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.     

Characterization of vapor grown (001) GaAs1−xPx layers by selective photo-etching

The technique of selective photo-etching known for GaAs was successfully applied to (001)GaAs_0.6P_0.4 and it can also be used for the characterization of phosphorus-rich GaAs_1?xP_x layers and for GaP. Using this technique misfit dislocations in GaAs_1?xP_x up to now mainly examined with the transmission electron microscope (TEM) in the GaAs_1?xP_x transition layer could be examined in a density range which is hardly accessible for observation with the TEM. Misfit dislocations are also observed in GaP and GaAs_1?xP_x layers when these are heavily doped with nitrogen. The dislocation density of GaAs_1?xP_x layers of constant composition is compared with predictions from a model of Abrahams et al. for the dislocation density of GaAs_1?xP_x as a function of the rate of grading. Within this model our results can only be explained when the asymmetry factor m between the length to distance ratio of misfit dislocations in the transition layer is about two orders of magnitude greater than the value previously derived from TEM studies of GaAs_1?xP_x layers.     

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.     

Dependence of the surface topology and raman scattering spectra of Ge x Si1−x /Si films on the composition variation over the layer thickness

The surface topology and Raman scattering spectra of Ge _x Si_{1 ? x} /Si(100) films are investigated in dependence of the composition variation over the film thickness. It is shown that the character of the Ge content variation in the Ge _x Si_{1 ? x} alloy at the constant cumulative Ge fraction in the film (x _int = 0.5) affects the surface morphology of the grown Ge _x Si_{1 ? x} /Si layer. The heterostructures were grown by molecular-beam epitaxy.     

Morphological control of MgxZn1−xO layers grown on Ga:ZnO/glass substrates for photovoltaics

Mg_xZn_1?xO has been used in various photovoltaic cells because its energy bandgap can be tailored by controlling the Mg composition in this ternary compound. The Mg_xZn_1?xO layers with different surface morphologies including two-dimensional (2-D) films and one-dimensional (1-D) nanostructures are preferred for conventional p–n junction solar cells and polymer–inorganic hybrid solar cells, respectively. The Mg_xZn_1?xO layers are sequentially grown on Ga-doped ZnO (GZO) transparent conductive electrode using metalorganic chemical vapor deposition (MOCVD). The effect of the buffer layers on Mg_xZn_1?xO surface morphology is investigated. It is observed that Mg_xZn_1?xO deposited at ～500 °C on a low-temperature (～250 °C) ZnO buffer layer is in the form of 2-D dense and smooth films, whereas, on a high-temperature (～520 °C) ZnO buffer layer is in the form of 1-D nanostructures. Based on the structure characterization results, a growth mechanism in terms of nucleation and texturing is proposed to explain the buffer layer effect.     

Growth and characterization of Fe1 ? x M x VO4 single crystals (M = Al, Cr, Co, Ga)

Triclinic Fe_{1 ? x} M _x VO₄ single crystals (M = Al, Ga, Co, Cr) have been grown by the flux method from systems based on PbO-V₂O₅. Their crystallographic parameters are determined by powder X-ray diffraction. Fe_{1 ? x} Ga _x VO₄ single crystals (x = 0?C0.3) with a volume more than 1cm³ are grown using the seeding technique. The temperature and field dependences of magnetization and magnetic susceptibility of the grown Fe_{1 ? x} Ga _x VO₄ and Fe_{1 ? x} Al _x VO₄ single crystals (x = 0.3 in the solution-melt) are reported. It is shown that the magnetizations of these crystals exceed that of FeVO₄, and both of their antiferromagnetic phase transitions are shifted to lower temperatures.     

Electron spin resonance of Mn2+ IN As–Se and As–Te glasses

The electron spin resonance of Mn has been studied in As_xSe_100?x with 0 ? x ? 70 and As_xTe_100?x with 40 ? x ? 70. All samples, except those with x < 20 in As_xSe_100?x, exhibit six hyperfine lines centered at g = 4.3. A g = 2.0 line is observed in As–Se with largely scattered linewidth by samples, but not in As–Te unless oxygen contamination is included in the samples. The g = 4.3 line in As–Se is closely related to a formation of As₂Se₃-type layer structure and interpreted as being caused by Mn situated at the interlayer position and surrounded by four Se atoms in an arrangement of rhombic symmetry. In As–Te, a similar model by four Te atoms is valid in composition near As₂Te₃, but the surrounding Te is replaced by As as As content increases. The g = 2.0 line is concluded to come from phase-separated antiferromagnetic particles of Mn–O and MnSe. The linewidth is scattere by differences in the relative amounts of the two kinds of particles and in particle size.     

X-ray diffraction studies of the structure of LaA1Ga metallic glasses

The LaLa and LaA1 partial atomic distribution functions have been determined for glassy La_1?xA1_x from X-ray diffraction studies of isomorphous alloys of La_1?xA1_x, La_1?x(A1Ga), and La_{1 ?x}Ga_x for x = 0.20, 0.24 and 0.28. The atomic short range order of these La based metallic glasses is quite different from that of typical amorphous transition metal-metalloid alloys and from dense random packing models. A relatively short and well defined LaA1 nearest neighbor distance suggests some covalency in the bonds between unlike atoms and possible chemical ordering in the alloys.     

Studies on the AsSbSe glass system

Results are reported of measurements of the glass transition temperature (T_g), conductivity and density (d) for glasses of the AsSbSe system, with compositions (As, Sb)₄₀Se₆₀ and As_xSb₁₅Se_85?x. The results are compared with glasses of similar compositions of the As_xSe_100?x about compositions in the Ge_xSb₁₅Se_85?x and As_xSe_100?x glasses, in the case of As_xSb₁₅Se_85?x glasses, the As-rich compositions exhibit higher values of T_g and d compared to the stoichiometric composition As₂₅Sb₁₅Se₆₀. These results are discussed in the light of a chemically ordered structural arrangement in these glasses.     

One- and two-phonon Raman spectra of amorphous AsSe systems

The polarization dependence of the one- and two-phonon Raman scatterings of the amorphous semiconductor As_xSe_1?x systems for 0 ? x ? 0.5 was measured. The analysis of the Raman data by the superposition of the vibrations of AsSe₃ pyramids and SeSe bonds suggests the existence of the intermediate range interaction in the As_xSe_1?x systems.Two phonon Raman spectra for 0.3 ? x are explained by the combination plus overtone of the first order Raman scatterings. For x ? 0.2, however, the overtone processes become predominant.The dependence of the Raman spectra on the As concentration suggests that the chemical orderings are predominant.     

Devitrification characteristics of GexSe1−x glasses

Amorphous Ge_xSe_1?x compounds have been prepared and carefully characterized in the range 0 ? x ? 0.43 (density and micro-hardness measurements and DTA experiments). By heating samples from the amorphous phase, the crystallization process has been investigated for 0.15 ? x ? 0.30. An immiscibility gap in the vitreous region is determined by optical microscopy, from phase-separation observations; moreover, a new metastable crystalline phase appearing as an intermediate step between phase separation and stable GeSe₂ is described.     

On the effect of structural and symmetrical features of potassium titanyl phosphate crystals with different contents of niobium,antimony, and zirconium on the second-harmonic intensity

A model is proposed, which shows that, at small deviations from the centrosymmetric state of the atomic structure, the quadratic nonlinear susceptibility of a crystal monotonically decreases with approach of the degree of central symmetry \(\eta _{\overline 1 } \)[φ(r)] of the electric potential function of the crystal structure to unity. The quadratic nonlinear susceptibility of K_{1 ? x} Ti_{1 ? x} Nb _x OPO₄ (x = 0, 0.02, 0.04, 0.11), K_{1 ? x} Ti_{1 ? x} Sb _x OPO₄ (x = 0.01, 0.07, 0.17), and KTi_{1 ? x} Zr _x OPO₄ (x = 0.03, 0.04) crystals has been measured. The degree of central symmetry \(\eta _{\overline 1 } \)[φ(r)] has been calculated for the structures of K_{1 ? x} Ti_{1 ? x} Nb _x OPO₄ (x = 0, 0.04, 0.11), K_{1 ? x} Ti_{1 ? x} Sb _x OPO₄ (x = 0.01, 0.07, 0.17), and KTi_{1 ? x} Zr _x OPO₄ (x = 0.03, 0.04) crystals. It is shown that, at \(\eta _{\overline 1 } \)[φ(r)] > 0.7, the relationship between the quadratic nonlinear susceptibility of the investigated crystals and the degree of their central symmetry \(\eta _{\overline 1 } \)[φ(r)] is in qualitative agreement with the proposed model.     

Growth and structure of photosensitive Pb1−x Mn x Te(Ga) epitaxial films

The growth and structure of (1-1.5)-μm-thick Pb_1?x Mn _x Te(Ga)(x = 0.06) films with 0.4?0.9 at % of gallium, grown on BaF₂(111) and Pb_1?x Sn _x Te (x = 0.2) (100) substrates by molecular beam epitaxy, have been investigated. It is established that the films are crystallized into an fcc structure, and their growth planes are (111) and (100), according to the substrate orientation. The optimal conditions for obtaining high-resistivity photosensitive p-and n-type films with a perfect crystal structure (W _1/2 = 80″?100″) have been determined.     

Infrared spectra of amorphous thallium-selenium alloys

Amorphous thallium-selenium alloys (a-Tl_xSe_1?x) have been studied by Fourier transform infrared spectroscopy within the composition range 0 ? x ? 0.53. Differential scanning calorimetry measurements were used to check the selenium content and the homogeneity of the samples. For the stoichiometric composition a-TlSe there is strong evidence of short-range order similar to that found in the crystalline phase, c-TlSe. The spectra of both amorphous and crystalline TlSe display features associated with bond-bending and bond-stretching modes of distorted tetrahedra at ～97 cm^?1 and ～163 cm^?1, reflecting the similarity of tetrahedral coordination within the [(TlSe₂)^?1]n chains. At the Se-rich side with x ? 0.24, a superposition of typical infrared bands of a-TlSe and amorphous selenium (a-Se) is observed. The results are discussed in terms of a structural model for a-Tl_xSe_1?x.     

Structural mechanisms of superionic conductivity in M 1−x R x F2+x single crystals

The relationship between the superionic transport in fluorite phases M _{1 ? x} R _x F_{2 + x} (M = Ca, Sr, or Ba; R are rare earth elements) and their defect structure has been analyzed. The superionic conductivity of M _{1 ? x} R _x F_{2 + x} crystals is provided by the high concentration of charge carriers. However, the carrier concentration is several tens of times lower than the concentration of anionic defects, which is explained by the presence of defect regions (DRs), which partially block carriers. The dependence of the superionic conductivity of M _{1 ? x} R _x F_{2 + x} phases on the RF₃(x) content has a percolation nature. Crystals of these phases are divided into two groups with respect to the percolation threshold: x _{p, 1} = 2–3 mol % RF₃ and x _{p, 2} = 7–8 mol % RF₃. The corresponding DR volumes are 3000–4000 ų (x _{p, 1}) and 500–700 ų (x _{p, 2}). The x _{p, 1}, and x _{p, 2} values correlate, respectively, with the octahedral cubic {M _{14 ? p} R _p F_{68 ? 69}} and tetrahedral {M _{4 ? p} R _p F₂₆} clusters, which are DR cores. The DR model and cluster structure are indicative of the heterogeneity of nonstoichiometric M _{1 ? x} R _x F_{2 + x} crystals at the nanoscale level with respect to the chemical composition and the electrical and crystallochemical (coordinations of M and R) characteristics.     

Coloring elimination in Sr1 − x Ce x F2 + x crystals in the visible spectral range during growth from melt

Crystals of the Sr_{1 ? x} Ce _x F_{2 + x} compositions close to the congruent one (x ～ 0.3) are fabricated by the vertical directional crystallization. It is shown that the use of CF₄ to form a fluorinating atmosphere during growth leads to additional spurious absorption in the crystals in the range 350–600 nm. The use of PbF₂ and ZnF₂ for fluorination makes it possible to obtain colorless Sr_{1 ? x} Ce _x F_{2 + x} crystals of the desired optical quality from melt. The thermal conductivity of crystal with x ～ 0.28 in the temperature range 80-500 K lies within 1.50 ± 0.03 W m^?1 K^?1. High ionic conductivity makes the Sr_{1 ? x} Ce _x F_{2 + x} crystals promising for application in solid-state ionics.     

Crystallization and local order of bulk AsxTe1−x glasses

The crystallization and the local order of the bulk As_xTe_1?x (0.2 ? × ? 0.8) glasses have been investigated by DSC, X-ray and Mössbauer methods. During the crystallization a metastable phase has been observed between 0.35 ? × ? 0.5 and it was identified as fcc AsTe. The local surrounding of these As_xTe_1?x glasses characterized by the ¹²⁵Te Mössbauer measurements were compared with those of the stable monoclinic As₂Te₃, hexagonal Te and metastable fcc AsTe.     

Electron spin resonance study of hydrogenated microcrystalline silicon–germanium alloy thin films

Paramagnetic defects of undoped hydrogenated microcrystalline silicon–germanium alloys (μc-Si_1?xGe_x:H) grown by low temperature (200 °C) plasma-enhanced chemical vapor desposition (PECVD) have been measured by electron spin resonance (ESR) and compared with those of hydrogenated amorphous silicon–germanium (a-Si_1?xGe_x:H). The spin density of μc-Si_1?xGe_x:H increases with Ge content and shows a broad maximum of ～10¹⁷ cm^?3 at x ～ 0.5, which reasonably accounts for the decreased photoconductivity. While the Ge dangling bond defects prevail in a-Si_1?xGe_x:H for Ge-rich compositions, we detected no ESR signal in μc-Si_1?xGe_x:H for x > 0.75 where an electrical change occurs from weak n- to strong p-type conduction. These results indicate that dangling bonds are charged in large densities due to the presence of the acceptor-like states in undoped μc-Si_1?xGe_x:H.     

Fluorite M1 − x R xF2 + x phases (M = Ca, Sr, Ba; R = rare earth elements) as nanostructured materials

The structural defects (M ²⁺ and R ³⁺ in the noncubic environment of F^?, interstitial F^?, and anion vacancies) in nonstoichiometric M _{1 ? x} R_xF_{2 + x} crystals with the CaF₂ structure form {M ₈[R ₆F_68-69]} superclusters of nanometer linear dimensions. This fact allows one to classify the M _{1 ? x} R _xF_{2 + x} phases as nanostructured materials (NSM). The superclusters concentrate rare-earth ions (R ³⁺ = RE). In a M _{1 ? x} R _xF_{2 + x} crystal with the fluorite cation motif, two chemically different parts can be separated: the R ³⁺-depleted matrix and the R ³⁺-enriched clusters. The M _{1 ? x} R _xF_{2 + x} phases are the first NSM among fluorides; they constitute a new type of these materials in which different chemical compositions of the matrix and nanoinclusions are combined with their isostructurality and coherent conjugation of the crystal lattices. Superclusters can also form associates with linear dimensions of tens or hundreds of angstroms. A model is suggested which describes the main characteristic of such NSMs. These materials behave as single crystals in X-ray, neutron, and electron diffraction experiments. The influence of microheterogeneity on some physical properties of the M _{1 ? x} R _xF_{2 + x} phases is also considered.