title: Approaching metal oxide high-k dielectrics and semiconductors by solution-processing of molecular precursors
Creators: name: Koslowski, Nico
Publisher: name: UNSPECIFIED
Year: 2021

description: Functional metal oxide semiconductors (MOS) and high-k dielectrics possess a tremendous potential to replace the conventional silicon/silicon dioxide material combination for next-generation electronics, due to the realization of low-voltage operations and enhanced TFT performance characteristics. Thereby, solution-processing enables a low-cost and large-area fabrication of metal oxide based thin-film transistors as well as a simple approach to fine-tune the electrical properties by the modification of the materials composition and improvement of the processing conditions. Since the employment of frequently used precursors such as metal salts usually require high decomposition temperatures as well as the addition of stabilizers or other additives, we synthesized well-defined single-source molecular precursors in the course of this thesis, in order to achieve high quality functional metal oxide thin films. These precursors display various desirable chemical properties including air-stability, high solubility in aqueous or organic solvents, the requirement of reduced decomposition temperatures and the ability to process via photoactivation and thus certainly could help to cope with the industrial demand for future flexible electronics. During this thesis, the low-temperature solution-processing of dielectric AlxOy and YxOy thin films was achieved by the employment of the precursor compounds tris[(diethyl-2-nitromalonato)]aluminum (Al-DEM-NO₂) and bis[(diethyl-2-nitromalonato)]nitrato yttrium (Y-DEM-NO₂), possessing nitro-functionalized diethylmalonato ligands, which are enhancing the exothermic decomposition behavior. Capacitors processed at 250 °C already display decent dielectric performance characteristics and slightly elevated temperatures of 350 °C result in capacitors possessing excellent dielectric properties with dielectric constants (k) of 11.9 and 14.9, respectively as well as very low leakage current densities of J ˂ 10⁻⁹ A cm⁻² at 1 MV cm⁻¹ and satisfying electrical breakdown fields EB &gt; 2 MV cm⁻¹. Consequently, the formation of the respective metal oxide thin film allows the fabrication of TFT devices, exhibiting decent electrical performance characteristics, when combined with a solution-processed IZO semiconductor and processed at 350 °C. TFTs based on the amorphous AlxOy dielectric are exhibiting a field-effect mobility µₛₐₜ of 7.1 cm² V⁻¹ s⁻¹, threshold voltage Vₜₕ of 8.7 V and a current on/off ratio Ion/off of 1.4 × 10⁵, while TFTs based on YxOy thin films exhibit a µₛₐₜ of 2.1 cm² V⁻¹ s⁻¹, Vₜₕ of 6.9 V and Ion/off of 7.6 × 10⁵. Furthermore, the utilization of the malonato precursors of aluminum and yttrium enabled the successful synthesis of various compositions of the amorphous ternary yttrium aluminium oxide dielectric YAlxOy. Thereby, the inclusion into the AlxOy host lattice reaches its limit at an incorporation of 30 mol-% yttrium (30-YAlxOy). As a result, the 30-YAlxOy composition exhibits the best overall dielectric performance, among the investigated compositions, with k=12.5, J=1.1 x 10⁻⁹ A cm⁻² at 1 MV cm⁻¹, EB &gt; 3.7 MV cm⁻¹ and displaying almost no capacitive frequency dispersion. Consequently, the implementation of the 30-YAlxOy dielectric results in TFT devices, exhibiting good electrical performance characteristics with a µₛₐₜ of 2.6 cm² V⁻¹ s⁻¹, Vₜₕ of 12.4 V and Ion/off of 1.8 x 10⁷. Besides, the malonato precursors were also investigated towards their potential for the photoactivation of metal oxide dielectrics. As a result, the Al-DEM-NO₂ precursor undergoes the desired photolytic decomposition by employing irradiation in the deep-UV range (λ=160 nm). The resulting AlxOy dielectric thin films, processed at 150 °C, possess excellent dielectric properties (k=9.0, d=52 nm, J=1.7 x 10⁻⁹ A cm⁻² and EB=4.1 MV cm⁻¹) and thus are feasible for the deposition on plastic substrates compatible with flexible electronics. We could also successfully demonstrate the aqueous combustion synthesis of dielectric metal oxide thin films by employing well‐defined urea nitrate coordination compounds of aluminium and yttrium with the compositions [Al(CH₄N₂O)₆](NO₃)₃ and [Y(CH₄N₂O)₄(NO₃)₂](NO₃). Thereby, capacitors based on the amorphous AlxOy, exhibit a very high areal capacity value of 184 nF cm⁻² and very low leakage current density of J ˂ 10⁻⁹ A cm⁻² at 1 MV cm⁻¹, processed at a moderate PDA temperature of 250 °C, while YxOy thin films only start to perform at processing temperatures T ≥ 300 °C, in accord with the respective DSC analysis confirming the exothermic decomposition of the precursors, due to the urea-nitrate “fuel-oxidizer” reaction of the metal nitrates and urea molecules. Additionally, this approach was transferred to the generation of multinary amorphous semiconducting IGZO, by employing urea-nitrate compounds of indium, gallium and zinc. As a result, IGZO-based TFTs show an active TFT performance at processing temperatures as low as 200 °C. TFTs annealed at 300 °C display good device performance characteristics with a µₛₐₜ of 1.7 cm² V⁻¹ s⁻¹ and Ion/off &gt;10⁷. Furthermore, we investigated indium-free, semiconducting ZTO thin films, by introducing the novel Sn(II)-oximato precursor in combination with the established Zn-oximato precursor. As a result, EPR spectroscopy reveals a higher defect concentration for SnO₂ in comparison to ZnO and thus a precursor ratio with higher Sn content of Sn:Zn = 7:3 is necessary to obtain the optimum overall TFT performance with µₛₐₜ of 5.18 cm² V⁻¹ s⁻¹, Vₜₕ of 7.5 V and Ion/off of 6 x 10⁸, processed at 350 °C.