Transport In Semiconductors Research Articles

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.

Generalized Scharfetter–Gummel schemes for electro-thermal transport in degenerate semiconductors using the Kelvin formula for the Seebeck coefficient

Many challenges faced in today's semiconductor devices are related to self-heating phenomena. The optimization of device designs can be assisted by numerical simulations using the non-isothermal drift-diffusion system, where the magnitude of the thermoelectric cross effects is controlled by the Seebeck coefficient. We show that the model equations take a remarkably simple form when assuming the so-called Kelvin formula for the Seebeck coefficient. The corresponding heat generation rate involves exactly the three classically known self-heating effects, namely Joule, recombination and Thomson–Peltier heating, without any further (transient) contributions. Moreover, the thermal driving force in the electrical current density expressions can be entirely absorbed in the diffusion coefficient via a generalized Einstein relation. The efficient numerical simulation relies on an accurate and robust discretization technique for the fluxes (finite volume Scharfetter–Gummel method), which allows to cope with the typically stiff solutions of the semiconductor device equations. We derive two non-isothermal generalizations of the Scharfetter–Gummel scheme for degenerate semiconductors (Fermi–Dirac statistics) obeying the Kelvin formula. The approaches differ in the treatment of degeneration effects: The first is based on an approximation of the discrete generalized Einstein relation implying a specifically modified thermal voltage, whereas the second scheme follows the conventionally used approach employing a modified electric field. We present a detailed analysis and comparison of both schemes, indicating a superior performance of the modified thermal voltage scheme.

Can Thermodynamics Guide Us to Make Better Solar Cells?

Thermodynamics has provided a powerful tool to study radiation and its conversion into useful work. Starting from the so-called Shockley's paradox, this article discusses the thermodynamic view of fundamental losses to photovoltaic conversion, and how thermodynamics enters the charge-carrier transport in semiconductors and heat-exchange processes at p-n junctions. Turning to photon flows, considerations based on detailed balance and reciprocity provide a comprehensive picture of the voltage produced by the solar cell in the presence of nonradiative recombination. We shall use these tools to examine several topics under recent discussion, including photon recycling and hot-carrier conversion based on thermoelectricity.

Composite transport mechanism enhancing thermoelectric performance of Ag-doped Mg3Sb2

The first principle calculations are employed to investigate the Ag-doped Mg3Sb2. Comparing the formation energy and the bonds length between the Ag atom and its nearest Sb atoms, it is found that Ag atom is more favorable to substitute Mg atom in the anionic [Mg2Sb2]2− layers. Comparing the electronic energy band structures between the un-doped and Ag-doped Mg3Sb2, the Femi level across the valence band through Ag doping. The double transport mechanism are induced by the doping, at low temperature, materials exhibit metallic properties, and when the temperature is higher than 422 K, the thermal excitation of electrons increases gradually, metallic and semiconductor transport mechanism coexist. As the temperature continue rising, the metallic conduction mechanism returns to dominant. The composite transport mechanism are verified in our calculations, and are found in temperature dependence of the electronic conduction and Seebeck coefficient. Our calculations explain the previous experiments.

Spin transport in n-type 3C–SiC observed in a lateral spin-pumping device

3C–SiC is a promising platform for semiconductor spintronics because it consists of light group-IV elements and has a zinc blende structure. To demonstrate spin transport in this attractive semiconductor, we conducted an experiment of spin-pump-induced spin transport through n-type 3C–SiC. A spin current is injected from Ni80Fe20 into the SiC channel under ferromagnetic resonance of the Ni80Fe20. The DC electromotive force caused by the inverse spin Hall effect in an adjacent metal detector attached to the SiC is detected as a manifestation of the spin current transport. This indicates 1.2-μm-long spin transport through the n-type 3C–SiC at room temperature. This achievement is the first step in the investigation of the physics of 3C–SiC for further spintronics study and its application.

Hybrid Thermal Transport Characteristics of Doped Organic Semiconductor Poly(3,4-ethylenedioxythiophene):Tosylate

Understanding thermal transport in doped organic semiconductors would promote the development of flexible electronics and organic thermoelectrics. Temperature-dependent thermal conductivity of poly...

Imaging material functionality through three-dimensional nanoscale tracking of energy flow.

The ability of energy carriers to move between atoms and molecules underlies biochemical and material function. Understanding and controlling energy flow, however, requires observing it on ultrasmall and ultrafast spatio-temporal scales, where energetic and structural roadblocks dictate the fate of energy carriers. Here, we developed a non-invasive optical scheme that leverages non-resonant interferometric scattering to track tiny changes in material polarizability created by energy carriers. We thus map evolving energy carrier distributions in four dimensions of spacetime with few-nanometre lateral precision and directly correlate them with material morphology. We visualize exciton, charge and heat transport in polyacene, silicon and perovskite semiconductors and elucidate how disorder affects energy flow in three dimensions. For example, we show that morphological boundaries in polycrystalline metal halide perovskites possess lateral- and depth-dependent resistivities, blocking lateral transport for surface but not bulk carriers. We also reveal strategies for interpreting energy transport in disordered environments that will direct the design of defect-tolerant materials for the semiconductor industry of tomorrow.

Cationic Vacancy Mediated Conductivity and Charge Transport in Non‐Stoichiometric Epitaxial BaTi0.75Nb0.25O3 Films

High‐quality epitaxial BaTi0.75Nb0.25O3 (BNTO) films with controllable Ba/(Nb + Ti) ratio are fabricated by pulsed laser deposition, and a systematic investigation on their conductivity and charge transport behaviors is carried out. The Ba/(Nb + Ti) ratio can be significantly regulated from 0.96 to 0.53 when the laser fluence varies from 0.5 to 2.0 J cm−2. Resistivity–temperature measurements indicate that all the BNTO films exhibit semiconductor transport behaviors at temperatures from 50 to 400 K. Their conductivity increases monotonically with the decrease in the Ba/(Nb + Ti) ratio. For films with a high Ba/(Nb + Ti) ratio of 0.96, the small polaron (SP) hopping dominates in the temperature range from 200 to 400 K. Nevertheless, the SP hopping can only be observed at temperatures ranging from 285 to 400 K for the films with a Ba/(Nb + Ti) ratio of 0.53, implying that the reduction of Ba vacancy is favorable for SP hopping transport in BNTO films, especially at low temperature. Hall effect measurements indicate a higher carrier mobility observed in the films with lower Ba/(Nb + Ti) ratio, revealing the promotion of carrier mobility by Ba vacancy. The work reveals that cationic vacancy plays an important role in the conductivity and charge transport mechanisms in perovskite oxide materials, such as BNTO films.

Effect of transient space–charge perturbation on carrier transport in high-resistance CdZnTe semiconductor**Project supported by the National Natural Science Foundation of China (Grant No. 61874089), the Fund of MIIT (Grant No. MJ-2017-F-05), the 111 Project of China (Grant No. B08040), the NPU Foundation for Fundamental Research, China, and the Research Found of the State Key Laboratory of Solidification Processing (NWPU), China.

The polarization effect introduced by electric field deformation is the most important bottleneck of CdZnTe detector in x-ray imaging. Currently, most of studies focus on electric field deformation caused by trapped carriers; the perturbation of electric field due to drifting carriers has been rarely reported. In this study, the effect of transient space–charge perturbation on carrier transport in a CdZnTe semiconductor is evaluated by using the laser-beam-induced current (LBIC) technique. Cusps appear in the current curves of CdZnTe detectors with different carrier transport performances under intense excitation, indicating the deformation of electric field. The current signals under different excitations are compared. The results suggest that with the increase of excitation, the amplitude of cusp increases and the electron transient time gradually decreases. The distortion in electric field is independent of carrier transport performance of detector. Transient space–charge perturbation is responsible for the pulse shape and affects the carrier transport process.

Thermal Transport in 2D Semiconductors—Considerations for Device Applications

AbstractThe discovery of graphene has stimulated the search for and investigations into other 2D materials because of the rich physics and unusual properties exhibited by many of these layered materials. Transition metal dichalcogenides (TMDs), black phosphorus, and SnSe among many others, have emerged to show highly tunable physical and chemical properties that can be exploited in a whole host of promising applications. Alongside the novel electronic and optical properties of such 2D semiconductors, their thermal transport properties have also attracted substantial attention. Here, a comprehensive review of the unique thermal transport properties of various emerging 2D semiconductors is provided, including TMDs, black‐ and blue‐phosphorene among others, and the different mechanisms underlying their thermal conductivity characteristics. The focus is placed on the phonon‐related phenomena and issues encountered in various applications based on 2D semiconductor materials and their heterostructures, including thermoelectric power generation and electron–phonon coupling effect in photoelectric and thermal transistor devices. A thorough understanding of phonon transport physics in 2D semiconductor materials to inform thermal management of next‐generation nanoelectronic devices is comprehensively presented along with strategies for controlling heat energy transport and conversion.

Electronic structure and transport in amorphous metal oxide and amorphous metal oxynitride semiconductors

Recently, amorphous oxide semiconductors have gained significant interest due to their low-temperature processability, high mobility, and high areal uniformity for display backplanes and other large area applications. A multication amorphous oxide, amorphous indium gallium zinc oxide (a-IGZO), has been extensively researched and is now being used in commercial applications. It has been proposed that, in a-IGZO semiconductors, overlapping In-5s orbitals form the conduction path and the carrier mobility is limited due to the presence of multiple cations, which creates a potential barrier for the electronic transport. A multianion approach toward amorphous semiconductors has been suggested to overcome this limitation, and has been shown to achieve Hall mobilities up to an order of magnitude higher compared to multication amorphous semiconductors. In the present work, we compare the electronic structure and electronic transport in a multication amorphous semiconductor, a-IGZO, and a multianion amorphous semiconductor, amorphous zinc oxynitride (a-ZnON) using computational methods. Our results show that, in a-IGZO, the carrier transport path is through the overlap of outer s-orbitals of mixed cations, whereas, in a-ZnON, the transport path is through the overlapping Zn-4s orbitals, which is the only type of metal cation present. We also show that for multicomponent ionic amorphous semiconductors, the electron transport can be explained in terms of the orbital overlap integral, which can be calculated simply from the structural information. The orbital overlap integral has a direct correlation with the carrier effective mass, which is calculated using computationally expensive first-principles density functional theory based methods.

Doping of Two-Dimensional Semiconductors: A Rapid Review and Outlook

ABSTRACTDoping, as a primary technique to modify semiconductor transport, has achieved tremendous success in the past decades. For example, boron and phosphorus doping of Si modulates the dominant carrier type between p-type and n-type, serving as the backbone for the modern microelectronic technologies. Doped III-V semiconducting systems exhibit phenomenal optoelectronic properties. Magnesium doped gallium nitride plays an important role to build efficient blue light-emitting diode (LED), which won Nobel Prize in physics in 2014. The rise of two-dimensional (2D) materials sheds light on their potential in next generation electronic, optoelectronic, and quantum applications. These properties can further be controlled via doping of 2D materials, however, many challenges still remain in this field. Here, we present a rapid review on the recent achievements and challenges in the metastable and substitutional doping of 2D materials, followed by providing an outlook on integrating 2D materials into more advanced electronic architectures.

Percolation description of charge transport in amorphous oxide semiconductors

The charge transport mechanism in amorphous oxide semiconductors (AOS) is a matter of controversial debates. Most theoretical studies so far neglected the percolation nature of the phenomenon. In this article, a recipe for theoretical description of charge transport in AOSs is formulated using the percolation arguments. Comparison with the previous theoretical studies shows a superiority of the percolation approach. The results of the percolation theory are compared to experimental data obtained in various InGaZnO materials revealing parameters of the disorder potential in such AOS.

Percolation description of charge transport in the random barrier model applied to amorphous oxide semiconductors

Charge transport in amorphous oxide semiconductors is often described as the band transport affected by disorder in the form of random potential barriers (RB). Theoretical studies in the framework of this approach neglected so far the percolation nature of the phenomenon. In this article, a recipe for theoretical description of charge transport in the RB model is formulated using percolation arguments. Comparison with the results published so far evidences the superiority of the percolation approach.

Modelling and simulation of SEU in bulk Si and Ge SRAM

In this work, the random-walk drift-diffusion (RWDD) model has been coupled to a circuit simulator to investigate single event upsets (SEU) induced by alpha particles in SRAM cells with silicon or germanium as bulk material. The impact of semiconductor charge generation and transport properties on the SEU mechanisms is quantified and discussed in a first-order approach considering ideal materials and generic devices. Our results suggest that the radiation response of Ge-based SRAM should be similar to the one observed for Si-SRAM, the benefits of higher mobilities at circuit level for germanium offsetting the negative impact of a relatively low energy value for electron-pair creation on transient current pulse magnitudes.

Quantum localization and delocalization of charge carriers in organic semiconducting crystals

Charge carrier transport in organic semiconductors is at the heart of many revolutionary technologies ranging from organic transistors, light-emitting diodes, flexible displays and photovoltaic cells. Yet, the nature of charge carriers and their transport mechanism in these materials is still unclear. Here we show that by solving the time-dependent electronic Schrödinger equation coupled to nuclear motion for eight organic molecular crystals, the excess charge carrier forms a polaron delocalized over up to 10–20 molecules in the most conductive crystals. The polaron propagates through the crystal by diffusive jumps over several lattice spacings at a time during which it expands more than twice its size. Computed values for polaron size and charge mobility are in excellent agreement with experimental estimates and correlate very well with the recently proposed transient localization theory.

Donor–Acceptor‐Conjugated Polymer for High‐Performance Organic Field‐Effect Transistors: A Progress Report

AbstractPolymeric semiconductors have demonstrated great potential in the mass production of low‐cost, lightweight, flexible, and stretchable electronic devices, making them very attractive for commercial applications. Over the past three decades, remarkable progress has been made in donor–acceptor (D–A) polymer‐based field‐effect transistors, with their charge‐carrier mobility exceeding 10 cm2 V−1 s−1. Numerous molecular designs of D–A polymers have emerged and evolved along with progress in understanding the charge transport physics behind their high mobility. In this review, the current understanding of charge transport in polymeric semiconductors is covered along with significant features observed in high‐mobility D–A polymers, with a particular focus on polymeric microstructures. Subsequently, emerging molecular designs with further prospective improvements in charge‐carrier mobility are described. Moreover, the current issues and outlook for future generations of polymeric semiconductors are discussed.

Charge-particles transport in semiconductors characterized by a generalized Langevin equation with a fractional noise

From the view of intrinsic noise in semiconductors, the model of charge-particles transport in one-dimensional semiconductors described by a generalized Langevin equation (GLE) driven by a intrinsic fractional Gaussian noise is proposed in this paper. The analytical expression of the average charged particle velocity, the differential mobility, the conductivity of charge-particles, the two-time correlation function of the velocity fluctuation and the power spectral of total current density fluctuation have been derived. Furthermore, the model of motion for the charge-particle subjected to an electric field and a magnetic field governed by GLE is introduced. The expressions of mean values and variances of charged particle velocity, Fokker–Planck equation of charge-particle motion, the asymptotic behaviors of the relaxation functions have been obtained. Under the Hall effect, the Hall angle, the Hall coefficient, as well as the electrical resistivity also have been studied.

Interplay between in-plane and flexural phonons in electronic transport of two-dimensional semiconductors

Out-of-plane vibrations are considered as the dominant factor limiting the intrinsic carrier mobility of suspended two-dimensional materials at low carrier concentrations. Anharmonic coupling between in-plane and flexural phonon modes is usually excluded from the consideration. Here we present a theory for the electron-phonon scattering, in which the anharmonic coupling between acoustic phonons is systematically taken into account. Our theory is applied to the typical group V two-dimensional semiconductors: hexagonal phosphorus, arsenic, and antimony. We find that the role of the flexural modes is essentially suppressed by their coupling with in-plane modes. At dopings lower than 10$^{12}$ cm$^{-2}$ the mobility reduction does not exceed 30\%, being almost independent of the concentration. Our findings suggest that compared to in-plane phonons, flexural phonons are considerably less important in the electronic transport of two-dimensional semiconductors, even at low carrier concentrations.

Modulating the Charge Transport in 2D Semiconductors via Energy-Level Phototuning.

The controlled functionalization of semiconducting 2D materials (2DMs) with photoresponsive molecules enables the generation of novel hybrid structures as active components for the fabrication of high-performance multifunctional field-effect transistors (FETs) and memories. This study reports the realization of optically switchable FETs by decorating the surface of the semiconducting 2DMs such as WSe2 and black phosphorus with suitably designed diarylethene (DAE) molecules to modulate their electron and hole transport, respectively, without sacrificing their pristine electrical performance. The efficient and reversible photochemical isomerization of the DAEs between the open and the closed isomer, featuring different energy levels, makes it possible to generate photoswitchable charge trapping levels, resulting in the tuning of charge transport through the 2DMs by alternating illumination with UV and visible light. The device reveals excellent data-retention capacity combined with multiple and well-distinguished accessible current levels, paving the way for its use as an active element in multilevel memories.