Function Of Superlattice Period Research Articles

Effect of external perturbations on phonon-polariton modes and photonic band gap in piezoelectric crystal

The behavior of polariton dispersion as a function of superlattice period and temperature in a novel piezoelectric superlattice (PSL) by using Lithium Niobate is analyzed theoretically. By implementing the temperature coefficients of each acoustic and optical parameters, the dielectric permittivity is determined. The modes are shifted and the polaritonic gap decreases with temperature. The transmission properties of a one-dimensional novel photonic crystal having novel piezoelectric superlattice as one of the layers and air as another layer have been investigated by means of transfer matrix method. The photonic band gaps can be tuned by altering the thicknesses of the layers, incidence angle, the number of periods and temperature. When the thickness of the PSL layer changes from μm to mm, the photonic band gap is found to be shifted from THz to GHz region. The evolution of these results provides a guideline for designing optoacoustic devices, filters, and sensors.

Molecular dynamics simulation of thermal conductivity of GaN/AlN quantum dot superlattices

AbstractWe calculated thermal conductivity of GaN/AlN quantum dot superlattices by molecular dynamics simulation. The results of investigation of the effect of quantum dots on thermal conductivity as a function of superlattice period are presented in this paper. An empirical potential function of Stillinger‐Weber potential was used for simulations. Thermal conductivity was obtained by Green‐Kubo's equation. The results show that the values of thermal conductivity parallel to the wetting layers decreased due to the effect of quantum dots. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phonon–polariton in a piezoelectric superlattice

In a series of works Zhu et al. [Phy. Rev.Lett. 90 (2003) 0532903] have investigated the possibility of acoustic polaritons in a novel superlattice system consisting of piezoelectric materials. In the present work that two excitations are pointed out are: one due to piezo polaritons and the other due to the optic phonon mode. The behavior of the photonic gaps as a function of superlattice period is investigated systematically.

Maximum of Dielectric Permittivity Caused by Structural Transition in Ferroelectric BaTiO3-SrTiO3 Superlattices

We invoke the onset of dislocations along the BaTiO3-SrTiO3 interface as reported by Wunderlich et al. to explain the non-monotonic behaviour of the dielectric permittivity as a function of superlattice periodicity and the less than four-fold in-plane symmetry at the dielectric maximum. At a periodicity of about 10/10, depending on composition and growth mechanism, several groups report a maximum of dielectric permittivity. In addition to that we observe in-plane symmetry less than tetragonal for 10/10 superlattices by HR-XRD, in contrast to initial low-resolution data from Tabata et al. thus challenging the assumption of unrelaxed strain all the way through the superlattice. The aim of this article is to link both effects to the increasing volume fraction of conducting layers close to the interface in series with the superlattice layers.

Dielectric enhancement and Maxwell–Wagner effects in ferroelectric superlattice structures

In an attempt to reproduce the functional properties associated with relaxor electroceramics, pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced at stacking periodicities of a few unit cells, consistent with relaxor behavior. However, enhancement in dielectric constant was generally associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency shows that fine-scale superlattices conform to Maxwell–Wagner behavior. This suggests that the observed enhancement of the real part of the dielectric constant is an artifact produced by carrier migration to interfaces within the dielectric. A comparison of this data with that already published on dielectric superlattices suggests that previous claims of an enhancement in dielectric constant may also be attributed to the Maxwell–Wagner effect.

Experimental and Theoretical Investigation into the Dielectric Behaviour of Ferroelectric Thin Film Superlattices

ABSTRACTPulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced, and temperature migration of the peak in dielectric constant as a function of frequency was observed, at stacking periodicities of a few unit cells. However, such “relaxor-like’ features were found to be associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency showed that fine-scale superlattices conform to Maxwell-Wagner behaviour, indicating that the observed features may be an artefact of increased carrier mobility. Modelling showed that both dielectric enhancement and frequency relaxation could readily be reproduced by Maxwell-Wagner formalism.

Experimental and Theoretical Investigation into the Dielectric Behaviour of Ferroelectric Thin Film Superlattices

AbstractPulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced, and temperature migration of the peak in dielectric constant as a function of frequency was observed, at stacking periodicities of a few unit cells. However, such ‘relaxor-like’ features were found to be associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency showed that fine-scale superlattices conform to Maxwell-Wagner behaviour, indicating that the observed features may be an artefact of increased carrier mobility. Modelling showed that both dielectric enhancement and frequency relaxation could readily be reproduced by Maxwell-Wagner formalism.

Investigation into the dielectric behavior of ferroelectric superlattices formed by pulsed laser deposition

In an attempt to reproduce the functional properties associated with relaxor electroceramics, pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was enhanced at stacking periodicities of a few unit cells, consistent with relaxor behavior. However, enhancement of the dielectric constant was found to be associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency shows that fine-scale superlattices conform to Maxwell–Wagner behavior. This suggests that the observed enhancement of the real part of the dielectric constant is an artefact produced by carrier migration. A comparison of this data with that already published on dielectric superlattices suggests that previous claims of an enhancement in dielectric constant may also be due to the Maxwell–Wagner effect. The onset of Maxwell–Wagner behavior was attributed to increasing density of defect zones associated with discontinuities in the superlattice structures. In an attempt to exaggerate the influence of such zones, deliberate delays between deposition of successive dielectric layers were introduced. This resulted in reproduction of several features normally associated with relaxors: enhancement of dielectric constants by over an order of magnitude; strong frequency dispersion around and below Tm; migration of Tm with frequency. However, these features were again associated with relatively high loss.

Oscillatory behavior in the electrical resistivity of transition-metal superlattices

The electrical resistivity of Co/Ni and Cu/Ni superlattices exhibit resistivity oscillations as a function of superlattice period {Lambda}, while Ag/Pd does not. Moreover, the resistivity of Co/Ni superlattices shows quantum size effects and a resistivity enhancement with {Lambda}, as a function of total film thickness d. These results are consistent with a model which assumes d-electron localization in the superlattice structure. {copyright} {ital 1998} {ital The American Physical Society}

Interface and layer thickness dependence of the effective mass in [formula omitted] superlattices studied by high field cyclotron resonance

The effective mass has been studied in a series of semiconducting and semimetallic InAs GaSb superlattices as a function of superlattice period and band gap. The mass is found to be determined almost entirely by the superlattice period, and is relatively insensitive to both the ratio of the InAs to GaSb thickness, and the structure of the interface layer, which was grown as both a monolayer of InSb and GaAs. Using pulsed magnetic fields of up to 180 T it was possible to observe spin-split cyclotron resonance at room temperature for all samples studied. On cooling the spin-splitting was found to be temperature dependent. This is attributed to the importance of electron-electron interactions which couple the two spin transitions as observed recently in high purity GaAs GaAlAs heterojunctions at low temperatures.

Thermoelectric figure of merit of quantum wire superlattices

The electrical conductivity, the thermoelectric power, and the electrical contribution to the thermal conductivity of quantum wire superlattices have been studied. The effects of tunneling through the barriers due to finite potential off-sets and of the thermal currents through the barrier layers are shown to be essential to describe properly the thermoelectric figure of merit of realistic quantum wire superlattices. The figure of merit exhibits a maximum as a function of superlattice period which, for large barrier off-sets, is found to be substantially enhanced over that for the bulk material and also larger than that for quantum well superlattices.

Effect of superlattice structure on the thermoelectric figure of merit.

The electrical conductivity, the thermoelectric power, and the electrical contribution to the thermal conductivity of superlattices have been studied within the envelope-function approach for the electrons. The effects of tunneling through the barriers due to finite potential offsets and of the thermal currents through the barrier layers are shown to be essential to describe properly the thermoelectric figure of merit of realistic superlattices. The figure of merit is calculated as a function of superlattice period, potential barrier offset, and barrier width. It is found that the figure of merit has a maximum as a function of superlattice period and that its value there can be somewhat larger than that of the corresponding bulk. For larger periods the figure of merit is generally less than that of the correponding bulk.

Magnetotransport in Mo/Ni superlattices

We have measured the electrical resistivity, magnetoresistance (MR), and Hall resistivity of Mo/Ni superlattices with modulation wavelengths Λ between 13.8 Å and 5000 Å in the temperature range 1–300 K and in magnetic fields up to 50 kG. The samples with Λ less than 680 Å exhibit a resistance minimum below 15 K and a logarithmic temperature dependence below this minimum. The perpendicular MR ⊥ is negative and the parallel MR ‖ is positive for large layer thickness. The Hall voltage saturates at high fields and the saturation value exhibits a maximum as a function of superlattice period.

Exciton binding energies and the valence-band offset in mixed type-I-type-II strained-layer superlattices.

Strained CdTe-${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te superlattices are of mixed type: electrons and heavy holes are confined to the CdTe layers (type I) while light holes are confined to the ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te layers (type II). In this paper we calculate the exciton binding energy (EBE) as a function of superlattice period for both type-I (spatially direct) and type-II (spatially indirect) excitons. For the heavy-hole (type-I) exciton the binding energy is larger than the bulk value, and varies only slowly with the period down to small periods, where the exciton acquires a three-dimensional character and our calculation breaks down. For the light-hole (type-II) exciton the binding energy at large period is much smaller, due to the spatial separation of electron and hole. As the period decreases, the binding energy increases steadily to reach its bulk value for vanishingly small period. Given the EBE's, we can fit the already published data on the exciton transition energies with a single adjustable parameter, the ``average valence-band offset'' (averaged over the heavy and light holes). This is the algebraic sum of the chemical-offset and the hydrostatic-strain contribution, and is found to be (2\ifmmode\pm\else\textpm\fi{}4)% of the difference in band gap between the barrier and well. This value lies in the range predicted theoretically.

Enhancement of Photocarrier Lifetime Due to Charge Separation in a-Si:H/a-Ge:H Superlattices

ABSTRACTThe lifetime of photogenerated electrons in a-Ge:H/a-Si:H multilayer structures with layers about 100Å thick, is enhanced by two orders of magnitude above the value in bulk a-Ge:H. This lifetime enhancement effect is explained by charge separation produced by layers, due to the assymmetry in the conduction and valence band offsets at the interfaces. The peak in the electron lifetime as a function of superlattice periodicity is determined by a trade-off between the electron tunneling rate into the a-Si:H barriers and electrostatic repulsion of holes due to photo-induced space charge.