Stability Studies of ALD Films and Infiltrated Hybrid Materials
Fairach, Selma Raquel
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Aluminum oxide (alumina) thin films deposited through atomic layer deposition (ALD) are of great interest in chemical barrier and corrosion protection applications. However, the stability of ALD alumina in aqueous solutions is still not fully understood. Due to its metastable amorphous phase, the hydration and degradation behavior of ALD alumina films behaves differently from its crystalline Al2O3 counterpart. A full understanding of why these films hydrate and/or dissolve requires the exploration of different deposition conditions and ion content in solutions used. This thesis will discuss efforts to further elucidate the hydration and dissolution behavior of ALD alumina films. For this study, alumina thin films were ALD deposited onto silicon substrate at 150 °C using trimethylaluminum (TMA) and H2O. These films were then studied in Type 1 DI water and different concentrations of NaCl solutions at room temperature. Films were gently dried using a nitrogen gun and thickness was measured using a Cauchy ellipsometry model. After 15 days of immersion in Type 1 DI water, significant thickness growth is observed at twice (27 days) and 2.5 times (33 days) the normalized thickness. Similar hydration is not observed in salt-containing aqueous solutions nor upon exposure to air. This thesis will discuss the possible effects of CO2 dissolution and carbonate formation as well as ionic species on the hydration and dissolution processes of these alumina films. Similar stability applications are observed in the infiltration of hybrid organic-inorganic electronic devices through vapor phase infiltration (VPI). Spiro-OMeTAD is one of the most-studied hole transport layer (HTL) materials in perovskite solar cells, but it is known to degrade quickly due to thermal effects as well as gold diffusion into the layer. This thesis will discuss how infiltration on TiCl4 and H2O into Spiro-OMeTAD layers can change the thermal properties and prevent early degradation. Upon infiltration of TiOx, the formation of crystals observed on Spiro-OMeTAD layers decrease, and at 10 hours of infiltration, the glass transition temperature of the film is found to decrease almost 15 °C. Similar behavior is not found in samples that are thermally controlled, indicating that this behavior is not an annealing effect. This thesis will discuss all the different thermal stability results with varying infiltration times and precursors used.