Chemical Influence of Carbon Interface Layers in Metal/Oxide Resistive Switches.

Thin layers introduced between a metal electrode and a solid electrolyte can significantly alter the transport of mass and charge at the interfaces and influence the rate of electrode reactions. C films embedded in functional materials can change the chemical properties of the host, thereby altering the functionality of the whole device. Using X-ray spectroscopies, here we demonstrate that the chemical and electronic structures in a representative redox-based resistive switching (RS) system, Ta2O5/Ta, can be tuned by inserting a graphene or ultrathin amorphous C layer. The results of the orbitalwise analyses of synchrotron Ta L3-edge, C K-edge, and O K-edge X-ray absorption spectroscopy showed that the C layers between Ta2O5 and Ta are significantly oxidized to form COx and, at the same time, oxidize the Ta layers with different degrees of oxidation depending on the distance: full oxidation at the nearest 5 nm Ta and partial oxidation in the next 15 nm Ta. The depth-resolved information on the electronic structure for each layer further revealed a significant modification of the band alignments due to C insertion. Full oxidation of the Ta metal near the C interlayer suggests that the oxygen-vacancy-related valence change memory mechanism for the RS can be suppressed, thereby changing the RS functionalities fundamentally. The knowledge on the origin of C-enhanced surfaces can be applied to other metal/oxide interfaces and used for the advanced design of memristive devices.

Structural, Optical, and Magnetic Properties of Erbium-Substituted Yttrium Iron Garnets.

We synthesized a series of slightly erbium-substituted yttrium iron garnets (Er:YIG), Y3-x Er x Fe5O12 at different Er concentrations (x = 0, 0.01, 0.05, 0.10, and 0.20) using a solid-state reaction and investigated their structural, magnetic, and optical properties as a function of Er concentration. The volume of the unit cell slightly increased with Er concentration and Er atoms predominately replaced Y atoms in the dodecahedrons of YIG. The optical properties exhibited certain decreases in reflectance in the 1500-1600 nm wavelength range due to the presence of Er3+. Despite the many unpaired 4f electrons in Er3+, the total magnetic moments of Er:YIG showed similar trends with temperatures and magnetic fields above 30 K. An X-ray magnetic circular dichroism study confirmed the robust Fe 3d magnetic moments. However, the magnetic moments suddenly decreased to below 30 K with Er substitution, and the residual magnetism (M R) and coercive field (H C) in the magnetic hysteresis loops decreased to below 30 K with Er substitution. This implies that Er substitution in YIG has a negligible effect on magnetic properties over a wide temperature range except below 30 K where the Er 4f spins are coupled antiparallel to the majority Fe 3d spins. Our studies demonstrated that above 30 K the magnetic properties of YIG are retained even with Er substitution, which is evidence that the Er doping scheme is applicable for YIG-based magneto-optical devices in the mid-infrared regime.

Atomic and Electronic Manipulation of Robust Ferroelectric Polymorphs.

Polymorphism allows the symmetry of the lattice and spatial charge distributions of atomically thin materials to be designed. While various polymorphs for superconducting, magnetic, and topological states have been extensively studied, polymorphic control is a challenge for robust ferroelectricity in atomically thin geometries. Here, the atomic and electric manipulation of ferroelectric polymorphs in Mo1- x Wx Te2 is reported. Atomic manipulation for polymorphic control via chemical pressure (substituting tungsten for molybdenum atoms) and charge density modulation can realize tunable polar lattice structures and robust ferroelectricity up to T = 400 K with a constant coercive field in an atomically thin material. Owing to the effective inversion symmetry breaking, the ferroelectric switching withstands a charge carrier density of up to 1.1 × 1013 cm-2 , developing an original diagram for ferroelectric switching in atomically thin materials.

Octahedral Symmetry Modification Induced Orbital Occupancy Variation in VO2.

Octahedral symmetry is one of the parameters to tune the functional properties of complex oxides. VO2, a complex oxide with a 3d1 electronic system, exhibits an insulator-metal transition (IMT) near room temperature (∼68 °C), accompanying a change in the octahedral structure from asymmetrical to symmetrical. However, the role of octahedral symmetry in VO2 on the IMT characteristics is unclear. Crystal and electronic structure analyses combined with density-functional-theory calculations showed the bandwidth-controlled IMT characteristics of monoclinic VO2 with high octahedral symmetry. The expanded apical V-O length for a high octahedral symmetry of a VO2 film increased the bandwidth of the conduction band by depressing V 3d-O 2p hybridization. As a result, the interdimer hopping energy increased and thereby decreased the IMT temperature, although the short V-V chain enhanced electron correlation. These findings suggest that octahedral symmetry can control the IMT characteristics of VO2 by changing the orbital occupancy.

Direct Evidence of Electronic Interaction at the Atomic-Layer-Deposited MoS2 Monolayer/SiO2 Interface.

The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS2 and adjacent SiO2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS2-SiO2 interface.

Propagation Control of Octahedral Tilt in SrRuO3 via Artificial Heterostructuring.

Bonding geometry engineering of metal-oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin-film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO3/SrTiO3 (SRO/STO) superlattices. In particular, the propagation of RuO6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

Phase identification of vanadium oxide thin films prepared by atomic layer deposition using X-ray absorption spectroscopy.

The chemical and local structures of vanadium oxide (VO x ) thin films prepared by atomic layer deposition (ALD) were investigated by soft X-ray absorption spectroscopy. It is shown that the as-deposited film was a mixture of VO2 and V2O5 in disordered form, while the chemistry changed significantly after heat treatment, subject to the different gas environment. Forming gas (95% N2 + 5% H2) annealing resulted in a VO2 composition, consisting mostly of the VO2 (B) phase with small amount of the VO2 (M) phase, whereas O2 annealing resulted in the V2O5 phase. An X-ray circular magnetic dichroism study further revealed the absence of ferromagnetic ordering, confirming the absence of oxygen vacancies despite the reduction of V ions in VO2 (V4+) with respect to the precursor used in the ALD (V5+). This implies that the prevalence of VO2 in the ALD films cannot be attributed to a simple oxygen-deficiency-related reduction scheme but should be explained by the metastability of the local VO2 structures.

X-ray spectroscopy study on the electronic structure of Sn-added p-type SnO films.

The electronic structure of the Sn-added p-type SnO thin film was examined using x-ray absorption spectroscopy (XAS). Sn was intentionally added to a pristine SnO film, and the film was annealed to form p-type SnO. Sn L1- and L3-edge XAS was used to examine the oxidation states of the Sn-added p-type SnO. Compared to the case of the reference SnO, the spectrum of the Sn-added SnO (after annealing) partly contained the lineshape for SnO2, suggesting that the oxidation of Sn + SnO was progressed such that the film became preferably SnO2 + SnO rather than Sn + SnO2. O K-edge XAS, x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE) were also used to scrutinize the electronic structure. The direct bandgap of the annealed film was estimated to be ~3.6 eV, consistent with the reported SnO2 bandgap, while that of the as-deposited Sn-added SnO was <2.5 eV. The large bandgap after annealing suggests that the metallic Sn was no longer in existence and manifested the functionality of the annealed Sn + SnO as a p-type semiconductor.

Precision Interface Engineering of an Atomic Layer in Bulk Bi2Te3 Alloys for High Thermoelectric Performance.

Grafting nanotechnology on thermoelectric materials leads to significant advances in their performance. Creation of structural defects including nano-inclusion and interfaces via nanostructuring achieves higher thermoelectric efficiencies. However, it is still challenging to optimize the nanostructure via conventional fabrication techniques. The thermal instability of nanostructures remains an issue in the reproducibility of fabrication processes and long-term stability during operation. This work presents a versatile strategy to create numerous interfaces in a thermoelectric material via an atomic-layer deposition (ALD) technique. An extremely thin ZnO layer was conformally formed via ALD over the Bi0.4Sb1.6Te3 powders, and numerous heterogeneous interfaces were generated from the formation of Bi0.4Sb1.6Te3-ZnO core-shell structures even after high-temperature sintering. The incorporation of ALD-grown ZnO into the Bi0.4Sb1.6Te3 matrix blocks phonon propagation and also provides tunability in electronic carrier density via impurity doping at the heterogeneous grain boundaries. The exquisite control in the ALD cycles provides a high thermoelectric performance of zT = 1.50 ± 0.15 (at 329-360 K). Specifically, ALD is an industry compatible technique that allows uniform and conformal coating over large quantities of powders. The study is promising in terms of the mass production of nanostructured thermoelectric materials with considerable improvements in performance via an industry compatible and reproducible route.

Ferroelectric Polarization Rotation in Order-Disorder-Type LiNbO3 Thin Films.

The direction of ferroelectric polarization is prescribed by the symmetry of the crystal structure. Therefore, rotation of the polarization direction is largely limited, despite the opportunity it offers in understanding important dielectric phenomena such as piezoelectric response near the morphotropic phase boundaries and practical applications such as ferroelectric memory. In this study, we report the observation of continuous rotation of ferroelectric polarization in order-disorder-type LiNbO3 thin films. The spontaneous polarization could be tilted from an out-of-plane to an in-plane direction in the thin film by controlling the Li vacancy concentration within the hexagonal lattice framework. Partial inclusion of monoclinic-like phase is attributed to the breaking of macroscopic inversion symmetry along different directions and the emergence of ferroelectric polarization along the in-plane direction.

Field-Effect Device Using Quasi-Two-Dimensional Electron Gas in Mass-Producible Atomic-Layer-Deposited Al2O3/TiO2 Ultrathin (<10 nm) Film Heterostructures.

We report the field-effect transistors using quasi-two-dimensional electron gas generated at an ultrathin (∼10 nm) Al2O3/TiO2 heterostructure interface grown via atomic layer deposition (ALD) on a SiO2/Si substrate without using a single crystal substrate. The 2DEG at the Al2O3/TiO2 interface originates from oxygen vacancies generated at the surface of the TiO2 bottom layer during ALD of the Al2O3 overlayer. High-density electrons (∼1014 cm-2) are confined within a ∼2.2 nm distance from the Al2O3/TiO2 interface, resulting in a high on-current of ∼12 µA/µm. The ultrathin TiO2 bottom layer is easy to fully deplete, allowing an extremely low off-current, a high on/off current ratio over 108, and a low subthreshold swing of ∼100 mV/decade. Via the implementation of ALD, a mature thin-film process can facilitate mass production as well as three-dimensional integration of the devices.

In2Ga2ZnO7 oxide semiconductor based charge trap device for NAND flash memory.

The programming characteristics of charge trap flash memory device adopting amorphous In2Ga2ZnO7 (a-IGZO) oxide semiconductors as channel layer were evaluated. Metal-organic chemical vapor deposition (MOCVD) and RF-sputtering processes were used to grow a 45 nm thick a-IGZO layer on a 20 nm thick SiO2 (blocking oxide)/p++-Si (control gate) substrate, where 3 nm thick atomic layer deposited Al2O3 (tunneling oxide) and 5 nm thick low-pressure CVD Si3N4 (charge trap) layers were intervened between the a-IGZO and substrate. Despite the identical stoichiometry and other physicochemical properties of the MOCVD and sputtered a-IGZO, a much faster programming speed of MOCVD a-IGZO was observed. A comparable amount of oxygen vacancies was found in both MOCVD and sputtered a-IGZO, confirmed by x-ray photoelectron spectroscopy and bias-illumination-instability test measurements. Ultraviolet photoelectron spectroscopy analysis revealed a higher Fermi level (E F) of the MOCVD a-IGZO (∼0.3 eV) film than that of the sputtered a-IGZO, which could be ascribed to the higher hydrogen concentration in the MOCVD a-IGZO film. Since the programming in a flash memory device is governed by the tunneling of electrons from the channel to charge trapping layer, the faster programming performance could be the result of a higher E F of MOCVD a-IGZO.

Composition, Microstructure, and Electrical Performance of Sputtered SnO Thin Films for p-Type Oxide Semiconductor.

p-Type SnO thin films were deposited on a Si substrate by a cosputtering process using ceramic SnO and metal Sn targets at room temperature without adding oxygen. By varying the dc sputtering power applied to the Sn target while maintaining a constant radio frequency power to the SnO target, the Sn/O ratio varied from 56:44 to 74:26 at the as-deposited state. After thermal annealing at 180 °C for 25 min under air atmosphere using a microwave annealing system, the films were crystallized into tetragonal SnO when the Sn/O ratio increased from 44:56 to 57:43. Notably, the metallic Sn remained when the Sn/O ratio was higher than 55:45 at an annealed state. When the ratio was lower than 55:45 at the annealed state, the incorporated Sn fully oxidized to SnO, making the films useful p-type semiconductors, whereas the films became metallic conductors at higher Sn/O ratios. At the Sn/O ratio of 55:45 at the annealed state, the film showed the highest Hall mobility of 8.8 cm2 V-1 s-1 and a hole concentration of 5.4 × 1018 cm-3. Interestingly, the electrical conduction behavior showed trap-mediated hopping when the Sn metal was cosputtered, whereas the single SnO film showed regular band conduction behavior. The residual stress effect could interpret such property variation originated from the sputtering power and postoxidation-induced volumetric effects. This report makes a critical contribution to the in-depth understanding of the composition-structure-property relationship of this technically important thin film material.

Crystal and Magnetic Structures of La2CoPtO6 Double Perovskite.

We investigated the crystal structure and magnetic structure in cobaltite-platinate double perovskite of La2CoPtO6, employing various techniques of X-ray diffraction, neutron diffraction, and the extended X-ray absorption fine structure analysis. It is shown that the crystal symmetry is maintained as P21/n in the temperature range of <500 °C, whereas the lattice constants and the Co-Co distances undergo a continuous structural evolution toward the high-symmetry phases with increasing temperature. The Co-O bonds were overall longer and had a larger degree of structural and thermal disorders than the Pt-O bonds. As for the magnetism, an antiferromagnetic order is stabilized in the Co2+ sublattice at temperatures below 28 K. It is demonstrated that owing to the substantial distortions (quantified by a distortion parameter Σ > 0.03 Å) of the Co-Co networks, the system is not subject to spin frustration effect. Details in the magnetic structure are determined; at 12 K, the Co magnetic moment was (0.8, 0, 2.7) Bohr magneton, the magnetic propagation vector was (-0.5, 0, 0.5), and the magnetic symmetry was preferably Γ1(Ag).

Structural Analyses of Phase Stability in Amorphous and Partially Crystallized Ge-Rich GeTe Films Prepared by Atomic Layer Deposition.

The local bonding structures of GexTe1-x (x = 0.5, 0.6, and 0.7) films prepared through atomic layer deposition (ALD) with Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Te precursors were investigated using Ge K-edge X-ray absorption spectroscopy (XAS). The results of the X-ray absorption fine structure analyses show that for all of the compositions, the as-grown films were amorphous with a tetrahedral Ge coordination of a mixture of Ge-Te and Ge-Ge bonds but without any signature of Ge-GeTe decomposition. The compositional evolution in the valence band electronic structures probed through X-ray photoelectron spectroscopy suggests a substantial chemical influence of additional Ge on the nonstoichiometric GeTe. This implies that the ALD process can stabilize Ge-abundant bonding networks like -Te-Ge-Ge-Te- in amorphous GeTe. Meanwhile, the XAS results on the Ge-rich films that had undergone post-deposition annealing at 350 °C show that the parts of the crystalline Ge-rich GeTe became separated into Ge crystallites and rhombohedral GeTe in accordance with the bulk phase diagram, whereas the disordered GeTe domains still remained, consistent with the observations of transmission electron microscopy and Raman spectroscopy. Therefore, amorphousness in GeTe may be essential for the nonsegregated Ge-rich phases and the low growth temperature of the ALD enables the achievement of the structurally metastable phases.

Electrochemical Tantalum Oxide for Resistive Switching Memories.

Redox-based resistive switching memories (ReRAMs) are strongest candidates for the next-generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta2 O5 -based devices by anodizing. This method allows to grow high-quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical-oxide-based devices demonstrate superior properties, i.e., endurance of at least 106 pulse cycles and/or 103 I-V sweeps maintaining a good memory window with a low dispersion in ROFF and RON values, nanosecond fast switching, and data retention of at least 104 s. Multilevel programing capability is presented with both I-V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.

Formation of ferromagnetic Co-H-Co complex and spin-polarized conduction band in Co-doped ZnO.

Magnetic oxide semiconductors with wide band gaps have promising spintronic applications, especially in the case of magneto-optic devices. Co-doped ZnO (ZnCoO) has been considered for these applications, but the origin of its ferromagnetism has been controversial for several decades and no substantial progress for a practical application has been made to date. In this paper, we present direct evidence of hydrogen-mediated ferromagnetism and spin polarization in the conduction band of ZnCoO. Electron density mapping reveals the formation of Co-H-Co, in agreement with theoretical predictions. Electron spin resonance measurement elucidates the ferromagnetic nature of ZnCoO by the formation of Co-H-Co. We provide evidence from magnetic circular dichroism measurements supporting the hypothesis that Co-H-Co contributes to the spin polarization of the conduction band of hydrogen-doped ZnCoO.

Interfacial Metal-Oxide Interactions in Resistive Switching Memories.

Metal oxides are commonly used as electrolytes for redox-based resistive switching memories. In most cases, non-noble metals are directly deposited as ohmic electrodes. We demonstrate that irrespective of bulk thermodynamics predictions an intermediate oxide film a few nanometers in thickness is always formed at the metal/insulator interface, and this layer significantly contributes to the development of reliable switching characteristics. We have tested metal electrodes and metal oxides mostly used for memristive devices, that is, Ta, Hf, and Ti and Ta2O5, HfO2, and SiO2. Intermediate oxide layers are always formed at the interfaces, whereas only the rate of the electrode oxidation depends on the oxygen affinity of the metal and the chemical stability of the oxide matrix. Device failure is associated with complete transition of short-range order to a more disordered main matrix structure.

Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(NtBu)(NEt2)3, Ta(NtBu)(NEt2)2Cp, and H2O.

The growth characteristics of Ta2O5 thin films by atomic layer deposition (ALD) were examined using Ta(NtBu)(NEt2)3 (TBTDET) and Ta(NtBu)(NEt2)2Cp (TBDETCp) as Ta-precursors, where tBu, Et, and Cp represent tert-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta2O5 films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta2O5 films were 0.77 Å cycle-1 at 250 °C and 0.67 Å cycle-1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt2) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta-Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta2O5 film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta2O5 film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta2O5 film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta2O5 films was 2.1-2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases.

Effect of Zinc Incorporation on the Performance of Red Light Emitting InP Core Nanocrystals.

This report presents a systematic study on the effect of zinc (Zn) carboxylate precursor on the structural and optical properties of red light emitting InP nanocrystals (NCs). NC cores were assessed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), energy-dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HRTEM). When moderate Zn:In ratios in the reaction pot were used, the incorporation of Zn in InP was insufficient to change the crystal structure or band gap of the NCs, but photoluminescence quantum yield (PLQY) increased dramatically compared with pure InP NCs. Zn was found to incorporate mostly in the phosphate layer on the NCs. PL, PLQY, and time-resolved PL (TRPL) show that Zn carboxylates added to the precursors during NC cores facilitate the synthesis of high-quality InP NCs by suppressing nonradiative and sub-band-gap recombination, and the effect is visible also after a ZnS shell is grown on the cores.