Luminescence due to exciton-exciton collision in GaAs

Emission spectra of high-purity GaAs have been studied at 4.2 K under N₂ laser excitation. The slope of the low-energy tail of the main band has been found to fit well with the theoretical prediction for radiative Auger recombination of free excitons. Measurements under electric field support the dominant contribution of this process in the luminescence of highly excited GaAs.     

Spontaneous luminescence due to high density electron-hole plasma in GaAs under nano- and pico-second pulse excitation

Spontaneous luminescence due to high density electron-hole plasma in GaAs is observed at 4.2 K under nano- and pico-second pulse excitation. From the pico-second time-resolved spectra, it is found that the hot carriers are cooled down rapidly within 150 psec, and the changes of spectra are not appreciable in the later stage. One may consider, together with results of the spectral shape analysis, that the electron-hole liquid formation is improbable at least within the time range observed.     

Luminescence from compressed electron-hole plasma in germanium

We have studied the luminescence spectra from compressed electron-hole plasma in pure germanium. The spectra show a satisfactory thermalization of the charge carriers and lead to a reasonable value of the compressibility. These features encourage to further experimental efforts in the studies of the Mott transition in highly excited semiconductors.     

Impact-ionized electron-hole plasma instability in GaAs

The possibility of the excitation of impact-ionized electron-hole plasma oscillatory instability in GaAs with frequency up to 10¹² Hz is shown. The linear and nonlinear stages of the instability are investigated.     

Luminescence of direct-and indirect-gap electron-hole plasma in TlBr

Luminescence of TlBr crystals highly excited by a nanosecond pulsed-dye laser (3.4 eV) at the bath temperature ∼ 8.5 K was studied. Two emission lines labeled A (∼ 2.98 eV) and B (∼ 2.62 eV) were found, which show typical behavior of the electron-hole plasma recombination radiation. The A-line is assigned to the recombination of e-h pairs in the direct gap (X⁺₆−X^-₃) and the B-line to the simultaneous recombination in the indirect gap (X⁺₆−R^-₆). Condensation of carriers into an electron-hole liquid was not observed.     

Theory of spin-polarized electron-hole liquid

The properties of an electron-hole liquid with different densities of spin-up and spin-down carriers have been calculated for a simple semiconductor model and CdS. The spin polarization lowers both equilibrium density and binding energy. Exchange leads to a different energy gap for each spin direction which can be detected in the circular polarization of the luminescence.     

Luminescence lineshape of free excitons and electron-hole drops in Si

We study experimentally and theoretically the lineshape of the TO-LO and TA phonon assisted luminescence of free excitons and electron-hole drops in Si. We show that, in the case of the TA replica, the appropriate electron-phonon matrix element should be taken k-dependent, contrary to previous investigations.     

Experimental evidence for electron-hole liquid in ZnO

Measurements of optical gain in ZnO have been performed as a function of excitation intensity and temperature. The results are interpreted in terms of formation of an electron-hole liquid at temperatures below 70K. The liquid has a binding energy of 22 meV and a low temperature density of 0.98 × 10¹⁸cm^-3.     

Correlated ejection of electron-hole drops from a continuously generated electron-hole liquid in Ge

The photocurrent power spectra were measured in a Ge photodiode as a function of the incident optical intensity at 1.8 K and 4.3 K. The observed spectra showed a peak at low frequencies superimposed on a continuum which has a cut-off at high frequency. The interpretation of the results implies that the electron-hole drops coming from a same region of the photoexcited liquid are ejected periodically.     

Luminescence due to isoelectronic impurities in CuCl

Luminescence spectra have been investigated for CuCl doped with the isoelectronic halogen impurities, Br and I. Broad emission bands characterized by a multiphonon structure were found at around 4200 and 4010 Å in Br- and I-doped samples, respectively, at 4.2 K. From the observed features of these luminescences, they are ascribed to the radiative decay of excitons bound at halogen impurities in CuCl. The roles of these isoelectronic impurities in CuCl are discussed in relation to a theory of excitons in a binary mixed crystal.     

Luminescence of high density electron-hole plasma in CdSe at elevated temperature

Luminescence of high density electron-hole plasma in CdSe is observed in the 77–300 K temperature range by picosecond pulse excitation. With increasing temperature from 77 K the stimulated emission band is replaced by the spontaneous emission band. Temperature changes of spectral features of these two bands and also their time dependence after pulse excitation are consistent with the theoretical consideration.     

Luminescence of a quasi-two-dimensional electron-hole liquid and excitonic molecules in Si/SiGe/Si heterostructures upon two-electron transitions

The low-temperature photoluminescence of Si/Si_0.91Ge_0.09/Si heterostructures in the near-infrared and visible spectral ranges is investigated. For the structure in which the barrier in the conduction band formed by the SiGe layer is transparent to electron tunneling, the broad luminescence line observed in the visible range is analyzed by comparing its shape with the numerical convolution of the spectrum of near-infrared recombination radiation originating from the electron-hole liquid. The comparison demonstrates that, at high excitation levels, the visible-range emission is caused by two-electron transitions in a quasi-two-dimensional spatially direct electron-hole liquid. Furthermore, the combined analysis of the photoluminescence spectra in the near-infrared and visible ranges yields the binding energy of a quasi-two-dimensional free biexciton in the SiGe layer of these heterostructures. In the structures with a wide SiGe layer that is not tunneling-transparent to electrons, a spatially indirect (dipolar) electron-hole liquid is observed.     

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)     

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.     

Luminescence of arsenic vacancy-related defects in GaAs

Photoluminescence measurements at 77°K on solution grown and heat treated GaAs specimens are shown to exhibit a luminescent band near 8900 A. This band is suggested to be related to arsenic vacancies.     

Two-dimensional electron-hole liquid in the strong magnetic field

The Matsubara diagram technique is used to study the formation and thermodynamic properties of an electron-hole liquid (EHL) in the two-dimensional system in the transverse strong magnetic field. The system has discrete energy spectra, i.e. a motion of particles becomes “zero-dimensional”. Therefore, the exchange interaction alone is sufficient to form EHL, the correlation correction being negligible.     

Two-dimensional electron-hole pair diffusivities in thin GaAs/AlGaAs Quantum Wells

Diffusivities of two-dimensional electron-hole pairs in thin GaAs/AlGaAs Quantum Wells (QWs) are studied experimentally and theoretically as functions of temperature and well-width. With growing well-widths, increasing diffusivities are observed for fixed Al-contents. Experimental diffusivities for the lateral carrier motion in continuously as well as in interrupted-grown thin QWs of different barrier Al-content are presented for T>150 K. Increasing diffusivities are observed for rising temperatures in the range T190 K. A comparison of the experimental data and results of theoretical model calculations indicates that the increase is partly related to thermal dissociation of excitons into free carrier pairs. The effective diffusivity of this two-component system is calculated using a system of rate equations and considering acoustic-deformation-potential scattering, polar-optical scattering and barrier-alloy-disorder scattering.The experimental data were obtained at: 4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, W-7000 Stuttgart 80, Germany