In this work, we experimentally explore and compare FET gate stacks with and without an inner metal plane between a linear dielectric (SiO 2 ) and a ferroelectric layer (Si-doped HfO 2 ) operating in the negative capacitance (NC) regime. The use of nanosecond-range pulses enables us to observe hysteresis-free NC and reconstruct the ${S}$ -shaped polarization-voltage curves. The devices with the inner metal plate show a higher equivalent NC value, which offers the potential for a higher differential amplification in a NC-FET. However, such a NC region is observed over a smaller range of electric field and polarization, which leads to hysteresis. Moreover, the presence of a metal layer in between the ferroelectric and the insulator favors domain formation resulting in destabilization of the NC effect. For the gate structure where the ferroelectric and the insulator are in contact, the ${S}$ -shaped polarization-voltage curve shows a better agreement with Landau–Ginzburg–Devonshire formalism for the monodomain state. The uniform polarization closely mimicking the monodomain state is possible due to the polarization imprint occurring due to the structural asymmetry. By nanometer resolution polarization mapping via off-resonance piezoelectric force microscopy (PFM), we corroborate the presence of imprint, which can intrinsically stabilize one ferroelectric state. Overall, the article provides an experimental demonstration that the absence of the inner metal plane in the gate structures stabilizes the NC regime favorable for hysteresis-free NC-FET.
Dielectric breakdown of (Pb,La)(Zr,Ti)O3(PLZT) film capacitors with Pt and SrRuO3 electrodes, in its initial or virgin state and after voltage cycling that causes polarization fatigue, is studied by constant-current measurements. It is shown that for different types of PLZT capacitors, the breakdown onset is controlled by the critical charge flown through the film (charge to breakdown) rather than the voltage applied to the capacitor. The charge to breakdown for the asymmetrical Pt/SrRuO3/PLZT/Pt capacitors is found to be much higher that that for the Pt/PLZT/Pt system. Polarization fatigue caused by bipolar voltage cycling provokes a substantial decrease of charge to breakdown. These results can be interpreted in terms of percolation model used for breakdown in SiO2, where the charge to breakdown is associated with injected-carrier-assisted creation of the critical concentration of defects required for formation of the breakdown paths. In this article we propose two possible mechanisms for interpretation of the observed variation of the PLZT capacitor breakdown performance. One mechanism is based on different contributions of PLZT film grains and grain boundaries to the total current flowing through the system. The second mechanism is related to the different kinetics of generation and relaxation of defects in the band gap of the PLZT film. The decrease of the charge to breakdown induced by polarization fatigue is explained by generation of defects during the fatigue bipolar polarization reversals.
The region-by-region polarization switching in ferroelectric Pb(Zr,Ti)O3 thin films sandwiched between Pt electrodes has been directly observed using piezoelectric scanning probe microscopy. A resolution improved by one order-of-magnitude compared to the standard piezoelectric response imaging technique for ferroelectric capacitors was achieved by reducing the top electrode thickness to 10–15nm through polishing. It was demonstrated that the individually switched regions correspond to single grains or clusters of grains where the grain boundaries act as frontiers limiting the propagation of the switched state. The study of the propagation of the reversed polarization state as a function of voltage applied shows a rather discontinuous growth of the switched areas, the movement of the domain walls being triggered abruptly by different threshold voltages. This result agrees with the earlier proposed nucleation-limited switching model. The observation of the frozen regions that do not switch even at higher voltages provides significant insight into the “bits-failure” problem in submicron ferroelectric capacitors used for nonvolatile memory applications.
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.
We report the universal boosting impact of a true negative capacitance (NC) effect on digital and analog performances of Tunnel FETs (TFETs), mirrored for the first time in near hysteresis-free experiments and exploiting the S-shaped polarization characteristics. Well behaved InGaAs TFETs with a minimum swing of 55 mV/dec at room temperature are combined with high-quality single crystalline PZT capacitors, placed in series with the gate. When fully satisfying the exact NC matching conditions by a single crystalline ferroelectric that can perform a mono-domain state, a hysteresis-free (sub-10mV over 4 decades of current) NC-TFET with a sub-thermionic swing and an SS min of 40 mV/dec is demonstrated. In other devices, improvement in the subthreshold swing, down to 30 mV/dec, and analog current efficiency factor, up to 180 V -1 , are achieved in NC-TFETs with a hysteresis as small as 30 mV. Importantly, the I 60 FoM of the TFET is improved up to 2 orders of magnitude. The supply voltage is thereby reduced by 50%, down to 300 mV, providing the same drive current. Our results show that NC can open a new direction as a universal performance booster in the FET design by significantly improving the low I 60 and low overdrive of TFETs.
We propose and experimentally demonstrate top-gated complementary n- and p-type black phosphorous field effect devices (FETs) by engineering the workfunction of pre-patterned electrodes embedded in a SiO 2 bottom layer. Pre-patterned electrodes offer the advantages of reducing the exposure time of exfoliated flakes to oxidant agents with respect to top-contacted devices and maximizing the accessible area for sensing applications. The presented devices are realized by mechanical exfoliation of multilayer black phosphorous flakes on top of pre-patterned embedded source and drain contacts. A capping layer consisting of 15-nm thick Al 2 O 3 is deposited to prevent flakes degradation and serves as top gate dielectric. The silicon substrate can be exploited as back gate to program the FETs threshold voltage. We deposited both Au and Ag embedded contacts to investigate the impact of electrodes workfunction on BP FETs polarity. Au contacted devices show p-type conduction with ON/OFF current ratio 140 and holes mobility up to 40 cm 2 V -1 s -1 . Devices with Ag contacts exhibit prevalent n-type conduction with ON/OFF ratio 1700 and electron mobility 2 cm 2 V -1 s -1 . The reported results represent a substantial improvement with respect to reported alternative implementations of black phosphorous FETs with pre-patterned, non-embedded electrodes. Moreover, we demonstrate that Ag is a promising metal for electron injection in black phosphorous FETs.
Control of magnetic domain walls (DWs) and their propagation is among the most promising development directions for future information-storage devices. The well-established tools for such manipulation are the spin-torque transfer from electrical currents and strain. The focus of this paper is an alternative concept based on the nonvolatile ferroelectric field effect on DWs in a ferromagnet with carrier-mediated exchange coupling. The integrated ferromagnet/ferroelectric structure yields two superimposed ferroic patterns strongly coupled by an electric field. Using this coupling, we demonstrate an easy-to-form, stable, nondestructive, and electrically rewritable switch on magnetic domain wall propagation.
