(PrBa2Cu3O7t−δ)1 superlattice

Physica C 183 ( 1991 ) 252-256 North-Holland

Unit cell-by-unit cell grown superlattice

(YBa2Cu307_6)l/(PrBa2Cu307_6) 1

K. Kamigaki a, T. Terashima b, K. S h i m u r a b, y . B a n d o b and H. Terauchi a a Department of Physics, Kwansei-Gakuin University, Nishinomiya 662, Japan b InstituteforChemicalResearch, Kyoto University, Ufi611, Japan Received 19 September 1991

Perfect unit cell-by-unit cell growth of RBa2Cu307_ a (R; rare-earth metals) was first demonstrated by means of X-ray diffraction. The supedattice of [ (YBa2CuaO~_a)~/(PrBa2Cu3OT_a)l ] io was grown by means of reactive evaporation (RE) with reflection high-energy electron diffraction (RHEED) intensity oscillations. No aperiodicity and no intermixing of YBa2Cu307_ 6 and PrBa2Cu30~_~ due to the interfacial roughness were found, indicating that the RE-growth monitored in situ by RHEED oscillations makes the exact unit cell growth possible following the two-dimensional growth manner.

It is expected that good c-axis oriented ultrathin epitaxial films can elucidate the role of the two-dimensionality for the high-To superconducting mechanism. A few groups fabricated the multiheterostructures consisting of ultrathin R B a 2 C u 3 0 7 _ 6 (R=Y, Pr) films using magnetron sputtering [ l ] and laser ablation [ 2,3 ] methods. They discussed the superconducting nature from the viewpoint of the dimensionality. In such discussions, the detailed structural characterization is essential. The fabrication and the characterization of the ultimate superlattice of the (YBa2Cu307_ 6) ~/ (PrBa2Cu307- a) ~ provides most direct information with respect to the layer controllability and the crystal imperfection due to the interconnection. The X-ray studies on such superlattices were carried out, but the superstructure peaks are fairly broad compared with fundamental ones of the average lattices [ 3,4 ]. This fact implies that the unit cell-by-unit cell growth is not attained so well and the resultant interfacial roughness introduces very serious breaking of the superlattice coherence. In fact, the transmission electron microscopy of such films revealed the existence of undulations in the layers [ 5 ]. Recently, the reflection high-energy electron diffraction (RHEED) intensity oscillations in the oxide film growth was first observed in our group [ 6 ]. Using RHEED oscillations, the unit cell-by-unit cell

control in oxide film growth was successful and well established. In this work, we fabricated the superlattice of (YBa2Cu307_ a) 1/ (PrBa2Cu307- 6) 1 ( l 0 periods) by monitoring the RHEED intensity oscillations. The excellent unit cell controllability and high crystalline quality were demonstrated by X-ray diffraction. The superlattice of [ (YBa2Cu307_a) t/(PrBa2Cu3OT_6) l ] lo was grown on a { 100} SrTiO3 substrate by means of a reactive evaporation (RE) method with observing the RHEED intensity oscillations. The superconducting nature was described elsewhere [ 7 ]. X-ray diffraction measurements were carried out using a conventional double-axis diffractometer and a Rigaku RU-300 (60 kV, 300 mA) Xray generator with a Cu rotating anode. Cu La radiation was monochromatized by a pyrolytic graphite crystal and higher harmonic components of the incident X-ray beam such as the half wavelength (2/ 2) component were suppressed by an X-ray pulse height analyser. The contribution of the 2/2 component to the "2 component was estimated to be 1 × 10 - 7 b y using the (200) reflection of MgO, and those of other higher components were negligible. Typical RHEED intensity oscillations during the growth of the superlattice of ( Y B a 2 C u 3 0 7 ) I / ( P r Ba2Cu307_a)l are shown in fig. I. The oscillations in the 5th to 7th periods were exhibited. Individual

0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

K. Kamigaki et al. / (YBazCu3OT_~J(PrBa2CusOz_~l superlattice

s~ start

lattice, only (00L) peaks were observed, where the (003) peak was buried under the very strong (001)s substrate peak. (The notation S denotes the substrate.) This implies that this film grows perfectly along the c-axis. The epitaxial relationship was in 2 vestigated also by grazing incidence X-ray diffraction (GIXD) measurements [9] in addition to the RHEED investigations. The GIXD spectra traced along the [ 100 ] and [ l l0 ] directions showed no (00L) peaks and no extra peaks of the misaligned grains, which was consistent with the previous results in single heterostructures [ 10 ]. This suggests that the supcdattice satisfies the epitaxial relationship of the (001) [ 100]F//(001 ) [ 100Is, where F denotes the film crystal. The mosaicness of the (001 ) plane around the surface normal was obtained to be ~ 0.5 ° from the rocking curve widths taken around the (400) and the (330) reflections, where they consisted of two and three peaks, respectively, due to the (110) twin. The superstructure peaks clearly appeared in the middle of the (00L) peaks of the average lattice. This means that the 2/2 components of the (00L) reflections appear at the same positions as the superstructure peaks. For instance, the 2/2 peaks of the

20S start

6th

& start

7th start

start

start

i

. . . .

"r

~"~0

~(~l

YBC;O

YBCO

YB~

TIME Fig. 1. Typical RHEED intensity oscillations during the growth of the superlattice of [ (YBa2Cu307_6) l/(PrBa2Cu3OT_~) l ] io.

oscillations between the arrows marked with "start" and "stop" correspond to the unit cell growth of RBaECu3OT_6 [6]. It is noted that after each unit cell growth period, growth interruption was performed for improvement of the surface smoothness

[8]. The X-ray scattering spectrum scanned along the direction normal to the crystal surface is shown in fig. 2. Focusing on the reflections of the "average"

10 6

[(YBCO)I (Pr SCO)l"110

o

253

A

==

g N

~. 10 s

- 4 0.40"

>Ul Z

Z

10 4

i

i

i

i

i

5

10

15

20

25

2-THETA

(deg.)

Fig. 2. X-ray scattering spectrum scanned along the direction normal to the crystal surface in the supeflattice of the [ (YBa2Cu307-6),/ (PrBa2Cu3OT_ 6)~ ] lO. Superstructure peaks were clearly observed in the middle of the (00L) peaks of the "average" lattice. Laue oscillations were also observed. Note that the peak widths of the superlattice and the average lattice are the same.

254

K. Kamigaki et al. /(YBa2Cu3)z_a)l(PrBazCu307_~)l superlattice

(003) (and (001)s) and the (005) reflections appeared at 20~ 12 ° and 19 ° , respectively, which are just the same positions as the superstructure peaks. As mentioned above, the amplitude of the 2/2 contribution to the 2 component is estimated to be about 1 × 10- 7 in the present diffractometry. The observed (005) peak intensity was about 1 X 106 counts/200 s and that of the (003) peak was of the same order, implying that the 2/2 contributions of (00L) peaks were negligible ( ~ 0.1 counts/200 s). Furthermore, in even very strong substrate peaks, the contribution was estimated to be ,-, 10 counts/200 s, which implies that these superstructure peaks are intrinsic in the present superlattic e. It was found that the peak widths of the superstructure peaks at 20~ 12 ° and 19 ° and that of the (002) peaks of the average lattice were entirely the same (0.40 ° without deconvolution of an instrumental resolution). This fact suggested that there was no fluctuation of periodicity and no intermixing of YBa2Cu307_a and PrBa2Cu307_6 due to the interfacial roughness. Previously, it was revealed that RBa2Cu307_6 grew in the unit cell-by-unit cell manner [6], i.e. the minimum layer unit was 1 unit lattice. This implies that the amplitude of the surface roughness, if it exists, is larger than one unit size. This drastically breaks the superlattice coherence because the superlattice unit consists of only two units, resuiting in the drastic increase of the peak widths and drastic decrease of the intensities of superstructure reflections. For the estimation of the mosaicness of the (001) plane along the surface normal, the rocking curves were taken around the (005) peak and the super structure peak between the (005) and the (006) reflections. It was found that the rocking curve widths of the (005) and the superstructure peaks were both 0.18 °. These small peak widths may be attributed to the nature of PrBa2Cu307_6. For instance, the mosaicness of 300 A thick YBa2Cu307_dSrTiO3 is about 0.5 °, while that of 250 A thick PrBa2Cu307_a/SrTiO3 is less than 0.1 °. The observed and calculated X-ray spectra in the 20 range of 10-20 ° are exhibited in fig. 3. The intensities were normalized to the (002) reflections. In the calculation, the atomic coordinations of YBa2Cu307_6 and PrBa2Cu307_a given by Le Page et al. [ 11 ] using single-crystal X-ray analysis were

employed and the repeated number of the superlattice was N = 10. Although it seemed that these were not so reliable compared with those of the recent Rietveld analysis using high-resolution neutron diffraction, they are useful enough in this simple calculation. Various factors such as the Lorenz-polarization, Debye-Waller and scattering volume factors were not taken into account. However, the 20 region discussed here is relatively small and the exclusion of such factors does not give serious interruption. It was found that the calculated spectrum reproduced the observed one, although slight differences were seen. One of the origins of such differences may be attributed to the bulk structural parameters. The lattice parameter c of the average lattice was obtained to be 11.684/k from the (00L) peaks, which was almost the same as that of bulk YBa2Cu307_a. On the other hand, that of the 250 A-thick PrBa2Cu307_6 epitaxial films grown on SrTiO3 was obtained to be 11.725 A. Furthermore, it was revealed that the in-plane lattice parameters of the average lattice were 3.873 A and 3.917 A. These are similar with those of the 250 A PrBa2Cu307_~ on SrTiOa. These imply that the PrBa2Cu307_6 in the superlattice is in almost bulk state and only YBa2Cu3Ov_6 is in a distorted state. This concept introduces the lattice contraction of YBa2Cu307_6 along the c-direction following the Poisson effect and such lattice contraction may induce the structural modification. Another origin is that this film is in a mixed state of [ (YBa2CuaO7_6) 1/(PrBa2Cu307_6) l ] io superlattice regions and the coexistence regions of YBa2Cu307_6 and PrBa2Cu3Ov_6 domains due to the interfacial roughness. Two cases can be considered. One is that the coexisted regions are embedded threedimensionally in the superlattice. Another is that the coexisting regions laterally distribute (like column formation). In the former, the superlattice coherence length along the stacking direction was not defined uniquely because coexistence of the YBa2Cu307_a and PrBa2Cu307_6 destroyed partly the periodic alternation of YBa2Cu307_a and PrBa2Cu307_6, and introduced the various coherence lengths. This results in the deterioration of the Laue oscillations due to the overlap of oscillations with different periodicity, and in the increase of widths of the superstructure peaks. The appearance of clear Laue oscillations and the same peak width between

255

K. Kamigaki et al. /(YBa2Cu3Oz_a)l/(PrBa2Cu~O7_a)~ superlattice I

I

(002) 10 o

N=IO

.~. • •

*

* .

--

observed calculated

L~

F10-1

10 o

10-2

10-I

z

| o

10

12

14

16

18

20

10 -2

t

2 - T H E T A (deg.)

Fig. 3. Observed and calculated X-ray spectra around the ( 002 ) reflection. X-ray intensities were normalized at (002) reflections in both spectra and the instrumental resolution was not convoluted in the calculated spectrum. It was found that a simple calculation reproduced the observed spectrum. the (00L) and the superstructure peaks implied that even if there were coexistence regions their volume fraction was very small. It is reasonable that the surface roughness produced in each growth period does not break the periodic lattice-alternation seriously, since this superlattice has a small repetition number and the roughness effect is not cumulative with such small repetition number. In the latter only the enhancement o f the (00L) peaks occurred without degradation o f the Laue oscillations and without broadening the peak widths. This means that this model can explain the observed intensities very easily. It was known that a similar system as the latter was proposed in GaAs-AIAs heterostructures as lateral superlattices, named "tilted superlattice" [ 12 ] or "fractional layer superlattice" [13], although they have one-dimensional modulation periods defined exactly. These superlattices cannot be attained without a terraced substrate and the special technique for the monolayer-controlled layer-by-layer growth. That is, in the layer-by-layer growth mode, it is impossible to obtain such structure naturally, This model is therefore unreal and ruled out. Recently, it was revealed that the intrinsic island

growth led to a gradual roughening o f the growing surface [ 5 ]. The growth interruption after every one unit cell layer growth, however, can suppress such roughening as evidenced by the intensity recovering during the interruption in the R H E E D intensity oscillations [7 ]. Furthermore, in this film, the film thickness is very small ( ~ 234 ~ ) . These things make possible the perfect unit cell alternation without imperfection. In summary, a X-ray diffraction study was performed on the superlattice o f [(YBa2Cu307_a)1/ (PrBa2Cu3OT_~)l]to grown by means of the RE method coupled with R H E E D intensity oscillations. It was revealed that exact unit cell-by-unit cell growth was established in the RE growth with the R H E E D monitor. Excellent epitaxy and crystallinity were also revealed. These indicate that RE-grown films make it possible to discuss the intrinsic superconducting natures depending on their film thickness.

References

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256

K. Kamigaki et aL / (YBaeCua)z_~)t(PrBa~Cu307_~) l superlattice

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