Investigation On Ultrathin Amorphous Oxide Semiconductors And The Related Thin-film Transistors

In this era,thin-film transistors(TFTs)based on amorphous oxide semiconductors(AOSs)with exceptional properties of low preparation temperature,high carrier mobility,excellent uniformity and high optical transparency have grabbed the attention of researchers.Since AOS TFTs have been increasingly applied in a variety of high-end flat panel displays(FPDs),such as active matrix organic light emitting diode displays(AMOLED)and active matrix liquid crystal displays(AMLCD)etc.Presently,up gradation of high-tech products based on FPDs and AOS TFTs is in high demand.AOS TFTs with a smaller dimension,higher field-effect mobility and better stability is need of hour.Based on this consideration,ultra-thin-film transistors(UTFTs)using ultrathin AOS films(thickness,several nanometers)were prepared.Meanwhile,the effects of film thickness,film composition and annealing temperature on the properties of ultrathin AOS films and behaviors of the related UTFTs were systematically investigated.The research contents of this thesis are summarized as follows:Firstly,amorphous ZnGe0.05SnO(a-ZnGe0.05SnO)films with different thicknesses were prepared by pulsed laser deposition(PLD).The effects of thickness on the properties of amorphous oxide films and their TFTs were investigated.The oxygen content rises and the transmittance increases with decreasing film thickness.Over all,as the film thickness decreases,the field effect mobility increases first and then decreases,the threshold voltage decreases first and then increases.However,the film with a thickness of 4.3 nm and the related TFT depict many abnormal phenomena.To explain these phenomena,we have proposed an interface interaction model.This model can provide good guidance for the preparation of ultrathin AOS films and the related UTFTs.Furthermore,only when the film thickness is less than 4.3 nm(such as 3.1 nm),high-quality ultrathin AOS films can be obtained,leading to the TFTs operating in the enhancement mode with high filed-effect mobility.Secondly,ultrathin a-ZnGexSnO films with different Ge contents(a-ZnSnO at x=0)were prepared by PLD.Here,Ge is used as an oxygen vacancy(VO)suppressor.The effect of Ge content on properties of ultrathin a-ZnGexSnO films and the related UTFTs were investigated in detail.As the Ge content increases,the oxygen vacancy content first increases and then decreases,causing the threshold voltage to decrease first and then increase,while the field effect mobility is greatly reduced.As the film thickness is only about 3.2 nm,oxygen is easily diffused into the film by annealing,leading to a very low Vo content of ultrathin a-ZnSnO film.Then,the introduction of Ge has little effect of reducing the Vo concentration and will lead to decreasing filed-effect mobility.Among all the UTFTs,the a-ZnSnO UTFT demonstrates the best performance and the best stability,indicating that VO suppressors are unnecessary for AOS UTFTs.Thirdly,ultrathin a-ZnSnO films and the related UTFTs were prepared by PLD.The effects of annealing temperature on the properties of films and behaviors of UTFTs were investigated.The ultrathin a-ZnSnO film has a large specific surface area.As the annealing temperature increases,the film roughness decreases with changes in surface morphology and the resistivity of the film decreases,which will affect the performance of UTFTs.The UTFT annealed at 200 ? depicts no switching properties due to the high resistivity of the film,while other UTFTs with the annealing temperature varying from 250 to 450 ? exhibit good switching properties and high filed-effect mobility operating in the enhancement mode.The performance and stability of the UTFTs vary with the annealing temperature.Amongst them,the a-ZTO UTFT annealed at 350 ?depicts the best performance and moderate stability,the a-ZTO UTFT annealed at 450 ?exhibit the best stability and moderate performance.Fourthly,ultrathin a-InZnO and a-ZnSnO films and the related UTFTs were prepared by PLD.The properties of the two nanofilms and behaviors of the related UTFTs were comparably studied.Both nanofilms depict high transimitance(>95%),and the related UTFTs exhibit high field-effect mobility operating in the enhancement mode.And we have taken a new approach to compare the VO content in AOSs of different systems,which explains their different properties and behaviors well.Compared with ultrathin a-ZnSnO film,ultrathin a-InZnO film has lower VO content and then lower carrier concentration.Moreover,the 4d105s0 orbital radius of In3+ is larger than that of Sn4+.All these properties contribute to the superior performance of a-InZnO UTFTs compared to that of a-ZnSnO UTFTs.