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We report an experimental study indicating ultrafast creation and annihilation of space-charge domains in a semiconductor superlattice under the action of a THz field. Our experiment was performed for an InGaAs/InAlAs superlattice with the conduction electrons undergoing miniband transport. We applied to a superlattice a dc bias that was slightly smaller than a critical bias necessary for the formation of space-charge domains caused by a static negative differential conductivity. Additionally subjecting the superlattice to a strong THz field, resulted in a dc transport governed by the formation of domains if the frequency of the field was smaller than an upper frequency limit (~3 THz). From this frequency limit for the creation and annihilation of domains we determined the characteristic time of the domain buildup. Our analysis shows that the buildup time of domains in a wide miniband and heavily doped superlattice is limited by the relaxation time due to scattering of the miniband electrons at polar optic phonons. Our results are of importance for both an understanding of ultrafast dynamics of pattern formation in nanostructures and the development of THz electronic devices.

We studied the effects caused by Joule heating in GaAs/AlAs superlattices. We measured the current–voltage characteristics of superlattice mesas of different cross sections. Each current–voltage characteristic showed a corresponding peak current followed by a region of negative differential conductivity, according to miniband transport in the superlattice. We found that Joule heating in the mesa reduced the peak-current density. Current jumps in the region of negative differential conductivity disappeared for large cross-section mesas indicating a suppression of electron domains. The results are of significance for the development of high-power superlattice oscillators for submillimeter radiation.

We study electron miniband magneto-transport in GaAs/AlAs superlattices with AlAs barriers of two monolayers. Here, the two-dimensional AlAs barrier comprises an in-plane distribution of AlAs islands. We use magneto-transport to probe both the rate and the anisotropy of the electron scattering associated with the islands.

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.

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.

In order to investigate the modification of excitation and decay of core holes by condensation and adsorption in the simplest possible cases, as well as the coupling to atomic motion, we have studied the Ar2p and Kr3d excitation regions in Ar and Kr mono- and multilayers on Ru(001). Using synchrotron radiation from BESSY (Berlin), total and Auger electron yields and yields of desorbing ions and neutrals, as function of photon energy, as well as decay electron spectra for specific primary excitations have been measured. The main results are: Multilayers: Energies for resonant core excitations are shifted to higher values by 0.6 to 1 eV compared to free atoms; for Kr, surface (smaller shifts) and bulk excitations can be distinguished. Autoionization and normal Auger spectra are clearly different, as in the gas phase. Besides desorption of neutral atoms as in the valence region, singly and doubly charged ions as well as ionic clusters are desorbed. Monolayers: The shift of resonant excitation energies relative to the gas phase is smaller than for condensed layers. Decay spectra for resonant and non-resonant excitations are identical, as for strongly coupled chemisorbates, proving that charge exchange with the metal is fast compared to core decay. As in the valence region, only neutral atoms desorb. The results shed light on the screening and charge transfer behaviour and on the mechanisms of stimulated desorption which are operative in them.

We report millimeter wave generation by a self-sustained current oscillation in a doped InGaAs/InAlAS wide miniband superlattice. The superlattice (miniband width 160 meV) showed, at room temperature, a current voltage characteristic with negative differential conductance. Coupled to a high-frequency circuit, the superlattice generated millimeter waves, at a frequency (55 GHz) which was tunable by half a percent by changing the bias voltage. The power (0.3 mW) corresponded to an efficiency (i.e., ratio of microwave power to dc power input) of 0.3%. We attribute the microwave generation to a current oscillation caused by traveling dipole domains.

Presents experimental evidence for ultra-fast domain formation in a semiconductor superlattice. An InGaAs/InAlAs superlattice device was mounted in a corner cube. The superlattice device showed a region of negative differential conductance in the current-voltage characteristic for voltages larger than an onset voltage. When the superlattice device was irradiated with radiation smaller than 1.2 THz, the onset voltage shifted, with increasing THz radiation power, to lower voltages. Taking an analysis based on a drift-diffusion model into account, we suggest that dipole domains form within about 400 fs.

We observed that the propagation of a dipole domain in a GaAs/AlAs superlattice can be controlled by an external high-frequency field. The doped GaAs/AlAs superlattice showed a negative differential conductance giving rise to propagating dipole domains. Depending on the frequency of the high-frequency field, either frequency-locked or quasiperiodic propagating domain states occurred. The behavior can be described by a ``devil's staircase,'' where in the locked state the ratio of the external frequency and the domain transit frequency is a rational number. A theoretical analysis indicates that the high-frequency field controls the domain propagation by a modification of both the domain velocity and the shedding of the domains from the cathode.

Frequency multiplication of microwave radiation by use of space charge domains in a GaAs/AlAs superlattice is reported. A microwave field triggered the formation and annihilation of domains giving rise to the generation of harmonics. The new method of frequency multiplication, which is demonstrated for 20 GHz radiation, should be applicable to the generation of radiation up to almost 1 THz.

We investigate the electronic properties of wide miniband (\ensuremath{\sim}100 meV) GaAs/AlAs superlattice structures in which the tunnel barriers are formed from planar arrays of AlAs islands. By studying the effect of an in-plane magnetic field on the miniband conduction, we determine the strength and anisotropy of the rate of electron scattering by the island arrays.