Flat Silicon Substrates Research Articles

Plasma Polymerization on Mesoporous Surfaces: n-Hexane on Titanium Nanoparticles

Plasma treatment of porous materials has huge potential in many applications where chemical modification of interior structure or its loading with polymeric matrices is required. Plasma polymers are often assumed to grow conformally on solid supports; however, their innate roughness may interfere with the surface topography at mesoscale. Here, model mesoporous coatings were prepared by deposition of titanium nanoparticles with the average size of 50 nm onto flat silicon substrates. The nanoparticles were fabricated by magnetron sputtering in the configuration of a gas aggregation source. The porous coatings were subsequently subjected to deposition of soft hydrocarbon plasma polymers from n-hexane. In the early stage of the deposition, negligible increase of thickness is observed as the plasma polymer fills the inner free space of the coatings. The topographical features expand in lateral dimension, but the RMS roughness does not change. In the late growth regime, the inner voids become filled and the plasma polymer grows on top of the coatings. The growth proceeds with preferential filling of the valleys between the surface asperities, thus leading to smoothening of the surface. The decrease of roughness is manifested in negative growth exponent. Equality between the absolute values of the local and global roughness exponent evidences about self-affine growth dynamics. Nevertheless, the set of the critical exponents (the growth exponent β = −0.16, the dynamic exponent 1/z = 0.16, the roughness exponent |α| = 1.0) indicates that the deposition does not match any of universality classes of local growth. Local diffusion coupled with nonlocal mass transport due to re-emission of the incoming species can explain the kinetic smoothening observed. The technology allows for fabrication of mesoporous coatings with precisely adjustable pore size or straightforward production of nanocomposite thin films with nanoparticles homogeneously embedded in the matrix of plasma polymer.

Effects of Pillar Height and Junction Depth on the Performance of Radially Doped Silicon Pillar Arrays for Solar Energy Applications

The effects of pillar height and junction depth on solar cell characteristics are investigated to provide design rules for arrays of such pillars in solar energy applications. Radially doped silicon pillar arrays are fabricated by deep reactive ion etching of silicon substrates followed by the introduction of dopant atoms by diffusion from a phosphorus oxide layer conformally deposited by low‐pressure chemical vapor deposition. Increasing the height of the pillars has led to doubling of the efficiency from 6% for flat substrates to 12% for 40 μm high pillars with a 900 nm junction depth because of an increase in the total junction area and lower optical reflection. For higher pillars, the current density and efficiency is decreased, which is attributed to the increasing presence of defect states at the surface introduced during the etching process. This effect can be counteracted by an Al2O3 passivation layer on the pillar surface. An optimum efficiency of 13% is found for a junction depth of 790 nm for 40 μm pillar height. At increased junction depths, the efficiency is decreased due to the ever thinner undoped core of the pillars, causing pillars with a large junction depth to become less efficient than flat silicon substrates.

Titanium nitride films for micro-supercapacitors: Effect of surface chemistry and film morphology on the capacitance

Electrochemical capacitors (EC) in the form of packed films can be integrated in various electronic devices as power source. A fabrication process of EC electrodes, which is compatible with micro-fabrication, should be addressed for practical applications. Here, we show that titanium nitride films with controlled porosity can be deposited on flat silicon substrates by reactive DC-sputtering for use as high performance micro-supercapacitor electrodes. A superior volumetric capacitance as high as 146.4 F cm−3, with an outstanding cycling stability over 20,000 cycles, was measured in mild neutral electrolyte of potassium sulfate. The specific capacitance of the films as well as their capacitance retentions were found to depend on thickness, porosity and surface chemistry of electrodes. The one step process used to fabricate these TiN electrodes and the wide use of this material in the field of semiconductor technology make it promising for miniaturized energy storage systems.

Mesoporous Silica Nanoparticles under Sintering Conditions: A Quantitative Study.

Thin films made of mesoporous silica nanoparticles (MSNs) are finding new applications in catalysis, optics, as well as in biomedicine. The fabrication of MSNs thin films requires a precise control over the deposition and sintering of MSNs on flat substrates. In this study, MSNs of narrow size distribution (150 nm) are synthesized, and then assembled onto flat silicon substrates, by means of a dip-coating process. Using concentrated MSN colloidal solutions (19.5 mg mL(-1) SiO2), withdrawal speed of 0.01 mm s(-1), and well-controlled atmospheric conditions (ambient temperature, ∼ 70% of relative humidity), monolayers are assembled under well-structured compact patterns. The thin films are sintered up to 900 °C, and the evolution of the MSNs size distributions are compared to those of their pore volumes and densities. Particle size distributions of the sintered thin films were precisely fitted using a model specifically developed for asymmetric particle size distributions. With increasing temperature, there is first evidence of intraparticle reorganization/relaxation followed by intraparticle sintering followed by interparticle sintering. This study is the first to quantify the impact of sintering on MSNs assembled as thin films.

Efficient Multiterminal Spectrum Splitting via a Nanowire Array Solar Cell.

Nanowire-based solar cells opened a new avenue for increasing conversion efficiency and rationalizing material use by growing different III–V materials on silicon substrates. Here, we propose a multiterminal nanowire solar cell design with a theoretical conversion efficiency of 48.3% utilizing an efficient lateral spectrum splitting between three different III–V material nanowire arrays grown on a flat silicon substrate. This allows choosing an ideal material combination to achieve the proper spectrum splitting as well as fabrication feasibility. The high efficiency is possible due to an enhanced absorption cross-section of standing nanowires and optimization of the geometric parameters. Furthermore, we propose a multiterminal contacting scheme that can be fabricated with a technology close to standard CMOS. As an alternative we also consider a single power source with a module level voltage matching. These new concepts open avenues for next-generation solar cells for terrestrial and space applications.

Optical reflectance of alkali-textured silicon wafers with pyramidal facets: 2D analytical model

Abstract

High current field emission from individual non-linear resistor ballasted carbon nanotube cluster array

Field emission (FE) electron sources based on carbon nanotubes (CNTs) have the potential to serve as cold cathodes for various vacuum microelectronic and nanoelectronic devices. Emission currents are extremely sensitive to variation in emitter geometry and local surface states, both of which are difficult to synthesize uniformly when fabricating a CNT field emission array (FEA). Such non-uniformities cause unstable emission, limiting the current output. Here, we propose a method for simulating and fabricating a high performance CNT-FEA with emission units that are individually connected to a single crystalline silicon pillar (SP), which acts as an non-linear ballast resistor. Results showed that the driving field for this CNT-FEA was greatly reduced relative to CNT-FEAs on a flat silicon substrate. This improvement was due to the high aspect ratio of the CNT clusters combined with SPs. The FE behavior demonstrated that the emission current was limited by the non-linear resistors (NLRs). Emitted currents density over 1.65A/cm2 at a low extraction field of 5.8V/μm were produced by a 1mm2 emmiting area. The proposed technology may be used to fabricate cathodes capable of reliable, uniform, and high current emission.

Evolution of magnetism on a curved nano-surface.

To design custom magnetic nanostructures, it is indispensable to acquire precise knowledge about the systems in the nanoscale range where the magnetism forms. In this paper we present the effect of a curved surface on the evolution of magnetism in ultrathin iron films. Nominally 70 Å thick iron films were deposited in 9 steps on 3 different types of templates: (a) a monolayer of silica spheres with 25 nm diameter, (b) a monolayer of silica spheres with 400 nm diameter and (c) for comparison a flat silicon substrate. In situ iron evaporation took place in an ultrahigh vacuum chamber using the molecular beam epitaxy technique. After the evaporation steps, time differential nuclear forward scattering spectra, grazing incidence small angle X-ray scattering images and X-ray reflectivity curves were recorded. In order to reconstruct and visualize the magnetic moment configuration in the iron cap formed on top of the silica spheres, micromagnetic simulations were performed for all iron thicknesses. We found a great influence of the template topography on the onset of magnetism and on the developed magnetic nanostructure. We observed an individual magnetic behaviour for the 400 nm spheres which was modelled by vortex formation and a collective magnetic structure for the 25 nm spheres where magnetic domains spread over several particles. Depth selective nuclear forward scattering measurements showed that the formation of magnetism begins at the top region of the 400 nm spheres in contrast to the 25 nm particles where the magnetism first appears in the region where the spheres are in contact with each other.

Controlled Doping Methods for Radial p/n Junctions in Silicon

P/n and n/p junctions with depths of 200 nm to several micrometers have been created in flat silicon substrates as well as on 3D microstructures by means of a variety of methods, including solid source dotation (SSD), low‐pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition, and plasma‐enhanced chemical vapor deposition. Radial junctions in Si micropillars are inspected by optical and scanning electron micro­scopies, using a CrO3‐based staining solution, which enables visualization of the junction depth. When applying identical‐doping parameters to flat substrates, ball grooving, followed by staining and optical microscopy, yields similar junction depth values as high‐resolution scanning electron microscopy imaging on stained cross‐sections and secondary ion mass spectrometry depth profilometry. For the investigated 3D microstructures, doping based on SSD and LPCVD give uniform and conformal junctions. Junctions made with SSD‐boron doping and CVD‐phosphorus doping could be accurately predicted with a model based on Fick's diffusion law. 3D‐microstructured silicon pillar arrays show an increased efficiency for sunlight capturing. The functionality of micropillar arrays with radial junctions is evidenced by improved short‐circuit current densities and photovoltaic efficiencies compared with flat surfaces, for both n‐ and p‐type wafers (average pillar arrays efficiencies of 9.4% and 11%, respectively, compared with 8.3% and 6.4% for the flat samples).

Fabrication of metamaterial absorber using polymer brush – gold nanoassemblies for visualizing the reversible pH-responsiveness

Poly(2-dimethylaminoethyl methacrylate) was grafted onto flat silicon substrates via atom transfer radical polymerization to form bottom layers. Two fabrication processes, very-large-scale integration and reactive ion etching, were applied in sequence to generate 400 nm-scale hole arrays of tethered poly(2-dimethylaminoethyl methacrylate) layers as sub-wavelength hole array on the Si surfaces, followed by the immobilization of surface functionalized gold nanoparticles by means of physical interaction (electrostatic attraction, entanglement, and hydrogen bonding) to generate a metamaterial absorber. Hole dimensions of the metamaterial absorber shrank reversibly and significantly upon changing the pH from 10.7 and 3.3 due to swelling behavior, leading to a significant change in surface plasmon resonance. We found that the change in the surface plasmon resonance of the metamaterial absorber, related to the dimensions of its holes upon pH treatment, resulted in a color change from purple to orange, observable by the naked eye at a fixed angle of 10°. The maximum shift of the absorptive peak position was greater than 300 nm, and these shifts corresponded to the color changes in the solutions at various values of pH.

Silver substrates for surface enhanced Raman scattering: Correlation between nanostructure and Raman scattering enhancement

The fabrication of substrates for Surface Enhanced Raman Scattering (SERS) applications matching the needs for high sensitive and reproducible sensors remains a major scientific and technological issue. We correlate the morphological parameters of silver (Ag) nanostructured thin films prepared by sputter deposition on flat silicon (Si) substrates with their SERS activity. A maximum enhancement of the SERS signal has been found at the Ag percolation threshold, leading to the detection of thiophenol, a non-resonant Raman probe, at concentrations as low as 10−10M, which corresponds to enhancement factors higher than 7 orders of magnitude. To gain full control over the developed nanostructure, we employed the combination of in-situ time-resolved microfocus Grazing Incidence Small Angle X-ray Scattering with sputter deposition. This enables to achieve a deepened understanding of the different growth regimes of Ag. Thereby an improved tailoring of the thin film nanostructure for SERS applications can be realized.

Ultrasonic force microscopy on poly(vinyl alcohol)/SrTiO(3) nano-perovskites hybrid films.

Atomic Force Microscopy (AFM) and Ultrasonic Force Microscopy (UFM) have been applied to the characterization of composite samples formed by SrTiO3 (STO) nanoparticles (NPs) and polyvinyl alcohol (PVA). The morphological features of the STO NPs were much better resolved using UFM than contact-mode AFM topography. For high STO concentrations the individual STO NPs formed nanoclusters, which gathered in microaggregates. The STO aggregates, covered by PVA, exhibited no AFM frictional contrast, but were clearly distinguished from the PVA matrix using UFM. Similar aggregation was observed for NPs in the composite samples and for NPs deposited on top of a flat silicon substrate from milliQ water solution in the absence of polymer. In the hybrid films, most STO nanoparticles typically presented a lower UFM contrast than the PVA matrix, even though stiffer sample regions such as STO should give rise to a higher UFM contrast. STO NPs with intermediate contrast were characterized by an UFM halo of lower contrast at the PVA/STO interface. The results may be explained by considering that ultrasound is effectively damped on the nanometer scale at PVA/STO interfaces. According to our data, the nanoscale ultrasonic response at the PVA/STO interface plays a fundamental role in the UFM image contrast.

Rolling, sliding and torsion of micron-sized silica particles: experimental, numerical and theoretical analysis

The contact mechanics of individual, very small particles with other particles and walls is studied using a nanoindenter setup that allows normal and lateral displacement control and measurement of the respective forces. The sliding, rolling and torsional forces and torques are tested with borosilicate microspheres, featuring radii of about 10μm. The contacts are with flat silicon substrates of different roughness for pure sliding and rolling and with silicon based, ion-beam crafted rail systems for combined rolling and torsion. The experimental results are discussed and compared to various analytical predictions and contact models, allowing for two concurrent interpretations of the effects of surface roughness, plasticity and adhesion. This enables us to determine both rolling and torsion friction coefficients together with their associated length scales. Interestingly, even though normal contacts behave elastically (Hertzian), all other modes of motion display effects due to surface roughness and consequent plastic deformation. The influence of adhesion is interpreted in the framework of different models and is very different for different degrees of freedom, being largest for rolling.

Gold nano-island arrays on silicon as SERS active substrate for organic molecule detection

Gold islands forming highly controlled arrays have been fabricated by two potential step electrochemical deposition method using nanopatterned Si surface templates. In the present work, the Raman scattering studies realized using 11-mercaptoundecanoic probe molecule showed that such structures exhibit an enhanced Raman signal compared with nanostructured physical deposited thin gold film on flat silicon substrate and can be valued as surface-enhanced Raman scattering substrates. Besides the more appropriate management of nano-island arrays distribution, the high ratio of their Raman signals can be explain by the epitaxial-like growth mechanism of the metallic nano-islands, clearly showed by X-ray diffraction studies. Furthermore, the substrates enabled reproducibility and stability detection due to the chemically assembling of organothiol molecules, the X-ray photoelectron spectroscopy studies confirming formation of the thiolate species which corresponds to AuS bonds, and also, the unwanted ‘hot-spots’ are missing, which make them suitable for high sensitivity biosensing applications.

Direct growth of III–V quantum dots on silicon substrates: structural and optical properties

This paper reports the recent progress obtained by direct growth of III–V semiconductor quantum dots (QDs) on flat and prepatterned silicon substrates. For the flat surfaces, we discuss the basic growth studies addressing mainly morphological and structural properties of QD-like structures using transmission electron microscopy. For the growth on prepatterned substrates, we report on the optimization of the electron beam lithography process to fabricate sub-100 nm holes in silicon and the MBE growth on prepatterned Si (1 0 0) surfaces. Finally, we report the observation of bright photoluminescence emissions from single InAs/GaAs QDs grown directly on flat silicon substrates.

Electrohydrodynamic direct-writing ZnO nanofibers for device applications

Interest towards adapting and developing nanofiber technology for device applications has been expanded in recent years. Here, we describe the use of electrohydrodynamic direct-writing (EDW) for the precise deposition of orderly functional nanofibers as relevant device architectures. Individual ZnO nanofibers can be obtained and assembled into complex patterns via EDW and subsequent thermal treatment. Grid arrays consisting of ZnO nanofibers were fabricated on a flat silicon substrate by controlling the trajectory of substrate. Furthermore, the precise deposition of ZnO nanofibers on three-dimensional silicon substrates including microbeams and pillars was demonstrated, illustrating the integration capability with MEMS structures and device platforms. A single ZnO nanofiber field-effect transistor was also fabricated and characterized. Our results show promise for EDW as a potential method to fabricate micro/nanodevices, especially electronics in nanoscale.

Carbon nanotube cathode with capping carbon nanosheet

Here, we report a vertically aligned carbon nanotube (VACNT) film capped with a few layer of carbon nanosheet (FLCN) synthesized by chemical vapor deposition using a carbon source from iron phthalocyanine pyrolysis. The square resistance of the VACNT film is significantly reduced from 1500Ω/□ to 300Ω/□ when it is capped with carbon nanosheet. The VACNT capped with carbon nanosheet was transferred to an ITO glass substrate in an inverted configuration so that the carbon nanosheet served as a flexible transparent electrode at the bottom and the VACNT roots served as emission tips. Because all of the VACNTs start growing from a flat silicon substrate, the VACNT roots are very neat and uniform in height. A field emission test of the carbon nanosheet-capped VACNT film proved that the CNT roots show better uniformity in field emission and the carbon nanosheet cap could also potentially serve as a flexible transparent electrode, which is highly desired in photo-assisted field emission.

In Situ Deposition Monitoring by a Tilted Fiber Bragg Grating Optical Probe: Probing Nucleation in Chemical Vapour Deposition of Gold

Abstract The gold(I) compound [Au(N i Pr) 2 CNMe 2 ] 2 was used as a chemical vapour deposition precursor to deposit gold metal films between 200 – 225 °C on both flat silicon substrates as well as on silica optical fibers. The deposited gold film was nanocrystalline with a preferred (111) orientation and had marginal conductivity. The film shows good purity and has a very high growth rate of 222 nm/min (37 A/s) when measured on the optical fiber. The growth of the gold film on the optical fiber was monitored by the attenuation of a guided light mode diffracted into the fiber's cladding by a tilted Bragg grating (TFBG) inscribed in the fiber core. The initiation of the gold film growth was found to coincide with an increase in the attenuation of polarized light propagating in the fiber and monitored at wavelengths near 1559 nm. This attenuation peaked when the film granularity maximized losses to the cladding mode and then disappeared when the film became thicker and continuous. Further evidence of a continuous film was indicated by a wavelength shift of 0.2 nm of the cladding mode resonance wavelengths. A growth rate of 222 nm/min (37 A/s) was found during the first 50 seconds of deposition, tapering off to an overall growth rate of 39 nm/min (6.5 A/s). The use of a TFBG as a sensor was shown to be a valuable method to monitor film nucleation, growth and uniformity.

Nonlinear length dependent electrical resistance of a single crystal zinc oxide micro/nanobelt

We have systematically investigated the intrinsic electrical property of a single crystal zinc oxide (ZnO) micro/nanobelt (MB/NB) using a conductive atomic force microscopy (AFM) technique. By mounting one end of the MB/NB on a flat nonconductive silicon substrate, a platform for performing electrical property characterization using conductive current AFM is established. The quantitative characterization of the electrical resistance of the MB/NBs was performed by acquiring I-V curves for the MB/NB in between the electrode and the conductive AFM tip. The resistance of the single crystalline ZnO MB/NB was measured to be exponentially dependent on the length of the MB/NB. A systematic model based on the anisotropic velocity of the carriers in the crystal planes has been proposed and fits the experimental measurement well. This research reveals that the electrical resistance shows a nonlinear length dependence in the semiconducting single crystal MB/NB, which is significantly different from the bulk counterpart. Understanding such a property could definitely improve the design and the performance of next generation electrical nanodevices.

Continuous and Nanostructured TiO2 Films Grown by dc Sputtering Magnetron

The growth of Anatase nanostructured films using dc reactive magnetron sputtering and post-annealing treatment is reported. TiO2 has been deposited on Porous Anodic Alumina Films used as templates which were previously grown in phosphoric acid solution and etched to modify their pore diameters. This synthesis via results in the formation of vertically aligned and spatially ordered TiO2 nanostructures replicating the underlying template. Previously, the growth optimization of TiO2 thin films deposited by dc magnetron sputtering on flat silicon substrates was done. The crystalline structure and Ti in-depth concentration profile were determined by grazing incidence X-ray diffraction and Rutherford backscattering spectrometry, respectively. The surface morphology of the samples was explored by mean of a Field Emission Gun scanning electron microscope. Optical properties of the nanostructured samples were studied by using the reflectance spectra received in the UV-visible range. In these spectra different band gap values and complex light absorption features were observed.