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Microstructure of edge-type Josephson junctions with PrBa2Cu3O7−χ barrier layer
PHYSICA
Physica C 198 (1992) 278-286 North-Holland
Microstructure of edge-type Josephson junctions with PrBazCu307_x barrier layer O.I. L e b e d e v , A . L . V a s i l i e v a n d N . A . K i s e l e v Institute of Crystallography, Russian Academy of Sciences, 117333 Moscow, Russia L.A. M a z o , S.V. G a p o n o v , D . G . P a v e l i e v a n d M . D . S t r i k o v s k y Institute of Applied Physics, Russian Academy of Sciences, 603600 N.Novgorod, Russia Received 22 April 1992
HREM investigations of edge Josephson junctions (EJJ) with PrBa2Cu307_x barrier layer (PB) were performed. All layers (superconducting YBaECU307_x (Y 1) and (Y2), insulating PrBa2Cu307_x (PI) and barrier (PB) were obtained by laser ablation. The edges were formed by ion sputtering using a fotoresist mask. EJJ shows Josephson conductivity at To= 77 K, giving Jc= 104 A/cm 2 at U~= 50 pV. Cross-sectional images show that Y1, PI and PB layers are single crystalline with the c-axis normal to the substrate surface. The Y2 layer in the regions of a multilayered structure is polycrystalline. The PB/Y 1 interface is characterised by APB line boundaries; it is inclined to the substrate by 20-35 °,
1. Introduction Edge-type Josephson j u n c t i o n s ( E J J ) seem to be most p r o m i s i n g for fabricating controllable EJJs on the basis o f H T S C films. The c-axis o r i e n t e d films, having a high value o f critical current density (Jc) and low noise are most suitable in this case. In edgetype connections o f these films the Josephson current direction lies in the ab Y B C O plane, i.e. in the direction o f higher Y B C O coherence length, m a k i n g it easier to o b t a i n a superconducting junction. EJJs have been the subject o f a n u m b e r o f studies [ 1-4 ]. Barrier layers were f o r m e d from non-superconducting YBCO using the oxyfluoride process [ 1 ], the PrBa2Cu307_x layers [ 2 ], N b - d o p e d SrTiO3 [ 3 ] and from cubic non-superconducting Y B C O layers [ 4 ]. The coherence length in YBa2fu307_x is short a n d anisotropic; thus the EJJ properties are defined by the contact microstructure (fig. 1 ): Y 1 / P B a n d Y 2 / P B interfaces, Y I a n d Y2 layers adjacent to the contact area, PB barriers, their m u t u a l orientation, lattice defects, a m o r p h o u s areas, etc. The defects that can affect the EJJ properties have d i m e n s i o n s c o m p a r a b l e to the crystal lattice a n d could be identified most effectively by high resolu-
St
I Y2 I'll
c lc
PI
Y1
I
SrTiOa Fig. 1. Schematic representation of the EJJ structure: Y 1 is a superconducting layer, P 1 an insulating layer, PB a barrier layer, and Y2 a superconducting layer. tion electron microscopy ( H R E M ) . H R E M was successfully used for studying expitaxial YBCO films on step-edge SrTiO3 [ 5 ]. In this p a p e r H R E M data o f an EJJ analysis with a PrBa2Cu3OT_x barrier layer is reported.
2. Experiment The EJJ structure is schematically shown in fig. 1. All layers were obtained by laser ablation on a SrTiO3 substrate [ 6]. A deposition system with a Nd-glass
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
O.L Lebedev et al. / Microstructure of edge-type Josephson junctions
laser ( ~ 1 Joule output energy in 30 ns pulse), equipped by the plasmadynamical system of droplets separation, was used. The system provided films with the following parameters: T¢ (zero resist a n c e ) = 8 9 - 9 1 K, J¢ (77 K ) > 106 A / c m 2, surface droplets concentration < 2 × 104 cm -2. The structure was formed in two stages. In the first stage the superconducting YBa2CuaO7_x (Y1) and insulating (PI) layers were deposited in one vacuum cycle. Either a PrBaECU307_x or a non-superconducting YBa2Cu307_ x layer, deposited from the same target at lower substrate temperature ( ~ 5 0 0 ° C ) served as an insulator. The edges were formed by ion sputtering (At +, 2 keV) using a photoresist mask. After removal of the photoresist the specimens were ultraviolet treated for restoring the edge surface properties, lost during the ion bombardment. After
2 79
that, the 10-25 nm thick PB barrier layer and the Y2 layer (about 200 nm thick) were deposited in an other vacuum cycle. EJJs with a 20 nm layer demonstrate Josephson conductivity at To= 77 K, giving a Jc value of 103l 0 4 A / c m 2 at a characteristic voltage U¢ ~ 50 ~tV. Detailed information on the EJJs formation will be given in a separate paper. Cross-sections of films and Josephson junctions were prepared for HREM. After mechanical treatment, grinding and polishing to about 30 ~tkm thickness the specimens were ion thinned by Ar + ions with 5 keV energy at 8-10 ° angles relative to the specimen surface. Precautions were taken to reduce the film sputtering rate during ion thinning, because the etching rate of the SrTiO3 substrate is much lower compared to the HTSC film (beam shields were used
Fig. 2. Cross-sectional low magnification images of the EJJs, (a) Y2 and PB layers are partially removed by ion etching, (b) the dashed line is parallel to the inclined interface between barrier and other layers and shows changing of the interface angle near the top of the Y1 layer.
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O.L Lebedev et aL / Microstructure of edge-type Josephson junctions
to prevent sputtering along the interface). The overall etching time for such specimens was about 300 h. The HREM study was carried out on a Philips EM430 ST microscope at 200 and 250 kV.
3. Results and discussion
Cross-sectional low magnification images of EJJ are shown in fig. 2(a) and (b). In fig. 2(a) the Y2 and PB layers are partially removed by ion etching during specimen preparation. Due to contrast difference between YBCO and PrBCO (also observed in ref. [6 ] ) the edge constant is visualized by an arrow in fig. 2(b). The PB/Y1 interface in the region of the edge contact (fig. 2 ( b ) ) is inclined by 20-25 ° relative to the substrate. The angle increases to 35 ° with growing distance from the substrate. The inclination angle variations may be caused by different
ion etching rates of PI and Y 1 layers in the process of the step fabrication. The inclined interfaces are wavy with unevenness up to several nm. One might have expected partial substrate surface sputtering and step appearance after ion etching during the process of EJJ edge formation by the ion bombardment. However, this was not observed (fig. 2 ( b ) ) , probably due to high etching stability of SrTiO3. Lattice resolution images reveal epitaxial growth of the PrBCO intermediate layer and the Y2 film (fig. 3). The c-axis orientation in the PB intermediate layer and the Y2 film remains normal to the substrate. The PB layer may also be identified by defect appearance on the Y1/PB interface. Prior to the PB layer deposition the vacuum cycle was broken. Renewing the deposition process after lithography and ion etching results in defects appearing on the Y1/ P boundary, marking the interface. The subsequent
Fig. 3. Cross-sectionallattice resolution image of the EJJ. The PB layer is arrowed.
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281
Fig. 4. High-resolution cross-sectional image of the EJJ. (a) Y 1/PB interface is arrowed. The bright area in the right part of the interface contains domains with moir6 fringes and lightly inclined (001) planes possibly due to some changes in oxygen content in these domains. (b) Enlarged part of the P B / Y 1 interface (big arrows) with APB-like defects (one of the l/3 steps is shown with white arrows). (c) Enlarged part of the PB/Y1 interface (big arrows). The uninterrupted atomic plane parallel to the (001) having bright contrast is indicated with white arrows.
282
O,L Lebedev et al. 1 Microstructure of edge-type Josephson junctions :'~"T
Fig. 4. Continued.
formation of Y2 on PB takes place in a single vacuum cycle and the defect concentration on the PB/ Y2 interface is significantly lower (fig. 3). HREM images (fig. 4 (a, b) ) show that these defects are 1/ 3c PrBCO lattice shifts relative to Y1, resembling antiphase boundaries (APB) observed in a study [ 7 ]. However, in some parts of the Y1/Pb interface these defects are absent (fig. 4(c) ). The elimination of the 1/3c shift occurs due to nonstoichiometry defects in the PB layer. The 5-8 nm areas with moir6 fringes in a Y 1 layer were observed in the region 20 nm along the YI/PB interface. The (001 ) planes in this region are slightly inclined relative to the (001) planes of the rest part of the Y 1 film. The moir6 type fringes and the plane inclination could be caused by the appearance of domains with another oxygen content formed after different treatments of the edge surface. Amorphous layers are not observed on the interfaces. Electron diffraction (ED) investigations also confirm the small misorientation of these layers in the EJJ area. An ED pattern taken from the EJJ region (fig. 5 ) with light spots doubling and streaks can be observed. The reflections of Y1 and P are superimposed. EM image analysis gives a 25-30 nm thickness evaluation of the PB layer (fig. 3). The Y2 film in the edge contact region is characterized by a high APB concentration and stacking faults-double rows of
Fig. 5. Electron diffraction pattern taken from the EJJ area. The spot split due to small (001) planes inclination near the interfaces is arrowed.
CuO (fig. 3). Such stacking defects are often ended on APBs. Figures 6 and 7 are cross-sectional images of the layered structures on different sides of the EJJ. Figure 6 is a micrograph of a two-layered part of Jo-
O.L Lebedev et al. / Microstructure of edge-type Josephson junctions
283
Fig. 6. High-resolution cross-sectional image of the two-layers part o f the EJJ.
Fig. 7. Cross-sectional image of the multilayers structures near EJJ. The Y2/PB and PB/PI interfaces are shown with black arrows. The inclusion on the top of the PB surface is arrowed.
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O.L Lebedev et al. /Microstructure of edge-type Josephsonjunctions
sephson junction (Y2-PB/SrTiO3). The PB layer has a darker contrast and is characterized by a higher defect concentration, probably due to bad layer stoichiometry. Defect areas with moir6 patterns appear on the interface, possibly as a result of the substrate surface unevenness and following the superimposition of the substrate and PrBCO images or intermediate layer appearance. Nevertheless, the interface contains no amorphous layer on the SrTiO3 surface caused by ion thinning. The PB layer c-axis is oriented normal to the substrate. The Y2 layer orientation is the same and its non-stoichiometric defect concentration is lower than in the PB layer. Figure 7 is a low magnification image of the PB structure viewed in the other side of the EJJ. The Y 1, PB and Y2 layers are easily distinguished. The Y1 film is single crystalline, with the c-axis normal to the substrate surface, containing defects. The Y 1 layer thickness is 0.15 Ixm. The PI insulating layer grows epitaxially on Y 1, its thickness is about 100 nm. The APB concentration in the PB layer is lower compared to Y1. The next layer is 0.25 ~tm thick Y2; it
is polycrystalline with grains both c and a ( b ) oriented. Figure 8 is a grain boundary (GB) image. The GB is coherent. The APB concentration in the Y2 film is higher than in Y 1 and PB. Crystalline grains of unidentified phase 10-100 nm in size are observed on the Y2/PB interface (fig. 9). The reason of the high defect concentration grains misorientation may be an unsufficiently fiat PB surface or its contamination after making for the lithography process. There was no masking material on the inclined surface of the EJJ resulting in a lower defects concentration in the Y2 film. Figure 9 shows the lower part of an EJJ multilayered structure - a (110) YBCO/SrTiO3 (100) cross-section. The (001) layers in the right and central part of the micrograph are shifted by c/3 relative to the c-axis - a characteristic feature of an APB. The SrTiO3 surface is uneven and could be one of the reasons of the APB appearance. Another reason, even though less probable, is that the APBs are screw dislocation boundaries. Screw dislocations were found in YBCO films using tunneling microscopy [ 5,6 ].
Fig. 8. High-resolutionimage of the Y2 layer. Several YBCOgrains in different orientations are shown. Grain boundaries are coherent.
O.L Lebedev et al. / Microstructure of edge-type Josephson junctions
285
Fig. 9. High-resolution image of the (001) SrTiO3/Y 1 interface. APBs are indicated with big arrows. Some of the 1/3 plane shift on APBs are arrowed. The inclination of adjacent crystalline layers near APBs (areas indexed "s" for example) is visible. These layers are shown by pairs of small arrows.
Fig. 10. High-resolution image of the SrTiO3/Y 1 interface. The SrTiO3 surface is inclined at 3 ° from (001) SrTiO3. Some of the 1/3 plane shift on the APBs is arrowed. The inclination of adjacent crystalline layers near APBs (areas indexed "s" for example) is visible. These layers are shown by pairs of small arrows.
T h e layers are slightly i n c l i n e d r e l a t i v e to e a c h o t h e r ( ~ 3 ° ) in areas i n d e x e d " s " . Such patterns, in particular, m a y be o b s e r v e d f r o m s o m e parts o f screw d i s l o c a t i o n s o r f r o m s u p e r i m p o s e d areas o f t w o dislocations. F i g u r e 10 shows the Y B C O / S r T i O 3 interface. In this case the surface is i n c l i n e d f r o m the ( 0 0 1 ) SrTiO3 p l a n e ( ~ 3 ° ). T h e A P B s are also o b s e r v e d in this interface. T h e c o n c e n t r a t i o n o f A P B s in such
areas is slightly higher, c o m p a r e d to t h e case w i t h o u t i n c l i n e d substrate surface. 4. S u m m a r y T h e m i c r o s t r u c t u r e o f an E J J w i t h a PrBazCuaO7_x b a r r i e r layer, o b t a i n e d b y laser a b l a t i o n was defined. T h e edge c o n t a c t i n c l i n a t i o n was estim a t e d to be 2 0 - 3 5 °. At this i n c l i n a t i o n angle the c-
286
O.L Lebedev et al. / Microstructure of edge-type Josephson junctions
axis o f all layers in the edge contact region is oriented n o r m a l to the substrate, ensuring the best electrophysical properties o f the layers in the EJJ region. The interface as well as substrate areas lithographically treated are free from a m o r p h o u s layers, i.e. from a d d i t i o n a l barriers. At the same t i m e the PB layer, grown after breaking the v a c u u m cycle, cont a i n e d m o r e defects than the P I layer, f o r m e d immediately on the Y 1 layer. Possibly, this is a result o f higher defect concentration on the P 1 surface after v a c u u m loss a n d ion thinning. The c o n t a m i n a t i o n o f the fiat part o f PI film surface with photoresist could be the reason o f the polycrystallinity a n d high APB concentration on the Y2 film. A step in the substrate, which was expected to app e a r in the process o f EJJ edge f o r m a t i o n by ion b o m b a r d m e n t was not observed, p r o b a b l y due to the higher etching stability o f SrTiO 3 c o m p a r e d to YBCO a n d PrBCO. APBs were observed, which m a y be interpreted as b o u n d a r i e s between neighboring screw dislocations or could be taken to be the result o f substrate surface unevenness. The Y 1 layer APB density is significantly lower c o m p a r e d to Y2. Evidently, the higher roughness o f the PB layer defines a higher concentration o f APBs.
H R E M investigations showed that laser ablation allows one to o b t a i n a defined type o f EJJs with practically perfect structure. The obtained data gives hope to the possibility o f fabricating controllable devices by changing the Pr b a r r i e r layer thickness.
References [1 ] R.B. Laibowitz, R.H. Koch, A. Gupta, G. Koren, W.J. GaUagher,V. Foglietti, B. Oh and J.M. Viggiano, Appl. Phys. Lett. 56 (1990) 686. [2] J. Gao, W.A.M. Aarnink, G.J. Gerritsma and H. Rogalla, Physica C 171 (1990) 126. [3] D.K. Chin and T. Van Dyzer, Appl. Phys. Lett. 58 (1991) 753. [4] B.D. Hunt, N.C. Foote and LJ. Bajuk, Appl. Phys. Lett. 59 ( 1991 ) 982. [ 5 ] C.L Jia, B. Kabius, K. Urban, K. Herrman, G.J. Cui et al., Physica C 175 (1991) 545. [6 ] A.L. Vasiliev, N.A. Kiselev, A.M. Dovidenko, S.V. Gaponov and M.A. Kaljagin, Sverchprovodimost (Physics, chemistry, technics) 3 (1990) 557. [7] W. Zandbergen and G. Thomas, Phys. Status Solidi A 107 (1988) 825. [8] A.L. Vasiliev, N.A. Kiselev, A.I. Golovashkin et al., KSF 7 (1987) 59 (in Russian).
Physica C 198 (1992) 278-286 North-Holland
Microstructure of edge-type Josephson junctions with PrBazCu307_x barrier layer O.I. L e b e d e v , A . L . V a s i l i e v a n d N . A . K i s e l e v Institute of Crystallography, Russian Academy of Sciences, 117333 Moscow, Russia L.A. M a z o , S.V. G a p o n o v , D . G . P a v e l i e v a n d M . D . S t r i k o v s k y Institute of Applied Physics, Russian Academy of Sciences, 603600 N.Novgorod, Russia Received 22 April 1992
HREM investigations of edge Josephson junctions (EJJ) with PrBa2Cu307_x barrier layer (PB) were performed. All layers (superconducting YBaECU307_x (Y 1) and (Y2), insulating PrBa2Cu307_x (PI) and barrier (PB) were obtained by laser ablation. The edges were formed by ion sputtering using a fotoresist mask. EJJ shows Josephson conductivity at To= 77 K, giving Jc= 104 A/cm 2 at U~= 50 pV. Cross-sectional images show that Y1, PI and PB layers are single crystalline with the c-axis normal to the substrate surface. The Y2 layer in the regions of a multilayered structure is polycrystalline. The PB/Y 1 interface is characterised by APB line boundaries; it is inclined to the substrate by 20-35 °,
1. Introduction Edge-type Josephson j u n c t i o n s ( E J J ) seem to be most p r o m i s i n g for fabricating controllable EJJs on the basis o f H T S C films. The c-axis o r i e n t e d films, having a high value o f critical current density (Jc) and low noise are most suitable in this case. In edgetype connections o f these films the Josephson current direction lies in the ab Y B C O plane, i.e. in the direction o f higher Y B C O coherence length, m a k i n g it easier to o b t a i n a superconducting junction. EJJs have been the subject o f a n u m b e r o f studies [ 1-4 ]. Barrier layers were f o r m e d from non-superconducting YBCO using the oxyfluoride process [ 1 ], the PrBa2Cu307_x layers [ 2 ], N b - d o p e d SrTiO3 [ 3 ] and from cubic non-superconducting Y B C O layers [ 4 ]. The coherence length in YBa2fu307_x is short a n d anisotropic; thus the EJJ properties are defined by the contact microstructure (fig. 1 ): Y 1 / P B a n d Y 2 / P B interfaces, Y I a n d Y2 layers adjacent to the contact area, PB barriers, their m u t u a l orientation, lattice defects, a m o r p h o u s areas, etc. The defects that can affect the EJJ properties have d i m e n s i o n s c o m p a r a b l e to the crystal lattice a n d could be identified most effectively by high resolu-
St
I Y2 I'll
c lc
PI
Y1
I
SrTiOa Fig. 1. Schematic representation of the EJJ structure: Y 1 is a superconducting layer, P 1 an insulating layer, PB a barrier layer, and Y2 a superconducting layer. tion electron microscopy ( H R E M ) . H R E M was successfully used for studying expitaxial YBCO films on step-edge SrTiO3 [ 5 ]. In this p a p e r H R E M data o f an EJJ analysis with a PrBa2Cu3OT_x barrier layer is reported.
2. Experiment The EJJ structure is schematically shown in fig. 1. All layers were obtained by laser ablation on a SrTiO3 substrate [ 6]. A deposition system with a Nd-glass
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
O.L Lebedev et al. / Microstructure of edge-type Josephson junctions
laser ( ~ 1 Joule output energy in 30 ns pulse), equipped by the plasmadynamical system of droplets separation, was used. The system provided films with the following parameters: T¢ (zero resist a n c e ) = 8 9 - 9 1 K, J¢ (77 K ) > 106 A / c m 2, surface droplets concentration < 2 × 104 cm -2. The structure was formed in two stages. In the first stage the superconducting YBa2CuaO7_x (Y1) and insulating (PI) layers were deposited in one vacuum cycle. Either a PrBaECU307_x or a non-superconducting YBa2Cu307_ x layer, deposited from the same target at lower substrate temperature ( ~ 5 0 0 ° C ) served as an insulator. The edges were formed by ion sputtering (At +, 2 keV) using a photoresist mask. After removal of the photoresist the specimens were ultraviolet treated for restoring the edge surface properties, lost during the ion bombardment. After
2 79
that, the 10-25 nm thick PB barrier layer and the Y2 layer (about 200 nm thick) were deposited in an other vacuum cycle. EJJs with a 20 nm layer demonstrate Josephson conductivity at To= 77 K, giving a Jc value of 103l 0 4 A / c m 2 at a characteristic voltage U¢ ~ 50 ~tV. Detailed information on the EJJs formation will be given in a separate paper. Cross-sections of films and Josephson junctions were prepared for HREM. After mechanical treatment, grinding and polishing to about 30 ~tkm thickness the specimens were ion thinned by Ar + ions with 5 keV energy at 8-10 ° angles relative to the specimen surface. Precautions were taken to reduce the film sputtering rate during ion thinning, because the etching rate of the SrTiO3 substrate is much lower compared to the HTSC film (beam shields were used
Fig. 2. Cross-sectional low magnification images of the EJJs, (a) Y2 and PB layers are partially removed by ion etching, (b) the dashed line is parallel to the inclined interface between barrier and other layers and shows changing of the interface angle near the top of the Y1 layer.
280
O.L Lebedev et aL / Microstructure of edge-type Josephson junctions
to prevent sputtering along the interface). The overall etching time for such specimens was about 300 h. The HREM study was carried out on a Philips EM430 ST microscope at 200 and 250 kV.
3. Results and discussion
Cross-sectional low magnification images of EJJ are shown in fig. 2(a) and (b). In fig. 2(a) the Y2 and PB layers are partially removed by ion etching during specimen preparation. Due to contrast difference between YBCO and PrBCO (also observed in ref. [6 ] ) the edge constant is visualized by an arrow in fig. 2(b). The PB/Y1 interface in the region of the edge contact (fig. 2 ( b ) ) is inclined by 20-25 ° relative to the substrate. The angle increases to 35 ° with growing distance from the substrate. The inclination angle variations may be caused by different
ion etching rates of PI and Y 1 layers in the process of the step fabrication. The inclined interfaces are wavy with unevenness up to several nm. One might have expected partial substrate surface sputtering and step appearance after ion etching during the process of EJJ edge formation by the ion bombardment. However, this was not observed (fig. 2 ( b ) ) , probably due to high etching stability of SrTiO3. Lattice resolution images reveal epitaxial growth of the PrBCO intermediate layer and the Y2 film (fig. 3). The c-axis orientation in the PB intermediate layer and the Y2 film remains normal to the substrate. The PB layer may also be identified by defect appearance on the Y1/PB interface. Prior to the PB layer deposition the vacuum cycle was broken. Renewing the deposition process after lithography and ion etching results in defects appearing on the Y1/ P boundary, marking the interface. The subsequent
Fig. 3. Cross-sectionallattice resolution image of the EJJ. The PB layer is arrowed.
0.I. Lebedev et al. /Microstructure of edge-type Josephson junctions
281
Fig. 4. High-resolution cross-sectional image of the EJJ. (a) Y 1/PB interface is arrowed. The bright area in the right part of the interface contains domains with moir6 fringes and lightly inclined (001) planes possibly due to some changes in oxygen content in these domains. (b) Enlarged part of the P B / Y 1 interface (big arrows) with APB-like defects (one of the l/3 steps is shown with white arrows). (c) Enlarged part of the PB/Y1 interface (big arrows). The uninterrupted atomic plane parallel to the (001) having bright contrast is indicated with white arrows.
282
O,L Lebedev et al. 1 Microstructure of edge-type Josephson junctions :'~"T
Fig. 4. Continued.
formation of Y2 on PB takes place in a single vacuum cycle and the defect concentration on the PB/ Y2 interface is significantly lower (fig. 3). HREM images (fig. 4 (a, b) ) show that these defects are 1/ 3c PrBCO lattice shifts relative to Y1, resembling antiphase boundaries (APB) observed in a study [ 7 ]. However, in some parts of the Y1/Pb interface these defects are absent (fig. 4(c) ). The elimination of the 1/3c shift occurs due to nonstoichiometry defects in the PB layer. The 5-8 nm areas with moir6 fringes in a Y 1 layer were observed in the region 20 nm along the YI/PB interface. The (001 ) planes in this region are slightly inclined relative to the (001) planes of the rest part of the Y 1 film. The moir6 type fringes and the plane inclination could be caused by the appearance of domains with another oxygen content formed after different treatments of the edge surface. Amorphous layers are not observed on the interfaces. Electron diffraction (ED) investigations also confirm the small misorientation of these layers in the EJJ area. An ED pattern taken from the EJJ region (fig. 5 ) with light spots doubling and streaks can be observed. The reflections of Y1 and P are superimposed. EM image analysis gives a 25-30 nm thickness evaluation of the PB layer (fig. 3). The Y2 film in the edge contact region is characterized by a high APB concentration and stacking faults-double rows of
Fig. 5. Electron diffraction pattern taken from the EJJ area. The spot split due to small (001) planes inclination near the interfaces is arrowed.
CuO (fig. 3). Such stacking defects are often ended on APBs. Figures 6 and 7 are cross-sectional images of the layered structures on different sides of the EJJ. Figure 6 is a micrograph of a two-layered part of Jo-
O.L Lebedev et al. / Microstructure of edge-type Josephson junctions
283
Fig. 6. High-resolution cross-sectional image of the two-layers part o f the EJJ.
Fig. 7. Cross-sectional image of the multilayers structures near EJJ. The Y2/PB and PB/PI interfaces are shown with black arrows. The inclusion on the top of the PB surface is arrowed.
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O.L Lebedev et al. /Microstructure of edge-type Josephsonjunctions
sephson junction (Y2-PB/SrTiO3). The PB layer has a darker contrast and is characterized by a higher defect concentration, probably due to bad layer stoichiometry. Defect areas with moir6 patterns appear on the interface, possibly as a result of the substrate surface unevenness and following the superimposition of the substrate and PrBCO images or intermediate layer appearance. Nevertheless, the interface contains no amorphous layer on the SrTiO3 surface caused by ion thinning. The PB layer c-axis is oriented normal to the substrate. The Y2 layer orientation is the same and its non-stoichiometric defect concentration is lower than in the PB layer. Figure 7 is a low magnification image of the PB structure viewed in the other side of the EJJ. The Y 1, PB and Y2 layers are easily distinguished. The Y1 film is single crystalline, with the c-axis normal to the substrate surface, containing defects. The Y 1 layer thickness is 0.15 Ixm. The PI insulating layer grows epitaxially on Y 1, its thickness is about 100 nm. The APB concentration in the PB layer is lower compared to Y1. The next layer is 0.25 ~tm thick Y2; it
is polycrystalline with grains both c and a ( b ) oriented. Figure 8 is a grain boundary (GB) image. The GB is coherent. The APB concentration in the Y2 film is higher than in Y 1 and PB. Crystalline grains of unidentified phase 10-100 nm in size are observed on the Y2/PB interface (fig. 9). The reason of the high defect concentration grains misorientation may be an unsufficiently fiat PB surface or its contamination after making for the lithography process. There was no masking material on the inclined surface of the EJJ resulting in a lower defects concentration in the Y2 film. Figure 9 shows the lower part of an EJJ multilayered structure - a (110) YBCO/SrTiO3 (100) cross-section. The (001) layers in the right and central part of the micrograph are shifted by c/3 relative to the c-axis - a characteristic feature of an APB. The SrTiO3 surface is uneven and could be one of the reasons of the APB appearance. Another reason, even though less probable, is that the APBs are screw dislocation boundaries. Screw dislocations were found in YBCO films using tunneling microscopy [ 5,6 ].
Fig. 8. High-resolutionimage of the Y2 layer. Several YBCOgrains in different orientations are shown. Grain boundaries are coherent.
O.L Lebedev et al. / Microstructure of edge-type Josephson junctions
285
Fig. 9. High-resolution image of the (001) SrTiO3/Y 1 interface. APBs are indicated with big arrows. Some of the 1/3 plane shift on APBs are arrowed. The inclination of adjacent crystalline layers near APBs (areas indexed "s" for example) is visible. These layers are shown by pairs of small arrows.
Fig. 10. High-resolution image of the SrTiO3/Y 1 interface. The SrTiO3 surface is inclined at 3 ° from (001) SrTiO3. Some of the 1/3 plane shift on the APBs is arrowed. The inclination of adjacent crystalline layers near APBs (areas indexed "s" for example) is visible. These layers are shown by pairs of small arrows.
T h e layers are slightly i n c l i n e d r e l a t i v e to e a c h o t h e r ( ~ 3 ° ) in areas i n d e x e d " s " . Such patterns, in particular, m a y be o b s e r v e d f r o m s o m e parts o f screw d i s l o c a t i o n s o r f r o m s u p e r i m p o s e d areas o f t w o dislocations. F i g u r e 10 shows the Y B C O / S r T i O 3 interface. In this case the surface is i n c l i n e d f r o m the ( 0 0 1 ) SrTiO3 p l a n e ( ~ 3 ° ). T h e A P B s are also o b s e r v e d in this interface. T h e c o n c e n t r a t i o n o f A P B s in such
areas is slightly higher, c o m p a r e d to t h e case w i t h o u t i n c l i n e d substrate surface. 4. S u m m a r y T h e m i c r o s t r u c t u r e o f an E J J w i t h a PrBazCuaO7_x b a r r i e r layer, o b t a i n e d b y laser a b l a t i o n was defined. T h e edge c o n t a c t i n c l i n a t i o n was estim a t e d to be 2 0 - 3 5 °. At this i n c l i n a t i o n angle the c-
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axis o f all layers in the edge contact region is oriented n o r m a l to the substrate, ensuring the best electrophysical properties o f the layers in the EJJ region. The interface as well as substrate areas lithographically treated are free from a m o r p h o u s layers, i.e. from a d d i t i o n a l barriers. At the same t i m e the PB layer, grown after breaking the v a c u u m cycle, cont a i n e d m o r e defects than the P I layer, f o r m e d immediately on the Y 1 layer. Possibly, this is a result o f higher defect concentration on the P 1 surface after v a c u u m loss a n d ion thinning. The c o n t a m i n a t i o n o f the fiat part o f PI film surface with photoresist could be the reason o f the polycrystallinity a n d high APB concentration on the Y2 film. A step in the substrate, which was expected to app e a r in the process o f EJJ edge f o r m a t i o n by ion b o m b a r d m e n t was not observed, p r o b a b l y due to the higher etching stability o f SrTiO 3 c o m p a r e d to YBCO a n d PrBCO. APBs were observed, which m a y be interpreted as b o u n d a r i e s between neighboring screw dislocations or could be taken to be the result o f substrate surface unevenness. The Y 1 layer APB density is significantly lower c o m p a r e d to Y2. Evidently, the higher roughness o f the PB layer defines a higher concentration o f APBs.
H R E M investigations showed that laser ablation allows one to o b t a i n a defined type o f EJJs with practically perfect structure. The obtained data gives hope to the possibility o f fabricating controllable devices by changing the Pr b a r r i e r layer thickness.
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