Values Of Charge Carrier Mobility Research Articles

Performance Comparison of Single- and Dual-Gate Carbon-Nanotube Thin-Film Field-Effect Transistors

In this paper, we present the performance comparison of a planar dual-gate semiconducting-nanotube thin-film transistor (SNTFT) with single-gate SNTFTs. A thin film of 95% enriched semiconducting single-walled carbon nanotubes (SWCNTs) is deposited on the amino-silane-modified hafnium oxide (HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) surface. HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> deposited by radio-frequency sputtering is used as the dielectric material for the back gate of the SNTFT, and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is used as the dielectric material for the top gate. The performance of SNTFTs with the gate structures located at the top, bottom, or both the top and the bottom (dual gate) of the SWCNT thin-film channel is compared. All SNTFTs have exhibited a p-type output characteristic behavior with distinct saturation and a linear region of operation. The electrical characterization of these devices shows that the dual-gate SNTFT out performs the single-gate devices in terms of subthreshold slopes, threshold voltage, and on-off current ratios. The dual-gate SNTFT exhibits a subthreshold slope of 280 mV/dec, a threshold voltage of -0.65 V, and a maximum on-off drain current ratio of 1.2 × 103. A maximum transconductance value of 5.89 μS and a charge carrier mobility value of 2.26 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V · s obtained for the dual-gate SNTFT are greater compared with the corresponding values of single-gate SNTFTs having identical dimensions.

Columnar Mesophases Based on Zinc Chlorophyll Derivatives Functionalized with Peripheral Dendron Wedges

A chlorophyll derivative with a central zinc ion, a methoxy functionality at its 3(1)-position, and functionalized with a second-generation dendron (3,4-3,4,5)12G2-CH(2)OH at its 17(2)-position was synthesized starting from natural chlorophyll a (Chl a). This compound exhibits liquid crystalline (LC) behavior and its mesomorphic properties have been characterized by differential scanning calorimetry (DSC), polarisation optical microscopy (POM), powder X-ray diffraction (XRD), and scanning probe microscopy (SPM). A combination of powder XRD, high resolution scanning tunneling microscopy (STM), and atomic force microscopy (AFM) experiments revealed the formation of nano-segregated well-ordered columnar tubular superstructures consisting of about five molecules in the column stratum. These self-assembled columns are further self-organized into a two-dimensional oblique unit cell lattice. Semiconducting behavior of this compound has been studied by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) method and charge carrier mobility values of ∼10(-2) cm(2) V(-1) s(-1) are observed. Such organized columnar superstructures constructed from semisynthetic zinc chlorins are reminiscent of the tubular organization of the bacteriochlorophyll dyes in the light-harvesting chlorosomal antennae of green sulphur bacteria.

Surface states and photo-induced charge transfer on oxygen-terminated chemical vapor deposition diamond

Time-of-Flight (ToF) experiments were carried out to study the electrical transport and charge trapping properties of oxidized freestanding single crystalline (100) CVD diamond crystals. It is shown that the diamond surface and properties of the electrical contacts are of extreme importance, influencing measured charge carrier mobility values. While sputtered Al contacts yield textbook ToF spectra and electron mobilities μe exceeding 1860 cm2/Vs, thermally deposited contacts suffer from enhanced trapping. We propose a model in which the observed polarization effects can be explained by charge transfer to oxygen-related surface states, which interact with surface adsorbates, according to the well-known transfer-doping model.

Impact of Linking Topology on the Properties of Carbazole Trimers and Dimers

Synthesis and properties of new glass-forming 2-, 2,7-substituted carbazole dimers and trimers exhibiting ambipolar charge carrier transport are reported. The results of thorough evaluation of thermal, optical, and photoelectrical properties of the carbazole derivatives are presented. Additionally, a comparative experimental and theoretical analysis of the 2,7-substituted carbazole trimer with the 3,6-substituted analogue is performed. 2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole was found to show very high electron mobility (μe = 2 × 10−3 cm2 V−1 s−1 at an electric field of 1.8 × 105 V/cm) and reasonably high hole mobility (μh = 8 × 10−4 cm2 V−1 s−1 at an electric field of 1.8 × 105 V/cm). 2-Substituted carbazole dimers also demonstrate bipolar conductive properties although with significantly lower charge carrier mobility values. The obtained carrier mobility values for the 2,7-substituted carbazole compound were found to be nearly 2 orders of magnitude higher than those for 3,6-substituted carbazole counterpart. The pronounced differences in carrier mobility are justified by DFT calculations indicating extended π-conjugation and a twice as large transition dipole moment in the 2,7-disubstituted carbazole derivative. This is also supported by the optical properties of the 2,7-disubstituted carbazole compound showing up to 5 times larger radiative relaxation rates, 3-fold increase in the fluorescence quantum yield and absorbance, as well as the reduced electron−vibronic system coupling as compared to the 3,6-substituted analogue. The prolonged molecular geometry and extended π-conjugation of the 2,7-substituted carbazole derivative facilitate more ordered and dense molecular packing and thus enhance intermolecular coupling in the solid films contributing to more efficient carrier hopping.

Top‐Gate Organic Field‐Effect Transistors with High Environmental and Operational Stability

Over the past several years, great progress has been made in the development of organic fi eld-effect transistors (OFETs). Prototypes of electronic devices such as drivers for fl at-panel displays, [ 1 ] complementary circuits, [ 2 , 3 ] radio-frequency identifi cation tags, [ 4 ] and chemical or biological sensors [ 5 , 6 ] have already been demonstrated. While charge-carrier mobility values have improved [ 2 , 3 , 7–9 ] with comparable values for both n and p -channel transistors, long-term environmental and operational stability remain two major issues that need to be resolved before OFETs can realize their full commercial potential. Recently, much effort has been devoted to improve the stability of OFETs. [ 10–18 ] For instance, to improve the environmental stability of OFETs, air-stable organic semiconductors have been synthesized [ 10 , 11 ] or encapsulation layers have been developed. [ 12 , 13 ] On the other hand, achieving operational stability is still a major challenge faced by OFETs as well as other fi eld-effect transistor (FET) technologies, such as those based on a -Si:H, poly-Si, and metal-oxide semiconductors. The operational stability of a FET is in general related to dipolar orientation and charge trapping/de-trapping events at all its critical interfaces and in the bulk of the semiconductor and gate dielectric. [ 14–18 ] The degradation of the performance of a FET during operation is refl ected by changes of its current-voltage characteristics that result from changes of mobility ( μ ), of threshold voltage ( V th ), or variations of the capacitance density ( C in ) of the gate dielectric. The dynamics of the physical and/or chemical mechanisms producing these changes, intrinsic or extrinsic, affect the performance of a FET on different time scales. [ 14 ] The stability of a FET is determined by the total effects produced by several physical and/or chemical processes, but in general, one tends to dominate over the others. This has caused current approaches to improve the stability to focus on mitigating individual processes. [ 15–18 ] Furthermore, the stability of OFETs has been primarily evaluated in devices with a bottom-gate geometry. OFETs with a top-gate geometry are relatively rare because the choice of gate dielectric material is limited since its deposition can potentially damage the organic semiconductor layer underneath. The use of an amorphous fl uoropolymer, CYTOP,

Charge carrier mobility in poly[methyl(phenyl)silylene] studied by time-resolved terahertz spectroscopy and molecular modelling

Time-resolved terahertz spectroscopy and combination of quantum chemistry modeling and molecular dynamics simulations were used for the determination of charge carrier mobility in poly[methyl(phenyl)silylene]. Using time-resolved THz spectroscopy we established the on-chain charge carrier drift mobility in PMPSi as 0.02 cm(2) V(-1) s(-1). This value is low due to the formation of polarons: the hole is self-trapped in a potential formed by local chain distortion and the transient THz conductivity spectra show signatures of its oscillations within this potential well. This view is supported by the agreement between experimental and calculated values of the on-chain charge carrier mobility.

Charge carrier mobility in sulphonated and non-sulphonated Ni phthalocyanines: experiment and quantum chemical calculations

The objective of this interdisciplinary paper was to study theoretically and experimentally the electronic part of charge carrier transport in the class of sodium salts of sulphonated Ni phthalocyanine as candidates for p-type channels in organic field-effect transistors. These materials were selected because of their enhanced solubility as compared to their non-sulphonated counterparts. The values of the field-effect charge carrier mobility determined on the OFET structures using NiPc(SO3Na)x films were much higher than the charge carrier mobility obtained on the respective device prepared from non-substituted phthalo- cyanine. In order to explain differences between charge carrier mobility of sulphonated and non-sulphonated Ni phthalocyanines, quantum chemistry studies of molecular aggregates were performed. Quantum chem- istry modeling of the semiconductive molecular systems is new and progressive - we highlighted factors at the molecular level which led to the enhancement of the charge carrier mobility in systems containing SO3Na groups.

Microwave hall effect measurements on biopolymers

A 33-GHz microwave Hall effect detection system is described which is capable of measuring charge carrier mobility values larger than about 3 × 10-5 m2/V sec. An important advantage of this electrodeless technique for measurements on biological materials is that macroscopic defects do not mask the desired microscopic electronic properties. For several polypeptides and protein samples the results are consistent with a conduction mechanism involving the hopping or tunneling of charge carriers between localized energy states. However, for collagen-methylglyoxal samples a relatively large p-type Hall effect has been observed which not only indicates the possibility of conduction in a band of extended states, but also gives strong support to the concept that methylglyoxal can act as an electron acceptor in charge-transfer interactions with protein molecules.

High Temperature Thermoelectric Properties of Nano-Bulk Silicon and Silicon Germanium

AbstractPoint defect scattering via the formation of solid solutions to reduce the lattice thermal conductivity has been an effective method for increasing ZT in state-of-the-art thermoelectric materials such as Si-Ge, Bi2Te3-Sb2Te3 and PbTe-SnTe. However, increases in ZT are limited by a concurrent decrease in charge carrier mobility values. The search for effective methods for decoupling electronic and thermal transport led to the study of low dimensional thin film and wire structures, in particular because scattering rates for phonons and electrons can be better independently controlled. While promising results have been achieved on several material systems, integration of low dimensional structures into practical power generation devices that need to operate across large temperature differential is extremely challenging. We present achieving similar effects on the bulk scale via high pressure sintering of doped and undoped Si and Si-Ge nanoparticles. The nanoparticles are prepared via techniques that include high energy ball milling of the pure elements. The nanostructure of the materials is confirmed by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. Thermal conductivity measurements on the densified pellets show a drastic 90% reduction in the lattice contribution at room temperature when compared to doped single crystal Si. Additionally, Hall effect measurements show a much more limited degradation in the carrier mobility. The combination of low thermal conductivity and high power factor in heavily doped n-type nanostructured bulk Si leads to an unprecedented increase in ZT at 1275 K by a factor of 3.5 over that of single crystalline samples. Experimental results on both n-type and p-type Si are discussed in terms of the impact of the size distribution of the nanoparticles, doping impurities and nanoparticle synthesis processes.

Room-temperature discotic liquid-crystalline coronene diimides exhibiting high charge-carrier mobility in air

Six N,N′,5,11-tetrasubstituted coronene-2,3,8,9-tetracarboxydiimides have been synthesised incorporating 3,4,5-tri(n-dodecyloxy)phenyl or 2-(n-decyl)-n-tetradecyl groups in various positions. Differential scanning calorimetry, polarised optical microscopy, and X-ray diffraction indicate that all form columnar discotic mesophases from around room temperature to around 200 °C. Charge-carrier mobility values, which energetic considerations suggest are electron mobility values, have been determined in non-aligned samples cooled from the isotropic melt using the space-charge-limited current technique. The highest mobility, 6.7 cm2V−1 s−1, was found in N,N′-bis(n-2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecylfluorooctyl)-5,11-bis(3-[{3,4,5-tri(n-dodecyloxy)phenyl}carbonyloxy]-n-propyl)coronene-2,3,8,9-tetracarboxydiimide, which X-ray diffraction suggests is the most highly ordered of the materials examined.

Growth and electrical properties of N,N′-bis(n-pentyl)terrylene- 3,4:11,12-tetracarboximide thin films

A n-type semiconductor molecule N,N′-bis(n-pentyl)terrylene-3,4:11,12-tetracarboximide (TTCDI-5C) was synthesized. Theoretical calculations predict several advantages in electrical properties, including large adiabatic electron affinity and small reorganization energy. The molecule was deposited on SiO2 surfaces and the structure of the resultant thin film was studied. Grain size and thin film crystallinity improve as the temperature increases. Top-contact organic field effect transistors (OFETs) using TTCDI-5C as the semiconductor layer were fabricated using SiO2 as the gate dielectric. Values of charge carrier mobility up to 7.24×10−2cm2V−1s−1 and current on/off ratios higher than 104 were obtained, demonstrating the potential of TTCD-5C for use in OFETs.

Efficient High Area OFETs by Solution Based Processing of a π-Electron Rich Donor

We report on the preparation of high performance field-effect transistors (FETs) based on large areas of aligned films of a TTF derivative, namely, tetrakis-(octadecylthio)-tetrathiafulvalene (TTF-4SC18). TTF-4SC18 assembles into one-dimensional stacks in which the long alkyl chains promote intermolecular π−π overlapping due to their extremely closely packed nature. The films were prepared from solution by zone-casting, a simple technique that does not require the use of preoriented substrates. The films were characterized by AFM and X-ray, indicating an extremely high crystalline quality. The TTF molecules are tilted with respect to the substrate surface and are well-aligned in the casting direction. More than 40 FETs were measured, showing a remarkable reproducibility of their performance. The average charge carrier mobility value measured along the casting direction was about 0.006 cm2/V s for a channel length L = 100 μm and about 0.01 cm2/V s for L = 80 μm and L = 50 μm. The FET mobilities determined in the direction perpendicular to the orientation were ca. 1 order of magnitude lower. We found that all the devices after annealing exhibited an enhanced performance with FETs mobilities about 1 order of magnitude higher. The best devices revealed a charge carrier mobility close to 0.1 cm2/V s with an on/off ratio of the order of 104.

Organic FET devices: structure–property relationship in evaporated films of three fluorenone derivatives

The transport properties of three fluorenone derivatives used as active layers in organic field effect transistor (OFET) have been studied. A full structural characterization, evidencing the packing features due to the fluorenone moiety, has been carried out. To rationalize the charge carrier mobility values, it is shown that in addition to structural considerations the redox behavior of molecules has to be taken into account. Structure–property relationships are identified, which might drive the future development of oligomers suitable for OFET applications.

Liquid crystals containing a 2,6-disubstituted anthracene core—mesomorphism, charge transport and photochemical properties

The synthesis of 2-(4-phenyl)-6-anthracenes and 4,4′-bisphenyl-2,6-anthracenes, disubstituted by saturated (decyloxy) or unsaturated ((Z)-dec-4-enyloxy) chains, is reported. All the materials are liquid crystals and exhibit smectic A and/or C phases. As compared to the saturated chains, the unsaturated ones provide a significant decrease of the transition temperatures. The presence of the central “lath-like” anthracene core in the rigid aromatic moiety is found to induce some molecular π-stackings, at short range, in the mesophases. This molecular peculiarity is thought to be in a large part responsible for the high charge-carrier mobility values measured (i.e. 2 × 10−3 cm2 V−1 s−2), despite the disordered character of the mesophases. Preliminary examination of photodimerisation revealed that the formation of photoadducts was very poor in the smectic phases as compared to the same experiment run in solution. This result was attributed to the instability of the anthracene photodimers at the too high temperatures of the mesophases.

Charge carrier mobilities of fluorene-based polymers

Charge carrier mobilities of fluorene-based polymers were investigated using TOF (time of flight) and TrEL (transient electroluminescence) method. Single, double and triple bonds were introduced to the main chain of the fluorene polymers to investigate the effect of main chain structure on the charge carrier mobilities. It showed that the triple bonds introduced polymers generally have lower charge carrier mobilties than the other polymers. Especially, the polymer with two adjacent acetylene groups showed very low charge carrier mobility value, which was about a thousand times lower than that of the double bond introduced polymer.

Electrical transport properties of cadmium substituted copper ferrites

Electrical transport properties such as electrical conductivity ( σ) and thermoelectric power ( S) of cadmium substituted copper ferrites, forming the chemical formula Cu 1− x Cd x Fe 2O 4, where x=0.2, 0.4, 0.6, 0.8 and 1.0 have been investigated from room temperature to well beyond the Curie temperature. Plots of log( σT) vs. 10 3/ T are linear and show a transition near the Curie temperature except in the case of cadmium ferrite. Based on the Seebeck coefficient ( S), the ferrites under investigation have been classified as n-type semiconductors. The values of charge carrier concentration and mobility have been computed from experimental values of Seebeck coefficient and electrical conductivity. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region. An attempt is made to explain the conduction mechanism in these ferrites. The properties of cadmium substituted copper ferrites have been correlated with these of zinc substituted copper ferrites, cadmium and zinc being two non-magnetic divalent ions occupying essentially tetrahedral A sites when substituted in ferrites.

Mesomorphic phthalocyanines, tetraazaporphyrins, porphyrins and triphenylenes as charge-transporting materials

The columnar mesomorphism of triphenylene-, phthalocyanine-, tetraazaporphyrin- and porphyrin-based compounds is briefly reviewed and the potential use of this particular supramolecular structure for charge-conducting devices is discussed. Suitably substituted triphenylenes exhibit ideal material properties for photoconducting devices, and well-aligned layers have been shown to give a very high charge carrier mobility of 0.1 × 10-4 m 2 V -1 s -1 (time-of-flight measurements). From a molecular point of view, porphyrin systems should be superior to triphenylenes, but the properties of their mesophases do not support the advantageous homeotropic alignment of the columnar stacks. However, pulse radiolysis time-resolved microwave conductivity measurements revealed the highest charge carrier mobility value of 0.28 × 10-4 m 2 V -1 s -1 for mesomorphic phthalocyanines.

Electrical transport properties of zinc-substituted cobalt ferrites

Electrical conductivity ( σ) and thermoelectric power ( s) studies of zinc-substituted cobalt ferrites are undertaken as a function of composition and temperature. On the basis of the Seebeck coefficient ( S), the ferrites under investigation have been classified as n-type and p-type semiconductors. Electrical conductivity of all the ferrites is found to increase with increasing temperature with a change of the slope at magnetic transition ( T c). The activation energy in the ferrimagnetic region is in general less than that in paramagnetic region. The values of charge carrier concentration and mobility are also computed and discussed as a function of composition and temperature. An attempt is made to explain the conduction mechanism in these ferrites.

Electrical conductivity and thermoelectric power of lithium‐cadmium ferrites

AbstractThe electrical conductivity (σ) and thermoelectric power (Q) of polycrystalline lithiumcadmium ferrites having the chemical formula Li0.5–x/2CdxFe2.5–x/2O4 where (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) have been investigated as a function of temperature. Lithium‐cadmium ferrite with x = 0.4 is found to possess minimum electrical conductivity and lowest Seebeck coefficient. Plots of log (σT) versus 103T are almost linear and have shown a transition near the Curie temperature except in the case of cadmium ferrite. The Seebeck coefficient is negative for all the compositions showing that these ferrites behave as n‐type semiconductors. The values of charge carrier concentration and mobility have also been computed. The properties of cadmium‐substituted lithium ferrites have been correlated with those of zinc‐substituted lithium ferrites, cadmium and zinc being two non‐magnetic divalent ions occupying essentially tetrahedral A sites when substituted in ferrites.

Electrical conduction in copper vanadate

A.C. and d.c. electrical conductivities, thermoelectric power and dielectric constant of copper vanadate (CuV2O6) have been measured in the temperature range 300–1000 K in order to discuss the electrical conduction in the compound. The extrinsic conduction, which takes place below ∼500 K, has been explained by small polaron hopping mechanism while intrinsic conduction, which takes place above ∼500 K, has been explained by large polaron band mechanism in view of the values of activation energy and charge carrier mobility in the temperature ranges 300–500 K and 500–1000 K.