Transport Properties Of Films Research Articles

Influence of annealing on the local structure, magnetic and transport properties of Mn-doped SiC films

Mn-doped SiC films are deposited on Si (1 0 0) substrates by the RF-magnetron sputtering technique. The influence of annealing on the local structure, magnetic and transport properties of the films is investigated systematically by x-ray diffraction, x-ray photoelectron spectroscopy, x-ray absorption fine structure, transport and magnetic measurements. The results show that for the as-deposited and 800 °C annealed films, the doped Mn atoms occupy the C sites of the 3C-SiC lattice, only semiconducting behaviour is observed in the whole temperature range and the transport property is governed by Mott variable range hopping behaviour, suggesting that the carriers are strongly localized. After 1200 °C annealing, the majority of Mn atoms form Mn4Si7 secondary phases and a crossover from semiconducting to metallic transport behaviour with transition temperatures occurring at 134 K is detected in the film. Magnetic characterization shows that all the films are ferromagnetic with a Curie temperature of above 300 K and the saturated magnetic moment increases monotonically with increasing annealing temperature. It is speculated that the ferromagnetism of the films is intrinsic and can be ascribed to rely on the percolation of bound magnetic polarons for the as-deposited and 800 °C annealed films. After 1200 °C annealing, the carbon-incorporated Mn4Si7 compounds can be considered to be the origin of ferromagnetism in the film.

The effects of substrate temperature and oxygen pressure on the crystal strcture and transport properties of Bi2Sr2Co2Oy thermoelectric films deposited by pulsed laser deposition

We fabricate Bi2Sr2Co2Oy films on c-Al2O3 by pulsed laser deposition and investigate the effects of substrate temperature and oxygen pressure on the crystal stucture and the transport properties of the films. The resulting single phase c-axis Bi2Sr2Co2Oy films obtained under the optimal condition have a room temperature resistivity of about 2.9 m/cm and a seebeck coefficient of 110 V/K, leading to a larger power factor than that of the single crystal. In addition, a negative magnetroresistance of 40% at 2K and 9T is observed in the films.

Post annealing effect on transport properties of La 0.67Ca 0.33MnO 3 films grown on vicinal cut substrates

La 0.67Ca 0.33MnO 3 thin films have been grown on 10°, 15°, and 20° vicinal cut SrTiO 3 (1 0 0) substrates by pulse laser deposition. The single phase and the least textured growth have been studied by X-ray diffraction analysis. The post annealing effect with high temperature and high oxygen pressure on the transport properties of films has been investigated by resistance versus temperature measurements. Films with post annealing show large enhancement of metal–insulator transition temperature T p about 20–30 K towards higher temperature and obvious decrease of resistance, which is attributed to the refilling of oxygen, the change of Mn–O–Mn angle and the improvement of crystallinity by the post annealing effect. Specially, film on 20° vicinal cut substrate exhibits the biggest range gap of peak resistance drop, which may originate from more defects caused by steps at this tilt angle and many of these defects are removed after post annealing.

Growth and transport properties of oriented bismuth telluride films

Oriented n-type bismuth telluride thin films with various layered nanostructures have been fabricated by radio-frequency (RF) magnetron sputtering. The crystal structures and microstructures of the films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The transport properties including carrier concentration, mobility, Seebeck coefficient and in-plane electrical conductivity were measured, which showed strong microstructure-dependent behaviors. The relationship between morphologies and transport properties of the films was explored. The optimal morphology and transport properties of films were obtained at the substrate temperature of 350 °C under the pressure of 1.0 Pa with oriented layered structure. Based on these results, a formation mechanism of these nanostructures is proposed and discussed. The interfaces and grain boundaries formed in these layered structures are beneficial to the reduction in thermal conductivity, which could result in potential TE films with high ZT value.

Electronic transport on carbon nanotube networks: A multiscale computational approach

Carbon nanotube networks are one of the candidate materials to function as malleable, transparent, conducting films, with the technologically promising application of being used as flexible electronic displays. Nanotubes disorderly distributed in a film offers many possible paths for charge carriers to travel across the entire system, but the theoretical description of how this charge transport occurs is rather challenging for involving a combination of intrinsic nanotube properties with network morphology aspects. Here we attempt to describe the transport properties of such films in two different length scales. Firstly, from a purely macroscopic point of view we carry out a geometrical analysis that shows how the network connectivity depends on the nanotube concentration and on their respective aspect ratio. Once this is done, we are able to calculate the resistivity of a heavily disordered networked film. A comparison with an experiment offers us a way to infer about the junction resistance between neighbouring nanotubes. Furthermore, in order to guide the frantic search for high-conductivity films of nanotube networks, we turn to the microscopic scale where we have developed a computationally efficient way for calculating the ballistic transport across these networks. While the ballistic transport is probably not capable of describing the observed transport properties of these films, it is undoubtedly useful in establishing an upper value for their conductivity. This can serve as a guideline in how much room there is for improving the conductivity of such networks.

Bi0.99Sb0.01 Thin Film Transport Properties and Size Effect

Bi0.99Sb0.01 Thin Film Transport Properties and Size Effect

Electrical and Structural Real-Time Changes in Thin Thermoelectric (Bi0.15Sb0.85)2Te3 Films by Dynamic Thermal Treatment

A recent trend in thermoelectrics is miniaturization of generators or Peltier coolers using the broad spectrum of thin-film and nanotechnologies. Power supplies for energy self-sufficient micro and sensor systems are a wide application field for such generators. It is well known that thermal treatment of as-deposited p-type (Bi 0.15 Sb 0.85 ) 2 Te 3 films leads to enhancement of their power factors. Whereas up to now only the start (as-deposited) and the end (after annealing) film stages were investigated, herein for the first time, the dynamical changes of sputter-deposited film properties have been observed by real-time measurements. The electrical conductivity shows a distinct, irreversible increase during a thermal cycle of heating to about 320°C followed by cooling to room temperature. The interpretation of the Seebeck and Hall coefficients points to an enhancement in Hall mobility after annealing. In situ x-ray diffractometry shows the generation of an additional Te phase depending on temperature. This is also confirmed by energy-dispersive x-ray microanalysis and the corresponding mapping by scanning electron microscopy. It is presumed that the Te enrichment in a separate, locally well-defined phase is the reason for the improvement in the integral film transport properties.

Tunneling and charging effects in discontinuous superparamagneticNi81Fe19/Al2O3multilayers

The magnetic and transport properties of films based on discontinuous layers of ${\text{Ni}}_{81}{\text{Fe}}_{19}$ (Py) embedded in ${\text{Al}}_{2}{\text{O}}_{3}$ were investigated. In films with nominal Py thicknesses 6 and $8\text{ }\text{\AA{}}$ superparamagnetic particles with median diameters ${D}_{\text{med}}=2.8$ and 3.1 nm and distribution widths ${\ensuremath{\sigma}}_{\text{D}}=1.2$ and 1.3 nm were formed. Current voltage $(IU)$ curves were measured with the current perpendicular to the film plane. The analyses show that the charge transport occurs via tunneling; with the charging energy supplied by thermal fluctuations at high temperature, $T\ensuremath{\ge}100\text{ }\text{K}$, and by the electric field at low temperature, $T<10\text{ }\text{K}$. The separation of the two regimes allows independent estimates of the mean charging energy $⟨{E}_{\text{C}}⟩\ensuremath{\approx}40\text{ }\text{meV}$ for both samples; from the resistance $R$ versus $T$ analyzed in an effective-medium model at high temperature and from $I$ versus $U$ at 4 K. In order to obtain a consistent description of the transport properties, the size distributions must be included to account for the deviation from the single size behavior $R\ensuremath{\sim}\text{exp}({E}_{\text{C}}/{k}_{\text{B}}T)$ at high $T$. The scaling parameter in the relation $I\ensuremath{\propto}{(\frac{U}{{U}_{\text{th}}}\ensuremath{-}1)}^{\ensuremath{\gamma}}$, where ${U}_{\text{th}}$ is the threshold for conduction, is estimated to $\ensuremath{\gamma}\ensuremath{\approx}2$ at 4 K. The superparamagnetic relaxation of the particles becomes blocked below a temperature $T\ensuremath{\approx}20\text{ }\text{K}$ respective 30 K for 6 and $8\text{ }\text{\AA{}}$. The magnetic field $(B)$ dependence of the resistance $R(B)$ displays a single maximum of the ratio $\text{MR}=[R(B)\ensuremath{-}R(2\text{ }\text{T})]/R(2\text{ }\text{T})$ in zero field at room temperature and a characteristic splitting of the peak at 4 K, attributed to the blocking. The maxima, $\ensuremath{\approx}0.9\mathrm{%}$ for $6\text{ }\text{\AA{}}$ and 1.1% for $8\text{ }\text{\AA{}}$, are positioned at fields about a factor of two to three higher than the coercive fields of the samples.

Effect of PVA addition on formation of spray-deposited Sm0.5Sr0.5CoO3 thin films on CeO2 substrates

Nano-crystalline films of Sm0.5Sr0.5CoO3 (SSC) have been formed on CeO2 substrates by spraying stoichiometric aqueous solution containing Sm, Sr, and Co ions. Effect of polyvinyl alcohol (PVA) addition as a complexing agent in spray solution on stoichiometry, crystallite size, morphology, and transport properties of film are studied. The results showed that the SSC cathode had maximum crystallite size for 40% PVA addition. Electrical performance of film decreases with decrease in the particle size, while the electronic to ionic predominance transition temperature decreases with decreasing particle size. These films are studied for their potential application as a cathodic material in developing intermediate temperature solid oxide fuel cells.

Transverse photothermal beam deflection technique for determining the transport properties of semiconductor thin films

Nonradiative transitions occurring in semiconductors result in thermal emissions carrying information on the material’s thermal and electronic properties. A simple one-dimensional theoretical model is devised which accounts for the photothermal signal variations due to nonradiative transitions occurring in semiconductor thinfilms. The theory was verified by determining the transport properties of p-type silicon wafer. We could get the thermal diffusivity, minority carrier lifetime, surface recombination velocity, and minority carrier mobility of CuInS2 thin films, thereby proving the efficiency and simplicity of photothermal beam deflection technique for real time characterization of semiconductor thin films. The film fabrication history, composition, and post deposition treatments play crucial role in determining the transport properties and the effect of these conditions on transport properties of the film as well as on the solar cell parameters is discussed.

Influence of the Polymerization Potential on the Transport Properties of Polypyrrole Films

Polypyrrole films have been prepared by potentiostatic electrochemical polymerization at low temperatures. The cyclic voltammograms and the electronic transport properties of the films are investigated as a function of the polymerization potential. As the potential increases from 520 mV to 1.2 V, the oxidation peak moves to larger voltages, while above 1.2 V, the peak voltage drops again. The film conductivity drops monotonously as the polymerization potential is increased. However, the localization length of the current-carrying states, which characterizes the temperature dependence of the conductivity, correlates with the oxidation peak and shows a minimum for films polymerized at 1.2 V. Furthermore, we show that, with an independent doping step after polymerization, the conductivity of the films can be increased by up to 50%. A maximum conductivity of 1360 S/cm has been observed.

Dynamic in situ observations of electrical and structural changes in thin thermoelectric (Bi0.15Sb0.85)2Te3 films

Thin films of (Bi0.15Sb0.85)2Te3 were prepared by dc magnetron sputter deposition on different substrates. It is well known that thermal treatment of as-deposited p-type (Bi0.15Sb0.85)2Te3 films leads to an enhancement of the power factor. Whereas up to now only the initial (as deposited) and the final (after annealing) film stages have been investigated, here, the dynamic changes of sputter-deposited film properties have been observed by in situ measurements. The enhancement of the power factor shows a significant dependence on thermal treatment. The best thermoelectric films have been prepared at a substrate temperature of 170 °C, with a power factor of 24.4 μW/(cm K2). The changes in the Seebeck and Hall coefficients are caused by the enhancement in the Hall mobility after annealing. In situ x-ray diffractometry shows the generation of additional Te in dependence of the temperature. This is also confirmed by energy-dispersive x-ray microanalysis and the corresponding mapping in a scanning electron microscope. It is supposed that the locally well-defined Te enrichment is the reason for the improvement in the integral film transport properties.

Electron scattering in Au films containing Co clusters

We present a study of the transport properties of films consisting of Co n clusters embedded in a Au matrix for different Co cluster concentrations. The films were prepared by low-energy cluster beam deposition of Co clusters, combined with simultaneous thermal evaporation of Au. By varying the deposition rates of both Co clusters and Au, films with different Co concentrations were obtained. We investigated the influence of the Co concentration on the low-temperature resistance behavior, thus probing the dominant scattering mechanisms determining the film resistance. The change in low-temperature resistance behavior reveals a transition from films with a low Co concentration, where the low-temperature resistance is dominated by phonon scattering to increasingly ferromagnetically behaving films, in which electron–electron interactions are dominant. The Debye temperature of the films was found to scale with the Co concentration in the films. The low-temperature electron mean free path was determined, which allowed to investigate the influence of the film granularity on the resistance. The temperature dependence of the resistivity reflects the increasing granularity as Au is gradually replaced by Co grains.

Effect of 200 MeV Ag15+ ion irradiation on structural and electrical transport properties of Fe3O4 thin films

Thin films of Fe3O4 have been deposited on single crystal MgO(100) and Si(100) substrates using pulsed laser deposition. Films grown on MgO substrate are epitaxial with c-axis orientation whereas, films on Si substrate are highly 〈111〉 oriented. Film thicknesses are 150nm. These films have been irradiated with 200MeV Ag ions. We study the effect of the irradiation on structural and electrical transport properties of these films. The fluence value of irradiation has been varied in the range of 5×1010ions/cm2 to 1×1012ions/cm2. We compare the irradiation induced modifications on various physical properties between the c-axis oriented epitaxial film and non epitaxial but 〈111〉 oriented film. The pristine film on Si substrate shows Verwey transition (TV) close to 125K, which is higher than generally observed in single crystals (121K). After the irradiation with the 5×1010ions/cm2 fluence value, TV shifts to 122K, closer to the single crystal value. However, with the higher fluence (1×1012ions/cm2) irradiation, TV again shifts to 125K.

Mass Transport and Electrode Accessibility Through Periodic Self-Assembled Nanoporous Silica Thin Films

Ordered nanoporous silica films have attracted great interest for their potential use to template nanowires for photovoltaics and thermoelectrics. However, it is crucial to develop films such that an electrode under the nanoporous film is accessible to solution species via facile mass transport through well-defined pores. Here, we quantitatively measure the electrode accessibility and the effective species diffusivity for nearly all the known nanoporous silica film structures formed by evaporation-induced self-assembly upon dip-coating or spin-coating. Grazing-angle of incidence small-angle X-ray scattering was used to verify the nanoscale structure of the films and to ensure that all films were highly ordered and oriented. Electrochemical impedance spectroscopy (EIS) was then used to assess the transport properties. A model has been developed that separates the electrode/film kinetics and the film transport properties from the film/solution interface and bulk solution effects. Accounting for this, the accessible area of the nanoporous film coated FTO electrode (1-theta) is obtained from the high-frequency data, while the effective diffusivity of the ferrocene dimethanol (D(FDM)) redox couple is obtained from intermediate frequencies. It was found that the degree of order and orientation in the film, in addition to the symmetry/topology, is a dominant factor that determines these two key parameters. The EIS data show that the (211) oriented double gyroid, (110) oriented distorted body center cubic, and (211) distorted primitive cubic silica films have significant accessibility (larger than 26% of geometric area). However, the double-gyroid films showed the highest diffusivity by over an order of magnitude. Both the (10) oriented 2D hexagonal and (111) oriented rhombohedral films were found to be highly blocking with only small accessibility due to microporosity. The impedance data were also collected to study the stability of the nanoporous silica films in aqueous solutions as a function of pH. The distorted primitive silica film showed much faster degradation in pH 7 solution when compared to a blocking film such as the 2D hexagonal. However, silica films maintained their structure at pH 2 for at least 12 h.

Structural investigation of thin tetracene films on flexible substrate by synchrotron X-ray diffraction

Structural properties of tetracene thin films grown by vacuum sublimation on a flexible Mylar © substrate have been investigated by means of synchrotron X-ray diffraction. The films are polycrystalline and are made up of crystalline domains oriented with the (0 0 l) planes almost parallel to the substrate and completely misoriented around the surface normal. Two crystallographic phases (α and β thin film phases) have been identified. They differ for the d h k l interplanar spacing, both larger than that of the bulk. As a comparison, results from tetracene films grown on SiO 2 have been reported to investigate the different charge transport properties of films grown on Mylar and on SiO 2 substrates.

Correlation between morphology and transport properties of composite films: Charge transport in composites

Metal/dielectric composite films consisting of metal objects located in dielectric matrix are investigated by computer simulation. The complete computer experiment is devoted to the study of correlation between structural properties and electrical characteristics of composite films. In the present analysis transport properties of films are calculated near the metal–dielectric transition when the basic mechanism of charge transport is the tunnel effect. The conductivity of composite film is disseminated into individual percolation paths influenced by object arrangements in the composite film.

Growth and characterization of Sn doped ZnO thin films by pulsed laser deposition

AbstractSn:ZnO thin films with different Sn concentrations were grown by pulsed laser deposition (PLD) onto single‐crystal Si(001) substrates at an oxygen pressure of 2 × 10–2 mbar and substrate temperature of 600 °C. The targets used were high density Sn:ZnO pellets with different Sn concentrations produced by mixing ZnO and SnO2 by conventional ceramic routes. A deep structural and electrical characterization was carried out in order to determine the role of an increasing Sn nominal concentration on the ZnO film transport properties. Only films with a nominal 0.1 at% Sn show an improvement of the transport properties, lower resistivity and higher donor concentration, with respect to pure ZnO thin films. For films with larger Sn nominal concentrations segregated SnZnO phases appear that lead to larger film resistivities and no increase in donor concentration. The 0.1 at% Sn film is accordingly a good candidate to study the possible room temperature ferromagnetism when co doping with Mn. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Fe2O3 on the transport and magnetic properties of half metallic Fe3O4

We present a systematic study of the transport and magnetic properties of half metallic Fe3O4 films grown on Si (100) substrates with a Fe2O3 buffer layer using electron beam deposition technique. Transmission electron microscope images show the presence of small grains that are well separated by grain boundaries in our polycrystalline films. We observed that the Verwey transition does not appear for our Fe3O4 films and tunneling of spin polarized electrons across grain boundaries dominates the transport properties of the films. Magnetic measurements show a reduced magnetization in our films which does not saturate even at high fields. Coercivity of the films increases with decreasing temperature, suggesting the existence of additional anisotropy at low temperatures. Magnetoresistance curves show linear behavior at high fields which may be attributed to second order tunneling through intermediate states in the grain boundaries. At lower fields, the magnetoresistance behavior is governed by direct tunneling.

Comparison of pressure, magnetic-field, and excess manganese effects on transport properties of film and bulk ceramic La–Ca manganites

The pressure, magnetic-field, and excess manganese effects on transport and magnetoresistance effect (MRE) are studied in both epitaxial films and bulk ceramics of the manganites (La0.7Ca0.3)1−xMn1+xO3−y (x=0–0.2). A comparison of the electrical behavior in both kinds of samples of similar composition at hydrostatic pressures of up to 1.8GPa and in magnetic fields of up to 8kOe is performed. The pressure and magnetic-field effects are shown to increase with increasing manganese content. Experimental data show that the effects of pressure and magnetic field on the temperatures of both the metal-insulator transition (TMD) and the MRE peak (TMR) are considerably stronger in the films than in the ceramics. The hydrostatic pressure increases TMD and TMR. It was shown that for both types of samples the magnetoresistance effect is affected in opposite ways by pressure and magnetic field. A direct correlation is established between TMD and conductivity bandwidth as well as between MRE and concentration of charge carriers at applied pressure. The differences in the values of pressure effect on resistance, MRE and TMD temperature in the films and ceramics are connected with both granular structure of ceramics and the oxygen nonstoichiometry in ceramic and film samples of the same content as well as with the film strain induced by lattice mismatch between the film and the substrate. The origin of pressure-magnetic-field effects is analyzed in the framework of the double exchange interaction and the small polaron hopping and variable-range hopping models.