Fifteen years have passed since graphene was first isolated on the substrate from bulk graphite. During that period, 2D layered materials with intrinsic band gaps have been realized. Although many exciting results have been reported for both their fundamental physics and applications, the discussion of 2D electron device application to the future integrated circuit is still based on the expectation of the inherently high properties that 2D materials ideally possess. This review article focuses on the gate stack property, which is one of most important building blocks in the field effect transistor. Starting from the comparison of the 2D/SiO2 interface properties with the conventional SiO2/Si interface properties, recent advances in the studies of gate stack properties for bilayer graphene and MoS2 FETs are discussed. In particular, the advantages and disadvantages of the 2D heterostructures with 2D insulator of h-BN are emphasized. This review may provide conceptual and experimental approaches for controlling the 2D heterointerface properties.
We investigate a two-dimensional (2D) heterostructure consisting of few-layer direct bandgap ReS2, a thin h-BN layer and a monolayer graphene for application to various electronic devices. Metal-insulator-semiconductor (MIS)-type devices with two-dimensional (2D) van-der-Waals (vdW) heterostructures have recently been studied as important components to realize various multifunctional device applications in analogue and digital electronics. The tunnel diodes of ReS2/h-BN/graphene exhibit light tuneable rectifying behaviours with low ideality factors and nearly temperature independent electrical…
Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) devices with a thin, high-k gate dielectric and a 9-atom wide (0.95 nm) armchair GNR as the channel material, we demonstrate FETs with high on-current (Ion >1 uA at Vd = -1 V) and high Ion/Ioff ~10^5 at …
Clockwise to anti-clockwise hysteresis crossover in current-voltage transfer characteristics of field effect transistors (FETs) with graphene and MoS$_2$ channels holds significant promise for non-volatile memory applications. However such crossovers have been shown to manifest only at high temperature. In this work, for the first time, we demonstrate room temperature hysteresis crossover in few-layer MoS$_2$ FETs by using a gate-drain underlap design to induce a differential response from traps at the MoS$_2$-HfO$_2$ channel-gate dielectric interface to applied gate bias. The appearance of in…
Silicon nanowires (Si NW) are ideal candidates for solution processable field effect transistors (FETs). The interface between the nanowire channel and the gate dielectric plays a crucial role in the FET performance, and it can be responsible for unwanted effects such as hysteresis of the I-V characteristics due to threshold voltage shift when the gate voltage is applied. Using gate-voltage bias stress measurements we show that a large hysteresis of up to 40V in Si NW FETs with SiO2 dielectric is mainly due to the holes traps at the nanowire/SiO2 interface. An approach for reducing this hyster…
Heterostructures made by stacking different materials on top of each other are expected to exhibit unusual properties and new phenomena. Interface of the heterostructures plays a vital role in determining their properties. Here, we report the observation of a two-dimensional (2D) spinodal interface in graphene-molybdenum carbide (α-Mo2C) heterostructures, which arises from spinodal decomposition occurring at the heterointerface, by using scanning tunneling microscopy. Our experiment demonstrates that the 2D spinodal interface modulates graphene into whispering gallery resonant networks filled …
Layered two-dimensional (2D) semiconducting transition metal dichalcogenides (TMD) have been widely isolated, synthesized, and characterized recently. Numerous 2D materials are identified as the potential candidates as channel materials for future thin film technology due to their high mobility and the exhibiting bandgaps. While many TMD filed-effect transistors (FETs) have been widely demonstrated along with a significant progress to clearly understand the device physics, large contact resistance at metal/semiconductor interface still remain a challenge. From 2D device research point of view,…
We implemented a nanoelectronic interface between graphene field effect transistors (FETs) and soluble proteins. This enables production of bioelectronic devices that combine functionalities of the biomolecular and inorganic components. The method serves to link polyhistidine-tagged proteins to graphene FETs using the tag itself. Atomic Force Microscopy and Raman spectroscopy provide structural understanding of the bio/nano hybrid; current-gate voltage measurements are used to elucidate the electronic properties. As an example application, we functionalize graphene FETs with fluorescent protei…
The use of a foreign metallic cold source (CS) has recently been proposed as a promising approach toward the steep-slope field-effect-transistor (FET). In addition to the selection of source material with desired density of states-energy relation (D(E)), engineering the source: channel interface for gate-tunable channel-barrier is crucial to a CS-FET. However, conventional metal: semiconductor (MS)-interfaces generally suffer from strong Fermi-level-pinning due to the inevitable chemical disorder and defect-induced gap states, precluding the gate-tunability of the barriers. By comprehensive ma…
We develop a generalized theory for the scattering process produced by interface roughness on charge carriers and which is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape obtained on Ge/GeSi heterointerfaces obtained by atom probe tomography, we have been able to define the full set of interface parameters relevant to the scattering potential, including both the in-plane and axial correlation inside real diffuse interfaces. Our experimental findings indicate a partial coherence of the interface roughness along …
Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMD) interfaced to gallium nitride (GaN) are excellent material systems to realize broadband light emitters and absorbers. The surface properties of the polar semiconductor, such as GaN are dominated by interface phonons, thus the optical properties of the vertical heterostructure depend strongly on the interface exciton-phonon coupling. The origin and activation of different Raman modes in the heterostructure due to coupling between interfacial phonons and optically generated carriers in a monola…
Neutron reflectometry (NR) has emerged as a unique technique for the investigation of structure and magnetism of thin films of both biologically relevant and magnetic materials. The advantage of NR with respect to many other surface-sensitive techniques is its sub-nanometer resolution that enables structural characterizations at the molecular level. While in the case of bio-relevant samples, NR can be used to probe thin films at buried interfaces, non-destructively, even adopting a complex sample environment. Whereas the polarized version of NR is best suited for revealing the interface magnet…
Ferroelectric field-effect transistors (Fe-FETs) with ferroelectric hafnium oxide (FE HfO2) as gate insulator are being extensively explored as a promising device candidate for three-dimensional (3D) NAND memory application. FE HfO2 exhibits long retention over 10 years, high endurance over 1012 cycles, high speed with sub-ns polarization switching, and high remnant polarization of 10-30 μC/cm2. However, the performance of Fe-FETs is known to be much worse than FE HfO2 capacitors, which is not completely understood. In this work, we developed a comprehensive Fe-FET model based on a charge bala…
Vertically stacked van der Waals heterostructures are a lucrative platform for exploring the rich electronic and optoelectronic phenomena in two-dimensional materials. Their performance will be strongly affected by impurities and defects at the interfaces. Here we present the first systematic study of interfaces in van der Waals heterostructure using cross sectional scanning transmission electron microscope (STEM) imaging. By measuring interlayer separations and comparing these to density functional theory (DFT) calculations we find that pristine interfaces exist between hBN and MoS2 or WS2 fo…
By stacking various two-dimensional (2D) atomic crystals [1] on top of each other, it is possible to create multilayer heterostructures and devices with designed electronic properties [2-5]. However, various adsorbates become trapped between layers during their assembly, and this not only affects the resulting quality but also prevents the formation of a true artificial layered crystal upheld by van der Waals interaction, creating instead a laminate glued together by contamination. Transmission electron microscopy (TEM) has shown that graphene and boron nitride monolayers, the two best charact…
We have studied the domain wall resistance in W/Ta/CoFeB/MgO heterostructures. The Ta layer thickness is varied to control the type of domain walls via changes in the interfacial Dzyaloshinskii Moriya interaction. We find a nearly constant domain wall resistance against the Ta layer thickness. Adding contributions from the anisotropic magnetoresistance, spin Hall magnetoresistance and anomalous Hall effect describe well the domain wall resistance of the thick Ta layer films. However, a discrepancy remains for the thin Ta layer films wherein chiral Néel-like domain walls are found. These result…
Straight, axial InAs nanowire with multiple segments of GaInAs were grown. High resolution X-ray energy-dispersive spectroscopy (EDS) mapping reveal the distribution of group III atoms at the axial interfaces and at the sidewalls. Significant Ga enrichment, accompanied by a structural change is observed at the GaInAs/InAs interfaces and a higher Ga concentration for the early grown GaInAs segments. The elemental map and EDS line profile infer Ga enrichment at the facet junctions between the sidewalls. The relative chemical potentials of ternary alloys and the thermodynamic driving force for li…
The magnetoelectric effect predicted in topological insulators makes heterostructures that combine magnetic materials and such insulators promising candidates for spintronics applications. Here, we theoretically consider a setup that exhibits two well-separated interfaces between a topological insulator and a ferromagnetic insulator. We show that there is a topological magnetic dipole-dipole interaction stemming from long-range Coulomb interactions. We analytically derive the magnetization dynamics at the two interfaces and discuss how the long-range coupling can be applied to non-locally indu…
We experimentally investigate the origin of the two-magnon scattering (TMS) in heavy-metal (HM)/ferromagnet (FM)/oxide heterostructures (FM = Co, Ni81Fe19, or Fe60Co20B20) by varying the materials located above and below the FM layers. We show that strong TMS in HM/FM/oxide systems arises primarily at the HM/FM interface and increases with the strength of interfacial spin-orbit coupling and magnetic roughness at this interface. TMS at the FM/oxide interface is relatively weak, even in systems where spin-orbit coupling at this interface generates strong interfacial magnetic anisotropy. We also …
First-principles density functional calculations are performed in C-BN heterojunctions. It is shown that the magnetism of the edge states in zigzag shaped graphene strips and polarity effects in BN strips team up to give a spin asymmetric screening that induces half-semimetallicity at the interface, with a gap of at least a few tenths of eV for one spin orientation and a tiny gap of hundredths of eV for the other. The dependence with ribbon widths is discussed, showing that a range of ribbon widths is required to obtain half-semimetallicity. These results open new routes for tuning electronic …
The idea to utilize not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (10$^{12}$ Hz) emission spectroscopy, we demonstrate optical generation of spin-polarized electric currents at the interfaces of metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of th…
Heterostructures involving graphene and bismuth, with their ability to absorb light over a very wide energy range, are of interest for engineering next-generation opto-electronics. Critical to the technological application of such heterostructures is an understanding of the underlying physics governing their properties. Here, using first-principles calculations, we study the interfacial interactions between graphene and bismuth thin-films. Our study reveals non-intuitive phenomena associated with the moiré-physics of these superlattices. We show a preservation of graphene-derived Dirac cones i…
Combined diverse two-dimensional (2D) materials for semiconductor interfaces are attractive for electrically controllable carrier confinement to enable excellent electrostatic control. We investigated the transport characteristic in heterointerface of multilayer molybdenum disulfide and hexagonal boron nitride (MoS$_2$/h-BN) to reveal that the charge transfer switching (CTS) is highly dependent on both the local gate constriction and bias in the channel. Notably, the CTS is shown to be controlled at a molecular level through electrotunable gated constriction. The resulting significant change i…
The origin of the two-dimensional electron gas (2DEG) in the interface between $γ$-Al$_2$O$_3$ (GAO) and SrTiO$_3$ (STO) (GAO/STO) as well as the reason for the high mobility of the 2DEG is still in debate. In this paper, the electronic structures of [001]-oriented GAO/STO heterostructures with and without oxygen vacancies are investigated by first-principle calculations based on the density functional theory. The calculation results show that the necessary condition for the formation of 2DEG is that the GAO/STO heterostructure has the interface composed of Al and TiO$_2$ layers. For the heter…
Two-dimensional (2D) materials are highly promising for tunnel field effect transistors (TFETs) with low subthreshold swing and high drive current because the shorter tunnel distance and strong gate controllability can be expected from the van der Waals gap distance and the atomically sharp heterointerface formed independently of lattice matching. However, the common problem for 2D-2D TFETs is the lack of highly doped 2D materials with the high process stability as the sources. In this study, we have found that p+-WSe2 doped by charge transfer from a WOx surface oxide layer can be stabilized b…
Fabrication of custom-built heterostructures based on stacked 2D materials provides an effective method to controllably tune electronic and optical properties. To that end, optimizing fabrication techniques for building these heterostructures is imperative. A common challenge in layer-by-layer assembly of 2D materials is the formation of bubbles at the atomically thin interfaces. We propose a technique for addressing this issue by removing the bubbles formed at the heterostructure interface in a custom-defined area using the heat generated by a laser, equipped with raster scanning capabilities…
The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. Herein we show that metallic interfaces can be realized in SrTiO3-based heterostructures with various insulating overlayers of amorphous LaAlO3, SrTiO3 and yttria-stabilized zirconia films. On the other hand, samples of amorphous La7/8Sr1/8MnO3 films on SrTiO3 substrates remain insulating. The interfacial conductivity results from the formation of oxygen vacancies near the interface, suggesting that the redox reactions on the surface of …
Two-dimensional (2D) layered materials are promising for replacing Si to overcome the scaling limit of recent ~5 nm-length metal-oxide-semiconductor field-effect transistors (MOSFETs). However, the insulator/2D channel interface severely degrades the performance of 2D-based MOSFETs, and the origin of the degradation remains largely unexplored. Here, we present the full energy spectra of the interface state densities (Dit) for both n- and p- MoS2 FETs, based on the comprehensive and systematic studies, i.e., thickness range from monolayer to bulk and various gate stack structures including 2D h…
Controlling the strain level in nanowire heterostructures is critical for obtaining coherent interfaces of high crystalline quality and for the setting of functional properties such as photon emission, carrier mobility or piezoelectricity. In a nanowire axial heterostructure featuring a sharp interface, strain is set by the materials lattice mismatch and the nanowire radius. Here, we show that introducing a graded interface in nanowire heterostructures offers an additional parameter to control strain. For a given interface length and lattice mismatch, we first derive theoretically the maximum …
The problem for interface solutions in the quantum theory of heterostructures comprising narrow-gap semiconductors is reformulated in the language of commutative diagrams. By this way the theory of interface states in such heterostructures is naturally factorized in the two subproblems: (i) Criterion for the existence of interface states, (ii) Their localization in the common energy gap; the solution of both of them being presented by the requirement for commutativity of the relevant diagrams. It is shown that these two problems are dual in the sense of categorical duality, the passing from th…
We have studied the magnetic layer thickness dependence of the orbital magnetic moment in magnetic heterostructures to identify contributions from interfaces. Three different heterostructures, Ta/CoFeB/MgO, Pt/Co/AlO$_x$ and Pt/Co/Pt, which possess significant interface contribution to the perpendicular magnetic anisotropy, are studied as model systems. X-ray magnetic circular dichroism spectroscopy is used to evaluate the relative orbital moment, i.e. the ratio of the orbital to spin moments, of the magnetic elements constituting the heterostructures. We find that the relative orbital moment …
The structures of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures relating to the interface-induced perpendicular magnetic anisotropy (PMA) were investigated using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray magnetic circular dichroism. We found that Al atoms from the Co2FeAl layer significantly interdiffuse into MgO, forming an Al-deficient Co-Fe-Al/Mg-Al-O structure near the Co2FeAl/MgO interface. This atomic replacement may play an additional role for enhancing PMA, which is consistent with the observed large perpendicular orbital magneti…
We examine the possibility of intrinsic interface states bound to the plane of In-Sb chemical bonds at InAs/AlSb interfaces. Careful parameterization of the bulk materials in the frame of the extended basis spds^* tight-binding model and recent progress in predictions of band offsets severely limit the span of tight-binding parameters describing this system. We find that a heavy-hole like interface state bound to the plane of In-Sb bonds exists for a large range of values of the InSb/InAs band offset.
We apply the envelope function approach to abrupt heterostructures starting with the least action principle for the microscopic wave function. The interface is treated nonperturbatively, and our approach is applicable to mismatched heterostructure. We obtain the interface connection rules for the multiband envelope function and the short-range interface terms which consist of two physically distinct contributions. The first one depends only on the structure of the interface, and the second one is completely determined by the bulk parameters. We discover new structure inversion asymmetry terms …
In this paper, density functional theory calculations are used to explore the electronic and atomic reconstruction at interfaces between III-III/I-V oxides. In particular, at these interfaces, two dimensional electron gases (2DEGs) with twice the interfacial charge densities of the prototypical LaTiO3/SrTiO3 heterostructure are observed. Furthermore, a significant decrease in the band effective masses of the conduction electrons is shown, suggesting that possible enhancements in electron mobilities may be achievable. These findings represent a framework for chemically modulating 2DEGs, thereby…
Depositing disordered Al on top of SrTiO$_3$ is a cheap and easy way to create a two-dimensional electron system in the SrTiO$_3$ surface layers. To facilitate future device applications we passivate the heterostructure by a disordered LaAlO$_3$ capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth.
The standard model of enhanced ionic conductivities in solid electrolyte heterostructures follows from a continuum mean-field description of defect distributions that makes no reference to crystalline structure. To examine ionic transport and defect distributions while explicitly accounting for ion-ion correlations and lattice effects, we have performed molecular dynamics simulations of a model coherent fluorite heterostructure without any extrinsic defects, with a difference in standard chemical potentials of mobile fluoride ions between phases induced by an external potential. Increasing the…
Topological interface states in multivalley systems are studied to unravel their valley sensitivity. For this purpose, multivalley IV-VI topological crystalline insulator (TCI) heterostructures are explored using magneto-optical Landau level spectroscopy up to 34 teslas. We characterize the topological interface states emerging from the distinct L-valleys in Pb1-xSnxSe multi quantum wells grown along the [111] direction. It is shown that the shape of the 2D Fermi surfaces of topological interface states residing at the TCI/trivial insulator interfaces are strongly affected by the valley anisot…
Interface-Induced Superconductivity in Magnetic Topological Insulator-Iron Chalcogenide Heterostructures
The Pennsylvania State University（宾夕法尼亚州立大学）；Rutgers University（罗格斯大学）；University of Washington（华盛顿大学）；Chinese Academy of Sciences（中国科学院）；Materials Research Institute, The Pennsylvania State University（宾夕法尼亚州立大学材料研究所）；National High Magnetic Field Laboratory（国家高磁场实验室）；Paul Scherrer Institute（保罗谢尔研究所）；National Institute of Standards and Technology（美国国家标准与技术研究院）
The experimental observation of quantum anomalous Hall effect (QAHE) in magnetic topological insulators has stimulated enormous interest in condensed-matter physics and materials science. For the purpose of realizing high-temperature QAHE, several material candidates have been proposed, among which the interface states in the CdO/ferromagnetic insulator heterostructures are particularly interesting and favorable for technological applications. Here, we report the experimental observation of the interfacial ferromagnetism and anomalous Hall effect in the Fe3O4/CdO/Fe3O4 heterostructures grown v…
Organic semiconductors based on small conjugated molecules generally behave as insulators when undoped, but the hetero-interfaces of two such materials can show electrical conductivity as large as in a metal. Although charge transfer is commonly invoked to explain the phenomenon, the details of the process and the nature of the interfacial charge carriers remain largely unexplored. Here we use Schottky-gated heterostructures to probe the conducting layer at the interface between rubrene and PDIF-CN2 single crystals. Gate-modulated conductivity measurements demonstrate that interfacial transpor…
Interfaces between heavy metals (HMs) and antiferromagnetic insulators (AFIs) have recently become highly investigated and debated systems in the effort to create spintronic devices able to function at terahertz frequencies. Such heterostructures have great technological potential because AFIs can generate sub-picosecond spin currents which the HMs can convert into charge signals. In this work we demonstrate an optically induced picosecond spin transfer at the interface between AFIs and Pt using time-resolved THz emission spectroscopy. We select two antiferromagnets in the same family of fluor…
LaAlO$_{3}$/SrTiO$_{3}$ heterostructure with polar and non-polar constituents has been shown to exhibit interface metallic conductivity due to fractional charge transfer to the interface. The interface reconstruction by electron redistribution along (001) orientation, in which half of an electron is transferred per two dimensional unit cell to the adjacent planes, resulting in a net transfer of half of the charge, to both the interface and top most atomic planes, has been ascribed to a polar discontinuity at the interface in the Polar Catastrophe model. This avoids the divergence of electrosta…
Bulk photovoltaic (BPVE) effect in crystals lacking inversion symmetry offers great potential for optoelectronic applications due to its unique properties such as above bandgap photovoltage and switchable photocurrent. Because of their large spontaneous polarizations, ferroelectric materials are ideal platforms for studying BPVE. However, identifying the origin of experimentally observed photovoltaic response is often challenging due to the entanglement between bulk and interface effects, leading to much debate in the field. This issue is particularly pronounced in vertical heterostructures, w…
We explore the interface properties of perovskite heterostructure CsPbBr$_3$/CsPbI$_3$ through first-principles calculations. The structural interface is formed by the bonding of Cs-Br and Cs-I with bond length of $\sim$4.106 and 3.922 Å. The upshift of Goldsmith tolerance factor in the range $0.8<t<1$ from $t<0.8$ is revealed for the bi-layer interface, from bulk, reflecting the structural rearrangement from anisotropy to isotropy in confinement. The band gap arises mainly due to the energy difference of I-5p orbital than that of Br-4p at the valence band and Pb-6p at the conduction band. Hea…
Performing an analysis within density functional theory, we develop insight into the structural and electronic properties of the oxide heterostructure LaAlO3/SrTiO3. Electrostatic surface effects are decomposed from the internal lattice distortion in order to clarify their interplay. We first study the interface relaxation by a multi-layer system without surface, and the surface effects, separately, by a substrate-film system. While elongation of the TiO6 octahedra at the interface enhances the metallicity, reduction of the film thickness has the opposite effect due to a growing charge depleti…
A comprehensive study of drain current dispersion effects in $β$-Ga$_2$O$_3$ FETs has been done using DC, pulsed and RF measurements. Both virtual gate effect in the gate-drain access region and interface traps under the gate are most plausible explanations for the experimentally observed pulsed current dispersion and high temperature threshold voltage shift respectively. Unpassivated devices show significant current dispersion between DC and pulsed IV response due to gate lag effect characterized by time constants in the range of 400~$μs$ to 600~$μs$. An activation energy of 99~$meV$ is estim…
