Study Of Tin Oxide-based Semiconductor Films And Devices

With the rapid development of display technology in large size,ultra-high resolution and flexible wearable application,it is critical to develop low-cost and high-performance thin-film transistor（TFT）driving backplane technology.Metal oxide semiconductors are considered to a new generation of display backplane driving technology due to their advantages such as high mobility,high transparency,good uniformity,and low-temperature manufacturing process and so on.Electronic products are closely related to our lives with the development of science and technology.At the same time,a large amount of e-waste is also produced,which causes a huge threat to our health and living environment.Therefore,it is great significant to develop low-cost and eco-friendly oxide TFTs for display technology.Tin oxide（SnO₂）is a low-cost,eco-friendly semiconductor material due to its non-toxic,cheap,and high chemical stability.However,it is difficult to achieve good performance of SnO₂ TFT because it contains high carrier concentration.In this thesis,low-cost and eco-friendly SnO₂-based semiconductor films and devices were developed by theoretical calculation,composition ratio,interface engineering and fabrication process optimization.And the solution-processed SnO₂-based films was also explored.The main research works are shown as follows:（1）The low-cost and non-toxic Sielement was selected to dope and regulate carrier concentration of SnO₂ film.The Si-doped SnO₂ material was studied by using first-principles calculation and magnetron sputtering,respectively.The results of theoretical calculation showed that the conduction band minimum of SiSnO was composed of Sn5s and Si3s states,and the valence band maximum consisted of O2p state.The bandgap value of SnO₂ was enlarged and the transmittance of SnO₂ film in the infrared spectrum and visible light range was increased as Sidoping concentration increased.It might be caused by the introduction of Si3s state that makes the conduction minimum shift toward high energy level.In addition,the number of electron losses could be reduced by the formation of Si-O covalent bond,implying that the carrier concentrations of SnO₂ film could be reduced.The experimental results showed that the optical band gap of the SnO₂ film was enlarged and its carrier concentration was suppressed with an increase of Siconcentration.It was approximated to the theoretical calculation results.Meanwhile,TFT with the SiSnO film as the active layer were prepared,which could achieve good"switching"characteristics.It indicated that a new SiSnO semiconductor material was successfully designed.（2）The optimization of electrode materials and the quality improvement of the contact interface between the semiconductor layer and the electrode were proposed to reduce contact resistance.For electrode materials,the effect of ITO,Ti and Mo electrodes on the device performance were investigated.It is found that the formation of non-ohmic contact between Ti and SiSnO film led to deterioration of device performance.The device with ITO or Mo electrode could obtain good electrical properties because the ohmic contact between ITO or Mo and SiSnO film was formed.In consideration of flexible wearable display application,the contact characteristics of Mo electrode and SiSnO film was further studied due to the better flexibility of metal electrode.For the existence of diffusion between Mo and SiSnO film,an induced self-passivation interface was proposed to further improve the performance of the device.A rich oxygen layer was formed on the surface of SiSnO film annealed at 300℃ in air ambient,which induced the formation of a new interface layer at the SiSnO/Mo interface and ohmic contact.It was beneficial to carrier transport and improve device performance.The saturation mobility,current switching ratio and subthreshold swing were 6.78 cm²/V s,5.99×10⁶and 0.82 V/decade,respectively.（3）A high-density passivation layer was adopted to isolate the adsorption of water and oxygen molecules,and the photoresist residue on the back-channel surface was eliminated in order to achieve good device performance.The saturation mobility of the device was 4.2 cm²/V s,the current on-off ratio was 6.94×10⁹,and the subthreshold swing was 0.23 V/decade.The shift of threshold voltage of SiSnO TFT under positive and negative bias conditions was 1.42V and-1.48 V,respectively.To further facilitate the practical application of the SiSnO TFT,the instability mechanisms of device caused at different bias stress conditions（gate bias,drain bias and illumination）were explored.The instability of the device under the gate bias condition was mainly caused by the charge trapping mechanism.The instability of the device under the drain bias stress condition was caused by the lateral electric field.The reason of the device under light irradiation was that the neutral oxygen vacancies inside the semiconductor would be transformed to ionized oxygen vacancies.（4）For the requirement of flexible wearable display technology,the feasibility of flexible SiSnO TFT was explored.Based on the research work in Chapter 3,the polyimide film was selected as substrate,which was compatible with the farication process of SiSnO TFT.And the flexible SiSnO TFT was successfully fabricated.To broaden the application of the flexible SiSnO TFT,the modulation of the internal density of states in SiSnO film was proposed by oxygen partial pressure in order to obtain high quality of SiSnO film.The device could achieve good electrical performance without thermal annealing treatment.When the oxygen partial pressure was 0.18mtorr,the saturation mobility was as high as 7.59 cm²/Vs,the current on-off ratio was 1.59×10⁷,and the subthreshold swing was 1.46 V/decade.（5）The fabrication of SnO₂-based films was proposed by using sol-gel method,which would effectively reduce the preparation cost and bulid the foundation for printing technology in the future.The influence of substrate treatment,rotation speed and annealing temperature on the surface morphology and quality of SnO₂ thin films.A smooth,flat and high-quality SnO₂film was achieved by substrate with oxygen plasma treatment,spin coating with low rotation speed+high rotation speed and high annealing temperature.Based on the results of solution-processed SnO₂ film,the effect of Zr doping concentration on the properties of SnO₂ film was further studied.The transmittance of all the film was high.With the increase of Zr doping concentration,the carrier concentration of the film was first increased and then decreased,and the mobility was gradually decreased.