Accumulative Polarization Reversal in Nanoscale Ferroelectric Transistors

The electric-field-driven and reversible polarization switching in ferroelectric materials provides a promising approach for nonvolatile information storage. With the advent of ferroelectricity in hafnium oxide, it has become possible to fabricate ultrathin ferroelectric films suitable for nanoscale electronic devices. Among them, ferroelectric field-effect transistors (FeFETs) emerge as attractive memory elements. While the binary switching between the two logic states, accomplished through a single voltage pulse, is mainly being investigated in FeFETs, additional and unusual switching mechanisms remain largely unexplored. In this work, we report the natural property of ferroelectric hafnium oxide, embedded within a nanoscale FeFET, to accumulate electrical excitation, followed by a sudden and complete switching. The accumulation is attributed to the progressive polarization reversal through localized ferroelectric nucleation. The electrical experiments reveal a strong field and time dependence of the phenomenon. These results not only offer novel insights that could prove critical for memory applications but also might inspire to exploit FeFETs for unconventional computing

On the Control of the Fixed Charge Densities in Al₂O₃‑Based Silicon Surface Passivation Schemes

A controlled field-effect passivation by a well-defined density of fixed charges is crucial for modern solar cell surface passivation schemes. Al₂O₃ nanolayers grown by atomic layer deposition contain negative fixed charges. Electrical measurements on slant-etched layers reveal that these charges are located within a 1 nm distance to the interface with the Si substrate. When inserting additional interface layers, the fixed charge density can be continuously adjusted from 3.5 × 10¹² cm^–2 (negative polarity) to 0.0 and up to 4.0 × 10¹² cm^–2 (positive polarity). A HfO₂ interface layer of one or more monolayers reduces the negative fixed charges in Al₂O₃ to zero. The role of HfO₂ is described as an inert spacer controlling the distance between Al₂O₃ and the Si substrate. It is suggested that this spacer alters the nonstoichiometric initial Al₂O₃ growth regime, which is responsible for the charge formation. On the basis of this charge-free HfO₂/Al₂O₃ stack, negative or positive fixed charges can be formed by introducing additional thin Al₂O₃ or SiO₂ layers between the Si substrate and this HfO₂/Al₂O₃ capping layer. All stacks provide very good passivation of the silicon surface. The measured effective carrier lifetimes are between 1 and 30 ms. This charge control in Al₂O₃ nanolayers allows the construction of zero-fixed-charge passivation layers as well as layers with tailored fixed charge densities for future solar cell concepts and other field-effect based devices

Dually Active Silicon Nanowire Transistors and Circuits with Equal Electron and Hole Transport

We present novel multifunctional nanocircuits built from nanowire transistors that uniquely feature equal electron and hole conduction. Thereby, the mandatory requirement to yield energy efficient circuits with a single type of transistor is shown for the first time. Contrary to any transistor reported up to date, regardless of the technology and semiconductor materials employed, the dually active silicon nanowire channels shown here exhibit an ideal symmetry of current–voltage device characteristics for electron (n-type) and hole (p-type) conduction as evaluated in terms of comparable currents, turn-on threshold voltages, and switching slopes. The key enabler to symmetry is the selective tunability of the tunneling transmission of charge carriers as rendered by the combination of the nanometer-scale dimensions of the junctions and the application of radially compressive strain. To prove the advantage of this concept we integrated dually active transistors into cascadable and multifunctional one-dimensional circuit strings. The nanocircuits confirm energy efficient switching and can further be electrically configured to provide four different types of operation modes compared to a single one when employing conventional electronics with the same amount of transistors

Electric Field Cycling Behavior of Ferroelectric Hafnium Oxide

HfO₂ based ferroelectrics are lead-free, simple binary oxides with nonperovskite structure and low permittivity. They just recently started attracting attention of theoretical groups in the fields of ferroelectric memories and electrostatic supercapacitors. A modified approach of harmonic analysis is introduced for temperature-dependent studies of the field cycling behavior and the underlying defect mechanisms. Activation energies for wake-up and fatigue are extracted. Notably, all values are about 100 meV, which is 1 order of magnitude lower than for conventional ferroelectrics like lead zirconate titanate (PZT). This difference is mainly atttributed to the one to two orders of magnitude higher electric fields used for cycling and to the different surface to volume ratios between the 10 nm thin films in this study and the bulk samples of former measurements or simulations. Moreover, a new, analog-like split-up effect of switching peaks by field cycling is discovered and is explained by a network model based on memcapacitive behavior as a result of defect redistribution

Investigation of Embedded Perovskite Nanoparticles for Enhanced Capacitor Permittivities

Growth experiments show significant differences in the crystallization of ultrathin CaTiO₃ layers on polycrystalline Pt surfaces. While the deposition of ultrathin layers below crystallization temperature inhibits the full layer crystallization, local epitaxial growth of CaTiO₃ crystals on top of specific oriented Pt crystals occurs. The result is a formation of crystals embedded in an amorphous matrix. An epitaxial alignment of the cubic CaTiO₃ ⟨111⟩ direction on top of the underlying Pt {111} surface has been observed. A reduced forming energy is attributed to an interplay of surface energies at the {111} interface of both materials and CaTiO₃ nanocrystallites facets. The preferential texturing of CaTiO₃ layers on top of Pt has been used in the preparation of ultrathin metal–insulator–metal capacitors with 5–30 nm oxide thickness. The effective CaTiO₃ permittivity in the capacitor stack increases to 55 compared to capacitors with amorphous layers and a permittivity of 28. The isolated CaTiO₃ crystals exhibit a passivation of the CaTiO₃ grain surfaces by the surrounding amorphous matrix, which keeps the capacitor leakage current at ideally low values comparable for those of amorphous thin film capacitors

Local Ion Irradiation-Induced Resistive Threshold and Memory Switching in Nb₂O₅/NbO_<i>x</i> Films

Resistive switching devices with a Nb₂O₅/NbO_<i>x</i> bilayer stack combine threshold and memory switching. Here we present a new fabrication method to form such devices. Amorphous Nb₂O₅ layers were treated by a krypton irradiation. Two effects are found to turn the oxide partly into a metallic NbO_<i>x</i> layer: preferential sputtering and interface mixing. Both effects take place at different locations in the material stack of the device; preferential sputtering affects the surface, while interface mixing appears at the bottom electrode. To separate both effects, devices were irradiated at different energies (4, 10, and 35 keV). Structural changes caused by ion irradiation are studied in detail. After successful electroforming, the devices exhibit the desired threshold switching. In addition, the choice of the current compliance defines whether a memory effect adds to the device. Findings from electrical characterization disclose a model of the layer modification during irradiation

Ferroelectricity in Simple Binary ZrO₂ and HfO₂

The transition metal oxides ZrO₂ and HfO₂ as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO₂ thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO₂–ZrO₂ mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group <i>Pbc</i>2₁, whose noncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics

Compact Nanowire Sensors Probe Microdroplets

The conjunction of miniature nanosensors and droplet-based microfluidic systems conceptually opens a new route toward sensitive, optics-less analysis of biochemical processes with high throughput, where a single device can be employed for probing of thousands of independent reactors. Here we combine droplet microfluidics with the compact silicon nanowire based field effect transistor (SiNW FET) for in-flow electrical detection of aqueous droplets one by one. We chemically probe the content of numerous (∼10⁴) droplets as independent events and resolve the pH values and ionic strengths of the encapsulated solution, resulting in a change of the source–drain current <i>I</i>_SD through the nanowires. Further, we discuss the specificities of emulsion sensing using ion sensitive FETs and study the effect of droplet sizes with respect to the sensor area, as well as its role on the ability to sense the interior of the aqueous reservoir. Finally, we demonstrate the capability of the novel droplets based nanowire platform for bioassay applications and carry out a glucose oxidase (GOx) enzymatic test for glucose detection, providing also the reference readout with an integrated parallel optical detector

Switching Kinetics in Nanoscale Hafnium Oxide Based Ferroelectric Field-Effect Transistors

The recent discovery of ferroelectricity in thin hafnium oxide films has led to a resurgence of interest in ferroelectric memory devices. Although both experimental and theoretical studies on this new ferroelectric system have been undertaken, much remains to be unveiled regarding its domain landscape and switching kinetics. Here we demonstrate that the switching of single domains can be directly observed in ultrascaled ferroelectric field effect transistors. Using models of ferroelectric domain nucleation we explain the time, field and temperature dependence of polarization reversal. A simple stochastic model is proposed as well, relating nucleation processes to the observed statistical switching behavior. Our results suggest novel opportunities for hafnium oxide based ferroelectrics in nonvolatile memory devices

Bipolar Electric-Field Enhanced Trapping and Detrapping of Mobile Donors in BiFeO₃ Memristors

Pulsed laser deposited Au-BFO-Pt/Ti/Sapphire MIM structures offer excellent bipolar resistive switching performance, including electroforming free, long retention time at 358 K, and highly stable endurance. Here we develop a model on modifiable Schottky barrier heights and elucidate the physical origin underlying resistive switching in BiFeO₃ memristors containing mobile oxygen vacancies. Increased switching speed is possible by applying a large amplitude writing pulse as the resistive switching is tunable by both the amplitude and length of the writing pulse. The local resistive switching has been investigated by conductive atomic force microscopy and exhibits the capability of down-scaling the resistive switching cell to the grain size