Carrier Electron Mobility Research Articles

Electronic structure and carrier mobilities of twisted graphene helix

We have investigated the effect of twisting on electronic band structure, effective mass and carrier mobilities of three prototypes of AGNRs (N=6, 7 & 8) using Density functional theory combined with Deformation potential theory and Effective mass approximation. It is shown that the influence of twisted modes electron-phonon interaction is smaller than stretching modes, nonetheless, twisting has a profound effect on effective mass and mobilities. Similar to earlier reported conclusion in which the ideal N=3n wide HAGNR potentially exhibit an electron mobility comparable to intrinsic graphene, we also found that the ideal N=3n+2 HAGNRs hole mobility reside more closely to intrinsic graphene which could be increased further through Fluorine passivation. Thus, the control of the ribbon width along with passivation and extent of deformation are of paramount importance for determining the n-type or p-type of ribbons. Also, because of strong response to torsional strain, the N=8 F passivated AGNRs are the most appropriate for mechanical and high frequency switching. Our results suggest that twisting a ribbon can be considered as a good alternative way for controlled manipulation of the band structure and carrier mobilities for applications in mechanical switching devices.

Tunable Properties of Novel Ga2O3 Monolayer for Electronic and Optoelectronic Applications.

A novel two-dimensional (2D) Ga2O3 monolayer was constructed and systematically investigated by first-principles calculations. The 2D Ga2O3 has an asymmetric configuration with a quintuple-layer atomic structure, the same as the well-studied α-In2Se3, and is expected to be experimentally synthesized. The dynamic and thermodynamic calculations show excellent stability properties of this monolayer material. The relaxed Ga2O3 monolayer has an indirect band gap of 3.16 eV, smaller than that of β-Ga2O3 bulk, and shows tunable electronic and optoelectronic properties with biaxial strain engineering. An attractive feature is that the asymmetric configuration spontaneously introduces an intrinsic dipole and thus the electrostatic potential difference between the top and bottom surfaces of the Ga2O3 monolayer, which helps to separate photon-generated electrons and holes within the quintuple-layer structure. By applying compressive strain, the Ga2O3 monolayer can be converted to a direct band gap semiconductor with a wider gap reaching 3.5 eV. Also, enhancement of hybridization between orbitals leads to an increase of electron mobility, from the initial 5000 to 7000 cm2 V-1 s-1. Excellent optical absorption ability is confirmed, which can be effectively tuned by strain engineering. With superior stability, as well as strain-tunable electronic properties, carrier mobility, and optical absorption, the studied novel Ga2O3 monolayer sheds light on low-dimensional electronic and optoelectronic device applications.

Growth mechanism and transport properties of chemically deposited PbxCd1-xS thin film’s photoelectrochemical (PEC) solar cell

The dc conductivity and thermoelectric power of PbxCd1-xS (0 ≤ x ≤ 1) thin films were fabricated and investigated for the solar cell efficiency. The deposition parameters such as time, concentration, temperature, and pH of the solution were optimized to get superior quality thin films. The optical properties revealed that the band gap of the films decreased as the composition parameter ‘x’ increased. XRD results show the films are polycrystalline with an individual hexagonal (CdS) and cubic (PbS) structure. FESEM revealed that the films have uniform surface morphology over the entire substrate. The crystallite size measured by XRD and FESEM studies was found to be varied with composition ‘x’. EDXS shows a compositional analysis of prepared films.The four probe technique is used to estimates the electrical conductivity (σ) and thermal activation energy at room temperature. The TEP data are analyzed to calculate electron carrier density (n) and mobility (μ) of PbxCd1-xS thin films at a temperature of 45 °C to 110 °C. In the photoconduction study, it is observed that dark current rises linearly with applied voltage and photocurrent varies with applied voltage. PbxCd1-xS thin film’s PEC solar cell was characterized by determining the open-circuit voltage (Voc), short circuit current (Isc), maximum power (Pmax), fill factor (ff) and efficiency (ɳ %). The area 0.2 cm2 is selected for the sample cell and was illuminated by a 90 mW/cm2 solar radiation. From the I–V higher magnitudes fill factor 37.02% and conversion efficiency 0.10940% is obtained for composition Pb0.2Cd0.8S.

Fabrication of Cost-Effective Reduced Graphene Oxide Based Parallel Grids Embedded Dye-Sensitized Solar Mini-Modules

Cost-effective, parallel grids embedded dye-sensitized solar mini-modules were fabricated successfully by utilizing reduced graphene oxide (rGO) and rGO-silver (rGO-Ag) nanocomposites as grid current collectors. The pure rGO grids embedded dye-sensitized solar mini-module showed a maximum power conversion efficiency of 4.10% with improved fill factor, circumvented iodide/triiodide electrolyte corrosion, and reduced the module power loss significantly. Electrochemical impedance spectroscopy studies revealed that the 1:1 wt% ratio rGO-Ag nanocomposite grids improved the module's electron diffusion length, carrier mobility and lifetime, and produced a higher open-circuit voltage (Voc) as compared with the pure silver grids in the dye-sensitized solar mini-module.

First-principles prediction of the electronic property, carrier mobility and optical absorption in edge-modified pristine sawtooth penta-graphene nanoribbons (SSPGNRs)

Based on the first-principle calculations, we systematically study the electronic property, carrier mobility and optical absorption spectra in the monohydrogen(H), dihydrogen(2H) and Al edge-modified pristine sawtooth penta-graphene nanoribbons (SSPGNRs), respectively. Our results show that the SSPGNRs have suitable band gap, high carrier mobility and good optical absorption. The charge carrier mobility of SSPGNRs can be as high as 104 cm2 V−1 s−1, behave like n-type semiconductor, the different edge modifications enable SSPGNRs to convert from n-type to p-type semiconductor, and specifically, spin-splitting occurs in the band structure of the 2H-SSPGNR. Furthermore, the edge modifications can change the optical absorption that the absorption spectrum of Al-SSPGNR increases in visible and ultraviolet light regions obviously. The spin and optoelectronic properties of SSPGNRs with different edge modifications suggest its potential applications in spin electronics, optoelectronic devices and solar cells, which also provide some theoretical guidance for regulating the properties of SSPGNRs by edge modifications.

Exploring Ambipolar Semiconductor Nature of Binary and Ternary Charge-Transfer Cocrystals of Triphenylene, Pyrene, and TCNQ

Applications of organic cocrystal systems to obtain semiconductor materials with low band gap, balanced electron and hole carrier mobility, low cost, solution processability, air stability, and eas...

Large piezoelectric coefficients combined with high electron mobilities in Janus monolayer XTeI (X = Sb and Bi): A first-principles study

The absence of both the inversion symmetry and out-of-plane mirror symmetry together with spin–orbit coupling (SOC) can induce novel electronic and piezoelectric properties. In this work, the piezoelectric properties along with carrier mobilities of Janus monolayer XTeI (X=Sb and Bi) are studied by density functional theory. By using generalized gradient approximation (GGA) plus SOC, they are found to be indirect gap semiconductors with the Rashba spin splitting. The piezoelectric tensors of Janus monolayer XTeI (X=Sb and Bi) are reported by using the density functional perturbation theory. Due to lacking both the inversion symmetry and out-of-plane mirror symmetry for Janus monolayer XTeI (X=Sb and Bi), both in-plane and out-of-plane piezoelectric effects can be observed, and the large piezoelectric coefficients are predicted (e.g., d11=12.95pm/V for SbTeI and d11=8.20pm/V for BiTeI), which are comparable and even higher than the ones of many other two-dimensional materials and other well-known bulk piezoelectric materials, especially for out-of-plane piezoelectric coefficients. With GGA+SOC, the high electron carrier mobilities are obtained, and the electron mobility of BiTeI along armchair direction reaches up to about 1319cm2V−1s−1. The carrier mobility shows a rather pronounced anisotropy between electron and hole/armchair and zigzag directions. It is found that tensile strain can improve the piezoelectric coefficients d11 of Janus monolayer XTeI (X=Sb and Bi). For example, at 4% strain, the d11 of SbTeI (BiTeI) is up to 20.12 pm/V (11.48 pm/V), compared with unstrained 12.95 pm/V (8.20 pm/V). Our works imply Janus monolayer XTeI (X=Sb and Bi) have potential applications in flexible electronics and piezoelectric devices, and can stimulate further experimental works.

Carbon nanofiber based CuO nanorod counter electrode for enhanced solar cell performance and adsorptive photocatalytic activity

Dye-sensitized solar cells (DSSCs) are known as new generation solar cell of photovoltaic technologies (PV), and have become hotspot topic in the PV research. DSSCs include four main components such as photoanode, counter electrode, dye, and electrolyte. The counter electrode is a vital component of DSSCs which has a significant effect on the solar cell performance and cost of the devices. In this research, CuO nanorod/carbon nanofiber (CuO/CNF) thin films are produced with the diameter of nanorods and vary from 10 to 50 nm as counter electrodes (CEs) for DSSCs. The applications of as-synthesized materials were also investigated in the field of photocatalysis. It was shown that CuO/CNF CE–based solar cells exhibited 6.5% solar cell efficiency (η) under solar irradiation. We demonstrate that CuO nanorods can be good alternatives for expensive platinum as it is composed of inexpensive and abundant materials and prepared by a simple fabrication process. CNF, exhibiting high carrier electron mobility, was employed to improve the catalytic and electrical properties of resulting CuO/CNF CEs. The power conversion efficiency of the DSSC was enhanced by almost 29% in the case of CuO/CNF CEs. In addition, the synthesized CuO/CNF nano-heterostructures exhibited superior photocatalytic activity toward the degradation of textile dye (Orange II) and antibiotic (tetracycline) compared to raw CuO. The enhanced efficiency can be ascribed to the reduction of Eg~band-gap energy and to the synergetic effect of adsorption and photocatalysis.

A simple molecular design for tunable two-dimensional imine covalent organic frameworks for optoelectronic applications.

Two-dimensional covalent organic frameworks (2D-COFs) belong to a new class of molecular materials that have attracted huge attention in recent years due to their analogous nature to graphene. In this work, we present a systematic study of the electronic structure, carrier mobility and work function of imine based 2D-COFs. We identify these 2D-COFs as a new class of semiconducting materials with tunable electronic/optoelectronic properties and significant mobility. The results show that by rationally doping 2D-COFs at the molecular level, it is possible to control their structural and optoelectronic responses. Cohesive energy calculations revealed that all the studied 2D-COFs are thermodynamically stable. Also, the calculated binding energy of 2D-COFs on HOPG was found to be less than 1 eV, which indicates that the COFs do not interact strongly with HOPG, and it will not affect their electronic properties. Additionally, we have synthesized a 2,4,6-pyrimidinetriamine based 2D-COF and experimentally measured its band gap using scanning tunnelling spectroscopy. The experimentally measured band gap is found to be in good agreement with theoretical results.

Tuning the mechanical and electronic properties and carrier mobility of phosphorene via family atom doping: a first-principles study

Formation energy and cohesive energy are used to evaluate the thermodynamic stability of substitutionally doped phosphorene.

Characteristics of AlGaN/GaN high electron mobility transistor temperature sensor

Semiconductor temperature sensors have been widely used in medical, industrial, aviation and civil fields due to their advantages such as high sensitivity, small size, low power consumption and strong anti-interference ability. However, most Si-based temperature sensors are not suitable for the application in high-temperature environments. The new AlGaN/GaN heterojunction material not only has a wide band gap, but also has a high two-dimensional electron gas concentration and carrier mobility. Therefore, the device made with it not only has good electrical properties, but also can be applied in ultra-high environments. In this paper, a temperature sensor based on gateless AlGaN/GaN high electron mobility transistor structure was fabricated and its temperature-dependent electrical properties were characterized. The temperature dependence of current-voltage characteristics of the device were tested from 50 to 400 °C. The sensitivity of the device was studied as a function of the channel aspect ratio of the device. The stability of electrical properties was characterized after heating in air and nitrogen at 300—500 °C for 1 hour. The theoretical and experimental results show that as the aspect ratio of the device increases, the sensitivity of the device increases. At a fixed current of 0.01 A, the average sensitivity of the device voltage with temperature changes is 44.5 mV/°C. Meanwhile, the good high temperature retention stability is shown during stability experiments.

A New Three-Dimensional Subsulfide Ir2In8S with Dirac Semimetal Behavior

Dirac and Weyl semimetals host exotic quasiparticles with unconventional transport properties, such as high magnetoresistance and carrier mobility. Recent years have witnessed a huge number of newly predicted topological semimetals from existing databases; however, experimental verification often lags behind such predictions. Common reasons are synthetic difficulties or the stability of predicted phases. Here, we report the synthesis of the type-II Dirac semimetal Ir2In8S, an air-stable compound with a new structure type. This material has two Dirac crossings in its electronic structure along the Γ-Z direction of the Brillouin zone. We further show that Ir2In8S has a high electron carrier mobility of ∼10 000 cm2/(V s) at 1.8 K and a large, nonsaturating transverse magnetoresistance of ∼6000% at 3.34 K in a 14 T applied field. Shubnikov de-Haas oscillations reveal several small Fermi pockets and the possibility of a nontrivial Berry phase. With its facile crystal growth, novel structure type, and striking electronic structure, Ir2In8S introduces a new material system to study topological semimetals and enable advances in the field of topological materials.

Intriguing electronic structures and carrier mobilities of two-dimensional GaN nanosheets: Thickness and surface effects

Two-dimensional GaN nanosheets have emerged as a promising candidate material for the application of low-dimensional optoelectronic devices, but their fundamental electronic structure and intrinsic carrier mobility are extremely sensitive to the phase structure and thickness. Herein, we perform systematic first-principles calculations within hybrid density-functional theory to investigate the thickness and surface effects on electronic properties and carrier mobilities in two-dimensional GaN nanosheets. Our results show that the reduction of thickness leads to a series of phase transitions of pristine GaN nanosheets and surface reconstruction of hydrogenated GaN nanosheets for the minimization of surface dangling bonds. The calculated electronic structures reveal that the band gaps and band-edge states of GaN nanosheets are determined by three competition mechanisms among the quantum confinement effect, surface effect, and Stark effect, resulting in a large band-gap range from 1.32 to 4.35 eV. Owing to the influence of thickness and surface effects, intrinsic carrier mobilities of GaN nanosheets cover a wide range from 400 to 6314 cm2 V−1 s−1 for electrons and from 11 to 366 cm2 V−1 s−1 for holes, respectively. The findings suggest the possibility to obtain suitable band gap and high carrier mobilities in GaN nanosheets for optoelectronic applications by the control of thickness and phase engineering.

Weak antilocalization and two-carrier electrical transport in Bi1−xSbx single crystals (0%≤x≤17.0%)

Experimental investigation of the weak antilocalization (WAL) effect on the Hall resistivity is quite rare, because the WAL is known to have no influence on the Hall effect in a single-band system. We challenge this view in a system that has both electrons and holes by deriving a two-band model modified by the WAL effect and by applying it to the low-field magnetoresistance (MR) and Hall resistance (HR) of $\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}$ single crystals $(0\ensuremath{\le}x\ensuremath{\le}17.0%)$. Simultaneous occurrence of a dip in MR and nonlinearity in HR suggests that the $\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}$ is a rare three-dimensional system, in which WAL and two distinct charge carriers interplay. The modified two-band model describes all the main features of MR and HR that are not captured by the conventional theory. From the quantitative analysis based on the modified theory, the values of key parameters, such as carrier density and mobility of electrons and holes, are estimated. The modified two-band model provides a solid framework for understanding electrical transport phenomena of a material with strong spin-orbit interaction and multiple distinct charge carriers.

First-principle prediction of the electronic property and carrier mobility in boron arsenide nanotubes and nanoribbons

In this work, the electronic structure and carrier mobility of single-walled boron arsenide nanotubes (BAsNTs) have been systematically studied by using Boltzmann transport equation with the relaxation time approximation. We found that the ionic characteristic of B–As bond results in the dipole shells in the optimized BAsNTs. It is predicted that both zigzag BAs nanotubes (ZNTs) and armchair BAs nanotubes are semiconductors, and the strong σ*–π* hybridization in small ZNTs leads to a rapid drop of bandgap with a decrease of radius. Interestingly, as the size (n) of the NTs decreases, the hole mobility (μh) of ZNTs has an evident 3p (p is an integer) oscillation but electron mobility (μe) basically falls down, which falls even faster when the radius gets smaller. Comparing the carrier mobility between BAsNTs and its unzipping nanoribbons, we found that rolling BAs nanoribbons (BAsNRs) into BAsNTs would increase the μe but decrease the μh. The different behavior of the carrier mobility in BAsNRs and BAsNTs results from their distinct bond features of edge states, which vary with different widths (for BAsNRs) or radii (for BAsNTs).

8.1: Invited Paper: Advanced Process and Structure for High Performance Oxide Thin‐Film Transistors

The required carrier mobility of metal oxide thin‐film transistors (TFTs) has been increasing rapidly to meet the demands of the ultra‐high‐resolution, large panel size and three dimensional visual effects as a megatrend of flat panel display. However, the typical field‐effect mobility of IGZO TFTs in the practical production line is less than 10 cm2/Vs, which is still not enough to drive the high‐end flat panel displays with ≥ 300 ppi, more than 60 inch and high frame rate ( ≥ 240 Hz). Approaches to improve the mobility of electron carriers in metal oxide TFT would involve the optimization of cation composition, stacked channel structure and the lattice ordering‐induced crystallization. In this paper, we presented our recent efforts toward the high performance and good reliability, which included the non‐IGZO composition, double channel structure such as ZTO/IZO and ZTO/ITO, the metal‐induced crystallization at a low temperature, and atomic layer deposited IGZO TFTs.

Exploring the electronic, charge transport and lattice dynamic properties of two-dimensional phosphorene

Two-dimensional nanostructures are emerging materials for device applications. Development of the field require materials with high carrier mobility and sufficient band gap. Two-dimensional black phosphorus (phosphorene) is a novel material with potential applications in nanoelectronics. Herein, employing density functional theory, the electronic structure, carrier mobility, structural stability, and Raman active modes of phosphorene are studied. Using PBE functional, it is found that phosphorene shows a direct band gap 0.95 eV. From deformation potential theory, the mobility of phosphorene is found to be of the order of 103 cm2 V−1 s−1, and is found to be nearly same for electrons along x- and y-direction (μx ~ μy), whereas, it is significantly larger for holes (μy = 105.36μx) along y-direction. Also, no softmodes ware seen in phonon dispersion which indicates that phosphorene is dynamically stable material. The vibrational modes at Γ-point shows that all even parity modes are Raman active.

Realizing nearly-free-electron like conduction band in a molecular film through mediating intermolecular van der Waals interactions

Collective molecular physical properties can be enhanced from their intrinsic characteristics by templating at material interfaces. Here we report how a black phosphorous (BP) substrate concatenates a nearly-free-electron (NFE) like conduction band of a C60 monolayer. Scanning tunneling microscopy reveals the C60 lowest unoccupied molecular orbital (LUMO) band is strongly delocalized in two-dimensions, which is unprecedented for a molecular semiconductor. Experiment and theory show van der Waals forces between C60 and BP reduce the inter-C60 distance and cause mutual orientation, thereby optimizing the π-π wave function overlap and forming the NFE-like band. Electronic structure and carrier mobility calculations predict that the NFE band of C60 acquires an effective mass of 0.53–0.70 me (me is the mass of free electrons), and has carrier mobility of ~200 to 440 cm2V−1s−1. The substrate-mediated intermolecular van der Waals interactions provide a route to enhance charge delocalization in fullerenes and other organic semiconductors.

Theoretical investigations on charge transport properties of tetrabenzo[a,d,j,m]coronene derivatives using different density functional theory functionals (B3LYP, M06-2X, and wB97XD)

Charge transport rate is one of the key parameters determining the performance of organic electronic devices. Based on density functional theory, exchange-correlation functionals which adequately account for non-covalent interactions, such as M06-2X and wB97XD, should significantly improve the accuracy of charge transport rate calculations for large systems with non-covalent interactions. In this work, the B3LYP hybrid functional, the variant hybrid functional M06-2X, and the long-range-corrected wB97XD functional were used to perform geometry optimizations and charge transport rate calculations on 11 variants of tetrabenzo[ a,d,j,m]coronene, including tetrabenzo[ a,d,j,m]coronene itself and its tetra-substituted and octa-substituted derivatives. Our results indicate that the molecular geometries of these benzocoronene semiconductors are large quasi-planar conjugated π systems, and the incorporation of different substituents significantly affects their frontier molecular orbitals. The hole carrier mobility ( µ+) and electron carrier mobility ( µ−) of the methoxy-substituted derivatives (TBC(OCH3)4 and TBC(OCH3)8) were relatively low. The results of the tetrabenzo[ a,d,j,m]coronene molecules studied were consistent with using the aforementioned M06-2X, wB97XD, and B3LYP methods. We found that the octa-substituted derivatives (TBCF8, TBCCl8, TBC(CH3)8, and TBC(CN)8) could be used as p-type organic semiconductor materials.

Overcoming Mobility Lifetime Product Limitations in Vertical Bridgman Production of Cadmium Zinc Telluride Detectors

Cadmium zinc telluride (CZT) possesses excellent material properties for a wide range of applications where room temperature operability, durability, and high efficiency are required. However, because CZT is a challenging material to produce in useful quantities, the growth and fabrication costs have remained high, creating an economic challenge for vendors. While the traveling heater method (THM) is the predominant means of commercial CZT crystal growth, the vertical Bridgman method (VB) is an attractive alternative due to its relatively fast growth rate. However, VB grown CZT has yet to compete with THM grown CZT, particularly in terms of charge collection efficiencies, where the charge collection efficiency is characterized by the single carrier electron mobility lifetime (μτe) product. Despite efforts to overcome this discrepancy, the μτe product in VB grown CZT has remained an order of magnitude lower than THM. Eliminating this difference would bring VB one step closer to outpacing THM in terms of economic feasibility. This paper discusses the development of a unique technique that combines the advantages of both growth methods to better understand this discrepancy and the underlying mechanisms behind it. CZT ingots were grown from melt via VB with highly off-stoichiometric concentrations of tellurium (Te). Melt mixing via accelerated crucible rotation (ACRT) was applied to compensate for any negative effects associated with off-stoichiometry, i.e. flux inclusions. CZT material has been produced at growth rates commensurate with VB (one ingot/week) and with charge collection efficiencies commensurate with THM (mid 10−2 cm2/V) in long bars typical of commercial applications.