Effect Of Interface Roughness Research Articles

Effects of interfacial roughness on phonon transport in bilayer silicon thin films

Effects of interfacial roughness on phonon transport in bilayer silicon thin films

Effect of interface roughness on superconducting transition temperatures of Nb/Co multilayers

Superconductor (SC)/ferromagnet (FM) Nb/Co multilayers have been produced by magnetron-sputtering with a 100nm thickness of Nb and 5, 10, and 20nm of Co. The superconducting properties have been investigated by electric transport measurements. It was found that the thicker Co layers decrease the superconducting transition temperature (Tc) less than the thinner ones. In order to understand this unexpected behavior, the microstructure of the layers has been investigated by means of Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and by energy-dispersive X-ray spectroscopy line scan analyses in scanning TEM (STEM) mode. It was found that the decisive parameter which determines the effect of magnetic layers on Tc of the superconducting layers is not only the roughness (R), but the ratio of the roughness to thickness (dCo) of the magnetic Co layer, δ=R/dCo. For δ>1 the magnetic stray field of the magnetic layers is the main reason for the Tc reduction.

The effect of interface roughness scattering on Si SOI FinFET with Ando's and extended Prange and Nee model

A new multi-subband interface roughness scattering (IRS) model is incorporated into a 3D Finite Element Monte Carlo simulator with 2D Schrödinger equation based quantum corrections. The model takes advantage of wavefunctions and energy levels obtained in solutions of Schrödinger equation on slices across the channel to calculate the respective form factors. The new 3D code is then used to forecast the performance of 10.7 nm gate length SOI Si FinFETs with rectangular cross-section and rounded corners. We found that the multi-subband IRS is much stronger at large electron kinetic energies resulting in a drive current of 600 mA/μm2 that is 25% reduction when compared to 3D Ando model.

Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells

The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer–polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semicrystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈140 °C) at temperatures of 90, 100, and 110 °C for time periods from 2 min up to 250 min. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1 nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.

Effects of interface roughness on the annealing behaviour of laminated Ti-Al composite deformed by hot rolling

A laminated Ti-Al composite has been fabricated by hot compaction and hot rolling of alternate layers of commercial purity Ti and Al sheets with a thickness of 200 μm. The hot compaction temperature was 500°C and in a following step the composite has been reduced 50% in thickness by hot rolling. The fully consolidated composite has been annealed at 300°C and 500°C for different length of time. As a result of the differences in crystal structure and mechanical properties between Ti and Al protrusions and retrusions formed at the interface. A heterogeneous interface has thereby been created. The heterogeneity affected the recovery kinetics of the aluminium phase which at 300°C was faster near the interface than in the middle of the Al layer. This effect of a heterogeneous interface is of relevance when optimizing the thermomechanical processing of the composite to obtain high strength and formability for application.

Effects of interface roughness scattering on device performance of indirectly pumped terahertz quantum cascade lasers

The impacts of interface roughness (IR) scattering on device performance of indirectly-pumped (IDP) terahertz quantum cascade lasers are studied. Three different active region designs with almost the same lasing frequency at threshold and comparable oscillator strength are experimentally investigated and the measurement data are analyzed and compared with numerical simulation. The simulation results show that all structures suffer from the detrimental effect of intersubband roughness scattering in terms of threshold current density, and probably operating temperature. The intrasubband IR scattering time could also to be a limiting factor in the IDP structures due to the employed high energetic barrier.

Electron-Transport Properties of a ZnMgO/ZnO Hetero Structure and the Effect of Interface Roughness and ZnMgO Thickness

The electron-transport properties of a Zn\(_{1-x}\)Mg\(_x\)O/ZnO hetero structure were studied by use of an ensemble Monte Carlo technique. There was no intentional doping in the profile and the entire charge is because of polarization of charges at the interface. Interface roughness, and the intensity of acoustic and optic phonon scattering were used in the ensemble Monte Carlo method to find the transport properties of the two-dimensional electron gas (2DEG). The low-field mobility characteristics of the structure were determined as a function of temperature, the thickness of the ZnMgO layer, and the interface roughness. The interface roughness was found to have a very large effect on the drift mobility of carriers, especially at low temperatures. It was observed that as the thickness of the ZnMgO increases the concentration of electrons increases whereas the mobility of the electrons decreases. The low-field mobility values obtained from ensemble Monte Carlo simulations were in agreement with published experimental data.

Stochastic Interdigitation as a Toughening Mechanism at the Interface between Tendon and Bone

Reattachment and healing of tendon to bone poses a persistent clinical challenge and often results in poor outcomes, in part because the mechanisms that imbue the uninjured tendon-to-bone attachment with toughness are not known. One feature of typical tendon-to-bone surgical repairs is direct attachment of tendon to smooth bone. The native tendon-to-bone attachment, however, presents a rough mineralized interface that might serve an important role in stress transfer between tendon and bone. In this study, we examined the effects of interfacial roughness and interdigital stochasticity on the strength and toughness of a bimaterial interface. Closed form linear approximations of the amplification of stresses at the rough interface were derived and applied in a two-dimensional unit-cell model. Results demonstrated that roughness may serve to increase the toughness of the tendon-to-bone insertion site at the expense of its strength. Results further suggested that the natural tendon-to-bone attachment presents roughness for which the gain in toughness outweighs the loss in strength. More generally, our results suggest a pathway for stochasticity to improve surgical reattachment strategies and structural engineering attachments.

Effects of interfacial roughness on the planar Hall effect in NiFe/Cu/IrMn multilayers

This paper reports that the planar Hall effect in NiFe/Cu/IrMn multilayers was strongly influenced by the Cu spacer thickness (t Cu), which was due to the variation of interfacial roughness. With t Cu increasing, a peculiar change of planar Hall voltage was observed. The reason for the voltage behaviors was that the interfacial roughness influenced the spin-asymmetry of spin-polarized electrons in ferromagnetic metals. The diffuse scattering to the electrons turned to specular scattering when the interface became flat, leading to the variation of resistivity change (Δρ). As the increase in t Cu, the extremum field was reduced because of the weaken exchange coupling between NiFe and IrMn layers.

Effects of interface roughness on photoluminescence full width at half maximum in GaN/AlGaN quantum wells

Low temperature photoluminescence (PL) measurements have been performed for a set of GaN/AlxGa1−xN quantum wells (QWs). The experimental results show that the optical full width at half maximum (FWHM) increases relatively rapidly with increasing Al composition in the AlxGa1−xN barrier, and increases only slightly with increasing GaN well width. A model considering the interface roughness is used to interpret the experimental results. In the model, the FWHM's broadening caused by the interface roughness is calculated based on the triangle potential well approximation. We find that the calculated results accord with the experimental results well.

The influence of the AlN barrier thickness on the polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

The electron mobility scattering mechanisms in AlN/GaN heterostuctures with 3 nm and 6 nm AlN barrier thicknesses were investigated by temperature-dependent Hall measurements. The effect of interface roughness (IFR) scattering on the electron mobility was found to be enhanced by increasing AlN barrier thickness. Moreover, using the measured capacitance-voltage and current-voltage characteristics of the fabricated heterostructure field-effect transistors (HFETs) with different Schottky areas on the two heterostuctures, the variations of electron mobility with different gate biases were investigated. Due to enhanced IFR scattering, the influence of polarization Coulomb field (PCF) scattering on electron mobility was found to decrease with increasing AlN barrier layer thickness. However, the PCF scattering remained an important scattering mechanism in the AlN/GaN HFETs.

Modeling range dependent sediment and interface roughness effects on propagation loss with finite elements

At the Target and Reverberation Experiment (TREX) off the coast of the Florida panhandle in May 2013, propagation loss was measured over a track with significant sediment and interface roughness variability. In this study, the finite element method is applied to model the effects of sediment and interface roughness variability on transmission loss. Finite elements provide a full wave solution to the Helmholtz equation to the accuracy of the discretization. Therefore, it provides both forward and backward propagation fields. Where available, data will be taken from TREX environment measurements. Additionally, the results will be compared with an energy loss model which relies on the product of the range dependent reflection coefficient. [Work supported by ONR, Ocean Acoustics.]

Spontaneous emission of Bloch oscillation radiation under the competing influences of microcavity enhancement and inhomogeneous interface degradation

A theory for the spontaneous emission (SE) of terahertz radiation for a Bloch electron traversing a single energy miniband of a superlattice (SL) in a cavity, while undergoing elastic scattering is presented. The Bloch electron is accelerated under the influence of a superimposed external constant electric field and an internal inhomogeneous electric field, while radiating into a microcavity. The analysis of the SE accounts for both the spectral structure of nonharmonic miniband components and the Bloch oscillation degradation effects arising from elastic scattering due to SL interface roughness. The interface roughness effects are decomposed into contributions arising from independent planar and cross-correlated neighboring planar interfaces; parametric numerical estimates show that the cross-correlated contribution to the SE relaxation rate is relatively small, representing less than roughly 10% of the total relaxation rate. It is shown that the degradation effects from SL interface roughness can be more than compensated for by the enhancements derived from microcavity-based tuning of the emission frequency to the cavity density of states peak. The theoretical approach developed herein has general applicability beyond its use for elastic scattering due to interface roughness. As well, the results obtained in this analysis can be useful in the development of SL-based Bloch-oscillator terahertz devices.

A high carrier injection terahertz quantum cascade laser based on indirectly pumped scheme

A Terahertz quantum cascade laser with a rather high injection coupling strength based on an indirectly pumped scheme is designed and experimentally implemented. To effectively suppress leakage current, the chosen quantum cascade module of the device is based on a five-well GaAs/Al0.25Ga0.75As structure. The device lases up to 151 K with a lasing frequency of 2.67 THz. This study shows that the effect of higher energy states in carrier transport and the long-range tunnel coupling between states that belong to non-neighbouring modules have to be considered in quantum design of structures with a narrow injector barrier. Moreover, the effect of interface roughness scattering between the lasing states on threshold current is crucial.

Impact of interface roughness on the performance of broadband blackbody absorber based on dielectric-metal film multilayers

We report on factors affecting the performance of a broadband, mid-IR absorber based on multiple, alternating dielectric / metal layers. In particular, we investigate the effect of interface roughness. Atomic layer deposition produces both a dramatic suppression of the interface roughness and a significant increase in the optical absorption as compared to devices fabricated using a conventional thermal evaporation source. Absorption characteristics greater than 80% across a 300 K black body spectrum are achieved. We demonstrate a further increase in this absorption via the inclusion of a patterned, porous anti-reflection layer.

Influence of Topcoat-Bondcoat Interface Roughness on Stresses and Lifetime in Thermal Barrier Coatings

Failure in Atmospheric Plasma-Sprayed (APS) thermal barrier coatings (TBCs) is associated with the thermo-mechanical stresses developing due to the thermally grown oxide (TGO) layer growth and thermal expansion mismatch during thermal cycling. The interface roughness has been shown to play a major role in the development of these induced stresses and lifetime of TBCs. Modeling has been shown as an effective tool to understand the effect of interface roughness on induced stresses. In the previous work done by our research group, it was observed that APS bondcoats performed better than the bondcoats sprayed with High Velocity Oxy-Fuel process which is contrary to the present literature data. The objective of this work was to understand this observed difference in lifetime with the help of finite element modeling by using real surface topographies. Different TGO layer thicknesses were evaluated. The modeling results were also compared with existing theories established on simplified sinusoidal profiles published in earlier works. It was shown that modeling can be used as an effective tool to understand the stress behavior in TBCs with different roughness profiles.

Surfactant influence on interface roughness and magnetoresistance value in Fe/Cr multilayers

Effect of interfacial roughness on magnetoresistance (MR) value in Fe/Cr multilayers was studied. The Fe/Cr multilayers were prepared on a MgO(100) substrate by molecular beam deposition technique and doped with Bi and In surfactants. The morphology of the layers, especially the interface structure, was studied using conversion electron Mössbauer spectroscopy (CEMS). CEMS spectra indicate an increase in the width of interfaces of modified samples; this effect is stronger for the sample doped with In. Auger electron spectroscopy shows the segregation of the surfactant to the surface. Magnetic and magnetotransport properties of the samples were studied by the magneto-optic Kerr effect and MR measurements. The observed hysteresis loops confirm in all samples antiferromagnetic arrangement of Fe layers sandwiched between Cr while the value of MR is larger for the doped samples. It was observed that roughening of the interfaces resulted in an increase of the MR effect.

Minimum thermal conductivity in superlattices: A first-principles formalism

The thermal conductivity of silicon-germanium superlattices is computed from density-functional perturbation theory using relaxation times that include both anharmonic and interface roughness effects. A decrease in the group velocity of low-frequency phonons in addition to the interface-disorder-induced scattering of high-frequency phonons drives the superlattice thermal conductivity to below the alloy limit. At short periods, interplay between decrease in group velocity and increase in phonon lifetimes with increase in superlattice period leads to a minimum in the cross-plane thermal conductivity. Increasing the mass mismatch between the constituent materials in the superlattice further lowers the thermal conductivity below the alloy limit, pointing to avenues for higher efficiency thermoelectric materials.

Influence of interface roughness on the exchange bias of Co/CoO multilayers

The effect of interface roughness on the magnetic properties of ferromagnetic/antiferromagnetic multilayer has been studied by comparing the interfaces within identical multilayer (lower vs. upper) or from different Co/CoO multilayers (different deposition sequence). It has been found that for identical multilayer, the upper Co/CoO interfaces grow rougher and show stronger exchange bias than the lower Co/CoO interfaces. Structural analyses indicate that the successive layer deposition gives rise to a cumulative roughness at the upper interface, which affects the magnetic properties of ferromagnetic layer and its coupling to the antiferromagnetic layers. The interface roughness strengthens the interfacial exchange coupling by the increase of defect-generated uncompensated antiferromagnetic spins; such spins form coupling with the spins from Co layer at the interface. Due to the different coupling strength of each Co layer to the neighboring CoO layers, the multilayer showed distinct switching fields during the magnetization reversal process. Our results imply the possibility to control the strength of exchange bias, coercivity, and related magnetic properties by tuning the interface roughness.

Viscoplastic Model with Zero-Thickness Interface Elements for Lateral Earth Pressure on Wall Due to Line Load

This work presents a viscoplastic model for predicting earth pressure distribution generated behind an instrumented wall due to line loads in addition to backfill. Eight-nodded two-dimensional quadrilateral elements are adopted for idealization of wall and soil, whereas, the interface between them is idealized by six-nodded zero thickness interface elements. The nonlinear behavior of soil is modeled as viscoplastic or time-dependent with Mohr–Coulomb criterion, while the wall is modeled as elastic material. The solution employed is based on 2 × 2 Gauss integration rule and incremental procedure with iterations using the explicit time-marching scheme. The results indicate significant effects of initial stresses, interface roughness, and both position and magnitude of line loads on lateral earth pressure distribution along the wall, in addition to, the applicability of the model and the efficiency of the computer program developed.