Nonequilibrium Carrier Transport Research Articles

Fractional Model of Multiple Trapping with Charge Leakage: Transient Photoconductivity and Transit–Time Dispersion

The model of multiple trapping into energy-distributed states is a successful tool to describe the transport of nonequilibrium charge carriers in amorphous semiconductors. Under certain conditions, the model leads to anomalous diffusion equations that contain time fractional derivatives. From this perspective, the multiple-trapping model can be used to interpret fractional transport equations, formulate initial and boundary conditions for them, and to construct numerical methods for solving fractional kinetic equations. Here, we shortly review the application of fractional multiple-trapping equations to problems of transient photoconductivity relaxation and transit–time dispersion in the time-of-flight experiment and discuss the connection of the multiple-trapping model with generalized fractional kinetic equations. Different types of charge leakage are discussed. The tempered fractional relaxation is obtained for recombination via localized states and distributed order equations arise for the non-exponential density of states presented as a weighted mixture of exponential functions. Analytical solutions for photocurrent decay in transient photoconductivity and time-of-flight experiments are provided for several simplified situations.

Revealing charge carrier dynamics and transport in Te-doped GaAsSb and GaAsSbN nanowires by correlating ultrafast terahertz spectroscopy and optoelectronic characterization

Recent advances in the growth of III-V semiconductor nanowires (NWs) hold great promise for nanoscale optoelectronic device applications. It is established that a small amount of nitrogen (N) incorporation in III-V semiconductor NWs can effectively red-shift their wavelength of operation and tailor their electronic properties for specific applications. However, understanding the impact of N incorporation on non-equilibrium charge carrier dynamics and transport in semiconducting NWs is critical in achieving efficient semiconducting NW devices. In this work, ultrafast optical pump-terahertz probe spectroscopy has been used to study non-equilibrium carrier dynamics and transport in Te-doped GaAsSb and dilute nitride GaAsSbN NWs, with the goal of correlating these results with electrical characterization of their equilibrium photo-response under bias and low-frequency noise characteristics. Nitrogen incorporation in GaAsSb NWs led to a significant increase in the carrier scattering rate, resulting in a severe reduction in carrier mobility. Carrier recombination lifetimes of 33 ± 1 picoseconds (ps) and 147 ± 3 ps in GaAsSbN and GaAsSb NWs, respectively, were measured. The reduction in the carrier lifetime and photoinduced optical conductivities are due to the presence of N-induced defects, leading to deterioration in the electrical and optical characteristics of dilute nitride NWs relative to the non-nitride NWs. Finally, we observed a very fast rise time of ∼2 ps for both NW materials, directly impacting their potential use as high-speed photodetectors.

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During high-power optical picosecond pumping of the GaAs layer of the Alx Ga1−x As−GaAs−Alx Ga1−x As heterostruc- ture, stimulated picosecond emission arises in it. It has been experimentally revealed how, upon saturation of the emission gain, the maximum energy density of its spectral component and the time to reach this density depend on the photon energy, on the parameters of the amplification and relaxation of the component. It follows from these dependences that the indicated density and time are influenced by the slowing down of the transport of nonequilibrium carriers in the energy space. The slowdown is caused by the interaction of carriers with emission. It has been found that as the diameter of the active region approaches zero, the measured characteristic relaxation time of the component tends to the theoretical universal residual relaxation time prolonged due to the indicated transport slowdown.

Analysis of nonequilibrium phenomena on electric characteristics of advanced MOSFETs by using a quantum energy transport model

AbstractIn this paper, we have investigated nonequilibrium effects for advanced MOSFETs by using a device simulator with quantum energy transport (QET) model. The QET model allows to simulate nonequilibrium carrier transport as well as quantum confinement. The QET model includes the mobility model as a function of carrier temperature in order to consider the nonlocal effects. We have simulated advanced MOSFETs down to 20 nm gate length using the QET model. The QET model is compared with the quantum drift diffusion (QDD) model which includes a mobility model with local assumptions. It is found that the nonlocal mobility model is needed to simulate the advanced MOSFETs with less than 40 nm.

Influence of structural rigidity of Cu(II)/Mo(VI) complexes on the photoconductivity of film polymeric composites

Novel film polymeric composites based on the non-photoconducting polyvinyl butyral with heterometallic complexes (NH4)4n{[Cu(en)(H2O)][P2Mo5O23]}n·3.5nH2O (1) and (NH4)2n{[Cu(en)2][Cu(en)(H2O)][P2Mo5O23]}n·3nH2O (2) [en = ethylenediamine] have been prepared. Both compounds are similar and contain Strandberg anions [P2Mo5O23]6– linked to Cu-diamine species. The main structural difference between complexes is the presence of additional bridging [Cu(en)2] fragments in 2, which provide an extra connectivity between the neighboring polyoxometalates. However, the different saturations and relaxation times of the photoresponse, as well as different activation energies, are shown. The variability of the photoconducting properties can be explained by the influence of a chain rigidity on the transport of non-equilibrium charge carriers.

Modelling of nanoscale multi-gate transistors affected by atomistic interface roughness

Interface roughness scattering (IRS) is one of the major scattering mechanisms limiting the performance of non-planar multi-gate transistors, like Fin field-effect transistors (FETs). Here, two physical models (Ando’s and multi-sub-band) of electron scattering with the interface roughness induced potential are investigated using an in-house built 3D finite element ensemble Monte Carlo simulation toolbox including parameter-free 2D Schrödinger equation quantum correction that handles all relevant scattering mechanisms within highly non-equilibrium carrier transport. Moreover, we predict the effect of IRS on performance of FinFETs with realistic channel cross-section shapes with respect to the IRS correlation length (Λ) and RMS height (). The simulations of the n-type SOI FinFETs with the multi-sub-band IRS model shows its very strong effect on electron transport in the device channel compared to the Ando’s model. We have also found that the FinFETs are strongly affected by the IRS in the ON-region. The limiting effect of the IRS significantly increases as the Fin width is reduced. The FinFETs with channel orientation are affected more by the IRS than those with the crystal orientation. Finally, Λ and are shown to affect the device performance similarly. A change in values by 30% (Λ) or () results in an increase (decrease) of up to in the drive current.

The space charge limited current and huge linear magnetoresistance in silicon

Huge magnetoresistance in space charge regime attracts broad interest on non-equilibrium carrier transport under high electric field. However, the accurate fitting for the current-voltage curves from Ohmic to space charge regime under magnetic fields has not been achieved quantitatively. We conjecture that the localized intensive charge dynamic should be taken into consideration. Here, by introducing a field-dependent dielectric constant, for the first time, we successfully simulate the current-voltage curves of covalent crystal silicon wafers under different magnetic fields (0–1 Tesla). The simulation reveals that the optical phonon, instead of the acoustic phonon, plays a major role for the carriers transport under magnetic fields in space charge regime.

Direct-current triboelectricity generation by a sliding Schottky nanocontact on MoS2 multilayers.

The direct conversion of mechanical energy into electricity by nanomaterial-based devices offers potential for green energy harvesting 1-3 . A conventional triboelectric nanogenerator converts frictional energy into electricity by producing alternating current (a.c.) triboelectricity. However, this approach is limited by low current density and the need for rectification 2 . Here, we show that continuous direct-current (d.c.) with a maximum density of 106 A m-2 can be directly generated by a sliding Schottky nanocontact without the application of an external voltage. We demonstrate this by sliding a conductive-atomic force microscope tip on a thin film of molybdenum disulfide (MoS2). Finite element simulation reveals that the anomalously high current density can be attributed to the non-equilibrium carrier transport phenomenon enhanced by the strong local electrical field (105-106 V m-2) at the conductive nanoscale tip 4 . We hypothesize that the charge transport may be induced by electronic excitation under friction, and the nanoscale current-voltage spectra analysis indicates that the rectifying Schottky barrier at the tip-sample interface plays a critical role in efficient d.c. energy harvesting. This concept is scalable when combined with microfabricated or contact surface modified electrodes, which makes it promising for efficient d.c. triboelectricity generation.

20th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON 20)

The 20th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON 20) was held at the Hyatt Regency Hotel and Conference Center in downtown Buffalo, New York, USA from July 17–21, 2017. The meeting serves as the premier forum for researchers working on the study of nonequilibrium phenomena in semiconductors, from all parts of the globe, and was organized locally by faculty at the University at Buffalo and the University of Rochester.EDISON is an international conference series with biennial meetings that cover recent progress in the field of electron dynamics in solid-state materials and devices. Initially established with the title Hot Carriers in Semiconductors (HCIS), the first meeting took place in Modena, Italy in 1973. The name was changed to International Conference on Nonequilibrium Carrier Dynamics in Semiconductors at the 1997 Berlin meeting, and again at the 2009 meeting in Montpellier to the current form. Since then, EDISON conferences have been held in Santa Barbara, USA (2011); Matsue, Japan (2013); Salamanca, Spain (2015); and Buffalo, USA (2017).The EDISON 20 program included 12 invited talks, 50 oral contributions, 116 accepted abstracts, and one special seminar. In total, 152 participants from 22 countries gathered to share and exchange ideas and recent results. Approximately 45% of the participants were from North America, 30% from Europe, and 25% from the Asia Pacific region. Fully 30% of the participants were students, and there was a very good balance between experimental and theoretical approaches. Topics with the most contributions were: Electronic and optical properties of graphene and other 2D materials; Nonequilibrium carrier transport; and Terahertz phenomena in semiconductor materials and devices.I thank all the members of the Program Committee and the International Advisory Committee for their valuable input, as well as Professor Jonathan Bird and Alex Desha for their many hours devoted to organizing a successful EDISON conference. Also critical to the success of the conference is financial support, so I thank the School of Engineering and Applied Sciences, the Vice President for Research and Economic Development, the Department of Physics, and the Department of Electrical Engineering at the University at Buffalo, as well as the U.S. Department of Energy, Kurt J. Lesker Company, and the Hajim School of Engineering & Applied Sciences at the University of Rochester for their sponsorship of EDISON 20. Finally, I thank all the scientists who contributed to the preparation and peer-review of these proceedings.Erik Einarsson, EditorEDISON 20 Publications ChairUniversity at Buffalo

Unique characteristics of nonequilibrium carrier transport dynamics in an undoped GaAs/n-type GaAs epitaxial structure

We have investigated nonequilibrium carrier dynamics in an undoped GaAs/n-type GaAs epitaxial structure at room temperature using reflection-type pump–probe spectroscopy at different pump photon energies and Monte Carlo simulation. It was found that the transport process of photogenerated electrons in the undoped layer is characterized by the valance between the quasiballistic motion passing through the undoped layer and the intervalley scattering from the Γ valley to the L one as a function of the excess energy of photogenerated electrons. The Γ–L scattering component exhibits threshold-like appearance and then gradually increases with increasing excess energy.

High-Current Gain Two-Dimensional MoS2-Base Hot-Electron Transistors

The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ∼ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications.

Photovoltaic properties of films of polyvinyl alcohol-xanthene dye composites

It has been found that films of polymer composites based on polyvinyl alcohol doped with a xanthene dye possess photovoltaic properties, and the photovoltaic response varies little at the dye concentration greater than 10 wt % with respect to the weight of the polymer. The photovoltaic effect is due to photogeneration of charge carriers in the dye aggregates and transport of nonequilibrium charge carriers among them. It is concluded that, in order to reduce the production cost of photoactive media for photovoltaic cells, it is possible to use cheap polymers with a low concentration of organic dyes.

Coherent Control of Nanoscale Ballistic Currents in Transition Metal Dichalcogenide ReS2.

Transition metal dichalcogenides are predicted to outperform traditional semiconductors in ballistic devices with nanoscale channel lengths. So far, experimental studies on charge transport in transition metal dichalcogenides are limited to the diffusive regime. Here we show, using ReS2 as an example, all-optical injection, detection, and coherent control of ballistic currents. By utilizing quantum interference between one-photon and two-photon interband transition pathways, ballistic currents are injected in ReS2 thin film samples by a pair of femtosecond laser pulses. We find that the current decays on an ultrafast time scale, resulting in an electron transport of only a fraction of one nanometer. Following the relaxation of the initially injected momentum, backward motion of the electrons for about 1 ps is observed, driven by the Coulomb force from the oppositely moved holes. We also show that the injected current can be controlled by the phase of the laser pulses. These results demonstrate a new platform to study ballistic transport of nonequilibrium carriers in transition metal dichalcogenides.

MBE growth, structural and transport properties of alternately-strained ZnSSe/CdSe superlattices with effective band-gap 2.5–2.6 eV

Short-period ZnSySe1−y/CdSe superlattices (SLs) with an effective energy gap Eg=2.5–2.6eV (T=300K) are grown by molecular beam epitaxy pseudomorphically on GaAs (001), and their structural properties are studied by using X-ray diffraction (XRD) and transmission electron microscopy. Both sulfur content and thickness of the ZnSySe1−y SL barriers have been determined via analysis of the XRD curves, taking account of the thickness of the CdSe SL layers estimated independently from the low-temperature (T=77K) PL spectra of the single CdSe quantum dot (QD) layer (nominal thickness of 2.8 monolayer) embedded into each SL. The evaluated SL parameters are shown to be in good agreement with the intended ones. The efficient non-equilibrium carrier transport in the SLs along the growth direction at 300K is demonstrated by photoluminescence (PL) spectroscopy through the relative temperature variation of the intensities of the PL peaks originated from the buried CdSe/ZnSe QD layers and the ZnSSe/CdSe SLs.

Band filling effects on temperature performance of intermediate band quantum wire solar cells

Detailed studies of solar cell efficiency as a function of temperature were performed for quantum wire intermediate band solar cells grown on the (311)A plane. A remotely doped one-dimensional intermediate band made of self-assembled In0.4Ga0.6As quantum wires was compared to an undoped intermediate band and a reference p-i-n GaAs sample. These studies indicate that the efficiencies of these solar cells depend on the population of the one-dimensional band by equilibrium free carriers. A change in this population by free electrons under various temperatures affects absorption and carrier transport of non-equilibrium carriers generated by incident light. This results in different efficiencies for both the doped and undoped intermediate band solar cells in comparison with the reference GaAs p-i-n solar cell device.

The nature of the plateau on the time-of-flight curves in molecularly doped polymers

The transient current curves for a polar molecularly doped polymer were compared for the nearsurface and bulk generation of charge carriers. The horizontal plateau on the first curve was expected to transform into a tilted straight line under the quasiequilibrium transport conditions, but it did not. Instead, for bulk generation, the curve was a hyperbola, on which the slope decreased almost sevenfold during the existence of the plateau. This behavior points to nonequilibrium charge carrier transport. The appearance of a plateau in our case is explained by the effect of the defective surface layer, as indicated in our previous communications.

Effect of temperature, electric and magnetic field on spin relaxation in single layer graphene: A Monte Carlo simulation study

In this article, we employ the semiclassical Monte Carlo approach to study the spin polarized electron transport in single layer graphene channel. The Monte Carlo method can treat non-equilibrium carrier transport and effects of external electric and magnetic fields on carrier transport can be incorporated in the formalism. Graphene is the ideal material for spintronics application due to very low Spin Orbit Interaction. Spin relaxation in graphene is caused by D'yakonov-Perel (DP) relaxation and Elliott-Yafet (EY) relaxation. We study effect of electron electron scattering, temperature, magnetic field and driving electric field on spin relaxation length in single layer graphene. We have considered injection polarization along z-direction which is perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. This theoretical investigation is particularly important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene.

Characteristics of ultrafast optical responses originating from non-equilibrium carrier transport in undoped GaAs/n-type GaAs epitaxial structures

We have investigated the characteristics of ultrafast optical responses originating from a carrier transport process in undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structures with the use of a reflection-type pump-probe technique at room temperature. The built-in electric field in the i-GaAs top layer, whose strength is controlled by its thickness d, accelerates the transit of photogenerated carriers through the i-GaAs layer. We systematically observed that the decay time of a carrier-induced reflectivity change shortens with an increase in built-in electric field strength resulting from a decrease in d: 6.1, 12, and 28 kV/cm for d = 1200, 500, and 200 nm, respectively. In the i-GaAs/n-GaAs sample with d = 200 nm, which has the highest built-in electric field strength, the decay time is much shorter than the oscillation period of longitudinal optical (LO) phonon. From the spectrally resolved detection of the reflected light, it was found that the energy relaxation of the photogenerated carriers by the LO-phonon scattering hardly occurs in the i-GaAs layer, which indicates a quasiballistic transport. This finding demonstrates that the i-GaAs/n-GaAs structure with the non-equilibrium carrier transport process is useful for ultrafast optical applications.

Field injection of low-energy electrons into the ZnSe/CdSe/ZnSe heterostructure with the use of an ultra-high-vacuum tunneling microscope

1126 In lasers with electronbeam pumping, the semi� conductor crystal transforms the energy of the acceler� ated electron beam into the laser emission with a wavelength corresponding to the band gap of the used semiconductor. Pumping with the fast electron beam is a universal method of lasing in semiconductor mate� rials with any band gap Eg and any starting resistivity. Such lasers make it possible to generate optical pulses with a power of several tens of megawatts in a broad spectral range from the midIR to nearUV range. The main advantage of the lasers with electron Abstract—The field emission injection of lowenergy electrons ( Ee ≈ 10 eV) into the ZnSe/CdSe/ZnSe het� erostructure has been considered. The probe of the ultrahighvacuum tunneling microscope has been used as a field emitter. It has been shown that the energy of injected electrons is sufficient for impact ionization in ZnSe. The impact ionization creates a high concentration of nonequilibrium carriers in the nearsurface ZnSe layer. The transport of nonequilibrium carriers in the heterostructure under study has been simulated. The electric field of the nearsurface space charge an d surface recombination have been taken into account. The calculation has demonstrated that filling the active region of CdSe with nonequilibrium carriers is highly

Structural and optical optimization of ZnSe‐based laser heterostructures with graded index waveguide

AbstractWe report on structural and photoluminescence (PL) studies of wide gap II‐VI laser heterostructures involving the graded index waveguide (GIW) based on short period Zn(Mg)SSe/ZnSe superlattices (SLs) and the active region comprising single or multiple electronically‐coupled CdSe/ZnSe QD sheets. Precise compensation of elastic stresses in the SL waveguide and optimization of the ZnSe/GaAs initial growth stage have resulted in good crystalline quality of the laser structures and reduction of the extended defect density down to 104 cm‐2. Express monitoring of the defect density by using the photoluminescence microscope was supported by transmission electron microscopy studies. PL data have demonstrated efficient transport of nonequilibrium carriers through the GIW SLs to the active region (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)