Study On Stability Of Metal-Oxide Thin-Film Transistors

At present,metal-oxide thin-film transistor(MO-TFT)has been widely used in large size display panels,but there are still many problems in the application of small and medium size ultra-high-definition display devices,such as the instability under the influence of bias stress with light or heat.This paper focuses on the study of MO-TFTs'stability.Through theoretical analysis,material design,device preparation process optimization,the problem of MO-TFTs'poor stability under the influence of bias stress with light or heat has been improved,which explores a new path for the large-scale application of MO-TFT.The main research contents and progress of this paper are as follows:(1)Pulse laser depesotion(PLD)technology is used as the active layer preparation process and the typical ternary oxide semiconductor-indium zinc oxide(IZO)is used as the active layer.The effects of process conditions on the device performance and stability were studied.It is found that the deposition process and the ratio of Zn/In have great influence on the performance of the device.When n _Zn/n _In=4:6,the device performance can be the best.During the deposition process,the increase of oxygen partial pressure will reduce the electron mobility of the device.As result,the threshold voltage(V_th)moves forward,which indicates that oxygen can inhibit the formation of oxygen vacancies(V_O)and reduce the free electron concentration.The mobility of the best IZO TFT is 34.5 cm²V^-1s^-1,and the V_th of the device is-0.17 V.Under positive/negative bias stress(P/NBS),the V_th drift(?V_th)are small,which are+0.51 V(PBS)and-0.63 V(NBS)respectively.However,the influence of heat or light on the bias stability is serious.Under positive/negative thermal gate bias stress(P/NBTS)at 80?,the?V_th are+0.76V(PBTS)and-2.61 V(NBTS),respectively.The reason is that higher temperature makes the defect state have higher activity,which leads to the increasing probability of carriers trapped by the defects of active layer/gate insulator interface.The V_th under positive/negative grating bias(P/NBIS)of 15 W/m² white light has a serious negative drift(|?V_th|>10 V).Also,the cut-off current(I_off)increases by about two orders of magnitude,which is caused by the ionization of V_O.(2)In order to improve the thermal or illuminated bias stability of IZO,Ni O,a p-type oxide,is further doped into IZO.As a p-type semiconductor,Ni O is doped into IZO to introduce acceptor defects,annihilating photogenerated carriers therefore improving light stability.The experimental results show that when Ni O is doped into IZO,the stability under NBTS and NBIS is significantly improved compared with that of IZO TFT:the?V_th under NBTS at 80°C is-2.41 V,and the stability under 15 W/m² white light NBIS is significantly improved.The improvement of NBIS stability is due to the introduction of acceptor defects by Ni O doping,which can annihilate part of photogenerated carriers and reduce the drift of V_th.(3)A new semiconductor material,Cd In Sc O,is designed,which improve the stability of TFT greatly.It is found that the electron mobility of the In Sc O TFT device doped with 1 wt.%Cd O(Cd In Sc O-1)is 24.0 cm²V^-1s^-1,and the V_th is-0.28 V.Also,the thermal or illuminated stability test of Cd In Sc O-1 TFT shows excellent results.The?V_th under P/NBTS at 80°C are+0.12 V and-0.10 V respectively,and the o Vth under 15 W/m² white light P/NBIS are+0.15V and-0.16 V respectively.The?V_th under PBIS of red light(650 nm),green light(550 nm)and blue light(450 nm)are+0.12 V,+0.12 V and+0.15 V respecitvely,which show excellent PBIS stability of TFT.Meanwhile,the?V_th under NBIS of red light,green light and blue light are-0.09 V,-0.36 V and-0.91 V,respectively.The main reason is that the energy level of V_O in Cd In Sc O is deep level,closing to the valence band maximum(VBM)and higher energy photons are needed to excite electrons in V_O.The shorter the light wavelength is,the easier it is to excite photogenerated carriers.(4)The calculation of Cd In Sc O system based on the first principle show that the Sc atoms tend to aggregate,while the Cd atoms tend to separate from each other.1 wt.%Cd has ltte effect on the effective mass of Cd In Sc O system,while Cd²⁺accords with electronic structure of(n-1)d¹⁰ns⁰(n?5),which can ehance the transportation capacity of semiconductor;With the increase of doping amount,the effect of Cd O on the effective mass of cdinsco system can not be ignored,which makes the carrier transport capacity decrease so that the mobility decrease;Due to the upward repulsion between the O p states and the Cd d states and most of the holes combine with V_O to form Cd-V_O pairs,the density of states near the VBM decreases.Therefore,the Cd In Sc O TFT doped with 1 wt.%Cd has high stability.Through the calculation of dielectric constant,it is found that the gap state caused by Cd O doping was compensated by V_O state,which verifies the generation of Cd-V_O pair.