A. V. Bobyl

Porous silicon and its applications in biology and medicine

Development of safe container materials for targeted and controlled drug delivery to the right site in the body is one of the most important aspects of modern biotechnologies. In the last decade, a significant progress has been achieved in the study of nanostructured drug carriers, but the use of many nanomaterials is fraught with the enormous risk because of their high toxicity. The real breakthrough became the use of porous silicon, which has such important properties as biocompatibility, bioavailability, and biodegradability, which makes it possible to use it for solving a wide range of biological and medical problems in the field of diagnosis and treatment of diseases, implantology, and biomolecular screening.

Specific features of epitaxial-film formation on porous III–V substrates

Porous GaAs (100) and (111) substrates with nanostructured (∼10 nm) surface profiles are obtained in which pores branching in the 〈111〉 direction form a dense network with a volume density of ∼60% under the surface at a depth of ∼(50–100) nm. The surface of the substrates and the structure of GaSb layers grown on these substrates are studied. A decrease of 22% in the lattice-parameter mismatch at the GaSb/GaAs(porous) interface compared with that at the GaSb/GaAs(monolithic) interface is observed. Ideas about the chemical mechanisms of pore formation in III–V crystals are developed, and relations connecting the structure of porous layers to the composition of electrolytes and anodization conditions are established. It is shown that the dependence of the layers’ growth rate on lattice elastic strain can be conducive to an enhanced overgrowth of pores and to a transition to planar growth.

Deep microrelief measurement and stereo photography in scanning electron microscopy

Fundamental differences between standard photography and micro-object imaging in a scanning electron microscope are analyzed. Basic ways of taking stereograms with a scanning electron microscope by the object rotation method and the observing technique are described. It is shown that stereo photography correctly estimates the spatial arrangement of features in objects with an intricate microrelief and makes it possible to very accurately calculate the depth of this relief (which sometimes exceeds the field of vision at large magnifications) in wide limits (from 0.5 to 100.0 μm). Using the developed surface of Li4Ti5O12 oxide and GaAs whiskers as examples, a stereographic computational method based on linear measurements of the same area on the object before and after rotation is demonstrated.

Study of the light-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters

The photo-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters with an initial efficiency of 10.4% under light flux densities of 1 and 10 kW m−2 (AM1.5G) is studied. It is shown that the stabilized state is reached after 500 h of exposure to the standard light-flux density and after 300 min at a flux 10 times higher in density. In both cases, the efficiency decreases by 1.2–1.4 abs. %. The experimentally measured spectral and current-voltage characteristics of the photovoltaic converters are used to determine the nonequilibrium carrier lifetimes and to calculate variation dependences of the dangling-bond concentration in i-α-Si:H and i-μc-Si:H layers. The dependences are approximated in terms of the floating-bond model. The calculated dangling-bond concentrations after various exposure times are used to simulate the dependences of the photovoltaic-converter parameters on light exposure. The results obtained show good coincidence between the simulated degradation rates of the current and efficiency of a tandem photovoltaic cell and the experimental data.

Temperature Dependence of the Contact Resistance of Ohmic Contacts to III-V Compounds with a High Dislocation Density

A new mechanism describing the rise in the contact resistance ρc of ohmic contacts to n-n+-n++-GaAs(GaP, GaN, InP) structures with increasing measurement temperature T, experimentally observed in the temperature range 100–400 K, is suggested on the basis of a theoretical analysis of the temperature dependence of ρc. Good agreement between the experimental and theoretical ρc(T) dependences is obtained and explained for a case where there is a high density of dislocations (on which metallic shunts are localized) in the near-contact region of the semiconductor.

Method for optimizing the parameters of heterojunction photovoltaic cells based on crystalline silicon

An approach is proposed to calculate the optimal parameters of silicon-based heterojunction solar cells whose key feature is a low rate of recombination processes in comparison with direct-gap semiconductors. It is shown that at relatively low majority-carrier concentrations (Nd ∼ 1015 cm–3), the excess carrier concentration can be comparable to or higher than Nd. In this case, the efficiency η is independent of Nd. At higher Nd, the dependence η(Nd) is defined by two opposite trends. One of them promotes an increase in η with Nd, and the other associated with Auger recombination leads to a decrease in η. The optimum value Nd ≈ 2 × 1016 cm–3 at which η of such a cell is maximum is determined. It is shown that maximum η is 1.5–2% higher than η at 1015 cm–3.

Analysis of the possibility of high-efficiency photovoltaic conversion in tandem heterojunction thin-layer solar cells

The possibility of creating tandem heterojunction-with-intrinsic-thin-layer (HIT) solar cells possessing photovoltaic conversion efficiency greater than that of the best existing single-junction HIT structures is analyzed. It is established that, because of small carrier lifetimes and high degrees of compensation, the use of amorphous silicon in tandem HIT cells cannot provide for record high conversion efficiency. Key parameters of the material for a widegap p-n junction on the frontal side of tandem solar cells are determined that will allow a photovoltaic conversion efficiency above 25% under AM1.5 conditions to be reached.

Surface of porous silicon under hydrophilization and hydrolytic degradation

Analysis of the IR absorption spectra is used to trace changes in the chemical composition of surface layers of mesoporous silicon crystals during the course of their hydrophilization via oxidation in hydrogen-peroxide solutions and as a result of indirect, by intermediate bromination, and direct nucleophilic substitution of bound hydrogen with a hydroxyl. The spontaneous process of atomic rearrangement with the transfer of oxygen atoms from adsorbed OH groups to lower-lying atomic layers of the crystalline skeleton, which yields Si-H bonds on its surface: -Si-Si-OH→-Si-O-Si-H, is revealed. The sequence of elementary processes that facilitate the hydrolytic degradation of porous silicon in weakly alkaline media is considered. The role played by the deformation of chemical bonds in a porous crystal in promoting the hydrolysis of silicon is noted. It is shown that the surface modification of porous silicon via bromination and subsequent treatment in water makes it possible to substantially increase the rate of its hydrolytic degradation in weakly alkaline solutions that are similar in pH values to biological fluids.

Simulation of daytime variations in the characteristics of a-Si:H solar cells

The time dependences of the key characteristic of a-Si:H solar cells over daylight hours are theoretically simulated. The model is used to calculate the time dependences for an arbitrary geographic latitude in the interval 30°–60° and arbitrary day of the year. The calculated results are illustrated for a geographic latitude of 45° and equinox. The relative variations in the characteristics of the a-Si:H solar cells are valid with a relatively high accuracy for the solar cells based on alternative semiconductors provided that their efficiency ranges form 7 to 20%.

Investigation of the atomic, crystal, and domain structures of materials based on X-ray diffraction and absorption data: A review

Ways to gain and analyze experimental data obtained by X-ray techniques used in material examination are described. Emphasis is on the methods of extended X-ray absorption fine structure, X-ray diffraction, and X-ray low-angle scattering.