Germanium-based Field Effect Transistor With ZrO₂ Gate Dielectric

For the purpose of continuous scaling of CMOS roadmap in the future advanced integrated circuit,the metal oxide semiconductor field-effect transistor(MOSFET)has recently introduced new channel materials and novel gate dielectrics in terms of functionality,switching speed,power dissipation and cost in chips.The germanium(Ge)has the higher hole mobility compared to silicon(Si),which is very suitable for p-channel devices.The application of a high-?dielectric for the gate dielectric can continuously reduce the gate length of MOSFET while reduces the gate leakage current.A ferroelectric field-effect transistor(Fe FET)using ferroelectric as the gate dielectric can be applied to ferroelectric nonvolatile memory and neurosynaptic devices.ZrO₂ material has a high dielectric constant and shows antiferroelectricity property.Therefore,this thesis investigates ZrO₂ as the gate dielectric to systematically study Ge-based MOSFETs and Fe FETs.The primary achievements are as follows:1.Ge p MOSFET and p Fin FET with ZrO₂ dielectricFirst,the effects of post-metallization annealing(PMA)and post-deposition annealing(PDA)on the electrical properties of Ge p MOSFETs with ZrO₂ gate dielectric are studied.For transistors without PDA treatment,as the PMA temperature increases,the ZrO₂ will crystallize.The crystallization of ZrO₂ contributes to the increase of the permittivity of ZrO₂and the decrease of the density of interface states(D_it),resulting in a reduced capacitance equivalent thickness(CET),a better subthreshold swing(SS)and high effective hole mobility(?_eff).It is demonstrated that Ge p MOSFETs with a PDA treatment at 400°C have a lower CET and a steeper SS compared to devices without PDA.Subsequently,high-mobility Ge p MOSFETs with a 2.5 nm crystalline ZrO₂ gate dielectric are realized and compared with devices with O₃/ZrO₂,amorphous ZrO₂,and Al₂O₃/ZrO₂gate dielectrics.?_eff can be slightly enhanced by O₃ oxidation of the Ge surface prior to ZrO₂deposition.An interfacial Al₂O₃ passivation layer enhances?_eff and reduces D_it,but increases the CET.Passivation-free Ge p MOSFETs with 2.5 nm-thick crystalline ZrO₂ have a CET of0.73 nm.For a high Q_invof 1×10¹³ cm^-2,passivation-free Ge p MOSFETs achieve a?_eff of190 cm²/Vs,which is higher than the mobilities previously reported for unstrained Ge p MOSFETs with CET less than 1 nm in the literature.Finally,Ge p Fin FETs on(100)-oriented Ge OI wafer with ZrO₂ dielectric are realized.Transistors with fin channel along[110]direction demonstrate the improved drive current and channel?R_tot/?L_G compared to the devices along[100]direction.At a Q_inv of 5×10¹²cm^-2,Ge OI Fin FETs along[110]direction have 60%and 10%improved?_eff in comparison with[100]devices and Si university mobility,respectively.2.ZrO₂ Ferroelectric FETs for Non-volatile Memory ApplicationWe demonstrate for the first time ZrO₂ ferroelectric field-effect transistors for embedded non-volatile memory applications.Multiple sweeps of polarization versus voltage measurement demonstrate that a metal/ZrO₂/Ge capacitor is entirely free of wake-up effect and has significantly improved fatigue characteristics compared to a Hf ZrO_x control device.Thanks to relatively small remnant polarization and a high-quality ZrO₂/Ge interface,up to10⁷ cycles program/erase endurance,10 ns program/erase speed,and>10-year data retention at 85°C are achieved.Simultaneously,the impacts of post-rapid thermal anneal(RTA)and thickness of ZrO₂ on the polarization(P)and electrical characteristics of Ta N/ZrO₂/Ge capacitors and Fe FETs areinvestigated,respectively.After the RTA ranging from 350 to 500°C,Ta N/ZrO₂/Ge capacitors with 2.5 and 4 nm-thick amorphous ZrO₂ film exhibit the stable P.It is proposed that the ferroelectric behavior originates from the migration of the voltage-driven dipoles formed by the oxygen vacancies and negative charges.Fe FETs with 2.5 nm,4 nm,and 9 nm ZrO₂ demonstrate the decent memory window(MW)with 100 ns program/erase pulses.A4-nm-thick ZrO₂ Fe FET has significantly improved fatigue and retention characteristics compared to devices with 2.5 nm and 9 nm ZrO_2.The retention performance of the ZrO₂Fe FET can be improved with the increase of the RTA temperature.An MW of?0.46 V is extrapolated to be maintained over 10 years for the device with 4 nm ZrO₂.3.ZrO₂ Ferroelectric FET for Neurosynaptic Device ApplicationUsing ZrO₂ Fe FET to gradually adjust the ferroelectric polarization switching to realize neurosynaptic devices,successfully demonstrated the short-term plasticity and long-term plasticity of synapses.Short-term potentiation of synaptic was observed under a single small pulse.Under a series of positive or negative pulse stimulation,long-term potentiation or long-term depression of synaptic function is obtained respectively.Under repeated pulse stimulation,synaptic devices can transform from short-term plasticity to long-term plasticity.By adjusting the time interval of stimulation pulses of presynaptic neurons and postsynaptic neurons,the pulse time-dependent plasticity(STDP)synaptic characteristics are demonstrated.