Infrared absorption in a high density electron-hole plasma in germanium at low temperatures

Measurements of the absorption in the temperature range 1.8–9 K have been made for wavelengths between 3.1 and 4.2μm, in germanium highly excited by a 1.06μm laser. The shape of the absorption curve is noted to be similar to that observed by Pokrovskii and Svistunova at lower excitation levels, and somewhat broader than that calculated by Smith, Chen and McGill for exciton absorption. A cross section of 4.5 (±1.1) × 10^-17 cm² at 3.39μm is deduced from normalization with the theoretical curves of Smith et al.     

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.     

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.     

Cooling of high density electron-hole plasma

We generalize for high density electron-hole plasma, the previous theories [12] of temperature cooling of non-equilibrium hot plasma. Especially we take into account the cooling by emission of mixed longitudinal optical phonon and plasmon modes, these quasiparticles described by a non-equilibrium distribution function. We show that a strong slowing of the plasma cooling occurs, at high electron-hole density. We calculate for CdSe the temperature kinetics of plasma created by Yag laser (pulse duration 30 ps).     

Formation of the electron-hole droplet cloud in germanium

We describe luminescence imaging experiments that probe the nature of the phonon wind which transports electron-hole droplets in Ge. The participation of non-equilibrium phonons emanating near the excitation region is supported by: (a) sharp features in the droplet spatial distribution (b) a dependence of the average cloud density on the excitation photon energy, and (c) a rapid initial buildup of the cloud.     

High density electron-hole plasma in Si induced by femtosecond pulses

High electron and hole (e-h) densities of about 10²² cm^-3 have been produced in silicon, using 620 nm wavelength laser pulses of 100 fs duration. These density values are determined by measuring the dependence of the pulse self-reflectivity on its energy. By comparison with a fully non-linear model of light propagation, we show that dissipation processes inside the plasma are dominated by e-h collisions, with a characteristic time of 3 × 10^-16s. The onset of melting within 100 fs and its nature, considering the high plasma density, are also discussed in view of scattered light measurements.     

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.     

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.     

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.     

Analysis on experimental valence charge density in germanium at RT and 200 K

The electronic structure and hence the valence charge distribution of germanium at 296 and 200 K has been elucidated from structure factors measured by X-ray diffraction experiment using maximum entropy method (MEM) and multipole model. The methods adopted here are used to extract the fine details of the charge density distribution in the valence region. The charge density evaluated using both the models along the bond path and at the mid bond positions are compared and found to confirm the covalent bond existing in the solid. Topology of the charge density in the crystal is analysed and the critical points determined reveal unique spatial arrangement of valence charge in the direction normal to the bonding direction. The Laplacian of the charge density is also analysed for the understanding of the spatial distribution and the partitioning of the valence charge. The local charge concentration and the mapping of the electron pairs of the Lewis and valence shell electron pair repulsion (VSEPR) models have been done using electron localization function (ELF) and localized orbital locator (LOL).     

Energy relaxation of an electron-hole plasma in semiconductors

We calculate the energy relaxation of an electron-hole plasma created by a short laser pulse in semiconductors like Si and GaAs in two cases: (i) when the carrier-carrier collision time is much shorter than the carrier-phonon one, so that a carrier temperature T_c exists. We give the variation of T_c with time; (ii) when there is no carrier temperature and the initial energy distribution is a peaked function of width Δ. We give the time evolution of the system when Δ is much larger and much smaller than the phonon energy.     

Temperature dependent properties of the condensed electron-hole state in germanium

The energy of the condensed electron-hole state in germanium shows a blue shift when the temperature is raised, and it does not depend on excitation intensity. From the temperature dependence of the luminiscence intensity an ‘activation energy’ of 2.3 meV is determined for the condensed phase-free exciton transition. All experimental details can be explained by a two phase model. The experiments cannot be interpreted by the radiative recombination of biexcitons.     

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.     

Photoluminescence and electric conductivity in germanium at low temperatures

We have studied the exciton and electron-hole droplet (EHD) luminescence in optically irradiated germanium at temperatures between 1.8 and 4.2 K in the presence of an electric field. Simultaneously the electric conductivity was measured. The sample material was high-purity Ge (N _A –N _D =7·10¹⁰ cm^–3) andp-doped Ge withN _A =3·10¹⁴ cm^–3. In the high-purity Ge samples the exciton and EHD-luminescence intensity decreased nearly linearly as a function of the applied electric current, whereas the dependence upon the electric field was more complicated. Our results could be explained by a model in which carrier annihilation at the contacts following a rapid drifting process plays a dominant role (drift model). In thep-doped Ge samples the current-dependence of the luminescence intensity was qualitatively similar. However, here the drift model is not strictly valid any more because of the reduced carrier mobility and the generation of additional carriers by impurity impact ionization. During variation of the electric field, the luminescence intensity and the electric current show hysteresis. Here the capture of the moving carriers by the EHD appears to play an important role, in addition to the EHD-nucleation process.     

Density-dependent luminescence properties of the electron-hole liquid in highly excited germanium

Luminescence from electron-hole liquid (EHL) has been observed in pure germanium with electron-hole pair densities up to 1.8 × 10¹⁹cm^?3 at 1.6K by use of a high power-pulsed laser. From the fit of the theoretical line shapes, the Fermi energy and ground-state energy of EHL are experimentally obtained as a function of electron-hole pair density.     

Accurate measurement of the work function of electron-hole drops in germanium

The work function of electron-hole drops in Ge relative to free excitons is measured by an extremely simple and accurate technique. The threshold excitation power for drop condensation is determined as a function of temperature by observation of the sharp increase in shot noise associated with the drops in a collector junction. The resulting value of the work function is φ = 17.2 ± 1.0 K.     

Low-frequency degenerate surface modes of an electron-hole plasma

The computations of Flahive and Quinn¹ of the dispersion curves of low frequency degenerate surface (DS) modes propagating along the magnetic field in an electron-hole plasma are extended to higher values of the wavenumber. We find that beyond a certain value of the wavenumber the DS mode re-enters the allowed region of surface wave propagation and tends to an asymptotic frequency ω_R (<ω_LH). These low frequency resonances of an electron-hole plasma are discussed with reference to the experimental observations.     

Decay and recondensation of electron-hole liquid in germanium under CO2 laser pulse irradiation: Investigation by microwave conductivity measurements

In microwave conductivity investigations of photoexcited germanium at low temperatures under CO₂ laser pulse irradiation the evaporation of EHL and e-h plasma formation have been observed. This plasma irreversibly vanishes at high CO₂ laser intensities I_CO2 >4 × 10⁵ W cm^?2 but recondenses at low intensities. It was found that complete and irreversible disappearance of EHL is due to the e-h plasma throw out to the crystal boundaries by phonon wind, generated in 10.6 μm radiation absorption whereas at I_CO2 > 10⁶ W cm^?2 it is connected with the crystal lattice heating over the condensation critical temperature. A theoretical analysis of the CO₂ laser produced phonon wind interaction with e-h plasma is briefly presented. By comparing with experimental data on recondensation process the phonon wind efficiency is estimated.