Radiative recombination of free excitons in GaAs quantum wells

We have measured the radiative lifetime of excitons in GaAs quantum wells under resonant excitation at 10 K using time resolved luminescence spectroscopy with 6 ps time resolution. The luminescence decay has two components: a fast one with a time constant τ₁ (∼ 17 - 40 ps) and a slow one with a time constant τ₂ (∼ 80 - 300 ps). τ₂ is the lifetime of thermalised excitens at 10 K. τ₁ is due to two mechanisms in parallel: the radiative recombination of excitons with k_u < K₀ and the scattering by acoustical phonens into non radiative exciton states (k_u > k₀ and J = 2). The variation with temperature of τ₁ gives the lifetime of the excitons at k = 0, τ₀, which varies between 20 and 50 ps depending on the sample.     

Effect of dielectronic recombination excitation on solar line intensities

The intensities of lines emitted from the solar atmosphere and observed on earth are calculated by considering electron-impact excitation followed by radiative emission. In order to determine the contribution of dielectronic-recombination -excitation (DRE), the calculations have been repeated with the inclusion of DRE as a process for excitation, along with electron-impact excitation; the excited electron is assumed to deexcite through radiative emission. The DRE is found to contribute significantly and the contribution generally increases with a decrease in wavelength and an increase of ionic charge.     

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.     

Diffusion of excitons in CdS-Se and ZnSe-Te solid solutions at high excitation levels

It is shown that, with strong pulsed excitation, the intensity of the exciton recombination band in the fluctuation tail of the density of states in the limit of large times in the presence of traps is described by the asymptote of a solution to the diffusion equation. The critical diffusion index corresponds to a “normal” process in the CdS-Se solid solution and to “anomalous” diffusion in the case of ZnSe-Te. Fiz. Tverd. Tela (St. Petersburg) 40, 892–893 (May 1998)     

Hyperbolic excitons in semiconductors

The energy and damping of the quasi-stationary state corresponding to a hyperbolic exciton in a semiconductor crystal are calculated. It was assumed that the screened Coulomb potential describes the interaction between the electron and hole. The resonance conditions due to the hyperbolic exciton are determined.     

Relaxation processes at high excitation levels and Bose-Einstein condensation of excitons in ZnTe crystals

The singularities of low-temperature emission spectra obtained for ZnTe crystals excited resonantly by a strongly monochromatic source are investigated. The possibility of attaining the coherent luminescent emission due to the exciton-exciton interaction under these conditions is proved. The appearance of the non-polarized needle-like peak and the processes of inelastic interaction of polaritons of the upper polariton branch (UPB) as well as the disappearance of the Raman component are interpreted as the spectral attributes of Bose-Einstein condensation of UPB polaritons.     

Surface state excitons in semiconductors

We calculate binding energies and wave-functions of surface state excitons. Our approach is based on the effective mass approximation, and assumes a small but finite surface state penetration depth. Central cell corrections are included. We obtain binding energies in the tenth of eV range, in agreement with recent observations on GaAs (110) and Si (111) 7 × 7, for surface masses of order unity.     

Recombination-assisted creation of cation excitons and cross-luminescence quenching in CsCl crystals at high excitation densities

At high excitation densities, recombination-assisted creation of cation excitons, which transfer energy efficiently to the anion sublattice to initiate the luminescence of anion excitons and impurity centers, has been observed in CsCl crystals. At the same time, the creation of cation excitons competes with the electron recombination with cation holes and quenches the cross-luminescence. The intensity ratio of the cross-luminescence to exciton-impurity luminescence is different for crystal irradiation with γ rays and heavy particles.     

Self-induced transparency of excitons in semiconductors

Self-induced transparency of excitons is analysed in a model where intraband processes are described by density matrices and interband processes by coherent pair amplitudes. We determine the dispersion law of the carrier wave and the exciton wave function. The theory predicts: (a) a forbidden energy gap centered at the exciton line which broadens with increasing intensity, (b) a critical dependence of the effect on intensity, (c) a strong influence of intensity and carrier frequency on the exciton wave function.     

Production of coherent states of excitons in semiconductors by means of the recombination of free carriers

It is shown that the modern experimental techniques make it possible to produce a coherent Bose condensate of excitons in semiconductors by the direct recombination of electrons from the conduction band and holes from the valence band. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 436–440 (10 March 1997)     

Core excitons in Ga-V compound semiconductors

The major chemical trends in the energies of Ga 3d core excitons in GaP, GaAs, and GaSb are accounted for by a theory which treats only the central-cell part of the electron-hole interaction.     

Stability of high-density one-dimensional excitons in carbon nanotubes under high laser excitation

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.     

Radiative recombination at deep impurity centres in AlN:O

The ionization energy of the deep donor states due to oxygen and of the acceptor levels due to compensating V_A1 vacancies were determined from the optical properties of AlN:O. The theoretical calculations together with the experimental data (the emission spectra, the excitation of luminescence spectra and EPR data) indicate that the deepest donor level belongs to the ion O^?.