The electron-hole drop and the impurity-induced semiconductor-metal transition

It is shown that metallic condensation should occur in Si even under condition of heavy doping. The density of delocalized donor-electrons strongly influences the equilibrium density of holes within the drop and in that fashion the half-width of the electron-hole recombination line. It is therefore possible to derive properties of the semiconductor-metal transition from a study of the electron-hole drop in doped samples. This is confirmed by an application to Si : P.     

Excitation wavelength dependence of electron-hole drop velocities in Germanium

We show experimentally that the velocities of electron-hole drops in Germanium are independent of the wavelength of optical excitation. This result implies that the phonons emitted during the thermalization of electron-hole pairs have no effect on droplet motion and also permits us to draw conclusions about the decay of high frequency phonons. This leaves the “phonon wind” as originally proposed by Keldysh as the most likely cause of droplet motion.     

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.     

Deep-impurity effects on dimensional resonance of a large electron-hole drop in germanium

Alfvén wave dimensional resonance of a large electron-hole drop is studied in detail in strained Ge doped with Zn impurities. Impurity scattering rate within the large drop is 2.7 × 10¹⁰ s⁻¹ for holes. This is larger than 1.2 × 10¹⁰ s⁻¹ for electrons. The difference causes an imbalance of microwave response between electrons and holes and enhances a helicon-like nature in magneto-plasma modes.     

On the low energy tail of the electron-hole drop recombination spectrum

The luminescence line shape of electron-hole drops in germanium is calculated assuming that the final state of the radiative transition is lifetime broadened due to Auger processes in the degenerate bands. This level broadening can account for the existence of the experimentally observed low energy tail in the recombination spectrum. The theoretical line shape is in good agreement with experimental results.     

Measurement of electron-hole drop size in doped germanium by light scattering

We report measurements of sizes of electron-hole droplets in germanium with antimony. Between nominally pure crystals and those doped to a level of 3.5 × 10¹⁵ Sb cm^?3, the droplet radius increases by a factor of two. We believe that this result can only be understood in terms of a comprehensive model in which droplet size and motion are related.     

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.     

On the interaction of an electron-hole drop with the crystal surface

Treating the crystal as an isotropic elastic continuum, we show that deformation-potential coupling should lead to a local, crater-type lattice distortion centered at the electron-hole-drop (EHD). Surface restraints on this displacement field generate attractive forces between the drop and the crystal surface. This result is discussed in connection with recent experiments on EHD-motion in Ge.     

Microwave heating and evaporating of a large electron-hole drop in pure Ge

At the large microwave power level, we have observed a sharp increase of the m.w. absorption in magnetic field and an appearance of steps in the m.w. absorption kinetics. These steps are connected with the intense evaporation of drops caused by the m.w. field, which provides the existence of the dense cloud of free carriers surrounding the drop. The density of the cloud has been found to depend strongly on magnetic field.     

Time delay in formation of a γ-phase electron-hole drop in germanium

An explanation is given for the long time delay in the formation of gamma phase EHD in germanium. The Cerenkov radiation of phonons by moving alpha drops at velocity mach 0.2–0.3 is calculated to be the controlling process in the time delay.     

Sign reversals of the carriers and the volume change in Al-Cu-Fe quasicrystalline alloys

New results of dilatometric experiments with rapidly quenched Al-Cu-Fe quasicrystalline alloys in the course of their thermal annealing are presented. It is established that the icosahedral (I) phases with different types of carriers exhibit different signs of volume changes with ordering. The observed effect is a direct experimental proof of the fact that structural defects in icosahedral quasicrystals are electrically active centers.     

Rapid formation kinetics of a long-lived electron-hole drop in Ge under pulsed excitation

The buildup of a large long-lived electron-hole drop in stressed Ge is studied using its Alfvén wave resonances. This microwave method permits rapid time resolution of the formation process after a short intense (100 nsec, 7 W) laser excitation pulse. A delay of about 1 μsec is observed before the drop radius rapidly increases from zero to ～ 100 μm within the next μsec. This is consistent with the interpretation that the photo-produced electron-hole pairs are quickly accelerated to the strain-induced potential minimum which is well within the crystal.     

Effect of deep impurity of recombination lifetime of a large electron-hole drop in strained germanium

Lifetime shortening of a large electron-hole drop in zinc-doped Ge is reported by using time-resolved photoluminescence technique. The lifetime decreases remarkably with the concentration of a double acceptor Zn up to ∼10¹⁶ cm⁻³. This makes a striking contrast to the result by Chen et al. for ordinary electron-hole droplets in Ge doped with shallow impurities. Additional information from microwave Alfvén wave dimensional resonance of a large drop is also presented.     

Phonon wind induced anomalous dependence of the electron-hole drop luminescence in Ge at low magnetic fields

Anomalous variations of EHD luminescence in Ge in low magnetic fields depending on intensity and orientation of excitation are reported. Angular dependent drop size distribution, quantum efficiency $\text{Q}$ and dilution of the initial plasma are derived and shown to result from the anisotropy of the phonon wind. Magnetic field inhibited escape of Auger carriers increases $\text{Q}$ by 20%. Strongly enhanced surface recombination due to magnetic field induced confinement of the carriers is deduced.     

Electro-migration in a two-band metal and in Hall field; Application to an electron-hole drop

The Das-Peierls semi-classical model for a theory of electro-migration in metals is extended to a two-band system of electron and hole carriers. It is shown how the “effective charge” on the electro-migrating ion can be obtained from electro-migration experiments in Hall field. An experiment of this type is proposed to determine the charge on an electron-hole drop recently observed in some semi-conductors.     

On the liquid drop model mass formulae and charge radii

An adjustment to 782 ground-state nuclear charge radii for nuclei with N, Z 3 \ge8 leads to R₀ = 1.2257 A^1/3\ensuremath R_0 = 1.2257 A^{1/3} fm and s \sigma = 0.124 fm for the charge radius. Assuming such a Coulomb energy E_c = \frac35 e²Z²/1.2257 A^\frac13\ensuremath E_c = \frac{3}{5} e^2Z^2/1.2257 A^{\frac{1}{3}} , the coefficients of different possible mass formulae derived from the liquid drop model and including the shell and pairing energies have been determined from 2027 masses verifying N, Z 3 \ge8 and a mass uncertainty ￡ \le150 keV. These formulae take into account or do not the diffuseness correction ( Z²/A\ensuremath Z^2/A term), the charge exchange correction term ( Z^4/3/A^1/3\ensuremath Z^{4/3}/A^{1/3} term), the curvature energy, the Wigner terms and different powers of I = (N - Z)/A . The Coulomb diffuseness correction or the charge exchange correction term play the main role to improve the accuracy of the mass formulae. The different fits lead to a surface energy coefficient of around 17-18MeV. A possible more precise formula for the Coulomb radius is R₀ = 1.2332A^1/3 + 2.8961/A^2/3 - 0.18688A^1/3I\ensuremath R_0 = 1.2332A^{1/3} + 2.8961/A^{2/3} - 0.18688A^{1/3}I fm with s \sigma = 0.052 fm.     

Influence of charge relaxation on the capillary oscillations of a charged viscous spheroidal drop

A dispersion relation is derived for the spectrum of capillary modes of a charged spheroidal drop of a viscous liquid with allowance for charge relaxation. It is shown that the finite charge transport rate leads to lowering of the instability growth rates for various capillary modes of a spheroidal drop of a low-viscosity liquid. As the degree of deformation of the drop increases, the magnitude of the absolute change in the growth rate caused by the finite rate of charge redistribution decreases. Zh. Tekh. Fiz. 69, 28–36 (August 1999)     

On the correlation of “hot” and “cold” electron-hole drop densities in uniaxially stressed silicon

“Hot” and “cold” electron-hole drops were studied in 〈100〉-stressed silicon by intense optical pulse excitation and gated photoluminescence measurements. We obtain varying relative densities of hot and cold EHD electrons either by choosing the delay time of the gate appropriately or by using samples of different impurity concentrations. In all cases, the spectra can entirely be fitted assuming only a homogeneous EHD-phase consisting of both, hot and cold electrons. This result is in agreement with recent theoretical work which — contrary to germanium — finds phase separation in stressed silicon not to be very likely.     

Influence of charge carrier diffusion on the stability of a charged drop of a finite-conductivity liquid

A dispersion relation for the capillary oscillations of a spherical drop of a viscous incompressible liquid with a charge transfer finite rate is derived and analyzed with emphasis on the role of diffusion. It is shown that diffusion has the strongest influence on the stability of rapidly damped quasi-periodic motions of a low-conductivity liquid. The instability growth rate of capillary oscillations grows with the charge diffusion coefficient and decreases with rising conductivity of the liquid.     

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.