Utilization of XANES to understand local electronic and geometric structure of ALD functional thin films [electronic resource]
Keywords: Chemistry.

With an ever increasing global demand for energy, it is paramount that energy conversion devices become more efficient or energy consuming devices use less power. The key to making these improvements is to gain a better fundamental understanding of materials. More specifically if one is able to arrive at the structural solution for a particular material, then its various properties could be predicted and tuned. To come to this level of understanding requires tools that are equally precise in the fabrication of the materials as they are in the characterization of them. Atomic layer deposition (ALD) is a deposition technique that can create highly conformal films of varying chemistries with sub-nm precision due to its self-limiting nature. Typically, these fabricated films are characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction (XRD), etc. If the films are amorphous, contain nano-crystallites or are buried in nanostructures, then even some of these characterization techniques are no longer feasible. X-ray absorption near edge structure (XANES) however, may still be used to gain detailed information of the electronic and geometric structure of the thin film. XANES is a powerful characterization technique capable of revealing oxidation states, coordination chemistry, molecular orbitals, band structures, local displacement and chemical short-range order information. It can be performed on samples that are in the solid, liquid, or gas phase and can be performed in a wide range of temperatures and pressures. In this thesis, the combination of these two techniques are used to gain unprecedented insight into several functional thin films. Together the two techniques allow one to control the bonding environment and finely characterize nanomaterials at the atomic level. I will show how this synergistic combination can lead to the fundamental understanding of properties and their link to processing parameters. Specifically, this thesis provides understanding of interface effects, composition, thickness and electrical properties as well as insight on reaction mechanisms revealed by novel in-situ measurements. For second generation solar cells, the price of electricity can be decreased either by increasing the efficiency or by increasing its lifetime. The cost of the solar cell is no longer a big advantage over the first generation Si solar cells. Si solar cell cost has gone down over the years and increases in efficiency are now negligible (the efficiency of single crystal Si solar cells sits at ≈25%). The most important parameter to reduce cost in a second generation and emerging solar cell technology is to increase the efficiency. For ALD lead sulfide (PbS) Quantum dots (QDs) in a Quantum dot sensitized solar cell (QDSSC) architecture we gain atomic level insight at junctions between PbS QDs and metal oxide nano-materials, learning that distortions away from a cubic structure induced by the interface, increases the band gap. Going into ternary oxides, we next explore Zn(O, S), a commonly used buffer layer in thin film solar cells (TFSC). Through composition, thickness, interface and ALD deposition sequence control, the role of each on efficiency will be elaborated. Not unlike in solar cells, charge transport also needs to be tuned in dielectric materials used in dynamic random access memory (DRAM) applications. To be a viable option for DRAM application the dielectric film has to be high-k, ultrathin, electrically insulating and have the ability to uniformly coat high aspect ratio structures. ALD of conventional group IV high-k metal oxides like ZrO2 and HfO2 won't be able to meet future requirements for the semiconductor industry. ALD BaTiO3 (BTO) is promising candidate due to its very high dielectric constant. Due to its amorphous nature, XANES was utilized to track changes in geometric structure as a function of cationic composition, where a structure property relationship was established to link its electrical behavior. Finally, the two techniques are combined for in-situ measurements where we study the semiconductor ZnS. In-situ ALD/XANES measurements were performed for the first time on this system, where our work led us to propose a new mechanism for ALD sulfides grown on oxide interfaces.