The Investigation Of Negative Capacitance Field Effect Transistors Based On Ferroelectric Material

Negative Capacitance Field-Effect Transistor?NCFET?,one of the most promising candidates for ultralow power applications,has been attracting lots of interests,due to its capability of achieving sub-60 mV/decade subthreshold swing?SS?via the negative capacitance?NC?effect induced by the ferroelectric film inserted into gate stack.Although the validity of NCFET has been initially established,there is still a lack of exploration and understanding of its typical electrical characteristics.In this dissertation,the typical electrical characteristics of NCFETs was experimentally investigated,aiming at paving the road for its practical applications.This dissertation includes four parts:1)The characterization of the typical properties of NCFETs,2)The investigation on the design principle of high-performance NCFET,3)The exploration of the frequency dependent characteristics of NCFET,and 4)The clarification of the physical mechanism of NCFET.1.The characterization of the typical properties of NCFETsGe and GeSn channel HfZrO_x?HZO?NCFETs were fabricated and characterized systematically.With rapid thermal annealing temperature(T_Annealing)of HZO?6.5 nm?at450 ^oC,Ge NCFET demonstrates sub-60 mV/decade steep SS,sub-40 mV hysteresis,and22%enhancement in I_DS compared to the control MOSFET.The typical characteristics of NCFET including,NDR phenomena in I_DS-V_DS curves and capacitance peaks in C_G-V_G curves are first experimentally observed in both Ge and GeSn NCFETs.It is experimentally demonstrated that the minimum intensity of NDR in NCFETs with t_FE=6.6,4.5 and 3.7 nm are-10,-4,and⁰?S/?m,respectively,which means the NDR in NCFET is positively correlated with the degree of capacitance matching.The experimental results evidently confirm the feasibility of NCFET and NC effect via its improved electrical performance and typical phenomena.2.The investigation on the design principle of high-performance NCFETIt is theoretically reported that the hysteresis of NCFET can be eliminated by increasing the negative ferroelectric capacitance(|C_FE,N|)to underlying MOS capacitance(C_MOS)ratio.The remnant polarization?P_r?and coercive field?E_c?of HZO,which determines the|C_FE,N|[=?2/3????×(P _r/E_c×t_{HZ O})],can be tunned by changing T_Annealing.Our experiments revealed that,as T_Annealing varies from 350 to 450 ^oC,HZO exhibits a significant increase in the ratio of P_r/E_c,which results in the improvement of the magnitude of|C_FE,N|,therefore contributing to the reduction of hysteresis of the ferroelectric NC transistors.The most effective way to adjust the ratio of|C_FE,N|/C_MOS is to change the coverage area of the ferroelectric film on the gate region of the NC transistor.As the area of HZO is increased,the increased|C_FE,N|produces the much better capacitance matching between|C_FE,N|and the C_MOS,contributing to the elimination of hysteresis of the NCFET.In addition,the ratio of|C_FE,N|/C_MOS also can be tuned by the variation of the thickness of the ferroelectric film(t _FE),which has the similar impact on the electrical performance of NCFET.In conclusion,it is experimentally investigated and clarified the design principle of high-performance NCFET based on the rule of capacitance matching.3.The exploration of the frequency dependent characteristics of NCFETWhether the NC transistors can achieve sub-60 mV/decade SS while maintaining hysteresis-free characteristics at high frequency is critical for the ultralow power logic applications.We reported the first investigation of frequency dependence of performance in Ge HZO NCFET.For Ge NCFETs with internal gate,pulse measurement produces a significant increase in hysteresis compared to the DC measurement,but devices still remain sub-60 mV/decade SS.Ge NCFETs without internal gate show the little dependence of hysteresis,SS,and capacitance characteristics on pulse measurement?from DC to 1?s?.Ge NCFET without internal gate still achieves sub-30 mV/decade SS and 110mV hysteresis with 1?s pulse measurement,and significant peak in gate capacitance due to the NC effect at MHz.The different frequency dependences of performance in NC devices with and without internal gate might be attributed to the difference in the mechanism of capacitance matching.4.The clarification of the physical mechanism of NCFETWe experimentally investigated the nature of NC effect via the comparison between the measured voltage gains and the calculated results considering the depolarization field.It is demonstrated that the measured and calculated results show good consistency,revealing the fact that NC effect originates from the depolarization field of the ferroelectric film.In addition,it is also experimentally investigated the impact of polarization?P?switching behaviors on hysteretic and near hysteresis-free NCFETs.Compared with the typical abrupt P switching in hysteretic devices,near hysteresis-free NCFETs show the gradual and continuous response of P under applied voltage,indicating that the mechanism underlying near hysteresis-free characteristics is incomplete dipoles flipping,rather than complete dipoles switching.