Research On Strain Modulations Of Optical Properties Of Flexible ZnO Thin Films

Due to the excellent mechanical properties,more and more devices are beginning to develop in the direction of flexibility.As a wide band gap semiconductor,ZnO is widely used in flexible devices such as flexible transistors for good transparency in the visible light and high conductivity after doping.Commonly,during the preparation and use of these ZnO-based thin-film devices,they are subjected to mechanical behaviors such as stretching and bending.These mechanical effects have an important influence on the stability and reliability of thin-film devices.At the same time,using these mechanical behaviors to modulate the characteristics of the thin film,thereby improving the performance of the thinfilm devices.In this dissertation,the optical properties of flexible ZnO thin films are modulated by strain,created by stretching and bending with experiments and theoretical calculations.The main research contents are as follows:First,the measurement and characterization methods of flexible ZnO thin films are studied.The principle of XRD measurement and the method of using XRD to obtain film strain are explained.Spectroscopic ellipsometry is chosen to characterize the optical properties of ZnO films.The Muller matrix ellipsometer used to characterize anisotropic materials is introduced and the anisotropy modeling theory in ellipsometry is also described.Through the above anisotropic characterization method,the optical constant of flexible substrate PET is extracted.Secondly,the optical properties of flexible ZnO thin films during uniaxial stretching are characterized.The process of growing flexible ZnO thin film by magnetron sputtering is introduced.Combining stretching stage and ellipsometer,strain-optical characteristics of PET substrate are studied,which can be used for the next optical characterization of ZnO film.The in-situ stretching characterization of XRD shows the stress and strain of the ZnO film under tension.Finally,a strain-optical measurement experimental device was used to measure the optical properties of the ZnO film during uniaxial stretching,and the relationship between the refractive index of the ZnO film and the tensile strain is obtained.At last,using the magnetron sputtering technology to grow strained ZnO films on a pre-bent flexible substrate is explained.The XRD measurement results show that the substrate pre-bending method can produce adjustable strain in the film.The optical constants and the optical band gaps of the ZnO films under bending strain are obtained using Mueller matrix ellipsometer.The relationship between strain and refractive index and strain and optical band gap are given.Then,the band structure,bandgap and optical constants of ZnO under uniaxial strain are studied by the first principle method.The consistency between the theoretical calculation results and the experimental results proves the accuracy and reliability of the experimental results.The experimental phenomena and theoretical calculation results in this paper can provide experimental and theoretical guidance for the influence of mechanical behavior on the manufacturing process and use of flexible thin-film devices,and promote the application of strain engineering in the control of the optical and electrical properties of flexible nanofilms,thereby improving reliability and stability of thin-film devices.Further,the performances of flexible devices can be improved.