J. MATER. CHEM., 1994, 4( lo), 1585-1589 YBCO and BSCCO Thin Films Prepared by Wet MOCVD 0. Yu. Gorbenko," V. N. Fuflyigin,"* Yu. Yu. Erokhin," I. E. Graboy," A. R. Kaul," Yu. D. Tretyakov," G. Wahlb and L. Klippeb" Chemistry Department, Moscow State University, 119899 Moscow, Russian Federation IOPW, Technische Universitaet Braunsch weig, Braunschweig, Bienroder Weg 53, Germany The newly developed technique of low-pressure single aerosol source MOCVD (wet MOCVD) has been applied to prepare thin films of YBa,Cu,O, and Bi,Sr,CaCu,O,. The influence of the deposition rate, po,-T conditions, film stoichiometry on the phase composition, orientation and superconducting properties of the films was studied and compared for both systems. Thin films of YBa,Cu,O, with high-Tc(end) =92 K,jc(77 K) =1.7 x 1O6 A cm -2; Bi,Sr,CtaCu,O, with Tc(end) =84 K, jc(77K) =1 x 1O4 A cm -2 were deposited.On the basis of the crystal chemistry approach the conditions for the various orientations were established. MOCVD is recognized as a good technique for the deposition of high-T, and j, superconducting films of YBa2Cu30, and Bi,Sr,Ca, -lCu,O, (n=2,3).'-7 One drawback of the technique is the precise control of the film stoichiometry. In the case of MOCVD with individual sources of precursor vapour a small change of the source temperature or gas flow through the source leads to a considerable change in the film stoichiometry and morphology, leading to the deterioration of the supercon- ducting properties of the film^.^.^ For the last three years the problem promoted a search for various single-source MOCVD technique^.'^^'^ Wet MOCVD'2-14 involves deposition from vapour precursors produced by the evaporation of an organic solution nebulized in a carrier gas flow.This method was found to combine the reproducibility of the vapour composi- tion with the flexibility of composition control. The mechan- ism of the process in comparison with conventional MOCVD is complicated by the influence of the solvent vapour.15 Combustion of the solvent vapour can lead to an indefinite increase in the temperature in the deposition zone of the reactor and to formation of carbon dioxide and water vapour. In this connection the adjustment of the deposition conditions to the area of the thermodynamic stability of the supercon- ducting phases is necessary, taking into consideration the partial pressure of carbon dioxide and water vapour.16 The goals of this paper are to study the features of YBCO and BSCCO thin films deposited by wet MOCVD and to compare the characteristics of the deposition process for both systems.Experimental The experimental set-up is shown in Fig. 1. An ultrasonic nebulizer was used to produce an aerosol of the organic solution with a mean dimension of the droplets <5 pm. The aerosol was transported by the carrier gas to the heated zone where the evaporation of the solvent and precursors took <reservoir ?atedpipes T = 270 "C ultrasonic source water Fig. 1 CVD system with ultrasonic evaporator place.Vapours of the precursors thus obtained reached the substrate where a film was formed. A cold-wall reactor with a radiofrequency heater on the substrate was used to niinimise the combustion of the solvent.16 itPreviou~ly,'~*~~ was shown that diglyme (CH,0CH2CH20CH2CH20CH,) is an appropriate solvent for this technique due to (1) its rather high boiling point (162"C), (2) its low viscosity and (3) the high solubility (0.1 mol-l) of thd chelates (thd =2,2,6,6-tetramethyl-heptanedion-3,5-ate) in it. Moreover diglyme forms adducts with the less volatile thd complexes of the alkaline-earth metals.17 This is the probable reason for the observed higher stability of the diglyme solutions than solutions in alkanes.The temperature of the transport line where the evaporation of the solvent and precursors takes place must be in tfie range 240-270 "C: at higher temperatures decomposition of the precursors was observed; at lower temperatures it was rmposs- ible to achieve complete evaporation of the precursors at the nebulization rate used. Parameters for the process are summa- rized in Table 1. A mobile mass spectrometer system MSQlOOl was used to measure the composition of the gas phase during thci depos- ition run. The effects of the total pressure, deposition tccmpera- ture, solution and O2 feeding and variation of solvent were studied in separate deposition runs. XRD patterns of the films were measured with a Siemens D5000 four-circle diffractometer.SEM was accomplished with a CAMSCAN4 equipped with EDX and WDX systems for microprobe analysis. The stoichiometry of the films was determined with a precision of at least 5 mol% for each metallic component. Ac magnetic susceptibility and screening measurements were used to determine T, andj,." Table 1 Parameters of the deposition process parameter deposition temp./"C total pressure/mbar partial pressure of oxygenjmbar total gas flow/l h-' concentration of solution/molI-single-crystal substrate deposition rate/pm h-' solution feeding rate/ml h-' YBCO BSCCO 700-820 700-830 5-30 15-35 0.1-5 5-25 20-40 0.01-0.1 (lOO)YSZ, (100)MgO. (100)SrTi03, (100)LaA103, (100)NdGa03 0.25-5 0.25-3 3-10 YSZ, yttria-stabilized zirconia (Zro,83Yo,l,02~y).Results and Discussion Deposition Rate and po,-T Conditions A high deposition rate is known to be very important for the technology of HTSC thin films. Wet MOCVD easily permits the attainment of higher deposition rates (up to 10 pm min-') than conventional CVD since a much higher evaporation rate of the precursors can be achieved. However, the rate of the superconducting phase formation can be lower than the deposition rate. This is essential for BSCCO films since the kinetics of 2212 and 2223 phase formation are extremely slow, as was found to be the main problem for the CVD of BSCC0.7 Nearly stoichiometric Y 123 thin films deposited at 700-820°C with rates up to 3 pm h-' contained only the HTSC phase according to XRD data.A deposition rate of < 1 pm h-' and a temperature of 750-830 "C were necessary to prepare 2212 thin films with T, values of 75-84K. The increase of the deposition rate up to 2.5-3 pm h-' resulted in the formation of films containing both 2201 and 2212 phases. If the deposition rate is too high a layer containing basically the 2201 phase grows from the vapour phase. One can suggest the following sequence of phase transformations in this system: vapour--$ Bi,( Sr,Ca),CuO, -+ Bi,Sr,CaCu,O, -+Bi2Sr2Ca2Cu,0, (2201 phase) (2212 phase) (2223 phase) The po,-T conditions determine the range over which super- conducting phases exist.lg po,-T diagrams for the pure HTSC phases are shown in Fig. 2. One can see that the upper temperature of 2212 phase existence is significantly lower than that of the Y123 phase at the same po,.So the possibility of varying the deposition temperature for bismuth-containing superconductors is more limited than for the Y123 phase because a high rate of phase formation should be also provided. 0.5-0.8 pm 2212 films with T,(end)=75-84 K were deposited at 750-820°C with a rate of 0.5-0.8 pm h-l. At the lower temperature it was impossible to deposit single- phase films at such a rate. Films thus obtained always contained an admixture of the 2201 phase. Note that epitaxial Y123 thin films were deposited by conventional MOCVD under po,T conditions close to the line of the Cu,O-CuO equilibrium.20A higher partial pressure of oxygen was necessary for thin films of the Y123 phase to be obtained on (1OO)YSZ with the highest c texture by wet MOCVD.The corresponding po,-T conditions are marked by asterisks in Fig. 2. The conditions for epitaxial growth of 2212 films lay in the range of higher partial pressures of oxygen (Fig. 2). The deposition in the range close to the line of the Cu,O-CuO 2DF 2212-\\ +++ -3.0 I I I I I I 7 8 9 10 11 12 13 lo4 KIT Fig. 2 po, z's. T diagrams for Y123 and 2212 phases. *, Conditions of epitaxial growth of the Y123 phase by wet MOCVD; +, conditions of epitaxial growth of the 2212 phase by wet MOCVD. J. MATER. CHEW, 1994, VOL. 4 equilibrium resulted in films with worse superconducting properties, e.g. a film deposited at 800 'C and po2=8 mbar had T,=65 K (deposition rate=0.8 pm h-I) and the optimal conditions for the growth of the epitaxial 2212 films are po2 = 15-25 mbar, Td=780-820 "C.A possible reason for this is the inevitable deviation of the film cation stoichiometry from the 2212 composition that reduces the temperature of the decomposition according to the data of ref. 19 and 21. For example at po, =0.21 bar the stoichiometric 2212 phase decomposes at 875-880 "C; however, a small excess of copper and calcium reduces the temperature of appearance of the liquid phase to 857 "C. Stoichiometry The stoichiometry of the films is a very important factor for both systems.20-21 The stoichiometry determines the phase composition and as a consequence the superconducting properties of the films, the critical current density being the most sensitive parameter.The basic dependences found in the conventional MOCVD of Y123 are also valid for wet MOCVD. Ba enrichment resulted in drastic deterioration of the film texture and a decrease in T, and j,." A small Cu enrichment (up to Y :Ba :Cu =1: 2 :4) leads to enhancement of the c texture and a higher j, [j,(77 K)= 1 x lo6 A cm-,] with T, being nearly constant and equal to 90 92 K. The stoichiometry effects for BSCCO MOCVD have not been studied previously. We found that an excess of Bi and alkaline-earth elements (Bi,+,Sr,Ca1+,Cu,0~,, x =0.1, 0.15) led to the formation of non-textured thin films with low T, values (20-40K). Films of the 2212 phase on (100) SrTiO, with the best properties [T,=80-84 K, jJ77 K) up to 1 x lo4 A ern-,] contained an excess of calcium and copper.The appearance of the liquid phase is considered to promote the formation of superconducting phases.,, Nevertheless, the influence of the liquid phase on the properties of HTSC thin films is not simple. Crystal growth is usually accelerated in these films even if the liquid phase fraction is small. On the other hand, aggregation of secondary phases, chemical inter- action with the substrate and formation of drops which solidify separately under cooling (Fig. 3) occur if the fraction of the liquid is high. This results in a poor film morphology and a low transition temperature. In this connection it is interesting to compare the samples with an excess of Bi and with an excess of Cu and Ca.According to the phase diagram23 an excess of bismuth results in the formation of a greater amount of the liquid phase than an excess of calcium and copper. BSCCO thin films containing an excess of bismuth (Bi,~,Sr,CaCu,,,O,) had rather poor morphologies with traces 740 760 780 800 820 840 deposition ternperature/"C Fig. 3 Dependence of 2212 film orientation on the deposition temperature: intensity =Ix/(Z200+ I,,, + +Ioo8 +I,,,,), I, =I,oo (u), (1008f~oo,,) (61, (I,,, +1115) (4 J. MATER. CHEM.. 1994, VOL. 4 of melting. The T,s of these film were lower than those of films containing an excess of copper and calcium ( Bi,Srl~,,Ca,,,Cu,~,,O,). The latter also had a better mor- phology (Fig.3). Orientation of HTSC Films Anisotropy of the superconducting properties of HTSC films means that only films with definite types of orientation are of interest for applications. The orientation of the HTSC films was influenced by practically all the main parameters in the deposition process: deposition temperature, total pressure, oxygen partial pressure, rate of deposition, substrate material. The formation of films with a predominant a orientation at low deposition temperatures was observed for both superconducting phases studied (Fig. 4). The increase in the deposition temperature led to the growth of the contribution of the c orientation. At high deposition rates highly textured films could not be prepared.A decrease in the deposition rate promoted the formation of c-oriented films (Fig. 5). These results are in agreement with the fa~t~~*~~ that the c-orientation is therniodynamically preferred since the equilibrium Gibbs energy of formation of the c-oriented grains is greater than that of a-oriented grains. Formation of the a orientation is more favourable kin- etically for Y123 as well as for 2212 phases. According to the data obtained by RHEED26 chaotic distribution of metal (a1 H10pm H10pm Fig. 4 hlorphology of the BiHTSC 1 pm films: (a) Bi2.2Sr2CaCu2,20,, (h)Bi2Sr1.85C~1.3CU2.1S0~ @ 0.66 0 ,4 0 0.45 0.90 1.35 1.80 2.25 deposition rate/ym h-' Fig.5 Influence of the deposition rate on 2212 film Orientation: intensity=z~/('200+z113 +Ills f IOOS +IOOIO), zs=1200 (a),(I,13+1115) (4,(Zoos +I00,o) (c) atoms on the surface of the substrate takes place during the first moments of deposition, then the formation oj a two-dimensional net corresponding to the ac plane can be reached by smaller displacements of the atoms than the formlation of the two-dimensional net corresponding to the ab plane.This is due to the structure of facets: the ab plane is bu,ilt from atoms of only two elements, meanwhile the ac (hc) plane is built from atoms of three or four elements (Fig. 6). During the growth of the following layers the process is repeated. Additionally, one can easily see that the atomic layers which are parallel to the ab or ac planes are not electrically neutral.Thus the HTSC phase cannot grow in the c directicm layer- by-layer but by blocks containing neutral columns of atomic layers. For example, the thinnest neutral column has the height of the unit cell in the case of the Y123 phase.2s Such a column includes two layers for growth in the a directlion and six layers for growth in the c direction (Fig. 6). In thc case of the 2212 phase such piles would include two and seven atomic layers, respectively. The latter are too large to form easily. Thus at high deposition rates the growth of predomiriantly a-oriented films is preferred (Fig. 5). At the same time the necessary conditions for c-orientation are a low deposition rate and a high diffusion mobility of the components of the film.Generally higher mobility can be reached at ]he high temperatures, especially in the presence of a liquid phase. Even if no liquid phase forms, if the deposition temperature is closer to the liquid-phase appearance temperaturll: then a higher diffusion mobility is also achieved. This apprclach was applied earlier to obtain thin films of Y123,l where composi- tion deviations result in a change in the liquid-phase appear- ance temperature of ca. 50°C. The influence of the stoichiometry of the deposited films on the orientation of the films is also important. The \ ariation of stoichiometry which can promote the formation of the liquid phase resulted also in growth of the mainly c-oriented HTSC films. The presence of the liquid phase m'tkes the transport of the components in the film easier and r'avoured growth in the thermodynamically preferred orientation. It was established that an excess of copper increased the conuibution of the c orientation in comparison with stoichiomet ric films of Y123.The influence of the excess of copper may be seen from two viewpoints. First, the additional amount af copper can induce the formation of a non-equilibrium as well as an equilibrium liquid phase. Secondly, an excess of Cu, which is Considered to be the most mobile component in thi:, system, favours the formation of a two-dimensional lattice correspond- ing to the facet ab since this facet can be built only by copper and oxygen atoms. On the other hand, it was also found that an excess of J.MATER. CHEM., 1994, VOL. 4 vapour c orientation a orientation Fig.6 Formation of blocks corresponding to the different types of the films orientation. Positions of the Y123 phase unit cell relative to the substrate surface are shown. Oxygen atoms are omitted. copper or bismuth resulted in the growth of predominantly c-oriented 2212 films (Fig. 7); however, an excess of bismuth or alkaline-earth element favoured the a orientation. So the same approach can be applied to the consideration of the influence of the stoichiometry on the orientation of BSCCO thin films. Nevertheless, note that the role of the liquid phase is probably greater in this case than in the case of the Y123 film. This is because the liquid phase can be in equilibrium with the superconducting phase at the deposition temperature of BSCCO films.The orientation of the thin films is also affected by the crystal structure of the single-crystal substrate. The presence of nearly coincident site lattices is necessary for the appearance of a definite type of film orientation. In the case of the Y123 phase, c/3=b. So the better lattice match between the film and the substrate is for the c-orientation of Y123 phase and also favours the a orientation of this phase, but for 2212 and 2223 phases there is no such relation between c and a(b). Substrates with the lattice parameters close to a of the 2212 or 2223 (SrTi03) phases consequently favour only the c orientation of the film. In ref. 16 it was established that the influence of the solvent on the deposition process is versatile: (1) a thermal effect produced by solvent oxidation; (2) cracking of the solvent molecules leading to contamination by carbon; (3) an increase in CO, partial pressure; (4)a decrease of 0, partial pressure; (5) adsorption of solvent molecules on the film surface.Processes (1)-(4) are very important for wet MOCVD in the hot-wall reactor; however, for deposition in the cold-wall reactor, intensive oxidation of the solvent was not observed according to mass-spectrometric measurements (Fig. 8). 1.1 1.2 1.3 1.4 1.5 1.6 1.7 (Bi + Cu)/(Sr + Ca) Fig.7 Influence of 2212 film stoichiometry on the film orienta-tion: intensity =1XA1200 +Ill3 +1115 +I,,,+~OO,,), 1, =(I008+loolo) (4,IZoo(b); & =780 "C,substrate ( 100) MgO 0.01- E c m--.+ .+-0 + f 0.001- v) + 2 a- m.- 2c Q (a 1 x 0.0001It- I N I 1 I 1 Fig. 8 Dependence of pco2 for 'cold' (a)and 'hot' (b) wall reactor on the molar ratio of oxygen to diglyme according to mass-spectrometric measurements Conclusions A flexible and simple MOCVD technique has been developed to prepare superconducting YBCO and BSCCO films. 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