Papers by Keyword: Superlattice

Abstract: In multi-layer structures impurity scattering is effectively reduced by the modulation doping in order to achieve high charge carrier mobility and, as a consequence, better device performance. In this paper, the thermoelectric properties of superlattices when electrons are scattered by strongly screened ionized impurities are discussed. In low-temperature and strong screening circumstances, dependence of the thermopower, power factor, and figure of merit on the superlattice period, miniband width, and screening radius is found. For the specified superlattice parameters and ionized impurity concentration, the figure of merit reaches the value of 2.6. The thermopower of the superlattices five times exceeds that of bulk samples.

Abstract: We report our experiments based on the interfaces of a 5-period superlattice, containing GaAsP(3Å)/GaAs (190Å) heterostructures grown by molecular beam epitaxy (MBE). The atomic arrangement at the interfaces of GaAsP/GaAs is investigated using high resolution transmission electron microscopy (HRTEM). Our results indicate that the superlattice was grown coherently with strained layers. We propose that the atomic arrangement at the interface is GaP, assuming that phosphorus incorporation occurs primarily via substitution due to desorption of arsenic at the surface for substrate temperatures above 500°C. The incorporation of phosphorus has been investigated using fast Fourier transform (FFT) patterns and shows a form of strain distribution near the heterointerface. The FFT patterns of the superlattice reveal that strain distributes mostly near the interface and gradually decreases along the direction of growth. Phosphorus diffused into a GaAs layer changes the lattice constant in the growth direction, which reduces strain in the superlattice.

Abstract: Superlattice formation in thin layers of oxidizing crystals and the effect of near-surface proton saturation on structure ordering, formation and periodical distribution of quantum wells have been discussed. The paper shows, it is necessary to develop non-Euclidean approach to the crystal’s internal geometry and consider, in consecutive order, the question of the four-dimentional Riemannian space into three-dimentional Eucliden space interpretation (R_E interpretation).

Abstract: The results on protonation in solutions and melts of salts and acids, as well as structural changes associated with the formation of nanocomposition structure of materials are presented. It is shown by structural methods that proton localization is invariant to the volume in the protonated layer and is accompanied by changes between oxygen distances, enlargement of the unit cell and transition to the rhombic phase. Having the maximum crystal-chemical activity, protons create a hexagonal lattice in accordance with the features of equipotential pictures of their nonequilibrium electrostatic fields. The increase in the integral intensity of reflexes observed on neutronograms of protonated LiNbO₃ (102), (111), (113) it is associated with the ordering of protons in the hexagonal oxygen sublattice of the initial phase.

Abstract: Tuning the bandgap of superlattice structures creates devices with unique optical, electronic and mechanical properties. Designing solar cells with superlattice structures increases the range of light energy absorbed from the solar spectrum in the device. A superlattice is a nanostructure composed of alternating thin layers of two materials. The thickness of the constituent materials alters the optical bandgap of the superlattice. This paper discusses a mathematical model which computes the effective bandgap of a CdTe/PbTe superlattice based on a given thickness of the CdTe and PbTe films. The output of this model is verified by fabricating superlattices with different thickness and measuring their effective bandgaps. The electrochemical atomic layer deposition method is used to fabricate the superlattice structures. The advantage of this method over other vacuum techniques is that it is inexpensive and operates at room temperature. This paper also discusses a method to mitigate the lattice mismatch between the substrate and the superlattice. The optical bandgaps, crystallinity, grain size and chemical composition of the structures are measured using a spectrometer, diffractometer, transmission electron microscope and scanning electron microscope, respectively. The bandgaps of the fabricated superlattices were in agreement with the simulated values. This model can be used for designing the bandgaps of superlattices which can be incorporated in solar cells.

Abstract: Based on a Kronig-Penney model the electronic properties of heteropolytypic superlattices consisting of lamellas of 3C-and 4H-SiC polytypes with thicknesses below ten nanometers are analysed. Due to the large difference in the electron negativity and the resulting high barriers between the different polytypic lamellas an increased number of minizones compared to other materials are formed. The field strength for the appearance of the negative differential resistance in the heteropolytypic superlattice is lower than the critical fields in the silicon carbide polytypes.

Abstract: Ferroelectric superlattices with polarization perpendicular to the surface or interface are studied within the framework of the Landau-Ginzburg theory. An interface energy is introduced in the free energy to describe the effect of mixing and local polarization coupling at interface. Internal electric field is considered in the model. For superlattices grown on substrate, the influence of substrate on the properties of ferroelectric superlattices is required. This brief review is a sequel to the previous review article [1], which summarizes the recent development in Landau-Ginzburg theory developed for studying ferroelectric superlattices over approximately the last three years.

Abstract: We report on the observation of the THz electroluminescence in 6H-SiC and 8H-SiC n⁺–n^-–n⁺ structures of hexagonal crystals with natural superlattice, caused by applied electrical field along the lattice and natural superlattice axis. It is shown that there are the terahertz electroluminescence correspond to the narrow lines at 5.3–12.7 meV. The emission channel can be well explained by the optical intraladder transitions in the Bloch oscillations regime.

Abstract: Effects of both the phase and the amplitude inhomogeneities of different dimensionalities on the Greens function and on the one-dimensional density of states of spin waves in the sinusoidal superlattice have been studied. Processes of multiple scattering of waves from inhomogeneities have been taken into account in the self-consistent approximation.

Abstract: Structural features of interfaces in [Co/Cu]_n superlattices obtained by magnetron sputtering have been studied by nuclear magnetic resonance (NMR). Modification of interface structural characteristics and magnetoresistive properties of the superlattices with the increase of the number of [Co/C] bilayers is analyzed. Correlation between magnetoresistance and interface structural characteristics has been revealed.