Oscillations In Semiconductor Superlattices Research Articles

Noise-enhanced chaos in a weakly coupled GaAs/(Al,Ga)As superlattice.

Noise-enhanced chaos in a doped, weakly coupled GaAs/Al_{0.45}Ga_{0.55}As superlattice has been observed at room temperature in experiments as well as in the results of the simulation of nonlinear transport based on a discrete tunneling model. When external noise is added, both the measured and simulated current-versus-time traces contain irregularly spaced spikes for particular applied voltages, which separate a regime of periodic current oscillations from a region of no current oscillations at all. In the voltage region without current oscillations, the electric-field profile consist of a low-field domain near the emitter contact separated by a domain wall consisting of a charge accumulation layer from a high-field regime closer to the collector contact. With increasing noise amplitude, spontaneous chaotic current oscillations appear over a wider bias voltage range. For these bias voltages, the domain boundary between the two electric-field domains becomes unstable and very small current or voltage fluctuations can trigger the domain boundary to move toward the collector and induce chaotic current spikes. The experimentally observed features are qualitatively very well reproduced by the simulations. Increased noise can consequently enhance chaotic current oscillations in semiconductor superlattices.

Intermittency route to chaos and broadband high-frequency generation in semiconductor superlattice coupled to external resonator.

We investigate the onset of broadband microwave chaos in the miniband semiconductor superlattice coupled to an external resonator. Our analysis shows that the transition to chaos, which is confirmed by calculation of Lyapunov exponents, is associated with the intermittency scenario. The evolution of the laminar phases and the corresponding Poincare maps with variation of a supercriticality parameter suggest that the observed dynamics can be classified as type I intermittency. We study the spatiotemporal patterns of the charge concentration and discuss how the frequency band of the chaotic current oscillations in semiconductor superlattice depends on the voltage applied.

Numerical method for hydrodynamic modulation equations describing Bloch oscillations in semiconductor superlattices

We present a finite difference method to solve a new type of nonlocal hydrodynamic equations that arise in the theory of spatially inhomogeneous Bloch oscillations in semiconductor superlattices. The hydrodynamic equations describe the evolution of the electron density, electric field and the complex amplitude of the Bloch oscillations for the electron current density and the mean energy density. These equations contain averages over the Bloch phase which are integrals of the unknown electric field and are derived by singular perturbation methods. Among the solutions of the hydrodynamic equations, at a 70K lattice temperature, there are spatially inhomogeneous Bloch oscillations coexisting with moving electric field domains and Gunn-type oscillations of the current. At higher temperature (300K) only Bloch oscillations remain. These novel solutions are found for restitution coefficients in a narrow interval below their critical values and disappear for larger values. We use an efficient numerical method based on an implicit second-order finite difference scheme for both the electric field equation (of drift-diffusion type) and the parabolic equation for the complex amplitude. Double integrals appearing in the nonlocal hydrodynamic equations are calculated by means of expansions in modified Bessel functions. We use numerical simulations to ascertain the convergence of the method. If the complex amplitude equation is solved using a first order scheme for restitution coefficients near their critical values, a spurious convection arises that annihilates the complex amplitude in the part of the superlattice that is closer to the cathode. This numerical artifact disappears if the space step is appropriately reduced or we use the second-order numerical scheme.

Theory of spatially inhomogeneous Bloch oscillations in semiconductor superlattices

In a semiconductor superlattice with long scattering times, damping of Bloch oscillations due to scattering is so small that nonlinearities may compensate it and Bloch oscillations persist even in the hydrodynamic regime. To demonstrate this, a Boltzmann-Poisson transport model of miniband superlattices with inelastic collisions is proposed and hydrodynamic equations for electron density, electric field and the complex amplitude of the Bloch oscillations are derived by singular perturbation methods. For appropriate parameter ranges, numerical solutions of these equations show stable Bloch oscillations with spatially inhomogeneous field, charge, current density and energy density profiles. These Bloch oscillations disappear as scattering times become sufficiently short. For sufficiently low lattice temperatures, Bloch and Gunn type oscillations mediated by electric field, current and energy domains coexist for a range of voltages. For larger lattice temperatures (300 K), there are only Bloch oscillations with stationary amplitude and electric field profiles.

Operation of a semiconductor superlattice oscillator

We report an experimental study of a semiconductor superlattice oscillator and present an analysis of the origin of gain. The oscillator generated microwave radiation (at frequencies around 60 GHz). An analysis of the results suggests that the oscillator operated in a pure charge accumulation mode that can occur in a medium with a negative differential mobility. We relate the negative differential mobility to miniband transport. Additionally, we propose a microwave-terahertz double oscillator that may be suitable to realize a terahertz Bloch oscillator.

Acoustic analog of electronic Bloch oscillations and Zener tunneling

The observation of Bloch oscillations in sound propagation through a multilayer of two different fluidlike components is predicted. In order to obtain the equivalent to the acoustic analog of a Wannier-Stark ladder [E. E. Mendez et al., Phys. Rev. Lett. 60, 2426–2429 (1988)], a set of cavities with increasing thickness is employed. Bloch oscillations were theoretically predicted as time-resolved oscillations in transmission in direct analogy to electronic Bloch oscillations in semiconductor superlattices [J. Feldmann et al., Phys. Rev. B 46, R7252–R7255 (1992)]. Finally, an experimental setup is proposed to observe the phenomenon by using arrays of cylindrical rods in air, which acoustically behaves as a fluidlike system with effective sound velocity and density [D. Torrent et al., Phys. Rev. Lett. 96, 204302 (2006)]. For the proposed system, Bloch oscillations and Zener tunneling are confirmed by using multiple scattering simulations. [Work supported by MEC of Spain.]

Semiconductor-superlattice oscillator with superlattices connected in parallel and series as active elements for generation of millimetre waves

We report on a semiconductor-superlattice oscillator with a superlattice electronic device (SLED) containing superlattices connected in parallel and series as active elements for generation of millimetre waves. We present experimental results indicating that parallel connected superlattices can lead, compared to a single superlattice as the active element, to an increase of the output power of the oscillator. Studies of blocks of parallel connected superlattices in series lead us to propose using a SLED with parallel and serially connected superlattices as an active element. Our superlattice oscillator, with different arrangements of superlattice mesas in a quasiplanar design, operated in the frequency range of around 100 GHz. It was electrically tunable and delivered microwave power of about 0.1 mW.

Proposal of a Microwave-Driven Semiconductor Superlattice Oscillator for Generation of THz Radiation

We present the proposal of a microwave-driven semiconductor superlattice oscillator. We show that the interplay of a microwave pump field with a synchronous harmonic field can make a semiconductor superlattice to a gain medium for the harmonic field. Placing the superlattice in a resonator for the harmonic field allows the operation of an oscillator. The gain mechanism is based on Bloch oscillations of miniband electrons. The gain is mediated either by the interaction of the high-frequency field with the single electrons or with space charge domains or with both. The microwave-driven superlattice oscillator should be suitable for generation of coherent radiation up to several THz.

Self-stabilization of chaotic domain oscillations in superlattices by time-delayed feedback control

Semiconductor superlattices driven into the regime of negative differential conductivity often exhibit complex or even chaotic self-sustained current oscillations due to travelling field domains. For a reliable operation of a superlattice as an electronic oscillator, such unpredictable and irregular conditions should be avoided. In this work we present a theoretical analysis, proposing a novel scheme to stabilize high-frequency domain oscillations in semiconductor superlattices by a time-delayed feedback loop.

Miniband transport and oscillations in semiconductor superlattices

We present and analyse solutions of a recent derivation of a drift-diffusion model of minibandtransport in strongly coupled superlattices. The model is obtained from a single-minibandBoltzmann–Poisson transport equation with a BGK (Bhatnagar–Gross–Krook) collisionterm by means of a consistent Chapman–Enskog expansion. The reduced drift-diffusionequation is solved numerically and travelling field domains and current oscillations areobtained. A broad range of frequencies can be achieved, depending on the modelparameters, in good agreement with available experiments on GaAs/AlAs superlattices.

Publisher’s Note: Self-stabilization of high-frequency oscillations in semiconductor superlattices by time-delay autosynchronization [Phys. Rev. E68, 066208 (2003)

Publisher’s Note: Self-stabilization of high-frequency oscillations in semiconductor superlattices by time-delay autosynchronization [Phys. Rev. E<b>68</b>, 066208 (2003)

Self-stabilization of high-frequency oscillations in semiconductor superlattices by time-delay autosynchronization

We present a scheme to stabilize high-frequency domain oscillations in semiconductor superlattices by a time-delayed feedback loop. Applying concepts from chaos control theory we propose to control the spatiotemporal dynamics of fronts of accumulation and depletion layers which are generated at the emitter and may collide and annihilate during their transit, and thereby suppress chaos. The proposed method only requires the feedback of internal global electrical variables, viz., current and voltage, which makes the practical implementation very easy.

Magneto‐Bloch oscillations, anomalous Hall velocity and nonlinear magnetotransport in semiconductor superlattices

We investigate transient coherent magneto-Bloch oscillations (MBO's) of electronic wave packet photo-excited in a biased semiconductor superlattice (SSL) in a crossed or tilted magnetic field, as well as temporal transition to nonlinear magnetotransport in the superlattice. The avoided crossing between the full-miniband MBO's and bottom-miniband cyclotron oscillations in SSL is described in general for tilted electric and magnetic fields. It is shown that the full-miniband MBO's in a biased SSL in a crossed magnetic field give rise to anomalous Hall velocity which is inverse to the electric field, depends in general on the excitation conditions and determines the shift of the MBO's frequency with respect to the frequency of Bloch oscillations without magnetic field. The electric-field-induced transition from the ordinary to anomalous Hall drift velocity determines the value of a peak stationary carrier velocity υ(p) and the corresponding fields. The general criterion for the value of the electric field at which the υ(p) is reached is found.

Feasibility of a semiconductor superlattice oscillator based on quenched domains for the generation of submillimeter waves

We discuss the feasibility of a semiconductor superlattice oscillator which exploits the quenching of propagating dipole domains for the generation of submillimeter waves. We studied the dynamics of electrons in a semiconductor superlattice by performing a simulation based on a drift-diffusion model, taking into account feedback from a resonant circuit. The simulation delivers propagating dipole domains which are quenched before they reach the anode. The periodic formation and quenching of domains creates a self-sustained oscillation of the current through the superlattice. The frequency of the oscillation can be more than three times higher than without feedback. We suggest that with already existing superlattices an oscillator working in the quenched domain mode can be realized up to almost 500 GHz.

Coupled bloch-phonon oscillations in semiconductor superlattices

We investigate coherent Bloch oscillations in GaAs/AlxGa1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].

Chaos-induced orbit delocalization and complex Bloch oscillations in semiconductor superlattices

We show how semiconductor superlattices can be used to provide a new type of quantum chaotic system that is accessible to experiment. When a magnetic field is tilted relative to the superlattice axis, the semiclassical orbits of electrons confined to a single miniband undergo a controllable transition from stable Bloch oscillations to chaotic orbits. The onset of chaos delocalizes the electron paths and generates complex trajectories that extend across many periods of the superlattice. We show that the chaotic motion has a fundamentally different origin to that reported for previous semiconductor structures.

Dephasing effects induced by weak disorder in superlattices

We investigate the dephasing dynamics of Bloch oscillations in semiconductor superlattices by means of a very simple model including weak disorder and applied electric fields. A thorough numerical study of our model allows us to claim that small, unintentional well width fluctuations can be responsible for fast dephasing of Bloch oscillations at low temperatures. We show that the lifetime of Bloch oscillations is controlled by a characteristic time which depends on the degree of disorder and is independent of the electric field. This result is further supported by the excellent agreement between our model calculations and several recent experiments, and leads to specific new predictions.

Rabi oscillations in semiconductor superlattices

We investigate the dynamics of a semiconductor superlattice driven by an ac electric field by solving numerically the one-dimensional time-dependent Schr\"odinger equation including interface roughness effects. Rabi oscillations between minibands are clearly identified under resonant conditions, but we show that their lifetime is controlled by a characteristic time that depends on the superlattice deviation from perfection and is independent of the electric field. Experiments for the observation of the predicted effects are proposed.

Generation of millimeter waves with a GaAs/AlAs superlattice oscillator

We report on a semiconductor superlattice oscillator for generation of millimeter waves (frequency 65 GHz). The main element of the oscillator is a doped short-period GaAs/AlAs superlattice with negative differential conductance. The oscillator is due to current oscillations caused by charge density domains. The oscillator delivered, at an efficiency of 0.2% for the conversion of electrical power to radiation power, a power of 100 μW in a bandwidth of the order of 200 kHz.

Influence of LO-Phonon Emission on Bloch Oscillations in Semiconductor Superlattices

The intraband phase coherence of electronic Bloch wavepackets in semiconductor superlattices is partially conserved when electrons undergo energy relaxation by LO-phonon emission. The decay time of the intraband coherence as probed by terahertz-emission spectroscopy is found to be unaffected by excitation below or above the threshold for LO-phonon emission. On the other hand, the population relaxation probed by femtosecond optical transmission spectroscopy is drastically accelerated when the threshold is reached. Electrons which have undergone LO-phonon emission form a more localized distribution in k-space giving rise to a stronger THz signal.