Charge Transport In Semiconductors Research Articles

Two-phonon scattering in nonpolar semiconductors: A first-principles study of warm electron transport in Si

The ab-initio theory of charge transport in semiconductors typically employs the lowest-order perturbation theory in which electrons interact with one phonon (1ph). This theory is accepted to be adequate to explain the low-field mobility of non-polar semiconductors but has not been tested extensively beyond the low-field regime. Here, we report first-principles calculations of the electric field-dependence of the electron mobility of Si as described by the warm electron coefficient, $\beta$. Although the 1ph theory overestimates the low-field mobility by only around 20%, it overestimates $\beta$ by over a factor of two over a range of temperatures and crystallographic axes. We show that the discrepancy in $\beta$ is reconciled by inclusion of on-shell iterated 2-phonon (2ph) scattering processes, indicating that scattering from higher-order electron-phonon interactions is non-negligible even in non-polar semiconductors. Further, a ~20% underestimate of the low-field mobility with 2ph scattering suggests that non-trivial cancellations may occur in the perturbative expansion of the electron-phonon interaction.

Coupled Electromagnetic and Hydrodynamic Modeling for Semiconductors Using DGTD

A hydrodynamic model (HDM) solver based on discontinuous Galerkin time domain finite element method (DGTD-FEM) has been developed in order to simulate the transient charge transport in semiconductors. The bipolar transport equations have been numerically solved together with the Poisson equation to realize ballistic charge transport in semiconductor devices. Furthermore, the developed solver is coupled with an FEM-based full-wave Maxwell solver in order to model the behavior of semiconductors under external electromagnetic illumination. This multiphysics coupled solver is capable of simulating transient behavior of photoactive semiconductor devices that are illuminated by externally modulated light at high frequencies. With the time domain HDM solver, the inertia effects and ballistic transport are also accounted in the accurate transient simulations of high-frequency photoactive devices.

Charge transport in semiconductors assembled from nanocrystal quantum dots

The potential of semiconductors assembled from nanocrystals has been demonstrated for a broad array of electronic and optoelectronic devices, including transistors, light emitting diodes, solar cells, photodetectors, thermoelectrics, and phase change memory cells. Despite the commercial success of nanocrystal quantum dots as optical absorbers and emitters, applications involving charge transport through nanocrystal semiconductors have eluded exploitation due to the inability to predictively control their electronic properties. Here, we perform large-scale, ab initio simulations to understand carrier transport, generation, and trapping in strongly confined nanocrystal quantum dot-based semiconductors from first principles. We use these findings to build a predictive model for charge transport in these materials, which we validate experimentally. Our insights provide a path for systematic engineering of these semiconductors, which in fact offer previously unexplored opportunities for tunability not achievable in other semiconductor systems.

EPIC STAR: a reliable and efficient approach for phonon- and impurity-limited charge transport calculations

A computationally efficient first-principles approach to predict intrinsic semiconductor charge transport properties is proposed. By using a generalized Eliashberg function for short-range electron–phonon scattering and analytical expressions for long-range electron–phonon and electron–impurity scattering, fast and reliable prediction of carrier mobility and electronic thermoelectric properties is realized without empirical parameters. This method, which is christened “Energy-dependent Phonon- and Impurity-limited Carrier Scattering Time AppRoximation (EPIC STAR)” approach, is validated by comparing with experimental measurements and other theoretical approaches for several representative semiconductors, from which quantitative agreement for both polar and non-polar, isotropic and anisotropic materials is achieved. The efficiency and robustness of this approach facilitate automated and unsupervised predictions, allowing high-throughput screening and materials discovery of semiconductor materials for conducting, thermoelectric, and other electronic applications.

A facile and novel route to improve TIPS pentacene based organic thin film transistor performance with elastomer

In this study, we report a facile route to regulate the crystallization and enhance charge transport of small-molecule organic semiconductors by blending with a rubbery polymer additive polyisobutylene (PIB). Using a solution-processed semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) to mix with the PIB polymer, we show that the addition of PIB led to densely-arranged, lengthened TIPS pentacene needles with excellent long-range uniformity. Bottom-gate, top-contact organic thin-film transistors (OTFTs) based on TIPS pentacene/PIB hybrid film demonstrated a field-effect mobility of up to 0.15 cm2/Vs, and particularly, a 3-fold and 2-fold enhancement of average mobility and performance consistency (denoted by average mobility to standard deviation ratio). In addition, the effect of hexamethyldisilazane (HMDS) surfactant treatment on the semiconductor charge transport was studied. We ascribe such improved performance of TIPS pentacene/PIB blend OTFTs to the collaborative outcome of reduced crystal misorientation, diminished defects and charge trap centers, as well as a higher areal coverage. The effective method as demonstrated in this work can be generally implemented to attune the crystal growth and promote charge transport of newly-developed small-molecule organic semiconductors.

Highly Efficient Charge Transport in a Quasi‐Monolayer Semiconductor on Pure Polymer Dielectric

AbstractThe study of monolayer organic field‐effect transistors (MOFETs) provides an effective way to investigate the intrinsic charge transport of semiconductors. To date, the research based on organic monolayers on polymeric dielectrics lays far behind that on inorganic dielectrics and the realization of a bulk‐like carrier mobility on pure polymer dielectrics is still a formidable challenge for MOFETs. Herein, a quasi‐monolayer coverage of pentacene film with orthorhombic phase is grown on the poly (amic acid) (PAA) dielectric layer. More significantly, charge density redistribution occurs at the interface between the pentacene and PAA caused by electron transfer from pentacene to the PAA dielectric layer, which is verified by theoretical simulations and experiments. As a consequence, an enhanced hole accumulation layer is formed and pentacene‐based MOFETs on pure polymer dielectrics exhibit bulk‐like carrier mobilities of up to 13.7 cm2 V−1 s−1 from the saturation region at low VGS, 9.1 cm2 V−1 s−1 at high VGS and 7.6 cm2 V−1 s−1 from the linear region, which presents one of the best results of previously reported MOFETs so far and indicates that the monolayer semiconductor growing on pure polymer dielectric could produce highly efficient charge transport.

Effect of hydrostatic pressure on the electron-phonon scattering in GaAs

Electron-phonon (e-ph) interaction in semiconductors is responsible for many phenomena such as electron mobility and hot electron thermalization, which are important to compute charge transport in semiconductors. In semiconductors, the application of hydrostatic pressure modifies the character of conduction band valleys, changing the effective mass of carriers, and, consequently, changing the carrier mobility. This, in turn, may limit electron transport in the semiconductor. Here, we present the effect of hydrostatic pressure on the e-ph interaction in GaAs, which goes from a direct to an indirect bandgap semiconductor at high pressures. A detailed study of e-ph self-energy and the resulting scattering rates is presented for hydrostatic pressure ranging between 0 and 9.2 GPa. Individual contributions from phonon modes to the total scattering rates indicate that short wavelength LO phonon scattering is found to be the dominant mechanism at low pressures; and at higher pressures, acoustic as well as optic phonons contribute toward the overall e-ph scattering. Finally, relaxation times are evaluated and we find that at pressures above 2.9 GPa, electron relaxation times are an order of magnitude lower than at equilibrium. These results will influence transport properties at higher pressures.

Impact of solvent exposure on the structure and electronic properties of CH3NH3PbI3−xClx mixed halide perovskite films

Organic semiconductor charge transport layers are important constituents of perovskite-based solar cells. To assess the suitability of potential solvents for the deposition of the charge transport layers on perovskite surfaces, the effect of solvent exposure on the properties of methyl ammonium lead mixed halide CH3NH3PbI3–xClx films is investigated by grazing incidence X-ray diffraction, atomic force microscopy, ultraviolet–visible absorption spectroscopy, and photoelectron spectroscopy. While exposure to dimethylformamide (DMF) and water instantly dissolves the perovskite film, exposure to chlorobenzene (CB) and chloroform (CF) does not detectably affect the perovskite bulk properties. However, the electronic properties of the perovskite surface are substantially modified by the solvent exposure, resulting in an increased work function and less n-type appearance.

Modeling of diamond field emitter arrays for a compact source of high brightness electron beams

Many applications, such as compact accelerators and electron microscopy, demand high brightness electron beams with small source size and ultralow emittance. Diamond emitters manufactured with semiconductor processes can be employed in such compact beam sources. The micrometer-scale pyramid structure of the emitter allows enhancement of the external field compared to that at the substrate, leading to electron emission with small beam size. We investigate the dependence of the field enhancement on the shape of the emitter and the resulting emission characteristics. The beam formation and dynamics are simulated with the LSP [D. Welch, D. Rose, R. Clark, T. Genoni, and T. Hughes, Comput. Phys. Commun. 164, 183 (2004)] particle-in-cell code to obtain the macroscopic observables. To account for the semiconductor charge transport in the bulk material and the tunneling through the surface, a first-principle semiclassical Monte Carlo emission model is developed and applied to the diamond pyramid. Using this Monte Carlo emission model and the result from the geometric field enhancement calculation, we construct a simple model to qualitatively explain the measured emission characteristics. A comparison between our model and experiments indicates that the beam current is mostly emitted at the apex of the emitter.

A new approach to numerical simulation of charge transport in double Gate-MOSFET

We propose and describe in detail an effective numerical algorithm for finding the stationary solution of charge transport problem in a DG-MOSFET. Hydrodynamical models describing the process of charge transport in semiconductors are sets of nonlinear PDE's with small parameters and specific conditions on the boundary of transistor that essentially complicates the process of numerical simulations. We construct a new algorithm based on the stabilization method and ideas of approximation without saturation and pseudo-spectral methods that enables one to overcome all of the mentioned difficulties. The proposed algorithm enables us to obtain the solution for different geometrical characteristics of DG-MOSFET and boundary conditions (including the non-symmetric cases) with extremely small values of dimensionless doping density and dielectric constant that are used in practice.

Simulation of charge transport in micro and nanoscale FETs with elements having different dielectric properties

The hydrodynamical model is used for description of the process of charge transport in semiconductors with a high rate of reliability. It is a set of nonlinear partial differential equations with small parameters and specific conditions at the boundaries of field effect transistors (FETs), which essentially complicates the process of finding its stationary solutions. To overcome these difficulties in the case of FETs with elements having different dielectric properties, a fast pseudospectral method has been developed. This method was used for advanced numerical simulation of charge transport in DG-MOSFET.

Verification of Exciton Effects in Organic Solar Cells at Low Temperatures Based on a Modified Numerical Model

There are many models for researching charge transport in semiconductors and improving their performance. Most of them give good descriptions of the experimental data at room temperature. But it is still an open question which model is correct. In this paper, numerical calculations based on three modified versions of a classical model were made, and compared with experimental data for typical devices at room or low temperatures. Although their results are very similar to each other at room temperatures, only the version considering exciton effects by using a hydrogen-like model can give qualitative descriptions to recent experimental data at low temperatures. Moreover, the mobility was researched in detail by comparing the constant model and temperature dependence model. Then, we found the performance increases with the mobility of each charge carrier type being independent to the mobility of the other one. This paper provides better insight into understanding the physical mechanism of carrier transport in semiconductors, and the results show that exciton effects should be considered in modeling organic solar cells.

Conductance fluctuations in InAs quantum wells possibly driven by Zitterbewegung

The highly successful Dirac equation predicts peculiar phenomena such as Klein tunnelling and Zitterbewegung (ZB) of electrons. From its conception by Erwin Schrödinger, ZB has been considered key in understanding relativistic quantum mechanics. However, observing the ZB of electrons has proved difficult, and instead various emulations of the phenomenon have been proposed producing several successes. Concerning charge transport in semiconductors and graphene, expectations were high but little has been reported. Here, we report a surprisingly large ZB effect on charge transport in a semiconductor nanostructure playing “flat pinball”. The setup is a narrow strip of InAs two-dimensional electron gas with strong Rashba spin–orbit coupling. Six quantum point contacts act as pinball pockets. In transiting between two contacts, ZB appears as a large reproducible conductance fluctuation that depends on the in-plane magnetic field. Numerical simulations successfully reproduced our experimental observations confirming that ZB causes this conductance fluctuation.

Negative Isotope Effect on Field‐Effect Hole Transport in Fully Substituted 13C‐Rubrene

Isotopic substitution is a useful method to study the influence of nuclear motion on the kinetics of charge transport in semiconductors. However, in organic semiconductors, no observable isotope effect on field‐effect mobility has been reported. To understand the charge transport mechanism in rubrene, the benchmark organic semiconductor, crystals of fully isotopically substituted rubrene, 13C‐rubrene (13C42H28), are synthesized and characterized. Vapor‐grown 13C‐rubrene single crystals have the same crystal structure and quality as native rubrene crystals (i.e., rubrene with a natural abundance of carbon isotopes). The characteristic transport signatures of rubrene, including room temperature hole mobility over 10 cm2 V−1 s−1, intrinsic band‐like transport, and clear Hall behavior in the accumulation layer of air‐gap transistors, are also observed for 13C‐rubrene crystals. The field‐effect mobility distributions based on 74 rubrene and 13C‐rubrene devices, respectively, reveal that 13C isotopic substitution produces a 13% reduction in the hole mobility of rubrene. The origin of the negative isotope effect is linked to the redshift of vibrational frequencies after 13C‐substitution, as demonstrated by computer simulations based on the transient localization (dynamic disorder) scenario. Overall, the data and analysis provide an important benchmark for ongoing efforts to understand transport in ordered organic semiconductors.

Operation of Ge- and GaAs-tunnel diodes under the influence of electron beams

Nuclear radiation plays a very negative role in the semiconductor devices functionality, mainly when particular semiconductor devices are exposed to an extreme type of radiation. Tunneling is an important aspect of charge transport in semiconductor and molecular devices. So, the effect of electron irradiation on the current–voltage (I–V) characteristics of Germanium (Ge) and Gallium Arsenide (GaAs) tunneling diodes are reported at room temperature before and after irradiation. Electrons exposure, up to 3.73 My, of the tunnel diodes leads to a pronounced change in their electrical characteristics where the rate of change of the peak- and valley-currents, for Ge and GaAs tunnel diodes, due to electron exposure are shown to be about +53.6, +142 µA/MGy and +29.4, +53.6 µA/MGy, respectively. On the other hand, for the same irradiation doses, the rate of change of the valley- and forward-voltages and output power are shown to be about −44.9, −15.9 and −6.7 mW/MGy, for Ge tunnel diodes, respectively. While, GaAs samples, reported values of −81, −83 mV/MGy and −11.6 mW/MGy are observed. Besides, the peak to valley current ratio of both Ge- and GaAs TDs are proved to decrease due to electrons exposure, with damping ratios of about 78 and 81%, respectively.

Exploitation of the Maximum Entropy Principle in Mathematical Modeling of Charge Transport in Semiconductors

In the last two decades, the Maximum Entropy Principle (MEP) has been successfully employed to construct macroscopic models able to describe the charge and heat transport in semiconductor devices. These models are obtained, starting from the Boltzmann transport equations, for the charge and the phonon distribution functions, by taking—as macroscopic variables—suitable moments of the distributions and exploiting MEP in order to close the evolution equations for the chosen moments. Important results have also been obtained for the description of charge transport in devices made both of elemental and compound semiconductors, in cases where charge confinement is present and the carrier flow is two- or one-dimensional.

Enhanced P3HT/ZnO Nanowire Array Solar Cells by Pyro-phototronic Effect.

The pyro-phototronic effect is based on the coupling among photoexcitation, pyroelectricity, and semiconductor charge transport in pyroelectric materials, which can be utilized to modulate photoexcited carriers to enhance the output performance of solar cells. Herein, we have demonstrated the largely enhanced output performance of a P3HT/ZnO nanowire array photovoltaic cell (PVC) by using the pyro-phototronic effect under weak light illuminations. By applying an external cooling temperature variation, the output current and voltage of the PVC can be dramatically enhanced by 18% and 152% under indoor light illumination, respectively. This study realizes the performance enhancement of pyroelectric semiconductor materials-based solar cells via a temperature-variation-induced pyro-phototronic effect, which may have potential applications in solar energy scavenging and self-powered sensor systems.

Conjugated Polymer Alignment: Synergisms Derived from Microfluidic Shear Design and UV Irradiation.

Solution shearing has attracted great interest for the fabrication of robust and reliable, high performance organic electronic devices, owing to applicability of the method to large area and continuous fabrication, as well as its propensity to enhance semiconductor charge transport characteristics. To date, effects of the design of the blade shear features (especially the microfluidic shear design) and the prospect of synergistically combining the shear approach with an alternate process strategy have not been investigated. Here, a generic thin film fabrication concept that enhanced conjugated polymer intermolecular alignment and aggregation, improved orientation (both nanoscale and long-range), and narrowed the π-π stacking distance is demonstrated for the first time. The impact of the design of shearing blade microfluidic channels and synergistic effects of fluid shearing design with concomitant irradiation strategies were demonstrated, enabling fabrication of polymer-based devices with requisite morphologies for a range of applications.

DSMC method consistent with the Pauli exclusion principle and comparison with deterministic solutions for charge transport in graphene

A new algorithm for Monte Carlo simulations of charge transport in semiconductors is devised in order to properly deal with Pauli's exclusion principle in the degenerate case. Applications are presented in the case of monolayer graphene and comparisons with solutions of the Boltzmann equation obtained by using a discontinuous Galerkin method furnish a cross-validation of the proposed approach.

A Panorama of Electrical Conduction Models in Dielectrics, With Application to Spacecraft Charging

Conduction models in disordered materials are described, with a special focus on the transient behavior appearing on a broad timescale as a consequence of disorder. Multiple trapping models, hopping models, or random walks coupled with waiting-time distributions are commonly used to describe charge transport in semiconductors. Important concepts have been introduced in this field, such as demarcation energy, percolation, or transport energy. Dispersive transport appears as a consequence of the disorder, together with a memory effect, that may be described using various mathematical tools. The interest of this research field concerning the charging behavior of materials used in spacecraft applications, especially polymers, is underlined and a practical model of the insulator is suggested.