Intimate Valence Alternation Pairs Research Articles

Mechanisms of the photodarkening in chalcogenide glasses: Strains or defects?

The photodarkening is observed in many chalcogenide glasses, while its mechanism remains controversial. There are few reports which discuss the whole process. We consider the mechanism through ab initio molecular-orbital calculations for the simplest systems, elemental S and Se clusters. The results manifest that valence-alternation pair (VAP) defects are unstable in the dark, thus it cannot be an origin. On the other hand, intimate VAPs can be quasi-stable, while those affect only band-edge states, which is contradictory with observed spectral modifications. By contrast, strains in van-der-Waals (VDW) distances are likely to broaden the lone-pair electron band. A refined bond-twisting model is able to trace such a process, including annealing stage. Quantitatively, the maximal red-shift of ∼0.1 eV can be related with quasi-stable strains containable in a medium-range volume of ∼1 nm3, which can be thermally released through atomic twisting motions over angular barriers with a height of ∼1 eV.

Conduction mechanism and dielectric properties of a Se80Ge20−x Cd x (x = 0, 6 and 12 at.wt%) films

AC conductivity and dielectric properties of Se80Ge20−x Cd x (0 ≤ x ≤ 12 at.wt%) in thin film forms are reported in this paper. Thin films were deposited from the prepared compositions by thermal evaporation technique at 10−5 Torr. The films were well characterized by X-ray diffraction, differential thermal analysis and energy-dispersive X-ray spectroscopy. The AC conductivity and dielectric properties have been investigated for the studied films in the temperature range 293–393 K and over a frequency range of 102–105 Hz. The experimental results indicate that both AC conductivity σ AC(ω) and dielectric constants depend on temperature, frequency and Cd content. The frequency exponent s was calculated, and its value lies very close to unity and is temperature independent. This behavior can be explained in terms of the correlated barrier hopping between centers forming intimate valence alternation pairs. The density of localized states N(E F) at the Fermi level is estimated. The activation energy ΔE(ω) was found to decrease with increasing frequency. The maximum barrier height W m for the studied films was calculated from an analysis of the dielectric loss e 2 according to the Guintini equation. Its values agree with that proposed by the theory of hopping of charge carriers over potential barrier as suggested by Elliott for chalcogenide glasses. The variation of the studied properties with Cd content was also investigated.

Vapour growth and characterization of beta indium sesquitelluride crystals

Physical Vapour Deposition (PVD) provides stoichiometric crystals of different morphology, depending upon the materials, geometry of ampoules, temperature profiles, growth parameters and kinetics of crystallization. The crystal forms such as needles, platelets and spherulites of beta indium sesquitelluride (β-In2Te3) were produced by controlling the temperature of source and growth zones. The X-Ray Diffraction (XRD) and chemical analysis of the spherulitic crystals confirmed zinc blende structure with beta phase. Their resistivity (135.16Ωcm) at room temperature (300K) was determined by van der Pauw method. The temperature dependence of DC conductivity was investigated using the conventional two-probe technique. The variation of dielectric constant (ε1) and dielectric loss (tanδ) with temperature has been studied for different frequencies (1kHz–1MHz). The AC conductivity, σac(ω) was found to vary with angular frequency as ωs, where s is the frequency exponent. The values of s lie very close to unity and show a slight decrease with increase in temperature, which indicate a Correlated Barrier Hopping (CBH) between centres forming Intimate Valence Alternation Pairs (IVAP). The activation energy for conduction ranges from 0.187eV to 0.095eV. The microhardness of β-In2Te3 spherulites is found to be 353.5kg/mm2, which is higher than that of other semiconducting chalcogenides. The results thus obtained on crystals grown from vapour phase open up ample possibilities for radiation detector applications.

Electrical Conductivity and Dielectric Properties of Se85Te15−x Sb x (x = 0 at.%, 2 at.%, 4 at.%, and 6 at.%) Thin Films

The alternating-current (ac) conductivity and dielectric properties of Se85Te15−x Sb x (x = 0 at.%, 2 at.%, 4 at.%, and 6 at.%) films are reported in this work. Thin films were deposited by thermal evaporation under base pressure of 10−5 Torr. The films were well characterized by x-ray diffraction, differential scanning calorimetry, and energy-dispersive x-ray spectroscopy. The ac conductivity and dielectric properties have been investigated for the studied films in the temperature range from 297 K to 333 K and over the frequency range from 102 Hz to 105 Hz. The experimental results indicate that the ac conductivity \( \sigma_{\rm{ac}} (\omega ) \) and the dielectric constant depend on temperature, frequency, and Sb content. The frequency dependence of \( \sigma_{\rm{ac}} (\omega ) \) was found to be linear with a slope lying very close to unity and is independent of temperature. This behavior can be explained in terms of correlated barrier hopping between centers forming intimate valence-alternation pairs. The density of localized states N(E F) at the Fermi level is estimated. The activation energy \( \Updelta E(\omega ) \) was found to decrease with increasing frequency. The dielectric constant e 1 and dielectric loss e 2 were found to decrease with increasing frequency and increased with increasing temperature over the ranges studied. The maximum barrier height W m for the studied films was calculated from an analysis of the dielectric loss e 2 according to the Guintini equation. The values agree with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The variation of the studied properties with Sb content was also investigated.

Double-walled carbon nanotube/polymer nanocomposites: Electrical properties under dc and ac fields

The electrical properties of the poly(methyl methacrylate:carbon nanotubes nanocomposites) have been investigated by direct current conductivity and complex impedance spectroscopy methods. The direct current conductivity results of the poly(methyl methacrylate):carbon nanotube as a nanocomposites show that the electrical conductivity property of the poly(methyl methacrylate) changes from insulating state to semiconducting state with incorporation of double wall carbon nanotube DWCNTs into insulating polymer matrix. The alternating current conductivity mechanism of the nanocomposites is controlled by the correlated barrier hopping mechanism. The correlated barrier hopping CBH model for intimate valence alternation pairs IVAP's describes the conduction mechanism of PMMA doped with (1%) DWCNTs, while correlated barrier hopping CBH model for non-intimate valence alternation pairs describes the conduction mechanism of PMMA doped with (5% and 8%) DWCNTs. The real part of the complex impedance decreases with the increase of the applied frequency which revealed that the PMMA:DWCNT nanocomposites behaves like semiconducting materials. The complex impedance Nyquist plots for PMMA doped with different concentration DWCNTs over are characterized by the appearance of a single semicircular arc whose radii of curvature decreases with increasing the temperature. Cole and Cole plots show the presence of temperature dependent electrical relaxation phenomena in the PMMA:DWCNT nanocomposites. The obtained electronic parameters confirm that PMMA:DWCNTs exhibit organic semiconductor behavior.

Electrical conductivity and dielectric properties of bulk glass Se 55Ge 30As 15 chalcogenide

Measurements of conductivity (AC and DC), dielectric constant ε′( ω) and dielectric loss ε′′( ω) have been made for bulk glassy samples of Se 55Ge 30As 15 alloy in the frequency range from 10 kHz to 10 MHz and within the temperature range from 350 to 500 K. The DC conductivity shows an activated behavior above 370 K, giving a conduction activation energy Δ E=0.83 eV. The AC conductivity σ ac ( ω) was found to be temperature independent and vary as ω s with s=0.994, where s is the frequency exponent. The charge-transfer process explained via CBH model with centers of intimate valence alternation pairs (IVAPs) type, giving a maximum barrier height W M=2.23 eV and density of localized states N ( E F)=1.67×10 20 cm −3. The dielectric constants behavior, reveal that the tested bulk Se 55Ge 30As 15 chalcogenide exists in the form of molecular dipoles which remain frozen at low temperatures and, as the temperature is increased, the molecules attain freedom of rotation at temperatures above 370 K. The experimental results also gave a direct evidence of the existence of a Debye-type relaxation having a wide distribution of relaxation times. The most probable value of a spread of relaxation times have been estimated as τ=4.13×10 −6 s.

Structural and electrical properties of thermally evaporated cobalt phthalocyanine (CoPc) thin films

Thin films of CoPc of various thickness have been deposited onto glass substrates using thermal evaporation technique at room temperature. The dark electrical resistivity measurements were carried out at different temperature range (298–423 K). An estimation of mean free path () of charge carriers in CoPc thin films and bulk resistivity, was attempted. Measurements of thermoelectric power confirm that CoPc thin films behave as p-type semiconductors. The ac conductivity (σac) has been investigated in the frequency range (102–106 Hz) and temperature range (298–407 K). σac is found to be proportional to ωs where s ≈ 0.879 which is frequency and temperature independence. The ac conductivity interpreted by the correlated barrier hopping (CBH) model with centers of intimate valence alternation pairs type with a maximum barrier height, WM ≈ 1.594 eV.

Photo-induced Metastable State of S8Clusters in Liquid Sulfur

AbstractIt is well known that liquid sulfur undergoes a polymerization transition from S8 rings to the mixture of S8 rings and polymeric chains at Tp=159°C. In addition to the transition, it has been known that there is a transformation from S8ring to a molecular species other than S8ring and polymeric chain below Tpsince 19thcentury. Such molecular species can also be obtained by the illumination of light. Several models of the molecular form have been presented. However, the conclusive model which explains all experimental results has not been settled yet. Recently we measured transient photo-induced optical absorption in liquid sulfur and Munejiri et al. performed first-principles molecular-dynamics simulations to clarify the microscopic origin of the experimental results. The results of the simulations presented a plausible molecular form as the molecular species, which is a tadpole-shaped S8 cluster. We show that the molecular form clearly explains the previous experimental results. A possibility of the formation of an intimate valence alternation pair in the small cluster is also discussed.

A Fourier-transform photoluminescence study of radiative recombination mechanism in chalcogenide glasses

In this paper, we report photoluminescence (PL) studies carried out using Fourier-transform method in glassy Se (a-Se) and Ge-based Ge 20Se 80 and Ge 20Se 70Te 10 glasses. It was observed that the PL spectra of a-Se as well as that of Ge-based glasses exhibit fine features. The details of these fine features are extracted by deconvoluting the experimental spectra and are explained based on phenomenological defect models. Utilizing the deconvoluted information, the radiative recombination transitions occurring at ‘valence-alternation pairs’ are distinguished from those occurring at ‘intimate valence-alternation pairs’ and are illustrated with the help of a configurational coordinate diagram representation.

Defects in liquid selenium

We use the data generated in recent first-principles molecular-dynamics simulations of liquid selenium at the temperatures 570, 870, and 1370 K to investigate defects in the liquid. The defects represent disruptions of the chain structure associated with onefold and threefold atoms ${(C}_{1}$ and ${C}_{3}$ defects). We stress that for a full understanding we need to analyze the defects from the viewpoints of atomic coordination, defect dynamics, and electronic structure, and we develop analysis techniques to do this. We find that localized electronic states at the top of the valence band and the bottom of the conduction band are associated with ${C}_{1}$ and ${C}_{3}$ defects, respectively. At 570 K, the concentration of defects is very low, and they exist as ${C}_{1}$ and ${C}_{3}$ defects in bound pairs (intimate valence-alternation pairs), but at high temperatures the defects are isolated and are mainly of ${C}_{1}$ type. The defect concentrations are used to determine the mean length of Se chains, which we find to be much smaller than the values deduced from NMR measurements at lower temperatures, but in reasonable agreement at 1370 K. Analysis of the defect dynamics shows that the residence time of defects on individual atoms becomes extremely short---comparable with the vibrational period---at high temperature.

Evidence for valence alternation, and a new structural model of amorphous selenium

A molecular-dynamics simulation has been carried out for amorphous selenium. The simulation used 64 atoms in a constant volume simple cubic cell. The pair correlation function, g( r), and structure factor, S( Q), were computed and compared with experimental and previous theoretical studies. The average coordination number is exactly 2. Only one defect, an intimate valence alternation pair type defect was produced. The electronic density of states and the localization of the electronic state also manifest some properties of valence alternation pair defect type. The results give new evidence for valence alternation in a-Se.

Electronic and Lattice Structures of Isolated Se Chains and Defects in Them. II

Electronic and lattice structures of terminal and internal defects in a Se chain are systematically studied with the semi-empirical model developed previously. Structures of the defects are as follows: negative ion 1 C 1 - , a little altered helix; neutral radical 2 C 1 0 , and positive singlet ion 1 C 1 + , planer structures with four atoms; positive triplet ion 3 C 1 + , an acute triangle; n-type polaron 2 P - , locally relaxed helix; p-type polaron 2 P + and self-trapped triplet exciton 3 Ex, planer zig-zag structures with five atoms; intimate valence alternation pair (IVAP), a ramification of a Se atom. The IVAP is of C 3 + -C 1 - type. Interactions of a Se chain with Li, H and O are studied as a model of the effects of the wall on a Se chain enclosed in a mordenite channel (M-Se). A Se chain makes unusual charge transfer complexes with Li. Hydrogen binds to an internal Se making it C 3 + type or blocks a terminal Se. Based on these calculations we propose a model for photo-induced events in M-Se.

Experimental evidence for the existence of intimate valance alternation pair defects in GeO 2 glasses

Thermally stimulated depolarization current (TSDC) technique was employed to study the electrical properties of a series of high purity GeO 2 glasses with principal impurities being Cl(1600ppm), OH(5–200 ppm), and Na(3–30 ppm). A single TSDC peak was observed the intensity of which was invariant against changes in the concentration of Cl, OH and Na. This peak is attributed to the polarization caused by intrinsic dipolar defects in the glass structure. The concentration of these remains unchanged with variations in melt equilibration temperature of the glass (T φ ) and fluctuations in sodium, hydroxyl and chlorine contents. The concentration of dipoles computed from the experimental peak charge is in agreement with previous theoretical estimates of the concentration of intimate valence alternation pairs.

Xerographic properties of single- and double-layer photoreceptors based on amorphous selenium-tellurium alloys

Single- and double-layer xerographic photoreceptors were fabricated by vacuum deposition of a-Se1-xTex alloy films on aluminium substrates. Double-layer photoreceptors comprised of a thin (a few micrometres) a-Se1-xTex alloy serving as a photogeneration layer (PGL) on a thick (50 to 60 μm) a-Se layer acting as a charge transport layer (TL). Xerographic dark discharge experiments on a-Se films indicated that the decay of the surface potential over the time scale of observation (∼ 500 sec) is essentially due to bulk thermal generation of holes, and their consequent sweep-out and depletion. Hole emission occurs from discrete midgap localized states and it is field-assisted. When a-Se is alloyed with tellurium the dark discharge becomes more rapid due to an increase in both the volume density and energy spread of the midgap hole emission centres with tellurium concentration. In double-layer photoreceptors, with a tellurium content over 3 wt%, although the PGL was only a few micrometres in thickness, the dark discharge was mainly due to the tellurium-rich PGL. The repetition of the xerographic cycle over many cycles leads to the saturation of the surface residual voltage which was used to determine the concentration Nt of deep hole traps. For a-Se, Nt (1.9 x 1013cm−3) was found to be comparable to the concentration of midgap- hole emission centres, which suggested that amphoteric neutral centres (possibly intimate valence alternation pairs) are involved in hole capture and emission. The saturated residual voltage Vr∞ in a-Se1-xTex alloy films increased with the tellurium content up to 6 to 8 wt% Te due to tellurium-introduced deep traps. In double layers, the saturated residual voltage was more than that expected from the properties of monolayers, due probably to the presence of deep hole traps at the PGL-TL interface. Xerographic time-of-flight (XTOF) experiments confirmed the previous results from the electroded TOF experiments that tellurium addition increases the density of shallow traps. Combining the TOF data with the saturated residual voltage measurements gives a hole capture coefficient Ct of ∼ 1.2 x 10−8 cm3 sec−1 for pure a-Se. Chlorine doping of a-Se resulted in a very rapid dark discharge and a fall in the saturated residual voltage.

Radiative recombination in amorphous phosphorus

The authors have investigated radiative recombination in a-P at low temperatures using optically detected magnetic resonance (ODMR) and standard luminescence techniques. The ODMR results show that two distinct regions can be identified in the photoluminescence (PL) spectrum, corresponding to excitonic and pair emissions. The exciton emission has a maximum near 1.1 eV, and the pair emission band peaks near 1.4 eV in bulk a-P material. The results are compared with the existing PL and time-resolved spectroscopy (TRS) date on a-P, and a consistent model can be formed involving exciton and pair recombination at intrinsic centres. From arguments relating to the exciton-pair energy separation, it is inferred that intimate valence alternation pairs (IVAPS) form suitable candidates for the recombination centres.

A model for field-sensitive interface states

Oxygen derived point defects called Intimate-Valence-Alternation-Pairs (IVAP) apear to be able to account for the time, temperature, and field dependence of the generation of field-sensitive interface states. The theory and properties of IVAP defects in SiO2 are described. A model for the observed changes in the capacitance voltage characteristics of Metal-Oxide-Semiconductor structures based on the existence of these defects is proposed. Finally the model is compared to existing data from the literature on the buildup of interface states caused by charge injection or ionizing radiation.

Exciton and Pair Recombination at Intimate Valence-Alternation Pairs ina-As2S3

Optically detected magnetic resonance in $a$-${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ has shown that the emission consists of a high-energy triplet exciton recombination overlapping a low-energy pair process. The results are consistent with recombination at axial defects such as (${D}^{+}, {D}^{\ensuremath{-}}$) intimate valence-alternation pairs.

A theoretical study of defects in amorphous arsenic

Abstract Clusters of about 55 atoms have been built to model various defects which may be present in amorphous arsenic (a-As). These clusters have been relaxed and their electronic properties calculated by two complementary methods—by a self-consistent linear combination of atomic orbitals (LCAO) method, and by attaching Bethe lattices to the cluster surfaces and then calculating the electronic density of states (DOS) by the recursion method. Both methods are initially applied to a perfect 56-atom cluster to show that they are valid for calculating the DOS of a-As. They are then used to study the electronic states associated with isolated twofold and fourfold coordinated atoms, neighbouring twofold and fourfold coordinated atoms (‘intimate valence alternation pairs') and the vacancy. We have in addition examined the stability of the vacancy and find it is probably unstable. We therefore conclude that the optically induced E.S.R. signal observed in a-As is consistent with the presence of valence alternation pairs while the thermally generated E.S.R. signal is probably due to the presence of neutral twofold coordinated centres.

Paramagnetic centres associated with bonding defects in v-SiO2

Abstract Paramagnetic centres are produced in vitreous (v)-SiO2 by the trapping of photogenerated electrons and holes at the intrinsic bonding defect centres associated with over- and under-coordinated oxygen atoms, C3 + and C1- respectively. Capture of a hole by the C1- converts that centre to C1 0, the oxygen hole centre (OHC), whilst the capture of an electron by the C3 + centre converts it to a C3 0 which reconstructs forming the E′ centre, a threefold coordinated neutral silicon atom T3 0. The C3 +, C1- bonding defects occur as near neighbours in the network structure, so-called intimate valence-alternation pairs or I.V.A.P.s, so that an I.V.A.P. centre may trap either a hole or an electron, or both carriers. The unusual annealing behaviour of the OHC E.S.R. signal is shown to be related to centres which initially capture both an electron and a hole, and then convert from a spin-inactive configuration (T3 +, Cl −) to the spin-active configuration (T3 +, C1 0) through the thermal release of one electron.

Electrical and dielectric properties of As10Ge15Te75Ag x glasses

Abstract This paper analyses the effect of the addition of varying amounts of silver in concentrations from 0 to 10.0 at.% to As10Ge15Te75 glass on d.c. and a.c. conductivities, dielectric constant and various allied parameters in the temperature range 83-383 K and the frequency range from d.c. to 50 MHz. The activation energy of d.c. conductivity increases with silver content up to 1.0 at.% and shows a reversal in the trend with higher percentages (5.0 and 10.0 at.%). The a.c. conductivity and memory switching behaviour are not altered by the addition of silver. The dielectric data suggest a dipole-type relaxation mechanism. The dielectric data corroborate the observed conductivity data in suggesting a probable increase in Te-chain breakages leading to an increase in the number of charged dangling bonds with silver content up to 1.0 at.% and, at higher percentages of silver, alloy formation occurs resulting in the reversal of the trend. Most of the data can be explained by the ‘charged dangling bond’ model of Mott, Davis and Street (1975) and ‘valence-alternation pairs’ (VAPs) and ‘intimate valence-alternation pairs’ (IVAPs) suggested by Kastner, Adler and Fritzsche (1976).