Structural transformations and optical properties of As<Subscript>2</Subscript>S<Subscript>3</Subscript> chalcogenide glasses

The effects of melt temperature T _i and quenching rate V _i on the structure and optical properties of As₂S₃ glasses is studied. It is found that the glass band gap increases with T _i and V _i , whereas a decrease is observed in the glass density, refractive index (from 2.71 to 2.48), and two-photon absorption coefficient (from 0.37 to 0.15 cm/MW), which is accompanied by an increase in the optical-breakdown damage threshold.     

Temperature dependence of the optical energy gap for the CdS<Subscript>x</Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> quantum dots

A temperature dependence of the optical energy gap E _g (T) for the CdS_xSe_1?x quantum dots synthesized in a borosilicate glass matrix was investigated in the range of 4.2–500 K. It was demonstrated that this dependence reproduced the dependence E _g (T) for bulk crystals and is described by the Varshni formula for \(\bar r > a_B \) over the entire temperature range. Here, \(\bar r\) is the average dot radius, and a_B is the Bohr radius for the exciton in a bulk crystal. With the transition to quantum dots with \(\bar r > a_B \), a decrease in the thermal coefficient of the band gap and a deviation from the Varshni dependence were observed in the temperature range of 4.2–100 K. The specific features observed are explainable by a decrease in the resulting macroscopic potential of the electron-phonon interaction and by modification of the vibration spectrum for dots as their volume decreases.     

A Modified 8<Emphasis Type="Italic">f</Emphasis> Geometry with Reduced Optical Aberrations for Improved Time Domain Terahertz Spectroscopy

We present a modified 8f geometry for time domain terahertz (THz) spectroscopy (TDTS) experiments. We show, through simulations and data, that a simple rearranging of the off-axis parabolic mirrors, which are typically used to focus and direct THz radiation in TDTS experiments, results in a nearly 40 % reduction in the THz focal spot diameter. This effect stems from significant reduction of the principle optical aberrations which are enhanced in the conventional 8f geometry but partially compensated in the modified 8f experimental setup. We compare data from our home-built TDTS spectrometer in the modified 8f geometry to that of previous iterations that were designed in the conventional 8f geometry to demonstrate the effect.     

Electrical and thermoelectric properties of <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te

Temperature dependences of the Hall coefficient R, electrical conductivity σ, and thermopower α₀ are investigated in the range of 4–300 K. The specific features observed in temperature dependences R(T), σ(T), and α₀(T) are interpreted in the context of a model with two types of charge carriers.     

Preparation and optical properties of the co-doped ZnTe single crystals

The ZnTe:Co single crystals are obtained by diffusion Co-doping. The spectra of optical density in the range of 2.4–0.38 eV are investigated. It is found that Co-doping of the crystals shifts the absorption edge to the low-energy region. The similarity of optical absorption spectra of the ZnTe:Co and ZnS:Co crystals is established. The observed absorption lines of ZnTe:Co are attributed to the optical transitions of electrons from the level of the ground state ⁴ A ₂(F) of the Co²⁺ ion to the levels of excited states ⁴ T ₁(P), ⁴ T ₁(F), and ⁴ T ₂(F) split by the spin-orbit interaction.     

Electron mobility in the inversion layers of fully depleted SOI films

The dependences of the electron mobility μ_eff in the inversion layers of fully depleted double–gate silicon-on-insulator (SOI) metal–oxide–semiconductor (MOS) transistors on the density N _e of induced charge carriers and temperature T are investigated at different states of the SOI film (inversion–accumulation) from the side of one of the gates. It is shown that at a high density of induced charge carriers of N _e > 6 × 10¹² cm^–2 the μeff(T) dependences allow the components of mobility μ_eff that are related to scattering at surface phonons and from the film/insulator surface roughness to be distinguished. The μ_eff(N _e ) dependences can be approximated by the power functions μ_eff(N _e) ∝ N _e ^?n . The exponents n in the dependences and the dominant mechanisms of scattering of electrons induced near the interface between the SOI film and buried oxide are determined for different N _e ranges and film states from the surface side.     

Microstructural Investigation,Raman and Magnetic Studies on Chemically Synthesized Nanocrystalline Ni-Doped Gadolinium Oxide (Gd<Subscript>1.90</Subscript>Ni<Subscript>0.10</Subscript>O<Subscript>3−<Emphasis Type="Italic">δ</Emphasis></Subscript>)

Nanocrystalline Ni-doped gadolinium oxide (Gd_1.90Ni_0.10O_3?δ, GNO) is synthesized by co-precipitation method. The as-prepared sample is annealed in vacuum at 700°C for 6 h. Analyses of the x-ray diffractogram by Rietveld refinement method, transmission electron microscopy and Raman spectroscopy of GNO recorded at room temperature confirmed the pure crystallographic phase and complete substitution of Ni-ions in Gd₂O₃ lattice. Magnetization (M) as a function of temperature (T) and magnetic field (H) is measured by a superconducting quantum interference device magnetometer, which suggests the presence of ferromagnetic/antiferromagnetic phases together with a paramagnetic phase. From the M–T curve it can be shown that the ferromagnetic phase dominates over para-/antiferromagnetic phases in the temperature range of 300–100 K, but from 100 K to 50 K, the antiferromagnetic phase dominates over ferro-/paramagnetic phases. Hysteresis loops recorded at different temperatures indicate the presence of weak ferro-/antiferromagnetism, which dominates in the low field region (～ 4000 Oe), above which magnetization increases linearly. The sharp increase of magnetization in M–T curve observed in the temperature range of 50–5 K confirms the presence of dominating ferromagnetic plus paramagnetic phase over antiferromagnetic part. For the first time a combined formula generated from three-dimensional (3D) spin wave model and Johnston formula is proposed to analyze the coexistence of different magnetic phases in different temperature ranges. Interestingly, the combined formula successfully explains the co-existence of different magnetic phases along with their contribution at different temperatures. The onset of ferromagnetism in Gd_1.90Ni_0.10O_3?δ is explained by oxygen vacancy mediated F-centre exchange (FCE) coupling mechanism.     

Transport coefficients of <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> in a two-band model of the electron spectrum

The Hall factor and thermoelectric properties of an n-Bi₂Te_2.7Se_0.3 solid solution with the roomtemperature Seebeck coefficient |S| = 212 μV/K have been studied in the temperature range 77–350 K. The observed temperature dependences demonstrate a number of specific features, which were earlier found in samples with a lower electron density N. The effect of these specific features on the thermoelectric figure of merit Z appears to be more favorable for the sample under study: this sample is most efficient in the temperature range 120–340 K, and the average value of ZT is 0.71. It is found that a rise in the density N enhances the factor responsible for the effective mass decreasing as the temperature increases. This effect appears when the analysis is carried out in terms of a single-band parabolic model with N = const(T). This finding suggests that the most probable reason for the unusual behavior of these properties is the complex structure of the electron spectrum. Temperature dependences obtained from calculations of the transport coefficients show good agreement with the experimental data for two samples of the mentioned composition with different electron densities. The calculations have been performed in terms of a two-band model and an acoustic scattering mechanism and take into account the anisotropy and nonparabolicity of the light-electron spectrum.     

Fine Structure of the long-wavelength edge of exciton-phonon absorption and hyperbolic excitons in silicon carbide of 6<Emphasis Type="Italic">H</Emphasis> polytype

The fine structure of the long-wavelength edge of the polarization spectra of exciton-phonon absorption in moderate-purity n-type 6H-SiC crystals with a concentration of uncompensated donors N_D?N_A=(1.7–2.0)×10¹⁶ cm^?3 at T=1.7 K was studied. The analysis of new special features found at the absorption edge and the reliable detection of the onset of exciton-phonon steps related to the emission of phonons from acoustical and optical branches allowed highly accurate determination of a number of important parameters such as the band gap, the exciton band gap, the exciton binding energy, and the energies of spin-orbit and crystal-field splitting of an exciton. For the first time, transitions with the emission of LA phonons to the 1S exciton state with an M₁-type dispersion law were detected in E ∥ Z(C) polarization (the electric-field vector is parallel to the optical axis of the crystal). This observation supports the previously predicted “two-well” structure of the conduction band minimum in 6H-SiC.     

Observation of Antiferromagnetic Magnons and Magnetostriction in Manganese Oxide Using Terahertz Time-Domain Spectroscopy

We observed antiferromagnetic magnons in manganese oxide (MnO) using terahertz time-domain spectroscopy (THz-TDS). The antiferromagnetic resonance (AFMR) frequency shows the behavior of a first-order phase transition near the Néel temperature (116 K). The temperature dependence of the AFMR frequency is explained well by a molecular field approximation including biquadratic exchange. It was found that the relaxation rate of the AFMR signal shows an abrupt increase near the Néel temperature, where the antiferromagnetic long-range order disappears. The change $\Delta n$ in refractive index was obtained from the peak shift in the THz electric field. The temperature dependence of $\Delta n$ can be explained by the lattice and magnetostrictive contributions to this refractive index change. The measurement of refractive index by THz-TDS is very useful for optically opaque materials such as MnO.     

Temperature Dependence of Crystal Structure and THz Absorption Spectra of Organic Nonlinear Optical Stilbazolium Material for High-Output THz-Wave Generation

A stilbazolium material comprising 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST), which has a large nonlinear optical susceptibility, was studied for application in terahertz (THz)-wave generation. The temperature-dependent structure of the DAST crystal was measured by using powder X-ray diffraction from ?100 to 200 °C, indicating a volume expansion of 4.6 %. The lattice constants show anisotropic thermal expansion. Also, the temperature dependence of THz absorption spectra was measured by terahertz time-domain spectroscopy (THz-TDS) in the temperature range varying from ?80 to 88.1 °C. A strong absorption peak was found at around 1 THz, shifting slightly toward a lower frequency with increasing temperature. The temperature dependence of the THz spectra was compared with that of X-ray diffraction. The shifting of THz-vibrational frequencies of the DAST crystal suggests that the change in its lattice structure is temperature dependent.     

Simulation of Electronic Center Formation by Irradiation in Silicon Crystals

We present the results of a study on localized electronic centers formed in crystals by external influences (impurity introduction and irradiation). The main aim is to determine the nature of these centers in the forbidden gap of the energy states of the crystal lattice. For the case of semiconductors, silicon (Si) was applied as model material to determine the energy levels and concentration of radiation defects for application to both doped and other materials. This method relies on solving the appropriate equation describing the variation of the charge carrier concentration as a function of temperature n(T) for silicon crystals with two different energy levels and for a large set of N ₁, N ₂ (concentrations of electronic centers at each level), and n values. A total of almost 500 such combinations were found. For silicon, energy level values of ε ₁ = 0.22 eV and ε ₂ = 0.34 eV were used for the forbidden gap (with corresponding slopes determined from experimental temperature-dependent Hall-effect measurements) and compared with photoconductivity spectra. Additionally, it was shown that, for particular correlations among N ₁, N ₂, and n, curve slopes of ε ₁/2 = 0.11 eV, ε ₂/2 = 0.17 eV, and α = 1/2(ε ₁ + ε ₂) = 0.28 eV also apply. Comparison between experimental results for irradiation of silicon crystals by 3.5-MeV energy electrons and Co⁶⁰ γ-quanta revealed that the n(T) curve slopes do not always coincide with the actual energy levels (electronic centers).     

Study of deep levels in GaAs <Emphasis Type="Italic">p–i–n</Emphasis> structures

An experimental study of the capacitance–voltage (C–V) characteristics and deep-level transient spectroscopy (DLTS) of p⁺–p⁰–i–n⁰ structures based on undoped GaAs, grown by liquid-phase epitaxy at two crystallization-onset temperatures T_o (950 and 850°C), with optical illumination switched off and on, are performed. It is shown that the p⁰, i, and n⁰ layers of epitaxial structures are characterized by the presence of defects with deep donor- and acceptor-type levels in concentrations comparable with those of shallow donors and acceptors. Interface states are found, which manifest themselves in the C–V characteristics at different measurement temperatures and optical illumination; these states form an additive constant. A distinct temperature dependence of the steady-state capacitance of the structures is revealed. It is found that the injection of minority carriers under an applied positive filling pulse and optical recharging lead to modification of the structure and, correspondingly, the DLTS spectra of the p⁺–p⁰–i–n⁰ structures. It is revealed that the p⁺–p⁰–i–n⁰ GaAs structures grown at T_o = 850°C are characterized by a lack of interface states and that the recharging of acceptor-type deep traps under illumination does not change the C–V characteristics. The conventionally measured DLTS spectra reveal the presence of two hole traps: HL5 and HL2, which are typical of GaAs layers.     

Energy parameters of two-electron tin centers in PbSe

Relations that make it possible to use an experimentally measured temperature dependence of carrier concentration to determine the Hubbard energy U and temperature dependence of the Fermi level F for two-electron tin centers in lead selenide are derived. A study of Pb_1?x?ySn_xNa_ySe solid solutions shows that their Fermi level in the temperature region 100–600 K lies below the valence band top E _v and that their F(T) dependences are linear, with extrapolation to T = 0 yielding E _V ?F = 210±10 meV. The Hubbard energy of the two-electron tin centers in PbSe is found to be U = ?80±20 meV.     

Energy spectrum of charge carriers in TlIn<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Yb<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Te<Subscript>2</Subscript> solid solutions

The temperature dependences of the electrical conductivity σ(T), the Hall coefficient R(T), and the thermoelectric coefficient α(T) are investigated in TlIn_1–xYb_xTe₂ (0 < x < 0.10) solid solutions at 80–1000К. From the kinetic parameters, the effective masses of electrons and holes are determined. The obtained experimental data on σ(T) and α(T) are interpreted within the context of a model with one and two types of charge carriers. It is established that, since x = 0.05, the TlIn_1–xYb_xTe₂ solid solutions belong to the class of narrow-gap semiconductors that have high matrix elements of interaction.     

Nature of a temperature hysteresis of effective shear modulus in single-crystal silicon

The behavior of the effective shear modulus G _ef and internal friction in Si is comprehensively studied in the temperature range in which an anomalous hysteresis G _ef (T) exists. It is shown that this hysteresis is caused by mobile dislocations and that the pinning of these dislocations by radiation defects results in total blocking of the hysteresis. Metastable dynamics of the transformations responsible for the anomalous behavior of G _ef (T) are detected.     

Investigation of the ZnS-CdHgTe interface

The ZnS-Cd_xHg_1?xTe interface was investigated using the capacitance-voltage characteristics of MIS structures in experimental samples. During fabrication of the n⁺-p junctions based on p-Cd_xHg_1?xTe, the density of states within the range N _ss =(1–6)×10¹¹ cm^?2 eV^?1 at T=78 K was obtained. The experiments showed that the conditions in which n⁺-p junctions are fabricated only slightly affect the state of the ZnS-CdHgTe interface. The negative voltages of the at bands V _FB , even if immediately after deposition of the ZnS films V _FB >0, point to the enrichment of the ZnS-p-CdHgTe near-surface layer with majority carriers, specifically, holes. This led to a decrease in the leakage current over the surface. During long-term storage (as long as ～15 years) in air at room temperature, no degradation of differential resistance R _d , current sensitivity S _i , and detectivity D* of such n⁺-p junctions with a ZnS protection film was observed.     

Is It not Time to Go back to Tamman’s <Emphasis Type="Italic">T</Emphasis><Subscript>g</Subscript>?

Based on the analysis of numerous experimental data, it is shown that the currently generally accepted glass transition temperature T_g is not correct. It was the result of borrowing from G. Tamman the symbol T_g, which corresponds to the temperature at which a liquid in a viscous flow is transformed into a solid brittle glassy state, and using it instead of the symbol Tw. The latter corresponded to the bending temperature (for Tamman) on the property–temperature curve of glass-forming substances lying above Tamman’s T_g. The disclosed physico-chemical nature of the temperature curve Tw (currently, generally taken to be T_g), which is the temperature of the reverse direction of the interconversion of nanofragment patterns (of polymorphs) of the high- and low-temperature polymorphs that coexist in a vitreous substance, is based on the application of polymer-polymorphing representations of the structure of glass-forming substances. In the past few decades, the discovery of the preendothermal effect located, just as Tamman’s T_g, below the standard T_g, confirms the correctness of Tamman’s T_g characterized by the increased specific heat of the heated glass.     

Electronic Processes in CdIn<Subscript>2</Subscript>Te<Subscript>4</Subscript> Crystals

The results of investigations of electrical, optical, and photoelectric properties of CdIn₂Te⁴ crystals, which were grown by the Bridgman method are presented. It is shown that electrical conductivity is determined mainly by electrons with the effective mass m_n = 0.44m₀ and the mobility 120–140 cm²/(V s), which weakly depends on temperature. CdIn₂Te₄ behaves as a partially compensated semiconductor with the donor-center ionization energy E_d = 0.38 eV and the compensation level K = N_a/N_d = 0.36. The absorption-coefficient spectra at the energy hν < E_g = 1.27 eV are subject to the Urbach rule with a typical energy of 18–25 meV. The photoconductivity depends on the sample thickness. The diffusion length, the charge-carrier lifetime, and the surface-recombination rate are determined from the photoconductivity spectra.     

Analysis of the magnetic properties of (La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.93</Subscript>MnO<Subscript>3</Subscript> manganites at high temperatures

For the first time, the Faraday method is used to measure the temperature dependence of paramagnetic susceptibility χ(T) of (La_{1 ? x} Sr _x )_0.93MnO₃ (x = 0.2, 0.25, or 0.3) manganites in the temperature interval 60–850°C. It is demonstrated that the dependences have two kinks and three linear sections. The kink of curve χ^?1(T) is related to polymorphic transformations (Q′ → Q* and Q* → R) that take place in the crystal lattices of the samples. The main magnetic characteristics of the samples are determined with the least-squares processing of curve χ^?1. Is is demonstrated that dependence χ^?1(T) obeys the Curie-Weiss law. The energy state of the magnetoactive manganese atom in the Q′-and Q*-phase samples is close to the energy state of a free Mn²⁺ ion. In the R phase, this state is close to the state of a free Mn³⁺ ion.