Ideal Superlattice Research Articles

Disorder-Induced Degradation of Vertical Carrier Transport in Strain-Balanced Antimony-Based Superlattices

We investigate carrier transport in gallium-free strained-balanced $\mathrm{In}\mathrm{As}/{\mathrm{In}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}\mathrm{Sb}$ type-II superlattices in the presence of positional and compositional disorder. We use a rigorous nonequilibrium Green's function model based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure. Layer-thickness fluctuations, nonuniform antimony composition, and segregation throughout the superlattice stack lead to as-grown disordered structures that are quite different from the abrupt interface ideal superlattices. We find that regardless of its nature and cause, disorder significantly affects vertical-carrier-transport properties, by impeding the coherent propagation of carriers in the minibands. In particular, the minority-carrier hole mobility is fundamentally limited by the nonideal properties of the superlattice, namely the layer-thickness fluctuation and the nonuniform antimony distribution. Furthermore, upon reducing the temperature, holes become fully localized and transport occurs by hopping, which explains published measured detector data that demonstrates the quantum efficiency, exhibiting a very strong temperature dependence that degrades as the temperature is reduced. As a result, photodetectors that employ holes as minority carriers will be limited in performance, especially for long-wavelength infrared applications at low temperature. However, we find that minority-carrier electron mobility is largely unaffected by disorder, indicating the $p$-type absorbing layer as the preferred option.

Designing Hyperchaos and Intermittency in Semiconductor Superlattices.

Weakly coupled semiconductor superlattices under dc voltage bias are excitable systems with many degrees of freedom that may exhibit spontaneous chaos at room temperature and act as fast physical random number generator devices. Superlattices with identical periods exhibit current self-oscillations dueto the dynamics of charge dipole waves but chaotic oscillations exist on narrow voltage intervals. They disappear easily due to variation in structural growth parameters. Based on numerical simulations, we predict that inserting two identical sufficiently separated wider wells increases superlattice excitability by allowing wave nucleation at the modified wells and more complex dynamics. This system exhibits hyperchaos and varieties of intermittent chaos in extended dc voltage ranges. Unlike in ideal superlattices, our chaotic attractors are robust and resilient against noises and against controlled random disorder due to growth fluctuations.

Strain-induced band modulation of thermal phonons in carbon nanotubes

We show that carbon nanopeapods are an ideal artificial superlattice to induce band modulation of thermal phonons. Based on spectral energy density analysis, we found that a periodic deformation in carbon nanotubes induced by fullerene encapsulation leads to zone folding of phonon dispersions up to near the maximum frequency. The zone-folding effect gives rise to phonon band gaps caused either by Bragg reflection or mode hybridization, and this reduces group velocity. This was quantified to play the leading role in reduction of thermal conductivity by the fullerene encapsulation. The strain modulation of thermal phonon bands opens a possibility to control material thermal conductivity.

Controlling the harmonic conversion efficiency in semiconductor superlattices by interface roughness design

In semiconductor superlattices, when Bragg oscillating electrons interact with an input electromagnetic field, frequency multiplication is possible. An ideal superlattice has a purely antisymmetric voltage current response and can thus produce only odd harmonics. However, real world superlattices can also have even harmonic response and that increases the range of possible output frequencies. These effects have been recently explained with a predictive model that combines an Ansatz solution for the Boltzmann Equation with a Nonequilibrium Green’s Functions approach. This predictive tool, coupled with recent progress on GHz input sources, support the growing interest in developing compact room temperature devices that can operate from the GHz to the THz range. The natural question to ask is what efficiencies can be expected. This paper addresses this issue by investigating power-conversion efficiency in irradiated semiconductor superlattices. Interface imperfections are consistently included in the theory and they strongly influence the power output of both odd and even harmonics. Good agreement is obtained for predicted odd harmonic outputs with experimental data for a wide frequency range. The intrinsic conversion efficiency used is based on the estimated amplitude of the input field inside the sample and thus independent of geometrical factors that characterize different setups. The method opens the possibility of designing even harmonic output power by controlling the interface quality.

Role of O defects at the BiMnO3/SrTiO3 interface

We use first principles calculations to study ideal and O deficient BiMnO3/SrTiO3 superlattices. The ideal superlattice is characterized by parallel alignment of the Mn and Ti magnetic moments at the n-interface, while an antiparallel alignment has been reported experimentally. O defects at the n-interface are found to favor the MnO2 and BiO layers over the TiO2 layer. The band gap of the superlattice is strongly reduced when the MnO2 layer is O deficient and states are observed at the Fermi energy when the BiO layer is O deficient. Only in the latter case the Mn and Ti magnetic moments at the n-interface align antiparallel. Therefore, O defects in the BiO layer turn out to be essential for reproducing the experimental interface magnetism and for understanding its mechanism.

Transmission of Terahertz Acoustic Waves through Graphene-Semiconductor Layered Structures

We present a theoretical study of the acoustic properties of graphene-semiconductor layered structures. The transmission coefficient for longitudinal acoustic waves through the structure is evaluated by using the usual transfer matrix method. We find that the finite thickness of the graphene layer can affect significantly the transmission spectrum of the proposed structure. The features of the sound transmittance depend strongly on the number of the graphene layers. For mul-ti-layer graphene-semiconductor structures, the sound transmission spectrum looks very similar to that for an ideal superlattice. For such structures, terahertz acoustic forbidden gap can be observed even when a thick semiconductor layer is considered. These results are the consequence of the Bragg’s condition for sound waves. This study is relevant to the exploration of the acoustic properties of graphene-based layered structures and to the application of graphene as high-frequency acoustic devices.

High Thermal Conductivity in Short-Period Superlattices

The thermal conductivity of ideal short-period superlattices is computed using harmonic and anharmonic force constants derived from density-functional perturbation theory and by solving the Boltzmann transport equation in the single-mode relaxation time approximation, using silicon-germanium as a paradigmatic case. We show that in the limit of small superlattice period the computed thermal conductivity of the superlattice can exceed that of both the constituent materials. This is found to be due to a dramatic reduction in the scattering of acoustic phonons by optical phonons, leading to very long phonon lifetimes. By variation of the mass mismatch between the constituent materials in the superlattice, it is found that this enhancement in thermal conductivity can be engineered, providing avenues to achieve high thermal conductivities in nanostructured materials.

SiGe quantum wells for uncooled long wavelength infra-red radiation (LWIR) sensors

We demonstrate a novel single-crystalline high-performance thermistor material based on SiGe quantum well heterostructures. The SiGe/Si quantum wells are grown epitaxially on standard Si [001] substrates. Holes are used as charge carriers utilizing the discontinuities in the valence band structure. By optimizing design parameters such as the barrier height (by variation of the germanium content) and the fermi level Ef (by variation of the quantum well width and doping level) of the material, the layer structure can be tailored. Then a very high temperature coefficient of resistivity (TCR) can be obtained which is superior to the previous reported conventional thin film materials such as vanadium oxide and amorphous silicon. In addition, the high quality crystalline material promises very low 1/f-noise characteristics promoting an outstanding signal to noise ratio as well as well defined and uniform material properties.High-resolution X-ray diffraction was applied to characterize the thickness and Ge content of QWs. The results show sharp oscillations indicating an almost ideal super lattice with negligible relaxation and low defect density.The impact of growth temperature on the thermistor material properties was characterized by analyzing how the resulting strain primarily affects the performance of the TCR and 1/f noise. Results illustrate a value of 3.3 %/K for TCR with a low 1/f noise.

Light propagation in layered photonic crystals with admixture layers

Abstract The virtual-crystal approximation is used for numerical simulation of a polariton spectrum transformation in composite materials, consisting of alternating silicon and liquid crystal layers and randomly included admixture layers. The character of dependence of the bandgap width and the refractive index upon the concentra-tion of admixture layers is discussed. It is shown that the energy structure of this imperfect superlattice can be significantly altered by implantation of appropriate defect layers. Keywords: photonic crystal, Si - liquid crystal system, admixture layer, bandgap width. PACS: 78.66.-w UDC: 535.3 1. Introduction Propagation of electromagnetic waves in thin films and layered crystalline media is cur-rently drawing a close attention of researchers. The works [1–4] give the account of the related researches carried out for photonic crystals and [5] – for some composite materials based on silicon and liquid crystals. On the one hand, the interest towards these objects is due to their significance for electronics. On the other hand, this is because a notable ad-vance of technology that allows growing of ultra-thin films and periodic structures with controlled characteristics. There have been numerous theoretical and experimental studies (see, e.g., [6] and references therein) on exciton-like excitations in ideal dielectric superlattices. A general theory of optical waves in anisotropic crystals, including those formed of macroscopic layers, has been discussed in Ref. [7]. The authors [8] have investigated dispersion of po-laritons in a superlattice with two admixture layers. At the same time, a considerable in-terest has been focused on non-ideal superlattices with an arbitrary number of admixture layers, as well as on dependence of polariton spectrum on the concentration of corre-sponding defects. Further development of the theory of layered structures evidently re-quires considering more complex models like superlattices with randomly included ad-mixture layers of variable composition. A better understanding of how the optical proper-

Transformation of the polariton spectrum of nonideal topologically ordered superlattices

The transformation of the polariton spectrum of a nonideal superlattice (a one-dimensional crystal with two elements, layers, in a unit cell) caused by the presence of randomly embedded foreign (with respect to an ideal superlattice) impurity layers is investigated in the framework of the virtual crystal approximation. The obtained specific features of the energy gap as a function of concentration for different values of the polariton branch number testify that the energy structure of the superlattice can be changed significantly by embedding certain impurities into it.

Thermal boundary resistance at an epitaxially perfect interface of thin films

At the interfaces in an ideal epitaxial superlattice, it may be expected that there exists no thermal boundary resistance (TBR) due to thermal motions because the interfaces are atomically perfect. However, recent researches reported that the TBR still exists at the epitaxial interfaces of superlattices. Our previous study suggested the model, which was named as the C–M model, to predict accurately the TBR at an interface by considering a mass ratio between two species. In this study, we incorporated the effect due to an intermolecular potential well ratio into the previous model. The updated C–M model was based on the classical theory of a wave reflection and transmission, and provided an excellent agreement with the results of the molecular dynamics (MD) simulation. Furthermore, it suggests no TBR condition at an interface in superlattices.

Étude structurale par rayonnement synchrotron de superréseaux d'oxydes pérovskites

Perovskite oxides are analyzed by using high resolution synchrotron radiation diffraction. The diffraction profiles are fitted, taking into account the structural and chemical disorders which depart from the ideal defect-free superlattice. Two types of superlattices, (BaTiO 3 /SrTiO 3 ) 15 and (La 0.7 Sr 0.3 MnO 3 /SrTiO 3 ) 15 , are synthesized by metal-organic chemical vapor deposition (MOCVD) on oriented Nb-doped SrTiO 3 substrates. High-angle diffraction data show the excellent crystallinity and epitaxy of the films. For the (BaTiO 3 /SrTiO 3 ) n multilayers we observe a pseudo tetragonal symmetry with no difference between a and b axes, and they present a significant strontium/baryum interdiffusion. The (La 0.7 Sr 0.3 MnO 3 /SrTiO 3 ) n multilayers and their substrates almost have the same a=b parameter. Along the c axis, La 0.7 Sr 0.3 MnO 3 and SrTiO 3 have different lattice parameters. The crystallographic coherence of the stacking is higher in these samples.

The Role of Intermixing in the Phase Determination of the Interlayer Exchange Coupling in Multilayers: Application to Fe-Cr Superlattices

The phase of the short-range interlayer exchange coupling oscillations in Fe n Cr m superlattices is expressed in terms of the intermixing at the Fe-Cr as well as at the Cr-Fe interfaces. The interdiffusion is modeled through stochastic algorithms, which presupposes floating of the atoms on the surface of the sample during the epitaxial growth. It automatically leads to the different chemical and magnetic structure of Fe on Cr and Cr on Fe interfaces. Self-consistent calculations of the magnetic moments are performed on the basis of the periodic Anderson model. Although short-range (2 monolayers (ML)) oscillations of exchange coupling were detected for all considered structures, its amplitude and phase strongly depend on the interface alloying. Introduction of the same intermixing at both interfaces does not change the phase as compared to the ideal superlattices with sharp interfaces. However different interdiffusion leads to the ~ -phase shift in accordance with experimental results for the Fe-Cr-Fe trilayers grown on an Fe whiskers. Distribution of magnetic moments on Fe atoms contains several distinct peaks but their position and relative area weakly depend on the alloying and interlayer exchange coupling in the superlattice. On the contrary, magnetic moments on Cr atoms are very sensitive to the intermixing and their behavior determines the strength and the phase of exchange coupling oscillations.

Structural and Dielectric Properties of (BaTiO 3 /SrTiO 3 ) 15 Superlattices Grown by MOCVD

(BaTiO 3 /SrTiO 3 ) 15 superlattices were grown by metal organicchemical vapor deposition (MOCVD) on <001> oriented LaAlO 3 and Nb-doped SrTiO 3 substrates. The source was a pulsed liquid-injection delivery system, with a unique precursors solution for each compound. The thickness of the layers was controlled by adjusting the amount of liquid injected. Different sets of samples were prepared with varying interlayer thickness. The satellite peaks observed on the X-ray diffraction patterns indicate a superlattice structure. High-angle diffraction using X-ray synchrotron radiation was used to assess the structural and chemical disorder and the interface state with respect to the ideal defect-free superlattice. Electrical measurements were performed on the multilayers, using the SrTiO 3 :Nb substrate as a bottom electrode. As the period of the (BaTiO 3 /SrTiO 3 ) 15 superlattices increases (and also the total thickness), the dielectric constant increases. It reaches a value of 620 for (BaTiO 3 36 u.c./SrTiO 3 36 u.c.) 15 .

Suppression of Bulk Defects in Antimonide Superlattice Infrared Photodiodes

AbstractWhile physical properties of ideal antimonide superlattices (ASL) indicate that they should significantly outperform mercury cadmium telluride (MCT) based infrared photodiodes for low dark current applications in the long and very long wave-infrared (LWIR and VLWIR), this potential has not yet been fully realized. Even though measured Auger and tunneling rates in ASL's are reduced as predicted, overall carrier lifetimes remain much shorter, and dark currents much higher than expected. The large carrier losses are the result of defects in the ASL structure, with contributions measured from large bulk defects and surface channels along mesa sidewalls, and the remaining component believed to be due to midgap states.In this presentation we report on several studies of epitaxial growth parameters and their influence on defect formation. X-ray photoelectron spectroscopy analysis of oxide desorption from GaSb substrates shows the presence of both antimony and gallium oxides, along with their decomposition and desorption behavior with anneal temperature. A study of buffer growth shows that defect density and size are critically dependent on growth temperature, with an optimal growth window between 480 and 500 °C.. Side-by-side GaSb buffer growths on vicinal ((100) + 1 ° (111)) and flat (100) substrates show that while growing on vicinal material can suppress mound formation, it does not yield epilayers as flat as can be obtained on (100) substrates grown under optimal conditons. Finally, the ratio of As to In flux during superlattice growth can be used to tune the lattice parameter both above and below that of GaSb, with strain-related defects appearing when the mismatch reaches roughly 0.1%.

Coherent dynamics in one-dimensional multiband semiconductor superlattice driven by DC–AC electric fields

We investigate the electron dynamical processes taking place in nanodevices under high-frequency DC–AC fields by solving numerically the one-dimensional time-dependent Schrödinger equation including some ingredients of actual SLs, namely, finite interband coupling, multiband scattering. We found that Rabi oscillations between minibands are clearly identified under theoretical resonant conditions derived by an ideal two-band superlattice model, the resonance conditions have broadened, and the amount of broadening is about four times the Rabi oscillation frequency, and also that static electric field can weaken the fast dephasing effect of interface roughness on Rabi oscillations in superlattice. Experimental conditions for the observation of the predicted effects are discussed.

Electron localization and bloch oscillations in quantum-dot superlattices under a constant electric field

It is demonstrated that the electron spectrum in ideal two-dimensional and three-dimensional quantum-dot superlattices (SLs) under a constant electric field can be either discrete or continuous depending on the field orientation with respect to the SL crystallographic axes. In the latter case, the width of the resulting transverse miniband depends exponentially on the crystallographic index corresponding to the direction of the field. The electron localization area undergoes dramatic variations with the field orientation in the vicinity of the directions corresponding to the continuous energy spectrum. The Bloch oscillations in this kind of SL are considered. It is established that the scattering of oscillating electrons can be strongly suppressed by an appropriate choice of the field strength and direction.

Spin-wave susceptibility of partially randomized ferromagnetic superlattices

This paper is a theoretical investigation of the effect of inhomogeneities in the period of a ferromagnetic superlattice on the high-frequency superlattice susceptibility. The calculations are done for a model in which the uniaxial magnetic anisotropy is taken as the physical parameter that characterizes both the ideal superlattice and a partially randomized superlattice. It is found that as the inhomogeneities become more intense, the two resonance peaks corresponding to the splitting of the spectrum at the edge of the Brillouin zone of the superlattice broaden, move closer to each other, and finally merge into one. The height of this peak increases and the peak width decreases as the intensity of the inhomogeneities increases further. The effect of inhomogeneities on the susceptibility differs dramatically in the two limits of short-and long-wave inhomogeneities: in the latter case (in contrast to the former) the dependence of the separation of the susceptibility peaks on the intensity and correlation properties of the inhomogeneities is nonmonotonic. The possibility of observing these effects in spin-wave resonance experiments involving multilayer magnetic films is also discussed.

Optoelectronics of semiconductor superlattices

We discuss the linear and nonlinear optical properties of ideal and disordered semi conductor superlattices and their modification due to homogeneous electric fields applied along the growth direction. The disorder under consideration is caused by monolayer fluctuations of the well widths and the finite number of wells in short-period superlattices. We show how to derive the multiband semiconductor Bloch equations used for the theoretical modeling of these systems. For ideal superlattices we investigate the field dependence of linear absorption and the density dependent absorption. The dephasing due to electron-phonon interaction of the THz radiation emitted by Bloch-oscillating carriers is shown to be strongly field dependent. It is demonstrated that the THz-signals can be dominated by either free-carrier motion or beating between excitonic resonances, depending on the excitation conditions. Afterwards the influence of the disorder on the linear absorption, the THz emission and four-wave-mixing signals is discussed. For the linear absorption we show that its influence on the low energy side can be much weaker than on structures above the band edge. Here, we also present results for type II and doping superlattices. In these systems locally indirect excitons are shown to occur which differ in their field dependent behaviour from the direct excitons in type I superlattices. For THz radiation the disorder is shown to lead to a dephasing rate that is linear in the applied electric field. Finally, four-wave-mixing signals for some idealized finite superlattices are used to discuss the complex nature of disorder-induced dephasing.

Reply to ‘‘Comment on ‘Temperature limits on infrared detectivities of InAs/InxGa1−xSb superlattices and bulk Hg1−xCdxTe’ ’’ [J. Appl. Phys. 74, 4774 (1993)

Our calculations of the relative performance of ideal superlattice and HgCdTe (MCT) photovoltaic long wave infrared detectors, criticized in the previous Comment, do include radiative lifetimes. They also find the Auger-7 lifetime to be shorter than the radiative lifetime for p doping levels of 1017 cm−3. This agrees with other calculations and with the majority of experiments. The issue of detector thickness raised in the comment is not relevant since neither MCT nor the superlattices has an intrinsic advantage in this respect. The superlattices are further favored relative to MCT by lower tunneling currents, higher uniformity, and materials processing advantages. We suggest the superlattice system to be promising for the eventual realization of high-performance detectors.