N. N. Sibeldin

Electron-hole liquid and excitonic molecules in quasi-two-dimensional SiGe layers of Si/SiGe/Si heterostructures

The electron-hole liquid (EHL) in SiGe layers of Si/Si1 − xGex/Si quantum-confinement heterostructures is discovered. It is composed of quasi-two-dimensional holes in the quantum well formed by the SiGe layer and quasi-three-dimensional electrons, which occupy a wider region of space centered on this layer. The densities of electrons and holes in the EHL are determined to be p0 ≈ 8.5 × 1011 cm−2 and n0 ≈ 4.8 × 1018 cm−3, respectively. It is demonstrated that the gas phase consists of excitons and excitonic molecules. The conditions on the band parameters of the structure under which the formation of the EHL of this kind and biexcitons is possible are formulated.

Luminescence of a quasi-two-dimensional electron-hole liquid and excitonic molecules in Si/SiGe/Si heterostructures upon two-electron transitions

The low-temperature photoluminescence of Si/Si0.91Ge0.09/Si heterostructures in the near-infrared and visible spectral ranges is investigated. For the structure in which the barrier in the conduction band formed by the SiGe layer is transparent to electron tunneling, the broad luminescence line observed in the visible range is analyzed by comparing its shape with the numerical convolution of the spectrum of near-infrared recombination radiation originating from the electron-hole liquid. The comparison demonstrates that, at high excitation levels, the visible-range emission is caused by two-electron transitions in a quasi-two-dimensional spatially direct electron-hole liquid. Furthermore, the combined analysis of the photoluminescence spectra in the near-infrared and visible ranges yields the binding energy of a quasi-two-dimensional free biexciton in the SiGe layer of these heterostructures. In the structures with a wide SiGe layer that is not tunneling-transparent to electrons, a spatially indirect (dipolar) electron-hole liquid is observed.

Addressing the exciton fine structure in colloidal nanocrystals: the case of CdSe nanoplatelets

We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton splitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques.

Nonlinear emission dynamics of a GaAs microcavity with embedded quantum wells

The emission dynamics of a GaAs microcavity at different angles of observation with respect to the sample normal under conditions of nonresonant picosecond-pulse excitation is measured. At sufficiently high excitation densities, the decay time of the lower polariton emission increases with the polariton wavevector; at low excitation densities the decay time is independent of the wavevector. The effect of additional nonresonant continuous illumination on the emission originating from the bottom of the lower polariton branch is investigated. The additional illumination leads to a substantial increase in the emission intensity (considerably larger than the intensity of the photoluminescence excited by this illumination alone). This fact is explained in terms of acceleration of the polariton relaxation to the radiative states due to scattering by charge carriers created by the additional illumination. The results obtained show that, at large negative detunings between the photon and exciton modes, polariton-polariton and polariton-free carrier scattering are the main processes responsible for the filling of states near the bottom of the lower polariton branch.

Heating and evaporation of a two-dimensional electron–hole liquid by heat pulses

The dynamics of low-temperature (T = 5 K) photoluminescence spectra of Si/Si1-xGex/Si heterostructures (x = 0.045) under the influence of a stream of nonequilibrium phonons (heat pulses) propagating in the structure is investigated. The rapid evaporation of the electron–hole liquid in the quantum well of the structure is observed as the liquid is heated by nonequilibrium phonons. It is established that an increase in the exciton-gas density in the quantum well is caused by the evaporation of the electron–hole liquid and by an increase in the rate of exciton capture by the quantum well. It is shown that the interaction with nonequilibrium phonons results in the dissociation of bound-exciton complexes in the Si layers, which is accompanied by an increase in the exciton concentration and lifetime.

Kinetics of accumulation of excess holes under photoexcitation and their relaxation in GaAs/AlGaAs shallow quantum wells

The kinetics of accumulation of long-lived excess holes that appear in GaAs/Al0.05Ga0.95As shallow quantum wells under above-barrier photoexcitation and their relaxation is studied by time-resolved photoluminescence spectroscopy. The establishment of a steady state in the nonequilibrium electron-hole system under various combinations of above-barrier and intrawell excitation is also investigated. It is found that the temperature dependence of the excess-hole relaxation time (their lifetime in the quantum wells) exhibits activation behavior with two activation energies. It is established that excitons produced by nonresonant intrawell excitation undergo efficient cooling as they scatter off accumulated long-lived holes.

Accumulation of the excess of one type of charge carriers and the formation of trions in GaAs/AlGaAs shallow quantum wells

The dynamics of excitons and trions in GaAs/AlGaAs heterostructures with shallow quantum wells is studied in time-resolved photoluminescence experiments carried out with different repetition rates of picosecond pump pulses for the cases of intrawell, above-barrier, and“two-color” excitation. It is established that excess charge carriers of one type accumulated in the quantum wells under above-barrier excitation play a key role in the formation and dynamics of the exciton-trion system and determine its composition and kinetic properties. The lifetime of excess charge carriers in the quantum wells, estimated from the experimental data, exceeds 10 μs.

Effect of the excitation level on the thermal quenching and dynamics of the photoluminescence of GaAs/AlGaAs shallow quantum well structures

The effect of the excitation level on the dynamics of heavy-hole exciton photoluminescence in tunneling-isolated GaAs/AlxGa1 − xAs (x = 0.05) shallow quantum wells at temperatures of 5 to 70 K is investigated. It is shown that the exciton lifetimes depend strongly on the excitation level, while the activation energies characterizing the thermal escape of nonequilibrium charge carriers from the wells virtually do not.

Dipolar Biexcitons in Lateral Traps in Si/SiGe/Si Heterostructures

Si/Si1–xGex/Si heterostructures with large-scale (micrometer-size) lateral potential fluctuations at the upper SiGe/Si-cap heterointerface are grown. These potential fluctuations are caused by partial strain relaxation in the SiGe layer. Low-temperature photoluminescence (PL) spectra show that these fluctuations form lateral traps where photoexcited nonequilibrium charge carriers are accumulated and bind into dipolar excitons, which ultimately recombine. At temperatures below 6 K, a new narrow line with a width considerably less than that of the dipolar exciton PL line emerges in the spectra as the level of excitation increases. It is shown that this line is associated with the recombination of dipolar biexcitons in large-scale traps.

PHASE TRANSITIONS IN A TWO-DIMENSIONAL SYSTEM OF DIPOLAR EXCITONS IN A DOUBLE-WELL SIGE/SI HETEROSTRUCTURE

The transition from a dipolar electron–hole liquid (EHL) to a spatially direct one in two-dimensional layers of a type II (buffer Si1–yGey)/tSi/sSi1–xGex/tSi/(cap Si1–yGey) heterostructure is studied via photoluminescence spectroscopy at helium temperatures and high excitation levels. This transition occurs when the sSi1–xGex barrier layer for electrons (a quantum well (QW) for holes), that separates electron QWs (tSi layers) gets thinner. The basic parameters of both types of EHLs and the lifetime of dipolar excitons are determined.