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AbstractNarrow band emissions at 2.6–2.8 THz are observed out of liquid helium cooled 1 mm disk chips prepared of a wafer with the very low n type doped weak barrier GaAs–GaAlAs superlattice of 1000 periods. The emissions are at about 8.0–18.0 V pulsed voltage applied to the chips in region of the chips positive DC differential conductivity that guaranties absence of inhomogeneous electric field domains in the chips. The emission frequency bands are estimated with a cyclotron resonance filter; the measurements show that the band width is of about that of the THz quantum cascade laser. By using long voltage pulses the chip heating above 100 K is achieved without substantial change in emission power. We speculate that the emission is super luminescence (amplification) of whispering gallery modes in the chips as a result of inverted Wannier-Stark level transitions under bias. The results are the first world demonstration of THz stimulated emission in a simple superlattice within region of positive DC differential conductivity; they give strong impetus for development of THz and higher frequency sources based on such simple superlattices; the sources should well compete with the THz quantum cascade lasers in particular at elevated temperatures.

The results of complex analysis of the parameters of the GaAs/AlGaAs semiconductor superlattice with a complex unit cell using mathematical modeling and experimental methods of transmission electron microscopy, energy-dispersive analysis, and spectroscopy of photoluminescence and photocurrent are presented. Suppression of the intraband static negative differential conductivity under strong interaction conditions between minibands and the inharmonic electron-dispersion law is shown.

The results of studies of MOCVD growth regularities of GaAs/AlGaAs superlattices with narrow forbidden minibands are presented. The spectra of photoluminescence and X-ray diffraction are measured, the concentration distribution profiles of components are determined by secondary-ion mass spectrometry, and the concentration distribution of charge carriers are determined by the method of capacitance profiling. The relation of the growth modes of heterostructures to their crystalline characteristics and luminescent and electrical properties are studied. The photoluminescence measurements indicate the high quality of the superlattices. X-ray diffraction and the data on secondary ions confirm the high periodicity of the superlattices grown in the optimized modes. The nonlinear current-voltage characteristics with a region of negative differential conductivity at moderate voltages and subsequent current increase at higher voltages because of tunneling between the minibands is found for the superlattices grown in the optimal growth modes. Current oscillations at frequencies of ∼60 MHz were observed in the region of negative differential conductivity. The negative differential conductivity and oscillations confirm the presence of the electron localization effect in moderate electric fields in the first conductivity miniband, which emerges due to the Bragg reflection of carriers in the superlattice.

The main requirements for optimization of the MCT-based (CdxHg1 − xTe-based) structures for an increase in the wavelength of the stimulated radiation from them under optical pumping are discussed. The emergence of stimulated radiation in MCT range of 2–2.5 μm at room temperature for optimized MCT structures grown on GaAs substrates using the method of molecular beam epitaxy is shown experimentally. The obtained experimental data are the first results of observation of stimulated radiation from the MCT structures at these wavelengths at room temperature. The gain factor in the active medium measured for this case is very large and attains values of 50 cm−1, which allows one to hope that a further advance to a longer wavelengths is possible.

The results of experimental investigations for the I–V characteristics are presented for superlattices based on GaAs/AlxGa1−xAs with thin barriers in the electric-field region with an intense interminiband tunneling. The regular features for the I–V characteristics are found in the voltage range adequate to the static positive differential conductivity in the superlattice. On the basis of calculations for the Wannier-Stark levels, it is established that the observed features are associated with the resonant tunneling between these levels belonging to quantum wells located at a distance from 6–13 superlattice periods from each other. It is noted that the similar resonant delocalization of Wannier-Stark wave functions can lead to the existence of dynamic negative differential conductivity for the laser type of an appreciable value in such superlattices.

Current-voltage characteristics have been measured and low-frequency current instabilities have been studied for GaAs/Al x Ga1−x As superlattices with narrow forbidden minibands. At relatively low electric fields, a saw-like structure for current-voltage characteristics with alternating portions of positive and negative differential conductivity and spontaneous generation of low-frequency current oscillations with a complex frequency spectrum (varying from discrete to continuous) are observed. It is shown that the observed specific features of electron transport are caused by the spatial-temporal dynamics of electric-field domains (dipoles and monopoles). The effects of the bifurcation, hysteresis, and multistability of current-voltage characteristics are also observed. At high fields, regular features are observed and identified in the current-voltage characteristics; these features are caused by resonance tunneling of electrons between the levels of the Wannier-Stark ladders belonging to quantum wells separated by several periods.

The experimental observation of stimulated radiation of optically pumped CdxHg1 − x Te-based heterostructures in the wavelength range of 1.4–4.5 μm is reported. In the experiments, graded-gap Cd xHg1 − x Te samples grown on GaAs and Si substrates by molecular beam epitaxy were used. Superluminescence of such structures was observed at 77–300 K under the pulsed pumping of the samples by a Nd:YAG laser at a wavelength of 1.064 μm. At room temperature, stimulated radiation was observed at wavelengths of 1.4–1.7 μm. The obtained experimental data are the first results on the observation of stimulated radiation from graded-gap Cd x Hg1 − x Te structures on Si and GaAs substrates at these wavelengths at room temperature.

The experimental data on observation of spontaneous and stimulated emission from thin epitaxial CdxHg1−x Te films optically pumped by Nd: YAG laser radiation are reported. A simple theoretical model is suggested to describe the initiation of population inversion under these conditions. The parameters realized under the experimental conditions are theoretically estimated.

A method for diagnostics of the interaction of electron states in multilayer heterostructures in a dc electric field is suggested. The method is based on measurements of the resonance response of the harmonic amplitude of the current in a heterostructure to modulated laser radiation. The method is used for studying tunneling between the Wannier-Stark levels in the superlattice with strongly coupled quantum wells.

Stimulated emission of radiation from CdxHg1−xTe samples pumped by a Nd:YAG laser at temperature T≈77 K is obtained. Both spontaneous and stimulated emission is observed for the wavelengths λ in the range 3300–3600 nm. Experimental emission spectra are presented. The special features of the spectra and the possible applications of the structures are discussed.