Memory Properties Of Field Effect Transistors Based On Molybdenum Disulfide/Bismuth-based Ferroelectric Materials

Bismuth ferrite(BiFeO₃,BFO)and bismuth titanate(Bi₄Ti₃O₁₂,BTO)have the advantages of high permittivity and high polarization,so they have great application prospects in semiconductor devices.Researchers combined ferroelectric materials with field effect transistors to invent ferroelectric field effect transistors(FeFETs).It realizes the function of data memory.FeFETs have the advantages of non-volatile data memory,fast data read or write speed,low operating power consumption of the devices,long service life and compatibility with chip manufacturing process flow,which make FeFETs always attract the attention of researchers.At the same time,the two-dimensional Mo S₂has an atomic-level thickness,which can be integrated in field-effect transistors as a conductive channel.And when two-dimensional Mo S₂is used as the channel of the transistors,it has the characteristics of high mobility,large I_on/I_offratio and suppression of short-channel effect.It has great potential in the development of next-generation integrated circuits.In this work,bismuth ferrite doped with La,Mn and bismuth titanate doped with La were prepared by a sol-gel method.The two ferroelectric materials prepared were tested by various instruments.The morphology,optical properties and electrical properties were studied.Afterwards,FeFETs were prepared by doped bismuth ferrite and bismuth titanate as the ferroelectric layer and two-dimensional molybdenum sulfide as the conductive channel.The output characteristic curves,transfer characteristic curves and memory performance of these two kinds of FeFETs were studied.The main research results include the following three points:(1)Bi_1-xLa_xFe_0.92Mn_0.08O₃(BLFMO,0?x?0.15)films were successfully prepared by a sol-gel method.The prepared thin films had the best performance and were suitable as a ferroelectric layer of FeFETs through test analysis when x=0.15.In this work,the optimal sol-gel method for BLFMO thin films was developed.BTO and BLFMO thin films were prepared on heavily doped silicon substrates with the optimal sol-gel method,respectively.XRD,AFM,SEM and Raman spectroscopy were used to analyze the effect of doping on BFO crystal,the influence of lattice structure,surface morphology and optical properties.At the same time,the hysteresis loop and polarization reversal velocity of the sample were analyzed.It was found that the remanent polarization of the sample reached 30?C/cm~2and the reversal velocity was about 0.1 ms when x=0.15,which provided a theoretical basis for the preparation of FeFET.(2)FeFETs with a back-gate structure were prepared with two-dimensional Mo S₂as the conductive channel,aluminum oxide as the dielectric layer and BLFMO as the ferroelectric layer and these devices feature nonvolatile multi-bit memory.It is known that the devices have excellent electrical properties such as memory windows and I_on/I_offratio after analyzing the transfer characteristic curves of the prepared bismuth ferrite-based FeFETs.It is known that the source-drain electrode of the device is ohmic contact with the channel after analyzing its output characteristic curve.In addition,Si/Si O₂/Al₂O₃/Mo S₂structure transistors were prepared as a comparison device.By comparing the transfer curves,it can be known that the memory windows of the present FeFETs are caused by ferroelectric polarization.At the same time,voltage pulses of different amplitude and directions are applied to the gate of FeFETs.The analysis shows that the prepared FeFETs have good retention characteristics and stable operation characteristics.After applying different amplitudes of gate voltages to FeFETs,it can be observed that the devices have the function of multi-bit memory.the working principle of the multi-bit memory function of the devices are analyzed,which provides a new idea for improving the memory density of FeFETs.Finally,multiple devices were fabricated using the same preparation process.The results showed that each device has a larger memory window,proving that the preparation process of our bismuth ferrite-based FeFETs has a high success rate of good products.(3)Bi_3.25La_0.75Ti₃O₁₂films were prepared by a sol-gel method.They were used as the ferroelectric layer of FeFETs,and their non-volatile memory were successfully prepared.The lattice structure and grain size of the film samples were analyzed.We analyze the surface morphology and surface roughness of the films.It is found that these samples have relatively smooth surfaces and low root mean square roughness.In terms of electrical properties,it can be known that they have high permittivity and high remanent polarization from the hysteresis loop of the samples,which are suitable for application in FeFETs.Then,we used Bi_3.25La_0.75Ti₃O₁₂films as the ferroelectric layer of FeFETs,also used Al₂O₃as the dielectric layer and two-dimensional Mo S₂as the channel.We tested the output and transfer characteristic curves of the devices.It suggests that the devices have a higher I_on/I_offratio and ohmic contact as well as,while also achieving an ideal memory windows.In addition,the devices have obvious multi-bit memory function after applying voltage pulses.The FeFETs prepared in the above experiments have the characteristics of non-volatile memory,high I_on/I_offratio and low power consumption.It has great application prospects in the next generation of ferroelectric memories.In summary,we study the morphological characteristics,lattice structure,optical and electrical properties of doped BFO and BTO ferroelectric films.The BFO and BTO-based FeFETs were prepared,respectively.Various characterizations and electrical measurements were performed on these two kinds of FeFETs to analyze the performance of these devices in detail,providing new ideas for the development of new FeFETs.