Research On Effects Of Active Layer Modification On Thin Film Transistor Performance

Nowadays,various electronics have been developed as necessaries in peoples' daily lives.Among many components in electronic devices,thin-film transistors?TFTs?are the most important part,as TFTs play important roles in microprocessors,memory cards,phone chips,active matrix flat panel displays and other electronic devices.In order to replace the traditional silicon based semiconductors and realize high performance,large area electronics with flexibility and transparency in visible light region,developing new semiconducting materials has become a research highlight.Among these,due to various advantages of organic semiconductors,including their large number of species,easy synthesis,multi-functionalities and low temperature processing,they have gained great potentials in the applications of flexible circuits,sensors,and smart labels.However,organic semiconductors usually suffer from inferior stabilities and low mobilities.On the other hand,as semiconductors or conductors which are transparent in visible light region,metal oxides can maintain high mobility and stability even at amorphous state,making them compatible with flexible substrates.Thus,metal oxides can be used in next generation transparent and flexible electronics.Nowadays,TFTs with metal oxides as semiconducting layer have been commercialized already,while to make high performance metal oxide based TFTs,high cost and high energy consuming fabrication processes,such as radio frequency sputtering,pulse laser deposition and atomic layer deposition,are usually utilized.To overcome these short comes in both organic and metal oxide semiconductors,in this dissertation,various active layer modification methods were utilized to fabricate high performance thin-film transistors?TFTs?,including dielectric modification and dielectric/semiconductor interface modifications in organic TFTs?OTFT?,and functional polymer modification in metal oxide semiconductors towards high performance metal oxide TFTs.There are mainly three parts in this study:1.Influences of functional dielectric and dielectric surface modification on the performance of OTFTs were studied.First,OTFTs with high crystalline copper phthalocyanine?CuPc?active layers were fabricated by inserting an ultrathin pentacene buffer layer between the dielectric and CuPc layers.Charge carrier mobility of the OTFT with a buffer layer presented a much higher value of ⁰.20 cm2/Vs,compared to that of 0.27×10-2 cm2/Vs without buffer layer.Second,controllable memory window of around 0 ? 10 V can be realized by modifying the thickness and combination of triple-layer polymer dielectrics which are consist of poly?methyl methacrylate?and poly?vinyl alcohol?.Third,high performance unipolar inverters based on a significant variation of threshold voltage?VT?of OTFTs is realized by introducing UV/ozone?UVO?treatment to polystyrene dielectric,were fabricated.2.Influence of polyethylenimine?PEI?doping on the performance of metal oxide based TFT,were studied.PEI doping of In₂O₃ films not only frustrated crystallization and controlled the carrier concentration,but more importantly,acted as electron dopant and/or scattering center depending on the polymer doping concentration.The electron donating capacity of PEI combined with charge trapping and variation in the matrix film microstructure yielded,for an optimum PEI doping content,electron mobilities as high as ⁹ cm2/Vs on a 300 nm SiO2 gate dielectric,an excellent on/off ratio of 107,and an application optimal VT of-3.9 2 V.Importantly,these metrics exceed those of the pure In₂O₃ matrix with a maximum mobility ⁴ cm2/Vs.Furthermore,it was shown that this approach is extendible to other oxide compositions such as IZO,IGO and IGZO.3.Influences of functional dielectric and dielectric surface modification on the performance of OTFT based gas sensors were studied.First,ammonia gas sensors based on pentacene OTFTs were fabricated using different polymers as dielectric.Compared with those incorporating poly?vinyl alcohol?,poly?4-vinylphenol?or poly?methyl methacrylate?dielectric,a low detect limitation of 1 ppm and enhanced recovery property were obtained for OTFTs with polystyrene as gate dielectric.Second,a nitrogen dioxide gas sensors based on copper phthalocyanine OTFTs with simple and low-cost UVO treated polystyrene dielectric were realized.By introducing carbon-oxygen functional groups,the sensitivities of OTFT based sensors are tremendously improved.In this dissertation,to overcome the relative low mobility,inferior stability,and high-cost in organic and metal oxide based TFTs,various active layer modification methods were utilized to fabricate high performance TFTs,which pave the way for the extensive applications of TFTs in opto-electronic devices.