Solution-Processed Metal Oxide Gate Dielectrics and Their Implementations in Zinc Oxide Based Thin Film Transistors
Creators: name: Dikko, Umar
Publisher: name: Lancaster University
Year: 2023

description: 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 &lt; 550 °C were
 
 found to be amorphous with surface roughness of &lt; 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.