Graduate Thesis Or Dissertation

Synthesis and characterization of aluminum oxide based materials - from molecule to device

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  • A scalable synthesis of the "flat" tridecameric inorganic cluster [Al₁₃(μ₃-OH)₆(μ-OH)₁₈(H₂O)₂₄]¹⁵⁺ has been realized by treating an aqueous aluminum nitrate solution with zinc-metal powder at room temperature. Single crystals and polycrystalline samples are readily obtained in yields exceeding 55% relative to the starting reagent Al(NO₃)₃. Products have been characterized by X-ray diffraction and solid-state ²⁷Al MAS and MQMAS NMR. Furthermore, we report a new integrated platform that combines: (i) an atom- & step-economical electrolytic synthesis of Al-containing nanoclusters in water with strict pH control; and (ii) an improved femtosecond stimulated Raman spectroscopic method covering a broad spectral range (350 to 1400 cm⁻¹), aided by ab-initio computations, to elucidate cluster structures and formation mechanisms of the clusters in real time. Using this platform, a new and unique view of flat [Al₁₃(μ₃-OH)₆(μ₂-OH)₁₈(H₂O)₂₄](NO₃)₁₅ cluster formation is observed, in which three distinct stages are identified. The first stage involves the formation of a hypothetical [Al₇(μ₃-OH)₆(μ₂-OH)₆(H₂O)₁₂]⁹⁺ structure as an important intermediate towards the flat Al₁₃. Once the scalable synthesis has been developed, aqueous solution precursor made from "flat" Al₁₃ clusters are used for depositing high quality aluminum oxide thin films. Film structure, morphology, composition, and density at different annealing temperature are characterized by X-ray diffraction, AFM, SEM, TEM, FTIR, and X-ray Reflectivity. Optical properties of the films are investigated by spectroscopic ellipsometry. Simple metal-insulator-semiconductor capacitor test structure is used to evaluate the dielectric properties of the aluminum oxide thin films. After annealing at 500 °C, thin film exhibits low leakage current density (< 10 nA·cm⁻² at 1 MV·cm⁻¹) and high breakdown field (> 6 MV·cm⁻¹). As a gate dielectric layer in thin film transistors with amorphous zinc tin oxide active channel, solution processed aluminum oxide layer exhibit dielectric properties similar to high quality SiO₂ gate dielectrics, i.e. low gate leakage current (nA level from -10 V to 30 V) and small clockwise hysteresis. Finally, thin film dielectric material Al(PO₄)₀.₆O₀.₆·xH₂O, or "AlPO" is examined to explore a low-temperature dehydration alternative for the solution-deposited aluminun-oxide based films. As an amorphous oxide insulator, AlPO has been incorporated into thin-film transistors (TFT) via aqueous processing. It is found that the films must be heated above 600 °C to force dehydration and eliminate the mobile protons that cause unstable device operation. Here, we suggest that this dehydration temperature is largely dictated by rearrangements and densification near the surface of the film, as it is heated. A considerable quantity of water (and associated ions) becomes physically trapped in the bulk of the film. High temperatures are then required to promote diffusion and water loss across this surface "crust". A hypothesis is that an appropriate very thin layer of a material having a lower dehydration temperature could be used to inhibit the densification and drying of AlPO in the near-surface region, thereby facilitating continuous water loss at relatively low temperature. Therefore, we choose solution-deposited HfO₂ films to alter the AlPO top surface. This material combination effectively decreases the dehydration temperature of AlPO (at about 250 °C), leading to dramatically changes in the dielectric behavior.
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