Electrical conductivity and band structure of thin polycrystalline EuS films

A study of the electrical resistance of thin polycrystalline EuS films (0.4–0.8 μm thick) in the temperature range 120–480 K has provided the basis for a model of the band structure of this substance. It has been shown that the main impurity levels in thin polycrystalline EuS films are those related with localized states near the conduction band bottom, as well as the E _i donor levels of Eu ions outside regular lattice sites. The “tail” of the localized states extends in energy up to at least ?0.45 eV.     

Study of the electrical properties of thin SmS films at high pressures

The pressure-induced shift of impurity levels under hydrostatic compression (?1.9 × 10^?2 meV/MPa) at T = 300 K has been derived from measurements of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films on glass substrates at different pressures. The difference between the pressure-induced shifts of impurity levels in thin films and single crystals has been attributed to the effect of elastic properties of the substrate material. It has been shown that the semiconductor-metal phase transition in SmS films does not occur at pressures of up to 1000 MPa, because the impurity levels triggering the mechanism of phase transition at such pressures are not in the conduction band.     

Structural,optical and electrical properties of laser irradiated Pb doped Ga–Se chalcogenide thin films

Using the Transverse Electrical Excitation at Atmospheric Pressure (TEA) nitrogen laser, we had irradiated the amorphous thin films of Ga₁₀Se₈₁Pb₉ chalcogenide glass and the results have been discussed in terms of the structural aspects of Ga₁₀Se₈₁Pb₉ glass. The observed changes are associated with the interaction of the incident photon and the lone-pairs electrons which affects the band gap. The X-ray structural characterization revealed the amorphous nature of as prepared films and polycrystalline nature of the laser irradiated films. The optical band gap of these thin films is measured by using the absorption spectra as a function of photon energy in the wavelength region 400–1200 nm. It is found that the optical band gap decreases while the absorption coefficient increases with increasing the irradiation time. The decrease in the optical band gap has been explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase in exposure time. The dc conductivities and activation energies of these thin films are measured in temperature range 303–403 K. It has been found that the activation energy in Ga₁₀Se₈₁Pb₉ chalcogenide thin films decreases whereas the dc conductivity increases at each temperature by increasing the irradiation time.     

Effect of the size of x-ray coherent-scattering regions on the electrical parameters of semiconducting SmS

The relation of the electrical properties of semiconducting samarium sulfide (SmS) to the size of coherent scattering regions for x-rays is considered. The dependence of the carrier concentration on the size of coherent scattering regions in single-crystal and polycrystalline SmS samples, as well as in thin samarium sulfide films, is established experimentally. A satisfactory agreement of the calculated curves with the experiment suggests that the size of coherent scattering regions has a dominant effect on the concentrations of charge carriers and defect samarium ions in SmS. The effect of thermal shocks on the size of coherent scattering regions in SmS single crystals and polycrystals, as well as in polycrystalline Sm_0.4Eu_0.6S samples, is investigated.     

Influence of the mechanical effect and impurity doping on the endothermic effect in SmS

The effect of heat absorption has been found for powder samples of samarium sulfide (SmS) in a temperature range of 235–250 K. The comparison with a similar effect in single-crystal SmS has been performed and thermodynamic parameters of the process have been determined. It has been shown that the collective injection of electrons into the conduction band from impurity donor levels is responsible for the appearance of this effect in both cases. The shift of the endothermic peak upon powdering toward lower temperatures and an increase in the enthalpy are associated with a decrease in the characteristic size of the X-ray coherent scattering regions and, as a consequence, with an increase in the concentration of impurity donor levels.     

Electrical conductivity of SmS polycrystals

The electrical conductivity of SmS polycrystals has been studied in the temperature range 300?C870 K. It has been shown that, at 300 K ?? T ?? 700 K, the concentration of conduction electrons is determined by electron transfer from impurity donor levels, and at T > 700 K, by that from the samarium 4f levels.     

Thermal effects in Sm1 + x S in the homogeneity range

It is found that polycrystalline Sm_{1 + x} S semiconductors absorb heat in the temperature range 240?C260 K. The thermodynamic parameters of the heat absorption effect are determined. It is shown that this effect is due to the collective transfer of electrons from impurity donor levels to the conduction band.     

Electrical conduction mechanism in amorphous Se80In20−xPbx films

DC conductivity measurements on thin films of a-Se₈₀In_20−xPb_x (where x=0, 2, 6 and 10) are reported in the temperature range 200–400 K. The density of states near the Fermi level is calculated using the DC conductivity (Mott parameters). The conduction in the low-temperature region is found to be due to variable range hopping while that in the high-temperature region is due to thermally assisted tunneling of the carriers in the localized states near the band edge.     

Influence of indium content on the optical, electrical and crystallization kinetics of Se100−xInxthin films deposited by flash evaporation technique

Thin films of Se _100−xIn_x (x=10, 20 and 30 at%) have been prepared by the flash evaporation technique. The effect of the indium content on optical band gap of the Se_100−x In_x films has been investigated by the optical characterization. The optical band gap values of the Se_100−x In_x thin films were determined and are found to decrease with increasing indium content. This indium content changes the width of localized states in the optical band gaps of the thin films. It was found that the optical band gap, E_g, of the Se_100−x In_x films changes from 1.78 to 1.37 eV with increasing indium content from 10 to 30 at%, while the width of localized states in optical band gap changes from 375 to 342 meV. The temperature dependence of the dark electrical conductivity were studied in the temperature range 303-433 K and revealed two activation energies providing two electrical conduction mechanisms. The activation energy of the Se_100−x In_x films in the high temperature region changes from 0.49 to 0.32 eV with increasing indium content from 10 to 30 at%, while the hopping activation energy in the lower temperature region changes from 0.17 to 0.22 meV. The change in the electrical conductivity with time during the amorphous-to-crystalline transformation is recorded for amorphous Se_100−xIn_x films at two points of isothermal temperatures 370 and 400 K. The formal crystallization theory of Avrami has been used to calculate the kinetic parameters of crystallization.     

The mechanism of the appearance of an electromotive force on heating of SmS single crystals

We analyze the experimental variation of the concentration of conduction electrons in semiconducting SmS single crystals with increasing temperature within a shallow-impurity model. It is shown that the appearance of an electromotive force is due to accumulation of the critical concentration of free electrons, which results in screening of the Coulomb potential of Sm²⁺ impurity ions that are responsible for the creation of donor levels with an activation energy of 0.045 eV in the band gap of SmS single crystals.     

Inclusion of polyaniline in electrodeposited bismuth sulphide thin films: Synthesis and characterization

Structural, electrical and optical properties of polyaniline (PAni) doped Bi₂S₃ composite thin films prepared by electrodeposition method are reported. X-ray diffraction pattern indicates its polycrystalline nature and crystallite size increases with increase in the concentration of PAni. FTIR studies reveal that the dopant PAni has affected the absorption phenomenon in the IR region of the Bi₂S₃ thin films. The optical band gap energy is found to be 1.91 eV for as-deposited Bi₂S₃ thin film and it decreases with increase in the concentration of PAni. The morphology of the doped films changes due to the addition of PAni. Electrical studies indicate that the conductivity increases with increase in the concentration of PAni. The conduction results from a hopping due to localized states in the temperature range 300–358 K. Above 358 K, the conduction process is explained by the traps at grain boundaries of partially depleted grains.     

Some comments on the electronic properties of the surface space charge layer of copper phthalocyanine thin films

A simple analysis, using a theory of the surface space charge layer of semiconductors, of the published values of the work function φ and surface ionization energy Φ_s of copper phthalocyanine (CuPc) thin films was performed. Using a well known position of the Fermi level E_F within the band gap E_g the values of its absolute band bending eV_s and surface electron affinity X_s were determined. A small negative value of the absolute band bending eV_s = −0.17 ∓ 0.15 eV has been interpreted by the existence of the filled electronic surface states localized in the band gap below the Fermi level E_F. Such states were predicted theoretically for thin films and the crystalline surface of CuPc, and attributed to surface lattice defects of a high concentration.     

Thickness-dependent electrical and optical properties of Ge8Sb2Te11 thin films

The amorphous Ge₈Sb₂Te₁₁thin films with varying thickness are thermally deposited on well-cleaned glass substrate from its polycrystalline bulk. Absence of any sharp peak confirms the amorphous nature of deposited films. Thickness-dependent electrical and optical properties including dc-activation energy, sheet resistivity, optical band gap, band tailing parameter, etc. of Ge₈Sb₂Te₁₁thin films have been studied. The optical parameters have been calculated from transmission, reflection and absorbance data in the spectral range of 200–1100 nm. It has been found that optical band gap and band tailing parameter decreases with the increase in Ge₈Sb₂Te₁₁thin films thickness. The dc-activation energy and sheet resistivity decreases while the crystallization temperature of the amorphous Ge₈Sb₂Te₁₁ films increases with the increase in thickness of the films. The decrease of the sheet resistivity has been substantiated quantitatively using the classical size-effect theory. These results have been explained on the basis of rearrangements of defects and disorders in the amorphous chalcogenide system.     

Study of aluminium-modified Ge2Sb2Te5 thin films for the applicability as phase-change storage device material

Electrical and optical studies have been carried out on aluminium-modified Ge₂Sb₂Te₅ thin films to check its applicability as an active material in optical and electrical memory storage devices. Five polycrystalline bulk samples were prepared with compositions: Al_x(Ge₂Sb₂Te₅)_1?x; x = 0, 0.08, 0.14, 0.21, 0.25. Amorphous thin films were deposited from the polycrystalline bulk by thermal evaporation. Temperature-dependent resistance shows the increase in crystallization temperature of Ge–Sb–Te films on aluminium addition. Activation energy for conduction, conductivity, optical band gap, coefficient of refraction and extinction coefficient are studied with respect to Al content in both amorphous and crystalline phases of Ge–Sb–Te alloy films.     

Observation of impurity conduction associated with the L-minima in n-GaSb(Te)

Hydrostatic pressure measurements of Hall coefficient and resistivity in n-GaSb(Te) have been carried out over the temperature range 1.4 K–300 K. The results give direct evidence for impurity conduction related to the Te donor states associated with the L conduction band minima.     

Influence of composition on electrical and optical properties of new chalcogenide thin films from Ge-Se-Tl system

Bulk Ge₂₀Se_80−xTl_x (x ranging from 0 to 15 at%) chalcogenide glasses were prepared by conventional melt quenching technique. Thin films of these compositions were prepared by thermal evaporation, on glass and Si wafer substrates at a base pressure of 10⁻⁶ Torr. X-ray diffraction studies were performed to investigate the structure of the thin films. The absence of any sharp peaks in the X-ray diffractogram confirms that the films are amorphous in nature. The optical constants (absorption coefficient, optical band gap, extinction coefficient and refractive index) of Ge₂₀Se_80−xTl_x thin films are determined by absorption and reflectance measurements in a wavelength range of 400-900 nm. In order to determine the optical gap, the absorption spectra of films with different Tl contents were analyzed. The absorption data revealed the existence of allowed indirect transitions. The optical band gap showed a sharp decrease from 2.06 to 1.79 eV as the Tl content increased from 0% to 15%. It has been found that the values of absorption coefficient and refractive index increase while the extinction coefficient decreases with increase in Tl content in the Ge-Se system. These results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. DC electrical conductivity of Ge₂₀Se_80−xTl_x thin films was carried out in a temperature range 293-393 K. The electrical activation energy of these films was determined by investigating the temperature dependence of dc conductivity. A decrease in the electrical activation energy from 0.91 to 0.55 eV was observed as the Tl content was increased up to 15 at% in Ge₂₀Se_80−xTl_x system. On the basis of pre-exponential factor, it is suggested that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges.     

Why localized and extended impurity band states can coexist and be separated

It has been assumed that in the impurity band metallic regime there exists an energy E_c below which all states are localized and above which all states are extended. This discontinuous model contradicts the results of recent experiments. Using the quantum theory of measurement I resolve these difficulties as my title indicates.     

The electrical properties of Cu2Te

The conductivity, Hall effect, and thermo-emf are reported for thin films and polycrystalline bulk specimens of Cu₂Te, which is found to be an impurity semiconductor whose forbidden band has a width of 0.9–1.0 eV. The compound dissociates on vacuum evaporation, so it is very difficult to produce thin films.I am indebted to Professor D. N. Naskedov for his interest and for valuable comments.     

Semiconducting properties and band structure of polycrystalline Bi4Mo20O62 compounds

Transport coefficient measurements (electrical conductivity, thermoelectric power and Hall effect) have been performed on compacted bars of Bi₄Mo₂₀O₆₂ in polycrystalline form over the temperature range (130–500 K) Experimental results are discussed in comparison to those obtained recently on Sb₄Mo₂₀O₆₂ They are interpreted on the basis of the same _p-type semiconductor model with two inverted deep levels near the midgap Conduction mechanisms are governed by acoustical-phonon scattering of the carrriers The top of the valence band is assumed to be formed from the d_xy orbitais of some of the Mo atoms leading to narrow bonding bands, while the donor and acceptor levels may be formed from the nonbonding d_xy orbitais of some of the Mo atoms of the distorted octahedron framework EPR measurements are discussed on the basis of this model and in comparison to the EPR results for Sb₄MO₂₀O₆₂ It is assumed that more than one narrow energy level (localized d states located below the valence band) contributes to the area of the line The XPS spectrum obtained on Bi₄Mo₂₀O₆₂ is also discussed.     

Modifications in structural and optical properties of Mn-ion implanted CdS thin films

In this paper, we report on modifications in structural and optical properties of CdS thin films due to 190 keV Mn-ion implantation at 573 K. Mn-ion implantation induces disorder in the lattice, but does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. The optical band gap was found to decrease with increasing ion fluence. This is explained on the basis of band tailing due to the creation of localized energy states generated by structural disorder. Enhancement in the Raman scattering intensity has been attributed to the enhancement in the surface roughness due to increasing ion fluence. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d (⁴T₁ → ⁶A₁) transition of tetrahedrally coordinated Mn²⁺ ions.