| Currently,thin film transistors(TFTs)have been widely used as switching and driving elements in flat panel displays.Traditional silicon-based TFTs have been unable to meet the emerging technical requirements of display devices such as high resolution,high responsivity,and flexibility.The main reason is that silicon-based materials have many shortcomings.For example,amorphous silicon TFTs have low mobility and poor stability.The grain boundary of polysilicon will reduce the uniformity of TFTs performance.Compared with silicon-based materials,amorphous oxide semiconductors(AOSs)have been extensively studied because of their good photoelectric properties and uniformity.In particular,indium gallium zinc oxide(IGZO)has been a research hotspot in the field of TFTs since 2004.However,with the in-depth research,IGZO also exposed many problems that are difficult to solve.For example,its relatively narrow optical band gap(Eg～3.2eV)causes IGZO TFT to easily exhibit unstable device performance under ultraviolet illumination.At the same time,IGZO TFT also exhibits dissatisfactory electrical stability under the influence of other external factors.Therefore,it is very necessary to find an AOS material which is more suitable for the active layer of TFTs.By replacing Ga element in IGZO with Al element to form indium aluminum zinc oxide(IAZO)material,the Eg width and modulation range of the material can be significantly increased,which helps to enhance the illumination stability of semiconductor films and related devices.At the same time,the replacement of the expensive and rare metal element Ga by A1 will greatly reduce the manufacturing costs of films and devices.In addition,the Al-O bond has a higher bonding energy,which helps to enhance the ability to tune the carrier concentration,thereby improving the electrical stability of the semiconductor film and related devices.Therefore,IAZO is a promising TFT material.At present,IAZO TFTs still have three main problems.First,there are few studies on the basic fabrication process of IAZO TFTs,especially related to RF magnetron sputtering.And the device performance needs to be improved.Second,the overall preparation temperature of IAZO TFTs is relatively high,which is not conducive to their application in the field of flexible electronics.Third,the application research of high-κ gate dielectric materials such as Al2O3 and HfO2 in IAZO TFTs is relatively scarce.In this thesis,a series of systematic and in-depth researches were carried out targeting at the above-mentioned problems.This thesis first investigated the effects of annealing treatment,substrate temperature,and sputtering power on the performance of IAZO films and TFTs prepared by RF magnetron sputtering,thereby optimizing the basic fabrication process conditions of IAZO TFTs.Next,the IAZO films were treated with UV-ozone to realize the low-temperature preparation of IAZO TFTs,and the bilayer IAZO films structure was used to realize the room-temperature preparation of IAZO TFTs.Finally,the growth conditions and post-treatment process of the Al2O3 and HfO2 gate dielectric layers were explored and optimized,thereby obtaining good device performance of IAZO TFTs.The specific research contents of this thesis are as follows:1.Exploration and optimization of basic fabrication process for IAZO films and TFTsFirst of all,the effects of annealing treatment on the properties of sputtered IAZO films and different electrode materials on the performance of IAZO TFTs were investigated.This research indicated that annealing treatment can effectively tune the carrier concentration and other electrical properties of IAZO films.For IAZO TFTs,a Schottky contact was formed between Au and IAZO.Although IAZO TFT exhibited a higher saturation mobility(μsat),the larger hysteresis value would severely restrict its further research and application.In contrast,an Ohmic contact was formed between Ti and IAZO.The corresponding IAZO TFT showed a very ideal hysteresis value,but the mobility and other properties of this device still needed to be improved.Then,by investigating the effect of the sputtering substrate temperature on the performance of IAZO films and TFTs,we observed that the IAZO film prepared at room temperature exhibited relatively better photoelectric properties,and the IAZO TFT also showed more outstanding overall electrical performance.Meanwhile,the IAZO TFT fabricated by sputtering at room temperature exhibited satisfactory positive bias stability.Finally,we investigated the effect of sputtering power on the performance of IAZO films and TFTs,and found that low power(90W)was more suitable for preparing IAZO film with excellent photoelectric properties and TFT with higher electrical properties.This was mainly because the IAZO films deposited at low power existed the least defects.In addition,in the above three parts of the research,the annealing conditions were constantly optimized,and the overall performance of the IAZO film and TFT had been significantly improved.Therefore,after exploring and optimizing the basic preparation process of IAZO films and TFTs,we confirmed that IAZO was indeed a suitable active layer material.The transmittance of IAZO in visible region was extremely high and its Eg(about 4.1 eV)was much larger than IGZO,indicating that IAZO has better illumination stability.At the same time,the maximum Hall mobility of IAZO film exceeded 70cm2/Vs,indicating that it has potentially excellent TFT performance.Through the research in this part,we have fabricated IAZO TFT with excellent performance.It simultaneously exhibited a μsat of 12.08cm2/Vs,an on-off current ratio(Ion/Ioff)of 7.75×107,as well as a smaller hysteresis value(-0.19V)and threshold voltage(VT=-1.38V).2.Low temperature and room temperature preparations of IAZO films and TFTsFirst,we investigated the effect of UV-ozone treatment on the performance of IAZO films and TFTs.The results show that the surface morphology of the IAZO films became smoother and flatter after UV-ozone treatment,and it can effectively tune the carrier concentration and other electrical properties of IAZO films.When the UV-ozone treatment time was 5min,the IAZO film can obtain the highest Hall mobility(29.4cm2/Vs).At the same time,IAZO TFT also showed relatively optimal device performance,especially the higher μsat(8.76cm2/Vs)and Ilon/Ioff(6.44×108).In addition,the 5min UV-ozone treated IAZO TFT also showed good negative bias illumination stability,and the threshold voltage variation under 3000s was only-0.51V.Therefore,UV-ozone treatment can effectively realize low temperature fabrication of IAZO TFTs.Next,we investigated IAZO TFTs with dual active layer structure.When the bottom layer(high electron concentration)/top layer(low electron concentration)thicknesses were respectively 20/10nm,the IAZO film can obtain the highest Hall mobility(23.3cm2/Vs).And the IAZO TFT(Gate dielectric layer is SiO2)using this active layer structure also exhibited relatively optimal electrical performance,including higher μsat(12.05cm2/Vs)and Ion/Ioff(1.05×108),as well as smaller VT(4.10V),hysteresis(0.22V)and subthreshold swing(SS=0.93V/dec).At the same time,this device also showed relatively ideal negative bias illumination stability,positive bias illumination stability and laser illumination reliability.On this basis,we combined the Ta2O5 gate dielectric layer fabricated by sputtering at room temperature with the above-mentioned optimal dual active layer structure,thereby realizing the room temperature fabrication of high-performance IAZO TFT.The μsat of this device reaches 19.56cm2/Vs,as well as SS and VT are as low as 81 mV/dec and 1.24V,respectively.At the same time,the maximum threshold voltage variations of this device under positive bias illumination stress and laser illumination stress were only-0.78V and 0.05V,respectively.3.High-K dielectric layers grown by ALD used to fabricate IAZO TFTsFirst of all,this thesis studied the effect of ALD growth temperature on the performance of A1203 films and IAZO TFTs.When the ALD growth temperature was 150℃,the Al2O3 film had more metal-oxygen binding bonds,as well as fewer internal defects and oxygen-related surface adsorbates.The Al2O3 film also exhibited a smoother surface morphology at this temperature,and its surface root-mean-square roughness was only 0.18nm.And Al2O3 grown at 150℃ had the largest relative-dielectric-constant(6.5)and the highest breakdown voltage(approximately-25V).As for the device performance,the IAZO TFT using the Al2O3 gate dielectric layer grown at 150℃ showed relatively optimal electrical performance,including higher μsat(11.39cm2/Vs)and Ion/Ioff(1.25×108),as well as smaller VT(0.32V)and SS(0.13V/dec).At the same time,this device also exhibited ideal positive bias stability and laser illumination reliability.Next,this thesis investigated the effects of UV-ozone treatment on the performance of HfO2 films and IAZO TFTs.After UV-ozone treatment,the HfO2 film had more metal-oxygen binding bonds,as well as fewer internal defects and oxygen-related surface adsorbates.And the root-mean-square roughness of the HfO2 film surface was optimized from 0.101nm to 0.051nm.Compared with Al2O3,HfO2 prepared in this thesis had a higher relative dielectric constant(over 15).After comparing the device performance,it was found that the IAZO TFT with UV-ozone treated HfO2 gate dielectric layer exhibited better output and transfer characteristics,and its overall electrical performance had also been improved,including higher μsat(13.64cm2/Vs)and Ion/Ioff(1.18x107),as well as smaller VT(0.67V)and SS(0.11V/dec). |
