GaAs Multilayers Research Articles

Signatures of magnetism control by flow of angular momentum

Exploring new strategies to manipulate the order parameter of magnetic materials by electrical means is of great importance not only for advancing our understanding of fundamental magnetism but also for unlocking potential applications. A well-established concept uses gate voltages to control magnetic properties by modulating the carrier population in a capacitor structure1–5. Here we show that, in Pt/Al/Fe/GaAs(001) multilayers, the application of an in-plane charge current in Pt leads to a shift in the ferromagnetic resonance field depending on the microwave frequency when the Fe film is sufficiently thin. The experimental observation is interpreted as a current-induced modification of the magnetocrystalline anisotropy ΔHA of Fe. We show that (1) ΔHA decreases with increasing Fe film thickness and is connected to the damping-like torque; and (2) ΔHA depends not only on the polarity of charge current but also on the magnetization direction, that is, ΔHA has an opposite sign when the magnetization direction is reversed. The symmetry of the modification is consistent with a current-induced spin6–8 and/or orbit9–13 accumulation, which, respectively, act on the spin and/or orbit component of the magnetization. In this study, as Pt is regarded as a typical spin current source6,14, the spin current can play a dominant part. The control of magnetism by a spin current results from the modified exchange splitting of the majority and minority spin bands, providing functionality that was previously unknown and could be useful in advanced spintronic devices.

Homotopy Phases of FQHE with Long-Range Quantum Entanglement in Monolayer and Bilayer Hall Systems.

Correlated phases in Hall systems have topological character. Multilayer configurations of planar electron systems create the opportunity to change topological phases on demand using macroscopic factors, such as vertical voltage. We present an analysis of such phenomena in close relation to recent experiments with multilayer Hall setups including GaAs and graphene multi-layers. The consequences of the blocking or not of the inter-layer electron tunneling in stacked Hall configurations are analyzed and presented in detail. Multilayer Hall systems are thus tunable topological composite nanomaterials, in the case of graphene-stacked systems by both intra- and inter-layer voltage.

Broadband operation of rolled-up hyperlenses

This work is related to an earlier publication [Schwaiger et al., Phys. Rev. Lett. 102, 163903 (2009)], where we demonstrated by means of fiber-based transmission measurements that rolled-up Ag-(In)GaAs multilayers represent three-dimensional metamaterials with a plasma edge which is tunable over the visible and near-infrared regime by changing the thickness ratio of Ag and (In)GaAs, and predicted by means of finite-difference time-domain simulations that hyperlensing occurs at this frequency-tunable plasma edge. In the present work we develop a method to measure reflection curves on these structures and find that they correspond to the same tunable plasma edge. We find that retrieving the effective parameters from transmission and reflection data fails, because our realized metamaterials exceed the single-layer thicknesses of $5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, which we analyze to be the layer thickness limit for the applicability of effective parameter retrieval. We show that our realized structures nevertheless have the functionality of an effective metamaterial by supplying a detailed finite-difference time-domain study which compares light propagation through our realized structure (17-nm-thick Ag layers and 34-nm-thick GaAs layers) and light propagation through an idealized structure of the same total thickness but with very thin layers [2-nm-thick Ag layers and 4-nm-thick (In)GaAs layers]. In particular, our simulations predict broadband hyperlensing covering a large part of the visible spectrum for both the idealized and our realized structures.

Dynamical X-ray Diffraction from ZnS y Se1−y /GaAs (001) Multilayers and Superlattices with Dislocations

High-resolution x-ray diffraction is a valuable nondestructive tool for structural characterization of semiconductor heterostructures, and the diffraction profiles contain information on the depth profiles of strain, composition, and defect densities in device structures. Much of this information goes untapped because the lack of phase information prevents direct inversion of the diffraction profile. The current practice is to use dynamical simulations in conjunction with a curve-fitting procedure to indirectly extract the profiles of strain and composition. These dynamical simulations have been based on perfect, dislocation-free laminar crystals, rendering the analysis inapplicable to structures having dislocation densities greater than about 106 cm−2. In this work we present a dynamical model for Bragg x-ray diffraction in semiconductor device structures with nonuniform composition, strain, and dislocation density, which is based on the Takagi–Taupin equation for distorted crystals and accounts for the angular and strain broadening of dislocations. We show that the x-ray diffraction profiles from ZnS y Se1−y multilayers and superlattices are strongly affected by the depth distribution of the dislocation density as well as the composition, suggesting that it should be possible to extract the profiles of composition, strain, and dislocation density by the analysis of measured diffraction profiles.

Epitaxial Growth and Characterization of Zinc-blende CrAs/GaAs/MnAs/GaAs Multilayers

We report on the growth, structural and transport properties of zinc-blende CrAs/GaAs/MnAs/GaAs multilayers on GaAs(001) substrates. Structural analyses using in-situ reflection high-energy electron diffraction and exsitu cross-sectional transmission electron microscopy confirmed the realization of a zinc-blende crystal structure. Room temperature Hall measurements reveal that the as-grown multilayer exhibits p-type conductivity with a very low resistivity of <TEX>$0.052\;\omega{cm}$</TEX>, a high carrier concentration of <TEX>$6.2X10^{19}\;cm^{-3}$</TEX> and a Hall mobility of <TEX>$1.8\;cm^2/Vs$</TEX>. We also observed a clear decrease of the resistivity in samples after low temperature annealing.

Selective etching of InGaAs∕GaAs(100) multilayers of quantum-dot chains

We report selective chemical etching as a promising procedure to study the buried quantum dots in multiple InGaAs∕GaAs layers. The dot layer-by-dot layer etching is demonstrated using a mixed solution of NH4OH:H2O2:H2O. Regular plan-view atomic force microscopy reveals that all of the exposed InGaAs layers have a chain-like lateral ordering despite the potential of significant In–Ga intermixing during capping. The vertical self-correlation of quantum dots in the chains is observed.

Resonant Raman Scattering and Atomic Force Microscopy of InGaAs∕GaAs Multilayer Nanostructures with Quantum Dots

The transition from two-dimensional (2D) pseudomorphic growth to the three-dimensional (3D) (nanoisland) growth in In x Ga 1 - x As/GaAs multilayer structures grown by molecular-beam epitaxy was investi- gated by atomic force microscopy, photoluminescence, and Raman scattering. The nominal In content x in In x Ga 1 - x As was varied from 0.20 to 0.50. The thicknesses of the deposited In x Ga 1 - x As and GaAs layers were 14 and 70 monolayers, respectively. It is shown that, at these thicknesses, the 2D-3D transition occurs at x ♢ 0.27. It is ascertained that the formation of quantum dots (nanoislands) does not follow the classical Stranski-Krastanov mechanism but is significantly modified by the processes of vertical segregation of In atoms and interdiffusion of Ga atoms. As a result, the In x Ga 1 - x As layer can be modeled by a 2D layer with a low In content ( x 0.60). For multilayer In x Ga 1 - x As structures, lateral alignment of quantum dots into chains oriented along the direction can be implemented and the homogeneity of the sizes of quantum dots can be improved. © 2005 Pleiades Publishing, Inc. 110 ()

Charge and spin distribution in ferromagnetic Mn-doped InGaAs∕GaAs multilayers

The search for materials tailored for spintronic devices led recently to the study of III–V diluted magnetic semiconductors (DMS). These materials are found to be closer to room-temperature ferromagnetism than their II–VI counterparts. Following this pathway, heterostructures based on III–V DMS were found to have higher Curie temperature. In this work, we propose geometrical arrangements that maximize spin separation combining strained and DMS III–V layers.

High anisotropy of lateral alignment in multilayered (In,Ga)As∕GaAs(100) quantum dot structures

A formation process for long chains of quantum dots during the molecular-beam epitaxial growth of (In,Ga)As∕GaAs(100) multilayers is presented. The morphology evolution monitored by atomic force microscopy for a series of (In,Ga)As layers demonstrates that the highly anisotropic lateral alignment of dots is gradually developed as the result of the strain field interaction mediated by the GaAs spacer coupled with the anisotropic surface kinetics that occurs during capping the dots. The dot-chain structure, providing unique properties of its own, is demonstrated to serve as a template for the spatially controlled growth of strained quantum dots in general.

Magnetization process of Ni/Si/Ni/GaAs multilayer and its magnetoresistance

Abstract Magnetization process and magnetoresistance measurement demonstrate existence of antiparallel spin state in a Ni/Si/Ni/GaAs multilayer. This occurred because of five times larger coercivity of Ni/GaAs than that of the Ni/Si. The hysteresis loop of Ni/Si/Ni/GaAs multilayer is found to be quite equal to the algebraic sum of that of the Ni/GaAs and Ni/Si bilayers, where the total Ni contents of the two bilayers are equal to that of the multilayer. The bottom Ni layer of the multilayer is mostly influenced by the underneath GaAs but not by the Si.

Zincblende CrSb/GaAs multilayer structures with room-temperature ferromagnetism

CrSb (1 ML)/GaAs (5 nm) multilayers with period up to 4 have been grown on GaAs substrates by solid-source molecular-beam epitaxy at 250°C. Reflection high-energy electron diffraction reveals zincblende characteristics throughout the growth of multilayer structures. High-resolution cross-sectional transmission electron microscopy also indicates that the crystal structure of the multilayers is zincblende and with no dislocations at the interfaces. The presence of room-temperature ferromagnetism is confirmed by magnetization measurements.

MBE-Grown GaAs Films on GaAs(111)A Substrates Misoriented toward [21¯1¯

An experimental investigation is carried out into the electrical properties of doped MBE-grown GaAs films on GaAs(111)A substrates that may be misoriented toward a given direction, with silicon being the only dopant. Single n- and p-type films and planar pn junctions are produced on perfectly oriented substrates, the junctions being tested as part of diodes. In addition, δ-Si doped GaAs multilayers are grown on substrates misoriented by 0.5°, 1.5°, or 3° toward [21¯1¯]. The sheet-resistivity anisotropy of the multilayers is revealed and measured.

Magnetic relaxations in metallic multilayers

The intrinsic damping mechanism in metals caused by incoherent scattering of itinerant electron-hole pair excitations by phonons and magnons will be reviewed. The unique features of magnetic relaxations in multilayers were studied by ferromagnetic resonance (FMR) using magnetic single, Au–Fe–GaAs(001), and double layer Au–Fe–Au–Fe–GaAs(001) structures prepared by molecular beam epitaxy. The magnetic relaxation in single-layer films is described by the Gilbert damping with no extrinsic contributions to the FMR linewidth. These films provided an excellent opportunity to investigate nonlocal damping. The main result of these studies is that ultrathin Fe films in magnetic double layers acquire an additional interface Gilbert damping. This is in agreement with recent predictions of nonlocal interface damping which is based on the transport of spin angular momentum between the ferromagnetic layers. Measurements of the nonlocal Gilbert damping offer a possibility to carry out quantitative studies of the relaxation torques caused by nonlocal spin momentum transfer. Numerical simulations of magnetization reversal and stationary precession for an applied perpendicular current in Au–Fe–Au–Fe–GaAs(001) multilayers will be shown.

Dopant atom clustering and charge screening induced roughness of electronic interfaces in GaAsp-nmultilayers

The roughness of the electronic interfaces of p-n GaAs multilayers is investigated by cross-sectional scanning tunneling microscopy. Two physically different contributions to the roughness are found, both much larger than the underlying atomically sharp ``metallurgical'' interface. The roughness arises from the individual electrostatic screening fields around each dopant atom near the interface and from a clustering of dopant atoms. The latter leads to charge-carrier-depleted zones extending locally through the entire nominally homogeneously doped layer for layer thicknesses close to the cluster dimension, hence limiting the precision of the spatial and energetic positioning of the Fermi energy in nanoscale semiconductor structures.

Characterization of a Quantum Well in an Si/Si1– x Ge x /Si Heterostructure by X-ray Diffractometry

Rocking curves with a pronounced pendulum-solution pattern are obtained for Si1 – xGex/Si specimens with a single quantum well (QW) by double-crystal x-ray diffractometry. These oscillations are typical of the rocking curves for GaAs multilayers with QWs. With Si–Ge multilayers, they are obtained for the first time. The Ge depth profiles of the QW are reconstructed from the rocking curves. As a result, the thicknesses of both QW interfacial layers are estimated.

Electronic and magnetoresistive properties of MnAs(0001)/GaAs(111) heterostructures

Electronic band-structure calculations are carried out for the MnAs(0 0 0 1)/ GaAs(1 1 1) multilayers by using the linearized muffin-tin-orbital method within the atomic sphere approximation. The exchange splitting of Mn 3d states between spin-up and spin-down bands as well as the magnitude of local magnetic moment of Mn atoms are reduced remarkably for the Mn sites near the heterointerface compared with those in the bulk of MnAs. On the other hand, the spin-polarization at the Fermi level is enhanced for the MnAs monolayers near the heterointerface.

Interlayer conduction for bulk quantum Hall systems (TMTSF) 2X

Interlayer resistance measurements have been carried out in the quantum Hall states for the bulk quantum Hall system (TMTSF) 2 X for X= ClO 4 . We found that the temperature and field dependence for the ClO 4 salt is surprisingly very different from that for the AsF 6 salt. The interlayer resistance in the quantum Hall states is much smaller than theoretical prediction. The analyses indicate the presence of bulk conductivity, which is consistent with the observation of the finite inplane conductivity in the quantum Hall states. The difference from the GaAs multilayers is also discussed.

Structural characterization of LPOMVPE grown AlAs/GaAs multilayers

The characterization of the AlAs\\GaAs multilayers based on the modelling of the X-ray scattering is discussed. The structural parameters regarding the thickness of the bilayer components and the vertical and lateral variations (interface roughness) were assessed from low and high angle scan modes. The diffuse scattering analysis showed that both lateral and vertical correlation of the interface roughness develops in this LPOMVPE grown system.

Facet evolution of CCl 4-doped [formula omitted] multilayers during metalorganic chemical vapor deposition on patterned GaAs substrates

We have investigated the facet evolution of Al 0.5 Ga 0.5 As GaAs multilayers on mesa- or groove-patterned GaAs substrates during metalorganic chemical vapor deposition (MOCVD) as a function of growth temperature and CCl 4 flow rate. Instead of the conventional evolution of a (433)A side wall facet for undoped Al 0.5 Ga 0.5 As GaAs multilayers on a mesa, we have observed (511)A or (411)A facet evolutions during MOCVD growth in the presence of CCl 4. In addition, the ratio of the growth rates of both GaAs and Al 0.5Ga 0.5As on the side wall facets and the (100) mesa top are remarkably enhanced compared to the growth without CCl 4. Additionally fast planarization on a groove, which is desirable for device application, has been observed. The growth rate ratio shows strong dependencies on growth temperature and CCl 4 flow rate indicating that the CCl 4 supply rate can be an adequate parameter for lateral growth control. Using these characteristics, we have fabricated CCl 4-doped quantum wires (QWRs) on V-groove structures. Extremely enhanced thickness of the QWR on a V-groove due to CCl 4 effect is observed.

Ionization in GaAs multilayers

Ionization in GaAs/AlGaAs multilayer p-i-n structures is studied theoretically using the semianalytic trajectory method, and the results are compared with published experimental data. The theory predicts a transition in breakdown behaviour for layer widths of the order of 250 A as observed experimentally. It is also found that the alpha / beta ratio can be engineered in the structures.