Difference In Electron Mobilities Research Articles

Difference in electron mobility at 4H–SiC/SiO2 interfaces with various crystal faces originating from effective-field-dependent scattering

Although the channel resistance is partially reduced by suppressing 4H–SiC/SiO2 interface trapping, interface scattering still presents a problem. To clearly extract the effective-field (Eeff) dependence of the dominant scattering, a body biasing technique was adopted, under the condition that the charge density is constant to fix the screening effect. The electron mobilities were observed to be several fold higher for a-, m-, and 03¯38¯ faces than for Si- and C-faces. This result is primarily due to a magnitude difference in the Eeff-dependent scattering; thus, the difference is emphasized at higher Eeff values. Physical parameters to reproduce the observed mobility were estimated by simulating Coulomb and roughness scattering.

Semi-classical electronic transport properties of ternary compound AlGaAs2: role of different scattering mechanisms

Using Rode’s iterative method, we have investigated the semi-classical transport properties of the n-type ternary compound AlGaAs2. Four scattering mechanisms have been included in our transport calculation, namely, ionized impurity, piezoelectric, acoustic deformation and polar optical phonon (POP). The scattering rates have been calculated in terms of ab initio parameters. We consider AlGaAs2 to have two distinct crystal geometries, one in tetragonal phase (space group: ), while the other one having body centered tetragonal crystal structure (space group: ). Higher electron mobility has been observed in the body centered tetragonal phase, thereby making it more suitable for high mobility device application, over the tetragonal phase. In order to understand the differences in electron mobility for these two phases, curvatures of the E-k dispersion of the conduction bands for these phases have been compared. At room temperature, the dominant contribution in electron mobility was found to be provided by inelastic POP scattering. We have also noted that mobility is underestimated in relaxation time approximation compared with the Rode’s iterative approach.

Tunable self-organization in n-type liquid crystalline dibenzocoronene tetracarboxdiimides for high photoconductivity

ABSTRACT A comprehensive study on the self-organised mesomorphism and charge-transport properties of three n-type liquid crystalline (LC) dibenzocoronene tetracarboxdiimide derivatives (Dibenzo-CDI 1, 2, 3) bearing branched alkyl chains with varied side-chain length and branching point (α or β site) has been carried out. All three Dibenzo-CDI compounds possessed high solubility in common organic solvents and low-lying lowest unoccupied molecular orbital energy levels below −4.0 eV indicating their air-stable electron-transport capacity under ambient conditions. It is discovered that the side-chain length has a moderate influence on the molecular arrangement and thus leads to only limited variation of electron-transport properties. In contrast, the branching position plays a critical role on tuning molecular packing and orientation in the bulk mesophases, thereby resulting in dramatic differences in electron mobilities. Remarkably, Dibenzo-CDI 3 demonstrates good photoconductivity by time-of-flight method with high electron mobility up to 0.075 cm2/Vs, close to the highest value reported in certain n-type LC derivatives. All these results reveal that LC Dibenzo-CDI derivatives are potential candidates for high-performance solution-processable electronics, such as air-stable n-type organic field-effect transistors (OFETs) and solar cells.

The importance of intramolecular conductivity in three dimensional molecular solids.

Recent years have seen tremendous progress towards understanding the relation between the molecular structure and function of organic field effect transistors. The metrics for organic field effect transistors, which are characterized by mobility and the on/off ratio, are known to be enhanced when the intermolecular interaction is strong and the intramolecular reorganization energy is low. While these requirements are adequate when describing organic field effect transistors with simple and planar aromatic molecular components, they are insufficient for complex building blocks, which have the potential to localize a carrier on the molecule. Here, we show that intramolecular conductivity can play a role in controlling device characteristics of organic field effect transistors made with macrocycle building blocks. We use two isomeric macrocyclic semiconductors that consist of perylene diimides linked with bithiophenes and find that the trans-linked macrocycle has a higher mobility than the cis-based device. Through a combination of single molecule junction conductance measurements of the components of the macrocycles, control experiments with acyclic counterparts to the macrocycles, and analyses of each of the materials using spectroscopy, electrochemistry, and density functional theory, we attribute the difference in electron mobility of the OFETs created with the two isomers to the difference in intramolecular conductivity of the two macrocycles.

Effects of p-(Trifluoromethoxy)benzyl and p-(Trifluoromethoxy)phenyl Molecular Architecture on the Performance of Naphthalene Tetracarboxylic Diimide-Based Air-Stable n-Type Semiconductors.

N,N'-Bis(4-trifluoromethoxyphenyl) naphthalene-1,4,5,8-tetracarboxylic acid diimide (NDI-POCF3) and N,N'-bis(4-trifluoromethoxybenzyl) naphthalene-1,4,5,8-tetracarboxylic acid diimide (NDI-BOCF3) have similar optical and electrochemical properties with a deep LUMO level of approximately 4.2 eV, but exhibit significant differences in electron mobility and molecular packing. NDI-POCF3 exhibits nondetectable charge mobility. Interestingly, NDI-BOCF3 shows air-stable electron transfer performance with enhanced mobility by increasing the deposition temperature onto the octadecyltrichlorosilane (OTS)-modified SiO2/Si substrates and achieves electron mobility as high as 0.7 cm(2) V(-1) s(-1) in air. The different mobilities of those two materials can be explained by several factors including thin-film morphology and crystallinity. In contrast to the poor thin-film morphology and crystallinity of NDI-POCF3, NDI-BOCF3 exhibits larger grain sizes and improved crystallinities due to the higher deposition temperature. In addition, the theoretical calculated transfer integrals of the intermolecular lowest unoccupied molecular orbital (LUMO) of the two materials further show that a large intermolecular orbital overlap of NDI-BOCF3 can transfer electron more efficiently than NDI-POCF3 in thin-film transistors. On the basis of fact that the theoretical calculations are consistent with the experimental results, it can be concluded that the p-(trifluoromethoxy) benzyl (BOCF3) molecular architecture on the former position of the naphthalene tetracarboxylic diimides (NDI) core provides a more effective way to enhance the intermolecular electron transfer property than the p-(trifluoromethoxy) phenyl (POCF3) group for the future design of NDI-related air-stable n-channel semiconductor.

Two-dimensional π-expanded quinoidal terthiophenes terminated with dicyanomethylenes as n-type semiconductors for high-performance organic thin-film transistors.

Quinoidal oligothiophenes (QOT), as classical n-type semiconductors, have been well-known for a long time but with non-optimal semiconducting properties. We report here the design and selective synthesis of new two-dimensional (2D) π-expanded quinoidal terthiophenes, 2DQTTs, with proximal (2DQTT-i) and distal (2DQTT-o) regiochemistry for high-performance n-channel organic thin-film transistors (n-OTFTs) featuring high electron mobility, solution processability, and ambient stability. The elegant combination of thieno[3,4-b]thiophene [TT, donor (D)] and 5-alkyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione [TPD, acceptor (A)] units with relatively large π-surface endows these 2DQTTs with distinctive 2D structural characteristics and flat configuration stabilized by weak intramolecular S-O/S weak interactions. Furthermore, the A-D-A-D-A electronic structure maintains an adequately low LUMO energy level. These 2DQTTs are shown to exhibit outstanding semiconducting properties with electron mobilities of up to 3.0 cm(2) V(-1) s(-1) and on/off ratios of up to 10(6) (2DQTT-o) in ambient- and solution-processed OTFTs. Investigations on thin-film morphology reveal that the microstructure of 2DQTTs is highly dependent on the orientation of the fused thiophene subunits, leading to differences in electron mobilities of 1 order of magnitude. X-ray diffraction studies in particular reveal increased crystallinity, crystalline coherence, and orientational order in 2DQTT-o compared to 2DQTT-i, which accounts for the superior electron transport property of 2DQTT-o.

The growth behavior and properties of atomic layer deposited zinc oxide films using hydrogen peroxide (H2O2) and ozone (O3) oxidants

Two types of zinc oxide (ZnO) films were grown by thermal atomic layer deposition (ALD), each using hydrogen peroxide and ozone gas oxidants. Diethylzinc (DEZ) was used as the zinc precursor for all experiments. The use of hydrogen peroxide oxidant resulted in elevated growth rates by approximately 70% at relatively low temperatures below 200°C, in comparison with films grown using ozone gas. It is suggested that the use of hydrogen peroxide induces the formation of hydroxyl compounds on top of the growing film between each ALD cycle, which promotes better surface adsorption of the DEZ precursor molecules, thus enhancing the surface reaction rate. On the other hand, ozone-assisted growth is rather close to a thermally activated process, where the growth rate is observed to increase gradually with substrate temperature. At a relatively low growth temperature of 150°C, the electrical resistivity of the ZnO films grown using hydrogen peroxide was lower than the films grown using ozone, by approximately three orders of magnitude. The free carrier density was observed to be the major parameter affecting the resistivity, the latter decreasing with growth temperature. The differences in electron mobility and concentration were correlated to the microstructure and atomic bonding states examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).

High-mobility solution-processed zinc oxide thin films on silicon nitride

A high effective electron mobility of 33 cm2 V–1 s–1 was achieved in solution-processed undoped zinc oxide (ZnO) thin films. The introduction of silicon nitride (Si3N4) as growth substrate resulted in a mobility improvement by a factor of 2.5 with respect to the commonly used silicon oxide (SiO2). The solution-processed ZnO thin films grown on Si3N4, prepared by low-pressure chemical vapor deposition, revealed bigger grain sizes, lower strain and better crystalline quality in comparison to the films grown on thermal SiO2. These results show that the nucleation and growth mechanisms of solution-processed films are substrate dependent and affect the final film structure accordingly. The substantial difference in electron mobilities suggests that, in addition to the grain morphology and crystalline structure effects, defect chemistry is a contributing factor that also depends on the particular substrate. In this respect, interface trap densities measured in high-κ HfO2/ZnO MOSCAPs were about ten times lower in those fabricated on Si3N4 substrates. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Critical Role of Alkyl Chain Branching of Organic Semiconductors in Enabling Solution-Processed N-Channel Organic Thin-Film Transistors with Mobility of up to 3.50 cm2 V–1 s–1

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from ~0.001 to >3.0 cm(2) V(-1) s(-1). Incorporating a NDI-DTYM2 core with three-branched N-alkyl substituents of C(11,6) results in a dense in-plane molecular packing with an unit cell area of 127 Å(2), larger domain sizes of up to 1000 × 3000 nm(2), and an electron mobility of up to 3.50 cm(2) V(-1) s(-1), which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities.

Temperature and Electric Field Dependent Electron Transport in Cationic and Anionic Conjugated Polyelectrolytes

The temperature and electric field dependence of electron mobilities in conjugated polyelectrolytes (CPEs) with structure variations of the ionic functionality and counterions were investigated to gain insight into the influence of these parameters on charge transport. Electron mobilities were determined by using electron-only diodes, and the temperature and electric field dependence were modeled with the Gaussian disorder model, the correlated Gaussian disorder model, and the polaronic correlated Gaussian disorder model for comparison. The transport characteristics were found to be dependent on the type of charge, as well as the choice of counterion, with the former baring a stronger influence on transport. Anionic CPEs are more susceptible to differences in the calculated energetic disorder than their cationic counterparts. Examination of cationic CPEs with different counterions show that, although the energetic disorder is similar, large differences in electron mobilities can be obtained. These observa...

Effect of Epitaxial Layer Crystal Quality on DC and RF Characteristics of AlGaN/GaN Short-Gate High-Electron-Mobility Transistors on Sapphire Substrates

The DC and RF characteristics of short-gate high-electron-mobility transistors (HEMTs) formed on four AlGaN/GaN wafers grown on sapphire substrates with different crystal quality have been investigated. Atomic force microscopy observation revealed many pits and trenches on the AlGaN surface, and the morphology of each sample was distinct. There were also differences in electron mobility and sheet carrier concentration. However, the gate length dependences of the measured transconductance and cut-off frequency were virtually the same. For a more detailed investigation, we subtracted the source resistance and AlGaN thickness contributions from measured DC and performed a delay time analysis for the RF characteristics. The results indicate that the intrinsic performance of HEMTs was independent of the surface morphology and that effective electron velocity ranged from 1.4 to 1.8×107 cm/s.

Muon sites and diffusion in doped lithium oxide

We present muon-spin relaxation results for pure and doped lithium oxide, and deduce the muon site and the diffusion rates. Samples were doped with fluoride and hydroxide ions both of which carry a nuclear moment. These ions with charge −e substitute in place of an oxide ion with charge −2e, so must be accompanied by a corresponding vacancy on the Li + sublattice. The static muon relaxation rate is the same in all samples, showing that the muons are not trapped at or near the anion impurity sites. Muons occupy Li + vacancy sites at around 150 K and this suggests that the vacancies and anion impurities are well separated. The variation of hop rate with temperature is also almost identical for all the samples. This suggests that the impurities do not play any role in the hopping mechanism at this length scale. The doped samples show a significantly larger diamagnetic muon fraction than the pure material, probably a result of differences in electron mobility.

Influence of interstitial carbon defects on electron transport in strained Si1−yCy layers on Si(001)

We present experimental results on Hall mobilities of electrons in tensile strained Si1−yCy layers with a substitutional carbon yS=0.4%, but different concentrations of interstitial carbon. Although the lattice distortion due to misfit strain and hence, the band alignment are identical for all investigated samples, we find differences in electron mobility of nearly a factor 2 due to the varying concentration of interstitial carbon. For the highest interstitial C concentration (1×1020 cm−3), it was not even possible to obtain any reliable electrical data. We demonstrate that it is not sufficient to consider only strain in evaluating electrical properties of C containing layers. Specific growth conditions can lead to very different electrical properties due to the different amounts of interstitial C, even for pseudomorphically strained layers with the same lattice mismatch and band alignment.

Electron delocalization in alpha -nitrogen.

A new technique has been developed for measuring electron drift mobility in crystals on a microscopic scale through its effect on muonium (p, e ) atom formation via transport of electrons to thermalized positive muons (p, +). Electron transport mechanisms are shown to be fundamentally different in the n and P phases of solid nitrogen, giving at least 5 orders of magnitude difference in electron mobilities. Contrary to reported results of macroscopic time-of-flight measurements, excess electrons appear to be delocalized in n -Nq. PACS numbers: 71.25.— s, 73.20.Jc, 76.75.+i

Comment on electron mobility in the liquid isomeric pentanes

We comment on the large differences in electron mobilities that often exist between structural isomers of nonpolar organic liquids. Focusing on the isomers of liquid pentane as an example, we discuss the relevance and correlation of the first peak observed in the x-ray structure factor S(k) with the observed differences in electron mobility.

Novel extension of the trap model for electrons in liquid hydrocarbons

Abstract A novel extension for the trap model of electron mobilities in liquid hydrocarbons is described. The new model assumes: (a) two main types of electron trap exist in liquid hydrocarbons, one is deep and the second is shallow; (b) these traps are the same in all liquid alkanes. The difference in electron mobilities in different alkanes is accounted for by the difference in the frequency of electron trapping in each state. The probability of trapping in each state has been evaluated from the known structures of the normal alkanes. Electron mobilities in normal alkanes (C3-C10) show a very good correlation with the probability of trapping in deep traps, suggesting that the C-C bonds are the main energy sinks of the electron. A mathematical formula which expresses the electron mobility in terms of the probability of trapping in deep traps has been found from the Arrhenius relationship between electron mobilities and probability of trapping. The model has been extended for branched alkanes and the relatively high electron mobilities in globular alkanes has been explained by the fact that each branch provides some degree of screening to the skeleton structure of the molecule resulting in reduction of the probability of electron interaction with the molecular skeleton.