Size distribution of electron-hole drops in Ge

We investigate the size distribution of electron-hole drops collected in an illuminated p-n junction. The observed size distribution are of complex nature with several pronounced peaks revealing drop sizes of about 10⁸ electron-hole pairs (drop radius $\text{R ? 5}$ μm). An observed suppression of drop formation at low temperatures indicate that supersaturation effect are present.     

Non-isothermal nucleation of electron-hole drops in Ge

The delay effect of electron-hole drop nucleation caused by the presence of free carriers (electrons and holes) has been discovered in Ge. The phenomenon is discussed in the model of non-isothermal nucleation.     

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.     

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.     

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.     

The far-infrared absorption spectrum of electron-hole drops in silicon

We report on the first observation of the absorption spectrum of electron-hole drops in silicon. The absorption at 14°K consists of a broad peak at 34.2 ± 0.2 meV in the far-infrared spectral range. The lineshape can be well fit with a model that uses Mie theory of light absorption by small particles. The model includes both intra and interband terms. We find a plasma frequency of 51.9 ± 0.4 meV and from this we calculate a electron-hole density in the drops of (3.37 ± 0.06) × 10¹⁸ cm^?3.     

Volume and surface effects in the luminescence decay of electron-hole drops in Ge

Measurements of the luminescence time decay of electron-hole drops (EHD) in Ge are interpreted by taking into account the EHD volume recombination and the evaporation of free excitons (FE) at the EHD surface. From this study we get the EHD binding energy at T = 0 which is φ(0) = 15 ± 2 K. This result is consistent with the spectroscopic value of φ(0) if one takes into account the exchange splitting of FE.     

New magnetic phase transition between electron-hole drops and free-excitons in pure Ge

We have discovered a new magnetic phase transition between free-excitons and electron-hole drops in high purity Ge near the critical point of the liquid-gas phase diagram. The critical magnetic field is found to be H_c ≈ 0.4 T. For H?H_c the electron-hole drops are stable to higher temperatures by about 1 K with respect to zero field.     

Determination of nonradiative decay rate in electron-hole drops in Ge at 1.6°K

We show that microwave photoconductivity measurements of optically excited carriers in Ge at 1.6°K can be used to determine the importance of nonradiative recombination within electron-hole liquid drops. Our results show that the nonradiative lifetime is 80 μsec from which we calculate a radiative efficiency of 0.5 ± 0.1 for the condensed phase.     

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.     

Size effects in small electron-hole drops

The self-consistent formulation of density functional theory is used to calculate the single-particle properties of electron-hole droplets in Ge with between 10 and 120 pairs. Results are presented for the recombination luminescence lineshape, the electron and hole density profiles and the work function. As in the physics of the nucleus there are strong effects associated with the filling of the various electron and hole shells. In particular, the luminescence line narrows and oscillates as N decreases for a drop with less than 120 pairs. If N = 70 the width is 70% of the bulk value; while for N = 50 it is 80%. The density profiles and work functions for different N are rather varied and show very strong shell effects.     

Auger recombination of electron-hole drops

Phonon-assisted Auger recombination is calculated for indirect band gap semiconductors in the strongly degenerate case. It follows a reciprocal lifetime τ^?1=Cn² with C=7.19×10³¹ cm⁶ sec^?1 for Si and C=2.94× 10^?31 cm⁶ sec^?1 for Ge. These results are in good agreement with experimental values of the decay of electron-hole drops. Therefore one can conclude that phonon-assisted Auger recombination is the essential nonradiative recombination process in this case.     

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.     

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.     

Ground state energy of small electron-hole drops

The ground state energy of small electron-hole drops is calculated for droplets ranging in size from 10 to 10,000 pairs. A new value for the bending energy of $\text{1.1×10}{}^{\mathrm{?10}}\text{erg}\text{cm}$ is derived. We also give a simple highly accurate formula for the total energy per pair. The surface energy is extracted from the total energy and found to agree well with a previous self-consistent calculation. The density at the center of the drop remains essentially constant over the entire range of N, indicating that the drop is not dramatically compressed by the surface tension.     

Motion of electron-hole drops in low magnetic fields

The effect of magnetic fields on the motion of electron-hole drops in germanium is studied. A non-uniform strain is used to provide a known and controllable driving force for drop motion in the sample plane. Contrary to the results of earlier experiments in which drop motion was normal to the sample plane, the results are consistent with conventional models of drop-phenomenon interaction and weak magnetic fields have no observable effects on this motion.     

Formation and decay of electron-hole drops in Si

Using a single photon counting technique we were able to measure with very high time resolution the rise and decay times of the luminescence of free excitons and electron hole drops in Si. It was found that the formation and decay of droplets can be described by kinetic equations. The experimental data concerning the time behavior of the free excitons cannot be described by analogous simple kinetic equations. Therefore it is concluded that droplets are formed directly from a dense plasma rather than from a free exciton gas.     

Explosive instabilities in electron-hole drops under microwave radiation

We consider the Kroemer experiments on explosive instabilities in EHD in Ge in the light of an extension of the Altukhov theory to encompass general values of ωτ and of the ratio of drop radius to rf penetration depth. We suggest that the drops were heated above their critical temperature, but by a monotonic path.     

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.     

Radiative recombination from electron-hole drops in n-doped GaP

The observation of an isoelectronic impurity induced radiative no-phonon transition from electron-hole drops in strongly excited N-doped GaP is reported. The binding energy and carrier density within the drops are compared with theoretical values.