GaAsSb/GaAs quantum well growth by MOCVD hydride epitaxy with laser sputtering of antimony

A new method of obtaining quantum-size GaAs_1−x Sb_x (x⩽0.45) layers is proposed. The method consists in laser vaporization of solid metallic antimony near the substrate directly in the reactor. The antimony concentration is set by the antimony sputtering time with the arsine flux shut off. The polarization of the photoluminescence of the obtained layers indicates the formation of quantum wires. The heterostructures obtained are used to fabricate laser diodes. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 84–88 (10 July 1998)     

Generalization of the effective mass method for semiconductor structures with atomically sharp heterojunctions

The Kohn-Luttinger method of envelope functions is generalized to the case of heterostructures with atomically sharp heterojunctions based on lattice-matched layers of related semiconductors with zinc-blende symmetry. For electron states near the Γ point in (001) heterostructures the single-band effective-mass equation is derived, taking into account both the spatial dependence of the effective mass and effects associated with the atomically sharp heterojunctions. A small parameter is identified, in powers of which it is possible to classify the various contributions to this equation. For hole states only the main contributions to the effective Hamiltonian, due to the sharpness of the heterojunctions, are taken into account. An expression is derived for the parameter governing mixing of states of heavy and light holes at the center of the 2D Brillouin zone. Zh. éksp. Teor. Fiz. 116, 1843–1870 (November 1999)     

Boundary conditions for narrow-gap heterostructures described by the Dirac equation

Boundary conditions are obtained for narrow-gap heterostructures formed by semiconductors whose energy spectrum is described by the Dirac equation. The case where the carrier mass exhibits axial anisotropy is analyzed. A helicity operator whose eigenvalue is conserved over the entire heterostructure is generalized for anisotropic heterostructures. Fiz. Tverd. Tela (St. Petersburg) 40, 1345–1346 (July 1998)     

Differential magnetoreflection spectroscopy of doped and undoped II–VI semiconductor quantum wells

A method has been developed for recording and analyzing the differential magnetoreflection (magnetotransmission) spectra of semiconductor structures with quantum wells. The method was used to determine the exciton g-factor in semimagnetic CdTe/(Cd, Mn)Te heterostructures with quantum wells. In nonmagnetic structures with quantum wells containing a two-dimensional electron gas, the excitonic damping depends on the spin state of the exciton. This effect is explained by the exchange contribution to exciton-electron scattering. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 44–49 (10 January 1997)     

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)     

Large range of omni-directional reflection in 1D photonic crystal heterostructures

A simple design of one-dimensional omni-directional reflector based on photonic crystal heterostructures structure has been proposed. The proposed structure consists of a periodic array of alternate layers of SiO₂ and Te as the materials of low and high refractive indices, respectively. The structure considered here has three stacks of periodic structures having five layers each. The lattice period of successive stack is increased by a certain multiple (say gradual constant, δ) of the lattice period of the just preceding stack. For numerical computation, the method of transfer matrix method (TMM) has been employed. It is found that such a structure has wider reflection bands in comparison to a conventional dielectric PC structure and the width of the omni-directional reflection (ODR) bands can be enlarged by increasing the value of the gradual constant δ.     

ZnO–ZnS heterostructures with enhanced optical and photocatalytic properties

ZnO–ZnS heterostructures were fabricated via using ZnO rods as template in different Na₂S aqueous solutions. These heterostructures are 5–6 μm in length and formed by coating ZnO rod with a layer of porous ZnS shell comprising primary crystals about 10 nm in diameter. Subsequently, intact ZnS polycrystalline tubes were obtained by removing the ZnO cores with 25% (wt) ammonia. The as-prepared products were characterized by scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX), Fourier transform infrared (FT-IR), and electrochemical impedance spectroscopy (EIS). It was found that the electron transfer between ZnS shell and ZnO core strongly affect the photoluminescence and photocatalytic performances of these heterostructures. The rapid transfer of photo-induced electrons from the ZnS shell to the ZnO core leads to enhanced ultraviolet emission. However, if this correlation was destroyed, then the corresponding heterostructure exhibits improved photocatalytic efficiency due to the reduced volume recombination of the charge carries and the multiple reflection effect. Finally, a model based on band-gap alignment was proposed to elucidate the underlying mechanism of the enhanced UV emission and photocatalytic activity of these unique heterostructures.     

Nonlinearities in the reflection and transmission spectra of the photonic bandgap heterostructures with n–i–p–i crystals

Nonlinear optical properties of photonic crystal heterostructures with embedded n–i–p–i superlattices are investigated. Self-consistent calculations of the transmission and reflection spectra near the defect mode are performed using the transfer-matrix method and taking into account the gain saturation. Analysis of features and output characteristics is carried out for one-dimensional photonic crystal heterostructure amplifiers in the GaAs–GaInP system having at the central part an active “defect” from doubled GaAs n–i–p–i crystal layers. The gain saturation in the active layers in the vicinity of the defect changes the index contrast of the photonic structure and makes worse the emission at the defect mode. Spectral bistability effect, which can be exhibited in photonic crystal heterostructure amplifiers, is predicted and the hysteresis loop and other attending phenomena are described. The bistability behavior and modulation response efficiency demonstrate the potential possibilities of the photonic crystal heterostructures with n–i–p–i layers as high-speed optical amplifiers and switches.      

Diagnostics of heterostructures of resonant-tunneling diodes during epitaxial growth. II. Monitoring techniques based on reflection method

It is shown that the reflection methods, in particular, the reflection anisotropy method, can be efficiently used for in situ studying and monitoring the growth of heterostructures with layers thinner than 10 monolayers. A change in the layer composition at direct GaAs/AlAs heterointerfaces of the active region of the resonant-tunneling diode is recorded by the reflection anisotropy method with a thickness resolution of ～1 monolayer immediately during the growth. To estimate the quality of the formed active region of the resonant-tunneling diode, comparative reflection anisotropy spectroscopy is used.     

Electron g factor in quantum wires and quantum dots

A theory of the Zeeman effect for electrons in one-and zero-dimensional semiconductor heterostructures is developed. A relation is established between the number of linearly independent components of the g-factor tensor and the point symmetry of a low-dimensional system. A specific calculation is performed for a spherical quantum dot and a cylindrical wire. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 41–45 (10 January 1998)     

Formation of multilayer strained-layer heterostructures by liquid epitaxy. I. Theoretical aspects of the problem and mathematical model

Problems concerned with the formation of multilayer strained-layer heterostructures by “capillary” liquid-phase epitaxy with forced hydraulic replacement of the solutions in the growth channel are analyzed. It is shown for short contact times between the solutions and the crystallization surface that the character of their flow in the channel plays an important role in the achievement of uniformity in the physical characteristics of the layers grown. Theoretical estimates of the hydrodynamic stability of solutions moving in narrow channels are performed for several III-V systems. A mathematical model, which permits simulation of the conditions under which strained-layer heterostructures are fabricated, is developed. It takes into account diffusive and convective mass transport in the liquid for various flow regimes in the capillary and the displacement of the heterogeneous equilibria in the system under the influence of elastic stresses. Zh. Tekh. Fiz. 67, 42–49 (July 1997)     

Resonance optical spectroscopy of long-period quantum-well structures

Theory of specular light reflection from long-period quantum-well structures taking into account the exciton contribution to dielectric polarization has been developed for an arbitrary relation between the background refractive index in the well, n _a, and barrier-material refractive index nb. General expressions for the optical reflection and transmission coefficients for a structure with N equidistant quantum wells are derived with the use of the Green’s function and transfer matrix methods. Normal and oblique light reflectance spectra from II-VI-based heterostructures were found to reveal a bright interference pattern caused by the difference between n _a and n _b. A comparison of the theory with experiment has yielded the dispersion of n _a and n _b within a broad wavelength range and the parameters of the quasi-two-dimensional heavyhole exciton (e1-hh1), namely, the resonant frequency and the radiative and nonradiative damping rates. Reflectance spectra from resonant Bragg and quasi-Bragg structures with real exciton parameters are calculated, and the effect on these spectra of the refractive-index difference and the deviation from the Bragg condition is analyzed. Fiz. Tverd. Tela (St. Petersburg) 39, 2072–2078 (November 1997)     

Magneto-optical phenomena in MnAs/CaF2/Si(111) epitaxial films in a transverse magnetic field

Field and angular dependences of the rotation of the plane of polarization in a transverse magnetic field H⊥k under normal reflection of light (λ=633 nm) have been studied in MnAs ferromagnetic epitaxial films grown by MBE on CaF₂/Si(111) substrates. The angle of rotation of the plane of polarization a is shown to be determined by contributions even and odd in the magnetization M. The odd contribution is associated with the deviation of the easy plane of magnetic anisotropy from the film plane, which originates from misorientation of the Si surface from the (111) plane, or from a presence of small regions of ( )-oriented MnAs. The even contribution is due to the optical anisotropy of films connected with quadratic-in-M terms in the dielectric permittivity tensor ɛ _ij of manganese arsenide. A method based on measuring the angular dependences of a in a rotating magnetic field is proposed to separate these contributions. Fiz. Tverd. Tela (St. Petersburg) 41, 110–115 (January 1999)     

Mechanism of structural changes of Si(111) surfaces subjected to low-energy ion pulses during molecular-beam epitaxy

Reflected high-energy electron diffraction (RHEED) and detection of the intensity oscillations of the specular reflection have been used to investigate morphological changes in Si(111) associated with the two-dimensional layer-by-layer mechanism of silicon growth from a molecular beam under conditions of pulsed (0.25–1 s) bombardment with low-energy (80–150 eV) Kr ions in the interval of small total radiative fluxes (10¹¹–10¹² cm²²), for which the density of radiation-generated defects is small in comparison with the surface density of the atoms. After pulsed ion bombardment an increase in the intensity of the specular reflection is observed if the degree of filling of the monolayer satisfies 0.5<θ<1. No increase in the intensity occurs during the initial stages of filling of the monolayer. The maximum amplitude increment of the oscillations is reached at θ≈0.8. The magnitude of the amplitude increment of the RHEED oscillations increases with temperature up to 400°C and then falls. At temperatures above 500°C amplification of the reflection intensity essentially vanishes. Experiments on multiple ion bombardment of each growing layer showed that the magnitude of the amplitude increment of the oscillations decreased as a function of the number of deposited layers (the order of the RHEED oscillation). A mechanism for the observed phenomena is proposed, based on the concept of surface reconstruction by pulsed ion bombardment accompanied by formation of a (7×7) superstructure, which corresponds to a decrease of the activation energy of surface diffusion of the adatoms. Within the framework of the proposed mechanism the results of Monte Carlo modeling agree with the main experimental data. Zh. éksp. Teor. Fiz. 114, 2055–2064 (December 1998)     

Microwave waveguide method for the measurement of electron mobility and conductivity in GaAs/AlGaAs heterostructures

The microwave waveguide method for contactless determination of the electron mobility and conductivity of thin active layers is reported. The method is based on relative measurements of the magnetic field dependences of the derivative of the reflection coefficient with respect to the magnetic field from a semiconductor wafer bridging the waveguide.Experiments are performed on GaAs/AlGaAs heterostructures at microwave frequency = 36.4 GHz and liquid nitrogen temperature. For the analysis of the experimental data the theoretical basis for arbitrary frequencies is developed. The main advantage of the proposed method is that this method enables one to determine material parameters - mobility and conductivity - without careful calibration of the microwave system and does not require the accurate measurements of the absolute values of the reflection coefficient and phase of the reflected wave.     

Schwinger’s effective lagrangian from reflection and transmission amplitudes

The reflection and transmission amplitudes are defined from the asymptotic form of the solution of Dirac equation of a charged fermion in the presence of uniform time independent external electromagnetic field (E, H). Schwinger’s effective lagrangian is derived from the reflection and transmission amplitudes. It is found that both the real and imaginary parts of the effective lagrangian agree with Schwinger’s expressions derived from the elegant method of proper time formalism.     

Influence of defects on the temperature dependence of the density of electrons in the two-dimensional and doped channels of selectively doped AlxGa1−x As/GaAs heterostructures

The densities of electrons in the two-dimensional and doped channels of selectively doped AlGaAs/GaAs heterostructures are calculated. It is shown that traps and surface states in the AlGaAs layer can change the sign of the temperature dependence of the electron density in the two-dimensional channel. Zh. Tekh. Fiz. 68, 140–142 (October 1998)     

Direct measurement of the spatial distribution of light intensity in a scattering medium

A direct nonperturbative measurement of the spatial distribution of the light intensity in a strongly scattering medium is performed using an optoacoustic method. It is shown that near a surface the intensity can be five times greater than the incident intensity, and the absolute maximum of the intensity is observed at a depth ℓ(1–R)(1–4.0R) determined by the photon transport mean free path ℓ and the effective light reflection coefficient R of the boundary separating the scattering and external media. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 187–191 (10 August 1999)     

Excitons in ZnP2 crystals in the electric field of a Schottky barrier

A study is made of the effect of electric fields on the exciton states of β-ZnP₂ crystals (T=77 K) in structures with Schottky barriers formed by depositing semitransparent electrically-conducting InSnO₂ films on the crystal surface. The observed changes in the exciton optical reflection spectra when an electrical potential is applied to a barrier are explained by the shift and broadening of the exciton level caused by the Stark effect. The experimental data are compared with calculations based on a theory of exciton optical reflection from planar spatially nonuniform structures. Fiz. Tverd. Tela (St. Petersburg) 40, 884–886 (May 1998)     

Selective reflection of resonance radiation from excited media

According to quantum electrodynamics, the cross section for resonant scattering of radiation on an aggregate of excited atoms can be written as a sum of positive definite terms. This type of structure is not consistent with the Fresnel formulas for the reflection coefficient of radiation from thermally excited media. The difference shows up on a macroscopic level and indicates that semiclassical radiation theory cannot be used. A study of the correlation between elastic scattering and stimulated emission processes clarifies the reason for the discrepancies. The resulting singularities require summing of Feynman diagrams which appear beginning in the sixth order of perturbation theory. A lower bound estimate for the reflection coefficient from a plane layer is given, including processes which violate the statistics of radiation. The contribution of stimulated emission processes caused by the initially scattered photon are examined specifically. An experiment is proposed which would settle the choice of theories. Zh. éksp. Teor. Fiz. 113, 521–538 (February 1998)