Study On Fabrication And Properties Of ZnO Thin-film Transistors With High-k Gate Dielectric

In the field of flat panel display,zinc oxide thin-film transistor(ZnO-TFT)has attracted much attention because of its high carrier mobility,low preparation temperature,good chemical stability,good uniformity in large area preparation,and high transmittance in visible light range.However,the threshold voltage and operating voltage of ZnO-TFT with low permittivity gate dielectric are generally too large,which result in high power consumption.These shortcomings restricted its use in portable mobile,wearable electronic systems.In this work,ZnO-TFTs with different process parameters and structural parameters were fabricated by using high-? material as gate dielectric,and their electrical properties and stability were studied.The main research works and results include:Bottom-gate structure ZnO-TFT with high-? NbLaO as gate dielectric was successfully fabricated on the ITO glass substrate by magnetron sputtering method.and the basic electrical properties of ZnO-TFT were characterized.The device showed good electrical characteristics,the mobility is 4~20 cm2/ V·s,the threshold voltage is 1~5 V,the subthreshold swing is 0.2~0.6 V/decade,and the on-off current ratio is higher than 106.In addition,the temperature characteristics of typical device was tested and analyzed.The results indicate that the operating temperature has a significant effect on the electrical characteristics of the device.In the testing temperature range,as the temperature increases,the mobility and on-off current ratio increases significantly,but the threshold voltage and subthreshold swing reduce.The effects of NbLaO gate dielectric annealing temperature on the quality of NbLaO and ZnO thin films,especially on the electrical characteristics and gate-bias stress of ZnO-TFT and its mechanism were studied systematically.Compared with the devices annealed at 200 ? and 400 ?,the mobility of the device annealed at 300 ? is slightly lower,but the off-current is minimum(~3.20×10-12 A),the subthreshold swing SS is minimum(~0.25 V/decade),the on-off current ratio is maximum(~1.21×107),and the stability of the device under gate bias stress is also better than those of the other deivces.The X-ray diffraction(XRD)and atomic force microscopy(AFM)were used to analyze the quality and surface morphology of NbLaO and ZnO films,and the I-V hysteresis and noise characteristics of the devices were tested.It is revealed that the effect of gate dielectric annealing temperature on the performance of the device is mainly due to the difference of gate dielectric density and surface roughness,which leads to the difference of ZnO film's crystallization properties and the charge states at the gate-dielectric/ semiconductor interface.ZnO-TFT with SiO2/NbLaO/SiO2 laminated gate-dielectric structure was fabricated and its basic electrical properties were characterized.The stability under positive gate bias stress was also tested and analyzed.Compared with the single-layer gate-dielectric structure,the mobility of the device is slightly higher,but the threshold voltage and the subthreshold swing increase.The increase of the mobility is probably due to the interfacial characteristic of ZnO/SiO2 was superior to the interface of ZnO/NbLaO,and the interfacial scattering of channel carriers is weakened,while the increase of the threshold voltage and the subthreshold swing is probably due to the presence of more defects at the SiO2/NbLaO interface which deposited by magnetron sputtering.This work also proposed and fabricated a ZnO-TFT with planar dual-gate structure.The dynamic control of the electrical characteristics of the device can be achieved by adjusting one of the two gate biases.The dual-input logic function of the device has also been tested and found that the device can implement a NOR logic function.