Pure Silicon Substrates Research Articles

Ferrofluid droplet impingement cooling of modified surfaces under the influence of a magnetic field

This paper documents the spreading characteristics and heat transfer of a single ferrofluid droplet impinging onto dry solid surfaces with and without a magnetic field. Three surfaces were investigated: a pure silicon substrate, a silicon substrate with a thin titanium film, and a superhydrophobic coating. Fluid dynamics and cooling results of a ferrofluid droplet were compared to that of pure water and water-surfactant droplets for the same Weber number (We = 61) and initial surface temperature (T0 = 95∘C). Our results show that the magnetic field (magnetic Bond number Bom = 2186) increased the droplet spreading diameter up to 12% and suppressed droplet rebound, thus improving the cooling performance of a ferrofluid droplet. The highest heat transfer rates were measured for the pure silicon substrate under a magnetic field with an average value of 14 W. The lowest cooling performance was observed for the superhydrophobic surface without a magnetic field with a peak value of 0.82 W. However, using the magnetic field, the superhydrophobic surface achieved 263% higher heat transfer rate than the non-magnetic case because of the suppression of droplet rebound. Moreover, the surface did not foul from ferrofluid deposition, an advantage for spray cooling applications when using any nanofluid.

Formation of SiC by Vacuum Carbidization on Porous Silicon

Planar-view TEM investigation revealed the formation of cubic silicon carbide layers on porous silicon by vacuum carbidization. The formation of cubic silicon layers in the form of a two-phase system was found. At the same time, the formed SiC layers on the mesoporous buffer layer are predominantly polycrystalline. Using the Rutherford backscattering method, it was found that the use of buffer layers of porous silicon makes it possible to obtain SiC layers of greater thickness than on a pure silicon substrate under similar conditions of vacuum carbidization. It is shown that an increase in the pore size in porous silicon layers leads to an increase in the thickness of the formed SiC layers. It has been shown by scanning electron microscopy that vacuum carbideization of porous silicon leads to formation of SiC grains in pores, partial overgrowth and sintering of pores. The dependence of the SiC grain size on the pore size was established.

The Effect of Laser Structuring of Carbon Nanotubes on the Proliferation of Chondroblasts and Mesenchymal Stem Cells

The density of cartilage cells (chondroblasts) proliferating on a silicon substrate coated with vertically oriented arrays of multi-walled carbon nanotubes (MWCNTs) was shown to be higher than on a pure silicon substrate. Electron microscopy showed that the cells in a nutrient medium affected the vertical position of the nanotubes in the array. A method for structuring the MWCNT arrays by 100-ns laser pulse scanning and abrasive water processing on planar substrates was developed. As a result of the structuring of the MWCNTs, the arrays become resistant to bending under the influence of the nutrient medium with mesenchymal stem cells. Structured MWCNT arrays were shown to have no toxic or pathological effect on the viability and morphology of stem cells. Thus, such materials can be suggested for use in cell-adhesive components of biomedical devices.

Optical Properties of Lithium Niobate (LiNbO3) Thin Film Doped with Ruthenium Oxide

We have succeeded in growing thin film of lithium niobate with variation of ruthenium oxide impurity (0, 2, 4, 6%) doped on pure silicon substrate (100) p-type with chemical solution deposition (CSD) method assisted with spin coating method. Lithium niobate thin films with variations of ruthenium oxide impurity and MESA surfactant precursors (methyl ester Soulfonat acid) with 2 M solubility was made using spin coating at a rotational speed of 8000 rpm for 30 seconds with three repeated coatings. The growth of thin film was carried out with an 850°C annealing temperature which is held for 8 hours under atmospheric conditions in the Furnace. The optical properties test was performed with the VIS-NIR Spectrophotometer tool and obtained energy gap values in the range of 2 to 3 eV using Kubelka method. The optical properties of thin films indicate that thin films of lithium niobate with variations of ruthenium oxide impurity (0, 2, 4, 6%) have the potential to become the embryo of light sensor materials.

Synthesis and spectral properties of preorganized BODIPYs in solutions and Langmuir-Schaefer films

Abstract In order to investigate the influence of molecular structure peculiarities of boron-dipyrrine dyes (BODIPYs) on their properties in solutions and supramolecular organization in Langmuir-Schaefer (LS) films, four new BODIPY dyes with various aliphatic, aromatic or mixed nature meso-subtituents were synthesized and investigated. Spectral characteristics (electronic absorption and fluorescence) of the synthesized compounds in organic solvents and LS-films were studied. Floating monolayers of the BODIPYs were formed from chloroform solutions placed onto water subphase in Langmuir-Blodgett through. Thin films were prepared using the Langmuir-Schaefer technique by the transfer of floating monolayers onto standard polished glass, ITO covered glass or pure silicon substrate. The variation of the dye structure we consider as a preorganization aiming to influence the structure of LS-films. The morphology and structure of the LS-films was examined by fluorescent microscopy, scanning electron microscopy, atomic force microscopy and small angle X-ray diffraction analysis. It was found that the introduced substituents have no substantial influence on the position of the absorption and fluorescence bands in dilute solutions. In contrast, the fluorescent characteristics of the LS-films significantly depend on the substituent nature. Therefore, this strategy could be used for the direct tuning of compounds fluorescent properties in LS-films. Concerning the LS-film surface characteristics it was proved that the films are homogeneous, without disruptions and only some widely-spaced microcrystals could be observed. With respect to the LS-film structure, the change of the substituents introduced to the BODIPY molecule did not influence the average given periodicity of layers (d = 0.3–0.4 nm). This value corresponds to a single-layer arrangement of BODIPY molecules located parallel to the substrate surface. Nevertheless, the diffraction peak intensities depended on the molecular structure of BODIPYs and therefore the structurization in thin films. Moreover, the combination of the rigid phenyl moiety with long alkyl chains in one compound completely suppresses the aggregation of molecules maintaining the intense fluorescence in thin films. On the basis of used range of experimental and calculation methods the intralayer and interlayer structures were proposed. Intermolecular hydrogen bond formation and π-π staking of the BODIPY cores were found to be the structure forming forces during the films manufacturing, resulting the differences in crystallinity of the materials. While the alkyl-substituents prevent the type of interactions and suppress the association of the dyes and formation of excimers. Compounds under investigation show a manifestation of the intense solvatochromic properties which allow their application as sensors, including naked eye sensorics for solution polarity. Besides, the obtained results broaden prospective of functional materials usage based on BODIPY thin films as components of optoelectronics.

Applicability of moissanite, a monocrystalline form of silicon carbide,to retrospective and forensic dosimetry

Although many electronic devices today are based on pure silicon substrates, a number of other substrates are used in various applications. The aim of this study was to characterize the dose response properties of monocrystalline silicon carbide (SiC) manufactured by Cree Inc. Thanks to its physical and electronic properties, SiC has been found to be widely useful as a substrate and wide-bandgap semiconductor in high-frequency, high-power, and high-temperature electronic devices, as well as in short wavelength, radiation resistant opto-electronic devices. Broadband thermoluminescence (TL) emission of gem-quality SiC (“moissanite”) shows that the material is sufficiently sensitive to permit the detection of a ∼ 0.7 Gy beta dose, with dose response linearity of the 120 and 270 ∘ C TL peaks over more than 3 orders of magnitude. The 120 ∘ C peak, which shifts from ∼ 110 ∘ C at very low doses to ∼ 160 ∘ C following a 10 3 times dose increase, shows second order kinetics. In addition, pre-dosing of the 230 ∘ C peak is seen as it becomes readily apparent only after irradiation followed by annealing. TL detection through the BG39 filter masks high-temperature peaks at low doses due to the strong thermal background for T > 350 ∘ C , but a 370 ∘ C peak does become discernible at high doses. TL glow curves in the UV and violet regions are distinctly different from those detected in the wide visible region, indicating a complex emission spectrum. Luminescence detected in both the violet and near-UV regions during optical stimulation with blue and infrared photons confirms that the material is suitable as an optically stimulated luminescence (OSL) dosimeter. Isothermal decay studies of the OSL and TL signals attest to its long-term thermal stability, an essential criterion for usefulness in retrospective dosimetry and forensically significant scenarios, such as the need to prove a suspected exposure to radiation. Based on the foregoing results, it is recommended that moissanite be added to the list of materials potentially useful in the retrospective dosimetry of the general population. In order to optimize our understanding of how various substrates of electronic devices might also be used for this purpose, further dosimetric studies are recommended for this as well as other forms of crystalline SiC.

Fabrication and Composition Control of NiTi Shape Memory Thin Films for Microactuators

AbstractMicroactuators fabricated with NiTi thin films take advantage of the shape memory effect's large energy density (∼5-10 joules/cm3) and high strain recovery (∼8%). Microelectromechanical Systems (MEMS) designed with these actuators can serve as biosensors, micro-fluidic pumps or optical switches. However, the fundamental mechanical properties of these shape memory NiTi films have not been fully characterized with micro-scale test structures. Equiatomic NiTi thin films were deposited by co-sputtering NiTi and Ti targets with the intension of fabricating such test structures. Dual cathodes allowed direct control of the film composition by adjusting the Ti cathode power. Energy Dispersive Spectroscopy (EDS) quantified the film composition relative to pure standards. A thin (∼50 nm) chromium film on a pure silicon substrate created excellent film adhesion. Oxidized Si wafers did not bond with the Cr and NiTi films. This deposition method enabled control of film composition and the necessary adhesion.

Structural Study of GaN Layers Grown on Carbonized Si(111) Substrates

A structural study of a fabricated GaN/AlN/SiC/Si structure is reported. The GaN layer is grown by molecular beam epitaxy (MBE) on carbonized Si substrates where SiC is covered by a thin AlN buffer layer before GaN deposition. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED) studies allow evaluating the structural quality of the obtained SiC and GaN. As a first step, Si carbonization in a rapid thermal chemical vapor deposition reactor (RTCVD) is used to obtain ultra-thin βSiC(111)//Si(111) structures where SiC exhibits good crystalline orientation, homogeneity and structural quality. The latter allows to obtain a thin SiC buffer between the Si substrate and the GaN layers. The grown GaN layer is thicker than one micron and results of electron diffraction indicate that the GaN layer is well aligned with the Si(111) substrate. The measured surface dislocation density is in the range of 10 9 cm-2, thus SiC/Si templates improve the GaN structural quality compared to pure silicon substrates.

Single Phosphorus Ion Implantation into Prefabricated Nanometre Cells of Silicon Devices for Quantum Bit Fabrication

In the near future, devices that employ single atoms to store or manipulate information will be constructed. For example, a solid-state quantum computer has been proposed that encodes information in the nuclear spin of shallow arrays of single 31P atoms (quantum bits or qubits) in a matrix of pure silicon. Construction of these devices presents formidable challenges. One strategy is to use single ion implantation, with the energy range of 10 to 20 keV, to load the qubits into prefabricated cells of the device with a period of a few tens of nanometres. We have developed a method of single ion implantation that employs detector electrodes adjacent to the prefabricated qubit cells that can detect on-line single keV ion strikes appropriate for the fabrication of shallow arrays. Our method of the sub-20 keV single ion detection utilizes a pure silicon substrate with a very high resistivity, a thin (5 nm) SiO2 surface layer, biased electrodes applied to the surface and sensitive electronics that can detect the charge transient from single keV ion strikes. We show that our detectors have a near 100% efficiency for keV ions, extremely thin dead layer thickness (∼5 nm) and a wide sensitive region extending laterally from the electrodes (greater than 15 µm) where the nanometre cells can be constructed. We compare the method with the other methods, such as those of measuring the secondary electrons or phonons induced by single ion impacts.

Protein adsorption on preadsorbed polyampholytic monolayers

The adsorption behaviour of five different globular proteins on pure silicon substrates and on preadsorbed polyampholytic monolayers has been investigated as a function of protein concentration. The prelayers were prepared by adsorption of the ampholytic diblock copolymer poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate)(PMAA-b-PDMAEMA). This polyampholyte adsorbs in densely packed micelles directly from aqueous solution. Ellipsometry was used to determine the amount of adsorbed polyampholyte and protein. While ATR-IR spectroscopy gives information about the adsorption and desorption behaviour of the preadsorbed polyampholytic layer, the lateral structures of the dried films were investigated by scanning force microscopy (SFM). The amount of protein adsorbed was found to be strongly influenced by the preadsorbed polyampholyte compared to the adsorption on the pure silicon substrates. No displacement of the polyampholyte by the proteins was detected. In most cases the protein adsorption was reduced by the preadsorbed polyampholytic layer. The observed trends are explained by the change in electrostatic and hydrophilic characteristics of the substrates. Furthermore, the entropy of adsorption has to be taken into account.

Epitaxial Ir layers on SrTiO 3 as substrates for diamond nucleation: deposition of the films and modification in the CVD environment

Iridium films on SrTiO3(001) have recently proven to be a superior substrate material for the heteroepitaxy of diamond thin films by chemical vapour deposition in the effort towards the realization of single crystal diamond films. In this paper we report on the growth and structural properties of iridium (Ir) films deposited by electron-beam evaporation on SrTiO3(001) surfaces varying the deposition temperature between 280 and 950°C. The films were studied by scanning electron microscopy, atomic force microscopy and X-ray diffraction. At the highest temperature film growth proceeds via three-dimensional nucleation, coalescence and subsequent layer-by-layer growth. The resulting films show a cube-on-cube orientation relationship with the substrate and a minimum mosaic spread of 0.15°. Towards lower deposition temperatures the orientation spread increases only slightly down to ∼500°C while the surface roughness, after passing through a maximum at ∼860°C, decreases significantly. For the lowest temperatures (below 500°C) the mosaic spread rises accompanied by the occurrence of twins until the epitaxial order is lost. Plasma treatment in the diamond deposition reactor at high temperature (920°C) yields low nucleation densities and modifies the Ir surface. At the same time {111} facets show a significantly higher structural stability as compared with {001} facets. Nucleation at 700°C results in highly aligned diamond grains with low mosaic spread and a vanishing fraction of randomly oriented grains, proving the superior properties of Ir films on SrTiO3 for diamond nucleation as compared with pure silicon substrates.

Analysis of photoemission lines in silicon nitrided layers formed by low-energy nitrogen ion implantation in silicon

The in-depth composition of silicon nitrided layers obtained by low-energy ion implantation of nitrogen atoms (6 keV; 3×10 17 ions cm -2) into silicon single crystal was analyzed using X-ray photoelectron spectroscopy coupled with ion milling. Besides elemental composition, the XPS lines allow the description of local morphology through a decomposition of spectra in a set of one to five components after each argon-ion etching step. In this way, this procedure is used for the characterisation of the interfacial region between the pure silicon substrate and the Si 3N 4 layer, which can be compared with the theoretical models: such as the random bonding model (RBM) or random mixture model (RMM). However, in this interfacial area, there is evidence of a pile-up of nitrogen atoms which correlates with its role in determining electronic properties.

Investigation of the oxidation kinetics of CoSi2 on (111)Si by transmission electron microscopy

Transmission electron microscopy has been applied to study oxidation kinetics of CoSi2 on silicon for both dry and wet oxidation. Care was taken to determine the activation energies of oxidation in the temperature and time regime where the islanding of CoSi2 does not occur. For dry oxidation, activation energies for parabolic and linear growth were found to be 1.91 and 2.01 eV (±0.1 eV), respectively. For wet oxidation, activation energies for parabolic and linear growth were found to be 1.75 and 1.68 eV (±0.1 eV), respectively. The activation energy of the parabolic rate constant is substantially different from those obtained previously. The difference is attributed to the occurrence of islanding during oxidation in the previous study. A comparison of oxidation kinetics of CoSi2, NiSi2, TiSi2 on silicon with pure silicon substrates indicated that the oxidation kinetics are practically the same for CoSi2 and NiSi2 in the parabolic growth regime, but substantially different from those of TiSi2 on silicon and pure silicon. The similarity in oxidation kinetics of cubic CaF2 structure CoSi2 and NiSi2 on silicon with small mismatches to silicon is correlated to essentially the same stress level in these two silicides during the oxidation.

Investigation of the oxidation kinetics of NiSi2 on (111)Si by transmission electron microscopy

A transmission electron microscopy study of oxidation kinetics of NiSi2 for both dry and wet oxidation has been carried out. Care was taken to determine the activation energies of oxidation in the temperature and time regime where the islanding of NiSi2 did not occur. For dry oxidation, activation energies for parabolic and linear growth were found to be 1.87 and 1.94 eV (with an error bar of ±0.1 eV), respectively. On the other hand, activation energies for parabolic and linear growth were found to be 1.72 and 1.59 eV (with an error bar of ±0.1 eV), respectively, for wet oxidation. The activation energy of parabolic rate constant is seen to be substantially different from those obtained previously. The difference is attributed to the occurrence and absence of islanding during oxidation in the previous and present study, correspondingly. Compared to the oxidation of TiSi2 and pure silicon, a model based on the dominant diffusing species through silicide, i.e., metal and Si for NiSi2 and TiSi2, respectively, is proposed to explain the substantial difference and closeness in linear activation energies of wet oxidation between NiSi2, TiSi2, and pure silicon, respectively.

YBa 2Cu 3O 7- x thin films integrated on silicon-on-sapphire substrates with high critical current densities

The deposition of YBCO thin films on buffer layered silicon-on-sapphire substrates was investigated. The critical current density of the superconducting thin films is 3.1 × 10 6 A cm -2 at 77 K. No degradation of j c with time on such substrates was observed after more than two months after the deposition. In contrast to thin films on pure silicon substrates, microcracks did not occur up to film thicknesses of 140 nm. The in-situ reduction of the SiO 2 during the initial stages of buffer growth and the subsurface re-oxidation of the silicon was investigated by XPS.

Quantitative electron probe microanalysis for the characterization of thin carbon-boron layers in fusion devices

The first wall of the fusion device TEXTOR at the Forschungszentrum Julich has been coated in situ with an amorphous hydrogen rich carbon/boron film (a-C/B:H) which reduces plasma impurities caused by the plasma surface interaction. The results of the coating process of the 35 m2 large inner wall surface have been controlled by a recently developed modification of the quantitative electron probe microanalysis, which has been applied to 12 samples from specified positions inside the tokamak. The quantification itself is based on a Monte Carlo simulation of electron trajectories providing very accurate results for X-ray intensities emitted by elements present in the electron bombarded sample. The Monte Carlo results are used in the present work to calibrate the measured X-ray intensities emitted by boron and carbon from the a-C/B:H layers deposited on pure silicon substrates. As a result the total deposited mass of the layer per area unit as well as the composition of the layers (except hydrogen) could be determined very accurately. The relative errors were less than 7%. The limit of detectability were found to be in the range of one monolayer for boron as well as for carbon.

Dechanneling at an interface of finite width

Si crystals were implanted with 2.5 × 10 16 cm 2 boron ions. A surface layer of about 250 nm was melted several times with a pulsed laser causing an almost homogeneous distribution of the B atoms in the molten layer. The B depth distribution and lattice site were determined by nuclear reaction analysis. The B containing layer was found to be epitaxial with the pure silicon substrate and contracted in the direction perpendicular to the surface. This contraction causes a change in direction of all inclined planes and strings at the interface of the B containing layer and the substrate. In this work we study dechanneling of α particles at the interface. The dechanneling effect at the interface turned out to be very sensitive to the α-particle energy. This energy dependence was reproduced by Monte Carlo simulations. The amount of dechanneling at the interface is both determined by the magnitude of the contraction and the width over which the boron concentration drops from its constant value to zero. In principle it should be possible to determine both the width of the interfacial region and the magnitude of the contraction from dechanneling measurements alone. However it turned out not to be straightforward to simulate all axial planar dechanneling data correctly with only one set of parameters.

Sources of loss processes in phonon generation and detection experiments with superconducting tunneling junctions

Observing the phonon yield, i.e. the ratio of the experimental phonon signal amplitude and the corresponding calculated value, phonon losses within the generation-detection system can be localized and determined quantitatively. With tin junctions on pure silicon substrates immersed in liquid helium the phonon yield is 3–5%. Under vacuum conditions the yield rises to 10–12% indicating strong phonon transmission to the helium bath. The experimental lifetime for 280 GHz phonons in the silicon substrate is longer than 65 µs indicating negligible volume losses and losses at the free substrate surface. It is further shown, that volume losses inside the phonon generator and detector are small compared to the total loss of about 90%. By phonon reverberation measurements we find evidence that the main sources for phonon losses are localized at the boundaries of the tunneling junctions to the substrate. This is supported by an increase of the phonon yield with improved polishing from about 9% (mechanical), 10% (chemical) to 12% (sputter etching). A SIMS analysis indicates the presence of carbonhydrates and probably of water in the boundaries. This layer of extraneous molecules together with the nonideal surface structure of the substrate and the evaporated films weakens the mechanical bonding between the tunnel junctions and the substrate and is possibly causing strong phonon splitting by anharmonic forces.