, and varies from a few Q to a few
geometry hundred tiveThe barriers lowf~.Typical is leakage 0.3 warrant mm junctions Xjunction that 0.5 mm. the formed current area inwith conduction the the planar naacross the junction is caused primarily by the tunneling process. Reproducible current—voltage characteristics were observed, and are discussed in the following. As for the quasiparticle tunneling characteristics, below the 7~(R= 0) of Y1Ba2Cu3 07 ~ a gap-like structure 20 mY developed in the tunneling conductance, with additional asym_
—
tion at different temperatures.
conductors are very short and highly anisotropic. Secondly, the surface does not exhibit superconductivity consistent with the bulk of materials. In order to better characterize the S/I inter-
metnc modulations up to 50 mY. There is another feature of at ±4 mY appearing below 28 K. The data of dynamic resistance versus bias are shown in fig. 5, and are in good agreement with the etched bulk single crystal data [16]. In contrast to the BCS tunneling density of
Fig. 6. Current—voltage characteristics at low bias at 1.5 K for a junction of a resistance of 80 1?. The x axis scale is 0.5 mY per large division for (a) and (b). The y axis is (a) 10 RA and (b) 2 RA.
J. Kwo
/
Growth and properties of high 1~films in Y,Ba
states of conventional superconductors, tunneling results of the high 7 cuprate of Y1Ba2Cu307_~ showed reproducibly three unusual features: (1) a weak, underdeveloped gap structure; (2) presence of finite states inside the gap persistent to low temperature; (3) an asymmetric, linear normal state conductance versus bias. Although many of these features could be attributed to materials-related imperfections near the surface, the reproducibility and consistency of these recent data suggest that the unusual properties could be intrinsic to the high 7~cuprates. Junctions of lower resistance show at temperature below T~,of Pb the development of supercurrent at zero bias and associated hysteretic sub-gap structure with an I~Rvarying from 500 to 800 ttY [15]. The present I~Rproduct is about 10%—20% of the maximum theoretical value. Typical I—V characteristics at low bias at T = 1.5 K are shown in fig. 6 for a junction of a resistance of 80 Q and a critical current 1~of 5 jiA. The I—V shows a nearly complete Stewart McCumber hysteresis with numerous Fiske steps due to geometric resonances. The supercurrent modulates with magnetic field and microwave radiations in a manner expected for DC and AC Josephson effects. Although the present experimental evidence is highly suggestive of Josephson tunneling, the possibility due to a high I~Rshorts is not ruled out cornpletely.
6. Conclusions Molecular beam epitaxy aided with an activated oxygen source has successfully produced Y1Ba2 Cu307_~ epitaxial films of excellent structural and superconducting properties. The present results in single layer films provide a basis for future fabrications of sophisticated multilayer structures. Such approach has now opened up an exciting field of “molecular engineering of oxides”, which will be vital for both basic research and device work. Our tunneling study observed for the first time the Josephson current between a high ternperature superconductor and a conventional superconductor in a planar junction. This result
2Cu3O7_~perouskite by MBE
971
establishes an important criterion for theoretical postulations of high temperature superconductivity. Furthermore, it is also a major advance toward developing a Josephson technology based on all high 7 superconductors.
Acknowledgements The author would like to thank valuable collaborations with M. Hong, T.A. Fulton, D.J. Trevor, R.M. Fleming, A.F. Hebard, P.L. Gammel, A.E. White, A.R. Kortan, R.C. Farrow, and K.T. Short, and excellent technical assistance from J.P. Mannaerts.
References [11P.P. Freitas and T.S. Plaskett, Phys. Rev.
B36 (1987) 5723. [2] R. Bormann and J. Nolting, Appi. Phys. Letters 54 (1989) 2148. [3] J. Kwo, M. Hong, D.J. Trevor, R.M. Fleming, A.E. White, R.C. Farrow, A.R. Kortan and K.T. Short, Appl. Phys. Letters 53 (1988) 2683. [4] J. Kwo, M. Hong, DJ. Trevor, R.M. Fleming, A.E. White, R.C. Farrow, A.R. Kortan and K.T. Short, in: Science and Technology of Thin Film Superconductors, Eds. R.D. McConnell and S.A. Wolf (Plenum, New York, 1989) p. 101. [51J. Kwo, M. Hong, D.J. Trevor, R.M. Fleming, A.E. White, J.P. Mannaerts, R.C. Farrow, A.R. Kortan and K.T. Short, Physica C162—164 (1989) 623. [6] J. Kwo, M. Hong, T.A. Fulton, P.L. Gammel and J.P. Mannaerts, in: Processing of Films for High 7 Superconducting Electroncis, SPIE Proc., Vol. 1187 (1989) p. 57. [7] J. Kwo, in: Thin Film Growth Techniques for Low-Dimensional Structures, Eds. R.C. Farrow, S.S.P. Parkin, P.J. Dobson, J.H. Neave and A. Arrott (Plenum, New York, 1986) p. 337. [8] J. Kwo, in: Proc. Conf. 2nd ISTEC Workshop on Superconductivity, Kagoshima, May 1990. [9] The Ni substrate block was suggested by John Talvachio at Westinghouse R&D Center, Pittsburgh, PA. [10] T. Terashima, Y. Bando, K. lijima, K. Yamanioto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Letters 65 (1990) 2684. [11] A.T. Fiory, A.F. Hebard, Mankiewich and R.E. Howard, Appl. Phys. Letters P.M. 52 (1988) 2165. [121 T. Teras~a, K. lijima, K. Yamamoto, Y. Bando and H. Mazaki, Japan. J. AppI. Phys. 27 (1988) L91. [13] T. Venkatesan, X.D. Wu, B. Dutta, A. Inam, M.S. Hegde,
972
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/
Growth and properties of high T~films in Y
D.M. Hwang, C.C. Chang, L. Nazar and B. Wilkens, Appl. Phys. Letters 54 (1989) 581. [14] X.X. Xi, G. Linker, 0. Meyer, E. Nold, B. Obst, F. Ratzel, R. Smithey, B. Strehlau, F. Waschenfeld and J. Geerk, Z. Physik 74 (1989) 13.
1Ba2Cu3O7 ~ perovskite by MBE
[15] J. Kwo, T.A. Fulton, M. Hong and P.L. Gammel, Appl. Phys. Letters 56 (1990) 788. [16] M. Gurvitch, J.M. Valles, Jr., A.M. Cucolo, R.C. Dynes, J.P. Garno, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. Letters 63 (1989) 1008.
965
Growth and properties of high 7, films in Y1Ba2Cu3O7_~perovskite by molecular beam epitaxy J. Kwo AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA
Thin film synthesis of the high temperature superconductors in Y1Ba2Cu3O7_~perovskite has been successfully carried out by the advanced molecular beam epitaxy technique. The development of an efficient activated oxygen source led to in-Situ growth of high quality films of Y1 Ba 2Cu307_ and Dy1Ba2Cu 307_ without high temperature annealing. Excellent superconducting properties have been achieved: the T~,~ normal state resistivity p, and critical current density J~equal to the best records reported to date for Y1Ba2Cu307_~films. This paper reviews in-situ MBE growth, structural and superconducting characteristics of the films. The results of tunneling in planar junctions of Y1Ba2Cu307_~/nativebarrier/Pb are also presented in the context of quasiparticle tunneling and Josephson behavior.
1. Introduction Recent discovery of high temperature superconductivity in the layered perovskite oxides has generated much interest in synthesizing high ~ superconducting films for both fundamental and device research. Many experimental studies suggest that the occurrence of superconductivity is directly connected with itsbeam layered, intergrowth structure. The molecular epitaxy (MBE) technique is best known for its capabilities of low temperature and layer-by-layer growth, as has been demonstrated successfully for semiconductor materials. It is therefore anticipated that the MBE method is ideally suited for the thin film synthesis of high 7~oxide superconductors. Furthermore, it provides opportunities of fabricating artificial oxide heterostructures or metastable phases tailored on atomic scale. Studies of these synthetic, layered oxides may shed light on the theoretical mechanism responsiblc for high T~superconductivity, which, in turn, offer promise of producing even higher 7~materials. The paper reviews our recent successful application of the molecular beam epitaxy method to produce high T~Y1Ba 2Cu 307_ and Dy1Ba 2Cu3 07_ thin films on a number of substrates includ0022-0248/91/$03.50 © 1991
—
ing MgO, SrTiO3, and LaAIO3. The growth techniques are described, along with the design and operation of an activated oxygen source. Based on in-situ observations by RHEED, a model is proposed for the growth mode of the perovskites. Typical superconducting properties are the following: 7~(R= 0) = 92 K, i.~7~(inductive) = 0.4 K, ~(iOO2.K)Furthermore, = 55 jt~2 cm, tunneling and J~(77 K) = 4.5 10~ studies onX thin A/cm film junctions consisting of Y 1Ba 2Cu 307_ s/native barrier/ Pb indicated that the in-situ film surface exhibits quasiparticle tunneling characteristics similar to etched bulk single crystal. Josephson behavior was observed for the first time in this type of planar junctions.
2. MBE growth aided with an activated oxygen source Two basic requirements are demanded for the MBE growth of high 7~perovskites; namely, precisc compositional control, and an efficient activated oxygen source for oxidation [1—3].In ordinary MBE growth process, the first requirement is met with relative ease. However, for our present research it remains as a challenging task
Elsevier Science Publishers BY. (North-Holland)
966
J. Kwo
/
Growth and properties of high 7~films in YjBa
due to a large chemical unit cell containing multiple cation species. Discussions on this subject will be given later in the section. The necessity of activated oxygen species follows from the oxidation characteristics intrinsic of the YiBa2Cu3O7_~ compound. The thermodynamic phase diagram of oxygen partial pressure versus temperature determined for Y1Ba 2Cu 307_ indicates that for a given oxidation state there exists at a given temperature a minimal oxygen partial pressure below which that phase becomes unstable [1,2]. For instance, at a growth temperature of 650°C, the minimal oxygen pressure of forming Y1Ba 2Cu 306 is 6.0 x 10 ~ Torr. However, the molecular flow regime generic to MBE requires that the mean free path be greater than typical source to substrate distance (— 20—40 cm), and such condition implies an operating pressure less than 1 x i0-~ Torr. The abundance of activated oxygen species enables oxidations of the cation elements at an oxygen partial pressure of several orders lower, while still sustaining the long free path necessary for MBE. The in-situ growth of oxides in our work was achieved by reacting metal flux with neutral oxygen radicals (primarily atomic oxygen and excited molecular oxygen) at the substrates held at an elevated temperature [3—6].The design of our activated oxygen source is based on the concept of a down-stream plasma excited by microwave discharges contained in a fast flow reactor. Due to a distance 20 inch long between the discharge and the substrates, it is essential to achieve a high flow rate exceeding 10 SCCM to minimize wall recombinations. The initial design using a simple straight tubing with a small hole near the end experienced a loss of activated species by as much as 60% in the flow path due to a low flow rate [3,4]. The latest design shown in fig. 1 has succeeded to retain the activated oxygens by as high as 87% of those initially produced near the discharge [5,6]. Furthermore, the usage of a quartz ring containing multiple holes allows uniform depositions over a 1.0 inch diameter area. The pressure near the discharge is — 2 x 10_I Torr, and the oxygen pressure near the substrates is low 10 ~ Torr. The overall chamber pressure during growth is maintamed at low 10~~ Torr. The activated oxygen
2Cup7_ perovskite by MBE
I
HEATER
[~~J CRYOPUMP
/11
// I /~ j~\ / /
°2
e-
\H~/ y
Ba
CRYOPUMP
\J7” Cu
Fig. 1. The design of an activated oxygen source using a fast flow tube reactor and the internal system configuration.
flux near the substrates is estimated to be about 2 x 1016 species/s cm~. Metal cations are evaporated from individual thermal sources using a combination of effusion cells and e-beam evaporators. The coevaporation method is used, and no shuttered growth is employed thus far. The alkaline earths like Ba are evaporated from effusion cells at 600 °C. The flux detection and control of e-beam evaporation are made by Inficon Sentinel monitors based on electron impact emission spectroscopy. While this method proved to be very successful for pure metal depositions [7], the interference of the metal cation signal (especially for Y) with a large oxygen signal from the background pressure makes the rate control of Y quite difficult. The deposition of Cu is less affected because its light emission occurs at a different wavelength. An alternative solution is to use effusion cells to evaporate the rare earths as well as Cu. Recently, Dy1Ba 2Cu 307_ thin films were produced by evaporating Dy from an effusion cell held at — 1150°C [8]. The Dy flux was kept constant within 1% by regulating the cell temperature, and varied very little with the 02 pressure. The precise compositional control is necessary for producing a highly ordered superconducting structure free of impurity phases. A number of provisions were also made in the construction of the sample manipulator to be compatible with a highly oxidizing environment. Refractory materials like molybdenum and tantalum were avoided because they are subject to oxidation easily, and their oxides are highly volatile at 650 °C.The substrate block material is now replaced by nickel due to its less volatile oxide —
J. Kwo
/ Growth and properties of high
than Mo, and a better thermal conductor than other oxidation resistant materials like stainless steel [9].The heater is made of 0.020 inch rhenium wire wound into a flower pattern. The substrate temperature is measured by an infrared pyrometer. Consistent readings between the pyrometer and the thermal-couple methods have been obtamed once the Ni block surface became oxidized after multiple depositions. Good thermal contact between the substrates and the sample block is crucial for growing the perovskites due to poor thermal conductivity of the substrate materials. Presently Ag pastes are used to provide the thermal contact. The substrate temperature at which the best films were produced occurred in a range of 650—700 °C. Following the deposition, the films were cooled to 200°Cin an oxygen pressure 4 times higher than during growth, while keeping the plasma running continuously,
3. Structural properties Reflection high energy electron diffraction (RHEED) was used during growth to optimize the deposition conditions [3]. Sharp streaky pattern accompanied with Kikuchi arcs were generally seen. Typical diffraction patterns along azimuthal [100] and [110] axes are shown in figs. 2a and 2b. This observation, in general, implies that the growth takes place in a layer by layer mode,
T~films in 1’~Ba
2Cu3O7 _ ~ perouskite by MBE
967
producing a highly ordered, atomically smooth film surface. Furthermore, the alignments of the in-plane axes of Y1Ba2Cu 307 films with those of the underlying substrates of MgO(100) and SrTiO3(100) are evidence for epitaxial growth. The overall crystal structure was examined by X-ray diffraction [3]. Under present deposition conditions, growth on epitaxial substrates of (100) orientation including MgO, SrTiO3, and LaA1O3 produces c-axis oriented Y3Ba2Cu307_~ films. There is little evidence for a-axis oriented grains. Typical rocking curve of the (005) Bragg reflection is less than 0.3°. The detailed growth mechanism of the oxides still remains an interesting subject of study. Recently, strong oscillations of the specular RHEED intensity during growth of several perovskites including Y1Ba 2Cu 307 were observed by Terashima et al using the reactive evaporation method [10]. Furthermore, one period of oscillation corresponds to the height of one minimal unit cell which satisfies charge neutrality. The result is shown in fig. 3. In contrast to the layer-by-layer growth mode commonly known for metals or semiconductors, the growth of the perovskites is described by the unit-by-unit fashion. Presumably, it is related to the strong character of the ionic bond of oxides. This observation has an important implication that the oxide growth occurs by the formation of a 2D nuclei which is the minimum unit satisfying electrical neutrality. Moreover, it also suggests that within each growth unit the
Fig. 2. RHEED patterns along azimuthal (a) [100J and (b) [110] axes of a Y1Ba 2Cu 307
_
_
film 1000
A thick grown on
MgO(100).
968
J. Kwo
/
Growth and properties of high T~films in 1’~Ba
2Cu3O7 —
perovskite by MBE
(a)
start
C
-o
~
TIME (Arb. Unit)
Ba
Cu
,.fs1~ 02
Cu
(b)
Ba
j Cu02
Cu 02
~
~Ju1.7A
Fig. 3. (a) RHEED specular intensity as a function of the deposition time and (b) a proposed model for the Y1Ba2Cu3O7_~film growth, after Terashima et al. [10].
stacking sequence starts and ends with specific cation-oxide layers. 4. Superconducting properties
07
MBE grown Y1Ba2Cu3O7_~and Dy1Ba2Cu3 films showed excellent superconducting _
properties including 2~(R= 0), J~,and normal state resitivity p. Depositions on a variety of substrates including MgO(100), SrTiO3(100), and LaA1O3(100) have produced comparably good results, although the best superconducting transport properties were usually found for SrTiO3(100) of closest lattice match (— 1.0%) with Y1Ba2Cu307_~ [8]. In comparison, the very large lattice mismatch
J. Kwo
/
200
Growth and properties of high T~films in
I
‘ I
I
A
1000
I
FILM
60
15 0
(c) (Dl (a)
0 90 92 94 96
~
>-
-
(a) SrTiO (100) (b) LaAIO (100) Cc) MgO (100)
0 I
0
I
I I
100
200
I
300
1(K) Fig. 4. Resistivity versus temperature for Dy1Ba2Cu307_~ films 1000 A thick grown on (a) SrTiO3(100), (b) LaA1O3(100), and (c) MgO(100). The superconducting transitions in details are plotted in the inset.
9%) between Y1Ba 2Cu307 ~ and MgO conduces 7~,onset by — 2 K, and a higher 100 to K) abylower10%. p( The temperature dependence of the normal state resistivity for typical Dy1Ba2Cu3O7....~films 1000 A thick is shown in fig. 4 for three different substrates. The p versus T curves show a linear temperature dependence above T~,a common feature for current conductions along the Cu—Ox planes in the cuprates. The ratios of p(300 K)/p(100 K) in all three cases are 3.0, and the p versus T curves extrapolate to zero resistivity at T = 0. The detailed superconducting transition is plotted in the inset for each substrate. Notable difference in the shape of the transition was observed. Specifically, the 7(R = 0) is 92.3, 91, and 90.5 K for films grown on SrTiO3, LaA1O3, and MgO, respectively. The 10%—90% transition widths are 1.2 K for SrTiO3, and LaA1O3, and is only 0.5 K for MgO. The value of normal state resistivity p(lOO K) has been commonly used in the high T~field as a figure of merit to assess the materials quality. Note that p(l00 K) of the film grown on SrTiO3 is as low as 55 ~sQcm, which equals to the best value reported for bulk single crystals at present. (—
_
—
—
969
Screening measurements on Dy1Ba2Cu307_~ films showed a sharp superconducting transitions at 90 K, and a width less than 0.4 K [11]. The
mean field BCSthat behavior T~of 90 K.only Itwas also concluded such witha behavior is true for
/~~(b) a)
100
50
2Cu3O7_ ~ perovskite by MBE
penetration surement followed depth derived a temperature from thedependence screening meaof a
///..
40 20
________
88
//,/
}‘~ Ba
good film qualities in the absence of cracks or inhomogeneities [11]. The critical current densities, .i~,at 77 K in zero field were measured by the transport method. The typical 2 for values SrTiOare 4.5 x 106 and 2.0 x 106 A/cm 3(100) and MgO(100), respectively. Our J~results equal to the highest record reported by other groups using different growth techniques including reactive evaporation, pulsed laser deposition, and inverted cylindrical magnetron sputtering [12—14]. Thinner films generally show slightly poorer superconducting transport properties due to the lattice clamping effect and an increasing tendency of forming a tetragonal structure near the interface. For instance, reducing the film thickness to 500 A has led to an increase in the resitivity by about = 0) on decreases 88.5 K,10%. for The films1~(R grown SrTiO to 90.6 and 3 and MgO, respectively. Continuing reduction of the film thickness to below 100 A gives rise to substantially broadened transitions, especially for films grown on MgO. For a 90 A Y1Ba2Cu3O7_5 film on MgO, the zero resistivity is not reached until 70 K [6]. .
5. Tunneling studies of planar junctions The high t perovskites exhibit dramatic variations in conductivities with minor variations in the cation compositions and oxygen stoichiometry. This unusual property has offered new exciting possibilities of fabricating novel electronic devices in configurations of S/I/S or S/N/S oxide heterostructures. In comparison, such freedom in materials choices for devices was not available for traditional Josephson technology using low 7, Nb based superconductors. However, the progress toward fabricating all high 1 tunneling devices has been hampered due to the following difficulties. First, the coherence lengths of the high 7~super-
J. Kwo / Growth and properties of high 7 films in YjBafJu
970
3O7 —
I
I
I I
~
-
-
I
-
100K,
.,_,,,,,,,,—‘---..
-
70K C
1
30K V V
8K
I
150
I
I
I
100
50 0 -50 -100 -150 VOLTAGE (mV) Fig. 5. Quasipartscle tunneling data of dynamic resistance versus bias for a Y1Ba2Cu307~film/native barrier/Pb Junc-
perovskite byMBE
faces and to obtain meaningful tunneling results, in the first phase of study we focused the efforts on a simpler junction configuration consisting of a Y1Ba2Cu3O7_~thin film, native barrier, and a low 7~superconductor, Pb [15]. Low leakage tunnel barriers were formed by native insulating oxides of the in-situ grown Y1Ba2Cu3O7_~film surface exposed to room air or annealed at 450°Cfor 10 mm in 02. At the moment, the exact nature of the barrier is not clear, and still awaits further surface chemical analysis. However, empirically, the junction resistance appears to correlate with the film resistivity, and varies from a few Q to a few
geometry hundred tiveThe barriers lowf~.Typical is leakage 0.3 warrant mm junctions Xjunction that 0.5 mm. the formed current area inwith conduction the the planar naacross the junction is caused primarily by the tunneling process. Reproducible current—voltage characteristics were observed, and are discussed in the following. As for the quasiparticle tunneling characteristics, below the 7~(R= 0) of Y1Ba2Cu3 07 ~ a gap-like structure 20 mY developed in the tunneling conductance, with additional asym_
—
tion at different temperatures.
conductors are very short and highly anisotropic. Secondly, the surface does not exhibit superconductivity consistent with the bulk of materials. In order to better characterize the S/I inter-
metnc modulations up to 50 mY. There is another feature of at ±4 mY appearing below 28 K. The data of dynamic resistance versus bias are shown in fig. 5, and are in good agreement with the etched bulk single crystal data [16]. In contrast to the BCS tunneling density of
Fig. 6. Current—voltage characteristics at low bias at 1.5 K for a junction of a resistance of 80 1?. The x axis scale is 0.5 mY per large division for (a) and (b). The y axis is (a) 10 RA and (b) 2 RA.
J. Kwo
/
Growth and properties of high 1~films in Y,Ba
states of conventional superconductors, tunneling results of the high 7 cuprate of Y1Ba2Cu307_~ showed reproducibly three unusual features: (1) a weak, underdeveloped gap structure; (2) presence of finite states inside the gap persistent to low temperature; (3) an asymmetric, linear normal state conductance versus bias. Although many of these features could be attributed to materials-related imperfections near the surface, the reproducibility and consistency of these recent data suggest that the unusual properties could be intrinsic to the high 7~cuprates. Junctions of lower resistance show at temperature below T~,of Pb the development of supercurrent at zero bias and associated hysteretic sub-gap structure with an I~Rvarying from 500 to 800 ttY [15]. The present I~Rproduct is about 10%—20% of the maximum theoretical value. Typical I—V characteristics at low bias at T = 1.5 K are shown in fig. 6 for a junction of a resistance of 80 Q and a critical current 1~of 5 jiA. The I—V shows a nearly complete Stewart McCumber hysteresis with numerous Fiske steps due to geometric resonances. The supercurrent modulates with magnetic field and microwave radiations in a manner expected for DC and AC Josephson effects. Although the present experimental evidence is highly suggestive of Josephson tunneling, the possibility due to a high I~Rshorts is not ruled out cornpletely.
6. Conclusions Molecular beam epitaxy aided with an activated oxygen source has successfully produced Y1Ba2 Cu307_~ epitaxial films of excellent structural and superconducting properties. The present results in single layer films provide a basis for future fabrications of sophisticated multilayer structures. Such approach has now opened up an exciting field of “molecular engineering of oxides”, which will be vital for both basic research and device work. Our tunneling study observed for the first time the Josephson current between a high ternperature superconductor and a conventional superconductor in a planar junction. This result
2Cu3O7_~perouskite by MBE
971
establishes an important criterion for theoretical postulations of high temperature superconductivity. Furthermore, it is also a major advance toward developing a Josephson technology based on all high 7 superconductors.
Acknowledgements The author would like to thank valuable collaborations with M. Hong, T.A. Fulton, D.J. Trevor, R.M. Fleming, A.F. Hebard, P.L. Gammel, A.E. White, A.R. Kortan, R.C. Farrow, and K.T. Short, and excellent technical assistance from J.P. Mannaerts.
References [11P.P. Freitas and T.S. Plaskett, Phys. Rev.
B36 (1987) 5723. [2] R. Bormann and J. Nolting, Appi. Phys. Letters 54 (1989) 2148. [3] J. Kwo, M. Hong, D.J. Trevor, R.M. Fleming, A.E. White, R.C. Farrow, A.R. Kortan and K.T. Short, Appl. Phys. Letters 53 (1988) 2683. [4] J. Kwo, M. Hong, DJ. Trevor, R.M. Fleming, A.E. White, R.C. Farrow, A.R. Kortan and K.T. Short, in: Science and Technology of Thin Film Superconductors, Eds. R.D. McConnell and S.A. Wolf (Plenum, New York, 1989) p. 101. [51J. Kwo, M. Hong, D.J. Trevor, R.M. Fleming, A.E. White, J.P. Mannaerts, R.C. Farrow, A.R. Kortan and K.T. Short, Physica C162—164 (1989) 623. [6] J. Kwo, M. Hong, T.A. Fulton, P.L. Gammel and J.P. Mannaerts, in: Processing of Films for High 7 Superconducting Electroncis, SPIE Proc., Vol. 1187 (1989) p. 57. [7] J. Kwo, in: Thin Film Growth Techniques for Low-Dimensional Structures, Eds. R.C. Farrow, S.S.P. Parkin, P.J. Dobson, J.H. Neave and A. Arrott (Plenum, New York, 1986) p. 337. [8] J. Kwo, in: Proc. Conf. 2nd ISTEC Workshop on Superconductivity, Kagoshima, May 1990. [9] The Ni substrate block was suggested by John Talvachio at Westinghouse R&D Center, Pittsburgh, PA. [10] T. Terashima, Y. Bando, K. lijima, K. Yamanioto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Letters 65 (1990) 2684. [11] A.T. Fiory, A.F. Hebard, Mankiewich and R.E. Howard, Appl. Phys. Letters P.M. 52 (1988) 2165. [121 T. Teras~a, K. lijima, K. Yamamoto, Y. Bando and H. Mazaki, Japan. J. AppI. Phys. 27 (1988) L91. [13] T. Venkatesan, X.D. Wu, B. Dutta, A. Inam, M.S. Hegde,
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Growth and properties of high T~films in Y
D.M. Hwang, C.C. Chang, L. Nazar and B. Wilkens, Appl. Phys. Letters 54 (1989) 581. [14] X.X. Xi, G. Linker, 0. Meyer, E. Nold, B. Obst, F. Ratzel, R. Smithey, B. Strehlau, F. Waschenfeld and J. Geerk, Z. Physik 74 (1989) 13.
1Ba2Cu3O7 ~ perovskite by MBE
[15] J. Kwo, T.A. Fulton, M. Hong and P.L. Gammel, Appl. Phys. Letters 56 (1990) 788. [16] M. Gurvitch, J.M. Valles, Jr., A.M. Cucolo, R.C. Dynes, J.P. Garno, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. Letters 63 (1989) 1008.