E. P. Dodin

Tunable hot hole FIR lasers and CR masers

Abstract Basic principles for arranging population inversion and stimulated emission in hot hole systems at low temperature are discussed. Recent experimental results on observation of emissions from intersubband lasers, CR NEMAGs and CR lasers made from p-Ge are given.

Transport in GaAs/AlxGa1−xAs superlattices with narrow minibands: Effects of interminiband tunneling

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.

Stimulated emission at transitions between Wannier–Stark ladders in semiconductor superlattices

New intraband semiconductor lasers—Wannier–Stark lasers—based on simple GaAs (150 Å, quantum well)/GaAlAs (19 Å with an aluminum fraction of 12%, barrier) superlattices have been demonstrated. The amplification mechanism in these lasers is based on population inversion between the ground Wannier–Stark level in the superlattice quantum wells and the weakly populated upper Wannier–Stark level in the wells two, three, or four periods down in the applied potential. Multiple regions of intense stimulated microwave emission near voltages of 8, 13, and 20 V (i.e., in the vicinity of resonances between these Wannier–Stark levels of the superlattice) have been discovered in the laser chips. The stimulated emission emerges in the circuit formed by the chip and its wiring. The emission from one of the chips at a temperature of up to 150 K (near 20 V applied to the chip) occurs at a frequency of about 7.3 GHz and has an estimated power of up to 1 W. It has been shown that the negative conductivity responsible for the emission still persists at 300 K but the emission is unseen owing to high losses in the circuit at this temperature. The superlattice wafer has been grown by metalorganic chemical vapor deposition. It consists of 1000 periods and a stop layer, to produce a metal–superlattice–metal terahertz resonator. Terahertz radiation has not been observed owing to a low amplification, as compared to losses in the resonator. According to the performed experiments, calculations, and discussions, such superlattices as radiation sources in gigahertz, terahertz, and higher frequency ranges could compete with quantum cascade lasers under appropriate optimization of their parameters.

Transport in GaAs/AlxGa1−xAs superlattices with narrow forbidden minibands: Low-frequency negative differential conductivity and current oscillations

Current-voltage characteristics have been measured and low-frequency current instabilities have been studied for GaAs/AlxGa1−xAs 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.

Towards Wannier-Stark THz superlattice laser

Basing on measurements, computer simulations and calculations of transport properties in GaAs-GaAlAs superlattices (SL) with narrow (weak) barriers we propose and elaborate scheme of the SL Wannier-Stark (W.-S.) THz lasers. The laser transitions (tunable by applied voltage) are between levels of the first and the second W.-S. ladders corresponding to wells separated two-three periods apart and being nearby the resonant condition. The resonance provides high value of the transition matrix element that gives value of THz amplification coefficient up to 200-300 cm-1 for the doping 5–10·1015 cm−3. This value is substantially higher than the one for THz quantum cascade lasers and allows to use in the lasers simple n+−SL-n+ waveguide. Some observations on transport in the SLs are given.

Structural, optical, and current investigations of superlattices with a complex AlGaAs-based unit cell

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.

MOCVD-grown heterostructures with GaAs/AlGaAs Superlattices: Growth features and optical and transport characteristics

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.

Optical resonance identification of long-range electron tunneling between superlattice levels in an electric field

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.

Transport Properties and Terahertz Emission in Narrow Minigap GaAs-GaAlAs Superlattices

Low temperature transport and THz emission in GaAs-GaAlAs narrow minigap superlattices are studied. The THz emission due to interminiband transition, the Bloch oscillation, bremsstrahlung and reststrahlen (in heated by current pulse lattice) are observed by Ge:Ga, Ge:Au, n-GaAs and n-InSb detectors.