Fabrication And Electrical Properties Of Dual Active Layer Thin Film Transistors

Thin film transistors（TFTs）play an indispensable role in the display field.Compared to traditional polycrystalline silicon TFTs,metal oxide TFTs have higher carrier mobility and lower preparation costs.Especially the mature indium gallium zinc oxide（IGZO）TFT has enormous advantages in high mobility,large area reliability,etc.However,it still cannot meet the display requirements of higher resolution,greater responsiveness,and lower power consumption.Moreover,rare metal materials such as indium and gallium are expensive and toxic,making alternative materials such as tin zinc oxide（ZTO）a research hotspot.At present,the field effect mobility and long-term stability of ZTO TFT still need to be improved.To improve the performance of the ZTO TFT,two aspects of improving the performance of the film and improving the structure of the device can be considered:first,the internal mechanism affecting the electrical performance of ZTO TFT is studied,and the carrier transport is studied by doping method.Among them,lithium metal（Li）doping provides a good way to improve the performance of ZTO TFT devices.Because the strong Binding energy between lithium and oxygen reduces the number of oxygen vacancies in the active layer and the number of trap sites near the interface between the active layer substrate,which can improve the carrier concentration in the film and further improve the electrical performance of devices.The second is to use a dual active layer structure to improve the device performance.In recent years,research on improving device performance with dual active layer structures of different oxide semiconductor materials has shown a rapid upward trend.ZTO:Li/ZTO thin film transistors were fabricated by RF magnetron sputtering using ZTO thin films as upper active layer and Li-doped ZTO（ZTO:Li）thin films as lower active layer.The performance of dual-active-layer thin-film transistors（daft）is studied when each active layer acts as a channel independently,the impact mechanism is analyzed and summarized as follows:Single-layer ZTO thin film transistors are fabricated by RF magnetron sputtering.There are many studies on the effect of annealing treatment on ZTO thin films and their properties,but the internal micro-mechanism is still not clear.Therefore,XPS was used to analyze the different states of oxygen and the valence states of tin in the films at different annealing temperatures.The results show that the proportion of metal to oxygen bond is the most,and the proportion of oxygen vacancy is the least,the content of Sn⁴⁺decreases and the content of Sn²⁺increases at 600℃annealing.At this point,ZTO thin-film transistors have the best electrical performance,including saturation mobility(μ_SAT)of 12.64 cm²/Vs,threshold voltage(V_TH)of-6.61 V,Subthreshold Swing（SS）of 0.79 v/decade,I_ON/I_OFFof 1.87×10⁹,the off current is2.08 p A.The results show that the oxygen vacancy has an important effect on the electrical properties of the device,and provide a theoretical basis for the following experiments.Secondly,in order to obtain the optimal gas atmosphere for ZTO:Li TFT,the influence of Ar/O ratio on the properties of ZTO:Li TFT was studied.The experimental results show that the electrical properties of TFT devices tend to deteriorate with the increase of oxygen to argon ratio in the process of magnetron sputtering.When the Ar:O ratio is 100:0,the mobility is 20.58 cm²/Vs,the threshold voltage is-12.34 V,the switching current ratio is 1.19×10⁹,and the subthreshold swing is 1.11 V/decade.This lays a foundation for the fabrication of the dual-active-layer thin film transistors.ZTO:Li/ZTO TFT devices were fabricated by RF magnetron sputtering using ZTO:Li thin films with argon to oxygen ratio of 100:0 as the lower layer and ZTO thin films as the upper layer.Dual-active-layer oxide thin-film transistor devices have been studied more and more,but the mechanism is not clear.Therefore,XPS is used to analyze the different states of oxygen in each layer of the double-layer TFT device.It is found that the content of metal-oxygen bond in the lower film is more than that in the upper film,and the content of oxygen vacancy is less than that in the upper film.Dual-active-layer thin-film transistors combined with two-layer materials exhibit superior performance over single-layer devices,includingμ_SATof 13.98 cm²/Vs,V_THof-13.54 V,SS of 0.84 V/decade,and I_ON/I_OFFof 1.13×10⁹.