DC conduction processes and electrical parameters of the organic semiconducting zinc phthalocyanine, ZnPc, thin films

Electronic conduction in thermally evaporated thin films of organic semiconductor zinc phthalocyanine (ZnPc) has been investigated in a broad temperature range using gold Ohmic contacts. Electronic conduction by charge carrier hopping was dominated at low temperatures and for all applied voltages. At higher temperatures and at voltages just below 2 V conduction was found to obey Ohm's law, while at higher voltages space-charge-limited conduction (SCLC) was the dominated mechanism, which was controlled by hole-trapping states distributed exponentially within the band gap.In freshly prepared samples adsorbed oxygen was responsible for lower hole mobility and higher charge carrier concentrations. Prolonged heating of ZnPc films at 425 K resulted in lower defect state density, and thus reduced trap concentration and higher charge carrier mobility.     

Molecular junction of n-thiophenes sandwiched between two Au (111) electrodes

In this work, the electron-transport properties of the molecular junction of oligothiophenes sandwiched between two Au (111) electrodes are studied based on the combination of the density functional theory and non-equilibrium Green’s function (NEGF) approach. From the calculation of electron properties, it is revealed that by increasing the number of thiophenes rings the (highest occupied molecular orbital-lowest unlocked molecular orbital) gap and the total energy decreases. Also, the transmission coefficient at zero voltage and the current–voltage curve for the thiophene molecules is calculated. The results indicate that the electrical conductivity and the value of band gap decrease exponentially with increasing the length of the molecules. Moreover, by simulating this molecular wire, we were able to obtain a current in the range of micro-amperes, which is a good current in the electronic application. However, it is known that the linear-response conductance is overestimated about an order of magnitude or more by the NEGF?+?DFT approach when semi-local approximate functional like PBE is used.     

Electronic transport properties of silicon chains sandwiched between graphene electrodes: first-principles study

The effects of orientation and silicon chain length on the electronic transport properties for linear silicon chains sandwiched between two graphene electrodes are investigated by using non-equilibrium Green’s functions combined with density functional theory. Our results demonstrate that the conductance of single silicon chains can hardly be affected by its orientation, as there is negligible difference between the conductance of tilted and un-tilted chains, and the conductance is impacted greatly by the length of chains, i.e. the transmission coefficient is doubled for double chains. The equilibrium conductance of single silicon chains shows even-odd oscillating behavior, and its tendency decreases with the increase of the chain length. The non-equilibrium electronic transport properties for all types of chains are also calculated, and all current–voltage curves of silicon chains show a linear character. The frontier molecular orbitals, the total and projected density of states are used to analyse the electronic transport properties for all types of chains.     

Controllable growth of nanocomposite films with metal nanocrystals sandwiched between dielectric superlattices

Nanocomposite films, with Ni nanocrystals (NCs) (5–8 nm) embedded within SrTiO₃/BaTiO₃ superlattice (period: 30 nm), were prepared by laser molecular beam epitaxy. In situ reflection high-energy electron diffraction was employed to study the role of lattice strain in the self-organization of NCs and the epitaxial growth of SrTiO₃/BaTiO₃ superlattice. It was found that the strain from large lattice mismatch drove the self-organization of Ni NCs. Also the SrTiO₃/BaTiO₃ epitaxial growth was achieved on the NC-dotted irregular interface, because the epitaxial growth occurred preferably at sites with low strain. The fine alternation of the two processes would provide a possible route to engineer controllably the nanocomposite microstructure.     

Dielectric response of aligned semiconducting single-wall nanotubes

We report measurements of the full intrinsic optical anisotropy of isolated single-wall carbon nanotubes (SWNTs). By combining absorption spectroscopy with transmission ellipsometry and polarization-dependent resonant Raman scattering, we obtain the real and imaginary parts of the SWNT permittivity from aligned semiconducting SWNTs dispersed in stretched polymer films. Our results are in agreement with theoretical predictions, highlighting the limited polarizability of excitons in a quasi-1D system.     

Dielectric enhancement of excitons in semiconducting quantum wires

The energy of the exciton ground state in a semiconducting cylindrical quantum wire surrounded by a dielectric has been calculated using a variational technique accounting for the effect of dielectric enhancement. The effect of dielectric enhancement in such a system has been clearly demonstrated. Exciton parameters have been calculated for an intercalated leadiodide-based quasi-one-dimensional semiconductor and GaAs wires in asbestos nanotubes. Zh. éksp. Teor. Fiz. 111, 274–282 (January 1997)     

Laser-induced synthesis of compound semiconducting films

Compound semiconducting films are synthesized by laser irradiation of sandwich films of the constituants, under various conditions (time of irradiation, laser power, temperature, etc.). Experimental data give evidence of the roles of various parameters. After a presentation of the experimental results, the fundamental physico-chemical mechanisms of laser material interactions are analyzed. Particular emphasis is given to effects that need to be treated by using very-far-from-equilibrium concepts.     

Interesting transmission and reflection from a magnetic film sandwiched between two identical metal films

Using the 4×4 transfer matrix method, we study the optical properties of a designed structure composing one magnetic film sandwiched between two identical metal films. There is a special resonant effect. At one special wavelength, the incidence of one linearly polarized light result in one pure left-handed circular polarization or one pure right-handed circular polarization with the same intensity.     

Dynamical properties of thin ferromagnetic semiconducting films

A Green's function technique is used to investigate the temperature dependence of the spin-wave energies of ferromagnetic semiconducting thin films including the damping effects. It is shown that the frequencies of thin ferromagnetic films are smaller, whereas the damping effects are larger compared to the bulk.     

Photoconductivity studies of Te-substituted Sn-Sb-Se semiconducting films

Amorphous thin films of chalcogenide Sn₁₀Sb₂₀Se_70?X Te _X (0≤X≤8) composition were deposited using the thermal evaporation technique. The dark conductivity measurement showed a thermally activated conduction process with single activation energy in a studied temperature regime. Photoconductivity showed no maxima in the measured temperature regime revealing that the material belongs to the type II photoconductor. The observed small difference between activation energy for photoconduction ΔE _ph and dark conduction ΔE accounts for low photosensitivity of the material. The intensity variation of the photocurrent obeys the power law with the exponent γ～0.56–0.64 revealing the dominant bimolecular recombination mechanism in the studied compositions. Transient photoconductivity revealed that initial rise of the photocurrent becomes slow with tellurium content in the sample. The change in the shape of the transient photocurrent with composition is qualitatively explained based upon change in defect statistics introduced by the tellurium content in the sample. The decay process after the initial decay was found to be nonexponential and is described with a differential life time of charge carrier that showed a decreasing trend with the tellurium content in the sample.     

Thermopower of doped semiconducting hydrogenated amorphous carbon films

The thermopower S(T) and electrical conductivity have been measured from 25 to 250C for semiconducting a-C:H films doped with boron or phosphorus. S has the expected sign (positive for B-doping and negative for P-doping), is low for all films (10–50 μ V/K), and increases nearly linearly with T. This behavior, along with that observed for the electrical conductivity, is consistent with conduction via hopping at or near the Fermi level which has been shifted via doping from near mid-gap into broad bands of tail states at the appropriate band edges.     

Ac electrical parameters of Al-ZnPc-Al organic semiconducting films

The ac electrical parameters of thermally evaporated zinc phthalocyanine, ZnPc, semiconducting thin films was measured in the temperature range of 180–390 K and frequency between 0.1 and 20 kHz. Aluminum electrode contacts were utilized to sandwich the organic ZnPc semiconducting films. Capacitance and loss tangent decreased rapidly with frequency at high temperatures, but at lower temperatures a weak variation is observed. An equivalent circuit model assuming ohmic contacts could qualitatively and successfully explains capacitance and loss tangent behavior. The ac conductivity showed strong dependence on both temperature and frequency depending on the relevant temperature and frequency range under consideration. Ac conductivity σ (ω) is found to vary with ω, as ω ^s with the index s ≤ 1.35 suggesting a dominant hopping conduction process at low temperatures (< 250 K) and high frequency. The conductivity of some samples did not increase monotonically with temperature. This behavior was attributed to oxygen exhaustion of the sample as its temperature is increased. The ac conductivity behavior at low temperatures of ZnPc films could be described well by Elliott model assuming hopping of charge carriers between localized sites.     

TbDyFe薄膜对三明治膜巨磁阻抗效应的影响

将600℃退火后的超磁致伸缩材料(Tb0.27Dy_0.73)_0.3Fe_0.7薄膜作为Ni_80.2Fe_14.1Si_0.2Mn_0.4Mo_5.1三明治膜的基底，制备出四层膜.结果表明：附加的磁致伸缩并没有减小材料的巨磁阻抗(GMI)效应，而由于磁场下磁致伸缩材料的应力效应影响了三明治膜中的各向异性场，使三明治膜的GMI效应增大了4倍.再将制备态的四层膜在280℃下真空退火，退火态四层膜也增大了三明治膜的GMI效应，但可能由于磁致伸缩向磁性层中的扩散，其GMI效应相对于制备态四层膜则有所降低. 关键词： 巨磁阻抗(GMI)效应 三明治膜 TbDyFe薄膜 各向异性场     

Optical properties of semiconducting iron disilicide thin films

The iron silicides samples were prepared by annealing of iron films evaporated onto silicon wafers and capped with amorphous silicon thin overlayers. Semiconducting FeSi₂ phase is formed by annealing at the temperatures from 550°C to 850°C. The optical properties of the FeSi₂ layers have been deduced from reflectance and transmittance measurements carried out in the temperature range of (77–380) K. The spectral dependence of the absorption coefficient favours direct allowed transitions with forbidden energy gap of 0.87eV at the room temperature. The application of a simple three-parameter semiempirical formula to the temperature dependence of the direct energy gaps leads to the following best fit parameters: the band gap at zero temperature E _g (0) = (0.895 ± 0.004)eV, the dimensionless coupling parameter S = 2.0 ± 0.3, and the average phonon energy <hw> = (46 ± 8)meV. By examining all the reported triplets of parameters for -FeSi₂ fabricated by different techniques and thermal processes, an obvious discrepancy can be found for the lattice coupling parameter and average phonon energy, although the bandgaps at 0 K are very similar. Unlike the theoretical prediction and the earlier reported result, our results do not show any evidence of a particularly strong electron-phonon interaction, which would give the lower carrier mobilities. -FeSi₂ seems to be an intriguing material where states with energies near the band edges permit ambiguous interpretation of the character of transitions. From optical model for the thin film-substrate system we found the index of refraction to be (5–5.9) in the photon energy interval from 0.65 to 1.15eV. There is also indication of an additional higher-energy absorption edge at l.05eV.     

Effect of distance between acceptor and donor on optical properties of composite semiconducting polymer films

The excitation energy transfer from poly(N-vinylcarbazole) (PVK) to tris(8-hydroxyquinoline) aluminum (Alq₃) in composite films was investigated by adding an inert polymer, poly(methyl methacrylate) (PMMA). The energy transfer efficiency calculated from the photoluminescence (PL) excitation spectra is consistent with that from the time-resolved PL decay data of the composite films. We have found a linear relationship between the two kinds of the distances, which are calculated according to volume density and the Förster theory. Experimental results and analyses provide a facile method to infer the energy transfer efficiency and the distance between the donor and the acceptor molecules in the composite films.     

Dielectric properties of evaporated films

The evidence for vacancy-pair formation in alkali halide crystals is reviewed. Existing information on the dielectric properties and structure of thin films is discussed with a view to using the high defect concentrations in vacuum-deposited films to confirm vacancy-pair orientation effects. Experimental measurements of the dielectric constant and dissipation factor of evaporated films have been made at room temperature over a range of frequencies from 100 c.p.s. to 100 kc/s. Even under vacuum these films show pronounced losses at low frequencies which are attributed to excess vacancy concentrations. Such losses are contrary to the theory of ideal lattices and are not shown by single crystals.

In spite of the excess defect concentrations no evidence of vacancy-pair orientation has been found. Ageing effects have been observed in all cases, the losses decreasing with time as the defect concentration decreases. In the alkali halides the magnitude of the losses depends on the cation mobility but they decrease at a rate determined by the anion mobility. This leads to a proposed dielectric relaxation mechanism in which individual crystallites form the polarizable units, becoming polarized by migration of the cation vacancies towards one end. The losses decrease as the defects are gradually eliminated by simultaneous condensation of positive and negative vacancies at grain boundaries. This is essential to maintain electrical neutrality and the rate is determined by the diffusion of the slower anion vacancies. The measured rates are in agreement with anion activation energies obtained by tracer methods. These results cannot be explained by vacancy-pair formation even if it is assumed that vacancy pairs can form but are incapable of orientation and hence we must conclude that there is little or no vacancy-pair formation.

Measurements at atmospheric pressure show that moisture has a pronounced effect in all cases, producing dielectric losses which completely obscure the vacancy effects. The changes in dielectric properties during and after adsorption cannot be explained as conductivity effects and are in complete opposition to any modification of the Maxwell-Wagner theory. The ageing effects show that after the initial adsorption, the water molecules migrate over crystallite surfaces to positions where they are more strongly bound and contribute to the dielectric polarization by a form of hindered rotation closely analogous to mechanisms proposed for ice. At these equilibrium sites, H-bonds are frequently formed between the adsorbed molecules and the halide ions of the crystal lattice.     

Electrical and optical properties of semiconducting camphoric carbon films

In situ variation in resistance of camphoric carbon versus time of pyrolysis, temperature of pyrolysis and effect of sintering are studied to perceive the time required for the completion of pyrolysis and the activation energy from the electrical conductance plot. Variation in the electrical conductance versus temperature and activation energies derived from these measurements, are reproducible when film is thermally treated below 750 °C. Thermal treatment above 750 °C changes the anatomy of the film causing a change in the conductance profile as well as decreases its band gap to 0.1 eV. Camphor pyrolyzed at 650 °C gives semiconducting carbon with optical band gaps 1 eV (direct) and 0.8 eV (indirect). Increase in pyrolysis temperature also shifts G-band of Raman spectrum from 1605 to 1586 cm⁻¹ i.e., towards value corresponding to graphitic carbon. SEM micrograph of camphoric film shows absence of any carbon nanobeads or fibers as normally observed with camphoric carbon pyrolysed in this temperature range.     

Magnetism due to oxygen vacancies and/or defects in undoped semiconducting and insulating oxide thin films

Room temperature ferromagnetism was observed in HfO₂, TiO₂, and In₂O₃ films grown on yttrium-stabilized zirconia, LaAlO₃, and MgO substrates, respectively. While the magnetic moment is rather modest in the case of In₂O₃ films, it is very large in the other two cases. Thin film form, which might create necessary defects and/or oxygen vacancies, must be the main reason for undoped semiconducting and insulating oxides to become ferromagnetic. From the results, a serious question arises if a transition-metal doping indeed plays any essential role in producing ferromagnetism (FM) in non-magnetic oxides.