Negative Differential Mobility Research Articles

Dynamics of field-driven domain-wall propagation in ferromagnetic nanowires

Ferromagnetic nanowires are likely to play an important role in future spintronic devices. Magnetic domain walls, which separate regions of opposing magnetization in a nanowire, can be manipulated and used to encode information for storage or to perform logic operations. Owing to their reduced size and dimensionality, the characterization of domain-wall motion is an important problem. To compete with other technologies, high-speed operation, and hence fast wall propagation, is essential. However, the domain-wall dynamics in nanowires has only been investigated in the last five years and some results indicate a drastic slowing down of wall motion in higher magnetic fields. Here we show that the velocity-field characteristic of a domain wall in a nanowire shows two linear regimes, with the wall mobility at high fields reduced tenfold from that at low fields. The transition is marked by a region of negative differential mobility and highly irregular wall motion. These results are in accord with theoretical predictions that, above a threshold field, uniform wall movement gives way to turbulent wall motion, leading to a substantial drop in wall mobility. Our results help resolve contradictory reports of wall propagation velocities in laterally confined geometries, and underscore the importance of understanding and enhancing the breakdown field for practical applications.

Monte Carlo study of terahertz generation from streaming distribution of two-dimensional electrons in a GaN quantum well

We study terahertz (THz) generation from anisotropic streaming distribution of a two-dimensional electron gas (2DEG) in a GaN/AlGaN quantum well using the ensemble Monte Carlo method. Under proper electric fields, electrons do not have enough energy for intersubband and intervalley transitions. They make nearly cyclic motion in momentum space within the first subband, which gives rise to a spindle-shaped electron distribution. This effect is improved due to large polar optical phonon energy, strong electron–phonon interaction and much sharper increase of the polar optical phonon emission rate in GaN 2DEG. We demonstrate that electrons show dynamic negative differential mobility at the transit-time frequency and its harmonics at low temperatures due to this cyclic motion. The resonant frequency can be tuned by simply varying the applied electric field. We may thus use this effect to develop voltage-tunable THz masers.

Ensemble Monte Carlo transport simulations for semiconducting carbon nanotubes

We report on ensemble Monte Carlo transport simulations for semiconducting, single-wall, zigzag carbon nanotubes. The basis for the Monte Carlo simulations is provided by electronic structure calculations within the framework of a simple tight-binding model that takes the effect of the tube curvature on the band structure into account. The principal scattering mechanisms considered are due to the electron–phonon interactions involving longitudinal acoustic and optical phonons. Using ensemble Monte Carlo simulations, the steady-state and transient characteristics are explored. The steady-state velocity saturates due to optical-phonon scattering, and negative differential mobility is obtained for large electric fields. The results also show interesting transient phenomena that are caused by the limited phase space of these dynamically one-dimensional structures.

Monte Carlo Calculation of High Frequency Mobility and Diffusion Noise in Nitride-Based Semiconductors

Monte Carlo simulations of high-field transport in semiconductor nitrides, GaN and InN, is used to calculate the velocity-field characteristics and the high-frequency behavior of the differential mobility, spectral density of velocity fluctuations, and noise temperature. It is found that due to very short relaxation time scales of nitrides, the characteristic frequencies associated with extrema and cutoff decay of the negative differential mobility, etc. are shifted to higher frequency range with respect to the case of standard A3B5 compounds. This property is favorable for applications of nitrides in the THz frequency range.

Anomalous mobility of Brownian particles in a tilted symmetric sawtooth potential

Overdamped motion of Brownian particles in a 1D periodic system with a simple symmetric sawtooth potential subjected to both unbiased thermal noise and spatially nonhomogeneous three-level colored noise is considered analytically. Upon application of a tilting force the particles exhibit anomalous transport properties, namely, absolute negative mobility, negative differential mobility, and the phenomenon of hypersensitive differential response. It is established that the mobility (differential mobility included) depends nonmonotonically on the parameters (switching rate, amplitude, and temperature) of nonequilibrium and thermal noises. The necessary conditions for various anomalous transport properties are found.

Additional phonon modes and close satellite valleys crucialfor electron transport in hexagonal gallium nitride

Conventional models of electron transport in hexagonal GaN crystals predicting electron drift velocity peak value up to 3.2×107cm∕s at 140–220kV∕cm and a pronounced negative differential mobility at higher fields are revised. The new model is suggested accounting for the additional low-energy optical phonon modes (∼26meV) and the satellite valley location close (400meV) to the conduction band bottom. Electron scattering on these and conventional (∼92meV) LO-phonon modes together with the fast intervalley exchange is shown to limit electron drift velocity (<1.9×107cm∕s at T=300K), in excellent agreement with the time-of-flight experiment.

Gunn oscillations in GaN channels

Gallium nitride with its high negative differential mobility threshold is an appealing material for high power millimetre-wave oscillators as a Gunn diode. By means of extensive ensemble Monte Carlo simulations, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz is studied in detail. Their operations are checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations cease. The degeneracy effects due to the Pauli exclusion principle are also considered, but their effects are seen to be negligible within the realistic bounds of the Gunn diode operation.

Frequency multiplication using induced dipole domains in a semiconductor superlattice

We theoretically studied the possibility of frequency multiplication using propagating dipole domains which are induced in a semiconductor superlattice by microwave radiation. We have investigated the dynamics of electrons in a superlattice submitted to both a static voltage and a microwave field by performing a simulation based on a drift-diffusion model and incorporating current-limiting boundary conditions. The motion of electrons in the superlattice was governed by an Esaki–Tsu drift velocity field characteristic with a negative differential mobility above a critical electrical field. The simulation delivered, for a static voltage larger than a critical voltage, the periodic formation and annihilation of propagating dipole domains and, as a consequence, a reduction of the direct current through the superlattice. Our simulation showed that an additional microwave field can periodically induce and subsequently quench domains giving rise to a strongly anharmonic current. The anharmonicity of the current is the origin for the generation of higher harmonics of the microwave field. Both the formation and annihilation of a domain can take place within a time of about 1 ps suggesting that the mechanism of domain induction and quenching can be used for generation of radiation up to almost 1 THz .

Corrugation-type instability of the free motion of 180° domain walls in uniaxial ferromagnets

Investigation of the spectrum of oscillations of a 180° domain wall (DW) freely moving in a uniaxial ferromagnet shows that the one-dimensional structure becomes unstable with respect to surface distortions in the region of negative effective mass (negative differential mobility). The upper critical value of the perturbation wave vector, above which the DW corrugation is not developed and the wave vector of a perturbation mode with the maximum increment are determined.

Negative differential mobility in pentacene

AbstractDespite narrow bands and large energy gaps, which result in a negligible number of intrinsic carriers, high‐quality molecular crystals, such as pentacene and naphthalene, show high mobilities at low temperatures. As a result, injected carriers can have their “temperature” increased by easily attainable electric fields. Their mobility is then expected to decrease with an increasing field, both because of the increased scattering rate and the increase in the effective mass due to the nonparabolicity of the energy bands. Using Poisson's equation, we investigate the effect on the current voltage characteristic of the mobility decrease being so steep over some field range that the drift velocity (vd) decreases with an increasing field; that is, the sample displays negative differential mobility (NDM) over that range. NDM is expected for samples in which the effective mass increases. We find that, with increasing voltage, V, after the Ohm's law region, there is a region in which J ∝ V2, as expected for a space‐charge‐limited value of J. When V is large enough to cause heating of the carriers, J increases less rapidly than V2, tending toward a linear dependence on V in the hot carrier region. Thus, V/J tends to saturate at high fields. It is shown that the data of Schön et al. for pentacene, when properly interpreted, show saturation of V/J rather than that of vd, as they claimed. The direct measurement of vd versus the field by the time‐of‐flight method for naphthalene at low temperatures by Warta and Karl was interpreted by them as indicating saturation of vd. We point out that the determination of vd by the time‐of‐flight method in a situation with varying mass and trapping is not straightforward. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2595–2600, 2003

Electron momentum and energy relaxation rates in GaN and AlN in the high-field transport regime

Momentum and energy relaxation characteristics of electrons in the conduction band of GaN and AlN are investigated using two different theoretical approaches corresponding to two high electric-field regimes, one up to 1--2 MV/cm values for incoherent dynamics, and the other at even higher fields for coherent dynamics where semiballistic and ballistic processes become important. For the former, ensemble Monte Carlo technique is utilized to evaluate these rates as a function of electron energy up to an electric-field value of 1 MV/cm (2 MV/cm) for GaN (AlN). Momentum and energy relaxation rates within this incoherent transport regime in the presence of all standard scattering mechanisms are computed as well as the average drift velocity as a function of the applied field. Major scattering mechanisms are identified as polar optical phonon (POP) scattering and the optical deformation potential (ODP) scattering. Roughly, up to fields where the steady-state electron velocity attains its peak value, the POP mechanism dominates, whereas at higher fields ODP mechanism takes over. Next, aiming to characterize coherent dynamics, the total out-scattering rate from a quantum state (chosen along a high-symmetry direction) due to these two scattering mechanisms are then computed using a first-principles full-band approach. In the case of POP scattering, momentum relaxation rate differs from the total out-scattering rate from that state; close to the conduction-band minimum, momentum relaxation rate is significantly lower than the scattering rate because of forward-scattering character of the intravalley POP emission. However, close to the zone boundary the difference between these two rates diminishes due to isotropic nature of intervalley scatterings. Finally, a simple estimate for the velocity-field behavior in the coherent transport regime is attempted, displaying a negative differential mobility due to the negative band effective mass along the electric-field direction.

Semiclassical transport and phonon scattering of electrons in semiconducting carbon nanotubes

Current flow, considering a semiclassical electron--electric-field interaction and electron scattering by acoustic phonons, is studied in semiconducting zig-zag carbon nanotubes. The $\ensuremath{\pi}$-electronic band structure and the phonon spectrum of the nanotube are both calculated from graphene by the zone-folding method. Scattering rates are calculated using first-order perturbation theory and the deformation-potential approximation, while the selection rules for the electron-phonon interaction are developed based on the conservation of crystal momentum. The steady-state transport properties of electrons in small-diameter nanotubes are simulated using the Monte Carlo method. Results show negative differential mobility occurring at smaller threshold fields as the tube diameter increases. The peak drift velocity is also found to depend on the tube diameter, and reaches values as high as $5\ifmmode\times\else\texttimes\fi{}{10}^{7}\mathrm{c}\mathrm{m}/\mathrm{s}$ in the largest tube considered with a diameter of $\ensuremath{\cong}4.6\mathrm{nm}.$ The simulated low-field mobility is as large as in graphite, $\ensuremath{\cong}2\ifmmode\times\else\texttimes\fi{}{10}^{4}{\mathrm{cm}}^{2}/\mathrm{V}/\mathrm{s},$ for the larger tubes, but decreases as the tube diameter decreases.

Comparative full-band Monte Carlo study of Si and Ge with screened pseudopotential-based phonon scattering rates

In a previous article [J. Appl. Phys. 92, 5359 (2002)], we presented a combination of a full-band Monte Carlo method using an advanced band structure and a variable Brillouin zone discretization, with phonon scattering rates based on the screened pseudopotential considering the positions of the atoms in the elementary cell. To make the method suitable for sufficiently fast applications, such as device simulations, the simplest wave number dependent approximation was introduced. It contains an average of the cell structure factor, and only two fit parameters: The acoustic and the optical deformation potentials. As the pseudopotential, the Ashcroft model potential is chosen, and screening is taken into account using the Lindhard dielectric function. In the present article, based on the study of the influence of the two deformation potentials on the electron and hole drift velocities in Si and Ge, we show how to select the deformation potentials. Depending on the targeted agreement with experimental results, the pairs of deformation potentials for electrons and holes can be used uniformly for a wide temperature range or separately for different temperatures. For Ge, we achieve remarkable quantitative agreement with the temperature, field, and orientation dependencies of experimental electron and hole drift velocities in the wide temperature range from 77 to 300 K with a single set of the two deformations potentials for each carrier type. A detailed comparative simulation of the transport properties in Ge and Si at different temperatures is presented which is comprised of the steady-state dependence of the drift velocity on the electric field, the low-field mobility, and transient transport. Peculiarities of the drift velocity-field dependencies, such as the anisotropy, and a negative differential mobility are discussed in terms of the different band structures in connection with the field dependence of the simulated distribution functions. For doped materials, ionized impurity scattering is included. The resulting dependencies on the doping level are consistent with experimental values.

Frequency multiplication of microwave radiation by propagating space-charge domains in a semiconductor superlattice

We report on frequency multiplication of microwave radiation by propagating space-charge domains in a semiconductor superlattice; the domains were due to a negative differential mobility of miniband electrons. We irradiated an arrangement of two GaAs/AlAs superlattices, mounted in a rectangular waveguide system, with microwave radiation (frequency near 70 GHz) and observed the generation of harmonics; the conversion of radiation power to the third harmonic showed a remarkable efficiency (5%). A theoretical study, based on a drift-diffusion model for the miniband electrons, suggests that the microwave radiation has driven the formation and annihilation of space-charge domains and that the resulting anharmonic current was the source of the frequency-multiplied radiation. Our results indicate that frequency multiplication by space-charge domains in a semiconductor superlattice can be exploited for efficient generation of submillimeter-wave radiation.

Full band Monte Carlo study of high field transport in cubic phase silicon carbide

A full band Monte Carlo study of the electron transport in 3C–SiC is presented based on an ab initio band structure calculation using the local density approximation to the density functional theory. The scattering rates and impact ionization transition rates have been calculated numerically from the ab initio band structure using both energy dispersion and numerical wave functions. This approach reduces the number of empirical parameters needed to a minimum. The two empirical coupling constants used have been deduced by fitting the simulated mobility as a function of lattice temperature to experimental data. The peak velocity was found to be approximately 2.2×107 cm/s with a clear negative differential mobility above 600 kV/cm. The electron initiated impact ionization coefficients were found to be 2–10 times stronger than the reported values for the hole initiated impact ionization.

Superheterodyne amplification of sub-millimeter electromagnetic waves in an n-GaAs film

A superheterodyne amplification of sub-millimeter electromagnetic waves in GaAs due to the negative differential mobility is analyzed. The nonlinearity arises from the modulation of concentration, due to a space charge wave, and from the ponderomotive force, due to electromagnetic waves. The case of resonant interaction of two traveling counter propagating electromagnetic waves and the space charge wave in a thin n-GaAs film placed into a dielectric waveguide is considered. The case of 2D electron gas is also analyzed. The simulation of this nonlinear interaction shows a certain increment of the amplification.

Monopole Charge Domain in High-Gain Gallium ArsenidePhotoconductive Switches

Considering that semi-insulating galliumarsenide photoconductive switches can be triggered into the high gainmode and no reliable theories can account for the observed transientcharacteristics, we propose the monopole charge domain model to explainthe peculiar switching phenomena occurring in the high gain mode and wediscuss the requirements for the lock-on switching. During operation onthis mode, the applied field across the switch and the lock-on fieldare all larger than the Gunn threshold field. Our developed monopolecharge domain is based on the transferred-electron effect, but thedomain is only composed of large numbers of electrons piled up due tothe negative differential mobility. Using the model and taking thephysical mechanism of the avalanche impact ionization and recombinationradiation into consideration, we interpret the typical phenomena ofthe lock-on effect, such as the time delay between the beginning of opticalillumination and turning-on of the switch, and the conduction mechanism of thesustaining phase. Under different conditions of bias field intensityand incident light energy, the time delay of the switching iscalculated. The results show that the physical mechanisms of impactionization and recombination radiation occurring in the monopole chargedomain are responsible for the lock-on switching.

Full-Band Monte Carlo Simulation of Electron Transport in 3C-SiC

A full band Monte Carlo study of the electron transport in 3C-SiC is presented based on ab initio band structure calculation using the Local Density Approximation (LDA) to the Density Functional Theory (DFT). The scattering rates and impact ionization transition rates have been calculated numerically from the band structure using both energy dispersion and wave functions. Coupling constants for the phonon interactions have been deduced by fitting the simulated mobility as a function of lattice temperature to experimental data. The saturation velocity was found to be approximately 2.2*107 cm/s with a visible negative differential mobility above 600 kV/cm. The electron initiated impact ionization coefficients were found to be 2 to 10 times stronger than reported values for the hole initiated impact ionization

Monte Carlo Calculations of THz Generation in Nitrides

The electromagnetic power generation associated with the optical-phonon transit-time resonance of a nitride-based maser is analyzed by Monte Carlo simulations under small- and large-signal regimes. Numerical results show that a dynamic negative differential mobility occurs in a wide frequency range with an optimal generation frequency easily tunable in the terahertz range by varying the static electric field strength. The generation phenomena are present up to the liquid nitrogen temperature with reasonably high values of the amplification coefficient. The efficiency of the amplification and generation is found to depend nonmonotonously on static and microwave electric field amplitudes, generation frequency, and doping level, so that for each generation frequency there exists an optimal range of parameter values. A comparative study among the performances of different nitrides is also reported.

Monte Carlo Calculations of THz Generation in Nitrides

The electromagnetic power generation associated with the optical-phonon transit-time resonance of a nitride-based maser is analyzed by Monte Carlo simulations under small- and large-signal regimes. Numerical results show that a dynamic negative differential mobility occurs in a wide frequency range with an optimal generation frequency easily tunable in the terahertz range by varying the static electric field strength. The generation phenomena are present up to the liquid nitrogen temperature with reasonably high values of the amplification coefficient. The efficiency of the amplification and generation is found to depend nonmonotonously on static and microwave electric field amplitudes, generation frequency, and doping level, so that for each generation frequency there exists an optimal range of parameter values. A comparative study among the performances of different nitrides is also reported.