The impact of bismuth incorporation into the epitaxial layer of a (Ga,Mn)As dilute ferromagnetic semiconductor on its magnetic and electromagnetic properties is studied in very thin layers of quaternary (Ga,Mn)(Bi,As) compound grown on a GaAs substrate under a compressive misfit strain. An addition of a small atomic fraction of 1% Bi atoms, substituting As atoms in the layer, predominantly enhances the spin-orbit coupling strength in its valence band. The presence of bismuth results in a small decrease in the ferromagnetic Curie temperature and a distinct increase in the coercive fields. On the other hand, the Bi incorporation into the layer strongly enhances the magnitude of negative magnetoresistance without affecting the hole concentration in the layer. The negative magnetoresistance is interpreted in terms of the suppression of weak localization in a magnetic field. Application of the weak-localization theory for two-dimensional ferromagnets by Dugaev et al. to the experimental magnetoresistance results indicates that the decrease in spin-orbit scattering length accounts for the enhanced magnetoresistance in (Ga,Mn)(Bi,As).

Incorporation of Bi into GaAs-(Ga,Al)As-Ga(As,Bi) core-shell nanowires grown by molecular beam epitaxy is studied with transmission electron microscopy. Nanowires are grown on GaAs(111)B substrates with Au-droplet assisted mode. Bi-doped shells are grown at low temperature (300 degrees C) with a close to stoichiometric Ga/As flux ratio. At low Bi fluxes, the Ga(As,Bi) shells are smooth, with Bi completely incorporated into the shells. Higher Bi fluxes (Bi/As flux ratio similar to 4%) led to partial segregation of Bi as droplets on the nanowires sidewalls, preferentially located at the nanowire segments with wurtzite structure. We demonstrate that such Bi droplets on the sidewalls act as catalysts for the growth of branches perpendicular to the GaAs trunks. Due to the tunability between zinc-blende and wurtzite polytypes by changing the nanowire growth conditions, this effect enables fabrication of branched nanowire architectures with branches generated from selected (wurtzite) nanowire segments.

Pulsed current-induced magnetization reversal is investigated in the layer of (Ga,Mn)(Bi,As) dilute ferromagnetic semiconductor (DFS) epitaxially grown under tensile misfit strain causing perpendicular magnetic anisotropy in the layer. The magnetization reversal, recorded through measurements of the anomalous Hall effect, appearing under assistance of a static magnetic field parallel to the current, is interpreted in terms of the spin-orbit torque mechanism. Our results demonstrate that an addition of a small fraction of heavy Bi atoms, substituting As atoms in the prototype DFS (Ga,Mn)As and increasing the strength of spin-orbit coupling in the DFS valence band, significantly enhances the efficiency of current-induced magnetization reversal thus reducing considerably the threshold current density necessary for the reversal. Our findings are of technological importance for applications to spin-orbit torque-driven nonvolatile memory and logic elements. Published under an exclusive license by AIP Publishing.

The contribution of the Mn 3d states to the valence band of Ga_0.98Mn_0.02Sb, an important factor determining the properties of this system, including the mechanism responsible for the magnetic characteristics, has been revealed by photoelectron spectroscopy. The resonant photoemission experiment, carried out for photon energies close to the Mn 3d -> 3p excitation, allowed us to identify the spectral feature corresponding to emission from the Mn 3d states. The scanning of the valence band along the [100] direction of the Brillouin zone, by the angle-resolved photoemission experiment, showed that these states contributed to a dispersionless structure at the binding energy of 3.6 eV (with respect to the Fermi energy). The revealed shape of the Mn 3d contribution is consistent with the supposition that the p-d exchange interaction prevails as a mechanism supporting ferromagnetism in Ga_1-xMn_xSb.

Hexagonal boron nitride has already been proven to serve as a decent substrate for high quality epitaxial growth of several 2D materials, such as graphene, MoSe2, MoS2 or WSe2. Here, we present for the first time the molecular beam epitaxy growth of MoTe2 on atomically smooth hexagonal boron nitride (hBN) substrate. Occurrence of MoTe2 in various crystalline phases such as distorted octahedral 1T' phase with semimetal properties or hexagonal 2H phase with semiconducting properties opens a possibility of realization of crystal -phase homostructures with tunable properties. Atomic force microscopy studies of MoTe2 grown in a single monolayer regime enable us to observe impact of growth conditions on formation of various structures: flat grains, net of one-dimensional structures, quasi continuous monolayers with bilayer contribution. Comparison of the distribution of the thickness with Poisson distribution shows that tested growth conditions favorite formation of grains with monolayer thickness. The diffusion constant of MoTe2 grown on hBN can reach order of 10-6 cm2/s for typical growth conditions. Raman spectroscopy results suggest a coexistence of various phases with domination of 2H MoTe2 for samples grown at lower temperatures. XPS measurements confirm the stoichiometry of MoTe2.

Thin crystalline layers of TaAs Weyl semimetal are grown by molecular beam epitaxy on GaAs(001) substrates. The (001) planes of the tetragonal TaAs lattice are parallel to the GaAs(001) substrate, but the corresponding in-plane crystallographic directions of the substrate and the layer are rotated by 45 degrees. In spite of a substantial lattice mismatch (about 19%) between the GaAs(001) substrate and TaAs epilayer, no misfit dislocations are observed at the GaAs(001)/TaAs(001) interface. Only stacking fault defects in TaAs are detected by transmission electron microscopy. Thorough X-ray diffraction measurements and analysis of the in situ reflection high-energy electron diffraction images indicate that TaAs layers are fully relaxed already at the initial deposition stage. Atomic force microscopy imaging reveals the columnar structure of the layers, with lateral (parallel to the layer's surface) columns about 20 nm wide and 200 nm long. Both X-ray diffraction and transmission electron microscopy measurements indicate that the columns share the same orientation and crystalline structure.

Challenges and opportunities arising from molecular beam epitaxial growth of topological crystalline insulator heterostructures composed of a rock-salt SnTe(001) layer of varying thickness (from 80 nm to 1000 nm) and a zinc blende 4 mu m thick CdTe(001) buffer layer grown on a commercial GaAs(001) substrate with 2 degrees off-cut toward the [100] direction were studied with a focus on crystal lattice strain, unit cell symmetry breaking and surface quality. The results indicate that the CdTe buffer is almost fully relaxed whereas in SnTe layers slight anisotropic relaxation is observed that varies from 86.2% to 98.3% with the layer thickness increasing. The relaxation process involves formation of misfit dislocations, mainly of Lomer-type (consisting of two associated 60 degrees dislocations), both at CdTe/GaAs and SnTe/CdTe interfaces. Azimuthal spatial distribution of defects is anisotropic due to a disparity of 60 degrees dislocation mobility toward orthogonal [-110] and [110] crystallographic directions. This results in a monoclinic distortion of the SnTe unit cell, as observed especially in the layers grown without additional Te molecular flux. A reflections selection method is proposed to measure such crystal deformations. Qualitatively new morphology of the SnTe surface of a reduced symmetry with nanoripple-like structures oriented close to the 100 (or, rarely, to 120) crystallographic in-plane direction is observed. The possible mechanism of their formation is dislocation-driven while their extended shape and predominant crystalline orientation may be influenced by the anisotropy of defect azimuthal distribution. Due to the magnitude of measured lattice strain (similar to 10(-3)) the monoclinic distortion in SnTe(001) layers is expected to be large enough to affect their physical properties, e.g., offering the way of controlling the crystal-symmetry-protected surface states (deformation-induced opening of the energy gap in the spectrum of metallic topological surface states). Thus, it may serve as an additional degree of freedom in designing topological spintronic devices.

Here we report on detailed studies of the magnetic properties of the wurtzite (Ga,Mn)As cylindrical shells. Ga0.94Mn0.06As shells have been grown by molecular beam epitaxy at low temperature as a part of multishell cylinders overgrown on wurtzite (Ga,In)As nanowires cores, synthesized on GaAs (111)B substrates. Our studies clearly indicate the presence of a low temperature ferromagnetic coupling, which despite a reasonably high Mn contents of 6% is limited only to below 30 K. A set of dedicated measurements shows that despite a high structural quality of the material the magnetic order has a granular form, which gives rise to the dynamical slowdown characteristic to blocked superparamagnets. The lack of the long range order has been assigned to a very low hole density, caused primarily by numerous compensation donors, arsenic antisites, formed in the material due to a specific geometry of the growth of the shells on the nanowire template. The associated electrostatic disorder has formed a patchwork of spontaneously magnetized (macrospin) and nonmagnetic (paramagnetic) volumes in the material. Using high field results it has been evaluated that the total volume taken by the macrospins constitute about 2/3 of the volume of the (Ga,Mn)As whereas in the remaining 1/3 only paramagnetic Mn ions reside. By establishing the number of the uncoupled ions the two contributions were separated. The Arrott plot method applied to the superparamagnetic part yielded the first experimental assessment of the magnitude of the spin-spin coupling temperature within the macrospins in (Ga,Mn)As, TC = 28 K. In a broader view our results constitute an important contribution to the still ongoing dispute on the true and the dominant form(s) of the magnetism in this model dilute ferromagnetic semiconductor.

The lattice mismatch between interesting 2D materials and commonly available 3D substrates is one of the obstacles in the epitaxial growth of monolithic 2D/3D heterostructures, but a number of 2D materials have not yet been considered for epitaxy. Here, we present the first molecular beam epitaxy growth of a NiTe2 2D transition-metal dichalcogenide. Importantly, the growth is realized on a nearly lattice-matched GaAs(111)B substrate. Structural properties of the grown layers are investigated by electron diffraction, X-ray diffraction, and scanning tunneling microscopy. Surface coverage and atomic-scale order are evidenced by images obtained with atomic force, scanning electron, and transmission electron microscopy. Basic transport properties were measured confirming that the NiTe2 layers are metallic, with a Hall concentration of 10(20) to 10(23) cm(-3), depending on the growth conditions.

Diluted ferromagnetic semiconductors combining ferromagnetic and semiconducting properties in one material provide numerous new functionalities, attractive for basic studies and potentially useful for novel device applications. The tailoring of the electronic structure in analogy to conventional semiconductors has yet to be explored. Here, we demonstrate the conservation of broken inversion symmetry and band structure tailoring for high-quality molecular-beam-epitaxy-grown (In,Ga,Mn)As films with 3% In plus 2.5% or 5.6% Mn using hard-x-ray photoelectron diffraction (hXPD) and momentum microscopy. Photon energies of 3-5 keV ensure that the results are not corrupted by surface effects, which are known to be strong in semiconductors. The missing inversion center of the GaAs host lattice leads to fingerprint-like hXPD signatures of As and Ga sites. For both concentrations, Mn predominantly occupies Ga substitutional sites. Momentum microscopy reveals a shift of the chemical potential with increasing Mn doping and a highly dispersing band, crossing the Fermi level for high Mn concentration. The Mn doping induces a pronounced modification of the spin-orbit split-off band.