Multiplasmon laser gain in low-dimensional systems

The light absorption and laser gain in quantum wells are calculated using the cumulant expansion method and the fluctuation-dissipation theorem with allowance made for the strong Coulomb interaction of charge carriers. It is shown that the multiplasmon transitions result in a smoothening of the absorption spectrum and a shift in the absorption edge toward the long-wavelength range. The theoretical laser gain spectra are in agreement with the available experimental data. For In_0.05Ga_0.95As quantum wells, the laser gain g = 50 cm^?1 is reached at an electron density nd₀ = 1.64 × 10¹² cm^?2.     

Multicomponent dense electron gas as a model of Si MOSFET

We solve a 2D model of N-component dense electron gas in the limit N→∞ and in the range of the Coulomb interaction parameter N ^?3/2?r _s ?1. The quasiparticle interaction on the Fermi circle vanishes as ?²/Nm. The ground-state energy and the effective mass are found as series in powers of r _s ^2/3 . In the quantum Hall state on the lowest Landau level at integer filling 1?ν<N, the charge-activation-energy gap and the exchange constant are Δ=log(r _s N^3/2)?ω_H/ν and J=0.66?ω_H/ν.     

Electrical properties of the SiC/Si composite and the biomorphic SiC ceramic fabricated from spanish beech wood

A study has been made of the dependences of the electrical resistivity and the Hall coefficient on the temperature and magnetic field for the SiC/Si composite fabricated from spanish beech wood and bio-SiC, a high-porosity material formed by chemical extraction of silicon from this composite. The main charge transport parameters of these materials have been determined and analyzed. It has been shown that electric transport in bio-SiC is effected by n-type carriers with a high concentration of ～10¹⁹ cm^?3 and a low mobility of ～1 cm² V^?1 s^?1. The relations obtained have been analyzed by invoking the theory of quantum corrections to conductivity.     

Polarization dependence of intersubband absorption and photoconductivity in p-type SiGe quantum wells

We present a detailed study of the polarization dependence of subband absorption and photoconductivity in Si/SiGe quantum wells. For samples with a hole concentration ofp_s2.8×10¹² cm², bothp- ands-polarized absorptions have been observed and transitions to several excited states are clearly identified by comparison with self-consistent Luttinger-Kohn type calculations. The photoconductivity is surprisingly insensitive to the polarization, which indicates the importance of the subsequent transport process on the photocurrent responsitivity.     

Charge exchange and excitation cross sections at collisions of alpha particles with be-like impurity oxygen ions in a plasma

The charge exchange and excitation cross sections at collisions of alphas with O⁴⁺(1s ²2s ²) impurity atoms in a hot plasma for striking energies E _c varying from 20 keV to 2 MeV are determined for the first time. The cross sections are calculated using the method of close-coupling equations with 13 singlet four-electron quasi-molecular states taken as a basis. The partial cross sections of charge transfer to the 1s, 2s, and 2p states of a He⁺ ion and for O⁴⁺(1s ²2s ²) → O⁴⁺(1s ²2lnl’) (n = 2, 3) electronic excitation of an oxygen ion are found. The maximal value of the charge exchange total cross section roughly equals 2.2 × 10^?16 cm² at E _c ≈ 0.7 MeV. The excitation total cross section has a maximum of ≈ 7.7 × 10^?16 cm² at E _c ≈ 80 keV for single-electron excitation and ≈6.5 × 10^?16 cm² at E _c ≈ 0.7 MeV for two-electron excitation.     

Partial cross sections for Rb− photodetachment in the region of the Rb(5p 2 P 1/2 3/2) thresholds and their analysis by multichannel quantum defect theory

A crossed ion-laser beam apparatus has been used to measure accurate relative total and partial cross sections for photodetachment from Rb^? ions with high photon energy resolution (0.1–0.6 cm^?1) in the region of the Rb(5p ² P _1/2,3/2) thresholds (photon energy range 16,350–16,820 cm^?1). Satisfactory fits to these data by multichannel quantum defect theory (MQDT) have been obtained, resulting in a reliable set of seven MQDT parameters. The electron angular distribution parameter for the Rb(5_s) channel was found to beβ(5s)=2, independent of photon energy. MQDT predicts a sharp, window-type variation of?(5s) around the minimum of the 5s-cross section below the Rb(5p ² P _1/2) threshold (where an accurate experimental measurement ofβ(5s) was not possible) and a similar behaviour ofβ(6s) in the case of Cs^?photodetachment below the Cs(6p ² P _1/2) threshold.     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Capture cross-section and density of deep gap states in a−SiHx schottky barrier structures

Recombination of charge carriers in a?SiH_x Schottky barriers with density of states near mid-gap ranging from 2.8×10¹⁵?7×10¹⁶cm^-1eV^-1 is attributed to recombination centers with hole capture cross-section of 1.3×10^-15cm².     

Anomalous hall effect in Mn δ-doped GaAs/In0.17Ga0.83As/GaAs quantum wells with high hole mobility

Magnetic and magnetotransport properties of GaAs(δ〈Mn〉)/In_0.17Ga_0.83As/GaAs quantum wells with different Mn concentrations are studied. The delta-doped manganese layer has been separated from the GaAs quantum well with a spacer with an optimal thickness (3 nm), which has provided a sufficiently high hole mobility (≥10³ cm²V^?1 s^?1) in the quantum wells and their effective exchange with Mn atoms. It is found that the anomalous Hall effect (AHE) is exhibited only in a restricted temperature range above and below the Curie temperature, while the AHE is not observed in quantum wells with quasi-metallic conductivity. Thus, it is shown that the use of the AHE is inefficient in studying magnetic ordering in semiconductor systems with high-mobility carriers. The features observed in the behavior of the resistance, magnetoresistance, and Hall effect are discussed in terms of the interaction of holes with magnetic Mn ions with regard to fluctuations of their potential, hole transport on the percolation level, and hopping conduction.     

Excitonic state in quantum wells formed from “above-barrier” electronic states

Lines corresponding to localized excitonic states formed from “above-barrier” electron and/or hole states (specifically, excitation lines of excitons formed by an electron localized in a QW and a free heavy hole) have been observed in the photoluminescence excitation spectra of GaAs/Al_0.05Ga_0.95As structures with quantum wells (QWs), each containing one single-particle size-quantization level for charge carriers of each type. A computational method is proposed that permits finding the binding energy and wave functions of excitons in QWs taking the Coulomb potential into account self-consistently. The computed values of the excitonic transition energies agree quite well with the experimental results. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 613–619 (10 November 1999)     

PhotoleitfÄhigkeit von CaWO4-Einkristallen

Photoconductivity has been detected in CaWO₄-single crystals. By means of a pulse method the mobility of the charge carriers could be determined. The very small mobilities (Μ ⁺=0.1 cm²/Vs,Μ ^?≈ 5 cm²/Vs) are explained in the picture of “small polarons”.     

Photoelectron-photon coincidence studies of the A$̃and B$̃excited electronic states of X-CC-CN+, X = CH3, CD3, Cl,Br, I

Fluorescence quantum yields and lifetimes of the above given cations in selected levels within their lowest excited electronic states have been measured by a photoelectron—photon coincidence technique. These data, obtained under collision-free conditions, lead to the radiative and non-radiative rate constants as a function of the internal energy. The symmetry of the A$\text{\$}\text{?}$state is ²A₁ (X = CH₃, CD₃), ²Σ⁺ (X = Cl), but ²Π (X = Br, I) and the corresponding k_r values for these two groups, 1–2 × 10⁶s^?1 and 2 × 10⁷s^?1 respectively, reflect the different nature of the transitions. Other essential features of the results are discussed.     

An influence of deposition temperature on structural,optical and electrical properties of sprayed ZnO thin films of identical thickness

Nanocrystalline ZnO thin films were deposited at different temperatures (T_s = 325 °C–500 °C) by intermittent spray pyrolysis technique. The thickness (300 ± 10 nm) independent effect of T_s on physical properties was explored. X-Ray diffraction analysis revealed the growth of wurtzite type polycrystalline ZnO films with dominant c-axis orientation along [002] direction. The crystallite size increased (31 nm–60 nm) and optical band-gap energy decreased (3.272 eV–3.242 eV) due to rise in T_s. Scanning electron microscopic analysis of films deposited at 450 °C confirmed uniform growth of vertically aligned ZnO nanorods. The films deposited at higher T_s demonstrated increased hydrophobic behavior. These films exhibited high transmittance (>91%), low dark resistivity (～10^?2 Ω-cm), superior figure of merit (～10^?3 Ω^?1) and low sheet resistance (～10² Ω/□). The charge carrier concentration (η -/cm³) and mobility (μ – cm²V^?1s^?1) are primarily governed by crystallinity, grain boundary passivation and oxygen desorption effects.     

ESR studies of trans-(CH)x during photoexcitation

A series of spin resonance experiments on samples illuminated with light at photon energies ?ω>E_g are reported. The results set an upper limit on the quantum efficiency for photoproduction of unpaired spins, γ_s? 2×10^-7,implying that the photo-induced excited states and the photo-generated carriers are spinless and are to be associated with charged solitons.     

Spectroscopy of the <Superscript>5</Superscript>H superheavy hydrogen isotope

The formation of the ⁵H superheavy hydrogen isotope was experimentally sought in the reactions induced by stopped π^? mesons absorbed by ⁹Be nuclei. Peaks in missing-mass spectra were observed in two reaction channels, ⁹Be(π^?, pt)X and ⁹Be(π^?, dd)X, and were attributed to the ⁵H resonance states. The lowest state has parameters Er=5.5±0.2 MeV and Г=5.4±0.5 MeV [E_r is the resonance energy measured from the (triton + two neutrons) threshold]. Therefore, ⁵H is bound more weakly than ⁴H. Excited states of ⁵H were also observed. All three resonance levels (E_1r=10.6±0.3 MeV, Г_1r=6.8±0.5 MeV; E_2r=18.5±0.4 MeV, Г_2r=4.8±1.3 MeV; E_3r=26.7±0.4 MeV, Г_3r=3.6±1.3 MeV) can decay into five free nucleons.     

Low-lying states of trinuclear mixed-valence cluster: [Fe3S4]0

Low-lying energy states of the [Fe₃S₄]⁰ cluster have been calculated by taking into account the double exchange, superexchange and vibronic interaction. It was found that the adiabatic potential of the excited state withS=0 corresponds to the full delocalization of the “excess” charge. From the analysis of experimental data of Mössbauer spectroscopy and the temperature dependence of the magnetic susceptibility the double exchange parametert≥4000 cm^?1 and the vibronic interaction parameter λ₂/2k cm^?1 have been estimated.     

Electron-hole plasma in pulse photoexcited single quantum wells

We report photoluminescence studies of MOCVD grown, GaAsAl_xGa_1?xAs single quantum wells which were intensly excited with a pulse dye laser at T=2K. For a well width of d～40Å, the spectra are interpreted as due to the radiative recombination of a hot electron-hole plasma confined to the well. The density of charge carriers and their temperature depend upon the excitation intensity, and vary in the range of 10¹¹–10¹³ cm^?2 and 100–500K for an absorbed photon flux of 10¹³–10¹⁶ photons-cm^?2 per pulse, respectively. The observed spectral features are identified as the e1-hh1 and e1-lh1 transitions and two additional bands which are tentatively assigned to transitions involving virtual bound states of either the electron or the hole. The electron-hole plasma spectra of the d～40Å sample are strongly polarized perpendicular to the well quantization axis. For wider wells (d～80 and 150Å) smaller photoexcited carrier densities were observed for the same absorbed photon flux. It is thus concluded that the capture efficiency of the well is small.     

Restricted Three Body Problem in Semiconductor Heterostructure

The three-body restricted problem for X ^? and X ⁺ trions when a spatially separated exciton and electron or hole are located in the parallel quantum wells (QW) is reduced to the 2D three body problem for the exciton and the projection of the electron or hole on the plane of the excitonic QW. In the limit of a large spatial separation of the QWs the eigenfunctions and energy spectrum for X ^? and X ⁺ trions are obtained analytically. The 2D Wigner crystallization of the trions in the coupled QWs is discussed.     

Conductivity peculiarities in epitaxial layers of cadmium selenide

Electrophysical properties are studied in epitaxial cadmium selenide films produced by condensation on common mica and fluorophlogopite substrates over a wide range of substrate temperatures (T_s). It is shown that under conditions close to equilibrium highly perfect layers are produced with charge carrier mobility up to 300 cm²/V · sec and concentration ～ 10¹⁶ cm^?3. The temperature dependence of carrier concentration and mobility are studied in undoped and doped CdSe films. The values of the intercrystallite energy barriers are determined in layers condensed at differing T_s. It is shown that at T_s>630?C the charge carrier diffusion mechanism is close to that of a monocrystal, while for a barrier diffusion mechansim (T_s<630?C) the character of the energy barriers for the cubic and hexagonal phases in CdSe is somewhat different. Donor-level ionization energy and ionized center concentration are determined.     

Semiempirical investigation of perturbations of the g factors of electronic-vibrational-rotational levels of hydrogen: III. The r 3Π g − and s 3Δ g − states of the H2 and D2 molecules

The g factors of rovibrational levels of the (4d)r ³Π _g ^? and (4d)s ³Δ _g ^? states of the H₂ and D₂ molecules have been obtained for the first time. These values were found within the nonadiabatic model taking into account the interaction of the 4dπ³Π_g and 4dδ³Δ_g states in the pure precession approximation using semiempirical values of the expansion coefficients of the wave function in an adiabatic basis, which was obtained for the first time for the states of the triplet 4d complex of terms of the hydrogen molecules, and the results of numerical calculation of the overlap integrals of the vibrational wave functions of these states. It is established that the interference effects of the interaction between the 4dπ³Π _g ^? and 4dδ³Δ _g ^? states lead to significant (up to 7 times for the r ³Π _g ^? state of the H₂ and D₂ molecules and 70 and 8 times for the s ³Δ _g ^? state of the H₂ and D₂ molecules, respectively) differences between the nonadiabatic values of the g factors and the corresponding adiabatic values. It is found that the perturbed values of the g factors are much closer to the values corresponding to the case of Hund’s d coupling of angular momenta than to the values corresponding to the b coupling. It is established that the perturbations of the g factors of rovibrational levels of the states of the 4d complex of terms are much greater (up to 2 times for the ³Π _g ^? states and 350 times for the ³Δ _g ^? states) than the perturbations of the same characteristics for the 3d complex of terms of the hydrogen molecule with the same vibrational and rotational quantum numbers.