High-performance Negative-capacitance Field-effect Transistors Based On Doped HfO₂ Ferroelectric Thin Films And Two-dimensional Channel Materials

With the continuous development of Moore’s Law,the integration of chips continues to rise,and the size of devices continues to develop in the direction of miniaturization.However,when the CMOS process reached the 10-nm technology node,the further reduction of device sizes is hindered.One of the main problems is that it is difficult to continuously reduce the power consumption of devices.One of the ways to reduce the power consumption of devices is to reduce its subthreshold swing（SS）.According to the Boltzmann limit,the minimum SS of conventional field-effect transistors is only 60 mV/dec.The negative-capacitance field-effect transistor（NCFET）,which can obtain an internal voltage amplification by means of so-called negative-capacitance effect through inserting a ferroelectric thin film into its gate stack,is one of the candidates to prepare ultra-low power consumption devices in the future,since it can break through the Boltzmann limit and achieve a SS below 60 mV/dec.The development of silicon-based technology has reached its physical limit.Two-dimensional transition metal sulfides,e.g.molybdenum disulfide and tungsten disselenide,possess some excellent characteristics such as no dangling bonds on the surface,almost no tunneling effect,and significant suppression of short channel effects,due to their atomic-level ultra-thin thickness,high carrier mobility,suitable band-gap width and excellent CMOS compatibility.It is one of the ideal channel materials to replace silicon-based materials and prepare nanoscale CMOS devices.This thesis combines doped HfO₂ ferroelectric thin films,which has a negative-capacitance effect,with two-dimensional materials to prepare high-performance NCFETs.The specific research contents are as follows:Firstly,Hf_0.5Zr_0.5O₂（HZO）thin film was prepared by atomic layer deposition（ALD）.The ferroelectricity of the HZO thin film under different annealing temperatures and thicknesses conditions were investigated.The corresponding transistors were prepared by using HZO/Al₂O₃ as stacked gate dielectric and MoS₂ as channel material.With the increase of annealing temperature from 350°C to 650°C,the ferroelectricity（remanent polarization intensity/coercive field,P_r/E_c）of the HZO films increases first and then decreases.At the annealing temperatures of 600～650°C,the HZO films exhibit the best ferroelectricity.At the same time,as the thickness of the HZO film increases,its ferroelectricity（P_r/E_c ratio）is gradually enhanced.On this basis,the corresponding HZO/Al₂O₃/MoS₂ negative-capacitance transistor is prepared and its SS is lowered to 33.1 mV/dec.Then,CF₄ plasma is used to treat the HZO film,and the F ion is used to passivate the surface dangling bonds and oxygen vacancies in the gate stack,which reduces the surface roughness of the gate dielectric,improves the interface quality,and increases the k value and ferroelectricity of the HZO film.Compared with the untreated HZO film,the P_r/E_c of the treated HZO film was changed from5.74 p F/cm to 8.39 p F/cm,corresponding to the negative capacitance transistor SS reduce from37.4 mV/dec to 17.8 mV/dec.In order to reduce the hysteresis of MoS₂ transistor,a backgate MoS₂ negative capacitance transistor with 2 nm HfO₂ as the encapsulation layer is prepared,and a backgate MoS₂ negative capacitance transistor without hysteresis is prepared by using the shielding effect of high k coating layer.Then,in order to further enhance the ferroelectric properties of thin films,the negative capacitance transistor characteristics of Hf,Zr and Al mixed oxide HfZrAlO（HZAO）as ferroelectric layer are studied.When a certain amount of Al₂O₃ was added into HZO thin film,HZAO thin film had more orthointersection phase crystallization.Compared with HZO thin film,the P_r/E_c of HZAO thin film changed from 8.14 p F/cm to 9.21 p F/cm.The SS of the prepared NCFET is further reduced to 12.8 mV/dec.In order to further simplify the preparation of transistors,this paper studied the single layer of HZAO as a negative capacitor layer and positive capacitor matching layer of negative capacitor transistor,through the Al content and annealing temperature of HZAO thin film were studied,show that when the Al content is 5%,When the annealing temperature is 650 to 700°C,Hf_0.475Zr_0.475Al_0.05O_y film has the strongest ferroelectric property and P_r/E_c is reached.Using single-layer HZAO as the grid medium,the corresponding NCFETs are prepared,and the SS of the device reaches 20.4 mV/dec.And realized hysteresis free,compared with the traditional HZO/Al₂O₃ structure NCFETs,further simplified the device preparation process.Finally,considering the further practical application of NCFETs,a backgate negative capacitance inverter with WSe₂ as a channel material and ferroelectric HZAO film as gate medium was prepared in this paper due to the fact that WSe₂ can achieve bipolarity without doping:（1）First,the contact metal of p-type WSe₂ was studied and the optimal contact metal was found to be Pt,the SS of it reaches 22.5 mV/dec,the mobility rate is 13.1 cm²/Vs.Hysteresis free p-type NCFET;（2）Then,we studied the contact metal of n-type WSe₂ and found that the optimal contact metal was Ti,and the obtained NCFET achievedthe SS of 26.6mV/dec.And the mobility is up to 8.9 cm²/Vs.And realize the characteristics of hysteresis free;（3）On the basis of（1）and（2）,p-WSe₂-NCFET and n-WSe2-NCFET were connected in series to prepare a negative capacitance inverter with a maximum voltage gain of 55.The results show that the negative capacitor can be used not only in transistors,but also in small logic devices and small analog circuits.