Cs/GaAs(100) surface: Two-dimensional metal or Hubbard insulator?

The evolution of the characteristic electron energy loss spectra accompanying the deposition of cesium on GaAs(100) surfaces with various superstructural reconstructions is studied experimentally. It is shown that the appearance of loss peaks in the GaAs band gap for Cs coverages θ_Cs>0.5 monolayers is due to resonances in the longitudinal and transverse polarizability of two-dimensional metallic clusters of adatoms. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 537–542 (25 October 1999)     

Transmission spectra of thin CdS and CdSe crystals near higher-order isotropic points

Summary For photon energies below the absorption edge in CdS, CdSe and other II–VI crystals the polariton dispersion curves forE⊥c andE‖c (c-axis in a wurtzite crystal) corss at some points called isotropic points (IP). The occurrence of isotropic points provides the possibility of mode coupling between ordinary and extraordinary waves. Since the consequences of mode coupling on the optical properties for photon energies near the lowest IP lying much below the first excitonic state were widely discussed in recent years, more attention is now paid to isotropic points lying near the band gap and related to then=2,3, … excitonic states (?higher isotropic points?). Making use of Stahl's real density matrix approach we derive the polariton dispersion relationsk _⊥ (ω), andk _‖ (ω), for CdS and CdSe bulk crystals and determine the positions of IP's due to the crossing of theB-polariton with higherA-excitonic resonances. By the method of multiple internal reflection we calculate the transmission spectra for various crystal thicknesses (between 3 μm and 0.5 mm) and coupling mechanisms. The calculated transmission shows sharply peaked structures centred at the isotropic points.     

Ultrafast vibronic phase transitions induced in semiconductors by femtosecond laser pulses

The intermode anharmonic interaction in the theory of ultrafast (t∼10⁻¹³ s) vibronic phase transitions induced on semiconductor surfaces (Si, GaAs) by femtosecond laser pulses is calculated. The conditions for plasma-induced transitions either to a state of chaotic disorder in the positions of the atoms (“cold liquid”) or into a state with crystal symmetry different from the initial symmetry (a new crystalline phase) are determined. It is shown that a NaCl-type structure is realized in GaAs for a transition of the second type, the transition being due to the instability of the longitudinal optical phonon branch. The corresponding numerical estimates are made for Si and GaAs. Fiz. Tverd. Tela (St. Petersburg) 41, 1462–1466 (August 1999)     

Infrared radiation from hot holes during spatial transport in selectively doped InGaAs/GaAs heterostructures with quantum wells

The infrared radiation from hot holes in In_xGa_1−x As/GaAs heterostructures with strained quantum wells during lateral transport is investigated experimentally. It is found that the infrared radiation intensities are nonmonotonic functions of the electric field. This behavior is due to the escape of hot holes from quantum wells in the GaAs barrier layers. A new mechanism for producing a population inversion in these structures is proposed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 478–482 (10 October 1996)     

Manifestation of magnetically induced spatial dispersion in the cubic semiconductors ZnTe, CdTe, and GaAs

Nonreciprocal birefringence due to magnetically induced spatial dispersion was observed in the T _d-class cubic semiconductors ZnTe, CdTe, and GaAs near the fundamental absorption edge. The dispersion of the parameters A and g, describing the contributions from terms of the type B _ik_j to the diagonal and off-diagonal components of the permittivity tensor ε _ij(ω,B,k), is determined for ZnTe and CdTe. Analysis of the dispersion and anisotropy of the nonreciprocal birefringence shows that in ZnTe, CdTe, and GaAs, in contrast to magnetic semiconductors of the type Cd_1−x Mn_xTe, it is due excitonic mechanisms. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 514–519 (10 April 1999)     

Donor-acceptor recombination in type-II GaAs/AlAs superlattices

A study is reported of steady-and nonsteady-state photoluminescence of intentionally undoped and uniformly silicon-doped type-II (GaAs)₇(AlAs)₉ superlattices grown by MBE simultaneously on (311)A-and (100)-oriented GaAs substrates. It has been established that at elevated temperatures (160>T>30 K) the superlattice spectra are dominated by the line due to the donor-acceptor recombination between donors in the AlAs layers and acceptors located in the GaAs layers. The total carrier binding energy to the donor and acceptor in a pair has been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1734–1739 (September 1998)     

Polariton reflectance spectra from thin ZnSxSe1−x layers

A study of reflectance spectra from thin ZnS_xSe_1−x solid-solution layers in the region of excitonic resonances is reported. It has been found that an increase in sulfur concentration in the layers increases the inhomogeneous broadening of the quantized polariton lines. It has been established that the inhomogeneous line broadening in a reflectance spectrum depends on the magnitude of exciton-photon mixing; it is small in the long-wavelength region where the photon component of the polariton is large, and large at short wavelengths where the mechanical component dominates. Fiz. Tverd. Tela (St. Petersburg) 40, 867–868 (May 1998)     

Optical phonons in quantum-wire structures

The optical vibrational modes in GaAs/AlAs structures grown on a (311)A-oriented GaAs surface are investigated. It is found that the line corresponding to the fundamental TO vibrational mode localized in a GaAs quantum wire is split into two lines with different directions of the polarization vector. The dispersion of the TO phonons of GaAs in the (311) direction is determined from the IR spectra of periodic structures. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 107–110 (25 January 1998)     

Raman spectroscopy of resonance exciton tunneling in GaAs/AlAs superlattices in electric fields

Optical-resonance-Raman scattering by acoustic phonons is used to study the effect of an electric field on the state of excitons in GaAs/AlAs superlattices. When the energy of the exciting photon coincides with the energy of an exciton bound to Wannier-Stark states of a heavy hole and electron with Δn=0,±1, the acoustic Raman scattering is enhanced. Oscillations in the intensity of the Raman spectrum in the electric field are explained by resonance delocalization of the exciton ground state as it interacts with Wannier-Stark states of neighboring quantum wells or with Wannier-Stark states of a higher electron miniband. Fiz. Tverd. Tela (St. Petersburg) 40, 827–829 (May 1998)     

On the spontaneous-coherence-transfer-induced sign change of a sub-natural-width nonlinear resonance

A simple theoretical model is proposed which describes the positive sign of the nonlinear absorption resonances in the experiments of Akul’shin et al. [Phys. Rev. A 57, 2996 (1998)]. An analytical expression for the linear response to a weak probe field is obtained in the weak-saturation limit. It is shown that the positive sign of sub-natural-width resonances is due to the spontaneous transfer of low-frequency coherence from an excited level to the ground state. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 776–781 (10 June 1999)     

Splitting of transverse optical phonon modes localized in GaAs quantum wires on a faceted (311)A surface

The energy splitting of fundamental localized transverse optical (TO1) phonon modes in GaAs/AlAs superlattices and quantum wires grown by molecular-beam epitaxy on a faceted (311)A GaAs surface is observed by Raman spectroscopy. The form of the Raman scattering tensor makes it possible to observe the TO_x and TO_y modes separately, using different scattering geometries the y and x axes are the directions of displacement of the atoms and are directed parallel and transverse to the facets on the (311)A surface). Enhancement of the splitting of the TO1_x and TO1_y modes is observed as the average thickness of the GaAs layers is decreased from 21 to 8.5 Å. The splitting is probably due to the effect of the corrugation of the GaAs/AlAs (311)A hetero-interface on the properties of localized phonon modes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 45–48 (10 July 1997)     

Localized optical phonons in GaAs/AlAs superlattices grown on (311)A and (311)B surfaces

A study is reported of optical vibrational modes in [311]-grown GaAs/AlAs superlattices. An analysis of the TO and LO localized modes observed in IR reflectance spectra showed that the difference between the TO and LO mode frequencies in superlattices grown on (311)A and (311)B surfaces is due to the different localization lengths of these modes. The dispersion of transverse optical phonons in GaAs derived from IR reflectance spectra is in a good agreement with Raman scattering data. Fiz. Tverd. Tela (St. Petersburg) 40, 550–552 (March 1998)     

External photoelectric effect and high-efficiency photocathodes in the soft-x-ray region of the spectrum

Two approaches to calculating the “true” x-ray photoemission from high-efficiency photocathodes are analyzed in the photon energy range 1–10 keV. Analytic expressions are derived for the quantum yield of true x-ray photoelectrons. The calculated values of quantum yield are compared with experimentally measured values. Fiz. Tverd. Tela (St. Petersburg) 39, 1665–1671 (September 1997)     

Giant population inversion of hot electrons in GaAs/AlAs type heterostructures with quantum wells

The heating of electrons in an Al_xGa_1−x As/GaAs (x>0.42) heterostructure in a lateral (directed along the heterointerfaces) electric field is studied. Population inversion on the size-quantization subbands of the Γ valley of GaAs and a giant population inversion between the X-valley states of Al_xGa_1−x As and Γ-valley states of GaAs are predicted. The possibility of using these inversions for achieving stimulated IR emission is discussed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 73–77 (10 July 1998)     

Photostimulated mesoscopic current fluctuations in a-Si based microstructures

Photostimulated two-level switchings of a vertical current are observed in thin (L=200 Å) amorphous silicon layers with a submicron electrical contact area (S=0.5×0.5 mm). The switching frequency increases with the photon flux. The conductivity fluctuations are explained on the basis of a microscopic nonlocal Staebler-Wronski effect. It is shown that the elementary event of formation of a metastable defect is accompanied by a reorganization of the system over a long distance of ∼2500 Å. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 674–677 (25 November 1996)     

Spin-wave resonances in nonuniformly strained films of FeBO3

Spin-wave resonances are investigated in thin films of the antiferromagnet FeBO₃ with magnetic anisotropy of the easy-plane type. It is observed that high-order resonances are observed only when nonuniform stresses are created in the sample. In nonuniformly stressed samples the antiferromagnetic resonance lines are broadened, and against the background of this broadening spin-wave resonances are visible whose positions are well described within the free-particle approximation. Using this method it is possible to resolve resonances with wave numbers ≃1.5×10⁵ cm⁻¹. The presence of strong uniaxial elastic stresses is established experimentally in nonuniformly strained films. A contact mechanism for exciting spin-wave resonances is discussed. Zh. éksp. Teor. Fiz. 112, 564–573 (August 1997)     

Negative persistent photoconductivity in GaAs (δ-Sn) structures

The effect of illumination with various wavelengths λ (770 nm<λ<1120 nm) on the conductivity of GaAs structures with tin δ-doping of the vicinal faces was investigated in the temperature range 4.2–300 K. Negative persistent photoconductivity was found in strongly doped samples. It was shown on the basis of the results of investigations of the Hall and Shubnikov-de Haas effects that the negative photoconductivity is due to a large decrease in the electron mobility with increasing electron density. The decrease of electron mobility is explained by ionization of DX centers, which destroys the spatial correlation in the distribution of positively charged donors and negatively charged DX centers. Zh. éksp. Teor. Fiz. 116, 2130–2139 (December 1999)     

Effect of neutron irradiation on the IR absorption spectra of cupric oxide single crystals

A strong increase of the absorption coefficient with photon energy increasing from 0.1 to 1.0 eV is observed in the spectra of CuO single crystals irradiated with neutrons to a fluence of 5×10¹⁸ cm⁻². The difference of the absorption coefficients before and after irradiation depends on the wavelength as λ⁻². The effect of neutron irradiation on CuO is qualitatively similar to that of neutrons on other semiconductors (for example, GaAs) and differs from that obtained by irradiating CuO with charged particles. Zh. Tekh. Fiz. 69, 98–99 (December 1999)     

Generation of 1015–1017 eV photons by ultrahigh-energy cosmic rays in the Galactic magnetic field

It is shown that the expected dip in the diffuse photon spectrum above the threshold of e ⁺ e ⁻ pair production, i.e., at energies 10¹⁵–10¹⁷ eV, may be absent due to the synchrotron radiation by the electron component of the extragalactic ultrahigh-energy cosmic rays (UHE CRs) in the Galactic magnetic field. The mechanism we propose requires small (<2×10⁻¹² G) extragalactic magnetic fields and a large photon fraction in the UHE CRs. For a typical photon flux expected in top-down scenarios of UHE CRs, the predicted flux in the region of the dip is close to the existing experimental limit. The sensitivity of our mechanism to the extragalactic magnetic field may be used to improve existing bounds on the latter by two orders of magnitude. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 487–492 (25 October 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Valence band photoemission from in-situ grown GaAs(100)-c(4 × 4)

The electron structure of GaAs(100)-c(4 × 4) has been studied by means of angular-resolved photoelectron spectroscopy for photon energies (20–40) eV. The sample was prepared by molecular beam epitaxy in-situ at the BL41 beamline of the MAX I storage ring of the Max-lab in Lund. Photon energy variation helped in separating dispersing bulk features from nondispersing surface features in the energy distribution curves recorded at normal emission. Two sets of peaks were related to bulk transitions from the two topmost E(k _⊥) branches of the valence band of GaAs and one more set came from the surface state in the center of the 2D Brillouin zone. Good agreement was found between experimental bulk dispersion branches and theoretical calculations based on realistic final state dispersion. The surface state peak, hardly visible at 20 and 22 eV photon excitations, gets clearly enhanced at higher excitation energies. In contrast to earlier measurements of this kind, two major differences have been found: (i) clearly developed surface state peak just below the top of the v alence band, (ii) absence of a large peak in the electron energy distribution at around −6.5eV below the valence band top. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.