Dikko, Umar and Adamopoulos, George (2023) Solution-Processed Metal Oxide Gate Dielectrics and Their Implementations in Zinc Oxide Based Thin Film Transistors. PhD thesis, Lancaster University.
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Abstract
Thin-film transistors (TFTs) based on oxide semiconductors are a promising technology for a host of large-volume electronic applications. Whilst progress on solution-processed oxide semiconductors has been rapidly advancing, research efforts towards the development of new dielectrics has been relatively slow, with most of the reported work performed using conventional dielectrics based in SiO2. As a result, the majority of oxide transistors reported to date operate at relatively high voltages and hence consume significantly more power. In order to circumvent this bottleneck, recent work has been focussing on the development of low-voltage oxide transistors, including the use of high-k dielectrics, and several candidates have already been investigated and were mostly deposited by costly vacuum-based techniques. This thesis investigates the properties of high-k metal oxides dielectrics as well as their implementation in TFTs, deposited by spray pyrolysis, a simple and versatile technique that combines high yield and large area compatibility. In particular, the structural, optical, surface and electronic properties of tantalum aluminate (TaAlOx), hafnium titanate (HfTiO4) and zirconium silicate (ZrSiO4) were studied as along with their performance as gate dielectric for TFTs implementing ZnO semiconducting channels. In all cases, stochiometric TaAlOx, HfTiO4 and ZrSiO4 films deposited at < 550 °C were found to be amorphous with surface roughness of < 1 nm. The optical bandgap varies between 4.9 eV and 8.8 eV, 5.8 eV and 3.8 eV, and 5.8 eV and 8 eV for TaAlOx, HfTiO4 and ZrSiO4 films respectively. Their dielectric constant values vary between 24 and 7, 14 and 60, and 23 and 4.2 while their leakage current density at 1 MV/cm were between 10^-6 A/cm^2 and 10^-10 A/cm^2, 10^-7 A/cm^2 and 10 A/cm^2, and 10^-5 A/cm^2 and 10^-4 A/cm^2 respectively. Particularly, the stoichiometric TaAlOx, HfTiO4 and ZrSiO4 films exhibited the bandgap of 5.4 eV, 4.4 eV, 6.1 eV, dielectric constant of 13, 30, 12 and leakage current density at 1 MV/cm of 10^-8 A/cm^2, 0.3 A/cm^2, 10^-7 A/cm^2 respectively. The performance of ZnO – based TFTs employing stoichiometric TaAlOx, HfTiO4 and ZrSiO4 gate dielectric showed promising characteristics such as low voltage operation of 4 V, high electron mobility of 16 cm^2/Vs, 7 cm^2/Vs, 57 cm^2/Vs, high current modulation ratio of 10^5, 10^7, 10^6, low subthreshold swing of 0.56 V/dec, 0.17 V/dec, 0.28 V/dec, interface trap density of 7.7x10^12 cm^-2 ,2.1x10^12 cm^-2, 10^13 cm^-2 and threshold voltage of 3.2V, 0.6V, 0.1 V respectively. In addition, the effect of post-deposition annealing (at 800 °C for 30 mins in air) on HfTiO4 films were investigated. Stochiometric HfTiO4 films were crystalline of an orthorhombic structure, surface roughness of 1.95 nm, optical bandgap of 4.36 eV, dielectric constant of 38 and leakage current density of 5 mA/cm^2 at 1 MV/cm. These remarkable findings significantly demonstrated the achievement of a high- performance high-k metal oxide gate dielectrics as alternatives to the conventional SiO2 for future integration into wide areas of electronic application.