D. A. Komarov

Use of radiation effects for a controlled change in the chemical composition and properties of materials by intentional addition or substitution of atoms of a certain kind

This study is a continuation of works [1–12] dealing with the field developed by the authors, namely, to widen the possibilities of radiation methods for a controlled change in the atomic composition and properties of thin-film materials. The effects under study serve as the basis for the following two methods: selective atom binding and selective atom substitution. Such changes in the atomic composition are induced by irradiation by mixed beams consisting of protons and other ions, the energy of which is sufficient for target atom displacements. The obtained experimental data demonstrate that the changes in the chemical composition of thin-film materials during irradiation by an ion beam of a complex composition take place according to mechanisms that differ radically from the well-known mechanisms controlling the corresponding chemical reactions in these materials. These radical changes are shown to be mainly caused by the accelerated ioninduced atomic displacements in an irradiated material during irradiation; that is, they have a purely radiation nature. The possibilities of the new methods for creating composite structures consisting of regions with a locally changed chemical composition and properties are demonstrated for a wide class of materials.

Controlled modification of superconducting properties of NbN ultrathin films under composite ion beam irradiation

In this work, the results of studying the microstructure and superconducting properties of ultrathin films on the basis of NbN in the initial state and after modification by being subjecting to composite ion beam irradiation with the energy ∼(1–3) keV are presented. HRTEM analysis showed that the initial films on the sapphire substrate in orientation “c-cut” are characterized by a grain size essentially exceeding the film thickness, while on the other substrates the size of grains corresponds to the thickness of film. Using XPS analysis, it was shown that in the initial films the atomic ratio of Nb and N is 0.51/0.49, respectively, the percentage of oxygen being lower than 5%. For ultrathin films 5 nm in thickness, the critical temperature of transit to superconducting state (Tc) is found to be ∼13.6 K and the density of critical current is jc ∼ 8MA/cm2. In the work it is experimentally determined that the irradiation of NbN films by composite ion beams leads to the controlled modification of its superconducting properties due to the process of selective substitution of nitrogen atoms on the oxygen atoms.

Formation of metal nanowires using techniques of the selective removal of atoms and investigation into the electric properties of metal nanowires

The electric properties of films of the metals reduced from oxides using the selective removal of atoms (SRA) under proton irradiation were investigated. The electric characteristics of SRA films were found to correspond to characteristics of the deposited films of pure metals. The possibility of creating nanowires in a matrix of initial oxide under irradiation through a PMMA mask was shown. Individual nanowires and pairs of nanowires from bismuth were created. The electric characteristics of both nanowires and leak currents between closely located nanowires were measured. The possibility of creating two isolated nanowires at a distance of 70 nm was shown.

Development of ion-beam technique for manufacturing silicon nanowires

A new technique for manufacturing silicon nanowires on the surface of a conventional silicon wafer using ion-beam irradiation through a lithographic mask has been proposed. The conditions needed for synthesizing silicon oxide using the irradiation of the silicon substrate by protons with an energy of about 1 keV have been studied. The possibility of synthesizing silicon oxide in the region of the geometric shadow under the monocrystalline silicon nanowire has been demonstrated.

Formation of monocrystalline silicon nanowires using low-energy ion irradiation

In this work we have proposed a technique which can be used to fabricate monocrystalline silicon nanowires on a silicon wafer surface by forming specially designed extended structures with a negative slope angle of side walls and their subsequent oxidation by irradiation with low-energy oxygen ions at a temperature of 400°C. The shape and sizes of the structures were chosen first so that the process of oxidation would not modify the central part of the nanowire and keep the monocrystallinity and doping level of the initial silicon and, second, to provide the electrical insulation of the nanowire from the wafer. The dependence of the oxidation depth on the temperature and duration of plasma irradiation as well as the electrical properties of the resulting oxide are studied.

Study of the evolution of the atomic composition of thin NbN films under irradiation with mixed ion beams by methods of electron energy loss spectroscopy

The variation in the atomic composition of ultrathin NbN films under irradiation by mixed ion beams to a doze of 4 dpa (for nitrogen) is experimentally studied by methods of electron energy loss spectroscopy with a transmission electron microscope in the transmission scan mode on cross-cut samples. The behavior of the substitution of nitrogen atoms by oxygen atoms has been established; it is characterized by changing the composition of the conducting part of the film from NbN to NbNO.

The use of ion irradiation for converting superconducting thin-film NbN into niobium oxide Nb2O5

It is shown experimentally that the use of ion irradiation allows one to convert superconducting thin-film niobium nitride into dielectric niobium oxide in a controllable manner. The conversion of NbN into Nb2O5 throughout the entire thickness of the film is demonstrated via transmission electron microscopy and layer-by-layer XPS analysis. This conversion is followed by a corresponding increase in the film thickness with no signs of sputtering and thus provides the possibility of forming dielectric regions of the required sizes and shapes in the process of fabrication of various functional cryoelectronic elements.

Electron Energy-Loss Spectroscopy Study of the Change in the Free-Electron Density in Thin Superconducting NbN Films under Ion-Beam Irradiation

The change in the free-electron density in ultrathin (5 nm) superconducting NbN films in the initial state and after irradiation by O+ ions to doses of (0.1–0.9) × 1017 cm–2 has been studied by electron energy-loss spectroscopy (EELS). The analysis has been performed on cross section samples prepared by the focused ion beam method, using plasmon oscillations with energies up to 50 eV. The radiation-induced replacement of nitrogen atoms with oxygen atoms in niobium nitride is found to change the electrical properties of the material, which leads to a decrease in the free-electron density with an increase in the irradiation dose.

Investigation of radiation-induced transformations in thin NbN films by analytical electron microscopy

This work demonstrates implementation of low energy electron energy loss technique (EELS) in scanning transmission electron microscopy (STEM) to investigate the changes of free electron density at room temperature in ultra-thin NbN films under composite ion beam irradiation up to the deses of ~3 d.p.a. for nitrogen atoms. It was found the constant value of the free electron density ~1.6 1029 m-3 in this dose range while the irradiated material was characterized by metal type of electrical conductivity.