FTIR-spectroscopy of (GaAS)n/(AlAs)m superlattices

The optical properties of (GaAs)_n/(AlAs)_m superlattices in the infra-red spectral region have been studied. The confinement of optical phonons has been observed in both GaAs and AlAs layers of superlattices under investigation. The superlattice modes caused by the coupling between LO phonons and collective intersubband excitations have been found in doped superlattices. Macroscopic and microscopic calculations have been used for the analysis of experimental results. Good agreement with experiment has been obtained.     

FP-LMTO study of structural,electronic and optical properties of wurtzite (CdS)n/(CdSe)n superlattices

In this paper, we have conducted a first-principles study of the structural, electronic and optical properties of (CdS)_n/(CdSe)_n superlattices (where n is numbers of monolayers) in the wurtzite phase (B4), using the Full-Potential Linear Muffin-Tin Orbital (FP-LMTO) method within the Local Density Approximation (LDA) technique, in order to describe the exchange correlation energy. The calculated electronic properties indicate that all (CdS)_n/(CdSe)_n superlattices configurations, possess a semiconductor behavior with same energy gaps. We have seen more carefully and accurately that the different superlattices configurations have no effect on the electronic properties; in particular, we did not observe any dependence between the band gap behavior and the used layers.     

Properties of BaTiO3/BaZrO3 ferroelectric superlattices with competing instabilities

Properties of (BaTiO₃)₁/(BaZrO₃) _n ferroelectric superlattices (SLs) with n = 1?7 grown in the [001] direction are calculated from first principles within the density functional theory. It is revealed that the quasi-two-dimensional ferroelectricity occurs in these SLs in the barium titanate layers with a thickness of one unit cell; the polarization is oriented in the layer plane and weakly interacts with the polarization in neighboring layers. The ferroelectric ordering energy and the height of the barrier separating different orientational states of polarization in these SLs are sufficiently large to provide the formation of an array of independent polarized planes at 300 K. The effect of the structural instability on the properties of SLs is considered. It is shown that the ground state is a result of simultaneous condensation of the Γ₁₅ polar phonon and phonons at the M point (for SLs with even period) or at the A point (for SLs with odd period); it is a polar structure with out-of-phase rotations of the octahedra in neighboring layers, in which highly polarized layers are spatially separated from the layers with strong rotations. The competition between the ferroelectric and structural instabilities in biaxially compressed SLs manifests itself in that the switching on of the octahedra rotations leads to an abrupt change of the polarization direction and can cause an improper ferroelectric phase transition to occur. It was shown that the experimentally observed z-component of polarization in the SLs can appear only as a result of the mechanical stress relaxation.     

Phonon-plasmon interaction in tunneling GaAs/AlAs superlattices

The phonon-plasmon interaction in tunneling GaAs _n /AlAs _m superlattices (m=5and 6≥n≥0.6 monolayers) was studied by Raman scattering spectroscopy. The interaction of optical phonons localized in GaAs and AlAs layers with quasi-three-dimensional plasmons strengthens as the thickness of GaAs quantum wells decreases and the electronic states in the superlattices become delocalized due to tunneling. It is assumed that the plasmons also interact with the TO-like phonon modes localized in quantum islands or in thin ruffled layers.     

形变超晶格Si/Ge的能带结构

用经验的紧束缚方法对短周期的(Si)_n/(Ge)_m形变超晶格的电子态进行了计算。结果表明,由于布里渊区折迭的要求,只有当n+m=10时超晶格才可能产生直接能隙。对周期为n+m=10的超晶格,Γ,N,△处的导带谷间的相对位置对直接能隙的形成具有决定作用,而n的大小与衬底的组分对此有极大影响。(Si)₆/(Ge)₄和(Si)₈/(Ge)₂超晶格在Si_1-xG 关键词：     

Optical and electronic properties of (GaAs)n / (InAs)n(001) superlattices: LMTO-ASA approach

The optical and electronic properties of (GaAs)_n/(InAs)_n superlattices are calculated by means of LMTO-ASA method. The too small band gap problem of bulk material and superlattices is corrected by adding to the effective potentials an additional external potential that is sharply peaked at the atomic sites. The results show that the optical properties of GaAs/InAs(001) superlattices are about average of that of two bulks of GaAs and InAs.     

Investigation of the structure and luminescence properties of CdF2-CaF2 : Eu superlattices on Si(111)

A complex investigation of CdF₂-CaF₂ : Eu superlattices with different bilayer thicknesses (2.0–17.5 nm) grown by molecular beam epitaxy on Si(111) has been carried out. The structural perfection of the layers and interfaces of the superlattices have been estimated from the X-ray diffractometry and reflectometry data. The possibility of producing short-period pseudomorphic superlattices with a period of approximately 2 nm has been established. It has been shown that these superlattices are characterized by a larger root-mean-square roughness amplitude of the interfaces as compared to the long-period superlattices. The specific features of cathodoluminescence spectra have been analyzed as a function of the superlattice period. It has been revealed that, with a decrease in the superlattice period, the intrinsic luminescence intensity of fluorides increases in comparison with the intensity of the luminescence associated with the emission of Eu²⁺ impurity ions; in this case, several Eu³⁺ luminescence bands appear in the spectrum. The possibilities of electron probe microanalysis for determining the ratio of thicknesses of individual layers in short-period superlattices have been demonstrated.     

Determination of the roughness of heteroboundaries from photocurrent spectra of short-period AlAs/GaAs superlattices

Photocurrent spectroscopy is used to study the nature of the roughness of heteroboundaries in (AlAs)_m/(GaAs)_n short-period superlattices (m=3−5, n=10−13) grown by molecularbeam epitaxy. The formation of minibands broadens the optical spectra of superlattices in comparison with isolated quantum wells; therefore to analyze the degree of perfection of the boundaries we used the decay of the minibands into a series of discrete Wannier-Stark levels in an electric field parallel to the superlattice axis. Exciton lines were observed in the photocurrent spectra in an electric field corresponding to direct and indirect (in space) transitions between the Wannier-Stark levels. Comparison of experimental data with calculation indicates that even in the better structures, in addition to monotonic variation of the thickness of the layers over area, roughnesses in the heteroboundaries one monolayer in height are present with characteristic lateral dimension not exceeding 10 nm. Fiz. Tverd. Tela (St. Petersburg) 39, 2085–2089 (November 1997)     

(Si)n/(Ge)n超晶格几何结构

本文用Keating模型计算了Si_1-xGe_x(x=0—1)作衬底、沿(100)方向生长的(Si)_n/(Ge)_n(n=1—6)应力超晶格的几何结构,并讨论了衬底对超晶格生长的影响,计算结果发现对于(Si)_n/(Ge)_n超晶格,用适当的Si_1-xGe_x作衬底有利于超晶格的生长。 关键词：     

Raman scattering from GaAsAlAs monolayer-controlled superlattices

Measurements of Raman scattering were performed on (GaAs)_n?(AlAs)_n monolayer-controlled superlattices with n = 1, 2, 3 and 4 grown by molecular beam epitaxy. The zone-folding effect for longitudinal acoustic phonons and frequency shift of longitudinal optic phonons were observed. The experimental results agree well with the calculated ones according to the elastic and linear chain models.     

Intervalley electron scattering by phonons in (GaAs)<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript>(AlAs)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>(001) ultrathin superlattices

The electron scattering by short-wavelength and long-wavelength phonons in (GaAs) _m (AlAs) _n (001) superlattices with ultrathin layers (n, m = 1, 2, 3) has been investigated using the pseudopotential method and the phenomenological model of bonding forces. The deformation potentials have been found for intervalley electron transitions in the conduction bands of the superlattices and solid solutions of the corresponding compositions. It has been shown that, owing to the localization of the wave functions in the quantum wells Γ, L, and X, the intensity of intervalley electron transitions in the superlattices, as a whole, is higher than that of similar transitions in the solid solutions. As the content of light Al atoms in the superlattices increases, the deformation potentials monotonically increase for the X-X transitions and decrease for the L-L and X-L transitions. The potentials of the Γ-X and Γ-L transitions change nonmonotonically depending on the layer thickness due to the pronounced quantum-well effects in the deep Γ quantum wells of GaAs. The deformation potentials averaged over phonons and related valleys in the superlattices are close to the corresponding potentials in the solid solutions.     

Electronic structure of (BP)n/(BAs)n (0 0 1) superlattices

An accurate ab initio full potential linear muffin-tin orbital method has been used to investigate the structural, electronic and optical properties of BP, BAs and their (BP)_n/(BAs)_n superlattices (SLs). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The calculated structural properties of BP and BAs compounds are in good agreement with available experimental and theoretical data. It is found that BP, BAs and their alloys exhibit an indirect fundamental band gap. The fundamental band gap decreases with increasing the number of monolayer n. The optical properties show that the static dielectric constant significantly decreases in superlattices compared to their binary compounds.     

Spectroscopic studies of real space indirect symmetric GaAs/AlAs short period superlattices

The band structure and the optical transitions of symmetric ultra-short period (GaAs) _m /(AlAs) _n superlattices (n=m is the number of monolayers) have been studied using different spectroscopic techniques, namely photoacoustic spectroscopy, reflectivity and luminescence, photoluminescence excitation and high excitation intensity time-resolved luminescence. Direct observation of the transition from type I to type II energy band alignment is reported for superlattices whose configuration consists of more than 12 monolayers (i.e. m=n>12). The radiative recombination processes associated with the real space indirect transitions have been investigated as a function of the density of photogenerated electron-hole pairs, revealing an unusual density-dependent behaviour. A tentative interpretation in terms of a condensed electron-hole state at the indirect gap is given, which accounts for the long decay time of the type II luminescence at high excitation rate.     

Raman scattering studies of (GaP)n/(InP)n (n = 1, 1.7, 2) short‐period superlattice structures

We report Raman scattering from (GaP)_n/(InP)_n (n = 1, 1.7, 2) short‐period superlattice (SPS) structures to study the effect of lateral composition modulation (LCM) on the behavior of optical phonons. Cross‐sectional transmission electron microscope images revealed that LCM was formed with complex pattern in the n = 1.7 and n = 2 samples grown at 490 °C. Interestingly, both the InP‐ and the GaP‐like longitudinal optical (LO) phonon energies increased systematically as the number of monolayers was increased from n = 1 to n = 2. A significant broadening of the phonon line shapes was also observed for the n = 1.7 and n = 2 samples. In contrast, for samples grown at 425 °C, both the increase of the LO phonon energies and the broadening of the phonon line shapes were observed only when n = 1.7. Our results demonstrate that the optical phonons in the (GaP)_n/(InP)_n SPS structures are significantly affected in the occurrence of LCM related to the growth temperature and the number of monolayers.Copyright © 2009 John Wiley & Sons, Ltd.     

Quasi-two-dimensional ferroelectricity in KNbO<Subscript>3</Subscript>/KTaO<Subscript>3</Subscript> superlattices

First-principles density functional theory is used to calculate the phonon spectrum in the paraelectric phase, the ground-state structure and polarization distribution in the polar phase, and energies of ferro- and antiferroelectrically ordered phases of free-standing (KNbO₃)₁(KTaO₃) _n ferroelectric superlattices with n = 1–7. It is established that quasi-two-dimensional ferroelectricity with polarization oriented in the layer plane, which weakly interacts with polarization in neighboring layers, appears in potassium niobate layers with a thickness of one unit cell in the superlattices. The possibility of using of such ferroelectric superlattices as a medium for three-dimensional information recording is shown.     

Coherent crystallization of bismuth under strong excitation by ultrashort laser pulses

By means of coherent control, fully symmetric bismuth phonons A _1og are investigated at helium temperature depending on the degree of crystal excitation. It is shown that coherent phonons with large amplitude show phase rigidity that is absent at small amplitude. Such a change in the phase properties of crystal lattice excitations can be understood using the concept of a coherent crystal, the achievement of which results from condensation of phonon modes arising with increasing degree of excitation.     

X-ray diffraction investigation of BaTiO3/(Ba,Sr)TiO3 superlattices

Samples of BaTiO₃/(Ba_{1 ? x} Sr _x )TiO₃ (BT/BST-x) superlattices have been studied using X-ray diffraction analysis. A complete parallel orientation of all the studied films and the substrate has been revealed, and the modulation periods ?? of the superlattices, the parameters of the unit cell averaged over the ?? period, and the parameters of individual components of the superlattices have been determined.     

X-Ray diffraction and Raman spectroscopy studies of superlattices BaTiO3/(Ba0.5,Sr0.5)TiO3/SrTiO3

The structural characteristics of the BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃/SrTiO₃ superlattice on a (001) MgO substrate have been studied using X-ray diffraction. The modulation period and unit cell parameters of layers forming the superlattice have been measured. The sizes of coherent scattering regions and average microstrains in the direction perpendicular to the surface have been estimated. The obtained characteristics are compared to those of the two-layer BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃ superlattice. The Raman spectra demonstrate a substantial shift of the soft E(TO) mode in the three-layer superlattice as compared to the position in the two-layer superlattice. The effects observed are associated with a substantial increase in the temperature of the phase transition of the three-layer superlattice to the paraelectric phase.     

Pump pulse duration dependence of coherent phonon amplitudes in antimony

Coherent optical phonons of A_1k and E_k symmetry in antimony have been studied using the femtosecond pump–probe technique. By varying the pump-pulse duration and keeping the probe duration constant, it was shown that the amplitude of coherent phonons of both symmetries exponentially decreases with increasing pulse width. It was found that the amplitude decay rate for the fully symmetric phonons with larger frequency is greater than that of the doubly degenerate phonons, whereas the frequency and lifetime for coherent phonons of both symmetries do not depend on the pump-pulse duration. Based on this data, the possibility of separation between dynamic and kinematic contributions to the generation mechanism of coherent phonons is discussed.