Effect of CVD process parameters on phase and chemical composition of BSCCO thin films

Superconducting BSCCO thin films were obtained with high deposition rate (about 35 nmlmin) at temperatures of 720-8100C by MOCVD-technique. Characteristics of evaporation process of precursors were determined. The influence of deposition temperature and oxygen e a l pressure on superconducting phase formation and chemical composition of the films was studied. For recent years many studies were performed on the fabrication of thin films of high Tc oxide superconductors [I-91. Superconducting thin films have possibilities for application in submillimeter-wave and microwave devices, bolometers and cryoelectronic devices such as SQUID'S. The preparation techniques for BSCCO films reported in literature include sputtering 131, electron beam evaporation 141, laser ablation [5], pyrolysis of organic salts solutions [6] and chemical vapour deposition [1,7,8]. MOCVD due to an advantage in high deposition rate and possibility of coating of large surfaces is evidently more promising for fabrication of thin films on filaments or ribbons for electrotechnical application. MOCVD of BSCCO thin films was reported by Yamane et al.[l], Zhang et al.[7], Endo et al.[8]. Nevertheless, some fundamental questions are still not clear. There are at least three superconducting phases in the Bi-Sr-Ca-Cu oxide system: Bi2Sr2CuOx (2201-phase, Tc=4K), Bi2Sr2CaCu2OX (2212-phase, Tc = 80K), Bi2Sr2Ca2Cu3OX (2223-phase, Tc = 1 1OK) [7]. All these phases are non-stoichiometric with respect both to metals and oxygen content; critical parameters essentially depend on phase stoichiometry. Five component phase diagram of Bi-Sr-Ca-Cu-0 system is not completely studied, this fact does not allow to predict phase relations under Po2-T conditions of CVD process. Moreover the process of deposition is strongly incongruent and deviation of film composition from that of vapour depends on used Po2-T conditions. Typical deposition rates did not exceed 10 nmlmin [9] what is probably due to slow kinetics of HTSC-phase formation at generally used deposition temperature of 8000C. More rapid deposition is desirable for practical applications. These considerations stress the necessity of process parameters optimization at high deposition rate what is the goal of this paper. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993349 JOURNAL DE PHYSIQUE IV BSCCO films were prepared in vertical hot-wall reactor with sharp temperature gradient furnace. Source materials (bismuth triphenyl BiPh3 and 2,2,6,6-tetramethylheptandionates-3,5 of copper Cu(thd)2, strontium Sr(thd)2 and calcium Ca(thd)2) were placed in separated evaporators. Temperatures of sources were 110-1150C for BiPh3, 130-1350C for C~(thd)~, 235-2400C for Sr(thd)2 and 185-1950C for Ca(thd)2. Vapours of precursors were transported by Ar-gas flow to reactor inlet where mixing with oxygen took place. Total argon flow was in the range of 30-69 lph, oxygen flow was 1-40 lph. Deposition was carried out at overall pressure of 10-15 torr and temperature varied from 7200C to 8lWC. Deposition rate was about 35 nmlmin, single crystalline (001) MgO was used as a substrate. After deposition cooling in oxygen flow (0.5h) was created. Thickness of the films measured with Tally Step profilometer was about 1 pm. Films composition was determined by electron-probe microanalysis. X-ray characterization was made using Stadi P ("Stoe") diffractometer. Measurements of electrical resistance were carried out by ac four-probe technique. 3.Results and discussion. 3.1. EvaporcMion qfprecursors. The problem of temporal stability of precursors at evaporation temperature is well known for Ba(tI~d)~ [lo]. This problem is also suspected to be important for Ca(thd)2 and for Sr(thd)2. The latter being the least volatile compound among the precursors used for BSCCO system limits the deposition rate. Thus it is important to establish the upper temperature limit of stable Sr(thd)2 evaporation. Kinetics curves of Sr(thd)2 and Ca(thd)2 evaporation at different temperatures and at constant carrier-gas flow are given on Fig. 1. These data show that the isothermal evaporation rate of Sr(thd)2 is practically constant during two hours ( fourfold more than real time of deposition run) in the temperature range of 225-24WC. At higher temperature (2450C) appreciable decrease of evaporation rate was observed. Thus maximal constant evaporation rate of Sr(thd)2 (0.01 mmollmin) can be achieved at 240%. Ca(thd)2 is rather stable within all used temperature range.