High-field Electron Transport Research Articles

The intensive terahertz electroluminescence induced by Bloch oscillations in SiC natural superlattices

We report on efficient terahertz (THz) emission from high-electric-field-biased SiC structures with a natural superlattice at liquid helium temperatures. The emission spectrum demonstrates a single line, the maximum of which shifts linearly with increases in bias field. We attribute this emission to steady-state Bloch oscillations of electrons in the SiC natural superlattice. The properties of the THz emission agree fairly with the parameters of the Bloch oscillator regime, which have been proven by high-field electron transport studies of SiC structures with natural superlattices.

High-field electron transport in semiconducting zigzag carbon nanotubes

Electron transport in semiconducting zigzag carbon nanotubes is studied by solving the Boltzmann transport equation using the single-particle Monte Carlo technique. The electronic band structure is based on a standard nearest-neighbour tight-binding parameterization, and the phonon spectrum is calculated using a fourth nearest-neighbour force constant model. The electron–phonon scattering probabilities are calculated within a tight-binding formalism. The steady-state drift velocities for the semiconducting zigzag nanotubes (8, 0), (10, 0), (11, 0), (13, 0), and (25, 0) are computed as functions of electric field strength and temperature, and the results are analysed here. The results show the presence of negative differential resistance at high electric fields for some of the nanotubes. The drift velocity and the low-field mobility reach a maximum value of ≅ 4.67 × 107 cm s−1 and≅ 4 × 104 cm2 V −1 s−1, respectively, for a (25, 0) nanotube.

Boundary-enhanced momentum relaxation of longitudinal optical phonons in GaN

The influence of the channel geometry on the high-field electron transport properties has been investigated in n-type GaN at room temperature. It is found that electrons in narrow channels drift much faster than those in wide channels. This discrepancy is attributed to the boundary-enhanced momentum relaxation of longitudinal optical (LO) phonons, which increases the electron drift velocity through enhancing the electron energy dissipation while weakening the momentum relaxation. In narrow channels, the effective LO-phonon momentum relaxation time is about ∼0.67 ps. The suggested theoretical analysis coincides well with the important features of the experimental results.

Calculation of High Field Electron Transport Properties in GaSb and GaAS Using a Monte Carlo Method

Calculation of High Field Electron Transport Properties in GaSb and GaAS Using a Monte Carlo Method

Steady-State and Transient Electron Transport in ZnO: Recent Progress

ABSTRACTWe briefly review some recent results on the steady-state and transient electron transport that occurs within bulk wurtzite zinc oxide. These results were obtained using an ensemble semi-classical three-valley Monte Carlo simulation approach. They showed that for electric field strengths in excess of 180 kV/cm, the steady-state electron drift velocity associated with bulk wurtzite zinc oxide exceeds that associated with bulk wurtzite gallium nitride. The transient electron transport that occurs within bulk wurtzite zinc oxide was studied by examining how electrons, initially in thermal equilibrium, respond to the sudden application of a constant electric field. These transient electron transport results demonstrated that for devices with dimensions smaller than 0.1 μm, gallium nitride based devices will offer the advantage, owing to their superior transient electron transport, while for devices with dimensions greater than 0.1 μm, zinc oxide based devices will offer the advantage, owing to their superior high-field steady-state electron transport.

Steady-state and transient electron transport within bulk wurtzite zinc oxide

We study the steady-state and transient electron transport that occurs within bulk wurtzite zinc oxide using an ensemble semi-classical three-valley Monte Carlo simulation approach. We find that for electric field strengths in excess of 180 kV/cm, the steady-state electron drift velocity associated with bulk wurtzite zinc oxide exceeds that associated with bulk wurtzite gallium nitride. We also present evidence to suggest that the negative differential mobility exhibited by the velocity-field characteristic associated with bulk wurtzite zinc oxide is not related to transitions to the upper valleys. The transient electron transport that occurs within bulk wurtzite zinc oxide is studied by examining how electrons, initially in thermal equilibrium, respond to the sudden application of a constant electric field. From these transient electron transport results, we conclude that for devices with dimensions smaller than 0.1 μm, gallium nitride based devices will offer the advantage, owing to their superior transient electron transport, while for devices with dimensions greater than 0.1 μm, zinc oxide based devices will offer the advantage, owing to their superior high-field steady-state electron transport.

Comparison of High Field Electron Transport Properties in Wurtzite Phase of ZnO, GaN and SiC

Comparison of High Field Electron Transport Properties in Wurtzite Phase of ZnO, GaN and SiC

Large-Signal Analysis of Terahertz Generation in Submicrometer GaN Diodes

The conditions for microwave power generation in a submicrometer GaN diode, with a relatively lightly doped active channel, coupled to an external resonant circuit are investigated. Applying a high-field electron transport model based on the local quasi-static approximation, we show that oscillations in group III-nitride diodes can be supported in the terahertz-frequency range near the limited space-charge accumulation regime. The shape of the diode voltage and electronic current waveforms are examined in terms of the circuit parameters and operating frequencies over the bandwidth of active generation. Based on a Fourier series analysis of the diode voltage and current, the generated power and dc-to-RF conversion efficiency at the fundamental and the second or higher order harmonic frequencies are estimated. The calculation results clearly indicate that submicrometer GaN diodes (channel doping of 1 × 1017 cm-3) can achieve large output powers (> 1 W) in the absence of Gunn domain formation, over a wide range of frequencies, near 0.5 THz.

On the possibility of a shunt-stabilized superlattice terahertz emitter

High field electronic transport through a strongly coupled superlattice (SL) with a shunting side layer is numerically studied using a drift-diffusion model that includes both vertical and lateral dynamics. The bias voltage corresponds to an average electric field in the negative differential conductivity region of the intrinsic current-field curve of the SL, a condition that generally implies space charge instability. Key structural parameters associated with both the shunt layer and SL are identified for which the shunt layer stabilizes a uniform electric field profile. These results support the possibility to realize a SL-based terahertz oscillator with a carefully designed structure.

Theory of high field carrier transport and impact ionization in wurtzite GaN. Part I: A full band Monte Carlo model

High field electron and hole transport in wurtzite phase GaN is studied using an ensemble Monte Carlo method. The model includes the details of the full band structure derived from nonlocal empirical pseudopotential calculations. The nonpolar carrier-phonon interaction is treated within the framework of the rigid pseudoion approximation using ab initio techniques to determine the phonon dispersion relation. The calculated carrier-phonon scattering rates are consistent with the electronic structure and the phonon dispersion relation thus removing adjustable parameters such as deformation potential coefficients. The impact ionization transition rate is computed based on the calculated electronic structure and the corresponding wave-vector dependent dielectric function. The complex band structure of wurtzite GaN requires the inclusion of band-to-band tunneling effects that are critical at high electric fields. The electric-field-induced interband transitions are investigated by the direct solution of the time dependent multiband Schrödinger equation. The multiband description of the transport predicts a considerable increase in the impact ionization coefficients compared to the case in which tunneling is not considered. In the second part of this work it will be shown that the proposed numerical model correctly predicts the carrier multiplication gain and breakdown voltage of a variety of GaN avalanche photodetectors that have been recently fabricated by several research groups.

High-field electron transport of wide quantum wells under in-plane magnetic fields

High-field transport properties of a bilayer two-dimensional electron gas in a wide quantum well (QW) subjected to an in-plane magnetic field have been investigated by ensemble Monte Carlo simulations. The electron energy spectrum was calculated self-consistently. Due to the effect of Hartree potential, a coupled two-dimensional electron gas appears in the wide QW. The in-plane magnetic field induces significant modifications in bonding and antibonding subbands of the QW. The high-field transport behavior is significantly different from the low-field case. With increasing the magnetic field, the high-field drift velocity increases to its maximum value and then decreases monotonically. The magnetic-field-dependent behavior of drift velocity is qualitatively explained as the competition between the magnetic-field-induced depopulation of the antibonding subband and the electron effective mass enhancement.

Terahertz generation in GaN diodes operating in pulsed regime limited by self-heating

The conditions for pulsed regime operation of terahertz power generation in vertical nanoscale GaN-based diodes are investigated via self-consistent simulation of the high-field electron transport in the active channel and thermal transport in the entire device structure. The combined electrothermal model allows for a detailed analysis of the dynamical local distributions of the electric field, drift-velocity, and lattice temperature. We show that stable generation is achievable with a self-heating limited output power of 2.25 W at an operation frequency of 0.71 THz for a pulse width of 3 ns with a few tens of nanosecond duty cycle.

Low-field and high-field electron transport in zinc blende InN

We report on the electron transport in zinc blende InN simulated by the ensemble Monte Carlo method. To obtain the relevant band structure parameters, ab initio calculations have been carried out. Then, Monte Carlo transport simulations at room temperature and over a wide range of carrier concentrations have been performed. We obtain a steady-state peak drift velocity around 3.3×107 cm/s at an electric field of 55 kV/cm. For low-doped material, a room-temperature low-field mobility of about 6000 cm2/V s is calculated. A comparison with wurtzite InN does not reveal an advantage for the zinc blende InN phase regarding the electron transport.

COMPARISON OF STEADY-STATE AND TRANSIENT ELECTRON TRANSPORT IN InAs, InP AND GaAs

An ensemble Monte Carlo simulation is used to compare high field electron transport in bulk InAs , InP and GaAs . In particular, velocity overshoot and electron transit times are examined. For all materials, we find that electron velocity overshoot only occurs when the electric field is increased to a value above a certain critical field, unique to each material. This critical field is strongly dependent on the material, about 400 kVm-1 for the case of GaAs , 300 kVm-1 for InAs and 700 kVm-1 for InP . We find that InAs exhibits the highest peak overshoot velocity and that this velocity overshoot lasts over the longest distances when compared with GaAs and InP . Finally, we estimate the minimum transit time across a 1 μm GaAs sample to be a bout 3 ps. Similar calculations for InAs and InP yield 2.2 and 5 ps, respectively. The steady-state and transient velocity overshoot characteristics are in fair agreement with other recent calculations.

Coupled force-balance and particle-occupation rate equations for high-field electron transport

It is pointed out that in the framework of balance-equation approach, the coupled force-balance and particle-occupation rate equations can be used as a complete set of equations to determine the high-field transport of semiconductors in both strong and weak electron-electron interaction limits. We call to attention that the occupation rate equation conserves the total particle number and maintains the energy balance of the relative electron system, and there is no need to introduce any other term in it. The addition of an energy-drift term in the particle-occupation rate equation [Phys. Rev. B 71, 195205 (2005)] is physically inadequate for the violation of the total particle-number conservation and the energy balance. It may lead to a substantial unphysical increase of the total particle number by the application of a dc electric field.

Dielectric functions, elasto-optical effects, and critical-point parameters of biaxially stressed Si1−yCy alloys on Si (0 0 1)

The complex dielectric functions ϵ(ℏω) from 0.74 to 6.6 eV of pseudomorphically strained Si1−yCy (0 < y < 0.014) alloys grown on Si (0 0 1) were determined using spectroscopic ellipsometry. Interference effects due to surface overlayers and multiple reflections at the substrate/epilayer interface were subtracted. We also report the critical-point parameters (amplitude, energy, broadening, phase angle and dimension) of the E′0, E1, E2 and E′1 interband transitions. While the E1 energy gap increases linearly with increasing C content, in good agreement with a continuum elasticity model (taking into account the effects of biaxial stress and alloying with C based on a linear interpolation of the Si and diamond E1 energies), the E′0 gap stays approximately constant and the E2 gap shows a significant decrease. The amplitudes of all critical points decrease by about 50% and the broadenings increase by about 50–80% when adding 1.4% C. The phase angles remain approximately the same as in Si, except for E′1. The changes in the critical-point parameters can be understood due to the lattice relaxation (four Si nearest neighbours move towards C) and the strong alloy scattering, with obvious implications for ultrafast or high-field electronic transport in such alloys. Since the Si1−yCy alloys are under a tensile biaxial stress, the measured ordinary dielectric function is also affected by piezo-optical effects, which were calculated using literature data for bulk Si.

Low- and high-field transport studies for semiconducting carbon nanotubes

We calculate low- and high-field electron transport characteristics for semiconducting, single-wall, zigzag carbon nanotubes with diameters ranging from small to relatively large. The Boltzmann transport equation is solved directly using an iterative technique and indirectly by the ensemble Monte Carlo method. The basis for the transport calculation is provided by electronic structure calculations within the framework of a simple tight-binding model. Scattering mechanisms considered are due to the electron–phonon interactions involving longitudinal acoustic, longitudinal optic, and radial breathing-mode phonons. Results show significant effects of tube diameter on both low- and high-field transport characteristics.

Negative-mass transport in semiconductor diodes

An analysis of high-field electron transport in a semiconductor diode is presented using a simple kinetic model. A cosine band structure is assumed for the conduction band and the diode is taken to be short enough for the rate of energy-relaxation collisions to be negligible. The drift velocity of the electrons is determined by momentum-relaxation collisions independent of the applied electric field and the band structure, and transfer or tunneling to other valleys or bands is ignored. The analysis is applied to GaN, and conditions for the appearance of a negative differential resistance (NDR) associated with the negative-mass states in the band are derived for the regime of diode lengths and fields in which Bloch oscillations do not occur. It is shown that NDR in this regime occurs over a limited range of diode lengths and fields. Lengths and fields corresponding to breakdown conditions are explicitly avoided.

On the magnetic field profile in a high-power planar magnetron discharge

A simple analytical model is used to describe the effect of the Hall current on the external magnetic field in a high-power planar magnetron. The model predicts that when the Hall current reaches a sufficient value, the magnetic field profile changes its configuration from transverse (arch) into quasi-longitudinal. According to the model, plasma diamagnetism might be a cause of anomalously high cross field electron transport observed in circular planar magnetrons.

Conductance versus bias voltage characteristics of multi-walled carbon nanotubes

High-field electron transport in multi-walled carbon nanotubes (MWNTs) is studiedemploying a mechanically controllable break junction (MCBJ) technique. Bridging ofMWNTs over Au electrodes is accomplished by repeatedly breaking Au contactsin a nanotube-dispersed solution. Observed conductance versus bias voltage(G–V) characteristics show that the conductance of MWNTs linearly increases with the bias upto 3 V. Also, the important role of the nanotube/electrode contact resistance onthe electron transport through MWNTs is pointed out from measurements ofG–V characteristics at various electrode distances.