On the structure of amorphous GexTe1−x systems

Calculations have been made for the quadrupole splitting of a $\text{3}\text{2}$ spin state of Te¹²⁵ in an amorphous Ge_xTe_1?x system. The results favour the existence of a threefold coordinated black phosphorus structure with an excess of TeTe chains for x-values between o and 0.5; beyond 0.5, threefold coordinated GeTe and an excess of amorphous Ge coexist.     

Infrared structural studies of GeySe1−y glasses

Bulk and thin-film specimens of Ge_ySe_1?y have been made the range 0 < y < 0.50. Transmission and reflectivity measurements at 77 and 300 K are reported in the range 50–400 cm^?1. Mode assignment are made and an analysis is presented which supports the chain crossing model for y<0.33. The origin of several small spectral features is discussed in terms of crystal-like stacking within a volume of characterization dimension 10–20 Å.     

Change of short-range order with temperature and composition in liquid GexSe1−x as shown by density measurements

Density measurements were performed on melts of the binary chalcogenide system Ge_xSe_1?x (0 ? x ? 0.5) up to 1000°C. The isotherm of molar volumes V_m at 750°C shows a relative maximum near GeSe₂. Molar volumes of the system behave linearly between GeSe and Se at 1000°C. V_m's of melts between x = 0.30 and x = 0.35 decrease at high temperatures on heating. The anomalous density behaviour of the melts clearly shows a change of short-range order from a less to a more densely packed structure, caused by the development of a pσ-bonding system. Within the composition range $\text{0 ? x ?}\text{1}\text{3}$ the short-range order at lower temperatures is determined mainly by $\text{GeSe}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}$ tetrahedra linked directly corner-to-corner or via Se atoms. At higher temperatures pσ bonds arise more and more, even in melts rich in selenium. Within the composition range $\text{1}\text{3}\text{? x ? 0.5}$ the short-range order is mainly determined by a distorted octahedral configuration, even at lower temperatures. From the short-range orders of melts and from crystalline structures of GeSe₂ and GeSe, the tendency of glass formation from the melt is discussed in detail.     

Effect of annealing on an amorphous GexTe1−x matrix with Te cristallites

Annealing shows very different behaviour for Te crystallites in an amorphous Ge_xTe_1?x matrix as a function of x (x = 0.1 … 0.5). For x ? 0.2, annealing at increasing temperatures increases the number (size) of the Te crystallites with subsequent GeTe + Te crystallization. However for x ? 0.3 there is first a disappearance of Te crystallites, then an appearance of GeTe crystallites, and subsequently GeTe + Te crystallization. Crystallites of either Te or GeTe act as extrinsic defects which add to the intrinsic ones. Activatioon energy decreases (increases) and conductance increases (decreases) as the number of defects increases (decreases). In all cases the final metallic state is obtained only when both GeTe and Te crystallites are present.     

X-ray investigation of the crystallization of chalcogenide glasses of the type (As2Se3)1−x:(Tl2Se)x

Amorphous and crystalline states of As₂Se₃, (As₂Se₃)₃ : Tl₂Se and As₂Se₃ : Tl₂Se have been studied using X-ray diffraction techniques. Structural changes arise during the process of annealing in the temperature range between their softening and melting points are reported and their rates investigated. The crystallization temperatures were found to be 105 ± 5 °C, 135 ± 5 °C and 180 ± 5 °C respectively. The unit cell parameters are identified for each of the three resulting crystalline phases, that for As₂Se₃ : Tl₂Se being orthorhombic while the other two are monoclinic.     

Composition dependence of the refractive index and its photo-induced variation in the binary glass systems: Ge1−xSex and As1−xSex

The composition dependence of the refractive index and its photo-induced variation have been calculated for the Ge_1?xSe_x and As_1?xSe_x binary glass systems, using the random network model. It is found that the refractive index has a minimum in Ge_1?xSe_x at x = 0.8, while it shows a monotomic increase with increasing As content in As_1?xSe_x glasses. The refractive index of the Ge_1?xSe_x system decreases with illumination and the variation Δn of GeSe₂ is ?0.0316, while the refractive index of the As_1?xSe_x system increases with illumination and the variation of As₂Se₃ is 0.01. These results are in agreement with the experimental results reported by several workers.     

Infrared studies of Se-based polynary chalcogenide glasses (II): YxZxSe100−2x (Y = Ge,As; Z = As,Te)

The infrared (IR) absorption spectra for Y_xZ_xSe_100?2x glasses (Y = Ge, As;Z = As, Te), x = 2.5 and 5.0 are measured in the wavenumber region 700-60 cm^?1 at room temperature. These IR spectra are explained by comparing with the IR spectra already reported for the binary glasses such as Ge–Se, As–Se and Se–Te. In Ge_xAs_xSe_100-2x glasses (x ? 5.0), the main spectral features as well explained by both the spectra of Ge_xSe_100?x and As_xSe_100?x glasses. Main structural units in these glasses are considered to be GeSe₄ tetrahedra and AsSe₃ pyramids, and Se₈ rings and Se_n chains which are the units in pure glassy Se. In Ge_xTe_xSe_100?2x glasses (x ? 5.0) and IR band which cannot be explained by either the spectra of Ge_xSe_100?x or Se_100?xTe_x glasses appears at 210 cm^?1. This band is considered to be due to Ge–Te bonds. The IR spectra of As_xTe_x Se_100?2x glasses (x ? 5.0) are well explained by both the spectra of As_xSe_100?x and Se_100?xTe_x glasses. It is concluded that As and Te atoms combine with Se atoms in the forms of AsE₃ pyramids and Se₅Te₃ mixed rings, respectively.     

Infrared studies of Se-based polynary chalcogenide glasses (III): YxZxSxSe100−3x (Y = Ge,As; Z = As,Te)

The infrared (IR) absorption spectra for Y_xZ_xS_xSe_100?3x glasses (Y = Ge, As; Z = As, te), with x = 2.5 and 5.0, are measured in the wavenumber region 700-60 cm^?1 at room temperature. These IR spectra are qualitatively explained by comparing with the IR spectra of the binary and ternary glasses. In Ge_xAs_xS_xSe_100?3x glasses (x ? 5.0), the main spectral features are explained by both spectra of the two ternary glasses Ge_xS_xSe_100?2x and As_xS_xSe_100?2x. In Ge_xS_xTe_xSe_100?3x glasses (x ? 5.0), the main spectral features are well explained by both spectra of the two ternary glasses Ge_xS_xSe_100?2x and Ge_xTe_xSe_100?2x. On the other hand, main spectral features in As_xS_xTe_xSe_100?3x glasses (x ? 5.0) are well explained by both spectra of the ternary glasses As_xS_xSe_100?2x and the binary glasses Se_100?xTe_x. In these glasses with low concentrations (x ? 5.0) some chemical bonds are confirmed and some structural units estimated.     

A thermodynamic and kinetic investigation of amorphous (1−x)As2Se3 · xSb2Se3 alloys

The heats of formation of amorphous (1?x)As₂Se₃ · xSb₂Se₃ (x = 0 to 0.4) referred to crystalline As₂Se₃ and Sb₂Se₃ were measured by liquid metal solution calorimetry. The values of heats of formation of amorphous (1?x)As₂Se₃ · xSb₂Se₃ decreased from 1.39 ± 0.03 kcal · (g-at)^?1 at x = 0 to 1.27 ± 0.04 kcal · (g-at)^?1 at x = 0.4.The glass transition temperature and the temperatures of the maximum rates of crystallization and fusion were measured by differential scanning calorimetry. The glass transition temperature increased and the temperatures of the maximum rates of crystallization and fusion decreased with increasing Sb₂Se₃ content.The relaxation process in amorphous (1?x)As₂Se₃ · xSb₂Se (x = 0.3) was investigated by measuring changes in microhardness, small-angle X-ray scattering and heat capacity with time of annealing at several temperatures ranging from room temperature to 413 K. With increasing annealing time the microhardness, the height and the temperature of the glass transition peak increased whereas the intensity of small-angle X-ray scattering decreased. These changes reflect relaxation towards a more stable structure of smaller molecular mobility. The changes in the enthalpy with annealing time and the activation energy spectra for relaxation were derived from the heat capacity data. The effects of temperature and time of annealing on the various properties are explained in terms of structural changes and relaxation kinetics.     

Investigation of microphase separation in (K2OP2O5)x − (WO3)1−x glasses by electron microscope studies of surface replicas

Microphase sn the glass system (K₂OP₂O₅)_x ? (WO₃)_1?x for x = 0.48 and 0.33 was investigated. In both systems evidence of phase separation on a scale of 100 Å to 200 Å was found. The samples stayed completely amorphous during the cooling process but the two separate phases present in the glasses could unfortunately not be identified. These results coupled with conductivity versus mole percent of WO₃ measurements suggest that the mechanism of phase separation is of the spinodal type.     

Raman and infrared spectra of As2Sx chalcogenide glasses with x ⩽ 3

Raman laser and far infrared spectra of As₂S_x glasses with x ? 3 are given and discussed. The purpose of the work is to bring a vibrational spectroscopic contribution to the study of these glasses and to the hypothesis, still under discussion in the literature, that they might be constituted either by a homogeneous vitreous phase or by a mixture of As₂S₃ and As₄S₄.Our results confirm a phase separation, with formation of ß-As₄S₄, below a certain value of x, which depends not only on the preparation method of the samples but also on other factors such as melting time. Laser irradiation of ß-As₄S₄ modifies its Raman spectrum. Such a phenomenon is attributable to two principal factors, either a partial polymerization or formation of a species richer in arsenic. A structural and formation model of the As₂S_x glasses is given, starting from a more generalized structural model of vitreous As₂S₃ which is an accord with the vibrational results and those by the diffraction method in the literature.     

ac Conductivity of 4.5 TiO2−x · 2 P2O5

The ac conductivity of a member of the family of glasses 4.5 TiO_2?x · 2 P₂O₅ has been measured between 77 and 300 K, and up to 100 kHz. The dc conductivity was measured over only part of this temperature range. The measured ac conductivity can be represented by σ_ac = σ₀ + σ₁ω^s, with s < 1, and temperature dependent. A similar equation describes the ac dielectric constant, ?_ac = ?₀ + ?₁ω^s?1, where $\text{?}{}_1\text{= σ}{}_1\text{tan}\text{1}\text{2}\text{sπ}$. A simple proportionality of s to temperature holds at low temperature; at the higher temperatures, the T-dependence of s is no longer simple. The observed behaviour of the ac properties of this glass is in general accordance with a recently proposed model for systems where transport occurs by hopping. The over-all behaviour is comparable to other transition metal glasses.Using the model and treating the carriers as polarons yields an expression for s in terms of temperature. Values for the polaron radius and the effective dielectric constant are then extracted from the measurements. These values are in good agreement with values for similar systems obtained by other means.     

Photo-acoustic spectroscopy of SixSe1−x glasses

The optical properties near the fundamental absorption edge has been studied for a series of Si_xSe_1−x glasses using photoacoustic spectroscopy. The compositional dependence of the bandgap E_O, derived from these measurements, is presented and contrasted with the GeSe and the SiS systems. This data is qualitatively explained with a model which accounts for differing numbers of homopolar and heteropolar and heteropolar bonds as the composition is varied. Additional support for this interpretation is found in the compositional behavior of the glass transitions of these alloys.     

Photoconductivity of oxychalcogenide glasses in the system As2Se3CdO

The photoconductivity of oxychalcogenide glasses in the system As₂Se₃CdO was investigated. When 2–3 mol % CdO was added, the photoresponse peak of the parent glass As₂Se₃ in the vicinity of 740 nm became broader and a little weaker. The addition of more than about 4 mol % CdO brought about a sharp and strong peak at 720 nm and a broad peak at 860 nm. X-ray diffraction and electron microscopic observations revealed the presence of crystalline CdSe in the glass matrix, indicating that the reaction As₂Se₃ + 3CdO → As₂O₃ + 3CdSe took place in the melting process of these glasses. Tempeature and light intensity dependences of the photocurrent lead to the conclusion that the above spectral photoresponse is greatly affected by the presence of the dispersed crystalline CdSe.     

Change of bonding system in liquid SexTe1−1 alloys as shown by density measurements

Density measurements have been performed on melts in the binary chalcogenide system Se_xTe_1?x up to a temperature of 950°C. Anomalous behaviour of the density was observed. At higher temperatures the density increased on raising the temperature up to a maximum whose position shifts to lower temperatures with increasing concentration of tellurium. This anomalous density behaviour clearly shows an increase of the average coordination number of atoms in the melt. This increase of CN is caused by a change of the bonding system because of the tendency of Te to form pσ bondings. It was assumed that there exist two distinct structures in thermodynamical equilibrium at each temperature according to a law of mass action, for which cooperation effects have to be taken in account. At low temperatures Te atoms easily accept a loosely packed structure I with the aid of selenium. At high temperatures even Se atoms develop a more densely packed structure II with the aid of Te.     

AlxGa1−xN bulk crystal growth: Crystallographic properties and p–T phase diagram

The recent results on the growth of the Al_xGa_1−xN bulk single crystals (0.22≤x≤0.91) from solution in liquid Ga under high nitrogen pressure are discussed. We focus on the influence of temperature and the choice of the Al source on the crystal growth. The experiments involving different sources of aluminum such as Al metal, pre-reacted polycrystalline Al_yGa_1−yN and AlN powder are compared. The best results were achieved using pre-reacted polycrystalline Al_yGa_1−yN or/and AlN. Single-crystal structure refinement data of these Al_xGa_1−xN crystals are presented. We also update the p–T phase diagram of (Al,Ga)N compound at high N₂ pressure for various Al content, which is the basis for (Al,Ga)N synthesis.     

Mössbauer spectra in x PbO · (1 − x) B2O3 doped with Fe2O3

A Mössbauer spectroscopic study has been made on x PbO · (1 ? x) B₂O₃ containing 10 mol% Fe₂O₃ in melt before quenching. X has been varied between 0 and 0.85 in steps of 0.05. In samples with χ ≥ 0.25, a quadrupole doublet was observed. The corresponding isomershift and quadrupole spliting have shown a dependence on χ. A second type of spectrum has been observed in samples with χ < 0.25. It consisted of a six finger hyperfine pattern convulating over a central doublet spectrum. The corresponding internal field and the relative peak intensities are lower than observed in polycrystalline Fe₂O₃. The possible explanations for both types of spectrum are given.     

The leaching behavior of PbOSiO2 glasses

The leached layer of PbOSiO₂ glasses formed by diluted nitric acid solution has been investigated by ellipsometry and Auger electron spectroscopy (AES). The leaching behavior of PbOSiO₂ glasses in 10^?4 N aqueous solution of NHO₃ at 30°C was measured in real time using a Nikon auto-ellipsometer.The results were applied by curve fitting to the two-layer model from the concentration profile obtained by AES, and the refractive index profile against the film thickness was determined.The leached layer is inhomogeneous and consists of a low refractive index region and a transition region. The gradient of the refractive index in the former region is extremely small and the refractive index becomes nearly constant between 1.42 and 1.44. The shape of latter region becomes stable with its thickness at 100–310 Å, and moves in the direction of depth without changing the shape as the leaching proceeds.     

Investigation of recombination and trapping processes in 3As2Se32Sb2Se3 amorphous films by photoconductivity method

Photoconductivity experiment has been performed on high resistivity ($\text{π ? 10}{}^9\text{Ω-}\text{cm}$) 3As₂Se₃2Sb₂Se₃ amorphous films in the temperature range from 278°K to 308°K, as a function of light intensity, I₀. The results are that at intermediate light levels the photocurrent varies as I₀^0.7 and at high light levels the photocurrent is directly proportional to I₀. A simple recombination and trapping model is developed to interpret the observed photoconductivity data. From the temperature dependence of the photocurrent, an effective trap level located 0.3 eV below the band edge is deduced.