Evidence for reversible oxygen ion movement during electrical pulsing: enabler of emerging ferroelectricity in binary oxides

Ferroelectric HfO2-based materials and devices show promising potential for applications in information technology but face challenges with inadequate electrostatic control, degraded reliability, and serious variation in effective oxide thickness scaling. We demonstrate a novel interface-type switching strategy to realize ferroelectric characteristics in atomic-scale amorphous binary oxide films, which are formed in oxygen-deficient conditions by atomic layer deposition at low temperatures. This approach can avoid the shortcomings of reliability degradation and gate leakage increment in scaling polycrystalline doped HfO2-based films. Using theoretical modeling and experimental characterization, we show the following. (1) Emerging ferroelectricity exists in ultrathin oxide systems as a result of microscopic ion migration during the switching process. (2) These ferroelectric binary oxide films are governed by an interface-limited switching mechanism, which can be attributed to oxygen vacancy migration and surface defects related to electron (de)trapping. (3) Transistors featuring ultrathin amorphous dielectrics, used for non-volatile memory applications with an operating voltage reduced to ±1 V, have also been experimentally demonstrated. These findings suggest that this strategy is a promising approach to realizing next-generation complementary metal-oxide semiconductors with scalable ferroelectric materials.

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Introduction
The semiconductor industry, a cornerstone of modern technology, is currently navigating through a pivotal phase.The scaling of complementary metal-oxide semiconductor (CMOS) technology, long the bedrock of electronic devices, is encountering significant challenges [1,2].As we approach the physical limits of traditional CMOS scaling, the industry is compelled to seek alternative avenues to enhance chip performance and functionality.Emerging at the forefront of this technological evolution are novel devices and architectures, designed to transcend the constraints of CMOS scaling [3].This includes the exploration and integration of innovative mechanisms and technologies onto the CMOS platform.Among these, steepslope transistors and advanced non-volatile memory systems stand out, not only offering improvements in system performance but also paving the way for revolutionary computing paradigms such as in-memory computing [4][5][6][7][8][9].A critical aspect of this transformation lies in the development and integration of gate oxides.These materials are proving instrumental in boosting the performance of transistor devices, which are now being constructed from a variety of materials.Particularly noteworthy are ferroelectric materials, which have emerged as frontrunners in the race to develop nextgeneration non-volatile memory (NVM).The unique ability of ferroelectric materials to maintain two distinct remnant polarization states without the need for an external electric field makes them highly suitable for memory storage applications [10][11][12][13].In the realm of memory device configurations, the one-transistor-one-capacitor (1 T-1 C) model is gaining traction [14,15].This approach effectively mimics the functionality of a 6T-SRAM, offering a blend of efficiency and performance [16].
Ferroelectric oxides, especially atomic layer deposition (ALD)-grown doped-HfO 2 FE films, are garnering attention for their superior compatibility over conventional perovskitebased FE materials.These oxides are not only promising in the context of energy-efficient electronics but also align well with the existing CMOS technology framework [17][18][19].HfO 2 has been successfully employed in high-κ metal gate technology for logic transistors [20,21].Its compatibility and scalability within the CMOS framework make it an ideal candidate for integrating ferroelectric devices into mainstream semiconductor components.Research into ferroelectric HfO 2 -based devices has been extensive, covering a wide range of applications.This includes next-generation memory devices and various logic devices such as ferroelectric random-access memory [22,23], ferroelectric tunnel junctions [24,25], ferroelectric field-effect transistors [26][27][28][29][30], and negative capacitance fieldeffect transistors [31][32][33].Each of these applications opens new possibilities in the semiconductor domain.However, challenges remain in the widespread adoption of these materials.Issues include insufficient electrostatic control, compromised reliability, and serious variation of effective oxide thickness (EOT) scaling in terms of very-large-scale integration [11].
Recent research has elucidated ferroelectric-type behaviors in amorphous dielectric films (the ferroelectricity is unaffected by the crystallization state).Cheema et al have introduced a groundbreaking concept of emergent ferroelectricity in subnanometer binary oxide films on silicon substrates [34].Their research notably demonstrates ferroelectric-like polarization switching in ZrO 2 films with thicknesses as low as 5 Å and 1 nm.There inevitably exist amorphous phase fractions in such ultra-thin films deposited at low temperatures.Takagi et al reported that the ferroelectric properties of La 2 O 3 almost disappear at low temperatures, which have been shown to lead to a reduction in the subthreshold swing, supporting the contribution of ion drift [35,36].Our research has similarly observed ferroelectric characteristics in amorphous dielectrics, particularly in Al 2 O 3 , ZrO 2 , and HfO 2 .These have been demonstrated in NVM and synaptic applications [37][38][39][40], showing no wake-up effect, high endurance, low operating voltage, and compatibility using low-temperature processes.However, it is hard to clearly connect this observed hysteresis and ferroelectricity in classical ferroelectric films with conclusive contributions from specific phases [39].Therefore, it is imperative to note that the classification of amorphous materials as ferroelectrics is subject to ongoing scientific debate.
In resistive random-access memory (RRAM), there exists a class of switching mechanisms based on ionic migration, which is distinct from the conductive filament mechanism and shows dependence on the cell area.This mechanism is frequently observed in oxide-based RRAM devices.The migration of oxygen vacancies under an applied electric field can lead to a change in the resistance at the interface between the metal electrode and the oxide layer.This mechanism is particularly notable in materials such as TiO 2 and HfO 2 [41,42].Theoretical models have substantiated that the distribution or migration of charges at specific metal-oxide interfaces, such as Schottky contacts, can indeed lead to ferroelectric hysteresis phenomena.Moreover, these theoretical findings emphasize that such behavior exhibits a significant correlation with frequency [43].However, a consensus has not been reached on this mechanism, and we are attempting to obtain unambiguous evidence of ferroelectricity.
In this work, we intentionally introduce emerging ferroelectric materials: ultrathin binary oxide films (AlO x , ZrO x , HfO x , SiO x , etc).The emergence of ferroelectricity in ultrathin oxide films has been verified by electric and optical measurements.The electrical properties, as well as the frequency and amplitude dependency of the external electric field for oxygen vacancy, are systematically characterized.We have realized non-volatile mobile-ionic capacitive memory in the TaN/ZrO x /TaN metal-insulator-metal (MIM) structure.Furthermore, non-volatile memory with an operating voltage reduced to ±1 V was also experimentally demonstrated.The demonstrated ferroelectricity in ultrathin binary oxide films with an ultra-low leakage current and low thermal budget could pave the way to future memory devices that are fully CMOS compatible with advanced process nodes.

Sample fabrication
MIM-and metal-oxide-semiconductor (MOS)-type capacitors with ZrO x , AlO x , HfO x , and SiO x binary oxides as dielectrics have been fabricated.These binary oxide films were deposited by ALD under oxygen-deficient conditions to tune the concentration of oxygen vacancies.ZrO x and HfO x binary oxide films were deposited at 250 • C using TEMAZr, TEMAHf, and H 2 O as precursors of Zr, Hf, and O, respectively.AlO x and SiO x binary oxide films were deposited at 300 • C using TMA, TDMAS, and H 2 O as precursors of Al, Si, and O, respectively.TaN was deposited by reactive sputtering.Here, to favor the formation of the TaO x N y interfacial layer, the pressure and flow of N 2 have been adjusted and in situ oxidation was adopted.The oxygen flow rate was controlled using an optical emission monitoring system.The capacitors were fabricated by lithography patterning and dry etching without the treatment of rapid thermal annealing (RTA).Amorphous SiO x mobile-ionic field-effect transistors (MIFETs) were fabricated using a conventional CMOS process, starting with an n-type Ge substrate.The source and drain (S/D) regions were doped by BF 2+ implantation at an energy of 30 keV with a dose of 1 × 10 15 cm −2 .Then, dopant activation was performed by RTA in N 2 ambient at 400 • C for 30 s.The SiO x gate stack was formed by ALD under the same conditions as the MIM.Subsequently, a 100 nmthick TaN gate layer was deposited by reactive sputtering.After gate patterning via lithography and etching processes, 20 nm-thick nickel S/D metal electrodes were formed by a lift-off process.

Characterization
The P-V, cycling, transient polarization loss, and retention measurements were conducted using dynamic hysteresis, fatigue, leakage current, pulse, and retention measurement modules using the TF Analyzer 3000, respectively.The cycling frequencies varied from 100 Hz to 1 MHz, and the measurement pulses were all performed at 1 kHz unless otherwise specified.The positive-up, negative-down (PUND) waveform is used for the transient polarization loss measurement.The rise time of the triangular pulses was fixed at 250 µs in both PUND and retention tests.High-resolution transmission electron microscopy (HRTEM) was performed with a FEI Tecnai G2 F20 operated at 200 kV.Morphology and piezoresponse force microscopy (PFM) measurements were performed using Asylum Research (MFP-3D Infinity).The electrical measurements were performed using a Keithley 4200A-SCS semiconductor parameter analyzer in the ambient atmosphere.

Modeling method
A model for MIM capacitors considering mobile oxygen vacancies and oxygen ions is developed.By solving the ion drift-diffusion equation coupled with the Poisson equation, the distribution of mobile ions and the charge at the electrode as a function of time or applied voltage can be obtained.
The proposed model for MIFET consists of three coupled parts, namely the one-dimensional ion drift-diffusion (IDD) equations for the concentration distribution of mobile ions in the dielectric, the two-dimensional Poisson's equation for potential (ϕ) in the whole region of the MOS, and the twodimensional non-equilibrium Green's function (NEGF) for the charge density profile (ρ) in the semiconductor channel region.

Results and discussion
MIM-and MOS-type capacitors with ultrathin ZrO x , AlO x , HfO x , and SiO x binary oxides as dielectrics have been fabricated.Note that binary oxides were deposited in oxygendeficient conditions by ALD with a thickness range of 2.5-3.6 nm.As shown in figures 1(a)-(d), the amorphous binary oxides are confirmed by the HRTEM image, which is consistent with the fabrication process in that crystalline annealing was prevented.Figure 1(e) shows the measured polarizationvoltage (P-V) curves of these capacitors by applying a triangular waveform at 1 kHz with various sweeping voltages.The obvious ferroelectric-like behavior with a single-loop hysteresis and a non-zero effective remnant polarization can be observed.A larger applied voltage V is required to achieve a fixed polarization for a thick binary film capacitor.The displacement current-voltage characteristics of the TaN/ZrO x /Ge MIS structure are applied to a triangular pulse with a sweeping voltage of 2.5 V, which is shown in figure S1.This kind of behavior is quite different from HfO 2 -based ferroelectrics, which exhibit a polarization switching current peak [44].The leakage current J leak shown in the inset is significantly smaller than J dis , indicating that ferroelectricity in amorphous oxide films is not induced by leakage current (figure S1).The polarization value is relatively small, less than 4 µC cm −2 , compared to that of the reported HfO 2 -based FE devices [20], in which the ferroelectric behavior originates from the orthorhombic (o-) phase of the crystalline structure.ZrO x , AlO x , HfO x , and SiO x are binary oxides that have traditionally been classified as paraelectric materials, not expected to exhibit ferroelectric properties, and typically involve a crystalline structure with a non-centrosymmetric arrangement of atoms.The emergence of ferroelectric characteristics in these amorphous materials is not only of scientific interest but also opens up new avenues for research and applications.
To eliminate the impact of non-switching components including leakage and paraelectric response, PUND measurements were conducted to extract the switching current component of the TaN/ZrO x /TaN capacitor.As shown in figure 2(a), the current responses in two continuous pulses exhibited variations, leading to finite polarization and hysteresis.Additionally, it was noted that the displacement current in the ferroelectric-type P-V characteristics significantly deviated from the typical ferroelectric behavior [19].An optimized second-harmonic generation method has been explored for ferroelectric domain characterization in ZrO x thin films (figure 2(b)).To further corroborate the ferroelectric-like characteristics of ZrO x devices, a PFM test was performed on ZrO x layered onto TaN/Si samples.The PFM amplitude and phase plots with clear contrast are presented in figure 2(c).Notably, the phase contrast indicates the presence of opposing ferroelectric dipoles written onto the surface of ZrO x on TaN.The ferroelectricity of other ferroelectric materials has also been demonstrated through the above tests [45].Notably, the phenomenon of ferroelectric-like effects in amorphous materials can be induced through various mechanisms, such as space charge polarization effects [46], metal-semiconductor Schottky contacts [47], and the modulation of oxygen vacancy dipoles [48].The role of oxygen vacancies, in particular, has been a subject of considerable interest, as they have long been considered to have a significant relationship with ferroelectric properties.The evolution from paraelectric to ferroelectric switching behavior in TaN/AlO x /Si and TaN/ZrO x /TaN capacitors can be observed by adjusting the pulse time ratio of precursor sources (supplementary figure S2).The ferroelectric switching behavior begins to emerge under the formation of the oxygen-deficient layer, indicating that the ferroelectric behavior is related to oxygen vacancies.O 18 has been used as a tracer in this study to clarify the mechanism of ferroelectric behavior.Six-cycle ZrO x with a precursor of O 18 has been deposited at the upper TaN/ZrO x interface in the fabrication of TaN/ZrO x /TaN capacitors.Figure 2(d) shows the depth profile for O 18 and O 16 atoms for the TaN/ZrO x /TaN samples with an initial state and two polarized states by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS).All spectra were normalized and aligned based on the upper TaN/ZrO x interface.The concentration of O 18 at the lower TaN/ZrO x interface decreases during a negative pulse compared to its initial state.Following the application of a positive pulse, the O 18 content at the lower TaN/ZrO x interface increases once more.Distinct changes in the O 18 /O 16 ratio at the lower TaN/ZrO x interface suggest that the ferroelectric polarization is accompanied by the migration of oxygen ions.A subnanometer thick oxide layer can be observed at the interface between TaN and ZrO x , indicating that the TaN film undergoes oxidation through the chemical reduction-oxidation reaction acting as a reservoir of oxygen ions [46].This oxide layer can introduce mobile ions and positively charged oxygen vacancies (V O 2+ ). Figure 3(a) displays the extracted P PUND -V hysteresis curves at various frequencies from 100 kHz to 1 kHz.It is observed that the P PUND -V loops of the device strongly depend on the measurement frequency, and polarization will increase at a lower frequency.Similar properties can also be obtained in TaN/ZrO x /Ge MOS capacitors applied to triangular waveforms at frequencies in the range from 100 Hz to 100 kHz, as seen in figure S3.Such a charge response seems to differ from the properties of polarization in conventional ferroelectric materials [49,50], whereas the ion drift mechanism can explain this phenomenon.In contrast, a variety of these devices with time-dependent characteristics will have the potential for artificial intelligence applications [51].To explain the ferroelectric-like phenomenon in amorphous dielectrics, a simulation framework considering mobile V O 2+ and oxygen ions (O 2− ) is developed, as shown in figure S4.The simulated P PUND -V hysteresis curves with various frequencies are plotted in figure 3(b), and they show similar characteristics to the experimental results.The negative slope of the P-V curve is due to the fact that there is still an external electric field strong enough to cause the drift of oxygen to be higher than the diffusion of oxygen, resulting in the increase of the accumulated O 2− charge near the metal/dielectric interface.The faster the frequency, the greater the negative slope, which is not valid due to the comparable slow ion motion to keep up until the frequency of 100 kHz.The experimental and simulated results of P PUND -V hysteresis curves for various sweeping voltage amplitudes are also shown in figures 3(c) and (d).It should be noted that the diffusion rate and the mobility of positive and negative ions should be different, which results in different saturated regions in the P PUND -V curves.The simulations successfully reproduce the qualitative behavior of the experimental data, confirming the migration of mobile ions in the dielectric under an applied electric field, leading to ferroelectric behavior.We also confirm that amorphous binary oxide ferroelectric samples exhibit an interface-type conducting path by considering the area dependence of the cell resistance, which has also been reported in previous work on electrochemical migration of oxygen vacancies (supplementary figure S5) [47].
In addition, it should be noted that V O 2+ ions act not only as mobile ions but also as trapped charge centers, as described in previous studies [48].V O 2+ ions are known to be present in the interfacial layer or dielectric materials and can be influenced by external electric fields.The trapping or detrapping of V O 2+ ions can be achieved by applying an electric field pulse, meaning that the V O 2+ ions can be injected into the interfacial layer or released back into the dielectric material.The contribution of trapping-detrapping effects has also been investigated by P-V and I-V measurements with a triangular waveform at 1 kHz after voltage pulse modulation.As shown in figure 4(a), ferroelectric polarization switching has been observed at the initial state.When a reset trianglewave pulse with an amplitude of 2.5 V and frequency of 10 kHz is applied, the V O 2+ will be neutralized by electrons trapped by the charge centers, leading to the paraelectric P-V hysteresis (figure 4(b)).On the other hand, when applying a set pulse with 100 Hz and gradually increasing amplitudes after the reset procedure, the electrons trapped in neutralized oxygen vacancies will be detrapped.The neutralized oxygen vacancies will transform back to V O 2+ , which can migrate in the dielectric, leading to a transition from paraelectrictype to ferroelectric behaviors.This process can be reversible.Due to the significant correlation between ferroelectricity and V O 2+ concentration in amorphous ZrO x films, it is feasible to attain reversible multi-ferroelectricity with varying polarization through the modulation of V O 2+ concentration.Multiple states of ferroelectric hysteresis can be obtained by the change of the set pulse amplitudes, which would be related to the ion migration barriers of the trap energy levels (figures 4(c)-(f)) [52].This gradually increasing pulse amplitude contributes to more ion redistribution, and further results in ferroelectriclike hysteresis in mobile-ion-based devices.This process can be used to reconfigure the material's electrical properties and could have important applications in the development of next-generation electronic devices in synaptic computing and configurable architecture.The capacitances corresponding to the multiple P-V states are measured at 1 kHz, as shown in figure 3(g).Multiple levels of capacitance are achieved, indicating that the device's capacitance can be controlled by the reversible modulation of oxygen vacancies.Such behavior is due to the fact that the concentration of V O 2+ can directly affect the dielectric properties of the material.When these ions are incorporated into the dielectric material, they can influence the polarization and charge distribution within the material, leading to changes in the dielectric constant and capacitance [53].Interestingly, multiple resistance states have also been observed from the I-V characteristics in figure 4(h), showing that the high resistance state corresponds to the low capacitive state (LCS) and the low resistance state corresponds to the high capacitive state (HCS), which is corresponding with the modulation of field-induced ion migration barrier height and width based on an equivalent circuit model of a capacitor and a resistor [54].The coexistence of oxygen vacancies and O negative ions plays a crucial role in the interface-type conduction path.They are often involved in the electrochemical migration process at the interface between the metal electrode and the oxide.This migration can alter the resistance of the interface, leading to the resistive switching effect [37].Additionally, the cycle-to-cycle variability of the multiple capacitive states is also measured (figure 4(i)).While V O 2+ ions can be entirely trapped in the interfacial layer, leading to a consistently stable zero-concentration state, the opposite scenario occurs when V O 2+ ions are fully released into the dielectric material, resulting in a stable maximum concentration state.Between these two states, the concentration of V O 2+ ions may be unstable and exhibit certain fluctuations, which may also be related to the kinetics of the hopping process [55].The capacitances of maximum capacitive states (C HCS ) and minimum capacitive states (C LCS ) exhibit an impressive C HCS /C LCS ratio as high as 1250.This phenomenon could have important implications for the development of emerging memcapacitive devices for energy storage [56].
To explore the impact of mobile ions in amorphous dielectrics on field-effect transistors (FETs), we developed a model for the MIFET.The structure of the MIFET under study is depicted in figure 5(a).The simulation framework will be illustrated in detail in a later section 2.3.A key assumption in our model is that the movement of V O 2+ ions is negligible compared to that of O 2− ions, and the charge of V O 2+ near the interfaces of the channel/dielectric and dielectric/gate remains constant.We analyzed the simulated drain current-gate voltage (I D -V GS ) curves for FETs, both with and without mobile ions, under a 10 kHz triangular waveform V GS ranging from 2.5 V to −2.5 V, as shown in figure 5(b).Notably, the hysteresis I D -V GS curves, indicative of mobile ion influence, are also extractable through simulation.The drain voltage (V DS ) in these simulations was set at −0.05 V. Our model successfully predicts the clockwise ferroelectric-type hysteresis observed in these curves.Analyzing the distribution of mobile ions along the channel of the MIFET reveals that it can be concluded that the surface potential should normally be lower/ higher than the gate potential, implying that the accumulation of mobile ions is maintained at a higher/lower level.This observation suggests that mobile ions, controlled by the V GS , can dominate the surface potential of the channel, resulting in ferroelectric-type hysteresis.Furthermore, it is anticipated that MIFET will enhance the subthreshold swing in both forward and reverse scans, which is consistent with the experimental results reported by Endo et al in their studies on La 2 O 3 devices [35].MIFETs achieve a strong modulating efficiency of surface potential as a result of the mobile ions.The amplification effect of the semiconductor surface potential caused by ion movement in the dielectric will produce a steep sub-kT/q subthreshold swing.To validate our model and confirm the effects of mobile ion behaviors in MIFETs, SiO x -based MIFETs on n-Ge (001) substrates using the gate last process have been fabricated.A schematic of the MIFET and the corresponding HRTEM image of the gate stack of the SiO x MIFET are shown in figures 5(c) and (d), respectively.Similar to the gate stack structure above, ultrathin SiO x film was deposited by strategically modulating the ALD condition.A GeO x interfacial layer between SiO x and Ge adding ∼0.3 nm to the capacitance equivalent thickness was confirmed via capacitance measurement [37].The P-V hysteresis loops measured for the TaN/SiO x /Ge MOS capacitor, as shown in figure 5(e), also exhibit ferroelectric-type behavior with a polarization of approximately 2 µC cm −2 .Moreover, this minimal polarization may enhance memory endurance since, as shown in figure S6, no remnant polarization degradation is observed after 10 8 cycles of sweeping for TaN/SiO x /Ge MOS capacitor.This improvement is attributed to just a few µC cm −2 of polarization of the SiO x film, which typically protects the interfacial layer with a lower dielectric constant and breakdown electric field from the breakdown due to the easily accessible balanced charge for such metal/ferroelectric/dielectric/semiconductor gate stacks [37].Furthermore, figure 5(f) displays the measured I D -V GS curves of the SiO x MIFET, with a V DS of −0.05 V.The clockwise I-V loops indicate the diffusing back and forth of mobile ions in the dielectric, similar to the ferroelectric switching in ferroelectrics.Additionally, the memory window (MW) of the MIFETs under Program/Erase (PGM/ERS) pulses with varying pulse widths and amplitudes were characterized to assess their potential use in NVM.Frequency-dependent pulse voltage transfer characteristics of SiO x MIFETs were demonstrated, as seen in figure 5(g), which shows the I D -V GS curves of the fabricated MIFETs under different PGM/ERS conditions.Notably, the MW increases with both the pulse width and amplitude, attributed to the constrained movement of mobile ions within the dielectric film.Thanks to the low dielectric constant of SiO 2 film, an MW of approximately 0.4 V was achieved with a ±1 V pulse for PGM/ERS, a significantly lower operating voltage compared to that reported for doped-HfO 2 ferroelectric field-effect transistors.
Notably, the ability of amorphous films to display such behavior expands the potential applications of these materials in many aspects, particularly in the realm of high-end built-in memory technologies.These can accumulate: (1) ultra-low leakage current, with 0.4 pA µm −1 at a gate voltage of 1 V compared with its doped HfO 2 counterpart; (2) a reduced thermal budget without annealing, which is most beneficial for the ongoing advanced integration of back end of line; (3) enhanced scalability as thin as 2 nm in terms of EOT scaling of very-large-scale integration; (4) increased endurance due to the comparatively small polarization density P r , usually about 2 µC cm −2 , which inevitably will reduce the MW needed to be balanced (such low polarization density can also contribute to operation voltage reduction due to the charge balance between the ferroelectric film and channel for continuity); and (5) low operating voltage.These ultrathin binary oxide films (AlO x , ZrO x , HfO , SiO x , etc) are commonly used gate stack materials, which provide a wide range of κ values to choose from for diverse applications, such as high κ for EOT scaling and low κ for reducing operating voltage.Therefore, the incorporation of amorphous films that exhibit ferroelectric-like properties is poised to drive substantial improvements in in-memory technology.

Conclusion
In summary, the present results demonstrate that ferroelectricity can be engineered in conventional amorphous high-κ dielectrics by simply adjusting the oxygen level during the low-temperature ALD deposition.This relies on the fact that different ferroelectric memory-specific characterization tests have been performed to ensure the existence of ferroelectricity along with long-range displacement of oxygen ions in oxygendeficient amorphous oxides.By controlling the trappingdetrapping and migration of oxygen vacancies with different voltage pulse schemes, we have realized capacitance memory with the MIM structure, which exhibits high C HCS /C LCS , multi-state operation, and excellent endurance.A transistor with amorphous dielectrics is demonstrated for NVM applications with an ultra-low operating voltage of ±1 V, which can foster future hardware solutions along with ultra-low leakage in advanced technology nodes.Therefore, the presented approach expands the research subject of conventional ferroelectricity to engineer many widely used, extremely thin binary oxides for logic or memory transistors for future CMOS technologies.

Future perspectives
In this work, the emergence of ferroelectricity in binary oxide materials has been developed, achieving ferroelectricity in conventional amorphous dielectric materials by simply adjusting the oxygen level during low-temperature ALD deposition.The discovery of emerging ferroelectricity in amorphous binary oxides opens up a new path for non-volatile storage technology solutions, which could avoid the shortcomings of reliability degradation and gate leakage increment in scaling polycrystalline doped HfO 2 -based films.Based on amorphous dielectrics, an NVM device with low-temperature process compatibility, low leakage current, excellent reliability and low operating voltage can be realized.Although a series of characterization tests and simulation analyses has been conducted, understanding of the mechanism behind the emergence of ferroelectricity in amorphous dielectrics remains limited.To advance the application of this novel ferroelectric material, further research on the theoretical mechanism must be carried out.

Figure 1 .
Figure 1.(a)-(d) HRTEM image of the emerging ferroelectric materials-based MIM and MOS capacitor with amorphous ultrathin binary oxide films (ZrOx, AlOx, HfOx, SiOx).(e) Measured P-V characteristics of the ferroelectric materials based on MIM and MOS capacitors with different binary oxide films at 1 kHz.Ferroelectric P-V curves can be observed.

Figure 2 .
Figure 2. (a) PUND measurement of a MIM capacitor at 1 kHz.The displacement current is confirmed.(b) Detection of local SHG in a ZrOx thin film.(c) Amplitude and phase images of PFM measurement for the ZrOx/TaN sample.Phase change indicates the opposite polarity.(d) TOF-SIMS composition profiles of O 18 and O 16 normalized for TaN/ZrOx/TaN samples.Relative concentration changes of the O 18 /O 16 ratio at the TaN/ZrOx interface suggest that polarization is accompanied by the migration of oxygen ions.

Figure 3 .
Figure 3. (a) Extracted P PUND -V curves of MIM capacitors with various frequencies, showing strong frequency dependence.(b) Simulated P PUND -V hysteresis curves for various frequencies.(c) The measured and (d) the simulated P PUND -V hysteresis curves for various sweeping voltage amplitudes.

Figure 4 .
Figure 4. (a)-(f) P-V and I-V curves under different trapping-detrapping processes of oxygen vacancies modulated by reset and set pulse.Paraelectric-type and ferroelectric-type characteristics can be reversibly switched.(g) Multiple capacitive states are achieved and are measured at 1 kHz for (a)-(f) due to the modulation of oxygen vacancies.(h) Multiple resistance states are obtained.The high resistance state corresponds to the low capacitive state and the low resistance state corresponds to the high capacitive state, respectively.(i) Cycle-to-cycle variability of multiple capacitive states.

Figure 5 .
Figure 5. (a) The schematic of the simulated MIFET.(b) Comparison of I D -V GS curves between the simulated results of MIFET and the normal FET without mobile ions at V DS = −0.05V. (c) Schematic cross-section of SiOx-based MIFET.(d) HRTEM images of the fabricated SiOx MIFET gate stack, showing the amorphous SiOx film with a thickness of 4.5 nm.(e) Measured P-V characteristics of the TaN/SiOx/Ge gate stack.(f) Measured I D -V GS curves of the SiOx MIFET at V DS = −0.05V. (g) Memory window of SiOx MIFET under various PGM/ERS conditions; the operating voltage can reach below ±1 V.