Si Nanopillars Research Articles

Controlling the spontaneous emission of the telecom O-band centers in Silicon-on-Insulator with coherent dipole-quadrupole interactions on a silicon pillar lattice

The G-center in silicon-on-insulator (SOI) has emerged as a relevant single photon source due to the zero-phonon line at 1278 nm, matching the O-band of optical telecommunication wavelengths. Due to the weak emission of the G-center from planar SOI, it is necessary to enhance its emission, in terms of collection efficiency and fluorescence enhancement, to further study its optical properties for application in quantum technologies. Here we design a fabrication-ready SOI Mie-resonator made of a lattice of silicon nanopillars on silica to achieve the spontaneous emission enhancement of single G-centers. Using Si nanopillars lattice on silica with period and dimensions optimized, owing to the coherent superposition of the electric dipolar and magnetic quadrupolar electromagnetic Mie-scattering moments of the individual pillars to the periodic lattice, we show the G-center emission/decay rate enhancement averaged over it's possible dipole orientations is about 13 times compared to bulk. This corresponds to a fluorescence enhancement of about 125 times compared to bulk and about 5-6 times compared to an optimized single pillar with photon collection with a confocal objective. These results provide a fabrication pathway for out-of-plane single photon collection from the SOI G-center or other telecom O-band single-photon sources for their potential deployment in CMOS-compatible quantum optical technologies.

Dual-spectral narrow bandwidth surface lattice resonance sensors

Dual-spectral narrow bandwidth surface lattice resonance (SLR) sensors consisting of semiconductor/SiO2 nanoparticle dimer arrays are proposed to achieve two high figures of merit (FOM). We demonstrate that FOM can be optimized by the change of semiconductor nanoparticles’ shape. The best shape of Si nanoparticles with high FOM is pillar because of the narrowest FWHM of SLR. The FOM of Si/SiO2 nanopillar dimer array can reach a value of 180 due to the high sensitivity and narrow bandwidth. Besides, the influences of different structural parameters (such as the gap of the Si nanopillars in each dimer, Si nanopillars’ diameter and height) on FOM of Si/SiO2 nanopillar dimer arrays are investigated. In the trade-off between the strong SLR and the high FOM, the gap of 50[Formula: see text]nm, the Si nanopillars’ diameter of 80[Formula: see text]nm, and the Si nanopillars’ height of 100[Formula: see text]nm, are chosen as suitable values. Furthermore, a comparison of FOM between Si, Ge, GaAs nanopillars with fixed sizes is implemented to select the appropriate materials. The best semiconductor material to obtain a high FOM is Si because of a narrower FWHM and higher sensitivity. This work is of great significance for the design of multi-spectral SLR sensors.

Formation of Si nanopillars through partial sacrificing in super passivation reactive ion etching

The vertical gate-all-around (VGAA) metal-oxide-semiconductor field-effect transistor (MOSFET) holds remarkable potential in the three-dimensional (3D) integrated circuits (ICs), primarily owing to its capacity for vertical integration. The Si nanopillar, a crucial channel in the VGAA MOSFET, is conventionally shaped via the reactive ion etching (RIE) system employing SF6/O2. Past studies have indicated that high O2 gas conditions in RIE often result in Si grasses irregular nanostructures, such as nanospikes on the bottom surface, due to over-passivation. However, this study revealed that ultrahigh O2 proportions (>70%), especially when combined with low chamber pressure, inhibit the development of Si grasses in the RIE system (termed as super passivation). Nevertheless, this scenario leads to the segmentation of the Si nanopillar. To address this issue, a proposed partial sacrificing method, achieved by sacrificing the upper segment of the nanopillar through prolonged processing time and reduced mask size, successfully yielded Si nanopillars without Si grasses. Furthermore, an empirical model was developed to elucidate how experimental parameters influence etching characteristics, encompassing etching rate and Si nanopillar shape, through a systematic examination of the RIE etching process. This research significantly contributes to the production of VGAA MOSFETs and 3D ICs.

Polarization and frequency multiplexed multi-channel focusing terahertz vortex with a birefringent dielectric metasurface

We propose an approach to generate polarization and frequency multiplexed focusing vortex beams using a birefringent dielectric metasurface in terahertz (THz) range. The designed metadevice consists of Si nanopillars with different sizes. The transmission efficiency of each unit cell under orthogonal polarized illumination can exceed 70% at both operating frequencies 1 THz and 1.2 THz. Combining the functions of beam deflection, focusing, and vortex beam generation in the interference holography design strategy, polarization and frequency multiplexed multi-channel focusing THz vortex beams with different topological charges and generation positions can be achieved, which can greatly improve the transmission capacity of THz communications. Combining its compact and efficient features with multiplexing methods, our designed metadevice has enormous potential for application in THz vortex generation and information processing.

Mid-infrared large-aperture metalens design verification and double-layer micro-optical system optimization

In this article, we explored the design and simulation techniques of large-aperture metalenses, and the optimization design methods of metalenses. For verification and experimental demonstration, a centimeter-scale aperture single-layer metalens and a field of view optimized metalens doublet,composed of subwavelength-spaced Si nanopillars with an operating wavelength of 3.77 µm were designed and manufactured. Finally, the focusing performance of the two under narrow-band laser irradiation was characterized, and an imaging demonstration of the metalens doublet was performed under an optical bandwidth of 250 nm (3500-3750 nm). We envision that the calculation, design, sample manufacturing and demonstration research on large-aperture metalens presented here will provide an important reference for the design and verification of large-aperture metasurface lenses or special metasurface devices in the future, such as large-aperture compact multifunctional metalens optical equipment for low-load special application systems like airborne, spaceborne, missile, satellite and deep sea.

Fabrication of Metal Contacts on Silicon Nanopillars: The Role of Surface Termination and Defectivity.

The application of nanotechnology in developing novel thermoelectric materials has yielded remarkable advancements in material efficiency. In many instances, dimensional constraints have resulted in a beneficial decoupling of thermal conductivity and power factor, leading to large increases in the achievable thermoelectric figure of merit (ZT). For instance, the ZT of silicon increases by nearly two orders of magnitude when transitioning from bulk single crystals to nanowires. Metal-assisted chemical etching offers a viable, low-cost route for preparing silicon nanopillars for use in thermoelectric devices. The aim of this paper is to review strategies for obtaining high-density forests of Si nanopillars and achieving high-quality contacts on them. We will discuss how electroplating can be used for this aim. As an alternative, nanopillars can be embedded into appropriate electrical and thermal insulators, with contacts made by metal evaporation on uncapped nanopillar tips. In both cases, it will be shown how achieving control over surface termination and defectivity is of paramount importance, demonstrating how a judicious control of defectivity enhances contact quality.

Broadband anti-reflection coating for Si solar cell applications based on periodic Si nanopillar dimer arrays & Si3N4 layer

A structure of periodic Si nanopillar dimer array & Si3N4 layer which sits on Si substrates is presented to obtain a broadband high transmission and low reflection. We show numerically that the average reflection of this structure can reach 1.8%, and the average transmission can reach 93.1% in the 400–1100 nm range, due to the combined effects of the forward scattering effects of Si nanopillar dimers and Si3N4 layer’s anti-reflection effects. Si nanopillars’ diameter and height, Si3N4 layer’s height, the gap of dimers, and the period of the array have significant impacts on the transmittance and reflection. This work supplies a practicable way for decreasing broadband surface reflection and increasing the absorption of light for Si solar cell applications.

Cylindrical vector beam multiplexing holography employing spin-decoupled phase modulation metasurface

Abstract Cylindrical vector beams (CVBs) hold considerable promise as high-capacity information carriers for multiplexing holography due to their mode orthogonality. In CVB holography, phase holograms are encoded onto the wave-front of CVBs with different mode orders while preserving their independence during reconstruction. However, a major challenge lies in the limited ability to manipulate the spatial phase and polarization distribution of CVBs independently. To address this challenge, we propose a spin-decoupled phase modulation strategy by leveraging the propagation and geometric phase of composite phase metasurfaces. By exploiting the polarized Poincaré sphere, we show that CVBs can be decomposed into two circularly polarized components with orthogonal polarization states and conjugate phase distributions. This decomposition enables independent control of the phase and polarization distributions of CVBs by modulating the initial phase and phase difference of these two components. Consequently, two holograms with discrete spatial frequency distributions that carry opposite helical phases are encoded to modulate the wave-front of CVBs by the metasurface consisting of Si nanopillars. This allows for us to achieve successful four-channel CVB multiplexing holography. Benefiting from the non-dispersive nature of geometric phase, this metasurface exhibits a broad operating band spanning the entire visible light spectrum (443 nm–633 nm). These suggest that our proposed method offers comprehensive control over the spatial phase and polarization of CVBs, thereby holding significant potential for advancing their application in holography.

Observation of Capillary Condensation and Pattern Bending Phenomena in Si Nanopillars Using <i>In Situ</i> TEM

Capillary condensation, a ubiquitous phenomenon involving the heterogeneous nucleation of liquid droplets, has significant implications in various industrial, biological, and atmospheric processes. Strong capillary forces induced by highly curved menisci of condensates can have potentially significant impact on the structural integrity and functionality of nanodevices. While the influence of surface properties on the nucleation and growth of water droplets has been extensively studied at microscale, our understanding of water condensation at the nanoscale remains limited due to experimental challenges in imaging liquids at nanometer scales. In this study, we employ in situ liquid phase TEM imaging and for the first time present real-time observations of water condensation dynamics on arrays of vertical silicon (Si) nanopillars. Experimental and simulation results show that nucleation of water droplets occurs at the edges of the nanopillars and substrate, followed by the growth of an interfacial layer resembling a corona around the nanopillars. Subsequently, the formation of bridges between adjacent growing coronas leads to the development of symmetric and asymmetric bridged nanopillar geometries. Importantly, we find that the formation of bridges can induce bending and collapse of the nanopillars, depending on their aspect ratios. Overall, this study provides valuable insights into the nanoscale dynamics of capillary condensation and paves the way for advanced engineering applications and optimization of various technological processes.

SiNPLs/CoPc hybrid heterostructures based photodetector with low dark current and enhanced sensitivity

Fabrication of hybrid heterostructures, consisting of two different nanostructured materials are an efficient approach for further improving photodetecting performance over their individual counterparts. In this paper, we demonstrate the fabrication of an inorganic-organic hybrid heterostructure comprising of Si-nanopillars (SiNPLs) and cobalt phthalocyanine (CoPc). Photodetecting performance of SiNPLs/CoPc heterostructure is investigated in photoconductive mode. It exhibits a low dark current of 2.05×10−8 A and enhanced sensitivity (∼4.5 times) in contrast to pristine SiNPLs. Suppression of dark current with enhanced sensitivity is ascribed to the transfer of electrons from SiNPLs to CoPc layer and their trapping in disorder states present in CoPc layer. The process of electron transfer is confirmed by work function measurements. These results indicate that by employing heterostructure method, the trap states originating from the non-uniform molecular packing of organic semiconductors may be used to further improve the photodetecting performance of one-dimensional nanostructured materials. Additionally, the long-term stability of the SiNPLs/CoPc heterostructure was evaluated by exposing it to ambient conditions for 20 days.

Metasurfaces integrated with a single-mode waveguide array for off-chip wavefront shaping.

Integration of metasurfaces and SOI (silicon-on-insulator) chips can leverage the advantages of both metamaterials and silicon photonics, enabling novel light shaping functionalities in planar, compact devices that are compatible with CMOS (complementary metal-oxide-semiconductor) production. To facilitate light extraction from a two-dimensional metasurface vertically into free space, the established approach is to use a wide waveguide. However, the multi-modal feature of such wide waveguides can render the device vulnerable to mode distortion. Here, we propose a different approach, where an array of narrow, single-mode waveguides is used instead of a wide, multi-mode waveguide. This approach tolerates nano-scatterers with a relatively high scattering efficiency, for example Si nanopillars that are in direct contact with the waveguides. Two example devices are designed and numerically studied as demonstrations: the first being a beam deflector that deflects light into the same direction regardless of the direction of input light, and the second being a light-focusing metalens. This work shows a straightforward approach of metasurface-SOI chip integration, which could be useful for emerging applications such as metalens arrays and neural probes that require off-chip light shaping from relatively small metasurfaces.

Hydrophobic Si nanopillar array coated with few-layer MoS2 films for surface-enhanced Raman scattering

In this study, we covered Si nanopillar (NP) array with few-layer MoS2 films to convert their wettability characteristics from hydrophilic to hydrophobic for applications as a surface-enhanced Raman scattering (SERS) substrate. The Si NP array was fabricated using a semiconductor process. We then sulfurized and transferred MoO3 films coated onto the Si NP array to MoS2 films. The surface morphology and cross-sectional profile of the MoS2-coated Si NP array structure was examined using scanning electron microscopy and transmission electron microscopy. The SERS results indicate that the substrate exhibits a favorable enhancement factor of 1.76 × 103 and a detection limit of approximately 10−5M for Rhodamine 6G (R6G) utilized as the test molecule, attributed to the charge transfer (CT) mechanism at the interface between MoS2 and R6G. Contact angle measurements showed that the MoS2-coated Si NP array possesses a hydrophobic surface. Our results suggest that an MoS2-coated Si NP array with CT and hydrophobicity characteristics is extremely promising SERS substrates for SERS applications.

Fabrication of high-aspect-ratio SiO2 nanopillars by Si thermal oxidation for metalenses in the visible region

We propose a fabrication method of metalenses in the visible region with high-aspect-ratio SiO2 nanopillars by thermal oxidation of Si nanopillars. We first evaluated the expansion of the nanopillars in width due to thermal oxidation, which affects the phase shift on metalenses. Next, considering expansion due to thermal oxidation and processing errors, a metalens pattern was fabricated, and the pillar width distribution was measured. The highest aspect ratio was 8.7. Finally, the focusing of the fabricated reflective metalens was confirmed, which indicates that the proposed method can fabricate metalenses in the visible region with SiO2 nanopillars including transmissive metalens.

Birefringent dielectric multi-foci metalens for polarization detection

We propose a birefringent dielectric multi-foci metalens for polarization detection utilizing different transmission phases in two orthogonal directions, which consists of Si nanopillars with different sizes and more than 80% transmission efficiency. Implementing the superposition of the phase profiles in the x- and y-direction, the polarization states of the focal points include linear polarization state in the x-direction, linear polarization state in the y-direction, the incident polarization state, and the polarization state whose y-polarization component more shifted by π/2 compared with the incident polarization state. Based on the intensities of the deflected and converged focal points on the same focal plane, the full polarization information of the incident polarized light can be determined with an analyzer. The multiplexing design method with compact and efficient features renders this technique very attractive for polarization detection and information processing.

Fabrication of ultrahigh aspect ratio Si nanopillar and nanocone arrays

High aspect ratio (HAR) structures have many promising applications such as biomedical detection, optical spectroscopy, and material characterization. Bottom-up self-assembly is a low-cost method to fabricate HAR structures, but it remains challenging to control the structure dimension, shape, density, and location. In this paper, an optimized top-down method using a combination of pseudo-Bosch etching and wet isotropic thinning/sharpening is presented to fabricate HAR silicon (Si) nanopillar and nanocone arrays. To achieve these structure profiles, electron beam lithography and reactive ion etching were carried out to fabricate silicon pillars having a nearly vertical sidewall, followed by thinning or sharpening by wet etching with a mixture of hydrofluoric (HF) acid and nitric acid (HNO3). For the dry etching step using the pseudo-Bosch process, the sidewall angle is largely dependent on the SF6/C4F8 gas flow ratio, and it was found that a vertical profile can be attained with a ratio of 22/38. For the wet etching process, a very large HNO3/HF volume ratio is shown to give smooth etching with a slow and controllable etching rate. The final structure profile also depends on the pattern density/array periodicity. When the array period is large, silicon nanopillar is thinned down, and its aspect ratio can reach 1:135 with a sub-100 nm apex. When the pillar array becomes very dense (periodicity much smaller than height), a very sharp nanocone structure is obtained after wet etching with an apex diameter under 20 nm.

High aspect ratio arrays of Si nano-pillars using displacement Talbot lithography and gas-MacEtch

Structuring Si in arrays of vertical high aspect ratio pillars, ranging from nanoscale to macroscale feature dimensions, is essential for producing functional interfaces for many applications. Arrays of silicon 3D nanostructures are needed to realize photonic and phononic crystals, waveguides, metalenses, X-ray wavefront sensors, detectors. In particular, arrays of Si nanopillars are used as bio-interfaces in neural activity recording, cell culture, microfluidics, sensing and on-chip manipulation. Here, we demonstrate a strategy for realizing arrays of protruding sharp Si nanopillars, using displacement Talbot lithography combined with metal-assisted chemical etching (MacEtch) in gas phase. Such combination enables reliable and low cost pathway for fabrication of ordered nanopillars arrays on large scale. With the double exposure of a linear grating mask in orthogonal orientations and the lift-off technique, we realized a catalyst pattern of holes in a Pt thin film with a period of 1 μm and hole diameter in the range of 100–250 nm. MacEtch in gas phase by using vapor HF and oxygen from air allows to etch arrays of protruding Si nanopillars 200 nm-thick and aspect ratio in the range of 200 (pillar height/width) with an etching rate up to 1 μm/min. Gas-MacEtch has the advantage of no capillary stiction, no ion beam damage of the Si substrate, nanometric resolution and high fidelity of pattern transfer. In combination with controlled uniformity of feature size on large area, spatial frequency doubling and high resolution of Talbot lithography, the proposed method is an easy-to-scale-up processing that can support the fabrication of Si pillars arrays for many valuable applications both at micro and nano-scale.

Anisotropic behavior in the lithiation of a silicon nanopillar

There are reports on experimental observation of anisotropic expansion/swelling in the lithiation of crystal silicon (Si) nanopillars/nanowires. Such anisotropic expansion/swelling can cause the cracking of Si electrodes, leading to the capacity fading in Si-based lithium-ion batteries (LIBs). In this paper, we propose an anisotropic chemo-mechanical model that takes into account anisotropic characteristics of the physical properties of crystalline Si, including stiffness tensor (matrix), diffusion tensor and partial-molar volume tensor. Using direction-cosine matrix for the tensor transformation, we determine the orientation-dependent elastic constants, diffusion coefficients and partial molar volumes in the anisotropic chemo-mechanical model instead of using phenomenal parameters associated with experimental results. For the purpose of demonstration, we analyze the lithiated Si phase of Li15Si4, which exhibits cubic structure, and illustrate geometrical profiles of the lithiated Si nanopillars of [110] and [111] in longitudinal direction at Li15Si4 state. The numerical results reveal that the cross section of the [111]-oriented Si nanopillar remain circular and the cross section of the [110]-oriented Si nanopillar evolves to elliptical shape. Such results are in good accordance with the experimental observation reported by Lee et al. [P. Natl. Acad. Sci. 109(11) (2012) 4080–4085]. The numerical results also show that the anisotropic expansion likely reduces von-Mises stress in the [110]-oriented Si nanopillar, retarding mechanical degradation of the Si-based electrodes. This work provides a new insight into the anisotropic expansion and stress generation in crystalline electrodes.

Manipulation of optical bound states in the continuum in a metal-dielectric hybrid nanostructure

Optical bound states in the continuum (BICs) are spatially localized states with vanishing radiation, despite their energy embedded in the continuum spectrum of the environment. They are expected to greatly enhance light–matter interaction due to their long lifetime and high quality factor. However, the BICs in all-dielectric structures generally exhibit large mode volumes and their properties are difficult to manipulate. In this paper, we propose a metal–dielectric hybrid nanostructure where a silver film is inserted into the silicon (Si) substrate under the Si nanopillar array. We show that symmetry-protected BIC in this system can couple with surface plasmon polaritons (SPPs) to form a hybridized mode. Compared with previous symmetry-protected BICs in all-dielectric structures, the SPP-coupled BIC has a significantly decreased mode volume, and its corresponding electric field is strongly localized below the Si nanopillars. We also show that the SPP mode makes the original polarization-independent symmetry-protected BIC become polarization-dependent. In addition, we demonstrate that the silver film in the considered structure can induce a metal mirror effect. The destructive interference between the magnetic dipole inside the Si nanopillars and the mirror magnetic dipole in the silver film can lead to the formation of accidental BICs. Our hybrid structure provides a versatile platform for the manipulation of light–matter interaction in the nanoscale.

Metal–insulator–metal micro-capacitors for integrated energy storage up to 105 Hz

Metal–insulator–metal (MIM) micro-capacitors for use in integrated energy storage applications are presented. A new, simple and batch Si processing compatible method for the creation of high aspect ratio metallic 3D structures on the surface of a Si substrate is described. The method consists of creating an array of Si nanopillars and then depositing Al at a small angle off the vertical while rotating the sample. Using this method, the effective area of the samples is increased by a factor of 3.8. Various capacitors are created using the described 3D structures as the lower electrode, with anodic alumina and atomic layer deposited HfO2 as the dielectric. Al and Cu top electrodes are also investigated. Large values of capacitance densities as high as 3.2 μF cm−2 are achieved. All capacitors are demonstrated to possess small values of series resistances and stable operation up to a frequency of 105 Hz. These results make the presented MIM capacitors exceed the state-of-the-art while maintaining a simple and integrable fabrication scheme which renders them very interesting for energy storage applications where operational frequencies larger than 1 kHz are required, as is the case in several vibrational energy harvesters.

Adaptive moving environment for efficient molecular dynamics simulations of high-fluence ion irradiation

Ion-beam processing of materials is widely supported by atomistic simulations by means of molecular dynamics. Although the approach has given several valuable insights, it has a limited operational window in both time and length scales. In particular, for high-fluence ion irradiation with multiple consecutive cascades, the direct molecular dynamics method becomes prohibitively time consuming.In this work, we propose a speed-up algorithm for molecular-dynamics simulations of multiple consecutive collision cascades employing an adaptive moving environment model. In the model, the computational power is primarily focused on calculating the atomic movement in the propagating cascade regions, while thermally equilibrated regions outside the cascades are excluded. Up to a five-fold efficiency increase was seen with the adaptive moving environment compared to classical molecular dynamics, without any significant statistical difference in the results of multiple individual ion-cascade simulations in a heterostructure of alternating Si and SiO2 layers. Simulations of temperature-driven dynamic annealing during high-fluence ion irradiation of Si nanopillars at elevated temperatures using the adaptive moving environment showed similar trends as experiments with respect to temperature dependence. The model is included in the atomistic simulator toolkit, COSIRMA (COmputer Simulator for IRradiation of MAterials), and can easily be enabled through the user-friendly graphical interface.