Investigation of Surface Species and Reaction Mechanisms of Atomic Layer Deposition Processes Using In-Situ and Operando Surface-Enhanced Raman SpectroscopyPublic
Atomic layer deposition (ALD) is a gas-phase synthesis technique employed to manufacture thin films and metallic nanoparticles of various compositions and sizes, as well as individual, isolated species on various supports. An understanding of the dynamic surface chemistry that takes place during various ALD processes is vitally important for achieving high quality thin films, monodisperse nanoparticles, and well-defined isolated species on a multitude of supports. Previous characterization efforts on ALD processes have yielded information regarding growth rates, off-gassing products, film purity, and limited information regarding reaction mechanisms, thus paving the way for understanding what materials are feasible for ALD synthesis. There is still a gap, however, in the fundamental understanding of ALD processes and the surface species that facilitate these surface reactions. Surface-enhanced Raman spectroscopy (SERS), the in-situ spectroscopic technique employed herein, provides unique and valuable information involving surface species structure and thus provides a more complete fundamental understanding of ALD surface species and reaction mechanisms needed to fabricate sophisticated and well-defined materials. SERS is both surface sensitive and probes the lower wavenumber region (< 1000 cm-1) where structurally relevant vibrational modes that would otherwise be inaccessible occur. By utilizing a SERS that can (1) act directly as the nucleation site or (2) host nucleation sites near the plasmonic surface, one can perform ALD directly onto the surface and investigate the surface species via in-situ SERS. The work presented in this dissertation covers the investigation of surface species involved in two ALD processes, Al2O3 and TiO2 ALD. In the study of Al2O3 ALD, methylalumina species of a dimeric nature were observed on the surface for several cycles. Comparisons with an analogous chlorine-containing precursor showed the dimer to be stable under ALD conditions. The vibrational modes seen can be monitored for several cycles to analyze the distance dependence associated with the SERS substrate. In the TiO2 investigation, one cycle of ALD behavior and the corresponding species were observed via SERS. Subsequent TiO2 ALD cycles showed chemical vapor deposition (CVD) like behavior with unlimited growth of carbonaceous species.