Influence Of Ga Doping On Electrical Performance And Stability Of ZnO Thin-film Transistors Prepared By Atomic Layer Deposition

Metal oxide semiconductor field-effect transistors?MOSTFTs?have recently attracted a great deal of attention in ultra-high definition flat panel displays with high frame rates and large sizes.Compared to conventional amorphous silicon?a-Si?TFTs,metal oxide based TFTs have desired field-effect mobility(m_FE),which can meet the basic requirement of high-driving current for active-matrix organic light-emitting diode?AMOLED?displays.Furthermore,it provides a solution for the non-uniformity of polysilicon TFTs[6].So far,Zn O?ZO?,In Ga Zn O?IGZO?and In Zn O?IZO?have been widely studied.As for the channel layer materials of TFTs,Zn O is the most widely studied due to its abundance,simple composition,and environmental friendliness.However,instability under bias stress voltage of the pure Zn O TFTs remains a tremendous challenge because of the oxygen vacancy defects inside the channel layer.With the development of TFTs,multicomponent materials have been used to improve the electrical performance of TFTs,such as IGZO,IZO and Zn In Sn O?ZISO?.However,there is an issue of vital concern for the use of indium due to its high cost and environmental objection.Therefore,it is urgent to seek suitable doping elements to improve the stability of the oxide TFTs.Aiming at improving the instability of Zn O TFTs,GZO TFTs are prepared and studied by atomic layer deposition?ALD?in this work.The specific contents are as follows:First,GZO TFTs with the doping concentration of n _Ga:n _Zn=1:50 were prepared by ALD at 150 ?,and the annealing treatments were performed,with the annealing time fixing at 30 min and the annealing temperature varying form 200 ? to 500 ?.It was found that the device was still unable to work at the annealing temperature of 200 ?,not appearing the turn-off characteristic of a typical TFT.When the annealing temperature increased to 300 ?,however,the device exhibited the best electrical performance:a mobility?m_{F E}?of 8.1 cm²/Vs,a threshold voltage(V_th)of 6.3 V,a sub-threshold swing?SS?of 0.51 V/dec,and a current switching ratio for 10⁷.As the annealing temperature increases,the performance of the device begins to decrease,and them_FE from 8.1 cm²/Vs at 300 ? to 6.4 cm²/Vs at 400 ? to 4.5 cm²/Vs at 500?,the V_th and SS also gradually deteriorated.Next,in order to explore the applicability of GZO on flexible transparent substrates,the performance of n _Ga:n _Zn=1:150 GZO TFTs annealing at 200 ? was also studied,with the annealing time of 30,40,60,80,100,120,140,180,200,220 and 240 min.It can be seen from the results that the device annealed at the low temperature for a long time can also obtain the same good performance as the device annealed at 300 ? for 30 minutes.For example,the device annealed at 200 ? for 220 min achieves the best device performance at this temperature:them_FE is 23.4 cm²/Vs,the V_th is 3.2 V,the SS is 0.29 V/dec,and the current switching ratio is 10⁷.Second,in order to determine the optimal doping concentration,we prepared GZO TFTs with n _Ga:n _Zn=1:1,1:10,1:30,1:50,1:75 and 1:150 by ALD.From the fitted semiconductor parameters,it can be seen that the SS of all GZO TFTs is less than 0.6V/dec,showing the excellent interfacial state between the channel layer and the dielectric layer.In the series of doping concentrations,1:75 GZO TFTs show the best electrical properties:with am_FE of 16.2 cm²/Vs,a V_th of 6.0 V,a SS of 0.22 V/dec,the current switching ratio of 10⁷ and the lowest charge trap density(N_it)of 3.1×10¹².In order to study the effect of Ga doping on the stability of Zn O TFTs,the gate bias voltage V_GS=+10 V was applied to various concentrations of GZO TFTs and their transfer curves were measured in the original state.The results show that the stability of GZO TFTs are better than that of pure Zn O TFTs.Among those TFTs,1:75 GZO TFTs show the best positive bias stress?PBS?.Its threshold voltage deviation(?V_th)after continuous pressure application for 3600 s is only 0.14 V,while the?V_th of Zn O TFTs under conditions is as high as 0.89 V.Next,we placed the 1:75 GZO TFTs and Zn O TFTs under the same conditions in the air?temperature stable at 25 ?,humidity stable at 20%RH?to study the effect of Ga doping on the air stability of Zn O TFTs.Compared with the original transfer characteristic curves of the two types of TFTs and the transfer characteristic curves after 10 weeks,the Zn O TFTs are shifted to a greater extent.The above results show that Ga doping can not only improve the bias stability of Zn O TFTs,but also improve the air stability of Zn O TFTs.Finally,we measured the ultraviolet-visible spectroscopy spectra of 1:75 GZO and Zn O films.The transmission of 1:75 GZO is 85%,which is greater than 75%of the Zn O films.Moreover,from the transmission spectra fitting,the optical band gap of1:75 GZO is 3.20 e V,and the optical band gap of Zn O film is 3.11 e V,which shows that Ga doping can widen the band gap and improve transmittance of Zn O semiconductor materials.