Transparent thin film transistors (TFTs) have been the subject of extensive scientific research over the last couple of decades, for applications in displays and imaging, as their implementation in active-matrix liquid crystal displays backplanes is expected to improve their performance in terms of switching times and stability. To this end, several material systems have emerged as contenders to address this need for a high performance, low power, large-area electronics i.e. thin film silicon, organic semiconductors and metal oxides. The electronic limitations of thin film silicon are well documented, and although organic semiconductors have seen significant improvements in recent years, their persistent low mobility and instability means that they are unlikely to progress beyond niche applications.
This thesis is focused on the investigation of the physical properties of metal oxides and their implementation in TFTs. Metal oxide based TFTs were fabricated by spray pyrolysis, a simple and large-area-compatible deposition technique.
More precisely, the implementation of titanium-aluminate and niobium-aluminate both wide band gap and high-k gate dielectric metal oxides in solution processed ZnO-based TFTs was studied and high performance, low operational voltage devices were fabricated. ZnO-based TFTs employing stoichiometric Al2O3-TiO2 (k~13, Eg~4.5 eV) or Nb2O5-Al2O3 (k~13.5, Eg~5.1 eV) as gate dielectric exhibited low leakage currents, high on-off current modulation ratios, high field-effect mobilities and low subthreshold voltage swings.
Furthermore, the implementation of solution-processed crystalline indium-zinc oxide (c-IZO) as active channel material in TFTs was equally investigated and high-performance c-IZO-based TFTs employing Al2O3 were fabricated. The effects of metal cation doping in c-IZO matrix were investigated in particular, and c-IZO:X (X:Ga,Y,Zr,Nb) based TFTs were fabricated and their properties were assessed for each dopant. Amongst them, Yttrium doped c-IZO (c-YIZO)-based TFTs exhibited the best performance in terms of low off-state currents, high field-effect mobilities and low subthreshold voltage swings.