I, I I 2538 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989 COMPARISON OF YBa2Cu307 FILMS GROWN BY SOLID-STATE AND VAPOR-PHASE EPITAXY J. Talvacchio,* J. R. Gavaler.* J. Greggi.* M. G. Forrester,** and A. 1. Braginski* Westinghouse R&D Center Pittsburgh, Pennsylvania 15235 Abstract Epitaxial films of YBa~Cu307 (YBCO) have been grown on single-crystal SrTiO3 and MgO substrates by two different routes using rf and dc magnetron co-sputtering. In the first case, amorphous oxide films were deposited at a substrate temperature of 400'C and the crystalline film grew by solid-state epitaxy in an in-situ post-anneal at -850'C. In the second case, a substrate temperature of 600-650'C was used which was sufficient to crystallize the film as it was deposited from the vapor-phase. Reaction with the substrate was less for the films grown by vapor-phase epitaxy - even when they were annealed at 850'C - as shown by the transition temperatures of very thin films. Auger depth profile measurements, in-situ XPS analysis and transmission electron microscopy. that segregation of Ba at the free surface can occur in either type of film. The relative merits of each growth process are discussed for microwave applications and tunnel junction fabrication. XPS was used to show 1. Introduction The formation of all vapor-deposited YBCO thin films can be categorized as either a reaction of solid-state precursors or direct condensation from the vapor phase into the perovskite-related structure (tetragonal or orthorhombic) of YBa2Cu307-,. The former category currently incliitles all techniques in which a substrate temperature, T,. <400'C is used, the source of Ba is BaF2.I layered structures of Y. Ba. Cu (or their oxides) are deposited.2 or a solution of nitrates or metallorganic precursor compounds are dried on a s~bstrate.~.~ As-deposited crystalline films can be produced if sufficient oxygen - or sufficiently active oxygen - is available during deposition to be incorporated into the film. Most as-deposited crystalline films have been prepared by magnetron or diode sputtering and the necessary deposition conditions will be examined as part of this work (Table 1). However, co-e~aporation.~-~ laser plasma spraying.1° and ion-beam sputteringll have also been used successfully to grow such films. microwave applications and tunnel-junction development of high-transition-temperature (T,) superconductors. The important length scale for microwave applications is the magnetic penetration depth, A. Cu-0 planes) z 27 nm and X, the important length scale is the coherence length: tab 2 3.1 nm, tC z 0.4 nm.12 thickness of the layer of superconductor adjacent to the film's free surface and the interface with the substrate which must have optimized superconducting properties. inappropriate for these applications. solution-deposited films produced to date have low critical current densities. Films reacted in the solid state from layered structures, fluorides, or an amorphous metal undergo a volume c!;ange during reaction that leads to rough surfaces. Therefore, the films reported here in which YBCO is formed in a solid-state reaction were deposited as amorphous oxides. Specific film properties which will be compared are: reaction with the substrate and composition of the film This paper will discuss film properties that affect For T<<T , A,b(parallel to 180 nm.lf For tunneling. These length scales define the Some film deposition techniques appear to be For example, all * Supported in part by AFOSR Contract F49620-88-C-0039. ** Supported by AFOSR Contract F49620-88-C-0030. Manuscript received August 22, 1988. adjacent to the substrate, microstructure and surface roughness, surface composition, and dc resistivity. 2. Fabrication of YBCO Films All films were deposited by magnetron sputtering in an apparatus described e1~ewhere.l~ were deposited from metallic targets in an Ar/O, atmosphere in sub-monolayer layers onto a substrate holder which oscillated to face the targets in sequence at 8 cycles/min. Argon was introduced to the chamber from the dark-space shield of the Ba sputter gun and oxygen was introduced at the substrates. crystal rate monitors were fed back to the sputtering power supplies to control composition much more reproducibly than possible with constant sputtering power. was uniform across a 2-inch diameter substrate holder within the precision of electron microprobe measurements. Samples were generally annealed in-situ - that is. without exposure to air - in an 02 atmosphere.I4 Ex-situ anneals were used for some films when surface-sensitive properties were not being measured. Several deposition parameters were set differently for the two alternative fabrication routes. The most important. the substrate temperature, T,. was 400°C for amorphous film growth and 600-650°C for crystalline films. deposition rate had to be increased by 80% to maintain stoichiometry in the films deposited at higher T,. Deposition rates were a sensitive function of oxygen partial pressure for more than 0.3 mtorr 02, so a lower oxygen pressure - usually 0.03 mtorr - was used for amorphous film growth. For crystalline growth. the highest practical oxygen pressure was used. Before sputtering, the oxygen pressure was set t o -0.6 mtorr. pressure could be increased to -3 mtorr. A t higher pressures, the surface of the Ba target oxidized, indicated by a change in the plasma color from green to purple. The Ba deposition rate decreased dramatically, and indications appeared of oxygen ion sputtering of the substrate h01der.l~ Ion sputtering of the substrate holder was always present to some degree and all holders were pre-coated with YBCO to prevent contamination of the films by molybdenum from screws and clamps. 1 hr at 850'C and 30 min-1 hr at 400'C. As-deposited crystalline films grown on (llZ0) sapphire were identified as having a tetragonal structure by the relative intensities of (013). (103). and (110) x-ray diffraction peaks. on SrTi03(100) or MgO(100) substrates were always sufficiently textured that x-ray diffraction could not distinguish between tetragonal and orthorhombic structures. A mixture of the two structures was most likely obtained. Although as-deposited films were not superconducting, their room-temperature resistivity was as IOW as 2 mn-cm.16 Table 1 is a summary of the deposition conditions used by a number of researchers who have obtained as-deposited crystalline films by sputtering. is that it shows a lack of consensus regarding target composition and placement, and sputter-gas pressures. common deposition conditions are rather trivial: T, E 550-7OO'C. oxygen must be added to the sputter gas (in addition to oxygen from the target), and non-stoichiometric targets must have extra Cu to compensate for re-sputtering effects. The critical deposition issues are often not addressed. distance should be reported since it determines the extent of re-sputtering of the film surface. problem for planar sputtering geometries can be inferred from Briefly, Y. Ba. and Cu The deposition rates at three vibrating The composition The Cu During sputtering. the As-deposited amorphous films were annealed 30 min to Films grown The significance of this table The In 'particular. the effects of target-to-substrate The severity of this 001 8-9464/89/0300-2538$01 .WO 1989 IEEE ~~ - -~ 1----
Transcript
I, I I
2538 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989
COMPARISON OF YBa2Cu307 FILMS GROWN BY SOLID-STATE AND VAPOR-PHASE EPITAXY
J. Talvacchio,* J. R. Gavaler.* J. Greggi.* M. G. Forrester,** and A. 1. Braginski*
Westinghouse R&D Center Pittsburgh, Pennsylvania 15235
Abstract
Epitaxial films of YBa~Cu307 (YBCO) have been grown on single-crystal SrTiO3 and MgO substrates by two different routes using r f and dc magnetron co-sputtering. In the first case, amorphous oxide films were deposited at a substrate temperature of 400'C and the crystalline film grew by solid-state epitaxy in an in-situ post-anneal at -850'C. In the second case, a substrate temperature of 600-650'C was used which was sufficient to crystallize the film as it was deposited from the vapor-phase. Reaction with the substrate was less for the films grown by vapor-phase epitaxy - even when they were annealed at 850'C - as shown by the transition temperatures of very thin films. Auger depth profile measurements, in-situ XPS analysis and transmission electron microscopy. that segregation of Ba at the free surface can occur in either type of film. The relative merits of each growth process are discussed for microwave applications and tunnel junction fabrication.
XPS was used to show
1. Introduction
The formation of all vapor-deposited YBCO thin films can be categorized as either a reaction of solid-state precursors or direct condensation from the vapor phase into the perovskite-related structure (tetragonal or orthorhombic) of YBa2Cu307-,. The former category currently incliitles all techniques in which a substrate temperature, T,. <400'C is used, the source of Ba is BaF2.I layered structures of Y. Ba. Cu (or their oxides) are deposited.2 or a solution of nitrates or metallorganic precursor compounds are dried on a s ~ b s t r a t e . ~ . ~ As-deposited crystalline films can be produced if sufficient oxygen - or sufficiently active oxygen - is available during deposition to be incorporated into the film. Most as-deposited crystalline films have been prepared by magnetron or diode sputtering and the necessary deposition conditions will be examined as part of this work (Table 1). However, co-e~aporat ion.~-~ laser plasma spraying.1° and ion-beam sputteringll have also been used successfully to grow such films.
microwave applications and tunnel-junction development of high-transition-temperature (T,) superconductors. The important length scale for microwave applications is the magnetic penetration depth, A. Cu-0 planes) z 27 nm and X, the important length scale is the coherence length: tab 2 3.1 nm, tC z 0.4 nm.12 thickness of the layer of superconductor adjacent to the film's free surface and the interface with the substrate which must have optimized superconducting properties.
inappropriate for these applications. solution-deposited films produced to date have low critical current densities. Films reacted in the solid state from layered structures, fluorides, or an amorphous metal undergo a volume c!;ange during reaction that leads to rough surfaces. Therefore, the films reported here in which YBCO is formed in a solid-state reaction were deposited as amorphous oxides.
Specific film properties which will be compared are: reaction with the substrate and composition of the film
This paper will discuss film properties that affect
For T<<T , A,b(parallel to 180 nm.lf For tunneling.
These length scales define the
Some film deposition techniques appear to be For example, all
* Supported in part by AFOSR Contract F49620-88-C-0039. ** Supported by AFOSR Contract F49620-88-C-0030. Manuscript received August 22, 1988.
adjacent to the substrate, microstructure and surface roughness, surface composition, and dc resistivity.
2. Fabrication of YBCO Films
All films were deposited by magnetron sputtering in an apparatus described e1~ewhere.l~ were deposited from metallic targets in an Ar/O, atmosphere in sub-monolayer layers onto a substrate holder which oscillated t o face the targets in sequence at 8 cycles/min. Argon was introduced t o the chamber from the dark-space shield of the Ba sputter gun and oxygen was introduced at the substrates. crystal rate monitors were fed back to the sputtering power supplies to control composition much more reproducibly than possible with constant sputtering power. was uniform across a 2-inch diameter substrate holder within the precision of electron microprobe measurements. Samples were generally annealed in-situ - that is. without exposure to air - in an 02 atmosphere.I4 Ex-situ anneals were used for some films when surface-sensitive properties were not being measured.
Several deposition parameters were set differently for the two alternative fabrication routes. The most important. the substrate temperature, T,. was 400°C for amorphous film growth and 600-650°C for crystalline films. deposition rate had to be increased by 80% to maintain stoichiometry in the films deposited at higher T,. Deposition rates were a sensitive function of oxygen partial pressure for more than 0.3 mtorr 02, so a lower oxygen pressure - usually 0.03 mtorr - was used for amorphous film growth. For crystalline growth. the highest practical oxygen pressure was used. Before sputtering, the oxygen pressure was set to -0.6 mtorr. pressure could be increased to -3 mtorr. A t higher pressures, the surface of the Ba target oxidized, indicated by a change in the plasma color from green to purple. The Ba deposition rate decreased dramatically, and indications appeared of oxygen ion sputtering of the substrate h01der.l~ Ion sputtering of the substrate holder was always present to some degree and all holders were pre-coated with YBCO to prevent contamination of the films by molybdenum from screws and clamps.
1 hr at 850'C and 30 min-1 hr at 400'C. As-deposited crystalline films grown on (llZ0) sapphire were identified as having a tetragonal structure by the relative intensities of (013). (103). and (110) x-ray diffraction peaks. on SrTi03(100) or MgO(100) substrates were always sufficiently textured that x-ray diffraction could not distinguish between tetragonal and orthorhombic structures. A mixture of the two structures was most likely obtained. Although as-deposited films were not superconducting, their room-temperature resistivity was as IOW as 2 mn-cm.16
Table 1 is a summary of the deposition conditions used by a number of researchers who have obtained as-deposited crystalline films by sputtering. is that it shows a lack of consensus regarding target composition and placement, and sputter-gas pressures. common deposition conditions are rather trivial: T, E 550-7OO'C. oxygen must be added t o the sputter gas (in addition to oxygen from the target), and non-stoichiometric targets must have extra Cu t o compensate for re-sputtering effects. The critical deposition issues are often not addressed. distance should be reported since it determines the extent of re-sputtering of the film surface. problem for planar sputtering geometries can be inferred from
Briefly, Y. Ba. and Cu
The deposition rates at three vibrating
The composition
The Cu
During sputtering. the
As-deposited amorphous films were annealed 30 min to
Films grown
The significance of this table
The
In 'particular. the effects of target-to-substrate
The severity of this
001 8-9464/89/0300-2538$01 .WO 1989 IEEE
~~ - -~ 1----
the references in Table 1 in which substrates are placed perpendicular to the target. Our own experience with a single YBa2Cu307 target in a planar geometry is that re- sputtering of the substrates and substrate holder contaminated our films even without 0, added to the sputtering gas for an Ar pressure in the range of 5 to 100 mtorr.
annealing temperature needed to transform tetragonal films to the orthorhombic structure. attributed to the facts that T, measurements can be dominated by a minority phase and the tetragonal and orthorhombic structures cannot be distinguished in standard "28" x-ray diffraction scans on textured films. Our highest Tc's were obtained from films post-annealed in an O2 atmosphere at 850°C.
A lack of consensus is also apparent in Table 1 for the
Part of the disparity can be
3. Electrical Properties - Ultrathin Films
Examples of resistivity plotted as a function of temperature are shown in Fig. 1 for two of our YBCO films. Important differences in the temperature dependence of resistivity of amorphous and crystalline as-deposited films were evident in very thin films. films 100 nm thick were semiconducting. Sec. 4 and documented in many other reports show that there is significant reaction and interdiffusion between the film and substrate. However. as-deposited crystalline films 100 nm thick were superconducting after a post-anneal and had positive temperature coefficients of resistivity for T>Tc - even for films annealed at 850°C. Films 100 nm thick deposited on MgO(100) also had T, (zero resistance) = 30-35K. The T,'s of 100 nm thick films are slightly higher but in reasonable agreement with the Tc's of films deposited at 600-65O'C reported in references 17. 18. and 19. but much lower than values obtained in ultrathin films by Bando et aL6 Epitaxial growth of YBCO at a substrate temperature of 630°C is shown in the RHEED patterns of Fig. 2 to be clearly established at a film thickness of 10 nm. 400'C. did not exhibit a RHEED pattern. measurements of the 10 nm solid-state epitaxial films showed that the major source of contamination was Ti diffusion from the substrate.
Crystallized amorphous Data presented in
In-situ annealed 10 nm thick films deposited at XPS
2539
I I I I I
(al p loo0
' U :
E m -
f m - -
- - $ m -
- - E -
0 100 m 300 Temperdure ( K)
Fig. 1 - Resistivity versus temperature for (a) a 100 nm thick film which was crystalline as-deposited, and (b) a 500 nm thick film which was amorphous as- deposited. Both films were annealed in an 0, atmosphere at 850°C for 1 hr.
Fig. 2 - 9 kV RHEED patterns from (a) a SrTi03(100) substrate at a [ O l l ] azimuth, and (b) a 10 nm thick film deposited at 630'C. are similar to spots obtained from the 2x2 reconstruction of SrTi03(100). but relative Y3d. Ba3d. C U ~ , , Ti2 , and Sr3d photoelectron counts confirmed that [he film was r10 nm thick.
Extra spots in (b)
I
!
Table 1. Fabrication of as-deposited crystalline Y BCO films by sputtering.
First Author Sputtering Targets Ar / 0 2 T s Distance Rate Thickness As-deposited Tannea~ ILaboratory 1 Technique JComposition) (mtorr) [cm) [nm/min) (microns] Substrates Structure, T, Icm
This work 2 rf. 1 dc magn. Y.Ba.Cu 2013 600-630 11 3-4.5 0.1-0.8 SrTiOj tetragonal 850'C. 80K Miura (Toshiba)" rf magnetron Y.Ba2Cu03.C~ 2.512.5 560 2.2 0.4 SrTiOj 80K
a Substrates perpendicular to target. Bias sputtering. Sputtering from inside edge of annular target.
I
2540
4. Film/Substrate Reaction
The reaction between SrTi03 substrates and YBCO films was studied by Auger spectroscopy depth profiles. 3(a) is a profile of an as-deposited amorphous film after annealing at 850'C. formed at the interface was also observed in a TEM analysis. consisted of randomly-oriented and equi-axed crystalline grains. The presence of this disordered layer apparently did not interfere with the nucleation of an epitaxial film (Sec. 5) during solid-state crystallization, so we speculate that the Ba segregation to the interface must have occurred after the YBCO crystalline structure formed. are nearly identical to Auger depth profiles reported by Wu et al. for annealed YBCO films which had been amorphous as-deposited by laser ablation.36 The thickness of the transition layer between the part of the film with Y:Ba:Cu = 1:2:3 and the BaO interface layer is comparable to the magnetic penetration depth. For microwave applications, the losses in this transition layer and dielectric losses in the BaO layer are expected to be very high. dielectric losses also mandate that MgO substrates should be used rather than SrTi03.37
Figure 3(b) shows that the film/substrate interface is much sharper for a YBCO film deposited at 600'C and annealed at 850'C. throughout the film. data (not shown) for films grown on MgO at 600'C and post-annealed at 850'C. Wu et al. showed a similar result for laser-ablated films which were crystalline as-deposited on SrTi03.*
Figure
The 4 0 0 nm thick BaO layer which
The TEM analysis showed that the BaO layer
The data in Fig. 3(a)
Considerations of
A ratio, Y:Ba:Cu 1: 1:2:3 is maintained We have obtained similar depth profile
5. Microstructure
X-ray diffractometer data from Y BCO films crystallized from the vapor phase or from an amorphous oxide indicated that the film texture was the same for either fabrication route. Most grains in the films were oriented with the a- axis, [loo]. parallel to the growth direction and a smaller amount of c-axis, [Ool]. growth. Figs. 4(a) and (b) show typical results for "a>c" growth. and exclusively a-axis growth. respectively. The highest critical current densities, J,. are obtained for c-axis growth.l* The highest J,'s
O l M a w r n a w , n a hpth fmm Surface I nml
80 I I I ( ( I I I I 1
- (b)
V
hpth (mm Surface I nml
Fig. 3 - Auger spectroscopy depth profiles for YBCO films annealed at 850'C: and (b) crystalline as-deposited.
(a) amorphous as-deposited,
srm,
id Fig. 4 - Portions of x-ray diffractometer scans showing
predominantly a-axis growth, and exclusively a-axis growth.
measured in these predominantly a-axis films were 5x10' A/crn2 at 4.2K and 2x104 at 77K. planar tunneling, a-axis growth is clearly preferable to have the benefit of a longer coherence length. applications, it is not clear which orientation is better. Preliminary measurements published in Ref. 38 found lower surface resistances in c-axis films. measurements of our films are reported in Ref. 39.
The x-ray diffractometer scans shown in Fig. 4 only identified growth texture. TEM data showed that the films were both polycrystaliine and epitaxial. That is. a mosaic structure formed in which (almost) all grains were oriented with respect to the SrTi03(100) substrate, but the c-axes of individual grains could lie Darallel to [OlO] or [OOl] directions
However, for
For microwave
Surface resistance
~~ ~. in the SrTjO,.
that the mosaic structure was common to both solid-state Figure 5 contains two TEM micrographs which show
and vapor-phase epitaxial growth. Differences were observed in grain size and shape. Films crystallized in the solid state had roughly equiaxial grains 0.5 to 1.0 pm wide. grains in as-deposited crystalline films had an aspect ratio of -4:l with a 0.5 pm longer edge and the c axis parallel to the shorter edge. Figure 5(b) shows an example. found in both types of films. of a grain oriented 45' away from the
The
others.
(a)
Fig. 5 - TEM micrographs showing characteristic grain sizes, shapes, and orientation for post-annealed films which had been crystallized (a) from an amorphous oxide, and (b) duiing deposition.
1
2541
6. Surface Composition and Structure
It is well known that exposure to air will degrade the surface of YBCO to a depth greater than the coherence length - the critical length for tunneling. an earlier publication that the surface is degraded during crystallization of an amorphous film - even for complete1 in-situ processing - by segregation of Ba to the surface.l0 The same phenomenon has since been observed for films deposited a t 600-65O'C during post-annealing in 0, a t 600°C. Barium se regation can be avoided by using a rapid-ramp anneal!8 The segregation appears to be driven by oxygen deficiency. the oxygen-deficiency of the tetragonal structure or the presence of some amorphous oxide must be invoked to explain the segregation.
planar tunnel junctions. had smoother surfaces - usually featureless a t lo5 magnification in an SEM. crystalline films 0.5 pm thick had a dense "basket-weave" appearance with feature sizes and orientations which were the same as observed by TEM. thickness due to this structure were approximately 100 nm.
We have shown in
In as-deposited crystalline films, either
Surface morphology is also important for fabrication of Films grown by solid-state epitaxy
The surfaces of as-deposited
Local variations in film
Conclusions
As-deposited crystalline films clearly form better film/substrate interfaces than films crystallized in the solid state, and are the better fabrication alternative for thin-film microwave applications. The improved interfaces with the substrate are related to the chemical stability of the crystalline phase and not simply due to reduced processing temperatures. base electrodes for high-T, tunnel-junction development. although crystallized amorphous films are smoother. general and rather trivial deposition conditions needed for growing crystalline YBCO films can be specified a t this time.
Either fabrication alternative could be used as
Only
Acknowledgments
The authors acknowledge the assistance of H. C. Pohl. A. M. Stewart, R. F. Farich. D. Detar. T. Mullen, and R. T. Blackham.
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