Thin films for superconducting electronics: Precursor performance issues, deposition mechanisms, and superconducting phase formation-processing strategies in the growth of Tl₂Ba₂CaCu₂O₈films by metal-organic chemical vapor deposition

Abstract

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 nm/min 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 °C in 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 × 105 A/cm2 at 77 K, and surface resistances as low as 0.4 mΩ (40 K, 10 GHz).

Synthesis and Characterization of Volatile, Fluorine-Free β-Ketoiminate Lanthanide MOCVD Precursors and Their Implementation in Low-Temperature Growth of Epitaxial CeO2 Buffer Layers for Superconducting Electronics

“…Film Growth Experiments. MOCVD of CeO 2 thin films was carried out in the low-pressure, horizontal, cold-wall MOCVD system described elsewhere . The metal−organic precursor and the reactant gas (100 sccm of O 2 ; sccm = standard cubic centimeters per minute) vapor streams were mixed immediately upstream of the reactor.…”

“…However, the success of an MOCVD process depends critically on the availability of volatile, thermally stable precursors which exhibit constant vapor pressure and the capacity to selectively form the desired phase at the substrate surface. Low-melting precursors are preferred because solid compounds can be handled with ease at room temperature, while the liquid form at reservoir operating temperatures affords constant surface area for stable vapor delivery to the reactor. − …”

“…Thus far, many growth techniques including MBE, , pulsed laser deposition, , sputtering, − and electron beam evaporation , have been utilized for CeO 2 film growth. However, as noted above, molecule-based metal−organic chemical vapor deposition (MOCVD) techniques offer many attractive features such as conformal coverage, ability to coat complex shapes, simplified apparatus, and lower growth temperatures for thin film growth and device fabrication. − Nevertheless, MOCVD has not been fully optimized for CeO 2 deposition. Although low-temperature film deposition is requisite for multilayer device fabrication, all CeO 2 MOCVD processes reported prior to this contribution have required very high growth temperatures (680−800 °C). ,,, Efforts have been made to improve these CeO 2 growth parameters by using special reactors and/or other methods; , however, high deposition temperatures are still required.…”

Analysis of the fluoride effect on the phase-selective growth of TlBa2Ca2Cu3O9−x thin films: Phase evolution and microstructure development

“…33 Although the 1223 pellet methods only produced films of Tl-2212, a slight reduction of the thallium content from the 1223 stoichiometry resulted in traces of Tl-2223, a comparatively thallium-poor phase. II, with the goal of producing phase-pure samples of Tl-1223.…”

“…2. Barium fluoride is frequently obtained as a byproduct of MOCVD processes employing fluorine-containing metalorganic precursors, 33 and for TBCCO phases, fluoride has been found to dissipate during high-temperature annealing, presumably via the reaction BaF 2 + Tl 2 O → 2TlF + BaO. 7.…”