Electric field dependence of conductivity in thin amorphous films and bulk glasses of the system Ge4Se6−xTex

The electric field dependence of the conductivity of amorphous Ge₄Se_6−xTe_x (x = 0, 0.5, 1.0 or 1.5) is strongly influenced by the way in which the samples are prepared. Bulk samples are compared with thin films of identical composition prepared by vacuum evaporation at relatively low temperature or by flash evaporation. Differences in the shape of the field dependence of the conductivity are discussed in terms of a conductivity model which takes into account the Poole-Frenkel effect with intrinsic defects of the valence alternation pair type and hopping contributions from charge transport via localized states at the band edges. It is shown that the preparation technique influences the density of intrinsic defects and localized states at the band edges resulting in a different field dependence of the conductivity.     

Engineering the Charge Transport of Ag Nanocrystals for Highly Accurate,Wearable Temperature Sensors through All‐Solution Processes

All‐nanocrystal (NC)‐based and all‐solution‐processed wearable resistance temperature detectors (RTDs) are introduced. The charge transport mechanisms of Ag NC thin films are engineered through various ligand treatments to design high performance RTDs. Highly conductive Ag NC thin films exhibiting metallic transport behavior with high positive temperature coefficients of resistance (TCRs) are achieved through tetrabutylammonium bromide treatment. Ag NC thin films showing hopping transport with high negative TCRs are created through organic ligand treatment. All‐solution‐based, one‐step photolithography techniques that integrate two distinct opposite‐sign TCR Ag NC thin films into an ultrathin single device are developed to decouple the mechanical effects such as human motion. The unconventional materials design and strategy enables highly accurate, sensitive, wearable and motion‐free RTDs, demonstrated by experiments on moving or curved objects such as human skin, and simulation results based on charge transport analysis. This strategy provides a low cost and simple method to design wearable multifunctional sensors with high sensitivity which could be utilized in various fields such as biointegrated sensors or electronic skin.     

Mesoscopic 3D Charge Transport in Solution-Processed Graphene-Based Thin Films: A Multiscale Analysis

Graphene and related 2D material (GRM) thin films consist of 3D assembly of billions of 2D nanosheets randomly distributed and interacting via van der Waals forces. Their complexity and the multiscale nature yield a wide variety of electrical characteristics ranging from doped semiconductor to glassy metals depending on the crystalline quality of the nanosheets, their specific structural organization ant the operating temperature. Here, the charge transport (CT) mechanisms are studied that are occurring in GRM thin films near the metal-insulator transition (MIT) highlighting the role of defect density and local arrangement of the nanosheets. Two prototypical nanosheet types are compared, i.e., 2D reduced graphene oxide and few-layer-thick electrochemically exfoliated graphene flakes, forming thin films with comparable composition, morphology and room temperature conductivity, but different defect density and crystallinity. By investigating their structure, morphology, and the dependence of their electrical conductivity on temperature, noise and magnetic-field, a general model is developed describing the multiscale nature of CT in GRM thin films in terms of hopping among mesoscopic bricks, i.e., grains. The results suggest a general approach to describe disordered van der Waals thin films.     

The effect of adsorbed oxygen on the conductivity of perylenetetracarboxylic acid <Emphasis Type="Italic">N,N</Emphasis>-dimethyldiimide films

The effect of adsorbed oxygen on the conductivity of vacuum-deposited perylenetetracarboxylic acid N, N-dimethyldiimide films with variable surface topography was studied using the method of cyclic thermodesorption. The results are interpreted in terms of a two-level hopping conductivity model. Microscopic parameters of the electron hopping transport (the radius of electron localization in the intrinsic and impurity states and the concentration of localization centers) are evaluated. The regions of oxygen concentrations in which the electron transport proceeds by hopping via intrinsic states, or impurity localized electron states, or a combined system involving the states of both types are determined.     

The effect of adsorbed oxygen on the conductivity of lead phthalocyanine films

The effect of adsorbed oxygen on the conductivity of lead phthalocyanine films obtained by laser deposition in vacuum was studied by the method of cyclic thermodesorption. The results are explained in terms of the hopping conductivity model. It is suggested that the electron transport proceeds by hopping via both intrinsic and impurity centers differing by the radius of electron localization. It is shown that, using the method of cyclic thermodesorption in combination with the hopping conductivity model, it is possible to evaluate the electron transport parameters, such as concentration of the localization centers and the radius of electron localization in the intrinsic and impurity states, and to determine the type of centers (intrinsic versus impurity) involved in the hopping transport.     

Structural,dielectric, and electrical studies on thermally evaporated CdTe thin films

Sandwich structures of cadmium telluride (CdTe) thin films between Ag electrodes were prepared by thermal evaporation technique at a vacuum of ~2 × 10⁻⁵ torr. Structural characterization of these thin films was performed using X-ray diffraction (XRD) studies. The effect of temperature and frequency on the electrical and dielectric properties of these films was studied in detail and reported in this article. The experimental study indicates that for the CdTe thin film the dielectric constant and dielectric loss increases with temperature and decreases with frequency. However, A.C. conductivity increases both with temperature and frequency. The data of complex impedance measurements over the same range of temperature and frequency are used to describe the relaxation behavior of the CdTe film. Our results indicate that the transport behavior of carriers in CdTe thin films is consistent with the correlated barrier hopping (CBH) model.     

Novel approach to the study of electrical conduction in bromine-doped polyacetylene

Using X-ray analysis we showed that on doping with iodine or bromine trans-polyacetylene loses its crystalline character. The optical and electrical properties of bromine-doped films (approximately 1000 Å thick) were investigated and the formation of charge transfer complexes was quantitatively correlated to the production of charge carriers and to the conductivity σ. Our results indicate that the most probable conduction mechanism which obtains at room temperature in halogen-doped polyacetylene is three-dimensional disordered hopping. The density of states N(E_F), the hopping distance R and the carrier localized wavefunction α were estimated for this organic semiconductor.     

The substrate temperature dependent electrical properties of titanium dioxide thin films

Titanium dioxide thin films were obtained by a dc sputtering technique onto heated glass substrates. The relationship between the substrate temperature and the electrical properties of the films was investigated. Electrical resistivity measurements showed that three types of conduction channels contribute to conduction mechanism in the temperature range of 13–320 K. The temperature dependence of electrical resistivity between 150 and 320 K indicated that electrical conduction in the films was controlled by potential barriers caused by depletion of carriers at grain boundaries. The conduction mechanism of the films was shifted from grain boundary scattering dominated band conduction to the nearest neighbor hopping conduction at temperatures between 55 and 150 K. Below 55 K, the temperature dependence of electrical resistivity shows variable range hopping conduction. The correlation between the substrate temperature and resistivity behavior is discussed by analyzing the physical plausibility of the hopping parameters and material properties derived by applying different conduction models. With these analyses, various electrical parameters of the present samples such as barrier height, donor concentration, density of states at the Fermi level, acceptor concentration and compensation ratio were determined. Their values as a function of substrate temperature were compared.     

Transport study of a novel polyfluorene/poly(p-phenylenevinylene) copolymer by various mobility models

The polyfluorene/poly(p-phenylenevinylene) copolymer based hole-only devices are fabricated and the current–voltage characteristics are measured as a function of temperature. The hole current is fitted well with space-charge limited and field-dependent mobility model, which provides a direct measurement of the hole mobility μ as a function of electric field E and temperature. The mobility is fitted with existing Gill’s model, Gaussian disorder model, correlated Gaussian disorder model and Brownian motion model. Energy hopping time and activation energy are obtained from Brownian motion model. Microscopic transport parameters are derived and a consistent picture of the influence of the molecular structure of the polymer on the charge transport is depicted. For the polyfluorene/poly(p-phenylenevinylene) copolymer, although with a high degree of irregularity in structure and larger energetic disorder, the two bulky structure favors charge delocalization and remove defect sites, results in a higher mobility. The results suggest space-charge limited and field-dependent mobility model combine with various mobility model, include Brownian motion model, is a useful technique to study charge transport in thin films with thicknesses close to those used in real devices.     

Trap State Effects in PbS Colloidal Quantum Dot Exciton Kinetics Using Photocarrier Radiometry Intensity and Temperature Measurements

Colloidal quantum dots (CQDs) have attracted significant interest for applications in electronic and optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells. However, a poor understanding of charge transport in these nanocrystalline films hinders their practical applications. The photocarrier radiometry (PCR) technique, a frequency-domain photoluminescence method spectrally gated for monitoring radiative recombination photon emissions while excluding thermal infrared photons due to non-radiative recombination, has been applied to PbS CQD thin films for the analysis of charge transport properties. Linear excitation intensity responses of PCR signals were found in the reported experimental conditions. The type and influence of trap states in the coupled PbS CQD thin film were analyzed with PCR temperature- and time-dependent results.     

Electrical transport properties of as evaporated and annealed amorphous GexSe1−x thin films

D.C. conductivity measurements were made on evaporated and annealed amorphous Ge_xSe_1−x thin films as a function of temperature (80–330K) and composition (x = 1.0, 0.9, 0.7, 0.5, 0.3, 0.1). It was observed that for Ge-rich films (x = 1.0, 0.9, 0.7) the conduction in the high temperature region takes place due to thermally assisted tunneling of charge carriers in localized states at the band edges and at low temperatures conduction is due to variable range hopping in localized states near the Fermi level. For Se-rich films (x=0.5, 0.3, 0.1) the conduction is intrinsic in the entire temperature range of measurements. It was observed that annealing reduces the density of states at the Fermi level. At the highest annealing temperature films become polycrystalline.     

Charge transport and ammonia sensing properties of flexible polypyrrole nanosheets grown at air–liquid interface

Flexible polypyrrole nanosheets (thickness ∼150 nm) grown at the air–liquid interface have been investigated for charge transport and NH₃ sensing application. Polypyrrole nanosheets films exhibited a uniform and dense morphology. Temperature dependent charge transport measurements revealed that the PPy films obey Mott's 3-D variable range hopping mechanism. The mobility values calculated using temperature dependent current voltage characteristics indicated them to obey Arrhenius behaviour. These films exhibited a reversible response towards NH₃ at room temperature. The sensor exhibited a sensitivity of ∼12% with a typical response and recovery times of 240 s and 50 min, respectively towards 50 ppm of NH₃. Raman studies indicated that there is an increase in the antisymmetrical C–N stretching upon exposure to higher concentration of NH₃ (500 ppm) and could be assigned to the interaction of NH₃ with the carbon backbone of PPy film. Our results clearly emphasize that these flexible PPy films could be used to realize flexible sensors.     

Sn doping effects on properties of ZnO thin films deposited by RF magnetron sputtering using a powder target

In the present study, tin doped ZnO thin films (ZnO:Sn) at different contents (0–3 wt%) were deposited onto glass substrates by RF magnetron sputtering using a powder compacted target at room temperature. The effect of Sn concentration on the structural, optical and electrical properties of the ZnO:Sn thin films were investigated. The X-ray diffraction analysis shows that the pure ZnO thin film exhibits a strong intensity of the (002) peak indicating a preferential orientation along the c-axis. For Sn doped ZnO thin films, there is a change in the orientation from the (002) plane to the (101) one. The undoped ZnO thin films have transmittance 85% in the visible range and slightly increased for 0.5 wt% of Sn, while it get decreased with further increasing the Sn doping concentration. The optical band gap energy get increased with increasing the doping concentration. Moreover, the electrical conductivity and conduction mechanism are also studied by impedance spectroscopy in the frequency range of 1KHz–13 MHz at various temperatures (633–743 K). The AC conductivity in ZnO thin films increased with angular frequency. The frequency exponent S decreases with increasing temperature. Such behavior suggests that the correlated barrier hopping (CBH) model may be suitable to explain the conduction mechanism in ZnO thin films. The activation energy values calculated from angular frequency and DC conductivity are in good agreement confirming that the conduction mechanism is thermally activated by hopping between localized states.     

Electrical conduction mechanism in solution grown thin polyvinylchloride (PVC) films

The electrical transport behaviour of thin polyvinylchloride (PVC) films deposited by the solution growth technique has been investigated. The current transport in PVC films of 2500 Å thickness at temperatures below 250 K is ascribed to hopping mechanisms. The weak temperature dependence of the conductivity is attributed to interchain hopping, whereas the strong temperature dependence is attributed to the trap hopping process. The conductivity of PVC films is increased on doping with iodine. This is interpreted on the basis of the formation of charge transfer complexes in the film. The activation energy for conduction increases from 0.7 to 1.22 eV at 315 K and decreases from 2.2 to 0.8 eV at 375 K on doping PVC films with iodine (0.7 g of iodine per 100 ml PVC solution). At high (≧5 × 10⁴ V cm^-1) fields and at higher (≧350 K) temperatures, the observed conduction behaviour can be described by the Schottky emission mechanism. The height of the Schottky barrier is found to depend on the type of the metal electrode and the direction of the current. The barrier height decreases with increasing iodine concentration in the PVC films.     

Transport phenomena in thin organic layers

Electronic transport phenomena in thin polycrystalline organic films were studied using transient currents and a.c. techniques. The results obtained for tetracene and p-terphenyl layers are very similar and may be explained by a hopping model. Additional effects are caused by inter-grain barriers. The heights of these barriers (0.07–0.08 eV) are reduced by an external electric field.     

Effect of substitution of Sn for Bi on structural and electrical transport properties of SnS thin films

In this work, results are reported concerning the effect of the Bi concentration on the structural and electrical transport properties of SnS thin films, grown through a chemical reaction of the metallic precursors with elemental sulfur (sulfurization) in a two-stage process. XRD measurements revealed that the samples deposited by sulfurization of Sn or Bi grow in the SnS and Bi₂S₃ phases, respectively, whereas those obtained by sulfurization of a Sn:Bi alloy grow with a mixture of several phases. Special emphasis was placed on studying through σ versus T measurements, the effect of the Bi concentration on the transport properties of SnS:Bi films. To identify the dominant transport mechanisms, the σ versus T curves were analyzed in two different temperature ranges. It was also found that in the range of temperatures greater than 300 K, the conductivity is predominantly affected by transport of free carriers in extended states of the conduction band, whereas in the range of temperatures below 250 K, the conductivity is dominated by the VRH (variable range hopping) transport mechanism.     

Transport layer at the boundary of two polymer films

Electron transport in a two-dimensional boundary layer between two films of an organic polymer has been studied. It is established that the electric conductivity of such layers is several orders of magnitude higher than the surface conductivity of individual polymer films involved in the experimental bilayer structure. Measurements of the temperature dependence of the conductivity showed that the transfer of charge carriers along the boundary layer can be described in terms of a model based on the thermoelectron emission and hopping transport over trap levels.     

N掺杂Cu2O薄膜的低温沉积及快速热退火研究

采用氧化亚铜(Cu_2O)陶瓷靶,利用射频磁控溅射沉积法在氮气和氩气的混合气氛下制备了N掺杂Cu_2O(Cu_2O∶N)薄膜,并在N_2气氛下对薄膜进行了快速热退火处理,研究了N_2流量和退火温度对Cu_2O∶N薄膜的生长行为、物相结构、表面形貌及光电性能的影响。结果显示,在衬底温度300℃、N_2流量12sccm条件下生长的薄膜为纯相Cu_2O薄膜;在N_2气氛下对预沉积薄膜进行快速热退火处理不影响薄膜的物相结构,薄膜的结晶质量随退火温度(450℃)的升高而显著改善;快速热退火处理能改善薄膜的结晶质量和缺陷,降低光生载流子的散射,增强载流子的传输,预沉积Cu_2O∶N薄膜经400℃退火处理后展示出较好的电性能,薄膜的霍尔迁移率(μ)为27.8cm~2·V~(-1)·s~(-1)、电阻率(ρ)为2.47×10~3Ω·cm。研究表明低温溅射沉积和快速热退火处理能有效改善Cu_2O∶N薄膜的光电性能。     

Study of electrical properties of CIGS thin films prepared by multistage processes

In this work, the dispersion mechanisms affecting the electric transport in CuIn_1−xGa_xSe₂ (CIGS) thin films grown by a chemical reaction of the precursor species, which are evaporated sequentially in two and three-stage processes are analyzed. It was found, through conductivity and Hall coefficient measurements carried out as functions of temperature, that the electrical conductivity of the CuIn_1−xGa_xSe₂ films is affected by the transport of free carriers in extended states of the conduction band as well as for variable range hopping transport mechanisms, each one predominating in a different temperature range.     

Electrical transport properties of thin film of a-Se87Te13 nanorods

We have studied the electrical transport properties of thin film of a-S₈₇Te₁₃ nanorods. Initially, the glassy alloy of S₈₇Te₁₃ is prepared by melt-quenching technique. The amorphous nature of this alloy is verified by using X-ray diffraction technique. The nanorods of a-S₈₇Te₁₃ are synthesised on a glass substrate under an ambient gas (Ar) atmosphere using physical vapour condensation system. The morphology and microstructure of these nanorods are studied using scanning electron microscopy and transmission electron microscopy. The temperature dependence of dc conductivity for these nanorods is also studied over a temperature range of 500–100?K. On the basis of the temperature dependence of dc conductivity, the conduction mechanism in these nanorods is investigated. The results reveal that the thermally activated process is responsible for the transport of carriers in the temperature range 500–300?K. While the conduction takes place via variable range hopping (VRH) for temperature region 300–100?K. It is therefore, suggested that three-dimensional Mott's variable range hopping (3D VRH) is the conduction mechanism responsible for the transport of charge carriers in the temperature region 300–100?K. Various Mott's parameters such as density of states, degree of disorder, hopping distance and hopping energy are estimated on the basis of best fitting to our experimental data for Mott's 3D VRH model.