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.

The interface formation between ferroelectric HfO2 layers and TiN bottom electrodes was studied by hard X-ray photoelectron spectroscopy and directly correlated to the electric polarization characteristics of the TiN/HfO2/TiN capacitors. We consistently deduced the interface chemistry from HfO2- and TiN-related core levels, dependent on the oxygen flow ṁ supplied before and during physical vapor deposition (PVD) growth of HfO2. The results underline the critical, twofold impact of oxygen supply on HfO2 and interface properties. When supplied before growth, the supplied oxygen stabilizes the TiN/HfO2 interface by oxidation and formation of a self-limiting (noninsulating) TiO2 intralayer. When supplied during growth, on the other hand, oxygen flows above a critical threshold reduce the oxygen vacancy concentration within the HfO2 film. We reveal a direct relation between the maximum ferroelectric remanent polarization and a critical threshold PVD oxygen exposure flow rate. The results allow for advancement of the PVD growth process in terms of a more flexible design of the ferroelectric HfO2 films with chemically stable TiN interfaces.

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.

On-Chip TaO x-Based Non-volatile Resistive Memory for in vitro Neurointerfaces.

Novel two-dimensional electron systems at the interfaces and surfaces of transition-metal oxides recently have attracted much attention as they display tunable, intriguing properties that can be exploited in future electronic devices. Here we show that a high-mobility quasi-two-dimensional electron system with strong spin-orbit coupling can be induced at the surface of a KTaO 3 (001) crystal by pulsed laser deposition of a disordered LaAlO 3 film. The momentum-resolved electronic structure of the buried electron system is mapped out by hard x-ray angle-resolved photoelectron spectroscopy. From a comparison to calculations, it is found that the band structure deviates from that of electron-doped bulk KTaO 3 due to the confinement to the interface. Fermi surface mapping shows a three-dimensional, periodic intensity pattern consistent with electron pockets of quantum well states centered around the points and the expectations from a Fourier analysis-based description of photoemission on confined electron systems. From the k broadening of the Fermi surface and core-level depth profiling, we estimate the extension of the electron system to be at least 1 nm but not much larger than 2 nm, respectively.