Remote Oxygen Scavenging of the Interfacial Oxide Layer in Ferroelectric Hafnium-Zirconium Oxide-Based Metal-Oxide-Semiconductor Structures.

Discovery of ferroelectricity in HfO2 has sparked a lot of interest in its use in memory and logic due to its CMOS compatibility and scalability. Devices that use ferroelectric HfO2 are being investigated; for example, the ferroelectric field-effect transistor (FEFET) is one of the leading candidates for next generation memory technology, due to its area, energy efficiency and fast operation. In an FEFET, a ferroelectric layer is deposited on Si, with an SiO2 layer of ∼1 nm thickness inevitably forming at the interface. This interfacial layer (IL) increases the gate voltage required to switch the polarization and write into the memory device, thereby increasing the energy required to operate FEFETs, and makes the technology incompatible with logic circuits. In this work, it is shown that a Pt/Ti/thin TiN gate electrode in a ferroelectric Hf0.5Zr0.5O2 based metal-oxide-semiconductor (MOS) structure can remotely scavenge oxygen from the IL, thinning it down to ∼0.5 nm. This IL reduction significantly reduces the ferroelectric polarization switching voltage with a ∼2× concomitant increase in the remnant polarization and a ∼3× increase in the abruptness of polarization switching consistent with density functional theory (DFT) calculations modeling the role of the IL layer in the gate stack electrostatics. The large increase in remnant polarization and abruptness of polarization switching are consistent with the oxygen diffusion in the scavenging process reducing oxygen vacancies in the HZO layer, thereby depinning the polarization of some of the HZO grains.

Local Epitaxial Templating Effects in Ferroelectric and Antiferroelectric ZrO2.

Nanoscale polycrystalline thin-film heterostructures are central to microelectronics, for example, metals used as interconnects and high-K oxides used in dynamic random-access memories (DRAMs). The polycrystalline microstructure and overall functional response therein are often dominated by the underlying substrate or layer, which, however, is poorly understood due to the difficulty of characterizing microstructural correlations at a statistically meaningful scale. Here, an automated, high-throughput method, based on the nanobeam electron diffraction technique, is introduced to investigate orientational relations and correlations between crystallinity of materials in polycrystalline heterostructures over a length scale of microns, containing several hundred individual grains. This technique is employed to perform an atomic-scale investigation of the prevalent near-coincident site epitaxy in nanocrystalline ZrO2 heterostructures, the workhorse system in DRAM technology. The power of this analysis is demonstrated by answering a puzzling question: why does polycrystalline ZrO2 transform dramatically from being antiferroelectric on polycrystalline TiN/Si to ferroelectric on amorphous SiO2/Si?

Tetravalent Doping in Fluorite-Based Ferroelectric Oxides for Reduced Voltage Operations.

First-principles calculations show a reduced energy barrier for polarization switching via a bulk phase transition by doping of hafnium-zirconium oxide (HZO). The tetragonal P42/nmc phase serves as a transition state for polarization switching of the polar orthorhombic Pca21 phase. Due to the high symmetry of the tetragonal phase, dopants can form more energetically favorable local oxygen bonding configurations in the tetragonal phase versus the orthorhombic phase. Significant bond strain is observed in the orthorhombic phase due to the low symmetry of the host crystal structure which decreases the relative stability of the doped orthorhombic phase compared to the doped tetragonal phase, thereby significantly lowering the barrier for switching but slightly affecting the polarization of the orthorhombic phase. Si is a promising dopant for an efficient ferroelectric device with minimal disturbance in the electronic structure parameters. Ge doping is suitable for stabilizing the tetragonal phase which shows a high k value.

Hafnium-zirconium oxide interface models with a semiconductor and metal for ferroelectric devices.

Density functional theory (DFT) is employed to investigate ferroelectric (FE) hafnium-zirconium oxide stack models for both metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) structures. The role of dielectric (DE) interlayers at the ferroelectric interfaces with metals and semiconductors and the effects of thickness scaling of FE and DE layers were investigated using atomic stack models. A high internal field is induced in the FE and DE layers by the FE polarization field which can promote defect generation leading to limited endurance. It is also shown that device operation will be adversely affected by too thick DE interlayers due to high operating voltage. These DFT models elucidate the underlying mechanisms of the lower endurance in experimental MIS devices compared to MIM devices and provide insights into the fundamental mechanisms at the interfaces.

Correction: Hafnium-zirconium oxide interface models with a semiconductor and metal for ferroelectric devices.

[This corrects the article DOI: 10.1039/D1NA00230A.].

Promising photovoltaic efficiency of a layered silicon oxide crystal Si3O.

Computational searching and screening of new functional materials exploiting Earth abundant elements can accelerate the development of their energy applications. Based on the state-of-the-art material search algorithm and ab initio calculations, we demonstrate a recently suggested stable silicon oxide with a layered structure (Si3O) as an ideal photovoltaic material. With many-body first-principles approaches, the monolayer and layered bulk of Si3O show direct quasiparticle gaps of 1.85 eV and 1.25 eV, respectively, while an optical gap of about 1.2 eV is nearly independent of the number of layers. Spectroscopic limited maximum efficiency (SLME) is estimated to be 27% for a thickness of 0.5 µm, making it a promising candidate for solar energy applications.

Symmetry Dictated Grain Boundary State in a Two-Dimensional Topological Insulator.

Grain boundaries (GBs) are ubiquitous in solids and have been of central importance in understanding the nature of polycrystals. In addition to their classical roles, topological insulators (TIs) offer a chance to realize GBs hosting distinct topological states that can be controlled by their crystal symmetries. However, such roles of crystalline symmetry in two-dimensional (2D) TIs have not been definitively measured yet. Here, we present the first direct evidence of a symmetry-enforced metallic state along a GB in 1T'-MoTe2, a prototypical 2D TI. Using scanning tunneling microscopy, we show a metallic state along a GB with nonsymmorphic lattice symmetry and its absence along another boundary with symmorphic symmetry. Our atomistic simulations demonstrate in-gap Weyl semimetallic states for the former, whereas they demonstrate gapped states for the latter, explaining our observation well. The observed metallic state, tightly linked to its crystal symmetry, can be used to create a stable conducting nanowire inside TIs.

A New Family of Two-Dimensional Crystals: Open-Framework T3 X ( T = C, Si, Ge, Sn; X = O, S, Se, Te) Compounds with Tetrahedral Bonding.

To accelerate development of innovative materials, their modelings and predictions with useful functionalities are of vital importance. Here, based on a recently developed crystal structure prediction method, we find a new family of stable two-dimensional crystals with an open-channel tetrahedral bonding network, rendering a potential prototype for electronic and energy applications. The proposed structural prototype with a space group of Cmme hosts at least 13 different freestanding T3 X compounds with group IV ( T = C, Si, Ge, Sn) and VI ( X = O, S, Se, Te) elements. Moreover, the proposed materials display diverse electronic properties ranging from direct band gap semiconductor to topological insulator at their pristine forms, which are further tunable by mechanical strain.

Enhanced Thermoelectric Properties in a New Silicon Crystal Si24 with Intrinsic Nanoscale Porous Structure.

Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high electric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si24 features highly anisotropic crystal structure with nanometer-sized regular pores. Here, based on first-principles study without any empirical parameter we show that the slightly doped Si24 can provide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compared with the cubic diamond phase of silicon. We ascribe the enhancement to the intrinsic nanostructure formed by the nanopore array, which effectively hinders heat conduction while electric conductivity is maintained. This can be a viable option to enhance the thermoelectric figure of merit without further forming an extrinsic nanostructure. In addition, we propose a practical strategy to further diminish the thermal conductivity without affecting electric conductivity by confining rattling guest atoms in the pores.

Computational study of pressure-driven methane transport in hierarchical nanostructured porous carbons.

Using the reflecting particle method together with a perturbation-relaxation loop developed in our previous work, we studied pressure-driven methane transport in hierarchical nanostructured porous carbons (HNPCs) containing both mesopores and micropores in non-equilibrium molecular dynamics simulations. The surface morphology of the mesopore wall was systematically varied by tuning interaction strength between carbon atoms and the template in a mimetic nanocasting process. Effects of temperature and mesopore size on methane transport in HNPCs were also studied. Our study shows that increased mesopore wall surface roughness changes the character of the gas-wall interaction from specular to diffuse, while the gas-gas interaction is diminished due to the decrease of adsorption density. Effects of the mesopore wall surface morphology are the most significant at low temperatures and in small channels. Our systematic study provides a better understanding of the transport mechanisms of light gases through carbon nanotube composite membranes in experiments.

Computational Study of Pressure-Driven Gas Transport in Nanostructured Carbons: An Alternative Approach.

We demonstrated a computationally efficient method in nonequilibrium molecular dynamics (NEMD) simulations to study pressure-driven gas transport in porous media. The reflecting particle method (RPM)14 was used to establish a steady-state gas flow along the transport channel, and the gas density in the feed chamber was properly adjusted to allow a constant pressure drop under various conditions by using a perturbation-relaxation loop developed here. This method was validated for methane flow through carbon nanotubes over a wide range of temperatures, giving results comparable to those of the commonly used dual control volume grand canonical molecular dynamics (DCV-GCMD) method but at least 20 times more efficient, even though the transport condition tested is favorable for the latter. This made it possible to perform systematic studies on the effects of temperature, pressure, and channel size on the transport behaviors. Our study shows that adsorption density varies significantly with temperature, which dramatically influences the transport mechanisms, especially in small channels at low temperatures and under high pressures. This newly developed NEMD method can be readily extended to study gas transport through channels with more complex surface morphology.

Effects of in-plane magnetization orientation on magnetic and electronic properties in a Bcc Co (001)/rock salt MgO (001)/Bcc Co (001) magnetic tunnel junction system: ab initio calculations.

Ab initio calculations were performed on a fully epitaxial bcc Co (001)/rock salt MgO (001)/bcc Co (001) magnetic tunnel junction system for two cases where the magnetization is parallel to bcc Co [100] and to bcc Co [110]. Structural optimization reveals that the two cases are equivalent systems and that the Co electrodes contract in the z-direction whereas the MgO insulating barrier expands. The magnetic moments of each monolayer vary slightly in each case; furthermore, only the magnetic moment at the surface of the Co atom shows any enhancement (12%). The layer decomposed density of states profiles reveals that the bonding character of the junction interface is derived mainly from the 2p-3d hybridization of the MgO and Co interfacial atoms.