Flat Silicon Substrates Research Articles

Pre‐patterned silicon substrates for the growth of III–V nanostructures

AbstractThis paper reviews the recent progresses obtained by direct growth of III–V semiconductor quantum dots (QDs) on pre‐patterned and flat silicon substrates. This combination allows us to study in detail the growth mechanisms of III–V materials on silicon substrates. For the flat surfaces, we concentrate on basic growth studies addressing mainly morphological properties of QD‐like structures with a main emphasis on surface preparation and growth parameters. For the pre‐patterned substrates, we report the optimization of electron beam lithography and dry etching processes to fabricate sub‐100 nm holes in pre‐patterned Si (100) substrates with controlled size, shape, and periodicity. The pre‐patterned silicon substrates underwent thorough ex situ chemical and in situ cleaning processes before the molecular beam epitaxy (MBE) growth. Finally, the MBE growth sequence of QDs on patterned silicon surface has shown highly selective formation of localized dome like nanostructures in patterned holes with 1 µm period. magnified imageA 3D AFM image of a single nanohole in silicon substrate with diameter and depth of about 70 nm.

Fluorine Based Superhydrophobic Coatings

Superhydrophobic coatings, inspired by nature, are an emerging technology. These water repellent coatings can be used as solutions for corrosion, biofouling and even water and air drag reduction applications. In this work, synthesis of monodispersive silica nanoparticles of ~120 nm diameter has been realized via Stöber process and further functionalized using fluoroalkylsilane (FAS-17) molecules to incorporate the fluorinated groups with the silica nanoparticles in an ethanolic solution. The synthesized fluorinated silica nanoparticles have been spin coated on flat aluminum alloy, silicon and glass substrates. Functionalization of silica nanoparticles with fluorinated groups has been confirmed by Fourier Transform Infrared spectroscopy (FTIR) by showing the presence of C-F and Si-O-Si bonds. The water contact angles and surface roughness increase with the number of spin-coated thin films layers. The critical size of ~119 nm renders aluminum surface superhydrophobic with three layers of coating using as-prepared nanoparticle suspended solution. On the other hand, seven layers are required for a 50 vol.% diluted solution to achieve superhydrophobicity. In both the cases, water contact angles were more than 150°, contact angle hysteresis was less than 2° having a critical roughness value of ~0.700 µm. The fluorinated silica nanoparticle coated surfaces are also transparent and can be used as paint additives to obtain transparent coatings.

Impact of rough silicon buffer layer on electronic quality of GaAs grown on Si substrate

The electronic and the structural properties of n-GaAs layers grown on rough surface of silicon substrate by molecular beam epitaxy (MBE) has been investigated by photoluminescence (PL), time resolved photoluminescence (TRPL) and high resolution X-ray diffraction (HRXRD). The relationship between electronic and structural properties of the n-GaAs layer was checked, showing that the defect density is a strong cause for trapping the minority carriers. The impact of introducing intermediate rough silicon layer between silicon substrate and n-GaAs layer on the electronic properties was observed, showing that the structure grown on rough Si involves higher lifetime than those developed on flat silicon substrate. Such structure could be used for economic solar cells fabrication.

Deposition of Amorphous Hydrogenated Carbon Nitride Films with a Dielectric Barrier Discharge

AbstractThin amorphous carbon nitride (CNx) films have been deposited by DBD plasma. Deposition experiments have been performed on flat silicon (100) substrates using a C2H6/N2 gas mixture and deposition times of up to 6 h. The C2H6/N2 gas ratio has been varied between 1:1 and 1:5 to observe the influence of the feeding gas mixture on the chemical composition of the deposited films. The experiments were performed at gas pressures of 300 and 500 mbar. The chemical composition and bond structure of the deposited CNx films were evaluated by X‐ray photoelectron spectroscopy and by attenuated total reflectance‐Fourier transform infrared spectrometry. The results show that the chemical composition of amorphous CNx thin films depends on the working gas composition. Films deposited with higher nitrogen gas concentrations show a larger N/C ratio, an increase of C=N and C≡N bonds, and become harder.magnified image

Layer-by-layer assembled smectite-polymer nanocomposite film for rapid fluorometric detection of aflatoxin B1

Aflatoxin B1 (AFB1) is a potent biological toxin produced by fungi Aspergillus flavus and Aspergillus parasiticus. Current quantification methods for AFB1 are mostly established on immunoaffinity columns which are both costly and labor intensive. Inspired by smectites’ high AFB1 adsorption capacity and affinity, we report the synthesis of a smectite-polyacrylamide (PAM) nanocomposite film for AFB1 detection and prove the feasibility of such a film for AFB1 quantification. A layer-by-layer assembly process was developed to achieve uniform morphology and thickness of the nanocomposite films on flat silicon substrates. Positive correlations between the peak fluorescence intensity of the adsorbed AFB1 and its concentration in the test solutions were obtained. The smectite-PAM nanocomposite film has shown similar AFB1 adsorption capabilities as the smectite, while exhibiting excellent structural stability in aqueous solutions. Our results suggest the feasibility of using the smectite-PAM nanocomposite film as a simple biosensor to achieve rapid fluorometric quantification of AFB1.

Influence of Molecular Weight Dispersity of Poly{2-(perfluorooctyl)ethyl acrylate} Brushes on Their Molecular Aggregation States and Wetting Behavior

The relationships between the molecular aggregation states and water repellency of the perfluoroalkyl (Rf) groups of poly{2-(perfluorooctyl)ethyl acrylate} (poly(FA-C8)) brush thin films with broad and narrow molecular weight dispersities (MWDs) were analyzed by grazing incidence wide-angle X-ray diffraction (GI-WAXD) and water contact angle measurements. MWD-controlled-poly(FA-C8) brush thin films were prepared by surface-initiated atom transfer radical polymerization on a flat silicon substrate in the absence and presence of an ionic liquid. In-plane diffraction profiles of poly(FA-C8) brush films with narrow MWD had peaks corresponding to the periodic lengths of bilayer lamellae at qxy = 2–6 nm–1. This indicated that the orientation of Rf groups were parallel to the surface of the silicon substrate. In contrast, the peak of the in-plane GI-WAXD for brush films with broad MWD brush was confirmed at qxy = 12.5 nm–1, indicating that the Rf groups were oriented perpendicular to the surface of the silicon s...

Dynamic covalent polymer brushes: reversible surface modification of reactive polymer brushes with alkoxyamine-based dynamic covalent bonds

Reactive poly(methacrylate)-based polymer brushes with radically exchangeable alkoxyamine units in the side chains were prepared on flat silicon substrates and their reactivity and surface properties were investigated. The reactive polymer brushes were prepared by surface-initiated atom transfer radical polymerization (ATRP) and the graft density of the brushes was estimated to be approximately 0.36 chains per nm2. Radical crossover reactions among the alkoxyamine units in the side chains of the polymer brushes and at the ends of the fluorinated polymer chains were carried out to produce polymer brushes with low surface free energies. Angular-dependent X-ray photoelectron spectroscopy (XPS) measurement suggested that the fluorinated polymer chains were predominantly attached to the outermost surface because of the size exclusion from the high graft density. In addition, the de-grafting reaction of the fluorinated polymer chains was also performed to confirm the reversibility of the radical crossover reactions. The surface properties of the polymer brushes after the grafting and de-grafting processes were characterized by XPS and contact angle measurements. The composition calculated from the XPS indicated that the reversible grafting of the fluorinated polymer chains proceeded successfully. The surface wettability of the polymer brushes also changed after the grafting of the fluorinated polymers. Furthermore, after the de-grafting of the fluorinated polymer chains, the peaks attributed to fluorine atoms completely disappeared from the XPS spectrum and the wettability of the surface after the de-grafting returned to that obtained before grafting.

Hydrophobicity of Teflon Coated Well-Ordered Silver Nanorod Arrays

ABSTRACTFrom wings of flies to plant leafs, hydrophobic surfaces are well-common in nature. Many of these surfaces have micro and nano hierarchical structures coated with low surface energy layer. In this work, we mimicked similar structure by fabricating Teflon coated periodic and well-ordered silver nanorod arrays and investigated the effect of nanorod separation on water contact angle (WCA). The silver nanorod arrays were deposited on patterned and flat silicon substrates using glancing angle deposition (GLAD) technique. Then a thin layer of Teflon was deposited on the silver nanorods by small angle deposition (SAD) technique. A systematic increase in water contact angle was observed with increasing nanorod separation which is attributed to the decreased area fraction of solid-liquid interface.

Chemical sensing based on double layer PHEMA polymer and platinum nanoparticle films

Abstract Chemical gas sensors have been fabricated from platinum and poly(2-hydroxyethylmethacrylate) (PHEMA) polymer. Sputtered platinum nanoparticles have been deposited on flat silicon substrates and a second layer of PHEMA polymer has been deposited on top using ink-jet printing. Density of the platinum nanoparticles and resistance of the device, which can both be tuned by controlling the deposition time, directly affect the response of the sensors as measured in the presence of ethanol and humidity. Chemi-resistive and chemi-capacitive sensors capable of detecting low gas concentrations (1000 ppm of humidity and 500 ppm of ethanol) have been made possible.

Synthesis of single-walled carbon nanotubes over a spin-coated Fe catalyst in an ethanol–PEG colloidal solution

Single-walled carbon nanotubes (SWCNTs) have been grown on flat silicon oxide substrates by the decomposition of methane. The spin-coated iron(III) nitrate with a concentration of 40 mmol/L diluted in a colloidal solution with ratio 1:1 (v/v) of absolute ethanol to PEG-400 was found to form iron nanoparticles that are small enough to grow SWCNTs. The role of PEG was to interact with iron ions and encapsulate the iron, thus preventing agglomeration, while the absolute ethanol isolated the PEG–iron micelles. The ratio of absolute ethanol to PEG controlled the viscosity and the distance between PEG–encapsulated iron micelles, and subsequently governed the size and density of the iron nanoparticles formed. To reduce the effect of surface tension that will disturb the uniformity of a thin colloid film, –OH groups were introduced on the silicon wafer by a piranha solution and the hydrogen bonds formed with PEG preserved the uniformity of the iron nanoparticle distribution during the synthesis of SWCNTs.

Rate-Dependent Adhesion Between a Spherical PDMS Stamp and Silicon Substrate for a Transfer-Assembly Process

The transfer-assembly process is a promising technique for fabricating high-performance flexible devices. An important element of the transfer assembly process is controlling the adhesion during the picking and placing steps. In this study, we developed an apparatus to measure the adhesive force between a spherical polydimethylsiloxane (PDMS) stamp and a flat silicon substrate. The apparatus consisted of a multi-axis stage for alignment, a vertical stage, and a loadcell. The adhesive force between the stamp and the substrate was measured for various unloading velocities during the picking step. The size of the contact area was simultaneously measured during the picking step by incorporating an optical microscope with a high-speed camera into the apparatus. The effect of the unloading velocity on the adhesion was carefully investigated. The actual transfer process was performed to demonstrate the effect of rate-dependent adhesion during the transfer assembly of single-crystal silicon devices. Practical guidelines for controlling the stamp velocity based on the transfer steps were developed from the obtained results.

Synthesis of individual ultra-long carbon nanotubes and transfer to other substrates

In this article, we report the synthesis of ultra-long carbon nanotubes (CNTs) by thermal chemical vapour deposition method. Ultra-long, individual and aligned CNTs were directly grown on a flat silicon substrate. The orientation of the nanotubes was found parallel to the gas flow direction. The ultra-long CNTs were grown with different transition metallic salts, such as nickel chloride, iron (III) chloride, cobalt acetate and ruthenium acetate, as the catalysts. The influence of the growth conditions, such as growth temperature, reactive gas flow on the length and alignment of the CNTs was studied in detail. By using different catalysts, ultra-long single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) were successfully grown. These ultra-long CNTs were transferred to other substrates by two methods. (1) The first method is to use polydimethylsiloxane as a stamp. (2) The second method is to use KOH as an etching agent. The diameter and length of the CNTs were characterised by transmission electron microscope, scanning electron microscope, atomic force microscope and Raman spectroscopy. The results indicate that the length of the CNTs can reach up to 4 mm. The diameter of the SWCNTs is in the range of 0.7–2.1 nm and the diameter of the MWCNTs is approximately 150 nm.

Fabrication of hydrophobic inorganic coatings on natural lotus leaves for nanoimprint stamps

Hydrophobic inorganic films were obtained by direct deposition of copper or silicon onto natural lotus leaves by ion beam sputtering deposition technique. Scanning electron microscopy observations showed a lotus-leaf-like surface structure of the deposited inorganic films. Hydrophobic nature of the inorganic films on lotus leaves had been improved compared to the inorganic films deposited on flat silicon substrates. Water contact angles measured on the lotus-leaf-like copper and silicon films were 136.3±8° and 117.8±4.4°, respectively. The hydrophobic lotus-leaf-like inorganic films had been repeated used as nanoimprint stamps. Negative structures of lotus-leaf-like inorganic films were obtained on the polystyrene resist layers.

Hydrogen etching of Si 3N 4 layers with plasma assisted hot wire CVD

In order to develop sustainable processes for clean manufacturing environment for thin film or other solar cell production, we studied the hydrogen etching of silicon nitride (Si 3N 4) films on flat crystalline silicon (c-Silicon) substrates. With an arrangement primarily constructed for hot wire CVD (HWCVD) deposition of thin silicon films also cleaning processes with atomic hydrogen were studied with a simplified three wire assembly. The three filaments could be biased independently by different potential. A variation of hydrogen pressure and flow was performed to find out conditions of high etching rates for the Si 3N 4 layers. The etching rate was simply determined by measuring the time for total removal of the film, since this could be easily detected by the change of the anti-reflection property. Etching rates of 0.1 nm/s have been obtained under 15 Pa and a flow of 50 sccm. An intensive study was carried out of the direct current (DC) plasma hot wire CVD conditions.

Multidirectional Hierarchical Nanocomposites Made by Carbon Nanotube Growth within Layer-by-Layer-Assembled Films

We demonstrate fabrication of multidirectional and hierarchical carbon nanotube (CNT) films on diverse substrates, using nanocomposite catalyst films prepared by layer-by-layer (LBL) assembly. CNT density and yield are controlled by the thickness of a montmorillonite clay/poly(dyallyldimethyl ammonium chloride (MTM/PDDA) support film. Using identical methods, few-walled CNTs are grown on flat silicon substrates, carbon fibers, and titanium wire mesh. On flat substrates, unique bilayer CNT forests, reminiscent of microscale “accordions”, form because of diffusion of the Fe catalyst through the support which is then split because of mechanical forces exerted by the growing CNTs. Electrochemical measurements of CNT-coated Ti wires demonstrate an 85-fold enhancement in specific capacitance, and 7.1 F/g for the CNTs alone. This novel approach to substrate engineering for CNT growth can create materials with unique and nonlinear properties by hierarchical ordering of CNTs at multiple length scales, and is scala...

Synthesis of vertically aligned carbon nanotubes and diamond films on Cu substrates for use in high-power electronic devices

Currently, most of the vertically aligned carbon nanotubes (VA-CNTs) and diamond films are mainly synthesised on flat silicon (Si) substrate. However, to achieve thermal dissipation in high-power electronic devices (HPEDs), the VA-CNTs and diamond films need to be attached to thermal dissipation metal substrates (like Cu, Ag, Al, etc.). In this paper, the fabrication process of the VA-CNTs and diamond films on Cu substrate is reported in detail. The VA-CNTs were synthesised by the thermal chemical vapour deposition (CVD) method. The VA-CNTs on Cu substrates were fabricated by two different methods: directly growing the VA-CNTs using thin catalytic metal layers such as Fe/Al or Cr/Al as a catalyst; transferring the VA-CNTs film that was pre-grown on Si substrate to Cu substrate. The diamond films were also directly grown on the Cu substrate by microwave plasma chemical vapour deposition (MPCVD). The grown VA-CNTs and diamond films were tested as the thermal dissipation media on a 0.5W InGaN LED chip. The VA-CNTs and diamond films greatly increased input current of the LED by more than 500 mA and 350 mA without reaching saturation. This is higher compared with that of the device packaged using normal commercial silver thermal paste. Initial experiment results on the LED demonstrated that the VA-CNTs and diamond films greatly improve the light's output power and that they are optimal choices for the thermal dissipation of HPED.

Surface grafting of thermoresponsive microgel nanoparticles

A monolayer of thermoresponsive microgel nanoparticles, containing poly(N-isopropylacrylamide) (PNIPAM), has been anchored to the surface of silicon wafers, glass slides, polyvinylidene fluoride (PVDF) fibers, and tungsten wires using a “grafting to” approach. The behavior of the synthesized grafted layers is compared with the behavior of the PNIPAM brushes (densely end-grafted layers). The comparison demonstrates that in many aspects the microgel grafted layer is comparable to PNIPAM brushes with respect to its thermoresponsive properties. Indeed, the grafted monolayer swells and collapses reversibly at temperatures below and above the lower critical solution temperature (LCST) of PNIPAM. For the flat silicon substrate, a wettability study of the grafted layer shows an approximately 20° increase in the advancing contact angle of water upon heating above the LCST of PNIPAM. Wettability data obtained for the tungsten wires indicate that the grafted microgel layer retains its ability to undergo morphological changes when exposed to external temperature variations on complex curved surfaces. Therefore, the microgel-grafted layer can be considered as a system capable of competing with the PNIPAM brushes.

Adhesion properties of nanoparticle-coated emulsion aggregation toner

Toner is the key material in printing and copying processes. Fundamental understanding of toner detachment and adhesion during the printing process is critical to improve both the efficiency of toner usage and the quality of print. To control their adhesion property, toner particles can be surface-coated with nanoparticle additives to modify their surface roughness, and consequently, to tune their adhesion properties. In this study, a technique based on the rolling resistance moment of the particle–substrate adhesion bond is used to quantify the effect of nanoparticle surface area coverage (SAC) on the effective work of adhesion of individual toner particles. Nanoparticle-coated model emulsion aggregation (EA) toner microparticles with the specified SAC levels of 0%, 10%, 50% and 100% were studied and the corresponding particle–substrate work of adhesion values were determined and compared. It is quantitatively demonstrated that the work of adhesion between a surface-coated toner particle and a flat silicon substrate decreases significantly with increasing nanoparticle SAC, which provides an effective means to tailor the adhesion performance of the EA toner. Also, based on the experimental data, for a nanoparticle-coated microparticle on a flat substrate, two possible modes of contact formation were identified and discussed.

Nanocrystals formation and intense green emission in thermally annealed AlN:Ho films for microlaser cavities and photonic applications

Plasma magnetron sputtered thin films of AlN:Ho deposited on flat silicon substrates and optical fiber were characterized and analyzed for structural changes after thermal annealing at 1173 K for 40 min, by atomic force microscopy (AFM). The films grown, at liquid nitrogen temperature, on silicon substrates were amorphous while those deposited around optical fiber were crystalline. The films were also investigated for any change in the luminescence when thermal activation was performed for 40 min in a nitrogen atmosphere. The AFM analysis identified the existence of crystalline structures in parts of the films after thermal annealing. The x-ray diffraction could not provide those results. The films around optical fiber were crystalline even deposited at liquid nitrogen temperature. Clearly, amorphous films are hard to achieve on smaller substrate size. Direct observation of green emission is possible with naked eye, when the thermally annealed films are studied under cathodoluminescence. The green emission occurs at 549 nm as a result from S52→I58 transition in Ho3+ that enhanced with thermal activation, making it a very useful candidate for photonic and optical devices applications.

Plasma Amino Acid Coatings for a Conformal Growth of Titania Nanoparticles

We report on the conformal synthesis of ultrathin films from the amino acid histidine on flat silicon substrates and 3D periodic polymer structures via plasma enhanced chemical vapor deposition. We demonstrate the efficient utilization of this functional amino acid nanocoating for the formation of individual titania nanoparticles with dimensions from 2 to 15 nm depending upon reduction conditions. The titania nanoparticles were grown directly on histidine-functionalized planar and 3D polymer substrates by a wet-chemistry method that showed uniform surface coverage that reached approximately 75%. This approach demonstrates the potential for modifying the optical properties of periodic porous polymeric structures via direct conformal growth of titania nanoparticles.