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Laser molecular-beam epitaxy has been used to form Co40Fe40B20 layers on Bi2Te3 topological insulator substrates, and their growth conditions have been studied. The possibility of growing epitaxial ferromagnetic layers on the surface of a topological insulator is demonstrated for the first time. The CoFeB layers have a body-centered cubic crystal structure with the (111) crystal plane parallel to the (0001) plane of Bi2Te3. 3D mapping in the reciprocal space of high-energy electron-diffraction patterns made it possible to determine the epitaxial relationships between the film and the substrate. [ABSTRACT FROM AUTHOR]

We study molecular beam epitaxy growth, morphology, crystal structure, and magnetic properties of Co nanoislands on CaF2/Si(111) surface. In order to have a full appreciation of complex growth kinetics at different stages, a comprehensive study of Co growth on CaF2 is carried out by atomic force, scanning electron, and transmission electron microscopies in the direct space, as well as by x-ray and electron diffraction in the reciprocal space. These experimental data are complemented by theoretical modeling. Magnetic properties are characterized by magneto-optical Kerr effect and superconducting quantum interference device magnetometries. Key effects influencing the Co growth on fluorite are addressed, including the sticking probability, the preferential nucleation sites, the size and shape time evolution, the dependence of Co morphology on temperature and Co exposure, and the coalescence mechanism. The two-stage deposition technique is developed, whereby the low-temperature seeding stage is used to facilitate Co nucleation, and the follow-up high-temperature deposition yields Co particles with high crystalline quality. Our results enable precise control over the resulting morphology, spatial ordering, and crystal structure affecting the magnetic properties. In particular, it is demonstrated that the transformation from dense to isolated Co nanoparticles leads to the change of the in-plane and out-of- plane magnetic anisotropy and also the sign of polar and longitudinal magneto-optical Kerr effects. We study molecular beam epitaxy growth, morphology, crystal structure, and magnetic properties of Co nanoislands on CaF2/Si(111) surface. In order to have a full appreciation of complex growth kinetics at different stages, a comprehensive study of Co growth on CaF2 is carried out by atomic force, scanning electron, and transmission electron microscopies in the direct space, as well as by x-ray and electron diffraction in the reciprocal space. These experimental data are complemented by theoretical modeling. Magnetic properties are characterized by magneto-optical Kerr effect and superconducting quantum interference device magnetometries. Key effects influencing the Co growth on fluorite are addressed, including the sticking probability, the preferential nucleation sites, the size and shape time evolution, the dependence of Co morphology on temperature and Co exposure, and the coalescence mechanism. The two-stage deposition technique is developed, whereby the low-temperature seeding stage is used to facilitate Co nucleation, and the follow-up high-temperature deposition yields Co particles with high crystalline quality. Our results enable precise control over the resulting morphology, spatial ordering, and crystal structure affecting the magnetic properties. In particular, it is demonstrated that the transformation from dense to isolated Co nanoparticles leads to the change of the in-plane and out-of- plane magnetic anisotropy and also the sign of polar and longitudinal magneto-optical Kerr effects. [ABSTRACT FROM AUTHOR]

Owing to the optimized growth technology of the 1-2 nm calcium fluoride films on n-(111)-silicon, metal/ tunnel-insulator/semiconductor phototransistors have been fabricated by the molecular beam epitaxy at the temperature 250°C. The characteristics of these transistors were measured in a wide range of voltages, and the proofs for current gain were found throughout the investigated range. The gain value exceeds 103 approaching the theoretically estimated value in this system. The stability and reproducibility of the device characteristics were satisfactory. The results support the candidacy of calcium fluoride for being a vital dielectric in silicon-based functional electronics. [ABSTRACT FROM AUTHOR]

Using molecular-beam epitaxy, Au/CaF2/ n-Si(111) structures were fabricated that exhibit lower currents at a given fluoride film thickness (1.5–2 nm) than those of all similar structures studied. At a positive voltage at the metal, the current is in agreement with that calculated within the model with conservation of the transverse component of the wave vector during tunneling transport. Relative contributions of electron and hole components were analyzed for forward and reverse biases. The effect of the nonuniform distribution of the insulator thickness over the area on measured currents was estimated. The thin CaF2 layers that were grown are potentially applicable as barrier layers in various devices of functional electronics. [ABSTRACT FROM AUTHOR]

The magnetic structure of MnF2 is determined by the neutron diffraction method in the metastable orthorhombic phase grown in the form of thin (∼1 μm) film on a CaF2 buffer layer by the molecular beam epitaxy method. The magnetic moments of Mn++ form a simple two-sublattice antiferromagnetic structure and are directed along the c crystallographic axis parallel to the film plane. Using the temperature dependence of magnetic reflections, a Néel temperature of 67.19(7) K and a critical index of 0.50(2), which corresponds to the mean field approximation, are determined. [ABSTRACT FROM AUTHOR]

Current-voltage characteristics of epitaxially grown Au/CaF2/Si(111) metal-insulator-semiconductor structures with thin (1.5–6 nm) pseudomorphic fluoride layer have been studied. It was found that CaF2 films in these structures are of better insulating quality than those in the devices reported previously. Typical breakdown field for the fluorite layers was about 8×106 V/cm and the tunnel current did not exceed the values predicted by simulations with realistic parameters. [ABSTRACT FROM AUTHOR]

Resonant tunneling diodes were implemented on Ge quantum dots fabricated using StranskiKrastanov growth mode on CaF[sub 2] matrix, lattice matched to Si(111) substrates. The negative differential conductance and conductance oscillations due to hole resonant tunneling through the zero-dimensional states of Ge quantum dots are clearly observed at room temperature. From the period of conductance oscillations, the energy separations between the states of the quantum dots with different sizes are estimated to be 40-50 meV (i.e., >kT=26meV at T=300 K). [ABSTRACT FROM AUTHOR]

Heterostructures of the CdF[sub 2]:Er/CaF[sub 2]/CaF[sub 2](111) type were grown by the method of molecular beam epitaxy. Doping with erbium was performed, for the first time in materials of this type, by subliming the metal from an effusion cell immediately during the cadmium fluoride layer growth. A special procedure of CaF[sub 2] substrate preparation for epitaxy was developed. Measurements of the lateral conductivity of the heterostructures by a two-point-probe technique showed that, depending on the dopant (erbium) concentration, the resistivity ranges from 2.5 × 10[sup 5] to 50 Ω cm. [ABSTRACT FROM AUTHOR]

Cobalt nano-structured ultrathin films were grown on orthorhombic MnF(2) by molecular beam epitaxy on CaF(2) epitaxial layers deposited on Si(111) substrates. The Co film was grown at room temperature. It was found to be polycrystalline, forming nano-islands with height≈diameter≤10 nm. X-ray absorption evidences the chemical stability of the Co/MnF(2) interface. Remarkably, x-ray magnetic circular dichroism (XMCD) demonstrates that the Co induces a net magnetization on the Mn ions close to the interface. The magnetic moments of these Mn ions couple antiparallel to the Co and rotate upon field reversal following the magnetization of the Co both below and high above the Néel temperature of MnF(2) (T(N) = 67 K). The density of coupled Mn moments is found to be temperature dependent, with an equivalent thickness of ~1.5 MnF(2) monolayers at 20 K, decreasing to about ~0.5 ML as the temperature is raised to 300 K. Interestingly, the intensity of the Mn XMCD signal appears to be related to the coercivity of the Co layer. This behavior is interpreted in terms of the competition between thermal fluctuations, exchange coupling between Co and Mn at the interface and, at low temperature, the antiferromagnetic order in MnF(2).