Study On The Mechanisms And Controllable Effects Of Doping In Zinc Oxide Based Thin-film Transistors

In recent years,a new generation of flat-panel displays represented by thin film transistor liquid-crystal displays(TFT-LCD)has become one of the important development direction of information industry.However,while the large-area flat panel display increase the requirement of high resolution and high frame frequency,the mobility and stability of TFTs,drive element,has to be improved.Because of the vast development potential in high mobility,optical transparency,low cost,and large-area fabrication,amorphous oxide semiconductor attracts the attention of people and is expected to become one of the main material of a new generation of flat panel display.Although current performance of amorphous oxide TFTsrepresented by amorphous indium gallium zinc oxide amorphous oxide TFTs has great development,for a new generation of flat-panel displays,its performance still needs to be further improve and promote to meet the requirements of the development.This paper mainly uses sol-gel method doping in amorphous TFTs to study the doping effect on mobility and stability.To fundamentally improve the performance of amorphous oxide TFTs,this paper studies the change of performance by doping different components and investigates the doping mechanism through the tests of different characterization techniques.The thesis mainly revolves around the following three parts:The first,the investigation of alkali metals doped amorphous indium zinc oxide(a-InZnO)TFTs.Sol-gel processed a-InZnOTFT is low cost,environmental protection,easy to fabrication and can be large-area preparation,which makes it has a great potential in the application of the flat display,but relatively low mobility(usually 1-10 cm2/V·s)of the a-InZnO TFT severely limits its practical application.To address this issue,we employed alkali metal dopants of lithium,sodium,and potassium,to obtain high-performance solution-processed a-InZnO TFTs with enhanced on-state channel current,improved subthreshold swing,and high field-effect mobility(34.5 cm2/V·s).In addition,alkali metals doped a-InZnO TFTs also improve bias stability over that of the original samples.It indicates that alkali metals doping could increase electron concentrations,improve interface quality,and significantly reduce oxygen vacancy defect states in the active channel layer of a-InZnO films.The analysis of testing and characterization results indicates that alkali metals doping could increase electron concentrations,improve interface quality,and significantly reduce oxygen vacancy defect states in the active channel layer of a-InZnO films.Thus,alkali metals doped a-InZnO TFTs by the sol-gel process could be a promising and low-cost route toward high-performance solution-processed a-InZnO-based TFTs.The second,the investigation of metal tungsten(W)doped a-InZnO TFTs.The bias stress stability of TFT is an important parameter for practical applications.Under the gate control,TFT open and shut off,which requests TFT still has the stable electrical characteristics under bias condition for a long time,to avoid performance degradation.The inferior bias stress stability of a-InZnOTFT is improved by gallium and W metal ion doping,which will form chemical bonds between the metal cation and oxygen to inhibit the generation of oxygen vacancies and improve the stability of TFTs.But metal ion doping will reduce the TFTs mobility.To address this issue,W-doped a-InZnO TFTs with low W doping concentration are fabricated to improve both field-effect mobility and bias stress stability.We find that low doping concentration of W could improve the mobility for both the reduced carrier traps produced by oxygen vacancies and the extra electrons.The mobility can be 30.5 cm2/N·s,which reach the request of commercial TFT backplanes.In addition,the pulsed ?-? method is employed,which indicates that a proper small amount of the W-doping can effectively reduce the interface trap density and hysteresis.This study brings out a promising method for fabricating high-performance oxide based TFTs with high stability by W doping.The third,the investigation of high-performance amorphous indium-gallium-zinc-oxide(a-InGaZnO)and indium-tin-oxide(ITO)nanowire(NW)compositeTFTs.By incorporating 0.5 wt%ITO NWs into the a-InGaZnO thin film,the composite TFTs can achieve an enhanced field-effect mobility which is mainly attributed to the fact that the effective channel length is shortened by the high conductivity NWs.Transparent a-InGaZnO and ITO NWs high performance composite film is a-InGaZnO thin film incorporated with the ITO NWs which are synthesized using a chemical vapor deposition method.When the ITO NWs are distributed randomly in the thin film,the a-InGaZnO/ITO NW composite TFTs obtain a high ?FE of 76.5 cm2/V·s and a high current density of 73.3 ?A/ltm with 10-?m channel length,and the transparency of ITO NWs results in the good transparency of composite film.The LFN measurement is carried out for the a-InGaZnO TFTs with the ITO NWs incorporated,whose results indicate the LFN in composite TFTs fits the classical 1/f noise model very well and low-frequency noise of the thin film transistor is significantly lower after doping.The results suggest that the incorporation of ITO NWs should also improve the interface quality,besides shortening the effective channel length.These results provide a new theoretical basis for the preparation of composite film.