The Study on Solution-Processed Oxide Thin-Film Transistors

Author:Li Yu Zhi

Supervisor:peng jun biao


Degree Year:2019





Thin film transistors(TFTs),as the basic components for electrical signal processing,control and transmission,are widely applied in emerging fields such as flat panel displays,flexible electronics and smart electronics.Metal oxide TFTs have received extensive attention in the past decade due to its advantages of high mobility(1-100 cm2 V-1 s-1),transparency to visible light and good uniformity over large areas.Today,oxide TFTs based on IGZO(indium-gallium-zinc-oxide)semiconductor have been paved into mass production.Nevertheless,the semiconductor is prepared by physical vapor deposition,which requires expensive vacuum and photolithography equipments,resulting high preparation cost of devices.Solution processing is a feasible way to greatly reduce the production cost of oxide TFTs without employment of vacuum equipments.The author of this thesis has done a series of basic researches on solution-processed oxide TFTs,and makes some progresses on direct photopatterning of oxide films,inkjet-printed short-channel TFTs,inkjet-printed self-aligned TFTs,all-inkjet printed TFTs and active matrix quantum-dot light-emitting diode(AMQLED)character display based on inkjet-printed oxide TFT backplane.Thus,it provides feasible ways for the application of solution-processed oxide TFTs in the future.The main results of this thesis include the following parts:First,an ultraviolet(UV)micro-patterning method based on aqueous oxide precursor films has been developed,which can pattern the oxide films without using photoresist.The method simplifies the process,reduces the cost,and lowers the annealing temperature of the oxide films.A study shows that the UV irradiation and deionized water treatment used in the patterning process not only converts In-Cl to In-OH,but also greatly reduces the Cl-impurity in the precursor film,thereby greatly lowering post-annealing temperature of the film and defects in the final oxide film.The field-effect mobility of the patterned InOx TFTs is ten times larger than that of the unpatterned ones.To further improve the device performance of low-temperature patterned InOx TFT,InOx semiconductor is doped by Li element.At the annealing temperature of 180°C,15 at.%Li-doped InOx:Li TFT shows excellent electrical performance with a saturation mobility of 8.35±1.25 cm2 V-1 s-1.The device passivated by polystyrene exhibits good stability under both positive and negative gate bias stresses.Second,the coffee-ring effect in inkjet printing is employed to prepare narrow-width hydrophobic polymer stripes.Furthermore,the short-distance isolation of printed oxide precursor inks for the source/drain electrodes is realized by the barrier effect and de-wetting effect of the hydrophobic coffee stripes,and printed short-channel oxide TFTs are futher realized.This method overcomes the limitation of poor position accuracy of the common inkjet printer system and the difficulty in controlling ink spread on substrates.The prepared ITO electrode pairs are uniform with a channel length less than 5μm.The maximum saturation mobility of the short-channel InOx TFT is 4.9 cm2 V-1 s-1,and the Ion/Ioff ratio is as large as2.7×109.Third,the hydrophobic polymer coffee stripe prepared by inkjet printing is employed as an etching barrier to achieve equal-width pattern of the gate and dielectric layers.Anodization is employed to grow dielectric layers covering on both sides of the gate electrode,thereby realizing full coverage of the gate by the dielectric.Furthermore,the dewetting effect of the hydrophobic polymer coffee stripe is utilized to realize self alignment between the source/drain electrodes and the gate electrode,and thereby further realizing printed self-aligned oxide TFTs.The parasitic capacitance of prepared self-aligned In0.95Sc0.05Ox TFT is as low as 0.37 pF,and the cut-off frequency of the device is up to 2.1 MHz.Fourth,a general-purpose“solvent printing”technology has been developed to solve the issues that oxide TFT stacking and trans-film printing patterns are difficult to control,and further realizes fully printed oxide TFT array for the first time.A study show that the hydrophobic pattern not only effectively limits the spread of oxide precursor ink and thus defines the pattern of printed oxide film,but also provides an easy way to regulate surface morphology of printed oxide film.By optimizing the dielectric layer and the structure of TFT,the fully inkjet-printed top-gate In0.95Ga0.05Ox TFTs with an average mobility of 10.8 cm2 V-1s-1 is realized,and the device exhibits excellent stability under both positive and negative gate bias stress.Fifth,the“solvent printing”technology is employed to realize printed top-gate oxide TFT backplane,and the solution-processed quantum-dot light-emitting diodes are further integrated on the printed TFT backplane,thereby realizing the first active-matrix quantum-dot light-emitting diodes(AMQLED)char display based on the inkjet-printed oxide TFT backplane.This work preliminarily verifies the feasibility of inkjet-printed oxide TFTs applied in display.