G. K. Rasulova

Current bistability and switching in weakly coupled superlattices GaAs/AlGaAs

Current–voltage characteristics of 30 period 390 A GaAs/110 A Al0,3Ga0,7As superlattices are studied. Features revealed in multistable current branches of the hysteresis loops made it possible to estimate gradual changes in the size and shape of the domain boundary and reconstruct the final shape of the domain boundary. It was concluded that a considerable current bistability observed in the investigated superlattices is due to the domain boundary expansion. Switching between bistable current states in each hysteresis loop is detected. The switching time depends on the direction of the current jump. The switching time from a high- to low-current state is about 10−7 s; the time of the reverse switching to the high-current state is about 10−6 s. The observed switching is attributed to the shrinkage and expansion of the domain boundary size.

Self-oscillations in weakly coupled GaAs/AlGaAs superlattices at 77.3 K

We report 77.3 K self-oscillations in a 30-period weakly coupled GaAs/Al0.3Ga0.7As (28/10 nm) superlattice (SL). A study of frequency spectra of self-oscillations under external sinusoidal force in frequency-locked regime at different fixed voltages showed that current oscillations detected in the structureless regions of the I-V characteristic of weakly coupled SLs are forced oscillations. It was established that oscillations of free oscillator arising when the voltage is fixed at the first negative differential conductivity region at the beginning of the plateaulike region in the I-V characteristic are forcing oscillations. It was also shown that in order to describe the mechanism of the current oscillations in weakly coupled SLs, the model of the current oscillations in resonant tunneling diodes can be successfully applied.

Dynamic characteristics of weakly coupled GaAs/AlGaAs superlattices

Dynamic characteristics of weakly coupled superlattices have been experimentally studied by investigation of the transient current responses and an oscilloscopic study of the hysteresis loop. The observed transient current responses are classified into two categories: the switching current response with different levels of the initial and final current, and the transient current response retaining the same value of the initial and final current. The study of the oscilloscope images of the current–voltage characteristics produced by different types of an external sweep allowed us to reproduce the current traces for closed cycle current oscillations. Damped current oscillations with a frequency of 83 kHz have been observed.

Higher harmonics in the current oscillations in weakly coupled GaAs/AlGaAs superlattices

It is shown that the self-sustained oscillations observed in weakly coupled GaAs/AlGaAs superlattices result from the superposition of sinusoidal oscillations of several coupled oscillators. Each oscillator is a single-barrier resonant tunneling diode. The coupled oscillators form an expanded boundary of an electric-field domain under conditions of negative differential conductance.

Mutual synchronization of two coupled self-oscillators based on GaAs/AlGaAs superlattices

The interaction of self-oscillators based on 30-period weakly coupled GaAs/AlGaAs superlattices is studied. The action of one self-oscillator on the other was observed for a constant bias voltage in the absence of generation of self-sustained oscillations in one of the oscillators. It is shown that induced oscillations in a forced oscillator appear due to excitation of oscillations in the system of coupled oscillators forming the electric-field domain wall at the frequency of one of the higher harmonics of a forcing oscillation.

Carrier density profile in weakly coupled GaAs/AlGaAs superlattices

The effect of doping level on the current-voltage characteristics of semiconductor heterostructures with GaAs/AlGaAs superlattices was studied by the electrochemical capacitance-voltage profiling method. It was shown that a high density of free electrons in the GaAs/AlGaAs superlattice screens the external electric field and inhibits the formation of a domain with a high electric field that is responsible for resonant tunneling in weakly coupled superlatticest.

Electric-field domain boundary instability in weakly coupled semiconductor superlattices

Damped oscillations of the current were observed in the transient current pulse characteristics of a 30-period weakly coupled GaAs/AlGaAs superlattice (SL). The switching time of the current is exponentially decreased as the voltage is verged towards the current discontinuity region indicating that the space charge necessary for the domain boundary formation is gradually accumulated in a certain SL period in a timescale of several hundreds ns. The spectral features in the electroluminescence spectra of two connected in parallel SL mesas correspond to the energy of the intersubband transitions and the resonance detuning of subbands caused by charge trapping in the quantum wells (QWs) residing in a region of the expanded domain boundary. The obtained results support our understanding of the origin of self-oscillations as a cyclic dynamics of the subband structure in the QWs forming the expanded domain boundary.

Generation in a microstrip-resonator-stabilized double-barrier resonant tunneling structure

Generation in a microstrip-resonator-stabilized double-barrier resonant tunneling structure based on GaAs/AlAs heterostructures has been investigated for the first time. The structures fabricated contain near-contact layers (spacers) that prevent impurities from penetrating into the active part of the structure and improve the temporal characteristics of the system. AuNiGe alloy microstrip contacts, which connect the structure with an external rf circuit, were prepared in a planar implementation, making it possible to minimize the RC delay time in the negative differential conductance region by decreasing the series resistance and capacitance of the structure. In structures with spacer layers, the negative differential conductance exhibits a complex behavior due to the influence of the space charge.

Resonance tunneling via excited subbands and the existence of a new type of electric-field domains in long-period superlattices

Successive resonance tunneling via excited states, accompanied by the appearance of resonance features on the multistability current-voltage branches, is observed in a superlattice. The data show a strongly non-equilibrium charge-carrier distribution over subbands with energies below the energy of an optical phonon and indicate that a new type of an electric-field domain forms, the structure of which is determined by resonance tunneling between excited subbands in neighboring quantum wells.