O. S. Lytvyn

Nanostructured Silver Substrates With Stable and Universal SERS Properties: Application to Organic Molecules and Semiconductor Nanoparticles

Nanostructured silver films have been prepared by thermal deposition on silicon, and their properties as SERS substrates investigated. The optimal conditions of the post-growth annealing of the substrates were established. Atomic force microscopy study revealed that the silver films with relatively dense and homogeneous arrays of 60–80-nm high pyramidal nanoislands are the most efficient for SERS of both organic dye and inorganic nanoparticles analytes. The noticeable enhancement of the Raman signal from colloidal nanoparticles with the help of silver island films is reported for the first time.

Raman study of self-assembled SiGe nanoislands grown at low temperatures

We report on Raman scattering measurements on Si-capped Ge quantum structures grown by molecular beam epitaxy on Si(001) at low temperatures. We find a double band structure in the Ge–Ge frequency range for nanoislands grown at substrate temperatures ranging in the interval 300–500 °C. Complementary information has been obtained from performing Raman scattering experiments on annealed samples. The results are interpreted in terms of a model that considers quantum structures (hut clusters) composed of a strained Ge core and a more relaxed SiGe shell.

Native oxide emerging of the cleavage surface of gallium selenide due to prolonged storage

The crystal structure and surface morphology of native oxide emerging on the surface of the (0001) cleavage of undoped and Cd-doped or Dy-doped single crystals of layered GaSe due to prolonged storage in air are studied. The factors that lead to the differences in the outward appearance of oxide films on the surface of undoped (dull surface) and doped samples (transparent films) are analyzed. The results of studies of electrical properties of the 〈gallium selenide〉-〈native oxide〉 systems are reported. It is shown that the films of native oxide on the GaSe surface feature current instability with an N-like current-voltage characteristic. Attention is paid to low values of effective permittivity of native oxide.

Raman Spectroscopy and Electroreflectance Studies of Self-Assembled SiGe Nanoislands Grown at Various Temperatures

SiGe nanoislands grown in a silicon matrix at temperatures of 300 to 600°C are studied using Raman spectroscopy and electroreflectance. For islands grown at relatively low temperatures (300–500°C), phonon bands are observed to have a doublet structure. It is shown that changes in the percentage composition, size, and shape of nanoislands and, hence, in the elastic stresses (depending on the growth temperature of the structures) have a significant effect on the energies of optical electronic interband transitions in the islands. As a consequence, the resonance conditions for Raman scattering also change. It is found that interdiffusion from the silicon substrate and the cover layer (determining the mixed composition of SiGe islands) is of importance even at low growth temperatures of nanostructures (300–400°C).

Magnetic properties and surface morphology of layered In 2 Se 3 crystals intercalated with cobalt

The magnetic properties of layered CoxIn2Se3 crystals electrochemically intercalated with cobalt in an external magnetic field and without a magnetic field and the morphology of the van der Waals surfaces of layers of these crystals have been investigated. It has been found that the ferromagnetic ordering at room temperature is observed only for CoxIn2Se3 crystals intercalated in an external magnetic field. These crystals are nanocomposite materials that consist of a layered matrix and arrays of nanorings and nanowires formed from Co nanocrystals on the van der Waals surfaces of the In2Se3 layers. Cobalt nanocrystals in CoxIn2Se3 crystals have a pyramidal equilibrium shape, which is characteristic of the face-centered cubic crystal structure, and their geometrical sizes are of the order of a few nanometers. The specific features of self-organization of cobalt magnetic nanostructures on the van der Waals surfaces of layered semiconductor crystals during their electrolytic intercalation in a magnetic field and the magnetic properties of these structures have been considered.

Photoinduced etching of thin films of chalcogenide glassy semiconductors

Photoinduced enhancement of the solubility of annealed films made of chalcogenide glassy semiconductors (ChGSs) in amine-based selective etchants has been observed. The etching rate increases with the illumination intensity, and its spectral dependence is correlated with absorption in the film at the absorption edge. It is demonstrated that the new photoinduced effect enables a photolithographic process (including interference lithography) to occur on ChGS layers, annealed at a temperature close to the glass-transition temperature of a chalcogenide glass by simultaneous illumination and selective etching of layers of this kind. A possible mechanism for the photoinduced etching of ChGSs is discussed.

Electrical properties of hybrid (ferromagnetic metal)—(layered semiconductor) Ni/p-GaSe structures

Two-barrier Ni/n-Ga2Se3/p-GaSe structures with nanoscale Ni-alloy grains caused by reactions at the “metal-layered semiconductor” interface were formed after growing Ni layers on the p-GaSe (0001) surface. Current–voltage and capacitance–voltage characteristics of hybrid structures were studied in the temperature range of 220–350 K. The dependence of the impedance spectra on the bias voltage was studied at various temperatures. The frequency dependences of the impedance at high frequencies (f = 106 Hz) are discussed in terms of the phenomena of spin injection and extraction in structures with an ultrathin spin-selective Ni/n-Ga2Se3 barrier and the effects of spin diffusion and relaxation in the semiconductor substrate. The room-temperature phenomena of the Coulomb blockade and negative differential capacitance were detected. These phenomena are explained based on an analysis of transport processes in a narrow region near the “ferromagnetic metal–semiconductor” interface, where nanoscale grains are arranged.

Thermal-resistant TiB x -n-GaP Schottky diodes

The effect of rapid thermal annealing on the parameters of TiBx-n-GaP Schottky barriers and interphase interactions at the TiBx-GaP interface are studied. It is shown that the contact TiBx-n-GaP system features an increased thermal stability without varying the electrical parameters of the Schottky barrier at temperatures as high as 600°C.

The influence of TiB2-thin film thickness on metal–GaAs structural characteristics

The X-ray diffraction and Atomic Force Microscope investigations of TiB 2/GaAs as-produced and annealed device structures has been carried out. The samples were obtained by magnetron sputtering on previously photon cleaned Czochralski-grown (001) GaAs substrates doped by Te up to concentration 10 18 cm -3 . The magnetron sputtering was carried out in the argon atmosphere at pressure in the chamber 5·10 -3 torr. The currents of sputtering were 0.3 and 0.4A, and thickness of TiB 2-films was from 10 nm up to 50 nm.. The samples annealing was carried out in a stream of hydrogen in the furnace at temperatures 400, 600, and 800 °C during 1 minute with heating velocity of 1800°C/min. It was shown that film thickness and magnetron sputtering current determines the film surface and interface structural parameters as well as a fe atures of processes of structural relaxation under short-term thermal annealing.

Surface structure of unoxidized and oxidized Bi2Se3 crystals

The structure and topography of unoxidized and oxidized surfaces of Bi2Se3 layered crystals have been studied by X-ray diffraction and atomic force microscopy. Air oxidation at 450°C has been shown to lead to the formation of Bi2O3 film. The structure and unit-cell parameters of the semiconductor and oxide have been determined. The surface of cleaved unoxidized Bi2Se3 has been found to be covered with nanostructures in the form of “hillocks.” They range widely in lateral and vertical dimensions and are randomly distributed over the sample surface. The surface oxide nanostructures have the form of islands of arbitrary shape. The oxide crystallites form ridges up to 2000 nm in height.