Showing papers on "Evaporation (deposition) published in 1995"

Polyaniline thin films for gas sensing

Abstract: Thin films of polyaniline have been deposited by spinning, evaporation and by the Langmuir-Blodgett technique. The films are shown to possess slightly different in-plane electrical conductivities, reflecting differences in their chemical structure and layer morphology. The conductivity is found to depend on the gas ambient. All types of polyaniline films are sensitive to H2S and NOx at concentrations down to 4 ppm. However, only spun and evaporated films are responsive to SO2.

Observation of a negative electron affinity for boron nitride

Abstract: This study reports UV‐photoemission (UPS) measurements made on boron nitride crystals and thin films. The materials examined are commercial grade c‐BN powder and thin films of BN deposited with ion beam assisted e‐beam evaporation and laser ablation. The thin film samples examined exhibited varying amounts of sp3 (cubic) and sp2 (hexagonal, amorphous) bonding as determined by FTIR measurements. The UPS measurements displayed the spectral distribution of the low energy photoemitted electrons and the total energy width of the spectra. These characteristics can be related to the electron affinity. The measurements on several of the BN powder and thin film samples revealed features in the emission spectra which are indicative of a negative electron affinity (NEA) surface.

X‐ray photoelectron spectroscopy study of the difference between reactively evaporated and direct sputter‐deposited TiN films and their oxidation properties

Abstract: X‐ray photoelectron spectroscopy (XPS) is used to study carefully prepared oxide‐free titanium nitride (TiN) films of nearly identical stoichiometry, grown by direct sputtering and reactive evaporation. The line shapes and peak positions of the Ti 2p and N 1s transitions are dependent upon the deposition method. The XPS results showed that oxynitrides are present following exposure of either type of film to dry oxygen; however, reactively evaporated films have a considerably higher oxidation rate. Differences in valence band and electron energy loss spectroscopy (EELS) spectra are correlated with the changes observed in the core levels. The EELS spectra detected the presence of a low energy loss, which is correlated with the TiN electron density of states closest to the Fermi level. The secondary electron emission yields for reactively evaporated TiN were also measured before and after dry oxygen and air exposure, and after heating. The maximum yields are about 10% lower for reactively evaporated films than for similarly treated previously measured sputter‐deposited layers.

Growth of conjugated oligomer thin films studied by atomic-force microscopy.

Abstract: We present a quantitative investigation by atomic-force microscopy of the growth of an organic thin film made of sexithienyl evaporated on mica in high vacuum. Different forms of aggregation in sexithienyl thin films are induced upon a systematic increase in the temperature of the substrate during the evaporation. For temperatures up to 150 \ifmmode^\circ\else\textdegree\fi{}C, the films consist of tightly packed grains whose size increases with temperature according to an Arrhenius behavior with \ensuremath{\sim}0.36 eV activation energy. The \ensuremath{\chi}-square grain-size distribution suggests that growth rate is controlled by diffusion. This mechanism is supported by a simple model calculation of the van der Waals interaction between a single T6 molecule and a (100) T6 surface, that yields energy barriers to translation and rotation of the molecule equal to 0.3 and 0.5 eV, respectively. At 200 \ifmmode^\circ\else\textdegree\fi{}C the film undergoes a morphological change to a lamellar structure and extended microcrystalline structures appear at higher temperatures. The presence of preferential directions suggests that orientational ordering is induced by the mica substrate. The possibility of obtaining ordered aggregates by a suitable choice of substrate and temperature during evaporation has relevance towards the realization of molecular devices with anisotropic properties.

Ion induced stress generation in arc‐evaporated TiN films

Abstract: The influence of Ti ion bombardment on the intrinsic stress and microstructure of TiN films during deposition by arc evaporation of Ti in pure N2 has been investigated. Ions with an average charge of +1.6 were accelerated from the arc discharge by a negative substrate bias Vs between 5 and 540 V which yielded a steady‐state substrate temperature between 300 and 600 °C, respectively. The compressive intrinsic stresses in the films, as determined by the x‐ray‐diffraction (XRD) sin2 ψ method after subtracting the thermal stress contribution at room temperature, changed abruptly from 1.9 to a maximum of 6.5 GPa as Vs increased from 5 to 100 V. The compressive stress then decreased monotonically to ∼1.6 GPa as Vs increased to 540 V. Broadening of XRD peaks (β) showed accompanying inhomogeneous strain with a maximum values for Vs=100 V. Cross‐sectional transmission electron microscopy showed a dense columnar film microstructure. Electron microdiffraction showed a distorted structure within the same columns for ...

Residual stresses in evaporated silicon dioxide thin films: Correlation with deposition parameters and aging behavior

Abstract: Silicon dioxide thin films have been prepared by electron‐gun evaporation under various deposition conditions. The residual stresses in SiO2 films were determined by measurements of the radius of curvature of Si and Ge substrates. The composition and density of films were deduced from Rutherford backscattering spectroscopy and elastic recoil detection analyses. The films were found to be stoichiometric (Si/O=1/2) under the deposition conditions investigated. The compressive residual stresses in films deposited at the base pressure (2×10−5 mbar) varied from −20 to −550 MPa as the substrate temperature increased from 20 to 285 °C. At a substrate temperature of 200 °C, the residual stresses varied from +70 to −180 MPa with decreasing oxygen pressure in the deposition chamber. The contribution of three types of stresses, namely thermal, intrinsic, and water‐induced stresses, can be distinguished. The stress component resulting from the absorption of water molecules in porous SiO2 films was obtained from varia...

Preparation and characterization of Cu(In1−xGax)3Se5 thin films

Abstract: Polycrystalline Cu(In1−xGax)3Se5 thin films were prepared by four source evaporation with controlling and shielding of the molecular beams from elemental sources. Ga content x, can be controlled by deposition times of CuIn3Se5 and CuGa3Se5 layers, which form Cu(In1−xGax)3Se5 films through the interdiffusion. X‐ray diffraction analyses showed that the films with x≲0.5 have an ordered vacancy chalcopyrite and the films with x≳0.5 have a zinc blende structure. The optical band gap of the films linearly increased from 1.23 eV (x=0) to 1.85 eV (x=1) with increasing Ga content. The conductivity of the films was about 10−6/Ω cm and about 10−7/Ω cm under and above x=0.3, respectively.

In Situ Fluorescence Probing of the Chemical Changes during Sol–Gel Thin Film Formation

Abstract: Pyranine (8-hydroxy-1,3,6-trisulfonated pyrene) was used as an in situ fluorescence probe to monitor the chemical evolution during sol-gel thin film deposition of silica by the dip-coating process. The sensitivity of pyranine luminescence to protonation/deprotonation effects was used to quantify changes in the water/alcohol ratio in real time as the substrate was withdrawn from the sol reservoir. The spatially resolved spectral results showed that preferential evaporation of alcohol occurred, and that the solvent composition in the vicinity of the drying line reached values in excess of 80 vol% water. Correlation of the luminescence results with the interference pattern of the depositing film allowed the solvent composition to be mapped as a function of film thickness. HE sol-gel process is a method of preparing inorganic oxide T glasses and ceramics from polymeric or colloidal sols at temperatures well below those used in traditional processing technique^.'-^ Interest in sol-gel thin films has grown in recent years as a number of applications have emerged in areas such as protective and optical coatings, high and low dielectric constant films, electrochromics, waveguides, ferroelectric thin films, sensors, and membrane^.^ The films can be produced readily by several different methods including spin coating and dip coating.5 Dip coating is perhaps more important technologically since a uniform coating can be deposited onto substrates of large dimensions and complex geometries. In the dip-coating process the substrate is immersed into the coating sol and then withdrawn at a constant rate. Brinker, Hurd, and co-workers have carried out extensive work, both experimental and theoretical, and have established how various physical and chemical parameters involved in dip coating affect the structure and properties of the final film.5 In the dip-coating process, a substrate is withdrawn slowly at constant speed (typically 10-20 cm/min) from a sol which contains polymeric or colloidal species in suspension (see

Photoluminescence of pyrenebutyric acid incorporated into silicone film as a technique in luminescent oxygen sensing

Abstract: The photoluminescence of pyrenebutyric acid (PBA) incorporated into a cast silicone film has been studied, and the structural features around the luminescent molecule are discussed. The III/I peak intensity ratio of the emission spectrum was higher for the film than for PBA solution, indicating that the PBA in the silicone film existed in a non-polar micro-environment. The III/I ratio was dependent on both the evaporation of the cast solvent and the drying conditions of the film, which was ascribed to the remaining trace cast solvent in contact with PBA. The long emission lifetime of PBA in the slow-evaporation film showed that its local molecular mobility is suppressed by tight molecular packing. The amount of PBA extracted from the film using water increased with increase in the polarity of the cast solvent, suggesting that different cluster and channel structures are formed depending upon the cast solvent used. The efficiencies of the emission quenching by oxygen were similar for the films prepared by different cast solvent. The linear Stern–Volmer plots for the quenching by oxygen showed that a luminescent oxygen sensor can be fabricated by a simple mixture-casting method. Other characteristics of using PBA films as oxygen sensors, such as, sensitivity, detection limit and response time etc., are also discussed.

Tetragonal germanium flims deposited by the cluster-beam evaporation technique

Abstract: Germanium (Ge) films have been fabricated by the cluster‐beam technique. In transmission electron microscope (TEM) micrographs, the film deposited on a substrate at room temperature appears smooth and uniform, while that deposited at liquid nitrogen temperature consists of nanostructures with diameters from 4 to 40 nm. The x‐ray diffraction (XRD) patterns show that the crystalline structure for either deposition temperature is not the ordinary diamond structure but is tetragonal. The absorption spectra are distinct from that of bulk Ge. The photo‐oxidized Ge nanostructures exhibit blue light emission, which is strong enough to be recognized with the naked eye even under a room light.

Optical constants and film density of TiNxOy solar selective absorbers

Abstract: Thin TiNxOy films were deposited on glass substrates in an activated reactive evaporation process. The influence of the reactive gas composition and pressure during deposition on the film density ρMass was investigated by grazing incidence x‐ray‐diffraction measurements. The density increases from 3.8 to 4.8 g/cm3 when lowering the total gas pressure ptotal from 10−3 h Pa down to 2.5×10−4 h Pa, whereas the ratio of the partial gas pressures of nitrogen and oxygen has no measurable effect on the film density. Data on the refractive index are presented, obtained from reflectance and transmittance measurements in the optical wavelength range (λ=0.3–2.5 μm). Bruggeman effective medium theory was applied to explain the optical behavior n(λ) as a result of the low density of the coatings. The high porosity of the films suppresses the metallic character of the phases TiN and TiO contained in the TiNxOy film. Its dielectriclike behavior in the infrared region is essential for low thermal emittances when deposited on copper to form a solar selective absorber.

The growth and properties of Pd and Pt on Al2O3/NiAl(110)

Abstract: Small metal particles have strongly size-dependent properties, which - in the case of supported particles - are also influenced by the particle substrate interaction. We have investigated such effects for Pd deposited on a thin alumina film grown on NiAl(i10) by probing the adsorption behaviour of CO. Structure and size of the metal islands were varied by employing different substrate temperatures during evaporation. We found that at 90 K small, disordered aggregates are formed, whereas deposition at 300 K results in larger crystallites with (111) facets. The CO thermal desorption spectra show a size-dependent behaviour which can be attributed to a higher degree of CO coordination with decreasing particle size. The results are compared with the corresponding behaviour of Pt. In contrast to Pd, Pt leads to a strong interaction with the substrate at 300 K. This gives rise to a desorption feature of CO in a temperature range which is typical for transition metal oxides.

Orientation selection in thin platinum films on (001) MgO

Abstract: Orientation selection in platinum films of ∼20 nm thickness deposited onto (001) MgO substrates by e‐beam evaporation was investigated through ion beam channeling and x‐ray diffraction. A mixture of crystallites having (111) and (001) orientation was observed in Pt films deposited over a range of substrate temperatures from 25 to 700 °C, with the (111) orientation dominant at low temperatures. The (111) orientation was present in these evaporated films at significantly higher substrate temperatures than reported for Pt films deposited by sputtering or pulsed laser deposition. Both orientations had strongly preferred in‐plane orientations: [110] Pt//[110] MgO for the (001)‐oriented crystallites and [110]Pt//[110] MgO for the (111). The orientation selection process was rationalized based on the expected relative interfacial energies for these two orientations.

Effect of substrate temperature on the deposition of polytetrafluoroethylene by an ionization‐assisted evaporation method

Abstract: Thin films were deposited from low molecular weight polytetrafluoroethylene source materials by an ionization‐assisted evaporation method, and their crystallinity and molecular orientation were studied by x‐ray diffraction and infrared spectroscopy. The film properties were largely influenced by the substrate temperature and the molecular weight of the source material. At lower substrate temperatures, the molecules were oriented with the chain axis parallel to the substrate surface, and the film had preferential crystal orientation with the (100) plane of the hexagonal structure parallel to the substrate. With increasing substrate temperature, the molecules lost parallel orientation and the film crystallinity decreased. Such changes took place upon increasing the substrate temperature from 180 to 240 °C when deposited from a source material with a molecular weight of 8500, and from room temperature to 130 °C when the source molecular weight was 1500.

Field evaporation of gold atoms onto a silicon dioxide film by using an atomic force microscope

Abstract: To investigate whether field evaporation of gold atoms is responsible for dot formation in an atomic force microscope (AFM) gold‐coated tip/vacuum/SiO2 film/p‐type Si substrate configuration, we have performed elemental analysis of the dots and measured the dependence of the threshold voltage on SiO2 thickness with both polarities for the dot formation. The experiments demonstrate that it is feasible to form gold dots on SiO2 films 17–107 A thick by adjusting the pulsed voltages applied to the gold‐coated AFM tip. Energy dispersive x‐ray spectroscopy (EDX) shows that the dots include gold. The threshold voltages increase almost linearly with the SiO2 thickness. Furthermore, the voltage with negative polarity is lower than that with positive polarity. These results provide evidence that the dot formation on the SiO2 film using AFM occurs by field evaporation.