C. Raptis

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.

On the structural stability of amorphous Se/CdSe multilayers: a Raman study

Abstract The effects of thermal annealing and laser beam illumination on the structure of amorphous Se/CdSe multilayers of up to 20 periods and various sublayer thicknesses (3.5, 5, 6.5 and 10 nm) have been studied by measuring the higher frequency (vibrational) Raman spectra. Three Raman bands have been observed in this spectral region at 209 cm−1 (CdSe), 237 cm−1 (Se) and 256 cm−1 (Se). After annealing, the intensity of these bands increases for all amorphous multilayers (AML) samples which, in a first approach, indicates an improvement of interface quality. In AML of thin ( 5 nm) sublayers the opposite effect is concluded for Se sublayers, and hardly any change for CdSe ones. A gradual increase of laser power density causes a likewise modification of Raman spectra, which is attributed to crystallization of Se sublayers; in similar low temperature (25 K) experiments (using even higher laser power densities) there is no indication of substantial crystallization, implying that the latter is mainly a thermal effect. From the threshold for crystallization, we observed that the structural stability of Se/CdSe AML is better, the smaller the sublayer thickness, an important result for electrophotographic applications.

Raman scattering boson peak and differential scanning calorimetry studies of the glass transition in tellurium?zinc oxide glasses

Raman scattering and differential scanning calorimetry (DSC) measurements have been carried out on four mixed tellurium-zinc oxide (TeO(2))(1 - x)(ZnO)(x) (x = 0.1, 0.2, 0.3, 0.4) glasses under variable temperature, with particular attention being given to the respective glass transition region. From the DSC measurements, the glass transition temperature T(g) has been determined for each glass, showing a monotonous decrease of T(g) with increasing ZnO content. The Raman study is focused on the low-frequency band of the glasses, the so-called boson peak (BP), whose frequency undergoes an abrupt decrease at a temperature T(d) very close to the respective T(g) values obtained by DSC. These results show that the BP is highly sensitive to dynamical effects over the glass transition and provides a means for an equally reliable (to DSC) determination of T(g) in tellurite glasses and other network glasses. The discontinuous temperature dependence of the BP frequency at the glass transition, along with the absence of such a behaviour by the high-frequency Raman bands (due to local atomic vibrations), indicates that marked changes of the medium range order (MRO) occur at T(g) and confirms the correlation between the BP and the MRO of glasses.

Stability of amorphous Se/Se100−xTex multilayers: A Raman study

The Raman spectra of amorphous Se/Se100−xTex multilayers (AMLs) of various compositions and sublayer thicknesses have been measured at room and low temperature (38 K) with the aim to assess the thermal and absorption effects of laser illumination on the structural stability of the AMLs. Under thermal treatment at room temperature (mediated by increasing gradually the power of the probing laser line), the AML stability (manifested by the rate of Se crystallization) increases with decreasing Se100−xTex sublayer thickness and with decreasing Te content. However, in single layers (2 μm thickness) of Se100−xTex, we have observed the opposite effect, that is, the stability of single layers increases with increasing Te content. This apparent contradiction is explained in terms of thermodynamic energy considerations stated previously [K. Tanaka et al., Mater. Res. Soc. Symp. Proc. 118, 343 (1988)] and of a higher crystallization temperature of Se sublayers in the AMLs than that of bulk Se of the single layer. In ...

A comparative raman study of the local structure in (Ge2S3) x (As2S3)1-x and (GeS2) x (As2S3)1−x glasses

The Raman spectra of glasses in the ternary (Ge2S3)x(As2S3)1−x and (GeS2)x(As2S3)1−x systems are reported. It is shown that the degree of disorder in the (Ge2S3)x(As2S3)1−x glasses increases with an increase in the Ge content. The differences in the spectra of glasses with the same molar content of As2S3 are discussed in terms of the changes occurring in the local structure due to deviations from the stoichiometry. The observed changes in the intensity of the boson peak indicate changes in the medium-range order structure of the glasses under study.

The boson peak in Raman spectra of AsxS1−x glasses

The Raman spectra of AsxS1−x glasses with x < 40 at % (Z < 2.4) have been studied in a wide temperature range (20–300 K). A well resolved boson peak is observed in the low-frequency portion of the spectrum, not withstanding the appearance of floppy modes in the glasses under study. It is shown that the boson peak is characterized by two parameters: intensity and the peak position. A comparison of the intensity variation for the boson peaks indicates that the degree of disorder increases as x decreases. This effect is caused by floppy modes in the glass network and by sulfur phase separation. Studies of reduced boson peaks in the Raman spectra of AsxS1−x glasses confirms the theoretical assumption that the shape of the peaks is independent of composition and temperature.

Photoinduced Structural Changes in Amorphous Chalcooenides Studied By Raman Spectroscopy

The use of Raman spectroscopy as probing technique for the study of structure and structural modifications in amorphous materials is briefly discussed, with emphasis being given to amorphous chalcogenides.