Bruce J. Hinds

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

Metal‐organic chemical vapor deposition/open flow thallium annealing route to epitaxial Tl2Ba2Ca2Cu3O10 thin films

Phase‐pure epitaxial Tl2Ba2Ca2Cu3O10 thin films have been grown on single crystal (110) LaAlO3 substrates using an improved metal‐organic chemical vapor deposition process. First, Ba‐Ca‐Cu‐Ox precursor films are grown on LaAlO3 (110) substrates using Ba(hfa)2⋅tet, Ca(hfa)2⋅tet, and Cu(hfa)2 (hfa=hexafluoroacetylacetonate; tet=tetraglyme) as volatile metalorganic precursors. Thallium is then incorporated into the films during a post‐anneal in the presence of a Tl2O3, BaO, CaO, CuO powder mixture at 820 °C for 12 h in a flowing 10% O2/Ar atmosphere. The films have a transport‐measured Tc=115 K and Jc=1.5×105 A/cm2 (80 K), while magnetic hysteresis measurements yield Jc=6×105 A/cm2 (77 K). Preliminary surface resistance measurements give Rs=0.35 mΩ at 5 K, 10 GHz.

In‐plane orientation control of (001)YBa2Cu3O7−δ grown on (001)MgO by pulsed organometallic beam epitaxy

Thin films of (001) YBCO are grown on epitaxially polished (001) MgO by pulsed organometallic beam epitaxy. The in‐plane orientation of the film is controlled by the thickness of a BaO layer, grown in situ, prior to the YBCO growth. For thin BaO layers (<≊7×1014 Ba/cm2) the films grown [110]YBCO∥[100]MgO. For thick BaO layers (≳≊11×1014 Ba/cm2) the films grow [100]YBCO∥[100]MgO. A mechanism that relates the change in YBCO in‐plane orientation to a change in the structure of the initial BaO layers with BaO thickness is described.

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).

Transmission electron microscopic characterization of metal-organic chemical vapor deposition-derived superconducting Tl2Ba2Ca1Cu2Oχ thin films on Au substrates: Evidence for Au-Cu alloy formation and texturing on a nonplanar metal substrate

Abstract The microstructure of superconducting Tl 2 Ba 2 Ca 1 Cu 2 O χ thin films grown on Au substrates by metal-organic chemical vapor deposition (MOCVD) has been investigated. Characterization techniques included X-ray diffraction, variable temperature magnetization, scanning and transmission electron microscopy (SEM and TEM, respectively) and energy dispersive X-ray spectroscopy (EDX) measurements. The films exhibit a magnetically derived T c ≈80 K and a high degree of texturing with the crystallite c -axis oriented perpendicular to the Au substrate surface as evidenced by enhanced (00 l ) X-ray diffraction reflections. SEM/ EDX and TEM/EDX provide evidence of Au-Cu alloy formation but no Au diffusion into the film. The superconducting grains exhibit good compositional uniformity and excellent alignment with the substrate, even though the Au substrates were not smooth and exhibited a moderate density of steps.

Materials for superconducting electronics: In situ growth of PrGaO3 thin films by metalorganic chemical vapor deposition

Phase‐pure thin films of the YBCO, BSCCO, TBCCO lattice‐matched and low dielectric‐loss perovskite insulator PrGaO3 have been grown in situ on single‐crystal (110) LaAlO3 , (001) SrTiO3 , and (001) MgO substrates by metalorganic chemical vapor deposition (MOCVD). Films were grown at temperatures between 750 and 800 °C using the volatile metalorganic β‐diketonate precursors M(dpm)3 (M=Pr, Ga, and dpm=dipivaloylmethanate). As assessed by x‐ray diffraction, the films grow epitaxially on LaAlO3 and SrTiO3 substrates with a high degree of (001) and/or (110) plane orientation parallel to the substrate surface. The films grown on MgO substrates are polycrystalline. Scanning electron microscopy reveals that the MOCVD‐derived PrGaO3 films have smooth, featureless surfaces on all three kinds of substrates.

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.

The growth of (001) YBa2Cu3O(7−δ) thin films on (001) MgO by pulsed organo—metallic beam epitaxy with controlled in-plane orientation

Abstract Thin films of (001) YBCO are grown on epitaxially polished (001) MgO by pulsed organo–metallic beam epitaxy. The in-plane orientation of the film is controlled by the thickness of a BaO layer, grown in situ, prior to the YBCO growth. For thin BaO layers ( ≈1.1×1015 Ba cm−2) the films grow [100]YBCO|[100]MgO. For YBCO films grown with thin BaO layers (>≈0.7 times 1015 Ba cm−2), ex situ, low energy Ar+ ion sputtering of the MgO surface prior to film growth will also induce a change of in-plane orientation from [110]YBCO|[100]MgO on epitaxial polished MgO to [100]YBCO|[100]MgO on Ar+ sputtered MgO. A mechanism that relates the change in YBCO in-plane orientation to a change in the structure of the initial BaO layers with BaO thickness is described.

Characteristic doping-dependent properties of HTS cuprate thin films prepared via MOCVD

Abstract Thin film samples of the Bi-2212 and Tl-2212 high- T c superconductors were prepared via metal-organic chemical vapor deposition with second generation precursors. The resulting epitaxial, c -axis oriented films were used for measurements of the in-plane resistivity versus temperature and room temperature thermoelectric power. The results for Bi-2212 thin films followed the established doping-dependent trends seen in other cuprate oxides and preliminary data for the Tl-2212 films show similar features as well. The results are compared to a generic normal state phase diagram for the cuprates.

The Influence of Weak Links and Oxygen Deficiency on Electrical Properties of Bi-2212 and Ti-2212 Hts Thin Films

Thin films of the Bi-2212 and T1–2212 compounds were prepared by MOCVD deposition techniques. Resistivity versus temperature and critical current density measurements were used to characterize the electrical properties. An analysis of the data based on a proposed model determined the influence of intragranular weak links. Thin film samples in both systems with near optimum oxygen doping showed a correlation between the slope and magnitude of the resistivity in the normal state. Samples with reduced oxygen content displayed a strong increase in the intragrain boundary resistance, consistent with weak link defects. The results agree with a similar analysis of YBCO samples and support a common mechanism for the development of weak links in cuprates.