Study On Self-assembled Passivation Layer Of Flexible Metal Oxide Thin-film Transistors

Flexible electronics are committed to integrating into daily life,and promoting the development of revolutionary applications,such as artificial skin,intelligent textiles and skin display screen.Metal oxide thin film transistors（TFTs）are suitable for flexible electronics due to their large-area uniformity,low processing temperature,excellent optical transparency,good electrical characteristics and high flexibility.However,these devices also exhibit poor electrical and environmental stability.The multifunctional self-assembled monolayer/multilayers（SAMs）can be deposited on the surface of TFTs by simple and low-cost process,which improve the stability and modulate the electrical performance of TFTs significantly without damage.Therefore,the study on SAMs passivation layers has attractive great interests in the field of flexible electronics..This thesis studies methods to improve the electrical performance and stability of metal oxide In Sn Zn O（ITZO）TFTs with low cost,low process temperature,low process complexity and no damage based on the self-assembled molecular layers.The electrical performance of ITZO TFTs is improved through the interfacial dipoles between SAMs and ITZO as well as the nitrogen doping by octadecylamine（ODA）SAMs.The excellent passivation effect of SAMs improves the bias stability and environmental stability of devices,while the ODA-based superhydrophobic protective layer further improves the reliability of ITZO TFTs under strong acid and long-term water immersion conditions.The main contents are as follows:（1）The effects of head groups in self-assembled molecules on electrical properties and stability of ITZO TFTs are studied.Three SAMs with three different head groups（-OH,-COOH,-NH₂）were deposited by vapor phase method,and their modification effects on devices were compared.The results show that the head group in SAMs molecules determine the effective coverage,surface morphology and the surface energy of of SAMs.The relative orientation and strength of the interface dipole between SAMs and ITZO films are controlled by head groups,leading to the variations of the threshold voltage and mobility.The ODA SAMs with-NH₂head groups exhibit the lowest surface energy and appropriate surface dipole.Therefore,the passivated TFTs have the best electrical performance and stability..Flexible ITZO TFTs with three SAMs shows good electrical properties at a low bending radius of 7.5 mm.（2）The property of ITZO TFTs is modified by nitrogen ODA self-assembled molecular layers using vapor phase method.The XPS results demonstrate that the nitrogen in ODA SAMs is doped into ITZO films.With the increase of deposition time,the nitrogen concentration increases gradually,leading to the variation of oxygen vacancy and adsorbed oxygen content in ITZO.This affects the electrical properties of TFT.With a deposition time of 1 hour,the oxygen vacancy concentration is the lowest,and the density of adsorbed oxygen is also maintained at a relatively low level in the back channel of ITZO TFTs,resulting in the optimal electrical performance and bias stability..The threshold voltage of the device is negatively shifted from 2.2 V to-0.6 V,the subthreshold swing is reduced from 0.233 V/dec to 0.152 V/dec,and the field effect mobility is increased from 14.03 cm²/（V.s）to 19.89 cm²/（V.s）.The shift of threshold voltage is reduced from 4.2 V to 0.8 V under a positive bias stress of 10 V.The effect of ODA SAMs on DOS distribution in the ITZO active layer is extracted and analyzed by low-frequency C-V method.It is found that nitrogen ODA SAMs can reduce the density of deep states and tail states under the optimal deposition conditions,directing to an improvement of the electrical performance of ITZO TFTs.（3）To improve the environmental stability of metal oxide TFT,a superhydrophobic layer based on ODA is deposited at the back channel of ITZO TFTs.The superhydrophobic layer forms an air layer at the interface between solid and liquid Through the low surface energy and high roughness surface structure.The air layer effectively reduces the contact area between water molecules and the surface of ITZO films,which inhibits the adsorption of water molecules on the surface.Thus the the devices with superhydrophobic layers exhibits stable electrical performance in the long-term water immersion environment the strong acid conditions.The superhydrophobic layers can maintain the superhydrophobic property under10000 bending cycles with a low bending radius of 7.5 mm,and the electrical characteristics of the covered devices hardly change.Furthermore,a multifunctional passivation layer with nitrogen doping and superhydrophobic interface is obtained by combining nitrogen ODA SAMs with the superhydrophobic layer.This passivation layer can optimize the electrical performance and comprehensively improve the bias stability and environmental stability of of flexible TFT.The multifunctional SAMs modification technology studied in this thesis improves the electrical performance,the bias stability and environmental stability of of ITZO TFTs,which provides theoretical guidance for the application of metal oxide TFT in the field of flexible electronics.