【作者】 张军；

【导师】 唐明华；

【作者基本信息】 湘潭大学 ， 微电子学与固体电子学， 2010， 硕士

【摘要】 铁电材料因为具有许多优良性质(如铁电性和压电性)，近年来被用于制作射频器件和非易失性存储器。微电子技术的发展使器件的尺寸下降到纳米级，在这个尺寸范围内忽略铁电材料的尺寸效应是不能接受的。铁电材料的尺寸效应与电极和铁电材料间的界面层有很紧密的联系。而界面层对铁电材料的可靠性(保持性能、印记和疲劳)有很大的影响。因此研究铁电材料的界面效应与尺寸效应对提高基于铁电薄膜材料的器件的性能是很有必要的。本文首先研究了界面层对铁电场效应晶体管性能的影响；然后用andau．．Ginzburg—Devonshire理论讨论了界面效应与尺寸效应的关系；最后研究了空间电荷在界面效应与尺寸效应中的作用。主要内容和结果如下：1．为了研究电极与铁电材料问界面层对铁电场效应晶体管的影响，构建了一个改进的铁电场效应晶体管模型。在模型中假设界面层有另一种介电／铁电相。通过这个模型，模拟了铁电场效应晶体管的C_v特性，，_Ⅳ特性和半导体衬底的表面势。模拟结果表明当界面层的厚度增加时，铁电场效应晶体管的性能变差。另一方面也说明界面层的存在与铁电场效应晶体管保持性能下降有一定的联系。2．用Landau．Gingzburg．Devonshir理论对铁电薄膜的界面层和薄膜厚度对铁电薄膜极化强度的影响进行了理论研究，界面层的影响用外推长度来衡量。考虑不同程度界面层的影响，模拟了不同厚度的铁电薄膜的极化强度分布、平均极化强度和极化强度最大值。得到了铁电材料的极化强度随薄膜厚度和外推长度的变化趋势。通过研究发现界面层抑制了界面附近的极化强度。研究还表明平均极化强度和极化强度最大值都随薄膜厚度的减小而减小，尤其是当薄膜厚度小于50nin时。研究结果表明界面效应是极化强度随薄膜厚度减小而减小的主要原因。3．用数值方法研究了空间电荷浓度和薄膜厚度对铁电薄膜内的电势和电场强度分布的影响。对Baudry的微分介电常数模型进行改进，得到一个新的介电常数表达式。通过研究不同厚度的铁电薄膜，发现铁电薄膜是完全耗尽还是部分耗尽与铁电薄膜的厚度和空间电荷的浓度有很大关系。数值结果还显示在电极界面层附近的电场强度随空间电荷浓度的增加而增强。同时还发现增加空间电荷的浓度使薄膜的介电常数增加，而使其极化强度减小。更多还原

【Abstract】 Recently,ferroeleetries have been used in RF devices and nonvolatile memoriesdue to their fascinating properties(such as ferroelectricity and piezoelectricity)．Micmelectronics technologies are pushing the size of devices down to nanorange．Atsuch scales，neglecting size effects becomes clearly unacceptable．It is reported thatsize effects are closely associated with the interface effects．On the other hand，interface effects have great impact on the reliability of ferroelectrics(retention,imprint and fatigue)．So it is necessary to study the interface effects and the sizeeffects to improve the performance ofdevices based on ferroelectric films．This paperfocuses on three parts．Firstly,we study the influence oftheinterface on the characteristics of metal--ferroelectric--insulator-semiconductorfield-effecttransistor(MFIS-FETs)．Thenwe studytheinterfaceandthickness effectson polarization in thin ferroelectric films using Ginzburg—Landau-Devonshire theory．Inthe end，we studythe space chargeandthickness effectsinferroelectricthinfilms．The main results are given as following：1．An improved MFIS-FET model is developed to study the effect of theferroelectric—electrode interface．By assuming the interface layer possesses asecondary phase，the capacitance-voltage(c-v)，current-voltagei-of the ferroelectric transistor and the surface potential of thesemiconductor are simulated．The modeling results show that the electricalcharacteristics of MFIS-FET become Worse as the interface layer thickness increases，inferringthe significant rolethatthethickness playsinthemodel．Itis alsoindicatedthatthe retentioncharacteristicofthedeviceisassociatedwiththeinterfacelayer．2．The effectsofinterface(representedbythe extrapolationlength)andthicknesson the polarization in ferroelectric thin films have been theoretically investigatedusing Landau--Gingzburg--Devonshire(LGD)theory．Considering different degree ofimperfectionoftheinterface，thepolarizationprofile，the averagepolarizationandthemaximal polarization of various film thicknesses have been numerically simulated．The general trend ofpolarization against the extrapolation length and the thickness isobtained．Itisfoundthatthepolarizationneartheinterfaceis dramatically suppresseddue to the interface effects．The simulated result also shows that the averagepolarization and maximal polarization decreases with thickness，especially when the thckness 1S below 50 nm．It 1S demonstrated that the interface effects are the majorreason ofthe reduction ofthe polarization along with the thickness decrease．3．Atheoreticalmodelis presentedto numerically examinethe effects of spacecharge concentration and thickness on potential and electric field distributions inferroelectric thin films．An expression of the relative dielectric constant is derived byimproving Baudry’S differential permittivity model．The nmcl-ical results indicatethatthefilm depletionis affected significantlybythethickness andthe space chargeconcentration．It is found that the increase of the space charge density wouldstrengthen the electric field in the vicinity ofthe electrode interface．We also foundthat the dielectric constant is enhanced and the film polarization is weakenedremarkably with increasing the space charge density for a given film thickness．更多还原