Carrier Drift Velocity Research Articles

Full-band Monte Carlo analysis of strain effects on carrier transport in GaN

The effects of strain on the carrier transport in gallium nitride (GaN) are investigated using a full-band Monte Carlo method combined with an empirical tight-binding method. The impacts on the carrier mobility, carrier drift velocity, and breakdown characteristics are discussed. Compressive uniaxial or tensile biaxial strain is beneficial for achieving higher hole mobility in vertical GaN devices due to the light-hole band being lifted above the heavy-hole band. Analysis of the breakdown phenomena indicates that strain does not degrade the breakdown characteristics in terms of its effect on the band structure.

Terahertz Generation through Coherent Excitation of Slow Surface Waves in an Array of Carbon Nanotubes

In this paper, we present a scheme for generating terahertz (THz) radiation using an array of parallel double-walled carbon nanotubes (DWCNTs) subjected to a direct current (DC). The longitudinal surface plasmon polaritons (SPPs) in the DWCNTs are coherently excited by two near-infrared laser beams with slightly different frequencies. Through numerical methods, we investigate the spectral characteristics of the SPPs in the presence of a DC current in the nanotubes. We identify high-quality plasmonic modes with a slowdown factor exceeding 300 in the terahertz frequency region. The amplification of these slow SPP modes is facilitated by the DC current in the DWCNTs, fulfilling a synchronism condition. This condition ensures that the phase velocity of the SPPs is closely matched to the drift velocity of the charge carriers, allowing for an efficient energy exchange between the current and the surface electromagnetic wave. The high-frequency currents on the nanotube walls in the DWCNT array enable the emission of THz radiation into the far field, owing to an antenna effect.

High frequency amplification of acoustic phonons in fluorine-doped single-walled carbon nanotubes

Herein, we report on a strong high-frequency induced amplification of coherent acoustic phonons in a non-degenerate fluorine-doped single-walled carbon nanotubes (FSWCNTs) by utilising a tractable analytical approach in the hypersound regime, ql≫1 (where q is the acoustic wavenumber and l is the carrier mean free path). The acoustoelectric gain obtained is highly nonlinear and is due to stimulated Cerenkov phonon emission by electrically driven carriers undergoing intraminiband transport and capable of performing Bloch oscillations. The transport process causes the carriers to undergo population inversion leading to intraminiband phonon-assisted processes. The generation rate (phonon emission) is expansive and surpasses phonon losses. The threshold field (Eo), at which attenuation switches over to amplification (gain) depends on the FSWCNT parameters (Δs&Δz), carrier drift velocity (vd=μEo), sound velocity (vs) and the ratio ζs,z. This result has potential for intense sources of reasonable acoustic phonons in the sub-THz regime and is vital for the generation of SASER (sound amplification by stimulated emission of acoustic radiation). The amplified phonons also have THz frequencies with wavelengths in the nanometer range, and depends on high spatial parameters which has potential applications for phonon filters and spectrometers.

Terahertz amplification and lasing by using transverse electric modes in a two-layer-graphene-dielectric waveguide structure with direct current

We study for the first time the interaction between the waveguide modes of graphene structure and freely propagating terahertz (THz) electromagnetic waves (this interaction takes place within the light cone). We revealed a new and rather unexpected physical phenomenon by showing that freely incident THz electromagnetic waves can resonate with the surface transverse electric (TE) modes of the graphene waveguide in virtue of these modes having their dispersions in the vicinity of the light cone. The dispersion and amplification of surface TE modes in a dielectric waveguide covered with two graphene layers biased by direct current (DC), as well the amplification and lasing of incident THz wave by excitation of TE mode resonances, are investigated. The DC flows perpendicular to the direction of the surface wave propagation and creates the capacitive complex conductivity of graphene at THz frequencies, which is necessary for the existence of surface TE modes in graphene. The real part of graphene conductivity can be negative at THz frequencies due to DC in graphene which leads to amplification and lasing of THz radiation. Such structure can be of great practical importance because an external THz wave can be amplified or generated in lasing process without using special coupling elements commonly needed for ensuring the interaction between external THz wave and surface waveguide modes. The use of a two-layer graphene structure makes it possible to reduce the charge–carrier drift velocity required for reaching the lasing threshold at those resonances, as compared to a structure with a single graphene layer.

Probing drift velocity dispersion in MAPbI3 photovoltaic cells with nonlinear photocurrent spectroscopy.

Conventional time-of-flight (TOF) measurements yield charge carrier mobilities in photovoltaic cells with time resolution limited by the RC time constant of the device, which is on the order of 0.1-1 µs for the systems targeted in the present work. We have recently developed an alternate TOF method, termed nonlinear photocurrent spectroscopy (NLPC), in which carrier drift velocities are determined with picosecond time resolution by applying a pair of laser pulses to a device with an experimentally controlled delay time. In this technique, carriers photoexcited by the first laser pulse are "probed" by way of recombination processes involving carriers associated with the second laser pulse. Here, we report NLPC measurements conducted with a simplified experimental apparatus in which synchronized 40ps diode lasers enable delay times up to 100 µs at 5kHz repetition rates. Carrier mobilities of ∼0.025cm2/V/s are determined for MAPbI3 photovoltaic cells with active layer thicknesses of 240 and 460nm using this instrument. Our experiments and model calculations suggest that the nonlinear response of the photocurrent weakens as the carrier densities photoexcited by the first laser pulse trap and broaden while traversing the active layer of a device. Based on this aspect of the signal generation mechanism, experiments conducted with co-propagating and counter-propagating laser beam geometries are leveraged to determine a 60nm length scale of drift velocity dispersion in MAPbI3 films. Contributions from localized states induced by thermal fluctuations are consistent with drift velocity dispersion on this length scale.

Study of Annihilation Photon Pair Coincidence Time Resolution Using Prompt Photon Emissions in New Perovskite Bulk Crystals.

Semiconductor-based radiation detectors can typically achieve better energy and spatial resolution when compared to scintillator-based detectors. However, if used for positron emission tomography (PET), semiconductor-based detectors normally cannot achieve excellent coincidence time resolution (CTR), due to the relatively slow charge carrier collection time limited by the carrier drift velocity. If we can collect prompt photons emitted from certain semiconductor materials, there are possibilities that the CTR can be greatly improved, and time-of-flight (ToF) capability can be achieved. In this paper, we studied the prompt photon emission (mainly Cherenkov luminescence) property and fast timing capability of cesium lead chloride (CsPbCl3) and cesium lead bromide (CsPbBr3), which are two new perovskite semiconductor materials. We also compared their performance with thallium bromide (TlBr), another semiconductor material that has already been studied for timing using its Cherenkov emissions. We performed coincidence measurements using silicon photomultipliers (SiPMs), and the full-width-at-half-maximum (FWHM) CTR acquired between a semiconductor sample crystal and a reference lutetium-yttrium oxyorthosilicate (LYSO) crystal (both with dimensions of 3 × 3 × 3 mm3) is 248 ± 8 ps for CsPbCl3, 440 ± 31 ps for CsPbBr3, and 343 ± 16 ps for TlBr. Deconvolving the contribution to CTR from the reference LYSO crystal (around 100 ps) and then multiplying by the square root of 2, the estimated CTR between two of the same semiconductor crystals was calculated as 324 ± 10 ps for CsPbCl3, 606 ± 43 ps for CsPbBr3 and 464 ± 22 ps for TlBr. This ToF capable CTR performance combined with an easily scalable crystal growth process, low cost and toxicity, as well as good energy resolution lead us to the conclusion that new perovskite materials such as CsPbCl3 and CsPbBr3 could be excellent candidates as PET detector materials.

Terahertz transverse electric modes in graphene with DC current in hydrodynamic regime

The dispersion, excitation, and amplification of electromagnetic transverse electric (TE) modes at terahertz (THz) frequencies in graphene in the hydrodynamic (HD) regime, with a direct electric current flowing perpendicular to the TE mode wavevector, were theoretically investigated. The expression for the nonlocal HD conductivity of graphene with a direct electric current flowing perpendicular to the TE mode wavevector was derived. The direct electric current in graphene leads to the capacitive nature of the graphene HD conductivity at THz frequencies, which makes TE modes exist in this frequency range. The excitation of TE modes in graphene by an incident THz wave was modeled for the attenuated total reflection geometry. A new physical mechanism of TE mode amplification in graphene effective for a low value of carrier drift velocity was predicted. THz lasing regimes with TE modes in graphene structure with direct electric current were found. The results of this work can be used to create miniature technologically feasible sources and amplifiers of THz radiation.

Graphene\u2019s non-equilibrium fermions reveal Doppler-shifted magnetophonon resonances accompanied by Mach supersonic and Landau velocity effects

Oscillatory magnetoresistance measurements on graphene have revealed a wealth of novel physics. These phenomena are typically studied at low currents. At high currents, electrons are driven far from equilibrium with the atomic lattice vibrations so that their kinetic energy can exceed the thermal energy of the phonons. Here, we report three non-equilibrium phenomena in monolayer graphene at high currents: (i) a “Doppler-like” shift and splitting of the frequencies of the transverse acoustic (TA) phonons emitted when the electrons undergo inter-Landau level (LL) transitions; (ii) an intra-LL Mach effect with the emission of TA phonons when the electrons approach supersonic speed, and (iii) the onset of elastic inter-LL transitions at a critical carrier drift velocity, analogous to the superfluid Landau velocity. All three quantum phenomena can be unified in a single resonance equation. They offer avenues for research on out-of-equilibrium phenomena in other two-dimensional fermion systems.

Ultrafast dynamics of carriers and phonons of photoinjected double-plasma in aluminium nitride

Aluminum nitride is attracting great interest of the industry and scientific community due to its interesting properties. In this paper is performed a theoretical study on the ultrafast transient transport properties of photoinjected carriers in wurtzite AlN subjected to electric fields up to 80 kV/cm. For this, the Nonequilibrium Statistical Operator Method was used. The evolution towards the steady state of drift velocity of carriers (electrons and holes) and nonequilibrium temperature (carriers and phonons) subpicosecond scale were determined.

Terahertz plasmon amplification in a double-layer graphene structure with direct electric current in hydrodynamic regime

The possibility of excitation and amplification of terahertz plasmons in a structure containing a layer of graphene with charge-carrier drift and a layer of graphene without carrier drift is theoretically investigated. The plasmon excitation in the graphene structure by an incident terahertz electromagnetic wave is calculated in the attenuated total reflection geometry. It is shown that, in graphene with charge-carrier drift, it is possible to achieve negative values of the real part of graphene conductivity for the phase velocity of plasmons exceeding the charge carrier's drift velocities, which indicates a non-Cherenkov amplification of terahertz plasmons in the double-layer graphene structure for practically achievable direct electric current values. Terahertz amplification originates due to the variation of the carrier mass density in graphene with terahertz electric field. In the case of weak deceleration of terahertz wave incident onto graphene structure, the value of the negative conductivity of graphene does not depend on the direction of the direct current in graphene (only the codirectional and counterdirectional currents with respect to the direction of plasmon propagation are considered). This is due to the insignificant spatial dispersion of the hydrodynamic conductivity of graphene in the case of weak deceleration of terahertz waves. The results of this work can be used to create compact terahertz radiation amplifiers operating at room temperature.

Hot carrier dynamics of photoinjected plasma in indium nitride

A theoretical study using the nonequilibrium statistical operator method is performed to investigate the ultrafast transient transport properties of photoexcited carriers in wurtzite InN subjected to electric fields up to 20 kV/cm. The evolution toward the steady state of the drift velocity of carriers (electrons and holes) and nonequilibrium temperature (carriers and phonons) is determined. The transient regime occurs at the subpicosecond scale with maxima and minima in the hole and electron drift velocities.

Room temperature negative differential resistance in clay-graphite paper transistors

Paper electronics have been fast developed in recent years due to their degradability and recyclability; however, most of them are barely equipped with logic functions, impeding them from becoming autonomous on-paper systems. To provide a working unit for logic implementations, a paper field-effect transistor (FET) with a clay-graphite channel that demonstrated negative differential resistance (NDR) is fabricated in this study. The device exhibited a p-type semiconductor characteristic with an optical bandgap of ∼1.97 eV according to photoluminescence analysis, resulting from considerable defective graphite mixed with clay in the channel. Considering that the NDR behavior is theoretically predicted to be tunable by the contact resistance, we fabricated FETs with different contact resistances and observed that the NDR behavior occurred in devices with relatively lower contact resistance. The NDR effect is also tunable by varying the gate voltage, which matches well with simulation results. The possible mechanism for the NDR behavior is the decrease in carrier drift velocity induced by band-to-band tunneling, associated with impurities-induced trapping states and gate-induced confined states. This work provides a promising and easy pathway to develop digital components using paper electronics in the near future.

Усиление терагерцовых электромагнитных волн в структуре с двумя слоями графена при протекании постоянного электрического тока: гидродинамическое приближение

The amplification of electromagnetic terahertz radiation in a structure with two layers of hydrodynamic graphene with a direct electric current is studied theoretically. The hydrodynamic conductivity of graphene is investigated. It is shown that the real part of the graphene conductivity can be negative in the terahertz frequency range at the drift velocities of charge carriers in graphene that are lower than the phase velocity of the electromagnetic wave. For small wavevectors of a terahertz wave incident on a graphene structure, the spatial dispersion insignificantly contributes to the hydrodynamic graphene conductivity. Because of this, the amplification efficiency does not depend on the direction of currents in each of the graphene layers. It is shown that graphene with direct electric current can be used to create THz amplifiers operating at room temperature.

Resonant amplification of surface plasmon polaritons with an electric current in a single-walled carbon nanotube lying on a spatially modulated substrate

We discuss the possibility of using a direct electric current pump in order to amplify a mid-infrared surface plasmon polariton (SPP) in a single-walled carbon nanotube (CNT) deposited on a substrate. Amplification is achieved providing that the phase velocity of the SPP and the drift velocity of the charge carriers in the nanotube are matched. Calculating the amplification characteristics involves numerical simulations that allow to account for the influence of the substrate on the dispersion and inhomogeneity of the plasmon mode field distribution across the CNT. The spectral behaviour of an amplified SPP is shown to exhibit a high sensitivity to variations of the refractive index of the substrate supporting the CNT. We propose a design for the resonant amplification of SPPs with a DC current pump, in which a distributed resonator is obtained with a periodical modulation of the refractive index of the substrate.

The Effect of the Carrier Drift Velocity Saturation in High-Power Semiconductor Lasers at Ultrahigh Drive Currents

Experimental light-current characteristics of high-power semiconductor lasers operating at ultrahigh drive currents have been studied in terms of numerical models taking into account the effect of the carrier drift velocity saturation. It was shown that the simplest diffusion-drift (DD) model underestimates the optical loss at high drive currents, which leads to overestimation of the peak optical power when compared with the experimental results. The inclusion of the carrier drift velocity saturation effect into the DD model results in that a strong-electric-field domain is formed at the interface with the active region on the side of the p-emitter already at low drive currents. The amplitude of this field substantially grows as the current increases further, which restricts application of this model to description of high-power semiconductor lasers at high drive currents. With the nonlocal energy-balance model used to describe the carrier transport in the laser heterostructure, it becomes possible to take into account the effect of carrier heating by the electric field. As a result of the carrier heating, the drift velocity decreases and the concentration of excess carriers in the waveguide layer grows, compared with the simplest DD model. The inclusion of the additional internal optical loss mechanism obtained in the energy-balance model provided a satisfactory coincidence of the calculated light-current characteristics with the experimental dependences.

Room-Temperature Amplification of Terahertz Radiation by Grating-Gate Graphene Structures

We report on experimental studies of terahertz (THz) radiation transmission through grating-gate graphene-channel transistor nanostructures and demonstrate room temperature THz radiation amplification stimulated by current-driven plasmon excitations. Specifically, with increase of the direct current (dc) under periodic charge density modulation, we observe a strong red shift of the resonant THz plasmon absorption, its complete bleaching, followed by the amplification and blue shift of the resonant plasmon frequency. Our results are, to the best of our knowledge, the first experimental observation of energy transfer from dc current to plasmons leading to THz amplification. We present a simple model allowing for the phenomenological description of the observed amplification phenomena. This model shows that in the presence of dc current the radiation-induced correction to dissipation is sensitive to the phase shift between THz oscillations of carrier density and drift velocity, and with increase of the current becomes negative, leading to amplification. The experimental results of this work as all obtained at room temperature, pave the way towards the new 2D plasmons based, voltage tuneable THz radiation amplifiers.

Hysteric photo-conduction and negative differential resistance in cesium lead bromide

For the perovskite halides, current voltage hysteresis is the biggest puzzle to be solved before industrialization in spite of promising features for future photo-voltaic applications. All the possible causes, from the classical (viz., morphology, defects, slow transient capacitance, etc.) to quantum (viz., spin–orbit interaction) ones, are investigated. However, its origin is still under debate, as possibilities showed some ambiguity on the science known until now. In the present work, we have studied the time dependent photo-conduction behavior of CsPbBr3 in continuous illumination of AM 1.5 G Sun light for 3 h. We observed a negative differential resistance for a forward scanned current–voltage curve in AM 1.5 G Sun light. Our investigations suggest that the photo-current voltage hysteresis is primarily affected by the thermionic-field emission, which slows down the drift velocity of hot charge carriers with field applications. This study will further lead the scientific community to investigate whether this slowdown in drift velocity is related to the Gunn effect or the Rashba effect.

Influence of Hot-Carriers on the On-State Resistance in Si and GaAs Photoconductive Semiconductor Switches Working at Long Pulse Width**Supported by the Rector’s Fund of China Academy of Engineering Physics (Grant No. YZJJLX2016002), and the National Natural Science Foundation of China (Grant Nos. 61504127 and U1530128).

We demonstrate that the transport of hot carriers may result in the phenomenon where an oscillated output current appears at the waveforms in a high-power photoconductive semiconductor switch (PCSS) working at long pulse width when the laser disappears or the electric field changes. The variational laser and electric field will affect the scattering rates of hot carriers and crystal lattice in high-power PCSS, and the drift velocity of hot carriers and also the on-state resistance will be changed. The present result is important for reducing the on-state resistance and improving the output characteristics of high-power Si/GaAs PCSS.

The effect of bias and frequency on amplitude to phase conversion of photodiodes

Fluctuation of the optical power incident on a photodiode will cause phase variation of the electrical signal. This phenomenon is known as amplitude-to-phase (AM-to-PM) conversion. The effects of bias voltage and test frequency on AM-to-PM conversion are described systematically in this paper. A larger null point of AM-to-PM coefficient and smaller phase variation have been observed when bias voltage increases or test frequency decreases. The variation of transit time which is caused by different carrier velocity is the main reason of the phase variation. The influence mechanism of bias on AM-to-PM conversion has been explained in detail by giving the relationship between the carrier drift velocity and the internal electric field related to bias. When the internal voltage exceeds a certain value, the transit time will be almost unchanged and the phase variation is small because of saturated drift velocity. These results can provide guidance to operate the optoelectronic link under optimal conditions.

Photothermal conversion assisted photocatalytic hydrogen evolution from amorphous carbon nitrogen nanosheets with nitrogen vacancies.

Amorphous carbon nitrogen (a-CN) has attracted a lot of attention due to its unique properties, different from those of its crystal form. Here, we demonstrate a near-infrared (NIR) photothermal conversion assisted photocatalytic hydrogen evolution from a-CN with nitrogen vacancies (a-CNN) nanosheets. Experiments suggest that sp2 hybridized C[double bond, length as m-dash]C structures can be created in a-CNN. These structures, just like small islands, disperse on a-CNN, leading to fluorescence quenching and a superior vis-NIR light absorption. Meanwhile, these structures, like "hot islands", can generate a stronger NIR photothermal conversion. A series of in situ characterization techniques are developed to clarify the detailed mechanism of photothermal conversion assisted photocatalytic hydrogen evolution. It is found that photothermal conversion can not only accelerate the drift velocity of the photo-induced carrier, but also increase the carrier concentration, which finally promotes the photocatalytic hydrogen evolution. Due to photothermal conversion assistance, the hydrogen production rate of a-CNN nanosheets is promoted to 3.1 mmol g-1 h-1 compared to 0.71 mmol g-1 h-1 for a-CN, in which the NIR photothermal conversion is proven to contribute a 16% promotion to the hydrogen production. These findings suggest that creating an NIR photothermal conversion of photocatalysts by constructing "hot islands" can greatly promote photocatalytic hydrogen production.