Electron-hole liquid in GaS

Photoluminescence spectra of “pure” GaS are recorded under high excitation levels and low temperatures between 1.9 and 10K. This work reports a new and broad band which dominates the spectrum at high excitation intensities and temperatures near 1.9K. Its dependence on the excitation level is highly superlinear. When temperature is raised from 1.9 to 10K the spectrum observed of low excitation levels is recovered. We interpret the band here observed as due to radiative recombination from electron-hole droplets. From best fitting between experimental spectrum and calculated theoretical curves the critical density and the condensation energy are estimated as, n_c = 4.5 × 10²⁰ cm^?3 and φ = 9.0 meV.     

Dynamics of the phase transitions in the system of nonequilibrium charge carriers in quantum-dimensional Si1 − x Ge x /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 Si_{1 ? x} Ge _x /Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si_{1 ? x} Ge _x 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.     

Exciton-contribution to the reflection spectrum at high excitation intensities in CdS

We report measurements of changes in reflection spectrum of CdS due to increasing the density of photoexcited carriers at temperatures above the critical temperature for electron-hole liquid formation. The contribution of the exciton resonance is seen to decrease and analysis of the lineshape indicates that this decrease is due to exciton-exciton collision and a change in exciton polarizability. These results are consistent with a transition from exciton to free electron-hole plasma (Mott transition) at a density of n ～ 2.5 x 10¹⁷ cm^-3.     

Spectroscopic study of thulium-activated double sodium yttrium fluoride Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Tm<Superscript>3+</Superscript> crystals: I. Intensity of spectra and luminescence kinetics

Na_0.4Y_0.6F_2.2:Tm³⁺ crystals with a thulium content from 1 to 100 at % have been grown by the Stockbarger-Bridgman method. The optical spectra of Na_0.4Y_0.6F_2.2:Tm³⁺ crystals were investigated in detail at room and low (10 K) temperatures, and the luminescence kinetics was analyzed using different excitation methods. The structure of the Stark splitting of thulium levels as “quasi-centers,” characterized by inhomogeneous broadening of the Stark components, is determined from analysis of the absorption spectrum at 10 K. The oscillator strengths of the transitions from the ground state to excited multiplets are determined from the absorption cross-section spectra at 300 K for ten transitions in the range 5000–38 500 cm^?1 and seven transitions in the range 5000–28 500 cm^?1. The transition intensity parameters Ω _t , obtained by the Judd-Ofelt method from the spectra due to the transitions to ten and seven excited levels, were found to be, respectively, (i) Ω₂ = 1.89 × 10^?20, Ω₄ = 2.16 × 10^?20, and Ω₆ = 1.40 × 10^?20 cm² and (ii) Ω₂ = 2.04 × 10^?20, Ω₄ = 2.01 × 10^?20, and Ω₆ = 1.44 × 10^?20 cm². These values of the intensity parameters were used to calculate the radiative transition probabilities and branching ratios and to estimate the multiphonon nonradiative transition probabilities for NYF:Tm. The luminescence decay kinetics from thulium radiative levels upon their selective excitation by nanosecond laser pulses has been studied and the lifetimes of thulium radiative levels in NYF crystals have been found.     

Spectral dependent kinetics of the EHD luminescence in As-doped Ge

Following pulsed laser excitation of As-doped Ge with impurity concentrations between 10¹⁵-10¹⁷cm^-3, we observe the electron-hole drop (EHD) and excitonic luminescence decay. The spectrum resolved no-phonon (NP) EHD luminescence kinetics are found to depend on its spectral position. “Plateaus” on the kinetic curves for the high energy side of NP-spectrum are observed at high excitation. The data suggest this luminescence is due to the states which are in dynamical equilibrium during some time after excitation pulse.     

Magnetoluminescence studies of excitonic scattering processes in high purity GaAs

The broad emission line at about 1.512 eV, which dominates the emission spectra of pure GaAs at intermediate excitation levels (1–300 kW/cm²) and low temperatures (<40 K) is investigated in magnetic fields up to magnetic flux densities of 10 T. The shift of the emission maximum in the magnetic field is exactly the same as recently reported for the free exciton. This demonstrates that at high excitation levels exciton-electron scattering is the dominant mechanism for the radiative decay of free excitons.     

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/Si_{1 − x} Ge _x /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 p ₀ ≈ 8.5 × 10¹¹ cm⁻² and n ₀ ≈ 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.     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Effect of interparticle interactions on radiative lifetime of photoexcited electron-hole system in GaAs quantum wells

The paper reports on an investigation of changes in the photoluminescence linewidth and lifetime of excitons and electron-hole plasma over a wide range of densities between 3×10⁷ and 3×10¹² cm⁻² at a temperature of 77 K in GaAs/AlGaAs quantum wells. The roles played by thermal ionization of excitons at low densities of nonequilibrium carriers, exciton-exciton and exciton-electron collisions, and ionization of excitons at high pumping power densities have been studied. Zh. éksp. Teor. Fiz. 112, 353–361 (July 1997)     

Electron-hole liquid in strained SiGe layers of silicon heterostructures

The electron-hole liquid has been found in strained SiGe thin films of Si/Si_1?x Ge_x/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.     

Photoinduced charge redistribution and its influence on excitonic states in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures

The photoinduced charge redistribution in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures under different conditions of optical excitation has been investigated using scanning probe microscopy and optical spectroscopy in the temperature range from 5 to 300 K. Excitation of the samples by radiation with a photon energy greater than the band gap of Zn(Cd)Se leads to the accumulation of electrons in quantum wells, which is detected using scanning spreading resistance microscopy. For moderate excitation densities (up to 25 W/cm²) and at temperatures ranging from 80 to 100 K, the density of a quasi-two-dimensional electron gas formed in quantum wells is several orders of magnitude higher than the density of electron-hole pairs generated by the excitation radiation. The excess electron concentration in the quantum well leads to a broadening of the exciton resonances and to an increase in the relative intensity of the donor-bound exciton emission line and also determines the increase in the luminescence quantum yield with increasing excitation intensity. An additional illumination with a photon energy less than the band gap of Zn(Cd)Se decreases the concentration of excess electrons in quantum wells. The influence of the additional illumination is observed at a temperature of approximately 100 K and almost completely suppressed at 5 K. The obtained results are explained in terms of the formation of a potential barrier for electrons at the ZnMgSSe/GaAs interface and by the specific features of recombination processes in the electron-hole system containing impurity centers with different charge states.     

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

The exciton condensation in a Si_1?x Ge _x solid solution layer of Si/Si_1?x Ge _x /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.     

Magnetically stabilized electron-hole liquid in indium antimonide

Luminescence spectra of sufficiently pure n-type indium antimonide crystals (N _D−N _A=(1–22)·10¹⁴ cm⁻³) in a magnetic field of up to 56 kOe, at temperatures of 1.8–2 K, and high optical pumping densities (more than 100 W/cm²) have been studied. More evidence of the existence of electron-hole liquid stabilized by magnetic field has been obtained, and its basic thermodynamic parameters as functions of magnetic field have been measured. When the magnetic field increases from 23 to 55.2 kOe, the liquid density increases from 3.2·10¹⁵ to 6.7·10¹⁵ cm⁻³, the binding energy per electron-hole pair rises from 3.0 to 5.2 meV, and the binding energy with respect to the ground exciton level (work function of an exciton in the liquid) rises from 0.43 to 1.2 meV. Zh. éksp. Teor. Fiz. 111, 737–758 (February 1997)     

Electron-hole plasma in pulse photoexcited single quantum wells

We report photoluminescence studies of MOCVD grown, GaAsAl_xGa_1?xAs single quantum wells which were intensly excited with a pulse dye laser at T=2K. For a well width of d～40Å, the spectra are interpreted as due to the radiative recombination of a hot electron-hole plasma confined to the well. The density of charge carriers and their temperature depend upon the excitation intensity, and vary in the range of 10¹¹–10¹³ cm^?2 and 100–500K for an absorbed photon flux of 10¹³–10¹⁶ photons-cm^?2 per pulse, respectively. The observed spectral features are identified as the e1-hh1 and e1-lh1 transitions and two additional bands which are tentatively assigned to transitions involving virtual bound states of either the electron or the hole. The electron-hole plasma spectra of the d～40Å sample are strongly polarized perpendicular to the well quantization axis. For wider wells (d～80 and 150Å) smaller photoexcited carrier densities were observed for the same absorbed photon flux. It is thus concluded that the capture efficiency of the well is small.     

Large electric-field-induced enhancement of resonant Raman scattering of a single quantum well

The electric-field enhancement of the resonant Raman efficiency of confined optical phonon modes in a single quantum well has been observed in an asymmetrical n-type triple-barrier GaAs/AlAs resonant tunneling structure. The measurement takes advantage of an outgoing resonance with the e₂-hh₁ exciton transition confined to the wide quantum well, and according to the polarization properties, the Fröhlich interaction dominates the scattering mechanism. A fifteenfold increase is found from zero field to 7.5 × 10⁴ V/cm, which results from break-down of the parity selection rules for the photon-exciton and the exciton phonon coupling mechanisms.     

Excitation spectroscopy,Raman scattering and the temperature dependence of the luminescence in highly excited red HgI2

The luminescence of red HgI₂ is investigated as a function of temperature, excitation intensity and wavelength. At high excitation intensity and low temperature an “M-band” emission dominates. This M-band is assigned to biexciton decay and bound exciton scattering with acoustic phonons (“acoustic wing”), this assumption being supported by the results of excitation spectroscopy. The energy of the biexciton is determined to be (4661 ± 1) meV. From the evaluation of Raman spectra, the phonon energies (1.9, 3.1 and 14.0 ± 0.2) meV are found. At higher temperatures two lines are observed, one of which is ascribed to exciton-free carrier scattering. Position and line shape are in good agreement with theoretical results. The other emission line is found to be due to scattering involving excitons or carriers bound to lattice defects.     

Electrical Conductivity of Noble Gas Plasmas in the Warm Dense Matter Regime

The electrical conductivity of noble gas plasmas is investigated by using the relaxation‐time approximation in the density range 10^–5–10 g cm^–3 and temperature range 10⁴–10⁵ K. The electrical conductivity calculated shows reasonable consistency with shock wave experiments and theoretical simulations at above 2 × 10⁴ K where the Coulomb interaction dominates. A nonmetal to metal transition in helium plasma is predicted at 2.4 g cm^–3 and shown reasonable agreement with the most recent shock wave experiment (above 1.9 g cm^–3). Furthermore, the insulator‐metal transition densities of all the noble gas plasmas are predicted and compared with available results (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li₆Gd(BO₃)₃ doped with Eu³⁺ and Ce³⁺ and Li₆Eu(BO₃)₃ crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce³⁺-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.     

Autosolitons in electron-hole plasma in p-Ge

The self-organization of an electron-hole plasma (EHP) heated by an electric field in pure p-Ge samples at T = 77 K has been studied experimentally. The derived current-voltage characteristics (CVCs) and the distributions of the electric field and IR emission of the hot carriers along the samples show that the segments of a steep rise or the S-shaped segments of the CVCs in samples with n-p junctions are related to the formation of longitudinal thermal-diffusion autosolitons (AS_‖); as a result, thin (d = 2–20 μm in diameter), melted-through current channels appear. Such AS_‖ are formed at high EHP densities (n ≥ 1 × 10¹⁶ cm⁻³), when the electron-hole scattering is dominant, and at electron temperatures T _e = (2–4.5)T ₀ (T ₀ is the lattice temperature). The saturation segments and the N-shaped segments in the CVCs are attributable to the generation of transverse thermal-diffusion high-field autosolitons (AS_⊥) in the form of narrow strata with electric field strengths = 1–20 kV cm⁻¹. High-field AS_⊥ are formed at EHP densities n = 5 × 10¹³−1 × 10¹⁶ cm⁻³, when the electron-phonon scattering is dominant, and at electron temperatures T _e ∼ Θ ≥ 5T ₀ (Θ is the Debye temperature). The generated longitudinal and transverse autosolitons have high temperatures (T _e ≥ 1000 K) and reduced carrier densities and can exist simultaneously in different parts of the sample. Original Russian Text ? M.N. Vinoslavskiĭ, P.A. Belevskii, A.V. Kravchenko, 2006, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2006, Vol. 129, No. 3, pp. 477–492.     

Time-resolved luminescence spectra of AgBr at high excitation densities

Time-resolved luminescence measurements in AgBr at 1.9K and cw excitation densities up to 100 kW/cm² reveal new emission bands with strong vibrational structure close to the free exciton. Besides the emission of excitons bound to isoelectronic I^- traps two different luminescence systems are separated by making use of their different lifetime and excitation behaviour. The results are discussed in view of the electron-hole-liquid recombination recently found in the investigated spectral range.