Anisotropy of Two-Dimensional Electron and Hole Mobilities in (0 0 1) GaAs/Al x Ga 1− x As Heterostructures Under Uniaxial Stress

Transport properties of two-dimensional electron and hole gas at (0 0 1) GaAs/Al x Ga 1 m x As heterointerface in [1 1 0] and [1 m 1 0] directions have been investigated for the first time under in plane uniaxial compression up to 5 kbar. Resistivity, Shubnikov-de Haas oscillations and Hall effect were measured and carrier densities and mobilities were determined. The in-plane uniaxial compression significantly modifies the energy spectrum of p-GaAs/Al x Ga 1 m x As that leads to strong anisotropy of hole mobility under compression. In the case of the n-type heterostructure uniaxial compression causes only change of the carrier concentration and the corresponding small change of the initial mobility anisotropy, determined by anisotropic surface roughness scattering.     

Effect of external uniaxial stress on the electronic properties and symmetry of p-GaAs/AlxGa1−xAs quantum wells

Abstract

The results of experimental and theoretical investigations of the energy spectrum and electronic properties of symmetric p-[001] Al_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As heterostruc-tures under uniaxial [110] compression are presented. The stress-induced piezoelectric field breaks the confining potential symmetry in the quantum well and lifts the degeneracy of the hole subbands. A redistribution of holes in the spin subbands of the ground state takes place, which is revealed in the different shifts of the Shubnikov-de Haas oscillation maxima corresponding to the different spin subbands. The [110] uniaxial compression significantly modifies the band structure, which leads to a strong aniso-tropy of the Fermi surface. The electrical resistance becomes strongly anisotropic under applied compression, decreasing in the direction parallel to the compression and increasing in the perpendicular direction.     

Piezoelectric effect and the long-term resistance relaxations induced by uniaxial compression in p-GaAs/AlxGa1−x as heterostructures

Long-term resistance relaxations induced by uniaxial compression in (001)p-GaAs/Al_0.5Ga_0.5 As heterostructures are observed, and the main properties of these relaxations are investigated: the dependence of their character on the direction of the uniaxial compression, the change in concentration of current carriers during the relaxation processes, and the quenching of the relaxations by temperature, illumination, and high electric fields. It is found that the character of the relaxation process is different for compression directions [110] and $[1\bar 10]$ : in the first case the concentration of two-dimensional holes in the quantum well decreases in the course of the relaxation, while in the second case it increases. A model is proposed in which the cause of the relaxations of the piezoresistance and the anisotropic character of these relaxations is assumed to be the piezoelectric field, the value of which, according to estimates, is |E _z| = 1.152 × 10⁴ V/cm per kbar.     

Negative and persistent photoconductivity in p-type Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As heterostructures under uniaxial stress

Abstract

Illumination of a double p-Al_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As heterostructure by a red light emitting diode results in a negative photoconductivity that, after the diode is switched off, slowly relaxes to a positive persistent photoconductivity, characterised by an approximately 1.5-fold increase of the two-dimensional hole concentration. This metastable state may be explained with a model in which deep electron traps are supposed to be located above the Fermi level on the inverted heterointerface. Under uni-axial compression the hole concentration in the persistent photoconductivity state, as well as in the dark state, demonstrates the same linear decrease.     

Two-dimensional electrons at the n-GaAs/Al x Ga1−x As heterointerface under uniaxial compression

The transport characteristics and quantum oscillations of magnetoresistance have been studied in n-(001)GaAs/Al _x Ga_1?x As heterostructures at liquid helium temperatures and uniaxial compressions up to 3.5 kbar. Under such loading conditions, the density of two-dimensional (2D) electrons at the heterointerface is determined primarily by a piezoelectric field induced in the [001] direction. The screening of this field leads to an increase in the 2D electron density at the heterointerface under compression along the [110] axis and to a decrease in this density, under compression along the $[1\bar 10]$ axis. The formation and subsequent redistribution of a compensating charge at the heterointerface are impeded, which leads to the development of relaxation processes in the stressed system at sufficiently high pressures. The pressure dependences of the anisotropic mobility and the effective cyclotron mass of 2D electrons show that their energy spectrum remains isotropic and the dispersion obeys a parabolic law in the pressure range studied.     

In situ magneto-optical Kerr effect study of uncovered Fe films on ZnSe(0 0 1)

The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2×2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 °C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1 T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from ∼5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe.     

Molecular-beam epitaxy on shallow mesa gratings patterned on GaAs(3 1 1)A and (1 0 0) substrates

We report on the morphology and properties of the surface formed by molecular-beam epitaxy on shallow mesa gratings on patterned GaAs(3 1 1)A and GaAs(1 0 0). On GaAs(3 1 1)A substrates, the corrugated surface formed after GaAs growth on shallow mesa gratings along is composed of monolayer high steps and (3 1 1)A terraces. These pattern induced steps which are different on opposite slopes play an important role in InAs growth on this novel template leading to distinct lateral modulation of the island distribution. On GaAs(1 0 0) substrates, growth on shallow mesa gratings along [0 1 1], [0 1 0] and is drastically sensitive to the pattern direction due to the difference of steps along [0 1 1] and .     

Structure and homoepitaxial growth of GaAs(6 3 1)

We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 °C reflection high-energy electron diffraction (RHEED) showed along the [−1 2 0] direction a 2× surface reconstruction for GaAs(6 3 1)A, and a 1× pattern was observed for GaAs(6 3 1)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(6 3 1)B surface pits were formed. For GaAs(6 3 1)A, 500 nm-thick GaAs layers were grown at 585 °C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5−9−3] direction. Transversal views of the bulk-truncated GaAs(6 3 1) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures.     

Response of fcc metals and L12 and D022 type trialuminides to uniaxial loading along [100] and [001]: ab initio DFT calculations

Ab initio density-functional calculations have been used to investigate the response of the face-centred cubic (fcc) metals Al and Cu, and of the L1₂- and D0₂₂-type trialuminides Al₃(Sc,Ti,V) to uniaxial loading along the [100] and [001] directions. The results obtained under uniaxial strains are compared to the response to biaxial (epitaxial) strains. The ideal tensile and compressive strengths and their limitation by shear instabilities along these deformation paths have been calculated. Although the response of both pure fcc metals could be expected to be very similar, our results show a fundamental difference: whereas for Cu a special invariant state with C ₂₂?=?C ₂₃, leading to a bifurcation from the tetragonal to an orthorhombic deformation path, is reached at a strain of 10%, for Al this state is reached only at a strain of 33% close to the critical strain defining the ideal tensile strength. The reaction of the L1₂-type trialuminides is comparable to the response of Al; no bifurcation to an orthorhombic deformation path is predicted. The response of the D0₂₂-type trialuminides is different from that of the L1₂-type phases because of the difference in the stacking of the atomic planes along the [001] direction. For D0₂₂-type trialuminides, the uniaxial compression along this direction or epitaxial tension in the (001) plane leads to the formation of a stress-free D0₃ structure, in complete analogy to the fcc???bcc transformations observed for the pure metals. Under uniaxial [100] loading the guiding symmetry along the deformation path is orthorhombic and leads to the formation of special structures under both tension and compression parts, which are related to the D0₃ structure in the same way as the parent D0₂₂-lattice is related to the L1₂ structure.     

Electron paramagnetic resonance study of the nuclear spin dynamics in an AlAs quantum well

The nuclear spin dynamics in an asymmetrically doped 16-nm AlAs quantum well grown along the [001] direction has been studied experimentally using the time decay of the Overhauser shift of paramagnetic resonance of conduction electrons. The nonzero spin polarization of nuclei causing the initial observed Overhauser shift is due the relaxation of the nonequilibrium spin polarization of electrons into the nuclear subsystem near electron paramagnetic resonance owing to the hyperfine interaction. The measured relaxation time of nuclear spins near the unity filling factor is (530 ± 30) min at the temperature T = 0.5 K. This value exceeds the characteristic spin relaxation times of nuclei in GaAs/AlGaAs heterostructures by more than an order of magnitude. This fact indicates the decrease in the strength of the hyperfine interaction in the AlAs quantum well in comparison with GaAs/AlGaAs heterostructures.     

Modification of the energy spectrum and magnetic breakdown in a system of 2D holes at the GaAs/Al<Subscript>0.5</Subscript> Ga<Subscript>0.5</Subscript>As heterojunction upon uniaxial compression

Uniaxial compression of the p-type GaAs/Al_0.5Ga_0.5 As heterostructures induces magnetic breakdown between the spin-split ground-state subbands of two-dimensional heavy holes. This phenomenon serves as direct experimental evidence of a strong qualitative modification of the energy spectrum of these structures upon uniaxial deformation. This modification has been revealed by numerical calculations, and, according to it, the subband spin splitting decreases upon compression, while the contours of the hole Fermi surface in both subbands touch one another in the compression direction in a pressure range of 2.5 kbar.     

Hole mobility enhancement in uniaxial stressed Ge dependence on stress and transport direction

Utilizing a six-band k.p valence band calculations that considered a strained perturbation Hamiltonian, uniaxial stress-induced valence band structure parameters for Ge such as band edge energy shift, split, and effective mass were quantitatively evaluated. Based on these valence band parameters, the dependence of hole mobility on uniaxial stress (direction, type, and magnitude) and hole transport direction was theoretical studied. The results show that the hole mobility had a strong dependence on the transport direction and uniaxial stress. The hole mobility enhancement can be found for all transport directions and uniaxial stess configurations, and the hole transport along the [110] direction under the uniaxial [110] compressive stress had the highest mobility compared to other transport directions and stress configurations.     

Vibrational properties of (0 0 1) III-V nitride superlattices

We study the acoustic waves of superlattices grown along the [0 0 1] direction and formed by slabs of cubic III-V nitride. We consider structures formed by combining AlN, GaN and InN in the zinc-blende structure because of the possible applications of these materials. The anisotropy of the materials has been taken into account and the different propagation directions including symmetry directions and general directions have been considered. The dispersion relations for these propagation directions have been obtained. The evolution of gaps along the propagation directions is studied. Total gaps for some propagation directions and frequency ranges are found in GaN-InN superlattices.     

Spin-engineering in the Co75Fe25/Cu(1 1 0) system

We present results on the growth and magnetic anisotropies of Co₇₅Fe₂₅ films grown on a Cu(1 1 0) single crystal. Angular dependent MOKE measurements show a thickness dependent, in-plane rotation of the easy axis of magnetisation of up to 60° from the [0 0 1] direction (towards [−1 1 0]). For a film thickness of 5 ML, just greater than that required for the onset of ferromagnetism, uniaxial anisotropy is observed with the easy axis along the [0 0 1] direction. As the film thickness increases this is seen to rotate in-plane towards the [−1 1 0] direction as the contribution from the cubic anisotropy constant grows. At a film thickness of 9 ML there is predominantly cubic anisotropy and at 10 ML the easy axis is rotated to 150° with respect to the [1 −1 0] axis, where it is stabilised.     

单轴[110]应力硅电子迁移率

基于k·p微扰法研究单轴[110]应力作用下硅的导带结构,获得单轴[110]应力硅的导带底能量及电子有效质量.在此基础上,考虑电子谷间、谷内及电离杂质散射,采用弛豫时间近似计算单轴[110]应力硅沿不同晶向的电子迁移率.结果表明：单轴[110]应力作用下硅的电子迁移率具有明显的各向异性.在[001]、[110]及[110]输运晶向中,张应力作用下电子沿[110]晶向输运时迁移率有较大的增强,由未受应力时的1 450 cm²·Vs^-1提高到2 GPa应力作用下的2 500 cm²·Vs^-1.迁移率增强的主要原因是电子有效质量的减小,而应力作用下硅导带能谷分裂导致的谷间散射几率的减小对电子迁移率的影响并不显著.     

Generalized susceptibilities and phonon anomalies in Pd and Pt metals

The generalized susceptibility, χ(q), in Pd and Pt for q along the [100], [110], [111], and [120] directions was determined from their APW and RAPW energy band structures, respectively, using the analytic tetrahedron linear energy scheme of Rath and Freeman. The band structures were previously found to yield Fermi surface radii, temperature dependencies of the static magnetic susceptibility, χ(T), resistivity, and a spin lattice relaxation, T₁T, in very good agreement with experiment. In the χ(q) calculations, we used 2048 tetrahedra in 1/48th irreducible BZ and the energy eigenvalues for bands 4, 5, and 6 which cross the Fermi energy as fitted to a Fourier series representation. The intraband parts of χ(q) at q = 0 for both metals are found to agree with the density of states at the Fermi energy to without 0.5%. Our results show that the dominant contribution to χ_intra arises from the dominant band 5 whose “jungle-gym” FS has strong nesting features; the main peak for Pd occurs at the same q value (= 0.65π/a) for q along the [0q0], [q, q, 0], and [q, q, q] directions. The locus of this main peak is a square in the (0, 0, 1) plane. The maximum of χ_intra for q along the [110] and [111] directions are 23% and 13%, respectively, higher than the value of χ(q) at q = 0. For q along the [010] and [120] directions, the peak is, however, lower than the value of χ_intra at q = 0. Hence, while phonon anomalies are predicted for the [110] and [111] directions, no anomaly is predicted for either the [100] or [120] direction. The predicted q value for the [110] anomaly, $\text{q}\text{= 0.65π/a}$ is close to the experimental value of ～0.7 π/a. Although there may be a hint of an anomaly at 0.56 [111] in the measurements, a more detailed investigation of this region is called for. For platinum, χ_intra for q along the [010], [110] and [111] directions has main peaks which occur at q = 0.68 π/a, 0.75 π/a, and 0.85 π/a, respectively. Here too, this main peak comes from the nesting of the jungle-gym Fermi surface which is not, however, as flat as that of palladium. Anomalies are predicted (although weaker in Pt than in Pd) along [110] and [111] but not along [100] and [120]. The [110] anomaly is close to the measured q value (～0.7–0.8 π/a). Also in agreement with experiment, we predict a weaker [110] anomaly for Pt than for Pd. In both Pd and Pt, weaker anomalies are predicted for the [111] direction than for the [110] direction.     

Theoretical study on the fine structure of PbFe0.5Nb0.5O3

The full potential linearized augmented plane wave method within the generalized gradient approximation was used to determine the tetragonal fine structure for PbFe_0.5Nb_0.5O₃, i.e., the equilibrium configuration of B-site compound cations (Fe and Nb). It was found that the displacement of atom Fe along [001] direction is 0.022 nm and that of Nb is 0.0020 nm. The ferroelectric instability of complex compound PbFe_0.5Nb_0.5O₃ is mainly coming from the displacement of atom Fe along [001] direction.     

Molecular dynamics simulation of the plastic behavior anisotropy of shock-compressed monocrystal nickel

Molecular dynamics simulations were used to study the plastic behavior of monocrystalline nickel under shock compression along the [100] and [110] orientations. The shock Hugoniot relation, local stress curve, and process of microstructure development were determined. Results showed the apparent anisotropic behavior of monocrystalline nickel under shock compression. The separation of elastic and plastic waves was also obvious. Plastic deformation was more severely altered along the [110] direction than the [100] direction. The main microstructure phase transformed from face-centered cubic to body-centered cubic and generated a large-scale and low-density stacking fault along the family of { 111 } crystal planes under shock compression along the [100] direction. By contrast, the main mechanism of plastic deformation in the [110] direction was the nucleation of the hexagonal, close-packed phase, which generated a high density of stacking faults along the [110] and[1?10] directions.     

Orientation dependence of structural transition in fcc Al driven under uniaxial compression by atomistic simulations

By molecular dynamics simulations employing an embedded atom method potential,we have investigated structural transformations in single crystal Al caused by uniaxial strain loading along the [001],[011] and [111] directions. We find that the structural transition is strongly dependent on the crystal orientations. The entire structure phase transition only occurs when loading along the [001] direction,and the increased amplitude of temperature for [001] loading is evidently lower than that for other orientations. The morphology evolutions of the structural transition for [011] and [111] loadings are analysed in detail. The results indicate that only 20% of atoms transit to the hcp phase for [011] and [111] loadings,and the appearance of the hcp phase is due to the partial dislocation moving forward on {111} fcc family. For [011] loading,the hcp phase grows to form laminar morphology in four planes,which belong to the {111} fcc family; while for [111] loading,the hcp phase grows into a laminar structure in three planes,which belong to the {111} fcc family except for the (111) plane. In addition,the phase transition is evaluated by using the radial distribution functions.     

Magnetic ordering in (110) Eu films in an applied magnetic field

Below its ordering temperature (T _N = 90 K), bulk bcc Eu has a helical magnetic state with propagation vectors along the three equivalent 〈100〉 directions. In contrast, epitaxial (110)Eu films exhibit a unique magnetic ordering: the domain with a magnetic helix propagating along the in-plane [001] direction vanishes on cooling, at the expense of other domains with helices propagating along [100] and [010]. This paper is devoted to the study of the stability of the magnetic domains in an external magnetic field using neutron scattering experiments and macroscopic magnetization measurements. The helix propagating along the [001] direction can be restored by the application of an external field along this direction. On the contrary, when a magnetic field is applied along an intermediate direction, specifically [10], the domain with a helix propagating along [001] is suppressed. Both effects depend on the film thickness. They are explained if one considers that, because of the low magnetic anisotropy of Eu, a helix with a propagation vector parallel to (or close to) the applied magnetic field is energetically more favourable than cycloidal structures with unchanged propagation vectors. Finally, the amplitudes of the propagation vectors and their directions (that are modified in films compared to bulk) do not vary under magnetic field.