We have analyzed epitaxial, c-axis oriented YBa2Cu3O7–delta thin films grown in situ by sequential ion-beam sputtering on (100) SiTiO3 and (100) MgO substrates. X-ray diffraction studies showed the presence of both homogeneous and inhomogeneous lattice distortions along the c-direction. The c-axis lattice parameters ranged from 11.72 to 12.00 Å. The broadening of the (00l) Bragg peaks in excess of the broadening due to finite film thickness was found to be due to inhomogeneous lattice distortions. The overall trend in the data shows an increase of the inhomogeneous strains with the enlargement of the c-axis lattice parameter. The inhomogeneous lattice distortions are interpreted as fluctuations in the c-axis lattice parameter. The resistive transitions were found to be correlated to the lattice distortions. We show correlations between the midpoint Tc and the c-axis lattice parameter and between the transition widths and the inhomogeneous lattice distortions

Johnson, W. L.

Kittl, J. A.

Nieh, C. W.

English

Caltech Authors - Main

Lattice distortions in YBa2Cu 3 0 7--s thin films grown in situ by sequential 
ion beam sputtering 
J. A. Kittl and W. L. Johnson 
W. M. Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, 
California 9112.5 
C. W. Nieh 
Hughes Research Laboratory, Malibu, California 90265 
(Received 13 September 1990; accepted for publication 25 January 1991) 
We have analyzed epitaxial, c-axis oriented YBa,CujO, _ 6 thin films grown in situ by 
sequential ion-beam sputtering on (100) SiTiOs and (100) MgO substrates. X-ray diffraction 
studies showed the presence of both homogeneous and inhomogeneous lattice distortions 
along the c-direction. The c-axis lattice parameters ranged from 11.72 to 12.00 A. The 
broadening of the (001) Bragg peaks in excess of the broadening due to finite film thickness 
was found to be due to inhomogeneous lattice distortions. The overall trend in the data 
shows an increase of the inhomogeneous strains with the enlargement of the c-axis lattice 
parameter. The inhomogeneous lattice distortions are interpreted as fluctuations in the c-axis 
lattice parameter. The resistive transitions were found to be correlated to the lattice 
distortions. We show correlations between the midpoint T, and the c-axis lattice parameter 
and between the transition widths and the inhomogeneous lattice distortions. 
In recent years, the in situ growth of high-T, super- 
conductor thin films and their applications to devices has 
been a subject of intense work. Several techniques were 
reported for in situ growth of YBa2Cu307 _ 6 thin films.‘-* 
Most of the attention was focused on films of optimized 
characteristics such as high-transition temperatures, sharp 
transitions, high-crystal quality, etc. The study of depar- 
tures from these ideal characteristics is, however, impor- 
tant in understanding the in situ growth of high-T, super- 
conductor films. 
The properties of sputtered “123” thin films, such as 
the transition temperature T,, degrade as the deposition 
temperature is lowered.‘-” This has been correlated to an 
expansion of the c-axis lattice parameter that increases as 
the deposition temperature is lowered.” The supercon- 
ducting transition temperatures decrease with the enlarge- 
ment of the c-axis lattice parameter as was shown for films 
grown in situ by magnetron sputtering,‘* sequential ion 
beam sputtering,” and e-beam evaporation.‘3 In films 
grown by sequential ion beam sputtering, a broadening of 
the (001) x-ray Bragg reflections that increases with the 
enlargement of the c-axis lattice parameter has also been 
reported. lo 
In this communication, we report an analysis of the 
lattice distortions and their correlations to characteristics 
of the superconducting transitions in “123” thin films 
grown in situ by sequential ion beam sputtering. The low- 
ering of T, with the enlargement of the c-axis lattice pa- 
rameter is shown to hold over a wide range of lattice pa- 
rameters. The broadening of the (001) x-ray lines was 
analyzed for several orders of diffraction. This analysis 
showed that stacking faults are not an important cause of 
the broadening. The “size” and “inhomogeneous lattice 
distortions” contributions to the broadening were evalu- 
ated. The size coefficients were consistent with the film 
thicknesses. The main cause of broadening of the (001) 
x-ray Bragg peaks in films with large c-axis lattice param- 
eters was found to be inhomogeneous lattice distortions. 
The resistive transitions were found to be wider for higher 
values of the inhomogeneous lattice distortion coefficient. 
These results suggest that the superconducting resistive 
transition midpoint and width are related to the fluctua- 
tions and mean value of the lattice distortions along the 
c-direction, which cause the (001) x-ray peak broadening 
(other than the broadening due to finite film thickness) 
and shift. 
Epitaxial, c-axis oriented YBa2Cu307 _ 8 thin films 
were grown by sequential ion beam sputtering from ele- 
mental Y, Ba, and Cu targets. The stacking sequence of the 
“123” compound was followed with the individual layer 
thicknesses nominally equal to one monolayer. Films rang- 
ing from 300 to 1600 A were grown on ( 100) SrTi03 and 
on (100) MgO substrates. The typical film thickness was 
400 A. Details on the deposition technique have already 
been reported.* The films have expanded c-axis lattice pa- 
rameters ranging from 11.72 to 12.00 A. The c-axis lattice 
parameters were evaluated by high-angle extrapolations us- 
ing the (001) series. Anneals in O2 at 400 “C produced 
changes in the c-axis lattice parameter of < 0.02 A, and no 
significant changes in the superconducting transitions. An- 
neals in O2 at 850 “C followed by anneals at 400 “C were 
effective in reducing the lattice parameter to approximately 
the bulk value of 11.68 A. 
Resistivity measurements were done by the standard 
AC four-point technique. Figure 1 shows the transition 
temperatures (midpoints) as a function of the c-axis lattice 
parameter. The tendency of T, to decrease with the expan- 
sion of the c-axis lattice parameter was found to hold for a 
wide range of lattice parameters. 
Analysis of our results and those of Eom et a1.13 show 
6710 J. Appl. Phys. 69 (9), 1 May 1991 0021-8979/91/09671 O-03$03.00 @  1991 American Institute of Physics 6710 
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100 I I I I 
90  
80 
g 70 
g 60 
50 
40 
q 
q II 
m  
m  
II 
q 
•I . 
u 
t I 30 
11.6 11.7 11.8 11.9 12.0 
C-AXIS LATTICE PARAMETER (A) 
FIG. 1. Plot of the transition temperature (midpoint) as a function of the 
c-axis lattice parameter. 
that the relation between T, and the c-axis lattice param- 
eter, and the effects of anneals, are the same in different in 
sifu growth techniques. This indicates that films grown by 
these various techniques present the same type of defects. 
X-ray diffraction 8-28 spectra were taken with a Sie- 
mens D500 diffractometer in a high-resolution geometry, 
using CuKa radiation. The x-ray scans were corrected for 
Ka and instrumental broadening. The full widths at half 
maximum of the (ON) Bragg peaks, AK, were plotted in 
reciprocal space as a function of their K values. The plots 
show a monotonic increase of AK with K. Figure 2 shows 
the plots for several films grown on MgO with different 
c-axis lattice parameters. 
The line broadening due to finite crystal size along the 
film surface normal (c-direction) is constant in reciprocal 
space and given by 
AKr0.9(2r/L), 
0.07 I 
- c-lattice parameter: 
0.06 - 0 11.78A 0 - 
0.05 l ii.81 A - 0 
. n 17.07A 0 
0.04 - o  11.91 A 
n . 
0.03 - n 0 
. 
l 
l 
. 0.02 - El q  - !J 
0.01 - P 
0.00 ’ 1  I I I 
0  1 2 3 4 
K (l/A) 
FIG. 2. Full widths at half max imum of the (001) x-ray Bragg peaks AK 
as a function of their K values, for several fi lms grown on MgO. The data 
were corrected for Ka and instrumental broadening. 
SrTida 
w 
ANNEALED 
t I 1 z 0 .0; 
11.6 11.7 11.8 11.9 12.0 
C-AXIS LATTICE PARAMETER (A) 
FIG. 3. Inhomogeneous strain E,,, as a function of the c-axis lattice 
parameter. 
where L is the crystal size. The line broadening due to 
inhomogeneous lattice distortions along the c-direction is 
given by 
where E=&/c is the local inhomogeneous lattice distortion 
along the c-direction, (E) = 0, and E,,, = (E’) 1’2 is the 
root mean square inhomogeneous lattice distortion.14 
The contributions to line broadening due to finite size 
and to inhomogeneous lattice distortions were evaluated by 
least square fits assuming particular peak shapes for each 
contribution. The best fits were obtained in most cases as- 
suming Cauchy size broadening and Gaussian lattice dis- 
tortions broadening. The values of crystal size obtained 
were consistent with the film thicknesses as measured by 
Rutherford backscattering spectrometry and by cross sec- 
tional transmission electron microscopy. 
Figure 3 shows a plot of the inhomogeneous strains, 
E IXW as a function of the c-axis lattice parameter. The error 
bars were evaluated by assuming Cauchy-Cauchy and 
Gaussian-Gaussian peak profiles. There is an overall ten- 
dency for films with larger c-axis lattice parameters to 
present higher inhomogeneous lattice distortions. 
Comparison of the x-ray spectra for films before and 
after annealing in O2 at 400 “C showed no change in the 
widths and positions of the (001) Bragg peaks. Films an- 
nealed at 850 “C in O2 showed a sharpening of the (001) 
Bragg peaks and a shift to the bulk “123” lattice constant, 
indicating a relaxation of the strains (Fig. 3). 
The resistivity data showed that films with large c-axis 
lattice parameters and high inhomogeneous strains had 
considerably broad transitions. The transition widths were 
best correlated to the inhomogeneous strains. Typically, 
films with E,,, of 0.15, 0.30, and 0.60% had transition 
widths of 5, 10, and 30 K, respectively (Fig. 4). A relation 
between the broadening of diffraction lines and the broad- 
ening of superconducting transitions was proposed in per- 
ovskite superconductors.‘5 These broadenings have been 
attributed to fluctuations in the lattice parameterI and to 
the proximity of phases with different oxygen ordering.15V16 
6711 J. Appl. Phys.,Vol.69,No.9,1 May 1991 Kittl,Johnson,and Nieh 6711 
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TRANSITION WIDTH (K) 
FIG. 4. Plot of inhomogeneous strains erms vs resistive transition widths, 
Off-axis x-ray diffraction studies in films grown by 
magnetron sputtering rule out a’substrate-induced stress as 
the main cause of the c-axis lattice parameter expansion.‘2 
We speculate that the c-axis lattice parameter expan- 
sion is related to defects trapped in the film during growth. 
Films grown at lower temperatures would present a higher 
density of defects, resulting in larger c-lattice parameters. 
This would explain the dependence of the c-lattice param- 
eter on the deposition temperature.” Furthermore, it is 
tempting to assume that both the homogeneous and inho- 
mogeneous lattice distortions are due to the same cause. 
The inhomogeneous lattice distortions would then be re- 
lated to the fluctuations in the density of defects, or to the 
microscopic strain fields associated with the defects. 
Results from deposition techniques that use activated 
oxygen sources suggest a relation between the oxygen con- 
tent of in situ grown films and the lattice expansion and 
degradation of Tc1',18 The effect of anneals and the rela- 
tion between the T, and the c-axis lattice parameter in in 
situ grown films are different than in bulk samples.” This 
indicates that the lattice expansion cannot be attributed to 
oxygen vacancies in the chain sites as in bulk samples, but 
may be due to other oxygen defects. Another possible de- 
fect structure was proposed’* consisting of substitutional 
disorder between the Y and Ba sublattices. 
In conclusion, we have analyzed “123” films grown in 
situ by sequential ion-beam sputtering. We found that the 
broadening of the (001) x-ray peaks is due to inhomoge- 
neous strains. We have shown that there is a relation be- 
tween the inhomogeneous and homogeneous lattice distor- 
tions along the c-direction, that can be interpreted as a 
relation between the fluctuations and mean value of the 
strain. We have found relations between the characteristics 
of the superconducting transition and the lattice distor- 
tions. Further studies are being carried out to elucidate the 
nature of the defect structure in the films. 
This research was supported by the National Science 
Foundation, Materials Research Groups, Grant DMR- 
8811795; the Hughes Research Laboratories, Malibu, Cal- 
ifornia; and a gift from the Ford Motor Co. through the 
Ford Aerospace Division, Newport Beach, California. 
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Lattice distortions in YBa2Cu3O7–delta thin films grown in situ by sequential ion beam sputtering

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Lattice distortions in YBa2Cu3O7–delta thin films grown in situ by sequential ion beam sputtering

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