Correlation between luminescent properties and structure organization in AlGaN/GaN superlattices annealed after erbium ion implantation

The evolution of the structure organization of MOCVD-grown AlGaN/GaN superlattices subjected to erbium ion implantation with an energy of 1 MeV and dose of 3 × 10¹⁵ cm^?2 and subsequent annealing is correlated with their photoluminescent properties. The structure organization is quantitatively estimated using parameter Δ (degree of violation of local symmetry), which is found via multifractal analysis of surface morphology patterns obtained with atomic force microscopy. It is shown that the implantation not only causes Ga segregation on the surface, but also changes the structure organization, which shows up in the finer grain structure compared with the starting one and disordering, as well as in an increase in Δ. As the annealing temperature rises from 700 to 800°C, Δ declines, indicating that the structure organization is improved, and the intensity of the dominating photoluminescence peak due to Er³⁺ ions (1.542 μm) grows. With a further increase in the annealing temperature to 1050°C, the structure organization degrades, domains get larger, voids 100–200 nm deep form, and the photoluminescence intensity drops. The formation of voids during high-temperature annealing is also substantiated by data for 230-keV proton scattering. It is thus established that the improvement of the superlattice structure organization activates erbium and causes the erbium-ion-related luminescence intensity to grow.     

Molecular-beam epitaxy and properties of heterostructures with InAs nanoclusters in an Si matrix

The formation of heteroepitaxial structures with InAs nanoclusters in an Si matrix during molecular-beam epitaxy and variations in the parameters of these structures due to thermal annealing are investigated by reflection high-energy electron diffraction (RHEED), medium-energy ion scattering (MEIS), and scanning electron microscopy (SEM). It is demonstrated that the deposition of InAs onto the Si(100) surface at certain temperatures brings about the formation of tetrahedral nanopyramids with {111} orientation of the lateral faces. It is revealed that InAs nanoislands can be epitaxially overgrown with silicon, which leads to gradual smoothing of the three-dimensional relief. After annealing under vacuum, the Si/InAs/Si(100) structures are stable at tem-peratures up to 700°C.     

Light-induced transformation of C60 films in the presence and absence of oxygen

The composition and structure of C₆₀ fullerite films prepared by discrete evaporation in quasiclosed volume, as well as their changes induced by laser irradiation, have been studied by ellipsometry and Rutherford backscattering. The starting film has a 150-Å thick stable top layer and a carbon to oxygen ratio of 10:1. Exposure of a film both to vacuum and to air results in formation of an insoluble photo-transformed phase, but in the second case the change in the refractive index implies the appearance of compounds with oxygen. The material does not undergo complete polymerization, although all structural changes cease at an irradiation dose of 10₄ photons per fullerene molecule. Treatment of the polymerized phase with organic solvents produces a porous structure, with the voids totaling 48% in the case of exposure in vacuum, and 30% when exposed in air.     

Study of YBa2Cu3O7−x films at various stages of their growth by medium-energy ion scattering

The initial stages in the formation and growth of yttrium-barium cuprate films have been studied in the course of magnetron sputtering of a ceramic target by combining medium-energy-ion scattering (MEIS) and scanning-electron microscopy. The growth mechanisms of YBa₂Cu₃O_7−x films on MgO and substrates having the perovskite structure, SrTiO₃ and LaAlO₃, at deposition temperatures of 700–780 °C and (Ar+O₂) pressure of ∼70 Pa were found to be essentially different. Simulation of MEIS spectra (H⁺ or He⁺ ions with initial energies of 150–250 keV) and comparison of these results with experimental data revealed that in the first case a film forms from pyramid-shaped islands and, in the second, it grows by a two-dimensional layer-by-layer process starting practically with the first monolayer. For the island mechanism, MEIS permitted determination of the substrate surface coverage and showed the growth of the YBa₂Cu₃O_7−x phase to be paralleled by formation of epitaxial nuclei of a Cu₂O phase. After the first, initial stage in the film formation, the second stage, regular growth within reduced thicknesses of 7–15 nm, sets in. This stage is characterized by a practically complete coverage of the substrate and a stable composition. The third stage, regular growth, of an apparently helical nature, was observed to set in at thicknesses above 100 nm. In this stage, the quality of film structure in the bulk and on the surface was found to be somewhat inferior to that of YBa₂Cu₃O_7−x single crystals and did not depend on substrate type. Fiz. Tverd. Tela (St. Petersburg) 41, 588–595 (April 1999)     

Combined MEIS/TEM study of structural defects in Si implanted with a high dose of 100 keV Si ions

Medium-energy ion scattering and transmission electron microscopy have been used to study the structural perfection of a Si single crystal implanted with 100 keV Si ions at a dose of 1×10¹⁷ cm^?2, which exceeds the amorphization threshold by two orders of magnitude. The implantation of Si ions was found to produce a high density of extended defects without amorphization of the Si layer. The increasing depth dependence of the full width at half-minimum of the dip in angular scans of backscattered protons, was observed in a Si layer containing a high density of extended defects, in contrast to the decreasing dependence in the perfect Si crystal.     

Electro- and Magnetotransport near a LaAlO3/SrTiO3 Interphase Boundary

Comparison of the channel spectra of medium energy ion scattering, visualized for LaAlO₃/(001)SrTiO₃ heterostructures with a thickness of the lanthanum aluminate layer of one to six unit cells, indicates that the lanthanum aluminate layer grows coherently on a TiO₂-terminated surface of a strontium titanate substrate. The resistance of the interphase boundary in the heterostructure with a thickness of the LaAlO₃ layer of six unit cells decreased with temperature. At T < 100 K, the positive magnetoresistance and Hall mobility of electrons increased sharply with decreasing temperature.

     

Initial stages of the growth of barium strontium titanate films on a semi-isolating silicon carbide substrate

The initial stages of the growth of ferroelectric barium strontium titanate films on single-crystal silicon carbide substrates have been studied for the first time. The choice of a substrate with high thermal conductivity has been due to the possibility of applying these structures in powerful microwave devices. The temperature ranges separating the mechanism of the surface diffusion of deposited atoms from the diffusion via a gaseous phase during the growth of multicomponent films have been determined. The studies show that the mass transfer by means of surface diffusion leads to the formation of small-height nuclei that cover a large area of the substrate, whereas the mass transfer via a gaseous phase leads to the formation of a “columnar” islandtype structure with small percentage of covering the substrate and larger island heights.     

Aerosol deposition of detonation nanodiamonds used as nucleation centers for the growth of nanocrystalline diamond films and isolated particles

The aerosol deposition of detonation nanodiamonds (DNDs) on a silicon substrate is comprehensively studied, and the possibility of subsequent growth of nanocrystalline diamond films and isolated particles on substrates coated with DNDs is demonstrated. It is shown that a change in the deposition time and the weight concentration of DNDs in a suspension in the range 0.001–1% results in a change in the shape of DND agglomerates and their number per unit substrate surface area N _s from 10⁸ to 10¹¹ cm⁻². Submicron isolated diamond particles are grown on a substrate coated with DND agglomerates at N _s ≈ 108 cm⁻² using microwave plasma-enhanced chemical vapor deposition. At N _s ≈ 10¹⁰ cm⁻², thin (∼100 nm) nanodiamond films with a root-mean-square surface roughness less than 15 nm are grown.     

Study of intercalated fullerene films by medium-energy ion scattering

This paper reports on a study of films of fullerenes and their halogen compounds, deposited on mica and silicon, by medium-energy ion scattering (MEIS). The film thickness nonuniformity was determined. A considerable increase in the halogen concentration was found at the interface, which is associated with the specific film deposition conditions. It was demonstrated that the composition of individual layers of a film can be determined using MEIS.     

Fast-ion channeling in fullerene crystals

The motion of H⁺ and He⁺ ions with energies of 230 and 2000 keV in C₆₀ and K₃C₆₀ crystals was calculated by the Monte Carlo method. Ion channeling was shown to occur in the 〈100〉 and 〈111〉 directions. The main parameters characterizing channeling were determined. Medium-energy ions were found to be preferable in the detection of channeling in C₆₀ films.