Magnesium Oxide (MgO).- Zincblende Magnesium Sulphide (?-MgS).- Zincblende Magnesium Selenide (?-MgSe).- Zincblende Magnesium Telluride (?-MgTe).- Zinc Oxide (ZnO).- Wurtzite Zinc Sulphide (?-ZnS).- Cubic Zinc Sulphide (?-ZnS).- Zinc Selenide (ZnSe).- Zinc Telluride (ZnTe).- Cubic Cadmium Sulphide (c-CdS).- Wurtzite Cadmium Sulphide (w-CdS).- Cubic Cadmium Selenide (c-CdSe).- Wurtzite Cadmium Selenide (w-CdSe).- Cadmium Telluride (CdTe).- Cubic Mercury Sulphide (?-HgS).- Mercury Selenide (HgSe).- Mercury Telluride (HgTe).

Handbook on physical properties of semiconductors

We present the first review of the current state of the literature on electronic properties and phase transitions in TlX and TlMX2 (M = Ga, In; X = Se, S, Te) compounds. These chalcogenides belong to a family of the low-dimensional semiconductors possessing chain or layered structure. They are of significant interest because of their highly anisotropic properties, semi- and photoconductivity, nonlinear effects in their I–V characteristics (including a region of negative differential resistance), switching and memory effects, second harmonic optical generation, relaxor behavior and potential applications for optoelectronic devices. We review the crystal structure of TlX and TlMX2 compounds, their transport properties under ambient conditions, experimental and theoretical studies of the electronic structure, transport properties and semiconductor–metal phase transitions under high pressure, and sequences of temperature-induced structural phase transitions with intermediate incommensurate states. The electronic nature of the ferroelectric phase transitions in the above-mentioned compounds, as well as relaxor behavior, nanodomains and possible occurrence of quantum dots in doped and irradiated crystals is discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/20/29/293202/pdf

Electronic properties and phase transitions in low-dimensional semiconductors

Investigation of the electric properties of semi-conducting materials in an applied ac electric fields gives information about the nature of charge transport and localized states in the forbidden gap. Layered crystals usually contain structural defects, such as dislocations and vacancies that may form a high density of localized states near the Fermi level. So, the current study was carried out for insight into the dielectric Properties of Tl2S layered single crystals. These properties were studied using the ac measurements in the low temperatures ranging from 77 to 300 K. The real part of dielectric constant e?, imaginary part of dielectric constant e?, the dissipation factor tan δ and the alternating current conductivity σac were measured in an applied ac electric field of frequencies extending from 2.5 to 50 kHz. Based on the dependencies of these dielectric parameters on both the frequency and temperature, the dielectric properties of the crystals under investigation were elucidated and analyzed. The ac conductivity was found to obey the power law σac(ω) = Aωs with which the values of the exponent s were evaluated to be less than unity in the range 0.21 ≥ s ≥ 0.19. Furthermore, it was found that the temperature dependence of ac conductivity follows the Arrhenius relation via which the impact of temperature on the electrical processes in an applied ac electric field was illustrated and analyzed. The influences of temperature and frequency on both the exponent s and band gap were also discussed in this investigation.

https://file.scirp.org/pdf/JMP20110100002_92465513.pdf

Impacts of Temperature and Frequency on the Dielectric Properties for Insight into the Nature of the Charge Transports in the Tl2S Layered Single Crystals

A study of the electrical conductivity of polypyrrole-polyoxyphenylene composites (PPy-POP and PPy-POP-MPcTS) prepared by in situ electropolymerization is presented, where MPcTS stands for tetrasulfonated tetrasodium metallophthalocyanine salts and is used as a second dopand for polypyrrole. The conductivity is studied as a function of temperature in the range 77≤T≤300 K. The temperature dependence of the total ac conductivity, in the frequency range 102-105 Hz, changes by approximately five orders of magnitude, showing a sub-linear dispersive behaviour. The temperature dependence of the dc conductivity gives evidence for a transport mechanism based on the Mott's variable-range hopping model in one dimension for the PPy-POP composite, which shows a crossover to three dimensions for the PPy-POP-MPcTS composites. Using this model we were able to calculate meaningful values for the density of states, hopping energy and hopping distance.

https://iopscience.iop.org/article/10.1088/0022-3727/34/11/323/pdf

Evaluation of the electrical conductivity of polypyrrole polymer composites

A study of the optical and electrical properties of a conducting polypyrrole–polyoxyphenylene composite, PPy–POP, prepared by insitu electropolymerization is presented. Electropolymerization was performed potentiostatically in a solution of pH 9 which contained the monomers pyrrole, allylphenol and sodium 4-hydroxybenzenesulfonate (4HBS), by applying a potential of 1.25 V vs. SCE. The films obtained were characterized optically by UV/VIS and IR spectroscopy and electrically by measurements of the temperature dependence of the ac and dc conductivity. FTIR measurements indicated that the polymer blend obtained consists of PPy and the insulating polymer poly-2-allyloxyphenylene (POP), whereas the third monomer, 4HBS, is incorporated into the PPy–POP film as dopant for the conducting PPy. Furthermore, optical characterizations show a light degree of overoxidation of PPy in the PPy–POP composite. In the UV/VIS spectra, the formation of both polaron and bipolaron electronic states of the band structure of PPy can be seen, but the IR spectra demonstrate the transition of the PPy structure from a conducting quinoid to benzoid type with increasing polymerization potential. This is accompanied by the introduction of a carbonyl group into the PPy backbone and a reduction of the conjugation length of the polymer chain, which has a strong influence on the conductivity of the polymer composite. Despite this overoxidation process, the PPy–POP film retains a conductive character which allows the growth of thick films. The temperature dependence of the ac and dc conductivity of PPy–POP was investigated. The total ac conductivity, σtot(ω), in the frequency range 102–105 Hz, changes by approximately four orders of magnitude in the range from 77 to 300 K, showing a sub-linear dispersive behavior. The temperature dependence of the dc conductivity of such a polymer composite can be described by Mott's variable range hopping (VRH) model according to σ=σ0 exp[-(T0/T)γ], with γ=1/2

Optical and electrical characterization of a conducting polypyrrole composite prepared by in situ electropolymerization

It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS2 and TlInS2 single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS2 and TlInS2 single crystals in the temperature range 110–150 K.

Investigation of dc hopping conduction in TlGaS2 and TlInS2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe2 single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe2 single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T⩽210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, NF=1.3×1019eV·cm3)−1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe2 single crystals increases from 130 A to 170 A as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe2 single crystals is investigated.

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The effect of gamma irradiation on the dielectric properties and ac conductivity of a TlInS2 single crystal with a layered structure has been investigated in the frequency range from 5 × 104 to 3.5 × 107Hz. It has been shown that gamma irradiation of the TlInS2 single crystal with a dose of 104–2.25 × 106 rad leads to a considerable increase in the dielectric loss tangent tanδ, the real part ɛ′ and imaginary part ɛ″ of the complex permittivity, and the ac conductivity σac across the layers. It has been established that, for all gamma irradiation doses, the TlInS2 single crystal is characterized by the dielectric loss due to electrical conduction up to a frequency of 107 Hz and by the relaxation loss at a higher frequency. Irradiation of the TlInS2 single crystal results in an increase in the dispersion of tan δ, ɛ′, and ɛ″. It has been demonstrated that, as the gamma irradiation dose is accumulated in the TlInS2 single crystal, the density of localized states near the Fermi level NF increases (from 5.2 × 1018 to 1.9 × 1019 eV−1 cm−3).

Effect of gamma irradiation on the dielectric properties and electrical conductivity of the TlInS2 single crystal

It is revealed that TlS single crystals exhibit a variable range hopping conduction along a normal to their natural layers at temperatures T ≤ 230 K in a dc electric field and a nonactivated hopping conduction at low temperatures in strong electric fields. Estimates are made for the density of states near the Fermi level (NF = 2.8 × 1020 eV−1 cm−3 and their energy spread (ΔW = 0.02 eV), the localization radius (a = 33 A), the average jump distance in the region of activated (Rav(T) = 40 A) and nonactivated (Rav(F) = 78 A) hopping conduction, and also the drop in the charge carrier potential energy along the jump distance in an electric field F: eFR = 0.006 and 0.009 eV at F = 7.50 × 103 and 1.25 × 104 V/cm, respectively.

Charge transfer over localized states in a TlS single crystal

Thermodynamic, kinetic, and dielectric properties of the As2S3–As2Se3 system have been investigated. The dependences of the Gibbs free mixing energy on the composition of the As2S3–As2Se3 system have been calculated. It has been found that glasses based on As2Se3 are formed in the concentration range up to 15 mol % As2S3. The glass–crystal transition based of As2Se3 samples has been studied in the framework of the linear isoconversion model. The As2S3–As2Se3 phase diagram has been approximated by the thermodynamic method. The concentration-temperature dependences of the Gibbs free energy of alloys (1 – x)As2Se3⋅ xAs2S3 have been constructed. The process of glass–crystal transition in alloys of the As2S3–As2Se3 system has been analyzed thermodynamically. The temperature dependence of the conductivity of the sample As2Se3 at constant current and dielectric properties of 0.8As2Se3 ⋅ 0.2As2S3 solid solutions in alternating electric fields with a frequency of f = 20–106 Hz have been studied. The nature of dielectric losses has been established, as well as the charge transfer hopping mechanism in the studied samples. The density of localized states near the Fermi level, the energy spread of these states, the average distance, and time of carrier jumps over the forbidden zone have been estimated.

M. M. Asadov

Papers

Investigation of dc hopping conduction in TlGaS2 and TlInS2 single crystals

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

Effect of gamma irradiation on the dielectric properties and electrical conductivity of the TlInS2 single crystal

Charge transfer over localized states in a TlS single crystal

Thermodynamic and Dielectric Properties of As2S3–As2Se3