Study On Interface Effects And Size Effects Of Ferroelectric Thin Films

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．...