Impurity band in SnBi<Subscript>4</Subscript>Se<Subscript>7</Subscript>: thermoelectric power and electrical resistivity measurements

Thermoelectric power and electrical resistivity measurements on polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ in the temperature range of 90–420 K are presented and explained assuming the existence of an impurity band. The variation of the electron concentration with temperature above 300 K is explained in terms of the thermal activation of a shallow donor, by using a single conduction band model. The density of states effective mass m ^*=0.15m ₀ of the electrons, the activation energy of the donors, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by acoustic phonon, optical phonon, and polar optical phonon as well as by alloy and ionized impurity modes. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the electrical resistivity with temperature between 420 and 90 K is explained.     

Conductivity anisotropy and localization of charge carriers in TlInTe<Subscript>2</Subscript> single crystals

The electrical resistivity of TlInTe₂ chain-structure semiconductors in directions parallel and perpendicular to the chains is analyzed as a function of temperature. It is demonstrated that, in both cases, the temperature dependences of the electrical resistivity in the temperature range under investigation are characterized by two portions associated with different mechanisms of electrical conduction. In the high-temperature range, the electrical conduction is predominantly provided by thermally excited impurity charge carriers in the allowed band. In the low-temperature range, the conduction occurs through charge carrier hopping between localized states lying in a narrow energy band near the Fermi level. The activation energy for impurity conduction is determined. The localization lengths and the density of localized states near the Fermi level, the spread in energies of these states, and the average carrier-hopping distances are estimated for different temperatures.     

Electrical properties of a-Ge<Subscript>x</Subscript>Se<Subscript>100-<Emphasis Type="Italic">x</Emphasis></Subscript>

In general, the conductivity in chalcogenide glasses at higher temperatures is dominated by band conduction (DC conduction). But, at lower temperatures, hopping conduction dominates over band conduction. A study at lower temperature can, eventually, provide useful information about the conduction mechanism and the defect states in the material. Therefore, the study of electrical properties of Ge_xSe_100-x in the lower temperature region (room temperature) is interesting. Temperature and frequency dependence of Ge_xSe_{100-x (x} = 15, 20 and 25) have been studied over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggested that the behaviour of germanium selenium system (Ge_xSe_100-x ) have been successfully explained by correlated barrier hopping (CBH) model.     

Impurity band conduction in CdHgTe crystals

Electrical conduction at 77 K in Cd_xHg_1−xTe, with the composition x ⩽ 0.2, is by electrons in the conduction band, by holes in the valence band and by holes in the impurity band. In samples with zero energy gap, x < 0.14, electrical conduction by holes in the valence band is comparable to electrical conduction by holes in the impurity band. In the open energy gap CdHgTe, electrical conduction by holes in the valence band is negligible in comparison to electrical conduction by holes in the impurity band. In CdHgTe samples, electrical conduction in the impurity band is described by the “Fermi Glass” model.     

Investigation of the electronic structure of La1−x(M2+)xCrO3, Cr2O3 and La2O3 by X-ray photoelectron spectroscopy

The electronic structures of lanthanum chromites, pure and doped with magnesium and strontium, have been studied in comparison with Cr₂O₃ and La₂O₃ through the use of X-ray photo-electron spectroscopy. The main peaks and satellites of inner and outer electrons are properly assigned. The band structure of LaCrO₃ is determined by using the XPS data and a calculation based on point charge model. The partially filled Cr(3d) band is localized. The conduction is mainly extrinsic. The measured Fermi-level is close to the valence band indicating a low activation energy in agreement with the results of conductivity measurements.     

Analysis of the charge transfer mechanisms responsible for the current-voltage characteristics of Ca<Subscript>4</Subscript>Ga<Subscript>2</Subscript>S<Subscript>7</Subscript>: Eu<Superscript>3+</Superscript> single crystals

The current-voltage characteristics of Ca₄Ga₂S₇: Eu³⁺ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca₄Ga₂S₇: Eu³⁺ single crystals are high-resistance semiconductors with a resistivity of ～10⁹ Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×10¹⁴ to 2.7×10¹⁵ cm^?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×10³ V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×10⁴ V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Ba0.5Sr0.5TiO3电子结构和光学性质的第一性原理研究

利用第一性原理研究了Ba_0.5Sr_0.5TiO₃的能带结构和光学性质.结果表明,导带和价带都来源于钛原子3d轨道和氧原子2p轨道的杂化.导带主要由钛原子的3d轨道贡献,价带主要由氧原子的2p轨道贡献.吸收系数为10⁵ cm^-1量级,且吸收主要集中在低能区.折射率为n（0）=2.1,结果与实验符合. 关键词： 第一性原理 能带结构 光学性质     

Hall effect in n-type GaS

Hall effect measurements have been performed for the first time on n-type GaS single crystals, grown buy the Bridgam-Stockbarger method. Electrical properties are dominated by a donor centre at 0.52 eV, with a concentration of 2.3 × 10¹⁶cm^–3. The conduction band density-of-states effective mass has been estimated to be 1.3m_e.The low temperature conduction is compatible with a hopping process with an activation energy of 0.15 eV.     

Decay of the homogeneous state in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiSe<Subscript>2</Subscript>

The stability of a homogeneous state in materials with a more than half-filled polaron band has been investigated for the Fe _x TiSe₂ system used as an example. It has been demonstrated that the factor limiting the stability of the homogeneous state of these materials is a change in the degree of filling of the conduction band due to the thermal broadening of the polaron band. This factor becomes substantial when the top of the polaron band intersects the Fermi level. It has been revealed that the decay of the homogeneous state leads to the precipitation of the intercalant. The morphology and structure of the precipitates formed in this decay in Fe _x TiSe₂ single crystals have been studied by transmission electron microscopy.     

氢化非晶硅的低温输运

从低温跳跃电导模型出发,计算低温下的跳跃频率,热发射率及输运能级随温度变化,分析讨论了非晶硅热激电导的低温峰。认为输运机制的变化是氢化非晶硅热激电导低温峰T_M独立于起始温度T₀的主要原因,T_M相应于输运机制变化的温度。 关键词：     

Thermoelectric power and ac conductivity of A-type Nd2O3

Measurement of thermoelectric power Θ of pressed pellets of A-type Nd₂O₃ from 550 to 1180K and electrical conductivity (σ) at dc, 50 Hz, 1.542 kHz and 3 kHz at different temperatures is reported. It is concluded that electrical conduction at high temperature (T>600K) in this solid is due to positive large polarons in O²⁻ : 2p (valence) band and negative intermediate polarons in Nd³⁺ : 5d (conduction band). The energy band gap of the solid has been found to be 2.44 eV. At low temperatures, conduction by hopping of charge carriers from one impurity centre to another has been predicted.     

Instability of monovalent ions of transitional metals in alkaline earth fluorides: CaF<Subscript>2</Subscript> and SrF<Subscript>2</Subscript> activated by Co<Superscript>2+</Superscript> or Ni<Superscript>2+</Superscript>

The ground state of monovalent ions of transitional metals in alkaline-earth fluorides is localized in the conduction band. It is shown by the example of monovalent Co and Ni ions in CaF₂ and SrF₂ crystals that this is responsible for the unstable state of these ions in cubic environments. A stable state is achieved by the monovalent ions shifting to off-center positions.     

Electrical Properties of Intercalated Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MoSe<Subscript>2</Subscript> Compounds in Constant and Alternating Fields

On intercalated Ag_xMoSe₂ samples, in addition to temperature measurements of the direct current electrical resistivity, measurements of the alternating current resistivity using the impedance spectroscopy technique are carried out in a wide frequency range and at different temperatures. The activation behavior of the d.c. conductivity, which increases with increasing silver content in the samples, is shown. The a.c. conductivity undergoes frequency dispersion, described by a “universal dynamic response” (UDR). It is shown that the relaxation processes during charge transfer in a variable field are accelerated with increasing silver content in the samples and with increasing temperature. The data obtained are analyzed using the models of the band and hopping conduction.     

Energy barrier between states of a bistable center in photochromic crystals CdF2:Ga and CdF2:In

The establishment of thermal equilibrium between photoinduced (shallow) and ground (deep) states of bistable DX centers in photochromic crystals CdF₂:In and CdF₂:Ga, which are used as real-time holographic media, is studied based on the notions of chemical kinetics. Two mechanisms of mutual transformation of shallow and deep centers—the tunnel mechanism and the mechanism with the participation of free charge carriers—are considered. Equations describing the decay of a photoinduced shallow state are obtained. These equations take into account the distribution of electrons between the photoinduced and ground states and the conduction band. Analysis of the experimental kinetic curves of the decay of photoexcited shallow centers makes it possible to determine the activation energies and barrier height for thermally activated processes of mutual transformation of shallow and deep centers. In CdF₂:In and CdF₂:Ga, this barrier, which determines the decay kinetics of holograms, lies above the bottom of the conduction band by ～10 and ～500 meV, respectively.     

DFT and k · p modelling of the phase transitions of lead and tin halide perovskites for photovoltaic cells

3D hybrid organic perovskites, CH₃NH₃PbX₃ (X = halogen), have recently been used to strongly improve the efficiency of dye sensitized solar cells (DSSC) leading to a new class of low‐cost mesoscopic solar cells. CsSnI₃ perovskite can also be used for hole conduction in DSSC. Density functional theory and GW corrections are used to compare lead and tin hybrid and all‐inorganic perovskites. The room temperature optical absorption is associated to electronic transitions between the spin–orbit split‐off band in the conduction band and the valence band. Spin–orbit coupling is about three times smaller for tin. Moreover, the effective mass of relevant band edge hole states is small (0.17). The high temperature phase sequence of CsSnI₃ leading to the room temperature orthorhombic phase and the recently reported phases of CH₃NH₃MI₃ (where M = Pb, Sn) close to the room temperature, are also studied. Tetragonal distortions from the ideal cubic phase are analysed by a k · p perturbation, including spin–orbit effect. In addition, the non‐centrosymmetric phases of CH₃NH₃MI₃ exhibit a splitting of the electronic bands away from the critical point. The present work shows that their physical properties are more similar to conventional semiconductors than to the absorbers used in DSSC. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

低补偿度n-Hg1-xCdxTe的磁致金属-绝缘体相变和相变后的温度激活输运行为

报道了ｘ＝０．２１４组份、低补偿度（Ｋ《１）ｎ－Ｈｇ_1-xＣｄ_xＴｅ晶体在０．３─３０Ｋ温度范围，０─７Ｔ强磁场下的横向磁阻、电子霍耳迁移率、霍耳系数测量结果，观测到了磁致金属－绝缘体相变和相变后的温度激活输运行为。分析实验数据，提出：低补偿度、组份：ｘ＝０．２附近的ｎ－Ｈｇ_1-xＣｄ_xＴｅ，磁致金属－绝缘体相变（ＭＩＴ）发生的机理是载流子在浅施主杂质态上的磁冻结；发生磁冻结的前提是热冻出（ｔｈｅｒｍａｌ ｆｒｅｅｚｅ 关键词：     

Deconvolution of temperature dependence of conductivity,its reduced activation energy,and Hall-effect data for analysing impurity conduction in n-ZnSe

ABSTRACT

The temperature dependence of the reduced activation energy w?=?ε/k_BT of the conductivity σ has been utilised for determining the impurity conduction mechanism in doped semiconductors in many studies. Herein, the formula for deconvoluting w when plural conduction mechanisms appear is used to confirm the analysis of the data of the Hall-effect measurements on Al-doped n-ZnSe samples. The analysis is performed on the basis of an impurity-Hubbard-band model which includes ε ₂ conduction in the top Hubbard band as well as ε ₃ and Efros-Shklovskii (ES) variable-range hopping (VRH) conduction processes in the bottom Hubbard band. As the result of the analysis, transitions among the three hopping conduction mechanisms of ε ₂, ε ₃, and ES VRH are clearly shown in the temperature dependence of w as well as in that of the Hall mobility, which are hardly noticed in the temperature dependence of σ. In addition, the power-law exponent of the prefactor of ES VRH conductivity is determined through the fit to the temperature dependence of w to show that it decreases from ～ 1.5 to ～ 0 with increasing net donor concentration.     

Relaxation processes upon photoexcitation of semiconductor CdF<Subscript>2</Subscript> by laser pulses

The microwave-cavity-based technique is used to study the processes of photoionization of electrons from donor levels to the conduction band in semiconductor CdF₂ crystals doped with Y, In, or Ga. The samples were excited by periodic pulses of Nd-laser (λ = 1.06 μm, pulse width ～10 ns) in the temperature range 6–77 K. The transient processes were detected in the absorption and dispersion modes related to variation of the imaginary and real parts of the complex permittivity ?₁ ? i?₂ induced by the light pulses. The observed signals consisted of short peak at t ～ 0, approximately 40–70 ns in length, and a long tail with a duration of ～100 ms. The short peak is likely to be related to the stay of the photoexcited carriers in the conduction band, while the long tail is associated with the processes of excitation relaxation after the electrons coming back to the donor levels of the impurity band. The weak temperature dependence of the width of the peak at t ～ 0 is explained by the tunneling mechanism of relaxation of electrons through the energy (or, probably, spatial) barrier separating the bound and free states of the carriers in the semiconductor CdF₂.     

Single crystal polarized Raman spectra of the solid electrolyte Cu2HgI4; attempt frequencies for ion motion in Ag2HgI4

The polarized Raman scattering from small single crystals of Cu₂HgI₄ provided assignments for the more prominent Raman features to specific irreducible representations. The E symmetry assignment, mass dependence, and pressure dependence of the 36 cm^?1 band in Cu₂HgI₄ and 24 cm^?1 band in Ag₂HgI₄ indicate that these features approximate the attempt frequency for ion hopping. The unusually high pre-exponential factor in the Arrhenius expression for ion hopping is discussed in light of the observed attempt frequency; we conclude that despite the high activation energy the conduction mechanism is similar to other heavy-metal solid electrolytes.