N. S. Sokolov

Study of band offsets in CdF2/CaF2/Si(111) heterostructures using x‐ray photoelectron spectroscopy

The valence band offsets at the heterointerfaces of the CdF2/CaF2/Si(111) structure grown by molecular beam epitaxy were evaluated using x‐ray photoelectron spectroscopy, and the energy band diagram of this heterostructure was proposed. It was found that the interface of CdF2/CaF2 has large conduction band offset: 2.9 eV, and the energy level of CdF2 conduction band edge is below that of Si.

CdF2/CaF2 Resonant Tunneling Diode Fabricated on Si(111)

We propose use of a new material, CdF2, and a heterostructure composed of CdF2 and CaF2 for applications of Si-based quantum effect devices. The optimum growth temperature for each layer of a CaF2/CdF2/CaF2 heterostructure grown by molecular beam epitaxy on a Si(111) substrate was determined. Resonant tunneling diodes consisting of this heterostructure on Si were fabricated and negative differential resistance whose P/V current ratio was 24 at maximum was detected at room temperature.

Fluoride layers and superlattices grown by MBE on Si(111): dynamic RHEED and Sm2+ photoluminescence studies

Abstract The oscillations of reflection high-energy electron diffraction intensity during CaF 2 and SrF 2 epitaxial growth on the (111) surface of silicon and the fluorides have been investigated. From these studies the critical thickness of the fluoride pseudomorphic growth mode was measured for different growth conditions. These data allowed us to grow strongly strained coherent fluoride layers and superlattices. Studies of Sm 2+ ion photoluminescence in these fluoride structures at liquid-helium temperature have been shown to be useful for characterization of the crystalline quality of the films and for the measurement of elastic strain. High quantum yield of the luminescence enabled us to explore doped layers as thin as several monolayers.

Sm2+ Photoluminescence and X-Ray Scattering Studies of A- and B-Type Epitaxial CaF2 Layers on Si(111)

The crystalline quality and strain in molecular beam epitaxy (MBE)-grown CaF2:Sm/Si(111) structures with type-A (nonrotated) and type-B (rotated by 180° in the plane of the interface) epitaxial relationships were characterized by the bivalent samarium photoluminescence technique. It was found from the analysis of X-ray crystal truncation rod (CTR) scattering that the interface spacings along the [111] direction are (1.30±0.03)a0 for annealed samples with the type-A and (1.45±0.02)a0 with the type-B interface, where a0 is the Si(111) spacing. It was also confirmed that Ca atoms occupy T-sites at the interface (atop the atoms of the first Si layer) for the type-A interface. Both the photoluminescence and X-ray measurements indicate that the crystalline quality of type-A CaF2/Si(111) structures is inferior compared with that of type-B structures.

Thin yttrium iron garnet films grown by pulsed laser deposition: Crystal structure, static, and dynamic magnetic properties

Pulsed laser deposition has been used to grow thin (10–84 nm) epitaxial layers of Yttrium Iron Garnet Y3Fe5O12 (YIG) on (111)–oriented Gadolinium Gallium Garnet substrates at different growth conditions. Atomic force microscopy showed flat surface morphology both on micrometer and nanometer scales. X-ray diffraction measurements revealed that the films are coherent with the substrate in the interface plane. The interplane distance in the [111] direction was found to be by 1.2% larger than expected for YIG stoichiometric pseudomorphic film indicating presence of rhombohedral distortion in this direction. Polar Kerr effect and ferromagnetic resonance measurements showed existence of additional magnetic anisotropy, which adds to the demagnetizing field to keep magnetization vector in the film plane. The origin of the magnetic anisotropy is related to the strain in YIG films observed by XRD. Magneto-optical Kerr effect measurements revealed important role of magnetization rotation during magnetization reversa...

Electrical characterization and modeling of the Au/CaF2/nSi(111) structures with high-quality tunnel-thin fluoride layer

Au/CaF2/nSi(111) structures with 4–5 monolayers of epitaxial fluoride are fabricated and electrically tested. The leakage current in these structures was substantially smaller than in similar samples reported previously. Simulations adopting a Franz-type dispersion relation with Franz mass of mF∼1.2m0 for carriers in the forbidden band of CaF2 reproduced the measured current-voltage curves quite satisfactorily. Roughly, these curves could also be reproduced using the parabolic dispersion law with the electron mass of me=1.0m0, which is a material constant rather than a fitting parameter. Experimental facts and their comparison to modeling results allow qualification of the crystalline quality of fabricated structures as sufficient for device applications.

High-quality CdF2 layer growth on CaF2/Si(111)

Abstract Epitaxial CdF 2 was grown on the coherent or relaxed CaF 2 /Si(111) by molecular beam epitaxy. The structures were characterized by reflection high-energy electron diffraction, Rutherford backscattering spectroscopy and double-crystal X-ray diffraction. It was shown that the crystallinity of the CdF 2 layer grown on the CaF 2 /Si(111) structure, where the CaF 2 was coherently grown on the Si, was quite good so as to obtain 51 arc sec of full width at half maximum of X-ray diffraction.

Molecular beam epitaxy of CdF2 layers on CaF2(111) and Si(111)

Cadmium fluoride single crystal layers have been grown on CaF2/Si(111) or Si(111) substrates by molecular beam epitaxy. The structures are expected to have attractive electronic properties. The growth was monitored by reflections high energy electron diffraction (RHEED) techniques. A distinct (√3×√3) R30° superstructure has been observed on the CdF2(111) surface at growth temperatures below 150 °C. RHEED intensity oscillations indicate a two‐dimensional growth mode of CdF2.

Low-relaxation spin waves in laser-molecular-beam epitaxy grown nanosized yttrium iron garnet films

Synthesis of nanosized yttrium iron garnet (Y3Fe5O12, YIG) films followed by the study of ferromagnetic resonance (FMR) and spin wave propagation in these films is reported. The YIG films were grown on gadolinium gallium garnet substrates by laser molecular beam epitaxy. It has been shown that spin waves propagating in YIG deposited at 700 °C have low damping. At the frequency of 3.29 GHz, the spin-wave damping parameter is less than 3.6 × 10−5. Magnetic inhomogeneities of the YIG films give the main contribution to the FMR linewidth. The contribution of the relaxation processes to the FMR linewidth is as low as 1.2%.

Proximity effects and exchange bias in Co/MnF2(111) heterostructures studied by x-ray magnetic circular dichroism

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).