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., β‐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 (β‐diketonates, β‐ketoesterates, 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‐Tc superconductors. Novel copper, yttrium, cerum(IV), barium‐copper, yttrium‐copper and praseodynium‐copper species are reported. © 1998 John Wiley & Sons, Ltd.

A Logical Concept of Structure Prediction Derived from Supramolecular Polymers of Alkaline Earth Metal Halides Formed by Hydrogen Bonding and Complexation of the Metal Ion

“…[6] In our group, we are especially interested in alkaline earth compounds as their application might be found in the wide area of, for example, high-temperature superconductors and thin film oxides. [7] Various methods are available for the deposition of thin films on a substrate, for example, MO-CVD (metal ± organic chemical vapor deposition), dipping, spraying, or spinning. [8] In this context, we decided to focus on the dimensionality of the polymers formed by alkaline earth metal halides with both i) a complexing, but otherwise innocent, polyether oxygen donor ligand, and ii) a complexing as well as hydrogen-bonding H 2 O ligand.…”

Synthesis and characterization of new alkaline earth metal β-ketoiminates. The first structurally characterized strontium β-ketoiminate

“…Films of such materials can be deposited by a variety of chemical methods, the most popular ones being the sol-gel process or metal-organic chemical vapor deposition (MOCVD) techniques. The precursors generally used for MOCVD of such films are b-diketonates [1][2][3], alkoxides or aryloxides [1,2], metallocenes [1,2], and bis(trimethylsilyl)-or alkylamides [1,3]. The use of ketoiminates of group 2 is limited to barium [4] and magnesium [5] derivatives, although a wide range of bketoimines can easily be prepared from b-diketones and various amines [6].…”

“…2 The asymmetric unit contains two independent molecules. One of them is located on a C 3 1 Syntheses have been performed under argon using Schlenk tubes and vacuum line techniques.…”

“…The compounds were non-volatile (up to 250°C/2 · 10 À4 Torr). 2 The crystal of 1 (0.37 · 0.37 · 0.35 mm) was obtained at r.t. from a slightly saturated toluene solution and mounted on a glass fiber with Paraton oil. Data were collected with a Nonius Kappa CCD diffractometer (scan type mixture of u rotations and x scans).…”

“…Those observed for the Ca derivative 2, namely two types of ketoiminate ligands in a 1:2 integration ratio, could be in agreement with a structure of type 2A probably more favorable for reducing the strain around the smaller central metal. The solubility of the Ca and Sr derivatives in organic solvents is very high, thus, although their volatility is poor, they might be considered as precursors for liquid injection MOCVD delivery systems for which volatility is no longer crucial [2].…”