Channeling spectrometry in HTSC thin film analysis

Nuclear Instruments North-Holland

and Methods

in Physics Research

Channeling spectrometry J. Remmel,

J. Geerk,

G. Linker,

Nuclear Instruments 8 Methods in Physics Research Sectlorl f3

B64 (1992) 174-178

in HTSC thin film analysis 0. Meyer,

R.L. Wang

KernforschunRszrntrum Karlsruhe, Institut fiir Nukleare FestkCperphysik,

’ and Th. Wolf

2

P.O.B. 3640, D-7500 Krrrlsruhe, Germany

MeV He+ ion channeling and backscattering spectrometry performance of single crystalline REBa,Cu,O, (RE = Y. Eu, the substrate surfaces was reduced by annealing in oxygen depended on the substrate temperature during deposition, T,. 710 and 810°C respectively. These minima were attributed

was applied to analyze the substrate surface finish and the growth Gd) thin films on LaAIO, single crystal substrates. The disorder at atmosphere. The minimum yield values, x,~,“, of the HTSC films and revealed minima of 4 and 5% for the RE/Ba sublattices at T, of to pure u- and c-axis growth as confirmed by X-ray measurements. The disorder at the substrate-film interface decreased with increasing r,. Oxygen analysis by 3.05 MeV He+ ion resonant scattering resulted in best xrnln values of about 25%), while for YBaCuO single crystals the best values observed were about 6%.

1. Introduction Ion backscattering and channeling spectrometry is a well established technique for the analysis of thin films and material surfaces [1,2].Since the discovery of high temperature superconductance, HTSC, thin film deposition RBS analysis proved to be a very useful method to optimize the deposition parameters with respect to film composition and lateral homogeneity [3]. When cpitaxial film growth was realized, channeling became a powerful1 tool to analyze the structure and disorder at surfaces, interfaces and in the bulk. Energy dependent channeling measurements reveal information about defect structures present in films and single crystals. The channeling method combined with nuclear reactions or resonant scattering can be used to analyze the oxygen sublattice. This is very important bccausc the oxygen concentration and lattice site occupation is known to play a dominant role for the properties of HTSC. Furthermore, channeling is applied to optimize the parameters for substrate surface and bulk quality improvement. A nearly perfect substrate surface finish is an essential prcrcquisite for best film growth. In this way deposition parameters could bc optimized to achieve high quality epitaxial thin film growth [3]. Because of the high sensitivity of the HTSC materials to radiation damage [4], however, RBS analysis is no longer a nondestructive method. For this reason it is necessary to reduce damage by ion bombardement, especially in channeling experiments, to a minimum. In the following we present some recent results of channeling experiments mainly on single crystalline

’ On leave from Chinese * Kernforschungszentrum Physik. 0168-583X/92/$05.00

Academy of Science, Beijing, China. Karlsruhe, lnstitut fiir Technische

0 1992 - Elsevier

Science

Publishers

EuBa,Cu,O, thin films on LaAlO, substrates, which demonstrate the power of channeling spectrometry in supcrconductance research. Preparation and supcrconductance properties of the films have been reported elsewhere [3].

2. Experimental For the channeling analysis two Van de Graaff accelerators with cncrgics of 2.5 and 3.75 MeV, respectively, were used. The RBS yield was detected by silicon surface barrier detectors with areas of 100 and 450 mm2. To rcducc the damage by the analyzing ion beam, the detectors were mounted at a short distance of about 5 cm from the sample under a backscattering angle of 155 O. This arrangement corresponds to a solid angle of about 0.18 sr. The current used was kept low, 0.1-0.5 nA, to reduce pileup. The total charge accumulated for an angular scan with reasonable statistics was 200 nC, and for an aligned spectrum 2 uC. A 2 MeV Hc+ ion beam was used for routine analysis. For the analysis of the oxygen sublattice the resonant O(cw / a)0 scattering at 3.048 MeV was applied [1,5]. An enhancement of the 0 yield, as compared to the Rutherford backscattering yield, up to a factor of 16 could be reached.

3. Results 3.1. Substrates

A high quality substrate surface is a necessary uisite for cpitaxial film growth. It was demonstrated B.V. All rights reserved

req[3]

.I. Remmel

et al. / Channeling spectrometry

that the film growth depends strongly on the bulk and surface quality of the substrates. A number of different substrates were analyzed and used for HTSC thin film deposition, such as SrTiO,, MgO or Zr(Y)O,. For a summary see ref. [3]. LaAlO, with a lattice parameter of 0.3778 nm, corresponding to a lattice mismatch for the u-axis, b-axis and c-axis/3 for EuBa,Cu,O, (a = 0.3846, b = 0.3897, c = 1.1709 nm) of 1.8, 3.1, and 3.2%, respectively, was chosen for the deposition of the REBa#_t,O, (RE = Y, Eu, Gd) thin films. As an example for the improvement of the surface finish and bulk quality by annealing, we show in fig. 1 the channeling spectra of an as-received LaAlO, substrate before (fig. la) and after (fig. lb) annealing for 2 h at 950 o C in 1 atm. 0,. In this example the surface peak area (SPA) in La atoms per row was reduced from about 85 to 4. From Monte Carlo calculations, which include the backscattering contribution due to thermal vibrations, a value of 2 La atoms per row is obtained for a defect-free surface, so that the upper result corresponds to about two disordered monolayers after annealing. The minimum yield, JJ,,~“, was reduced in this procedure from about 23% before to 1.8% after annealing. The best commercially obtainable substrates had 1.1 disordered monolayers at the surface and minimum yields of l.l%, so a heat treatment was not applied in all cases. In EuBa,Cu,O, thin films deposited on as-received LaAlO, substrates, minimum yield values of 4% in the Eu/Ba sublattice could be reached. Other substrates

200

17.5

of HTSC thin films

with small lattice mismatches, such as NdGaO,, the subject of further investigations.

will be

3.2. Films For the optimization of the deposition parameters for thin films, channeling spectrometry plays an important role in growth analysis. The substrate temperature during deposition, T,, is one of the most important deposition parameters for epitaxial growth. The dependence of the quality of thin EuBa,Cu,O, films on T, has been studied here in detail. In previous X-ray measurements of YBa,Cu,O, films on SrTiO, [7], it was found that the crystalline direction of the films changes from c-axis growth to a-axis growth when the substrate temperature was reduced. In our experiments, the applied temperature range for EuBa,Cu,O, on LaAlO, can be divided into three regions: first, from 650 to 740” C for u-axis growth, second, from 740 to 800” C for an u/c-axis mixed growth, and third, from 800 to 8.50 o C for c-axis growth. The aligned spectra of a pure u-axis, a pure c-axis, and an u/c-oriented film are shown in fig. 2. It can be seen that low minimum yield values of 4 and 5% can be achieved for films of pure a- or pure c-axis orientation, respectively, indicating good epitaxial growth, whereas an enhanced xmin of 22% in the u/c mixed region reveals growth deteriorations if both kinds of grain orientation grow simultaneously in a film.

300

400 Channel

Fig. 1. Random

and aligned

backscattering

spectra

of a LaAIO, substrate atm 0,.

before

(a) and after (b) annealing

IV. HIGH-T

for 2 h at 950 o C in

SUPERCONDUCTORS

1

176

J. Remmel ct al. / Channeling spectrometry

of HTSC thin films

I

2 MeV He+-EuBaCuO 6

m CI .rl

56

li:“‘----j b)

350

300

400

450

Channel Fig. 2. Aligned

spectra

for (a) a-axis,

Fig. 2 shows in addition that oriented film in the interfacial than that for u-axis orientation. neling for u-axis films at the

(b) u/c

mix and (c) c-axis oriented thin EuBaZCu,O, spectrum is from the a-axis film.

the growth of a c-axis region is much better The enhanced dechaninterface may be ex-

films on LaAlO?.

plained by a higher amount of defects due to the relatively low substrate temperature. This assumption is supported by the results of channeling measurements we made on u-axis oriented films (not shown)

a/c-Axis Mix

c-Axis

5I

620

660

700 Deposition

Fig. 3. xmlln vs substrate

temperature.

The random

780

620

thin films on LaAIO,

substrates.

740 Temperature

T,, for EuBa,Cu,O,

(“Cl

J. Remmel et al. / Channeling spectrometry of HTSC thin films prepared using the template procedure. In this procedure a thin template layer is deposited at reduced T, while the bulk of the film grows at higher T,, preserving the a-axis orientation. Details are given in ref. [6]. For the template films the defect structure in the interfacial region depends strongly on the deposition temperature. The dechanneling in this region is decreased by a factor of about 2 if films prepared in one step at low 7; arc compared to those prepared in the template procedure with i?, = 800°C in the second step. The defect structure at the interface will be the subject of further investigations, including energy dcpendent measurements. The minimum yield values as a function of substrate temperature in the complete temperature range under investigation arc shown in fig. 3. It can be seen that optimum u-axis growth is achieved at 710 o C and optimum c-axis growth occurs at 820’ C, which is similar to the results reported previously [7,8]. 3.3. Oxygen sublattice The oxygen sublattice is of major importance for the new HTSC materials, because it is known to play a main role with respect to their superconducting properties. Moreover, the oxygen is the element with the highest mobility in this compound and therefore may be responsible for annealing steps observed below room temperature after low temperature ion irradiation [9]. The oxygen sublattice is, however, difficult to analyze

177

by RBS and channeling because of the small scattering cross-section for oxygen in comparison to the other components, and the large background arising either from the film or the substrate. There are, however, some possibilities to overcome these difficulties. One is to deposit thick films on low-mass substrates to separate the front backscattering edge of the oxygen from the substrate. A second one is to use resonant scattering, for example O(cu 1cu)O at 3.048 MeV He+ [l,lO]. We applied both methods. As an example of the first one, we quote data of 2 MeV He+ scattering on a 600 nm YBaCuO film on MgO. We measured a minimum yield value for the 0 sublattice of (30 f 7.5%). This large error is due to the large background from multiple scattering in the relatively thick film, and cannot be reduced by improvement of statistics, because the total dose has to be limited to reduce damage. To measure the oxygen sublattice more precisely, we used the resonant scattering method. We show as an example, in fig. 4, the spectra of a c- (fig. 4a) and an a-axis (fig. 4b) oriented EuBa,Cu,O, thin film. The minimum yield values for the oxygen sublattice are 26% for the c-axis and 57% for the a-axis oriented film, whereas for the Eu/Ba (Cu) sublattices, x,,,,” values of 6.5 (11) and 13% (15%) are measured, respectively. Minimum yield values around 60% in the 0 sublattice for a-axis, and around 40% for c-axis growth are typical. The minimum yield values for the c-axis film in fig. 4 correspond to a random fraction, Nd, of about 5% for the Eu/Ba, 10% for the Cu, and 24% for

300 Channel

Fig. 4. Random and aligned resonant scattering spectra at 3.05 MeV He+ for an u-axis (dotted (b)) EuBa&u,O, thin film on LaAIO,.

lines, (a)) and a c-axis (solid lines,

IV. HIGH-T SUPERCONDUCTORS

J. Remmrl et al. / Channeling spectrometry

178

of‘ HTSCthin films

Tilt Plane 26 deg

Ba-Sublattice Nd=lO% 0-Sublattice Nd=25%. 0" -3.0

-2.0

-1.0

0

1.0

2.0

Tilt Angle (degl Fig. 5. Angular

yield curves (symbols)

at a tilt angle of about 26” off the (100) plane in the Eu/Ba Monte Carlo simulations (solid lines).

the 0 sublattice. A Monte Carlo simulation of angular yield curves in a defective structure is given together with measured data in fig. 5. It shows a minimum yield value in the oxygen sublattice of about 300/O. Monte Carlo simulation of a perfect lattice at 30 K, however, yields a xmIn value for oxygen of 6c/c, which is in good agreement with measurements of the oxygen sublattice in a bulk single crystal. The discrepancy in the dechanneling for films and single crystals indicates that there is considerably more disturbance in the oxygen sublattice of the films. The higher x,,,~, in u-axis oriented films may be due to the lower deposition temperature.

4. Summary RBS analysis of substrate surfaces led to optimized annealing conditions to get high quality surfaces. RBS analysis of thin films made it possible to optimize deposition parameters to obtain reproducible single crystalline thin films with xmin values around 4% in the Ba sublattice. The template procedure yields better a-axis oriented films, especially with regard to their crystallinity at the interface region. Resonant scattering with 3.05 MeV He+ revealed additional defects in the 0 sublattice. This additional amount of defects may be attributed to distortions in the CuO sublattices of the films, and may be responsible for the high

critical ters.

currents

in these

(a) and the 0 sublattice

films acting

(b) with

as pinning

cen-

References [l] W.K. Chu, J.W. Mayer and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1987) and references therein. (21 L.C. Feldman, J.W. Mayer and ST. Picraux, Material Analysis by Ion Channeling (Academic Press. New York, 1982) and references therein. [3] J. Geerk, Cr. Linker and 0. Meyer. Mater. Sci. Rep. 4 (1989) 193. [4] 0. Meyer, F. Weschenfelder, J. Geerk. H.C. Li and G.C. Xiong, Phys. Rev. 837 (1988) 9757. [5] 0. Meyer, F. Weschenfelder, X.X. Xi, G.C. Xiong, G. Linker and J. Geerk. Nucl. Instr. and Meth. B35 (1987) 292. [61 R.L. Wang, J. Reiner. J. Remmel, E. Brecht, J. Geerk. 0. Meyer and G. Linker, Physica Cl80 (1991) 65. [71 G. Linker, X.X. Xi, 0. Meyer. Q. Li, and J. Geerk, Solid State Commun. 69 (1989) 249. L. 1x1 A. Inam. C.T. Rogers, R. Ramesh, K. Remschnig, Farrow, D. Hart, T. Venkatesan and B. Wilkens, Appl. Phys. Lett. 57 (1990) 2484. [91 G.C. Xiong, H.C. Li, G. Linker and 0. Meyer. Phys. Rev. 838 (1988) 240. for [lOI J.W. Mayer and E. Rimini, Ion Beam Handbook Material Analysis (Academic Press. New York, 1977) and references therein.