Fabrication And Performance Optimization Of Amorphous ZnOn Thin Film Transistor

Thin film transistors?TFTs?,as pixel driving and core semiconductor devices that constitute peripheral circuits,play a key role in flat panel display?FPD?technology.At present,most of the researches are ZnO-based thin film transistors.However,ZnO thin films are generally polycrystalline,with intrinsic oxygen vacancies?V_O?and poor device performance.Multi-component ZnO-based TFTs,such as In-Ga-Zn-O?IGZO?etc.cation-regulated TFTs have been commercialized.Through literature research,the anion-regulated amorphous zinc oxynitride thin film transistors?a-ZnON-TFTs?have been favored by some scientific research teams because of its high mobility,good stability,abundant raw materials and low preparation cost.In this paper,a-ZnON thin film was prepared by RF magnetron sputtering method using metal Zn?99.99%?target by optimizing the process parameters.Bottom-gate top contact type ZnON-TFT devices were prepared by lithographic mask wet etching process using the amorphous film as active layer.The effects of different thermal oxidation temperature,oxygen flow rate and nitrogen flow rate on the performance of ZnON-TFT devices were investigated.Firstly,the electrical properties of large-sized ZnON-TFT devices prepared by metal mask process and small-size ZnON-TFT fabricated by lithographic mask wet etching process were compared.Although the success rate of metal mask process is high,the leakage current of the device is as large as 10^-5 A.The active layer and metal electrodes are patterned by secondary lithography.After that,the leakage current of the device drops to the order of 10^-9 A.The active layer of the ZnON-TFT was prepared by thermal oxidation of Zn₃N₂.When the thermal oxidation temperature is below 200°C,the film phase is Zn₃N₂.There are many intrinsic defects in the film,the carrier concentration is high,and the devices are normally opening.when the thermal oxidation temperature is 300°C,the film is amorphous.The amorphous property makes the surface of the film flat and has good uniformity.The defect of donor states such as V_N and V_O are reduced,the carrier concentration is reduced to a suitable range for making TFT devices,and the gate voltage has an enhanced regulation for the channel current.The surface defect state density is small,and the device has the best performance parameters:?=2.25 cm²/V s,I_on/I_off=3.58×10⁵,SS=3.15 V/Dec,V_th=15.40 V,?V_th=+1.44 V.When the thermal oxidation temperature is higher than 300°C,the phase of the film is polycrystalline ZnO,the carrier concentration low.The gate voltage has a weak effect on the channel current regulation,and the device has no performance.The ZnON films were prepared by Ar/N₂/O₂ three-line mixed gas.The effects of different oxygen flow rates on the properties of ZnON-TFT devices were investigated.The results show that the phase of the samples changes from Zn₃N₂ to ZnO with the increase of oxygen flow rate before annealing.The film has the large roughness and many intrinsic defects,the carrier subject to scattering and trapping,and the overall performance of the device is poor.Overall,when the oxygen flow rate is 0.4 sccm,the device performance is the best,?=0.37 cm²/V s,I_on/I_off=1.21×10³,SS=10.97 V/Dec,V_th=3.13 V.After air annealing at 300°C,the surface roughness of the film is greatly improved compared with that before annealing.The internal defects of the film are reduced,the phase of the film also changed,and the device performance is improved to varying degrees.When the oxygen flow rate is 0 sccm and 0.3 sccm,the film is amorphous.Amorphous has good electrical properties,and the film with oxygen flow rate of0.3 sccm has moderate nitrogen content,the device has the best performance,?=21.59 cm²/V s,I_on/I_off=4.65×10⁵,SS=1.42 V./Dec,V_th=2.23 V,?V_th=+0.36 V.To further optimize the overall performance of ZnON-TFT devices,we investigated the effects of different nitrogen flow with the same ratio of nitrogen to oxygen on the performance of ZnON-TFT devices.With proper nitrogen incorporation,the intrinsic defect state of the film is reduced,the density of interface defect states decreased,the scattering and trapping effect on carriers weakened,and the comprehensive performance of the device is improved.Excessive nitrogen incorporation forms new nitrogen-related defects,and the performance of the device decreases with the increase of defect density in interface states.When the nitrogen-oxygen flow ratio is 60/0.4,the device performance best,?=33.83 cm²/V s,I_on/I_off=1.19×10⁵,SS=1.36 V/Dec,V_th=0.85 V,?V_th=+0.06 V.