A. A. Pruchkina

Dynamics of the phase transitions in the system of nonequilibrium charge carriers in quantum-dimensional Si1 − xGex/Si structures

The dynamics of the phase transition from an electron-hole plasma to an exciton gas is studied during pulsed excitation of heterostructures with Si1 − xGex/Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si1 − xGex layer. The electron-hole system decomposes into a rarefied exciton and a dense plasma phases for quantum wells with a germanium content x = 3.5% in the time range 100–500 ns after an excitation pulse. In this case, the electron-hole plasma existing in quantum wells has all signs of an electron-hole liquid. A qualitatively different picture of the phase transition is observed for quantum wells with x = 9.5%, where no separation into phases with different electronic spectra is detected. The carrier recombination in the electron-hole plasma leads a gradual weakening of screening and the appearance of exciton states. For a germanium content of 5–7%, the scenario of the phase transition is complex: 20–250 ns after an excitation pulse, the properties of the electron-hole system are described in terms of a homogeneous electron-hole plasma, whereas its separation into an electron-hole liquid and an exciton gas is detected after 350 ns. It is shown that, for the electron-hole liquid to exist in quantum wells with x = 5–7% Ge, the exciton gas should have a substantially higher density than in quantum wells with x = 3.5% Ge. This finding agrees with a decrease in the depth of the local minimum of the electron-hole plasma energy with increasing germanium concentration in the SiGe layer. An increase in the density of the exciton gas coexisting with the electron-hole liquid is shown to enhance the role of multiparticle states, which are likely to be represented by trions T+ and biexcitons, in the exciton gas.

Multiparticle states and the factors that complicate an experimental observation of the quantum coherence in the exciton gas of SiGe/Si quantum wells

The measured stationary and time-resolved photoluminescence is used to study the properties of the exciton gas in a second-order 5-nm-thick Si0.905Ge0.095/Si quantum well. It is shown that, despite the presence of an electron barrier in the Si0.905Ge0.095 layer, a spatially indirect biexciton is the most favorable energy state of the electron–hole system at low temperatures. This biexciton is characterized by a lifetime of 1100 ns and a binding energy of 2.0–2.5 meV and consists of two holes localized in the SiGe layer and two electrons mainly localized in silicon. The formation of biexcitons is shown to cause low-temperature (5 K) luminescence spectra over a wide excitation density range and to suppress the formation of an exciton gas, in which quantum statistics effects are significant. The Bose statistics can only be experimentally observed for a biexciton gas at a temperature of 1 K or below because of the high degree of degeneracy of biexciton states (28) and a comparatively large effective mass (about 1.3me). The heat energy at such temperatures is much lower than the measured energy of localization at potential fluctuations (about 1 meV). This feature leads to biexciton localization and fundamentally limits the possibility of observation of quantum coherence in the biexciton gas.

Excitonic luminescence of SiGe/Si quantum wells δ-doped with boron

Low-temperature photoluminescence of undoped and moderately δ-doped Si1−xGex/Si (x < 0.1) quantum wells has been studied. The influence of boron δ-layer on the excitonic luminescence and the luminescence caused by a dense electron plasma was demonstrated. The conditions under which the luminescence spectra of quantum wells are dominated by impurity-bound excitons (BE) have been established. Some unusual properties of these BE are explained in terms of type II band-offset in Si1−xGex/Si (x < 0.1) quantum wells, which favors a spatial separation of electrons and holes. It is shown that the temperature dependence of an excitonic emission in the quantum wells allows to calculate the BE-related density of states and, thus, can be used for contactless estimation of the impurity concentration in quantum wells.

Electronic spectrum of non-tetrahedral acceptors in CdTe:Cl and CdTe:Bi,Cl single crystals

The electronic spectra of complex acceptors in compensated CdTe:Cl, CdTe:Ag,Cl, and CdTe:Bi,Cl single crystals are studied using low-temperature photoluminescence (PL) measurements under both nonresonant and resonant excitation of distant donor–acceptor pairs (DAP). The wavelength modulation of the excitation source combined with the analysis of the differential PL signal is used to enhance narrow spectral features obscured because of inhomogeneous line broadening and/or excitation transfer for selectively excited DAPs. For the well-known tetrahedral (TD) AgCd acceptor, the energies of four excited states are measured, and the values obtained are shown to be in perfect agreement with the previous data. Moreover, splitting between the 2P3/2 (Г8) and 2S3/2 (Г8) states is clearly observed for AgCd centers located at a short distance (5–7 nm) from a hydrogen-like donor (ClTe). This splitting results from the reduction of the TD symmetry taking place when the acceptor is a member of a donor–acceptor pair. For ...

Effects of photoinduced charge redistribution on excitonic states in Zn(Cd)Se/ZnMgSSe quantum wells

Photoinduced charge redistribution processes in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well structures are studied using steady-state photoluminescence, photoreflectance, and scanning spreading resistance microscopy with an additional illumination. It is shown that an above-barrier optical pumping leads to the accumulation of electrons in the quantum wells. The resulting concentration of excess electrons in the quantum wells is several orders of magnitude higher than the concentration of photoexcited electron-hole pairs. These excess electrons induce broadening of excitonic resonances and, furthermore, cause an enhancement in the photoluminescence quantum yield and an increase in the relative intensity of the bound-exciton emission line. The additional below-barrier illumination at temperatures about 100 K leads to a decrease in the excess electron concentration in the quantum wells. The observed phenomena are explained in terms of a simple model considering the formation of a barrier in the conduction band near t...

Complex acceptors in CdTe:Cl investigated by differential spectroscopy

The acceptor states in CdTe:Cl single crystals are investigated using a technique based on the analysis of differential spectra of selectively excited donor-acceptor pair photoluminescence. The splitting of 2P3/2(Γ8) and 2S3/2(Γ8) states of the AgCd tetrahedral acceptor taking place upon a decrease in the donor-acceptor separation is found. The energies of seven excited states of the complex acceptor with an activation energy of ∼121 meV are determined.

Kinetics of Low-Temperature Microphotoluminescence of Exciton-Impurity Complexes in CdZnTe Single Crystals

Low-temperature (6.5 K) microphotoluminescence near the intrinsic absorption edge of CdZnTe alloy single crystals is studied under conditions of non-resonant and resonant excitation by picosecond pulses. Characteristic relaxation times for free excitons and exciton-impurity complexes of various types are determined. A significant (by a factor of 4–8) decrease in the photoluminescence signal decay time of exciton-impurity complexes on neutral donors during the transition to the resonant excitation mode is detected. The detected sharp decrease in the photoluminescence signal decay time can indicate the manifestation of collective effects in this system of emitters.

Isolated (Quantum) Emitters Generated with the Participation of Donor–Acceptor Pairs in the ZnSe/ZnMgSSe Heterostructure

Measurements of low-temperature (5 K) microphotoluminescence reveal the existence of isolated (quantum) emitters in ZnSe/ZnMgSSe quantum well with a radiation line experiencing abrupt displacements of its spectral position by several meV within 1–10 min. The unusual properties of the detected emitters are explained on the basis of a system with a large dipole moment in its ground state, e.g., a single donor–acceptor pair.

Radiative d–d transitions at tungsten centers in II–VI semiconductors

The luminescence spectra of W impurity centers in II–VI semiconductors, specifically, ZnSe, CdS, and CdSe, are studied. It is found that, if the electron system of 5d (W) centers is considered instead of the electron system of 3d (Cr) centers, the spectral characteristics of the impurity radiation are substantially changed. The electron transitions are identified in accordance with Tanabe–Sugano diagrams of crystal field theory. With consideration for the specific features of the spectra, it is established that, in the crystals under study, radiative transitions at 5d W centers occur between levels with different spins in the region of a weak crystal field.

Exciton emission of crystalline Zn(S)Se thin films arranged in microcavities based on amorphous insulating coatings

A technology for the production of fully hybrid microcavities on the basis of Zn(S)Se films and amorphous insulating SiO2/Ta2O5 coatings is proposed. The influence of all stages of the manufacturing cycle on the structure of exciton states in the Zn(S)Se films is demonstrated. This influence is reduced to four main effects: the appearance of a fine structure of emission lines related to free excitons; a decrease in the relative contribution of excitons bound at neutral acceptors to the exciton-emission spectrum; a shift of the emission lines related to exciton–impurity complexes and free excitons to lower frequencies; and a decrease in the splitting between emission lines related to heavy and light excitons. Samples of fully hybrid microcavities, in which the high structural and optical quality of Zn(S)Se films is retained, are fabricated.