Research On Synergistic Improvement Of Electrical Performance And Photo-bias Stability Of Indium Gallium Zinc Oxide Thin-film Transistor Devices

Thin-film transistors?TFTs?are core building blocks to control the millions of subpixels in the backplanes of liquid crystal displays and active-matrix organic light-emitting diode displays.InGaZnO?IGZO?is a more promising material compared with amorphous silicon and low temperature polysilicon owing to its room temperature processability,small channel leakage currents and high transmittance,so it has attracted the interest of researchers.However,how to improve the electrical performance and photo-bias stability of IGZO TFT devices synergistically is still an urgent problem to be solved.Firstly,a series of IGZO thin films were prepared by radio frequency magnetron sputtering with different O₂:Ar ratios.In the visible range of wavelength,the average transmittances of all films are above 80%.The optical band gap of the films increases with the increase of the O₂:Ar ratio,and reaches the maximum value when the O₂:Ar ratio is 1.11:10.As the O₂:Ar ratio increases,the resistivity of IGZO films increases from 10^-2?·cm to 10⁵?·cm,and the carrier concentration decreases from 10¹⁹ cm^-3 to 10¹¹ cm^-3.When the O₂:Ar ratio is 1.11:10,the carrier concentration of the film shows the lowest value.In addition,IGZO thin films with different O₂:Ar ratios were also prepared as semiconductor layers for thin-film transistors.The results show that the performance of the device is the optimal when the O₂:Ar ratio is 1.11:10.It can be noted that the most suitable O₂:Ar ratio is obtained for high performance thin-film transistor.After obtaining the optimal O₂:Ar ratio for the preparation of IGZO thin film,hafnium oxide with a relatively high dielectric constant is selected as the insulating layer material for transistors,and it is prepared by atomic layer deposition.IGZO TFTs with different thicknesses of hafnium oxide were prepared.The results show that the device has the best performance when the thickness of hafnium oxide is 200nm.Two Al₂O₃ passivation layers with a thickness of 2 nm were introduced both at the upper and lower interfaces of the hafnium oxide insulating layer forming sandwich structure to decrease the defects in the hafnium oxide.The results show that the performance of the device is significantly improved after the introduction of Al₂O₃ passivation layer.IGZO TFTs with different electrode materials of Al,ITO and Ti were prepared in order to select a suitable electrode material.It is found that Al is the most suitable electrode material.In order to improve the photo-bias stability of IGZO TFT,N₂ was simultaneously introduced into the chamber during the preparation of IGZO thin film.The electrical performance and stability of devices under different nitrogen flow rates were analyzed.Results show that the electrical performance of the devices decreases as we increase the nitrogen flow rate.In contrast,the stability of the devices becomes better first and then worse.IGZO TFT has the smallest threshold voltage shifts after the positive bias stress and negative bias illumination stress test of 10800 s,indicating that the the device has the best stability with the nitrogen flow rate of 0.3 sccm.The results indicate that the stability of IGZO TFT can be improved through a proper doping of nitrogen.In order to alleviate the decrease of the electrical performance after nitrogen doping,we introduced a ultrathin IGZO layer with high carrier concentration in the front channel?which was deposited in the atmosphere of Ar?.Furthermore,graded IGZO/IGZO:N TFT was prepared by regulating sputtering gas.The results show that the electrical performance of the device is significantly improved when the thickness of the high carrier concentration layer is optimized to 1.5 nm.The device shows excellent performance including a saturation mobility of 93.58 cm²/Vs,low threshold voltage of 0.38 V,large on/off current ratio of 1.67×10⁹ and small subthreshold swing of 73.76 m V/dec.In addition,the threshold voltage shifts of the IGZO TFT after positive bias stress and negative bias illumination stress test of 10800 s are as low as+0.1 V and-0.21 V.The results show that graded channel doping can improve the electrical performance and photo-bias stability of devices.In this paper,dual-channel layer IGZO/IGZO:N?1.5 nm/98.5 nm?TFT was prepared at the same time.It is found that the electrical performance and stability of as prepared TFT is worse than that of graded device.The synergistic improvements of both performance and stability are attributed to the well-controlled concentration and distribution of the V_O defects within the single IGZO channel,where orders of magnitude enhancement of the accumulated electron donors are induced by the V_O-rich front-channel region while keeping barrier-free full depletion through elimination of the junction interface.Meanwhile,the trapped electrons and photoexcited holes at the V_O-less channel bulk and back surface region are suppressed by N-doping.Our study shows that graded channel doping engineering could be a feasible way to synergistically boost the electrical performance and photo-bias stability of oxide TFTs.This method is simple and cost-effective,which is expected to be widely used in industrial fabrication in the future.