M. Gomez

The compositional dependence of the optical and electrical properties of hydrogenated amorphous Si-Ge films prepared by co-sputtering☆

Abstract Hydrogenated amorphous Si-Ge alloy films were prepared by co-sputtering from two half-disc targets. The films are about 1 μm thick and 12.5 cm long. The composition along the length of the film varies continuously from 70 to 5 at.% Si. The optical absorption and photoconductivity as a function of wavelength were measured. The energy gap as evaluated form these measurements decreases linearly from 1.65 eV for 70 at.% Si to 1.1 eV for 7 at.% Si. The photoconductance response ημτ varies markedly with composition, where η is the quantum efficiency for photocarrier generation, μ the mobility and τ the lifetime of the photocarriers. At the silicon-rich end ημτ = 2 × 10-6 cm2 V-1, decreases to 2 × 10-9 cm2 V-1 at around 40 at.% Si and increases again by one order of magnitude at the germanium-rich end. On the whole the characteristics of our sputtered films are comparable with those reported for similar alloys prepared by glow discharge. An important advantage of the sputtered films, however, is that it is possible to prepare a whole range of alloy concentrations in one run.

T-matrix approach for calculating local fields around clusters of rotated spheroids

A T-matrix formalism is used to calculate local electric fields around clusters of prolate spheroids in the long-wavelength regime. The calculations are performed as a function of interparticle distance as well as angle of orientation. The observed red shifts in the resonant wavelengths of the characteristic peaks are shown to obey an exponential relationship as a function of interparticle separation and a sinusoidal relationship as a function of angle of rotation of the spheroid. The behavior of the cluster is discussed and the two effects of separation and rotation are compared.

Relation between electroluminescence and photoluminescence in porous silicon

Abstract We present combined measurements of electroluminescence (EL) and photoluminescence (PL) in p-type porous silicon. The EL spectra were measured using an electrolyte contact for electron injection into the porous face of the sample. Upon applying the current, the EL intensity first rises with time, reaches a maximum, and then decays to zero. (The whole process takes about half an hour.) At the same time, the peak of the EL spectrum shifts from ≈850 nm in the beginning to ≈600 nm at the end of the process. The PL, which was measured simultaneously, peaked at ≈750 nm in the beginning and was much wider than all of the EL spectra. Towards the end of the EL process, the red part of the PL spectrum practically disappears. This shifts the PL peak towards the blue, to about the same wavelength as the EL peak (≈600 nm) and the spectrum becomes much narrower, comparable to the EL spectrum. The voltage across the sample during the EL process shows a moderate increase up to the point where the EL disappears, and then the voltage rises steeply. This behavior is associated with the build-up of a thin oxide layer on the porous surface. The combined results of EL and PL, and especially the disappearance of the red part in the photoluminescence spectrum at the end of the EL process, suggest that in addition to quantum confinement, localized surface states play an important role in the luminescence process, at least in the red part of the spectrum. Such states may be associated with adsorbed species and disappear upon oxidation.

Comparative analysis of the 1.54 μm emission of Er-doped Si/SiO2 films and the size distribution of the nanostructure

Abstract Er 3+ -doped nanocrystalline-Si/SiO 2 composite films were synthesized by RF co-sputtering of bulk-Si, SiO 2 , and Er 2 O 3 targets. The visible and 1.54 μm emission bands of the samples were measured, as well as their optical transmission. We varied the relative concentrations of Si, Er, and the annealing temperature. The nanoparticles size distributions of the samples were obtained from their optical transmission spectra. We analyzed the dependence of the characteristic 1.54 μm Er-emission intensity on the size and concentration of the nanocrystalline particles and the dependence of the emission on the preparation conditions.

Calculation of the aggregation and electrodynamic effects in granular systems

A calculation based on a multiple scattering model is presented in order to obtain the absorption of aggregated granular metals. The model is based on a T-matrix formalism in which the scattering units are clusters of nearest-neighbor metallic particles and the behavior of the scattering units is described by the orientational-averaged T-matrix of the clusters. The remainder of the system containing these scattering units will be considered in an effective medium calculation. The formalism is applicable to systems in which the sizes of the particles require the consideration of electrodynamic effects.

Multiple scattering renormalized T matrix theory for the dielectric constant of non-homogeneous thin films☆

Abstract A multiple scattering theory to obtain the effective dielectric constant for a granular metal film is developed considering correlation effects. A renormalization procedure is applied to obtain a field equation within a renormalized first smoothing approximation (RFSA), using T matrix formalism. From the field equation a dispersion relation is obtained that is expressed in terms of the dielectric constant of the metal corrected for size effects, the relative concentration of the constituents, the radius of the inclusions and a correlation length. Using the Maxwell Garnett model for the dielectric constant of the uncorrelated medium, the multiple scattering theory shows that correlation effects move the resonance in the imaginary part of the index of refraction predicted by the Maxwell Garnett model towards lower frequencies and broaden it. Comparison of the RFSA with experiments shows a better fit than do previous theories.

Yields, sensitivities and natural line shapes of high-energy Auger lines: Ta, W, Pt, Au, Pb and Bi

Abstract We present high-resolution measurements of the natural Auger line shapes of the two highest-energy MNN lines of Ta, W, Pt, Au, Pb and Bi. It was found that all the lines studied are quite narrow, having a width of about 2 eV. The main lines are accompanied by smaller satellite peaks. While these measurements are very suitable for determining the natural line shapes, they are inadequate for obtaining the line intensities. The atomic sensitivities were therefore measured with a resolution of 0.4% of the Auger energy (somewhat better than the commonly available resolution). With this resolution it is impossible to resolve the fine structure of the lines and also gross errors are incurred in the measurements of the atomic sensitivities. We corrected these measured sensitivities by applying our theoretical analysis developed previously. The correction factors for the sensitivities are around 10. These large factors represent also the errors incurred in the as-measured sensitivities if no correction is applied. The detailed line shapes, as derived from the high-resolution measurements, and the corrected sensitivities, as obtained from our analysis, are the true characteristics with which any theoretical calculations of Auger transitions should be compared.

Inhomogeneity in the Network Order of Device Quality a-Si:H

In this paper we show that the degree of order of the Si network in a-Si:H is increasing with two length scales from the surface into the bulk. The major manifestation of the disorder is the variation in the Si-Si bond-stretching rather than the variation in the width of the dihedral angle distribution. The results are interpreted in terms of the decrease of the hydrogen concentration from the free surface into the bulk.

IR spectroscopy of a-Si1 − xSnx: H films☆

Abstract Several hydrogenated amorphous Si 1 − x Sn x alloy films of thicknesses varying between 4000 and 8000 A were prepared on impurity-free silicon substrates using a reactive co-sputtering technique. The tin concentration varied among different samples from 0% to 10.7% and was determined using Auger depth profiling. The oxygen contamination increased with the tin concentration but it was found to be uniform throughout the thickness of the films. Samples were characterized using secondary ion mass spectrometry to detect the presence of various molecular species. The IR measurements showed vibrational bands characteristics of Si-H, Sn-H, Si-O and perhaps Sn-O. The presence of tin is found to facilitate the formation of more Si-H than SiH 2 bonds. The Si-O band shifts to 1075 cm −1 from 1030 cm −1 as the tin concentration is increased to 0.1%. In addition, a new peak at 1155 cm −1 appears in the mixed samples with tin concentration over 10%.

Corrections to the optical properties of cermets. I. Quantum size effects

Abstract The dielectric constant of metallic grains in real cermets is obtained using the formalism developed by Wood and Ashcroft, and the resonance structure derived from this formalism is smoothed out by using a log-normal distribution to average the dielectric constant over grain sizes. The results from the quantum theory for the metallic inclusions in Au/A12O3 cermet are compared with Drudes results against experimental data using mean field theory to obtain the effective medium dielectric constant of the cermet.