Vibrational modes of nanolines

Brillouin-light-scattering spectra previously have been shown to provide information on acoustic modes of polymeric lines fabricated by nanoimprint lithography. Finite-element methods for modeling such modes are presented here. These methods provide a theoretical framework for determining elastic constants and dimensions of nanolines from measured spectra in the low gigahertz range. To make the calculations feasible for future incorporation in inversion algorithms, two approximations of the boundary conditions are employed in the calculations: the rigidity of the nanoline/substrate interface and sinusoidal variation of displacements along the nanoline length. The accuracy of these approximations is evaluated as a function of wavenumber and frequency. The great advantage of finite-element methods over other methods previously employed for nanolines is the ability to model any cross-sectional geometry. Dispersion curves and displacement patterns are calculated for modes of polymethyl methacrylate nanolines with cross-sectional dimensions of 65?nm × 140?nm and rectangular or semicircular tops. The vibrational displacements and dispersion curves are qualitatively similar for the two geometries and include a series of flexural, Rayleigh-like, and Sezawa-like modes.     

Magnetic anisotropy of electrodeposited Co--P amorphous alloys

The development of magnetic anisotropy with regard to thickness in amorphous electrolytic Co-P alloys (15 at % P) has been observed in the range of thicknesses between 0.06-7.00μm. The study has been undertaken starting from the ratio Mr/Msof the hysteresis loops. The results obtained indicate that for thicknesses below 4000 Å the samples show an axis of easy magnetization in the plane. In the case of greater thicknesses, the axis of easy magnetization develops gradually out of the plane, reaching a perpendicular position near 10 μm. Our results suggest that what we are dealing with here is a shape anisotropy such as that proposed by Cargill, III et al.     

Corrosion and wear properties of electrodeposited amorphous chrome

Amorphous chrome was electrodeposited onto AISI 1025 steel substrates in a modified Sargent bath containing formic acid after electropolished in several electrolytic solutions. The average carbon content of the deposited layer was 2.0 wt%, which is higher than the carbon content of a chrome layer deposited in the modified Sargent bath. The hardness and wear resistance of the amorphous chrome layer are improved by annealing up to about 600 °C due to the precipitation hardening effect resulting from the formation of chromium carbide. The chrome layers electropolished in a modified Sargent bath for 2 min showed the longest neutral salt spray life amongst the chrome layers prepared in the same solution. Electropolishing in the modified Faust solution also improved the neutral salt spray life of chrome plated steel more than did the electropolishing in the plating bath. A chrome layer electropolished in the modified Faust solution for 2 min at a current density of 0.6 Acm-2 and 53 °C showed the highest corrosion resistance amongst the samples prepared in this study. The contamination of the plating bath with a modified Faust solution reduces the cathodic current efficiency. This revised version was published online in November 2006 with corrections to the Cover Date.     

An analysis of impact-induced deformation and fracture modes in amorphous glassy polymers

The finite deformation response of a planar block of polymer material subject to impact loading is analyzed using two constitutive models for glassy polymers, a reference Drucker–Prager type model and a physics-based macromolecular model, supplemented by a phenomenological model for craze initiation and widening. Full transient finite element analyses are carried out using a Lagrangian formulation of the field equations. The analyses allow an assessment of possible failure mechanisms under dynamic loading and the ability of the different models to predict such behavior. The results highlight the effect of the stress–strain behavior of polymers, notably the post-yield softening and large strain hardening, on localization of plastic flow. This behavior is adequately captured only by the macromolecular model.     

Spectroscopic ellipsometry analysis of amorphous silicon thin films for Si-nanocrystals

Hydrogenated amorphous and nano-crocrystalline silicon thin films were grown by very-high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD, 60 MHz). In this paper, we report the defects of nano-crystallites embedded in an amorphous matrix of hydrogenated silicon alloy (a-Si:H) thin film as investigated by spectroscopic ellipsometry (SE). The peak intensity and position of the imaginary part of the dielectric constant (epsilon2) as a function of the energy show various material states, including a-Si:H (3.5 eV) and nc-Si (4.2 eV), along with the absorption coefficient, thickness, optical gap, and the characteristics of the defects. The ratio of the characteristic Raman features, the TA/LO and LA/LO ratio, is related to the defect states in the films. It was correlated to the SE data. Following this, we look into the systematic change in the crystallinity of the film from the SE results. Quantized crystallinity values from the SE data show good agreement by more than 88.75% with the crystallinity information obtained through Raman spectroscopy.     

Mass spectrometric analysis during the vapor deposition of amorphous silicon

The gaseous species produced by the electron beam evaporation of silicon from conventional copper and carbon hearths were studied by utilizing a residual gas analyzer. At low evaporation rates about 13% of the silicon evaporates as molecules containing two to seven atoms. The percentage of clusters increases only slightly with rate, indicating that their heat of evaporation differs only slightly from that of the silicon atoms. Additional molecules, predominantly Si₂C, occur if vitreous carbon liners are used. The resulting carbon contamination of the amorphous silicon films (about 1.5 mol.%) was confirmed by electron microprobe analysis. A discussion is given concerning the influence of cluster deposition on the growth of amorphous silicon films.     

Metastable defects in hydrogenated amorphous silicon

The electronic structure of hydrogenated amorphous silicon (a-Si:H) is in a state of metastable equilibrium and can change upon application of external stimuli. We study the effect of thermal quenching and light soaking in lithium-doped a-Si:H, on its conductivity and thermopower. We present evidence showing that the metastable state obtained after fast quenching is different than that obtained after light exposure. Experiments on chalcogenides show that they are not affected by thermal quenching although they change upon light soaking. This is in contrast with lithium doped a-Si:H in which both effects are observed. Our experiments suggest that hydrogen present in a-Si:H plays an important role by controlling heterogeneities and potential fluctuations in a-Si:H. Light soaking appears to enhance these potential flucutations, whereas fast cooling seems to have little effect on them.     

Formation of amorphous electrodeposited Ni-W alloys and their nanocrystallization

Ni-W alloys containing about 5 to 30 at. % W were prepared by electrodeposition. The plating bath for the electrodeposition contained nickel sulfate, citric acid, sodium tungstate and ammonium chloride, and was operated at various bath concentrations and conditions of electrolysis. X-ray diffraction peaks of the deposited alloys broadened with increasing tungsten content and an amorphous pattern appeared at a tungsten content of about 20 at. %. Nanocrystallization behavior of the amorphous Ni-25.0 at. % W alloy has been studied by X-ray diffraction, high resolution transmission electron microscopy and small angle X-ray scattering. The Hall-Petch strengthening mechanism was observed for the hardness extending to a finest grain size of about 10 nm. When the grain size was less than about 10 nm, decrease of the hardness was observed. This decrease may be due to the significant increase of the intercrystalline volume fraction, especially the fraction associated with the triple junction.     

Auto-correlation function analysis of phase formation in iron ion-implanted amorphous silicon layers

High-resolution transmission electron microscopy (HRTEM) in conjunction with autocorrelation function (ACF) analysis have been applied to investigate the evolution of structural order in iron ion-implanted amorphous silicon layers. β-FeSi₂ nanocrystallites as small as 5 nm in size were detected in 600 °C annealed for 60 min a-Si layers. The embedded nanocrystalline β-FeSi₂ was found to grow in the interlayer with annealing temperature.     

Hydrogen-related mechanical stress in amorphous silicon and plasma-deposited silicon nitride

Hydrogen concentrations in amorphous silicon (a-Si) prepared by electron beam evaporation or plasma deposition as well as in plasma-deposited silicon nitride were measured as a function of annealing conditions using the photon-proton scattering method. Comparison with IR evaluations of SiH bonds shows the existence of “quasi-free” (IR inactive) hydrogen in a-Si. The amount of this component can be altered by annealing. The dependence of the mechanical stress on the annealing conditions was determined interferometrically, and the intrinsic contribution was separated in the case of plasma-deposited silicon nitride. As the hydrogen content decreases monotonically with increasing annealing temperature, the initial compressive stress of plasma-deposited a-Si or silicon nitride films changes to a tensile stress. For these films a linear correlation between the amount of mechanical stress and the number of SiH or NH bonds is shown. Evaporated a-Si films show a totally different stress behaviour from that of plasma-deposited a-Si. No correlation of stress with the hydrogen concentration was found for the evaporated a-Si films.     

High efficiency amorphous silicon solar cells with high absorption coefficient intrinsic amorphous silicon layers

This paper considers the intrinsic layer of hydrogenated amorphous silicon (a-Si:H) solar cells. The deposition temperatures (T_d) and electrode distances (between cathode and anode, E/S) are important factors for a-Si:H solar cells. Thus, this study examines the effects of deposition temperatures and electrode distances in the intrinsic layer of a-Si:H solar cells with regard to enhanced the short-circuit current density (J_sc) and thereby conversion efficiency. It is shown that the J_sc of a-Si:H solar cells can be increased by proper choice of T_d and E/S of the i-a-Si:H layers. The J_sc of the a-Si:H solar cells is largely dependent on light absorption of the i-a-Si:H layer. It is demonstrated that the absorption coefficient in an i-a-Si:H layer can be increased to provide higher J_sc under fixed thickness. Results show that the optimized parameters improve the J_sc of a-Si:H solar cells to 16.52 mA/cm², yielding an initial conversion efficiency of 10.86%.     

Film adhesion in amorphous silicon solar cells

A major issue encountered during fabrication of triple junction a-Si solar cells on polyimide substrates is the adhesion of the solar cell thin films to the substrates. Here, we present our study of film adhesion in amorphous silicon solar cells made on different polyimide substrates (Kapton VN, Upilex-S and Gouldflex), and the effect of tie coats on film adhesion.     

Langmuir-Blodgett films in amorphous silicon MIS structures

In this paper we describe the properties of MIS structures based on hydrogenated amorphous silicon (a-Si:H) and organic films deposited using the Langmuir-Blodgett technique. Results are reported for undoped a-Si:H passivated with an insulating film of cadmium stearate 80 nm thick. The deposition of the monolayers was found to be critically dependent on the surface condition of the semiconductor.The capacitance data display well-defined accumulation and depletion regions and suggest that inversion is obtained when the device is reverse biased. The conductance data are similar in shape to those observed for conventional MOS structures on crystalline silicon. However, hysteresis and frequency dispersion effects complicate their interpretation in terms of surface state densities.From this preliminary investigation we conclude that useful MOS devices incorporating both thick and thin insulating layers can be based on the a-Si:H/Langmuir-Blodgett film system.     

Vibrational specific heat enhancement from force-constant variations in amorphous solids

The fact that the restoring forces on atoms in amorphous solids fluctuate locally from their macroscopically averaged value is shown to result in an enhancement of the low-frequency density of vibrational states, lending to increased T ³ and T ⁵ specific heat contributions at low temperatures. This effect is distinct from localized tunneling-state contributions, but may be important for interpretations of thermal data in the study of tunneling states.     

Investigation of Sb diffusion in amorphous silicon

Amorphous silicon materials and its alloys became extensively used in some technical applications involving large area of the microelectronic and optoelectronic devices. However, the amorphous-crystalline transition, segregation and diffusion processes still have numerous unanswered questions. In this work we study the Sb diffusion into an amorphous Si film by means of Secondary Neutral Mass Spectrometry. Amorphous Si/Si_1−xSb_x/Si tri-layer samples with 5 at% antimony concentration were prepared by direct current magnetron sputtering onto Si substrate at room temperature. Annealing of the samples was performed at different temperatures in vacuum (p<10⁻⁷ mbar) and 100 bar high purity (99.999%) Ar pressure. During annealing a rather slow mixing between the Sb-alloyed and the amorphous Si layers was observed. Supposing concentration independent of diffusion, the evaluated diffusion coefficients are in the range of ∼10⁻²¹ m²s⁻¹ at 550 °C.     

Magnetic domain states in nano-sized Co nuclei electrodeposited onto monocrystalline silicon

In the present work, nano-sized magnetic nuclei of Co have been electrodeposited onto p-Si (111). The deposition follows a mechanism of progressive nucleation and growth controlled by diffusion. MFM studies showed that the transition between magnetic domain states is strongly dependent on the size and shape of the nuclei. A critical height h₀ is defined below which the nuclei presented always a single-domain configuration. The limiting lower boundary for the single-domain state calculated from the theory is quantitatively coincident with the experimental results.