Applied Surface Science

Published by Elsevier
Print ISSN: 0169-4332
Reflectance difference spectroscopy (RDS) is applied to follow in situ the preparation of clean and carburized W(1 1 0) surfaces and to study the temperature-induced transition between the R(15 × 3) and R(15 × 12) carbon/tungsten surface phases. RDS data for this transition are compared to data obtained from Auger-electron spectroscopy and low-energy electron diffraction. All techniques reveal that this transition, occurring around 1870 K, is reversible with a small hysteresis, indicating a first-order-like behaviour. The present results also prove a high surface sensitivity of RDS, which is attributed to the excitation of electronic p-like surface resonances of W(1 1 0).
Scanning tunneling microscopy (STM) lithography was utilized to investigate a 12-mer HS-ssDNA self-assembled Au (111) surface. Under low sample bias and high tunneling current, the repeated scanning resulted in the growth of nanostripes. The stripe orientation, the stripe width, and the spacer width between adjacent nanostripes were found to be dependent on their relative locations from dislocation points where two adjacent gold terraces overlap. The stripe and the spacer width also vary with the distance from these points. The results indicate that such stripes may reflect the strain distributions and the release pathway along the Au surfaces. The results also suggest that the presence of HS-ssDNA molecules enhances the lithography processes on the gold surface by acting as force transmitters.
Molecular depth profiling and three-dimensional imaging using cluster projectiles and SIMS have become a prominent tool for organic and biological materials characterization. To further explore the fundamental features of cluster bombardment of organic materials, especially depth resolution and differential sputtering, we have developed a reproducible and robust model system consisting of Langmuir-Blodgett (LB) multilayer films. Molecular depth profiles were acquired, using a 40-keV C(60) (+) probe, with LB films chemically alternating between barium arachidate and barium dimyristoyl phosphatidate. The chemical structures were successfully resolved as a function of depth. The molecular ion signals were better preserved when the experiment was performed under cryogenic conditions than at room temperature. A novel method was used to convert the scale of fluence into depth which facilitated quantitative measurement of the interface width. Furthermore, the LB films were imaged as a function of depth. The reconstruction of the SIMS images correctly represented the original chemical structure of the film. It also provided useful information about interface mixing and edge effects during sputtering.
The effect of incident angle on the quality of SIMS molecular depth profiling using C(60) (+) was investigated. Cholesterol films of ~300 nm thickness on Si were employed as a model and were eroded using 40 keV C(60) (+) at an incident angle of 40° and 73° with respect to the surface normal. The erosion process was characterized by determining at each angle the relative amount of chemical damage, the total sputtering yield of cholesterol molecules, and the interface width between the film and the Si substrate. The results show that there is less molecule damage at an angle of incidence of 73° and that the total sputtering yield is largest at an angle of incidence of 40°. The measurements suggest reduced damage is not necessarily dependent upon enhanced yields and that depositing the incident energy nearer the surface by using glancing angles is most important. The interface width parameter supports this idea by indicating that at the 73° incident angle, C(60) (+) produces a smaller altered layer depth. Overall, the results show that 73° incidence is the better angle for molecular depth profiling using 40 keV C(60) (+).
A novel hybrid mesoporous aluminosilicate sieve (HMAS) was prepared with fly ash and impregnated with zeolite A precursors. This improved the mercury adsorption of HMAS compared to original MCM-41. The HMAS was characterized by X-ray diffraction (XRD), nitrogen adsorption-desorption, Fourier transform infrared (FTIR) analysis, transmission electron microscopy (TEM) images and (29)Si and (27)Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. These showed that the HMAS structure was still retained after impregnated with zeolite A. But the surface area and pore diameter of HMAS decreased due to pore blockage. Adsorption of mercury from aqueous solution was studied on untreated MCM-41and HMAS. The mercury adsorption rate of HMAS was higher than that of origin MCM-41. The adsorption of mercury was investigated on HMAS regarding the pH of mercury solution, initial mercury concentration, and the reaction temperature. The experimental data fit well to Langmuir and Freundlich isotherm models. The Dublin-Radushkevich isotherm and the characterization show that the mercury adsorption on HMAS involved the ion-exchange mechanisms. In addition, the thermodynamic parameters suggest that the adsorption process was endothermic in nature. The adsorption of mercury on HMAS followed the first order kinetics.
Complementary surface and near-surface analytical techniques have been used to explore a brass (Cu-20Zn) surface before, during, and after exposure in air at 90% relative humidity. Volta potential variations along the unexposed surface are attributed to variations in surface composition and resulted in an accelerated localized growth of ZnO and a retarded more uniform growth of an amorphous Cu2O-like oxide. After 3 days the duplex oxide has a total mass of 1.3 μg/cm2, with improved corrosion protective properties compared to the oxides grown on pure Cu or Zn. A schematic model for the duplex oxide growth on brass is presented.
Cobalt Chromium (Co-Cr) alloys has been widely used in the biomedical arena for cardiovascular, orthopedic and dental applications. Surface modification of the alloy allows us to tailor the interfacial properties to address critical challenges of Co-Cr alloy in medical applications. Self assembled monolayers (SAMs) of Octadecylphosphonic acid (ODPA) have been used to form thin films on the oxide layer of the Co-Cr alloy surface by solution deposition technique. The SAMs formed were investigated for their stability to oxidative conditions of ambient laboratory environment over periods of 1, 3, 7 and 14 days. The samples were then characterized for their stability using X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and Contact Angle Measurements. Detailed high energy XPS elemental scans confirmed the presence of the phosphonic monolayer after oxidative exposure which suggested that the SAMs were firmly attached to the oxide layer of Co-Cr alloy. AFM images gave topographical data of the surface and showed islands of SAMs on Co-Cr alloy surface, before and after SAM formation and also over the duration of the oxidative exposure. Contact angle measurements confirmed the hydrophobicity of the surface over 14 days. Thus the SAMs were found to be stable for the duration of the study. These SAMs could be subsequently tailored by modifying the terminal functional groups and could be used for various potential biomedical applications such as drug delivery, biocompatibility and tissue integration.
Ultra-thin self-assembled monolayer (SAM)-oxide hybrid dielectrics have gained significant interest for their application in low-voltage organic thin film transistors (OTFTs). A [8-(11-phenoxy-undecyloxy)-octyl]phosphonic acid (PhO-19-PA) SAM on ultrathin AlOx (2.5 nm) has been developed to significantly enhance the dielectric performance of inorganic oxides through reduction of leakage current while maintaining similar capacitance to the underlying oxide structure. Rapid processing of this SAM in ambient conditions is achieved by spin coating, however, as-cast monolayer density is not sufficient for dielectric applications. Thermal annealing of a bulk spun-cast PhO-19-PA molecular film is explored as a mechanism for SAM densification. SAM density, or surface coverage, and order are examined as a function of annealing temperature. These SAM characteristics are probed through atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). It is found that at temperatures sufficient to melt the as-cast bulk molecular film, SAM densification is achieved; leading to a rapid processing technique for high performance SAM-oxide hybrid dielectric systems utilizing a single wet processing step. To demonstrate low-voltage devices based on this hybrid dielectric (with leakage current density of 7.7×10(-8) A cm(-2) and capacitance density of 0.62 µF cm(-2) at 3 V), pentacene thin-film transistors (OTFTs) are fabricated and yield sub 2 V operation and charge carrier mobilites of up to 1.1 cm(2) V(-1) s(-1).
Stent migration occurs in 10-40% of patients who undergo placement of esophageal stents, with higher migration rates seen in those treated for benign esophageal disorders. This remains a major drawback of esophageal stent therapy. In this paper, we propose a new surface modification method to increase the adhesion between self-expandable metallic stents (SEMS) and tissue while preserving their removability. Taking advantage of the well-known affinity between epoxide and amine terminated silane coupling agents with amine and carboxyl groups that are abundant in proteins and related molecules in the human body; we modified the surfaces of silicone coated esophageal SEMS with these adhesive self-assembled monolayers (SAMs). We utilized vapor phase silanization to modify the surfaces of different substrates including PDMS strips and SEMS, and measured the force required to slide these substrates on a tissue piece. Our results suggest that surface modification of esophageal SEMS via covalent attachment of protein-binding coupling agents improves adhesion to tissue and could offer a solution to reduce SEMS migration while preserving their removability.
Biodegradable polymers are of interest in developing strategies to control protein drug delivery. The protein that was used in this study is Keratinocyte Growth Factor (KGF) which is a protein involved in the re-epithelialization process. The protein is stabilized in the biodegradable polymer matrix during formulation and over the course of polymer degradation with the use of an ionic surfactant Aerosol-OT (AOT) which will encapsulate the protein in an aqueous environment. The release kinetics of the protein from the surface of these materials requires precise timing which is a crucial factor in the efficacy of this drug delivery system.Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was used in the same capacity to identify the molecular ion peak of the surfactant and polymer and use this to determine surface concentration. In the polymer matrix, the surfactant molecular ion peak was observed in the positive and negative mode at m/z 467 and 421, respectively. These peaks were determined to be [AOT + Na+] and [AOT-Na+]-. These methods are used to identify the surfactant and protein from the polymer matrix and are used to measure the rate of surface accumulation. The second step was to compare this accumulation rate with the release rate of the protein into an aqueous solution during the degradation of the biodegradable film. This rate is compared to that from fluorescence spectroscopy measurements using the protein autofluorescence from that released into aqueous solution.
Sample preparation is central to acquiring meaningful molecule-specific images with SIMS, especially when submicron lateral resolution is involved. The issue is to maintain the distribution of target molecules while attempting to introduce biological cells or tissue into the high vacuum environment of the mass spectrometer. Here we compare freeze-drying, freeze-etching, freeze-fracture and trehalose vitrification as possible strategies for these experiments. The results show that the prospects for successful imaging experiments are greatly improved with all of these methods when using cluster ion bombardment, particularly C(60) (+) ions, not only due to increased sensitivity of this projectiles, but also since it removes contamination overlayers without insult to the underlying chemistry. The emergence of 3-dimensional imaging capabilities also suggests that sample preparation should not perturb the 3-dimensional morphology of the cell, a situation not generally possible during freeze-drying. Hence, sample preparation and projectile type are strongly coupled parameters for bioimaging with mass spectrometry.
We report the use of SIMS imaging to quantify the relative difference in the amount of lipid between two sections, the plasma membrane and the cytoplasm, of single cells from two different populations. Cells were each labeled with lipophillic dyes, frozen, fractured and analyzed in a ToF-SIMS mass spectrometer equipped with a 40 keV C(60) (+) ion source. In addition to identifying cells from separate populations, the lipophilic dyes can be used as a marker for the outer leaflet of the cell membrane and therefore as a depth finder. Here, we show that it is possible to compare the amount of lipids with particular headgroups in the cell membrane of a treated cell to the membrane of a control cell. Following erosion of the cell membranes, the amount of the two specific lipid head groups in the cytoplasm of the treated cell can be compared to those lipids in a control cell. Here we take the first step in this experimental design and display the ability to analyze multiple sections of frozen cells following a single fracture.
There is an increased interest in how lipids interact with each other, especially in the lateral separation of lipids into coexisting liquid phases as this is believed to be an attribute of raft formation in cell membranes. ToF-SIMS has shown itself to be an excellent tool for investigating cellular and model membrane systems and will be perhaps the most powerful one for investigating raft formation. Results from our laboratory show the capability of ToF-SIMS at identifying unequivocally the content of coexisting liquid lipid phases. Using supported lipid monolayers we find that the inclusion of dipalmitoylphosphatidylethanolamine (DPPE) to a homogeneous dipalmitoylphosphatidylcholine (DPPC)/cholesterol phase results in the formation of cholesterol-rich domains [A.G. Sostarecz, C.M. McQuaw, A.G. Ewing, N. Winograd, J. Am. Chem. Soc. 126 (2004) 13882]. Also, for DPPE/cholesterol systems a single homogeneous DPPE/cholesterol phase is formed at ~50 mol% cholesterol, whereas DPPC/cholesterol systems form a single phase at 30 mol% cholesterol [C.M. McQuaw, A. Sostarecz, L. Zheng, A.G. Ewing, N. Winograd, Langmuir 21 (2005) 807]. Currently we are exploring the incorporation of sphingomyelin into phospholipid-cholesterol mixtures in an effort to gain a better understanding of its role in raft formation.
Phagocytosis is a major component of the animal immune system where apoptotic cellular material, metabolites, and waste are safely processed. Further, efficient phagocytosis by macrophages is key to maintaining healthy vascular systems and preventing atherosclerosis. Single-cell images of macrophage phagocytosis of red blood cells, RBCs, and polystyrene microspheres have been chemically mapped with TOF-SIMS. We demonstrate here cholesterol and phosphocholine localizations as relative to time and activity.
With the chemical imaging capability of ToF-SIMS, biological molecules are identified and localized in membranes without any chemical labels. We have developed a model membrane system made with supported Langmuir-Blodgett (LB) monolayers. This simplified model can be used with different combinations of molecules to form a membrane, and thus represents a bottom-up approach to study individual lipid-lipid or lipid-protein interactions. We have used ternary mixtures of sphingomyelin (SM), phosphatidylcholine (PC), and cholesterol (CH) in the model membrane to study the mechanism of domain formation and interactions between phospholipids and cholesterol. Domain structures are observed only when the acyl chain saturation is different for SM and PC in the mixture. The saturated lipid, whether it is SM or PC, is found to be localized with cholesterol, while the unsaturated one is excluded from the domain area. More complicated model membranes which involve a functional membrane protein glycophorin are also investigated and different membrane properties are observed compared to the systems without glycophorin.
A novel operation mode for time of flight-secondary ion mass spectrometry (ToF-SIMS) is described for a TOF.SIMS 5 instrument with a Bi-ion gun. It features sub 100 nm lateral resolution, adjustable primary ion currents and the possibility to measure with high lateral resolution as well as high mass resolution. The adjustment and performance of the novel operation mode are described and compared to established ToF-SIMS operation modes. Several examples of application featuring novel scientific results show the capabilities of the operation mode in terms of lateral resolution, accuracy of isotope analysis of oxygen, and combination of high lateral and mass resolution. The relationship between high lateral resolution and operation of SIMS in static mode is discussed.
Well oriented polyethylene (PE) films were prepared on glasses coated with Indium Tin oxide (ITO) and on Si(100) substrates by pulsed laser ablation. The films were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM). Nematic liquid crystal 4'-n-pentyl-4-cyanobiphenyl (5CB) is used to display the orientation features of the films. Based on the experiments, a picture is presented about the formation of the polymer films. It is expected that the as-deposited highly oriented polymer films will find various applications in integrated optics and liquid crystal displays
The paper describes an electrochemical model, which explains the continuous generation of Ba in oxide cathodes. In nondoped oxide cathodes electrolysis of BaO is, besides the exchange reaction from the activators in the cathode nickel, an important source of Ba during activation and life. By doping with rare earth oxides the conductivity of the oxide layer increases, which implies that the potential difference during current drawing over the oxide layer becomes lower and electrolysis of BaO is suppressed. This largely explains why doped oxide cathodes have a better life performance than nondoped cathodes. Furthermore, it is shown that the electronic conductivity of the (Ba,Sr)O layer largely controls the sensitivity for poisoning: the higher the conductivity, the larger the sensitivity for poisoning.
Since the invention of microfabricated field emission arrays (FEAs) several decades ago, numerous applications of FEAs have been proposed, and some are under development, such as ionizers for mass spectrometry, electron-beam lithography, surface-analyzing instruments, vacuum gauges, flat-panel displays and microwave tubes. The most prominent applications are involved with display and microwave devices. Monochrome field emission oscillographs have been produced and in some corporation, samples of color field emission display have also been exhibited. In microwave applications, theoretical and experimental research has been carried out in klystron amplifier and miniatured traveling wave tubes with effective power output obtained. In different applications, there are different requirements for FEAs. For example, displays need good emission uniformity in a large emission area, while microwave devices usually require relative high emission density and total emission current. However, even in the most dissimilar applications, emission stability of FEAs is always a fundamental requirement. In this paper an ageing process was used to improve the performance of FEAs.
In this paper, the authors describe an aging process to solve the unbalanced emission slump, which does not need any change in the electron gun design. The principle is to apply a high frequency (HF) magnetic field on the electron gun during the aging process. The effect is that the scanning electron beam and the HF magnetic field heat up the gun parts by electron bombarding and eddy current heating. In this way the grids are effectively degassed. A part of the desorbed gases is pumped by the Ba-getter in the tube, whereas another part is ionized by electron collision. These ionized gas molecules, notably Ar<sup>+</sup>, are partially trapped in gun parts. Therefore a lower residual gas pressure can be achieved. According to the theoretical and experiment results, we may conclude that the application of a magnetic ion trap during aging can improve the emission slump of the central beam and gas pressure in the tube significantly. The heating of the electron gun by an HF-coil or the DU in a pull back position during aging is rather easy to implement in a CRT factory.
By using radio frequency magnetron reactive sputtering system, oriented AlN films with different thickness were deposited on tungsten tips. Compared studies of field emission characteristics were performed between the bare and AlN coated W tips. The results showed that enhanced electron emission can be obtained from oriented AlN film on W tip. The hysteresis behaviors shown in current-electric field (I-E) curves during downward electric field sweeps were observed and the extent of hysteresis in I-E curves strongly depended on the thickness of the AlN films. The stability measurement of field emission current presented that the hysteresis can be attributed to the charging in AlN film as an insulator.
In this paper, we present a technique for fabricating carbon nanotubes (CNTs) field emission arrays in an integrated gate structure. Two kinds of such Spindt-type cathodes with vertically aligned carbon nanotubes emitters were fabricated. One process involved direct growth of CNTs from the bottom of micro-sized cells and the other process was to grow CNTs on pre-deposited Mo tips in Spindt-type arrays. The former had fairly good, consistent emitters with a number of CNTs self-“shaped” in a tip-like form, whereas the latter showed that the emitters comprised only a few or single CNTs on Mo tips. Preliminary current–voltage measurements of a 20 × 20 array with CNTs grown from the bottom of each cell showed an onset voltage of ∼30 V with a relatively large gate leakage current.
In order to improve the activation characteristics & emission ability of the conventional Ir-Coated impregnated tungsten cathodes, a new type of dispenser cathode with ternary alloy Ir/Re/W coating was developed. The improved cathodes show higher emission current density and faster activation characteristics than that of the conventional pure Ir-coated impregnated tungsten cathodes. XPS was used to analyze the element compositions on the surface of the cathodes coated with pure Ir and Ir/Re/W alloy. The results will be given in this paper.
Au-precoating on silicon substrate was observed to enhance the field emission characteristics of diamond films deposited by chemical vapor deposition technique. The emission current density increased substantially from 1 μA/cm<sup>2</sup> to 100 μA/cm<sup>2</sup>, while the turn on voltage decreased moderately from 14 MV/m to 10 MV/m. Raman spectra and electron diffraction in transmission electron microscopy (TEM) revealed that both diamond films deposited on Si or Au-coated Si (Au/Si) were nanosized crystals. The Au species were assumed to diffuse along grain boundaries, resulting in low resistance diamond films. Abundant supply of electrons via conducting grain boundaries was presumably the mechanism that enhanced the field emission of the diamond films grown on Au/Si substrates
Ba-oxide, Ba-dispenser and Ba-Scandate cathodes have been continuously improved in their emission performance in the past decades. Ba-oxide and Ba-dispenser cathodes are also the dominant types of thermionic cathodes used in most vacuum tube applications. When improvements in emissive properties are introduced, their impact on cathode life - where several years in a vacuum tube environment are typically required - also needs to be known. Hence, the investigation of cathode life-limiting effects is the basis of accelerated life predictions and of further cathode improvement. In this contribution, the main effects limiting the operating life of Ba/BaO-based thermionic cathodes are discussed, especially related to intrinsic dispensation and resupply to the emissive surface. Emission poisoning induced by adsorption of poisonous gases will not be addressed here. We will stress common points and point out the differences between the three types. (c) 2005 Elsevier B.V. All rights reserved.
Since the discovery of its excellent thermionic emission performances, 100 years ago by A. Wehnelt, the oxide-coated cathode has been the most widely used electron emitter in vacuum electronic devices: in vacuum valves up to the late 60's, and afterwards in CPT's and CDT's. Schematically, this cathode is composed of a 60 to 150 thick nickel alloy substrate called the base metal, covered with a 50 to 100 thick porous layer of alkaline-earth oxide. The exact definition of its "emitting system", i.e. base metal and emissive oxide, varies from one company to another. It is noteworthy that the emissive oxide is either a mixture of BaO, SrO or BaO, SrO and CaO, respectively called double or triple oxide having always sensitively the same respective composition. The base metal composition is of primary importance for the cathode emitting properties and is very often proprietary of the cathode manufacturing companies. Actually, the definition of base metal is more an "art" than a science because of the large number of parameters to take into account and of their potential interactions.
A field emitter array (FEA) containing emitters each with a built-in p-i-n junction for limiting and stabilizing emission current is fabricated. The junction is first formed over an area where the emitters are located. The field emitters are then formed by isotropic etching with each emitter containing at least one p-i-n junction. When a constant voltage is applied across the emitters and collectors, non-uniform emission of the emitters can be limited by the built-in p-i-n junctions and abrupt emission from one or few emitters of the array is prevented. The characteristics of the p-i-n FEA is also compared to that of the n-p FEA and n FEA. The p-i-n FEA shows an extremely stable emission and a saturation behaviour at high electric field region.
Summary form only given. A carbon nanotubes-deposited flexible film was prepared by electrophoretic method. A polyimide film coated with Titanium was used as a cathode and a stainless steel plate as an anode, respectively. The two electrodes were placed into a bath with solvent consisting of fine carbon nanotubes particles, Mg(NO<sub>3</sub>)<sub>2</sub> 6H<sub>2</sub>O and isopropylalcohol (IPA). Mixed solution was stirred for about 2-4 hours to assure the complete dissolution of Mg(NO<sub>3</sub>)<sub>2</sub> 6H<sub>2</sub>O and the dispersion of carbon nanotubes particles. DC electric field was applied to the two electrodes. The anode voltage was about 100V. After 1-2 minutes, the carbon nanotubes particles were deposited on the polyimide film coated with Titanium. The electron field emission properties of the prepared thin film were tested by a diode mechanism. The turn-on field of the flexible film was about 2.96V/μm. The current density of the flexible film was about 200μA/cm<sup>2</sup> at an electric field of 5.9V/μm. The morphology and structure of the carbon nanotubes-deposited flexible film were examined by scanning electron microscope (SEM) and Raman spectroscopy. The experimental results indicate that this film could be a promising material applicable to cold cathodes.
A method to fabricate the field-emission cathode of carbon nanotubes (CNTs) is proposed. We made the carbon nanotube cathode through the processing of powder metallurgy. Zinc powder and carbon nanotubes are mixed, pressed and sintered to form a bulk material. The surface of the sintered sample is etched by HNO<sub>3</sub> to remove a thin layer of zinc in order to make the tops of nanotubes to form the tips for field emission. The measurement shows that the cathode has typical I-V curve and F-N curve.
At this report, the producing technology of the field emission cathode based on ball-milled carbon fibers is represented. The method of the carbon fiber powder with the optimal particle size obtained is described. For the deposition on the cathode base screen-printing technique have been applied. The obtained cathode structure can provide the significant electric field amplification. The field emission properties of these cathodes are also described.
In this paper, we report a recent experimental observation of high field-emission current density (6.5 A/cm2) from carbon nanotube emitters. Carbon nanotubes are grown on Ni catalyst coated on Si substrates using chemical vapor deposition of C2H2 gas at 700 °C. Our research shows that it is beneficial for getting the high-current density in our experiment that Au film was deposited onto Si substrate before evaporating a Ni catalyst dot onto the substrate. We compared the field emission characteristics of CNTs cathodes grown on the Ni films with different thickness of 50, 70, 100 nm. The thickness of the Ni film is very important for the growth of CNTs, which will affect the field emission properties very much. The Fowler-Nordheim plot showed a good linear fit, indicating that the emission current of carbon nanotubes follows Fowler-Nordheim behavior. The calculated field enhancement factor was 2370.
Because of low work functions, the scandate cathode has a low operating temperature and high emission current density. However, it still has certain limit in application field for the shortcoming of emission uniformity and bad operating stability. After improving the manufacturing technology of tungsten sponge, stipulating reasonably the operating temperature, and adopting a stable module structure, the emission uniformity and capacity of the impregnated cathode has been ameliorated. If the power of heater change within ±5%, then the corresponding change of cathode temperature can be realized within 25-30°C and the fluctuation of emission current less than ± 2% in the application of TWTs in our company.
A new type of reservoir oxide cathode has been developed in IECAS. The emission characteristics of the cathode are tested. The results show the new cathode has larger emission current density, better resistance to poisoning at same operating condition, compared with those of conventional reservoir oxide cathode.
The emission and surface characteristics of the dispenser cathode coated with Os-W alloy and that coated with Os-W/Re are studied and compared. The dispenser cathode coated with Os-W/Re has applied in electron gun measurement system for making measurement of higher emission current and life test. It is found that the dispenser cathode coated with Os-W/Re has higher current density than that coated only with Os-W alloy. In electron gun the dispenser cathode coated with Os-W/Re has pulse current density of 30 A/cm<sup>2</sup> and life of more than 800 hours.
Selected-area deposition (SAD) of diamond films with selectivity greater than 200 was achieved on patterned SiN/Si surface, using microwave plasma CVD method. Among the important parameters that can modify the plasma characteristics, the total pressure and CH<sub>4</sub>-to-H<sub>2</sub> ratio were observed to influence the selected-area deposition behaviors most significantly. The number density of diamonds grown on Si surface decreased rapidly as the total pressure reduced from 75 torr to 60 torr. The selectivity of SAD diamond films increased as the CH<sub>4</sub>-to-H<sub>2</sub> ratio increased from 9:300 sccm to 15:300 sccm. These phenomena were accounted for by the decrease in proportion of ion species with CH<sub>4</sub>-to-H<sub>2 </sub> ratio such that the formation of sp<sup>3</sup> bonds on Si surfaces is suppressed. Scanning electron microscopy (SEM) and Raman spectroscopy indicated that the quality of diamonds grown on SiN surface was optimized for the SAD films deposited under 2500 W microwave power with CH<sub>4</sub>-to-H<sub>2</sub> = 15:300 sccm
Self-assembled high-density and high-uniformity InAs quantum dots (QDs) were grown on GaAs(001) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many 2-dimentional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. In addition, the QD density could be widely controlled by adjusting Sb supply amount. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.
The thermal stability of pure HfO(2) thin films is not high enough to withstand thermal processes, such as S/D activation or post-metal annealing, in modern industrial CMOS production. In addition, plasma nitridation technology has been employed for increasing the dielectric constant of silicon dioxide and preventing boron penetration. In this experiment, atomic layer deposition (ALD) technology was used to deposit HfO(2) thin films and inductively coupled plasma (ICP) technology was used to perform plasma nitridation process. The C-V and J-V characteristics of the nitrided samples were observed to estimate the effect of the nitridation process. According to this study, plasma nitridation process would be an effective method to improve the thermal stability of HfO(2) thin films.
Transmission-type GaAs photocathode are emerging from the laboratory as practical photo-sensing devices with vastly improved sensitivity and spectral range compared with other photocathode. It is widely used in the many fields such as semiconductor device, optical radiation measurement, camera device and low-level-light night vision. Measurement of parameters of photocathode, especially its spectral response, is of great importance on the technique of photocathode's performance. During the fabrication of photocathode, the technology of online spectral response measurement enables us to determine spectral response quickly and accurately. Through analyzing and comparing those measured response curves, much information about photocathodes, for example, the sensitivity, the photo-electron surface escape probability, the thickness of the active layer, the diffusion length of electrons and the back interface recombination velocity of electrons can be obtained which is useful in both the research and the fabrication of photocathodes (Zhiyuan Zhong and BenKang Chang, 1998; Andre et al, 1981).
The corrosion behavior of brass in 0.1 M NaCl was investigated with voltametric technique. For comparison the behaviors of pure components of the alloy was investigated under the same condition. Evaluation of the solution analysis with the voltametric curves shows that initial dissolution takes place at about −1.0 V (SCE) as ZnO and zinc ions. The dissolution of copper as ions is prevented by the exchange reaction between them. This prevention continues up to CuCl formation potential. Even in the passivity region of brass, dissolution of zinc continues with anodic polarization.
We obtain potentiodynamic curves of Al at different scanning rates beginning from −1.8 V in 0.l M NaCl at pH 4 and in 0.l M NaCl+0.06 M citrate at pH 4, 8 and 10; with or without buffered solution. The shapes of these curves show that the electrochemical and chemical dissolution and precipitation reactions on the Al surface occurs competitively depending on the pH of the solution, presence of buffer and on the scanning rate. This competition was also confirmed by i–t curves obtained at constant potential.
During the rapid progress in the integrated circuit industry up to the present, three groups of electronic systems, namely the calculators/electronic watches, the TV receivers/video cassette players and recorders, and the mainframe computers/personal computers, have played remarkable roles in expanding the integrated circuit market. Improvement in lithography by using sources with shorter wavelengths of light has been the first key technology during this progress. The second key process technology is the realization of the device structures at each generation of the integration, which have been successfully developed. For the 0.1 μm age in the future, lithography and new materials are possible key technologies. Related to the limits of miniaturization and integration, RC delay on the metal wiring, limit to the lithography and high-aspect-ratio holes in the insulator are discussed. The RC delay limits the operation speed of the integrated circuits. We propose a new method to raise the limit. The limit to lithography is basically ascribed to the accuracy of the electron-beam writing. Higher-density electron-beam systems have to be pursued. Finally, we show the trend of the aspect-ratios of the holes on the basis of the scaling approaches.
This investigation describes experiments on two sizes of p-channel metal-oxide-semiconductor field-effect-transistors (pMOSFETs), to study the negative bias temperature instability (NBTI) and hot-carrier (HC) induced degradation. This work demonstrates that the worst condition for pMOSFETs under HC tests occurs in CHC (channel HC, stressed at Vg = Vd) mode at high temperature. This study also shows that the worst degradation of pMOSFETs should occur in NBTI. This inference is based on a comparison of results for forward saturation current (Ids,f) and reverse saturation current (Ids,r) obtained in NBTI and HC tests.
Surface segregation on (100) oriented single crystal surfaces of the alloy Fe–25%Cr–2%Ni–0.14%Sb–N,S has been investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The experiments have been carried out in the temperature range from 500°C to 700°C in which a variety of segregation phenomena is observed. In the low temperature range, cosegregation of nitrogen and chromium occurs. Depth profiling and XPS studies suggest that the Cr and N segregation layer consists of (i) a two-dimensional surface compound and (ii) three-dimensional precipitates in the surface near region. A LEED investigation of the Cr and N saturated surface yields a clear (1×1) pattern after sufficiently long annealing times.At temperatures in excess of 600°C, segregated nitrogen and chromium are replaced by segregating antimony and nickel. Studies of the kinetics show that nickel and antimony jointly enrich at the sample surface; this proves that true cosegregation occurs. An XPS study of the antimony and nickel saturated surface shows that the antimony binding energy coincides with the value that had been measured for the binding energy of segregated antimony on pure iron. The corresponding LEED pattern shows reflex splitting which indicates domain formation.A further increase of the temperature to about 700°C causes segregation of sulphur displacing antimony from the surface. The sulphur covered surface exhibits a typical c(2×2) LEED pattern.
Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol–gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn1−xCoxO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet–visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn1−xCoxO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn1−xCoxO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn1−xCoxO (0 ≤ x ≤ 0.05), Zn0.90Co0.10O and Zn0.85Co0.15O are CoO, Co3O4 and Co2O3, with an increase of Co content, respectively.
The crystallization kinetics and morphology, growth mechanisms and stability of the phases of NiSi2±0.2 alloy films were studied. The alloys were co-evaporated on two substrates namely: (1) silicon single-crystal (Si(100)), (2) thermally oxidized silicon single-crystal (Si(100)). Crystallization, throughout the whole thickness of the film, was observed to occur at temperatures lower than 200°C. The activation energy depends upon the Si content in the thin alloy film as well as on the type of substrate. The activation energy of the alloys deposited on the Si substrate was found to be higher than that obtained for the inert one. After the crystallization is completed, for the films deposited on the silicon oxide, oriented {111} grain growth takes place. On the Si(100) substrate, epitaxial growth of NiSi2 was observed to occur (∼ 400°C) at the polycrystalline NiSi2/Si interface. This was accompanied by the appearance of twins as a misfit strain relief mechanism. Simultaneously, at the free surface, {111}-oriented grain growth occurs, which lowers the surface energy.
In this paper, the design of a fully integrated DC–5 GHz NMOS single-pole double throw (SPDT) transmit/receive (T/R) switch for radio-frequency (RF) applications in a 0.25-μm SiGe BiCMOS/RFCMOS technology, is presented. The switch insertion loss is <1.4 dB, the isolation is >30.1 dB, all over the 0–5 GHz band, and the return loss is >19.9 dB in the 0.8–1 GHz band and is >10.2 dB in the 0–0.8 GHz and 1–5 GHz bands.
0.7BiFeO3–0.3PbTiO3 (BFPT7030) thin films were deposited on SiO2/Si substrates by sol–gel process. The influence of heating rate on the crystalline properties of BFPT7030 thin films were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). XRD patterns of the films showed that a pure perovskite phase exists in BFPT7030 films annealed by rapid thermal annealing (RTA) technique. SEM and AFM observations demonstrated that the BFPT7030 films annealed by RTA at 700 °C for 90 s with the heating rate of 1 °C s−1 could show a dense, crack-free surface morphology, and the films’ grains grow better than those of the films annealed by RTA at the same temperature with other heating rates. XPS results of the films indicated that the ratio of Fe3+:Fe2+ is about 21:10 and 9:5 for the films annealed by RTA at 700 °C for 90 s with the heating rate of 1 and 20 °C s−1, respectively. That means the higher the heating rate, the higher the concentration of Fe2+ in the BFPT7030 thin films.
The growth, structure, surface morphology, optical properties and electrical resistivity studies on TiNx (0.4 < x ≤ 0.5) films is presented. The films of thickness 116–230 nm were grown on fused silica substrates by RF magnetron sputtering in 100% pure nitrogen atmosphere at ambient temperature and pressures from 12 to 25 mTorr. For the as-deposited films, the refractive index decreased from 1.86 to 1.6 with increasing N2 pressure from 12 to 25 mTorr. The absorption edge for the film deposited at 12 mTorr was 4.7 eV and it decreased to 3.5 eV on increasing the N2 pressure to 25 mTorr. Post-deposition annealing of the films at 873 K for 1 min did not cause any variation in the optical properties. The film deposited at 25 mTorr and annealed at 873 K showed a nanocrystalline peak corresponding to ɛ-Ti2N (3 1 1) with a crystallite size of 60 nm. Surface morphologies varied dramatically with N2 pressure. The electrical resistivity of the film deposited at 12 mTorr was 37 MΩ cm whereas it is 270 kΩ cm for the films deposited at 25 mTorr. Therefore, the current work provides signatures for the ɛ-Ti2N phase in terms of refractive index, optical absorption edge and electrical resistivity, that can be used to identify the presence of the sub-stoichiometric forms in a TiN film.
Secondary ion yields Y(Xiq) increase considerably when changing from atomic to molecular primary ions, whereas the parallel increase in the corresponding damage cross sections σ(Xiq) is much smaller. This results in a net increase of ion formation efficiencies E(Xiq)=Y/σ. For a more detailed understanding of the complex sputtering and ion formation processes, in particular for molecular primary ion bombardment, the secondary ion emission of well-defined polymethacrylate LB mono- and multilayers on Ag was investigated. For characteristic secondary ions Xiq emitted from these overlayers Y(Xiq) and σ(Xiq) for 11 keV Ne+, Ar+, Xe+, O2+, SF5+, C7H7+, C10H8+, C6F6+ and C10F8+ bombardment were determined and compared. The influence of primary ion energy was investigated in the energy range between 0.5 and 10 keV for Xe+ and SF5+ bombardment.For multilayers we found yield increases up to nearly a factor of 1000, when changing from Ne+ to SF5+ bombardment. We found a more pronounced yield and efficiency enhancement for multi than for monolayer coverages, a saturation of Y, σ and E enhancement for primary ions made of more than 6 heavy constituents at constant primary ion energy, no chemical effect on the secondary ion yields under static SIMS conditions (SF5+ / C7H7+ e.g.), and a pronounced decrease in secondary ion yields and secondary ion formation efficiencies for SF5+ primary ions with impact energies below 2 keV.
An investigation of the low-temperature operation of a 0.5 μm-gate Si:SiGe depletion-mode n-type modulation-doped field-effect transistor is presented. The investigated temperatures range from T=300 to 180 K. The benefits of cryogenic operation are discussed. Experimental indications of parallel conduction in the device are presented, as well as their dependence on operating temperature. Measured data are compared with two-dimensional device simulations in MEDICI™ carried out using mobility values from Monte Carlo material calculations.
Top-cited authors
Mark C. Biesinger
  • The University of Western Ontario
Chandrakant Lokhande
  • Shivaji University, Kolhapur
Toru Yamashita
  • Tomakomai National College of Technology
Peter Charles Hayes
  • eResearch Australasia - University of Queensland
Jiaguo Yu
  • China University of Geosciences