Raman scattering from optically pumped electron-hole plasma in insulating GaAs

We measured the single particle Raman Scattering from an optically pumped electron-hole plasma in bulk insulating GaAs, using IR radiation of 1.06 μm to both excite and probe the plasma. The relevant theory is derived in terms of a Landau generalized quasi-particle picture and we show how many-body effects are discernible in the spectrum. The results of the experiment lend independent support to the model of two-photon absorption in GaAs and allowed us to place a lower bound of 1 × 10¹⁶ cm^?3 for the threshold of saturation effects in the density of photo-generated electron-hole pairs in bulk GaAs.     

Nonresonant two-photon generation of excitonic matter in a CuCl pellet

A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor.     

Exciton-plasma transition in GaAs

In this work, we study the stability of excitons at high density, i.e. we calculate the reduction of the exciton binding energy due to exciton-exciton interactions in a high-density exciton gas. We derive first the effective electron-hole interaction in the presence of free carriers and excitons. We use the static approximation. The exciton binding energy is calculated by the variational technique. The computations are specialized to GaAs. We investigate the critical density when the exciton binding disappears, which corresponds to the exciton plasma transition. We conclude that this transition occurs at higher density than the reverse plasma exciton transition, determined by the standard criteria a₀q_D =1.19 [Rogers F. J., Graboske H. C., Jr. and Harword D. J., Phys. Rev.A1, 1577 (1970)].     

Reflectance anisotropy spectra of the diamond (100)-(2x1) surface: evidence of strongly bound surface state excitons

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)     

Dynamics of electron-hole plasma in semiconductor opening switches for ultradense currents

A physicomathematical model for calculating the dynamics of the electron-hole plasma in semiconductor opening switches for ultradense currents is developed. The model takes account of the real doping profile of a semiconductor p ⁺-p-n-n ⁺ structure and the following elementary processes in the electron-hole plasma: current-carrier diffusion and drift in high electric fields, recombination on deep impurities and Auger recombination, and collisional ionization in a dense plasma. The electrical pumping circuit of the opening switch is calculated by solving the Kirchhoff equations. The motion of the plasma in the semiconductor structure is analyzed on the basis of the model. It is shown that for ultrahigh pumping levels the interruption of the current in the opening switch occurs in the heavily doped regions of the p ⁺-p-n-n ⁺ structure and is due to saturation of the particle drift velocity in high electric fields. Zh. Tekh. Fiz. 67, 64–70 (October 1997)     

Diffusion of excitons and electron-hole drops in germanium

Diffusion of excitons and electron-hole drops is investigated in pure germanium, using a time-resolved cyclotron resonance method. The diffusion coefficient of excitons at 4.2 K is obtained to be ≈ 1000 cm²/sec. For electron-hole drops, when excitation is not so high, it is expected to be lower than ≈ 500 cm²/sec at 1.6 K.     

Excitonic photoluminescence in semiconductor quantum wells: plasma versus excitons

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies exciton and electron-hole plasma contributions. For low temperatures and low densities, the excitonic emission is extremely sensitive to details of the electron-hole-pair population making it possible to identify even minute fractions of optically active excitons.     

Observation of excitons in one-dimensional metallic single-walled carbon nanotubes

Excitons are generally believed not to exist in metals because of strong screening by free carriers. Here we demonstrate that excitonic states can in fact be produced in metallic systems of a one-dimensional character. Using metallic single-walled carbon nanotubes as a model system, we show both experimentally and theoretically that electron-hole pairs form tightly bound excitons. The exciton binding energy of 50 meV, deduced from optical absorption spectra of individual metallic nanotubes, significantly exceeds that of excitons in most bulk semiconductors and agrees well with ab initio theoretical predictions.     

Surface-sensitive NMR in optically pumped semiconductors

We present a scheme of surface-sensitive nuclear magnetic resonance in optically pumped semiconductors, where an NMR signal from a part of the surface of a bulk compound semiconductor is detected apart from the bulk signal. It utilizes optically oriented nuclei with a long spin-lattice relaxation time as a polarization reservoir for the second (target) nuclei to be detected. It provides a basis for the nuclear spin polarizer [IEEE Trans. Appl. Supercond. 14:1635, 2004], which is a polarization reservoir at the surface of the optically pumped semiconductor that polarizes nuclear spins in a target material in contact through the nanostructured interfaces.     

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)     

Optical properties of excitons in CdS semiconductor-insulator quantum wires

The characteristic features of the luminescence spectra of CdS semiconductor nanocrystals, crystallized in hollow channels in a dielectric template, are explained in terms of excitonic transitions in semiconductor-insulator quantum wires. The excitonic transition energies agree with the values calculated taking into account the effects of size quantization and the “dielectric enhancement of excitons” — the large increase in the electron-hole attraction as a result of the difference between the permittivities of the semiconductor and insulator. The theoretically computed binding energies of excitons in CdS quantum wires with a diameter of 10 nm reach 170 meV. It is shown that the excitonic transition energy is constant for a wide range of wire diameters. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 216–220 (10 August 1999)     

Dependence on plasma density of the conductivity of an electron-hole plasma in germanium at 2 K

High densities of electron-hole plasma have been optically injected into Ge at T ∼ 2K, and the plasma density inferred from measurements of the plasma absorption at 3.39μ. The behavior of the conductivity with increasing density and at different electric fields can be understood from a model of a plasma condensation into high density drops, with the plasma density in the drops ? 6 × 10¹⁶ c⁻³.     

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 Si_0.93Ge_0.07/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.     

Photon emission induced by elastic exciton-carrier scattering in semiconductor quantum wells

We present a study of the elastic exciton-electron (X-e⁻) and exciton-hole (X-h) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electron-hole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a two-dimensional Coulomb potential statically screened by the free carrier gas, we apply the variable phase method to obtain the excitonic wavefunctions, which we use to calculate the 1s exciton-free carrier matrix elements that describe the scattering of excitons into the light cone where they can radiatively recombine. The photon emission rates due to the carrierassisted exciton recombination in semiconductor quantum-wells (QWs) at room temperature and in a low density regime are obtained from Fermi’s golden rule, and studied for mid-gap and wide-gap materials. The quantitative comparison of the direct and exchange terms of the scattering matrix elements shows that fermion exchange is the dominant mechanism of the exciton-carrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electron-assisted and hole-assisted exciton recombinations.     

X-ray diffraction, vibrational and photoluminescence studies of the self-organized quantum well crystal H3N(CH2)6NH3PbBr4

We have prepared new semiconductor H₃N(CH₂)₆NH₃PbBr₄ crystals which are self-assembled organic-inorganic hybrid materials. The grown crystals have been studied by X-ray diffraction, infrared absorption and Raman spectroscopy scattering. We found that the title compound, abbreviated 2C₆PbBr₄, crystallizes in a two-dimensional (2D) structure with a P2₁/a space group. In the inorganic semiconductor sub-lattice, the corner sharing PbBr₆ octahedra form infinite 2D chains. The organic C₆H₁₈N₂⁺ ions form the insulator barriers between the inorganic semiconductor layers. Such a packing leads to a self-assembled multiple quantum well structure. Raman and infrared spectra of the title compound were recorded in the 50-500 and 400-4000 cm⁻¹ frequency regions, respectively. The assignment of the observed Raman lines was performed by comparison with the homologous compounds. Transmission measurements on thin films of 2C₆PbBr₄, obtained by the spin coating method, revealed a strong absorption peak at 380 nm. Luminescence measurements showed an emission line at 402 nm associated with radiative recombinations of excitons confined within the PbBr₆ layers. The electron-hole binding energy is estimated at 180 meV.     

Mott transition of excitons in coupled quantum wells

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.     

Wannier-Mott excitons on a helically-shaped one-dimensional semiconductor

Within a simplified model, we explore how bound electron-hole pair (exciton) states and optical transitions between them are affected by the geometry of a helically shaped one-dimensional semiconductor. Among the illustrated geometrical effects are variable enhancement of the binding energy for different excitons and the appearance of new excitonic states with spatially separated electron and hole positioned on different turns of the helix.     

Optically-pumped stimulated recombination radiation inp-type InSb in high magnetic fields

In this paper measurements of the frequency, linewidth and polarization of stimulated recombination radiation (SRR) fromp-type InSb are reported. The samples had low excess-carrier concentrations between 10¹⁴ and 10¹⁵ per cm³ and different lengths between 0.4 and 9 mm. They were held in magnetic fields up to 6T at temperatures of pumped liquid helium. The excitation was done optically by the radiation of a Q-switched CO-laser. We could observe a number of different stimulated processes:

1. band-to-band recombination (tuning between 1875 and 1980 cm^?1),
2. band-to-acceptor recombination (tuning between 1840 and 1930 cm^?1),
3. stimulated spin-flip Raman scattering (SFR) of the SRR by excited electrons,
4. SFR of the laser by excited electrons and its interaction with the SRR.

From the observed shift of the band gap by exchange and correlation energy the number of created electron-hole pairs can be calculated to be up to 10¹⁶ per cm³. The observed acceptor binding energy varies from 66 cm^?1 atB=0 to 71 cm^?1 at 4.5T.     

Exciton fission in excimer-forming crystal dynamics of excimer build-up

The optically induced fission of singlet excitons in both α- and β-perylene crystals is investigated at room temperature. In the monomeric β-perylene the fission thresold coincides with the energy of two triplet excitons whereas it is blueshifted by ～3500 cm^-1 in the excimer forming α-crystal. This indicates that the excimer is formed prior to the fission of the initially excited monomer state and it implies that the rate of excimer formation is exceeding 10¹² s^-1. Theoretical estimates led to k_exc ≈ 10¹²-10¹³ s^-1. Analysis of the experimental data shows also that two adjacent perylene molecules in the α-crystal, each of them in the T₁ electronic state, are bound with the binding energy B_2T1 ≈ 350 cm^-1.