A. A. Ignatov

High-frequency self-sustained current oscillation in an Esaki-Tsu superlattice monitored via microwave emission

Abstract We report the observation of a high-frequency self-sustained current oscillation in an n doped GaAs AlAs superlattice (at room temperature) that shows a negative differential conductance (Esaki-Tsu superlattice). The superlattice biased in the region of the negative differential conductance emitted microwave radiation at a fundamental frequency of about 6 GHz and at harmonics. We attribute the oscillation to space charge instabilities inside the superlattice caused by Bragg reflection of electrons in the lowest, wide miniband.

THz-field induced nonlinear transport and dc voltage generation in a semiconductor superlattice due to Bloch oscillations

We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.

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.

Frequency doubling and tripling of terahertz radiation in a GaAs/AlAs superlattice due to frequency modulation of Bloch oscillations

We report on frequency doubling and tripling of THz radiation in a voltage-biased GaAs/AlAs superlattice. By use of a corner cube antenna system, radiation from the Santa Barbara free-electron laser (frequency 0.7 THz) was guided into a superlattice mesa element and the second and third harmonic were coupled out of the mesa. Without bias only radiation of the third harmonic was generated, while the biased superlattice emitted radiation of both the second and third harmonic. We attribute the harmonic generation to frequency modulation of damped Bloch oscillations of the miniband electrons in the superlattice.

Ultrafast detection and autocorrelation of picosecond THz radiation pulses with a GaAs/AlAs superlattice

We used a wide miniband GaAs/AlAs superlattice (at room temperature) for detection and autocorrelation of picosecond THz radiation pulses (frequency 4.3 THz) from a free-electron laser. The detection was based on a THz-field induced change in conductivity of the superlattice, and the correlation on the nonlinearity of the conductivity change at strong THz-pulse-power. The nonlinear conductivity change was due to two effects, which we attribute to dynamical localization of miniband electrons and to ionization of deep impurity centers.

Suppression of current through an Esaki–Tsu GaAs/AlAs superlattice by millimeter wave irradiation

We report the observation of suppression of the dc current through an Esaki–Tsu GaAs/AlAs superlattice (that shows a negative differential conductance due to Bloch oscillations of miniband electrons) caused by irradiation with millimeter wave radiation (frequency 78 GHz). A theoretical analysis of the dc current indicated that the high‐frequency electric current followed the high‐frequency field with a response time smaller than the period (∼10−11 s) of the field. Our experiment, with the superlattice at room temperature, demonstrates that the Esaki–Tsu superlattice is suitable for ultrafast millimeter wave detection and other high‐frequency applications.

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.

Millimeter wave oscillator based on a quasiplanar superlattice electronic device

We report on a millimeter wave oscillator based on a quasiplanar superlattice electronic device (SLED). The SLED, a lateral structured GaAs/AlAs superlattice, showing, at room temperature, a negative differential conductance, was provided with two terminals lying in one plane and mounted in a waveguide structure. The oscillator delivered radiation, in a relative bandwidth of 10−5, that was tunable by about 10% around 70 GHz and had a power of 100 μW; depending on the voltage across the superlattice, additional oscillation lines (up to 180 GHz) appeared. We associate the generation of radiation with a current oscillation caused by traveling dipole domains in the superlattice.

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.

X-ray characterization of an Esaki-Tsu superlattice and transport properties

We have studied, by x-ray scattering, structural parameters of a short-period GaAs/AlAs superlattice and related the parameters to electric transport properties. Our results, for a superlattice consisting of 100 periods of about ten GaAs and seven AlAs monolayers in turn, indicate that an interface roughness of about 0.3 nm (thickness of one monolayer) may be responsible for a remarkable reduction of the electron mobility for the transport along the superlattice axis.