Open Access Dissertation
Date of Award
Chales H. Winter
Atomic Layer Deposition (ALD) is an advanced variant of Chemical Vapor Deposition (CVD) and is used to deposit smooth and conformal thin films for applications in the microelectronics industry. ALD requires metal precursors that are sufficiently volatile and thermally stable at elevated temperatures. Several group 5 ALD and CVD precursors were reported in the literature, which have been used to deposit TaN, NbN, Nb2O5, and Ta2O5 thin films. Synthesis of low and mid valent group 5 metal ALD precursors might give us access to deposit NbO2, Nb2O3, Ta2O3, and TaO2 thin films by low temperature ALD pathways upon applying mild oxidation conditions. To this end, attempts were made to synthesize niobium(IV) and tantalum(IV) complexes from pyrazolate, amidate, and α-iminoketone ligand systems. Pyrazolate compounds undergo reductive N-N bond cleavage on Nb(IV) and Ta(IV) metal centers and afford six membered metallacycles that are analogous to metallapyrimidines and metallapyrimidiniums. Two other pyrazolate ligands are found to be intact and the final oxidation state of the complexes were +5. Those metallacycles are nearly planer, conjugated, have 6 -electrons, and follow the Hückel formalism, hence formally aromatic. In order to estimate the aromaticity of those metallacycles against pyrazoles and pyrimidines, NICS calculations were performed. The metallacycles were found to be aromatic but only weakly aromatic than that of pyrimidines and pyrazoles.
In order to synthesize metal complexes that are in the +4 oxidation state, a novel class of Ta(IV) and Nb(IV) amidate compounds with a variety of substituent groups were synthesized and structurally characterized. All the compounds are monomeric and isolated by sublimation at temperatures between 110-160 C/0.05 Torr. Several compounds were crystallized in solution and X-ray crystal structures were obtained to address the structural features. TGA/DTA traces, melting points, and decomposition temperatures were used for the thermal stability assessment and preparative sublimation experiments were carried out to estimate the volatility properties. We have identified several compounds that are sufficiently volatile and thermally stable, which can be used as precursors for ALD to deposit group 5 metal oxide thin films.
Due to the fact that the limited availability of group 5 ALD precursors, we have synthesized niobium and tantalum compounds from α-iminoketonate ligands. The six-coordinate niobium and tantalum α-iminoketonate complexes were structurally characterized by X-ray crystallography and found that one of the α-iminoketone ligands undergo hydrogen abstraction at the α-carbon atom. Further evidence of hydrogen atom abstraction was obtained by 1H NMR experiments. In order to understand the reaction mechanism DFT calculations were performed on those compounds, several putative intermediates, and hypothetical gallium α-iminoketone analogs. The bond lengths of the possible intermediates and the experimental bond distances were compared and identified a plausible intermediate for the reaction. The resulted complexes were found out to be in the +5 oxidation state and contain two fully reduced α-iminoketone ligands and one iminoalkoxi ligand, which is a formal -radical monoanion.
Perera, Thuduwage Hiran, "Synthesis And Characterization Of Group 5 Metal Complexes Containing Pyrazolate, Amidate, And Related Ligands As Potential Precursors For Thin Film Growth By Atomic Layer Deposition" (2012). Wayne State University Dissertations. 612.