Transport Properties Of Thin Films Research Articles

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

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

Electrochemical Synthesis and Charge Transport Properties of CdS Nanocrystalline Thin Films with a Conifer-like Structure

Well-defined intersectional CdS nanocrystalline thin films with a conifer-like morphology were synthesized by a facile electrochemical process. The morphology, structure, and phase composition of CdS nanostructures were examined by X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, and energy-dispersive X-ray spectroscopy. The electrochemical deposition conditions influencing the synthesis of these conifer-like CdS nanocrystals, such as deposition temperature, deposition time, current density, and concentrations of the precursors, were studied systematically. The orientation-transformation mechanism of the growth of conifer-like CdS nanocrystals was proposed based on the results, which is beneficial for the shape-controlled synthesis of other shaped nanostructures. The charge transport properties of these thin films before and after annealing treatment were also studied via the space-charge-limited conduction model, from which the total density of trapping states and ...

Volume diffusion of Ytterbium in YAG: thin-film experiments and combined TEM–RBS analysis

In this study, we address volume diffusion of ytterbium in yttrium aluminum garnet (YAG) using thin-film single crystal diffusion couples. We employ analytical transmission electron microscopy (ATEM) as a tool for combined microstructural and microchemical analysis and compare the results to Rutherford backscattering (RBS) analysis. Given the high spatial resolution of the method, we focus on microstructural changes of the thin-film diffusant source during the diffusion anneal. We evaluate the potential influence of the associated changes in its transport properties on the evolution of concentration profiles in the single crystal substrate. This approach allows us to test the reliability of determination of volume diffusion coefficients from thin-film diffusion experiments. We found that for the chosen experimental setting, the influence of thin-film re-crystallization is small when compared with the experimental uncertainty and good estimates for the volume diffusion coefficients of Yb in YAG can be obtained using standard assumptions. Both Yb-concentration profiles analyzed with ATEM and with RBS give similar results. At 1,450°C and 1 bar, we infer log D Yb (m2/s) values of −19.37 ± 0.07 (TEM) and −19.84 ± 0.02 (RBS). Although the change in thin-film transport properties associated with successive crystallization during the diffusion anneal does not play a major role for our experimental setup, this effect cannot generally be ignored.

Persistent photocurrent and decay studies in CdS nanorods thin films

Structural and electrical transport properties of thin films of CdS nanorods synthesized by solvothermal process are studied through x-ray diffraction, scanning electron microscopy, transmission electron microscopy, activation energy measurements, and decay of photoconductivity. The films show thermally activated conductivity, high photosensitivity, and persistent photoconductivity, which decays initially fast followed by slow decay. The decay time constant for fast decay are of the order of a few seconds, whereas for slow decay it is in the range of about 100 s. The electrical transport is similar to that in the polycrystalline material. The slow decay of excess conductivity has been attributed to the presence of defects and local potential fluctuations within the nanorods.

Effects of oxygen depletion on photoinduced and transport properties of thin films of charge-ordered manganites

In this paper, we report a study of conductive and photoinduced properties of charge-ordered Bi0.4Ca0.6MnO3 thin films with different oxygen contents. The oxygen content of the films was varied following different annealing or deposition protocols. Change in film’s resistivity during annealing reflects oxygen dynamics at high temperatures. It was found that oxygen-deficient films exhibit a significant increase in the photoinduced resistivity changes and the lifetime of photoinduced conductive phase. Possible origin of this effect is discussed.

Phase separation induced by cation disorder and strain in (La,Y)2/3(Ca,Sr)1/3MnO3 films

Effects of cation disorder and strain on the surface morphologies and transport properties of thin films with different A-site cation disorder, which was achieved by Y and Sr codoping into La2/3Ca1/3MnO3, have been investigated. A metal-insulator transition (MIT) was observed on all films except that on MgO, which indicates that large strain increases insulator phase. With increasing cation disorder, the resistivity increases while MIT and Curie temperature decrease, which reveal that cation disorder suppresses the long-range ferromagnetic metallic phase. The spin-glass state and ferromagnetic-metallic state coexist in low temperature. Cation disorder induces more insulator phases in the large strain films. A Kondo-like transport behavior was observed in the large cation disordered films.

Effect of doping- and field-induced charge carrier density on the electron transport innanocrystalline ZnO

The charge transport properties of thin films of sol–gel processed undoped and Al-dopedzinc oxide nanoparticles with variable doping level between 0.8 and 10 at.% wereinvestigated. The x-ray diffraction studies revealed a decrease of the average crystallitesizes in highly doped samples. We provide estimates of the conductivity and the resultingcharge carrier densities with respect to the doping level. The increase of chargecarrier density due to extrinsic doping was compared to the accumulation of chargecarriers in field effect transistor structures. This allowed us to assess the scatteringeffects due to extrinsic doping on the electron mobility. The latter decreases from4.6 × 10−3 to4.5 × 10−4 cm2 V−1 s−1 with increasing doping density. In contrast, the accumulation leads to an increasing mobility up to1.5 × 10−2 cm2 V−1 s−1. The potential barrier heights related to grain boundaries between the crystallites werederived from temperature dependent mobility measurements. The extrinsic doping initiallyleads to a grain boundary barrier height lowering, followed by an increase due todoping-induced structural defects. We conclude that the conductivity of sol–gel processednanocrystalline ZnO:Al is governed by an interplay of the enhanced charge carrier densityand the doping-induced charge carrier scattering effects, achieving a maximum at 0.8 at.%in our case.

P-Type and n-type quaterthiophene based semiconductors for thin film transistors operating in air?

Abstract The solution and solid-state properties as well as the organic thin film transistor (OTFT) behavior of α , ω -perfluorohexyl-quaterthiophene ( DFH-4T ) are presented and compared to those of quaterthiophene ( 4T ) and α , ω -hexyl-quaterthiophene ( DH-4T ). UV/visible and fluorescence data showed the same weak effect of hexyl and perfluorohexyl substitutions on the optical properties of 4T core. Growth mechanism of DFH-4T based thin films deposited by vacuum deposition on heated Si/SiO 2 substrates has been investigated in details and demonstrated to be identical to that of DH-4T . The characterization in air of the charge transport properties of thin films based on DH-4T and DFH-4T revealed a switch from p- to n-type, respectively, depending on the nature of alkyl chains. A qualitative Schottky-type charge injection barrier model, based on HOMO and LUMO energy levels estimated from cyclic voltammograms and optical absorption spectra of DFH-4T relative to those of 4T and DH-4T , was introduced to explain such change in semiconducting properties observed under ambient conditions (temperature, light, air). In an attempt to answer to the question of “are p-type and n-type quaterthiophene derivatives appropriate semiconductors for thin film transistors operating in air?” we investigated the influence of the nature of the dielectrics to obtain OTFTs operating in air and showing environmentally stable mobility.

Stress relaxation of La 1/2Sr 1/2MnO 3 and La 2/3Ca 1/3MnO 3 at solid oxide fuel cell interfaces

Interfacial stress is thought to have significant effects on electrical and oxygen transport properties in thin films of importance in solid oxide fuel cell applications. We investigate how in-plane biaxial stress modifies the electronic structure of La 2/3Ca 1/3MnO 3 and La 1/2Sr 1/2MnO 3 thin films prepared by pulsed laser deposition on three different substrates to vary the in-plane stress from tensile to compressive. The electronic structure was probed by X-ray absorption spectroscopy of the Mn L 2,3-edge to characterize the interfacial disruption in this region in an element-specific, site-specific manner. The compressive or tensile interfacial strain modifies the relative concentrations of La and Sr in the interfacial region in order to achieve a better lattice match to the contact material. This atomic migration generates an interfacial region dominated by a compound with a single valency for the transition metal ion, resulting in a severe barrier to oxygen and electron transport through this region.

Weak localization and correlation effects in thin-film degenerate n-type silicon

The magnetoconductance of thin-film n-type silicon has been studied in order to provide insight into the low-temperature behavior of recent silicon-based semiconductor-metal hybrid structures. There is considerable interest in such structures as potential candidates for creating nonmagnetic read-head sensors for ultrahigh-density recording. The magnetoconductance of thin-film silicon was therefore analyzed as a function of magnetic field orientation at temperatures ranging from T=4.2 to 300 K. At low temperatures, the magnetoconductance consists of two components that are attributed to weak localization and correlation effects. Surprisingly, the thin-film transport properties behave two-dimensionally for the localization contribution, whereas the correlation contribution is isotropic. Similar two-dimensional localization behavior has previously been reported for silicon inversion layers but was not expected for silicon systems with a finite thickness. An analysis of the magnetoconductance as a function of magnetic field orientation has verified that the localization contribution depends only on the perpendicular to plane component of the field. The Hall coefficient was observed to vary with magnetic field, providing further evidence for a magnetoconductance governed by electron-electron interactions. Fitting of the data at T=4.2 K and for low magnetic fields provided values for the localization parameter α, the inelastic scattering time τi, and the two-dimensional effective electron screening constant F̃σ.

Thin manganite films for fast fault current limiter applications

The electrical transport properties of thin films of perovskite manganites deposited on various substrates are summarised and discussed, focusing on the exploitation of their features for fast fault current limiter applications. The influence of strong electric fields on the resistivity of thin La 0.67Ca 0.33MnO 3 and La 0.83Sr 0.17MnO 3 films was investigated using nanosecond duration and subnanosecond rise time electrical pulses. It was obtained that a strong electric field induces a substantial resistance decrease in the films at a threshold electric field value. The ratio of the film resistance at low electric field and at strong electric field is the highest for polycrystalline films prepared on lucalox substrates. It was demonstrated that a limiter operating at room temperature and having 10 V threshold voltage, with 0.7 dB initial losses in the non-limiting regime and 6 dB attenuation in the limiting regime could be designed using 100–400 nm thickness polycrystalline manganite films. The limiter for low temperature operation having the same initial losses could be designed with the following parameters: 4 V threshold voltage and 8 dB attenuation in the limiting regime.

Thermal transport properties of thin films of small molecule organic semiconductors

A series of harmonic Joule-heating experiments have been employed to determine the thermal conductivities of thin films of pentacene, N,N′-diphenyl−N,N′-di(3-methylphenyl)−(1,1′-biphenyl)-4,4′-diamine, and tris(8-hydroquinolinato)aluminum, three widely used organic semiconductors. Room-temperature thermal conductivity values of 0.51, 0.24, and 0.48W∕mK were measured for films of these three compounds, respectively. These values are over two orders of magnitude lower than those of inorganic semiconductors. While amorphous films were found to display only small thermal conductivity changes over the temperature range of 228–350 K, pentacene exhibited stronger variations that are typical of phonon-phonon scattering observed in polycrystalline semiconductors.

Effect of thermal treatment in oxygen, nitrogen, and air atmospheres on the electrical transport properties of zinc oxide thin films

The effects of thermal treatment in oxygen, air, and nitrogen gas atmospheres, at temperatures ranging from 573 to 1173 K, on the electrical transport properties of thin films of zinc oxide, prepared by sputtering deposition, have been investigated. These experiments have been carried out in preparation for future ion implantation doping studies in zinc oxide. As-prepared samples were slightly oxygen-deficient (Zn: 51%, O: 49%), due to a relatively low concentration of oxygen in the sputtering gas, and exhibited n-type conductivity (sheet resistance ∼3 × 10 2 Ω/square, sheet carrier concentration ∼8 × 10 14 cm − 2 , carrier concentration 4 × 10 19 cm − 3 , and mobility ∼30 cm 2/V s). Heat treatment in oxygen atmosphere led to a decrease in carrier concentration with increasing temperature. Upon annealing at 1173 K the carrier concentration was ∼1 × 10 17 cm − 3 and the mobility was ∼7 cm 2/V s, while heat treatment in nitrogen atmosphere, at the same temperature, resulted in almost no change in carrier concentration but with significant decrease in mobility to ∼1 cm 2/V s. Heat treatment in air led to values of carrier concentration and mobility, intermediate between those observed upon annealing in oxygen or nitrogen gases. These results suggest that the electrical transport properties of zinc oxide thin films are extremely dependent upon deposition conditions and post-deposition treatments and these effects should be carefully considered in any doping attempt by ion implantation.

Charge Carrier Transport in Neat Thin Films of Phosphorescent Iridium Complexes

The charge carrier transport properties of neat thin films of red-, green- and blue-phosphorescence Ir complexes have been investigated in terms of time-of-flight transient photocurrent measurement. Bis(2-(2'-benzo[4,5-a]thienyl)pyridinato-N,C3')iridium(acetylacetonate) (btp2Ir(acac)) (red-phosphorescence emitter) and iridium(III) bis[(4,6-di-fluorophenyl)-pyridinato-N,C2']picolinate (FIrpic) (blue-phosphorescence emitter) thin films exhibit bipolar charge transport. In contrast, the electron and hole transit signals of fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) (green-phosphorescence emitter) thin films are unobservable. The charge carrier transport properties of thin films of an Ir-complex-doped 4,4'-N,N'-dicarbazole-biphenyl (CBP) and the appropriateness of FIrpic as a triplet-exciton and hole blocking layer are shown.

Magnetic and transport properties of Co nanoparticles embedded in a carbon matrix

Co nanoparticles embedded in a carbon matrix have been prepared by gas condensation using a cluster gun. The magnetic, microstructural, and transport properties of the Co nanoparticles can be controlled by the volume fraction of Co particles, which is adjusted by the sputtering time of Co. With the decrease of the amount of Co, the transport properties of thin films showed a transition from a metallic to a semiconductorlike behavior. The sample with a semiconductorlike behavior showed a superparamagnetic behavior with a blocking temperature around 130K. The blocking temperature of the Co particles was found to decrease with the decrease of Co sputtering time, indicating a decrease in the average particle size because of the shorter sputtering time.

Control of growth and charge transport properties of quaterthiophene thin films via hexyl chain substitutions

Growth mechanism of quaterthiophene based organic thin films deposited by high vacuum deposition on Si:H(1 0 0) and SiO2 substrates have been investigated. Especially the influence of hexyl chain end-substitutions on the growth process and the electronic transport properties of organic thin films were studied in details. Highly crystalline films were obtained for both quaterthiophene (4T) and α,ω-hexyl-quaterthiophene (DH4T) based thin films. While unsubstituted 4T shows a typical island growth, an almost perfect layer-to-layer growth was found in the case of DH4T based films. It could be demonstrated that the change in the growth mode is directly related to the molecule structure, i.e. to the presence of hexyl chain substitutions on the 4T core, and leads to an increase of the grain size of one order of magnitude under the same evaporation conditions in the case of DH4T films. The characterization of the charge transport properties of thin films based on both molecules reveals one order of magnitude higher mobilities for the DH4T molecules. By using a simple model for charge transport in polycrystalline materials a linear dependence of the mobility on the grain size, independently from the molecule substitution, could be demonstrated. The results underline the importance of the control of the film morphology and give an impressive example of such a control in the case of hexyl end-substitutions of quaterthiophene.

Structure and transport properties of isomer-separatedC82

Structure of isomer-separated crystalline ${\mathrm{C}}_{82}$ has been studied by powder x-ray diffraction with synchrotron radiation. The Rietveld refinement was achieved by assuming a simple cubic lattice (space group $\mathrm{Pa}3\ifmmode\bar\else\textasciimacron\fi{}).$ The ${C}_{2}(a)$ symmetry for the ${\mathrm{C}}_{82}$ molecule was supported in this analysis. Transport properties of thin film of ${\mathrm{C}}_{82}$ have been studied by resistivity measurement. The thin film showed a narrow-gap semiconductorlike behavior with gap energy of 0.43 eV. The field effect transistor (FET) of a ${\mathrm{C}}_{82}$ thin film showed an $n\ensuremath{-}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{n}\mathrm{e}\mathrm{l}$ normally-on depletion-type behavior, and the mobility was $1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ whose value was one of the largest \ensuremath{\mu} among normally-on FETs with fullerenes. A hopping transport was found as channel conduction for the ${\mathrm{C}}_{82}$ FET above 150 K.

Structural, magnetic, and transport properties of thin films of the Heusler alloy Co2MnSi

Thin films of Co2MnSi have been grown on a-plane sapphire substrates from three elemental targets by dc magnetron cosputtering. These films are single phase, have a strong (110) texture, and a saturation magnetization of 4.95μB/formula unit at 10 K. Films grown at the highest substrate temperature of 715 K showed the lowest resistivity (47 μΩ cm at 4.2 K) and the lowest coercivity (18 Oe). The spin polarization of the transport current was found to be of the order of 54% as determined by point contact Andreev reflection spectroscopy. A decrease in saturation magnetization with a decrease in film thickness and different transport behavior in thinner films indicate graded disorder in these films grown on nonlattice matched substrates.

Effects of adsorption of alcohol and water on the electrical transport of carbon nanotubebundles

In situ studies on the effects of physisorption of various alcohol molecules(CnH2n+1OH;n = 1–4) on the transport properties of thin films of bundled single-walled carbonnanotubes reveal large increases in the thermoelectric power (TEP)S () and four-probe resistance R () at 40 °C. Interestingly, exposure to water causes virtually no change in the TEP, although theelectrical resistance shows a change of , typical for the alcohols. We observe a simple exponential response ofS(t) and R(t) to a sudden change in the molecular pressure. The characteristic time constants ( min) for S and R are found to be essentially the same for a given molecule, indicating thatthe changes in these transport properties track each other. In fact, plots ofS versus ΔR are linear, the slopes depending on the specific molecule. The transportresults are interpreted in terms of a Boltzmann model and the introductionof a new scattering channel for charge carriers in metallic tubes due toweak interactions with physisorbed molecules. The trends in the changes inS and R with adsorption of these polar molecules can be explained on the basis of the interplaybetween the adsorption energy and the molecular coverage on the nanotube surfaces.

Control of growth and charge transport properties of quaterthiophene thin films via hexyl chain substitutions

Growth mechanism of quaterthiophene based organic thin films deposited by high vacuum deposition on Si:H(1 0 0) and SiO 2 substrates have been investigated. Especially the influence of hexyl chain end-substitutions on the growth process and the electronic transport properties of organic thin films were studied in details. Highly crystalline films were obtained for both quaterthiophene (4T) and α,ω-hexyl-quaterthiophene (DH4T) based thin films. While unsubstituted 4T shows a typical island growth, an almost perfect layer-to-layer growth was found in the case of DH4T based films. It could be demonstrated that the change in the growth mode is directly related to the molecule structure, i.e. to the presence of hexyl chain substitutions on the 4T core, and leads to an increase of the grain size of one order of magnitude under the same evaporation conditions in the case of DH4T films. The characterization of the charge transport properties of thin films based on both molecules reveals one order of magnitude higher mobilities for the DH4T molecules. By using a simple model for charge transport in polycrystalline materials a linear dependence of the mobility on the grain size, independently from the molecule substitution, could be demonstrated. The results underline the importance of the control of the film morphology and give an impressive example of such a control in the case of hexyl end-substitutions of quaterthiophene.