Hole Droplets Research Articles

Bubble dynamics and atomization in evaporating polymeric droplets

We investigate the interaction of an aqueous polymeric droplet with a tuneable continuous laser in an acoustically levitated environment. The effect of laser irradiation intensity and polymeric concentration on various spatio-temporal parameters is unearthed using high-speed shadowgraphy and theoretical scaling analysis. We observe four temporal phases: droplet evaporation, vapour bubble growth followed by membrane inflation, bubble/membrane rupture through hole nucleation and droplet breakup. During the initial droplet evaporation phase, concentration build-up at the droplet surface beyond a critical limit leads to the formation of a skin layer. It is revealed that, at a given location inside the droplet, hot spots occur, and the maximum temperature at the hot spots scales linearly with irradiation intensity until a bubble nucleates. The low-intensity laser interaction leads to symmetric membrane inflation that eventually forms holes at droplet poles and cracks on the shell surface. On the contrary, high intensity causes early bubble nucleation followed by asymmetric membrane inflation that eventually ruptures through multiple hole formation. Furthermore, the growth and rupture of the membrane is followed by a catastrophic breakup of the droplet. Two dominant atomization modes are reported at significantly high irradiation intensities: stable sheet collapse and unstable sheet breakup. The evolution of droplets into a stable/unstable sheet follows universally observed ligament and hole dynamics. A regime map is shown to describe the influence of polymer concentration and irradiation intensity on the strength and mode of droplet atomization.

Multiphoton-excited exciton molecules in diamond

In this paper we report on observation of exciton molecules in the low-temperature photoluminescence spectra of diamond. The charge carriers are excited using multiphoton absorption of few-cycle near-infrared laser pulses. We observe that peaks corresponding to exciton molecules are well resolved and the formation of electron hole droplets is suppressed due to the initial carrier dynamics and low excited carrier densities.

Quasi-two-dimensional electron–hole droplets

The observation of a room-temperature stable liquid phase of electrons and holes in a quasi-two-dimensional photocell paves the way towards optoelectronic devices that harness collective phenomena.

Bayesian Spectroscopy of Admixed Photoluminescence Spectra with Exciton, Biexciton and Electron Hole Droplet States in a GaAs/AlAs Type-II Superlattice

We have analyzed a photoluminescence (PL) spectrum in a GaAs/AlAs type-II superlattice and have statistically confirmed stable existence of an electron-hole droplet (EHD) state. Although the PL spectrum consists of spectral components of the EHD, exciton and biexciton states, we have succeeded in decomposing the admixed spectrum into the respective components. In addition, posterior probability distributions of spectral parameters have been obtained in all spectral components from 30,000 times samplings through a replica exchange Monte Carlo method. By using mean values of their posterior probabilities, the PL spectrum can be completely reproduced by the sum of these spectral components. The stability energy of the EHD state has a sharp distribution in its posterior probability and has been confirmed to be enough larger than the ambient thermal energy. This fact is a strong evidence for the stable formation of the EHD state in the GaAs/AlAs type-II superlattice.

Dynamical Formation Process of Electron–Hole Droplets in a GaAs/AlAs Type-II Superlattice

We have investigated photoluminescence (PL) dynamics at 10 K in a GaAs/AlAs type-II superlattice from the viewpoint of the dynamical formation of electron–hole droplets (EHDs) from electron–hole plasma (EHP) in a gas phase. From systematic time-resolved PL (TRPL) spectra, we found that only a broad PL band attributed to the EHP is observed in an initial time region within ∼0.4 ns, whereas the exciton- and biexciton-PL bands appear at ∼0.7 ns. The peak energy and width of the broad PL band hardly depend on time after ∼14 ns, associated with the fact that the effective temperature of carriers agrees with that of the excitonic system. This suggests the formation of EHDs in thermal quasi-equilibrium. We estimated the stability energy relative to the biexciton energy to be ∼3.1 meV from the line-shape analysis of the TRPL spectra. This is consistent with the result obtained from the line-shape analysis of the steady-state PL spectrum. Consequently, the above temporal changes of the TRPL spectra demonstrate the...

Many-particle correlations in microscopic electron–hole droplets

Many-particle effects in two-component systems composed of two types of oppositely charged fermions are studied. The influence of mass ratio of the particles on physical properties of microscopic electron–hole droplets is investigated. We offer a theoretical model describing a quantum-mechanical system of a finite number of electron–hole pairs coupled by Coulomb interaction. The model is based on the Hartree–Fock approximation and the Random Phase Approximation with Exchange (RPAE). Within our theoretical approach we exclude center-of-mass motion from the spectrum of excited states of the system. Using our model we calculate the binding energies, static dipole polarizabilities and photoabsorption cross sections for the systems containing different numbers of particles with various ratios of electron and hole masses.

Collective states of two‐dimensional electron–hole system under the influence of Rashba spin–orbit coupling

AbstractThis study is concerned with a two‐dimensional (2D) electron–hole (e–h) system in a strong perpendicular magnetic field with special attention devoted to the Rashba spin–orbit coupling (RSOC). The influence of this interaction on the chemical potential of the Bose–Einstein condensed magnetoexcitons and on the ground‐state energy of the metallic‐type electron–hole liquid (EHL) is investigated in the Hartree–Fock approximation (HFA). The magnetoexciton ground‐state energy, and the energy of the single‐particle elementary excitations were obtained. We demonstrated that chemical potential is a monotonic function versus the value of the filling factor with negative compressibility, which leads to instability of the Bose–Einstein condensate of magnetoexcitons. The energy per one e–h pair inside the electron–hole droplets (EHD) is found to be situated on the energy scale lower than the value of the chemical potential of the Bose–Einstein condensed magnetoexcitons with wave vector ${\bf k} = 0$ calculated in the HFA.

Dynamics of electron–hole liquid condensation in CVD diamond studied by femtosecond pump and probe spectroscopy

We report on the femtosecond pump and probe study of the condensation dynamics of electron–hole liquid in CVD IIa type diamond. The parameters of electron–hole liquid created after carrier excitation by one- and two-photon absorption were determined on the basis of the analysis of spectral shape of time-integrated photoluminescence. The dynamics of condensation of carriers into electron–hole liquid was probed by measuring the time evolution of changes in infrared light absorption. These changes were interpreted in terms of the density dependent carrier scattering time according to the Drude model of free-carrier absorption. Measured condensation times range from 50 to 500ps depending on the initial carrier density (3×1016cm−3–5×1018cm−3) and the sample temperature (13–130K). The measured dynamics was reproduced very well by a rate equation model of electron–hole droplet formation for low photoexcited carrier densities. A ~10ps delay in initial stage of condensation observed for high carier densities was interpreted in terms of density dependent carrier cooling rate.

Effects of low-temperature annealing on polycrystalline silicon for solar cells

The polycrystalline silicon material grown by the edge-defined film-fed growth technique, and often used in solar cell production, is known to be carbon and dislocation rich. Aim of this work was to explore the effect of low-temperature annealing in vacuum on properties of these structural defects, often present in different solar-grade materials. Electrical measurements by deep level transient spectroscopy revealed the presence of the defects typically found in dislocated silicon. Detailed analysis further suggested that they are also carbon related, exhibiting quite unexpected behavior at such low-temperature annealing. Moreover, photoluminescence results showed electron–hole droplet condensation at dislocations after such low-temperature annealing. This further supports the hypothesis that point defects are incorporated at dislocation cores rather than in a cloud at its proximity.

Low‐temperature excitons produced by two‐photon excitation in high‐purity diamond crystals

AbstractWe have observed edge emission due to free excitons and electron–hole droplets in diamond by two‐photon excitation using subpicosecond laser pulses at 257 nm. We find that excitonic temperatures under two‐photon excitation are much lower than the case of one‐photon excitation. A minimum excitonic temperature of 3.4 K is extracted by spectral analysis. In order to understand the difference of the excitonic temperatures under two‐photon and one‐photon excitations, we discuss a heating mechanism which depends on the density distribution of the electron–hole pairs in the crystal along the laser beam. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of annealing under stress on defect structure of Si–Ge

Czochralski grown 1 1 1 oriented Si 0.986Ge 0.014 and Si 0.974Ge 0.026 samples, with interstitial oxygen concentration 8 × 10 17 cm −3, were processed for 5 h at 1270/1400 K (HT), under Ar pressure 10 5 Pa or 1.1 GPa (HP). Defect structure of Si–Ge was determined by synchrotron topography, high resolution X-ray diffractometry, infrared spectrometry and photoluminescence (PL) measurements. Topography of Si–Ge reveals so-called striations, related to Ge segregation. X-ray diffuse scattering is most pronounced for processed Si 0.974Ge 0.026; its intensity decreases with HT (HP). Processing, especially under HP, results in a changed lattice parameter, e.g. for Si 0.974Ge 0.026 processed at 1270 K under 1.1 GPa, a = 0.543444(5) nm while equals to 0.543301(5) nm for the as-grown sample. Intensity of PL at about 1.07 eV, related to the presence of electron hole droplets (EHD), decreased after processing at HT-(HP), evidencing out-annealing of point-like defects. Defects created in Si–Ge are in part related to tiny precipitates of oxygen. Dislocations are practically absent in processed Si–Ge. One can hope to apply HT-HP processing to improve structural perfection of Si–Ge.

Nonresonant two-photon generation of excitonic matter in a CuCl pellet

A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron–hole plasma and electron–hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor.

Electrical characterization of thin SOI wafers using lateral MOS transient capacitance measurements

A novel electrical evaluation method was proposed for crystal quality characterization of thin Si on insulator (SOI) wafers, which was done by measurement of minority carrier generation lifetime ( τ g) using transient capacitance method for lateral metal-oxide-semiconductor (MOS) capacitor. The lateral MOS capacitors were fabricated on three kinds of thin SOI wafers. The crystal quality difference among these three wafers was clearly shown by the τ g measurement results and discussed from a viewpoint of SOI fabrication. The series resistance influence on the capacitance measurement for this lateral MOS capacitor was discussed in detail. The validity of this method was confirmed by comparing the intensities of photoluminescence signals due to electron–hole droplet in the band-edge emission.

Enhanced luminescence from electron–hole droplets in silicon nanolayers

We have studied photoluminescence (PL) from the condensed phase in silicon-on-insulator samples with different Si layer thickness from 50 to 340 nm. Two major PL bands are observed at low temperatures, originating from free excitons (FE) and electron–hole droplets (EHD). It is found that with an increase of the excitation intensity the EHD PL shows a linear increase in the 50-nm-thick layer while a superlinear increase in the 340-nm-thick layer. The intensity ratio of the EHD PL to the FE PL in the 50-nm-thick layer is much larger than that in the 340-nm-thick layer under the same experimental conditions. The luminescence from the EHD is enhanced in thin Si nanolayers. These results suggest that highly dense electrons and holes are formed in the Si nanolayer and the interfaces act as the nucleation center of the EHD.

Maximum density hole droplets of an antidot in strong magnetic fields

We investigate a quantum antidot in the integer quantum Hall regime (the filling factor is two) by using a Hartree-Fock approach and by transforming the electron antidot into a system which confines holes via an electron-hole transformation. We find that its ground state is the maximum density droplet of holes in certain parameter ranges. The competition between electron-electron interactions and the confinement potential governs the properties of the hole droplet such as its spin configuration. The ground-state transitions between the droplets with different spin configurations occur as magnetic field varies. For a bell-shape antidot containing about 300 holes, the features of the transitions are in good agreement with the predictions of a recently proposed capacitive interaction model for antidots as well as recent experimental observations. We show this agreement by obtaining the parameters of the capacitive interaction model from the Hartree-Fock results. An inverse parabolic antidot is also studied. Its ground-state transitions, however, display different magnetic-field dependence from that of a bell-shape antidot. Our study demonstrates that the shape of antidot potential affects its physical properties significantly.

Ballistic phonon production in photoexcited Ge, GaAs, and Si

Phonon imaging and photoluminescence measurements are used to determine the frequency and spatial distribution of optically generated nonequilibrium phonons in Si, Ge, and GaAs at 1.7 K. At low excitation levels the thermalization of photoexcited carriers and the subsequent phonon down-conversion produce a broad frequency distribution of acoustic phonons that ``quasidiffuse'' in the crystal. These phonons produce a temporally broad heat pulse when detected at a distance from the excitation point. At moderate excitation levels (typically a 10-nS pulse with a power density of \ensuremath{\sim}20 ${\mathrm{W}/\mathrm{m}\mathrm{m}}^{2}$), the laser pulse produces a dense electron-hole plasma that can radically change the frequency distribution of nonequilibrium phonons. The plasma is a potentially rich source of low-frequency acoustic phonons, characterized by a temporally sharp heat pulse at a remote detector. The fraction of low-frequency phonons in the heat pulses is smallest in the direct-gap semiconductor GaAs, where rapid recombination depletes the populations of electrons and holes in just a few nanoseconds. More noticeable low frequency phonon components are seen in heat pulses in the indirect-gap semiconductors Ge and Si. At sufficiently high excitation densities (\ensuremath{\sim}60 ${\mathrm{W}/\mathrm{m}\mathrm{m}}^{2}$) in Ge, there is a suppression of the low-frequency phonon signal, which may result from phonon absorption within a cloud of electron hole droplets. An interesting alternative hypothesis is that the acoustic phonons created in the plasma are sufficiently dense to initiate phonon coalescence, whereby phonons are localized by phonon-phonon scattering over a relatively long period (500 ns). This localized ``hot spot'' could provide the phonon wind that drives the initial rapid expansion of the electron-hole plasma into the crystal.

Temporal decay measurement of condensate luminescence and its application to characterization of silicon-on-insulator wafers

We have investigated the temporal decay of luminescence due to electron–hole droplet (EHD) in silicon-on-insulator (SOI) wafers. The formation of EHD is realized under ultraviolet light excitation because of its shallow penetration depth and the presence of the buried oxide which acts as a diffusion barrier to carriers. The decay mechanism of the EHD luminescence is discussed on the basis of its dependence on the excitation intensity, surface/interface condition, temperature, and thickness of the superficial Si film as well as of time-resolved spectroscopic analysis. The dependence can be explained by assuming that the decay is determined by the concentration of free excitons and the intrinsic lifetime of the spatially localized EHD. We demonstrate that the formation of EHD plays an important role in the luminescence decay observed in a superficial thin film. Comparison of the EHD luminescence decay among SOI wafers fabricated by various techniques shows the great potential of this method for characterization of SOI wafers.

Luminescence of silicon thin film and SiGe multiple quantum wells realized on SOI

Abstract We have investigated photoluminescence (PL) of crystalline silicon-based layers sandwiched between SiO 2 barriers. We fabricate these structures on standard silicon on insulator (SOI) wafers realized by bonding techniques. Firstly, we realize silicon thin layers (50–200 nm) by thermal oxidation of the SOI free surfaces. At low temperature and under UV excitation, luminescence spectra is dominated by electron–hole droplet (EHD) or electron–hole plasma (EHP) recombination. We show that the behavior of the condensed phase is strongly modified by the spatial confinement of the carriers. Secondly, we grow by CVD Si / Si 1−x Ge x (x≈20%) multiple quantum wells (MQW) on SOI wafers. All the samples are capped by thermal SiO 2 layers. The main result is that luminescence from these samples is still observed up to room temperature; in comparison, the quenching temperature for similar Si/Si 1− x Ge x MQW directly grown on silicon substrates is 150 K. This result is due to the presence of the buried SiO 2 layer that avoids the thermal transfer of carriers from SiGe wells to the silicon substrate.

Excitonic processes in GaAs/AlAs type-II superlattices

In GaAs/AlAs type-II superlattices, the lowest-energy exciton consists of the X electron and Γ heavy hole that are confined in the AlAs and GaAs layers, respectively. The separation of the electron and hole both in real space and in momentum space leads to rich stages of the excitonic processes. Those are reviewed from the following aspects: the fundamental properties connecting with controllability, biexcitons, and condensation phases as bosons (excitonic superfluidity) and fermions (electron–hole droplet).

Phase diagram of the two-dimensional t-J model at low doping.

The phase diagram of the planar t--J model at small hole doping is investigated by finite size scaling of exact diagonalisation data of NXN clusters (up to 26). Hole-droplet binding energies, compressibility and static spin and charge correlations are calculated. Short range antiferromagnetic correlations can produce attractive forces between holes leading to a very rich phase diagram including a liquid of d-wave hole pairs (for $J/t\gtrsim 0.2$), a liquid of hole droplets (quartets) for larger J/t ratios ($J/t\gtrsim 0.5$) and, at even larger coupling J/t, an instability towards phase separation.