Ekkehard Schomburg

Self-sustained current oscillation above 100 GHz in a GaAs/AlAs superlattice

A GaAs/AlAs superlattice with a large miniband (120 meV) showed self-sustained current oscillation at a frequency of 103 GHz giving rise to microwave emission (power 0.5 mW). The emission line had a linewidth of about 1 MHz and was tuneable by about 800 MHz. An analysis suggests that the transport in the superlattice was mainly due to electrons in the lowest miniband and that the oscillation was caused by traveling dipole domains. We also observed frequency locking of the current oscillation attributed to a synchronization of domain propagation by the external high-frequency field.

Broadband semiconductor superlattice detector for THz radiation

We report on a broadband GaAs/AlAs superlattice detector for THz radiation; a THz field reduces the current through a superlattice, which is carried by miniband electrons, due to modulation of the Bloch oscillations of the miniband electrons. We studied the detector response, by use of a free electron laser, in a large frequency range (5–12 THz). The responsivity showed strong minima at frequencies of infrared active phonons of the superlattice. A theoretical analysis of the detector delivers an understanding of the role of phonons and gives a characterization of the responsivity.

Superlattice with hot electron injection: An approach to a Bloch oscillator

A semiconductor superlattice with hot electron injection into the miniband is considered. The injection changes the stationary distribution function and results in a qualitative change of the frequency behavior of the differential conductivity. In the regime with Bloch oscillating electrons and injection into the upper part of the miniband the region of negative differential conductivity is shifted from low frequencies to higher frequencies. We find that the dc differential conductivity can be made positive and thus the domain instability can be suppressed. At the same time the high-frequency differential conductivity is negative above the Bloch frequency. This opens another way to make a Bloch oscillator operating at THz frequencies.

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.

High-Frequency Impedance of Driven Superlattices

The complex impedance of a semiconductor superlattice biased into the regime of negative differential conductivity and driven by an additional gigahertz ac voltage is computed. From a simulation of the nonlinear spatiotemporal dynamics of traveling field domains we obtain strong variations of the amplitude and phase of the impedance with increasing driving frequency. These serve as fingerprints of the underlying quasiperiodic or frequency locking behavior. An anomalous phase shift appears as a result of phase synchronization of the traveling domains. If the imaginary part of the impedance is compensated by an external inductor, both the frequency and the intensity of the oscillations strongly increase.

Thermal breakdown, bistability, and complex high-frequency current oscillations due to carrier heating in superlattices

The impact of electron heating on vertical electrical transport in superlattices is shown to cause an S-shaped current–voltage characteristic in addition to the conventional N type occurring at lower fields. Our calculations are supported by experimental data. The combination of S- and N-type instabilities leads to a modified structure of the high-field domains associated with self-generated GHz oscillations.

Miniband transport in a GaAs/AlAs superlattice with submonolayer barriers in a static and THz electric field

Abstract We report on the observation of miniband transport in a GaAs/AlAs superlattice with AlAs barrier layers of submonolayer thickness. The current–voltage characteristic of the superlattice at static electric fields showed a negative differential conductance, which is related to miniband electrons performing Bloch oscillations. Under the influence of a THz field, the current through the superlattice was reduced due to a dynamic localisation of the miniband electrons. Our results showed that the novel superlattice can be used as THz detector with a dynamic range of six orders of magnitude and with a responsivity of 0.2 V/W.

Superlattice frequency multiplier for generation of submillimeter waves

We observed frequency multiplication of 65-GHz radiation up to the fifth harmonic (325 GHz; wavelength 0.9 mm) in a superlattice with negative differential conductance at room temperature. The efficiency of multiplication depended strongly on both the strength of a static field and the strength of the 65-GHz field. An analysis of the results shows that the nonlinear current, responsible for the frequency multiplication, was governed by the Esaki-Tsu current-voltage characteristic that describes the transport of carriers in large-miniband superlattices.

Nonlinear superlattice transport limited by Joule heating

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.

Superlattice detector as a fast direct detector and autocorrelator for terahertz radiation

We report on a GaAs/AlAs superlattice detector as a novel direct detector and autocorrelator for THz radiation. It is based on a doped wide-miniband GaAs/AlAs superlattice, with submonolayer AlAs barrier layers; the superlattice is operated at room temperature. THz radiation, generated by a free-electron laser and a mode locked p-Ge laser, was coupled into the superlattice via a corner cube antenna system. THz-irradiation of the biased superlattice resulted in a current reduction, which was monitored. The direct detector showed a fast response (20 ps, limited by the electronic circuit) and was robust against intense radiation pulses (peak power 10 kW). The responsivity was 100 times higher than the responsivity of detectors of comparable risetime and comparable robustness. Intense THz radiation caused a complete suppression of the current through the superlattice. This is the basis of the superlattice autocorrelator. The superlattice autocorrelator could resolve picosecond radiation pulses.