Optical and structural properties of Ge–Se bulk glasses and Ag–Ge–Se thin films

The optical and structural properties of Ge₂₀Se₈₀, Ge₂₅Se₇₅ and Ge₃₀Se₇₀ bulk glasses and Ag_x(Ge_0.20Se_0.80)_100−x thin films, where x = 0, 6, 11, 16, 20 and 23 at.% were studied. All samples were confirmed as amorphous according to XRD. The Raman spectra showed increase in 260 cm⁻¹ and 237 cm⁻¹ and decrease in 198 cm⁻¹ and 216 cm⁻¹ bands with different Se content in the bulk samples. The optical bandgap energy of bulk samples decreased (2.17–2.08 eV) and refractive index increased (2.389–2.426 at 1550 nm) with increasing Se content in bulk glasses. The Ge₂₀Se₈₀ thin films were prepared by vacuum thermal evaporation from Ge₃₀Se₇₀ bulk glasses. The Raman spectra of the films showed that peaks at 260 cm⁻¹ and 216 cm⁻¹ decreased their intensities with increasing Ag content in the thin films. The significant red shift of bandgap energy occurred upon different Ag content. The optically induced dissolution and diffusion resulted in graded refractive index profile along the film thickness caused by different Ag concentration. The refractive index increased from the substrate side to the top of thin films. The graded profile was getting more uniform with increasing content of silver in the thin film.     

Crystallization processes of Ag–Ge–Se superionic glasses

The interest in superionic systems has increased in recent years because of the potential application of these materials as solid electrolytes. In this field, amorphous materials present important advantages when compared to the crystalline solids: larger conductivity, isotropy and absence of grain boundaries. In this work, amorphous alloys of compositions (Ge₂₅Se₇₅)_100−yAg_y with y=10, 15, 20 and 25 at.% have been studied. Amorphous samples in bulk were obtained from the liquid by water quenching (melt-quenching technique). The crystallization kinetics of the amorphous alloys have been studied under continuous heating and isothermal conditions by means of differential scanning calorimetry. A glass transition and two exothermic transformations were observed in all the samples. The quenched samples and the crystallization products have been characterized by X-ray diffraction. The primary crystallization of the ternary phase Ag₈GeSe₆ and the secondary phase GeSe₂ was observed. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The first step of the crystallization of the Ag₈GeSe₆ phase in all the (Ge₂₅Se₇₅)_100−yAg_y samples is modelled taking into account the Johnson–Mehl–Avrami–Kolmogorov theory and considering that the changes in the composition only modify the viscosity of the undercooled liquid. The transformation diagrams (TTT and THRT) are calculated and the glass forming ability is analyzed. The experimental results are discussed and correlated with the structures proposed for the glass. The present results and conclusions are also compared with those reported by other authors.     

Application of Ag–Ge–Se based chalcogenide glasses on ion-selective electrodes

Chalcogenide glasses (ChG) have proven to be promissory materials for their application as sensitive membranes in ion-selective electrodes (ISE’s), which are used in electrochemical sensors in order to detect heavy metal ions in solutions. AgGeSe system is not an exception. ChG system Ag_x(Ge_0.25Se_0.75)_100−x with x between 10 and 25 at.% was investigated as membrane material in ion-selective electrodes. This system has shown sensitivity to the presence of Cu²⁺ and Fe³⁺ ions. The introduction of Cu or Fe to the ChG composition was made in order to explore the role of metal addition on selectivity towards Cu²⁺ or Fe³⁺ ions. The analytical properties such as reproducibility, linear range, sensitivity, detection limit and working pH range were studied and its dependence on the ChG composition was analyzed for the three systems. The cross-selectivity of some interfering ions was investigated.     

Structural relaxation and optical properties in amorphous Ge33As12Se55 films

Amorphous Ge₃₃As₁₂S₅₅ films prepared by ultra fast pulsed laser deposition (PLD) have been vacuum annealed over a range of different temperatures. Raman scattering measurements indicated that the features corresponding to Ge–Se and As–Se clusters increase in intensity with increasing annealing temperature (T_a) up to their respective glass transition temperature, and then decrease with further increasing T_a up to 300 °C. Optical property measurements showed that the refractive index deceases but the optical band gap increases with increasing T_a, and both of them could be fitted by the exponential function. The corresponding characteristic time extracted was found not to obey Arrhenius behavior, which is consistent with the existence of a broken network cut by cross-linking bonds and different clusters in films.     

High-field behaviour of amorphous AsSe films

Thin films of composition AsSe_χ where 0.8 ≤ χ ≤ 1.0 obtained by the evaporation of both crystalline and glassy As₂Se₃ have been examined for their high-field behaviour. The films exhibit two essentially linear I–V regions with a smooth nonlinear change over. While the initial linear region may perhaps correspond to a Poole-Frenkel emission, the very high-field linear region is likely to arise from a distortion of the band shapes and lowering of the mobility gap.     

Electro-optical characterization of Ge–Se–Te glasses

Chalcogenide bulk glasses Ge₂₀Se_80−xTe_x for x(0,15) have been prepared by systematic replacement of Se by Te. Selected glasses have been doped with Ho, Er and Pr, and samples have been characterized by transmission spectroscopy, measurements of dc electrical conductivity and low-temperature photoluminescence. Absorption coefficients have been derived from measured transmittance and estimated reflectance. Arrhenius plots of dc electrical conductivity, in the measured temperature range 300–460 K, are characterized by single activation energies roughly equal to the half of the optical gap. Activation energies deduced from Arrhenius plots reveal a systematic decrease with increasing Te content. Similarly, both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/or dominant luminescence band to longer wavelength due to Te → Se substitution. Samples doped with Ho and Er exhibit a strong luminescence at 1200 and 1540 nm due to ⁵I₆ → ⁵I₈ and ⁴I_13/2 → ⁴I_15/2 transitions of Ho³⁺ and Er³⁺ ions, respectively. Pr doped samples exhibit only a relatively weak luminescence peak at 1590 nm, which we tentatively assign to ³F₃ → ³H₄ transition of Pr³⁺ ions. Absorption of the base glass luminescence at 1460 and 1520 nm has been observed at low temperature on samples doped with Pr and Er, respectively.     

Photoinduced metal surface deposition in amorphous GeSAg films

Metallic silver photosurface deposition (PSD) and photoinduced stractural transformation in amorphous GeSAg films containing a large quantity of Ag have been studied. The photosensitivity of the films was greatly enhanced by the addition of gold, and also the morphology and distributing density of the deposits were markedly influenced. It has been found that the PSD effect can be reversed by a thermal annealing.     

Structural development in Ge-rich Ge–S glasses

The Raman spectra of Ge–S glasses in the Ge-rich region from Ge 33% to 46% have been investigated in order to know the structural development of the network glasses. From the detailed curve fits, we have found that there is an unassigned peak at 410 cm⁻¹ and it becomes larger with increasing Ge composition. To clarify the structural origin of the peak, we virtually constructed the atomic arrangement of the glassy state starting from the crystalline state through the liquid state and changed the composition gradually depleting the medium in sulfur. From the consideration of the structural modeling and the atomic orbital theory, we suggest that single Ge–S chain is a probable structural origin of the peak.     

Te-rich Ge–Te–Se glass for the CO2 infrared detection at 15 μm

Te_80−xGe₂₀Se_x glasses have been prepared along the GeSe₄–GeTe₄ axis using the classical method in silica tube under vacuum. A phase separation domain appears for composition around Te₄₀Ge₂₀Se₄₀. Our attention was turned toward the Te-rich compositions corresponding to 1 < x < 5 at.%. These glasses are transparent from 4 to about 20 μm without any purification of the starting elements. Furthermore the difference ΔT between the crystallization temperature T_x and the vitreous transition temperature T_g lies at about 110 °C that is to say 30 °C higher than for the GeTe₄ reference glass. Finally the introduction of a few percentages of Se makes the glasses much easier to prepare and more stable against crystallization, making them drawable as optical fibers for example. Taking into account their transparency window, encompassing the CO₂ absorption band around 15 μm, the Te_80−xGe₂₀Se_x with 1 < x < 5 at.% could become matchless composition for the CO₂ infrared detection as planed by the Darwin mission of the European Space Agency.     

Crystalline–amorphous and amorphous–amorphous transitions in phase-change materials

The transition from the crystalline state to amorphous state and back has been studied in the particular case of the GeSb₂Te₄ phase-change material by a computer simulation procedure. Modelling at the nanoscale indicates specific structural characteristics, especially the multiplicity of the amorphous phase as opposite to the uniqueness of the crystalline phase. In the particular case of the Si₁₂Ge₁₀As₃₀Te₄₈ switching glass two types of ordering have been pointed out and characterized.     

Effect of annealing temperature on the structural and optical properties of amorphous Sb2Te2Se thin films

Thin films of Sb₂Te₂Se were prepared by conventional thermal evaporation of the presynthesized material on Corning glass substrates. The chemical composition of the samples was determined by means of energy‐dispersive X‐ray spectrometry. X‐ray diffraction studies on the as‐deposited and annealed films revealed an amorphous‐to‐crystalline phase transition. The as‐deposited and annealed films at T _a = 323 and 373 K are amorphous, while those annealed at T _a= 423 and 473 K are crystalline with a single‐phase of a rhombohedral crystalline structure as that of the source material. The unit‐cell lattice parameters were determined and compared with the reported data. The optical constants (n , k ) of the investigated films were determined from the transmittance and reflectance data at normal incidence in the spectral range 400–2500 nm. The analysis of the absorption spectra revealed non‐direct energy gaps, characterizing the amorphous films, while the crystalline films exhibited direct energy gaps. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal stability and crystallization kinetics of new As–Ge–Se–Sb glasses

Thermal stability and crystallization kinetics of As₁₄Ge₁₄Se_72−xSb_x (where x = 3, 6, 9, 12 and 15 at.%) glasses are studied by the differential scanning calorimetry. The values of the glass transition temperature (T_g) and the peak temperature of crystallization (T_p) are found to be dependent on heating rate and antimony content. From the heating rate dependence of T_g and T_p the values of the activation energy for glass transition (E_t) and the activation energy for crystallization (E_c) are evaluated and their composition dependence discussed. Crystallization studies have been made under non-isothermal conditions with the samples heated at several uniform rates. Using a recent analysis developed for non-isothermal crystallization studies, information on some aspects of the crystallization process has been obtained. The stability calculations emphasized that the thermal stability decreases with increasing the Sb content.     

Surface morphology of as-deposited and illuminated As–Se chalcogenide thin films

The role of the composition and of the related changes of the structure in the formation of the surface of amorphous As_xSe_1−x (0 < x < 0.5) layers before and after light treatment was investigated by direct measurements of the surface roughness at nanometer-scale and surface deformations at micrometer-scale under influence of illumination. It was established that the surface roughness of the films, deposited by vacuum thermal evaporation, decreased with increasing As content, especially in compositions 0.1  x  0.3, where the maximum light stimulated surface deformations (localized expansion) occurs. Both relate to the rigidity percolation range and the maximum photoplastic effects, which are not directly connected to the known photodarkening effect, since it is minimal for these compositions.     

Local structural environment and intra-4f transition of rare-earth ion in chalcogenide glass: Comparison between Dy-doped Ge–As–S and Ge–Ga–S glasses

We have employed Dy L₃-edge extended X-ray absorption fine structure measurements to investigate the local structures of Dy doped in Ge–As–S and Ge–Ga–S glasses. It turned out that Dy–S distance on average is conspicuously longer in Ge–Ga–S glass despite the number of the nearest neighboring S atoms being nearly identical with each other. The enhanced rare-earth solubility of Ge–Ga–S glass is then explained in connection with decrease in covalence of Ga–S bonds compared with Ge–S or As–S bonds, and structural correlation between GaS₄ tetrahedra and Dy. The discrepancy in the local structural environments of trivalent Dy, however, results in no significant changes in the spectral lineshape and the mean energy of its intra-4f-configurational transitions.     

Charge transport in chlorine doped amorphous Se:Te xerographic photoreceptor films

Time-of-flight drift mobility experiments were carried out on a-Se_1?xTe_x (x = 0-0.1) with chlorine as an additive up to 70 atomic parts per million to investigate the charge transport mechanism in these xerographically important photoreceptor films. Hole drift mobility-temperature data indicate that hole transport in a-Se:Te alloys is controlled by a relatively discrete set of Te-introduced shallow traps (probably Te₁^? centres) at ～ 0.43 eV above E_v whose concentration increases nearly linearly with Te addition. Chlorine doping generates an additional set of traps (probably Cl₀^? centres) around the same energy as Te-induced traps in a similar fashion to the effect of Cl addition to a-Se. Electron drift mobility-temperature data for a-Se:Te alloys containing no Cl can also be interpreted by assuming Te-introduced electron traps at ～ 0.49 below E_c. There was no electron transport observable in Cl-doped a-Se:Te alloys.     

Transient photoconductivity in amorphous Se70Sb20Ag10 thin films

Se₇₀Sb₂₀Ag₁₀ bulk material was obtained using a conventional melt quenching technique. Thin films of a-Se₇₀Sb₂₀Ag₁₀ were prepared by vacuum evaporation technique in a base pressure of 10⁻⁴ mbar onto well-cleaned glass substrates. Dark, light conductivity and transient photoconductivity have been studied. The photoconductivity increases initially and then saturates with time when illuminated with low intensity light (<400 Lux); however, when illuminated with higher intensity light (>600 Lux) the photoconductivity increases initially, attains a maximum and then decreases with time. This behavior of photoconductivity has been studied at various temperatures. The results have been explained on the basis of Dember voltage and the interaction between photoexcited holes and the trapped electrons on the surface.     

Features in the photoluminescence line-shape of heavily Er-doped Ge–S–Ga glasses

Erbium doped chalcogenide glasses are of great interest in the integrated optoelectronic technology due to their Er³⁺ intra-4f emission at the standard telecommunication wavelength of 1.54 μm. In this paper, the photoluminescence (PL) of a series of (GeS₂)_x(Ga₂S₃)_100−x (x = 75 and 67) glasses doped with high amounts of Er₂S₃ (1.8, 2.1, 2.4 and 2.7 mol%) under excitation with 1064 nm light has been studied. A quenching PL effect at 1.22 аt.% Er-doped (GeS₂)₇₅(Ga₂S₃)₂₅ and 1.39 аt.% Er-doped (GeS₂)₆₇(Ga₂S₃)₃₃ glasses has been established. The relative changes in PL line-shape at around 1540 nm have been estimated by deconvoluting the spectra to Gaussian sub-bands centered at 1519 ± 1, 1537 ± 1, 1546 ± 1, 1555 ± 1 and 1566 ± 4 nm, which correspond to F₂₁, F₁₁, F₂₂, F₁₂ and F₁₃ transitions in the ⁴I_13/2 and ⁴I_15/2 energy levels and have intensity and manifestation that are strongly depend on the Er-doping level. The influence of gallium on the PL efficiency has been evaluated with a view to enhanced emission cross-section.     

Femtosecond optical Kerr effect study of amorphous chalcogenide films

The non-resonant third-order non-linear optical properties of some amorphous chalcogenide films were studied experimentally by the method of the femtosecond optical heterodyne detection of the optical Kerr effect. The real and imaginary parts of the complex third-order optical non-linearity could be effectively separated and their values and signs could be also determined. Amorphous chalcogenide films showed a very fast response in the range of 200 fs under ultrafast excitation. The ultrafast response and large third-order non-linearity are attributed to the ultrafast distortion of the electron orbitals surrounding the average positions of the nucleus of chalcogen atoms. The high third-order susceptibility and a fast response time of amorphous chalcogenide films make them promising materials for application in advanced techniques especially in optical switching.     

Influence of source composition on the properties of flash-evaporated thin films in the Cu–In–Se system

Using CuIn₂Se_3.5 as source material epitaxial layers are obtained on (111)A- und (100)-oriented GaAs substrates by flash evaporation in the substrate temperature range T_sub = 720—870 K. The composition of the films is between CuInSe₂ and CuIn₂Se_3.5. The structural properties of the films are similar to those for CuInSe₂ epitaxial layers and refer to a chalcopyrite-like structure. The films are always p-type conducting but different acceptor states are found in dependence on the substrate temperature.