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Pinning centres in YBa2Cu3O7- delta films
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1992 Supercond. Sci. Technol. 5 S125
(http://iopscience.iop.org/0953-2048/5/1S/024)
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Supercond. Sci. Technol. 5 (1992) S12S128. Printed i n the UK
Pinning Centers in Y Ba,Ca30,-, F i l m
I . Mannhart, I>. Ansclmctti", J . G . Bcdnorz, Ch. Gcrhcr, K . A . Miillcr ant1 I>.<?.
Scl~lorn
I13M Research Division, Zurich Research Laboratory, 8803 Ruschlikon, Switzcrland
"also at lnstitutc or Physics, linivcrsity o ~ l l a s c l 4056
,
Bascl, Switzcrlatid
ABSTRACT Pinning cciitcrs in cpitaxial YDa2Cii,07-,, fili~ishave bccn invcstigatcd by scanning
tunneling microscopy and transport studies. A n astonishing surfacc morphology has ~ C C I IIbund
.,v,,ic,i
.r h ' 1 :,,L!"&>
- , ~ . . I - a~ h:-h
.I -..:t..
- . I n 9 ---2
-I' I"--... A:nl-,~-+z-.%m "..A
-:--,I
h n l n o
?!lc
11, > L , C ; W u 1 1 1 1 1 - n ,,U, I > a , , \ , 1 1 1 1
GLL,-a,,,LL,
,,,g,, uti,,>,Ly
z,,,
L,,,
b
y
varying
t
l
i
c
growth
conditions
of
thc
dcnsity of' scrcw dislocations can bc controlled
YBa,Cu,O,+ films, allowing correlations bctween critical currcnts and screw dislocatiori tlcnsity
to be investigated. Filins with highcr scrcw dislocation dcnsitics are ohservcd to liavc higher
critical currcnt dcnsitics a n d a slower drop of .Ic ( I ! ) .
..n-r..-ntn*
1,1111,
, 1 1 1 1 L.7.
1. INTRODUCTION
.1.hc high critical currcnt densities of epitaxial films or higti-7; supcrconductors arc causctl b y strong
pinning with a spcctrum of pinning cncrgics [I]. ' I o d;it.c, howcvcr, no coniprchcnsivc pict.nrc of thc
physical natiirc of the pinning sourccs has cnicrgcd. By imaging sputtcrcd Ylk1,Cu,0,-,~ liinis using
scanning tunneling microscopy (S'IM)we havc ohscrvcd high dcnsitics or tlcrccts such as scrcw
dislocations and sinall holes in the cpitaxial films, as wcll as a sulxtantial surface roiighncss [2,3]. I n
this papcr, correlations o r the screw dislocation dcnsity with the critical cLirrcnt dcnsitics or the films
arc repoi-tcd and the role of pinning at thc scrcw dislocations and at other dcfccts is discusscd.
2. SAMPLE PREPARATION AND MEASUREMENT TECHNIQUES
1
The c-axis oricntcd YDa2<:u,~,-6 fihns wcrc grown hy dc-hollow catliodc magnetron spnltcring on
nominally (100) oriented Sr'l IO, substrates. As described in detail clscwl~cre [3], the sputtcring
clischaigz or ! > ~ j . ! ~ e
paraiiicteis
a siiiittciiiig pisssGuieorh5fi 31'yoiui (j,i/<j2 E 2 j ; i , a
V and 260.500 mA a n d a hcater block tcmpcraturc of750-780 "<:. 'Ihc f i l m s have a typical thickncss
o f 100-150 nm, a 7, of -87.88 K and a critical ctirrcnt density o f J C Y 1.5-7 x IO7 A/ctn' a t 4.2 K in
scir-ficid.
Thc as-grown films wcrc invcstigatcd with a lionicrnadc S'I'M at room tctnpcraturc in air using
mechanically prcpared I'to,8,$ro~2~ tips with a tunncling currcnt. o f 10-20 pA [2,3]. M a n y iniagcs of
each sample wcrc takcn at diKcrcnt locations and thc screw dislocation tlcnsity IZ of a film W R S derived
by avcraging over all imagcs obtaincd From the samplc.
Critical ciirrcnt dcnsities were mcasurcd using 8 pm x 100 p n bridgcs, patterned hy conventional
photolithography and wct ctcliing. Each sample for .I, rricasurcmcnt was cut fiom the sainc polished
substrak as the corrcsponding samplcs for microscopy and mounted Tor dcposition on tllc Iicatcr
block closc to the samples for microscopy. During thc .l,;(Tl) mcasurcincnts, donc by a 4-point
technique with a critcrion of I p V , niagiictk liJls o r p o l l 5 8 'Iwerc applied perpendicular to thc
transport currcnt and to the film surl'xc ( I [ / / c ) . Data wcrc takcn starting at high ficlds; hcfcrrc
measuring any data points the transport currcnt was rcduccd to mro. O n sevcral samplcs the J ,
lneasurcments wcrc repeated aftcr cycling to 300 K . 7'he scattcr in the rcsulting data give a Iowcr
hound on the precision o r the J, values or N 10%.
3. RESULTS
'fhe S'IM studies rcvealcd several fcaturcs of interest for pinning in thcsc films: First, the film show
an unexpcctcdly high density of dislocations (Fig. I): the dcnsity o r scrcw dislocations pcnctrating the
film surracc is typically around IO9 cnir2. Second, small reatures, about 5 nm x 5 nm in size, which
Seem to be holes more than 20 A deep ( I i g . 2) arc obscrvcd with an evcn higher density. Third, the
films have a typical surfacc roughncss (maximum hcight diKcrencc within one image, corresponding to
1/4 1 . d ) of 100-200 A (Fig. 1). It is noteworthy that the samc fcaturcs have also been reported by
otllcr groups, even for films grown by laser ablation or for films grown on other substrates such as
MgO C4.51.
~53.2048/92/01S125
+ 04 503.50 0 1992 lop Publishing Lld
5125
I?g. I . SI'M images of sputtcrcd Yna,<:u,O,-,
filtnr grown
at various temperatures (after
[3]). (a) 7.50 "<:;(h) 760 "C; (c)
770 " C ;(d) 780 "C.
Fig.
'I'hc
The
unit
2. SI'M image of a sputtered YRa2Cu30,_n film.
small dark spots are holes or insulating regions.
steps between the growth terraces are ahout one
cell high.
As described in detail elsewhere [3] we have found that thc dciisity of scrcw dislocations can he
controlled hy adjusting growth rate, growth temperature and suhstrate tilt from (100). As shown in
Fig. I , by var ing those parameters, films with screw dislocation densities between 5 x 10' cin-2 and
1.5 x IO9 cm- have been grown. Figure 3a shows the critical current density ,I, at 4.2 K in self-field
Y
for all films measured as a function of scrcw dislocation density together with a linear least squares
fit. 'The scatter in the data arises from inaccuracics i n the measurements of .I, and n, but may also be
caused by the wide span or deposition parainctcrs uscd, which are expected to alTcct not only the
screw dislocation density, but also other film propcrties such as the point defect concentration, which
have some influence on J,.
S126
Figure 3a indicates that for the samplcs invcstigatcd, a higher scrcw dislocation dcnTity accompanies a
higher critical current density at 4.2 K in self-ficld. A high screw dislocation dcnsity also lcatls to a
slower drop of .I, with increasing magnctic ficld as shown i n I?g. 3b. 'l'he magnctic ficld dcpcndencc
of the critical current dcnsity and the volunie pinning Forcc arc plottcd in 1:igs. 4 and 5 for two
samples selected on the hasis of Icast squaics fit calculations as bcing rcprcscntativc samples with
small and large n. As thcsc figures dcmonstratc, films with high scrcw dislocation dcnsitics may have a
volume pinning rorce of more t h a n a factor of two higher than tliosc with a sniallcr 1 2 . 1;uither
transport data can be found in [3].
, , , , , , , , , , , , ,
8
1.oo
0.90
~
.
c
m
m
Q:
6
9 4
.#'
3
I
I
I
I
I
I
I
,
I
1
1
2
,
,
(a)
1
0.70
,
6
4
pscrew
1
,
,
8
1
0
1
1
1
,
o.60
1
2
1
4
0
I
,
I
4
2
(b)
(lo8/cm2)
.
.
7
0
'#1
,
I
p 0 H :0.5T
e
. .
2
0
,
0.80
Y
7
'1
T I
I
,
6
I
,
,
I
,
,
I
8
1
0
1
2
1
4
pscrew (1O8/cm2)
I;ig. 3 . ( a ) Critical currcnt ticrisity a t 4.2 I< in scll:licld vs screw dislcmtion dcrisity for all li1rn.q
invcstigatcd. (b) Critical ciirrcnt dcnsity at 4.2 K ancl 0.5 ' I I/ c tinrlnalizcd to the self-licltl valuc vs
scrcw dislocation tlcnsitp for all films irivcstigotctl. I n (aj a n d (hj, thc straight lirics rcprcscnt Icast
squarcs fits; thc data points nurnhcrcd I and 2 rcfcr t.o t,lie sainplcs prcsentcd in Figs. 4 nntl 5 .
'7
7 ,
I
n
-'
0
1
2
3
4
L
8
I
!
5
6
7
8
(TI
pig, 4. ,Jc(1l) at 4.2 K of two characteristic
saniplcs with sci-cw dislocation dcnsitics of
0.5 x iox c n r 2 and of13.5 x ioy cm-2. 'fhc niagtictic ficltl was applied p,wallcl tn thc c-axis, pcrpcndiculai- to tlic transport currcnt ant1 to t.hc
snmplc surr'acc.
16
1
I
0
1
2
3
4
5
6
7
8
@ O H (Ti
Vig. 5. Magrictic ficld dcpcndciicc of tlic volume
pinning forcc for tlic two sainplcs i n r:ig. 4.
4. DISCUSSION
'fiicrc is gcncrnl :igrccmcnt i i i tlic litcratiirc t l m t in cpitaxial Ylla,('u,O,-d filnis pinning is
cllaractcrizcd hy a spcctruni 0 1 pintiiiig sitcs of'tlilrcrciit strcnglhr. I'inning b y thc film surface, by the
intcrracc t,o tlic siihstratc and by Ihc film cdgcs has I m n rcportcd [GI, 'l'hcsc liinning ccntcrs do not
seem to Iic strong cnoiigli to account for tlic .I( wliics o t ~ s c r ~ c'I'Iic
d . sanic npplics to pinning a t twin
plaiics. I'inniiig hy tlic Iagcrcd striicturc or Ylla,C:u,O,-s
is not ~ i p ~ d i c ~ ~tob lthe
e configuration
itivcstigntcd. Thcrc will hc sonic pinning duc t o t h c siirlacc rougliricss of tlic f i l m s , as tlic arcas
hct\vcon the growth spirals arc low cncrgy positions for the lliix litics. Hut surlicc roughticss is liOt
""r>,-rtr<l
g c i t l > y , bc.cr!!!sc the rcs!i!ti;ioe n;nning
forcc
SI=:,!! foi
I '--,'-\. to hc tllc. nlei!!
1'~""'~'
.-A
5127
two rcasons: First, due to thc small slopcs o f the growth spirals, thc potential rccovcrs only gradually
(SCCFigs. I and 2), and sccond, the flux lines may wandcr with easc in the vallcys bctwccn thc growth
spirals. Possible strong pinning sites include dislocations and point defccts. Pinning at dislocations
may bc strong if the flux lincs arc oricntcd parallcl to thc dislocation cores, which is the dominant
oricntation in oui- st,udics [3]. It has bccn suggcsted that dislocations provide strong elementary
pinning [7,8], hut thcy were considcrcd unlikely to bc a niajor source bccausc sirnplc cstimations
show that dislocation densities of thc order of the flux linc dcnsitics arc required, cqualing
IOR- IOq cn-* in self-field, which secnicd cxccssivcly high. Surprisingly, the screw dislocations
obscrvcd in the S'TM studies have this density, indicating that they may bc pinning sitcs. 'This idca is
corroborntcd by the corrclation bctwccn tlic scrcw dislocation dcnsity and thc critical current density
as shown in Figs. 3-5. but it is pointed out that tlic correlations givc no proof for a casual
rclationship. Above -0. I 'I the flux linc density greatly cxcccds tlic scrcw dislocation dcnsity, and the
screw dislocations cannot account Tor all the pinning rcquircd in that ficld rangc cvcn if collectivc
piniiing is taken into account. Ncvcrtlielcss, as shown in Fig. 5 , thc volume pinning rorce also
incrcascs with screw dislocation dcnsity in tlic high field rcginic, providing cvidcncc that in this field
:cgin;e <lc[cc:: c~rrc!ztcdin b'
o m w t h ta the screw distocatlons arc a !nain soiircc
pinning. !)$e:t
candidates arc point defccts or cdgc dislocations. 'The lattcr may rorrn a t prccipitatcs, or during the
coalcsccncc of niisorientcd growth sprials. For cxarnplc, 10" cdge dislocations per cni2 were observed
in thc coalcsccncc of Yl3a,Ci1,~0,_, nuclei on MgO suhstratcs in the carly stages of sputtcrcd film
growth 191. Also, iT thc iianomcter holcs arc o r sulficient dcpth, thcy will he strong pinning sites.
.IL..
5. SUMMARY
Using S I M , scrcw dislocations with densities in thc rangc of IO9 cni-* liavc bccn round i n epitaxially
films. The dcnsity of scrcw dislocations is sullicicnt to account
grown, c-.axis oricntcd YIki,(:u,O,-,
for the obscrvcd .I, at small magnetic fields, provided that thcy arc crcctivc pinning sitcs. 'Their
dcnsity can bc c&trollcd by acijusting growth paranictcrs such as tcmpcrature, growth rate or
suhstratc misorientation. Transport mcasurcmcnts indicate that tlic critical current dcnsitics of
sarnplcs with high scrcw dislocation densities arc iiighcr than currcnt densities of samplcs with kwcr
dislocat.ions.
'lhc authors gratcfully acknowledgc helpful discussions with E l l . Brandt, M. F e r m i , T. Frey, P.
Martinoli and S. Shindc as well as support from R.1';. Broom, A. Catana and the Laser Scicnic and
'Tcclinology dcpart.mcnt a t thc IDM Zurich Rcscarch Lahoratory.
References
Hagcn C W and Gricsscn R 1989 P / y s . Rev. r,etl. 62 2857-2860
<icrbcr Ch, Ansclmctti I>, Iktlnorz .I G, Mannhart .J and Sclilotn I> G 1991 Nulure 350
279-280
Schlom I) G, Ansclnlctti I>, I3cdnorz J C i , I h o m R,Cnt.ana A , I r c y 'I, Gerber Ch, Gunthcrodt
11-1, I m i g I I l', Mannhart .I and Mullcr K A , 'Screw Ilislocations in Sputtercd and I m e r
Ahlatcd YD(:O-I;ilms' to bc pnhlished in %. I'hys. fl
llawlcy M, Raistrick I I), Beet-y .I G and l.loulton R .I 1991 Science 251 1587-1589
I,ang I 1 I', Prey T a n d (iiinlhcrodt 1-1-1, 'Atomic Rcsolut.ion and Nanostructurc o f YBCO
Lascr-Ablated T h i n I;ilnir St.iirlirc1 by S I M ' to bc published in Ei~ropl~ys.
1,ell.
'!a!;a:a
S, ??n!age S M , !h:n C 3, F d c !? K, Gcbs!!c 7' !!, nrld0-i
"--''.-J
M !? 1989 P!?ysica ',C
162-164 1175-1 176; lloas B, Schultz, Sacniann-Ischcnko G 1990 P/iys. Rev. I x l r . 64 479-482
Chaudhari P, Dimos I>, Mannhart J 1989 Eurliev and Recerzl Aspects of St~percondr~clieily,cd.
Dcdnorz .I <; and Mullcr K A, 201-207 (Berlin, I~lcidclhcrgNew York: Springer)
Pan V M, I'ashitsky, E A 'Trctiatchenko 'The Electrostatic Mechanism o f Abrikosov Vortex
Pinning at Charged Point Ikfccts and Edge Dislocations in I-ligh-l'c Superconductors', preprint
Strcilrcr S K, Lairson R M. Ilom C 11, Clcniens R M , I3rauman J C, Gcballc 7' H 1991 P ~ J J S .
Rev. n 4 3 13007
SI28