Study On Device Physics And Fabrication Technology Of Flexible Transparent Electronics Based On Oxide Semiconductors

Flexible transparent electronic devices are key components in wearable electronics and the new era of internet of things(IoT).The entering of the next generation of electronic information technology,to a large extent,depends on the breakthrough of stable high performance yet low cost flexible transparent devices.Flexible transparent thin film transistors(TFTs),diodes and inverters have great potentials in wearable electronics,flexible displayers,radio-frequency identification tags,electronic-skins,implantable medical devices,etc.,owing to their mechanical robustness,high optical transmittance and electrical performance,superadd light-weight,small volume and low-cost fabrication.Different from the conventional devices those are based on silicon,sapphire and glass substrates,flexible transparent devices are mostly fabricated on polymer substrates at low temperature(<200°C).The problem lying in front of both the industry and academia circles is to seek better material quality and device performance in low temperature processinges.Zinc oxide(ZnO)and related materials can be synthesized at low temperatures via a variety of methods,thus are regarded as one kind of promising candidates for flexible transparent electronics.To begin with,we explored the fabrication process of high-performance flexible transparent ZnO-TFTs.Semiconductor,dielectric and transparent electrodes layers are deposited by radio-frequency magnetron sputtering and atomic layer deposition(ALD)technologies,respectively,and patterned via UV lithography and wet-etching.The objective of this part is to solve the critical problems resulting from the instability of the polymer substrates.It was found the polymer substrates would become much more stable after thermal pretreatment.With the conformal encapsulation of insulator films,the vapor and oxygen permeability,as well as the surface roughness of the polymer substrates,can be suppressed to a large extent.Distinguished from the traditional principles in crystal growth,a lower deposition temperature(near room temperature)is preferred in the deposition of organic and inorganic layers on polymer substrates to minimize the mechanical strain resulted from the huge mismatch between their thermal expansion coefficients.Besides that,the deformation of polymer substrates and the wrinkling and cracking of thin films on them are discussed and controlled in this part as well.Besides TFTs,the flexible transparent circuit also needs high-performance thin film diodes.However,research on flexible transparent diodes is quite limited despite the great progresses made in TFTs,mainly due to the lack of homogenous PN junction and the small Schottky barrier height(SBH)between oxide channel layer and transparent oxide electrodes.Based on the high-performance flexible transparent ZnO-TFTs,we proposed a novel field-effect diode structure and studied its device physics and application potentials.The field-effect diodes both have high rectification ratio over10~8 and are completely compatible with the fabrication processes of TFTs.Technology computer aided design(TCAD)simulations were employed to explore the device physics.The practical feasibility was demonstrated by building a rectification circuit with the field-effect diode.Unlike the HV devices based on GaN and SiC semiconductors,which are important in power electronics,ZnO based high-voltage(HV)devices are aimed at the energy management circuits in building intergrade photovoltaics(BIPVs)and triboelectric nanogenerators(TENGs).In the third part of this thesis,we carried out an investigation on the ZnO flexible transparent HV TFTs and HV diodes.We have successfully fabricated flexible transparent HVTFTs and HV diodes with high breakdown voltage of 150 V,the value of which could be adjusted via modulating the offset length.The HV AC voltage was successfully rectified into DC voltage and stored into supercapacitors,indicating its high compatibility with triboelectric nanogenerators(TENGs).Finally,we present a set of self-aligned lithography for the fabrication of flexible HV devices.The normal operation of HVTFTs and HV diodes relies on the length of offset region,which,to a great extent,effects the device performance.However,misalignment between the gate and drain patterns is unavoidable in the microfabrication process,especially those on flexible substrates which suffer from more deformation problems.Resolving this critical issue,we proposed the self-aligned technique to precisely control the offset length through modulating the dose in the proximity exposure process,which effetively improved the device uniformity and repeatability.