Showing papers on "Evaporation (deposition) published in 2008"

Patterned epitaxial vapor-liquid-solid growth of silicon nanowires on Si(111) using silane

Abstract: We have carried out a detailed study on the vapour-liquid-solid growth of silicon nanowires (SiNWs) on (111)-oriented Si substrates using Au as catalytic seed material Arrays of individual seeds were patterned by electron-beam lithography, followed by Au evaporation and lift-off SiNWs were grown using diluted silane as precursor gas in a low-pressure chemical vapor deposition system The silane partial pressure, substrate temperature, and seed diameter were systematically varied to obtain the growth rate of the NWs and the rate of sidewall deposition Activation energies of 19kcal∕mol for the axial SiNW growth and 29kcal∕mol for the radial deposition on the SiNW surface are derived from the data SiNW growth at elevated temperatures is accompanied by significant Au surface diffusion, leading to a loss of Au from the tips of the SiNWs that depends on the layout and density of the Au seeds patterned In contrast to NWs grown from a thin-film-nucleated substrate, the deterministic patterning of identical A

High sensitivity photonic crystal biosensor incorporating nanorod structures for enhanced surface area

Abstract: The surface area of a photonic crystal biosensor is greatly enhanced through the incorporation of a porous TiO 2 film possessing the structure of nanorods into the device The film is deposited by the glancing angle deposition technique in an e-beam evaporation system The sensitivity of high surface area sensors is compared with sensors without the high surface area coating Results for detection of polymer films, large proteins and small molecules indicate up to a four-fold enhancement of detected adsorbed mass density

On the origin of photoluminescence in indium oxide octahedron structures

Abstract: A sixfold decrease in photoluminescence signal intensity at 590nm with increase in deposition time from 3to12h has been observed in single crystalline indium oxide octahedron structures grown by vapor-phase evaporation method Electron paramagnetic resonance and energy dispersive x-ray analysis confirm that the concentration of oxygen vacancies increases with deposition time These results are contrary to the previous reports where oxygen vacancies were shown to be responsible for photoluminescence in indium oxide structures Our results indicate that indium interstitials and their associated complex defects other than oxygen vacancies are responsible for the photoluminescence in In2O3 microstructures

Fabrication of Arrays of Metal and Metal Oxide Nanotubes by Shadow Evaporation

Abstract: This paper describes a simple technique for fabricating uniform arrays of metal and metal oxide nanotubes with controlled heights and diameters. The technique involves depositing material onto an a...

Fabrication of Arrays of Metal and Metal Oxide Nanotubes by Shadow

Abstract: This paper describes a simple technique for fabricating uniform arrays of metal and metal oxide nanotubes with controlled heights and diameters. The technique involves depositing material onto an anodized aluminumoxide(AAO)membranetemplateusingacollimatedelectronbeamevaporationsource.Theevaporating material enters the porous openings of the AAO membrane and deposits onto the walls of the pores. The membrane is tilted with respect to the column of evaporating material, so the shadows cast by the openings of the pores onto the inside walls of the pores define the geometry of the tubes. Rotation of the membrane during evaporation ensures uniform deposition inside the pores. After evaporation, dissolution of the AAO in base easily removesthetemplatetoyieldanarrayofnanotubesconnectedbyathinbackingofthesamemetalormetaloxide. The diameter of the pores dictates the diameter of the tubes, and the incident angle of evaporation determines theheightofthetubes.Tubesupto1.5minheightand20-200nmindiameterwerefabricated.Thismethod is adaptable to any material that can be vapor-deposited, including indiumtin oxide (ITO), a conductive, transparent material that is useful for many opto-electronic applications. An array of gold nanotubes produced bythistechniqueservedasasubstrateforsurface-enhancedRamanspectroscopy:theRamansignal(permolecule) from a monolayer of benzenethiolate was a factor of510 5 greater than that obtained using bulk liquid benzenethiol.

Silicon-induced oriented ZnS nanobelts for hydrogen sensitivity

Abstract: Oriented ZnS nanobelts were grown on an Si substrate using hydrogen-assisted thermal evaporation under moist gas conditions. It was found that these ZnS nanobelts had a single crystal hexagonal wurtzite structure growing along the [0001] direction. They had a rectangular cross section with lengths of up to tens of micrometres, a typical width of 50–150 nm, and a thickness of ∼40 nm. A silicon-induced vapour–liquid–solid process was proposed for the formation of the ZnS nanobelts and their assembly. These oriented nanobelts have much faster response time to hydrogen gas than that of pure ZnO and Pd-sensitized ZnO, showing excellent hydrogen sensing properties.

Tuning of electrical and structural properties of metal-polymer nanocomposite films prepared by co-evaporation technique

Abstract: Nanocomposites consisting of Au and Ag nanoparticles embedded in Teflon AF 1600 (Teflon) and Nylon 6 (Nylon) matrices were prepared by a simultaneous vapor phase deposition of both the polymer and the metal. The composite films were deposited between two Au-Pd alloy electrodes prepared by sputtering onto kapton foil substrates enabling further electrical measurements. The electrical properties of the composites are strongly influenced by the metal filling factor and changes in the microstructure. At first, the dependence of the resistivity of the composites consisting of various Ag and Au nanoparticle concentrations was investigated. The resistivity is characterized by a threshold region with a critical metal filling factor. Changes in the microstructure, in particular, can occur as a result of an induced electric field in between the metal nanoparticles and a heat treatment. The I–V characteristics of Teflon AF composites for different Au concentrations were studied thoroughly. An increase in the slope of the I–V curve up to a certain voltage (breakdown voltage) was observed. This phenomenon is accompanied by the field induced tunneling of the charge carriers which enhances the conductivity. The change in conductivity was also analyzed for Nylon nanocomposites with various Au concentrations in the temperature range 20–180 °C. The observed temperature dependence is explained by activated electron tunneling between metal nanoparticles and by rearrangements in the microstructure (e.g. coalescence of metal nanoparticles).

Complementary inverter circuits based on p-SnO2 and n-In2O3 thin film transistors

Abstract: Thin film transistors (TFTs) of indium oxide (In2O3) and tin oxide (SnO2) were fabricated on SiO2 gate dielectric using reactive evaporation process. Structural investigation of the films revealed that In2O3 films were polycrystalline in nature with preferred (222) orientation and SnO2 films exhibited amorphous nature. The x-ray photoelectric spectroscopy measurements suggest that SnO2 films were oxygen rich and presume mixed oxidation states of Sn, namely Sn2+ and Sn4+. While the In2O3 based TFTs possess n-type channel conduction, SnO2 based TFTs exhibited anomalous p-type conductivity. Integration of these n- and p-type devices resulted in complementary inverter with a gain of 11.

Si-CdSSe core/shell nanowires with continuously tunable light emission.

Abstract: Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.

Field evaporation behavior during irradiation with picosecond laser pulses

Abstract: A field-ion specimen made of a low thermal diffusivity material, field evaporated using picosecond laser pulses incident from one side, is shown to develop different curvatures between the incident side and the “shadow” side of the specimen apex. Differences of approximately 2.6 and 1.5V∕nm in evaporation field were observed between the two regions of a type 304 stainless steel tip evaporated at 50K with pulsed laser intensities of 0.04 and 0.02nJ∕μm2ps, respectively. This indicates that diffraction of the laser beam cannot ensure uniform illumination and heating over the tip apex.

ZnO Nanotube Ethanol Gas Sensors

Abstract: This investigation reports on the growth of ZnO nanotubes on patterned Au/Al 2 O 3 /Au/RuO 2 templates by reactive evaporation and the fabrication of a ZnO nanotube ethanol gas sensor. The as-grown ZnO nanotubes were polycrystalline and had holes at their terminals and sidewalls. Injection of ethanol gas reduced the resistivity of the fabricated sensor. Introducing 100 ppm ethanol gas increased the measured sensitivities of the ethanol gas sensors from 34 to 87% as the temperature increased from 90 to 230°C. Additionally, the sensitivity of the sensor increased with the concentration of injected ethanol gas.

ZnS Nanowire Arrays: Synthesis, Optical and Field Emission Properties

Abstract: Nanowire arrays of cubic zinc sulfide are synthesized on zinc foil by a simple thermal evaporation route at relatively low temperature (<500 °C). A self-catalytic liquid–solid growth is proposed for the formation of the nanowires. Influence of the synthesis temperatures on the morphological features of the ZnS nanowires is studied. Growth sequence of the nanowires with synthesis time is also studied through scanning electron microscopy. These nanowires grow along the [111] direction and remain perpendicular to the surface of the Zn substrate. Room temperature photoluminescence measurement exhibit UV and blue emission peaks at around 350 and 397 nm, respectively. These ZnS nanowires exhibited good field emission properties similar to those of conventional field emitters.

Synthesis of tellurium nanostructures by physical vapor deposition and their growth mechanism

Abstract: One-dimensional tellurium nanostructures have been prepared by thermal evaporation in a tubular furnace under argon gas flow at a pressure of 1 atm. The depositions were found to occur along the length of the quartz tube with microrods deposited at higher temperatures (350–400 °C) and nanotubes at lower temperatures (<200 °C). It was observed that the growth of nanostructures is initiated by nucleation of spherical particles followed by growth of one-dimensional structures. Possible mechanisms for the growth of these nanostructures and the effect of source temperature on the structures obtained are discussed.

Electrical, structural, and chemical properties of HfO2 films formed by electron beam evaporation

Abstract: High dielectric constant hafnium oxide films were formed by electron beam (e-beam) evaporation on HF last terminated silicon (100) wafers. We report on the influence of low energy argon plasma ( ∼ 70 eV) and oxygen flow rate on the electrical, chemical, and structural properties of metal-insulator-silicon structures incorporating these e-beam deposited HfO2 films. The use of the film-densifying low energy argon plasma during the deposition results in an increase in the equivalent oxide thickness (EOT) values. We employ high resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and medium energy ion scattering experiments to investigate and understand the mechanisms leading to the EOT increase. We demonstrate very good agreement between the interfacial silicon oxide thicknesses derived independently from XPS and HRTEM measurements. We find that the e-beam evaporation technique enabled us to control the SiOx interfacial layer thickness down to ∼ 6 A. Very low leakage current density (<10−4 A/cm2) is measured at flatband voltage +1 V into accumulation for an estimated EOT of 10.9±0.1 A. Based on a combined HRTEM and capacitance-voltage (CV) analysis, employing a quantum-mechanical CV fitting procedure, we determine the dielectric constant (k) of HfO2 films, and associated interfacial SiOx layers, formed under various processing conditions. The k values are found to be 21.2 for HfO2 and 6.3 for the thinnest ( ∼ 6 A) SiOx interfacial layer. The cross-wafer variations in the physical and electrical properties of the HfO2 films are presented.

Evaporation Donor Substrate and Method for Manufacturing Light-Emitting Device

Abstract: One surface of a first substrate provided with at least light-absorbing layers separately formed, partition layers each formed between the light-absorbing layers and having an inverse taper shape, and material layers formed on the light-absorbing layers and on the partition layers so that the material layers are separated from each other is disposed to face a deposition target surface of a second substrate; light irradiation is performed from the other surface of the first substrate, only the material layers in regions overlapped with the light-absorbing layers are heated and evaporated to the deposition target surface of the second substrate.

Ultraviolet-Emitting ZnO Nanostructures on Steel Alloy Substrates: Growth and Properties

Abstract: ZnO nanostructures with a variety of morphologies and sizes, such as urchinlike structures composed of small-head nanonails, vertically aligned nanonails and nanocones, comblike nanostructures, hierarchical nanostructures, etc., have been synthesized via the simple thermal evaporation process onto the steel alloy substrate without the use of any metal catalyst or additives. It was observed from the detailed morphological and structural studies that substrate temperature, distances between source material and substrates, concentration of zinc vapors, zinc partial pressure, concentrations of reactant gases, and choices of substrates have serious impact on the morphologies and structural properties of as-grown products. Therefore, within certain reaction parameters, specific morphologies can be obtained. Detailed structural observations confirmed that the as-grown products are single-crystalline with the wurtzite hexagonal phase and grown along the (0001) directions. Raman-scattering and room-temperature pho...

Carbon monoxide gas-sensing properties of electron-beam deposited cerium oxide thin films

Abstract: Thin films of cerium oxide were deposited by electron-beam (e-beam) evaporation on unheated substrates. The films were annealed in air at 500 °C for 2 h, and were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy imaging and optical spectrophotometry. The films were investigated for the detection of carbon monoxide, and were found to be highly sensitive. The effect of bias voltage, operating temperature and gas concentration on the performance of the sensor is reported. The sensor response and recovery times were also measured, and were found to be highly efficient.

The roles of thermally evaporated cesium carbonate to enhance the electron injection in organic light emitting devices

Abstract: The properties of thermally evaporated cesium carbonate (Cs2CO3) and its role as electron injection layers in organic light emitting diodes were investigated. According to the ultraviolet photoemission spectra (UPS), the Fermi level of tris-(8-hydroxyquinoline)-aluminum (Alq3) after being doped with Cs2CO3 shifts toward or into the lowest unoccupied molecular orbital as a result of chemical reaction and charge transfer between Cs2CO3 and Alq3, which lowers the electron injection barrier and improves the current efficiency. As for whether Cs2CO3 being decomposed during the evaporation, we found that Cs2CO3 molecules were deposited on the substrates without decomposition, regardless of the evaporation rates, based on the signature features of carbonate groups and ionization energies measured in UPS spectra and the binding energy shifts of core level electrons. The reaction mechanisms between Cs2CO3 and Alq3 are also proposed. Since Cs2CO3 is not only used in the electron injection layer but also in converti...

Thermal Conductivity and Interfacial Thermal Resistance: Measurements of Thermally Oxidized SiO2 Films on a Silicon Wafer Using a Thermo-Reflectance Technique

Abstract: This article describes the development of a method to measure the normal-to-plane thermal conductivity of a very thin electrically insulating film on a substrate. In this method, a metal film, which is deposited on the thin insulating films, is Joule heated periodically, and the ac-temperature response at the center of the metal film surface is measured by a thermo-reflectance technique. The one-dimensional thermal conduction equation of the metal/film/substrate system was solved analytically, and a simple approximate equation was derived. The thermal conductivities of the thermally oxidized SiO2 films obtained in this study agreed with those of VAMAS TWA23 within ± 4%. In this study, an attempt was made to estimate the interfacial thermal resistance between the thermally oxidized SiO2 film and the silicon wafer. The difference between the apparent thermal resistances of the thermally oxidized SiO2 film with the gold film deposited by two different methods was examined. It was concluded that rf-sputtering produces a significant thermal resistance ((20 ± 4.5) × 10−9 m2·K·W−1) between the gold film and the thermally oxidized SiO2 film, but evaporation provides no significant interfacial thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1). The apparent interfacial thermal resistances between the thermally oxidized SiO2 film and the silicon wafer were found to scatter significantly (± 9 × 10−9 m2·K·W−1) around a very small thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1).

Fullerene (C60) decoration in oxygen plasma treated multiwalled carbon nanotubes for photovoltaic application

Abstract: Multiwalled carbon nanotubes (MWNTs) were functionalized by oxygen plasma treatment. Photoelectron spectroscopy study of oxygen plasma treated MWNTs (O-MWNTs) shows surface modification with hydroxyl and carboxyl groups. C60 decoration of MWNTs were carried out by thermal evaporation and more dense distribution of C60 was achieved on O-MWNTs. C60 decorated MWNTs were combined with poly(3-octylthiophene) for photovoltaic device fabrication. The device with C60 decorated O-MWNTs shows short circuit current density (Jsc), open circuit voltage (Voc), fill factor, and power conversion efficiency (η) as 1.68mA∕cm2, 0.245V, 27%, and 0.11%, respectively. It is expected that C60 provide large surface area for photoexcitons dissociation and efficient electron transportation, whereas MWNTs provide efficient hole transportation.

Nanostructured SnO2 films prepared from evaporated Sn and their application as gas sensors

Abstract: This paper describes the morphology, stoichiometry, microstructure and gas sensing properties of nanoclustered SnOx thin films prepared by Sn evaporation followed by a rheotaxial growth and thermal oxidation process. Electron microscopy was used to investigate, in detail, the evolution of the films as the oxidation temperature was increased. The results showed that the contact angle, perpendicular height, volume and microstructure of the clusters all changed significantly as a result of the thermal oxidation processes. Electron diffraction and x-ray photoelectron spectroscopy measurements revealed that after oxidation at a temperature of 600 °C, the Sn clusters were fully transformed into porous three-dimensional polycrystalline SnO2 clusters. On the basis of these results, a prototype SnO2 sensor was fabricated and sensing measurements were performed with H2 and NO2 gases. At operating temperatures of 150–200 °C the film produced measurable responses to concentrations of H2 as low as 600 ppm and NO2 as low as 500 ppb.