The Investigation Of ZrO₂ Based Ferroelectric Field-Effect Transistor

Ferroelectric field-effect transistors?FeFET?based on ferroelectric gate dielectric have attracted considerable interest for the applications of steep subthreshold swing?SS?negative capacitance logic devices and non-volatile memory?NVM?.Negative Capacitance Field-Effect Transistor?NCFET?has a steeper SS than does the traditional MOSFET due to its capability of achieving the internal gate voltage amplification induced by the ferroelectric film inserted into gate stack.FeFET is one of the most promising emerging NVMs with low power consumption and fast program/erase?P/E?speed.Ferroelectric materials development for NVMs and NCFETs has focused on polycrystalline doped HfO₂ due to its better thickness scalability and CMOS process compatibility.However,there still exists a fundamental limit for doped HfO₂ thickness scaling to meet the requirements of“post-Moore”logic devices and embedded memory.In addition,there are still many problems in the doped HfO₂ ferroelectric films,such as wake-up effect,endurance failure.In this dissertation,the effects of Zr composition and annealing temperature on the ferroelectricity of the HfZrO₂ film and electric characteristics of the NCFET devices were experimentally investigated.By incorporating a“Nanocrystal-Embedded-Insulator”?NEI?ferroelectric film in the gate stack,NCFETs?2.1 nm?and FeFET?3.6 nm?with smaller gate dielectric thickness have been realized.It is experimentally demonstrated that ZrO₂based NEI transforms from an anti-ferroelectric film to a ferroelectric film for the TaN/NEI/Ge?SiGe?structure.This dissertation includes four parts:1)the study of the ferroelectricity of HfZrO₂film and the electric performance of NCFETs with different Zr compositions,2)the investigation of ZrO₂ based NEI ferroelectric/anti-ferroelectric films,3)the characterization of ZrO₂ based NEI NCFET,and 4)the investigation of ZrO₂ based NEI FeFET NVM and synapse.1.Ferroelectricity of HfZrO₂ and electric performance of NCFETsHfZrO₂ films with different Zr compositions were prepared by atomic layer?ALD?technology.The properties of the HfZrO₂ film were characterized by X-ray photoelectron spectroscopy?XPS?,polarization-voltage?P-V?,capacitance-voltage?C-V?,and grazing incidence X-ray diffraction?GIXRD?.As the Zr composition increases,the remnant polarization?P_r?of the film increase first and then decrease,and the P_r reaches the maximum value when the Zr composition approaches 50%.Hf_0.33Zr_0.67O₂ exhibits the obvious anti-ferroelectric characteristics.It is indicated that Hf_0.67Zr_0.33O₂ NCFET achieves the reduced hysteresis compared to the devices with the higher Zr composition.The typical characteristics of NCFET including,sub-60 mV/decade SS,NDR phenomena in IDS-VDS curves,and capacitance peaks in CG-VG curves are observed in HfZrO₂ NCFETs.Multiple DC sweeping?10³?measurements show that the decent stability of the negative capacitance effect induced by the ferroelectric layer is achieved in NCFET.2.ZrO₂ based NEI ferroelectric/anti-ferroelectric filmNEI layers comprising ZrO₂ nanocrystals embedded in amorphous Al₂O₃ were successfully grown by ALD.The ferroelectric characteristics of the new ZrO₂ based NEI film and TaN/NEI/Ge capacitor were studied systematically by transmission electron microscope?TEM?,piezoelectric atomic force microscope?PFM?,P-V and PUND test.The anti-ferroelectric characteristics of the NEI films are proved by different capacitance structures,and the endurance characteristics were measured.ZrO₂ nanocrystals of different sizes and shapes were observed by high-resolution TEM analysis,and the nanocrystals can be smaller than 1 nm.From the diffraction patterns,the inter-planar spacing d within the nanocrystals is determined to be 0.173 nm,corresponding to?111?-oriented orthorhombic ZrO₂ phase or?101?-oriented tetragonal phase.The P-V curves of TaN/NEI?3.6 nm?/Ge sample shows that when the applied voltage is 4 V,the remnant polarization?P_r?of the NEI film is less than 1?C/cm²,which is significantly lower compared to the reported HfZrO₂,but exhibits a high coercive electric field?E_c??6.26 MV/cm?.The ZrO₂ based NEI film can achieve 10¹⁰ cycles endurance under a much larger E_c compared to doped-HfO₂ film.Neither wake-up nor imprint effect is oberved in NEI films.3.ZrO₂ based NEI NCFETGe channel NCFET with NEI thickness of 2.1 and 3.6 nm were studied.The typical characteristics of NCFET including,sub-60 mV/decade SS,NDR phenomena and capacitance peaks are observed during the measurement of the current,capacitance,SS,and transconductance of different thickness ZrO₂ based NEI NCFET.NCFET with 2.1 nm NEI has less significant characteristics improvement compared to the transistor with 3.6nm NEI,which is due to the volume and content of the NEI decreases with the decreases of the thickness.Multiple DC sweeping?10²?measurements show that the stability of the negative capacitance effect induced by the NEI layer is achieved in ZrO₂ based NCFET.With the heating temperature increases from 10 to 50 ^oC,the steep SS performance of the transistor does not degrade and the NDR effect degradation is not obvious.The proposed NEI FeFET design provides a pathway for scaling down the thickness of the ferroelectric/dielectric gate insulator layer to be compatible with FinFETs with very small fin pitches.4.The investigation of ZrO₂ based NEI FeFET NVM and synapseGe and SiGe channel FeFET NVM and neural network synapse devices with NEI thickness of 3.6 and 6 nm were studied.The NEI?3.6 nm?Ge pFeFET achieves a memory window of 0.14 V and 0.27 V with 100 ns and 1?s pulses,respectively,with a relatively low 1.6 V P/E voltage.This is the minimum P/E voltage of the non-volatile FeFET memory reported so far.The NEI?3.6 nm?nFeFET achieves a memory window of 0.63 V with±2 V/100 ns pulses.No memory window degradation is observed after 10⁴ P/E cycles for NEI?3.6 nm?pFeFET.Benchmarking of ZrO₂ based NEI FeFET memory against previously reported HfO₂-based FeFETs,ZrO₂ based NEI FeFETs have lower operating voltages while maintaining decent memory window.With partial polarization switching in the FE nanocrystals,the threshold voltage(V_TH)of a ZrO₂ based NEI FEFET can be finely controlled so that it functions as an analog synaptic device.The varying-amplitude and fixed-amplitude voltage pulses trains were applied to the ZrO₂ based FeFET synapse devices.Analog neural networks?NN?with sign back-propagation?SBP?algorithm are emulated to investigate the impact of device performance on NN online training accuracy.Under the varying-amplitude operating conditions?10 ns/0^±3 V?,ZrO₂ based NEI?3.6 nm?pFeFET synapse achieves large dynamic range(G_max/G_min>1000),small weight update non-linearity??_p/?_d=-0.01/0.01?and asymmetry factors,advantageous for analog-style NNs with online training.A convolutional NN is designed and emulated for a MNIST dataset,projecting an online training accuracy of 97%.In actual circuit design,for online learning,FeFET synapse devices are often programmed in parallel mode,which is simpler to operate and lower in power consumption.Under the fixed-amplitude operating conditions?10 ns/±3 V?,ZrO₂based NEI?3.6 nm?pFeFET synapses with large dynamic range(G_max/G_min>580)and high bias symmetry??_p/?_d=0.04/-0.14?are advantageous for convolutional NN with online training capability.An online training accuracy of 94.5%is achieved.ZrO₂ based NEI FeFET is the promising candidate for low power and high-performance non-volatile memory and synaptic applications.