Synergistic Enhancement Of Mobility And Stability Of Gradient Junctionless ZnO TFT

Thin film transistors（TFTs）are an important component of optoelectronic devices such as flat panel displays（FPDs）,liquid crystal displays（LCDs）,and active matrix light emitting diodes（AMOLEDs）,and perform key driving functions in display operations as the control element for pixel switching.Large size,high resolution,and high frame rate display technologies require TFTs with superior electrical properties and operating stability.The results of a large number of previous studies show that doping（anion doping,such as N doping;cation doping,such as Hf,Zr doping）is an effective means to improve the electrical performance or stability of oxide TFT devices.However,uniform doping can cause a trade-off between mobility and stability（trade-off）,so that the two cannot be improved synergistically.In order to solve the trade-off problem of mobility and stability of oxide TFT devices,a junctionless trench TFT with gradient distribution of carrier concentration is proposed.Based on this strategy,the factors controlling the carrier concentration variation of ZnO films are explored in this paper,and the optimal process parameters for uniform channel TFTs are determined.Subsequently,the gradient junctionless ZnO TFT devices with high mobility and high stability compatibility were prepared by using the working air pressure to regulate the carrier concentration in the channel layer and nitrogen doping in the back channel.The details of the study are as follows.（1）Extreme tuning of physical properties of ZnO thin films:ZnO thin films with different working air pressure and sputtering power were prepared to perform tuning of physical properties of ZnO thin films.It is found that the ZnO films have good optical transparency in the visible range.Structural characterization shows that the films all have（002）diffraction peaks with meritocratic orientation.The crystalline quality of the prepared ZnO films was good at an operating air pressure of 0.9 Pa and a sputtering power of 80 W.The electrical properties show that the carrier concentration decreases slightly and then increases exponentially(10¹⁰-10¹⁹ cm^-3)with increasing working pressure.The carrier concentration increases and then decreases in a small range with increasing sputtering power.It is demonstrated that the carrier concentration gradient of ZnO films can be regulated by changing the working air pressure.（2）Uniform channel ZnO TFT device preparation and transport characteristics analysis:Uniform channel ZnO TFTs with different sputtering air pressure（0.3 Pa-9Pa）,sputtering power（60 W-100 W）,active layer thickness（20 nm-100 nm）,and annealing temperature（100oC,200oC,300oC）were prepared,and their transport characteristics were analyzed to determine the optimal process parameters.It is shown that the corresponding devices have the optimal transport performance for the application in display devices at an operating air pressure of 0.9 Pa,sputtering power of 80 W,active layer thickness of 40 nm,and annealing temperature of 200oC.（3）Gradient junctionless trench ZnO TFT device preparation and transport characteristics analysis:Using the carrier concentration patterns of the films at different sputtering operating pressures,gradient junctionless ZnO:O_I|O_Ⅱ TFTs were prepared by depositing ultra-thin ZnO:O_I layers with large carrier concentrations at high operating pressures and successively depositing ZnO:O_Ⅱ layers with small carrier concentrations at low operating pressures.The optimal thickness of the front-channel ZnO:O_I ultra-thin layer is achieved when sputtering for 5 s.The electrical performance of the device is significantly improved compared with that of the uniform channel device.The superiority of the gradient structure was demonstrated by comparison with the double-layer junction channel devices.Subsequently,ZnO:O_I|O_Ⅱ|N TFTs were prepared by doping nitrogen into the back channel based on the gradient structure,and the electrical properties and stability of the devices were significantly improved relative to the undoped devices when the optimal nitrogen doping flow rate of 0.4 sccm was determined.In this study,gradient junctionless ZnO TFTs were successfully fabricated to achieve the compatibility of high mobility and high stability of the devices,which confirms the universality of the gradient channel strategy to synergistically enhance the mobility and stability of oxide TFT devices.