Trends in precursor design for conventional and aerosol‐assisted CVD of high‐Tc superconductors

Abstract

Conventional MOCVD techniques require molecules displaying volatility and constant vapor pressure. Metal oxide precursors, i.e., b-diketonates, or classical or functionalized metal alkoxides are mostly solids. The various approaches used to tailor volatility are discussed with barium derivatives as an example. The relationships between sublimation temperature and molecular weight suggest that volatility can be optimized on the basis of molecular weight.Aerosol-assisted CVD (AACVD) can use a larger range of precursors since volatility is no longer crucial. The solvent is an undesired ballast in a CVD process. High solubility of the precursors in the selected solvent is thus desirable. 'Stability' here includes the absence of precipitation which would change the stoichiometry of the feed solution for multicomponent oxides. Precipitation is often promoted by hydrolysis; stability toward moisture is thus desirable. The use of mixtures of precursors based on different ligands (b-diketonates, bketoesterates, alkoxides) can lead to ligand exchange reactions giving homometallic species, sometimes of low solubility, or mixed-metal species by self-assembly, thus improving solubility and stability toward moisture. These aspects are illustrated in compositions related to high-T c superconductors.

Processing and Applications of Aerosol‐Assisted Chemical Vapor Deposition

“…[10] In general, criteria for the selection of solvents for atomization are high solubility of the precursor, low vapor pressure, and low viscosity.…”

“…Indeed, AACVD techniques such as aerosolassisted metal-organic (AA-MO) CVD [31][32][33][34] and aerosol-assisted plasma (AA-P) CVD [5] were originally established to improve upon classical CVD processes. [10] Besides metal-organic com-pounds, metal salts, including nitrates, chlorides, and acetates, have been used as precursor chemicals in AACVDbased processes. AACVDbased methods can use a wider range of precursors since volatility is no longer crucial.…”

“…[98] Varying and unique surface morphology of the films can be obtained by changing the solvent of the precursor. [95] For example, ZnS/Mn films were deposited from Mn 2 (CO) 10 and Zn(SOCC(CH 3 ) 3 ) 2 TMEDA at 300°C with the controlled stoichiometry of Mn dopant levels at 1-8 %. [102] Ternary chalcogenide films were synthesized from a mixture of precursor chemicals with control over the composition by using; 1) precursors with matched reactivity, 2) aerosol-assisted delivery of precursor solution, and 3) a feed-rate-limited deposition condition.…”

SYNTHESIS, CHARACTERIZATION AND THERMAL BEHAVIOUR OF CALCIUM AND STRONTIUM 2, 2, 6, 6-TETRAMETHYL-3,5-HEPTANEDIONATE COMPLEXES. CRYSTAL STRUCTURE OF [Sr(μ, η2-thd)(η2-thd)(η2-MeOC2H4OH)]2

“…The two metals (Sr...Sr distance of 4.039(2) A) are connected via two thd Iigands which are bridging by only one oxygen atom. 25 The methoxyethanol ligand is displaced by diglyme and [Sr 2 (thd)4(diglyme) 2 (H-H 2 0)] (IR and 'H NMR evidences) is crystallized out quantitatively from an hexane solution of 1 containing ten equivalents of diglyme (solvents of commercial quality as for aerosol assisted CVD). By contrast, in the barium dimers [Ba(thd) 2 L] 2 (L = HOCHMeCH 2 NMe 2 , 18 2,2'-dipyridyl, 19 Et 2 0 20 ), both oxygen atoms are involved in bridging.…”

Tetraethylene Glycol Adducts of Alkaline Earth Halides

“…[17] The use of polyethylene glycol for the sol-gel and colloid synthesis of metal complexes, which are used as precursors in MOCVD techniques or spray pyrolysis has been described. [18][19][20][21] For these reasons we decided to investigate alkaline earth metal halide adducts of polyethylene glycols. These monotopic ligands are expected to give good precursors for testing i) the complexation and ii) the synthesis of oxidic materials.…”