Giant switchable non thermally-activated conduction in 180° domain walls in tetragonal Pb(Zr,Ti)O3.

Conductive domain walls in ferroelectrics offer a promising concept of nanoelectronic circuits with 2D domain-wall channels playing roles of memristors or synoptic interconnections. However, domain wall conduction remains challenging to control and pA-range currents typically measured on individual walls are too low for single-channel devices. Charged domain walls show higher conductivity, but are generally unstable and difficult to create. Here, we show highly conductive and stable channels on ubiquitous 180° domain walls in the archetypical ferroelectric, tetragonal Pb(Zr,Ti)O3. These electrically erasable/rewritable channels show currents of tens of nanoamperes (200 to 400 nA/µm) at voltages ≤2 V and metallic-like non thermally-activated transport properties down to 4 K, as confirmed by nanoscopic mapping. The domain structure analysis and phase-field simulations reveal complex switching dynamics, in which the extraordinary conductivity in strained Pb(Zr,Ti)O3 films is explained by an interplay between ferroelastic a- and c-domains. This work demonstrates the potential of accessible and stable arrangements of nominally uncharged and electrically switchable domain walls for nanoelectronics.

Nanowire Tunnel FET with Simultaneously Reduced Subthermionic Subthreshold Swing and Off-Current due to Negative Capacitance and Voltage Pinning Effects.

Nanowire tunnel field-effect transistors (TFETs) have been proposed as the most advanced one-dimensional (1D) devices that break the thermionic 60 mV/decade of the subthreshold swing (SS) of metal oxide semiconductor field-effect transistors (MOSFETs) by using quantum mechanical band-to-band tunneling and excellent electrostatic control. Meanwhile, negative capacitance (NC) of ferroelectrics has been proposed as a promising performance booster of MOSFETs to bypass the aforementioned fundamental limit by exploiting the differential amplification of the gate voltage under certain conditions. We combine these two principles into a single structure, a negative capacitance heterostructure TFET, and experimentally demonstrate a double beneficial effect: (i) a super-steep SS value down to 10 mV/decade and an extended low slope region that is due to the NC effect and, (ii) a remarkable off-current reduction that is experimentally observed and explained for the first time by the effect of the ferroelectric dipoles, which set the surface potential in a slightly negative value and further blocks the source tunneling current in the off-state. State-of-the-art InAs/InGaAsSb/GaSb nanowire TFETs are employed as the baseline transistor and PZT and silicon-doped HfO2 as ferroelectric materials.

Negative Capacitance as Universal Digital and Analog Performance Booster for Complementary MOS Transistors.

Boltzmann electron energy distribution poses a fundamental limit to lowering the energy dissipation of conventional MOS devices, a minimum increase of the gate voltage, i.e. 60 mV, is required for a 10-fold increase in drain-to-source current at 300 K. Negative Capacitance (NC) in ferroelectric materials is proposed in order to address this physical limitation of CMOS technology. A polarization destabilization in ferroelectrics causes an effective negative permittivity, resulting in a differential voltage amplification and a reduced subthreshold swing when integrated into the gate stack of a transistor. The novelty and universality of this approach relate to the fact that the gate stack is not anymore a passive part of the transistor and contributes to signal amplification. In this paper, we experimentally validate NC as a universal performance booster: (i) for complementary MOSFETs, of both n- and p-type in an advanced CMOS technology node, and, (ii) for both digital and analog significant enhancements of key figures of merit for information processing (subthreshold swing, overdrive, and current efficiency factor). Accordingly, a sub-thermal swing down to 10 mV/decade together with an enhanced current efficiency factor up to 105 V-1 is obtained in both n- and p-type MOSFETs at room temperature by exploiting a PZT capacitor as the NC booster. As a result of the subthreshold swing reduction and overdrive improvement observed by NC, the required supply voltage to provide the same on-current is reduced by approximately 50%.

Genuinely Ferroelectric Sub-1-Volt-Switchable Nanodomains in Hf xZr(1- x)O2 Ultrathin Capacitors.

The new class of fully silicon-compatible hafnia-based ferroelectrics with high switchable polarization and good endurance and thickness scalability shows a strong promise for new generations of logic and memory devices. Among other factors, their competitiveness depends on the power efficiency that requires reliable low-voltage operation. Here, we show genuine ferroelectric switching in Hf xZr(1- x)O2 (HZO) layers in the application-relevant capacitor geometry, for driving signals as low as 800 mV and coercive voltage below 500 mV. Enhanced piezoresponse force microscopy with sub-picometer sensitivity allowed for probing individual polarization domains under the top electrode and performing a detailed analysis of hysteretic switching. The authentic local piezoelectric loops and domain wall movement under bias attest to the true ferroelectric nature of the detected nanodomains. The systematic analysis of local piezoresponse loop arrays reveals a totally unexpected thickness dependence of the coercive fields in HZO capacitors. The thickness decrease from 10 to 7 nm is associated with a remarkably strong decrease of the coercive field, with about 50% of the capacitor area switched at coercive voltages ≤0.5 V. Our explanation consistent with the experimental data involves a change of mechanism of nuclei-assisted switching when the thickness decreases below 10 nm. The practical implication of this effect is a robust ferroelectric switching under the millivolt-range driving signal, which is not expected for the standard coercive voltage scaling law. These results demonstrate a strong potential for further aggressive thickness reduction of HZO layers for low-power electronics.

Effect of hysteretic and non-hysteretic negative capacitance on tunnel FETs DC performance.

This work experimentally demonstrates that the negative capacitance effect can be used to significantly improve the key figures of merit of tunnel field effect transistor (FET) switches. In the proposed approach, a matching condition is fulfilled between a trained-polycrystalline PZT capacitor and the tunnel FET (TFET) gate capacitance fabricated on a strained silicon-nanowire technology. We report a non-hysteretic switch configuration by combining a homojunction TFET and a negative capacitance effect booster, suitable for logic applications, for which the on-current is increased by a factor of 100, the transconductance by 2 orders of magnitude, and the low swing region is extended. The operation of a hysteretic negative capacitance TFET, when the matching condition for the negative capacitance is fulfilled only in a limited region of operation, is also reported and discussed. In this late case, a limited improvement in the device performance is observed. Overall, the paper demonstrates the main beneficial effects of negative capacitance on TFETs are the overdrive and transconductance amplification, which exactly address the most limiting performances of current TFETs.

Van der Waals MoS2/VO2 heterostructure junction with tunable rectifier behavior and efficient photoresponse.

Junctions between n-type semiconductors of different electron affinity show rectification if the junction is abrupt enough. With the advent of 2D materials, we are able to realize thin van der Waals (vdW) heterostructures based on a large diversity of materials. In parallel, strongly correlated functional oxides have emerged, having the ability to show reversible insulator-to-metal (IMT) phase transition by collapsing their electronic bandgap under a certain external stimulus. Here, we report for the first time the electronic and optoelectronic characterization of ultra-thin n-n heterojunctions fabricated using deterministic assembly of multilayer molybdenum disulphide (MoS2) on a phase transition material, vanadium dioxide (VO2). The vdW MoS2/VO2 heterojunction combines the excellent blocking capability of an n-n junction with a high conductivity in on-state, and it can be turned into a Schottky rectifier at high applied voltage or at temperatures higher than 68 °C, exploiting the metal state of VO2. We report tunable diode-like current rectification with a good diode ideality factor of 1.75 and excellent conductance swing of 120 mV/dec. Finally, we demonstrate unique tunable photosensitivity and excellent junction photoresponse in the 500/650 nm wavelength range.

Bent Ferroelectric Domain Walls as Reconfigurable Metallic-Like Channels.

Use of ferroelectric domain-walls in future electronics requires that they are stable, rewritable conducting channels. Here we demonstrate nonthermally activated metallic-like conduction in nominally uncharged, bent, rewritable ferroelectric-ferroelastic domain-walls of the ubiquitous ferroelectric Pb(Zr,Ti)O3 using scanning force microscopy down to a temperature of 4 K. New walls created at 4 K by pressure exhibit similar robust and intrinsic conductivity. Atomic resolution electron energy-loss spectroscopy confirms the conductivity confinement at the wall. This work provides a new concept in "domain-wall nanoelectronics".

Controlled stripes of ultrafine ferroelectric domains.

In the pursuit of ferroic-based (nano)electronics, it is essential to minutely control domain patterns and domain switching. The ability to control domain width, orientation and position is a prerequisite for circuitry based on fine domains. Here, we develop the underlying theory towards growth of ultra-fine domain patterns, substantiate the theory by numerical modelling of practical situations and implement the gained understanding using the most widely applied ferroelectric, Pb(Zr,Ti)O3, demonstrating controlled stripes of 10 nm wide domains that extend in one direction along tens of micrometres. The observed electrical conductivity along these thin domains embedded in the otherwise insulating film confirms their potential for electronic applications.

Cold-field switching in PVDF-TrFE ferroelectric polymer nanomesas.

Polarization reversal in ferroelectric nanomesas of polyvinylidene fluoride with trifluoroethylene has been probed by ultrahigh vacuum piezoresponse force microscopy in a wide temperature range from 89 to 326 K. In dramatic contrast to the macroscopic data, the piezoresponse force microscopy local switching was nonthermally activated and, at the same time, occurring at electric fields significantly lower than the intrinsic switching threshold. A "cold-field" defect-mediated extrinsic switching is shown to be an adequate scenario describing this peculiar switching behavior. The extrinsic character of the observed polarization reversal suggests that there is no fundamental bar for lowering the coercive field in ferroelectric polymer nanostructures, which is of importance for their applications in functional electronics.

Thermally induced cooperative molecular reorientation and nanoscale polarization switching behaviors of ultrathin poly(vinylidene fluoride-trifluoroethylene) films.

Ultrathin films of the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] have recently attracted intensive research interest due to their potential applications in emerging organic devices. As special geometry confinement systems, many aspects about their processing, microstructure, and performance are far from being well understood. Here, the cooperative molecular orientation, macroscopic ferroelectric properties, and nanoscale polarization switching behaviors of thermally crystallized ultrathin P(VDF-TrFE) films were investigated. With increasing annealing temperature, the films showed a distinct granule toward layered needle-network (LNN) morphology transition with deteriorated ferroelectricity at a critical point (T(cr)) around 140 °C. Accompanying this is that the polymer backbone first lay more parallel relative to the substrate, and then exactly at T(cr) it showed an abrupt standing-up reorientation. Interestingly, the polarization axis simultaneously showed just opposite orientation and reorientation. Nanoscale polarization switching characterization by using piezoresponse force microscopy and local ferroelectric hysteresis loops revealed a varied molecular orientation in the same needle grain and a polarization reversal constraint effect by the inhomogeneous LNN structure. On the basis of these observations, a tilted-chain lamellae structural model was proposed for the LNN film. The lying down of the polarization axis and the polarization reversal constrain effect well explain the inferior performance of the LNN film despite its higher crystallinity than that of the granular film. The results may shed some light on the understanding of the intercorrelation among the thermal crystallization, microstructure, and macroscopic performance of ultrathin polymer films.

Nonvolatile gate effect in a ferroelectric-semiconductor quantum well.

Field effect transistors with ferroelectric gates would make ideal rewritable nonvolatile memories were it not for the severe problems in integrating the ferroelectric oxide directly on the semiconductor channel. We propose a powerful way to avoid these problems using a gate material that is ferroelectric and semiconducting simultaneously. First, ferroelectricity in semiconductor (Cd,Zn)Te films is proven and studied using modified piezoforce scanning probe microscopy. Then, a rewritable field effect device is demonstrated by local poling of the (Cd,Zn)Te layer of a (Cd,Zn)Te/CdTe quantum well, provoking a reversible, nonvolatile change in the resistance of the 2D electron gas. The results point to a potential new family of nanoscale one-transistor memories.