A novel high-k gate dielectric material, i.e., hafnium-doped tantalum oxide (Hf-doped TaOx), has been studied for the application of the future generation metal-oxidesemiconductor field effect transistor (MOSFET). The film's electrical, chemical, and structural properties were investigated experimentally. The incorporation of Hf into TaOx impacted the electrical properties. The doping process improved the effective dielectric constant, reduced the fixed charge density, and increased the dielectric strength. The leakage current density also decreased with the Hf doping concentration. MOS capacitors with sub-2.0 nm equivalent oxide thickness (EOT) have been achieved with the lightly Hf-doped TaOx. The low leakage currents and high dielectric constants of the doped films were explained by their compositions and bond structures. The Hf-doped TaOx film is a potential high-k gate dielectric for future MOS transistors. A 5 àtantalum nitride (TaNx) interface layer has been inserted between the Hf-doped TaOx films and the Si substrate to engineer the high-k/Si interface layer formation and properties. The electrical characterization result shows that the insertion of a 5 àTaNx between the doped TaOx films and the Si substrate decreased the film's leakage current density and improved the effective dielectric constant (keffective) value. The improvement of these dielectric properties can be attributed to the formation of the TaOxNy interfacial layer after high temperature O2 annealing. The main drawback of the TaNx interface layer is the high interface density of states and hysteresis, which needs to be decreased. Advanced metal nitride gate electrodes, e.g., tantalum nitride, molybdenum nitride, and tungsten nitride, were investigated as the gate electrodes for atomic layer deposition (ALD) HfO2 high-k dielectric material. Their physical and electrical properties were affected by the post metallization annealing (PMA) treatment conditions. Work functions of these three gate electrodes are suitable for NMOS applications after 800°C PMA. Metal nitrides can be used as the gate electrode materials for the HfO2 high-k film. The novel high-k gate stack structures studied in this study are promising candidates to replace the traditional poly-Si-SiO2 gate stack structure for the future CMOS technology node.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 147-155). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Jack C. Lee. Vita. Includes bibliographical references. Available also from UMI Company.

The continual downscaling of the thickness of the SiO₂ layer in the complementary metal oxide semiconductor (CMOS) transistors has been one of the main driving forces behind the growth of the semiconductor industry for past 20-30 years. The gate dielectric works as a capacitor and therefore the reduction in thickness results in increase of capacitance and the speed of the device. However, this process has reached the limit when the further reduction of the SiO₂ thickness will result in a leakage current above the acceptable limit, especially for mobile devices. This problem can be resolved by replacing SiO₂ with materials which have higher dielectric constants (high-k). The leading candidates to replace SiO₂ as a gate material are hafnium dioxide and hafnium silicate. However, several problems arise when using these materials in the device. One of them is to find p and n type gate metals to match with the valence and conduction band edges of silicon. This problem can be rooted in lack of our understanding of the band alignment and its controlling mechanisms between the materials in the gate stack. Theoretical simulations using density functional theory can be very useful to address such problems. In this dissertation present a theoretical study of the band alignment between HfO₂ and SiO₂ interface. We identify oxygen coordination as a governing factor for the band alignment. Next, we discuss effects of Al incorporation on the band alignment at the SiO₂/HfO₂ interface. We find that one can tune the band alignment by controlling the concentration of Al atoms in the stack. We also perform a theoretical study of HfO₂/Metal interface in case of Rh. We identify Rh as a good candidate for a p-type gate metal due to its large work-function and the low oxidation energy. Finally, we report a study of the stability of oxygen vacancies across the gate stack. We model a gate stack composed of n-Si/SiO₂/HO₂/Rh. We find that oxygen vacancies are easier to create in SiO₂ than in HfO₂. Also, vacancies in HfO₂ modify the band alignment, while in SiO₂ they have no effect. / text

Compared with its inorganic counterpart, organic thin-film transistor (OTFT) has advantages such as low-temperature fabrication, adaptability to large-area flexible substrate, and low cost. However, they usually need high operating voltage and thus are not suitable for portable applications. Although reducing their gate–dielectric thickness can lower the operating voltage, it increases their gate leakage. A better way is making use of high-κ gate dielectric, which is the main theme of this research. Firstly, pentacene OTFTs with HfO2 gate dielectric nitrided in N2O or NH3 at 200 oC were studied. The NH3-annealed OTFT displayed higher carrier mobility, larger on/off current ratio, smaller sub-threshold swing and smaller Hooge?s parameter than the N2O-annealed device. All these advantages were attributed to more nitrogen incorporation at the dielectric surface by the NH3 annealing which provided stronger passivation of surface traps. The incorporation of lanthanum to hafnium oxide was demonstrated to realize enhanced interface in the pentacene OTFTs. Therefore, pentacene OTFTs with HfLaO gate dielectric annealed in N2, NH3, O2 or NO at 400 oC were investigated. Among the 4 devices, the NH3-annealed OTFT obtained the highest carrier mobility, smallest sub-threshold swing and smallest 1/f noise. All these should be attributed to the improved interface between the gate dielectric and the organic semiconductor associated with the passivation effects of the NH3 annealing on the dielectric surface. The processing temperature of OTFTs is a big concern because use of flexible or glass substrate is the trend in organic electronics. Therefore, the HfLaO gate dielectric was annealed in N2, NH3, or O2 at two different temperatures, 200 oC and 400 oC. For all the annealing gases, the OTFTs annealed at 400 oC achieved higher carrier mobility, which could be supported by SEM image that pentacene tended to form larger grains (thus less carrier scattering) on HfLaO annealed at 400 oC. Furthermore, the HfLaO film annealed at 400 oC achieved much smaller leakage because more thermal energy at higher annealing temperature could remove oxide defects more effectively. Fluorination of the HfLaO film (annealed in N2 or NH3 at 400 oC) in a plasma based on CHF3 and O2 was also proposed. For both annealing gases, the OTFT with a 100-s plasma treatment achieved higher carrier mobility and smaller 1/f noise than that without plasma treatment. All these improvements should be due to fluorine incorporation at the dielectric surface which passivated the traps there. By contrast, for longer time (300 s or 900 s) of plasma treatment, the performance of the OTFTs deteriorated due to damage of dielectric surface induced by excessive plasma treatment. Lastly, a comparative study was done on pentacene OTFTs with HfLaO or La2O3 as gate dielectric. For the same annealing gas (H2, N2, NH3, or O2 at 400 oC), the OTFT with La2O3 gate dielectric obtained lower carrier mobility, smaller on/off current ratio, and larger threshold voltage than that based on HfLaO. The worse performance of the OTFTs with La2O3 gate dielectric was due to the degradation of La2O3 film caused by moisture absorption. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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