Study And Fabrication Of N And W Doped InZnSnO Thin Film Transistors

Thin film transistors（TFTs）as key components are commonly applied in TFT-liquid crystal display（TFT-LCD）and active-matrix organic light emission diode（AMOLED）display circuits.Traditional Si-based TFTs can’t meet the requirements of the ultra-high-definition and large-scale displays due to their drawbacks.Amorphous metal oxide TFTs are considered as a promising alternative to Si-based TFTs because they show numorous advantages with high mobility and good uniformity.The current commecial In Ga Zn O（IGZO）TFT has a mobility of about 10 cm²/V·s.The InZnSnO（IZTO）TFT shows higher mobility than IGZO TFT.However,the mobility of the IZTO TFT is lower than 30cm²/V·s,which can’t meet the requirements of high-frame-rate and ultra-high-definition display.In order to solve the problem of the low mobility of the IZTO TFT,we proposed three oxide TFTs with the active layers of N-doped,Li and N co-doped and W doped IZTO,which were named to IZTO:N,IZTO:（Li,N）and IZTO:W TFTs,respectively.We investigated the effects of fabrication conditions and annealing conditions on their electrical characteristics and obtained IZTO TFTs with excellent overall performance and mobility over 40 cm²/V·s.The main contents and innovations are shown as follows:（1）N doped IZTO was proposed and IZTO:N TFTs were fabricated by RF magnetron sputtering.The elemental composition,optical performance,microstructure,surface roughness and defects of the active layer were characterized by SIMS,transmittance spectra,XRD,AFM and XPS,respectively.The active layer thickness,annealing temperature and sputtering oxygen flow rate were optimized and the effects of annealing temperature on the gate bias stress stability of the TFTs were studied.The optimized TFT had mobility of 37.6 cm²/V·s,threhold voltage of 1.4 V,sub-threshold swing of 0.38V/dec,on/off current of 4.9×10⁹ and off current of 2.4×10^-13 A.The overall electrical performance of the TFT is better than that of the most amorphous TFTs reported in the literature.The bias stability results indicated that the threshold voltage shifts of the TFT annealed at 275°C were-3.4 V and 13.6 V under the negative and positive bias stress,respectively.As the annealing temperature increased,the device became more stable because charge traps decreased,thus the value of the threshold voltage shift decreased.（2）Based on IZTO:N,Li and N co-doped IZTO was proposed and IZTO:（Li,N）TFTs were fabricated.The effects of sputtering oxygen flow rate,device annealing temperature,active layer annealing temperature and active layer thickness on the electrical properties of the TFTs were studied,as well as the stability over time in the air.The results of the electrical characteristics indicated that the TFTs sputtered without oxygen gas showed better properties than those sputtered with oxygen gas.Both the active layer annealing process and the device annealing process showed significant effects on the electrical properties of the TFT.The TFTs through the active layer annealing showed better properties than thoses through the device annealing.The performance of the devices improved as the temperature raised from 250°C to 300°C while degraded when the temperature exceeded 325°C.The performance of the device improved when the active layer thickness increased from 10 nm to 20 nm while degraded when the thickness continued to increase.The optimized IZTO:（Li,N）TFT showed the best overall properties with mobility of 38.9 cm²/V·s,threhold voltage of 3.2 V,sub-threshold swing of 0.60 V/dec,on/off current of 3.5×10⁹ and off current of 4.0×10^-13 A.The device can still work as a transistor after being exposed to air for 30 days while the mobility of the aged TFT decreased and the off current increased over time mainly due to the water absorption on the surface of the back channel.（3）W doped IZTO was proposed the IZTO:W TFTs were fabricated by RF magnetron sputtering.The active layer thickness,sputtering oxygen flow rate and annealing temperature were optimized.The effects of annealing atmosphere and sputtering nitrogen flow rate on the electrical characteristics of the devices were investigated,as well as the bias stress stability,stress recovery and stability over time in the air.The mobility and the sub-threshold swing were improved a lot when the device was annealed in the air and nitrogen.Especially nitrogen anneaing decreased the density of interface defects and sub-threshold swing.The electrical properties degraded when the TFTs were sputtered with nitrogen gas mainly because of the decrease of carrier density.The more nitrogen gas was introduced,the more serious the degradation was.The optimized IZTO:W TFT showed the best overall properties with mobility of 41.0 cm²/V·s,threhold voltage of 2.4 V,sub-threshold swing of 0.54 V/dec,on/off current of 6.8×10⁸ and off current of 1.9×10^-12 A.The device can still work after 60 days while showed degrade properties.The study on the bias stress stability and recovery found that the threshold voltage shift under stress mainly because of the charge trapping by traps.The threhold voltage shift was-9.4 and10.0 V under NBS and PBS,respectively.Compared with metal oxide TFTs without passivation of back channel,the shifts were in the reasonable range.