Thin Film Matrix Research Articles

Excitation Emission Matrix of Spin-coated Polymeric Thin Films and its Application to Polarized Fluorescence

We have reported the spectroscopic behaviors of polymeric pyridine derivatives, poly(vinylpyridines) (PVPys), using excitation emission matrix (EEM). As PVPys show complicated fluorescence spectra, EEM would be an effective tool to make the details of the photochemical reaction of PVPys clear. In this study, we apply EEM to polarized fluorescence measurement, and show 2-D mapping of emission anisotropy. We also show a new attempt of analyzing transition moment-distribution of the fluorophore in thin films using polarized fluorescence.

Thermal oxidation of tantalum films at various oxidation states from 300 to 700°C

This paper presents the combined use of mathematical modeling and Auger depth profiling to study and quantify the oxidation of Ta films over a wide range of temperatures. The thermal oxidation of tantalum films (∼700nm) is studied using direct measurements of species concentration by means of Auger depth profiling. The oxidation temperature range of this study extends from 300 to 700°C and the oxidation period varies from 5s to 12.5h. The Auger depth profiles revealed that the metallic film oxidizes to first form low valence oxides of Ta that progressively convert to tantalum pentoxide with increasing temperature and time. A first-order reaction diffusion model is used to quantify the diffusion of oxygen through a film that is evolving in composition. The Auger depth profiling and reaction-diffusion model are used to estimate the actual diffusivity values for oxygen in the evolving Ta/Ta-oxide thin-film matrix, rather than more conventional techniques that estimate either the initial diffusion of oxygen through a semi-infinite metal or give a depth- and time-integrated value for the diffusivity. A comparison between the actual diffusivity values estimated in this work and the depth- and time-integrated version using the same model revealed that the integrated values are higher than the actual diffusion values by greater than 300% for the temperature range tested. Moreover, these depth- and time-integrated values for diffusivity values match over the applicable temperature ranges the diffusivity values given in the literature, which are essentially integrated average values for Ta/Ta oxide matrix. Furthermore, using the Auger depth profiles, the oxide growth rates are quantified as a function of temperature and compared with available literature. The growth rate of the oxide that is observed to be logarithmic at 300°C is seen to have a parabolic growth at 500°C and then a multistep growth behavior (a combination of parabolic and linear growth) at 700°C. These growth rates and the transition from one growth type to another strongly correlate to the change in surface and film morphology and also the transition from amorphous to crystalline Ta2O5.

Electrical and optical properties of plasma-polymerized N, N,3,5-tetramethylaniline thin films

Plasma polymerization is a versatile and important technique for depositing uniform, pinhole-free and flawless thin films of organic materials. So plasma-polymerized N,N,3,5-tetramethylaniline (PPTMA) thin films were deposited onto glass substrates at room temperature by a capacitively coupled plasma polymerization system using TMA as a precursor. Infrared spectroscopy, elemental analysis and UV–visible (UV–vis) spectroscopy reveal that there are conjugations in the matrix of the PPTMA thin films. From UV–vis spectroscopy it is found that indirect energy gap varies from 1.49 to 1.86 eV with film thickness. Current density–voltage characteristics indicate that the conduction mechanism in PPTMA thin films is space charge-limited conduction (SCLC). The activation energies in the SCLC region are 0.21 ± 0.05 and 0.93 ± 0.08 eV at lower and higher temperature regions respectively. Electrical and optical measurements suggest that the top of valance band and the bottom of the conduction band may have gap states and the middle of the energy gap may be equal to the high-temperature activation energy.

Exciton related resonant Raman scattering from CdSe quantum dots in an amorphousGeS2 thin film matrix

CdSe quantum dots have been produced in an amorphousGeS2 thin film matrix by alternating thermal evaporation in vacuum and deposition of several CdSe andGeS2 layers, with the thickness of the latter being, typically, 20 times larger than that of the former. Ramanscattering spectra of these films have been measured over the temperature range 20–293 K using fourAr+ laser lines for the excitation and studied with emphasis on the position and intensity of thelongitudinal optical phonons of CdSe. A shift of these phonons to lower frequencies isseen which increases with decreasing CdSe layer thickness. On the other hand,resonant Raman effects have been observed as the phonon intensity depends stronglyon the nominal CdSe layer thickness, excitation energy and temperature. Theseresults imply formation of high quality CdSe quantum dots in the amorphousGeS2 thin film matrix and are interpreted in terms of resonant absorption in exciton electronicstates of these quantum dots.

Switchable reflectivity on silicon from a composite VO2-SiO2 protecting layer

Surfaces whose reflectivity can be thermally controlled were formed on single crystals of silicon by using ion beams to create a nanocomposite layer consisting of VO2 precipitates embedded in a thin-film matrix of amorphous SiO2. The surface nanocomposite layer was produced by first thermally oxidizing a Si crystal to produce an overlying SiO2 film with a desired thickness. Stoichiometric coimplantation of vanadium and oxygen ions and subsequent thermal processing were then employed to form embedded VO2 nanoparticles in the SiO2 film. The reflectivity of the vanadium dioxide particles undergoes large changes at the VO2 semiconductor-to-metal phase transition, thereby providing the mechanism for thermally controlling the reflectivity of the VO2∕SiO2∕Si layer and, effectively, the Si crystal surface. The VO2∕SiO2 nanocomposite layer was applied in a device configuration in which heating via current through the Si substrate is used to control the surface reflectivity.

Phase change induced degradation of amorphous vanadium oxide cathode thin film during charge-discharge

Amorphous vanadium oxide (V2O5) is a very good candidate as material for cathode thin film since it has a relatively high capacity. In addition, the room temperature deposition process is valuable in V2O5 thin film fabrication. Due to these advantages, much effort to grow amorphous V2O5 thin film has been made. In this research, we successfully grew amorphous V2O5 thin film using room temperature sputtering. Based on a Li/LiPON/V2O5 full cell structure, charge-discharge performances were measured according to cycling number. Even though the full cell structure showed an average capacity of 15 μAh/cm2 over more than 500 cycles, a capacity fade was shown after a few cycles. Many reports revealed that the phase change of V2O5 from amorphous to crystalline made this kind of capacity fade. In order to investigate this phenomenon, high-resolution transmission electron microscopy (HRTEM) was employed. The as-deposited V2O5 thin film consisted of a homogeneous amorphous without any grain-boundary and/or polycrystalline island. However, the microcrystalline V2O5 phase was randomly distributed in the amorphous V2O5 thin film matrix after 450 cycles by cross sectional TEM (XTEM). That is, some amorphous phase in the V2O5 thin film matrix changed to the crystalline phase. This crystalline phase strongly prevented the extraction of Li ions during the charge process, which induced the irreversible diffusion of Li ions from cathode to anode. From this result, a high efficiency thin film battery based on amorphous V2O5 can be fabricated by preventing amorphous-crystal phase transformation during cycling.

Two methods for modelling the propagation of terahertz radiation in a layered structure.

Modelling the interaction of terahertz(THz) radiation with biological tissueposes many interesting problems. THzradiation is neither obviously described byan electric field distribution or anensemble of photons and biological tissueis an inhomogeneous medium with anelectronic permittivity that is bothspatially and frequency dependent making ita complex system to model.A three-layer system of parallel-sidedslabs has been used as the system throughwhich the passage of THz radiation has beensimulated. Two modelling approaches havebeen developed a thin film matrix model anda Monte Carlo model. The source data foreach of these methods, taken at the sametime as the data recorded to experimentallyverify them, was a THz spectrum that hadpassed though air only.Experimental verification of these twomodels was carried out using athree-layered in vitro phantom. Simulatedtransmission spectrum data was compared toexperimental transmission spectrum datafirst to determine and then to compare theaccuracy of the two methods. Goodagreement was found, with typical resultshaving a correlation coefficient of 0.90for the thin film matrix model and 0.78 forthe Monte Carlo model over the full THzspectrum. Further work is underway toimprove the models above 1 THz.

Polyimide-matrix-based composite materials

A polycondensation solid-state-reaction-based polyimide (PI) matrix has been used for composite material formation. Two types of PI matrix have been employed – a bulk PI system prepared by pressing a mixture of the powdered precursors (4, 4′-oxidianiline (ODA) and pyromellitic dianhydride (PMDA)) and a thin-film matrix formed by vapor co-deposition of the same precursors, PI monomers. A variety of different “guests” (trimethylaminalan, phthalocyanine, carbon) and the methods for their incorporation into the PI matrix are presented. The influence of the “guest” concentration and the thermal-treatment conditions on the surface morphology are shown and discussed.

Determination of leaky modes in planar multilayer waveguides

In this letter, we present a numerically exact and simple novel technique for the determination of leaky modes in planar multilayer waveguides. The technique uses a dispersion equation provided by the thin-film transfer matrix method. Complex roots, that correspond to leaky modes, are searched on the basis of a smooth transition between the bound modes of a closed waveguide and the leaky modes of an open waveguide. The technique does not use any approximation.

Improved magnetic properties of self-assembled epitaxial nickel nanocrystallites in thin-film ceramic matrix

Nanocrystalline nickel particles were embedded in amorphous alumina and crystalline TiN matrices using a pulsed laser deposition process to investigate the effect of texturing on magnetic properties of nickel nanocrystallites. The crystalline quality of both the matrix and magnetic particles was investigated by cross-sectional high-resolution transmission electron microscopy. The embedded Ni nanocrystals were found to be epitaxial in the case of the TiN matrix and polycrystalline in Al2O3 amorphous matrix. The Ni nanocrystals on TiN/Si grow epitaxially because the TiN acting as a template grows epitaxially on Si substrate via domain epitaxy. On the other hand, Ni nanocrystals in the Al2O3 matrix are polycrystalline because of the amorphous nature of the alumina matrix. Magnetization versus temperature measurements have shown that the blocking temperature, above which the samples lose magnetization–field (M–H) hysteretic behavior, of the Ni–TiN sample (approximately 190 K) is significantly higher than that of Ni–Al2O3 sample (approximately 30 K) with a similar size distribution of embedded magnetic particles. A comparison of the values of coercivity (Hc) of the two samples, measured from M–H data, indicates that epitaxial Ni nanocrystals also exhibit significantly higher coercivity than polycrystalline Ni particles in amorphous alumina matrix. The high values of TB and Hc of Ni–TiN samples with respect to TB of N–A12O3 samples are believed to be associated with preferred alignment of nanocrystallites.

Microfabricated solid-state dissolved oxygen sensor

This paper describes the design, fabrication and testing of a microfabricated oxygen concentration sensor consisting of a microfabricated thin-film electrode matrix overlaid with a solid-state proton conductive matrix (PCM) and encapsulated in a bio-inert polytetrafluoroethelene (PTFE) film. Through cyclic voltammetry (CV) and voltage step (VS) measurements, the device was shown to have a linear response with respect to dissolved oxygen concentration over a range from 0 to 601μM (0–200atm% O2). The diffusion constant of the PCM was measured to be D1=1.7×10−10m2/s (±2.7×10−11m2/s, n=40), which translates to a response time constant of τ=0.7s (±0.11s, n=40). These results were consistent with those predicted by basic electrochemistry.

Spectroscopic Ellipsometry Studies of Nanocrystalline Silicon in Thin-Film Silicon Dioxide

ABSTRACTNanocrystalline silicon (n-Si) is formed in a silicon dioxide thin-film matrix by ion implantation followed by thermal annealing in forming gas at 1100 °C for 1 hour. The ion implantation is performed using multiple implants with different implantation energies and doses to create a quasi-flat concentration of silicon atoms throughout the silicon dioxide film. These samples are then analyzed using spectroscopic ellipsometry to characterize their linear optical properties. Implantations with small doses (5 × 1020 Si atoms/cm3) increase the refractive index by a small amount (δn∼0.006 at 600nm), while implantations with moderate dose (5 × 1021 Si atoms/cm3) have a larger increase in refractive index and exhibit optical absorption above ∼1.9 eV (650 nm).

Luminescence anisotropy of functionalized carbon nanotubes in solution

Luminescence anisotropy of functionalized single-wall (SWNT) and multiple-wall (MWNT) carbon nanotubes in solution and in polymeric thin-film matrix was investigated. The results show that the absorption and emission dipole moments are intrinsically collinear, corresponding to the limiting positive anisotropy. The observation of strong luminescence polarization for the functionalized carbon nanotubes in room-temperature chloroform is consistent with the polymeric nature of these luminescent species; and the partial depolarization in solution is rationalized in terms of the flexibility of the nanotubes.

The localized corrosion of Al at engineered Cu islands

Arrays of engineered copper islands on an aluminum thin-film matrix have been employed to investigate the role of copper in localized corrosion of Al-Cu alloys. When exposed to dilute NaCl solutions, the engineered samples corrode with a morphology similar to that observed in second-phase particles in real alloys. In-situ fluorescence microscopy allows the observation of oxygen reduction at copper islands during corrosion of the underlying aluminum thin-film matrix. The spacing between engineered copper islands was found to strongly influence the corrosion rate of the surrounding matrix.

Muon Spin Rotation and Relaxation Experiments on Thin Films

The recent development at the Paul Scherrer Institute of a beam of low energy muons allows depth dependent muon spin rotation and relaxation investigations in thin samples, multilayers and near surface regions (low energy μSR, LE-μSR). After a brief overview of the LE-μSR method, some representative experiments performed with this technique will be presented. The first direct determination of the field profile just below the surface of a high-temperature superconductor in the Meissner phase illustrates the power and sensitivity of low energy muons as near-surface probe and is an example of general application to depth profiling of magnetic fields. The evolution of the flux line lattice distribution across the surface of a YBa2Cu3O7 film in the vortex phase has been investigated by implanting muons on both sides of a normal-superconducting boundary. A determination of the relaxation time and energy barrier to thermal activation in iron nanoclusters, embedded in a silver thin film matrix (500nm), demonstrates the use of slow muons to measure the properties of samples that cannot be made thick enough for the use of conventional μSR. Other experiments investigated the magnetic properties of thin Cr(001) layers at thicknesses above and below the collapse of the spin density wave.

Size-dependent optical edge shifts and electricalconduction behaviour of RF magnetron sputtered CdTenanocrystals:TiO2 composite thin films

CdTe nanocrystals sequestered and passivated inan amorphous TiO2 thin film matrix have been prepared by RFsputtering from a composite TiO2:CdTe target. The CdTenanocrystal size and volume fraction increases from 15 to 40 nmand 2 to 20% respectively as the film thickness increases,typically from 0.05 to 0.25 µm. A systematic dependence of theoptical band edge on the CdTe nanocrystal size shows a strongquantum confinement effect. The optical edge shifts aresignificantly higher than the theoretical prediction based onsingle-particle confinement of decoupled electrons and holes.This is understood on the basis of nucleation-controlled growthof CdTe nanocrystals by direct vapour phase condensation, inwhich small nuclei are rapidly passivated by TiO2 depositingat much higher rates. The nano-sized CdTe growth island thus formedcomprises of several TiO2 passivated nanocrystals. Electricalconduction behaviour of these films show that tunnelling betweenthe CdTe nanocrystals is not a dominant mechanism, as athree-dimensional network is not realized due to smallthickness and lower coverage. The current transport isessentially space-charge-limited. The injection of electronsfrom nano-sized CdTe crystals follows spherical radial spacecharge flow which modifies the usual power law dependence fromquadratic to 3/2. The analytical description of the current conductionprocess in composite CdTe:TiO2 is discussed.

A low-energy muon study of thermal activation in single-domain iron particles

We present the first μSR study of the dynamic magnetic behaviour of iron clusters supported in a non-magnetic thin film matrix. Thin film samples afford the possibility of very tight control over the cluster size, but are too thin for study with conventional μSR owing to the relatively long (of order 0.5 mm) stopping range of 4 MeV muons. In this study, the thin film samples were a 500 nm thick, silver matrix containing iron clusters with an equivalent iron concentration of 0.1% by volume. SQUID magnetometry showed a mean cluster diameter of 2.8 nm. Transverse low-energy μSR measurements in a field of 25 mT, applied normal to the plane of the film, were made in the temperature range 4.7–300 K. At low temperatures, a relaxation rate of 0.4 μs −1 was observed and interpreted as due to a static Lorentzian distribution of dipolar fields with half-width 0.5 mT. At high temperatures, a motional narrowing of the field distribution, due to fluctuations in the orientation of the cluster moments, was observed. The fluctuations were found to follow an Arrhenius activation law, from which an activation energy of 51±9 K and an intrinsic cluster relaxation time of 12±4 ns were deduced.

Impedance matrix of thin metallic ferromagnetic films and SSWR in parallel configuration

Standing spin-wave resonance spectra of metallic ferromagnetic films in parallel configuration are calculated analytically for the general cases of different external microwave field amplitudes and different surface anisotropies at the two film surfaces, and expressed in terms of a surface impedance matrix. Relations to the transfer matrix introduced by Vittoria in a similar context and examples of computed absorption spectra for two typical materials with different exchange-conductivity parameters are discussed.

Superparamagnetic relaxation in iron nanoclusters measured by low energy muon spin rotation

Low energy (16 keV) muons were used to probe the dynamic magnetic behaviour of iron nanoclusters embedded in a silver thin film matrix. The silver film was 500 nm thick and contained a volume fraction of 0.1% iron. Measurements were made in a field of 25 mT, applied normal to the plane of the film, in the temperature range 4.7 K to 300 K. At temperatures above 20 K thermal activation of the cluster moments was seen as a narrowing of the field distribution sensed by the implanted muons. An intrinsic cluster relaxation time of τ0 = 12 ± 4 ns and an activation energy of 51 ± 9 K were deduced from fits to the data. SQUID magnetometry of thicker (1.5 µm) but otherwise identical films on graphite substrates showed the clusters to have a volume of the order of 10-26 m3, from which a cubic anisotropy constant of K = 2.3 ± 0.4 × 105 J m-3 was calculated. Remanence measurements showed no evidence of a preferred orientation for the magnetization of the cluster assembly.

Localized stress near and the thermal expansion of Al2Cu precipitates in an Al thin film matrix

The thermal expansion coefficient of Al2Cu precipitates embedded in an Al–Cu thin film matrix is determined by in situ x-ray diffraction measurements on free films. The phases present and the thermal expansion or contraction are simultaneously observed during a temperature ramp. The Al2Cu precipitate has a tetragonal structure. The thermal expansion coefficient of the Al2Cu phase in the a direction is significantly smaller than that of the surrounding Al matrix while the thermal expansion in the c direction is comparable to that of the Al matrix. Upon cooling from 500 °C, the mismatch of the thermal expansion between the precipitate and the Al matrix results in compressive and tensile stresses in the aluminum matrix near the precipitates and in compressive stresses in the precipitates. Those stresses are estimated to be on the order of 100 MPa.