Si Bilayer Research Articles

Influence of O content on exothermic and self-propagating characteristics of Ti/SiOx multilayer films

In this paper, the influence of O content on the exothermic and self-propagating characteristics of Ti/SiOx films is reported. 2 μm-thick multilayer films with various atomic ratios and bilayer thicknesses are prepared using a tertiary-source magnetron sputtering system with DC/RF multifunction cathodes. DC, DC, and RF modes are used for depositing Ti, Si, and SiO2 films, respectively, where cosputtering of Si and SiO2 is conducted to control O content in SiOx film layers. Threshold powers for exothermic reaction estimated by electric spark and laser irradiation depend on the Ti to Si ratio and bilayer thickness, whereas reaction propagation speed and maximum temperature mainly depend on the bilayer thickness and the Ti to Si ratio, respectively. The three characteristics are influenced by O content. The heat of reaction also shows both bilayer thickness and Ti to Si ratio effects, and it decreases with increasing O content. In the Ti/Si films, the exothermic reaction propagates completely into the films, whereas in the Ti/SiOx films, self-propagation of the exothermic reaction becomes difficult with increasing O content. The cause of the influence of O content on reaction propagation characteristics is discussed from the viewpoint of a change of heat balance with increasing O content in the Ti/SiOx films.

Band Flattening and Landau Level Merging in Strongly-Correlated Two-Dimensional Electron Systems

We review recent experimental results indicating the band flattening and Landau level merging at the chemical potential in strongly-correlated two-dimensional (2D) electron systems. In ultra-clean, strongly interacting 2D electron system in SiGe/Si/SiGe quantum wells, the effective electron mass at the Fermi level increases monotonically in the entire range of electron densities, while the energy-averaged mass saturates at low densities. The qualitatively different behavior of the two masses reveals a precursor to the interaction-induced single-particle spectrum flattening at the chemical potential in this electron system, in which case the fermion “condensation” at the Fermi level occurs in a range of momenta, unlike the condensation of bosons. In strong magnetic fields, perpendicular to the 2D electron layer, a similar effect of different fillings of quantum levels at the chemical potential—the merging of the spin- and valley-split Landau levels at the chemical potential—is observed in Si inversion layers and bilayer 2D electron system in GaAs. Indication of merging of the quantum levels of composite fermions with different valley indices is also reported in ultra-clean SiGe/Si/SiGe quantum wells.

Sliding ferroelectricity in two-dimensional MoA2N4(A = Si or Ge) bilayers: high polarizations and Moiré potentials

Strong sliding-ferroelectricity is predicted in high-mobility semiconducting MoSi2N4bilayers, and a small angle twist will induce strong Moiré potential and unique band alignment for exciton trapping.

Structural-colored silk based on Ti–Si bilayer

In this Letter, Ti–Si bilayer was deposited on white silk to achieve coloration of the silk. By controlling the thickness of the Ti layer and Si layer, the saturation and the hue of the color on the silk could be preciously modulated, respectively. The structural colors on the silk could cover the major colors in the International Commission on Illumination 1931 chromaticity diagram, and it exhibits good durability, which is demonstrated by rubbing and stretching treatments. The developed textile coloration method may provide an eco-friendly technology in the silk dyeing industry.

Design and analysis of Yb doped ZnO (YZO) and P–Si bilayer nano-stacked reflector for optical filter applications

Abstract In the present paper, a multilayer stack of Yb doped ZnO (YZO) and p-Si has been designed in which the total internal reflection of optical light is combined to form constructive interference. The mechanism behind light propagation in a stack is revealed, and factors affecting the reflectivity of the stacked reflector are analytically investigated. In the visible range, an optimized reflectivity up to 99.3% is reached from two bilayer stacks. The thickness of stack layers is optimized to validate Fresnel's Law and the same has been further authenticated by the experimental results obtained from the ellipsometer. The device structure is numerically analyzed using the full wave simulator CST microwave studio. Furthermore, the reflector bands are linearly reconfigured by angular manipulation of the incident light. The experimental and simulated outcome of the multilayer stacked reflector shows the highest reflectivity of 99.3% for 550 nm wavelength of visible frequency spectrum.

Improvement in Electrical Characteristics of ZnSnO/Si Bilayer TFET by W/Al₂O₃ Gate Stack

We have examined impacts of gate insulator (Al2O3 or HfO2) and gate electrode (TiN or W) on electrical performance of ZnSnO/Si bilayer tunneling fieldeffect transistors (TFETs). It is found from the capacitance-voltage ( ${C}$ - ${V}$ ) characteristics that the W gate is effective to reduce the counter-clockwise hysteresis. Additionally, the optimal temperature of post-metallization annealing (PMA) is different for each gate stack, and this optimization is critically important for the high ON-state current ( ${I} _{\mathrm{ ON}}$ ) and steep ON/OFF switching. The W/Al2O3/ZnSnO/Si bilayer TFET provides the hysteresis-free steep ON/OFF switching with the minimum sub-threshold swing ( SS ) of 65.4 mV/dec. and average SS of 72.0 mV/dec. in a gate-voltage swing of 0.3 V, which are 19% and 18% lower than the control TiN/Al2O3/ZnSnO/Si bilayer TFET. When the gate stack is replaced by W/HfO2/Al2O3, the TFET exhibits less steep ON/OFF switching in spite of thinning capacitance equivalent thickness (CET) from 5.9 and 2.3 nm. These results indicate the importance of improvement in the gate stack quality on the sub-threshold characteristics of the bilayer TFETs.

Reduction of process temperature for Si surface flattening utilizing Ar/H2 ambient annealing and its application to SOI-MISFETs with bilayer HfN high-k gate insulator

We have investigated the reduction of the process temperature for the Si surface flattening process by annealing in Ar/H2 ambient and its application to Si-on-insulator (SOI) metal-insulator-semiconductor field-effect transistors (MISFETs) with bilayer HfN high-k gate insulator. The surface rms roughness of 0.057 nm was realized for the p-Si(100) substrates by the annealing at 925 °C/10 min in Ar/1.0%H2 ambient. Although slip-line defects were observed in the isolated SOI region after the optimized flattening process, the device characteristics of the fabricated SOI-MISFETs with HfN1.3/HfN1.1/Si(100) bilayer gate insulator were found to have been improved by the surface flattening utilizing Ar/1.0%H2 annealing.

Subsurface damage and material removal of Al–Si bilayers under high-speed grinding using molecular dynamics (MD) simulation

By performing three-dimensional molecular dynamics (MD) simulations, the effects of the tool radius, depth of cut and grinding speed are thoroughly studied in terms of the workpiece deformation, material removal, dislocation movement, atomic trajectory, grinding temperature and average grinding force. The strength of ductile/brittle (Al/Si) bilayers is largely enhanced, because the interface can hinder the passage of dislocations. The interface in brittle/ductile (Si/Al) bilayers contributes to its ductility by increasing the movability of dislocations when gliding on it. The brittle to ductile transition of bilayers, which strongly depends on the interface debond energy, has a key role in controlling the dislocation slipping mechanism. The investigation also reveals that a larger tool radius, higher grinding speed or deeper depth of cut results in more chipping volume and higher grinding temperature in both bilayers. At the same machining parameters, the above changes in brittle/ductile (Si/Al) bilayers are more apparent than that in ductile/brittle (Al/Si) bilayers, since Si is stiffer and has a higher yield strength than Al.

Resistive switching control for conductive Si-nanocrystals embedded in Si/SiO2 multilayers

In this paper, we report on the enhanced control of resistive switching in multilayer Si/SiO2 structures, which permit the formation of Si nanocrystals with a typical size of 5.88 nm and overall good shape homogeneity. The deposition of a different number of Si and SiO2 bilayers (6, 8 and 10) allowed control of SET/RESET voltages in negative bias ranges 4.5–10 V and 6.3–13 V for six- and ten-bilayer devices, respectively. The corresponding resistance ratio between ON/OFF states varied in the ranges 107–105 for the aforementioned number of bilayers. Based on the result of XPS measurements, we suggest that the resistive switching in the studied system occurs due to the formation and annihilation of Si−Si and Si−O bonds, which serve as conductive pathways and isolating material, respectively.

Giant Enhancement in Rashba Spin‐Seebeck Effect in NiFe/p‐Si Thin Films

The spin‐Seebeck effect mediated thermoelectric energy conversion can provide an efficient alternative to traditional thermoelectrics for waste heat recovery. To achieve this goal, efficient spin to charge conversion using earth‐abundant materials is essential. Proximity induced Rashba effect arises from the charge potential mediated by structural inversion asymmetry, which has been reported in Si thin films and can be manipulated by controlling the thickness of Rashba layer. We demonstrate a giant Rashba spin‐Seebeck effect in NiFe/p‐Si (polycrystalline) bilayer thin films. The bilayer thin film specimens have p‐Si layer thickness of 5, 25, and 100 nm while keeping the NiFe layer thickness at 25 nm. The Rashba spin‐Seebeck coefficient has been estimated to be μV K−1 for 100 nm p‐Si, and increases by an order of magnitude to μV K−1 for 5 nm p‐Si. The measured spin‐Seebeck coefficient in 5 nm p‐Si specimen is one of the largest coefficients ever reported. The measured voltage of 100.3 μV is one of the largest reported spin‐Seebeck voltages, with smallest area of ≈160 × 10 μm2 used in any spin‐Seebeck measurement. This scientific and technological breakthrough using earth abundant elements brings the spin mediated thermoelectric energy conversion for waste heat recovery closer to reality.

Imaging of immunogold labeling in cells and tissues by helium ion microscopy.

Helium ion microscopy (HIM) scans samples with a fine ion beam exploiting the very short de Broglie wavelength of helium ions. Because the radiation induces only a small sample region to emit secondary electrons (SEs), very high resolution is expected. In order to explore the applications of SE-HIM in biology, COS7 kidney fibroblast cells and C2C12 myoblast cells cultured on a silicon (Si) nitride (SiN)/Si bilayer were dried and directly observed in high vacuum, without coating or staining. High contrast, high depth-of-field images were obtained revealing the nucleus, endoplasmic reticulum, cytoskeleton and putative mitochondria above a bright background from the support. Gold-tagged antibodies were employed to aid organelle identification. Signals from the gold tags were most clearly distinguishable by secondary electron (SE)-HIM when cells were grown on thin SiN film, and the minimum gap measured between gold particles showed the resolution to be 2 nm. Wheat germ agglutinin-gold labeling revealed clusters of gold particles ~50–200 nm in diameter on COS7 cells, which might represent assemblies of glycosylated proteins, suggesting the formation of membrane raft structures that include membrane proteins. SE-HIM also delivered high contrast images of unstained, uncoated, thin sections of Epon-embedded mouse kidney tissues mounted on a SiN/Si bilayer, revealing the details of sub-tissues and cell organelles. A charge-coupled mechanism explaining the observed SE-HIM contrast is proposed. Ionoluminescence-HIM was also performed targeting zinc oxide particles on cells. In conclusion, the high depth-of-field, high-resolution imaging achieved using HIM may have applications in various fields, including soft materials.

Fabrication and Deformation of 3D Multilayered Kirigami Microstructures.

Mechanically guided 3D microassembly with controlled compressive buckling represents a promising emerging route to 3D mesostructures in a broad range of advanced materials, including single-crystalline silicon (Si), of direct relevance to microelectronic devices. During practical applications, the assembled 3D mesostructures and microdevices usually undergo external mechanical loading such as out-of-plane compression, which can induce damage in or failure of the structures/devices. Here, the mechanical responses of a few mechanically assembled 3D kirigami mesostructures under flat-punch compression are studied through combined experiment and finite element analyses. These 3D kirigami mesostructures consisting of a bilayer of Si and SU-8 epoxy are formed through integration of patterned 2D precursors with a prestretched elastomeric substrate at predefined bonding sites to allow controlled buckling that transforms them into desired 3D configurations. In situ scanning electron microscopy measurement enables detailed studies of the mechanical behavior of these structures. Analysis of the load-displacement curves allows the measurement of the effective stiffness and elastic recovery of various 3D structures. The compression experiments indicate distinct regimes in the compressive force/displacement curves and reveals different geometry-dependent deformation for the structures. Complementary computational modeling supports the experimental findings and further explains the geometry-dependent deformation.

Performance enhancement of thin-film silicon solar cells by development of core component layers

High efficiency thin-film silicon (Si) solar cells were prepared on flexible substrates by the plasma-enhanced chemical vapor deposition method. To improve their performance, the microstructural, electrical, and optical properties of the core component layers including the metal rear reflectors, p- and n-type doped layers, and intrinsic absorber layers were controlled sophistically. To enable the use of flexible substrates with low heat resistance as well as to enhance light-scattering properties, nanotextured rear reflectors with a Ag/Al:Si bilayer structure were developed by dc magnetron sputtering at a low substrate temperature of below 150 °C. Highly crystalline n-doped seed layers (which effectively eliminate a defect-dense amorphous region formed in the initial growth stage of intrinsic nanocrystalline silicon (nc-Si:H) absorber layers) and p-type wide-bandgap nanocrystalline silicon carbide (nc-SiC:H) window layers (which reduce the parasitic absorption loss in the short-wavelength region and increase the open circuit voltage (VOC)) were successfully applied to enhance the performance characteristics of the solar cells. Through the combination of the developed core component layers, high conversion efficiencies of 8.84% (VOC = 0.53 V, JSC = 25.28 mA/cm2, and fill factor (FF) = 0.66, where JSC is the short circuit current) and 7.48% (VOC = 0.50 V, JSC = 21.07 mA/cm2, and FF = 0.71) were obtained for nc-Si:H solar cells fabricated on stainless steel (SUS) and polyimide substrates, respectively, at a low substrate temperature of below 150 °C. Flexible a-Si:H/nc-Si:H double-junction solar cells fabricated on SUS substrates showed a high conversion efficiency of 11.46% (VOC = 1.38 V, JSC = 11.53 mA/cm2, and FF = 0.72) when the nc-Si:H solar cells developed in this study were applied as a bottom cell.

Two-dimensional metallicity and ferromagnetic ordering in monoatomic-thick Ce layer on Si(111)

Novel properties found in the atom-thick metal overlayed semiconductor systems motivate the studies on the two-dimensional metal films. In this work, the electronic structures of Ce overlayer on a Si substrate have been studied by using first-principles approach. Monoatomic-thick Ce layer on Si(111) surface exhibits metallic behavior and ferromagnetic ordering with weak induced magnetic moments of the Si atoms in the first Si bilayer. Our results reveal the intralayer metallic bonding and the mixed covalent and ionic characteristic of the interfacial bonds in Ce-Si(111). The bonding picture is similar to that of the superconducting monolayer metals on Si(111) surface found in experiments, and elucidates the two-dimensional metallic nature of Ce overlayer with anisotropic band structures in the Ce-Si(111) interfacial systems.

Chemical etching of silicon carbide in pure water by using platinum catalyst.

Chemical etching of SiC was found to proceed in pure water with the assistance of a Pt catalyst. A 4H-SiC (0001) wafer was placed and slid on a polishing pad in pure water, on which a thin Pt film was deposited to give a catalytic nature. Etching of the wafer surface was observed to remove protrusions preferentially by interacting with the Pt film more frequently, thus flattening the surface. In the case of an on-axis wafer, a crystallographically ordered surface was obtained with a straight step-and-terrace structure, the height of which corresponds to that of an atomic bilayer of Si and C. The etching rate depended upon the electrochemical potential of Pt. The vicinal surface was observed at the potential at which the Pt surface was bare. The primary etching mechanism was hydrolysis with the assistance of a Pt catalyst. This method can, therefore, be used as an environmentally friendly and sustainable technology.

Silicon induced stability and mobility of indium zinc oxide based bilayer thin film transistors

Indium zinc oxide (IZO), silicon containing IZO, and IZO/IZO:Si bilayer thin films have been prepared by dual radio frequency magnetron sputtering on glass and SiO2/Si substrates for studying their chemical compositions and electrical characteristics in order to ascertain reliability for thin film transistor (TFT) applications. An attempt is therefore made here to fabricate single IZO and IZO/IZO:Si bilayer TFTs to study the effect of film thickness, silicon incorporation, and bilayer active channel on device performance and negative bias illumination stress (NBIS) stability. TFTs with increasing single active IZO layer thickness exhibit decrease in carrier mobility but steady improvement in NBIS; the best values being μFE ∼ 27.0, 22.0 cm2/Vs and ΔVth ∼ −13.00, −6.75 V for a channel thickness of 7 and 27 nm, respectively. While silicon incorporation is shown to reduce the mobility somewhat, it raises the stability markedly (ΔVth ∼ −1.20 V). Further, IZO (7 nm)/IZO:Si (27 nm) bilayer based TFTs display useful characteristics (field effect mobility, μFE = 15.3 cm2/Vs and NBIS value, ΔVth =−0.75 V) for their application in transparent electronics.

Alternative device structures for CIGS-based solar cells with semi-transparent absorbers

Abstract In this work, we discuss two alternative structures of CIGS-based solar cells, a bi-facial configuration and a back-wall configuration. One of the critical issues in these structures is fabrication of a transparent back contact which facilitates not only ohmic contact for holes in the valance but also an effective reflector for electrons in the conduction band. In order to meet these requirements of the transparent back contact, a p-type amorphous Si (p-a-Si)/intrinsic amorphous Si (i-a-Si) bi-layer was deposited onto the ITO back contact. Absorber films were formed by a metal precursor reaction in H2Se and H2S, and their thicknesses were adjusted to about 0.25 µm to enhance current collection from back-side illumination. The resulting devices with a bi-facial structure of SLG/ITO/p-Si/i-Si/Cu(In, Ga)(Se,S)2/CdS/i-ZnO/ITO/Ni-Al exhibited a comparable open-circuit voltage to a Cu(In, Ga)(Se,S)2 cell on a Mo substrate in tests with illumination of both sides. Back-wall cells with a structure of SLG/ITO/p-Si/i-Si/Cu(In, Ga)(Se,S)2/CdS/i-ZnO/Ag-reflector were also fabricated and improved current collection by greater light trapping was achieved compared to the bi-facial cell with back-side illumination.

An approach for the prediction of interfacial delamination of an a-Si3N4 /Si bilayer system

The prediction of interfacial delamination in multi-layered IC packaging has been a rather complex process as it is challenging to quantify the critical energies of interfacial adhesion experimentally. In this study, a combined approach based on molecular dynamics (MD) and finite element method (FEM) simulations is adopted to characterize and predict delamination in an a-Si3N4/Si bilayer system. The necessary material parameters that are required for the interfacial cohesive constitutive relation in FEM simulation are derived by MD simulation rather than experiment. The work of interfacial fracture obtained by the FEM simulation is about 0.11 J m−2. To validate the presented approach, an experimental nanoindentation test is also conducted to measure the work of interfacial fracture which is obtained to be about 0.085 J m−2. The results obtained by the MD-FEM simulations and the experiment show very good agreement. Therefore, the combined approach can also be applied to investigate other bi-materials or even multi-layered systems, thus facilitating the prediction of interfacial delamination and helping to improve the stability and reliability of the 3D multi-layered IC packaging.

Ion-beam-induced nanodots formation from Au/Si thin films on quartz surface

We report the synthesis of Si nanodots on quartz surface using ion irradiation. When a bi-layer of ultrathin Au and Si on quartz surface is irradiated by 500keV Xe-ion beam, the bi-layer spontaneously transforms into nanodots at a fluence of 5×1014ionscm−2. The spatial density and diameter of the nanodots are reduced with increase in applied ion fluence. The nanostructures exhibit photoluminescence in the visible range at room temperature where the intensity and wavelength depends upon ion fluence. The observed evolution seems to be correlated to ion beam mixing induced silicide formation at Au–Si interface.

Kerfless exfoliated thin crystalline Si wafers with Al metallization layers for solar cells

Abstract