Room Temperature Substrates Research Articles

Electroabsorption study of vacuum-evaporated films of Pt(II)octaethylporphyrin

Abstract Electric-field modulated absorption spectra in the range 270–580 nm have been studied in solid films of Pt(II)octaethylporphyrin (PtOEP) formed by vapour deposition onto room-temperature quartz substrates. The electroabsorption (EA) spectra were measured at first (1ω) and second (2ω) harmonic of fundamental frequency of modulating field (ω) applied to the sample in a sandwich cell arrangement with aluminum electrodes, Al/PtOEP/Al/quartz. The (2ω)EA spectra are interpreted in terms of the standard Stark spectroscopy analysis using a direct comparison of the (2ω)EA spectrum with first and second absorption derivatives. Both main absorption porphyrin bands (B and Q) give the first absorption derivative-like EA signals from molecular (Frenkel) excitons responding to an electric field with the similar values of an average polarizability change, Δ p ¯ ≅ 20 ± 5 A 3 / f 2 (f = 1.7 in the Lorentz field approximation). This result has been rationalized employing Gouterman’s “four orbital model”. A weakly resolved structure observed around 440 nm suggests a contribution of an intermolecular charge transfer state. The presence of an internal electric field, less than 105 V/cm, has been deduced from the relation between (1ω)EA and (2ω)EA spectra.

Brush plated CdSe films on high temperature substrates

CdSe is a direct band gap semiconductor belonging to the II–VI group and possessing excellent optoelectronic properties. Thin film transistors and image intensifier tubes have been fabricated with this material. It is also a very stable photo anode in wet photovoltaic cells. Several physical and chemical techniques are available for the growth of thin films of CdSe [1–5]. Though results on brush plated CdSe films have been reported earlier [6], the films were deposited on room temperature substrates. To our knowledge, this is the first report on CdSe films grown by the brush plating technique on substrates maintained at higher temperatures.

Fluorinated amorphous diamond-like carbon films deposited by plasma-enhanced chemical vapor deposition

Fluorinated amorphous diamond-like carbon films (a-C:F) have been prepared on room-temperature (100) Si substrates by using 13.56 MHz radio frequency plasma-enhanced chemical vapor deposition (rf PECVD), where methane (CH 4) and carbon tetrafluoride (CF 4) were used as gas precursors. The films were deposited as a function of the ratio of CF 4 to CH 4 flow rate (CF 4/CH 4) at 60 and 100 W rf power, respectively. During the film preparation, two processes, i.e., deposition coming from CH 4 and etching induced by CF 4, always coexist. The competition between both, mainly dependent on the CF 4/CH 4 ratio and the rf power, influenced the film surface, thickness, bonding states between C and F, microstructure, the incorporation level of F, and therefore the properties. The F content was found to rapidly increase to ∼12 at.% with the introduction of CF 4, leading to a sharp reduction in surface energy of the samples compared with that of diamond-like carbon (DLC) film. With increasing CF 4, the F content increased, concurrent with increase in CF, appearance to increase in CF 2 content, and variation from diamond-like to graphite-like in microstructure, as disclosed by X-ray photoelectron spectroscopy (XPS) and Raman analyses, respectively. Such change in the bonding configurations is also responsible for a continuing reduction in the film surface energy with increasing F incorporation.

Ion deposited nanoscale permalloy and Co films generated from laser excitation

A modified pulsed laser deposition system has been used to deposit permalloy and Co films from ions generated due to the impact of 248 nm excimer laser pulses. A synchronously pulsed magnetic field coil with a wide entrance throat and a tapered bore in conjunction with a set of permanent magnets has been used to capture and concentrate the ion and electron beam flux under vacuum conditions onto the substrates. Co and other films such as diamond like carbon (DLC) are strongly adherent when deposited onto even room temperature substrates. Permalloy films deposited onto Si (111) substrates with thicknesses from 10 to 50 nm exhibit anisotropic magnetoresistance of approximately 2% for magnetic fields applied in-plane parallel versus perpendicular to the current direction. Over 1 order of magnitude intensity modulation has been observed in x-ray reflectivity measurements for Co films, 7–15 nm thickness, deposited onto R-plane sapphire substrates and onto Si (111) substrates. In contrast to this Co films simultaneously deposited onto A-plane sapphire substrates hardly exhibited x-ray reflectivity fringes indicating a large surface roughness. Similarly DLC films deposited onto (111) Si substrates exhibited order of magnitude intensity modulations indicating a very small surface roughness. Scanning electron microscope measurements indicated smooth films were obtained with no discernible particulates.

Nanogranular Co–Al–O films prepared by laser ablation

AbstractNanogranular Co–Al–O films were prepared by pulsed laser deposition from Cox(Al2O3)1−x targets. Three targets, with different Co volume fractions, were used in order to study the influence of the film composition on magnetic and transport properties. The samples deposited on room temperature substrates exhibit a granular structure consisting of Co grains with the size of few nm embedded in an insulating Al2O3 matrix. The magnetic properties show a gradual transition from a low temperature blocked regime to a high temperature superparamagnetic state. It is confirmed by the disappearance of magnetic hysteresis at high temperatures and the temperature dependence of magnetization. Magnetoresistance (MR) higher then 9% at 20 K can be found in Co26Al23O51 film. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transmission electron microscopy investigation of pulsed-laser deposited hydroxylapatite thin films prepared by tripod and focused ion beam techniques

Hydroxylapatite-based ceramics are largely applied as coatings and thin films on metallic components of prostheses for inducing osteoblasts apposition and subsequent bone re-growth. We report for the first time the focused ion beam technique (FIB) as a milling method to prepare specimens for transmission electron microscopy investigations of HA thin films grown on metallic substrates. New FIB technique results were compared to those obtained by the thinning method of mechanical polishing with tripod. HA films were obtained by pulsed laser deposition on Si wafers and polished Ti–5Al–2.5Fe substrates coated with TiN buffer layers. A KrF* excimer laser ( λ=248 nm, τ FWHM≥20) was used for deposition. HA films were grown in vacuum (10 −4 Pa) on room temperature substrates, subsequently annealed at 550 °C in ambient air for 1 h. Processed structures were investigated by transmission electron microscopy (TEM), grazing incidence X-ray diffraction, selected area electron diffraction, and energy dispersive X-ray spectroscopy. We show that both tripod polishing and focused ion beam milling techniques well preserve the structural and chemical integrity of the thin layers, and their interfaces. Additionally, focused ion beam milling allows for preparation of long specimens (approx. 12 μm in our case). HA films were stoichiometric and had a nano-crystalline structure with an average grain size of 35 nm. Cross-section TEM patterns show a granular growth morphology of the films. Particulates trapped within the film were visualized. High resolution TEM analysis shows that the transition from cubic TiN to hexagonal HA is intermediated by a very thin amorphous calcium phosphate interlayer of ∼3–4 nm thickness.

Hydroxyapatite thin films grown by pulsed laser deposition and radio-frequency magnetron sputtering: comparative study

Hydroxyapatite (HA) thin films for applications in the biomedical field were grown by pulsed laser deposition (PLD) and radio-frequency magnetron sputtering (RF-MS) techniques. The depositions were performed from pure hydroxyapatite targets on Ti–5Al–2.5Fe (TiAlFe) alloys substrates. In order to prevent the HA film penetration by Ti atoms or ions diffused from the Ti-based alloy during and after deposition, the substrates were pre-coated with a thin buffer layer of TiN. In both cases, TiN was introduced by reactive PLD from TiN targets in low-pressure N 2. The PLD films were grown in vacuum onto room temperature substrates. The RF-MS films were deposited in low-pressure argon on substrates heated at 550 °C. The initially amorphous PLD thin films were annealed at 550 °C for 1 h in ambient air in order to restore the initial crystalline structure of HA target. The thickness of the PLD and RF-MS films were ∼1 μm and ∼350 nm, respectively. All films were structurally studied by scanning electron microscopy (SEM), grazing incidence X-ray diffraction (GIXRD), energy dispersive X-ray spectrometry (EDS) and white light confocal microscopy (WLCM). The mechanical properties of the films were tested by Berkovich nano-indentation. Both PLD and RF-MS films mostly contain HA phase and exhibit good mechanical characteristics. Peaks of CaO were noticed as secondary phase in the GIXRD patterns only for RF-MS films. By its turn, the sputtered films were smoother as compared to the ones deposited by PLD (50 nm versus 250 nm average roughness). The RF-MS films were harder, more mechanically resistant and have a higher Young modulus.

Observation of Crystallization Behavior in ITO Thin Films Prepared by RF-Magnetron Sputtering with and without External Heating

ITO films were deposited on the Corning 1737 glass by the RF-magnetron sputtering method with and without external heating as a function of deposition time. The as-grown ITO film, deposited on room temperature substrate without external heating for 10 min, was amorphous in nature. Continuous deposition over 20 min resulted in the development of crystallization of the films. The crystallization by prolonged deposition was initiated from the top surface of the film where energetic ions bombard. When the films were deposited with external heating, the crystallization was initiated from the beginning of the deposition at the surface of the substrate.

Optical and electrochromic properties of RF reactively sputtered WO 3 films

The effects of deposition conditions, including substrate temperature and sputtering gas pressure; on the optical and electrochromic (EC) properties of WO 3 films prepared by RF reactive sputtering were investigated. The maximum optical bandgap energy of 3.15 eV was obtained on a room temperature substrate, and decreased with increasing substrate temperature. However, the maximum refractive index of about 2.5 was obtained at a sputtering gas pressure of 5 mTorr and a substrate temperature of 500 °C, and decreased with decreasing substrate temperature and with increasing pressure. The decrease in bandgap energy and refractive index are thought to have been caused by an increase in WO 3 cluster size and a decrease in film density, respectively. Electrochromic (EC) response times of the WO 3 films were measured in an electrolyte of 1 N H 2SO 4 aqueous solution. Fast EC responses were obtained for the WO 3 films with wide bandgap energies and low refractive indices. The results indicate that the optical properties of WO 3 films are useful indicators of EC response.

Spectroscopic ellipsometry of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)

The optical functions of PTCDA have been determined by spectroscopic ellipsometry. The samples were prepared by thermal evaporation of PTCDA on glass substrates. The influence of the substrate temperature on the properties of PTCDA films has also been investigated. The lowest substrate temperature investigated here was room temperature. At room temperature and higher substrate temperatures, we have not observed ordering of the evaporated films, which would result in high anisotropy, as reported for PTCDA films grown by organic molecular-beam deposition at very low temperatures (so-called quasi-epitaxial growth). Therefore, the samples have been assumed to be isotropic. The obtained imaginary part of the dielectric function exhibits the expected features: a low-magnitude peak around 370 nm and a higher magnitude, two-peak structure in the 400–600 nm spectral region.

Hyperthermal molecular beam deposition of highly ordered organic thin films.

We use a seeded supersonic molecular beam to control the kinetic energy of pentacene (C22H14) during deposition and growth on Ag(111). Highly ordered thin films are grown at low substrate temperatures (approximately 200 K) at kinetic energies of a few electron volts, as shown by low energy He diffraction and x-ray reflectivity spectra. In contrast, deposition of thermal molecules yields only amorphous films. Growth at room or higher temperature substrates yields films of poorer quality irrespective of the depositing beam energy. We find that after the first wetting layer is completed, a new ordered phase is formed, whose in-plane lattice spacings match one of the bulk crystal planes. The high quality of the films can be interpreted as the result of local annealing induced by the impact of the impinging high-energy molecules.

Determination and analysis of optical constants for Ga 2Se 3 films near absorption edge

Ga 2Se 3 has been prepared in bulk and thin film forms. The composition of films has been checked using energy dispersive X-ray spectroscopy technique. X-ray diffraction measurements have shown that Ga 2Se 3 films evaporated at room temperature substrates were amorphous. The transmittance ( T) of Ga 2Se 3 thin films has been measured over the wavelength range (400– 900 nm ). The optical constants, the refractive index ( n) and the absorption index ( k) have been determined from the analysis of the transmittance data. Analysis of the refractive index ( n) yields the values of the long wavelength dielectric constant ( ε ∞), the average oscillator wavelength ( λ o), average oscillator strength ( S o), average oscillator energy E o, the refractive index dispersion parameter ( E o/ S o) and the dispersion energy ( E d). Analysis of absorption index ( k) yields both direct and indirect allowed transitions with optical energy gaps of 2.65 and 2.056 eV , respectively. The effect of annealing at different temperatures on optical constants is also investigated.

Thin films from fast clusters: golden TiN layers on a room temperature substrate

High quality titanium nitride films have been deposited on room temperature substrates by energetic cluster impact (ECI). Golden and very smooth TiN films can be produced, having a dense and nearly isotropic composition without any columnar structure. In the experiment, charged clusters—composed typically of several thousand TiN—are electrically accelerated and impinge with kinetic energies up to 20 keV onto a room temperature substrate. No other method can produce good TiN films at such a low temperature. Electron diffraction gives a nano-crystalline fcc structure and a slight 〈200〉 fiber texture. Elemental and chemical analysis was performed, as well as the mechanical properties measured.

Magnetic tunnel junctions utilizing diamond-like carbon tunnel barriers

We have devised a method whereby thin particulate-free diamond-like carbon films can be made with good adhesion onto even room-temperature substrates. The method employs a filtered ionized carbon beam created by the vacuum impact of a high-energy, approximately 1 J per pulse, 248 nm excimer laser onto a carbon target. The resultant deposition beam can be steered and deflected by magnetic and electric fields to paint a specific substrate area. An important aspect of this deposition method is that the resultant films are particulate free and formed only as the result of atomic species impact. The vast majority of magnetic tunnel junctions utilizing thin metallic magnetic films have employed a thin oxidized layer of aluminum to form the tunnel barrier. This has presented reproducibility problems because the indicated optimal barrier thickness is only approximately 13 Å thick. Magnetic tunnel junctions utilizing Co and permalloy films made by evaporation and sputtering have been fabricated with an intervening diamond-like carbon tunnel barrier. The diamond-like carbon thickness profile has been tapered so that seven junctions with different barrier thickness can be formed at once. Magnetoresistive (MR) measurements made between successive permalloy strip ends include contributions from two junctions and from the permalloy and Co strips that act as current leads to the junctions. Magnetic tunnel junctions with thicker carbon barriers exhibit MR effects that are dominated by that of the permalloy strips. Since these tunnel barriers are formed without the need for oxygen, complete tunnel junctions can be formed with all high-vacuum processing.

ELECTRONIC STRUCTURE OF Ag THIN FILMS ON A Ge(001) SURFACE

We have studied the electronic structure of Ag thin films of monoatomic height grown on a Ge(001) surface at 95 K by using photoelectron spectroscopy. All the surface states on the clean Ge surface vanished and Ag 5s derived state appeared in the bulk band gap just below the Fermi level. These suggest a strong interaction with the substrate. On the other hand, on three-dimensional (3D) islands formed on a room temperature substrate, small changes were found in Ge 3d spectrum from the clean surface. Each of the Ag 3D islands already had bulklike feature in the valence band structure. The results are consistent with recent observation by scanning tunneling microscopy.

Electrical and optical properties of amorphous Ga2Te3films

Ga 2 Te 3 has been prepared in bulk and thin film forms. The composition of films has been checked using energy dispersive X-ray (EDX) spectroscopy technique. X-ray diffraction (XRD) measurements have showed that the Ga 2 Te 3 films evaporated at room temperature substrates were amorphous. Investigation of the I - V characteristics in amorphous Ga 2 Te 3 films reveals that it is typical for a memory switch. The thickness dependence of the mean value of the switching voltage V th is linear in the investigated range and V th decreases exponentially with temperature in the range (298−393 K). The switching voltage activation energy ( e ) calculated from the temperature dependence of V th is found to be 0.277 eV. Electrical conduction activation energy () is found to be (0.564 eV). The agreement between the obtained value of the ratio (0.49) and its value derived theoretically (0.5) suggests that the switching phenomenon in amorphous Ga 2 Te 3 films is explained according to an electrothermal model for the switching process. The transmittance ( T ) of Ga 2 Te 3 thin films, has been measured over the wavelength range 400−2500 nm. From an analysis of the transmittance data, the optical constants, the refractive index ( n ) and the extinction coefficient ( k ), have been determined. Similarly the absorption coefficient ( α ) measurements, have been evaluated. Allowed nondirect transitions with optical energy gap () of 1.15 eV have been obtained.

Hot-Filament Chemical Vapor Deposition of Organosilicon Thin Films from Hexamethylcyclotrisiloxane and Octamethylcyclotetrasiloxane

A nonplasma technique, hot-filament chemical vapor deposition (HFCVD), is an alternative method for producing organosilicon films of novel structure. Films are deposited onto room-temperature substrates from the precursors hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane at high rates (>1 μm/min). Filament temperature can be used to control film structure, and the limited reaction pathways available via thermal decomposition make it possible to elucidate the chemistry of the growth process. During film growth, there appears to be competition between reaction pathways for the incorporation of cyclic and linear siloxane structures. For both and films, infrared, Raman, and nuclear magnetic resonance spectroscopies indicate the incorporation of ring structures consisting of three siloxane units. The concentration of these structures increases as filament temperature is raised and is especially pronounced for films deposited from In comparison, films grown from show a greater degree of incorporation of linear, unstrained structures over the range of filament temperatures studied. In contrast to plasma-deposited organosilicon films, cross-linking in HFCVD films occurs predominantly via silicon-silicon bonding and not from siloxane bonds with tertiary or quaternary silicon atoms. © 2001 The Electrochemical Society. All rights reserved.

High-Tc superconducting NbN films with low particulate density grown at 25 °C using pulsed laser deposition

The growth and characterization of high-Tc NbN films formed on room temperature substrates by pulsed laser deposition is described. The growth was performed at a high laser power density (>5 × 108 W/cm2), where the enhanced reactivity of species in the plume is proposed as the mechanism for increased nitrogen incorporation in films on unheated substrates. The Tcs were 16.2 K on MgO and 13 K on SiNx/Si substrates. In addition to electrical transport measurements, the films were characterized using RBS and x-ray diffraction. The particulate density on films grown at high power density was significantly reduced, which is correlated with the Nb target having a smoother morphology, characteristic of a quenched molten surface layer in the ablated area.

RHEED study of Nb thin film growth on Cu(1 1 1) and (1 0 0) single-crystals

Niobium (Nb) thin films were grown on (1 1 1) and (1 0 0) surfaces of Cu single-crystals using a special Nb evaporation source. The quality of the substrate surface and epitaxial parameters of the Nb over-layers were examined by reflection high energy electron diffraction. Two types of Nb deposit structure have been found and are dependent on the substrate temperature during the deposition. At room substrate temperature disordered Nb layers were observed while, at high substrate temperature (720 K), different azimuthal orientations of Nb(1 1 0) Nb epitaxial over-layers formed for both (1 1 1) and (1 0 0) Cu substrate surfaces.

Room temperature deposition of titanium carbide thin films using dense plasma focus device

Thin films of titanium carbide (TiC) were deposited on 304-stainless steel substrates using a dense plasma focus device. The conventional hollow copper anode of the 3.3 kJ Mather type plasma focus device was replaced by solid titanium anode for deposition of TiC thin films. The argon–acetylene admixture (in 7:3 ratio) was used as the filling gas at a pressure of 1.5 mbar. Substrates were placed along the anode axis at a distance of 11.5 cm from the top of the anode. Thin films of TiC were deposited using different numbers of focus shots, like 10, 20 and 30, at room temperature substrates. Deposited films were analyzed for their structure, surface morphology, surface profile, elemental composition and distribution using X-ray diffraction (XRD), scanning electron microscopy (SEM), surface profiling and energy dispersive X-ray spectroscopy (EDX). XRD patterns in all three cases are found to have diffraction peaks for (111), (200), (220), (311) and (222) TiC crystalline planes. Scanning electron micrographs show a smooth film surface with increasing clustering of TiC grains with the increase in number of focus deposition shots. EDX spectra confirm the presence of expected constituent elements with the gradual increase in TiK α peak (of the TiC thin film) relative to FeK α peak (of the substrate) with increasing numbers of focus deposition shots due to increasing film thickness. EDX mapping reveals a uniform distribution of TiC over the substrate surface. These results confirm the first ever deposition of TiC thin films using a dense plasma focus device.