Thallium barium calcium copper oxide thin films for superconducting electronics: Precursor performance, deposition mechanisms, phase formation, and trilayer structures by metal-organic chemical vapor deposition

Epitaxial {dollar}rm Tlsb{lcub}m{rcub}Basb2Casb{lcub}n-1{rcub}Cusb{lcub}n{rcub}Osb{lcub}2(n+1)+m{rcub}{dollar} (m = 1,2 n = 2,3: TBCCO) high temperature superconducting thin films are grown in a two-step process involving metal-organic chemical vapor deposition (MOCVD) of a BaCaCuO(F) thin film on (110) LaAlO{dollar}sb3{dollar}, followed by a reaction with Tl{dollar}sb2{dollar}O vapor in a carefully controlled post-anneal. The first MOCVD-step relies on Ba(hfa){dollar}sb2cdot{dollar}mep (hfa = hexafluoroacetylacetonate, mep = methylethylpentaglyme), Ca(hfa){dollar}sb2cdot{dollar}tet (tet = tetraglyme), and Cu(dpm)2 (dpm = dipivaloylmethanate) metal-organic precursors, with volatility readily quantified in a novel reduced pressure thermogravimetric analysis (TGA) method. Precursors based on fluorinated {dollar}beta{dollar}-diketonates coupled with an ancillary polyether ligands (including the liquid Ba(hfa){dollar}sb2cdot{dollar}mep) are demonstrated to be superior to the commonly used Ba(dpm){dollar}sb2{dollar} in terms of volatility and temporal stability. The study of deposition kinetics for this precursor system show a mass-transport limited process at temperatures as low as 350{dollar}spcirc{dollar}C (20nm/min growth rate). An interesting ligand exchange process results in the formation of Cu(hfa){dollar}sb2{dollar}. However this is effectively impeded by the use of H{dollar}sb2{dollar}O in the oxidizer stream, allowing for a well controlled MOCVD process. To form TBCCO films, the MOCVD-derived BaCaCuO(F) films are post-annealed in the presence of bulk {dollar}rm Tlsb2Basb2CaCusb2Osb8{dollar} (Tl-2212) at temperatures of 720-890{dollar}spcirc{dollar}C in flowing atmospheres ranging from 0% to 100% O{dollar}sb2.{dollar} Films are characterized by XRD, SEM, TEM, profilometry, and transport measurements. Tl-2212 films have T{dollar}sb{lcub}rm c{rcub}{dollar} as high as 105K and {dollar}rm Jsb{lcub}c{rcub} = 1.2times 10sp5A/cm2{dollar} at 77K. Surface resistances as measured by parallel plate method are as low as 400{dollar}muOmega{dollar} (40K and 10 GHz). These Tl-2212 films are also incorporated into an MOCVD process to form Tl-2212/MgO/Tl-2212 trilayer structures utilizing Mg(dpm){dollar}sb2{dollar} as a precursor. The concept of this MOCVD process for TBCCO trilayers is demonstrated, but the inherent surface morphology of TBCCO underlayer results in shorting between superconductor layers. Finally the conditions for TBCCO thin film growth are studied in a 2-zone thallination anneal, where Tl{dollar}sb2{dollar}O pressure, O{dollar}sb2{dollar} pressure and growth temperature are independently controlled. The Tl-2212 growth/stability line is found, as well as the use of Ag doping is explored utilizing Ag(hfa)PMe{dollar}sb3{dollar} MOCVD precursor. Tl-1223 phase films are grown with T{dollar}sb{lcub}rm c{rcub}{dollar} as high as 113K.