Interfacial Disorder Research Articles

Impact of Interfacial Disorder and Band Structure on the Resonant Conductance Oscillation in Quantum-Well-Based Magnetic Tunnel Junctions

Impact of Interfacial Disorder and Band Structure on the Resonant Conductance Oscillation in Quantum-Well-Based Magnetic Tunnel Junctions

Reducing Voltage Losses in Organic Photovoltaics Requires Interfacial Disorder Management

AbstractThanks to the introduction of non‐fullerene acceptors, efficiencies of organic photovoltaics are now approaching 20%. Closing the gap with inorganic photovoltaics requires minimizing voltage losses without penalizing charge extraction, for which microstructure control is crucial. However, the complex interplay between microstructure and charge generation, recombination, and extraction has so far not been unraveled. Here, a systematic study linking device performance to distinct microstructural features via machine learning is presented. Building bi‐layer devices allows to separately study the influence of aggregation and disorder on the energies and lifetimes of bulk and interfacial states. Unambiguous assignments of specific structural motifs to the device photophysics are thus possible. It is found that the control of aggregation‐caused quenching is decisive for the exciton splitting efficiency and thus the carrier generation. Furthermore, the static disorder at the donor–acceptor interface controls the nonradiative recombination by shifting the excited state population from the bulk toward the interface. Finally, the amount of disorder in the bulk is found decisive for charge extraction. The finding that charge generation, recombination, and extraction are controlled by distinct structural features, is the key to optimizing these motifs independently, which will pave the way for organic photovoltaics toward the detailed balance limit.

Effect of the alloyed interlayer on the thermal conductance of Al/GaN interface

Understanding the interfacial phonon transport is essential for optimizing the thermal management of microelectronics, especially for high energy density devices. Some calculations have suggested that introducing interfacial defects or disorders will increase the interfacial thermal conductance, which helps heat dissipation, while some studies suggested otherwise. In this work, we introduced substitutional impurities in GaN by growing a ∼2-nm-thick AlxGa1−xN or InxGa1−xN alloyed interlayer at the Al/GaN interface and measured the interface thermal conductance by time-domain thermoreflectance at room temperature. Our results show that substituting Ga atoms near the interface with either lighter Al atoms or heavier In atoms at a nominal concentration of 20% or less will not necessarily change the thermal conductance of the Al/alloy interface but is detrimental to the thermal transport across the total Al/GaN interface, which provides an experimental guideline for the thermal design of GaN-based devices.

Electrochemistry of Redox Active Ferrocene Covalently Attached to Glassy Carbon Electrodes

Formation of well-ordered redox active monolayers is desirable for systematic study of electron transfer for sensor applications and generation of tailored hybrid materials. We have covalently attached 6-bromohexylferrocene (Fc(CH2)6Br) to glassy carbon electrodes through a Grignard reaction and used cyclic voltammetry to characterize interfacial electron transfer and probe disorder in these modified electrodes. X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry confirmed attachment of the expected Fe(II) species and reversible electron transfer is observed. However, the full width at half max of the cyclic voltammogram features deviates from the ideal value of 90.6 mV and varies with surface coverage, indicating a disordered surface. We determine this disorder arises from phase segregation where some domains have a high loading of the hexylferrocene chains while others have a lower loading of hexylferrocene resulting in different local environments that are distinguishable by cyclic voltammetry.

Interfacial Disordering and Heterojunction Enabling Fast Proton Conduction.

The interfacial disorder is a general method to change the metal-oxygen compatibility and carrier density of heterostructure materials for ionic transport modulation. Herein, to enable high proton conduction, a semiconductor heterostructure based on spinel ZnFe2 O4 (ZFO) and fluorite CeO2 is developed and investigated in terms of structural characterization, first principle calculation, and electrochemical performance. Particular attention is paid to the interfacial disordering and heterojunction effects of the material. Results show that the heterostructure induces a disordered oxygen region at the hetero-interface of ZFO-CeO2 by dislocating oxygen atoms, leading to fast proton transport. As a result, the ZFO-CeO2 exhibits a high proton conductivity of 0.21 Scm-1 and promising fuel cell power output of 1070mWcm-2 at 510 °C. Based upon these findings, a new mechanism is proposed by focusing on the change of O-O bond length to interpret the diffusion and acceleration of protons in ZFO-CeO2 on the basis of the Grotthuss mechanism. This study provides a new strategy to customize semiconductor heterostructure to enable fast proton conduction.

Capping-layer-mediated lattice mismatch and redox reaction in SrTiO3-based bilayers

It is well known that the traditional two-dimensional electron system (2DES) hosted by the SrTiO3 substrate can exhibit diverse electronic states by modifying the capping layer in heterostructures. However, such capping layer engineering is less studied in the SrTiO3-layer-carried 2DES (or bilayer 2DES), which is different from the traditional one on transport properties but more applicable to the thin-film devices. Here, several SrTiO3 bilayers are fabricated by growing various crystalline and amorphous oxide capping layers on the epitaxial SrTiO3 layers. For the crystalline bilayer 2DES, the monotonical reduction on the interfacial conductance, as well as carrier mobility, is recorded on increasing the lattice mismatch between the capping layers and epitaxial SrTiO3 layer. The mobility edge raised by the interfacial disorders is highlighted in the crystalline bilayer 2DES. On the other hand, when increasing the concentration of Al with high oxygen affinity in the capping layer, the amorphous bilayer 2DES becomes more conductive accompanied by the enhanced carrier mobility but almost constant carrier density. This observation cannot be explained by the simple redox-reaction model, and the interfacial charge screening and band bending need to be considered. Moreover, when the capping oxide layers have the same chemical composition but with different forms, the crystalline 2DES with a large lattice mismatch is more insulating than its amorphous counterpart, and vice versa. Our results shed some light on understanding the different dominant role in forming the bilayer 2DES using crystalline and amorphous oxide capping layer, which may be applicable in designing other functional oxide interfaces.

Achieving Ultrahigh Electron Mobility in PdSe2 Field‐Effect Transistors via Semimetal Antimony as Contacts

AbstractEven though atomically thin 2D semiconductors have shown great potential for next‐generation electronics, the low carrier mobility caused by poor metal–semiconductor contacts and the inherently high density of impurity scatterings remains a critical issue. Herein, high‐mobility field‐effect transistors (FETs) by introducing few‐layer PdSe2 flakes as channels is achieved, via directly depositing semimetal antimony (Sb) as drain–source electrodes. The formation of clean and defect‐free van der Waals (vdW) stackings at the Sb–PdSe2 heterointerfaces boosts the room temperature transport characteristics, including low contact resistance down to 0.55 kΩ µm, high on‐current density reaching 96 µA µm−1, and high electron mobility of 383 cm2 V−1 s−1. Furthermore, metal–insulator transition (MIT) is observed in the PdSe2 FETs with and without hexagonal boron nitride (h–BN) as buffer layers. However, the layered h–BN/PdSe2 vdW stacking eliminates the interference of interfacial disorders, and thus the corresponding device exhibits a lower MIT crossing point, larger mobility exponent of γ ∼ 1.73, significantly decreased hopping parameter of T0, and ultrahigh electron mobility of 2,184 cm2 V−1 s−1 at 10 K. These findings are expected to be significant for developing high mobility 2D‐based quantum devices.

Size- and temperature-dependent thermal transport across a Cu−diamond interface: Non-equilibrium molecular dynamics simulations

Cu−diamond composites (CDCs) have greatly promising applications in thermal management for high-power electronics because of their outstanding thermophysical properties. Nonetheless, many fundamental mechanisms of interfacial thermal transport for CDCs remain poorly understood at present. Here we focus on investigating the size- and temperature-dependent thermal transport across a Cu−diamond interface using non-equilibrium molecular dynamics simulations. Results show that interfacial thermal conductance (ITC) is proportional to both the system size and ambient temperature. Especially, our predicted room-temperature ITC of 41.12 MW⋅m−2⋅K−1 at an infinitely long system is close to that of the experiments after an interface optimization. Additionally, the ITC at the system with a length of 323.2 Å is increased by over 2.5 times in the temperature range of 100−500 K, up to 36.39 MW⋅ m−2⋅ K−1. Detailed analyses of interfacial disorder and its concomitant effects, related to system size and temperature, are implemented for helping understand the significant improvement of ITC. The underlying mechanism is further uncovered by the phonon density of states as well as the spectral overlap factor at interfacial Cu and diamond. This study provides an important insight into the understanding of interfacial thermal transport in CDCs and a guideline for optimizing the design of CDCs in experiments.

Néel proximity effect at antiferromagnet/superconductor interfaces

Spin-splitting induced in a conventional superconductor weakens superconductivity by destroying spin-singlet and creating spin-triplet Cooper pairs. We demonstrate theoretically that such an effect is also caused by an adjacent compensated antiferromagnet, which yields no net spin-splitting. We find that the antiferromagnet produces N\'eel triplet Cooper pairs, whose pairing amplitude oscillates rapidly in space similar to the antiferromagnet's spin. The emergence of these unconventional Cooper pairs reduces the singlet pairs' amplitude, thereby lowering the superconducting critical temperature. We develop a quasiclassical Green's functions description of the system employing a two-sublattice framework. It successfully captures the rapid oscillations in the Cooper pairs' amplitude at the lattice spacing scale as well as their smooth variation on the larger coherence length scale. Employing the theoretical framework thus developed, we investigate this N\'eel proximity effect in a superconductor/antiferromagnet bilayer as a function of interfacial exchange, disorder, and chemical potential, finding rich physics. Our findings also offer insights into experiments which have found a larger than expected suppression of superconductivity by an adjacent antiferromagnet.

Mechanisms of Andreev reflection in quantum Hall graphene

We simulate a hybrid superconductor-graphene device in the quantum Hall regime to identify the origin of downstream resistance oscillations in a recent experiment [Zhao et al., Nature Physics 16, (2020)]. In addition to the previously studied Mach-Zehnder interference between the valley-polarized edge states, we consider disorder-induced scattering, and the appearance of the counter-propagating states generated by the interface density mismatch. Comparing our results with the experiment, we conclude that the observed oscillations are induced by the interfacial disorder, and that lattice-matched superconductors are necessary to observe the alternative ballistic effects.

Properties for Thermally Conductive Interfaces with Wide Band Gap Materials.

The goal of this study is to determine how bulk vibrational properties and interfacial structure affect thermal transport at interfaces in wide band gap semiconductor systems. Time-domain thermoreflectance measurements of thermal conductance G are reported for interfaces between nitride metals and group IV (diamond, SiC, Si, and Ge) and group III–V (AlN, GaN, and cubic BN) materials. Group IV and group III–V semiconductors have systematic differences in vibrational properties. Similarly, HfN and TiN are also vibrationally distinct from each other. Therefore, comparing G of interfaces formed from these materials provides a systematic test of how vibrational similarity between two materials affects interfacial transport. For HfN interfaces, we observe conductances between 140 and 300 MW m–2 K–1, whereas conductances between 200 and 800 MW m–2 K–1 are observed for TiN interfaces. TiN forms exceptionally conductive interfaces with GaN, AlN, and diamond, that is, G > 400 MW m–2 K–1. Surprisingly, interfaces formed between vibrationally similar and dissimilar materials are similarly conductive. Thus, vibrational similarity between two materials is not a necessary requirement for high G. Instead, the time-domain thermoreflectance experiment (TDTR) data, an analysis of bulk vibrational properties, and transmission electron microscopy (TEM) suggest that G depends on two other material properties, namely, the bulk phonon properties of the vibrationally softer of the two materials and the interfacial structure. To determine how G depends on interfacial structure, TDTR and TEM measurements were conducted on a series of TiN/AlN samples prepared in different ways. Interfacial disorder at a TiN/AlN interface adds a thermal resistance equivalent to ∼1 nm of amorphous material. Our findings improve fundamental understanding of what material properties are most important for thermally conductive interfaces. They also provide benchmarks for the thermal conductance of interfaces with wide band gap semiconductors.

Magnetic nanoprecipitates and interfacial spin disorder in zero-field-annealed Ni50Mn45In5 Heusler alloys as seen by magnetic small-angle neutron scattering.

Shell ferromagnetism is a new functional property of certain off-stoichiometric Ni-Mn-In Heusler alloys, with a potential application in non-volatile magnetic memories and recording media. One key challenge in this field remains the determination of the structural and magnetic properties of the nanoprecipitates that are the result of an annealing-induced segregation process. Thanks to its unique mesoscopic length scale sensitivity, magnetic small-angle neutron scattering appears to be a powerful technique to disclose the microstructure of such annealing-induced nanoprecipitates. In this study, the microstructure of a zero-field-annealed off-stoichiometric Ni50Mn45In5 Heusler alloy is investigated by unpolarized magnetic small-angle neutron scattering. The neutron data analysis reveals a significant spin-misalignment scattering, which is mainly related to the formation of annealing-induced ferromagnetic nanoprecipitates in an antiferromagnetic matrix. These particles represent a source of perturbation which, due to dipolar stray fields, gives rise to canted spin moments in the surroundings of the particle-matrix interface. The presence of anticorrelations in the computed magnetic correlation function reflects the spatial perturbation of the magnetization vector around the nanoprecipitates. The magnetic field dependence of the zero crossing and the minima of the magnetic correlation function are qualitatively explained using the law of approach to ferromagnetic saturation for inhomogeneous spin states. More specifically, at remanence, the nanoprecipitates act magnetically as one superdefect with a correlation length that lies outside the experimental q range, whereas near saturation the magnetization distribution follows each individual nanoprecipitate. Analysis of the neutron data yields an estimated size of 30 nm for the spin-canted region and a value of about 75 nm for the magnetic core of the individual nanoprecipitates.

Annealing temperature dependence of band alignment of NiO/β-Ga2O3

The band alignment of sputtered NiO on β-Ga2O3 was measured by x-ray photoelectron spectroscopy for post-deposition annealing temperatures up to 600 °C. The band alignment is type II, staggered gap in all cases, with the magnitude of the conduction and valence band offsets increasing monotonically with annealing temperature. For the as-deposited heterojunction, ΔE V = −0.9 eV and ΔE C = 0.2 eV, while after 600 °C annealing the corresponding values are ΔE V = −3.0 eV and ΔE C = 2.12 eV. The bandgap of the NiO was reduced from 3.90 eV as-deposited to 3.72 eV after 600 °C annealing, which accounts for most of the absolute change in ΔE V−ΔE C. Differences in thermal budget may be at least partially responsible for the large spread in band offsets reported in the literature for this heterojunction. Other reasons could include interfacial disorder and contamination. Differential charging, which could shift peaks by different amounts and could potentially be a large source of error, was not observed in our samples.

Inducing Dzyaloshinskii–Moriya interaction in symmetrical multilayers using post annealing

The interfacial Dzyaloshinskii-Moriya Interaction (iDMI) is an antisymmetric exchange interaction that is induced by the broken inversion symmetry at the interface of, e.g., a ferromagnet/heavy metal. Thus, the presence of iDMI is not expected in symmetrical multilayer stacks of such structures. Here, we use thermal annealing to induce the iDMI in a [Py/Pt]×10 symmetrical multilayer stack. Brillouin light scattering spectroscopy is used to directly evidence the iDMI induction in the annealed sample. Structural characterizations highlight the modified crystallinity as well as a higher surface roughness of the sample after annealing. First principles electronic structure calculations demonstrate a monotonic increase of the iDMI with the interfacial disorder due to the interdiffusion of atoms, depicting the possible origin of the induced iDMI. The presented method can be used to tune the iDMI strength in symmetric multilayers, which are the integral part of racetrack memories, magnonic devices as well as spin-orbitronic elements.

Ternary metal oxide substrates for superconducting circuits

Substrate material imperfections and surface losses are one of the major factors limiting superconducting quantum circuitry from reaching the scale and complexity required to build a practical quantum computer. One potential path towards higher coherence of superconducting quantum devices is to explore new substrate materials with a reduced density of imperfections due to inherently different surface chemistries. Here, we examine two ternary metal oxide materials, spinel (MgAl2O4) and lanthanum aluminate (LaAlO3), with a focus on surface and interface characterization and preparation. Devices fabricated on LaAlO3 have quality factors three times higher than those of earlier devices, which we attribute to a reduction in the interfacial disorder. MgAl2O4 is a new material in superconducting quantum devices, and even in the presence of significant surface disorder, it consistently outperforms LaAlO3. Our results highlight the importance of materials exploration, substrate preparation, and characterization for identifying materials suitable for high-performance superconducting quantum circuitry.

Dynamics of Interfacial Bubble Controls Adhesion Mechanics in Van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (vdWH) can result in novel functionality that crucially depends on interfacial structure and disorder. Bubbles at the vdWH interface can modify the interfacial structure. We probe the dynamics of a bubble at the interface of a graphene-hBN vdWH by using it as the drumhead of a NEMS device because nanomechanical devices are exquisite sensors. For drums with different interfacial bubbles, we measure the evolution of the resonant frequency and spatial mode shape as a function of electrostatic pulling. We show that the hysteretic detachment of layers of vdWH is triggered by the growth of large bubbles. The bubble growth takes place due to the concentration of stress resembling the initiation of fracture. The small bubbles at the heterostructure interface do not result in delamination as they are smaller than a critical fracture length. We provide insight into frictional dynamics and interfacial fracture of vdWH.

Elucidate interfacial disorder effects on the perpendicular magnetic anisotropy at Fe/MgO heterostructure from first-principles calculations

The interfacial perpendicular magnetic anisotropy (PMA) plays a key role in spintronic applications such as memory recording and computational devices. Despite robust PMA being reported at the Fe/MgO interface, there are still inconsistencies in the disorder effects on the interfacial magnetic anisotropy. Here we reported a comprehensive study of the influence of the interfacial disorder, including the underoxidization, overoxidization, and oxygen migration, on the PMA of the Fe/MgO interface using first-principles calculations. Compared to the pristine Fe/MgO interface, the underoxidation at the Fe/MgO interface keeps the interfacial PMA but reduces the interfacial anisotropy constant (K i ). The overoxidization and oxygen migration at the interface both reduce the K i and even switch the easy magnetization axis from the out-of-plane to in-plane direction at high oxygen percentage. In all the cases, the K i was found strongly correlated to the difference of the orbital magnetic moment along the in-plane and out-of-plane direction. Calculated layer-resolved and orbital-resolved K i revealed that the orbital coupling between the d xy and states of the interfacial Fe layer plays a key role in determining the interfacial magnetic anisotropy. This work provides deep insights into the oxidation effects on the interfacial magnetic anisotropy of Fe/MgO system and a possible avenue to tune the K i via interfacial engineering.

Low-Temperature Microwave Processed TiO2 as an Electron Transport Layer for Enhanced Performance and Atmospheric Stability in Planar Perovskite Solar Cells

Low-temperature (∼150 °C) solution processed planar perovskite solar cells (PSCs) using TiO2 as an electron transport layer (ETL) offer promise for a simple fabrication methodology and compatibility with polymeric substrates and perovskite-based tandem devices. However, the amorphous nature and presence of trap states on the low-temperature TiO2 surface hinder the effective carrier transport. Further, perovskite crystallization on ETL develops lattice strain resulting in the creation of unwanted defect centers. Herein, a low-temperature microwave processed compact TiO2 (MW-TiO2) film is reported that possesses lower surface oxygen vacancy defects and enhanced conductivity and promotes efficient electron extraction owing to the enhanced built-in potential (Vbi) at the MW-TiO2/perovskite interface. The suppressed heterogeneous nucleation of MAPbI3 crystals on the less defective MW-TiO2 surface relieves the interfacial strain, thereby making it a superior template for the growth of strain relaxed, high-quality perovskite films with a more n-type character having larger grains, resulting in suppressed interfacial/surface and bulk trap density. Further, MW-TiO2 mitigates the interfacial energetic disorder and Urbach energy owing to reduced strain, thereby boosting the open-circuit voltage (Voc) by 40 mV, while improved optoelectronic properties of MW-TiO2, lower interfacial charge transfer resistance, and high-quality perovskite films simultaneously improve the short-circuit current density (Jsc) and fill factor (FF) by 6.82 and 9.37%, respectively, over HT-TiO2 based devices. Compared to high-temperature (500 °C) annealed TiO2 based MAPbI3 planar PSCs, MW-TiO2 based devices exhibited a substantial performance enhancement of 22%, leading to the best efficiency of ∼18% and superior atmospheric stability (25 °C, 55% relative humidity) while maintaining 80% of its initial value after 2500 h. Experimental results are validated by device simulation studies with model accounting for trap-assisted interfacial and bulk recombination. Finally, MW processed flexible devices maintained over 80% of their initial power conversion efficiency (PCE) after 1000 bending cycles, thereby exhibiting excellent mechanical robustness. These results elucidate the critical role of MW-TiO2 in rendering improved performance, flexibility, and stability in low-temperature PSCs.

Spin Hall Magnetoresistance Effect from a Disordered Interface.

The spin Hall magnetoresistance (SMR) emerged as a reference tool to investigate the magnetic properties of materials with an all-electrical setup. Its sensitivity to the magnetization of thin films and surfaces may turn it into a valuable technique to characterize van der Waals magnetic materials, which support long-range magnetic order in atomically thin layers. However, realistic surfaces can be affected by defects and disorder, which may result in unexpected artifacts in the SMR, rather than the sole appearance of electrical noise. Here, we study the SMR response of heterostructures combining a platinum (Pt) thin film with the van der Waals antiferromagnet MnPSe3 and observe a robust SMR-like signal, which turns out to originate from the presence of strong interfacial disorder in the system. We use transmission electron microscopy (TEM) to characterize the interface between MnPSe3 and Pt, revealing the formation of a few nanometer-thick platinum-chalcogen amorphous layer. The analysis of the transport and TEM measurements suggests that the signal arises from a disordered magnetic system formed at the Pt/MnPSe3 interface, washing out the interaction between the spins of the Pt electrons and the MnPSe3 magnetic lattice. Our results show that the damaged interfaces can yield an important contribution to SMR, questioning a widespread assumption on the role of disorder in such measurements.

Strengthening the Intermolecular Interaction of Prototypical Semicrystalline Conjugated Polymer Enables Improved Photocurrent Generation at the Heterojunction.

The intra and intermolecular interactions (J- and H-type aggregation) in the conjugated polymer films are found to readily facilitate the electron and hole transport, respectively. However, how those different aggregation types influence the photocurrent generation at the heterojunction is still mysterious, especially for the newly developed semicrystalline conjugated polymers. Here, the prototypical copolymer PM6 is used to tune the relative content of aggregation types with various halogen-free processing solvents. Various measurements reveal that the toluene-processed PM6 film exhibits the increased H-aggregates and crystallinity in the π-π stacking direction compared to its o-xylene- and trimethylbenzene (TMB)-processed counterparts. This is partly resulted from the weak steric effect and good solubility in the PM6 solution prepared with toluene, which strengthens the intermolecular interaction of adjacent polymer segments. After analyzing the photovoltaic properties of PM6/Y6 bilayer devices, the faster charge carrier transport, smaller charge recombination, lower energy losses, and interfacial energetic disorder can be observed in the toluene-processed device, leading to the synergistically improved short-circuit current density (JSC ) and open-circuit voltage (VOC ). These findings indicate the control of the molecular packing structure in terms of aggregation types is a powerful strategy to promote the photocurrent generation process at the conjugated polymer-based heterojunction.