V. P. Ulin

Electrochemical pore formation mechanism in III-V crystals (Part II)

Chemical and electrical processes developing at the semiconductor-electrolyte interface under conditions of anodic polarization were analyzed. It was shown that dense chemisorption coatings are formed on the surface of III–V crystals at voltages of pore formation onset, and a degenerate inversion layer is formed on the semiconductor side. In this case, a drop of the largest part of the applied voltage in the adsorption layer creates the prerequisites for nucleophilic substitution reactions involving chemisorbed anions and coordination-saturated atoms under the crystal surface. The mechanisms of these reactions were considered as applied to sphalerite-structured crystals. The results of experimental studies of the structures and compositions of porous layers in III–V crystals formed in various electrolytes at various polarization voltages are explained on the basis of the obtained concepts.

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.

Unstrained epitaxial InxGa1−xAs films obtained on porous GaAs

Epitaxial layers of InGaAs solid solutions were grown on porous GaAs(100) substrates by liquid-phase epitaxy. A comparison between the compositions and thicknesses of these epitaxial layers with those of layers obtained under the same conditions on normal monolithic GaAs substrates suggests that the crystallization of epitaxial layers on porous substrates may be considered as the growth of free unstrained films.

Organic light-emitting diodes based on polyvinylcarbazole films doped with polymer nanoparticles

The optical and electrical properties of light-emitting diode structures with an active layer based on nanocomposite polyvinylcarbazole (PVK) films doped with nanoparticles of another light-emitting polymer, MEH-PPV, have been studied. It has been established that the size of MEH-PPV particles in the PVK matrix is of the order of 100 nm. The spectral range of photoluminescence of such structures can be changed by varying the ratio of PVK to MEH-PPV. The current-voltage characteristics of composite light-emitting diodes based on PVK: MEH-PPV films indicate p-type conductivity. It has been shown that a decrease in the MEH-PPV nanoparticle concentration in the PVK matrix shifts the threshold values of the bias for the onset of electroluminescence toward smaller values and makes the photoluminescence and electroluminescence spectra more similar to the spectrum of the white light-emitting diode. The influence of the form of the polymer and polymer nanoparticles on the mechanisms of injection and transport of charge carriers and the radiative recombination in the studied structures has been discussed.

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.

Study of processes of self-catalyzed growth of gaas crystal nanowires by molecular-beam epitaxy on modified Si (111) surfaces

The processes of growth of self-catalyzed GaAs crystal nanowires on Si (111) surfaces modified by three different methods are studied. For the technology of production of the GaAs nanowires, molecular-beam epitaxy is used. It is found that, in the range of substrate temperatures between 610 and 630°C, the surface density of nanowires and their diameter sharply increases, whereas the temperature dependence of the nanowire length exhibits a maximum at 610°C. An increase in the temperature to 640°C suppresses the formation of nanowires. The method that provides a means for the fabrication of purely cubic GaAs nanowires is described. A theoretical justification of the formation of the cubic phase in self-catalyzed GaAs nanowires is presented.

Sulfide passivation of InAs(100) substrates in Na2S solutions

Treatment of the InAs(100) surface with a 1M aqueous solution of sodium sulfide (Na2S) is found to result in the removal of a natural oxide layer from this surface with the formation of a continuous chemisorbed passivating layer of sulfur atoms that are coherently bonded to indium atoms of the crystal surface. No etching of the InAs surface in the sulfide solution occurs. The passivated InAs samples are characterized by a multiple increase in the photoluminescence intensity. The sulfide layer is desorbed from the InAs surface at temperatures of ∼400°C. This leads to the formation of a clean In-stabilized (100) surface with a (4 × 2) reconstruction. A simple technique is developed using sulfide passivation for preparing atomically smooth (2 × 4) growth surfaces of the InAs(100) substrates that are suitable for molecular-beam epitaxy of highly perfect layers of compounds based on CdSe.

Growth of cubic GaN by molecular-beam epitaxy on porous GaAs substrates

It is shown that GaN layers can be grown on (100)-and (111)-oriented porous single-crystal GaAs substrates by molecular-beam epitaxy with plasma activation of the nitrogen by an rf electron cyclotron resonance discharge. The resulting undoped epitaxial layers possessed ntype conductivity with a carrier concentration ∼1018. Data obtained by scanning electron microscopy and cathodoluminescence indicate that at thicknesses ∼2000 Å, continuous layers of the cubic GaN modification are obtained regardless of the substrate orientation.

Sulfide passivating coatings on GaAs(100) surface under conditions of MBE growth of 〈II–VI〉/GaAs

Atomic-force microscopy was applied to compare the topographies of naturally oxidized surfaces of GaAs(100) substrates and those substrates treated with aqueous solutions of sodium sulfide in various stages of their preparation for growth of ZnSe-based heterostructures by molecular beam epitaxy (MBE). It was found that annealing of oxidized substrates strongly disrupts the surface planarity and leads to the appearance of pits with density of 1010 cm−2. The pit density can be reduced by two orders of magnitude by treating the substrate surface with an aqueous solution of Na2S. Transmission electron microscopy demonstrated that sulfidation of GaAs substrates makes it possible to reduce the number of stacking faults at the ZnSe/GaAs interface to ∼3×105 cm−2 and, correspondingly, to improve the structural perfection of MBE-grown II–VI layers and heterostructures.

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.