Anisotropic Electron Mobility Research Articles

Control of highly anisotropic electrical conductance of tellurene by strain-engineering.

Tailoring the electronic anisotropy of two-dimensional (2D) semiconductors with strain-engineering is critical in nanoelectronics. Recently, 2D tellurene has been predicted theoretically and fabricated experimentally. It has potential applications in nanoelectronics, in particular, β-phase tellurene (β-Te) shows a desirable direct band gap (1.47 eV), high carrier mobility (2.58 × 103 cm2 V-1 s-1) and high stability under ambient conditions. In this work, we demonstrated, with first-principles density functional theory calculations, that the highly anisotropic electron mobility and electrical conductance of β-Te can be controlled by strain-engineering. The direction of electrical conductance of β-Te can be changed from the armchair to the zigzag direction at the strain between -1% and 0%. Meanwhile, we found that the bandgap of β-Te under strain experiences an indirect-direct transition with a conduction band minimum (CBM) shift from the X to Γ point. The significant dispersion of the bottom of the conduction bands along the Γ-Y direction switches to the X-Γ direction under uniaxial or biaxial strain which makes the rotation of the effective masses tensor. The qualitative rotation of the spatial anisotropic electron effective masses tensor by 90° also rotates the direction of the electrical conduction as the carrier mobility is inversely dependent on the effective masses. On the another hand, we also found that the deformation potential constant also plays an important role in the rotation of electrical conductance anisotropy. While anisotropic conductance of hole is impregnable under strain. In order to verify that β-Te can sustain large strain, we studied its stability and mechanical properties and found that β-Te shows superior mechanical flexibility with a small Young's modulus (27.46 GPa (armchair)-61.99 GPa (zigzag)) and large anisotropic strain-stress (12.89 N m-1 at the strain of 38% along armchair direction and 25.72 N m-1 at the strain of 26% along zigzag direction). The high anisotropic carrier mobility and superior mechanical flexibility of β-Te make it a promising candidate for flexible nanoelectronics.

Mobility Anisotropy in Black Phosphorus MOSFETs With HfO2 Gate Dielectrics

Precise measurements of the mobility anisotropy along high-symmetry crystal axes in black phosphorus (BP) MOSFETs are reported. Locally back-gated BP MOSFETs with 13-nm HfO2 dielectric and channel length ranging from 0.3 to 0.7 $\mu \text{m}$ are fabricated. A single BP flake of a uniform thickness is exfoliated and etched along armchair (AC) and zigzag (ZZ) crystal axes, and the orientations are confirmed using optical and transmission electron microscopy analyses. The hole and electron mobilities along each direction are extracted using the transfer length method. The AC-to-ZZ hole mobility ratio is found to increase from 1.4 (1.5) to 2.0 (2.9) as the sheet concentration increased from $5.1\times 10^{\textsf {11}}$ to $1.9\times 10^{\textsf {12}}$ cm−2 at room temperature (77 K). The room-temperature electron mobility anisotropy is found to be similar to that for holes with an AC-to-ZZ mobility ratio increasing from 1.4 to 2.1 from $5.1\times 10^{\textsf {11}}$ to $1.9\times 10^{\textsf {12}}$ cm−2 though electrons showed only a very weak temperature dependence. A Boltzmann transport model is used to explain the concentration- and temperature-dependent mobility anisotropies which can be well described using a charge center scattering model.

Indolocarbazole (IC) Derivatives as Promising p‐type Organic Semiconductors: A First‐Principle Study of Their Anisotropic Charge Mobilities

AbstractWe present a detailed theoretical study of the structural, charge transport, and optical properties of three indolocarbazole derivatives, viz., 11, 12‐Dihydroindolo[2, 3‐a]carbazole (DIC), 5, 10‐dimethyl‐5,10‐dihydrobenzo[a]indolo[2, 3‐c]carbazole (DMDBIC), and 2,11‐dimethoxydibenzo[2, 3:5,6]pyrrolizino[1, 7‐bc]indolo[1, 2,3‐lm]carbazole (DMDPIC). DFT calculations along with previously reported x‐ray crystal structures reveal that the studied compounds are planar and do not undergo significant changes in structure upon oxidation or reduction. The maximum hole anisotropic charge mobilities for DIC was found to be 0.338 cm2 V−1 s−1 at Φ=175.89° and 355.81° and the maximum electron mobility was 0.181 cm2 V−1 s−1 at Φ=85.94° and 265.85°. For DMDBIC, the predicted maximum anisotropic hole, and electron mobility was 0.501 cm2 V−1 s−1 and 0.775 cm2 V−1 s−1, respectively at Φ=56.72° and 236.63°, while for DMDPIC the predicted maximum anisotropic hole and electron mobility as 1.586 cm2 V−1 s−1 and 0.594 cm2 V−1 s−1, respectively at Φ=0° and 180.0°. The calculated mobilities show that these compounds may be suitable candidates as a p‐type organic semiconductor with better stability than acene compounds.

The inherent transport anisotropy of rutile tin dioxide (SnO2) determined by van der Pauw measurements and its consequences for applications

The anisotropic electron mobility of unintentionally doped, single crystalline, bulk, rutile SnO2(100) and (110) wafers is investigated by van der Pauw-Hall measurements. The room temperature average Hall electron mobility of μ ≈ 220 cm2/V s at a Hall electron concentration of n ≈ 1018 cm−3 suggests high-quality samples. The extracted 1.26 times higher mobility in the c-direction than perpendicular to it is in very good agreement with the corresponding anisotropy of the effective electron mass, which is 1.28 times higher perpendicular to c than parallel to c, suggesting rather isotropic scattering mechanisms. At temperatures below 100 K, a higher mobility anisotropy is found and tentatively attributed to low-angle grain boundaries with a surprisingly low energy barrier. Thus, the efficiency of mobility-sensitive applications, such as field effect transistors, increases by aligning the transport direction with the c-direction of the crystal. For transparent contact applications, such as Sb- or F-doped SnO2 (termed “ATO” or “FTO,” respectively), this benefit is expected to be even larger due to the increasing effective mass anisotropy with the increasing electron concentration.

Molecular Assembly-Induced Charge Transfer for Programmable Functionalities

The donor–acceptor interface within molecular charge transfer (CT) solids plays a vital role in the hybridization of molecular orbitals to determine their carrier transport and electronic delocalization. In this study, we demonstrate molecular assembly-driven bilayer and crystalline solids, consisting of electron donor dibenzotetrathiafulvalene (DBTTF) and acceptor C60, in which interfacial engineering-induced CT degree control is a key parameter for tuning its optical, electronic, and magnetic performance. Compared to the DBTTF/C60 bilayer structure, the DBTTFC60 cocrystalline solids show a stronger degree of charge transfer for broad CT absorption and a large dielectric constant. In addition, the DBTTFC60 cocrystals exhibit distinct CT arrangement-driven anisotropic electron mobility and spin characteristics, which further enables the development of high-penetration and high-energy γ-ray photodetectors. The results presented in this paper provide a basis for the design and control of molecular charge tr...

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.

Electronic and transport properties of n -type monolayer black phosphorus at low temperatures

We present a detailed theoretical study of the electronic and transport properties of monolayer black phosphorus (BP). This study is motivated by recent experimental activities in investigating $n$-type few-layer BP systems. The electron density of states, the screening length, and the low-temperature electron mobility are calculated for monolayer BP (MLBP). In particular, the electron transport mobilities along the armchair and zigzag directions are examined on the basis of the momentum-balance equation derived from a semiclassical Boltzmann equation. The anisotropic electron mobilities in MLBP along different directions are demonstrated where the electron-impurity scattering is considered. Furthermore, we compare the results obtained from two electronic band structures of MLBP and find that the simplified model can describe quite rightly the electronic and transport properties of MLBP. This study is relevant to the application of few-layer BP based electronic systems as advanced electronic devices.

Anisotropic electron mobility in fluorene-PPV and fluorene-MEH-PPV

ABSTRACTPolyfluorene copolymers are attractive semiconductor materials, in particular for applications in the organic electronics field. They are versatile to be chemically modified and allow a tuning of the emission to cover the entire visible spectrum. A better understanding of the fundamental aspects of the nature of electronic structure and charge transport properties contribute to the improvement of optoelectronic properties of polymeric materials. Here, we provide a structure–property relationship for models of fluorene-PPV and fluorene-MEH-PPV copolymers, using molecular quantum mechanics modelling. The anisotropy is discussed revisiting Mulliken's transition moment theory. Accordingly, our results show that electron mobility occurs preferentially intrachain for both copolymers. Moreover, the interchain electron mobility has the most propensity to occur via π-stacking interactions.

Effect of the phenoxy groups on PDIB and its derivatives

The anisotropic hole and electron mobilities in N,N′-3,4,9,10-perylenediimide-1,7-phenoxy (PDIB-2OPh) and N,Nʹ-3,4,9,10-perylenediimide (PDIB) were theoretically predicted using the Marcus–Hush theory. The substituent effect of phenoxy on their mobility rates, absorption spectra, electron affinities, and ionization potentials was explored. By comparing the simulated hole mobility in PDIB and PDIB-2OPh, it is found that the phenoxy rings act as spacers between adjacent stacking columns in the phenoxy-substituted derivatives. The increasement of the number of benzene oxygen groups leads to the absorption spectra red-shift of these molecular systems. This coincides with their change tendency of the adiabatic ionization potentials, vertical ionization potentials. However, the calculated adiabatic electron affinities and vertical electron affinities of N,N′-butyl-3,4,9,10-perylenediimide-1,6,7,12-phenoxy (PDIB-4OPh) are larger than those of PDIB;OPh. The steric effect in PDIB-4OPh is expected to cause space reversal and thus to changes in the properties of the molecule.

Morphological modulation of graphene-mediated hybridization in plasmonic systems.

We investigated the plasmonic response of a 2-dimensional ordered array of closely spaced (few-nm apart) Au nanoparticles covered by a large-area single-layer graphene sheet. The array consisted of coherently aligned nanoparticle chains, endowed with a characteristic uniaxial anisotropy. The joint effect of such a morphology and of the very small particle size and spacing led to a corresponding uniaxial wrinkling of graphene in the absence of detectable strain. The deposition of graphene redshifted the Au plasmon-resonance, strongly increased the optical absorption of the array and, most importantly, induced a marked optical anisotropy in the plasmonic response, absent in the pristine nanoparticle array. The experimental observations are accounted for by invoking a graphene-mediated resistive coupling between the Au nanoparticles, where the optical anisotropy arises from the wrinkling-induced anisotropic electron mobility in graphene at optical frequencies.

Electron transport properties in m-plane and c-plane AlN/GaN heterostructures with interface roughness and anisotropic in-plane strain scatterings

Anisotropic transport properties of a two-dimensional electron gas in nonpolar m-plane AlN/GaN heterostructures with the interface roughness coupled anisotropic in-plane strain scattering were investigated theoretically using a path-integral framework. The scattering potential was composed of the interface roughness and the effective field from the electron charge and the net piezoelectric polarization. We showed that the anisotropic biaxial strains generate only the net piezoelectric polarization along the [0 0 0 1]-direction and cause anisotropy in electron mobility with a magnitude lower than the -direction. We also showed that the anisotropy in electron mobility reduced with increasing electron density. Moreover, the anisotropic electron mobility disappeared when the anisotropic in-plane strain scattering was removed, and the relation for pure interface roughness scattering was reestablished. This formulation with existing roughness parameters gave a good description for the experimental results of polar c-plane AlN/GaN heterostructures.

An analytical model of anisotropic low-field electron mobility in wurtzite indium nitride

This paper presents a theoretical analysis of anisotropic transport properties and develops an anisotropic low-field electron analytical mobility model for wurtzite indium nitride (InN). For the different effective masses in the Γ–A and Γ–M directions of the lowest valley, both the transient and steady state transport behaviors of wurtzite InN show different transport characteristics in the two directions. From the relationship between velocity and electric field, the difference is more obvious when the electric field is low in the two directions. To make an accurate description of the anisotropic transport properties under low field, for the first time, we present an analytical model of anisotropic low-field electron mobility in wurtzite InN. The effects of different ionized impurity scattering models on the low-field mobility calculated by Monte Carlo method (Conwell–Weisskopf and Brooks–Herring method) are also considered.

First‐Principles Investigation of Anisotropic Electron and Hole Mobility in Heterocyclic Oligomer Crystals

Based on quantum chemistry calculations combined with the Marcus-Hush electron transfer theory, we investigated the charge-transport properties of oligothiophenes (nTs) and oligopyrroles (nPs) (n=6, 7, 8) as potential p- or n-type organic semiconductor materials. The results of our calculations indicate that 1) the nPs show intrinsic hole mobilities as high as or even higher than those of nTs, and 2) the vertical ionization potentials (VIPs) of the nPs are about 0.6-0.7 eV smaller than the corresponding VIPs of the nTs. Based on their charge-transport ability and hole-injection efficiency, the nPs have potential as p-type organic semiconducting materials. Furthermore, it was also found that the maximum values of the electron-transfer mobility for the nTs are larger by one-to-two orders of magnitude than the corresponding maximum values of hole-transfer mobility, which suggests that the nTs have the potential to be developed as promising n-type organic semiconducting materials owing to their electron mobility.

Angular-dependences of giant in-plane and interlayer magnetoresistances in Bi2Te3 bulk single crystals

Angular-dependences of in-plane and interlayer magnetotransport properties in n-type Bi2Te3 bulk single crystals have been investigated over a broad range of temperatures and magnetic fields. Giant in-plane magnetoresistances (MR) of up to 500% and interlayer MR of up to 200% were observed, respectively. The observed MR exhibits quadratic field dependences in low fields and linear field dependences in high fields. The angular dependences of the MR represent strong anisotropy and twofold oscillations. The observed angle-dependent, giant MR might result from the strong coulomb scattering of electrons as well as impurity scattering in the bulk conduction bands of n-type Bi2Te3. The strong anisotropy of the MR may be attributable to the anisotropy of electron mobility, effective mass, and relaxation time in the Fermi surface. The observed giant anisotropic MR in n-type Bi2Te3 bulk single crystals paves the way for Bi2Te3 single crystals to be useful for practical applications in magnetoelectronic devices such as disk reading heads, anisotropic magnetic sensors, and other multifunctional electromagnetic applications.

AlSb nucleation induced anisotropic electron mobility in AlSb/InAs heterostructures on GaAs

The influence of the growth conditions at the AlSb/GaAs interface on the electron mobility in AlSb/InAs heterostructures is investigated. We show that an excessive antimony flux during the initial stage of the AlSb buffer growth leads to a strong anisotropy of electron mobility in InAs between [110] and [1-10] crystallographic orientations. This anisotropy is attributed to the formation of trenches oriented along the [1-10] direction in the InAs channel. Transmission electron microscopy reveals that these trenches are directly related to twinning defects originating from the AlSb/GaAs interface.

Anisotropic transport properties in InAs/AlSb heterostructures

We have investigated the anisotropic transport behavior of InAs/AlSb heterostructures grown on a (001) InP substrate. An electrical analysis showed anisotropic sheet resistance Rsh and electron mobility μn in the two dimensional electron gas (2DEG). Hall measurements demonstrated an enhanced anisotropy in μn when cooled from room temperature to 2 K. High electron mobility transistors exhibited 27% higher maximum drain current IDS and 23% higher peak transconductance gm when oriented along the [1-10] direction. The anisotropic transport behavior in the 2DEG was correlated with an asymmetric dislocation pattern observed in the surface morphology and by cross-sectional microscopy analysis of the InAs/AlSb heterostructure.

Strain-enhanced electron mobility anisotropy in In xGa 1− xAs/InP two-dimensional electron gases

We systematically investigated electron mobility anisotropy in compressively strained, lattice-matched, and tensilely strained InGaAs quantum wells (QWs) grown on InP (0 0 1) by using Hall-bar devices with various current-flowing directions. Anisotropy of electron mobility, the highest along the [1 1 ¯ 0] direction and the lowest along [1 1 0], is systematically observed in all QWs, and well-fitted with a sinusoidal function of the current-flowing direction angle. The mobility anisotropy is minimum in the lattice-matched case and enhanced by both compressive and tensile strains in the QWs. We consider that random piezoelectric scattering, which is enhanced by the average normal strain in the QW, has anisotropy and plays an important role for the observed results.

Anisotropic High Electron Mobility and Photodynamics of a Self-Assembled Porphyrin Nanotube Including C60 Molecules

A cyclic porphyrin dimer (Ni2−CPDPy) linked by butadiyne moieties bearing 4-pyridyl groups includes a C60 molecule inside its cavity in solution to give a 1:1 inclusion complex (C60⊂Ni2−CPDPy). The charge-transfer (CT) band is observed at 645 nm in the UV−vis absorption spectrum of the solution of C60⊂Ni2−CPDPy. In the cyclic voltammogram of C60⊂Ni2−CPDPy, a small anodic shift of the porphyrin oxidation potential and a small cathodic shift of the fullerene reduction potential compared with their original redox potentials are indicative of CT interaction from the porphyrin to C60. In the crystal structure of C60⊂Ni2−CPDPy, a porphyrin nanotube is formed by the self-assembly of Ni2−CPDPy. Ni2−CPDPy molecules link together through nonclassical C−H···N hydrogen bonds and π−π interactions of the pyridyl groups along the crystallographic b axis. The included C60 molecules are linearly arranged in the nanotube to afford a supramolecular peapod. The charge-carrier mobility of the single crystal of C60⊂Ni2−CPDPy w...

Anisotropy of the Γ‐point effective mass and mobility in hexagonal InN

AbstractWe determine the anisotropic electron effective mass and mobility parameters in wurtzite InN thin films with free electron concentration N from 1.8 × 1017 cm–3 to 9.5 × 1018 cm–3 using Infrared Magneto‐optic Generalized Ellipsometry. The room‐temperature measurements were carried out with magnetic fields up to 4.5 T. For the Γ‐point we estimate m *⊥ = 0.047m 0 and m *‖ = 0.039m 0 for polarization perpendicular and parallel to the c ‐axis, respectively. Scattering by impurities or ionized donors may explain the decrease of mobility for polarization parallel to the c ‐axis from 1600 cm2/(Vs) to 800 cm2/(Vs) with increase in N , where the perpendicular mobility is further decreased, likely caused by additional grain boundary scattering. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropy of Electron Mobility in n-Type 15R-SiC Studied by Raman Scattering

Anisotropy of Electron Mobility in n-Type 15R-SiC Studied by Raman Scattering