Printed and low-temperature-processed indium oxide thin-film transistors for flexible applications: Dissertation

The convergence of printing and microelectronic technologies, often called printed electronics, is best embodied in printed thin-film transistor (TFT) semiconductor devices. TFTs, that are the key components in displays and flat panel X-ray sensors, are conventionally fabricated on rigid substrates from amorphous silicon (a-Si) using vacuum-processes and a high process temperature (> 250 °C). With the existing market pull for flexible and high-resolution organic light emitting diode (OLED) displays, novel semiconductor materials, such as organic and metal oxide (MO) semiconductors, are being developed to yield TFTs with flexibility and electrical performance beyond that of a-Si. Organic TFTs (OTFTs) can be printed and processed at low-temperature (<150 °C) on flexible substrates, whereas MO TFTs, that readily provide superior performance to both OTFTs and a-Si TFTs, are either vacuum-processed or require high-temperature processing (> 300 °C) when they are solution-processed. The thesis work focuses on the fabrication of MO semiconductors using printing processes and includes material, ink, and process development, as well as fabrication, characterization, and modelling of the printed MO TFT devices. We show in Publication [I] that thin, printed In2O3 layers can be used in enhancement-mode TFTs when the devices are stabilized using a post-contact annealing step at low-temperature. Moreover, we demonstrate, for the first time, that flexographyprinted In2O3 layers on flexible plastic substrate can be used in TFTs whose performance is beyond that of a-Si TFTs or printed OTFTs. In order to lower the annealing temperature of the MO materials, Publication [II] and Publication [III] introduce a low-temperature annealing method where low-wavelength far ultraviolet (FUV) exposure and thermal annealing are combined to reach the processing temperature (~150 °C) for an inkjet-printed In2O3 semiconductor that is compatible with low-cost plastic substrates. Finally, Publication [IV] demonstrates that high-gain depletion-load inverters can be fabricated via inkjet printing by exploiting the thicknessdependent electrical properties of the In2O3 semiconductors. In summary, this thesis demonstrates that MO semiconductors can be deposited using industrially-relevant printing processes and processed at low-temperature on flexible substrates. This could lead, in the future, to the use of printed MO TFTs in flexible applications, such as biosensors, flexible displays, large-area sensors, and integrated and radio-frequency circuits. The potential of these applications is also analysed in this thesis.