M. Frumar

Optically and thermally induced changes of structure, linear and non-linear optical properties of chalcogenides thin films

Abstract The spectral dependence of third-order non-linear susceptibilities, χ (3) , of amorphous As–S thin films was evaluated from changes of index of refraction using Miller’s rule. The exposure of thin fresh evaporated films and their annealing increases χ (3) , it increases also their homogeneity and level of their polymerization as can be judged from Raman spectra of the films. The large covalently bonded parts (‘molecules’) of the samples are apparently favorable for increase of χ (3) .

Pulsed laser deposition of pure and praseodymium-doped Ge–Ga–Se amorphous chalcogenide films

Abstract Pure and praseodymium-doped thin films of Ge30Ga5Se65 amorphous system were prepared by the pulsed laser deposition (PLD) technique. The composition of the prepared films was close to the composition of the used targets of bulk chalcogenide glass. The structure of the prepared films was also close to that of the targets as shown by the Raman spectra. The annealing of films shifted the position of the absorption edge to higher energies, which is explained by chemical homogenization of the films due to interaction of fragments of the evaporated material. Two luminescence bands near 1340 and 1610 nm were observed in the emission spectra of praseodymium-doped Ge–Ga–Se thin films. They were assigned to the radiative transitions between discrete energy levels of Pr3+ ions, 1G4–3H5 and 3F3–3H4, respectively. The luminescence intensity of ablated films was lower than that of bulk glasses. It increased after annealing of the films.

Thin amorphous chalcogenide films prepared by pulsed laser deposition

Abstract Amorphous Ge–Ga–Se thin films were prepared by pulsed-laser deposition (PLD), using different energy densities of the excimer KrF laser beam (λ=248 nm) on the glassy target. The chemical composition of prepared films was close to the composition of the chalcogenide glass targets. The structure of Ge–Ga–Se thin films was studied by Raman spectroscopy; GeSe4/2 tetrahedra, edge-sharing Ge2Se8/2 bi-tetrahedra, some Ge–Ge and Ge–Ga or Ga–Ga as well as Se–Se structural units were revealed. The optical properties (transmittance spectra, spectral dependence of index of refraction, optical gap, single oscillator energy, dispersion energy, dielectric constant) of the films were determined. The relations between the structure, optical parameters and energy density of the laser beam used for PLD are discussed.

Optical properties and phase change transition in Ge2Sb2Te5 flash evaporated thin films studied by temperature dependent spectroscopic ellipsometry

We studied the optical properties of as-prepared (amorphous) and thermally crystallized (fcc) flash evaporated Ge2Sb2Te5 thin films using variable angle spectroscopic ellipsometry in the photon energy range 0.54–4.13 eV. We employed Tauc–Lorentz (TL) model and Cody–Lorentz (CL) model for amorphous phase and TL model with one additional Gaussian oscillator for fcc phase data analysis. The amorphous phase has optical bandgap energy Egopt=0.65 eV (TL) or 0.63 eV (CL) slightly dependent on used model. The Urbach edge of amorphous thin film was found to be ∼70 meV. Both models behave very similarly and accurately fit to the experimental data at energies above 1 eV. The CL model is more accurate in describing dielectric function in the absorption onset region. The thickness decreases ∼7% toward fcc phase. The bandgap energy of fcc phase is significantly lower than amorphous phase, Egopt=0.53 eV. The temperature dependent ellipsometry revealed crystallization in the range 130–150 °C. The bandgap energy of amorph...

Raman study of photostructural changes in amorphous GexSb0.4−xS0.6

Abstract Raman spectra of amorphous GexSb0.4−xS0.6 films and bulk samples have been measured for several values of x. The observed Raman bands are due to heteropolar M–S (M=Ge, Sb) bonds in the GeS4 tetrahedra and SbS3 pyramids, as well as homopolar (defective) M–M bonds whose population increases with increasing x. Illumination of Ge-rich ternary films by band gap light induces photostructural changes, the most prominent of which being an increase of the relative population of M–M to M–S bonds. At larger intensities of illumination (by laser light), partial crystallization of Sb occurs in both films and bulk glasses and, again, this photostructural effect is larger in Ge-rich ternary samples. It is revealed that the SbS3 pyramidal population decreases after either of the above treatments, thus underlining the key role of the Sb–S bonds in the photostructural changes. Furthermore, the extent of these effects is discussed in terms of the defective bond population, the free volume and dimensionality of the glassy network.

Raman spectra and photostructural changes in the short-range order of amorphous As-S chalcogenides

Photoinduced changes of optical transmission, diffuse reflectance, refractive index and Raman spectra of As x S 100-x stoichiometric (x = 40) and non-stoichiometric (x = 38, 42) films, bulk glasses and glassy powders were measured. The photoinduced changes of the absorption edge, diffuse reflectance and refractive index were accompanied by changes in Raman band intensities and positions and, therefore, by changes of structure. The Raman spectra of freshly evaporated films, and spectra of As-rich, contain some bands of As 4 S 4 molecules whose content (e.g. ≥ 20 mol% in As 42 S 58 freshly evaporated films, ∼10 mol% for As 42 S 58 glasses) depends on the stoichiometry and state of the films (freshly evaporated, exposed or annealed films) or upon the state of samples (bulk, powdered glass, exposed or annealed powder). The As 4 S 4 content decreased by light exposure and also by annealing, while irradiation increases the content of As-As bonds. A model is proposed in which the photoinduced changes of the optical properties in As-S glasses are connected with photostructural changes, which change the short-range order, namely the densities of individual chemical bonds.

Photoinduced changes of the structure and physical properties of amorphous chalcogenides

Abstract The recent results of the study of photoinduced effects in chalcogenide glasses and layers were reviewed and discussed. The main attention was paid to changes of structure, optical transmissivity and reflectivity and to photoinduced optical anisotropy.

Photoinduced phenomena in amorphous As4Se3 pulsed laser deposited thin films studied by spectroscopic ellipsometry

Amorphous As4Se3 thin films were prepared by pulsed laser deposition technique. Variable angle spectroscopic ellipsometry was used for the study of reversible photoinduced phenomena in well-annealed films. Tauc–Lorentz and Cody–Lorentz models were employed for the analysis of ellipsometric data. Reversible photorefraction and photodarkening were identified by both models; however, Cody–Lorentz model is shown to be more appropriate for the description of amorphous chalcogenide thin films. The behavior of best fit Cody–Lorentz model parameters is discussed in detail.

Effect of cluster size of chalcogenide glass nanocolloidal solutions on the surface morphology of spin-coated amorphous films

Amorphous chalcogenide thin film deposition can be achieved by a spin-coating technique from proper solutions of the corresponding glass. Films produced in this way exhibit certain grain texture, which is presumably related to the cluster size in solution. This paper deals with the search of such a correlation between grain size of surface morphology of as-deposited spin-coated As33S67 chalcogenide thin films and cluster size of the glass in butylamine solutions. Optical absorption spectroscopy and dynamic light scattering were employed to study optical properties and cluster size distributions in the solutions at various glass concentrations. Atomic force microscopy is used to study the surface morphology of the surface of as-deposited and thermally stabilized spin-coated films. Dynamic light scattering revealed a concentration dependence of cluster size in solution. Spectral-dependence dynamic light scattering studies showed an interesting athermal photoaggregation effect in the liquid state.

Optical characteristics of pulsed laser deposited Ge-Sb-Te thin films studied by spectroscopic ellipsometry

Pulsed laser deposition technique was used for the fabrication of (GeTe)1−x(Sb2Te3)x (x = 0, 0.33, 0.50, 0.66, and 1) amorphous thin films. Scanning electron microscopy with energy-dispersive x-ray analysis, x-ray diffraction, optical reflectivity, and sheet resistance temperature dependences as well as variable angle spectroscopic ellipsometry measurements were used to characterize as-deposited (amorphous) and annealed (rocksaltlike) layers. In order to extract optical functions of the films, the Cody–Lorentz model was applied for the analysis of ellipsometric data. Fitted sets of Cody–Lorentz model parameters are discussed in relation with chemical composition and the structure of the layers. The GeTe component content was found to be responsible for the huge optical functions and thickness changes upon amorphous-to-fcc phase transition.