Biased Superlattice Research Articles

Analysis of electric-field domain formations and carrier transport phenomena in GaAs/AlAs asymmetric double-quantum-well superlattices

We evaluated the formation of electric-field domains (EFDs) in three kinds of asymmetric double-quantum-well (ADQW) superlattices (SLs) whose photoluminescence (PL) properties revealed various characteristics affected by EFD formation. In particular, anomalous comb-shaped PL branches were clearly observed. Each PL branch corresponds to one EFD, giving important information on EFD distribution in ADQW-SLs. We determined the strengths of all electric-fields in all the EFDs from their redshifted PL signals and plotted their reverse bias voltage dependence. We clearly identified the EFD distributions in the ADQW-SLs in which the resonant tunneling effect plays an important role to make carrier transport paths. These results demonstrate that the analysis of EFD distributions from PL properties is an effective tool to investigate the carrier transport phenomena in biased SLs.

Low-bias flat band-stop filter based on velocity modulated gaussian graphene superlattice

Transport properties of biased planar Gaussian graphene superlattice (PGGSL) with Fermi velocity barrier is investigated by transfer matrix method (TMM). It is observed that enlargement of bias voltage over miniband width breaks the miniband to WSLs leads to suppressing resonant tunneling. Transmission spectrum shows flat wide stop-band property controllable by external bias voltage with stop-band width of near 200meV. The simulations demonstrate that strong velocity barriers prevent tunneling of Dirac electrons leading to controllable enhancement of stop-band width. By increasing ratio of Fermi velocity in barriers to wells υc stop-band width increase. As wide transmission stop-band width (BWT) of filter is tunable from 40meV to 340meV is obtained by enhancing ratio of υc from 0.2 to 1.5, respectively. Proposed structure suggests easy tunable wide band-stop electronic filter with a modulated flat stop-band characteristic by height of electrostatic barrier and structural parameters. Robust sensitivity of band width to velocity barrier intensity in certain bias voltages and flat band feature of proposed filter may be opens novel venue in GSL based flat band low noise filters and velocity modulation devices.

Insight into the split and asymmetry of charge distribution in biased M-structure superlattice

The charge distribution in real space of an insertion variant based on an InAs/GaSb superlattice for an infrared detector is illustrated by in situ electron microscopy. The localization split of positive charge can be directly observed in the InAs/GaSb/AlSb/GaSb superlattice (M-structure) rather than in the InAs/GaSb superlattice. With the applied bias increasing from 0 to 4.5 V, the double peaks of positive charge density become asymmetrical gradually, with the peak integral ratio ranging from 1.13 to 2.54. Simultaneously, the negative charges move along the direction of the negative electric field. Without inserting the AlSb layer, the charge inversion occurs in both the hole wells and the electron wells of the InAs/GaSb superlattice under high bias. Such a discrepancy between the M-structure superlattice and the traditional superlattice suggests an effective reduction of tunneling probability of the M-structure design. Our result is of great help to understand the carrier immigration mechanism of the superlattice-based infrared detector.

Robust low-bias negative differential resistance in graphene superlattices

In this work, we present a detailed theoretical study on the low bias current–voltage (I–V) characteristic of biased planar graphene superlattice (PGSL), provided by a heterostructured substrate and a series of grounded metallic planes placed over a graphene sheet, which induce a periodically modulated Dirac gap and Fermi velocity barrier, respectively. We investigate the effect of PGSL parameters on the I–V characteristic and the appearance of multipeak negative differential resistance (NDR) in the proposed device within the Landauer–Buttiker formalism and adopted transfer matrix method. Moreover‚ we propose a novel venue to control the NDR in PGSL with Fermi velocity barrier. Different regimes of NDR have been recognized, based on the PGSL parameters and external bias. From this viewpoint‚ we obtain multipeak NDR through miniband aligning in PGSL. The maximum pick to valley ratio (PVR) up to 167 obtained for , the Fermi velocity correlation (ratio of Fermi velocity in barrier and well region), is 1.9 at bias voltages between 70–130 mV. Our findings have good agreement with experiments and can be considered in designing multi-valued memory‚ functional circuit, low power and high-speed nanoelectronic device applications.

Tunable negative differential resistance in planar graphene superlattice resonant tunneling diode

Here, we study the negative differential resistance (NDR) of Dirac electrons in biased planar graphene superlattice (PGSL) and investigate the transport characteristics by adopted transfer matrix method within Landauer-Buttiker formalism. Our model device is based on one-dimensional Kronig–Penney type electrostatic potential in monolayer graphene deposited on a substrate, where the bias voltage is applied by two electrodes in the left and right. At Low bias voltages, we found that NDR appears due to breaking of minibands to Wannier-Stark ladders (WSLs). At the critical bias voltage, delocalization appeared by WS states leads to tunneling peak current in current-voltage (I-V) characteristics. With increasing bias voltage, crossing of rungs from various WSL results in multi-peak NDR. The results demonstrate that the structure parameters like barrier/well thickness and barrier height have remarkable effect on I-V characteristics of PGSL. In addition, Dirac gap enhances peak to valley (PVR) value due to suppressing Klein tunneling. Our results show that the tunable PVR in PGSL resonant tunneling diode can be achievable by structure parameters engineering. NDR at ultra-low bias voltages, such as 100 mV, with giant PVR of 20 is obtained. In our device, the multiple same NDR peaks with ultra-low bias voltage provide promising prospect for multi-valued memories and the low power nanoelectronic tunneling devices.

Anharmonic Bloch oscillations of electrons in electrically biased superlattices

The phenomenon of anharmonic Bloch oscillations (i.e., oscillations with a frequency which is a multiple of the Bloch frequency) is considered. The energy-band structure of silicon-carbide polytypes where these oscillations are observed is calculated ab initio. A one-dimensional model potential making it possible to calculate the Stark wave functions in a biased superlattice is constructed for these polytypes. The transfer matrix method is used to find the set of energy levels (the so-called Stark ladder) and calculate the electron wave functions. It is shown that both radiative transitions between neighboring Stark levels (at the Bloch frequency) and transitions in the form of jumps via several levels, accompanied by emission at frequencies that are multiples of the Bloch frequency, may occur. The calculated probabilities of transitions between Stark-ladder levels increase with increasing applied field strength.

Strong Amplification of Coherent Acoustic Phonons by Intraminiband Currents in a Semiconductor Superlattice.

Sound amplification in an electrically biased superlattice (SL) is studied in optical experiments with 100fs time resolution. Coherent SL phonons with frequencies of 40, 375, and 410GHz give rise to oscillatory reflectivity changes. With currents from 0.5 to 1.3A, the Fourier amplitude of the 410GHz phonon increases by more than a factor of 2 over a 200ps period. This amplification is due to stimulated Čerenkov phonon emission by electrons undergoing intraminiband transport. The gain coefficient of 8×10^{3} cm^{-1} is reproduced by theoretical calculations and holds potential for novel sub-THz phonon emitters.

Anharmonic Bloch Oscillation of Electrons in Biased Superlattices

The oscillatory motion of electrons in a periodic potential under a constant applied electric field, known as Bloch oscillations (BO), is one of the most striking and intriguing quantum effects and was predicted more than eighty years ago. Oscillating electrons emit electromagnetic radiation and here we consider this BO effect for emission in the THz region. To date, it has been assumed that the Bloch oscillation of an electron is anharmonic oscillation, therefore with radiation emitted at the single Bloch frequency. We analyze scenarios when Bloch oscillations can be accompanied by the emission of radiation not only at the Bloch frequency but also with double and triple Bloch frequencies. The first scenario means that electrons could jump over neighboring Stark states. The second scenario of anharmonic emission is coupled to an opening of the minigap in the miniband.

A proposal for optical terahertz detection with externally biased nanopore superlattices

We propose and model a terahertz detector based on the transport properties of an excited carrier population in a nanopore structure. The present work supports the proposal with detailed electronic dispersion, optical absorption, and carrier transport.

Resonance structure of dynamic fractional Stark ladders in laser-driven biased superlattices

The resonance structure of an electronic Floquet state in a dynamic fractional Stark ladder (DFSL) is examined based on the scattering theory applied to a dressed potential resulting from renormalization of a laser–electron interaction to an original potential. Here, the DFSL is realized in laser-driven biased superlattices with a fractional matching ratio of a Bloch frequency to a laser frequency. It is revealed that, in contrast to a conventional understanding, the DFSL resonance position and lifetime tend to redshift and shorten, respectively, with an increase in strength of the laser field, and further, these show irregular changes in a limited region of the strength. The underlying physics is discussed in detail.

Excitation spectra of terahertz Bloch emission in semiconductor superlattices

We present theoretical and experimental studies of the amplitude of the terahertz (THz) light emitted by biased superlattices excited by fast optical pulses as a function of the pump photon energy. The comparison of the measured transient signal and of its dependence upon the excitation photon energy with a model calculation allows a clear separation between the contributions of the dissociated carriers and bound pairs to the emitted THz field.

Two miniband model for self-sustained oscillations of the current through resonant-tunneling semiconductor superlattices

A two miniband model for electron transport in semiconductor superlattices that includes scattering and interminiband tunnelling is proposed. The model equations for Wigner functions in a basis spanned by Pauli matrices include electron-electron scattering in the Hartree approximation and modified Bhatnagar-Gross-Krook collision terms. For strong applied fields, balance equations for the electric field and the miniband populations are derived using a Chapman-Enskog perturbation technique. These equations are then solved numerically for a dc voltage biased superlattice. Results include self-sustained current oscillations due to repeated nucleation of electric field pulses at the injecting contact region and their motion toward the collector. Numerical reconstruction of the Wigner functions shows that the miniband with higher energy is empty during most of the oscillation period: it becomes populated only when the local electric field (corresponding to the passing pulse) is sufficiently large to trigger resonant tunneling.

Intraband dynamics and terahertz emission in biased semiconductor superlattices coupled to double far-infrared pulses

This paper studies both the intraband polarization and terahertz emission of a semiconductor superlattice in combined dc and ac electric fields by using the superposition of two identical time delayed and phase shifted optical pulses. By adjusting the delay between these two optical pulses, our results show that the intraband polarization is sensitive to the time delay. The peak values appear again for the terahertz emission intensity due to the superposition of two optical pulses. The emission lines of terahertz blueshift and redshift in different ac electric fields and dynamic localization appears. The emission lines of THz only appear to blueshift when the biased superlattice is driven by a single optical pulse. Due to excitonic dynamic localization, the terahertz emission intensity decays with time in different dc and ac electric fields. These are features of this superlattice which distinguish it from a superlattice generated by a single optical pulse to drive it.

Resonant carrier dynamics in strongly biased superlattices

We study coherent electron dynamics in a biased undriven ideal semiconductorsuperlattice coupled to the continuum, near energy level anticrossings. In particular, weexamine the dependence of wavepacket dynamical characteristics on electric fielddetuning, and investigate mixed regimes involving a superposition of energy levelanticrossings showing both Rabi oscillations and resonant tunnelling. In earlierwork (Abumov and Sprung 2007 Phys. Rev. B 75 165421), Rabi and Zener resonances wereshown to have a common origin, and a criterion for the occurrence of either wasproposed. The present results allow a better understanding of the nature of aninterminiband resonance, which can be useful in the areas of microwave radiationgeneration and matter manipulation on the particle level, as well as demonstrating analternative approach to examining electron level structure of a finite superlattice.

Negative lateral conductivity of hot electrons in a biased superlattice

Nonequilibrium electron distribution in a superlattice subjected to a homogeneous electric field (biased superlattice with equipopulated levels) is studied within the tight-binding approximation, taking into account the scattering by optical and acoustic phonons and by lateral disorder. It is found that the distribution versus the in-plane kinetic energy depends essentially on the ratio between the Bloch energy and the optical phonon energy. The in-plane conductivity is calculated for low-doped structures at temperatures 4.2 K and 20 K. The negative conductivity is found for bias voltages corresponding to the Bloch-phonon resonance condition.

Interacting dynamic Wannier-Stark ladder driven by a periodic pulse train

The electronic structures of the Floquet states of the dynamic Wannier-Stark ladder (DWSL) are examined, where the DWSL is formed by driving the biased superlattices (SLs) by the periodic pulse train (PPT) with the electric field $F(t)$---with time $t$---and the temporal period $2\ensuremath{\pi}∕\ensuremath{\omega}$. For a strong $F(t)$, interminiband interactions, namely, the ac-Zener tunneling (ac-ZT), are predominantly caused in the DWSL. Such a system is termed the interacting DWSL. In order to understand the details of the Floquet states and the modulation patterns by alteration of a couple of the PPT laser parameters, the linear absorption spectra, ${\ensuremath{\alpha}}_{\mathit{abs}}({\ensuremath{\omega}}_{p};\ensuremath{\omega})$, of optical interband transitions invoked by the monochromatic probe laser ${f}_{p}(t)$ with the frequency ${\ensuremath{\omega}}_{p}$ are calculated, where the spectra are not only linear in ${f}_{p}(t)$ but also nonlinear in $F(t)$. The exciton effect is not included for the sake of simplicity. For the PPT driving with unit-pulse shapes largely deviated from the square and saw-toothed profiles, the spectra show unexpected dent structures, differing a great deal from the corresponding ac-ZT-free spectra basically similar to those of the original SLs just showing the ascending steplike structure. To deepen the understanding of this anomaly, the spectra of ${\ensuremath{\alpha}}_{\mathit{abs}}^{0}({\ensuremath{\omega}}_{p};\ensuremath{\omega})\ensuremath{\propto}\ensuremath{\partial}{\ensuremath{\alpha}}_{\mathit{abs}}({\ensuremath{\omega}}_{p};\ensuremath{\omega})∕\ensuremath{\partial}{\ensuremath{\omega}}_{p}$ are also calculated, whereby the dent structures become spectral dips showing the negative absorption. It is found that such anomalous behavior is attributed to the ac-ZT between different minibands that accompanies emission/absorption of the nonzero net number of photons with $J\ensuremath{\omega}$ (with $J$ a nonzero integer). This anomaly also shows the unusual time dependence in the dual-time optical susceptibility associated with ${\ensuremath{\alpha}}_{\mathit{abs}}^{0}({\ensuremath{\omega}}_{p};\ensuremath{\omega})$. Moreover, the possibility of existence of the negative absorption in the more realistic excitonic spectra is speculated.

Complex permittivity of a biased superlattice

We study the intersubband response in a superlattice subjected to a homogeneous electric field (a biased superlattice with equipopulated levels) within the tight-binding approximation. We consider the interplay between homogeneous and inhomogeneous mechanisms of broadening. We calculate the complex dielectric permittivity beyond the Born approximation for a wide spectral region, finding a low-frequency enhancement of the response. A detectable gain below the resonance is obtained for the low-doped GaAs-based biased superlattice in the terahertz spectral region. Conditions of the stimulated emission regime are discussed for metallic and dielectric waveguide structures. It is described that the appearance of a localized terahertz mode is described when a biased superlattice is placed at a vacuum-dielectric interface.

Intersubband transitions in a biased superlattice: THz gain and surface mode

AbstractIntersubband response in a biased superlattice with equipopulated levels is studied within the tight‐binding approximation and the complex dielectric permittivity is calculated beyond the Born approximation for a wide spectral region. The low‐frequency enhancement of the dispersion of permittivity is found and the conditions to obtain the stimulated emission regime is estimated. The enhancement of the emission due to the THz waveguide effect is also considered for the cases of the BSL placed between ideal metallic mirrors, the free‐standing BSL, and BSL at a vacuum‐dielectric interface. An appearance of the localized THz mode is described for the last case. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear Electron and Spin Transport in Semiconductor Superlattices

Nonlinear charge transport in strongly coupled semiconductor superlattices is described by Wigner-Poisson kinetic equations involving one or two minibands. Electron-electron collisions are treated within the Hartree approximation whereas other inelastic collisions are described by a modified BGK (Bhatnaghar-Gross-Krook) model. The hyperbolic limit is such that the collision frequencies are of the same order as the Bloch frequencies due to the electric field and the corresponding terms in the kinetic equation are dominant. In this limit, spatially nonlocal drift-diffusion balance equations for the miniband populations and the electric field are derived by means of the Chapman-Enskog perturbation technique. For a lateral superlattice with spin-orbit interaction, electrons with spin up or down have different energies and their corresponding drift-diffusion equations can be used to calculate spin-polarized currents and electron spin polarization. Numerical solutions show stable self-sustained oscillations of the current and the spin polarization through a voltage biased lateral superlattice thereby providing an example of superlattice spin oscillator.

Transverse magnetic mode along THz waveguides with biased superlattices

We study the propagation of transverse magnetic modes arising from a waveguide consisting on a GaAs-based superlattice located at vacuum–dielectric interface. The transverse mode is generated by the ultrafast intersubband response of the superlattice subjected to a high-frequency electric field. The superlattice is also subjected to a homogeneous bias potential to get a biased superlattice with equipopulated levels. The heterostructure is analyzed through the tight-binding approximation, and considering the level broadening caused by different scattering processes (homogeneous and inhomogeneous broadening mechanisms). We pay special attention to the dispersion relations of the complex dielectric permittivity because of real and imaginary parts of this function play a key role in wide miniband superlattices.