New tailored cuprates grown by pulsed laser deposition

PffiSlgA ELSEVIER

Physica C 341-348 (2000) 339 342 www,elsevier.nl/Iocate/physc

N e w t a i l o r e d c u p r a t e s g r o w n b y p u l s e d laser d e p o s i t i o n Bernard Mercey a, Trong-Duc Doan a, Philippe Lecoeur a, Wilfrid Prellier ~, Jean Frangois Hamet a, Paul A. Salvador b, Bernard Raveau a ~Laboratoire CRISMAT-ISMRA, UMR CNRS 6508, 14050 Caen France b Department of Materials Sciences and Engineering, Carnegie Mellon University, Pittsburgh, PA USA This paper reports on the use of pulsed laser deposition to grow tailored infinite-layer-derived structures. The goal is to modify the carrier by changing the nature of the charge reservoir intergrown with the infinite-layer structure. The challenges associated with the development of superlattices containing CaCuO2 are focussed upon. Both structural and electrical properties of materials with various charge reservoirs are presented.

1.

INTRODUCTION

Pulsed laser deposition (PLD) has been used extensively for the growth of superconducting cuprates like YBa2Cu3O7.~. Some groups have also used this method to stabilize metastable phases like the "infinite-layer" structure, which were heretofore stabilized using high pressure conditions [1]. Since the doping of this structure offers a possible route to new superconductors, we attempted to grow new superlattices comprised of infinite-layer blocks alternating with different charge reservoir blocks. Previous studies from our group [2,3] have shown that the intercalation of layers containing carbonate groups, with the general formula Ba2CuOzCO3, into the CaCuO2 infinite-layer structure could induce superconducting behavior. To extend this work to other charge reservoirs, we undertook to interleave two different charge reservoirs between infinite-layer blocks. The first one is the metallic oxide La4BaCusOt3, which when prepared as a thin film crystallizes as a cubic disordered perovskite [4]. The second reservoir is the insulating material Sr2(Cuo.sCr0.5)CuOx. The crystal structure of this material is similar to that of BazCuO2CO3, where strontium replaces barium and a mixed copper/chromium layer replaces the carbonate layer. This bulk structure is tetragonal with a=3.906A and c=8.193A [5]. This paper describes the development of materials grown by alternate deposition of these two materials with the infinitelayer phases.

2.

EXPERIMENTAL

Dense sintered ceramic targets, with the general formulas CaCuO2 (CCO), SrCuO2 (SCO), La4BaCusOt3.x (LBCO), and Sr2(CuosCr~5)CuO, (S2CrCO), were prepared via the standard ceramic synthesis methods [6]. For the PLD process, important deposition conditions were: a laser wavelength of k = 248nm, an energy density between 1.5 and 2 J/cm 2, a repetition rate of 2Hz, and a substratetarget distance of 5 cm. Films were deposited at a constant temperature on [001] SrTiO3 or [101] NdGaO3 single-crystal substrates. Superlattices built up from LBCO and CCO/SCO were grown between 550°C and 580°C on SrTiO3 substrates. Phases stabilized by alternate deposition from S2CrCO and CCO targets were grown between 650°C and 680°C on NdGaO3 substrates. Films were grown in a dynamic vacuum having an oxygen pressure ranging between 0.02 and 0.2 mbar. All films were cooled down at 20°C/rain in a static oxygen pressure of 500 mbar. A Seifert XRD 3000 diffractometer utilizing Cu Kcq radiation was used for the x-ray diffraction (XRD) characterization. The classical four-point method was used for resistance measurements in the temperature range 4-300K. Silver contacts were evaporated on the films through a mask and wires were soldered either using silver paste or ultrasonic bonding.

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B. Mercey et al./Physica C 341-348 (2000) 339-342

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3.

RESULTS S

3.1. Superlattices with LBCO Prior results have shown that epitaxial growth of SCO was possible on a LBCO layer [7], and viceversa. Various superlattices comprised of LBCO and SCO were routinely grown using the standard PLD process. A careful study of the XR.D patterns of the as-grown superlattices has shown that, depending on the deposition conditions, four phases could be stabilized within the superlattices. The SCO layers could be stabilized with or without e x c e s s oxygen. The SrCuO2+8 structures [8,9] were found to have a c-axis length of 3.65 A for 8 > 0, and 3.45A, as expected, for the "normal"(6 = 0) material. Similarly, the LBCO could be stabilized either with or without a deficit of oxygen. The oxygen deficient La4BaCusOi2 material [10,11] has a c-axis length of 3.97 A, while the La4BaCusOt3 material had the expected parameter of 3.877 A. The presence of excess oxygen in the Sr plane of the "infinite-layer" structure leads to an increase in the number of Cu 3+ inside the SrCuO2+8 structure, while oxygen deficit in the LBCO layer promotes the existence of Cu 2+ in the La4BaCusO~2 structure. Hence there is a redistribution of oxygen and trivalent copper that is dependent on the growth conditions (temperature and oxygen pressure) and superlattice structure [7]. Superlattices built up from these four structures were synthesized. A metal-like behavior is observed between 300 and 50K for the (LBCO)3(SCO)z superlattice with a 300K resistivity of 720 ~tf~-cm. However, no superconducting behavior is obtained for such superlattices. Ex-situ annealing under argon or oxygen did not lead to the observation of superconductivity. To prevent the formation of an infinite-layer phase having excess (apical) oxygen, and thereby decreasing the number of defects associated with the CuO2 planes, we would like to grow the superlattices LBCO/CCO. These superlattices could not be grown within our range of experimental conditions. Films grown by depositing alternate layers of LBCO and CCO exhibited no diffraction peaks. To overcome the problem of LBCO/CCO incompatibility, the growth of more complex superlattices was carried out. These more complex structures we called "mirror-like" [12] structures since they are built up from the three compounds LBCO, SCO, and CCO with a stacking sequence as follows: (LBCO)m/(SCO)n/(CCO)p/(SCO)n. Because the

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Figure 1. XRD pattern of an argon-annealed "mirror-like" structure. (Peaks of the substrate are labeled S). growth of the CCO phase is possible on the SCO phase, a large number of superlattices with various m, n, and p values have been obtained. The constancy of the deposition rate for each target allowed us to synthesize, via classical PLD, superlattices with the above-given general formula, including (LBCO)I(SCO)t(CCO)=(SCO)I. It is important to note that this new structure, which is built up from single unit cell blocks of distinct materials, could not be stabilized by classical preparation methods of solid state chemistry but it can be grown in a "classical" PLD system, even without the use of in-situ monitoring. The room temperature resistivity of this phase is = 1000/.tf)-cm and it does not present any superconducting behavior in the temperature range 4-300K. Annealing this artificially ordered material at 600°C in an oxygen pressure of 100 bar was carried out. The XRD pattern of the annealed film, presented in figure 1, demonstrates the stability of this material and the artificial order even upon post-annealing. While the room temperature resistivity decreased to 700 ~tf2-cm, superconducting behavior was not observed for this annealed superlattice. New related structures are currently being studied to induce superconductivity. 3.2. Towards S 2 C r C O / C C O Superlattices High-pressure experiments [5] have demonstrated that incorporation of S2CrCO as a charge reservoir is particularly efficient in promoting superconductivity in "infmite-layer"-derived structures. We should keep in mind that the growth of thin films using the PLD method may lead to phases that are different to those prepared by high pressure methods. The PLD grown carbonate-containing films exhibited predominantly pure carbonate layers

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B. Mercey et al./Physica C 341-348 (2000) 339-342

while the high-pressure materials exhibited an ordering between carbonate groups and copper within the layers [2,3]. Nevertheless, both phases were superconducting. It is, therefore, interesting to investigate if such differences would be observed in the chromate-containing phases. In our investigations the stabilization of complex intergrowth phases having the general formula (S2CrCO)m(CCO)n was quickly observed to be unfavorable using a single target of the same composition. To overcome this problem, a multi-target deposition system was used to design these materials artificially. Using a sintered target with the nominal composition Sr2(Cro.sCu0.s)CuOs+,, thin films having the same composition were grown by PLD at 680°C in an oxygen pressure of 0.25 mbar on a SrTiO3 substTate. The XRD pattem of such a film is presented in Figure 2. From this pattern, the lattice parameter along the direction perpendicular to the film is calculated to be 8.15 A, and it is in good agreement with the value measured for the c lattice parameter of the analogous high pressure phase (8.19 A [5]). This is an encouraging step towards superlattice growth since the basic chromate block appears to be stable as a c-axis film in our growth environment. Superlattice growth was therefore undertaken by alternating deposition of CCO layers and S2CrCO layers. However, this proved to be a difficult task. On SrTiO3 substrates, no film peaks were observed. However, crystalline films could be observed on NdGaO3 if a CCO layer was deposited as the first layer in the alternation sequence. The growth of this material was carried out using the same growth conditions as used for the S2CrCO films. Various deposition sequences lead to crystalline products as long as the ratio between the number of pulses on the CCO target was approximately 2 times larger than that on the S2CrCO target. Although an alternating deposition sequence was used, the diffraction patterns were very similar to those found for CCO, implying that a CCO-like structure is stabilized. Superlattice peaks were not observed in the diffraction patterns, but the relative intensities of the observed diffraction peaks varied with the stacking sequence. An important observation is that the resistivity of the films also depends on the stacking sequence. Figure 3 presents the variation of the resistivity with temperature for two different samples. Both samples exhibit high room temperature resistivity values and are semiconducting between 300 and ~100-150 K. However, a sharp decrease of the resis-

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Figure 2 XRD pattern of a S2CrCO film deposited on SrTiO3. (Peaks of the substrate are labeled S ; I is an impurity).

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Figure 3. Variation of the resistivity as a function of temperature for two stacking sequences, a : 15 pulses of CCO / 5 pulses of S2CrCO, b : 25 CCO/10 S2CrCO. tivity (6 orders of magnitude) is observed around 150 and 100 K for samples a and b, respectively. This decrease could be attributed to the existence of a superconducting phase whose structure and/or carrier concentration is not optimized. This could explain the high resistivity values, as well as the semiconducting behavior observed above the resistivity transition. 4.

CONCLUSIONS

Alternating deposition from multiple targets in standard PLD (without in-situ growth monitoring) was investigated and the results attest to the great potential of this deposition method for the growth of new oxides. Various charge reservoirs were selected to dope the "infmite layer" structure. While LBCO could be incorporated into novel structures, carrier

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concentrations have not been optimized. On the other hand, while chromate-containing layers could not be incorporated into well-defined superlattices, the incorporation of this phase with the infinite layer phase leads to encouraging resistivity behavior. A dramatic decrease is observed in the resistivity of several materials grown by alternating deposition methods. Experiments are still in progress to identify the origin of this transition, and to optimize the structure of these chromates. REFERENCES

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