Interface Inhomogeneity Research Articles

Spectral widths and Stokes shifts in InP-based quantum dots.

InP-based quantum dots (QDs) have Stokes shifts and photoluminescence (PL) line widths that are larger than in II-VI semiconductor QDs with comparable exciton energies. The mechanisms responsible for these spectral characteristics are investigated in this paper. Upon comparing different semiconductors, we find the Stokes shift decreases in the following order: InP > CdTe > CdSe. We also find that the Stokes shift decreases with core size and decreases upon deposition of a ZnSe shell. We suggest that the Stokes shift is largely due to different absorption and luminescent states in the angular momentum fine structure. The energy difference between the fine structure levels, and hence the Stokes shifts, are controlled by the electron-hole exchange interaction. Luminescence polarization results are reported and are consistent with this assignment. Spectral widths are controlled by the extent of homogeneous and inhomogeneous broadening. We report PL and PL excitation (PLE) spectra that facilitate assessing the roles of homogeneous and different inhomogeneous broadening mechanisms in the spectra of zinc-treated InP and InP/ZnSe/ZnS particles. There are two distinct types of inhomogeneous broadening: size inhomogeneity and core-shell interface inhomogeneity. The latter results in a distribution of core-shell band offsets and is caused by interfacial dipoles associated with In-Se or P-Zn bonding. Quantitative modeling of the spectra shows that the offset inhomogeneity is comparable to but somewhat smaller than the size inhomogeneity. The combination of these two types of inhomogeneity also explains several aspects of reversible hole trapping dynamics involving localized In3+/VZn2- impurity states in the ZnSe shells.

Experimental and numerical investigation of interface and mechanical properties of Al0.3CoCrFeNi/304L explosively weld composite plate

High entropy alloys (HEAs) have gained attention for their excellent properties, but research on combining them with conventional metals is limited. This study manufactured Al0.3CoCrFeNi/304L composite plates using explosive welding. Welding parameters were determined based on the theoretical weldability window. The detonation process and interface were analyzed using an advanced SPH-FEM algorithm. Interface morphology was characterized using OM, EDS, and EBSD, and mechanical properties were tested. Materials near the interface experienced extreme conditions, resulting in severe plastic deformation and wave interface generation. Interface inhomogeneity wase caused by unsteady detonation and inclined collision. Collision velocity predicted by the SPH-FEM model matched theoretical calculations, and the predicted interface agreed with experimental results. Increased explosive charge led to more intense interface undulation and a thicker diffusion layer. EBSD showed a single-phase structure with non-uniform elements at the nanometer scale. The interface exhibited the highest deformation and grain recrystallization, along with plastic deformation and texture. Mechanical tests revealed that larger wave interfaces had higher hardness, improved tension, bending, and impact toughness, but worse shear performance. Explosive welding proved effective for achieving high-quality bonding between HEAs and conventional metals, offering promising possibilities for HEA and traditional metal composites.

Effect of the complex air–sea interface on a hybrid atmosphere-underwater optical wireless communications system

Wireless optical communication (WOC) through the atmosphere–ocean system is fundamentally complex because of the diversity, concentration, types and vertical thickness of the constituents and poses a major challenge from the optical characteristics of bubbles at the air–sea interface and water constituents (phytoplankton, suspended sediments, and dissolved organic matter), and stratification (medium inhomogeneity). The present study aims to analyse the effects of atmospheric aerosols (their intensity, diversity, and vertical thickness), waves/bubbles at the air–sea interface, seawater constituents (phytoplankton, suspended sediments, and dissolved organic matter), and medium inhomogeneity along the path between a laser transmitter in the airborne platform and a receiver in the underwater platform using Monte Carlo technique. For the first time, our analysis showed that for a homogeneous atmosphere–ocean channel (7 km in the atmosphere column and 42 m in the ocean medium), the estimated normalized received power reduced to 17 dB and the time delay spread increased to 1.01 ns (considering a receiver of 7” and an aperture FOV of 180°) when compared to those values for a stratified atmosphere–ocean channel. The bubbles (both clean and coated) relative to the particulates in open oceanic water under high wind speeds have a significant impact on the spatio-temporal spread of the laser beam. There is also a well pronounced effect of coated bubbles on the received signal power in open ocean water. In contrast, the particulates have a higher impact on the received signal power than the clean/coated bubbles in turbid coastal water. These results suggest that the normalized received power when calculated for the cases of dense bubble populations under high wind speeds (at a wind speed 18 ms−1) reduced to 8–9 dB and increased to 0.0025 scintillation index. The coated bubbles at the air–water interface reduced the normalized received power by 1.41 dB compared to the clean bubbles. Further, our analysis indicated that the expansion of beam width and the reduction of beam divergence angle can be utilized to mitigate the performance degradation (power loss) caused by bubbles at the air–sea interface. These results and analyses will be helpful for a system designer to accurately define/decide the system parameters and build a functional communication system for the inhomogeneous atmosphere–ocean system.

Abnormal Raman Characteristics of Graphene Originating from Contact Interface Inhomogeneity.

The Raman peak position shift rate per strain (RSS) coefficient of graphene is crucial for quantitative strain measurement by Raman spectroscopy. Despite its essential role, the experimentally measured RSS values are found to be highly scattered and many times significantly lower than the theoretical prediction. Here, using in situ Raman spectroscopy with a tensile test system, we resolve this controversy by examining the Raman characteristics of graphene derived from chemical vapor deposition (CVD) transferred on polymer substrates. Our experiments show that the Raman 2D-peak position of CVD graphene can shift nonlinearly with applied strain, in contrast to its intrinsically linear trait. More importantly, the resultant RSS coefficient at the steady state is much lower than the theoretical prediction. By analyzing atomic force microscopy (AFM) phase images and full width at half-maximum (FWHM) of Raman spectra, we attribute the abnormal behavior to nanometer-scale inhomogeneity of the graphene/substrate contact interface. Assisted by a simplified discrete interface slip model, we correlate the evolution of nanometer-scale inhomogeneity with that of the apparent Raman response. The theoretical model provides a useful tool for understanding and optimizing the contact interface behavior of various two-dimensional materials on substrates; the revealed mechanism is critical for correct interpretation of data obtained by Raman or any other spectroscopies based on homogenized laser signals.

Gaussian Thermionic Emission Model for Analysis of Au/MoS2 Schottky-Barrier Devices

Schottky-barrier inhomogeneities are expected at the metal--transition-metal-dichalcogenide (TMDC) interface and this can impact device performance. However, it is difficult to account for the distribution of interface inhomogeneity as most techniques average over the spot area of the analytical tool (e.g., few hundred micrometers squared for photoelectron-based techniques), or the entire device measured for electrical current-voltage (I-V) measurements. Commonly used models to extract Schottky-barrier heights neglect or fail to account for such inhomogeneities, which can lead to the extraction of incorrect Schottky-barrier heights and Richardson constants that are orders of magnitude away from theoretically expected values. Here, we show that a Gaussian-modified thermionic emission model gives the best fit to experimental temperature-dependent current-voltage (I-V-T) data of van der Waals $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ interfaces and allow the deconvolution of the Schottky-barrier heights of the defective regions from the pristine region. By the inclusion of a Gaussian-distributed Schottky-barrier height in the macroscopic I-V-T analysis, we demonstrate that interface inhomogeneities due to defects are deconvoluted and well correlated to the impact on the device behavior across a wide temperature range from a room temperature of 300 K down to 120 K. We verify the Gaussian thermionic model across two different types of p-${\mathrm{Mo}\mathrm{S}}_{2}$ (geological bulk crystals and synthetic flux-grown crystals), and finally compare the macroscopic Schottky-barrier heights with the results of a nanoscopic technique, ballistic hole emission microscopy (BHEM). The results obtained using BHEM are consistent with the pristine $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ Schottky-barrier height extracted from the Gaussian-modified thermionic emission model over hundreds of nanometers. Our findings show that the inclusion of Schottky-barrier inhomogeneities in the analysis of I-V-T data is useful to elucidate the impact of defects (e.g., grain boundaries, metallic impurities, etc.) and hence their influence on device behavior. We also find that the effective Richardson constant, a material-specific constant typically treated as merely a fitting constant, is a useful parameter to check for the validity of the transport model.

Interface engineering of additively manufactured maraging steel-H13 bimetallic structures

This paper presents a study that aims to modify the interfacial characteristics of maraging steel-H13 bimetal structures through heat treatment procedures. Laser powder bed fusion (LPBF) was used to deposit maraging steel powder (MS1) on H13 hot work tool steel blocks. Subsequently, printed samples went through two different treatment cycles: (i) solution treatment, and (ii) aging treatment recommended for maraging steel. The microstructure, grain morphology, and mechanical response of MS1-H13 bimetals were assessed and compared with the as-built condition. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) techniques were used to analyze the interfacial modification upon heat treatment. Using uniaxial tensile testing and microhardness measurements along with 2D nanoindentation mapping, the mechanical properties of bimetal structures were correlated to the microstructural evolution of the samples during different heat treatments. Although the aging treatment was not effective to induce tangible microstructure changes, the solution treatment succeeded in the modification of MS1-H13 interface. A fully lath martensitic morphology was promoted across the interface, eliminating microstructure and hardness inhomogeneity as well as interface distinction between both sides. Finally, the solution treatment enhanced the mechanical properties of MS1-H13 bimetal structure by tripling the ultimate tensile strength.

Resistive switching in reactive electrode-based memristor: engineering bulk defects and interface inhomogeneity through bias characteristics

A novel work on several aspects of current conduction mechanism for resistive switching (RS) of a reactive electrode-based memristor (Al/ZnO/Al) against generally used inert electrode–based devices has been illustrated. Compliance current (Icc), i.e. maximum current through device and stop voltage (Vstop), i.e. maximum Vstop upon device, has been used as bias characteristics to evaluate the effect upon conduction behavior. Effect of change in Icc for constant Vstop and vice-versa upon high resistance state (HRS) and low resistance state (LRS) has been evaluated in terms of change in bulk defects, i.e. oxygen vacancy and interface inhomogenity, i.e. AlOx formation/dissolution. The device shows unipolar resistive switching (URS) at lower Icc (< = 0.1 mA) and transits irreversibly into bipolar resistive switching (BRS) at higher Icc (> = 1 mA). The dominant conduction behavior of the device is found to be homogenous resistive switching (HoRS). The conduction in HRS and LRS is found to be space-charge limited current conduction. Conductive atomic force microscopy results of the thin film switching layer support the HoRS mechanism and also illustrate the role of interfacial oxide in attributing BRS behavior in the Al/ZnO/Al resistive switch. Studies can be utilized in future to further understand the RS conduction mechanism in reactive electrode–based RS and can be applied for multi-state memory applications.

Interface/interphase effects on scattering of elastic P- and SV-waves from a circular nanoinclusion embedded in a solid viscoelastic matrix

This study aims to present an analytical investigation of the dynamic effects of the interfacial region on scattering of elastic P- and SV-waves by a circular nanoinclusion. The nanoinclusion is assumed to be surrounded by an interphase region and embedded in a polymer viscoelastic matrix. Wave function expansion method is utilized to solve the Navier wave equation in all three phases (fiber-interphase-matrix) and for the viscoelastic regions, the features of Havriliak-Negami model are used to account for the frequency-dependent material properties. Dynamic Stress Concentration Factor (DSCF) and Scattering Cross Section (SCS) results are obtained for both cases of incident waves. To focus on the interfaces, Gurtin-Murdoch model of surface elasticity is applied in modeling the interfaces. Additionally, effects of interphase inhomogeneity and viscoelasticity on the results are studied. The results indicate that considering the simultaneous effects of nanoscale interface parameter and inhomogeneity and viscoelasticity of the interphase has a considerable impact on the dynamic behavior of the nanoinclusion.

Evaluation of Schottky barrier source/drain contact on gate-all-around polycrystalline silicon nanowire MOSFET

Self-aligned nickel salicide of gate-all-around silicon nanowires (GAA Si NWs) MOSFET has been investigated using very-large-scale integration (VLSI) process. The nickel salicide (NiSix) characteristics including sheet resistance (Rs), phase transformation and Schottky barrier height (SBH) are discussed via line structures and Schottky diode, respectively. RTA temperature strongly affects Rs of NiSix and related with surface morphology. Below 700°C, near 4.5Ω/□ of smooth surface of NiSi Rs is obtained in comparison with 12.6Ω/□ of NiSi2 at 700°C. Both forward bias of current-voltage (I-V) and temperature-dependency of activation-energy methods have been used to characterize Schottky barrier height (SBH). For 400°C RTA sample, the SBH and ideality factor extracted from the forward bias of current-voltage (I-V) method are 0.38eV and 1.53, respectively. Besides, the SBH extracted from temperature-dependency of activation-energy methods is 2.9eV. The variation of SBH values are attributed to the thermionic-field emission (TFE) which the interface states and inhomogeneity induce slightly tunneling current. Moreover, various channel lengths of Schottky source/drain contacts of GAA Si NWs MOSFET devices were measured and results demonstrate that enough electrical performances combined with ambipolar behavior are available for future application in logical circuit and non-volatile memory.

Characterization of the spatial inhomogeneity of heterointerfaces in GaAs/AlGaAs quantum wells by photoreflectance spectroscopy

GaAs/AlGaAs quantum-well heterostructures with well widths from 20 to 35 nm are investigated by photoreflectance spectroscopy. The broadening of spectral lines related to band-to-band transitions increases with the transition energy and decreases with the well width. The observed decrease in the broadening parameter with increasing well width is explained in terms of spatial inhomogeneity of the heterointerfaces. According to the experimental data, the interface inhomogeneity in the structures under study is 0.34–0.39 nm (1.3–1.4 monolayers).

Effect of copper phthalocyanine (CuPc) interlayer on the electrical characteristics of Au/n-GaN Schottky rectifier

Electrical properties of Au/n-GaN Schottky rectifier with copper pthalocyanine (CuPc) interlayer were investigated using current–voltage (I–V), capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics. The barrier height obtained for the Au/CuPc/n-GaN Schottky diode was higher than that of the Au/n-GaN Schottky diode. This could be associated with the presence of the CuPc interlayer influencing the space-charge region of the Au/n-GaN structure. The Au/CuPc/n-GaN Schottky structure exhibits higher ideality factor, indicating the higher interface inhomogeneity in Au/CuPc/n-GaN as compared to Au/n-GaN Schottky structure. The density of interface states was extracted using I–V, C–V, and G–V characteristics. The results showed that the introduction of CuPc interlayer facilitated the reduction of interface state density (NSS) of Au Schottky contact to n-GaN. Particularly, the NSS obtained from frequency-dependent C–V characteristics was lower than that determined from forward I–V characteristics, which could be attributed to the inhomogeneous distribution of NSS at Schottky interface.

Variational bounds for the effective electroelastic moduli of piezoelectric composites with electromechanical coupling spring-type interfaces

The present work focuses on variational bounds for the effective electroelastic moduli of multiphase piezoelectric composites with thin piezoelectric interphase. Both the inhomogeneities and the matrix are assumed to be piezoelectric and transversely isotropic. The piezoelectric interphase is modeled as the spring-type interface with electromechanical coupling. The inhomogeneities are assumed to be spheroidal so that the reinforcement geometry is able to range from thin flake to continuous fiber. The effective properties of the piezoelectric composite with interfacial imperfection are defined and the principles of minimum internal energy and enthalpy are derived. These principles are applied to analytically obtain the upper and lower bounds for the effective electroelastic moduli. Unlike the Voigt–Reuss-type bounds for perfect interface, the present bounds depend not only on the material properties and volume fraction, but also on the interface parameters, inhomogeneity shape and orientation. An example of a two-phase composite is given for detailed discussion, where dependence of the electroelastic moduli and their bounds on the inhomogeneity shapes and orientations as well as the interface properties is provided and discussed. To qualitatively account for the dependence, analysis based on two possible mechanisms, i.e., the simple mixture rule of composite and the weakening effect by imperfect interface, are also provided.

Modelling the inhomogeneous SiC Schottky interface

For the first time, the I-V-T dataset of a Schottky diode has been accurately modelled, parameterised, and fully fit, incorporating the effects of interface inhomogeneity, patch pinch-off and resistance, and ideality factors that are both heavily temperature and voltage dependent. A Ni/SiC Schottky diode is characterised at 2 K intervals from 20 to 320 K, which, at room temperature, displays low ideality factors (n &amp;lt; 1.01) that suggest that these diodes may be homogeneous. However, at cryogenic temperatures, excessively high (n &amp;gt; 8), voltage dependent ideality factors and evidence of the so-called “thermionic field emission effect” within a T0-plot, suggest significant inhomogeneity. Two models are used, each derived from Tung's original interactive parallel conduction treatment of barrier height inhomogeneity that can reproduce these commonly seen effects in single temperature I-V traces. The first model incorporates patch pinch-off effects and produces accurate and reliable fits above around 150 K, and at current densities lower than 10−5 A cm−2. Outside this region, we show that resistive effects within a given patch are responsible for the excessive ideality factors, and a second simplified model incorporating these resistive effects as well as pinch-off accurately reproduces the entire temperature range. Analysis of these fitting parameters reduces confidence in those fits above 230 K, and questions are raised about the physical interpretation of the fitting parameters. Despite this, both methods used are shown to be useful tools for accurately reproducing I-V-T data over a large temperature range.

Soft Superconducting Gap in Semiconductor Majorana Nanowires

We theoretically consider the ubiquitous soft gap measured in the tunneling conductance of semiconductor-superconductor hybrid structures, in which recently observed signatures of elusive Majorana bound states have created much excitement. We systematically study the effects of magnetic and nonmagnetic disorder, temperature, dissipative Cooper pair breaking, and interface inhomogeneity, which could lead to a soft gap. We find that interface inhomogeneity with moderate dissipation is the only viable mechanism that is consistent with the experimental observations. Our work indicates that improving the quality of the superconductor-semiconductor interface should result in a harder induced gap.

The effect of interface modulation on phononic band gaps for longitudinal modes in semiconductor superlattices

We present the results of our theoretical investigation of the effect of a periodic interface modulation on the longitudinal vibrational modes in one-dimensional phononic crystals. Based on the plane wave method and the finite difference time domain method, our calculations show that the bottom and top of the first phononic gap can be controlled independently by adjusting the modulation introduced. The effective acoustic impedance of the modulated layers is demonstrated to be of key importance for the opening of phononic energy gaps. Band gaps for longitudinal modes can be expected to occur in a large range of interface inhomogeneity. The shape of the modulation proves not to have any significant effect on the investigated properties.

Film Thickness Dependence of Rectifying Properties of La1.85Sr0.15CuO4/Nb-SrTiO3 Junctions

In this work, p-n junctions are made from directly depositing optimal doped La1.85Sr0.15CuO4 (LSCO) films on n-type Nb-doped SrTiO3 substrates. Film thickness controlled rectifying behaviors are strikingly displayed. The starting points of the diffusion voltage reduction Vd–on change clearly with varying film thickness. Vd–on and TC coincide with each other when the film thickness is larger than 300 nm, indicating a close relation between the two parameters. However, when the film is very thin (< 350 nm) a departure between the two parameters was also observed. A possible reason for this is discussed within the framework of an inhomogeneous Schottky contact. Enhanced interface inhomogeneity due to the tensile strain appears to be the origin.

On barrier height inhomogeneities of Au and Cu/n-InP Schottky contacts

In this work, barrier height inhomogeneity and its origin are investigated for Au and Cu/n-InP metal/semiconductor contacts. The effective barrier height values of the Au and Cu/n-InP Schottky barrier diodes (SBDs) have been obtained as 0.480 and 0.404 eV from current–voltage ( I–V) characteristics using the thermionic emission theory. The barrier height values of the Au and Cu/n-InP SBDs have been obtained as 0.524 and 0.453 eV from the experimental reverse bias capacitance–voltage ( C–V) characteristics, respectively. The discrepancy between the barrier heights could be explained in terms of barrier height inhomogeneity approach. To investigate origin of the barrier height inhomogeneity, the interface atomic concentration and its distribution have been obtained by deep profile analysis using electron spectroscopy for chemical analysis (ESCA) data. The analysis shows that the interface is not abrupt. Thus, we could say lateral barrier height inhomogeneity could arise from the interface inhomogeneity.

Effect of DC bias on microstructural rearrangement of a-SiN:H films on PET substrate

Hydrogenated amorphous silicon nitride (a-SiN:H) films were deposited on flexible polyethylene terephthalate substrates at temperature as low as 100 °C by hot-wire chemical vapor deposition using SiH 4, H 2 and NH 3 precursors. Field emission scanning emission microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy and small angle X-ray scattering were employed to study structural and microstructural properties of a-SiN:H films. The rms surface roughness increased with increase of positive bias to substrate. Intermediate range order, porosity and interface inhomogeneity in amorphous of a-SiN:H films evaluated by acoustic and optical phonon of silicon network, Guinier plot and correlated length from Raman and SAXS characterizations. The fractal behavior of a-SiN:H domains approached the perfect symmetry and the intermediate range order of a-SiN:H films deteriorate with increase of the positive substrate bias. Both correlation length and void size of the a-SiN:H amorphous domain increased with increase of the substrate bias from 0 to +200 V.

Effect of composition inhomogeneity in a-SiO xN y thin films on their optical properties

The variation of optical properties of amorphous silicon oxynitride (a-SiO x N y ) thin films was studied as function of their elemental composition and the deposit homogeneity was checked by Auger profiling. The optical gap, determined from the transmission spectra, showed high values increasing with the oxygen concentration. The Urbach energy was also estimated and linked to the film structure and composition. Their refractive index was found to decrease linearly from 1.94 to 1.46 when the layer composition varies between silicon nitride and silicon oxide. Spectroscopic ellipsometry showed that all samples were transparent over the visible range and that the layer with intermediate composition exhibited interface inhomogeneity. These results were confirmed by Auger profiling.

Two circular inclusions with inhomogeneously imperfect interfaces in plane elasticity

This paper is concerned with the problem of two circular inclusions with circumferentially inhomogeneously imperfect interfaces embedded in an infinite matrix in plane elastostatics. Infinite series form solutions to this problem are derived by applying complex variable techniques. The numerical results demonstrate that the interface imperfection, interface inhomogeneity, and interaction among neighboring inclusions (fibers) will exert a significant influence on the stresses along the interfaces and average stresses within the inclusions.