Solution-processed SnO₂-based Films And Their Applications In Thin-film Transistors

Thin film transistor?TFT?is a vital component for the next generation flat panel display?FPD?.Compared with traditional silicon-based semiconductor materials,transparent In₂O₃-based metal oxide semiconductors,such as InGaZnO?IGZO?,are ideal materials for switching/driving TFT in novel display technology,which is because they combine electrically semiconducting with visible light transparency,and high carrier mobility,high uniformity,as well as low temperature processing.However,the growing use of IGZO materials is threatened by the scarcity and high price of In,prompting the search for alternative elements.It is generally believed that the spherical symmetrical s orbital of In³⁺is the main reason for its high mobility.Particularly,Sn⁴⁺and In³⁺have the same electronic structure([Kr]4d¹⁰5s⁰)and Sn element is abundant and cheap.Therefore,SnO₂-based oxide is considered as one of the most promising material to replace In₂O₃-based oxide,and has attracted ever-increasing attention.On the other hand,the rapidly developed solution processing technology equips with the advantages of low cost,simple operation and easy control chemical composition for thin film,which is of benefit to future low-cost and large-area electronics.In this article,the n-type transparent SnO₂-based thin films were prepared by solution spin-coating process,and the effects of different precursor solution concentration,annealing temperature,and Ga doping amount on the properties of SnO₂-based thin films and TFT are comprehensively investigated by a variety of characterization techniques.Following are the main research results:Firstly,the influences of precursor solution concentrations on SnO₂ thin films were inspected.It was found that the relationship between film thickness and precursor solution concentration was positively linearly correlated.The surface of all SnO₂ thin films were quite smooth and uniform with low roughness,meanwhile,the optical band gap and refractive index are almost constant,indicating that the concentration of precursor solution generally does not affect the optical properties and the surface morphology of the thin film.Secondly,the effects of annealing temperature?200-700 ^oC?on the SnO₂ thin films were systematically investigated.The results indicate that when the annealing temperature is lower than 300 ^oC,the solution-processed SnO₂ thin films are amorphous nature with uniform and smooth surface.While with further increase of annealing temperature,the thin film experiences the elimination of organic and Cl related residuals along with the transformation of Sn?OH?₄ to form crystalline SnO₂.Thirdly,the impacts of annealing temperature on SnO₂ TFT were studied.Note that the200 ^oC-and 250 ^oC-annealed SnO₂ TFT show no field effect performance,which attributed to the presence of many organic residues.With the increase of annealing temperature,removal of organics and Cl-related residues,as well as the formation of metal oxygen bonds lead to the increase of device mobility.However,with the continuous increase of temperature,the device mobility decreases due to the increase of film surface roughness and the presence of severe interfacial scattering of channel layer/dielectric layer.Furthermore,optimized SnO₂ TFT annealed at 450 ^oC exhibits the highest?_sat=0.52 cm²/V s,I_on/I_off=2.3×10⁴,S=5.06 V/decade,V_TH=-6.7 V.Fourthly,the effects of Ga doping on the SnGaO thin films were systematically studied.It is found that the influences of Ga-doping can be listed as follows:?1?degraded the crystallization of SnO₂-based films;?2?augment the transmittance with large E_g value;?3?decrease the formation of oxygen vacancies.Doped Ga acting as a carries suppresser in SnGaO can improve the I_on/I_off from¹0⁴ to¹0⁷ and modulate the?_sat from 5.84 to 0.41 cm²/V s.About all,the 450 ^oC-annealed with 45%Ga-doped SnGaO TFT exhibits excellent performance,with?_sat=4.26 cm²/V s,I_on/I_off=1.7×10⁷,V_TH=-4.4 V,S=1.01 V/decade with decent bias stress stability.