Study Of Oxide Thin-Film Transistors With Copper Electrodes

As display technology advances in size,resolution,and refresh rate,high mobility thin film transistor?TFT?arrays with low resistance electrodes are needed to improve the resistance-capacitance delay during signal transmission.Amorphous oxide semiconductor TFTs have attracted considerable attention on the flat panel display backplanes because of their high mobility and excellent uniformity.Meanwhile,copper?Cu?is considered an appropriate electrode material for its low resistivity.Therefore,this thesis focuses on the oxide TFT with Cu electrodes.Firstly,we explored the effect of deposition condition and the annealing temperature of amorphous In-Ga-Zn-O?a-IGZO?film on the performance of TFTs.Furthermore,we successfully prepared an a-IGZO TFT with excellent electrical properties,with a threshold voltage(V_th)of 0.9 V,a saturation mobility(?_sat)of 8.4 cm²/Vs,and a subthreshold swing?SS?of 0.16 V/dec,which laid the foundation for subsequent work.In addition,we studied the effect of sputtering conditions of Cu electrodes,post-annealing,and Cu₂O doping into IGZO on the performance of TFTs.We found that the diffusion of Cu electrodes can seriously degrade the device performance.Therefore,to prevent Cu diffusing into IGZO,we introduced Molybdenum?Mo?as an intermediate layer to constitute Cu/Mo source/drain electrode of a-IGZO TFTs.The experiments results revealed that even after annealing at 300°C,the 30 nm Mo layer can completely prevent the diffusion of Cu electrodes.The obtained Cu-Mo?100/30 nm?TFT possesses a high mobility of 7.8 cm²/Vs and a low subthreshold swing of 0.20 V/dec.Moreover,it exhibited good contact performance with short current transfer length L_T?8.1?m?,low contact resistance R_CW?151?cm?and effective contact resistance R_C-eff(6×10^-2??cm²).Moreover,we successfully fabricated a-IGZO TFTs with Cu/ITO?In-Sn-O?stacked electrodes.Experimental results show that a 30 nm thick ITO layer can completely prevent the diffusion of the annealed Cu electrode at 300°C,which greatly improved device performance compared with pure Cu electrodes.The device exhibited a V_th of 4.0 V,a?_sat of 7.9 cm²/Vs,and an SS of0.21 V/dec.While it showed good contact properties with an R_CW of 41?cm,an L_T of 2.3?m and an R_C-eff of 5×10^-3?cm² at V_G=30 V.As the display resolution continues to increase,the application of Cu stack electrodes will meet problems of low yield and high manufacturing costs due to the different etch rates of Cu and barrier layer.Therefore,it is very important to explore a new single-layer electrode wiring material for IGZO TFT.In this thesis,we report high-performance a-IGZO TFT with novel copper-chromium alloy?Cu-0.5wt.%Cr?as source/drain electrodes.After annealed at 300?,the device performance was greatly improved which showed a V_thh of 2.5 V,a?_satat of 8.7 cm²/Vs,an SS of 0.23 V/dec.X-ray photoelectron spectroscopy?XPS?analysis revealed that the improvement should be due to the interaction between Cr and a-IGZO during the annealing process,which improves the conductivity of the active layer.Compared with the pure Cu electrode device,the Cu-0.5wt.%Cr alloy electrodes device also exhibited good contact characteristics with an R_CW of 109.1?cm,an L_T of 7.7?m and an R_C-eff of 4.2×10^-2?cm² at V_G=30 V.To further improve the performance and stability of TFTs,we successfully fabricated high-mobility a-IGZO TFT based on alumina oxide?Al₂O₃?passivation layer?PVL?and Cu source/drain electrodes?S/D?.The mechanism of the high-mobility for a-IGZO TFT was proposed and experimentally demonstrated.XPS and transmission electron microscope results revealed that the conductivity of the channel layer was significantly improved due to the formation of metallic Indium nanoparticles on the back channel during Al₂O₃ PVL sputtering.Meanwhile,Ar atmosphere annealing induced the Schottky contact formation between the Cu electrodes and the channel layer.In conjunction with high conductivity channel and Schottky contact,the a-IGZO TFT based on Cu electrodes and Al₂O₃ PVL exhibited a remarkable mobility of 220.1 cm²/Vs.Based on the above studies,we have also successfully fabricated a high-mobility TFT with Nd-Al-In-Zn-O?NAIZO?as the active layer.The TFT showed good electrical properties with a V_thh of 3.6 V,a?_satat of 412.7 cm²/Vs,an SS of 1.5 V/dec.