Anisotropic Hole Mobility Research Articles

Origin of hole mobility anisotropy in 4H-SiC

Hole mobility anisotropy in 4H-SiC was investigated based on both experimental and theoretical approaches. First, the authors established a complete database of the anisotropic hole mobility along both directions parallel and perpendicular to the c-axis in 4H-SiC over the wide acceptor density and temperature ranges by preparing Hall bar structures on p-type SiC(112¯0) epitaxial layers. Empirical equations for the mobility along each direction vs the acceptor density and temperature were determined, which should be useful for the simulation and designing of any SiC devices. In addition to that, the anisotropy in the hole mobility was extracted from the experimental results, and its origin was discussed focusing on that in the effective mass (m∗) of holes. The obtained mobility ratio was far from the m∗ ratio at the valence band maximum, and an averaged m∗ along each direction was determined by theoretical calculation taking into account the energy distribution of holes. Consequently, the authors revealed that the anisotropic hole mobility is explained quantitatively by the anisotropic m∗ considering the E–k dispersion over the entire first Brillouin zone.

Two-dimensional β-noble-transition-metal chalcogenide: novel highly stable semiconductors with manifold outstanding optoelectronic properties and strong in-plane anisotropy.

In this work, five two-dimensional (2D) noble-transition-metal chalcogenide (NTMC) semiconductors, namely β-NX (N = Au, Ag; X = S, Se, Te), were designed and predicted by first-principles simulations. Structurally, the monolayer β-NX materials have good energetic, mechanical, dynamical, and thermal stability. They contain two inequivalent noble-transition-metal atoms in the unit cell, and the N-X bond comprises a partial ionic bond and a partial covalent bond. Regarding the electronic properties, the β-NX materials are indirect-band-gap semiconductors with appropriate band-gap values. They have tiny electron effective masses. The hole effective masses exhibit significant differences in different directions, indicating strongly anisotropic hole mobility. In addition, the coexistence of linear and square-planar channels means that the diffusion and transport of carriers should be anisotropic. In terms of optical properties, the β-NX materials show high absorption coefficients. The absorption and reflection characteristics reveal strong anisotropy in different directions. Therefore, the β-NX materials are indirect-band-gap semiconductors with good stability, high absorption coefficients, and strong mechanical, electronic, transport, and optical anisotropy. In the future, they could have great potential as 2D semiconductors in nano-electronics and nano-optoelectronics.

Single-layer ZnGaInS4: Desirable bandgap and high carrier separation efficiency for optoelectronics

Two-dimensional optoelectronic materials with desirable bandgaps and high carrier separation efficiency are in urgent need. Herein, single-layer ZnGaInS4 is designed and thoroughly studied by first-principles calculations. Our calculations reveal the excellent stability of single-layer ZnGaInS4, and the small cleavage energy demonstrates the feasibility to exfoliate single-layer ZnGaInS4 from bulk counterparts. At the HSE06 level, single-layer ZnGaInS4 has a moderate bandgap of 2.05 eV, and its valence band maximum and conduction band minimum are spatially separated, thus impeding the electron-hole pair formation and enhancing the photoelectronic performance. In the visible region, single-layer ZnGaInS4 exhibits an optical absorption coefficient of ∼ 105 cm−1. Notably, single-layer ZnGaInS4 possesses high electron carrier mobility and low hole carrier mobility, further affirming the effective carrier separation. Another attractive characteristic is that single-layer ZnGaInS4 has anisotropic hole mobility. The desirable bandgap and high separation efficiency of generated carriers, together with abundant optical absorption, promise single-layer ZnGaInS4 is a hopeful candidate in atomic-thick optoelectronic devices.

Molten Salts‐Driven Discovery of a Polar Mixed‐Anion 3D Framework at the Nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting

AbstractMixed‐anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn4Si2O7Cl2. We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon‐based connectors from a non‐oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H2 evolution and water oxidation. This work then paves the way to a new family of mixed‐anion solids

Molten Salts-Driven Discovery of a Polar Mixed-Anion 3D framework at the nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting.

Mixed-anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn4Si2O7Cl2. We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon-based connectors from a non-oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H2 evolution and water oxidation. This work then paves the way to a new family of mixed-anion solids.

Efficiently predicting directional carrier mobilities in organic materials with the Boltzmann transport equation.

Describing charge carrier anisotropy in crystalline organic semiconductors with ab initio methods is challenging because of the weak intermolecular interactions that lead to both localized and delocalized charge transport mechanisms. Small polaron hopping models (localized) are generally used to describe materials with small charge carrier mobilities, while periodic band models (delocalized) are used to describe materials with high charge carrier mobilities. Here, we prove the advantage of applying the constant relaxation time approximation of the Boltzmann transport equation (BTE) to efficiently predict the anisotropic hole mobilities of several unsubstituted (anthracene, tetracene, pentacene, and hexacene) and substituted (2,6-diphenylanthracene, rubrene, and TIPS-pentacene) high-mobility n-acene single crystals. Several density functionals are used to optimize the crystals, and the composite density functional PBEsol0-3c/sol-def2-mSVP predicts the most experimentally similar geometries, adequate indirect bandgaps, and the theoretically consistent n-acene charge transport mobility trend. Similarities between BTE and Marcus mobilities are presented for each crystal. BTE and Marcus charge carrier mobilities computed at the same geometry result in similar mobility trends, differing mostly in materials with more substitutions or structurally complex substituents. By using a reduced number of calculations, BTE is able to predict anisotropic carrier mobilities efficiently and effectively for a range of high-mobility organic semiconductors.

Laser-Induced Hole Coherence and Spatial Self-Phase Modulation in the Anisotropic 3D Weyl Semimetal TaAs.

Laser-induced electron coherence is a fascinating topic in manipulating quantum materials. Recently, it has been shown that laser-induced electron coherence in 2D materials can produce a third-order nonlinear optical response spatial self-phase modulation (SSPM), which has been used to develop a novel all-optical switching scheme. However, such investigations have mainly focused on electron coherence, whereas laser-induced hole coherence is rarely explored. Here, the observation of the optical Kerr effect in 3D Weyl semimetal TaAs flakes is reported. The nonlinear susceptibility (χ(3) ) is obtained, which exhibits a surprisingly high value (with = 9.9×10-9 e.s.u. or 1.4×10-16 m2 V-2 at 532nm). This cannot be explained by the conventional electron mobility, but can be well understood by the unique high anisotropic hole mobility of TaAs. The wind-chime model and χ(3) carrier mobility correlation adequately explain the results, suggesting the crucial role of laser-induced nonlocal ac hole coherence. These observations extend the understanding of SSPM from 2D to 3D quantum materials with anisotropic carrier mobility and from electron coherence to hole coherence.

Strong non-linear optical response of Sb2Se3 nanorods in a liquid suspension based on spatial self-phase modulation and their all-optical photonic device applications.

The field of nonlinear optics is constantly expanding and gaining new impetus through the discovery of fresh nonlinear materials. Herein, for the first time, we have performed spatial self-phase modulation (SSPM) experiments with an emerging anisotropic Sb2Se3 layered material in a liquid suspension for an all-optical diode and all-optical switching application. The third-order broadband nonlinear optical susceptibility (χ(3)single layer ∼ 10-9 esu) and nonlinear refractive index (n2 ∼ 10-6 cm2 W-1) of Sb2Se3 have been determined using a 671 nm laser beam. This result could be unambiguously explained by the anisotropic hole mobility of Sb2Se3. The linear relationship of χ(3) and carrier mobility emphasizes the establishment of nonlocal hole coherence, the origin of the diffraction pattern. Consequently, the time evolution of diffraction rings follows the 'Wind-Chime' model. A novel photonic diode based on Sb2Se3/SnS2 has been demonstrated using the nonreciprocal propagation of light. The self-phase modulation (SPM) technique uses laser lights of different wavelengths and intensities to demonstrate the all-optical logic gates, particularly OR logic gates. The exploration of nonlinear optical phenomena in Sb2Se3 opens up a new realm for optical information processing and communication. We strongly believe that this result will help to underpin the area of optical nonlinearities among its various applications.

MgXN2 (X = Hf/Zr) Monolayers: Auxetic Semiconductor with Highly Anisotropic Optical/Mechanical Properties and Carrier Mobility.

Two-dimensional (2D) semiconducting materials with distinct anisotropic physical properties have attracted intense interests. Herein, we show theoretical predictions that MgXN2 (X = Hf/Zr) monolayers are auxetic semiconductors with highly anisotropic electronic, optical, and mechanical properties. The density functional theory calculations coupled with a PSO algorithm (global-minimum search) suggest that both MgHfN2 (MgZrN2) monolayers exhibit orthorhombic symmetry (Pmma) and are direct-gap (indirect-gap) semiconductors with a bandgap of 2.43 eV (2.13 eV). Specifically, the MgHfN2 monolayer exhibits highly anisotropic hole mobility as well as very high electron mobility (∼104 cm2 V-1 s-1). G0W0+BSE calculations indicate that both monolayers bear notable optical anisotropy and relatively large exitonic binding energy (∼0.6 eV). In addition, both monolayers acquire remarkable mechanical anisotropy with a negative in-plane Poisson's ratio (∼-0.2) and high Young's modulus (∼260 N/m). The combination of highly anisotropic electronic, optical, and mechanical properties endows MgXN2 monolayers as potentially useful parts in multifunctional nanoelectronic devices.

Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect

In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene.

Two-dimensional electronic and charge-transport properties of a monolayer organic crystal: Impacts of the collinear transfer-integral correlations

Abstract Electronic transfer integrals between the frontier orbitals of adjacent molecules represent the most important parameter governing charge transport in organic crystals. Different transfer integrals may be significantly modulated by the same modes of molecular vibrations as a result of nonlocal electron-phonon couplings, leading to spatial correlations between the transfer integrals. Here, by employing a multiscale modeling approach, we investigate theoretically the impacts of transfer-integral correlations (TICs) on the two-dimensional electronic and charge-transport properties of a monolayer pentacene crystal. In particular, we focus on the collinear TICs that are present between molecular pairs along the same direction of molecular packings in the crystal. We reveal that the interplay of the collinear TICs along different directions leads to diverse landscapes of nonlocal electron-phonon couplings in reciprocal space, of which the impacts explicitly manifest themselves in the band structures at finite temperatures. We also evaluate the anisotropic hole mobility of the pentacene monolayer and demonstrate that the consequence of the collinear TICs can be reflected in the behaviors of the temperature-dependent band-like mobility observed in experiment. These findings provide an insight into how nonlocal electron-phonon couplings operate through TICs to impact the electronic and electrical properties of organic crystals in two dimension.

Intrinsic charge carrier mobility in single-crystal OFET by “fast trapping vs. slow detrapping” model

Abstract This study investigates the gate stress-induced mobility discrepancy in p-type single-crystal organic field-effect transistors (OFETs) on an octyltrichlorosilane (OTS)-modified SiO2/Si substrate. During measurements in atmosphere, anti-clockwise hysteresis was observed in the transfer curve, and the mobility calculated from the forward sweep was smaller than that calculated from the reverse sweep. Hysteresis has often been observed for OFETs but the mobility discrepancy has not been clearly understood. We formulated a “fast trapping vs. slow detrapping” model and suggested that the mobility values calculated from the reverse sweep represent the intrinsic property of the material. To verify the validity of this model, we investigated mobility anisotropy of an air-stable organic semiconductor, 2,7-bis(4-methoxyphenyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (DBOP-BTBT). By measuring the single-crystal OFET characteristics of many crystals with different orientations, we observed anisotropic hole mobility calculated from the reverse sweep. The mobility along the b-axis, which corresponds to the π-π stacking direction, was 13.9 cm2 V−1 s−1, and that along the a-axis was 6.2 cm2 V−1 s−1. However, we did not see clear anisotropy when mobility was calculated from the forward sweep due to a variation in the data. The threshold voltage from the reverse and the forward sweeps showed isotropic characteristics within the range from −60 to −70 V and from −50 to −60 V, respectively. These results indicate that the numbers of filled traps were different between the reverse and the forward sweeps at the interface, and confirm the validity of our model.

Diverse carrier mobility of monolayer BNCx: a combined density functional theory and Boltzmann transport theory study

BNCx monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNCx (x = 1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC2-1 is a material with very small band gap (0.02 eV) among all the structures while other BNCx monolayers are semiconductors with band gap ranging from 0.51 eV to 1.32 eV. The carrier mobility of BNCx varies considerably from tens to millions of cm2 V−1 s−1. For BNC2-1, the hole mobility and electron mobility along both x and y directions can reach 105 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNCx monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC4, its hole mobility along the y direction and electron mobility along the x direction unexpectedly reach 106 orders of magnitude, even higher than that of graphene. Our findings suggest that BNCx layered materials with the proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices.

Computational study on hole conduction in normal alkanes: Anisotropy and effect of dynamic disorder

Despite its importance, carrier conduction in electrical insulators is poorly understood. This work presents a computational study of hole conduction in single crystalline alkanes (n-C18H38 and n-C36H74). Hole mobilities are computed with the combination of molecular dynamics simulation, quantum chemical calculation, and the kinetic Monte Carlo method. The hole hopping rate is computed by the Fermi golden rule rate kernel without high temperature approximation. A strong correlation between the anisotropy of hole mobility and crystalline morphology is found. Hole mobilities in the direction of the c axis are more than an order of magnitude larger than those in the a − b plane. At room temperature, hole mobility is increased by roughly a factor of 10 due to the thermal motion of molecules. Computed anisotropic hole mobilities are in reasonable agreement with experimental values when the effect of dynamic disorder is taken into account. The results strongly indicate that hole transfer in crystalline alkane occurs in the phonon-assisted transport regime.

High, Anisotropic, and Substrate-Independent Mobility in Polymer Field-Effect Transistors Based on Preassembled Semiconducting Nanofibrils.

Achieving nanoscale control over the crystalline structure and morphology of electroactive polymer films and the possibility to transfer them onto any solid substrate are important tasks for the fabrication of high-performance organic/polymeric field-effect transistors (FETs). In this work, we demonstrate that ultrathin active layers preassembled at the water/air interface can possess high, anisotropic, and substrate-independent mobility in polymer FETs. By exploiting a modified approach to the Langmuir-Schaeffer technique, we self-assemble conjugated polymers in fibrillar structures possessing controlled thickness, nanoscale structure, and morphology; these highly ordered nanofibrils can be transferred unaltered onto any arbitrary substrate. We show that FETs based on these films possess high and anisotropic hole mobility approaching 1 cm2 V-1 s-1 along the nanofibrils, being over 1 order of magnitude beyond the state-of-the-art for Langmuir-Schaefer polymer FETs. Significantly, we demonstrate that the FET performances are independent of the chemical nature and dielectric permittivity of the substrate, overcoming a critical limit in the field of polymer FETs. Our method allows the fabrication of ultrathin films for low-cost, high-performance, transparent, and flexible devices supported on any dielectric substrate.

Single-Layered Hittorf's Phosphorus: A Wide-Bandgap High Mobility 2D Material.

We propose here a two-dimensional material based on a single layer of violet or Hittorf's phosphorus. Using first-principles density functional theory, we find it to be energetically very stable, comparable to other previously proposed single-layered phosphorus structures. It requires only a small energetic cost of approximately 0.04 eV/atom to be created from its bulk structure, Hittorf's phosphorus, or a binding energy of 0.3-0.4 J/m(2) per layer, suggesting the possibility of exfoliation in experiments. We find single-layered Hittorf's phosphorus to be a wide band gap semiconductor with a direct band gap of approximately 2.5 eV, and our calculations show it is expected to have a high and highly anisotropic hole mobility with an upper bound lying between 3000-7000 cm(2) V(-1) s(-1). These combined properties make single-layered Hittorf's phosphorus a very good candidate for future applications in a wide variety of technologies, in particular for high frequency electronics, and optoelectronic devices operating in the low wavelength blue color range.

First-Principles Investigation of Anistropic Hole Mobilities in Organic Semiconductors

We report a simple first-principles-based simulation model (combining quantum mechanics with Marcus-Hush theory) that provides the quantitative structural relationships between angular resolution anisotropic hole mobility and molecular structures and packing. We validate that this model correctly predicts the anisotropic hole mobilities of ruberene, pentacene, tetracene, 5,11-dichlorotetracene (DCT), and hexathiapentacene (HTP), leading to results in good agreement with experiment.

Large Area Liquid Crystal Monodomain Field-Effect Transistors

Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5' '-bis(5-alkyl-2-thienylethynyl)-2,2':5',2' '-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50 degrees with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm(2)/Vs and maximum electron mobilities around 0.002 cm(2)/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm(2)/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.