Self-Assembled Nanodielectrics and Combustion Processed Amorphous Metal Oxides as Unconventional Materials for Thin-Film Transistors

Stallings, Katie Lynn

doi:https://doi.org/10.21985/n2-7rtb-5n48

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Self-Assembled Nanodielectrics and Combustion Processed Amorphous Metal Oxides as Unconventional Materials for Thin-Film Transistors

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The demand for low cost, unconventional electronics requires new materials with unique characteristics that the traditionally used silicon-based technologies cannot provide. Metal oxide semiconductors, such has amorphous indium gallium zinc oxide (a-IGZO), have made impressive strides as alternatives to amorphous silicon for electronics applications. However, to achieve the full potential of these semiconductors, compatible unconventional gate dielectric materials must also be developed. To this end, solution-processable self-assembled nanodielectrics (SANDs) comprised of structurally well-defined and durable nanoscopic alternating organic (e.g., stilbazolium) and inorganic oxide (e.g., ZrOx, HfOx) layers offer impressive capacitances and low processing temperatures (T ≤ 200 °C). Amorphous metal oxides have already been utilized in manufactured displays, but conventional processing techniques require expensive high vacuum deposition or high annealing temperatures that are incompatible with low-cost plastic substrates. The combustion process was developed to provide a low temperature solution-based fabrication process. This research has focused on further developing these promising unconventional materials and processing method.In Chapter 2, the role of the organic layer component in the SAND structure is explored. SANDs are compatible with a wide variety of semiconductors and often enable superior thin-film transistor (TFT) device metrics in comparison to analogous inorganic dielectrics. This has been partly attributed to the interactions of the organic layers. To help determine the role of the stilbazolium derived organic layer (Chr) previously used, a new hydrocarbon chain based self-assembly molecule (Alk) was developed. By using Alk and Chr in differing organic layers, the effects of the highly polarizable Chr molecule’s built-in dipole on the overall SAND characteristics is better understood. The different layer identity and arrangement of the organic layers within the Zr-SAND structure is found to have a significant impact on the leakage behavior of the capacitors and the threshold voltage/turn-on voltage of pentacene transistors. Evidence of interactions between adjacent Chr organic layers enhancing these effects is also observed. Chapter 3 demonstrates the capabilities of SANDs to be applied to more complex device architectures. While SANDs have been paired with diverse semiconductors and yielded excellent device metrics, they have never been implemented in the most technologically relevant top-gated thin-film transistor (TFT) architecture. Here we combine solution-processed a-IGZO with solution-processed four-layer Hf-SAND dielectrics to fabricate top-gated TFTs, which exhibit impressive electron mobilities (µSAT = 19.4 cm2 V-1 s-1) as well as low threshold voltages (Vth = 0.83 V), subthreshold slopes (SS = 293 mV/dec), and gate leakage currents (10-10 A). Chapter 4 examines the performance and thermal stability of indium gallium oxide fabricated by pulsed laser deposition (PLD), and the solution-based combustion synthesis for both spin-coated and spray-coated films over a range of compositions. To leverage the existing knowledge base on amorphous oxides, this study directly compares one of the most promising solution processing techniques, combustion synthesis, with a more established PVD growth technique, pulsed laser deposition (PLD). The roles of processing technique and composition are understood by coupling structural studies, including analysis of local bonding and density, with existing literature of PVD amorphous oxide films. This work represents the first example of X-ray absorption spectroscopy analysis of spray combustion processed (Spray-CS) a-oxide films. A drop in TFT saturation mobility, with increasing Ga content, for PLD and spin-coated combustion processed (Spin-CS) devices is understood to be the disruption of carrier mobility imposed by the disparate local structure around Ga, as opposed to around indium. In contrast, saturation mobility for Spray-CS films is dominated by the processing method and cannot be tuned with composition. Similar to mobility trends, TFT on voltage rises with Ga content for Spin-CS and PLD-derived channel layers. Local structure studies reveal that the Spray-CS growth technique results in high In-O coordination levels, which explains the low carrier concentrations and resultant on voltage behavior. This work investigates, for the first time, the structural phase stability of solution-processed oxides through in situ glancing incidence X-ray diffraction annealing studies. Thermal phase stability, which informs processing parameters and device stability, is shown to increase with Ga content for all films.

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