Time-of-flight and photoconductivity studies of a-As2Se3 films

We have performed time-of-flight and transient photoconductivity experiments on similarly prepared films of a-As₂Se₃. When care is taken to avoid contact-induced effects in time-of-flight measurements, we find that both experiments give consistent results. We conclude that dispersive transport in a-As₂Se₃ is due to multiple trapping in an exponential distribution of localized states.     

Electrical transport in a-As2Se3 containing metallic impurities

Significant changes of the hole drift mobility are observed in a-As₂Se₃ containing concentrations less than ～ 10²⁰ cm^?3 of metallic impurities (Mn, Fe, Ni, Cu, Zn, Ga, In, Tl and Na). Depending upon metal concentration the results suggest a modification of the hopping transport channel in Ga, In and Tl doped samples and the buildup of new traps that are either isoenergetic with the intrinsic trap population (Tl, Ga, In, Cu) or lie deeper in the gap (Mn, Fe, Ni, Cu). Trap-limited hopping transport provides a consistent explanation of the experimental data.     

Electric field dependent photodecomposition of a-As2Se3

Electric field dependent photodensification of electroded, thin a-As₂Se₃ films is observed in the field range ?E?1.4 × 10⁵ V/cm. The applied field decreases the amount of ultimate photodensification, and, a non-transient increase in conductivity of the film occurs in the illuminated area. The field dependent optical absorption coefficient, α(R), cannot account for the decrease in photodensification. An extension to the concept of bond breaking in molecular solids is proposed to account for the observed phenomenon.     

A limit on the site inequivalence in g-As2Se3

X-ray photoelectron spectra of As and Se 3d in g-As₂Se₃ exhibit two distinct As sites, but only an excess Gaussian width of 0.4 eV for the Se sites. The second As site is attributed to the presence of As₄Se₄ clusters in agreement with earlier Raman spectroscopy, the broadening of the Se lines to the site inequivalence of the orpimental rafts.     

Geminate and non-geminate recombination in a-As2Se3

Combined zerographic measurements and a delayed-collection field technique has been applied to a-As₂Se₃. The data shows that the zero-field quantum efficiency is 0.45±0.02. The measured field and temperature dependence gives a best fit to the Onsager theory with $\text{r}{}_0\text{=80}\text{A}\text{?}$ for Φ₀=1. Non-geminate recombination was studied using a delayed-collection field technique as a function of the time delay of the collection field. The results indicate a remarkably long recombination lifetime≈1 sec at room temperature which is diffusion-controlled.     

Glass formation in the As2Se3-As2Te3-Sb2Te3 system

Chalcogenide glasses from the As₂Se₃-As₂Te₃-Sb₂Te₃ system were synthesized for the first time. The glass-forming region was determined by X-ray diffraction and electron microscopic analyses.The basic physicochemical parameters such as density (d), microhardness (HV) and temperatures of phase transformations (glass transition T_g, crystallization T_cr and melting T_m) were measured. Compactness and some thermomechanical characteristics such as volume (V_h) and formation energy (E_h) of micro-voids in the glassy network as well as the elasticity module (E) were calculated. The glass-forming ability was evaluated according to Hruby's criteria (K_G). The correlation between composition and properties of the (As₂Se₃)_x(As₂Te₃)_y(Sb₂Te₃)_z glasses was established and comprehensively discussed.     

Dielectric parameters in Se70Te30 and Se70Te28Zn2 chalcogenide glasses

Temperature and frequency dependence of dielectric constant (ε′) and dielectric loss (ε″) are studied in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈Zn₂. The measurements have been made in the frequency range (8-500 kHz) and in the temperature range 300 to 350 K. An analysis of the dielectric loss data shows that the Guintini's theory of dielectric dispersion based on two-electron hopping over a potential barrier is applicable in the present case.No dielectric loss peak is observed in glassy Se₇₀Te₃₀. However, such loss peaks exist in the glassy Se₇₀Te₂₈Zn₂ in the above frequency and temperature range. The Cole-Cole diagrams have been used to determine some parameters such as the distribution parameter (α), the macroscopic relaxation time (τ₀), the molecular relaxation time (τ) and the Gibb's free energy for relaxation (ΔF).     

Time resolved photoluminescence spectra of a-As2S3

Time resolved photoluminescence (PL) spectra of a-As₂S₃ are reported in the range 0–100 nsec after the excitation light pulse. After a very careful correction for apparatus spectral response, there is no large shift of the PL line as previously reported, but only an increase of the high energy side of the PL spectrum at short times. These results are discussed in terms of models with or without rearranged point defects.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Jahn-Teller band transition in La3S4 and La3Se4

La₃S₄ and La₃Se₄ undergo a cubic to tetragonal phase transformation at a temperature of 103 and 70 K respectively, the c/a ratio is 0.984 for La₃S₄ and 0.987 for La₃Se₄. In these compounds the conduction electron Fermi energy happens to be close to a band structure anomaly which drives the phase transition. We find some indication, that the anomaly might be of ?-type.     

Cation ordering and crystal structures in AGa2X4 compounds (CoGa2S4, CdGa2S4, CdGa2Se4, HgGa2Se4, HgGa2Te4)

Single crystals of some AGa₂X₄ compounds (CoGa₂S₄, CdGa₂S₄, CdGa₂Se₄, HgGa₂Se₄, HgGa₂Te₄) were prepared by chemical vapour deposition and flux method.The X-ray structural investigations indicated blende or defect chalcopyrite structures.A simple relationship is suggested between the c/a ratio and the cationic sublattice ordering.     

Interrelation between glass transition and reversible photostructural change in a-As2S3

The optical absorption edge has been observed to make a parallel shift to lower energy in a-As₂S₃ as the glass transition temperature, T_g, increases. The structural difference corresponding to different T_g and reversible photostructural change give the same linear relation between the edge shift and the volume change. The saturated position of the edge after long time illumination or annealing is determined only by the condition of last treatment and is independent of previous history. These results suggest that both structural changes are of the same kind and that in the glass there exists an equilibrium state under illumination, if crystallization is avoided.     

Magnetic and transport properties of Mn-doped Bi2Se3 and Sb2Se3

We report on the single crystal growth and thermoelectric and magnetic properties of Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals prepared by the temperature gradient solidification method. The composition and crystal structure were determined using electron probe microanalysis and θ–2θ powder X-ray diffraction studies, respectively. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the undoped sample due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin-glass and paramagnetic properties, respectively.     

Structure formation and mechanisms of DC conduction in thermally evaporated nanocrystallite structure ZnIn2Se4 thin films

ZnIn₂Se₄ is of polycrystalline structure in as synthesized condition. It transforms to nanocrystallite structure of ZnIn₂Se₄ film upon thermal evaporation. Annealing temperatures influenced crystallite size, dislocation density and internal strain. The hot probe test showed that ZnIn₂Se₄ thin films are n-type semiconductor. The dark electrical resistivity versus reciprocal temperature for planar structure of Au/ZnIn₂Se₄/Au showed existence of two operating conduction mechanisms depending on temperature. At temperatures >365 K, intrinsic conduction operates with activation energy of 0.837 eV. At temperatures <365 K, extrinsic conduction takes place with activation energy of 0.18 eV. The operating conduction mechanism in extrinsic region is variable range hopping. The parameters such as density of states at Fermi level, hopping distance and average hopping energy have been determined and it was found that they depend on film thickness. The dark current–voltage characteristics of Au/n-ZnIn₂Se₄/p-Si/Al diode at different temperatures ranging from 293–353 K have been investigated. Results showed rectification behavior. At forward bias potential <0.2 V, thermionic emission of electrons from ZnIn₂Se₄ film over a potential barrier of 0.28 V takes place. At forward bias potential >0.2 V, single trap space charge limited current is operating. The trap concentration and trap energy level have been determined as 3.12×10¹⁹ cm⁻³ and 0.24 eV, respectively.     

Selenium dioxide: Vibrational analysis of the 3130 Å1B2-X̃1A1 absorption system

The B absorption system of the three isotopic species ⁷⁸Se¹⁶O₂, ⁸⁰Se¹⁶O₂, and ⁷⁸Se¹⁸O₂ have been comparatively studied in the vapor phase. The 0_0⁰ band is at 31955.0/31957.4/31963.9 cm^?1, respectively. The main vibrational structure is due to 1_0ⁿ2_0^m progressions, with maximum intensity at n ～ 6. The stronger progressions are those with m = 0, 1, 2, 3. The progressions are severely perturbed. The molecule is bent in both of the combining electronic states. In the ground state for the (80, 16) species, ν₁″(a₁) = 922.6, ν₂″(a₁) = 372 cm^?1. In the electronically excited state, identified as ¹B₂, ν₁′, and ν₂′ have the approximate values 649 and 258 cm^?1, respectively. Evidence is presented in favor of a double-minimum potential function with respect to Q₃′, the band 3_0¹ being observed with appreciable intensity. The height of the potential barrier is about 600 cm^?1.     

Dielectric relaxation and delayed-collection field experiments in amorphous semiconductors

Delayed-collection field (DCF) experiments, reported recently, were interpreted by the authors as giving evidence for rapid recombination in several amorphous semiconductors including aSi:H and a-As₂Se₃. An analysis is made of the DCF experiment for such materials which display dispersive transport because of multiple trapping (MT). It is shown that when the delay time is long compared with the zero-delay transit time, the effective transit time becomes equal to the delay time. Under such circumstances, a decrease of the collected charge may be a result of MT alone, and does not necessarily provide evidence for rapid recombination. It is shown that the process of dielectric relaxation is modified in a MT system. The mechanism by which the electric field is screened is closely related to the DCF experiment.     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

Electronic conductivity and thermopower of Ca2NaMg2V3O12−x garnet

Results are reported from conductivity and thermoelectric power measurements on partially reduced Ca₂NaMg₂V₃O_12?x, with x < 5.10^?2, at temperatures of 300–1100 K. The conductivity is thermally activated with activation energies 0.26 ? E_a ? 1.28 eV for differently reduced samples. The thermopower is temperature independent in the 300–800 K region. These results are shown to be consistent with the adiabatic hopping of small polarons localised on the vanadium sublattice, where defect interactions result in the formation of multiple conduction pathways.     

Synthesis and characterization of new LnxSb2−xSe3 (Ln: Yb, Er) nanoflowers and their physical properties

New Ln_xSb_2−xSe₃ (Ln: Yb³⁺, Er³) based nanomaterials were synthesized by a co-reduction method. Powder XRD patterns indicate that the Ln_xSb_2−xSe₃crystals (Ln=Yb³⁺, Er³⁺, x=0.00-0.12) are isostructural with Sb₂Se₃. The cell parameters b and c decrease for Ln=Er³⁺ and Yb³⁺ upon increasing the dopant content (x), while a increases. SEM images show that doping of the lanthanide ions in the lattice of Sb₂Se₃ generally results in nanoflowers. UV-vis absorption and emission spectroscopy reveals mainly electronic transitions of the Ln³⁺ ions in case of Yb³⁺ doped nanomaterials. Emission spectra of doped materials, in addition to the characteristic red emission peaks of Sb₂Se₃, show additional emission bands centered at 955 nm, originating from the ²F_7/2→²F_5/2 transition (f-f transitions) of the Yb³⁺ ions. DSC curves indicate that Sb₂Se₃ has the highest thermal stability. The temperature dependence of the electrical resistivity of doped-Sb₂Se₃ with Yb³⁺ and Er³⁺ was studied.     

Dielectric dispersion in PbO-Sb2O3-As2O3:V2O5 glasses

Dielectric properties, viz. dielectric constant ε′, loss tan δ and a.c conductivity σ_ac (over a wide range of frequency and temperature) and dielectric breakdown strength of PbO-Sb₂O₃-As₂O₃ glasses doped with V₂O₅ (ranging from 0 to 0.5 mol%) are studied. Analysis of these results, based on optical absorption and ESR spectra, indicates that the insulating strength of the glasses is comparatively high when the concentration of V₂O₅ is about 0.3 mol% in the glass matrix.