Doctoral Dissertations

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Ziling Xue

Committee Members

David Young, David Joy, Jamie Adcock

Abstract

This dissertation describes research on the synthesis of a tantalum silyl complex, chemical vapor deposition (CVD) of Ti-Si-N ternary thin films, and mechanistic pathways in the CVD processes to form the Ti-Si-N ternary thin films and zirconium carbide (ZrC). An overview of the Ph.D. research appears in Chapter 1.

Chapter 2 describes the preparation of a thermally unstable bis(alkylidene) complex (RCH2)4Ta2(=CHR)2(Cl)2 (R = SiMe3, 1) and kinetic studies of its conversion to an unstable alkylidene-alkylidyne complex (RCH2)3Ta2(=CHR)(≡CR)(Cl)2 (3). The synthesis and characterization of the first silyl bis(alkylidyne) complex (RCH2)(R3Si)Ta(µ-CR)2Ta(CH2R)2 (4) are also reported.

Chapter 3 presents CVD of Ti-Si-N ternary thin films from the reaction of Ti(NMe2)4 with SiH4. Analyses of the thin films by XPS (X-ray photoelectron spectroscopy), RBS (Rutherford backscattering spectrometry), and SEM (scanning electron microscopy) showed that the films likely contained TiN and Si3N4. The reaction between Ti(NMe2)4 and SiH4 at room temperature yielded H2, HSi(NMe2)3 and a black solid. The reaction mixture, after heating, gave powders which were found to be likely ternary materials containing TiN and Si3N4. The formation of these two solid compounds in both the CVD processes and room temperature reaction (followed by heating), rather than a single phase Ti-Si-N solid, TiSin-Si3N4 or TiSin-TiN, is discussed with the use of a Ti-Si-N ternary phase diagram.

The reaction between Ti(NMe2)4 and SiH4, and the mechanistic pathways to TiN and Si3N4 were further investigated. These studies are described in Chapter 4. The reactions of M(NMe2)4 (M = Ti, Zr, Hf) with H2SiR'2 (R'2 = HPh, Ph2, MePh) were found to give aminosilanes and metal amide hydride species. Hydride amide complexes [(Me2N)3M(µ-H)(µ-NMe2)2]2M [M = Zr (7); Hf (8)] were isolated from the reactions of M(NMe2)4 (M = Zr, Hf) with H2SiPh2 and characterized by NMR and single-crystal X-ray crystallography. Ad initio quantum chemical calculations on the mechanism of the reactions of Ti(NR2)4 (R = Me, H) with SiH4 supported the experimental observations that these reactions preferentially gave aminosilanes and metal hydride amide species. An equilibrium, (Me2N)6ZrSi(SiMe3)3 (5b) + H2SiPh2 = (MeSi3)3SiZr-(H)(NMe2)2 (6) + HSi(NMe2)Ph2, was observed. This equilibrium suggested that the reactions between MNR2 and H-Si bonds to give M-H and Si-NR2 bonds were reversible. The results here indicated that the role of SiH4 in its reaction with Ti(NMe2)4 was to remove amide ligands as amidosilanes. The removal of amide ligands might be incomplete, and thus the reaction gave Ti amide hydride species as black solids. Subsequent heating of the black solids and aminosilanes perhaps yielded TIN and Si3N4, respectively, as the Ti-Si-N materials.

The mechanistic studies of CVD of ZrC from Zr(CH2CMe3)4 (11b) are described in Chapter 5. Ab initio MO calculations predicted that, unlike its titanium analogue, the first step in the thermolysis of Zr(CH2CMe3)4 (11b) was a γ-hydrogen abstraction reaction. Experimental studies were designed to confirm the theoretical prediction. Analyses of the volatile products in CVD of ZrC from Zr(CH2CMe3)4 (11b) and Zr(CD2CMe3)4 (11b-d8) showed that the major products were neopentane and isobutene in a ratio of 2.3 : 1. In addition, the ratio of neopentane-d2/neopentane-d6 produced in the CVD involving Zr(CD2CMe3)4 (11b-d8) was ca. 4.9. These results are discussed in terms of y-hydrogen abstraction as the preferred first step in the thermolysis of Zr(CH2CMe3)4 (11b).

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