Ge Chalcogenides Research Articles

Mechanisms of Heat Transfer in Thermoelectric Materials

The analysis of methods for calculating the components of thermal conductivity of solid materials is carried out. The problems of using these methods are identified and their assessment is given from the point of view of obtaining reliable results. Experimental data on the temperature dependences of thermoelectric parameters of the main thermoelectric materials (TEMs) used for the manufacturing of Peltier and Seebeck thermoelements are presented. Using the obtained data, we calculated the main components of the thermal conductivity of TEMs (phonon, electronic, and bipolar) in the temperature range from 250 to 1200 K for solid solutions based on Bi, Sb, Pb, and Ge chalcogenides, as well as SiGe obtained by various methods of directional crystallization. The relationship between the mechanisms of heat transfer in TEMs and the temperature dependences of electrical conductivity and thermoelectric power has been established. It is shown that the use of nanostructured TEMs is promising for increasing the thermoelectric figure of merit.

Stabilizing n‐Type Cubic GeSe by Entropy‐Driven Alloying of AgBiSe2: Ultralow Thermal Conductivity and Promising Thermoelectric Performance

AbstractThe realization of n‐type Ge chalcogenides is elusive owing to intrinsic Ge vacancies that make them p‐type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to SnSe at ambient conditions. The high‐symmetry cubic phase of GeSe is predicted to be stabilized by applying 7 GPa external pressure or by enhancing the entropy by increasing to temperature to 920 K. Stabilization of the n‐type cubic phase of GeSe at ambient conditions was achieved by alloying with AgBiSe2 (30–50 mol %), enhancing the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe2 concentration. The band gap of n‐type cubic (GeSe)1−x(AgBiSe2)x (0.30≤x≤0.50) has a value in the 0.3–0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe)1−x(AgBiSe2)x exhibits an ultralow lattice thermal conductivity (κL≈0.43 W m−1 K−1) in the 300–723 K range. The low κL is attributed to significant phonon scattering by entropy‐driven enhanced solid‐solution point defects.

Stabilizing n-Type Cubic GeSe by Entropy-Driven Alloying of AgBiSe2 : Ultralow Thermal Conductivity and Promising Thermoelectric Performance.

The realization of n-type Ge chalcogenides is elusive owing to intrinsic Ge vacancies that make them p-type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to SnSe at ambient conditions. The high-symmetry cubic phase of GeSe is predicted to be stabilized by applying 7 GPa external pressure or by enhancing the entropy by increasing to temperature to 920 K. Stabilization of the n-type cubic phase of GeSe at ambient conditions was achieved by alloying with AgBiSe2 (30-50 mol %), enhancing the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe2 concentration. The band gap of n-type cubic (GeSe)1-x (AgBiSe2 )x (0.30≤x≤0.50) has a value in the 0.3-0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe)1-x (AgBiSe2 )x exhibits an ultralow lattice thermal conductivity (κL ≈0.43 W m-1 K-1 ) in the 300-723 K range. The low κL is attributed to significant phonon scattering by entropy-driven enhanced solid-solution point defects.

Silver photodiffusion into Ge-rich amorphous germanium sulfide—neutron reflectivity study

Silver diffuses into chalcogenide films upon light exposure, and the kinetics of photodiffusion has been a subject of various investigations because of the difficulties in the in situ determination of the time-dependent Ag reaction and diffusion development in the chalcogenide layers. In this paper, we report the results of time-resolved neutron reflectivity measurement of Ag/Ge40S60/Si substrates under light exposure to clarify the kinetics of Ag photodiffusion into Ge-rich Ge chalcogenides. It reveals that Ag ions diffuse all over the Ge chalcogenide host layer once Ag dissolves into the layer without forming a metastable reaction layer unlike the case of S-rich Ge chalcogenide such as Ge20S80. The decay curve suggests that the Ag dissolution is determined by two types of Ag capturing chalcogen sites. Also, the observed relaxation time showed anomalous chalcogenide layer thickness dependence. This is attributed to an additional diffusion-driven accelerating factor, which is unique to the silver photodiffusion. Furthermore, we observed indicative changes in the formation of an inhomogeneous in-plane structure at the Ag/chalcogenide interface. This would be related to the nucleation and growth of the Ag-dissolved reaction product.

Glass transition and crystallization kinetics analysis of Sb–Se–Ge chalcogenide glasses

Differential thermal analysis (DTA) has been employed to investigate the effect of Ge addition on the glass transition behavior and crystallization kinetics of Sb10Se90−xGex (x = 0, 19, 21, 23, 25, 27) alloys. The three characteristic temperatures viz. glass transition (T g), crystallization (T c), and melting (T m) have been determined and found to vary with the heating rates and Ge content. Thermal stability and glass forming tendency have been evaluated in terms of ΔT (= T c − T g) and reduced glass transition temperature. The activation energies for glass transition and crystallization have been used to analyze the nucleation and growth process. The activation energy analysis also determines the suitability of alloys to be used in switching applications. Results have been interpreted in terms of bond energies and structural transformations in the investigated alloys.

Selection of Optimized Materials for CBRAM Based on HT-XRD and Electrical Test Results

Among emerging memory technologies that rely on the bistable change of a resistor, the conductive bridging random access memory (CBRAM) is of particular interest due to its excellent scaling potential into the sub-20 nm range and low power operation. This technology utilizes electrochemical redox reactions to form nanoscale metallic filaments in an isolating amorphous solid electrolyte. Ge chalcogenides are candidate materials for high performance solid electrolytes in combination with Ag as the preferred metal showing high mobility and switching speed. Due to the thermal budget for a back end of the line (BEOL) processing, the layer stack materials must withstand temperatures in the range of . Pure GeS was stable up to without crystallization. For GeSe, deleterious crystallization was observed. High temperature X-ray diffraction (HT-XRD) and electrical characterization with stepwise annealing were applied to characterize the thermal stability of Ag/GeSe and Ag/GeS material systems. The higher onset temperature for solid-state reactions found with HT-XRD in the Ag/GeS system is the key for the better electrical performance compared to the Ag/GeSe system. Even after thermal annealing with a peak temperature of , excellent and stable yield numbers of more than 90% for memory elements were achieved for the sulfide, which qualifies this material system for a low temperature BEOL process.

On the glass transition phenomenon in Se–Te and Se–Ge based ternary chalcogenide glasses

Abstract The present paper reviews the results of non-isothermal differential scanning calorimetry (DSC) measurements on some ternary glassy systems of Se–Te and Se–Ge chalcogenide alloys for evaluation of activation of glass transition. The activation energy of glass transition ( E g ) is calculated using Kissinger's relation, which is basically derived for amorphous to crystalline phase transition. The results show that E g values obtained from Kissinger's relation are in good agreement with the E g values which are obtained using Moynihan's relation based on the concept of thermal relaxation. This proves the applicability of Kissinger's relation for determination of activation energy of glass transition.

Temperature-Induced Density Anomaly in Te-Rich Liquid Germanium Tellurides:pversussp3Bonding?

The density anomaly of liquid Ge(0.15)Te(0.85) measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental densities. For box sizes ranging from 56 to 112 atoms, an 8 k-points sampling of the Brillouin zone is necessary to obtain reliable results. Contrary to other Ge chalcogenides, no sp(3) hybridization of the Ge bonding is observed. As a consequence, the negative thermal expansion of the liquid is not related to a tetrahedral bonding as in the case of water or silica. We show that it results from the symmetry recovery of the local environment of Ge atoms that is distorted at low temperature by a Peierls-like mechanism acting in the liquid state in the same way as in the parent solid phases.

Properties of molten Ge chalcogenides: an ab initio molecular dynamicsstudy

In this study, we perform first-principles molecular dynamics simulations of theeutectic alloy Ge15Te85 at five different densities and temperatures. We obtainstructures in agreement with the available diffraction data and obtain a new viewof the molten Ge chalcogenides. We show that the anomalous volume contractionobserved in the liquid 30 K above the eutectic temperature corresponds to asignificant change of the Ge–Te partial structure factor. The detailed structuralanalysis shows that volume variations observed upon melting in Ge15Te85, as inliquid GeSe and GeTe, can be explained in terms of the competition betweentwo types of local environment of the germanium atoms. A symmetricalcoordination octahedron is entropically favoured at high temperature, whilean asymmetrical octahedron resulting from the local manifestation ofthe Peierls distortion is electronically favoured at lower temperatures.

Re-entrant peierls distortion in IV–VI compounds

At room temperature, the local structure of crystalline group V elements (As, Sb,…) and their IV–VI isoelectronic compounds (GeSe,…) is governed by a Peierls distortion of the simple cubic or NaCl structure [1] which is a symmetry breaking electronic instability. The morphology of the distortion is determined by the filling ratio of the p-band; for a half-filled p- band, the sixfold coordination becomes 3 (short, covalent) +3 (long, van der Waals). In general, at high temperature, the structure recovers its higher coordination number. Neutron diffraction experiments have been made in the liquid state at λ=0.7 Å. It is observed that the Peierls distortion is still present in the liquid for most IV–VI compounds. This behavior is observed and discussed for a series of Sn and Ge chalcogenides: SnS, SnSe, GeS, GeSe and GeTe and their temperature evolution is discussed. GeSe and GeTe show an interesting re-entrant phase behavior. The heaviest IV-VI compound SnTe does not show a distorted state. We demonstrate that the hardness of the repulsive potential is a key parameter in this mechanism.

A study of network dimensionality in chalcogenide glass by low frequency Raman scattering

We have measured low frequency Raman spectra in Ge chalcogenide glasses. The dependence of spectral dimensionality of stretching fracton, d ̃ s , on the sample composition is obtained from low frequency reduced Raman spectrum and is correctly correlated to the actual network dimensionality of Ge chalcogenide glasses.The value of d ̃ s is supposed to be a parameter representing the degree of localization of lattice vibrations and an indicator of network dimensionality of network glasses. The findings on the interpretation of d ̃ s should be applied in future research of disordered systems.

Band-structure calculation for A4B6 layered crystals by the equivalent-orbital linear combination of atomic orbitals method

The band structures of GeSe, GeS, SnSe and SnS layered semiconductor compounds have been calculated by the EO LCAO method in a single-layer approximation. The Madelung energy of electron in the field of all lattice ions was taken into account. The results of band-structure and density-of-states calculations are in good agreement with earlier calculations carried out by the pseudopotential method. It is shown that, unlike Ge chalcogenides, the minimal energy gap for Sn chalcogenides is situated on the symmetry line Lambda (k, 0, 0) and not on V(0, 0, k). The effective masses of electrons and holes near band extrema have been calculated.

Photostructural changes in bulk chalcogenide glasses: An EXAFS study

EXAFS studies of reversible photostructural changes in bulk As and Ge chalcogenide glasses have been performed. The compositions studied were Ge33Se67, Ge40Se60, Ge40S60, As40S60 and As43S57. The magnitude of the photostructural change in both germanium and arsenic systems is larger in off-stoichiometry samples. In the AsS system the structural change is characterised by an increase in the AsSAs bond angle, whereas in the GeS (Se) systems a bond-breaking mechanism dominates. These data are consistent with neutron scattering and electron spin resonance data.

AN EXAFS STUDY OF Bi DOPING IN CHALCOGENIDE GLASSES

The role of Bi in causing a transition from p-type to n-type electrical behaviour in Ge chalcogenide glasses has been studied by EXAFS. It is found that the coordination number of the Bi is 3, independent of Bi content, but that the Debye-Waller factor increases by a factor of two at the p-n transition.

Mechanism for doping in Bi chalcogenide glasses.

The structural environment of Bi incorporated in Ge chalcogenide glasses has been found from extended x-ray absorption fine-structure data to consist of threefold coordination. An increase by a factor of 2 in the Debye-Waller factor occurs as the carrier type changes from $p$ to $n$. A mechanism involving the suppression of positively charged structural defects, and the consequent unpinning of the Fermi level, by the formation of partially ionic Bi-chalcogen bonds is proposed to account for the doping effect.

Defect states in Ge chalcogenides observed by photoluminescence and ESR

Photoluminescence studies of melt-quenched Ge x Se 1− x (0.2 < x < 0.43) glasses reveal two unresolved components with broad linewidths and large Stokes shifts. The lines have differing excitation spectra, composition dependences and decay lifetimes. Two distinct singly occupied localized states are also observed by optically induced ESR, and are distinguished by different composition dependences, microwave saturation, excitation wavelength dependences and annealing temperatures. These states are interpreted as defect states with negative correlation energy, originating from Ge and Se atoms. The annealing properties of luminescence fatigue and induced ESR indicate that at least one of the defects is common to both experiments. The annealing kinetics of ESR is reported and supports a model of recombination by tunnelling between singly occupied sites. A large increase in the luminescence linewidth is observed at the composition Ge 37Se 63. It is suggested that the reason is an anomalously large disorder potential that originates from the introduction of GeGe bonding states at the valence band edge.

Waveguiding properties of (Se,S)-based chalcogenide glass films and some applications to optical waveguide devices

Fundamental waveguiding properties of As-Se-S-Ge chalcogenide glass films and some applications to passive waveguide components are presented. The refractive index of the chalcogenide glass film annealed just below the glass transition temperature (200 degrees C) increases by light irradiation near the absorption edge. The refractive-index change amounted to 0.03 at 1.06 microm. The propagation loss of 0.4 dB/cm has been achieved at 1-microm thickness for As(40)Se(10)S(40)Ge(10) glass film. The photoinduced refractive-index change in the chalcogenide glass film was large enough to be applied to making passive waveguide components. The curved waveguide and optical directional coupler were designed as passive components.