Materials Chemistry and Physics 50 ( 1997) 166- 17 1 The effect of the reactant gas on YBa,Cu,O, film formation D.B. Buchholz, P.R. Markworth, J.A. Belot, T.J. Marks, R.P.H. Chang * NSF Science and Technoiogy Center for Superconductivity, Northwestern University, Evanston, IL 60208, USA Abstract The reactant gas used as the deposition ambient has a large effect on the composition and properties of cuprate superconductor films. In this study YBa,Cu@, thin films aredeposited by pulsedorganometallic beam epitaxy (POMBE). The barium precursor usedcontainsfluorine and it is therefore possible to incorporate unwanted fluorine into the deposited film. For our deposition process both the fluorine content and oxygen content of the film are highly dependent on the reactant gas used as the deposition ambient. Many of the reaction conditions that remove fluorine also deplete the film of oxygen. In this study a set of reaction conditions that produce YBazCu,O, films which are both fluorine-free and fully oxygenated are presented. Keywords: Reactant gas; Film formation; YBa,Cu,O, films 1. Introduction A fundamental consideration in an organometallic chemi- cal vapor deposition (OMCVD) process is the choice of organometallic precursors. For the deposition of the super- conducting oxide YBa,Cu,O,, the precursors of choice are most often the metal @-diketonates.Y (dpm) 3,Ba( dpm) z and Cu( dpm) 2 or Cu( acac) 2, where dpm = dipivaloylmethanate and acac = acetylacetonate [ I]. The aforementioned precur- sors are solids at room temperature and must be heated to obtain the vapor pressure (about I-10 Torr) needed for OMCVD [ 21. This is not a major problem for the yttrium and copper sources; sufficient vapor pressures of these metal precursors can be obtained at temperatures between 110 and 170 “C [ 1,3]. In this temperaturerangeY(dpm),,Cu(dpm), and Cu( acac), are all stable [ 4,5]. The barium precursor, however, must be heated to between 220 and 300 ‘C [ 1,3,6]. In this temperature range Ba(dpm), can thermally decom- pose and be transported as a mixture of oligomers. This results in a metallic barium transport rate that varies in time [ 71. To overcome the problem of Ba( dpm) 2 decomposition, a num- ber of attempts have been made to lower the sublimation temperature of the barium source. One has been to add an organic solvent such as teirahydrofuran IX] or dipivaloyl- methanate { 61 to the carrier gas. The precursor transport rate, however, is dependent on the partial pressure of the organic solvent in the carrier gas and variations in solvent partial pressure will result in a change in metal transport rate. The * Corresponding author. Tel.: + 1847 491 3537; fax: + 1 847 491 7820. 0254-0584/97/$17.00 0 1997 Elseviet Science S.A. AU rights reserved PIis0254-0584(97)01932-9 use of a solvent-doped carrier gas also has the undesirable effect of increasing the hydrocarbon concentration at the dep- osition surface and, thereby, increasing the possibility of incorporating carbon into the deposited film. A second option for lowering the barium source sublimation temperature has been to use fluorinated P-diketonate ligands to form more volatile barium organometallics [ 91. For the pulsed organometallic beam epitaxy (POMBE) system [lo] we have chosen to use a second generation fluorinated barium organometallic, Ba( hfa) z. tet, where hfa = hexafluoroacetylacetonate and tet = tetraglyme, that exhibits significantly improved thermal stability and volatil- ity over previously used sources such as Ba( hfa), [ 111. To obtain a vapor pressure sufficient for OMCVD, Ba( hfa),.tet needs only to be heated to between 130 and 150 ‘C, temper- atures well below the decomposition limit. The use of fluor- inated ligands, however, introduces fluorine into the chemistry of the CVD process and the possibility of incor- porating fluorinated species, such as BaF2, into the deposited film [ 121. Although minor amounts of fluorine may be slightly beneficial to the properties of YBa,Cu,O, [ 131, large amounts of fluorine will degrade electrical performance [ 141. The amount of fluorine incorporated into the film, however, will depend on the deposition conditions [ 121. The reactant gas used during film formation has a large effect on the composition and properties of the YBa&u,O, films. One film formation technique that requires the removal of fluorine involves the sequential evaporation of Y, BaFz and Cu layers as an amorphous film, followed by a post deposition anneal [ 151. The post deposition anneal is a two-