Si Layer Research Articles

Signature of an ultrafast photoinduced Lifshitz transition in the nodal-line semimetal ZrSiTe

Here we report an ultrafast optical spectroscopic study of the nodal-line semimetal ZrSiTe. Our measurements reveal that, converse to other compounds of the family, the sudden injection of electronic excitations results in a strongly coherent response of an $A_{1g}$ phonon mode which dynamically modifies the distance between Zr and Te atoms and Si layers. "Frozen phonon" DFT calculations, in which band structures are calculated as a function of nuclear position along the phonon mode coordinate, show that large displacements along this mode alter the material's electronic structure significantly, forcing bands to approach and even cross the Fermi energy. The incoherent part of the time domain response reveals that a delayed electronic response at low fluence discontinuously evolves into an instantaneous one for excitation densities larger than $3.43 \times 10^{17}$ cm$^{-3}$. This sudden change of the dissipative channels for electronic excitations is indicative of an ultrafast Lifshitz transition which we tentatively associate to a change in topology of the Fermi surface driven by a symmetry preserving $A_{1g}$ phonon mode.

Investigation of the thermal tolerance of silicon-based lateral spin valves

Improvement in the thermal tolerance of Si-based spin devices is realized by employing thermally stable nonmagnetic (NM) electrodes. For Au/Ta/Al electrodes, intermixing between Al atoms and Au atoms occurs at approximately 300 °C, resulting in the formation of a Au/Si interface. The Au–Si liquid phase is formed and diffuses mainly along an in-plane direction between the Si and AlN capping layers, eventually breaking the MgO layer of the ferromagnetic (FM) metal/MgO electrodes, which is located 7 µm away from the NM electrodes. By changing the layer structure of the NM electrode from Au/Ta/Al to Au/Ta, the thermal tolerance is clearly enhanced. Clear spin transport signals are obtained even after annealing at 400 °C. To investigate the effects of Mg insertion in FM electrodes on thermal tolerance, we also compare the thermal tolerance among Fe/Co/MgO, Fe/Co/Mg/MgO and Fe/Co/MgO/Mg contacts. Although a highly efficient spin injection has been reported by insertion of a thin Mg layer below or above the MgO layer, these thermal tolerances decrease obviously.

Origin of defect luminescence in ultraviolet emitting AlGaN diode structures

Light emitting diode structures emitting in the ultraviolet spectral range are investigated. The samples exhibit defect luminescence bands. Synchrotron-based photoluminescence excitation spectroscopy of the complicated multi-layer stacks is employed to assign the origin of the observed defect luminescence to certain layers. In the case of quantum well structures emitting at 320 and 290 nm, the n-type contact AlGaN:Si layer is found to be the origin of defect luminescence bands between 2.65 and 2.8 eV. For 230 nm emitters without such n-type contact layer, the origin of a defect double structure at 2.8 and 3.6 eV can be assigned to the quantum wells.

Unoccupied electronic states of 2D Si on Ag--Si(111)

Optimizing substrate characterization to grow 2D Si layers on surfaces is a major issue toward the development of synthesis techniques of the promising silicene. We have used inverse photoemission spectroscopy (IPES) to study the electronic band structure of an ordered 2D Si layer on the -Ag/Si(111) surface (-Ag). Exploiting the large upwards band bending of the -Ag substrate, we could investigate the evolution of the unoccupied surface and interface states in most of the Si band gap. In particular, the k ∥-dispersion of the -Ag free-electron-like S 1 surface state measured by IPES, is reported for the first time. Upon deposition of ∼1 ML Si on -Ag maintained at ∼200 °C, the interface undergoes a metal-insulator transition with the complete disappearance of the S 1 state. The latter is replaced by a higher-lying state U 0 with a minimum at 1.0 eV above E F. The origin of this new state is discussed in terms of various Si 2D structures including silicene.

Synthesis of superlattice heterostructure of germanium quantum dots in silicon spacer layers and its application in photovoltaic solar cells

Superlattice heterostructure of dislocation-free Ge quantum dots buried in Si spacer layers has been synthesized using ultra high vacuum-chemical vapor deposition technique. The heterostructure has been used as active layer in Si solar cells based upon vertical p-i-n junction configuration. Experiment shows that such solar cells exhibit strong repose to infrared radiation in contrast with the negligible response of solar cells with active layers of pure Si having the same thickness. A solar cell with 300 nm thick superlattice is shown to have thermodynamic efficiency of about 11.2%. This is correlated with the rather high quality of the heterostructure used in the present study as judged from photoluminescence spectroscopy and electron microscopy as well as strain and dark current measurements. It is concluded that such type of heterostructure may provide a pathway to develop efficient and cost effective solar cells. • Superlattice heterostructure of Ge quantum dots on Si has been synthesized. • Various techniques have been used to characterize the nanostructures. • The heterostructure has been used as active layer in Si (p-i-n)-type solar cells. • The thermodynamic efficiency of respective solar cell is determined.

Interfacial microstructure characterization and strength evaluation of Si3N4/Si3N4 joints with Si–Mg composite filler for high-temperature applications

Silicon-nitride (Si3N4) components were joined under vacuum at 1100 °C for 10 min using Si–Mg composite fillers with Mg contents (XMg) that ranged from 0 at.% to 59 at.%. The Si3N4/Si3N4 joints were fabricated via Si layer formation at the joint interface; the molten Si–Mg liquid was transformed into a solid Si layer after Mg-evaporation-induced isothermal solidification. The joint tensile strength at room temperature increased considerably when XMg exceeded the liquidus composition of 37 at.% because of the enhanced densification/thinning of the Si layer. In these cases, some Mg atoms reacted with Si3N4 to form a fine-grained MgSiN2-based layer, whereas relatively large (>0.1 μm) and smaller MgO precipitates (<10 nm) were observed in the Si layer. At a high XMg, the MgO precipitates were arranged in a network-like structure, which improved the fracture strength of the Si layer. The joints with a high strength at room temperature were examined using a three-point bending test at 1200 °C in air and endured a maximum fracture stress of ~200 MPa, which confirmed their potential for use in oxidizing atmospheres at least 100 °C above the bonding temperature.

High-Temperature Tribological Performance of Al2O3/a-C:H:Si Coating in Ambient Air

The study investigates thermal stability and high temperature tribological performance of a-C:H:Si diamond-like carbon (DLC) coating. A thin alumina layer was deposited on top of the a-C:H:Si coating to improve the tribological performance at high temperatures. The a-C:H:Si coating and alumina layer were prepared using plasma-activated chemical vapour deposition and atomic layer deposition, respectively. Raman and X-ray photoelectron spectroscopy were used to investigate the structures and chemical compositions of the specimens. The D and G Raman peaks due to sp2 bonding and the peaks corresponding to the trans-polyacetylene (t-Pa) and sp bonded chains were identified in the Raman spectra of the a-C:H:Si coating. Ball-on-disc sliding tests were carried out at room temperature and 400 °C using Si3N4 balls as counter bodies. The a-C:H:Si coating failed catastrophically in sliding tests at 400 °C; however, a repeatable and reproducible regime of sliding with a low coefficient of friction was observed for the Al2O3/a-C:H:Si coating at the same temperature. The presence of the alumina layer and high stress and temperature caused structural changes in the bulk a-C:H:Si and top layers located near the contact area, leading to the modification of the contact conditions, delivering of extra oxygen into the contact area, reduction of hydrogen effusion, and suppression of the atmospheric oxidation.

Guided-mode resonance in periodic surface textures on Si thin films induced by nanosecond laser irradiation

Laser-crystallization processes to convert amorphous-Si to poly-crystalline-Si are attracted considerable attention owing to their industrial applications, such as in active matrix organic light-emitting diode displays (AMOLEDs) and photovoltaic devices and general scientific interest. Multiple irradiation sequences were conducted with a 355 nm UV laser on amorphous-Si (40, 50, and 60 nm)/oxide(300 nm) layers on glass substrates. The resultant periodic surface textures on Si thin layers were characterized by resonance peaks in the transmittance spectra, depending on the polarization of the normal-incident light. By modeling the periodic textures as 1D linear gratings, the variations in peak intensities and positions were analyzed with respect to Si-layer thickness using the guided-mode resonance theory. In the case of transverse electric polarization, the electric field generated at resonance was entirely concentrated in the Si layer, leading to a red-shift with an increase in Si-layer thickness. Meanwhile, with transverse magnetic polarization, the magnetic field generated at resonance was mainly localized within the Si layer, but was partially distributed in the lower oxide layer when the Si-layer thickness was less than 45 nm and highly concentrated in the Si layer at Si-layer thicknesses greater than 45 nm. These results were consistent with the measured ripple positions and intensities. Thus, we propose that the observed resonance peaks can be used to characterize the surface ripples generated by laser crystallization, for e.g., in optical sensor devices on flexible substrates.

Evaluation of silicon carbide as a divertor armor material in DIII-D H-mode discharges

Silicon carbide (SiC) represents a promising but largely untested plasma-facing material (PFM) for next-step fusion devices. In this work, an analytic mixed-material erosion model is developed by calculating the physical (via SDTrimSP) and chemical (via empirical scalings) sputtering yield from SiC, Si, and C. The Si content in the near-surface SiC layer is predicted to increase during D plasma bombardment due to more efficient physical and chemical sputtering of C relative to Si. Silicon erosion from SiC thereby occurs primarily from sputtering of the enriched Si layer, rather than directly from the SiC itself. SiC coatings on ATJ graphite, manufactured via chemical vapor deposition, were exposed to repeated H-mode plasma discharges in the DIII-D tokamak to test this model. The qualitative trends from analytic modeling are reproduced by the experimental measurements, obtained via spectroscopic inference using the S/XB method. Quantitatively the model slightly under-predicts measured erosion rates, which is attributed to uncertainties in the ion impact angle distribution, as well as the effect of edge-localized modes. After exposure, minimal changes to the macroscopic or microscopic surface morphology of the SiC coatings were observed. Compositional analysis reveals Si enrichment of about 10%, in line with expectations from the erosion model. Extrapolating to a DEMO-type device, an order-of-magnitude decrease in impurity sourcing, and up to a factor of 2 decrease in impurity radiation, is expected with SiC walls, relative to graphite, if low C plasma impurity content can be achieved. These favorable erosion properties motivate further investigations of SiC as a low-Z, non-metallic PFM.

An optical modulator with ridge-type silicon waveguide based on graphene and MoS2 layers and improved modulation depth

In this paper, an optical modulator based on the hybrid structure is designed and analyzed in the wavelength range of 1.3 µm–1.8 µm. In this work, the proposed modulator consists of graphene, Molybdenum disulfide ( $${\mathrm{MoS}}_{2}$$ ), and Hexagonal Boron Nitride (h-BN) layers with Si layer on $${\mathrm{SiO}}_{2}$$ substrate. The Si layer on $${\mathrm{SiO}}_{2}$$ substrate has created a waveguide in the middle of the structure. Our investigations show that the strong coupling in this intermediate waveguide increases the light interaction with graphene, and finally increases the modulation depth (MD) of proposed modulator. In our proposed modulator, at telecommunication wavelength of 1.55 µm, the modulation depth equals 0.2 dB/µm, and the loss rate is 0.2 dB/µm. The highest modulation depth is 0.58 dB/µm, with loss of 0.58 dB/µm and occurs at wavelength of 1.8 µm.

Ground Plane Electrostatically Doped Junctionless Tunnel Field Effect Transistor: Process Immune Design for Suppressed Ambipolarity

The incorporation of a highly doped Si layer below the Si film, termed as the ground plane, in the electrostatically doped junctionless tunnel field effect transistor (ED-JLTFET) with high-K gate stack. This allows the conceptualization and realization of highly reduced ambipolar behaviour with enhancement in the drive current. The deployed GP sinks the electric field induced at drain side, increases the effective channel length for the short channel devices, prevents the direct source-to-drain tunneling and also the channel-to-drain tunneling in reverse direction at negative gate voltages. A significant reduction of sub-threshold slope and DIBL is also reported for the considered device structure. Using a calibrated exhaustive TCAD study, the detailed sensitivity analysis is also carried for the proposed device with the parametric sweep method. Further, the device analog/RF performance is also comprehensively examined for its high frequency operations. Interestingly, it is also observed that the deployment of this GP also results in better volume depletion. Moreover, due to electrostatically doped structure, the reported device is also expected to have a lower thermal budget, immune towards the random dopant fluctuations (RDFs) effects and tolerant towards the velocity degradation effects as well. Hence, the reported device is a potential candidate for implementing ultra low power steep switching transistors.

Simulation study about negative capacitance effects on recessed channel tunnel FET

A recessed-channel tunnel field-effect transistor (RCTFET) with intrinsic Si layer between gate and source/drain is proposed and its electrical characteristics are examined by technology computer-aided design simulation for lower subthreshold swing (SS) and higher on-off current ratio (I ON/I OFF) than conventional planar TFET. Although the SS and I ON/I OFF of RCTFET can be improved by optimizing the length of the intrinsic Si layer (L T), there is a trade-off in terms of turn-on voltage (V ON). To address this issue, a ferroelectric (FE) layer has been adopted to the gate stack for negative capacitance (NC) effects. Based on the study, the NC effects not only reduce V ON but also enhance the SS and I ON/I OFF characteristics. As a result, the optimized NC-RCTFET shows 3 times higher I ON and 23 mV dec−1 smaller average SS with 1 V lower V ON than the conventional RCTFET.

Efficient hydrothermal growth of high-performance MoS2/pyramid-Si photocathodes by surface hydrophilicity engineering

Compared with traditional precious metal catalysts, MoS2 is regarded as the promising hydrogen evolution reaction catalyst for Si photocathodes, because of its higher photochemical stability and lower hydrogen adsorption free energy. However, the layers of MoS2 are connected by van der Waals force, and the interaction force between the layers of MoS2 and Si is weak. Therefore, the homogeneous MoS2 nanosheets directly synthesized on Si by the hydrothermal method remain challenging. Herein, we report a facile strategy for the fast and efficient growth of vertically standing MoS2 on pyramid Si via surface hydrophilicity tailoring by inserting a thin TiO2 layer. Thanks to the improved light trapping and catalytic kinetics, the optimal MoS2/TiO2/Si shows a short-circuit photocurrent density of 9.79 mA/cm2 and an onset potential of 0.31 V vs reversible hydrogen electrode under 100 mW/cm2 Xe-lamp illumination. By comparing with different oxide buffer layers, surface hydrophilicity is found to be essential for promoting the homogenous nucleation and growth of vertically standing MoS2 by the hydrothermal method. These results not only provide valuable insight into the growth kinetics of transition metal chalcogenides in aqueous solution but also deliver an efficient route for assembling noble-metal free catalysts on silicon-based photoelectrodes.

Ultra-refined primary and eutectic silicon in rapidly solidified laser produced hypereutectic Al–Si alloys

ABSTRACT A series of laser-produced Al–Si surface layers of composition 35, 50, and 60 wt%Si was prepared and analysed for microstructure and hardness. Also, laser remelting at different scanning speeds was performed to obtain nano-sized silicon crystals. The results indicated that all the laser fabricated Al–Si layers consisted of fine primary silicon (PSi) crystals of average size 10 µm, surrounded by α-Al cells/dendrites and distributed in dendritic structure of α-Al and Al–Si eutectic. The results of laser remelting at speeds of 10 and 50 mm/s showed slight microstructural refinement. However, a significant reduction of the amount and size of the PSi to <1 µm as well as a favourable change in the morphology of the eutectic have resulted in the sample processed at a speed of 120 mm/s. The high cooling rate led to a dramatic reduction in the amount of the α-Al dendrites and triggers the eutectic silicon fibres to grew from the existing PSi crystals and ramify around its perimeter. The nanoindentation hardness increased as the interlamellar eutectic spacing decreased. The refinement of the PSi crystals and the modification of the eutectic silicon were attributed to the high density of twins which facilitate rapid and directional growth.

Vertical MBE growth of Si fins on sub-10 nm patterned substrate for high-performance FinFET technology* *Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0200504) and the National Natural Science Foundation of China (Grant No. 61927901).

A high quality epitaxial Si layer by molecular beam epitaxy (MBE) on Si (001) substrates was demonstrated to fabricate a channel with low density defects for high-performance FinFET technology. In order to study the effects of fin width and crystallography orientation on the MBE behavior, a 30 nm thick Si layer was deposited on the top of an etched Si fin with different widths from 10 nm to 50 nm and orientations of 〈100〉 and 〈110〉. The result shows that a defect-free Si film was obtained on the fin by MBE, since the etching damage was confined in the bottom of the epitaxial layer. In addition, the vertical growth of the epitaxial Si layer was observed on sub-10 nm 〈100〉 Si fins, and this was explained by a kinetic mechanism.

Terrace-confined guided growth of high-density ultrathin silicon nanowire array for large area electronics

Ultrathin silicon nanowires (SiNWs) are ideal 1D channels to construct high performance nanoelectronics and sensors. We here report on a high-density catalytic growth of orderly ultrathin SiNWs, with diameter down to and narrow NW-to-NW spacing of only ∼80 nm, without the use of high-resolution lithography. This has been accomplished via a terrace-confined strategy, where tiny indium (In) droplets move on sidewall terraces to absorb precoated amorphous Si layer as precursor and produce self-aligned SiNW array. It is found that, under proper parameter control, a tighter terrace-step confinement can help to scale the dimensions of the SiNW array down to the extremes that have not been reported before, while maintaining still a stable guiding growth over complex contours. Prototype SiNW field effect transistors demonstrate a high I on/I off current ratio ∼107, low leakage current of ∼0.3 pA and steep subthreshold swing of 220 mV dec−1. These results highlight the unexplored potential of catalytic growth in advanced nanostructure fabrication that is highly relevant for scalable SiNW logic and sensor applications.

A new route to designing a one-dimensional multiperiodic photonic crystal with adjustable photonic band gap and enhanced electric field localization

In this paper, photonic band gap (PBG) properties and electric field distribution of a one-dimensional multiperiodic photonic crystal were theoretically calculated based on a 2 × 2 transfer matrix method (TMM) using MATLAB software. Stacked layers of Nb2O5, Si and TiO2 were designed as a new model of the one-dimensional three-periodic photonic crystal. The results show that by changing parameters such as the layers order, number of periods, thickness of layers, defect layer thickness and the angle of incident, it is possible to engineer the width and position of PBG while also increasing the electric field intensity inside the photonic crystal. It is shown, that the [NS]2 [T]1 [NS]8 structure (N = Nb2O5, S = Si and T = TiO2) has a wider photonic band gap (PBG) allowing for applications such as laser eye protections. The [NS]2 [T]2 [NS]8 structure is found to be the highest in terms of localization of light in defect layer and being suitable for use in the photocatalysis field.

Uncertainty Quantification Analysis on Mechanical Properties of the Structured Silicon Anode via Surrogate Models

Silicon anode is the most promising candidate for next generation lithium ion batteries. A major drawback limiting its application is the significant volume change during lithiation-delithiation process, which may cause material pulverization and capacity degradation. A novel 3D bi-continuous nanoporous structured Si anode, consisting of porous metal scaffolds and thin Si coating layers, was proven to be an effective method to tackle this issue; however, uncertainty and non-uniformity, inherited from the fabrication process, will be inevitably introduced as important considerations for the performances of the Si anode. In this paper, uncertainty quantification (UQ) analysis is performed on the structured Si anode system to evaluate the influences of various design variables on its performances and to find the design optimization strategy. The biggest hurdle in the UQ study is the computational cost; to mitigate this challenge, a Gaussian Process based surrogate model is constructed using finite element simulation results as training data. It is found that the performances of the anode are rather sensitive to the geometric parameters, i.e. scaffold non-uniformity and Si layer thickness, whereas the mechanical properties of the materials are relatively less important. Furthermore, the optimal design is proposed to minimize the stress concentration in the Si anode.

Trimethoxymethylsilane as a solid-electrolyte interphases improver for graphite anode

To improve the cycling performance of graphite anode materials, we propose a functional electrolyte additive, trimethoxymethylsilane (TMSi), which contains a silyl ether functional group as part of its molecular structure. First principal calculation studies, in addition to ex situ analyses, demonstrated that electrochemical reduction of ethylene carbonate (EC) gives an anionic reduced EC product. Subsequent chemical reaction with TMSi then generates solid-electrolyte interphase (SEI) layers of Si–O and Si–C functionalized carbonate on the surface of the graphite anode, which prolongs and stabilizes the cycling performance of the cells. As a result, the cell cycled with TMSi-controlled electrolyte exhibits a cycling retention of 89.5%, whereas the cell cycled with standard electrolyte suffers from poor cycling retention (84.3%) after 100 cycles.

High-Overtone Thin Film Ferroelectric AlScN-on-Silicon Composite Resonators

This letter presents the first demonstration of thin-film ferroelectric Aluminum Scandium Nitride (AlScN)-on-silicon composite resonators, targeting high-overtone resonance modes in the sub-6GHz band with a high figure of merit (FoM). The resonators are based on sputtered ferroelectric AlScN films with Sc/(Al+Sc) ratio of ~30% and thickness of $\sim 1 {\mu }\text{m}$ . Two types of AlScN thickness-extensional (TE) resonators are co-fabricated on the same SOI platform; with and without a $3.55~\mu \text{m}$ -thick Si layer in the resonant stack. We show that although the passive Si device layer underneath the thin piezo-stack results in degradation of the electromechanical coupling coefficient $({k}_{t}^{2})$ , it boosts the quality factor $({Q})$ , provides structural robustness, and improves the overall ${Q} \times {k}_{t}^{2}$ FoM. The resonant frequency spectrum of the high-overtone TE modes of AlScN-on-Si composite resonator is analyzed and the dependency of ${k}_{t}^{2}$ on the Si device layer properties is studied. A high ${k}_{t}^{2}$ value of 11.7% at the 3rd-order TE resonant frequency of 2.4 GHz is reported, yielding a high ${k}_{t}^{2}\times {Q}_{\textit {max}}$ FoM of 84. The reported FoM shows $2\,\, {\times }$ improvement compared to the co-fabricated AlScN-only FBARs.