Silicon Pillars Research Articles

Localized states and quantum effect of photo-generated carriers in photovoltaic system

We have fabricated the multiple nanolayers impuritied on silicon pillars for Si solar cells to pick up photons in ultraviolet and infrared region of solar spectra, in which the localized states originated from nanosilicon doped with oxygen are built to avoid Auger recombination, and some interesting quantum phenomena in the localized states have been observed. The quantum effect of photo-generated carriers has been observed in I-V curve measurement on the photovoltaic sample prepared in oxygen by using nanosecond pulsed laser. More interesting, the twin states of quantum vibration are measured in the localized states originated from the impuritied nanosilicon, which provides a stable reservoir for electrons in the photovaltaic system. It should be noted that the amplitude change of the quantum vibration occurs under magnetic field with 0.33T on the sample prepared in oxygen, owing to the electron spin in the localized states. The photoluminescence (PL) spectra measured from 300 nm to 1700 nm exhibit the localized states in various regions in the photovoltaic system, in which the electrons can stand in the localized states with longer lifetime to be uneasy into Auger recombination.

Cantilever with High Aspect Ratio Nanopillars on Its Top Surface for Moisture Detection in Electronic Products

This work reports the patterning silicon pillars by metal‐assisted chemical etching (MACE) process as a post process on a silicon cantilever for a moisture detection. Although the cantilever is very fragile, the patterning of the pillar structures on the cantilever has been successfully demonstrated. The cantilever coated with a material absorbing water (such as polyimide and mesoporous silica) can use as a humidity sensor. Its bending is due to the surface stress change from water molecule absorption. However, the bending of the cantilever is usually at a small value. Here, the silicon cantilever with high aspect ratio pillars on its surface is proposed, which is expected for a larger bending of the cantilever during the water molecule absorption. The moisture detection utilizes the principle that the pillars stack together based upon the condensation behavior of a water vapor on their surfaces.

Large‐Area Wide‐Incident‐Angle Metasurface Perfect Absorber in Total Visible Band Based on Coupled Mie Resonances

A large‐area, ultrathin metasurface perfect absorber (MSPA) based on coupled Mie resonances from integrated arrays of silicon (Si) and metallic pillars is reported. In contrast to previous structures and mechanisms, coupled Mie resonances are generated from two pillars of dissimilar materials (dielectric and metallic) within an ultrathin structure such that the Mie resonances of dielectric and metallic patterns occur complementarily at different wavelengths to realize broadband absorption over the entire visible wavelength band. Double‐beam interference lithography, reactive ion etching, and sputter coated depositions are utilized to fabricate the proposed MSPA. With an appropriate arrangement of the patterned geometry, an average polarization independent absorption of 0.975 (0.958, experiment) in the entire visible band (from 400 to 760 nm) is achieved and remains high (0.930, simulation, and 0.912, experiment) when the incident angle increases to 70°. The proposed MSPA is thus an effective way to realize a large‐area, ultrathin metasurface absorber with a relatively simple and easy to fabricate structure.

Effect of Cu-doping on the visible photocatalytic performance of Cu<sub>2</sub>O immersion-plated on silicon nanoporous pillar array

In the past decades, semiconductor photocatalytic degradation was considered as one of the primary means to deal with the organic pollutants in water, because its final products, water and carbon dioxide, are harmless to environment. Owing to the high photocatalytic activity and corrosion stability, titanium dioxide (TiO2) has been widely studied and much promising progress has been made in the past years. However, TiO2 is a wide bandgap semiconductor (~3.2 eV), which decides that the generation of the active holes needed for photocatalysis can only be realized under the ultraviolet illumination accounting for only ~5% of the total solar energy. Considering that the solar energy distributed in the visible region is over 45%, the exploration on high-efficiency visible photocatalysts is of special significance. Actually, great efforts have been made to expand the active light wavelength of TiO2 from ultraviolet to visible regime, such as through doping or co-doping techniques or modifying semiconductor surfaces with other substances. Although the results from theoretical calculations is rather encouraging, the actual effectiveness through either element doping or surface modifying on tuning the energy band structure is far from satisfactory in experiments. Furthermore, the unstability of the semiconductors after doping or co-doping treatment is still a problem to be solved. Cuprous oxide (Cu2O) is a p-type and direct bandgap (2.17 eV) semiconductor material, which imply that it can directly utilize the visible light regime accounting for 45% of the solar energy. As a visible light photocatalyst, Cu2O has been studied and exhibited given photocatalytic activity. Meanwhile, the photocatalyst existed in nanopowder form would not be helpful for realizing cyclic utilization. So, preparing a high-efficiency, easy to be recycled photocatalyst under the visible light irradiation is of great importance. In the previous study, we reported the preparation and characterization of silicon nanoporous pillar array (Si-NPA), a silicon hierarchical structure featured by an array of regularly aligned, quasi-identical and highly nanoporous silicon pillars with strong light absorption across the whole visible region. These structural and optical properties imply that Si-NPA might be an effective photocatalyst to degrade organic pollutants under visible light irradiation. Its large specific surface area could provide sufficient space or active sites for pollutant adsorption, its strong visible region light absorption would be beneficial to fully utilize the sunlight energy, and its solid-state form would be helpful for realizing cyclic utilization compared with nanopowder photocatalysts. Considering the special structure and physical properties of Si-NPA, this work selected Si-NPA as substrates and the samples were obtained with different preparing parameters. Cu2O/Si-NPA, Cu2O:Cu/Si-NPA (with Cu2O as the main component), Cu:Cu2O/Si-NPA (with Cu as the main component) and Cu/Si-NPA were prepared by an immersion-plating method. The surface morphologies, microstructures and visible photocatalytic degradation performances to methyl orange (MO) were characterized. It is disclosed that for samples specified by 1.8 cm×2.0 cm, a photocatalytic degradation rate of ~53.6% to MO for Cu2O:Cu/Si-NPA was achieved under visible light (400–800 nm) with an optical power density of 10 mW/cm2, which was about 20% higher than Cu2O/Si-NPA. The efficiency promotion is attributed to the Cu-doping in Cu2O, which would be beneficial to decrease the probability of the photon-generated electrons captured by defects or recombined with holes and enhance their transport process from internal to material surface. Such an enhancement would produce the reactive oxygen species with a concentration that higher than pure Cu2O. These results might provide a promising way for further promoting the visible photocatalytic degradation rate of Cu2O to organic pollutant.

Preparation and characterization of Fe,Co,Si-pillared montmorillonites with aminosilanes as silicon pillars precursor

Fe,Co,Si-pillared montmorillonites with micro- and mesopores and high thermal stability were prepared by directly introducing transition metal ions (Fe and Co) and aminosilanes to the interlayer of calcium montmorillonite (Ca-Mt) from Inner Mongolia. The presence of Fe, Co and Si in the interlayer spaces of montmorillonite (Mt) and properties of these pillared Mt were investigated by a combination of X-ray diffraction, elemental analysis, N2 adsorption-desorption, thermal analysis, NH3 temperature-programmed desorption and H2 temperature-programmed reduction capacity tests techniques. The results indicated that the enhanced physicochemical properties of pillared Mt could be attributed to the formation of Fe, Co and Si composite pillars supporting the interlayer space. Particularly, aminosilane played an important role in improving the basal spacing and thermal stability of pillared montmorillonites.

CMOS compatible fabrication of micro, nano convex silicon lens arrays by conformal chemical vapor deposition.

We present a novel CMOS-compatible fabrication technique for convex micro-nano lens arrays (MNLAs) with high packing density on the wafer scale. By means of conformal chemical vapor deposition (CVD) of hydrogenated amorphous silicon (a-Si:H) following patterning of silicon pillars via electron beam lithography (EBL) and plasma etching, large areas of a close packed silicon lens array with the diameter from a few micrometers down to a few hundred nanometers was fabricated. The resulting structure shows excellent surface roughness and high uniformity. The optical focusing properties of the lenses at infrared wavelengths were verified by experimental measurements and numerical simulation. This approach provides a feasible solution for fabricating silicon MNLAs compatible for next generation large scale, miniaturized optical imaging detectors and related optical devices.

Silicon based On-chip Sub-Wavelength Grating Ring and Racetrack Resonator BioSensors

ABSTRACTIn this paper, we experimentally study the unique surface sensing property and enhanced sensitivity in subwavelength grating (SWG) based microring resonator biosensors versus conventional ring resonator biosensors. In contrast to a conventional ring, the effective sensing region in the SWG microring resonator includes not only the top and side of the waveguide, but also the space between the silicon pillars on the propagation path of the optical mode. It leads to an unique property of thickness-independent surface sensitivity versus common evanescent wave sensors; in other words, the surface sensitivity remains constantly high with progressive attachment of biomolecules to the sensor surface. To increase the robustness of performance of ring shaped circular SWG biosensors, we experimentally demonstrate silicon SWG racetrack resonators. A quality factor of 9800 and bulk sensitivity (S) is ∼429.7 nm/RIU (refractive index per unit) results in an intrinsic detection limit (iDL) 3.71×10-4 RIU in racetrack SWG biosensors while still retaining the accumulated surface thickness properties of circular rings.

Invar Electrodeposition for Controlled Expansion Interconnects

In this work, we developed an electroplating technique to produce uniform, low-internal-stress and controlled-expansion nickel iron alloy (36% of Nickel and 64% of Iron (INVAR)) films deposited on different substrates to fabricate redistribution layers (RDL), through silicon vias (TSV) interconnects, pillars, bumps, pads and free standing foils. The linear thermal expansion of the INVAR deposits was measured down to 0.4 ± 0.1 (x10−6, K−1) within a process window that corresponds to ultra-low internal stress (30 ± 2 MPa) and low surface roughness (Ra ∼2 nm). Thermal conductivity and specific heat capacity of the INVAR pinhole-free films were found to be in the range of 43 ± 5 W/mK and 0.45 ± 0.05 J/gK, respectively. Ultra-high ductility of about 40 ± 10 % has been obtained for free standing INVAR films, which possess high tensile strength of 500 ± 60 MPa and yield strength of 280 ± 50 MPa.

筋細胞内包ゲルファイバーの集積化による骨格筋組織の模倣

In this paper, we demonstrated mimicking of natural skeletal muscle tissue by integration of gel fiber containing muscle cells fabricated using microfluidic device. Gel fiber was mainly composed of sodium alginate gel and fibrin gel, its fibrin gel plays a key role as cell scaffold for cell growth and elongation. Gel fiber containing muscle cells was rolled up and transferred between silicon tube pillars to culture while apply tensile strength. After that, sodium alginate gel which inhibits cell growth and elongation was dissolved by immersing into 1μg/mL alginate lyase solution. As a consequence, shrinkage of integrated gel fiber containing muscle cells was confirmed at about twelve hours from culture, and anisotropic cell growth and elongation along integrated gel fiber direction was observed at the same time. This result represents mimicking of a part of natural skeletal muscle tissue and possibility of muscle contraction by culture in differentiation medium.

Growth of aluminum nitride on flat and patterned Si (111) by high temperature halide CVD

The aim of the present study is to evaluate the possibility of improving crystalline quality of AlN deposited on Si (111) substrates by implementation of substrate patterning. Growth on flat Si (111) substrates was conducted to investigate influence of deposition parameters on the growth behavior and quality of resulting AlN films. This investigation showed that SiC buffer layer is required to suppress disorientation of the AlN layer, optimum growth temperature is about 1260°C, Al-rich conditions favor more rapid coalescence of the nucleation island while N-rich conditions lead to the formation of a smooth surface. Transposition of the resulting AlN growth recipe on to patterned Si (111) substrates revealed reduction of the stress in AlN, bending of the threading dislocations and formation of a dislocation free area in the overgrown region. For the formation of a continuous layer on silicon pillars, we propose a novel technique based on the decrease of the mean free path of gaseous species leading to localized growth of AlN on the top of the pillars. Such continuous layer exhibit lower crack density compared to that on a flat substrate. Thus, the growth of AlN on pillar patterned Si substrate is seen to be a promising way for the further improvement of the AlN films quality.

Unique surface sensing property and enhanced sensitivity in microring resonator biosensors based on subwavelength grating waveguides.

In this paper, unique surface sensing property and enhanced sensitivity in microring resonator biosensors based on subwavelength grating (SWG) waveguides are studied and demonstrated. The SWG structure consists of periodic silicon pillars in the propagation direction with a subwavelength period. Effective sensing region in the SWG microring resonator includes not only the top and side of the waveguide, but also the space between the silicon pillars on the light propagation path. It leads to greatly increased sensitivity and a unique surface sensing property in contrast to common evanescent wave sensors: the surface sensitivity remains constantly high as the surface layer thickness grows. Microring resonator biosensors based on both SWG waveguides and conventional strip waveguides were compared side by side in surface sensing experiment and the enhanced surface sensing capability in SWG based microring resonator biosensors was demonstrated.

Gas sensor based on ZnO film/silica pillars

Silica submicron pillars are used as substrate for Zinc oxide (ZnO) gas sensor for the first time. The submicron pillars with the large surface ratio can improve the gas sensing performance obviously. Silicon pillars are fabricated by cesium chloride (CsCl) self-assembly lithography and inductively coupled plasma dry etching as substrate, the ZnO film is deposited on the pillars surface by RF magnetron sputtering. With this method, the pillar based gas sensor has the higher gas response, the shorter response and recovery time than the planar one with different working temperatures, different gas concentrations. 300 °C is the best working temperature, for planar gas sensor, the gas response is 22.81 for 1520 ppm ethanol, the response time is 55 s and the recovery time is 169 s. While for the pillar based one, the gas response is 28.20, the response time is 51 s and the recovery time is 92 s.

Monte Carlo simulations of atomic layer deposition on 3D large surface area structures: Required precursor exposure for pillar- versus hole-type structures

Due to its excellent conformality, atomic layer deposition (ALD) has become a key method for coating and functionalizing three dimensional (3D) large surface area structures such as anodized alumina (AAO), silicon pillars, nanowires, and carbon nanotubes. Large surface area substrates often consist of arrays of quasi-one-dimensional holes (into which the precursor gas needs to penetrate, e.g., for AAO), or “forests” of pillars (where the precursor gas can reach the surface through the empty 3D space surrounding the pillars). Using a full 3D Monte Carlo model, the authors compared deposition onto an infinite array of holes versus an infinite array of pillars. As expected, the authors observed that the required exposure to conformally coat an array of holes is determined by the height to width ratio of the individual holes, and is independent of their spacing in the array. For the pillars, the required exposure increases with decreasing center-to-center distance and converges in the limit to the exposure of an array of holes. Our simulations show that, when targeting a specific surface area enhancement factor in the range 20–100, a well-spaced pillar geometry requires a 2–30 times smaller precursor exposure than a hole geometry and is therefore more ALD friendly. The difference in required exposure is shown to depend on the initial sticking probability and structural dimensions.

Ordered Silicon Pillar Arrays Prepared by Electrochemical Micromachining: Substrates for High-Efficiency Cell Transfection

Ordered arrays of silicon nano- to microscale pillars are used to enable biomolecular trafficking into primary human cells, consistently demonstrating high transfection efficiency can be achieved with broader and taller pillars than reported to date. Cell morphology on the pillar arrays is often strikingly elongated. Investigation of the cellular interaction with the pillar reveals that cells are suspended on pillar tips and do not interact with the substrate between the pillars. Although cells remain suspended on pillar tips, acute local deformation of the cell membrane was noted, allowing pillar tips to penetrate the cell interior, while retaining cell viability.

Nanoscale plastic deformation mechanisms of single crystalline silicon under compression, tension and indentation

Mechanical behavior and underlying nanoscale plastic deformation mechanisms of single crystalline silicon under compression, tension and indentation are investigated through molecular dynamics in this work. Simulation results show that phase transformation from diamond cubic Si-I to [Formula: see text]-Sn is responsible for the plastic deformation behavior of Si both under compression and nanoindentation. A stress plateau is observed when the specimen is compressed uniaxially. Si-I to [Formula: see text]-Sn phase transformation has been demonstrated to be responsible for such stress plateau. Periodic boundary condition is found not suitable to study the tensile strength of silicon pillars. A pop-in behavior is observed in the force–displacement curve of nanoindentation. It has been proved that this pop-in region is induced by Si-I to [Formula: see text]-Sn phase transformation. Through tracking the atom stress, shear stress rather than normal stress is revealed to dominate the phase transformation process. During nanoindentation, to exclude size effect the substrate should be larger enough than the indenter.

Invar Electroplating for Controlled Expansion Interconnects

The current trends in convergent systems are to pack as many functions – digital, RF, optical, MEMS, LEDs, and other IoT devices – into a single package or module and current methods of metallizing the ever-shrinking components are rapidly becoming insufficient. A new set of metallization material technologies is necessary to meet these increasing expectations, methods that not only work with the substrates and materials of yesterday but are also ready for the future. The copper that is currently the workhorse of interconnect materials has a coefficient of thermal expansion (CTE) of 17 ppm/K while chip substrates’ CTE ranges from 2 to 7 ppm/K. The thermo-mechanical stresses induced in the package from the CTE mismatch between the die, the substrate and the buildup materials result in solder joint failure, die cracking, copper extrusion, delamination of the solder bumps and cracking of the build-up layers, leading to failure of the assemblies. Matching the CTEs of the chips and the substrates is required for future fine-pitch flip chip and wafer level package assembly in order to improve the solder joint reliability and fabricate stress-free 3D multichip systems. NANO3D Systems has developed an electroplating technique to produce uniform, low-internal-stress and controlled-expansion nickel iron alloy (INVAR) films deposited on different substrates, fabricating redistribution layers (RDL), through silicon vias (TSV), pillars, and free standing foils. INVAR electroplating was performed using a proprietary plating solution containing stabilizers that prevent the oxidation of iron, stress-reducers to reduce internal stress, and wetting agents to improve the surface morphology and eliminate pinholes. The thermal (thermal conductivity, linear thermal expansion, specific heat capacity) and mechanical (Young’s modulus, yield strength, tensile strength and internal stress) properties of the INVAR have been studied and demonstrated superior performance compared to currently available similar products. The composition of the INVAR (36 % of Nickel and 64% of Iron) electroplated films has been confirmed by utilizing SEM/EDX and SIMS. Linear thermal expansion of the films was tested using a technique based on bending of bonded layers due to thermal stress. Internal stress was studied using bent strip techniques. The linear thermal expansion down to 0.4±0.1 (x10-6, K-1) has been obtained for the INVAR plated films plated within a process window that corresponded to the lowest internal stress (30±2 (MPa)) and uniform morphology (Ra ~ 2 nm). The mechanical properties of the INVAR foils have been investigated by use of the Instron technique. The tensile strength of 500 ± 60 (MPa), yield strength of 280 ± 50 (MPa) and the ductility of 40 ± 10% have been measured for the pinhole-free INVAR films. The specific heat capacity of 0.45 ± 0.05 (J/gK)was measured by using differential scanning calorimetry (DSC) at 573 K, while thermal conductivity of as-plated INVAR was 43 ± 5 (W/mK). Smooth, uniform and flat RDL & pillars were fabricated by using through resist INVAR plating on silicon wafers. Optical profilometry studies of the morphology and thickness of plated INVAR on the silicon wafers show Ra ~ 2 nm and high thickness uniformity (of about 5 % & 3s). TSV fill of high aspect ratio features (>10:1) was also demonstrated. After being annealed at 300 0C for 2hr, SIMS studies showed no inter-diffusion in copper/Invar bi-layer foils. Trace impurities were <100 ppm in electroplated INVAR films. Analytical methods have been developed for plating solution analysis. UV-VIS and titration are the two main techniques employed for analysis of metals, organic acids, and buffers in solution. Standard curves were created by analyzing solutions with known concentrations of each analyte. Solutions of varying concentrations were prepared for analysis; low control limit (low), target, and upper control limit (high) concentrations. Blank solutions contained all components except for the one of interest. There was also a blank prepared with no metals at all to be used as the reference in UV-VIS analysis. Multiple analyses were collected for the standard solutions in order to perform a statistical analysis and confirm p/t <0.3. Using the standard curves concentrations of unknowns (active bath solutions) were calculated. The plating additives in INVAR electroplating bath such as stress-reducer, stabilizer and wetting agents were monitored by using cyclic voltammetric stripping (CVS) and FTIR techniques.

Field Emission from Titanium-Coated Carbon Nanotubes Grown on Micron-Sized Silicon Pillar Arrays

Carbon nanotubes (CNTs) have attracted great attention because of their unique physical properties such as high mechanical strength, thermal stability and electrical properties[1]. Because CNTs have high electron emission efficiency at low voltage due to their high aspect ratio, CNTs have been regarded as the most efficient electron emitter among various materials such as metal, semiconductor. However, the electric-field-screening effect of the dense CNTs and relatively high work function of CNTs (~ 5 eV) degrades the emission performance significantly[2,3]. CNTs grown on uneven substrate can reduce electric-field-screening effect and coating of CNTs with lower work function leads to enhanced field emission. In particular, it is reported that the coating Titanium (Ti) on CNTs affects the work function and the substantial change in surface morphology of CNTs most significantly[4].In this paper, we report the combined effect of suppression of electric-field-screening and lower work function on field emission characteristics by Ti-coated CNTs on micron-sized silicon (Si) pillar arrays. Micron-sized Si pillar arrays are formed using conventional photolithography process including reactive ion etching by mixture gas of SF6 and Ar. CNTs are grown on Invar(52% Fe-42% Ni-6% Co alloy) catalyst using C2H2 as a source gas by inductively-coupled plasma-enhanced chemical vapor deposition. Then, Ti with 5 nm thickness is optionally coated on CNTs by sputtering method and field emission behaviors are investigated in a diode configuration in the vacuum of ~10-7 Torr.Field enhancement factor for CNTs on Si pillar arrays was higher by 15% compared with that of CNTs on Si. The application of Ti on CNT surface reduced turn-on field by 30% further compared with that of pristine CNTs. In addition, coating of CNTs also improved emission stability by strengthening the adhesion between CNTs and substrate. We expect that the Ti-coated CNTs grown on micron-sized silicon pillar arrays can be excellent electron sources, providing a very stable current at very low fields by reduction of electric-field-screening effect and effective work function of emitters..

Invar Electroplating for Controlled Expansion Interconnects

NANO3D SYSTEMS (“NANO3D”) has developed an electroplating technique to produce uniform, low-internal-stress and controlled-expansion nickel iron alloy (36 % of Nickel and 64% of Iron (INVAR)) films on various substrates to fabricate redistribution layers (RDL), through silicon vias (TSV) interconnects, pillars, bumps, pads and free standing foils. The linear thermal expansion of the INVAR deposits was measured down to 0.4 ± 0.1 (x10-6, (K-1)) within a process window that corresponds to the ultra-low internal stress (30±2 (MPa)) and uniform morphology (Ra ~ 2 (nm)). The thermal conductivity and specific heat capacity of the INVAR pinhole-free films were found to be in the range of 43 ± 5 (W/mK) and 0.45 ± 0.05 (J/gK), respectively. Ultra-high ductility of about 40 ± 10 % has been obtained for the free standing INVAR films, which possess the high tensile strength of 500 ± 60 (MPa) and yield strength of 280 ± 50 (MPa).

(Invited) Silicon Emission Mechanism for Oxidation Process of Non-Planar Silicon

The advance in the understanding of the mechanism of the silicon oxidation process toward the fine control of 3D MOSFETs is explained. The silicon emission mechanism can be applied to the oxidation process of silicon pillars as well as that of planar silicon. Since the geometrical effect is inevitable for the 3D MOSFETs, the silicon emission mechanism is the key to achieve high-performance non-planar 3D MOSFETs.

High quality factor subwavelength grating waveguide micro-ring resonator based on trapezoidal silicon pillars.

Subwavelength grating waveguide-based micro-ring resonators (SWGMRs) are a promising platform for research in light-matter interaction. However, it is extremely difficult to achieve small radius SWGMR devices (e.g., 5 μm) with satisfying quality factors (e.g., ∼10,000). One major issue is the large bend loss of small radius SWGMRs. In this work, we report the use of trapezoidal silicon pillars instead of conventional rectangular silicon pillars as building blocks of SWGMRs. We found that an asymmetric effective refractive index profile created by trapezoidal silicon pillars can significantly reduce the bend loss and therefore increase the quality factors of SWGMRs. For the first time to the best of our knowledge, we have experimentally demonstrated a 5 μm radius SWGMR made of trapezoidal silicon pillars (T-SWGMR) with an applicable quality factor as high as 11,500, 4.6 times of that (∼2800) offered by a conventional SWGMR made of rectangular silicon pillars, which indicates an 81.4% reduction of the propagation loss. This approach can also be readily employed to enhance SWGMRs with larger radii. We have also experimentally demonstrated a 10 μm radius T-SWGMR with a quality factor as high as 45,000, which indicates a propagation loss as low as 6.07 dB/cm.