G. Dalba

Silicon nanocrystal formation in annealed silicon-rich silicon oxide films prepared by plasma enhanced chemical vapor deposition

Silicon-rich silicon oxide films deposited by plasma enhanced chemical vapor deposition with different total Si contents (from 39to46at.%) have been annealed at increasing temperature (up to 1250°C) in order to study the Si nanocrystal (Si-nc) nucleation as well as the structural changes induced in the amorphous embedding matrix. The comparison between x-ray absorption measurements in total electron yield mode, Raman spectroscopy, and photoluminescence spectra allowed us to gain insight about the Si nanocrystal formation, while the chemical composition and the nature of chemical bonds into the oxidized matrix was studied by Fourier transform infrared spectroscopy. A comprehensive picture of the nucleation process has been obtained, demonstrating the active role played by the hydrogen and nitrogen atoms in the formation of Si-nc and in the thermally induced evolution of the deposited films.

EXAFS Debye-Waller factor and thermal vibrations of crystals.

The EXAFS Debye-Waller factor depends on the correlation of atomic motion and can yield original information on the vibrational dynamics of crystalline solids. In this paper an introductory treatment of thermal disorder in EXAFS, based on thc cumulant approach, is given. Thc general relation between mean-square relative displacement (MSRD) measured by EXAFS and atomic thermal vibrations in harmonic approximation is explored. Strengths and limitations of the phenomenological Einstein- and Debye-correlated models are discussed. Some of the most significant results so far obtained are reviewed. A relatively simple method for estimating anharmonic corrections to the MSRD is proposed.

Short range order in AgI:Ag2O:B2O3 glasses: results from EXAFS and related techniques

Abstract The structure of the (AgI) x (Ag 2 O n B 2 O 3 ) 1−x superionic conducting glasses is being studied by X-ray absorption spectroscopy (EXAFS and related techniques). This paper reviews the results obtained up to date. The information obtained concerns the local environment of silver and iodine atoms and allows us to place constraints on the structural models consistent with the conductivity properties of the glasses.

Optical and X-ray absorption measurements on superionic (AgI)x(Ag2On B2O3)1−x glasses

Abstract The glasses (AgI) x (Ag 2 O n B 2 O 3 ) 1−x have been studied by optical and X-ray absorption spectroscopy at various x and n concentrations. The optical absorption of the n=4 glasses shifts linearly at the increasing of the AgI content x. Extrapolating to x=1, the corresponding value for α-AgI is crossed. The comparison of the EXAFS structures at the I L 3 edge of the n=3, x=0.3 glass and of β-AgI seems to indicate that the Ag-I coordination is substantially unmodified in the glass with respect to that in β-AgI. A further comparison of the EXAFS at the Ag L 3 edge allows to discuss the coordination Ag-O in the glass.

Local order in hydrogenated amorphous germanium thin films studied by extended x-ray absorption fine-structure spectroscopy

The effect of hydrogenation on the local order in amorphous germanium has been studied by EXAFS. Measurements have been carried out on sputtered a-Ge:H films with hydrogen concentrations of 0, 7, 10, and 15 at.%, as a function of temperature in the range 11 - 300 K. The first-shell EXAFS data were analysed by the ratio method based on cumulant expansion. The asymmetric distributions reconstructed from cumulants are in very good agreement with a parametrized distribution obtained by other researchers using calculated phase-shifts. For the unhydrogenated a-Ge (deposited at ), increases of the interatomic distance, at 11 K, static disorder, , and thermal disorder, , have been found with respect to those for c-Ge. Both the static and the thermal disorder are smaller than for an evaporated sample (deposited at ) previously studied. The insertion of hydrogen in a-Ge produces a sharp reduction of the interatomic distance, static disorder, and asymmetry of the distribution already at the lowest H concentration (7%); then these parameters decrease almost linearly when the hydrogen content increases. No appreciable influence of hydrogenation on the thermal disorder has been detected.

Chemical composition and local structure of plasma enhanced chemical vapor-deposited Si nanodots and their embedding silica matrix

X-ray absorption measurements in total electron yield mode have been carried out on Si nanodots embedded in amorphous silica produced by plasma enhanced chemical vapor deposition (PECVD). The amount of Si atoms composing the Si nanodots and the chemical composition of the amorphous host matrix has been determined thanks to the comparison with Rutherford backscattering spectrometry data. The influence of nitrogen, incorporated during the PECVD procedure, on the structure of the host silica matrix has been discussed.

X-ray absorption and diffraction studies of Pr3+, Tb3+ and Er3+-activated silica gels

Rare-earth (Pr3+, Tb3+, Er3+) doped silica xerogels were studied by x-ray absorption spectroscopy and x-ray diffraction. A change of the local environment around rare-earth ions upon xerogel densification at 900–950°C and co-doping with aluminum ions was determined from the rare-earths L3-edge EXAFS signals. The densification process induces a decrease of the coordination number and a compression and deformation of the first coordination shell, composed of oxygen atoms. The second coordination shell, composed of silicon and/or aluminum ions, also experiences some modification, which is attributed mainly to a shortening of the shell radius. No evidence of clustering of rare-earth ions upon densification was observed. X-ray diffraction data on Tb-doped gels confirm the EXAFS results.

Local structure and dynamics in AgI studied by EXAFS and molecular dynamics

Abstract EXAFS of AgI from 20 to 450 K has been analyzed by the cumulant method. The distribution of Ag-I distances agrees with the excluded-volume model of Boyce and Hayes. It is however suggested that in β phase the distribution can be accounted for by an anharmonic effective pair potential with temperature-independent shape. EXAFS reveals a high degree of correlation of vibrational motion of silver and iodine, both in β and α phases. I-Ag and I-I distributions obtained from EXAFS in β phase are compared with molecular dynamics (MD) calculations; the observed discrepancies suggest that EXAFS could be utilized to check the MD interatomic potentials.

XRD and EXAFS study of the local structure in some non-crystalline SbS compounds

Abstract The local structure of three non-crystalline SbS compounds - two vacuum evaporated thin films f-Sb 40 S 60 and f-Sb 28 S 72 and a rapidly quenched glass g-Sb 38 S 62 - has been studied by XRD and EXAFS. The local coordination around Sb atoms is not affected by the preparation procedures or the composition of the investigated samples: Sb atoms have 3 S as nearest neighbours, S atoms have 2 Sb; the mean distance between Sb atoms and their S nearest neighbours is almost equal for glass and thin films. Trigonal SbS 3 pyramids contribute the basic structural units which give rise to the disordered network through S bonds. On the confrary, static disorder of the basic units SbS 3 and medium-range correlations are affected both by the investigated preperation methods and compositions.

An optical study of Ni induced crystallization of a-Si thin films

The optical properties of nanocrystalline silicon (nc-Si), formed by nickel (Ni) induced crystallization of amorphous silicon (a-Si) films, are presented. Growth of nc-Si was characterized by Raman spectroscopy and UV?vis?NIR spectrophotometry. Significantly, the onset of crystallization occurred at 600??C within 15?min of annealing, as evidenced from the Raman peak centered at 514?cm?1. It is demonstrated that the shape of the optical absorption spectrum is a function of thickness, substrate temperature, topological disorder and metal content in the films. Ni doping of the films results in optical inhomogeneity in the films and therefore anomalous dispersion in the behavior of the refractive index. It is further shown that these parameters also influence the position of the Urbach edges. The present study shows that metal induced crystallization of a-Si does not require extended durations of annealing and that the crystallization process is accompanied by structural, chemical and microstructural inhomogeneity in the films.