Drift mobilities in amorphous AsSeTe

The electron and hole drift mobilities in As_xSe_100?x?yTe_y (0 ? x ? 7.1, 0 ? y ? 20.9 at.%) evaporated thin amorphous films and their temperature dependence were measured by means of the time-of-flight technique. The electron mobility decreases with increasing As content, while the hole mobility decreases upon addition of 6.4 at.% Te, then remains almost similar on further addition of Te. These effects of As and Te on the drift mobilities of carriers were shown to be independent of each other.     

Atomic structure and chemical order in binary Ge–Te and As–Te glasses: A Te K-edge X-ray absorption fine structure spectroscopic study

The nearest-neighbor coordination environments of Te atoms in Ge_xTe_100?x glasses with x = 15 and 20 and in As_xTe_100?x glasses with 40 ? x ? 65 have been studied with Te K-edge EXAFS spectroscopy. The average coordination number of Te atoms in all glasses is found to be ～2.0 and no violation of the 8-N rule is observed. The compositional makeup of the first coordination shell of Te atoms indicates that chemical order is largely preserved in both glass-forming binary systems. Sudden changes in the Te coordination environment and violation of chemical order are observed at the stoichiometric As₄₀Te₆₀ glass implying formation of a constrained network. The compositional dependence of the physical properties in both systems can be correlated to short-range chemical order.     

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.     

ESR study of Mn2+ in GeTe and GeTeSi glasses

The electron spin resonance spectra of Mn²⁺ ions have been studied in Ge_xTe_100?x with x = 15, 17.5 and 20, and Ge_20?xTe₈₀Si_x with 0 ?x? 20. All samples are found to exhibit six hyperfine lines centered at g = 4.3 with hyperfine interaction constant A = 56 × 10^?4cm^?1. The g = 4.3 line is interpreted as being caused by Mn²⁺ ions incorporated in the amorphous network and surrounded by four Te atoms in an arrangement of orthorhombic symmetry. Some of the samples of GeTe show a g = 2.0 line. This line also appears after heat treatment in air at temperatures above the glass transition temperature. It is concluded that the g = 2.0 line is caused by Mn²⁺ ions in phase separated microcrystalline or concentrated regions of MnO in the glass.     

Far-infrared transmission and bonding arrangement in Ge10Se90−xTex semiconducting glassy alloys

The far-infrared spectra of Ge₁₀Se_90−xTe_x where x = 0, 10, 20, 30, 40, 50 glassy alloys were measured in the wavenumber region 50-650 cm⁻¹ at room temperature. The results were explained in terms of the vibrations of the isolated molecular units. The addition of Te in Ge₁₀Se₉₀ has shown the appearance of GeTe₂ and GeTe₄ molecular units and vibrations of Se-Te bond as Se_8−xTe_x mixed rings. The assignment of various absorption bands has been made on the basis of absorption spectra of pure Se, binary Ge-Se, Ge-Te, Se-Te and ternary Ge-Se-Te glassy alloys. The far-infrared transmission spectrum has been found to shift a little towards lower wavenumber side with the addition of Te content to Ge₁₀Se₉₀. The addition of Te to Ge-Se system replacing Se has found to reduce the Se-Se bonds and Ge-Se bonds and leads to the formation of Se-Te, Ge-Te and Te-Te bonds.     

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.     

Thermally-induced structural and electrical effects in GeTe-based amorphous alloys

The thermal, structural electrical properties of bulk glasses based on GeTe compositions near the binary eutectic, Ge₁₅Te₈₅, are studied. Information regarding the non-crystalline state and the transformation from the non-crystalline to the crystalline state is reported. The particular alloys studied represent binary (Ge₁₇Te₈₃), ternary (Ge₁₅Te₈₀As₅) and quaternary (Ge₁₅Te₈₁Sb₂S₂) compositions. Structural information is obtained using X-ray diffraction techniques and density measurements. Thermal data are reported from differential scanning calorimetry (DSC), thermogravimetry (TGA) and mass spectrometry results. The electrical conductivity is measured as a function of temperature and, on the ‘as-prepared’ glasses, shows semi-conducting behavior with activation energies, E, of 0.43–0.48 eV. DSC, TGA and X-ray powder diffraction patterns indicate the samples crystallize as Te and GeTe in a two-step process, and melt at the binary eutectic temperature. The binary vaporizes as Te and GeTe in a two-step process. GeTeAs and GeTeSbS vaporize by essentially the same mechanism, with As evaporating (<300°C) before the Te, and Sb and S evaporating (420–480°C) after the Te but before GeTe. The results show that the properties of the bulk ‘as-prepared’ glasses are strikingly similar. Thermally-induced changes in the structural and electrical properties of bulk samples have been examined following a series of anneals (5 h, vacuum) at temperatures from 111°C to 190°C (glass transition temperature $\text{?125?133°}\text{C}$; crystallization temperature $\text{?206?228°}\text{C}$ as determined by DSC). DSC, TGA and mass spectrometry results have been correlated to electrical and structural changes. Results show that crystalline Te nucleates at the surface and forms a conductive surface layer. The conductivity of this surface layer is nearly temperature independent with E ≈ 10^?2 eV for all three alloys. Crystallization and the associated electrically conductive regions extend into the bulk material with further annealing. In these disordered alloys the additives As and Sb + S apparently do not act as electrical dopants in the sense of affecting the conductivity activation energy. The additives Sb + S however do retard crystallization of GeTe. The secondary crystallization product, GeTe, apparently changes the conduction mechanism to either a metallic or degenerate semiconductor type behavior.     

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.     

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.     

Xerographic spectroscopy of gap states in Se-rich amorphous semiconductors review

One of the most important parameters which determine the performance of many modern devices based on amorphous semiconductors is the drift mobility-lifetime product, μτ. There has been much interest in determination of charge-carrier ranges in amorphous semiconductors by various measurement techniques. Although the mobility, μ, can be measured by the conventional time-of-flight transient photoconductivity technique, the determination of the lifetime, τ, is often complicated by both experimental and theoretical limitations. The present article provides an overview of xerographic measurements as a tool for studying the electrical properties of amorphous semiconductors. First, details of the experimental set-up are discussed. Thereafter, the analysis and interpretation of dark discharge, the first cycle residual potential, cycled-up saturated residual potential are considered. It is shown that from such measurements the charge-carrier lifetime, τ, the range of the carriers, μτ, and the integrated concentration of deep traps in the mobility gap can be readily and accurately determined. Xerographic measurements on Se-rich amorphous photoconductors have indicated the presence of relatively narrow distribution of deep hole traps with integrated density of about 10¹³ cm^? 3. These states are located at ~ 0.85 eV from the valence band. A good correlation was observed between residual potential and the hole range, in agreement with the simple Warter expression. The capture radius is estimated to be r_c = 2–3 Å. Since r_c for pure a-Se and a-As_xSe_1 ? x is comparable to the Se–Se interatomic bond length in a-Se, it can be suggested that deep hole trapping centers in these chalcogenide semiconductors are neutral-looking defects, possibly of intimate valence-alternation pair (IVAP) in nature. The absence of any electron spin resonance signal (ESR) at room temperature seemed to be a strong argument in favor of this suggestion. Finally, photoinduced effects on xerographic parameters are discussed. It has been shown that photoexcitation of a-As_xSe_1 ? x amorphous films with band-gap light alters deep hole and electron states. During room-temperature annealing photosensitized states relax to equilibrium. Recovery process becomes slower with increasing As content. Qualitative explanation of the observed behavior may be based on associating the deep states with C₃⁺ and C₁^?intimate-valence-alternation–pair (IVAP) centers.     

A composition dependent thermal behavior of GexSe35−xTe65 glasses

Alternating differential scanning calorimetric (ADSC) studies have been performed to understand the thermal behavior of bulk Ge_xSe_35?xTe₆₅ glasses (17 ? x ? 25); it is found that the glasses with x ? 20 exhibit two crystallization exotherms (T_c1 & T_c2). On the other hand, those with x ? 20.5, show a single crystallization reaction upon heating. The exothermic reaction at T_c1 has been found to correspond to the partial crystallization of the glass into hexagonal Te and the reaction at T_c2 is associated with the additional crystallization of rhombohedral GeTe phase. The glass transition temperature of Ge_xSe_35?xTe₆₅ glasses is found to show a linear but not-steep increase, indicating a progressive, but a gradual increase in network connectivity with Ge addition. It is also found that T_c1 of Ge_xSe_35?xTe₆₅ glasses with x ? 20, increases progressively with Ge content and eventually merges with T_c2 at x ≈ 20.5 (〈r〉 = 2.41); this behavior has been understood on the basis of the reduction in TeTe bonds of lower energy and increase in GeTe bonds of higher energy, with increasing Ge content. Apart from the interesting composition dependent crystallization, an anomalous melting behavior is also exhibited by the Ge_xSe_35?xTe₆₅ glasses.     

The field effect in amorphous chalcogenides: An investigation of localized states and electronic transport

The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As₂Te₃, field effect screening by localized states at the Fermi level at low temperatures (～ 10¹⁹ cm^?3 eV^?1) and by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ～320 K; (2) in As₂SeTe₂, screening by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) with strongly type conductivity; (3) in As₂Se₂Te, screening by localized states at the Fermi level (～ 10¹⁹ cm^?3 eV^?1) with strongly p-type conductivity; and (4) in Sb₂Te₃, a very high density of localized states at the Fermi level (～ 2 × 10²⁰ cm^?3 eV^?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As₂Te₃ is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.     

Electrical properties of glassy AsxGe10Te90−x alloys

This paper analyses the electrical properties of glassy alloys of As_xGe₁₀Te_90?x, while reporting the conductivity and dielectric constant of As₅Ge₁₀Te₈₅ and As₁₅Ge₁₀Te₇₅ compositions in the temperature range 77–383 K and the frequency range from dc to 5 MHz. The dc conductivity has been shown to be of the form

The ratio σ₀₁/σ₀₂ is of the order of 10⁶. ΔE₁, the higher temperature activation energy, is dependent on the composition, while ΔE₂, the lower temperature activation energy, is less dependent on the composition. The dielectric constant has been found to be independent of temperature and frequency up to about 253 K. However, at higher temperatures, it becomes activated and proportional to log ω.Some common features of As_xGe₁₀Te_90?x are a kink in dc conductivity, a ω^0.8 relationship for ac conductivity, no evidence of variable-range hopping at low temperatures, field-dependent conductivity and memory switching. The data can be interpreted in terms of the dangling-bond theory of Mott and his collaborators. A high density of states of the order of 10²⁰eV^?1 cm^?3 near the Fermi level may be expected.     

Experimental studies of phase equilibria in high-temperature ternary immiscible metallic melts

Shear viscosity measurements were carried out for a liquid immiscible ternary system In–Se–Te in a temperature range from the monotectic to about 1200 K using an oscillating-cup viscometer. It was revealed that the ternary In₈₀Se_xTe_20−x (0 ⩽ x ⩽ 20 at.%) system may be considered as a set of quasibinary In–(Se/Te) alloys of almost critical concentrations. A variation of the Se to Te ratio at constant content of In changes the properties of coexisting liquids and affects the binodal temperature. The critical parameters describing the peculiarities of viscosity behavior in the vicinity of the critical point are evaluated. The analysis based on the dynamic theory of phase transitions for viscosity in the phase separation region is proposed. The results are compared with available data for immiscible dielectric solutions and metallic melts.     

Electrical properties of semi-insulating CdTe0:9Se0:1:Cl crystal and its surface preparation

CdTe_0.9Se_0.1:Cl crystals doped with chlorine at 5×10¹⁷ cm^?3 level were grown by the vertical Bridgman method. The composition of Se throughout the ingot was nearly constant at x=0.110±0.016. The electrical resistivity of CdTeSe:Cl was 4.5×10⁹ Ω cm. Chemical etchants were employed to obtain stoichiometric and flat surfaces for electrode deposition, and the effects of the etchants on CdTeSe surfaces were analyzed by photoluminescence (PL) and AFM with different bromine concentrations and etching times. The mobility-lifetime products of electron and hole in CdTeSe:Cl crystals were of the order of ～10^?2 cm²/V and its values are greater than those for CdTe crystals. The energy resolution of a CdTeSe:Cl detector was tested using a ²⁴¹Am radioactive source.     

Enthalpic structural relaxation in Te-Se glassy system

Differential scanning calorimetry (DSC) measurements were used to study enthalpic structural relaxation in the Te_ySe_(1−y) glassy system. The examined compositions were y = 0.1, 0.2 and 0.3. Single set of Tool-Narayanaswamy-Moynihan parameters was obtained from the curve-fitting procedure for each studied glass. A comparison with our previous measurements on a-Se was made and the development of particular TNM parameters with increasing Te content was discussed in terms of changes in molecular structure of the material. The curve-fitting results were further verified by several independent non-fitting methods.     

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.     

Photoelectrical properties of (Sb15As30Se55)100−xTex (0 ⩽ x ⩽ 12.5 at.%) thin films

This paper reports photoelectrical properties of (As₃₀Sb₁₅Se₅₅)_100?xTe_x amorphous chalcogenide films (0 ? x ? 12.5 at.%) through measurements of ‘steady state’ and ‘transient’ photocurrents. The composition dependence of the steady state photocurrent at room temperature shows that the photoconductivity increases while the photosensitivity decreases with increasing Te content. A study of photoconductivity of (As₃₀Sb₁₅Se₅₅)_100?xTe_x at different levels of light intensity reveals that, the photoconductivity increases exponentially with increase in light intensity. The Photocurrent (I_ph) when plotted against light intensity (G) follows a power law (I_ph = G^γ) the exponent γ for (As₃₀Sb₁₅Se₅₅)_100?xTe_x films has been found nearly 0.5 suggesting bimolecular recombination. The transient photoconductivity shows that the lifetime of the carrier decreases with increasing the light intensity. This decrease suggests that the photoconductivity mechanism in our samples was controlled by the transition trapping processes. The increase of Te content results in a monotonic decrease in the band gap and the free carrier life time of (As₃₀Sb₁₅Se₅₅)_100?xTe_x thin films. These results were interpreted on the basis of the chemical-bond approach.     

Electrical properties of a-Se85−xTe15Snx thin films

Se–Te alloys are an important system of chalcogenide glasses from application point of view. The incorporation of Sn additive alters the electrical properties of these alloys. The conductivity measurements have been done on the thin films of a-Se_85−xTe₁₅Sn_x (x = 0, 2, 4, 6 and 10 at.%) deposited using vacuum evaporation technique. Both dark (σ_d) and photoconductivity (σ_ph) show a maximum for x = 6 at.% of Sn, which, decreases on further Sn addition to the binary Se–Te alloy. The dark activation energy (ΔE_d) shows a minimum for x = 2 at.% of Sn, but increases on further Sn addition. There is a sharp decrease in photosensitivity (σ_ph/σ_d) on Sn addition to Se₈₅Te₁₅ alloy. The charge carrier concentration (n_σ) calculated with the help of dc conductivity measurements also show a maximum at x = 6 at.% of Sn. The results are explained on the basis of increase in the density of localized states present in the mobility gap on Sn incorporation.     

Impurity band conduction effects in liquid phase epitaxial Te-doped GaP grown from tin solution

GaP layers were grown by liquid phase epitaxy from tin solution on semi-insulating GaAs substrates with various amounts of Te added to the melt (x_Te = 10⁻⁴ …︁ 3 · 10⁻²). The Sn and Te concentrations in the layers were determined by chemical analysis as function of x. An analysis of the electrical measurements shows that the carrier transport in the layers is essentially determined by impurity band conduction effects.