Luminescence due to metallic electron-hole liquid in GaAs

Emission spectra of high-purity GaAs have been studied at 4.2K under very intense optical excitation. The results give the first experimental evidence for an important contribution of the electron-hole liquid phase in the luminescence of a direct-allowed semiconductor. Theoretical fit of the luminescence band-shape is satisfactory. The effects of applied electric field on the emission spectra are explained well using a concept of the metallic electron—hole plasma state.     

Bistable charge states in a photoexcited quasi-two-dimensional electron-hole system

The luminescence spectra of GaAs/AlGaAs quantum wells (QWs) with low-density quasi-two-dimensional electron and hole channels were studied. It was demonstrated that, at temperatures below some critical value (T _c ～30 K) and for an excitation power lying in a certain temperature-dependent range, two metastable charge states with two-dimensional charge densities differing in both magnitude and sign can occur in the system under the same conditions. The obtained experimental data agree well with the mathematical model allowing for the transfer of photoexcited carriers to the barrier followed by their tunneling into QW.     

Time-resolved spectra of spontaneous luminescence from high density electron-hole plasma in CdS

Pico-second time-resolved spectra of the spontaneous luminescence from high density electron-hole plasma in CdS are measured at 4.2 K suppressing the stimulation effect. It is found that after the pulse excitation hot carriers are cooled rapidly for the first 100 psec, and that therafter up to the 400 psec delay time the shape of spectra hardly changes but the spectral width depends on excitation density. Although it seems as if some kind of state like electron-hole liquid is formed, one cannot easily regard that state as the condensed electron-hole drop state.     

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.     

Optical gains associated with exciton collision processes and high density electron-hole plasma in CdSe

Optical gain spectra for CdSe are measured at 4.2 K changing excitation density. The gain in the 683–689 nm region is concluded to be due to exciton collision processes, in disagreement with the assignment to electron-hole liquid by other investigators. The gain due to high density electron-hole plasma, which has been found recently by the authors to be generated under very high excitation density, is observed in the 688–691 nm region.     

“Anomalous” spectral photoresistive field effect in CdS crystals caused by the screening of the electron-hole interaction

The changes observed in low-temperature (T = 77 K) near-band-edge photoconductivity spectra of CdS crystals in response to an external transverse electric field applied to the sample surface have been investigated. An analysis of the “anomalous” character of these changes for a number of crystals has revealed a significant role of the near-surface effects of screening of the electron-hole interaction in the formation of near-band-edge photoconductivity spectra of CdS crystals with a technological excess of cadmium near the surface. It has been shown that the depletion (enrichment) transverse electric field leads to a weakening (enhancement) of screening effects in the photoconductivity spectra of the CdS crystals.     

Collective excitations in electron-hole bilayers

We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.     

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.     

Radius of electron-hole drops in silicon

The radius of electron-hole drops at nucleation threshold is investigated as a function of temperature in Si using a small shift of their main luminescence line due to their surface energy. For T > 10 K, experiment and theory are consistent if we take into account a sticking coefficient (～ 4%) at the drop surface. For T ? 10 K our data indicate that the drop size is certainly small, but the measured radii are not likely. This suggests that other effects should be considered, such as the drop curvature energy for example.     

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.     

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.     

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)     

Temperature dependence of the radius of electron-hole drops in Ge

The temperature dependence of the radius of electron-hole drops in Ge is determined from measurements of the number of particles in the drops using the p-n junction technique. The drop radius is found to increase from 5.5μ at 1.7 K to 10μ at 3.2 K for an excitation intensity of 160 mWatt/mm². As a function of excitation level at 1.8 K the drop radius is found to increase from 2.9μ at 8 mWatt/mm² to 6.5μ at 300 mWatt/mm². Our data are compared to results available in this field.     

On the condensation of electron-hole droplets

The phase diagram at low temperature (T ? 1 K) of an electron-hole system, is drastically modified by life time effects: the system is made of multiexcitonic clusters of different sizes, the size distribution function having only one peak instead of two.     

The electron-hole system in GaSe at high densities

Absorption spectra in the limit of extremely high optical intensity in the region of the direct threshold present an induced transparency effect which is attributed to the disappearance of the bound excitonic states. This occurs at levels of excitation which decrease with increasing classical volume of each state. The effect is attributed to the evolution to a plasma state which appears to occur well above the Mott critical density for the metal-insulator transition.     

Evidence for high temperature electron-hole droplets in heavily doped silicon

The photoluminescence spectrum of silicon containing 2.0 × 10¹⁸ phosphorous atoms/cm³ is studied as a function of temperature in the range 1.5 K?T?80 K. A threshold temperature is not observed for the electron-hole droplet photoluminescence line. The results indicate that the impurity band becomes unstable with increasing temperature and disappears abruptly at about 51 K.     

Net negative charge of electron-hole drops in germanium

The giant fluctuating photocurrent has been observed in highly excited germanium samples with ohmic contacts at 1.6 K. From the observation of this fluctuating photocurrent spike with varying static electric field, it is found that the electron-hole drop is negatively charge as a whole.     

Excitons and electron-hole plasma in InSe

The spontaneous emission from either thin or thick samples of layered indirect-gap InSe semiconductor under dye laser excitation has been investigated either above or below the critical Mott density. The observed spectra consist of five bands in the near infrared region; four of them are localized in the energy range between the direct and the indirect gap and one only lies below the indirect energy gap. These bands can be associated with cooperative indirect and direct excitonic transitions and with electronhole plasma recombinations.     

Magneto-acoustic absorption by the electron-hole liquid in stressed germanium

We report the observation of magneto-acoustic absorption by the electron-hole liquid in a potential well in stressed germanium. This experiment confirms the metallic character of the liquid and yields direct values for the electron Fermi level ?_F = (2.6±0.1) meV and inter-carrier collision time τ = (6.0±0.5) × 10^-11sec at 1.8 K under a stress of approximately 5 kg/mm². From ?_F we deduce an electron density of n = (6.2) ± 0.4) × 10¹⁶cm^-3 at 1.8 K.