Surface acoustic wave controlled charge dynamics in a thin InGaAs quantum well

We experimentally study the optical emission of a thin quantum well and its dynamic modulation by a surface acoustic wave (SAW). We observe a characteristic transition of the modulation from one maximum to two maxima per SAW cycle as the acoustic power is increased which we find in good agreement with numerical calculations of the SAW controlled carrier dynamics. At low acoustic powers the carrier mobilities limit electron-hole pair dissociation, whereas at high power levels the induced electric fields give rise to efficient acousto-electric carrier transport. The direct comparison between the experimental data and the numerical simulations provide an absolute calibration of the local SAW phase.     

Photoconducive and photodielectric properties of heterostructures composed of poly-<Emphasis Type="Italic">N</Emphasis>-epoxypropylcarbazole and MEH-PPV films with a zinc octabutylphthalocyanine additive

The subject of investigation is poly-N-epoxypropylcarbazole and poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] films with zinc 2,3,9,10,16,17,23,24-octabutylphthalocyanine additives. The photoconducive and photodielectric properties of these heterostructures are studied in the absorption range of the metal complex. The photosensitivity of the structures depends on the triplet state of photogenerated electron-hole pairs, their dissociation in an external electric field, and charge carrier trapping. The increased photosensitivity of the heterostructures compared with that of constituent films is explained by a higher efficiency of dissociation of the photogenerated electron-hole pairs and a reduced influence of non-equilibrium carrier traps at the interfaces.     

Photocurrent generation in polymer-fullerene bulk heterojunctions

The photocurrent in conjugated polymer-fullerene blends is dominated by the dissociation efficiency of bound electron-hole pairs at the donor-acceptor interface. A model based on Onsager's theory of geminate charge recombination explains the observed field and temperature dependence of the photocurrent in PPV:PCBM blends. At room temperature only 60% of the generated bound electron-hole pairs are dissociated and contribute to the short-circuit current, which is a major loss mechanism in photovoltaic devices based on this material system.     

Dynamics of the electric field-assisted charge carrier photogeneration in ladder-type poly(para-phenylene) at a low excitation intensity

Electric field-assisted charge carrier photogeneration in a ladder-type methyl-substituted poly(paraphenylene) was investigated by ultrafast absorption spectroscopy at low excitation intensity. The dissociation of excitons into electron-hole pairs occurs from the vibrationally relaxed excited state throughout its lifetime and is caused by the applied electric field, rather than by existence of special "dissociation sites." These findings are of importance for material choice in device applications.     

Influence of random electric fields on luminescence yield and kinetics of insulators

In the present work we investigate theoretically the influence of random electric fields on electron-hole recombination in wide bandgap crystals. Effective Onsager radius and, therefore, electron-hole recombination rate are significantly modified by external electric fields. Electric field distribution functions for point defects and charged dislocations are evaluated analytically. Electron-hole recombination rate decreases with concentration of point defects and dislocations. In simple case of random fields created by charge carriers in highly excited regions the recombination rate is proportional to n 2/3 rather than n , where n is the concentration of excitations. Therefore modification of luminescence kinetics is most pronounced at initial stages of relaxation of highly excited regions.     

Charge separation in semicrystalline polymeric semiconductors by photoexcitation: is the mechanism intrinsic or extrinsic?

We probe charge photogeneration and subsequent recombination dynamics in neat regioregular poly(3-hexylthiophene) films over six decades in time by means of time-resolved photoluminescence spectroscopy. Exciton dissociation at 10 K occurs extrinsically at interfaces between molecularly ordered and disordered domains. Polaron pairs thus produced recombine by tunneling with distributed rates governed by the distribution of electron-hole radii. Quantum-chemical calculations suggest that hot-exciton dissociation at such interfaces results from a high charge-transfer character.     

Stabilization of electron-hole liquid in uniaxially strained germanium in a strong magnetic field

Luminescence spectra of uniaxially and uniformly strained high-purity germanium crystals at liquid-helium temperatures in a magnetic field of up to 14 T have been investigated. In strongly strained Ge crystals, a new line has been detected on the low-energy side of the excitonline in magnetic fields higher than 4 T. Studies of this line’s characteristics as functions of pressure, temperature, and magnetic field have led us to conclude that its presence is due to recombination of electron-hole pairs in an electron-hole liquid. The experimental data suggest that the metallic electron-hole liquid is stabilized in a strong magnetic field. By approximating the shape of the newly detected line using the model of metallic electron-hole liquid, we have obtained the electron-hole liquid density n _EHL(B) and Fermi energies E _Fe,h of electrons and holes. The liquid binding energy ø as a function of magnetic field has been estimated.     

Field-induced lifetime enhancement and ionization of excitons in GaAs/AlGaAs quantum wells

Picosecond and nanosecond luminescence studies of GaAs/AlGaAs quantum wells exposed to electric fields perpendicular to the layers are reported. The drastic red-shift of the photoluminescence with the electric field strength is accompanied by a strong increase of the electron-hole recombination lifetime. These features are most dominant for the wider wells due to the stronger polarization of the confined electron-hole pairs. Our observations are consistent with the model of the Quantum-Confined Stark Effect. In contrast, for high fields in the regime of ∼100 kV/cm field ionization limits the confinement of electrons and holes and leads to a strong decrease of the photoluminescence yield and lifetime with increasing field strength.     

Ordered structure formation in 2D mass asymmetric electron-hole plasmas

We study strong Coulomb correlations in dense two-dimensional electron-hole plasmas by means of direct path integral Monte Carlo simulations. In particular, the formation and dissociation of bound states, such as excitons, bi-excitons and many particle clusters, is analyzed and the density-temperature regions of their occurrence are identified. At high density, the Mott transition to the fully ionized state (electron-hole hexatic liquid) is detected. Particular attention is paid to the influence of the hole to electron mass ratio M on the properties of the plasma. For high enough values of M we observed the formation of Coulomb hole crystal-like structures.     

Numerical investigation of the theta pinch in intrinsic indiumantimonide

The electron-hole plasma in intrinsic InSb at 300 K can be compressed by electric and magnetic fields. In the theta-pinch configuration one applies, by a capacitor discharge over a one-winding coil around the sample, a magnetic field (0, 0,B _z), which induces an electric field (0,E _φ, 0); both fields cause the ambipolar motion (v _r, 0, 0) of the plasma. We describe the electron-hole plasma by means of a magnetohydrodynamical two-fluid model. The resulting system of differential equations is solved numerically under the assumptions of scalar hydrodynamic pressure and field-independent mobilities. The influence of the plasma properties on the theta pinch is studied as well as the dependence on the experimental conditions like radius of the sample, maximum value and risetime of the magnetic field.     

Influence of the Wavefunction Distribution on Exciton Dissociation in Electric Field

We show that the dissociation threshold of an exciton, a bound electron-hole pair, by an electric field is mainly determined by its energy: as expected, the dissociation voltage decreases with increasing exciton energy. However, within the multiplet of states belonging to a particular principal quantum number n, the dissociation voltage rises with increasing state energy, in contrast to the expectations based on energy arguments. This behavior is demonstrated for the yellow exciton states of Cu₂O and is attributed to the distribution of the wavefunction in the potential landscape, where the lower (higher) lying state in the multiplet is shifted away (towards) the tunnel barrier.     

Electron-hole coupled plasma mode in the magnetoreflection of graphite under high magnetic fields

A new structure which cannot be explained by any transitions between Landau levels was found in the far-infrared magnetoreflection spectra in graphite under high magnetic fields up to 45 T. The photon energy at which this structure is observed tends to zero at zero field, and it increases superlinearly with increasing magnetic field. The origin of this mode is interpreted in terms of the electron-hole coupled plasma which has never been observed in other substances before.     

含有共轭三键共聚物中光生载流子产生机制的探讨

本文以聚丁(对甲苯硫酸)-2,4-己二炔-1,6-二醇酯[polylbis(p-toluene sulphonate)ester of 2,4-hexadiyne-1,6-diole](PDA-TS)为例,对含有共轭三键的共聚物-聚丁二炔(polydiacetylene’s)(PDA’s)中光生载流子的产生机制,进行了初步探讨,认为其中的光生载流子分别由最初产生的电子-空穴对的分解和其后通过驰豫所形成的极化子-激子的离化而产生,对PDA-TS中三重激子的形成及其特点进行了详细的分析。 关键词：     

Effect of an electric field on the dissociation of electron-hole pairs interacting with triplet excitons in amorphous molecular semiconductors

The photoconductivity of amorphous molecular semiconductor films increases with the simultaneous photogeneration of singlet electron-hole pairs (EHPs) and triplet excitons but decreases when singlet EHPs are replaced by triplet EHPs. As the electric field increases, the influence of the triplet excitons on the photoconductivity of the films due to the dissociation of EHPs becomes less. It is concluded that as the electric field increases, the current-carrier mobility increases and the dissociation rate of EHPs becomes comparable to the spinconversion rate of EHPs interacting with triplet excitons. Fiz. Tverd. Tela (St. Petersburg) 39, 1020–1023 (June 1997)     

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.     

Internal-motion dependence of self-trapping of a Wannier exciton

The phase diagram of a Wannier exciton in the phonon fields is presented for 1s, 2s and 2p states of the internal (relative) motion on the basis of the adiabatic approximation. Differences in self-trapping among these states are revealed for an exciton with strong electron-hole Coulomb binding.     

Dependence on plasma density of the conductivity of an electron-hole plasma in germanium at 2 K

High densities of electron-hole plasma have been optically injected into Ge at T ∼ 2K, and the plasma density inferred from measurements of the plasma absorption at 3.39μ. The behavior of the conductivity with increasing density and at different electric fields can be understood from a model of a plasma condensation into high density drops, with the plasma density in the drops ? 6 × 10¹⁶ c⁻³.     

Luminescence of exciton-neutral donor complex in CdTe

Zeeman splittings and energy shifts of luminescence lines due to the exciton-neutral donor complex have been studied under magnetic fields in CdTe. Fine structures of the two-electron transitions associated with this complex are identified. The doublet structure of the initial state of the exciton bound to the neutral donor is assigned to the electron-hole spin-exchange interaction.     

Geometrical spin dephasing in quantum dots

We study spin-orbit mediated relaxation and dephasing of electron spins in quantum dots. We show that higher order contributions provide a relaxation mechanism that dominates for low magnetic fields and is of geometrical origin. In the low-field limit relaxation is dominated by coupling to electron-hole excitations and possibly 1/f noise rather than phonons.     

Kinetic theory of the hall effect in the self-magnetic field of a current

The Hall effect induced by the self-magnetic field of a current is considered in weak longitudinal fields in an electron-hole plasma on the basis of the solution for the kinetic equation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 108–112, September, 1972.