Ferroelectric Gates Research Articles

Doped HfO2-based ferroelectric-aided charge-trapping effect in MFIS gate stack of FeFET

The ferroelectric field effect transistor (FeFET) is a very promising candidate for low-power and non-volatile memory. However, the co-existing effect of ferroelectric polarization and interface charge trapping in the FeFETs is demonstrated and many efforts have been made to eliminate this charge-trapping effect, which is usually treated as a deleterious effect. In contrast, we have found that the charge-trapping effect can play a dominant role in ferroelectric gates. In this work, we have verified that the charge-trapping effect of the ferroelectric/insulator interface could induce a memory window as the main physical mechanism in the TiN/Hf0.5Zr0.5O2/SiO2/p-Si (MFIS) structure, in which the ferroelectric characteristics of HZO thin films was verified through a reverse-grown MFIS structure. We also demonstrated that 2.5 nm SiO2 is optimal for the charge tunneling effect and the device has the largest memory window. Moreover, in order to enlarge the memory window of MFIS capacitors, we utilized the stress-enhanced ferroelectric polarization characteristics of Hf0.5Zr0.5O2 to improve the charge-trapping effect. Such a finding demonstrates that the ferroelectric-aided charge-trapping devices are potential to be used in non-volatile memories.

Measuring Band Modulation of MoS2 with Ferroelectric Gates.

Electronic properties of two-dimensional (2D) materials can be significantly tuned by an external electric field. Ferroelectric gates can provide a strong polarization electric field. Here, we report the measurements of the band structure of few-layer MoS2 modulated by a ferroelectric P(VDF-TrFE) gate with contact-mode scanning tunneling spectroscopy. When P(VDF-TrFE) is fully polarized, an electric field up to ∼0.62 V/nm through the MoS2 layers is inferred from the measured band edges, which affects the band structure significantly. First, strong band bending in the vertical direction signifies the Franz-Keldysh effect and a large extension of the optical absorption edge. Photons with energy of half the band gap are still absorbed with 20% of the absorption probability of photons at the band gap. Second, the electric field greatly enlarges the energy separations between the quantum-well subbands. Our study intuitively demonstrates the great potential of ferroelectric gates in band structure manipulation of 2D materials.

Polarization Orientation in Lead Zirconate Titanate (001) Thin Films Driven by the Interface with the Substrate

The orientation of polarization in ferroelectric thin films is important for applications of these materials in nonvolatile memory and field-effect transistors with ferroelectric gates, and in catalysis and photocatalysis. One means of control is to choose a well defined substrate, but the influence of a substrate's electronic properties on polarization orientation is not well understood. In this investigation of model systems, the experimental findings are integrated with electrostatic models to determine the magnitudes and orientations of the interfacial electric fields that are useful for setting the polarization of the thin ferroelectric epilayer.

Time-Dependent Negative Capacitance Effects in Al2O3/BaTiO3 Bilayers.

The negative capacitance (NC) effects in ferroelectric materials have emerged as the possible solution to low-power transistor devices and high-charge-density capacitors. Although the steep switching characteristic (subthreshold swing < sub-60 mV/dec) has been demonstrated in various devices combining the conventional transistors with ferroelectric gates, the actual applications of the NC effects are still some way off owing to the inherent hysteresis problem. This work reinterpreted the hysteretic properties of the NC effects within the time domain and demonstrated that capacitance (charge) boosting could be achieved without the hysteresis from the Al2O3/BaTiO3 bilayer capacitors through short-pulse charging. This work revealed that the hysteresis phenomenon in NC devices originated from the dielectric leakage of the dielectric layer. The suppression of charge injection via the dielectric leakage, which usually takes time, inhibits complete ferroelectric polarization switching during a short pulse time. It was demonstrated that a nonhysteretic NC effect can be achieved only within certain limited time and voltage ranges, but that these are sufficient for critical device applications.

Giant enhancement in the ferroelectric field effect using a polarization gradient

Coupling of switchable ferroelectric polarization with the carrier transport in an adjacent semiconductor enables a robust, non-volatile manipulation of the conductance in a host of low-dimensional systems, including the two-dimensional electron liquid that forms at the LaAlO3 (LAO)-SrTiO3 (STO) interface. However, strength of the gate-channel coupling is relatively weak, limited in part by the electrostatic potential difference across a ferroelectric gate. Here, through application of phenomenological Landau-Ginzburg-Devonshire theory and self-consistent Poisson-Schrödinger model calculations, we show how compositional grading of PbZr1−xTixO3 ferroelectric gates enables a more than twenty-five-fold increase in the LAO/STO channel conductance on/off ratios. Incorporation of polarization gradients in ferroelectric gates can enable breakthrough performance of ferroelectric non-volatile memories.

Detangling extrinsic and intrinsic hysteresis for detecting dynamic switch of electric dipoles using graphene field-effect transistors on ferroelectric gates.

A transition in source-drain current vs. back gate voltage (ID-VBG) characteristics from extrinsic polar molecule dominant hysteresis to anti-hysteresis induced by an oxygen deficient surface layer that is intrinsic to the ferroelectric thin films has been observed on graphene field-effect transistors on Pb0.92La0.08Zr0.52Ti0.48O3 gates (GFET/PLZT-Gate) during a vacuum annealing process developed to systematically remove the polar molecules adsorbed on the GFET channel surface. This allows the extrinsic and intrinsic hysteresis on GFET/PLZT-gate devices to detangle and the detection of the dynamic switch of electric dipoles using GFETs, taking advantage of their high gating efficiency on ferroelectric gate. A model of the charge trapping and pinning mechanism is proposed to successfully explain the transition. In response to pulsed VBG trains of positive, negative, as well as alternating polarities, respectively, the source-drain current ID variation is instantaneous with the response amplitude following the ID-VBG loops measured by DC VBG with consideration of the remnant polarization after a given VBG pulse when the gate electric field exceeds the coercive field of the PLZT. A detection sensitivity of around 212 dipole per μm(2) has been demonstrated at room temperature, suggesting the GFET/ferroelectric-gate devices provide a promising high-sensitivity scheme for uncooled detection of electrical dipole dynamic switch.

Ferroelectric control of a Mott insulator

The electric field control of functional properties is an important goal in oxide-based electronics. To endow devices with memory, ferroelectric gating is interesting, but usually weak compared to volatile electrolyte gating. Here, we report a very large ferroelectric field-effect in perovskite heterostructures combining the Mott insulator CaMnO3 and the ferroelectric BiFeO3 in its “supertetragonal” phase. Upon polarization reversal of the BiFeO3 gate, the CaMnO3 channel resistance shows a fourfold variation around room temperature, and a tenfold change at ~200 K. This is accompanied by a carrier density modulation exceeding one order of magnitude. We have analyzed the results for various CaMnO3 thicknesses and explain them by the electrostatic doping of the CaMnO3 layer and the presence of a fixed dipole at the CaMnO3/BiFeO3 interface. Our results suggest the relevance of ferroelectric gates to control orbital- or spin-ordered phases, ubiquitous in Mott systems, and pave the way toward efficient Mott-tronics devices.

Electrical and memory window properties of Sr0.8-xBaxBi2.2Ta2-yZryO9 ferroelectric gate in metal-ferroelectric-insulator-semiconductor structure

Electrical characteristics of Sr0.8-xBaxBi2.2Ta2-yZryO9 ferroelectric films grown on HfO2/Si wafers by sol–gel spin coating technique were investigated from the viewpoint of application as ferroelectric gates in metal-ferroelectric-insulator-semiconductor (MFIS) stacks. It was observed that the leakage current density level was 10-8 A/cm2 under 14 V for moderate doping ratio. Determined memory windows from C-V characteristics of Sr0.8Bi2.2Ta2O9 (SBT) and Sr0.8-xBaxBi2.2Ta2-yZryO9 (x = 0.04, 0.08, 0.12 and y = 0.1, 0.2, 0.3) are 0.59, 0.65, 0.75, and 0.86 V at gate sweeping bias of 5 V, respectively. Some part of electronic properties of Sr0.8-xBaxBi2.2Ta2-yZryO9 with the objective to enhance memory window up to 45 % were discussed. It was interpreted that defects which are formed in Ba and Zr modified SBT affected the electronic processes like leakage current, memory window and charge trapping.

Polarization Screening in Multiferroic (Ga,Mn)As/P(VDF-TrFE) Transistors

Ferroelectric copolymers of polyvinylidene fluoride and trifluoroethylene are attractive gate materials because of a rather high remanent polarization, low dielectric permittivity, and easy integration on semiconductors due to the low processing temperature. Recently it was demonstrated that the functionalities of devices with ferroelectric polymer gates can be extended to magnetic field effect transistors with diluted magnetic semiconductor channels. Here we address the critical issue of polarization screening in such transistors quantifying the amount of polarization charge which controls the magnetic channel. The gate effect is shown to be limited by internal polarization screening rather than polarization retention loss.

Non-volatile ferroelectric control of ferromagnetism in (Ga,Mn)As

Multiferroic structures that provide coupled ferroelectric and ferromagnetic responses are of significant interest as they may be used in novel memory devices and spintronic logic elements. One approach towards this goal is the use of composites that couple ferromagnetic and ferroelectric layers through magnetostrictive and piezoelectric strain transmitted across the interfaces. However, mechanical clamping of the films to the substrate limits their response. Structures where the magnetic response is modulated directly by the electric field of the poled ferroelectric would eliminate this constraint and provide a qualitatively higher level of integration, combining the emerging field of multiferroics with conventional semiconductor microelectronics. Here, we report the realization of such a device using (Ga,Mn)As, which is an archetypical diluted magnetic semiconductor with well-understood carrier-mediated ferromagnetism, and a polymer ferroelectric gate. Polarization reversal of the gate by a single voltage pulse results in a persistent modulation of the Curie temperature of the ferromagnetic semiconductor. The non-volatile gating of (Ga,Mn)As has been made possible by applying a low-temperature copolymer deposition technique that is distinct from pre-existing technologies for ferroelectric gates on magnetic oxides. This accomplishment opens a way to nanometre-scale modulation of magnetic semiconductor properties with rewritable ferroelectric domain patterns, operating at modest voltages and subnanosecond times.

Nonvolatile gate effect in the PZT/AlGaN/GaN heterostructure

Field effect devices with ferroelectric gates can be a valuable alternative to existing ferroelectric capacitor memories combining random access, high speed, low power, non-destructive reading, and non-volatility. However, integrating the ferroelectric oxides with the semiconductor media presents a complicated issue impeding the progress in the field. AlGaN/GaN heterostructures offer a promising solution for the ferroelectric gate integration because of good chemical and thermal stability and possibility to fabricate structures with two-dimensional electron gas (2DEG) as close to the interface as 15–20 nm. In the present work we report on successful fabrication of the PZT/AlGaN/GaN heterostructure and demonstrate the possibility to modulate resistance of the 2DEG at the AlGaN/GaN interface using the ferroelectric gate. The effect of polarization reversal of 2DEG provoke a reversible non-volatile change in the resistance of 2DEG. These results suggest that ferroelectric gates integrated into GaN-based heterostructures may be potentially interesting for non-volatile memories with non-destructive reading operating in a wide temperature range.

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.

Ferroelectric Gates for Modulation of 2D Electron Gas at GaN/AlGaN Interfaces

AbstractThe Pb(Zr,Ti)O3 ferroelectric gate was successfully integrated into the GaN/AlGaN heterostructure with 2D electron gas. The processing conditions were optimized in a way to obtain high quality textured Pb(Zr,Ti)O3 films without destroying the 2D gas situated as close to the interface as 20nm. Study of transport properties in this system demonstrates a possibility to control the 2D gas by switching the spontaneous polarization in the gate. Concentration and mobility of electrons in the 2D gas were monitored using Hall effect and resistivity measurements in a wide temperature range from 4.2K to 300K. The polarization oriented in the direction “from bottom to top” provokes a partial depletion effect in the channel resulting in a conductivity decrease. A correlation between the depletion effect in 2D gas and the change of the spontaneous polarization with temperature has been observed. The depletion effect is found to be reversible so that the initial conductivity in 2D gas can be restored by inversing the spontaneous polarization in the gate. These results suggest that ferroelectric gates integrated into systems with 2D electron gas may be potentially interesting for a number of experiments and applications as a flexible and nondestructive way of making rewritable nanopatterns on semiconductor heterostructures.

Quantum-information processing with ferroelectrically coupled quantum dots

A proposal is described to construct a quantum-information processor using ferroelectrically coupled Ge/Si quantum dots. The spin of single electrons form the fundamental qubits. Small-diameter (10 nm) Ge quantum dots are optically excited to create spin-polarized electrons in Si. The static polarization of an epitaxial ferroelectric thin film confines electrons laterally in the semiconductor; spin interactions between nearest-neighbor electrons are mediated by the nonlinear process of optical rectification. Single-qubit operations are achieved through ``g-factor engineering'' in the Ge/Si structures; spin-spin interactions occur through Heisenberg exchange, controlled by ferroelectric gates. A method for reading out the final state, while required for quantum computing, is not described; approaches involving single-electron transistors may prove fruitful in satisfying this requirement.

Device physics of ferroelectric memories

Abstract A status report is given on high-dielectric (usually ferroelectric) thin-films of fine-grained ceramics utilized for ferroelectric memories, including active memory elements in nonvolatile random access memories (NV-RAMs), passive capacitors in dynamic random access memories (DRAMs), and ferroelectric gates in ferroelectric field-effect transistors (FEMFETs) configured for non-destructive read-out (NDRO) memories. Emphasis is upon breakdown fields, leakage current, fatigue, and retention. Both the BaxSr1-xTiO3 (BST) family and the SrBi2Ta2O9 family of layer-structure perovskites are discussed. The review ends with a list of unsolved physics questions about ferroelectric thin films that will be helpful in further engineering developments.