Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Many body effects contribute significantly to the energy states of electron-hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron-hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor <2, and similar effects are found in CdTe QWs. For filling factor >2 no internal transitions are observed. These measurements demonstrate the strong effects of electron-electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.     

Si–Si bond as a deep trap for electrons and holes in silicon nitride

A two-stage model of the capture of electrons and holes in traps in amorphous silicon nitride Si₃N₄ has been proposed. The electronic structure of a “Si–Si bond” intrinsic defect in Si₃N₄ has been calculated in the tight-binding approximation without fitting parameters. The properties of the Si–Si bond such as a giant cross section for capture of electrons and holes and a giant lifetime of trapped carriers have been explained. It has been shown that the Si–Si bond in the neutral state gives shallow levels near the bottom of the conduction band and the top of the valence band, which have a large cross section for capture. The capture of an electron or a hole on this bond is accompanied by the shift of shallow levels by 1.4–1.5 eV to the band gap owing to the polaron effect and a change in the localization region of valence electrons of atoms of the Si–Si bond. The calculations have been proposed with a new method for parameterizing the matrix elements of the tightbinding Hamiltonian taking into account a change in the localization region of valence electrons of an isolated atom incorporated into a solid.     

Ultrafast dynamics of surface electromagnetic waves in nanohole array on metallic film

We study the ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength holes array by means of transient photomodulation with ∼100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the dynamics and that is inconsistent with plasmon–polariton mechanism of extraordinary transmission. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron–phonon relaxation dynamics, involving non-equilibrium hot electrons and quasi-equilibrium phonons.     

Proposal of structures possessing high exciton concentration

The possibility of achievement of high exciton concentrations is analyzed. It is shown that high concentrations can be achieved in a three-layer thin molecular film due to the autoreduction processes taking place in it. Shortly, the appearance of high concentrations is the consequence of boundary conditions in film and of the magnitude of matrix elements of dipol-dipol interactions. The autoreduction takes place in the cases when matrix elements characterizing exciton transfer are less than statistical matrix elements. Based on numerical analysis, it was found that optical quanta concentrations of a three-layer film can achieve values of about 5×10⁻². The structures possessing so high concentration do not exist in nature, thus they have to be synthesised. For the current state of nanotechnology, it is not a problem. Fortunately achieving high concentrations requires only certain ratios of relevant characteristics of the film with a two-level exciton scheme, but not their single values.     

The direct exchange interaction for weakly delocalized multielectron systems in crystals

The multielectron theory of exchange interactions taking into account both electrostatic interactions between electrons (“potential” exchange) and electron transfer between magnetic centers (“kinetic” exchange) has been developed on the basis of Bogolubov's procedure using double irreducible operators. The final hamiltonian contains a number of isotropic and anisotropic terms. The exchange interaction parameter variations with the distance have been calculated. The terms determining the dependence on the occupancy of magnetic shells are shown to be of the same order of magnitude as the “potential” exchange and two orders of magnitude less than the “kinetic” exchange. It is shown that in the model adopted the mechanisms of direct exchange interactions always favour the antiferromagnetic spin ordering     

Screening by composite charged particles: the case of quantum well trions

We investigate the screening properties of a two-dimensional gas of charged excitons (trions). In a first approach to this complex problem, we determine the Hartree response of these composite charged particles within a random phase approximation, showing the effect of the trion internal structure. Only in the long wave-length limit, trions behave as point charges with mass equal to the sum of the three particle components. For finite wave-vectors, the trion screening strongly deviate from the point charge behavior and can even vanish completely at a nodal wave-vector, due to a compensation between the contribution of the two electrons and the hole within a trion. Predictions are presented for the screening of a Coulomb potential, the scattering by charged impurities and the properties of trionic plasmons.     

Electron and hole spectra of silicon quantum dots

In the framework of perturbation theory, the first several one-particle energies and wave functions for electrons and holes (six for each) in spherical silicon quantum dots are obtained in the envelope function approximation (kp method). It is shown that the model of an isotropic dispersion relation with the mean reciprocal effective mass is applicable for the ground state of holes in the valence band. Anisotropy of the dispersion relation, which takes place for bulk semiconductors, becomes significant for the electron ground state in the conduction band as well as for all excited (both electron and hole) states.     

Hidden symmetry and correlated states of electrons and holes in quantum dots

We describe hidden symmetry and its application to the construction of exact correlated states of electrons and holes in quantum dots. The hidden symmetry is related to degenerate single particle energy shells and symmetric interactions. Both can be engineered in a quantum dot. We focus on hidden symmetry involving spin singlet pairing of electrons and spin singlet pairing of holes. Detailed calculations for a third shell are presented to illustrate the mechanism of pairing.     

Influence of magnetic quantization on the effective electron mass in Kane-type semiconductors

Summary We study the effective electron mass at the Fermi level in Kane-type semiconductors on the basis of fourth order in effective mass theory and taking into account the interactions of the conduction electrons, heavy holes, light holes and split-off holes, respectively. The results obtained are then compared to those derived on the basis of the well-known three-band Kane model. It is found, takingn-Hg_1−x Cd _x Te as an example, that the effective electron mass at the Fermi level in accordance with fourth-order model depends on the Fermi energy, magnetic quantum number and the electron spin respectively due to the influence of band nonparabolicity only. The dependence of effective mass on electron spin is due to spin-orbit splitting parameter of the valence band in three-band Kane model and the Fermi energy due to band nonparabolicity in two-band Kane model. The same mass exhibits an oscillatory magnetic-field dependence for all the band models as expected since the origin of oscillations in the effective mass in nonparabolic compounds is the same as that of the Shubnikov-de Hass oscillations. In addition, the corresponding results for parabolic energy bands have been obtained from the generalized expressions under certain limiting conditions.     

Carrier spin polarization in ferromagnet-semiconductor (Ga,Mn)As/GaAs structures

The effect of ferromagnetic layers on the spin polarization of holes and electrons in ferromagnet-semiconductor superlattices with a fixed Mn δ-layer thickness of 0.11 nm and different GaAs interlayer thicknesses varying in the range from 2.5 to 14.4 nm and a fixed number of periods (40) is studied by means of hot-electron photoluminescence (HPL). Here, our study of the HPL demonstrates that the holes in δ-layers of (Ga,Mn)As DMS occupy predominantly the Mn acceptor impurity band. The width of the impurity band decreases with the increase of the interlayer distance. We also found that an increase in the GaAs interlayer thickness softens the magnetic properties of the ferromagnetic layers as well as reduces the carrier polarization. It is demonstrated that the hole spin polarization in the DMS layers and spin polarization of electrons in nonmagnetic GaAs are proportional to the sample magnetization.     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.     

Electron-hole diagrams versus exciton diagrams: the simplest problem on interacting excitons

We study the interaction of an exciton with a distant metal, which is the simplest problem on interacting excitons: The semiconductor and metal electrons being “different” species, we do not have to worry about the tricky consequences of Pauli exclusion between identical carriers, which appear in any other problem on interacting excitons. We show how the exciton absorption, in the presence of semiconductor-metal interaction, can be derived in a very simple and transparent way from an exciton diagram procedure, provided that we use the appropriate exciton-metal interaction vertex, which contains the scattering from an exciton state to another exciton state under a Coulomb excitation. We also show that the resolution of this problem using standard electron-hole diagrams is dreadfully complicated at the lowest order in the semiconductor-metal interaction already, preventing a full calculation of the exciton-metal coupling from this usual technique. Received 26 February 2001     

Temperature-Dependent Defect-Induced New Emission in ZnSe Crystal

The temperature-dependent photoluminescence behaviour of chemical vapour transport （CVT）-grown ZnSe crystal is investigated. A new emission band appears when temperature is reduced to 155K. It is shown that the new emission band is strongly related to defect emission peaked at around 2.1 eV. The emergence of the new emission band is accompanied by decreasing emission intensity of free exciton, as well as redshiR of defect emission with temperature decreases. The activated energy of the defect state is estimated to be 60.6 meV, which is approximately equal to the energy difference between the new emission and the free exciton emission at 155K.     

Impurity-stabilized zinc-blende phase of wurtzite compounds

We propose here a new approach to stabilizing the cubic zinc blende phase of semiconductors that are usually more stable in the hexagonal wurtzite phase. We show that this can be done by taking advantage of the valence and conduction band offsets between the cubic and the hexagonal phases. Due to this band offset, it will cost less energy to insert electrons by shallow donors, or insert holes by 3d acceptors in the zinc blende structure, thus stabilizing the cubic phase.     

A model of hopping and band DC photoconduction in doped crystals

Expressions for the screening length and the ambipolar diffusion length are derived, for the first time, for the case where hopping conduction and band conduction coexist in semiconductors with hydrogen-like impurities. A method is proposed for calculating the diffusion coefficient of electrons (holes) hopping between impurity atoms from data on the Hall effect, in the case where the hopping and band conductivities are equal. An interpretation is given of available experimental data on hopping photoconduction between acceptors (Ga) and donors (As) in p-Ge at T=4.2 K doped by a transmutation method. It is shown that the relative magnitude of the mobilities of electrons hopping between donors and holes hopping between acceptors can be found from the hopping photoconductivity measured as a function of the intensity of band-to-band optical carrier excitation.     

Interband Optical Transitions due to Donor Bound Excitons in Wurtzite InGaN Strained Coupled Quantum Dots： Strong Built-in Electric Field Effects

Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field （BEF） in the wurtzite InGaN strained coupled quantum dots （QDs）, the positively charged donor bound exciton states and interband optical transitions are investigated theoretically by means of a variational method. Our calculations indicate that the emission wavelengths sensitively depend on the donor position, the strong BEF, and the structure parameters of the QD system.     

A chemical-bond approach to doping,compensation and photo-induced degradation in amorphous silicon

We propose that during deposition of a-Si:H films a chemical equilibrium is established that relates the density of dangling-bond defects near mid-gap to the densities of electrons and holes in the conduction and valence band states. We develop the appropriate chemical reaction formalism and show that our model allows doping, compensation and photo-induced degradation to be treated within a single and unifying approach.     

The isometric group of non-rigid molecules

A new method for derivation of symmetry groups of non-rigid molecules is presented, based on the concept of isometry of nuclear configurations defined by a certain number of internal (structural) coordinates. The substitution group of the internal coordinates interrelating isometric nuclear configurations generates the internal isometric group by both permutations and intransitive imprimitive substitution groups, and together with the covering group it generates the full isometric group. Examples for these groups are given. It is shown that the rotation-internal motion hamiltonian is symmetric with respect to the isometric group. The relation between various approaches to the isometric group is investigated; the role of automorphisms is pointed out. The new approach is shown to give a self-consistent procedure for the quantum mechanical problem associated with the rotation-internal motion hamiltonian.     

Luminescence property studies of α-Al2O3 by means of nanosecond time-resolved VUV spectroscopy

The luminescence spectra of corundum monocrystals grown by different methods are investigated by means of a time-resolved spectroscopy method at temperatures 90 K and 300 K. The existence of fast and slow emission in the VUV luminescence spectra of irradiated and nonirradiated crystals was observed. We observed luminescence bands with a maximum at 326 nm produced by F ⁺ centers. A new type of fast luminescence at the band of 270 nm was found. This is known as cross-luminescence and is connected with the recombination of valence band electrons with the holes in the low ground band. It was shown that the band of 410 nm isn't due to to anionic centers (F-centers), but is determined by the short lifetime center of emission (F ^- -centers). Received 20 October 1998 and Received in final form 20 January 1999     

Influence of exciton-exciton interaction with spin paired charge carriers and exciton-lattice interaction on the surface properties of crystalline solids

Applying tight-binding approximation and spin pairing of like charge carriers in a pair of excitons created in a lattice, the possibility of forming a bound exciton-exciton state is studied. It is found that, provided there exists strong exciton-lattice interaction, such a bound state may be formed and its energy may lie within the valence band deforming the material into a crystalline solid with no energy gap. Lowering of the energy is calculated in naphthalene and anthracene crystals where some experimental results are known. The excess energy released after the formation of such bound state can be adequate, depending on the material, to desorb neutral atoms or eject of electrons from surfaces.