| The elevated-metal metal-oxide(EMMO)thin-tilm transisitors(TFTs)as one kind of amorphous indium-gallium-zinc oxide(a-IGZO)TFTs,exhibit better electrical performance because of the fewer defects in channel than the traditional a-IGZO TFTs.However,there were limited studies on the reliability of EMMO TFTs.Therefore,in this study the reliability experiments of EMMO TFTs under negative bias stress(NBS),positive gate bias stress(PBS),negative bias illumination stress(NBIS),positive gate bias stress(NBIS)and positive bias illumination stress(PBIS)were investigated and the corresponding degradation mechanisms were proposed.A new type of degradation phenomena with increased subthreshold swing(SS)and threshold voltage(VTH)after NBS were observed,which can recover in a short time.The degradation mechanisms under NBS is proposed to be the generation of singly charged oxygen vacancies(Vo+).It is observed the degradation phenomena of NBIS are similar to those of NBS and a two-stage recovery phenomenon was observed after NBIS was removed.The degradation mechanism under NBIS is proposed to be the creation of doubly charged oxygen vacancy(Vo2+)in addition to Vo+.Under PBS the transfer curve shifts to the positive gate bias direction,and the degradation mechanism is the accumulation of Vo2+ at the backchannel;the degradation phenomena with increased VFB and SS was observed under PBIS,and a two-stage recovery phenomenon was observed after PBIS was removed.We proposed the degradation mechanisms under PBIS is the generation of Vo+in addition to the accumulation of Vo2+at the back-channel.Finally,the explanations were confirmed by simulation with Silvaco TCAD.Additionally,there still needs some improvement in the mobility of EMMO TFT and there are greater demands to reduce the fabrication temperature by the substrate material because the traditional annealing is performed in oxygen atmosphere at 400℃.In this paper,rapid thermal annealing(RTA)is ultilized to form the conductive source/drain regions of EMMO TFT or improve the electrical characteristics of EMMO TFT that have formed conductive source/drain regions.After optimizing the RTA conditions performed to EMMO TFT that with or without conductive source/drain regions formed,it is determined that the temperature first rises to 200℃ at a rate of 30℃/s,then rises to 300℃ at a rate of 10℃/s and anneals for 5 s is the best RTA condition for EMMO TFT with conductive source/drain regions formed.The mobility is improved obviously after the annealing.The condition that with illumination shaded the temperature rises to 250℃ at a rate of 20℃/s and anneals for 40 s is the best RTA condition for EMMO TFT without conductive source/drain regions formed,where the mobility of the device after annealing is higher,the SS is smaller,and the linear relationship between the drain current and gate voltage is improved.The EMMO TFT with conductive drain/source regions formed by RTA have good reliability under direct current(DC)gate bias stresses.There is almost no degradation under PBS and NBS,but some degradations were observed when illumination with blue light was appleied.Comparing with the traditional EMMO TFTs,the RTA with smaller thermal budget provides a broader prospect to the EMMO TFTs with drain/source regions formed by RTA. |
