Thin Spacer Research Articles

An Assessment of the Lateral Selective Etching, with HCl, of High Versus Low Ge Content SiGe Layers

We have evaluated the feasibility of selectively etching, with gaseous HCl, high versus low Ge content SiGe layers. To that end, we have grown {20 to 25 nm thick Si / ~ 12-15 nm thick Si0.8Ge0.2 or Si0.75Ge0.25 / ~ 11-14 nm thick Si0.5Ge0.5} multilayers on SOI substrates. After a low temperature capping with SiN, such stacks were patterned into regular arrays of square mesas or lines with a stop on the buried oxide, to laterally etch the layers of interest. We first determined, at 500°C, 600 Torr, the (001) blanket HCl etch rates of SiGe 30% and 40%: 1.4 and 5.3 nm min.-1. We then switched over to patterned wafers. The lateral selective etching of Si0.5Ge0.5 versus Si0.8Ge0.2 (and Si) was feasible and the following features highlighted: (i) Si0.5Ge0.5 lateral etch rates along the <110> directions were much smaller than in the vertical [001] direction and (ii) because of micro-loading, lateral etch rates were definitely higher for squares than for lines (0.9-1.1 nm min.-1 for lines, to be compared with 1.7–2.1 nm min.-1 for squares). Meanwhile, the lateral selective etching of Si0.5Ge0.5 versus Si0.75Ge0.25 was far from being perfect, with much higher Si0.5Ge0.5 etch rates and a significant Si0.75Ge0.25 recessing at tunnel entrances. Si0.75Ge0.25 floors and ceilings were otherwise far more etched and thus tilted than Si0.8Ge0.2 ones. Inserting very thin Si spacers between Si0.5Ge0.5 cores and Si0.75Ge0.25 claddings partly solved those issues, with no recessing and lesser etchings at the entry points of tunnels.

Visible and near-infrared modulating tungsten suboxide nanorods electrochromic films in acidic aqueous electrolytes

Proton-based aqueous electrolytes can be used to achieve high performance electrochromic nanocrystal thin films due to their small ion size. However, acidic aqueous electrolyte systems have not yet been explored in near-infrared (NIR) absorbing plasmonic tungsten oxide nanocrystal films. Here, we demonstrate tungsten suboxide nanorod films with excellent visible and NIR modulation performance in the H+-based aqueous electrolytes, thanks to their mesoporous structure, nanosized domains, and open tunnel structure. Colloidally synthesized WO2.83 nanorods with an average width of 6 nm and length of 48 nm were converted to WO2.90 nanorod film via annealing in air, while still preserving open tunnels. These films exhibit fast switching speed (tc = 0.9 s, tb = 2.1 s), excellent cycling stability over 2500 cycles, wide optical modulation up to ΔT = 53.8 % in the NIR region, and a high coloration efficiency (CE) of 167 cm2 C⁻1 at 1300 nm. Additionally, introducing a thin spacer (25 μm) reduced intrinsic NIR absorption from water, thereby enhancing the NIR modulation properties. These highly performing aqueous proton-electrolytes-based electrochromic devices open new possibilities for implementing visible and NIR electrochromism.

Parametric analysis of reverse electrodialysis process for improved power generation: Influence of spacer thickness, feed solution flow rate, membrane, and concentration

Reverse electrodialysis (RED) technology produces electrical energy from a salinity gradient by transporting ions through ion-exchange membranes (IEM). Although research into this technology has recently seen considerable growth, there is still interest in improving its efficiency. As a result, a mathematical model to study the RED process was developed using Aspen Custom Modeler software, focusing on three membranes (Fumasep (FAD/FKD), Fujifilm Type 10, and Selemion (AMV/CMV)). A parametric analysis was conducted, considering spacer thickness, salinity gradient, and feed-solution flow rates. Evaluation of internal resistance, gross power density, and net power density was also performed. Results showed that using a thicker spacer (270 μm) in the high compartment and a thinner spacer (150 μm) with a high Reynolds number in the low compartment led to higher net power densities of 2.17 W·mcp−2 and 6.74 W·mcp−2 for Fumasep (FAD/FKD) membrane with (brackish/seawater) and (brackish/brine), respectively. Among the membranes tested, Fumasep (FAD/FKD) and Fujifilm Type 10 had similar results, with the former slightly exceeding the latter. However, the Selemion membrane (AMV/CMV) performed poorly in all tests.

A gate tunable transmon qubit in planar Ge

Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.

InAs/GaAs SK quantum dots stacking: Impact of spacer layer on optical properties

In this paper, we report the optical properties of vertically stacked multilayers of self-organized InAs/GaAs quantum dots QDs with different GaAs spacer layer thicknesses of 10 and 15 nm, using photoluminescence PL measurement. The 10 K PL spectrum exhibits double-emission peaks ambiguously identified in PL spectra where the excitation power dependence reveals that these emission peaks are attributed to fundamental ground state GS of large quantum dots in the low energy side, and first excited state 1ES transitions of large quantum dots in coincidence with fundamental ground state GS of smaller ones, in the high energy side. Both the excitation power dependence and the temperature dependence measurement results exhibited competition between the latter's optical transitions in the QDs, which becomes more prevalent at higher excitation powers. The main PL peak is quenched above 190 K, giving rise to “a 1D miniband”, ascribed to the electronic coupling between QDs along the stacking direction and tuning the emission wavelength around 1.3 μm.Our results emphasize the Key role of the vertically stacked InAs/GaAs QDs structures with thin GaAs spacer layers in optimizing design for long-wavelength devices.

Multi-wavelength and broadband plasmonic switching with V-shaped plasmonic nanostructures on a VO2 coated plasmonic substrate

In this paper, periodic arrays of identical V-shaped gold nanostructures and variable V-shaped gold nanostructures are designed on top of a gold-coated silicon dioxide (SiO2) substrate with a thin spacer layer of vanadium dioxide (VO2) to realize multi-wavelength and broadband plasmonic switches, respectively. The periodic array of identical V-shaped nanostructures (IVNSs) with small inter-particle separation leads to coupled interactions of the elementary plasmons of a V-shaped nanostructure (VNS), resulting in a hybridized plasmon response with two longitudinal plasmonic modes in the reflectance spectra of the proposed switches when the incident light is polarized in the x-direction. The x-direction is oriented along the axis that joins the V-junctions of all VNSs in one unit cell of the periodic array. On exposure to temperature, electric field, or optical stimulus, the VO2 layer transforms from its monoclinic semiconducting state to its rutile metallic state, leading to an overall change in the reflectance spectra obtained from the proposed nanostructures and resulting in an efficient multi-wavelength switching action. Finite difference time domain modelling is employed to demonstrate that an extinction ratio (ER) >12 dB at two wavelengths can be achieved by employing the proposed switches based on periodic arrays of IVNSs. Further, plasmonic switches based on variable V-shaped nanostructures—i.e. multiple VNSs with variable arm lengths in one unit cell of a periodic array—are proposed for broadband switching. In the broadband operation mode, we report an ER >5 dB over an operational wavelength range >1400 nm in the near-IR spectral range spanning over all optical communication bands, i.e. the O, E, S, C, L and U bands. Further, it is also demonstrated that the wavelength of operation for these switches can be tuned by varying the geometrical parameters of the proposed switches. These switches have the potential to be employed in communication networks where ultrasmall and ultrafast switches with multi-wavelength operation or switching over a wide operational bandwidth are inevitably required.

Silicon nanoparticle-based near-infrared surface enhanced fluorescence without any “dielectric spacer”

For the first time, herein, we report the theoretical investigation on silicon (Si) nanoparticle-based near-infrared (NIR) surface-enhanced fluorescence (SEF). Initially, the scattering spectra of single spherical-shaped Si nanoparticles of different sizes are plotted and characterized all the observed modes using the multipolar decomposition method. Later, the electric field intensity enhancement (EFIE) distribution inside and outside Si nanoparticles is plotted at the wavelengths of electric and magnetic type modes. Finally, the SEF enhancement (χSEF) is estimated by varying the excitation wavelength (λex), fluorescence wavelength (λem), and separation (d) between the fluorophore and Si nanoparticle of different sizes. The χSEF is found to vary from 1 to 3 orders of magnitude when the λem falls in the NIR region. In contrast to the plasmonic or metal nanoparticle-based SEF, the maximum χSEF is observed when d = 0. This indicates that the thin dielectric spacers between the fluorophores and Si nanoparticles are not required to obtain the enhancement in the case of the Si nanoparticle-based SEF technique. This can be considered as a significant advantage over the conventional metal nanoparticle-based SEF, where dielectric spacers are mandatory. Finally, the average SEF enhancement ⟨χSEF⟩ is also estimated.

Dynamic susceptibility of dipolar coupled magnetic vortices

We report a calculation of the dynamic susceptibility of a pair of ferromagnetic circular nanocylinders stacked along the common axis, one on the other, and separated by a thin nonmagnetic spacer. Our theoretical model considers the dipolar energy without restrictions on dipolar sums, along with the anisotropy and exchange energies. Our results indicate that the nanocylinders dipolar interaction may affect the susceptibility spectrum. We have found, for instance, that a 30nm thick, 70nm diameter Fe nanocylinder holds a single magnetic vortex, and the planar susceptibility (χxx) spectrum displays two low-frequency peaks (at 0.37 and 1.00 GHz). We have also found that the χxx susceptibility spectrum of dipolar-coupled vortices in a pair of Fe nanocylinders with a 5 nm spacer exhibits peaks at 0.3, 0.7, 1.2, and 1.4 GHz. Furthermore, the relative weight of the peaks is controlled by the degree of spatial localization of these excitations.

Proximity Effect-Induced Magnetoresistance Enhancement in a Fe3GeTe2/NbSe2/Fe3GeTe2 Magnetic Tunnel Junction.

The coupling between van der Waals-layered magnetic and superconducting materials holds the possibility of revealing novel physical mechanisms and realizing spintronic devices with new functionalities. Here, we report on the realization and investigation of a maximum ∼17-fold magnetoresistance (MR) enhancement based on a vertical magnetic tunnel junction of Fe3GeTe2 (FGT)/NbSe2/FGT near the NbSe2 layer's superconducting critical temperature (TC) of 6.8 K. This enhancement is attributed to the band splitting in the atomically thin NbSe2 spacer layer induced by the magnetic proximity effect on the material interfaces. However, the band splitting is strongly suppressed by the interlayer coupling in the thick NbSe2 layer. Correspondingly, the device with a thick NbSe2 layer displays no MR increase near TC but a current dependent on transport properties at extremely low temperatures. This work carefully investigates the mechanism of MR enhancement, paving an efficient way for the modulation of spintronics' properties and the achievement of spin-based integrated circuits.

Ultra-thin metal-free terahertz absorber for electromagnetic shielding

An ultrathin broadband absorber covering the frequency range of 0.1 to 10 terahertz (THz) and to THz with more than 80% and 90% absorption, respectively is designed and numerically analysed. The absorber structure utilizes a partial reflecting surface and reflector of graphene separated by a thin dielectric spacer providing the metal-free structure. Multiple resonances are generated in the absorber structure whose spectrum can be separated or merged to find the narrow or broad absorption spectrum. The proposed absorber operates with the combine effect of the electromagnetic (EM) resonances occurring in the graphene resonator and the destructive interference taking place in the dielectric spacer. The absorber operation is validated through the theory of electromagnetic coupled mode resonances and with the theory of multiple reflections. The narrow absorption peaks can allow the proposed absorber to be utilized as refractive index sensor and the broad absorption spectrum can allow it to be utilized in the EM shielding and packaging applications.

Time-Lapse GPR Measurements to Monitor Resin Injection in Fractures of Marble Blocks.

The objective of this study is to test the feasibility of time-lapse GPR measurements for the quality control of repairing operations (i.e., injections) on marble blocks. For the experimental activities, we used one of the preferred repairing fillers (epoxy resin) and some blocks from one of the world's most famous marble production area (Carrara quarries in Italy). The selected blocks were paired in a laboratory by overlapping one over the other after inserting very thin spacers in order to simulate air-filled fractures. Fractures were investigated with a 3 GHz ground-penetrating radar (GPR) before and after the resin injections to measure the amplitude reduction expected when the resin substitutes the air. The results were compared with theoretical predictions based on the reflection coefficient predicted according to the thin bed theory. A field test was also performed on a naturally fractured marble block selected along the Carrara shore. Both laboratory and field tests validate the GPR as an effective tool for the quality control of resin injections, provided that measurements include proper calibration tests to control the amplitude instabilities and drift effects of the GPR equipment. The method is accurate enough to distinguish the unfilled fractures from the partially filled fractures and from the totally filled fractures. An automatic algorithm was developed and successfully tested for the rapid quantitative analysis of the time-lapse GPR profiles collected before and after the injections. The whole procedure is mature enough to be proposed to the marble industry to improve the effectiveness of repair interventions and to reduce the waste of natural stone reserves.

Hysteresis of magnetization statics and dynamics in [Pt/Co] multilayer

The magnetic multilayer of Co separated by thin spacer layer of Pt was deposited by DC-magnetron sputtering. From the longitudinal magneto-optical Kerr effect based magnetometry and microscopy as well as magnetic force microscopy, the hybrid magnetization structure was deduced: the large size, micrometer scale magnetic domains with in-plane “core magnetization” patterned by nanometer scale domains with out-of-plane components. The hysteresis as a function of in-plane applied magnetic field of both: (i) magnetization curve measured by Superconducting Quantum Interference Device and (ii) dynamic responses measured by broadband Vector Network Analyzer spectroscopy were observed. The experimental results are well described by micromagnetic simulations. These magnetic history dependent effects were explained by magnetization cores, with in plane component, switching.

Highly Efficient Spin Transport in a Paramagnetic Insulator at Room Temperature

AbstractThe exploration of new materials exhibiting excellent spin transport properties, particularly those capable of efficiently transmitting pure spin currents at room temperature, is a crucial aspect of spintronics. This work reports the observation of room‐temperature spin transport in a paramagnetic insulator Gd3Ga5O12 (GGG). By measuring the longitudinal spin Seebeck voltage, spin Hall magnetoresistance, and anomalous Hall resistance in a Y3Fe5O12 (YIG)/GGG/Pt heterostructure at room temperature, the spin current is found to propagate in GGG up to a distance of 7 nm through the paramagnons, which are the collective excitations of the local spin within the paramagnet. Remarkably, this spin propagation phenomenon occurs at a small magnetic field and irrespective of whether the spin current injected into GGG is thermally induced from YIG or electrically generated through the spin Hall effect of Pt. Moreover, the inclusion of a thin GGG spacer layer increases the thermal spin current from YIG to Pt by 31%. Calculations based on the diffusive magnon transport model indicate high spin conductance at the GGG/Pt interface. These findings highlight the potential of a wide range of paramagnetic materials in facilitating spin transport and advancing the development of spintronic devices.

Strong Acousto-Plasmonic Coupling in Film-Coupled Nanoparticles Mediated by Surface Acoustic Waves

The interaction between phonons and localized plasmons in film-coupled nanoparticles designs can be exploited both for modulating the scattered electromagnetic field and the understanding of the mechanical vibrations at nanoscale. In this paper, we show by finite element numerical analysis an enhanced optomechanical interaction in a film-coupled gold nanoridges or pillars mediated by surface acoustic waves. The metallic nanoparticles are placed atop a multilayer structure consisting of a thin dielectric spacer covering a gold film layer on a silicon dioxide/or silicon substrate. Optical simulations reveal the existence of surface localized plasmons in the infrared range confined under the nanoparticles in the dielectric spacer and/or in between such particles. Optomechanical coupling between the plasmonic modes and localized phonons is evaluated from the shift in the plasmon eigenfrequency. It is found that the compressional, the in-phase compressional and the out-of-phase flexural modes, yield the highest coupling rates. Such phonons are excited by means of SAW launched from the system inlet in front of the particles. The findings in this paper could help design new generation of acousto-optic modulators monitored by fast coherent surface acoustics.

Nonreciprocal transmission of magnetoacoustic waves in compensated synthetic antiferromagnets

We investigate the interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a Pt/Co($2\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$)/Ru($0.85\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$)/Co($2\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$)/Pt compensated synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with equal thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interlayer dipolar coupling fields and interfacial Dzyaloshinskii--Moriya interaction (iDMI), the optical SW mode shows a large nondegenerate dispersion relation for counter-propagating SWs. Due to resonant SAW-SW interaction, we observe a nonreciprocal SAW transmission in the prepared piezoelectric/SAF hybrid device. We demonstrate that the nonreciprocity of the SAW transmission in symmetric SAFs with equal thicknesses of the magnetic layers can show a substantially different characteristic behavior in comparison to asymmetric SAFs or magnetic single layers with iDMI. For the prepared SAF, the nonreciprocal shift of the magnetoacoustic resonance fields and the magnetoacoustic SW excitation efficiency depend on the external magnetic field sweep direction. For one magnetic field sweep direction and angle of the magnetic field, the resonance fields of the waves propagating in one direction are larger than for the waves propagating in the opposite direction. In addition, the magnitude of the nonreciprocal field shift is at minimum if the external magnetic field is aligned perpendicular to the SW propagation direction. The experimental results are in agreement with a phenomenological SAW-SW interaction model.

Low Dark Current Operation in InAs/GaAs(111)A Infrared Photodetectors: Role of Misfit Dislocations at the Interface.

We demonstrate an extended short-wave infrared (e-SWIR) photodetector composed of an InAs/GaAs(111)A heterostructure with interface misfit dislocations. The layer structure of the photodetector consists simply of an n-InAs optical absorption layer directly grown with a thin undoped-GaAs spacer layer on n-GaAs by molecular beam epitaxy. The lattice mismatch was abruptly relaxed by forming a misfit dislocation network at the initial stage of the InAs growth. We found high-density threading dislocations (1.5 × 109 cm-2) in the InAs layer. The current-voltage characteristics of the photodetector at 77 K had a very low dark current density (<1 × 10-9 A cm-2) at a positive applied voltage (electrons flow from n-GaAs to n-InAs) of up to ∼+1 V. Simulation of the band structure revealed that the direct connection of GaAs and InAs and the formation of interfacial states by the misfit dislocations play significant positive roles in suppressing dark current. Under illumination with e-SWIR light at 77 K, a clear photocurrent signal was observed with a 2.6 μm cutoff wavelength, which is consistent with the bandgap of InAs. We also demonstrated e-SWIR detection at room temperature with a 3.2 μm cutoff wavelength. The maximum detectivity at 294 K exceeds 2 × 108 cm Hz0.5 W-1 for the detection of e-SWIR light at 2 μm.

Enhanced efficiency of launching hyperbolic phonon polaritons in stacked α-MoO3 flakes.

In this work, we reported a systemic study on the enhanced efficiency of launching hyperbolic phonon polaritons (PhPs) in stacked α-phase molybdenum trioxide (α-MoO3) flakes. By using the infrared photo-induced force microscopy (PiFM), real-space near-field images (PiFM images) of mechanically exfoliated α-MoO3 thin flakes were recorded within three different Reststrahlen bands (RBs). As referred with PiFM fringes of the single flake, PiFM fringes of the stacked α-MoO3 sample within the RB 2 and RB 3 are greatly improved with the enhancement factor (EF) up to 170%. By performing numerical simulations, it reveals that the general improvement in near-field PiFM fringes arises from the existence of a nanoscale thin dielectric spacer in the middle part between two stacked α-MoO3 flakes. The nanogap acts as a nanoresonator for prompting the near-field coupling of hyperbolic PhPs supported by each flake in the stacked sample, contributing to the increase of polaritonic fields, and verifying the experimental observations Our findings could offer fundamental physical investigations into the effective excitation of PhPs and will be helpful for developing functional nanophotonic devices and circuits.

Optimized emitter-base interface cleaning for advanced Heterojunction Bipolar Transistors

Heterojunction Bipolar Transistors needed for high-frequency applications require precise dopant control. In-situ doped epitaxies used during device fabrication rely on surface preparation to obtain an optimized doping profile. Defects due to imperfect cleaning relates to high emitter resistance and low-frequency noise.Base epitaxy is followed by the fabrication of thin L-shaped oxide spacers and in-situ arsenic-doped emitter epitaxy by Low Pressure Chemical Vapor Deposition. A cleaning step between spacers formation and emitter epitaxy is mandatory to remove residual contamination. Hydrofluoric acid (HF) wet cleaning, in-situ remote plasma cleaning (SiconiTM) and thermal treatments have been tested. A minimum etching budget is sought for limiting spacer consumption while adequately cleaning the interface.Time Of Flight-Secondary Ion Mass Spectroscopy (TOF-SIMS) and High-Resolution Transmission Electron Microscopy (HR-TEM) are used to characterize the interface and measure levels of arsenic, oxygen, fluorine and carbon. Emitter resistance and base-emitter breakdown voltage measurements are used to show the impact on electrical figures of merit. Siconi targeting 10 Å of thermal oxide removal followed by a thermal treatment (800 °C, 60 s) in the deposition chamber results the best process among the tested ones.

Multilayered membrane spacer: does it enhance solution mixing?

AbstractThe present review systematically investigates and illustrates the effect of multilayered membrane spacers on the features of fluid dynamics that influence all performance metrics. Multilayer spacers are frequently composed of three sets of filaments (i.e., top, middle, and bottom layers), which has the benefit of increasing mass transfer and decreasing membrane surface fouling when compared to ordinary monolayer (e.g., extruded spacer) and two‐layer spacers. The review found that the multilayer spacer's middle layer disperses primary flow to the thin side spacers placed near the membrane's surfaces. The thin side spacers will then form narrow passageways to keep the solution in situ for as long as mass transfer is achievable. The employment of thin spacers close to the membranes at satisfactory operational conditions (e.g., adequate flow velocity) results in swirling flows and incorporation of transverse and longitudinal eddies near to the membranes, reducing the boundary layer's width and making the associated ion concentration domain at the membranes much more consistent. The concept and implementation of multilayer geometry in feed channels appears to be promising, since a multilayered spacer can function at a lower maximum flow velocity than normal two‐layer spacers, saving operational energy while minimizing concentration gradients at the membrane surfaces. Furthermore, the multilayer structure's durability and mechanical strength may help to reduce membrane deformation and maintain long processes. Future studies might look at significantly reducing spacer thickness for industrial uses. © 2023 Society of Chemical Industry (SCI).

Nonreciprocal magnetoacoustic waves in synthetic antiferromagnets with Dzyaloshinskii-Moriya interaction

The interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a piezoelectric/magnetic thin film heterostructure yields potential for the realization of novel microwave devices and applications in magnonics. In the present work, we investigate the SAW-SW interaction in a Pt/Co(2 nm)/Ru(0.85 nm)/Co(4 nm)/Pt synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with different thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interfacial Dzyaloshinskii--Moriya interaction (iDMI) and interlayer dipolar coupling fields, the optical SW mode shows a large nondegenerate dispersion relation for oppositely propagating SWs. Due to SAW-SW interaction, we observe nonreciprocal SAW transmission in the piezoelectric/SAF hybrid device. The equilibrium magnetization directions of both Co layers are manipulated by an external magnetic field to set a ferromagnetic, canted, or antiferromagnetic configuration. This has a strong impact on the SW dispersion, its nonreciprocity, and SAW-SW interaction. The experimental results are in agreement with a phenomenological SAW-SW interaction model, which considers the interlayer exchange coupling, iDMI, and interlayer dipolar coupling fields of the SWs.