V. S. Krivobok

Condensation of excitons and the spectrum of multiparticle states in SiGe/Si quantum wells: The role of the barrier in the conduction band

It has been demonstrated that the barrier in the conduction band represented by the SiGe layer in SiGe/Si quantum wells affects the work function and equilibrium density of the quasi-two-dimensional condensed phase formed in these structures. The existence of a new recombination channel with unconventional characteristics is uncovered in the structures with barrier heights close to the critical value for the formation of the electron-hole liquid.

Optical and electrophysical properties of defects in high-purity CdTe

The electronic spectrum of defects formed in low-temperature synthesis and growth of high-purity CdTe[111]from the vapor phase of the starting components has been studied by the photoluminescence and photoconductivity methods, as well as using the analysis of the behavior with temperature of the conductivity. The studies have revealed in the crystals, in addition to the comparatively shallow centers involving primarily donors and acceptors contained in residual substitutional impurities, deep acceptor states with activation energies of 0.25, 0.60, and 0.86 eV, which differ in the character and magnitude of the localizing potential. While the deep centers at 0.60 and 0.86 eV display strong localization of electronic states, the center with the 0.25-eV activation energy is associated with defects for which the major part of the localizing potential extends uniformly in space over several unit cells. Such centers are assumed to originate from twinning-induced extended defects.

Channels of radiative recombination and phase transitions in a system of nonequilibrium carriers in a Si0.93Ge0.07/Si thin quantum well

The “exciton gas-plasma” transition (the Mott transition) in a Si0.93Ge0.07/Si thin quantum well is investigated using low-temperature photoluminescence. It is demonstrated that this transition is smooth and occurs in the concentration range from approximately 6 × 1010 to 1.2 × 1012 cm−2. At a temperature of 23 K and excitation densities of higher than 10 W/cm2, the shape and location of the luminescence line associated with the electron-hole plasma remain unchanged with an increase in the pump density. This can indicate the occurrence of an “electron-hole gas-liquid” transition. It is shown that, in the spectrum of the quantum well, the luminescence of boron-bound excitons dominates at liquid-helium temperatures and low excitation densities, whereas the free-exciton luminescence dominates at temperatures above 10 K. The influence of the homogeneous and inhomogeneous broadening on the electron-hole plasma and exciton luminescence is discussed.

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.

Compensation effect in undoped polycrystalline CdTe synthesized under nonequilibrium conditions

The compensation effect has been revealed in undoped polycrystalline CdTe synthesized during rapid crystallization. The revealed effect leads to an increase in the electrical resistivity to 108–1010 Ω cm at a background impurity concentration of ∼1015 cm−3 (GaCd and ClTe donors, unidentified acceptors). For some samples, this effect is accompanied by the appearance of persistent photoconductivity, which disappears at a temperature of ∼200 K. It has been shown that all the polycrystals studied are characterized by a three-level compensation mechanism in which the fundamental properties of the material are determined by deep donors and/or acceptors with a concentration of 1012 cm−3. Depending on the specific growth conditions, the electrical resistivity at room temperature is determined by deep centers with activation energies of 0.59 ± 0.10 and 0.71 ± 0.10 eV, which are supposedly related to intrinsic point defects, and deep centers with activation energies of 0.4 ± 0.1 eV, which belong to the DX center formed by the GaCd donor.

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.

Photoluminescence of CdTe grown in conditions considerably deviating from thermodynamic equilibrium

Undoped cadmium telluride produced in different conditions of fast crystallization of the vapor phase at temperatures of 420–600°C is studied by the low-temperature photoluminescence technique and electrical measurements. It is shown that, despite the relatively high rate of growth (∼1 μm s−1), the basic parameters of the lattice and band structure of the material are close to the corresponding parameters reported in publications for single-crystal CdTe. The conductivity type of the crystals grown in the study is controlled by hydrogen-like donors and acceptors associated with background impurities. It is found that, along with background impurities in n-type textures, there exist compensating acceptors with the nonstandard activation energies 48.5 ± 0.5, 98.0 ± 0.5, and 119.5 ± 0.5 meV. It is shown that the lowered temperature of growth, together with the excess tellurium content, yields a sharp increase in the concentration of deep electron states and isoelectronic defects with lowered symmetry.

Resonance spectroscopy of donor and acceptor centers in compensated cadmium telluride

Methods based on the selective excitation of low-temperature photoluminescence (LTPL) are applied to the analysis of the electron spectrum of defects created in strongly compensated cadmium telluride (CdTe). It is shown that dominant (with the highest concentration) acceptors have activation energies of 48.2 ± 0.4 meV, 97.9 ± 0.6 meV, and 119.7 ± 1.0 meV, which is not characteristic of known substitutional impurities in CdTe. For each of the acceptors listed above, the excitation energies of states are determined and preliminary conclusions are made about the symmetry of the centers. The observed structure of the excited states differs from the spectrum of standard substitutional acceptors and allows one to simulate the electron configuration of defects.

Fine structure of the emission spectrum of a two-dimensional electron–hole liquid in SiGe/Si quantum wells

The fine structure of the emission spectrum of a quasi-two-dimensional electron–hole liquid in shallow SiGe/Si quantum wells is observed experimentally. This fine structure is explained by the occurrence of steps in the density of states resulting from the coexistence of light and heavy holes in the liquid phase and by the interaction of charge carriers with charge-density oscillations in the liquid.