Oxygen pressure effects on superconducting (YBaCuO) thin films

Transcription

1 Oxygen pressure effects on superconducting (YBaCuO) thin films A. LARGETEAU 1,2, J. SIEJKA 3, J.C. MAGE 4, Y. LEMAITRE 4, B. MARCILHAC 4, C. CLERC 5, and G. DEMAZEAU 1,2 1 Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB-UPR CNRS 9048) - 87 avenue du Dr.A.Schweitzer Pessac cedex, France Ecole Nationale Supérieure de Chimie et de Physique de Bordeaux (ENSCPB) - 16 avenue Pey Berland Pessac cedex, France. 2 Université Bordeaux1 «Sciences-Technologies» cours de la libération Talence, France 3 Institut des Nanosciences de Paris - Université Pierre et Marie Curie (Paris 6) et Denis Diderot (Paris 7) - Campus Boucicaut rue Lourmel Paris - France 4 Thales Research&Technology - UMR137 CNRS/Thales - Domaine Corbeville Orsay France 5 C.S.N.S.M./CNRS, Orsay Campus, France Contact: Alain LARGETEAU largeteau@icmcb-bordeaux.cnrs.fr Abstract High oxygen pressures have been developed during these last fifty years in Materials chemistry mainly for stabilizing unusual oxidation states of transition metals or for controlling the oxygen stoichiometry in functional materials [1]. We have recently shown [2] that amorphous YBa 2 Cu 3 O 7+x thin film deposited by sputtering at temperature T 450 C have unusual oxidation state (x 1) in opposite to the films YBa 2 Cu 3 O 7 deposited at T 800 C on the single cristalline substrates (MgO or LaAlO3 ) which are crystalline and superconducting. The aim of this study was to improve the oxygen distribution in YBaCuO thin film using an annealing treatment (T 500 C) under oxygen pressures (P 200MPa) of a composite system formed by cathodic sputtering at T 800 followed by deposition at T 450 C. At first step a crystalline thin film of YBaCuO 7 on LaAlO3 substrate was formed followed by deposition on the top of amorphous YBaCuO 7+x thin film. Different physico-chemical characterizations have been developed after this high oxygen pressure treatment either for evaluating the oxygen depth distribution (RBS, Nuclear Reaction Analysis, ) or for following the physical properties (magnetic and electric measurements) of the YBaCuO superconducting thin film. 1

2 Introduction The aim of this study was to improve the oxygen distribution in YBaCuO thin film using an annealing treatment (T 500 C) under oxygen pressures (P 200MPa) of a composite system formed by cathodic sputtering at T 800 C followed by deposition at T 450 C. At first step, a crystalline thin film of YBa 2 Cu 3 O 7 on LaAlO 3 substrate was formed followed by deposition on the top of amorphous YBa 2 Cu 3 O 7+x thin film. In order to understand the effects of high pressure annealing on composite system included: crystalline of YBa 2 Cu 3 O 7 superconducting thin film covered by an amorphous YBa 2 Cu 3 O 7+x thin film we studied the effect of high pressure annealing on high quality superconducting YBa 2 Cu 3 O 7 thin film. Some results obtained on composite systems are also presented. The experiences are carried out at 450 C and 550 C in oxygen and nitrogen atmosphere. The atomic composition, the ration: Y:Ba:Cu, of the films are studied by Rutherford Back Scattering ( RBS) at 2 and 3 MeV ion beam ; the oxygen content of the films by the resonant back scattering spectroscopy at MeV ( non Rutherford) and the defects in the Ba and the oxygen sub lattice by the channelling effects. From the channelling data the thickness of the disorder layers formed on the top of crystalline films was also reported. The physical characterisation of the films is performed using two complementary techniques: 1- The measurements of the surface resistance Rs at 77K and 10GHz frequency. This technique is very sensitive to the presence of the non superconducting inclusions in the films (semiconductors, metals) and to the presence of the disorder in the films. 2- The SQUID measurements of the magnetic susceptibility of the films, between 20 and 100K, in order to estimate the critical current density Jc, and the transition temperature Tc. The data from atomic and structural analysis was correlated to the data provided by the physical analysis. I. Preparation of the samples I.1 Thin film of YBaCuO was deposed by cathodic sputtering in the mixture of Argon and oxygen atmosphere, using a polycrystalline target of YBa 2 Cu 3 O 7-ζ The substrates used are single crystal of MgO or LaAlO 3 heated during the deposition near 800 C. The molecules and atoms coming from the target (Y, Ba, Cu) are deposited on the heated substrate and form YBCO crystalline films. The plasma formed during the cathodic sputtering is highly oxidized and the oxygen content of the YBa 2 Cu 3 O 7-x films during deposition is near 7 with the value of x 0.1. After the deposition, the films are slowly cooled down in the oxygen atmosphere. The films formed are stoichiometric and supraconducting with Tc near 92 K. The effect of temperature during the cathodic deposition is very important; we have observed that for the temperature of deposition below 450 C, the deposed film is amorphous with an excess of oxygen at 100 C. This film is isolate in spite of the high content of oxygen. The films formed at 450 C and 500 C are particularly interesting; their oxygen content is similar to this of crystalline films YBa 2 Cu 3 O 7 but their structure is amorphous. In this work we present the results of two types of YBCO films : - The YBCO layer is crystalline and superconducting. The reference is YBCO7, - The under layer of YBCO7 is covered by a second layer of YBa 2 Cu 3 O 7, amorphous and semiconductor, with the oxygen content near 7. The reference of this second layer is YBCO7+X I.2 The crystalline and superconducting samples YBCO7, referenced as Theva are prepared by evaporation of the Y, Ba and Cu components in the low pressure oxygen 2

3 atmosphere on the MgO or LaAlO3 single crystalline substrates heated to ~650 C and in situ oxided in the oxygen. The films are stoichiometric and superconducting. The different samples were annealing in high pressure in oxygen or nitrogen atmosphere. The pressure equipment allows reaching 500 MPa for a temperature up to 500 C and a maximum of 50 MPa for a temperature of 900 C. The samples are deposited in an opening gold tube to protecting of thin film. II. Pressure effect on YBCO7 The aim is to determine the effect of the pressure treatment on the samples composed by the superconducting layer of YBa 2 Cu 3 O 7-x on MgO single crystal. Two types of samples are studied : prepared by the cathodic sputtering and referenced as AMY9; prepared by evaporation and referenced as Theva. The pressure treatments are: -annealing under 90MPa of oxygen at 450 C during 5h: Theva O 2, -annealing under 90MPa of nitrogen at 450 C during 5h: Theva N 2, -annealing under 130 MPa of oxygen at 550 C during 5h: AMY9, II.1 Atomic content ( ratios Y:Ba:Cu) and disorder of the YBCO films before and after high pressure treatment From the RBS at 2 and MeV the atomic composition (Y :Ba :Cu ratios) of YBCO thin films, the lattice disorder χmin and the number of the barium atoms out of the crystallographic sites in the layers are deduced. The disorder layer is formed on the top of the crystalline surface of samples Theva O 2 and Theva N 2. The layer AMY9 is decomposed during the annealing. A strong lost of barium is observed. The results are presented in Table 1. The total cationic composition of the films Theva, Theva O 2 and Theva N 2 are the same; however a strong disorder is introduced in the layers and the formation of disorder layer on the top of crystalline one is observed. The total atomic composition of the crystalline sub layer is the same for all Theva samples. The oxygen content of the disorder layer was measured by the resonance and the results are presented in II.2. Concerning the sign * and **, in the table, we have: * Value of x in the crystalline layer is determined by RBS, X rays and confirmed by the measurements of the physical parameters; the oxygen composition of the near surface layer is measured by the elastic back scattering resonance on oxygen at MeV. ** Oxygen composition is measured by the elastic back scattering resonance at the depth of 30 nm and it concerns 35 nm thick layer nm.. The oxygen content of the analysed samples is determined in the comparison to the reference one (Theva). Disorder in the oxygen sub lattice is determined by the ration of the α yields in the channelling, α channelling, to the α yield in the random geometry, α random. This ratio is named χmin 3

4 Sample description. Thickness of YBCO layer Theva 640 nm Theva O nm Theva N nm AMY9 420 nm Annealing conditions : Atmosphere Pressure Temperature Time O2 900 bar 450 C 5 h N2 900 bar 450 C 5 h O bar 550 C 3 h Atomic composition* From : RBS Total YBCO layer Disorder : χmin Crystalline sub layer 7.2 % 35 % Disorder upper layer Number of Ba out of crystallogra phic sites. (Atoms of Ba/cm 2 ) Thickness of disorder layer (nm) Oxygen ** From: Elastic resonance Near surface layer Composition (Depth of analysed layer (nm)) -x x. 75 % -x % 75 % 3 sub layers YBCO layer on the top: decomposed Y 1 Ba 3 Cu 3 O 11 All atoms 100% Table 1: Results of the measurements Y 1 Ba 2 Cu 3 O 6.2 All Atoms Y1Ba 3 Cu 3 O1 1 Disorder χmin (Depth of analysed layer (nm)) 47 % 75 % 70% 100% II.2 Oxygen content and disorder of the YBCO films before and after high pressure treatment The measurements are concerned the near surface layer of 35 nm thick situated at the depth of 30 nm. The oxygen content of the disorder layer for Theva and Theva O 2 are equal. The film treated in nitrogen atmosphere Theva N 2 has lost oxygen. It is highly probable that all film has lost oxygen. The sample AMY9 is decomposed. The strong oxidation is probably induced by the formation of BaO 2 on the surface and the decomposition of YBa 2 Cu 3 O 7-x. II.2 Nitrogen content of the films before and after high pressure treatment Samples The results of the measurement are given in the table 2. Reference: Si 3 N 4 /Si Thin layer of S i3 N 4 deposited on Si Theva N 2 Annealed in N 2 at α o counts dose: 20 µc Quantity of nitrogen atoms in the film ( 14 N/cm 2 ) (±3%) 16* ( ) Observations Number of 14 N atoms/cm 2 known with ±3% precision Quantity of nitrogen atoms in all film ( thickness~700 nm) 450 C, 900 bar, 5 h Theva 7* Background noise Table 2: Nitrogen content in Theva and Theva N 2 We can considerer that there is no incorporation of N 2 in the films. 4

5 II.3 Physical measurement The electrical measurement shows that the samples Theva, Theva N 2 and Theva O 2 are superconducting but the physical properties of samples annealed in O2 and particularly in N2 are strongly reduced. The magnetic measurement shows a difference between the three samples. These measurements have been made with SQUID and a magnetic field of 100G (Fig.1) 0,05 Theva Ref: red Theva O2 : green Theva N2: blue 0 Long Moment (EMU) -0,05-0,1-0,15-0,2-0, Temperature (K) Fig1 : Magnetic susceptibility versus temperature The values of magnetic susceptibility (Θ) at 20K, for Theva O 2 and Theva N 2 are smaller (absolute values) than that of Theva, showing the diminution of critical current density Jc in annealed samples. The values of Θ, between 20 and 50 K, decrease 4% for Theva and of 20% for Theva O 2 and Theva N 2. The values of Θ above 50K, decrease quickly with the temperature. We can speculate that : -between 20 and 50K the screening current is smaller than the critical current density Jc, for, the sample Theva, - close to 50K, the screening current is equal to Jc. II.4 Discussions, conclusions. Theva is homogeneous and has a stoichiometric composition : YBa 2 Cu 3 O 7 (RBS and NRA measurement). The oxygen content of Theva is equal to 7 in agreement with the measurement of the c parameter by X Rays. The cation content for Theva O 2 and Theva N 2 are equal to this of Theva :YBa 2 Cu 3 O 7. However a strong disorder is observed on the top of crystalline sub layer. The oxygen content for Theva N 2 is smaller that O 7 and is equal in the disorder upper layer to O 6.2 ; it is probable that all film is dis oxygenated. The nitrogen content is very low after high pressure treatment for Theva N 2 showing that the oxygen lost is not replaced by the nitrogen up take. 5

6 The YBCO structure and atomic composition of AMY9 is changed. The layer after the annealing is inhomogeneous; there are three sub layers, with a maximum of Ba and O content on the surface: formation of YBa 3 Cu 3 O 11. The barium losses are observed related probably to BaO 2 formation on the surface. The samples Theva O 2 and Theva N 2 are superconductor but their properties are deteriorated in comparison to the reference one. The results of the surface resistivity and SQUID measurement are better for the samples annealed at 450 C in oxygen than that in N 2 atmosphere. The decreasing of the critical current for Theva O 2 and Theva N 2 are due probably to the formation of disorder sub layers and to the defects formation in all layer. III. Pressure effect on YBCO7 covered by YBCO7+X amorphous films. The aim is to determine the effect of the pressure treatment on the samples composed by the superconducting layer of YBa 2 Cu 3 O 7-x on MgO single crystal and covered by a second layer of YBa 2 Cu 3 O 7+x, amorphous and semiconductor. The pressure treatments are: -annealing of the sample YBCO7/YBCO7+X (450) under 100 MPa (1Kbar) of oxygen at 450 C during 16h, -annealing of the sample YBCO7/YBCO7+X (500) under 200 MPa (2Kbar) of oxygen at 300 C during 16h, III.1 Oxygen content of the films Thin film YBCO7+X deposed on YBCO7 at 450 C and 500 C has the composition YBa 2 Cu 3 O 7+x with x~0.1. The measurements of the oxygen content, and of the crystallographic structure, after the annealing are in progress. III.2 Physical measurements The measurement by SQUID of the magnetic susceptibility of YBCO7/YBCO7+X (450) as prepared and after the high pressure annealing at 450 C in the O 2 atmosphere is presented on the Fig 2 ; for comparison the effect of annealing of the YBCO7/YBCO7+X (500) sample annealed at 300 C is also presented. The high pressure treatment improves the electrical properties of the films (Fig.2). The effect of annealing is better for the film annealed 450 C than that annealed at 300 C. 6

7 0,05 YBCO Cryst.+deposited at 450 C red Annealed in O2 at 300 C/16h/2Kbar green Annealed in O2 at 450 C at 450 C/16h/1Kbar blue 0 Long Moment (EMU) -0,05-0,1-0,15-0,2-0, Temperature (K) Fig.2 : Magnetic susceptibility versus temperature IV. Conclusion The observed results underline a strong effect of high pressure annealing both on the structure and the physical properties of YBCO films. The results involving on samples composed of double films seems interesting and further studies are in progress in order to explain such a behaviour. References [1] G. DEMAZEAU, D.Y. JUNG; Eur. J. of Solid State and Inorg. Chem. 32, (1995) [2] J.SIEJKA, J-C. MAGE, Y. LEMAITRE, B. MARCILLAC, Unusual oxidation state of amorphous YBaCuO thin films prepared by cathodic sputtering. To be published. 7