Richard J. McNeely

Thin films for superconducting electronics: Precursor performance issues, deposition mechanisms, and superconducting phase formation-processing strategies in the growth of Tl2Ba2CaCu2O8 films by metal-organic chemical vapor deposition

Epitaxial Tl2Ba2CaCu2O8 thin films with excellent electrical transport characteristics are grown in a two-step process involving metal-organic chemical vapor deposition (MOCVD) of a BaCaCuO(F) thin film followed by a postanneal in the presence of Tl2O vapor. Vapor pressure characteristics of the recently developed liquid metal-organic precursors Ba(hfa)2 • mep (hfa › hexafluoroacetylacetonate, mep › methylethylpentaglyme), Ca(hfa)2 • tet (tet › tetraglyme), and the solid precursor Cu(dpm)2 (dpm › dipivaloylmethanate) are characterized by low pressure thermogravimetric analysis. Under typical film growth conditions, transport is shown to be diffusion limited. The transport rate of Ba(hfa) 2 • mep is demonstrated to be stable for over 85 h at typical MOCVD temperatures (120 ‐ C). In contrast, the vapor pressure stability of the commonly used Ba precursor, Ba(dpm)2, deteriorates rapidly at typical growth temperatures, and the decrease in vapor pressure is approximately exponential with a half-life of ,9.4 h. These precursors are employed in a low pressure (5 Torr) horizontal, hot-wall, film growth reactor for growth of BaCaCuO(F) thin films on (110) LaAlO3 substrates. From the dependence of film deposition rate on substrate temperature and precursor partial pressure, the kinetics of deposition are shown to be mass-transport limited over the temperature range 350‐ 650 ‐ C at a 20 nmymin deposition rate. A ligand exchange process which yields volatile Cu(hfa)2 and Cu(hfa) (dpm) is also observed under film growth conditions. The MOCVD-derived BaCaCuO(F) films are postannealed in the presence of bulk Tl2Ba2CaCu2O8 at temperatures of 720‐890 ‐ Ci n flowing atmospheres ranging from 0‐100% O2. The resulting Tl2Ba2CaCu2O8 films are shown to be epitaxial by x-ray diffraction and transmission electron microscopic (TEM) analysis with the c-axis normal to the substrate surface, with in-plane alignment, and with abrupt film-substrate interfaces. The best films exhibit a Tc › 105 K, transport-measured Jc › 1.2 3 10 5 Aycm 2 at 77 K, and surface resistances as low as 0.4 mV

MOCVD precursor design issues. Recent advances in the chemistry and vapor pressure characteristics of Ba(hexafluoroacetylacetonate)2·polyether complexes

Abstract This contribution presents synthetic, structural and volatility results for a series of new Ba(hfa) 2 polyether (hfa=1,1,1,5,5,5-hexafluoropentane-2,4-dionate) MOCVD precursors. The major emphasis of this study is on the synthesis and characterization of new Ba(hexafluoroacetylacetonate) 2 ·polyether complexes as on well as elucidating relationships between volatility, melting point and molecular architecture using a variety of techniques, including reduced pressure thermogravimetric analysis (TGA). The results provide insights into tuning Ba precursor volatility characteristics based on straightforward modifications of the polyether structure.

Efficient route to TlBa2Ca2Cu3O9+x thin films by metal-organic chemical vapor deposition using TlF as a thallination source

Thin TlBa2Ca2Cu3O9+x films were grown on single crystal (110) LaAlO3 from metal-organic chemical-vapor deposition-derived Ba–Ca–Cu–Ox precursor films employing Ba(hfa)2⋅mep, Ca(hfa)2⋅tet, and Cu(dpm)2 (hfa=hexafluoroacetylacetonate; dpm=dipivaloylmethanate; tet=tetraglyme; mep=methylethylpentaglyme) as the volatile metal sources. Thallination is then accomplished by annealing the precursor films in the presence of a bulk BaO+CaO+CuO+TlF source at 885 °C in flowing O2. The presence of TlF is essential for nucleating the Tl-1223 phase. The resulting Tl-1223 films are nearly phase-pure, highly oriented, epitaxial by x-ray diffraction, and contain negligible fluoride by windowless energy-dispersive x-ray measurements. The films exhibit a transport measured Tc=103 K and Jc=4.4×104 A/cm2 (77 K, 0 T). Magnetic hysteresis measurements yield Jc=1.9×105 A/cm2 (30 K, 0.01 T) and show considerable flux pinning at low temperatures with Jc=1.4×105 A/cm2 (5 K, 4.5 T).

New structural aspects of Tl2Ba2CaCu2Oy epitaxial thin films grown by MOCVD on LaAlO3

Abstract The microstructure of a Tl 2 Ba 2 CaCu 2 O y (Tl-2212) superconducting thin film grown by metal-organic chemical vapor deposition (MOCVD) on a pseudo-cubic (001) LaAlO 3 substrate has been examined by analytical transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Over large regions, the film is epitaxial and Tl-2212 phase is found to be the major phase. The film/substrate interface is abrupt and smooth on the atomic scale but a strain-field is induced by the lattice mismatch between the film and the substrate. In addition to the intrinsic modulation structure of the Tl-2212 phase, a very different modulation structure has also been found. The space group for the Tl-2212 phase in this thin film was determined to be I4/mmm as usually reported. The loss of the (001) mirror plane is attributed to a defective Tl-2212 structure in which a considerable amount of Tl vacancies and Tl disorder occurred inhomogeneously in TlO layers. The reason for the Tl-deficiency is discussed.

Metal-organic chemical vapor deposition routes to films of transparent conducting oxides

This contribution reports the in situ growth of transparent, conducting Ga x In 2-x O 3 and Zn k In 2 O k+3 films by MOCVD (metal-organic chemical vapor deposition) techniques using In(dpm) 3 , Ga(dpm) 3 , and Zn(dpm) 2 (dpm = dipivaloylmethanate) as volatile precursors. In the former series, film microstructure in the x = 0.4 – 1.0 range is predominantly cubic with 25° C electrical conductivities as high as 1300 S/cm (n-type; carrier density = 1.2 × 10 20 cm −3 , mobility = 68 cm 2 /Vs) and optical transparency in the visible region greater than that of ITO. In the latter series, films in the composition range K = 0.16 – 3.60 were studied; the microstructural systematics are rather complex. Electrical conductivities (25° C) as high as 1000 S/cm (n-type; carrier density = 3.7 × 10 20 cm −3 , mobility = 18.6 cm 2 /Vs) for K = 0.66 were measured. The optical transparency window is significantly broader than that of ITO.

Novel Metal-Organic Chemical Vapor Deposition / T1F Annealing Route to Thin Films of Tl1Ba2Ca2Cu3O9+x

Thin films of TI1Ba2Ca2Cu3O9+x have been grown on single crystal (110) LaAlO3 by a metal-organic chemical vapor deposition process employing Ba(hfa)2.mep, Ca(hfa)2.tet. and Cu(dpm)2 (hfa = hexafluoroacetylacetonate; dpm = dipivaloylmethanate; tet = tetraglyme; mep =methylethylpentaglyme) as the volatile metal sources. A subsequent phase-selective annealing procedure accomplishes thallination using TIF in a bulk oxide pellet. The resulting films are nearly phase-pure and highly oriented and contain negligible fluoride by windowless energy-dispersive x-ray measurements. The films exhibit transport measured Tc = 103 K and Jc > 104 A/cm2 (77 K. 0 T) and at low temperatures retain Jc > 105 A/cm2 (5 K, 4.5 T) as measured by magnetic hysteresis.

MOCVD routes to Tl-2212/MgO/Tl-2212 trilayers — preliminary observations on growth and microstructural/electrical properties

Abstract The suitability of Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) and MgO for growing SIS trilayers by metal-organic chemical vapor deposition (MOCVD) is examined. Low temperature (500°C) epitaxial (100) MgO film growth on (110) LaAlO 3 by MOCVD using Mg(dpm) 2 (dpm=2.2.6.6-tetramethyl-3,5-heptanedionate) is demonstrated. Tl-2212 does not decompose under these MgO growth conditions, although decomposition occurs at lower O 2 partial pressures. To form trilayer structures. BaCaCuO(F) films are first grown on (110) LaAlO 3 by MOCVD using Ba(hfa) 2 · mep (hfa=1,1,1,5,5,5-hexafluoropentane-2,4-dionate; mep=2,5,8,11,14,17-hexaoxanonadecane), Ca(hfa) 2 tet (tet=2,5,8,11,14-pentaoxapentadecane) and Cu(dpm) 2 precursors and are then post-annealed in the presence of bulk Tl 2 Ba 2 CaCu 2 O N to form Tl-2212. MgO is then deposited by MOCVD. Finally, a top TBCCO layer is grown by MOCVD and subsequent Tl 2 O anneal to complete the trilayer. Both of the TBCCO layers are highly oriented, while the MgO layers are largely polycrystalline due to the rough underlying Tl-2212. Both TBCCO layers are superconducting ( T c ∼ 105 K).