Chemical vapor deposition (CVD) of two-dimensional materials has been an active area of research in recent years because it is a scalable process for obtaining thin films that can be used to fabricate devices. The growth mechanism for hexagonal boron nitride (h-BN) on metal catalyst substrates has been described to be either surface energy-driven or diffusiondriven. In this work, h-BN is grown in a CVD system on Ni single-crystal substrates as a function of Ni crystallographic orientation to clarify the competing forces acting on the growth mechanism. We observed that the thickness of the h-BN film depends on the Ni substrate orientation, with the growth rate increasing from the (100) surface to the (111) surface and the highest on the (110) surface. We associate the observed results with surface reactivity and diffusivity differences for different Ni orientations. Boron and nitrogen diffuse and precipitate from the Ni bulk to form thin multilayer h-BN. Our results serve to clarify the h-BN CVD growth mechanism which has been previously ascribed to a surface energy-driven growth mechanism.

The galvanostatic electrodeposition of epitaxial Fe x Ni 1−x films on n-GaAs͑001͒ substrates from aqueous metal ammonium sulfate solutions is reported. Structural measurements using X-ray diffraction and transmission electron microscopy indicate that the films have single crystalline or highly oriented body-centered cubic ͑bcc͒ or face-centered cubic ͑fcc͒ structure at Fe and Ni-rich compositions, respectively. The preparation of the substrate surface via ammonium hydroxide as well as the usage of an ammonium sulfate electrolyte buffer facilitates the initial nucleation of aligned metal islands on GaAs. The ratio of the Fe atomic concentration in the fcc films to that in the electrolyte is close to unity ͑1.1͒, whereas a preferential incorporation occurs for the bcc phase such that this ratio increases linearly with Fe atomic composition at a rate of 3.7. bcc Fe x Ni ͑1−x͒ /GaAs diodes have uniform Schottky barrier heights as measured by current-voltage measurements independent of Fe composition consistent with a high interfacial state density. The observed magnetic properties of Fe x Ni 1−x are consistent with single crystalline material with distributed inhomogeneities. The epitaxy via electrodeposition suggests the importance of ammonium sulfate for technical applications.

We electrically detect charge current induced spin polarization on the surface of molecular beam epitaxy grown Bi2Te3 thin film in a two-terminal device with a ferromagnetic MgO/Fe and a nonmagnetic Ti/Au contact. The two-point resistance, measured in an applied magnetic field, shows a hysteresis tracking the magnetization of the Fe. A theoretical estimate is obtained for the change in resistance on reversing the magnetization direction of Fe from coupled spin-charge transport equations based on quantum kinetic theory. The order of magnitude and the sign of the hysteresis is consistent with spin-polarized surface state of Bi2Te3.The three dimensional (3D) topological insulators (TIs) having insulating bulk and Dirac-type two dimensional (2D) surface states (SSs) with spin-momentum locking have potential for spintronics [1][2][3][4][5][6]. The dispersion relation of the SS guarantees that any charge current flow within these states will induce a non-zero spin accumulation on the 2D surface of a 3D TI. This current induced spin polarization of the SS, controllable by the magnitude and the direction of the current, can be used to torque a ferromagnet (FM) [4,5]. In recent experiments [7][8][9][10][11][12][13][14][15][16][17][18][19], the spin accumulation on the surface of 3D TIs Bi 2 Se 3 , (Bi x Sb 1−x ) 2 Te 3 , Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 , BiSbTeSe 2 , Bi 2 Te 2 Se and Sb 2 Te 3 , mostly grown by molecular beam epitaxy (MBE) or exfoliated, were electrically measured by the voltage probed with FM contact, where the voltage depends on the projection of SS spin polarization onto the FM magnetization direction.In this work, we detect the current induced spin polarization on the surface of an MBE grown Bi 2 Te 3 thin film using Fe contact deposited on the surface and separated by a thin MgO barrier. We also provide a theoretical estimate of the detected spin signal, i.e., the voltage probed with the FM contact. Previously, the voltage drop measured between a FM and a nonmagnetic (NM) contact placed on the surface of a TI was theoretically calculated either using non-equilibrium Green's function [20] or by solving the transport equations derived from Kubo formalism [21]. Here, we provide a different approach for the derivation of the coupled spin-charge transport differential equation based on quantum kinetic theory [22,23] in the diffusive limit. The experimentally measured spin signal matches well with the theory providing evidence for the spin polarized SS in our TI Bi 2 Te 3 thin film.The SSs of TIs are characterized by spin-momentum helically locked constant energy Fermi contour [1][2][3]. However, due to band-bending near the surface, a 2D electron gas (2DEG) can be formed from the quantum confinement of the bulk states in the band-bending potential with Rashba spin splitting arising from the gradient of the confinement potential [19,[24][25][26], and cannot be neglected a priori. Therefore, we obtain coupled spin and charge transport equations for SSs of a TI as well as a Rashba 2DEG. For ease of analysis, we...

We report magnetoresistance for current flow through iron/topological insulator (Fe/TI) and Fe/evaporated-oxide/TI contacts when a magnetic field is used to initially orient the magnetic alignment of the incorporated ferromagnetic Fe bar, at temperatures ranging from 100 K to room temperature. This magnetoresistance is associated with the relative orientation of the Fe bar magnetization and spin-polarization of electrons moving on the surface of the TI with helical spin-momentum locking. The magnitude of the observed magnetoresistance is relatively large compared to that observed in prior work.

Chromium selenide thin films were grown epitaxially on Al2O3(0001) and Si(111)-(7×7) substrates using molecular beam epitaxy (MBE). Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate a flat smooth film growth along the c-axis, and is very similar to that from a hexagonal surface. X-ray diffraction pattern confirms the growth along the c-axis with c-axis lattice constant of 17.39 Å. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature dependent resistivity. Antiferromagnetic nature of the film with a Néel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior. as observed from neutron diffraction studies [6][7][8][9]. Because of Cr vacancies in alternate layers, the moment associated with Cr atoms located on two different layers are different due to different neighboring environment and this leads to the complexity in the magnetic structure below TN.Previously, chromium selenide systems have been studied to investigate their suitability as thermoelectric material for intermediate-temperature applications [10][11][12][13][14], intermediate temperature power generation [13], electrochemical sensors [15], etc. Several groups have studied the structural, magnetic, electrical and thermoelectric properties of single crystal Cr2+xSe3-x compounds grown using solid state reaction method [10][11][12][13]16], soft chemical and hydrothermal synthesis [15,17,18] and chemical vapor transport method [4,14,[19][20][21]. However, the studies focus mostly on the improvement in thermoelectric properties of transition-metal-doped bulk samples of Cr2Se3. The epitaxial growth and different physical properties of Cr2Se3 thin films are yet to be explored in detail. Molecular beam epitaxy (MBE) is a highly specialized technique used to grow ultra-high purity large-area epitaxial thin films with abrupt interfaces and with precise control over their thicknesses. Compared to other growth techniques, MBE offers greater control to incorporate dopants in thin films. This makes it even more suitable growth method, as the electrical, magnetic and thermoelectric properties of this material system can be largely varied with addition of transition-metal/chalcogen dopants [1, 9-13, 21,22].In this work, we report the epitaxial growth of Cr2Se3 thin films under ultra-high vacuum (UHV) directly on Al2O3(0001) and Si(111)-(7×7) surfaces using MBE. Interestingly, we show that the growth occurs along (001) direction (c-axis). We present the details of growth, structural, electrical and magnetic properties characterized by several in situ and ex situ techniques, e.g., reflection high energy electron diffraction (RHEED), x-ray diffraction (XRD), scanning...