The mechanism of the remnant polarization (P r) growth during the first stage of ferroelectric HfO2-based memory cell operation (the wake-up effect) is still unclear. In this work, we reveal the microscopic nature of the P r growth in functional ferroelectric capacitors based on a polycrystalline 10 nm thick (111) out-of-plane textured Hf0.5Zr0.5O2 film during electric cycling. We observe the cycle-by-cycle evolution of the domain structure with the piezoresponse force microscopy (PFM). During the early stage of the wake-up, three types of domains are found: (i) normal domains (polarization aligned along the applied electric field), (ii) nonswitchable domains with upward and downward polarization, and (iii) domains with anomalous polarization switching (polarization aligned against the applied electric field) that are commonly surrounded by nonswitchable domains. Initially, nonswitchable and “anomalous” domains are 200–300 nm in width, and they occupy ∼70% of the capacitor area. During electric field cycling, these domains reduce in area, which is accompanied by the P r growth. We attribute the domain pinning and the anomalous polarization reversal to the internal bias field of the oxygen vacancies. The local density of the oxygen vacancies decreases during electric cycling, thus producing the reduction of the internal bias field. The correlation of the PFM data with both the results of the structural analysis of fresh and cycled Hf0.5Zr0.5O2 film by transmission electron microscopy and the performance of the ferroelectric capacitor indicates that after the first cycle of the wake-up the P r growth is not associated with phase transformations, but only with the transformation of the domain structure. The obtained results elucidate the physical mechanism of the emergence of P r during the wake-up of the ferroelectric HfO2-based memory cell.

While the conductance of a first-order memristor is defined entirely by the external stimuli, in the second-order memristor it is governed by the both the external stimuli and its instant internal state. As a result, the dynamics of such devices allows to naturally emulate the temporal behavior of biological synapses, which encodes the spike timing information in synaptic weights. Here, we demonstrate a new type of second-order memristor functionality in the ferroelectric HfO 2 -based tunnel junction on silicon. The continuous change of conductance in the p + -Si/Hf 0.5 Zr 0.5 O 2 /TiN tunnel junction is achieved via the gradual switching of polarization in ferroelectric domains of polycrystalline Hf 0.5 Zr 0.5 O 2 layer, whereas the combined dynamics of the built-in electric field and charge trapping/detrapping at the defect states at the bottom Si interface defines the temporal behavior of the memristor device, similar to synapses in biological systems. The implemented ferroelectric second-order memristor exhibits various synaptic functionalities, such as paired-pulse potentiation/depression and spike-rate-dependent plasticity, and can serve as a building block for the development of neuromorphic computing architectures.

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New interest in the implementation of ferroelectric tunnel junctions has emerged following the discovery of ferroelectric properties in HfO 2 films, which are fully compatible with silicon microelectronics technology. The coercive electric field to switch polarization direction in ferroelectric HfO 2 is relatively high compared to classical perovskite materials, and thus it can cause the migration of non-ferroelectric charges in HfO 2 , namely charged oxygen vacancies. The charge redistribution would cause the change of the tunnel barrier shape and following change of the electroresistance effect. In the case of ambiguous ferroelectric properties of HfO 2 ultrathin films, this oxygen-driven resistive switching effect can mimic the tunnel electroresistance effect. Here, we demonstrate two separate resistive switching regimes, depending on the applied voltage, in the same memristor device employing a ferroelectric Hf 0.5 Zr 0.5 O 2 (4.5 nm) layer. The first regime originates from the polarization reversal, whereas the second one is attributed to the accumulation/depletion of the oxygen vacancies at the electrode interface. The modulation of the tunnel barrier causes the enhancement of R OFF /R ON ratio in ∼20 times compared to the tunnel electroresistance effect. The developed device was used to formulate the criteria for unambiguous discrimination between the ferroelectric-and non-ferroelectric resistive switching effects in HfO 2 -based ferroelectric tunnel junctions.

Transistor structures comprising graphene and sub-wavelength metal gratings hold a great promise for plasmon-enhanced terahertz detection. Despite considerable theoretical effort, little experimental evidence for terahertz plasmons in such structures was found so far. Here, we report an experimental study of plasmons in graphene-insulator-grating structures using Fourier transform spectroscopy in 5-10 THz range. The plasmon resonance is clearly visible above the Drude absorption background even in chemical vapor deposited (CVD) graphene with low carrier mobility ∼ 10 3 cm 2 /(V s). We argue that plasmon lifetime is weakly sensistive to scattering by grain boundaries and macoscopic defects which limits the mobility of CVD samples. Upon placing the grating in close proximity to graphene, the plasmon field becomes tightly bound below the metal stripes, while the resonant frequency is determined by the stripe width but not by grating period. Our results open the prospects of large-area commercially available graphene for resonant terahertz detectors.Graphene-based optoelectronic devices benefit from high-speed operation 1,2 , broadband response 3 , and compatibility with on-chip optical interconnects 4 . Their major drawback is low electromagnetic wave absorbance by a single sheet of graphene. This problem is readily resolved via coupling of incident light to plasmons bound either to adjacent metal nanoparticles 5,6 or to graphene itself 7 . Unlike plasmons in metals, intrinsic graphene plasmons offer ultra-strong field confinement 8 and tuning of resonant frequency with gate voltage 9,10 .Resonant excitation of plasmons in graphene-based photodetectors becomes increasingly difficult when going from infrared to terahertz (THz) range 11 as the plasmon quality factor scales linearly with frequency. Despite considerable effort 12-14 evidence of plasmon-assisted THz detection in graphene are scarce and were reported only for high-quality encapsulated graphene 15 or epitaxial graphene on SiC 16 . Experimental demonstrations of terahertz plasmons in absorbance spectra of graphene, including scalable chemical vapor deposited (CVD) samples, are more numerous [17][18][19] . At the same time, most such experiments dealt with ribbon-patterned where collection of photocurrent is hindered and boundary scattering is enhanced.In this paper, we study the plasmonic properties of a basic building block of graphene-based terahertz detector 13,20 , the CVD graphene-channel field-effect transistor with a grating gate. We find that plasmonic contribution to absorption spectra is pronounced at 5 − 10 THz frequencies despite moderate carrier mobility ∼ 10 3 cm 2 /V s and short momentum relaxation time τ p ∼ 50 fs. We further argue that plasmon lifetime in CVD-graphene (as it enters the quality factor) exceeds the relaxation time as extracted from mobility, in contrast to reports for encapsulated graphene. We find that metal grating placed in immediate vicinity to graphene modifies the resonant plasmon frequencies. In particular, the recipro...

Ferroelectric hafnium oxide thin films—the most promising materials in microelectronics’ non-volatile memory—exhibit both unconventional ferroelectricity and unconventional piezoelectricity. Their exact origin remains controversial, and the relationship between ferroelectric and piezoelectric properties remains unclear. We introduce a new method to investigate this issue, which consists in a local controlled modification of the ferroelectric and piezoelectric properties within a single Hf0.5Zr0.5O2 capacitor device through local doping and a further comparative nanoscopic analysis of the modified regions. By comparing the ferroelectric properties of Ga-doped Hf0.5Zr0.5O2 thin films with the results of piezoresponse force microscopy and their simulation, as well as with the results of in situ synchrotron X-ray microdiffractometry, we demonstrate that, depending on the doping concentration, ferroelectric Hf0.5Zr0.5O2 has either a negative or a positive longitudinal piezoelectric coefficient, and its maximal value is −0.3 pm/V. This is several hundreds or thousands of times less than those of classical ferroelectrics. These changes in piezoelectric properties are accompanied by either improved or decreased remnant polarization, as well as partial or complete domain switching. We conclude that various ferroelectric and piezoelectric properties, and the relationships between them, can be designed for Hf0.5Zr0.5O2 via oxygen vacancies and mechanical-strain engineering, e.g., by doping ferroelectric films.

Composite multiferroics containing ferroelectric and ferromagnetic components often have much larger magnetoelectric coupling compared to their single-phase counterparts. Doped or alloyed HfO2-based ferroelectrics may serve as a promising component in composite multiferroic structures potentially feasible for technological applications. Recently, a strong charge-mediated magnetoelectric coupling at the Ni/HfO2 interface has been predicted using density functional theory calculations. Here, we report on the experimental evidence of such magnetoelectric coupling at the Ni/Hf0.5Zr0.5O2(HZO) interface. Using a combination of operando XAS/XMCD and HAXPES/MCDAD techniques, we probe element-selectively the local magnetic properties at the Ni/HZO interface in functional Au/Co/Ni/HZO/W capacitors and demonstrate clear evidence of the ferroelectric polarization effect on the magnetic response of a nanometer-thick Ni marker layer. The observed magnetoelectric effect and the electronic band lineup of the Ni/HZO interface are interpreted based on the results of our theoretical modeling. It elucidates the critical role of an ultrathin NiO interlayer, which controls the sign of the magnetoelectric effect as well as provides a realistic band offset at the Ni/HZO interface, in agreement with the experiment. Our results hold promise for the use of ferroelectric HfO2-based composite multiferroics for the design of multifunctional devices compatible with modern semiconductor technology.