Electron-hole Liquid Research Articles

Multicomponent Electron–Hole Liquid in Si/SiGe Quantum Wells

The density functional theory is used to calculate the energy of an electron–hole liquid in Si/Si1–xGex/Si quantum wells. Three one-dimensional nonlinear Schrodinger equations for electrons and light and heavy holes are solved numerically. It is shown that, in shallow quantum wells (small x), both light and heavy holes exist in the electron–hole liquid. Upon an increase in the Ge content, a transition to a state with one type of holes occurs, with the equilibrium density of electron–hole pairs decreasing by more than a factor of 2.

Quasi-Two-Dimensional Electron–Hole Liquid in Si/SiO2 Quantum Wells

To calculate the energy and equilibrium density of electron-hole pairs in SiO2/Si/SiO2 quantum wells (QWs), nonlinear Schrodinger equations for electrons and holes were numerically solved. Calculations were carried out for (100) and (111) silicon surfaces and various values of the QW width. It is shown that the binding energy of electron-hole pairs in a quasi-two-dimensional electron-hole liquid (EHL) is much higher than the binding energy in a three-dimensional EHL. The results of calculations are compared with the experimental results for a wide range of QW widths.

Quasi-Two-Dimensional Electron–Hole Liquid in Shallow SiGe/Si Quantum Wells

An analytical expression is obtained for the energy of a quasi-two-dimensional electron-hole liquid (EHL) in shallow quantum wells. It is shown that in the Si/Si1–xGex/Si structures with small x, the EHL contains light and heavy holes. With increasing x, the transition of EHL to a state with heavy holes occurs, and the equilibrium density of electron-hole pairs strongly decreases. The effect of an external electric field on the EHL properties is studied.

Intraexciton and Intracenter Terahertz Radiation from Doped Silicon under Interband Photoexcitation

Terahertz photoluminescence of boron- and phosphorus-doped silicon at low temperatures under interband photoexcitation is investigated. The lines of radiative transitions between free-exciton levels and between the levels of shallow impurity centers are observed. The intensities of these lines exhibit different dependences on temperature and excitation intensity. At temperatures near the temperature of liquid helium (T ~ 5 K), the terahertz radiation spectrum features a broad band (about 18–20 meV wide) with a peak at an energy of about 20–22 meV. This band is apparently associated with radiative transitions of nonequilibrium charge carriers from the states of the continuum to the state of an electron–hole liquid.

Multicomponent Structure of an Electron-Hole Liquid in Shallow SiGe/Si Quantum Wells

The fine structure of the emission spectrum lines is studied for a quasi-two-dimensional electronhole liquid (EHL) embedded in SiGe/Si quantum wells. It is shown that the patterns observed in the spectra of the two-particle (IR band) and four-particle (visible band) recombination can be explained by the presence of both heavy and light holes in the condensed phase. Comparison of the experimental data and calculated photoluminescence spectra of EHL allow determination of its main parameters, e.g., the equilibrium density, work function for pairs of particles, and light hole–heavy hole splitting at the G-point of the Brillouin zone for quantum wells that are 3.7–5.1% Ge.

Plasmonic Enhancement of the Four-Particle Emission Recombination Rate in a Quasi Two-Dimensional Electron-Hole Liquid

The plasmonic enhancement of four-particle emission from SiGe layer occurring in the yellow–green spectral region is accomplished using a quasi-two-dimensional electron–hole liquid localized in SiGe/Si quantum well and coupled with gold nanoparticles placed on the top of a quantum well cap layer. This emission is a result of the simultaneous recombination of two holes and two electrons from opposite valleys, accompanied by the transfer of energy to a single photon.

Theoretical Phase Diagram for the Room-Temperature Electron-Hole Liquid in Photoexcited Quasi-Two-Dimensional Monolayer MoS2.

Strong correlations between electrons and holes can drive the existence of an electron-hole liquid (EHL) state, typically at high carrier densities and low temperatures. The recent emergence of quasi-two-dimensional (2D) monolayer transition metal dichalcogenides (TMDCs) provides ideal systems to explore the EHL state since ineffective screening of the out-of-plane field lines in these quasi-2D systems allows for stronger charge carrier correlations in contrast to conventional 3D bulk semiconductors and enables the existence of the EHL at high temperatures. Here we construct the phase diagram for the photoinduced first-order phase transition from a plasma of electron-hole pairs to a correlated EHL state in suspended monolayer MoS2. We show that the quasi-2D nature of monolayer TMDCs and the ineffective screening of the out-of-plane field lines allow for this phase transition to occur at and above room temperature, thereby opening avenues for studying many-body phenomena without the constraint of cryogenics.

On the instability in the bilayer electron–hole plasma

We consider here the multi‐flavour electron–hole plasma formed with the charges of opposite signs separated spatially in a bilayer. The temperature of the system is assumed to be smaller than the degenerate temperature of both the electrons and holes. It is found that, if the charge density is sufficiently small, the system has a negative compressibility, and the homogeneous in‐layer charge distribution is thermodynamically unstable. As a result, the system breaks down into 2 co‐existing phases, the denser one being an electron–hole liquid. New sound excitation branch, inherent in the multi‐flavour 2‐dimensional plasma, proves to be unstable exactly for the same density region in which the system is thermodynamically unstable. As the inter‐layer distance increases, the plasmon spectrum of the electron–hole liquid becomes softer for finite momenta. Once the dispersion curve for the plasmon spectrum crosses the momentum axis, the plasmon spectrum becomes unstable. As a result, the system experiences a quantum phase transition leading to the formation of a charge density wave.

Electron–hole liquid in semiconductors and low-dimensional structures

The condensation of excitons into an electron–hole liquid (EHL) and the main EHL properties in bulk semiconductors and low-dimensional structures are considered. The EHL properties in bulk materials are discussed primarily in qualitative terms based on the experimental results obtained for germanium and silicon. Some of the experiments in which the main EHL thermodynamic parameters (density and binding energy) have been obtained are described and the basic factors that determine these parameters are considered. Topics covered include the effect of external perturbations (uniaxial strain and magnetic field) on EHL stability; phase diagrams for a nonequilibrium exciton-gas–EHL system; information on the size and concentration of electron–hole drops (EHDs) under various experimental conditions; the kinetics of exciton condensation and of recombination in the exciton-gas–EHD system; dynamic EHD properties and the motion of EHDs under the action of external forces; the properties of giant EHDs that form in potential wells produced by applying an inhomogeneous strain to the crystal; and effects associated with the drag of EHDs by nonequilibrium phonons (phonon wind), including the dynamics and formation of an anisotropic spatial structure of the EHD cloud. In discussing EHLs in low-dimensional structures, a number of studies are reviewed on the observation and experimental investigation of phenomena such as spatially indirect (dipolar) electron–hole and exciton (dielectric) liquids in GaAs/AlGaAs structures with double quantum wells (QWs), EHDs containing only a few electron–hole pairs (dropletons), EHLs in type-I silicon QWs, and spatially direct and dipolar EHLs in type-II silicon–germanium heterostructures.

Электронно-дырочная жидкость в полупроводниках и низкоразмерных структурах

Электронно-дырочная жидкость в полупроводниках и низкоразмерных структурах

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–y Ge y )/tSi/sSi1–x Ge x /tSi/(cap Si1–y Ge y ) 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.

Dipolar electron-hole liquid in a double-well SiGe/Si heterosystem

The transition from a dipolar to a spatially direct electron-hole liquid in two-dimensional layers of a type-II (buffer Si1-yGey)/tSi/sSi1-xGex/tSi/(cap Si1-yGey) heterostructure is investigated by photoluminescence spectroscopy at liquid-helium temperatures at high excitation levels. The transition takes place upon a reduction of the thickness of the sSi1-xGex layer, which forms a quantum well for holes in the valence band and a barrier in the conduction band separating the electron quantum wells (tSi layers). The main characteristics of both types of electron-hole liquid are determined. The lifetime of dipolar excitons is determined from photoluminescence kinetics measurements.

Charge Density Waves in the Electron–Hole Liquid in Coupled Quantum Wells

A many-component electron–hole plasma is considered in coupled quantum wells. The electrons and the holes are localized in the different wells. It is found in our previous works that the electron–hole liquid is the ground state of the system. In this paper it is shown that, as the separation between the wells increases, static charge density waves arise resulting in charge fluctuations which form a honeycomb lattice.

Stability of Quasi-Two-Dimensional Electron-Hole Liquid in Semiconductor Structures of the Type-II

Analytical expressions are obtained for the energy of a quasi-two-dimensional electron-hole liquid (EHL) and the threshold value of the barrier height for electrons, above which formation of the direct EHL is impossible. It is shown that the state with a quasi-two-dimensional EHL can be energetically favorable in semiconductors with the anisotropy of masses and (or) a large number of equivalent valleys. A comparison of the calculation results with the experimental data for the Si/SiGe/Si structure is made.

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.

Quantum phase transition of electron-hole liquid in coupled quantum wells

Many-component electron-hole plasma is considered in the Coupled Quantum Wells (CQW). It is found that the homogeneous state of the plasma is unstable if the carrier density is sufficiently small. The instability results in the breakdown into two coexisting phase - a low-density gas phase and a high-density electron-hole liquid. The homogeneous state of the electron-hole liquid is stable if the distance between the quantum wells l is sufficiently small. However, as the distance l increases and reaches a certain critical value, the plasmon spectrum of the electron-hole liquid becomes unstable. Hereupon, a quantum phase transition occurs, resulting in the appearance of the charge density waves of finite amplitude in both quantum wells. The strong mass renormalization and the strong Z-factor renormalization are found for the electron-hole liquid as the quantum phase transition occurs.

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.

Plasmonic enhancement of four-particle radiative recombination in SiGe quantum wells

The influence of gold nanoparticles deposited on the surface of a Si0.95Ge0.05/Si quantum-well heterostructure with a thin Si cap layer on the spectra of low-temperature recombination radiation of biexcitons and an electron–hole liquid confined in the quantum well is investigated. The spectra of both visible and near-infrared radiation are recorded from a region on the sample surface without nanoparticles and regions coated with nanoparticles of different areal densities. It is found that the presence of gold nanoparticles causes strong plasmonic enhancement of collective emission processes in which two holes simultaneously recombine with two electrons from opposite valleys of the conduction band, with the energy of the four particles being transferred to a single photon in the visible spectral range.

Pulsed photoconductivity in diamond upon quasi-continuous laser excitation at 222 nm at the formation of an electron–hole liquid

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm2 upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm2, sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.

Magnetic field stabilized electron-hole liquid in indirect-band-gapAlxGa1−xAs

An electron-hole liquid (EHL), a condensed liquidlike phase of free electrons and holes in a semiconductor, presents a unique system for exploring quantum many-body phenomena. While the behavior of EHLs is generally understood, less attention has been devoted to systematically varying the onset of their formation and resulting properties. We report on an experimental approach to tune the conditions of formation and characteristics using a combination of low excitation densities and high magnetic fields up to 90 T. Demonstration of this approach was carried out in indirect-band-gap $\mathrm{A}{\mathrm{l}}_{0.387}\mathrm{G}{\mathrm{a}}_{0.613}\mathrm{As}$. EHL droplets can be nucleated from one of two multiexciton complex states depending on the applied excitation density. Furthermore, the excitation density influences the carrier density of the EHL at high magnetic fields, where filling of successive Landau levels can be controlled. The ability to manipulate the formation pathway, temperature, and carrier density of the EHL phase under otherwise fixed experimental conditions makes our approach a powerful tool for studying condensed carrier phases in further detail.