| In recent years,thin-film transistors(TFTs)with amorphous indium gallium zinc oxide(a-IGZO)as the active layer have attracted much attention due to their superior characteristics.Elevated-metal metal-oxide(EMMO)TFT is one kind of a-IGZO TFTs,where the conductive source/drain regions and the intrinsic channel region are formed by oxygen annealing.Oxygen annealing needs to be carried out several times during the fabrication process and plays a crucial role in the performance of the EMMO TFTs.In this paper,the electrical characteristics and reliability of EMMO TFTs fabricated by different source/drain formation annealing processes are studied.Effects of the annealing processes on the property of the thin films are analyzed by X-ray photoelectron spectroscopy(XPS)etc.Finally,an improved annealing scheme is proposedBefore the last annealing process to form the source and drain,defects at the interface and the source/drain resistance limit the transfer characteristics of EMMO TFT,e.g.,low on-off current ratio(Ion//Ioff),negative threshold voltage(Vth)and large subthreshold swing(SS)EMMO TFT could form an intrinsic channel and conductive source/drain region with high defect concentration after the traditional oxygen annealing at 400 ?,Ion/Ioff increases,Vth is around-0.7 V and SS reduces to 180 mV/decade.When the oxygen annealing temperature is reduced to 200 ?,an obvious hump current is observed.Then the annealing atmosphere is changed to nitrogen,we observed short-circuited channel when the temperature is over 300 ?.However,when we reduce the temperature to 200=,the device characteristics is improved,Ion/Ioff increases to 1010,Vth is about-0.1 V and is around 140 mV/decade.Moreover,the short-circuit channel of the device(L=2 ?m)induced by the diffusion of oxygen defect from source/drain under high temperature is prevented.Argon annealing at 200 ? as the control experiment is also carried out,which verifies that nitrogen atmosphere plays an important role in improving the characteristics of the EMMO TFT.EMMO TFTs annealed in oxygen at 400 ? and nitrogen at 200 ? both exhibit good performance.We then studied the reliability of the EMMO TFTs prepared by these two annealing conditions under pulse gate bias stress and thermal stability experiments.The degradation of EMMO TFTs annealed in oxygen at 400 ? and nitrogen at 200? is very small under pulse gate stress,which indicates that these two annealing conditions both can reduce the trap states in the channel region.however,the channel of the EMMO TFTs annealed in oxygen at 400 ? is short-circuited after further annealing at 300 ? in nitrogen,while the devices annealed in nitrogen at 200 ? could keep the characteristics after the same nitrogen annealing at 300 ?,which indicates that the latter has better thermal stability.In order to improve the thermal stability of traditional EMMO TFT,we added nitrogen annealing at 200 ? in the preparation process,and the results showed that only when nitrogen annealing is performed as the last annealing step can effectively improve the thermal stability of the devices.We also prepared samples under different annealing conditions and characterized the change of oxygen vacancies and O-M bonds by XPS.It is observed that the oxygen vacancies in a-IGZO can be reduced after annealing in oxygen at 400 ? and annealing in nitrogen at 200 ?.When they are further annealed in at nitrogen 300 ? oxygen vacancies in the former increases significantly,while the latter keeps almost unchanged.Moreover,PPC experiment shows that the device annealed in nitrogen at 200 ? recover faster after the removal of the illumination,indicating that the fewer oxygen vacancies in active layer.We believe that the more stable N-M bond were formed during nitrogen annealing,which can effectively suppress formation of oxygen vacancies.Therefore,the reliability and thermal stability of the EMMO TFTs will be improved by introducing nitrogen annealing at 200 ? as the last annealing step in the fabrication process. |
PDF Full Text Request