| With the mature commercialization of active matrix display technology and rapid burst of flexible electronics,thin film transistors' structure designing,material choosing and electrical properties improving have become a field that researchers are consistently exploring.Thin film transistors(TFT)play an important role as switching device in those applications.And oxide semiconductor is the most promising candidate as the channel layer material compared to poly-crystal silicon,hydrogenated amorphous silicon and organic semiconductors,taking many factors including mobility,long-term stability,fabricating temperature,fabricating cost and flexibility into account.Within the family of oxide semiconductors,amorphous oxide semiconductors are the most potential ones,which is attributed to the fabricating temperature being quite close to the room temperature and great conformity of electrical properties because of the absence of grain boundaries.Among them,amorphous IGZO is most promising in terms of high mobility,good conformity,transparency and flexibility.In TFF devices,subthreshold voltage swing(SS)is the key parameter of measuring On/Off switching speed.A smaller SS represents higher On/Off switching speed.However,IGZO TFTs using all kinds of gate materials at present generally have a SS value higher of 100 mV/decade,which is quite inferior to 70?100 mV/decade of Sibased MOSFET and 60 mV/decade of two-dimensional material TFT.In order to reduce SS values of IGZO TFT,in experiments we designed and fabricated TFT devices of staggered top-gate structures based on amorphous IGZO channel,which were patterned by lithography and fabricated by means of several vapor deposition technologies including RF magnetron sputtering,electron beam vapor deposition and atomic layer deposition.We have explored approaches to reducing SS of IGZO TFT devices from aspects of gate material,mobility and device structure.In order to obtain higher dielectric constant of gate material,dielectric enhancement effect in interfacial polarization was deduced theoretically and simulated numerically.In summary,we draw conclusions in this article as followed:1)IGZO thin film was sputtered under the condition of 150 W power,0.5 Pa pressure,20 minutes of depositing and gas ratio Ar:O2=30:1.Elements composition of IGZO films characterized by XPS was In:Ga:Zn:O=1:0.98:0.55:2.91.Thickness of IGZO films was measured as 112 nm when sputtering for 20 minutes by ellipsometer.IGZO films annealed under 400? was detected as amorphous state by XRD.And the transforming temperature from amorphous state to crystalline state was between 400? to 500?.2)By measuring output characteristics and transfer characteristics,SS,mobility and threshold voltage were calculated.By comparing SS values of TFTs with different ratios of W/L and different mobilities,SS and mobility were correlated while SS and W/L were not.IGZO thin films were sputtered under three different oxygen partial pressure.When IGZO thin film was sputtered in the smaller oxygen partial pressure of 3.2%,a bigger mobility of 9.78 cm2/(V·s)was obtained along with a smaller SS value of 103 mV/decade.3)In IGZO TFTs fabricated under optimized conditions,SS values of TFTs with gate material of SiO2,A12O3 and ionic liquid were 136 mV/decade,103 mV/decade and 67 mV/decade respectively.Thus,using ionic liquid as gate material had reduced SS of IGZO TFTs significantly,making it close to SS value of two-dimensional material TFT.Dielectric constants of SiO2,Al2O3 and ionic liquid increased gradually,while SS values of IGZO TFT using them decreased gradually.4)In order to obtain higher dielectric constant of gate insulator,in this article we had applied Maxwell-Wagner model in interfacial polarization.By further numerical simulation,it is substantiated that in dual-layer system like oxide dual-layer,dielectric constant could be enhanced by adjusting resistivity of one layer.So higher dielectric constant of gate insulator could be obtained in this way. |
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