|
| ||
|
|
||
Carrier Mobility (carrier + mobility)
Kinds of Carrier Mobility Selected AbstractsOrientation Control of Linear-Shaped Molecules in Vacuum-Deposited Organic Amorphous Films and Its Effect on Carrier MobilitiesADVANCED FUNCTIONAL MATERIALS, Issue 3 2010Daisuke Yokoyama Abstract The molecular orientation of linear-shaped molecules in organic amorphous films is demonstrated to be controllable by the substrate temperature. It is also shown that the molecular orientation affects the charge-transport characteristics of the films. Although linear-shaped 4,4,-bis[(N -carbazole)styryl]biphenyl molecules deposited on substrates at room temperature are horizontally oriented in amorphous films, their orientation when deposited on heated substrates with smooth surfaces becomes more random as the substrate temperature increases, even at temperatures under the glass transition temperature. Another factor dominating the orientation of the molecules deposited on heated substrates is the surface roughness of the substrate. Lower carrier mobilities are observed in films composed of randomly oriented molecules, demonstrating the significant effect of a horizontal molecular orientation on the charge-transport characteristics of organic amorphous films. [source] Enhancement of Carrier Mobilities of Organic Semiconductors on Sol,Gel Dielectrics: Investigations of Molecular Organization and Interfacial Chemistry EffectsADVANCED FUNCTIONAL MATERIALS, Issue 3 2009Tommy Cahyadi Abstract The dielectric-semiconductor interfacial interactions critically influence the morphology and molecular ordering of the organic semiconductor molecules, and hence have a profound influence on mobility, threshold voltage, and other vital device characteristics of organic field-effect transistors. In this study, p-channel small molecule/polymer (evaporated pentacene and spin-coated poly(3,3,;-didodecylquarterthiophene) , PQT) and n-channel fullerene derivative ({6}-1-(3-(2-thienylethoxycarbonyl)-propyl)-{5}-1-phenyl-[5,6]-C61 , TEPP-C61) show a significant enhancement in device mobilities ranging from ,6 to ,45 times higher for all classes of semiconductors deposited on sol,gel silica gate-dielectric than on pristine/octyltrichlorosilane (OTS)-treated thermally grown silica. Atomic force microscopy, synchrotron X-ray diffraction, photoluminescence/absorption, and Raman spectroscopy studies provide comprehensive evidences that sol,gel silica dielectrics-induced enhancement in both p- and n-channel organic semiconductors is attributable to better molecular ordering/packing, and hence reduced charge trapping centers due to lesser structural defects at the dielectric-semiconductor interface. [source] Magnus' Green Salt Revisited: Impact of Platinum,Platinum Interactions on Electronic Structure and Carrier Mobilities,ADVANCED MATERIALS, Issue 15 2006E.-G. Kim Magnus' green salt is the prototype of a class of organic,inorganic hybrid semiconducting materials that combine attractive charge-transport properties and processability. By using density-functional-theory methods, the electronic structure of Magnus' green salt is investigated, in particular the nature of the interplatinum interactions (see figure). In conjunction with time-of-flight measurements of the carrier mobilities, key structure,property relationships for these materials are re-established. [source] Measurement of Charge-Density Dependence of Carrier Mobility in an Organic Semiconductor BlendADVANCED FUNCTIONAL MATERIALS, Issue 5 2010Christopher G. Shuttle Abstract Here, a new methodology for analyzing the charge-density dependence of carrier mobility in organic semiconductors, applicable to the low-charge-density regime (1014,1017,cm,3) corresponding to the operation conditions of many organic optoelectronic devices, is reported. For the P3HT/PCBM blend photovoltaic devices studied herein, the hole mobility µ is found to depend on charge density n according to a power law µ(n) , n,, where ,,=,0.35. This dependence is shown to be consistent with an energetic disorder model based upon an exponential tail of localized intra-band states. [source] Modeling Polymer Dielectric/Pentacene Interfaces: On the Role of Electrostatic Energy Disorder on Charge Carrier MobilityADVANCED FUNCTIONAL MATERIALS, Issue 20 2009Nicolas G. Martinelli Abstract Force-field and quantum-chemical calculations are combined to model the packing of pentacene molecules at the atomic level on two polymer dielectric layers (poly(methyl methacrylate) (PMMA) versus polystyrene (PS)) widely used in field-effect transistors and to assess the impact of electrostatic interactions at the interface on the charge mobility values in the pentacene layers. The results show unambiguously that the electrostatic interactions introduce a significant energetic disorder in the pentacene layer in contact with the polymer chains; a drop in the hole mobility by a factor of 5 is predicted with PS chains while a factor of 60 is obtained for PMMA due to the presence of polar carbonyl groups. [source] Influence of Dielectric Surface Chemistry on the Microstructure and Carrier Mobility of an n-Type Organic SemiconductorADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Parul Dhagat Abstract This paper examines the microstructure evolution of 3,4,9,10-perylene-tetracarboxylic bis-benzimidazole (PTCBI) thin films resulting from conditions imposed during film deposition. Modification of the silicon dioxide interface with a hydrophobic monolayer (octadecyltrichlorosilane (OTS-18)) alters the PTCBI growth habit by changing the unit cell contact plane. PTCBI films deposited on oxide surface have an orientation of (011), while films atop OTS-treated oxide surface have a preferred orientation of (001). The quality of the self assembled monolayer does not appear to influence the PTCBI growth preference significantly yet it enhances the carrier mobility, suggesting that charge traps are adequately passivated due to uniform monolayer coverage. High-quality monolayers result in n-type carrier mobility values of 0.05,cm2V,1s,1 Increasing the substrate temperature during PTCBI film deposition correlates with an increase in mobility that is most significant for films deposited on OTS-treated surface. [source] High Carrier Mobility up to 0.1,cm2,V,1,s,1 at Ambient Temperatures in Thiophene-Based Smectic Liquid Crystals,ADVANCED MATERIALS, Issue 5 2005M. Funahashi The synthesis and carrier-transport properties of oligothiophene liquid-crystalline semiconductors are reported. An asymmetrically substituted terthiophene and quaterthiophene give rise to a highly ordered smectic mesophase (see Figure) with an excellent carrier mobility of up to 0.1, cm2,V,1,s,1 over a wide mesophase temperature range, including ambient temperatures. This mobility is comparable to that of organic polycrystals. [source] Syntheses, Phase Behavior, Supramolecular Chirality, and Field-Effect Carrier Mobility of Asymmetrically End-Capped Mesogenic OligothiophenesCHEMISTRY - A EUROPEAN JOURNAL, Issue 14 2009Qingwei Meng Dr. Abstract Supramolecular chirality and liquid crystalline OFET: Achiral end-capped oligothiophenes can be tuned to exhibit supramolecular chirality with unique striped textures showing distinct circular dichroism signals as well as a highly ordered SmE phase that leads to high hole carrier mobility. A novel series of asymmetrically end-capped mesogenic oligothiophenes, with various oligothiophene core lengths, alkoxy tail lengths, and molecular polarities through introducing alkylsulfanyl or alkylsulfonyl functionalities as the terminal group, have been synthesized by palladium-catalyzed Suzuki cross-coupling and Kumada cross-coupling reactions as key steps. For the single end-capped oligothiophenes, CmO-Ar-OT(4)-H in which m=10, 12, 14, 16, and 18, all of these oligomers exhibited a broad temperature range of highly ordered smectic E and enantiotropic nematic phases, apart from the one with the longest octadecyloxy tail. For the double end-capped series C10O-Ar-OT(n)-R, R=Ph-SC6 or Ph-SO2C6 in which n=1, 2, 3, and 4, oligomers with more than one thiophene ring exhibited smectic A and smectic C phases, various crystal polymorphs and/or unusual low-temperature condensed phases. In the nonpolar, alkylsulfanylphenyl-substituted oligothiophene series, both the crystal/solid melting point and mesogenic clear point increased significantly with an increasing oligothiophene conjugation length. In the polar, alkylsulfonylphenyl-substituted oligothiophene series, all the oligomers showed increased melting points, but decreased mesogenic temperature intervals than those of their corresponding alkylsulfanyl counterparts. Remarkably, two different helical structures showing distinct striated textures or striped patterns were observed with a pitch of several to tens of micrometers under a polarized optical microscope upon cooling from their preceding fluidic smectic phases. The unusual twisted smectic layer structures in the thin solid films exhibiting distinct supramolecular chirality of both handednesses, revealed by circular dichroism measurements, were further confirmed by XRD analyses characterized by a sharp layer reflection together with its higher orders and diffuse wide-angle scatterings. In addition, initial studies showed that the highly ordered smectic phase of the single end-capped oligothiophenes can be utilized to improve field-effect charge mobility. C10O-Ar-OT(4)-H showed a hole mobility of 0.07,cm2,V,1,s,1 when deposited on octyltrichlorosilane-treated substrates at 140,°C and the on/off current ratios reached 5×105; on the other hand, its mobility was only 8×10,3,cm2,V,1,s,1 on the same substrate when deposited at room temperature. [source] Liquid Crystal Imidazolium Salts: Towards Materials for Catalysis and Molecular ElectronicsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2007Jean-Moïse Suisse Abstract 1,3-Bis(4-alkyloxyphenyl)-3H -imidazol-1-ium trifluoromethanesulfonates, alkyl = CH3(CH2)n,1, n = 8, 10, 12, 14 and 16, can be derived from the analogous 4-alkyloxyphenylamines. These imidazolium salts exhibit a lamellar liquid-crystal mesophase between 99 °C and 191 °C. The smectic-A phase was fully characterised by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. We report also the synthesis, lamellar crystal structure and catalytic activity of the PdII complex of the (deprotonated) carbene form of one of these salts. In addition, we measured the charged carrier mobilities in the mesophase.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Temperature-Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers,ADVANCED FUNCTIONAL MATERIALS, Issue 14 2010Patrick Pingel Abstract Previous investigations of the field-effect mobility in poly(3-hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium-MW: Mn,=,7,200 g mol,1; high-MW: Mn,=,27,000 g mol,1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) and organic field-effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra-grain mobility depends only weakly on MW (being in the order of 10,2 cm2 V,1 s,1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature-dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium-MW P3HT undergo a "pre-melting" significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short-chain P3HT is strongly inhibited by the presence of disordered domains, while in high-MW P3HT the low-mobility disordered zones are bridged via inter-crystalline molecular connections. [source] Emission Color Tuning in Ambipolar Organic Single-Crystal Field-Effect Transistors by Dye-DopingADVANCED FUNCTIONAL MATERIALS, Issue 10 2010Hajime Nakanotani Abstract The effect of dye-doping in ambipolar light-emitting organic field-effect transistors (LE-OFETs) is investigated from the standpoint of the carrier mobilities and the electroluminescence (EL) characteristics under ambipolar operation. Dye-doping of organic crystals permits not only tuning of the emission color but also significantly increases the efficiency of ambipolar LE-OFETs. A rather high external EL quantum efficiency (,0.64%) of one order of magnitude higher than that of a pure p -distyrylbenzene (P3V2) single crystal is obtained by tetracene doping. The doping of tetracene molecules into a host P3V2 crystal has almost no effect on the electron mobility and the dominant carrier recombination process in the tetracene-doped P3V2 crystal involves direct carrier recombination on the tetracene molecules. [source] Orientation Control of Linear-Shaped Molecules in Vacuum-Deposited Organic Amorphous Films and Its Effect on Carrier MobilitiesADVANCED FUNCTIONAL MATERIALS, Issue 3 2010Daisuke Yokoyama Abstract The molecular orientation of linear-shaped molecules in organic amorphous films is demonstrated to be controllable by the substrate temperature. It is also shown that the molecular orientation affects the charge-transport characteristics of the films. Although linear-shaped 4,4,-bis[(N -carbazole)styryl]biphenyl molecules deposited on substrates at room temperature are horizontally oriented in amorphous films, their orientation when deposited on heated substrates with smooth surfaces becomes more random as the substrate temperature increases, even at temperatures under the glass transition temperature. Another factor dominating the orientation of the molecules deposited on heated substrates is the surface roughness of the substrate. Lower carrier mobilities are observed in films composed of randomly oriented molecules, demonstrating the significant effect of a horizontal molecular orientation on the charge-transport characteristics of organic amorphous films. [source] Variable Temperature Mobility Analysis of n-Channel, p-Channel, and Ambipolar Organic Field-Effect TransistorsADVANCED FUNCTIONAL MATERIALS, Issue 1 2010Joseph A. Letizia Abstract The temperature dependence of field-effect transistor (FET) mobility is analyzed for a series of n-channel, p-channel, and ambipolar organic semiconductor-based FETs selected for varied semiconductor structural and device characteristics. The materials (and dominant carrier type) studied are 5,5,,,-bis(perfluorophenacyl)-2,2,:5,,2,:5,,2,,,-quaterthiophene (1, n-channel), 5,5,,,-bis(perfluorohexyl carbonyl)-2,2,:5,,2,:5,,2,,,-quaterthiophene (2, n-channel), pentacene (3, p-channel); 5,5,,,-bis(hexylcarbonyl)-2,2,:5,,2,:5,,2,,,-quaterthiophene (4, ambipolar), 5,5,,,-bis-(phenacyl)-2,2,: 5,,2,:5,,2,,,-quaterthiophene (5, p-channel), 2,7-bis((5-perfluorophenacyl)thiophen-2-yl)-9,10-phenanthrenequinone (6, n-channel), and poly(N -(2-octyldodecyl)-2,2,-bithiophene-3,3,-dicarboximide) (7, n-channel). Fits of the effective field-effect mobility (µeff) data assuming a discrete trap energy within a multiple trapping and release (MTR) model reveal low activation energies (EAs) for high-mobility semiconductors 1,3 of 21, 22, and 30,meV, respectively. Higher EA values of 40,70,meV are exhibited by 4,7 -derived FETs having lower mobilities (µeff). Analysis of these data reveals little correlation between the conduction state energy level and EA, while there is an inverse relationship between EA and µeff. The first variable-temperature study of an ambipolar organic FET reveals that although n-channel behavior exhibits EA,=,27,meV, the p-channel regime exhibits significantly more trapping with EA,=,250,meV. Interestingly, calculated free carrier mobilities (µ0) are in the range of ,0.2,0.8,cm2,V,1 s,1 in this materials set, largely independent of µeff. This indicates that in the absence of charge traps, the inherent magnitude of carrier mobility is comparable for each of these materials. Finally, the effect of temperature on threshold voltage (VT) reveals two distinct trapping regimes, with the change in trapped charge exhibiting a striking correlation with room temperature µeff. The observation that EA is independent of conduction state energy, and that changes in trapped charge with temperature correlate with room temperature µeff, support the applicability of trap-limited mobility models such as a MTR mechanism to this materials set. [source] Metastable Copper-Phthalocyanine Single-Crystal Nanowires and Their Use in Fabricating High-Performance Field-Effect TransistorsADVANCED FUNCTIONAL MATERIALS, Issue 23 2009Kai Xiao Abstract This paper describes a simple, vapor-phase route for the synthesis of metastable , -phase copper-phthalocyanine (CuPc) single-crystal nanowires through control of the growth temperature. The influence of the growth temperature on the crystal structures, morphology, and size of the CuPc nanostructures is explored using X-ray diffraction (XRD), optical absorption, and transmission electron microscopy (TEM). , -CuPc nanowires are successfully incorporated as active semiconductors in field-effect transistors (FETs). Single nanowire devices exhibit carrier mobilities and current on/off ratios as high as 0.4,cm2 V,1 s,1 and >104, respectively. [source] Transistor Paint: Environmentally Stable N -alkyldithienopyrrole and Bithiazole-Based Copolymer Thin-Film Transistors Show Reproducible High Mobilities without AnnealingADVANCED FUNCTIONAL MATERIALS, Issue 21 2009Junying Liu Abstract New solution processable 4-(2-hexyldecan)- 4H -bisthieno[2,3- d:3,,2,- b]pyrrole and 4,4,-dialkyl-2,2,-bithiazole-based copolymers (PBTzDTPs) are synthesized with excellent FET performance. These novel copolymers have considerable potential in printable electronics as they have high charge carrier mobilities, excellent air stability, good solution processibility, and no requirement for post-deposition thermal annealing, all requirements for this field of application. The thin film transistors fabricated from PBTzDTPs achieve field effect mobilities as high as 0.14,cm2 V,1 s,1 with current on/off ratios up to 106 without thermal annealing. In addition, the devices exhibit stable performance in air, showing no significant degradation over 60 days. Moreover, the polymers described here provide an excellent example of the systems in which higher mobility performance does not require higher crystalline, long-range ordered structures. Such a system appears to be particularly promising for rapid fabrication techniques, where kinetic conditions usually prevent the development of long-range order. [source] Comprehensive Modeling of Ion Conduction of Nanosized CaF2/BaF2 Multilayer HeterostructuresADVANCED FUNCTIONAL MATERIALS, Issue 1 2009Xiangxin Guo Abstract Molecular beam epitaxy-grown CaF2/BaF2 heterolayers are a demonstration of the potential of nanoionics. It has been shown that ion conductivities both parallel and perpendicular to the interfaces increase with decrease in interfacial spacing. This size effect was attributed to the thermodynamically necessary redistribution of the mobile fluoride ions (N. Sata, K. Eberl, K. Eberman, J. Maier, Nature 2000, 408, 946; X. X. Guo, I. Matei, J.-S. Lee, J. Maier, Appl. Phys. Lett. 2007, 91, 103102). On this basis, the striking phenomenon of an upward bending in the effective parallel conductivity as a function of inverse interfacial spacing for low temperatures (T,,,593,K) has been satisfactorily explained by application of a modified Mott,Schottky model for BaF2 (X.X. Guo, I. Matei, J. Jamnik, J.-S. Lee, J. Maier, Phys. Rev. B 2007, 76, 125429). This model was further confirmed by measurements perpendicular to the interfaces that offer complementary information on the more resistive parts. Here a successful comprehensive modeling of parallel and perpendicular conductivities for the whole parameter range, namely for interfacial spacings ranging from 6 to 200,nm and investigated temperatures ranging from 455 to 833,K, is presented. The model is based on literature data for carrier mobilities and Frenkel reaction constants and the assumption of a pronounced F, redistribution. Given the fact that an impurity content that was experimentally supported is taken into account and apart from minor assumptions concerning profile homogeneity, the only fit parameter is the space charge potential. In particular, it is worth mentioning that in BaF2 the low temperature Mott,Schottky space charge zone which is determined by impurities changes over, at high temperatures, into a Gouy,Chapman situation owing to increased thermal disorder. (The situation in CaF2 is of Gouy,Chapman type at all temperatures.) [source] Origin of the Reduced Fill Factor and Photocurrent in MDMO-PPV:PCNEPV All-Polymer Solar Cells,ADVANCED FUNCTIONAL MATERIALS, Issue 13 2007M. Mandoc Abstract The photogeneration mechanism in blends of poly[2-methoxy-5-(3,,7,-dimethyloctyloxy)-1,4 - phenylene vinylene] (MDMO-PPV) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3,,7,-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV) is investigated. The photocurrent in the MDMO-PPV:PCNEPV blends is strongly dependent on the applied voltage as a result of a low dissociation efficiency of the bound electron,hole pairs. The dissociation efficiency is limited by low carrier mobilities, low dielectric constant, and the strong intermixing of the polymers, leading to a low fill factor and a reduced photocurrent at operating conditions. Additionally, electrons trapped in the PCNEPV phase recombine with the mobile holes in the MDMO-PPV phase at the interface between the two polymers, thereby affecting the open-circuit voltage and increasing the recombination losses. At an intensity of one,sun, Langevin recombination of mobile carriers dominates over trap-assisted recombination. [source] Bis(carbazolyl)benzodifuran: A High-Mobility Ambipolar Material for Homojunction Organic Light-Emitting Diode DevicesADVANCED MATERIALS, Issue 37 2009Hayato Tsuji A new ambipolar material bis(carbazolyl)benzodifuran (CZBDF) shows well-balanced and high carrier mobilities for both holes and electrons (>10,3,cm2 V,1 s,1). This new material allows us to fabricate efficient p-i-n homojunction OLEDs that emit light across the full visible color range and perform at a level similar to state-of-the-art heterojunction devices. [source] Semiconducting Thienothiophene Copolymers: Design, Synthesis, Morphology, and Performance in Thin-Film Organic TransistorsADVANCED MATERIALS, Issue 10-11 2009Iain McCulloch Abstract Organic semiconductors are emerging as a viable alternative to amorphous silicon in a range of thin-film transistor devices. With the possibility to formulate these p-type materials as inks and subsequently print into patterned devices, organic-based transistors offer significant commercial advantages for manufacture, with initial applications such as low performance displays and simple logic being envisaged. Previous limitations of both air stability and electrical performance are now being overcome with a range of both small molecule and polymer-based solution-processable materials, which achieve charge carrier mobilities in excess of 0.5,cm2 V,1 s,1, a benchmark value for amorphous silicon semiconductors. Polymer semiconductors based on thienothiophene copolymers have achieved amongst the highest charge carrier mobilities in solution-processed transistor devices. In this Progress Report, we evaluate the advances and limitations of this class of polymer in transistor devices. [source] High-Performance Polymer-Small Molecule Blend Organic TransistorsADVANCED MATERIALS, Issue 10-11 2009Richard Hamilton A double-gate device is used to demonstrate that a blended formulation of semiconducting small molecules and a polymer matrix can provide high electrical performance within thin-film field-effect transistors (OTFTs) with charge carrier mobilities of greater than 2,cm2,V,1,s,1, good device-to-device uniformity, and the potential to fabricate devices from routine printing techniques. [source] Gate Dielectric Microstructural Control of Pentacene Film Growth Mode and Field-Effect Transistor Performance,ADVANCED MATERIALS, Issue 18 2007C. Kim Organic semiconductor/dielectric interfacial characteristics play a critical role in influencing organic thin-film transistor (OTFT) performance characteristics (see figure). Clear correlations between pentacene film deposition temperature, estimated polymer dielectric surface microstructural mobility, and the corresponding film growth mode, semiconductor phase composition, and carrier mobilities are established. [source] Magnus' Green Salt Revisited: Impact of Platinum,Platinum Interactions on Electronic Structure and Carrier Mobilities,ADVANCED MATERIALS, Issue 15 2006E.-G. Kim Magnus' green salt is the prototype of a class of organic,inorganic hybrid semiconducting materials that combine attractive charge-transport properties and processability. By using density-functional-theory methods, the electronic structure of Magnus' green salt is investigated, in particular the nature of the interplatinum interactions (see figure). In conjunction with time-of-flight measurements of the carrier mobilities, key structure,property relationships for these materials are re-established. [source] Liquid Crystalline Ordering and Charge Transport in Semiconducting MaterialsMACROMOLECULAR RAPID COMMUNICATIONS, Issue 14 2009Wojciech Pisula Abstract Organic semiconducting materials offer the advantage of solution processability into flexible films. In most cases, their drawback is based on their low charge carrier mobility, which is directly related to the packing of the molecules both on local (amorphous versus crystalline) and on macroscopic (grain boundaries) length scales. Liquid crystalline ordering offers the possibility of circumventing this problem. An advanced concept comprises: i) the application of materials with different liquid crystalline phases, ii) the orientation of a low viscosity high temperature phase, and, iii) the transfer of the macroscopic orientation during cooling to a highly ordered (at best, crystalline-like) phase at room temperature. At the same time, the desired orientation for the application (OLED or field-effect transistor) can be obtained. This review presents the use of molecules with discotic, calamitic and sanidic phases and discusses the sensitivity of the phases with regard to defects depending on the dimensionality of the ordered structure (columns: 1D, smectic layers and sanidic phases: 2D). It presents ways to systematically improve charge carrier mobility by proper variation of the electronic and steric (packing) structure of the constituting molecules and to reach charge carrier mobilities that are close to and comparable to amorphous silicon, with values of 0.1 to 0.7,cm2,·,V,1,·,s,1. In this context, the significance of cross-linking to stabilize the orientation and liquid crystalline behavior of inorganic/organic hybrids is also discussed. [source] Dielectric layers for organic field effect transistors as gate dielectric and surface passivationPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2008T. Diekmann Abstract Organic field effect transistors with the organic semiconductor pentacene, using silicon substrates, were successfully built on conventional inorganic dielectrics like silicon dioxide or silicon nitride. They can drive drain currents up to 15 mA. Beyond that, polymer films were investigated as gate dielectrics in order to achieve transistors on plastic films. On polyester substrates with an inorganic,organic gate dielectric, devices reach drain currents comparable to transistors on silicon dioxide and charge carrier mobilities of up to 0.35 cm2/V s. Analysis of the pentacene surface by atomic force microscopy showed pentacene crystallites achieving dimensions of more than 1.5 µm. The unprotected organic devices suffer from degradation due to water and oxygen incorporation. Therefore, the application of a hydrophobic polytetrafluoroethylene layer as capping layer is studied. Because of the reduced influence of water, a shift to positive threshold voltages is caused. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Potentiometry on pentacene OFETs: Charge carrier mobilities and injection barriers in bottom and top contact configurationsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2008R. Scholz Abstract In a combination of experimental techniques including electrical probes, potentiometry, and charge transient spectroscopy (QTS), we develop concepts how to quantify the potential drops at the contacts, the mobility in the channel region, and the density of states of deep traps in pentacene OFETs. For OFETs grown from unpurified pentacene on pre-patterned Au bottom contacts, a comparison between potentiometry and two-dimensional device simulations determines an injection barrier of 0.73 eV at the source contact and a hole mobility of 0.014 cm2 V,1 s,1 in the pentacene channel. Temperature-dependent QTS data reveal a trap level at about 125 meV from the hole transport band, indicating a relatively high density of unintentional dopants and therefore a high background density of majority charge carriers. In OFETs grown from purified pentacene onto a SiO2 gate dielectric and Au top contacts evaporated onto the pentacene channel without breaking the vacuum, potentiometry reveals a nearly perfect alignment of the metal work function with the hole transport level in the organic layer. The much lower density of deep traps in these samples raises the hole mobility to the range 0.1,0.2 cm2 V,1 s,1. A further improvement of the hole mobility and the resulting device performance can be achieved by a chemical treatment of the gate oxide with n-octadecytrichlorosilane (OTS). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Strain-engineered novel III,N electronic devices with high quality dielectric/semiconductor interfacesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003M. Asif Khan Abstract Since the early demonstration of 2D-electron gas [M. A. Khan et al., Appl. Phys. Lett. 60, 3027 (1992)] and a heterojunction field effect transistor (HFET) [M. Asif Khan et al., Appl. Phys. Lett. 63, 1214 (1993)] in III,N materials, rapid progress has been made to improve the DC and RF performance of GaN,AlGaN based HFETs. Stable and impressive microwave powers as high as 4,8 W/mm have been reported for device operation frequencies from 10 to 35 GHz. The key reason for these high performance numbers is an extremely large sheet carrier densities (>1 × 1013 cm,2) that can be induced at the interfaces in III,N hetereojunction [A. Bykhovsk et al., J. Appl. Phys. 74, 6734 (1993); M. Asif Khan et al., Appl. Phys. Lett. 75, 2806 (1999)]. These are instrumental in screening the channel dislocations thereby retaining large room temperature carrier mobilities (>1500 cm2/Vs) and sheet resistance as low as 300 ,/sq. These numbers and the high breakdown voltages of the large bandgap III,N material system thus enable rf-power approximately 5,10 times of that possible with GaAs and other competitor's technologies. We have recently introduced a unique pulsed atomic layer epitaxy approach to deposit AlN buffer layers and AlN/AlGaN superlattices [J. Zhang et al., Appl. Phys. Lett. 79, 925 (2001); J. P. Zhang et al., Appl. Phys. Lett. 80, 3542 (2002)] to manage strain and decrease the dislocation densities in high Al-content III,N layers. This has enabled us to significantly improve GaN/AlGaN hetereojunctions and the device isolation. The resulting low defect layers are not only key to improving the electronic but also deep ultraviolet light-emitting diode devices. For deep UV LED's they enabled us to obtain peak optical powers as high as 10 mW and 3 mW for wavelengths as short as 320 nm and 278 nm. Building on our past work [M. Asif Khan et al., Appl. Phys. Lett. 77, 1339 (2000); X. Hu et al., Appl. Phys. Lett. 79, 2832 (2001)] we have now deposited high quality SiO2/Si3N4 films over AlGaN with low interface state densities. They have then been used to demonstrate III,N insulating gate transistors (MOSHFET (SiO2) and MISHFET (Si3N4) with gate leakage currents 4,6 order less than those for conventional GaN,AlGaN HFETs. The introduction of the thin insulator layers (less then 100 Å) under the gate increases the threshold voltage by 2,3 V. In addition, it reduces the peak transconductance gm. However the unity cut-off frequency, the gain and the rf-powers remain unaffected as the gm/Cgs (gate-source capacitance) ratio remains unchanged. In addition to managing the defects and gate leakage currents we have also employed InGaN channel double heterojunction structures (AlInGaN,InGaN,GaN) to confine the carriers thereby reducing the spillover into trappings states. These InGaN based MOS-DHFETs exhibited no current-collapse, extremely low gate leakage currents (<10,10 A/mm) and 10,26 GHz rf-powers in excess of 6 W/mm. We have also demonstrated the scalability and stable operation of our new and innovative InGaN based insulating gate heterojunction field effect transistor approach. In this paper we will review the III,N heterojunction field-effect transistors progress and pioneering innovations including the excellent work from several research groups around the world. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Multicarrier analysis of magnetotransport data at low and high electric fieldsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2009O. V. S. N. Murthy Abstract We present some results on multicarrier analysis of magnetotransport data. Both synthetic as well as data from narrow gap Hg0.8Cd0.2Te samples are used to demonstrate applicability of various algorithms vs. nonlinear least square fitting, Quantitative Mobility Spectrum Analysis (QMSA) and Maximum Entropy Mobility Spectrum Analysis (MEMSA). Comments are made from our experience on these algorithms, and, on the inversion procedure from experimental R/,-B to S-, specifically with least square fitting as an example. Amongst the conclusions drawn are: (i) Experimentally measured resistivity (Rxx, Rxy) should also be used instead of just the inverted conductivity (,xx, ,xy) to fit data to semiclassical expressions for better fits especially at higher B. (ii) High magnetic field is necessary to extract low mobility carrier parameters. (iii) Provided the error in data is not large, better estimates to carrier parameters of remaining carrier species can be obtained at any stage by subtracting highest mobility carrier contribution to , from the experimental data and fitting with the remaining carriers. (iv)Even in presence of high electric field, an approximate multicarrier expression can be used to guess the carrier mobilities and their variations before solving the full Boltzmann equation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Deposition of SnO2:F Thin Films on Polycarbonate Substrates by PECVD for Antifouling PropertiesPLASMA PROCESSES AND POLYMERS, Issue S1 2007Marie Jubault Abstract SnO2:F thin films were deposited on polycarbonate and glass substrates by RF capacitively coupled plasma enhanced chemical vapor deposition (PECVD) using a mixture of tetramethyltin (TMT) [Sn(CH3)4], argon, oxygen as precursors. The effects of the substrate temperature, deposition time and doping on the resistivity and the morphology of the films have been studied. The as-deposited films appear to have higher carrier mobilities than amorphous ones, in the range of 5 and 7.5 cm2,·,V,1,·,s,1, which could be explained by the presence of nanocrystallites. In order to understand the nanostructure of the films, we studied the formation of nanoparticles and dust particles in the discharge. Finally, we have shown that the incorporation of less than 3% of F in the tin oxide layer could decrease the resistivity down to 3,·,10,3 ,,·,cm. [source] Tetrachloro-substituted Perylene Bisimide Dyes as Promising n-Type Organic Semiconductors: Studies on Structural, Electrochemical and Charge Transport PropertiesCHEMPHYSCHEM, Issue 1 2004Zhijian Chen Twisted ,-systems: The highly twisted 1,6,7,12-tetrachloro-substituted perylene bisimides possess an improved electron affinity. The nonplanar nature of these molecules facilitates a slipped brickstone-type rather than a columnar stacking of the ,-systems, with a potentially useful two dimensional contact feature. These compounds show isotropic charge carrier mobilities as high as up to 0.14 cm2,V,1 s,1 (see graphic). [source] Experimental analysis of Mn segregation in Bridgman-grown gallium antimonide: Dependency on the ampoule radiusCRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2004J. L. Plaza Abstract This work considered the segregation of manganese in vertical Bridgman-grown GaSb crystals for different ampoule diameters. Experimental data of the impurity distribution were obtained from atomic absorption spectrometry and also Hall measurements. It was demonstrated that the radial segregation is more pronounced in the case of thick ampoule diameters. Furthermore the manganese effective segregation coefficient, resistivity, carrier mobility and density were obtained in each case. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||||||||||||||