Study On The Functional Layer Modulation And Electrical Properties Of Oxide Thin-film Transistors

Display technology is the window of civilization and wisdom of modern society,and Thin-Film Transistors（TFTs）are known as the"grain"of modern display technology.TFTs are widely used in flat panel displays such as active matrix liquid crystal displays（AMLCD）and active matrix organic light emitting diode displays（AMOLED）as the core component of active switching and driving of display pixels,and the mobility and stability of TFTs are the key issues that determine the display technology towards industrialization.With the development of modern industrialization,there is an increasing demand for high resolution and high frame rate displays,which also puts forward higher and higher requirements for TFTs.Metal oxide semiconductor（MOS）,with its low cost,high mobility and easy to prepare large area,has become one of the main materials for the new generation of flat panel displays,and has a broad application prospect in the field of flexible,transparent and wearable electronics.At present,displays with IGZO metal oxide semiconductors as the active layer material for TFTs on the market have already achieved mass production,but the preparation of IGZO is mainly by physical vapor deposition method,which requires high vacuum equipment and photolithography,and these equipment and technologies are expensive and difficult to operate,limiting the rapid development of such displays.The solution method for the preparation of oxide semiconductors has attracted widespread attention because of its advantages such as low cost,bendability,and ease of large-area preparation.However,metal oxide thin film transistors（MOTFTs）prepared by the solution method have low mobility and poor stability compared to the vacuum method.In order to improve the mobility and stability of MOTFTs,as well as other electrical properties,the paper proposes functional layer tuning methods,including"co-doping"tuning by introducing additional metal cations in the insulating and active layers and"surface charge transfer doping"tuning on the conductive channel surface."The paper is organized around the following three parts The paper is organized around three main parts.（1）The modulation of the electrical properties of TFTs by"co-doping"metal cations in the active and insulating layers.A new strategy is proposed for the preparation of high-performance TFTs by co-doping indium oxide（In₂O₃）in the active layer and alumina（Al₂O₃）in the insulating layer with gallium（Ga）.The effects of Ga doping on the microstructure,thickness,surface morphology and electron density of In₂O₃ films,as well as on the oxygen defect state and electrical properties of Al₂O₃ films,were systematically investigated.The results show that Ga doping inhibits the crystallization of In₂O₃ thin films,reduces the thickness and surface roughness of In₂O₃ thin films,and most importantly,Ga doping reduces the electron density of In₂O₃ thin films,proving that Ga is an effective carrier inhibitor.For the Al₂O₃ films,Ga doping reduced the oxygen vacancy concentration and leakage current density,and increased the reflection index and dielectric constant.Finally,metal oxide TFTs with In₂O₃:Ga as the active layer and Al₂O₃:Ga as the insulating layer were prepared,which exhibited superior electrical properties and enhanced positive/negative bias stability compared with the devices without Ga doping.（2）Modulation of the electrical properties of In₂O₃ TFTs by"surface charge transfer doping".Doping is a powerful tool to regulate the charge carrier density in metal oxide thin film transistors.However,elemental doping of the crystalline semiconductor bulk phase leads to the disruption of the film microstructure,which greatly reduces the charge carrier mobility.Here,a surface charge-transfer doping method is proposed to effectively regulate the carrier concentration in In₂O₃ using an acceptor molecule without disrupting the lattice structure of In₂O₃.This part uses surface doping of 7,7,8,8-tetracyano-p-phenylenediquinodimethane（TCNQ）on the conducting channel formed by the active layer of In₂O₃ and the source-drain electrode.On the one hand,TCNQ can isolate the passivation of the active layer In₂O₃ by water and oxygen in the air and improve the stability of In₂O₃ TFTs.On the other hand TCNQ can induce some electrons at the interface with TCNQ in In₂O₃ to transfer into TCNQ,thus reducing the off-current of In₂O₃TFTs and achieving the purpose of improving the current switching ratio.In addition,the electron transfer from the active layer of In₂O₃ to the acceptor molecule TCNQ is demonstrated in the paper using spectroscopic techniques.Electrical tests show that the surface-doped TCNQ devices have better electrical performance and enhanced bias stability than the pristine devices.（3）Modulation of the electrical properties of Sn O₂ TFTs by"surface charge transfer doping".Since the ionic structure of Sn⁴⁺is similar to that of In³⁺and the reserves of tin are far more abundant than those of indium,Sn O₂ becomes one of the best choices to replace indium-based oxides.However,the mobility of Sn O₂ TFTs prepared by the solution method is low,and both mobility and stability of Sn O₂ are seriously affected due to its susceptibility to passivation by water and oxygen in air.This part uses the doping of PVA,an electron donor material,on the surface of the channel formed by the active layer and the source-drain electrode of Sn O₂.On the one hand,PVA can isolate the passivation of Sn O₂ by water and oxygen in the air and improve the stability of Sn O₂ TFTs.On the other hand PVA can provide electrons for Sn O₂ and improve the mobility of Sn O₂ TFTs.In addition,the controlled modulation of the threshold voltage of Sn O₂TFTs is achieved by precisely adjusting the concentration of PVA.