Thin Island Films Research Articles

Infrared absorption enhancement of C60 by overlaying thin silver island films.

Normal-incidence infrared absorption has been observed for silver overlaid C(60) thin films formed on surface-oxidized Si(111) substrates as a function of the silver or C(60) film thickness. The absorption spectra exhibit bands at 1429 and 1180 cm(-1) due, respectively, to the infrared active T(lu) (4) and T(lu) (3) modes of C(60) in multi-layers. Additionally, two bands appear at 1442 and 1370 cm(-1). The former band is caused by activation of the infrared inactive (Raman active) A(g) (2) mode via electron transfer from the silver to adsorbed C(60), and the latter is assigned to the T(1u) (4) mode red-shifted by the charge transfer. These bands are all enhanced in intensity dependent either upon the silver or C(60) thickness, i.e., the largest absorption enhancement is obtained for 25-monolayers-thick silver and 12-nm-thick C(60). Under atomic force microprobe inspection, the average size and height of the islands in the overlaid 25-monolayers-thick silver change with underlying C(60) film thickness. The influence of the C(60) film structure upon the silver film and in turn the absorption intensity is strongly suggested.

Unambiguous optical characterization of nanocolloidal gold films

The thin island film theory developed by Bedeaux and Vlieger [Optical Properties of Surface (Imperial College Press (2002)], is used to unambiguously analyze spectroscopic ellipsometry spectra of thin layers of nanocolloidal gold particles on silicon substrates covered by a natural oxide layer, for coverages up to 35%. Due to the absence of isotropy in three dimensions, standard effective medium theories cannot be applied. Image dipole effects as well as the lateral distribution of colloids have to be taken into account as described by Bedeaux and Vlieger. Additonally we show that an effective image dipole has to be taken into account if colloidal particles are adsorbed on a substrate that is terminated with a thin SiO2 layer with a thickness that is smaller than the radius of the gold nanoparticles (a = 6.7 nm). The natural oxide layer on our Si subtrates is only a fraction of this.

Nanoparticle formation induced by low-energy ion bombardment of island thin films

In order to explore whether low-energy ion bombardment of island thin films can be applied for preparation of nanoparticles supported on solid surfaces, morphological and compositional modifications of discontinuous silver layers deposited onto amorphous carbon or native oxide covered silicon substrates were followed by transmission electron microscopy (TEM) and electron spectroscopy during sputter etching with Ar+ ions. TEM micrographs confirmed the formation of isolated nanoparticles, while electron spectroscopic measurements supported the lack of chemical interaction and appreciable intermixing, although the latter is considered to be a major drawback of the method. Comparative studies carried out on the gold/silicon system with a strong tendency towards spontaneous alloying revealed that the unavoidable mixing due to ballistic effects can be compensated by chemically guided diffusion processes governed by the thermodynamic properties of the film/substrate pair. According to the theoretical description of ion-induced mixing, such a mechanism is feasible if the components of the film/substrate pair tend to segregate in equilibrium, which gives the necessary condition for the success of ion etching induced nanoparticle preparation.

Ostwald ripening in two dimensions: Time dependence of size distributions for thin-film islands

Thin film phase transitions can involve monomer deposition and dissociation on clusters (islands) and island coalescence. We propose a distribution kinetics model of island growth and ripening to quantify the evolution of island size distributions. Incorporating reversible monomer addition at island edges, cluster coalescence, and denucleation of unstable clusters, the governing population dynamics equation can be transformed into difference-differential or partial differential forms with an integral expression for coalescence. The coalescence kernel is assumed proportional to xνx′μ, where x and x′ are masses of coalescing clusters. The deposition and dissociation rate coefficients are proportional to cluster mass raised to a power, xλ. The equation was solved numerically over a broad time (θ) range for various initial conditions. The moment form of the equation algebraically yields asymptotic long-time expressions for the power law dependence of decreasing number concentration (θ−b) and increasing average cluster size (θb). When coalescence is negligible the power is b = 1/(1 − λ + d−1), where d = 2 or 3 is the dimensionality. If coalescence dominates the asymptotic time dependence, the power is b = 1/(1 − ν − μ).

Physical Properties of SiO2-doped Si Films and Electroluminescence in Metal/SiO2-doped Si/p-Si Diodes

The doping of SiO2 islands in thin Si films was carried out using rf magnetron sputtering. These films are referred to SiO2-doped Si films. These films display efficient optical absorption in the optical energy region of 1.4–2.0 eV and the absorption is comparable to that of a-Si:H films. Efficient photoluminesecence from Si network in these films was observed. Also, efficient electroluminescence from metal/SiO2-doped Si/p-Si diodes was observed. It is concluded that this luminescence originates from the optical transition between electron- bound states in films and injected holes from p-Si. These optical characteristics suggest that the SiO2-doped Si films are useful for photovoltatic applications.

Surface-Enhanced Infrared Absorption Spectroscopy of p-Nitrothiophenol on Vapor-Deposited Platinum Films

Surface-enhanced infrared absorption (SEIRA) has been observed by transmission spectroscopy on a vapor-deposited platinum substrate. Monolayers of p-nitrothiophenol ( p-NTP) adsorbed on thin island films of platinum were used to probe the enhancement and distortion of the absorption bands of p-NTP. The greatest enhancement was accompanied by distortion of the strongest band in the spectrum of p-NTP into a bipolar shape. The magnitude of enhancement was dependent on the size of the metal islands and their proximity to each other. The applicability of the surface selection rule to SEIRA was confirmed by the weakness of the antisymmetric NO2 stretching band in the spectra of the self-assembled monolayers. Although these preliminary transmission studies show only modest (∼10×) enhancement when p-NTP is vapor-deposited onto platinum films, the potential of SEIRA for transmission studies on platinum island films has been established.

Development of A Novel Dual-Layer Thick Ag Substrate for Surface-Enhanced Raman Scattering (SERS) of Self-Assembled Monolayers

A dual-layer, thick (∼70 nm) vapor-deposited Ag substrate has been developed that gives enhancement factors on the order of 104 for surface-enhanced Raman scattering (SERS) experiments. This substrate has a total thickness of ∼70 nm but also has an outermost surface morphology that approximates that of a thin Ag island film (AgIF) substrate. This is accomplished using a dual overlayer/underlayer structure in which a thick underlayer of 45-nm Ag is vapor-deposited onto a treated glass slide. This Ag underlayer is exposed to ambient conditions under which the surface chemisorbs oxygen, leading to the thermodynamically favorable formation of an active Ag2O interface. An overlayer of 25-nm Ag is vapor-deposited on top of this structure. The first Ag/Ag2O underlayer produces an active interface that decreases the diffusion of the Ag atoms from the second vapor-deposited overlayer, thereby forming Ag particles with shapes favorable for SERS enhancement. Atomic force microscopy results show that the Ag overlayer particles have ideal shapes for SERS enhancement with morphology comparable to thin, vapor-deposited AgIFs. X-ray photoelectron spectroscopy in the O(1s) region showed the presence of multiple forms of oxygen in the Ag/Ag2O underlayer; the main forms were identified as dissolved bulk oxygen and chemisorbed oxygen. Using self-assembled monolayers (SAMs) of 1-dodecanethiol, a SERS intensity increase of ∼400% was obtained for this new overlayer/underlayer Ag substrate when compared to SAMs formed on traditional thick Ag substrates. Enhancement factors calculated from the Raman intensity of trans-1,2-bis(4-pyridyl)ethane showed a SERS enhancement of approximately 104 for this new Ag/Ag2O/Ag substrate compared with the bulk. This compares favorably with the SERS enhancements obtained using thin AgIF substrates. The Ag/Ag2O/Ag substrates showed reproducible SERS intensities (RSD = 0.45−5%). The mechanisms responsible for the overall enhancement on this new substrate are proposed to be the ideal surface morphology of the Ag overlayer particles as well as a combined enhanced electromagnetic field produced by both the Ag/Ag2O underlayer and the Ag overlayer.

Relaxation of compressed elastic islands on a viscous layer

A recent technique can fabricate SiGe thin film islands on a glass layer, which itself lies on a silicon wafer. The islands initially have an inplane compressive strain. Upon annealing, the glass flows, and the islands relax. The resulting strain-free islands are used as substrates to grow epitaxial optoelectronic devices. This paper models the annealing process. A small island relaxes by inplane expansion. The glass being viscous, the relaxation starts at the island edges, and propagates to the island center. A large island, however, wrinkles at the center before the inplane relaxation arrives. Further annealing gives rise to one of two outcomes. The wrinkles may disappear when the inplane relaxation arrives, leading to a flat, strain-free island. Alternatively, the wrinkles may cause significant tensile stress in the island, leading to fracture. We model the island by the von Karman plate theory, and the glass layer by the Reynolds lubrication theory. The solid and the fluid couple at the interface by the continuous traction and displacement. Numerical simulations evolve the inplane expansion and the wrinkles simultaneously. We determine the critical island size, below which inplane expansion prevails over wrinkling. This critical island size depends on several experimental variables, and is much larger than the Euler buckling wavelength.

Magnetization processes in patterned media

Recent theoretical and experimental results on domain patterns and switching processes in patterned media are reviewed. For thin permalloy islands ∼0.5 to 4 μm in size the domain patterns observed correspond closely to theoretical expectations, showing a variety of closure domain configurations. Thin film islands ∼0.2 μm in size of hard magnetic materials are typically single domain in remanence. Coercivity fields show a wide dispersion in all cases, for both hard and soft materials.

Stable unidimensional arrays of coherent strained islands

We investigate the equilibrium properties of arrays of coherent strained islands in heteroepitaxial thin films of bidimensional materials. The model we use takes into account only three essential ingredients: surface energies, elastic energies of the film and of the substrate and interaction energies between islands via the substrate. Using numerical simulations for a simple Lennard-Jones solid, we can assess the validity of the analytical expressions used to describe each of these contributions. A simple analytical expression is obtained for the total energy of the system. Minimizing this energy, we show that arrays of coherent islands can exist as stable configurations. Even in this simple approach, the quantitative results turn out to be very sensitive to some details of the surface energy.

Formation of supported nanoparticles from island thin films during ion etching

Nanosized particles can be prepared either by deposition of material onto a substrate or by removing atoms from an already existing structure. While the former approach is well established and widely used, the latter is hardly explored, although it can be realized in such a simple way as reducing the particle sizes of an island thin film by low energy ion bombardment. In this work, the process of nanoparticle formation has been investigated during Ar + ion etching of silver layers deposited on native silicon oxide and amorphous carbon. TEM morphological studies have revealed gradual decrease in the size of the islands due to increasing ion dose. Electron spectroscopic measurements have supported the lack of chemical interaction and appreciable intermixing, although the latter is considered to be a major drawback of the method. On the contrary, more reactive systems such as gold on silicon have shown significant mixing induced by the ion beam, which suggests that chemical inertness of the island–substrate couple is a prerequisite for the success of nanoparticle preparation during ion etching.

Demonstration of surface plasmons in metal island films and the effect of the surrounding medium—An undergraduate experiment

We present a demonstration of the surface plasmon phenomenon as it occurs in thin metal island films. The metal films are deposited on glass microscope slides. The effect of the surface plasmon resonance may be observed visually on the slide without further apparatus. Heating the film changes the shape of the islands and therefore the resonant frequency of the surface plasmon and changes the color of the film. Placing the film in a dielectric medium changes the resonance condition for the surface plasmon again and changes the color again. We show this by coating the slides with commercially available liquids with different indices of refraction. We present a theoretical model that assumes the islands are oblate spheroids. There are enough details given so that the equations can be programed and the theoretical optical absorbance can be reproduced. We also present a modification to the theory so that the shift in resonant frequency can be calculated when the spheroids are immersed in the index fluids. We describe our apparatus for making thin films and our optical spectrometer system. We then present optical absorbance measurements of thin films of both Ag and Au in air and in two liquids with different indices of refraction.

A Comparison between Light Reflectometry and Ellipsometry in the Rayleigh Regime

The adsorption of Rayleigh particles is analyzed in terms of particle radius and surface coverage with thin island film theory with both scanning angle light reflectometry and ellipsometry around t...

Correlation between agglomeration of a thin film and reflow filling in a contact hole for sputtered Al films

Al reflow filling in contact holes by annealing after Al deposition at room temperature was investigated with relation to agglomeration properties of very thin films. A strong correlation between filling achievement and agglomeration property was observed when the underlayer TiN film property was changed. When rapid thermal nitridation (RTN) was carried out after reactive sputtering of TiN, excellent filling in submicron contact holes was achieved at annealing temperatures around 400 °C for the case of thick Al film, and agglomeration into small islands was observed for thin Al films. The activation energy of atomic mass transport for the filling process was 0.36 eV, which suggests a surface diffusion dominated filling process. On the other hand, when no RTN was carried out for TiN, reflow filling was achieved at temperatures around 600 °C which were close to the melting point of Al, and agglomeration into islands in thin films was suppressed at 400 °C. The activation energy of the latter case was 0.90 eV, which was between those of lattice diffusion and grain boundary diffusion. The correlation between filling achievement and agglomeration is ascribed to a large surface diffusion atomic flux in the case with RTN.

Diffusion and evaporation kinetics of large islands and vacancies on surfaces

The diffusion and evaporation kinetics of two-dimensional islands and vacancy islands on surfaces are studied over a wide range of island sizes. These kinetic processes are central in surface phenomena such as thin film coarsening, island aggregation, and coalescence on surfaces. Several studies have utilized scaling theories to infer the atomic level mechanisms responsible for the kinetics of island diffusion and evaporation. Using a dynamic Monte Carlo model, we study a model system where two-dimensional islands diffuse via an evaporation-condensation mechanism on a face-centered-cubic (100) surface. We examine the diffusion (evaporation) kinetics for isolated islands as a function of the island’s size in the range of 100 to 100 000 atoms. The diffusion coefficient and the island evaporation rate exhibit a power law scaling of the island size. We find crossover behavior in the scaling exponents between the regime of intermediate sized islands (between 100 and 1000 atoms) and large islands (greater than 1000 atoms). At high coverages, we also examine these quantities for vacancy islands. We find that intermediate island sizes exhibit unusual scaling behavior.

The kinetics of thin film island growth at initial stages

The simulation of the early stages of thin film formation is performed using a phenomenological approach considering the surface diffusion of the adatoms and the migration of islands. The model describes the kinetics of low surface coverage as well as of higher surface coverage. The effects of atom evaporation and island dissociation are not included in this model. The variation of the phenomenological coefficients gives the possibility to obtain the time dependence of the main characteristics such as surface coverage, island density and average island size describing initial stages of the film growth. The surface roughening during deposition can also be considered by the proposed model. The calculated curves are in good qualitative and quantitative agreement with the experimental results.

Optical study of continuous thin films and island films of Ir: Interband absorption and localization of conduction electrons of Ir

For continuous thin Ir films and Ir island films consisting of Ir particles larger than about 25 Å in diameter, optical absorption has been measured in the photon energy range of 0.5–6.5 eV. Interband absorption does not exist at about 1.1 eV. Absorption at about 3.1 eV is identified as interband absorption because its particle-size dependent change is similar to that of interband absorption of Cu particles. Interband absorption at about 4.4 eV is suggested to arise from transitions between bands 2 and 5 near L in the direction Q. Absorption due to plasma oscillations of conduction electrons is absent in the island films, showing that the conduction electrons are localized by a strong correlation interaction between the conduction electrons having strong d character. Strong d character based on large hybridization of s and d bands is responsible for the strong correlation interaction.

Optical absorption of island films of noble metals: wave length of the plasma absorption band

Experimental measurements of the positions of the optical plasma resonance band for thin island films of noble metals agree closely with positions calculated from a simple equation with no disposable parameters. The equation includes the volume fraction of particles, the refractive index of the substrate, and the real part of the dielectric constant of the metal. This equation is derived from the Lorentz–Lorenz or Clausius–Mossotti conditions. The particles in the films are right cylinders, and have polarizabilities consistent with the equation. There is no need to assume fitting parameters involving particle shapes and sizes and their distributions.

Simulation of the deposition and aging of thin island films

Two-dimensional atomistic simulations of the growth and post-deposition behaviour of island metal films are described. The program includes a module for the calculation of the film resistance on the basis of a charge limited tunnelling model. The dependence of the resistance on various deposition parameters is investigated. Examples of simulated post-deposition resistance drift are shown.

Temperature-dependent Monte Carlo simulations of thin metal film growth and percolation

A program for the simulation of the nucleation and growth of thin island films on amorphous substrates is presented. The program is based on the description of the phenomenon in terms of adsorption, diffusion, and reevaporation of single atoms on the substrate surface, which is represented as a triangular lattice of adsorption sites. A method of taking into account the interaction with neighboring particles in the evaluation of the diffusion activation barrier is proposed. The simulations are devoted to investigating the role of diffusion and reevaporation in the determination of the particular microstructure of the clusters. The effects of the substrate temperature on the morphology of the simulated island films are also presented. The cluster-size distribution and percolation threshold have been estimated and compared to experimental results available in the literature.