N. N. Sibel’din

Electron-hole liquid in strained SiGe layers of silicon heterostructures

The electron-hole liquid has been found in strained SiGe thin films of Si/Si1−xGex/Si heterostructures. The density and binding energy of the electron-hole liquid have been determined. Owing to the presence of internal strains in the SiGe layer, the density and binding energy are significantly smaller than the respective quantities for the electron-hole liquid in a bulk single crystal of the solid solution of the same composition. The critical temperature of the transition from the exciton gas to the electron-hole liquid is estimated using the experimental data. The Mott transition (from the exciton gas to electron-hole plasma) occurs above the critical temperatures for high excitation intensities.

Modulation of the resonant Rayleigh light scattering spectrum of GaAs/AlGaAs structures with quantum wells under above-barrier illumination

It is found that additional illumination by photons with energies above the band gap width in barrier layers leads to a strong (up to 40% in depth at the values of the illumination power used in this work) modulation of the light intensity elastically scattered upon resonant excitation of exciton states in quantum wells of GaAs/AlGaAs structures. Evidently, the effect observed is associated with the redistribution of oscillator strengths of exciton transitions due to the formation of three-particle exciton complexes (trions). These complexes arise through preferred capture of nonequilibrium like charge carriers (in our case, holes).

Excitonic state in quantum wells formed from “above-barrier” electronic states

Lines corresponding to localized excitonic states formed from “above-barrier” electron and/or hole states (specifically, excitation lines of excitons formed by an electron localized in a QW and a free heavy hole) have been observed in the photoluminescence excitation spectra of GaAs/Al0.05Ga0.95As structures with quantum wells (QWs), each containing one single-particle size-quantization level for charge carriers of each type. A computational method is proposed that permits finding the binding energy and wave functions of excitons in QWs taking the Coulomb potential into account self-consistently. The computed values of the excitonic transition energies agree quite well with the experimental results.

Morphological transformation of a germanium layer grown on a silicon surface by molecular-beam epitaxy at low temperatures

Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molecular-beam epitaxy at low temperatures (<350°C) and studied using photoluminescence and atomic force microscopy. It is found that the germanium layer undergoes a morphological transformation when its thickness becomes equal to approximately five monolayers: an island relief transforms into a smooth undulating relief.

Photoluminescence from germanium quantum wells and quantum dots in silicon grown by MBE at low temperature

The low-temperature (T=2 K) photoluminescence (PL) has been studied in Si/Ge structures grown by MBE at a low (250–350°C) temperature of Ge deposition. The luminescence spectra change dramatically when the average thickness of the Ge layer exceeds six monolayers. In this case, the PL line from the pseudomorphic layer (quantum well) retains its spectral position and increases in intensity at the expense of the luminescence line from islands (quantum dots), which then totally fades. The data obtained indicate a considerable difference between the epitaxial growth modes dominating in low and conventional (500–700°C) temperature ranges.

Phase transitions in nonequilibrium electron-hole systems of Si/SiGe/Si nanoheterostructures

The exciton condensation in a Si1−xGex solid solution layer of Si/Si1−xGex/Si heterostructures with the formation of electron-hole liquid has been investigated by low-temperature photoluminescence spectroscopy. In the temperature range above the critical temperature of the transition from an exciton gas to electron-hole liquid, a Mott transition from an exciton gas to electron-hole plasma has been found and investigated.

Lateral conductivity of p-type doped Si/Ge island structures

Numerical calculations of the conductance in structures with doping modulated along the current-flow direction are carried out taking into account band offsets at the interfaces between high-and low-resistivity regions. It is found that such structures exhibit S-shaped current-voltage characteristics; in the limiting case, there should be a negative-conductance region, with the abruptness of the heterojunction between the narrow-and wide-gap sections of the structure and the doping level being the critically important parameters in the theory. p-type Si/Si1−xGex island structures with different sizes of islands and different band offsets were grown by molecular-beam epitaxy. Theoretical results are compared with the data on lateral conductance of the grown structures.

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.

Photoluminescence of Si/Ge nanostructures grown by molecular-beam epitaxy at low temperatures

Multilayer Si/Ge nanostructures grown by molecular-beam epitaxy at low temperatures (250–300°C) of germanium deposition are studied using photoluminescence and atomic-force microscopy (AFM). It is assumed that, upon low-temperature epitaxy, the wetting layer is formed through the intergrowth of two-dimensional (2D) and three-dimensional (3D) nanoislands.

Exciton spectra of semiconductor superlattices in a parallel magnetic field

Low-temperature photoluminescence and photoluminescence excitation spectra of GaAs/AlGaAs semiconductor superlattices having different potential barrier widths (b=20, 30, 50, and 200 Å), i.e., degrees of tunnel coupling between quantum wells, are studied in magnetic fields up to 5 T oriented parallel and perpendicular to the layers of the structure. The changes in the qualitative character of the photoluminescence excitation spectra observed in a parallel magnetic field with increasing tunnel transparency of the barrier correspond to a transition from a quasi-two-dimensional to a quasi-three-dimensional electronic spectrum as a miniband develops in the superlattice. In the photoluminescence excitation spectra of the superlattice with b=50 Å, as the parallel magnetic field is increased, a new line appears in the violet wing of the spatially indirect exciton excitation line, which is absent in a perpendicular field. A similar line was also observed to arise in the photoluminescence spectra. It is shown that the indirect exciton luminescence line can be suppressed by both parallel and perpendicular magnetic fields.