Formation Of Interface States Research Articles

Boosting photoelectrochemical performance of CuFeO2/CuO photocathode by modulating heterojunction architecture and oxygen vacancies

This study aims to enhance the PEC properties of CuFeO2 nanosheets by constructing closely bonded CuFeO2/CuO heterostructures. CuO crystal-structured films were prepared using the sol-gel spin-coating method, wherein the annealing temperature was varied to modulate the content of oxygen vacancies. Compared to pristine CuFeO2 and two tandem architectures, the CuFeO2@VO-CuO core-shell architecture outperformed by yielding a photocurrent density of 68 μA/cm2, which marks an almost 8.5, 1.36 and 2.27 times enhancement. Experimental results verified solid atomic-level contact bonding at the CuFeO2@VO-CuO junction, thereby augmenting overall efficiency. Through the optimized fabrication process of the CuFeO2@VO-CuO core-shell heterostructure, carrier concentration and transport and electron-hole pair separation saw substantial improvement, further boosting its PEC performance. Critically, the presence of oxygen vacancies in the CuFeO2@VO-CuO heterostructure played a pivotal role. Oxygen vacancies eliminated potential traps for captured electrons at the interface and suppressed the formation of unoccupied interface states, reducing recombination of photogenerated electron-hole pairs. Moreover, oxygen vacancies facilitated conduction band alignment, promoting efficient separation and rapid transfer of photogenerated electron-hole pairs. In conclusion, the CuFeO2@VO-CuO core-shell heterostructure achieves efficient separation and transfer of photogenerated electron-hole pairs while effectively suppressing recombination, thanks to the synergy between the interfacial electric field and oxygen vacancies.

Topological interface state formation in an hourglass lattice sandwich meta-structure

Topological interface state formation in elastic structures has attracted extensive research interest. However, most previous studies only explored the topological properties of simple structures, such as plain beams and rods. This study explores the feasibility and methodology of producing topological interface states in hourglass lattice sandwich meta-structures. By extending the spectral element method (SEM), a theoretical framework is developed for analyzing such meta-structures. The results obtained by the analytical model agree well with the finite element (FE) simulation results. Based on the analytical model, we maintain the substrate plates and vary the radii of the struts that constitute the hourglass lattice to explore the topological change. The band inversion is identified in the first band gap by inspecting the band edge evolution pattern. The analysis also indicates that the second band gap does not support topological interface states. Subsequently, the vibration modes at band gap edges are examined to confirm the polarization transition. Based on these analyses, a topological sandwich meta-structure is designed. An ordinary meta-structure without using the topological design strategy is also constructed. Compared with the ordinary meta-structure, a solitary peak occurs on the transmittance curve of the topological meta-structure in the first band gap. The vibration mode at the solitary peak demonstrates that the transverse motion concentrates at the junction interface, exhibiting the typical energy localization behavior of topological interface states.

Structural strategy to enhance the quantum and photocatalytic efficiency of ZnO quantum dots by incorporation of interface states

ZnO quantum dots (QDs) are potential candidates for high-efficiency emitters and photocatalysts. High efficiency demands high recombination of carriers, while high photocatalytic efficiency demands charge carrier separation. Here, we report an unexplored approach for achieving these opposite properties simultaneously by incorporating interface states in the ZnO QDs through a new strategy. We have used TEOS with Zinc acetate as synthesis precursors to prepare SiO2 encapsulated 5–6 nm ZnO QDs (ZnO@SiO2 QDs). X-ray photoelectron spectroscopy (XPS), FTIR and negative photoconductivity test confirmed the formation of interface states due to Zn − O − Si bonds between the SiO2 matrix and ZnO quantum dots. The ZnO@SiO2 QDs possess remarkable photocatalytic dye degradation properties, e.g., stability, efficiency and recyclability that outperform even the uncoated and commercially available ZnO beside high quantum yield varying between 46.1 % to 56.4 %, simultaneously. The photocatalytic mechanism has been further verified using the radical scavenger test. Thus, the work outlines designers' material design perspectives and discusses the detailed photophysical mechanisms for guiding the development of next-generation high-efficiency emitters and photocatalysts.

Evidence of Hot-Hole Induced Degradation in Lateral NDRIFT MOSFET: Characterization and TCAD Analysis

The role of secondary hot holes in the degradation of n-channel lateral drift MOSFETs, possibly leading to gate oxide breakdown, is discussed. Trapping of positive charges in the gate and field oxides and the formation of interface states under hot-carrier stress (HCS) have been successfully monitored by means of direct current current-voltage (DCIV) measurements and through gate-drain capacitance measurements. Experimental data have been complemented with Technology Computer Aided Design (TCAD) analysis. A degradation model including both hot-electrons and secondary hot-holes induced degradation capable of interpreting and reproducing measured data is proposed. Simulations allow to gain insights in the kinetics laws governing charge trapping and interface states generation along the silicon/oxide interface. The drift kinetics of the ON-state resistance have been also characterized and reproduced by simulations.

Significantly enhanced varistor properties of CaCu3Ti4O12 based ceramics by designing superior grain boundary: Deepening and broadening interface states

Significantly enhanced varistor properties via tailoring interface states were obtained in Ca 1–2 x /3 Y x Cu 3 Ti 4 O 12 -SrCu 3 Ti 4 O 12 composite ceramics. The breakdown field was improved to 35.8 kV cm −1 and the nonlinear coefficient in 0.1–1 mA cm −2 was enhanced to 14.6 for Ca 0.67 Y 0.5 Cu 3 Ti 4 O 12 -SrCu 3 Ti 4 O 12 . Noticeably, the withstand voltage of single grain boundary reached up to 24 V while the reported ones were constant to about 3 V. Greatly improved properties were attributed to the formation of superior grain boundary rather than the reduced grain size. Surprisingly, with distinct discrepancy of nonlinear performance in the composites, the resistance and activation energy of grain boundary exhibited little differences. Based on the double Schottky barrier at grain boundary and the field-assisted thermal emission model, it was found that the excellent electrical nonlinearity arose from the formation of deeper and broader interface states at grain boundary. In this case, interface states were not easily entirely filled and the barrier could maintain its height under applied voltage. This work provides a novel routine for enhancing the varistor properties of CaCu 3 Ti 4 O 12 based ceramics by manipulating interface states at grain boundary.

Hybridization-Induced Inversion of Spin Polarization at Rubrene/Ferromagnetic Cobalt Interface

The formation of hybrid interface states plays a crucial role in spin injection from a ferromagnetic metal into organic semiconductors. Here, we investigate the energy-level alignment and spin-polarized hybrid interface states between cobalt and rubrene using photoemission spectroscopy. We reveal that the presence of charge injection from cobalt to the rubrene molecular layer is determined by XPS and UPS measurements. We also find a spin-polarized interface state between cobalt and rubrene at 0.5 eV below the Fermi level, which can be attributed to chemical interaction between rubrene and Co(001). This hybridization state can induce spin polarization inversion at the rubrene/Co(001) interface as measured by spin-polarized UPS. The spin polarization inversion at the rubrene/Co(001) interface can be interpreted by a Zener exchange-type mechanism. On the basis of these observations, we obtain a deeper understanding of the electronic structure of the rubrene/Co(001) spinterface and show the importance for the quantitative description of rubrene/Co(001)-based organic spintronic devices.

Local resonator stimulated polarization transition in metamaterials and the formation of topological interface states

Developing the acoustic-elastic analogy of a topological insulator has attracted extensive research attention in recent years. Designs developed in the literature strongly rely on reforming the host structure of a system to achieve topology transition. In this article, an innovative topological metamaterial is presented. Unlike topological phononic crystals, the proposed topological metamaterial has a uniform host structure without any phononic crystal features. The band inversion and polarization transition are achieved by manipulating the design of local resonators. It has been proven that by changing the coupling spring constant of local resonators, the mode topologies at the bound states associated with the band gaps can be converted. Three polarization transition points have been found from the bound evolution analysis, and all of them possess the capability of stimulating topological interface states in the corresponding band gaps. The Zak phase calculations have further ascertained the prediction about the topological interface states. The band structure and transmittance spectrum of a supercell lattice of the proposed topological metamaterial have been examined. The topological interface states, as well as the energy localization effects, have been successfully observed in all the three band gaps. The proposed metamaterial brings the convenience of creating topological interface states by carefully manipulating the local resonators only. The methodology presented in this work is thus of practical significance in transforming existing structures into topological systems without revising them.

Bi2Se3/Bi2Se3 and TlSe/TlSe junctions: enhanced coupling of topological interface states by intercalation

Based on first-principles calculations, we studied the coupling of topological interface states at the interface of Bi2Se3/Bi2Se3 and TlSe/TlSe junctions. From our calculations, the coupling leads to an energy gap at the Dirac point of topological interface states and the energy gap is an exponential decay function of interface spacing when the interface spacing is large enough. The law of exponential decay may be used to identify the formation of interface states from interface-bulk mixed states. It also suggests that the coupling is caused by quantum tunneling. The coupling of the interface states in TlSe/TlSe can be greatly enhanced by intercalating hexagonal BN, although hexagonal BN is often used as a 2D substrate or insulator. The intercalation of bilayer BN or single atomic Au layer can further enhance the interfacial coupling. Our calculations may be helpful for the application of junctions composed of topological insulators.

Effect of Hydrogen on Radiation-Induced Displacement Damage in AlGaN/GaN HEMTs

To explore the role of hydrogen in radiation degradation of AlGaN/GaN high-electron-mobility transistors (HEMTs), the performance degradation of the hydrogen untreated and pretreated devices is compared under the exposure of carbon ions. The energy of carbon ions is chosen as 7.6 MeV, and the maximum fluence reaches 4 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . By electrical character measuring, it is found that the positive shift of the threshold voltage and the decrease of the transconductance occur after to the irradiation. Hydrogen pretreatment accelerates the shift of threshold voltage and the decrease of the transconductance. The threshold voltage of the device is almost unchanged under 1-MeV electron irradiations. This means that the displacement damage leads to the shift of the threshold voltage. The height of Schottky barrier hardly changes in the hydrogen pretreated devices, showing that the ionization defects such as the formation of interface states under the gate are not the main cause of device degradation. Deep level transient spectrum (DLTS) results show that the irradiations cause the increase of gallium vacancies, and the hydrogen pretreatment inhibits its formation. According to the first principle calculation, the existence of hydrogen in GaN layer can reduce the formation energy of and the number of charges of the defects. It is speculated that hydrogen atoms participate in the evolution of radiation defects, which changes the types and number of defects in the devices.

Charge carrier confinement in hematite and hematite / MoTe2 nanocomposites; the role of interface states

We report on the charge carrier confinement of the hematite and Hematite/Molybdenum di-telluride (α-Fe2O3@MoTe2) nanocomposites with special focus on the formation and role of interface states. X-ray diffraction (XRD) data confirms the existence of α-Fe2O3 in trigonally distorted rhombohedral while the MoTe2 in 2H hexagonal phase. Interfacial behavior and local chemical structure of the α-Fe2O3 and α-Fe2O3@MoTe2 nanocomposites were observed from the Mössbauer spectroscopy data. The hyperfine field splitting of the composite sample confirms the existence of the Fe3+ ions in two different octahedral environments, where 10% of surface Fe ions are in chemical interaction with the MoTe2 layers thereby making interfacial states. UV–Vis diffuse reflectance spectroscopy was employed to determine the band gap and band tail states, which are the key parameters in controlling the localization behavior. The conductivity, relaxation behavior and charge carrier confinement were observed by employing impedance spectroscopy.

Impact of interface-state formation on the charge-carrier dynamics at organic-metal interfaces

We study the role of electronic interface states on the electron transfer dynamics between layers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and the (111) and (100) surfaces of silver. For this purpose, we investigate the change of the decay dynamics of the first (n = 1) image-potential state on these surfaces upon adsorption of an ordered monolayer of PTCDA by means of time-resolved two-photon photoemission (2PPE). We find that the already short lifetime of the (n = 1)-state on Ag(111) is only slightly further reduced by PTCDA adsorption, whereas a much stronger reduction by a factor of three is observed for adsorption on Ag(100) resulting in similar lifetimes for both orientations. We show by model calculations on the basis of an analytical one-dimensional pseudo-potential that the enhanced decay for PTCDA/Ag(100) can be attributed to the opening of an additional channel for electron-electron scattering by the formation of an interface state which is derived from the Shockley-type surface resonance of Ag(100).

Realization of Spin-dependent Functionality by Covering a Metal Surface with a Single Layer of Molecules.

An interface of molecule and metal has attracted much attention in the research field of nanoelectronics because of their high degree of design freedom. Here, we demonstrate an efficient spin-to-charge current conversion at the metal surface covered by a single layer of molecules. Spin currents are injected into an interface between metal (Cu) and lead(II) phthalocyanine by means of the spin pumping method. An observed voltage signal is caused by the inverse Edelstein effect, i.e., spin-to-charge current conversion at the interface. The conversion coefficient, inverse Edelstein length, is estimated to be 0.40 ± 0.06 nm, comparable with the largest Rashba spin splitting of interfaces with heavy metals. Interestingly, the Edelstein length strongly depends on the thickness of the molecule and takes a maximum value when a single layer of molecules is formed on the Cu surface. Comparative analysis between scanning probe microscopy and first-principles calculations reveal that the formation of interface state with Rashba spin splitting causes the inverse Edelstein effect, whose magnitude is sensitive to the adsorption configuration of the molecules.

Type-II Dirac point and extreme dispersion in one-dimensional plasmonic-dielectric crystals with off-axis propagation

Dirac cones exhibit linear crossing dispersion, which results in extraordinary topological properties in the energy bands. When the dispersion around the Dirac points is tilted, type-II or type-III Dirac points are present, which lead to intriguing transport phenomena. With the off-axis wave vector, a two-dimensional (2D) momentum space for one-dimensional (1D) crystals can be constructed. In this work, we revealed that the band structure in the extended 2D momentum space of carefully designed 1D plasmonic-dielectric crystals can be tuned to be highly tilted, forming type-II Dirac cones. In particular, an approximately upright dispersion can be achieved, which is referred to as a class-II Dirac point. Gapless interface states occur across the interface between two connected lattices formed by two distinct inversion-symmetry-broken crystals (i.e., type A and type B). The tilt of the band structure has a crucial influence on the formation of interface states and their sensitivity to the period, which results from the extraordinary properties of the surface impedance in the bulk gap. More importantly, class-II Dirac points exhibit peculiar topological properties. The wavelength area is divided into two regions, each of which can only support interface states for one configuration, type-AB or type-BA connections. Our results show that type-II Dirac points, especially those that are class II, have an underlying impact on the surface impedance and determine the existence of interface states.

Effects of the introduction of a chromium oxide monolayer at the C60/Fe(001) interface

The introduction of a two-dimensional oxide layer at the interface between an organic semiconductor and a ferromagnetic metal (spinterface) can help in tailoring the formation of spin-polarized hybridized interface states. Here, we consider the case of a Cr4O5 monolayer at the C60/Fe(001) interface, which is already known to feature the occurrence of spin-polarized states in the fullerene molecules. In this work, we employ scanning tunneling microscopy/spectroscopy and photoemission spectroscopy to show that the C60/Cr4O5/Fe(001) spinterface is characterized by the formation of a well-ordered fullerene monolayer and of strongly hybridized interface states. These experimental results are discussed in terms of state-of-the-art ab initio calculations of the structural, electronic, and magnetic properties at the interface.

Evaluations of minority carrier lifetime in floating zone Si affected by Si insulated gate bipolar transistor processes

This study evaluates minority carrier lifetime in floating zone Si affected by advanced and conventional Si IGBT (insulated gate bipolar transistor) processes. Si gate oxidations reduce lifetime due to the formation of interface states between the Si and the oxide. As a result, cross-sectional photoluminescence imaging shows that the lifetime around a trench is lower than that in the bulk region. The high temperature thermal treatment for B-base/P-emitter layer activation after gate oxidation improved the interface, thus resulting in the recovery of the lifetime. TEM observations reveal that the (110) trench surface shows irregular contrast while the (100) surface shows relatively smooth contrast, which is consistent with the lifetime result of “trench side wall surface (100)” > “trench side wall surface (110).” Our study conclusively clarifies that lifetime is correlated with the SiO2 surface state.

Peculiar covalent bonding of C60/6H-SiC(0 0 0 1)-(3 × 3) probed by photoelectron spectroscopy

High resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C60 on 6H-SiC(0 0 0 1)-, characterized by protruding Si-tetramers. The results show that C60 is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the pz orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least ) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C60 is attached to the substrate through a single covalent C60–Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C60 molecules to the substrate via the formation of donor-like interface states.

Topological interface states due to spontaneous symmetry breaking in a chain of anharmonic oscillators

In this work, we propose a one-dimensional system of coupled anharmonic oscillators with nonlinear coupling and study a spontaneous formation of interface excitations in the linearized spectrum of such periodic array. We show that during its evolution such system can undergo a topological transition from the disordered and topologically trivial phase into the nontrivial one. The topological transition is accompanied by the formation of the interface state in the spectrum of linearized excitations of the equilibrium state. Employing the technique based on the calculation of mean chiral displacement we demonstrate that the in-gap interface state exhibits a topological nature and is analogous to that in the well-known Su-Schrieffer-Heeger model. Our results demonstrate the formation of topological interface state induced by spontaneous symmetry breaking mechanism and open novel possibilities for designing various nonlinear electronic, photonic and polaritonic systems with disorder-robust control of information flows.

Nonstoichiometric Solution-Processed BaTiO₃ Film for Gate Insulator Applications.

Solution processed barium titanate (BTO) was used to fabricate an Al/BaTiO3/p-Si metal-insulator-semiconductor (MIS) structure, which was used as a gate insulator. Changes in the electrical characteristics of the film were investigated as a function of the film thickness and post deposition annealing conditions. Our results showed that a thickness of 5 layers and an annealing temperature of 650 °C produced the highest electrical performance. BaxTi1-xO3 was altered at x = 0.10, 0.30, 0.50, 0.70, 0.90, and 1.0 to investigate changes in the electrical properties as a function of composition. The highest dielectric constant of 87 was obtained for x = 0.10, while the leakage current density was suppressed as Ba content increased. The lowest leakage current density was 1.34×10-10 A/cm2, which was observed at x = 0.90. The leakage current was related to the resistivity of the film, the interface states, and grain densification. Space charge limited current (SCLC) was the dominant leakage mechanism in BTO films based on leakage current analysis. Although a Ba content of x = 0.90 had the highest trap density, the traps were mainly composed of Ti-vacancies, which acted as strong electron traps and affected the film resistivity. A secondary phase, Ba2TiO4, which was observed in cases of excess Ba, acted as a grain refiner and provided faster densification of the film during the thermal process. The absence of a secondary phase in BaO (x = 1.0) led to the formation of many interface states and degradation in the electrical properties. Overall, the insulator properties of BTO were improved when the composition ratio was x = 0.90.

Band Alignment of Al2O3 Layer Deposited NO and SO2 Exposed (001) H‐Diamond Heterointerfaces Studied by Synchrotron Radiation X‐Ray Photoelectron Spectroscopy

The interfacial energy band alignment of the interface between H‐terminated diamond, exposed to either NO or SO2, and an Al2O3 layer is studied using synchrotron radiation X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption near‐edge structure (XANES) measurements. From the results, type II band alignment is determined for both the Al2O3/NO/H‐diamond and the Al2O3/SO2/H‐diamond heterointerfaces. The valence band offsets are determined to be 3.7 eV for the Al2O3/NO/H‐diamond heterointerface and 3.5 ± 0.1 eV for the Al2O3/SO2/H‐diamond heterointerface. XPS spectra indicate that the NO and SO2 decompose on the H‐diamond surface, and XANES measurements demonstrate the formation of oxygen related interface states.

Significant Structural Distortion in the Surface Region of 4H-SiC Induced By Thermal Oxidation and Recovered By Ar Annealing

4H-SiC has attractive properties for power devices of high voltage applications, however, the performance of MOSFETs is often severely limited by the existence of electrically active defects at SiO2/SiC interface. The formation of interface states is often attributed to the carbon-related byproducts of thermal oxidation, however as suggested by theoretical study, it may also partly originate from the non-uniformity in electrical structure of 4H-SiC, due to the oxidation-induced local strains at the 4H-SiC surface [1]. In this study we systematically investigated the lattice distortions introduction in 4H-SiC surface region after various oxidation processes and the recovery of that directly by using surface sensitive in-plane x-ray diffractometry (XRD). 4º off-axis 4H-SiC (0001) wafers covered with 5 μm-thick n-type epitaxial layers were employed in this study. The wafers were cleaned by HF solution, then thermal oxidation process in dry oxygen at 1300ºC for various oxidation time were performed. For some of the samples, the thermally grown SiO2 layer was removed by chemical etching in HF solution, followed by annealing in Ar at 1150ºC or 1300ºC for various annealing time. The shallow incident angles less than or slightly more than the critical angle were used for in-plane XRD measurements to limit the x-ray penetration depth which can be widely tuned simply by changing the incident angle.In the in-plane XRD pattern, a peak corresponding to the diffraction of (1-100) planes was detected. The interplanar spacing was characterized for the result obtained with different incident angles. Before thermal oxidation, as expected, no significant peak position shift was observed even by changing the incident angle, since the lattice constant should be uniformly distributed in the epitaxial layer. In contrast, the peak position of oxidized 4H-SiC sample shifted to the lower angle for small incident angle measurement and gradually shifted back by increasing the incident angle [2]. In addition, when we changed the thermal oxide thicknesses by performing dry oxidation for different time, the shift of the peak position became larger for the longer oxidation time, and reaches to as large as +0.4% distortion for >40nm-thick oxide case [2]. The x-ray penetration depth where such a significant shift of the lattice constant was observed, was estimated to be only several hundreds of nanometers from the SiC surface, by a simple approximation of the penetration depth taking account of the critical angle and the x-ray absorption coefficient of 4H-SiC. These results clearly indicate that the dry oxidation process induces a significant lattice distortion locally in the surface region of 4H-SiC wafer. We also found that the lattice distortion of SiC surface remained even after the complete removal of SiO2 thin film by HF etching, though it was partially reduced. Thus the detected local strain in the surface region of SiC is distinguishable from an intrinsic strain due to the thermal expansion coefficient difference and atomic density difference of Si between SiC and SiO2 [3]. A further recovery of the lattice constant in the surface region was observed by Ar annealing. It should be noted that the relaxation occurs gradually by extending the Ar annealing time to a few hours even at 1300ºC, which is equivalent to the oxidation temperature in this study. This suggests a thermally-activated process inside SiC is required for the relaxation of the surface distortion. Actually a higher recovery rate was observed for 1300ºC than that for 1150ºC annealing. From these results we speculate that the observed surface distortion of 4H-SiC is caused by oxidation-induced defects which are accumulated only in the surface region and induce a significant strain locally in the surface region. Then the gradual relaxation by Ar annealing would be corresponding to the processes of decomposition of the defects followed by migration and desorption of the ejected atoms from the defects. Even though the relationship between the observed surface strain and the electrical structure of SiC surface is not clarified yet, such significant strain should have a certain impact on the electrical properties of SiO2/SiC interfaces.