С наступающим 2018 годом!

Коллектив кафедры атомной физики, физики плазмы и микроэлектроники поздравляет сотрудников и студентов Московского Университета с Новым Годом и желает творческих, научных и учебных успехов в наступающем году!

Дабы не подводить сухих итогов научной работы (с которыми можно ознакомиться в системе ИСТИНА), мы предлагаем подборку из 15 статей, опубликованных сотрудниками кафедры в высокорейтинговых журналах в 2017 году.

• Actinometry of o, n and f atoms / D. V. Lopaev, A. V. Volynets, S. M. Zyryanov et al. // Journal of Physics D - Applied Physics. — 2017. — Vol. 50. — P. 075202. [ DOI ]
  The applicability of actinometry for measuring the absolute concentration of O, N and F atoms in discharge plasma was studied. For this purpose, concentrations of these atoms were measured downstream of an ICP plasma by means of the actinometry method and of appearance potential mass-spectrometry (APMS). Comparison of the results showed good agreement between the two methods. Since the excitation cross sections of electron states O(3p 3P) and O(3p 5P) applied in actinometry are well tested, this allows using the APMS method for absolute calibration of the theoretical excitation cross sections for N and F atoms. As a result, total excitation cross sections sigma(total)of the atomic levels N(3p 4Po), F(3p 2Po)and F(3p 4Do) have been obtained for the first time. Since different types of electron energy distribution function (EEDF) were observed (Maxwellian, bi-Maxwellian and Druyvesteyn)the influence of these possible EEDF types on actinometric coefficients CX,Ar (X = O, N, F), that link the ratio of the atom and actinometer intensities IX/IAr with that of their concentrations [X]/[Ar], was also analyzed. It was shown that at the same ionization rate (effective electron temperature) the excitation rate constants kX,Ar are highly sensitive to the shape of EEDF,whereas actinometric coefficients CX,Ar depend on it only slightly. Dependence of actinometric coefficients on electron temperature CX,Ar(Te)is positive if the emitting level of the X-atom is lower than that of the actinometer, and negative if vice versa. The energy difference between the emitting states of O and Ar atoms is maximal (∼3 eV), so that CO,Ar(Te)is not constant for a whole range of electron temperatures typical for discharge plasmas (∼2–8 eV). For nitrogen atoms CN,Ar(Te)varies considerably with Te only when Te < 4 eV. In the case of fluorine atoms the energy difference of emitting F and Ar states is only ∼1 eV and coefficient CF,Ar(Te)is nearly constant in a wide region of Te > 1.5 eV. 
• Beyond moore’s technologies: operation principles of a superconductor alternative / I. I. Soloviev, N. V. Klenov, S. V. Bakurskiy et al. // Beilstein journal of nanotechnology. — 2017. — Vol. 8. — P. 2689–2710. [ DOI ] 
  The predictions of Moore’s law are considered by experts to be valid until 2020 giving rise to “post-Moore’s” technologies afterwards. Energy efficiency is one of the major challenges in high-performance computing that should be answered. Superconductor digital technology is a promising post-Moore’s alternative for the development of supercomputers. In this paper, we consider operation principles of an energy-efficient superconductor logic and memory circuits with a short retrospective review of their evolution. We analyze their shortcomings in respect to computer circuits design. Possible ways of further research are outlined.
• Broadband active electrically small superconductor antennas / V. K. Kornev, N. V. Kolotinskiy, A. V. Sharafiev et al. // Superconductor Science and Technology. — 2017. — Vol. 30, no. 10. — P. 103001. [ DOI ]
  Topical Review. New type of broadband active Electrically Small Antennas (ESA) based on Superconducting Quantum Arrays has been proposed and developed. These antennas are capable of providing both sensing and amplification of broadband electromagnetic signals with a very high Spurious-Free Dynamic Range (SFDR) up to 100 dB (and even more) with high sensitivity. The frequency band can range up to tens of gigahertz depending on Josephson junctions characteristic frequency set by fabrication. In this paper we review theoretical and experimental studies of the Superconducting Quantum Arrays (SQA) and the SQA based antenna prototypes of both transformer and transformer-less types. The evaluated ESA prototypes were fabricated using a standard Nb process with critical current density 4.5 kA/cm2. The measured device characteristics, design issues, comparative analysis of different ESA types, as well as requirements to the applicable interface are reviewed and discussed.
• Combined impact of 500 kev protons and oxygen plasma on polyimide films / L. S. Novikov, E. N. Voronina, V. N. Chernik et al. // Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms. — 2017. — Vol. 410. — P. 60–67. [ DOI ]
  This paper presents results of experimental investigation of combined impact of 500 keV protons with fluences of 10^15–10^16 cm^−2 and oxygen plasma with fluences of (0.8–3.5) 10^20 cm^−2 on polyimide films. Measured UV–vis transmission, Raman and XPS spectra of polyimide specimens before and after combined impact and data on the sample mass losses as a result of erosion due to oxygen plasma exposure are given. On the base of the obtained spectroscopic results, the changes in the polyimide structure caused by the proton and oxygen irradiation are analyzed and discussed.
• Current-phase relations in sisfs junctions in the vicinity of 0-π transition / S. V. Bakurskiy, V. I. Filippov, V. I. Ruzhickiy et al. // Physical Review B - Condensed Matter and Materials Physics. — 2017. — Vol. 95, no. 9. — P. 094522. [ DOI ]
  We consider the current-phase relation (CPR) in Josephson junctions with complex insulator-superconductor-ferromagnetic interlayers in the vicinity of the 0-π transition. We find a strong impact of the second harmonic on the CPR of the junctions. It is shown that the critical current can be kept constant in the region of 0-π transition, while the CPR transforms through multivalued hysteretic states depending on the relative values of tunnel transparency and magnetic thickness. Moreover, the CPR in the transition region has multiple branches with distinct ground states.
• Damage and etching of ultra low-k materials in fluorocarbon plasma at lowered temperatures / D. Lopaev, Y. A. Mankelevich, T. V. Rakhimova et al. // Journal of Physics D - Applied Physics. — 2017. [ DOI ]
  SiOCH ULK films with k-value from 2.5 to 2.1 and porosity from 24 to 40 % were etched in CHF3, CHF3+Ar, CF4 and CF4+Ar plasmas at +15…-120 oC with and without bias being applied. It was shown that the presence of Ar in gas mixture can significantly increase the damage of unetched ultra low-k (ULK) material (at sidewalls) due to the removal of –CH3 groups from the film by VUV photons. It was also shown that etching and damage of the sidewalls by F atoms can be partially prevented by lowering the temperature of the sample.
• Do multiple josephson junctions make better devices? / S. A. Cybart, H. Anna, K. Victor, C. P. Foley // Superconductor Science and Technology. — 2017. — Vol. 30, no. 9. — P. 090201–090206. [ DOI ]
• Multifold study of volume plasma chemistry in ar/cf4 and ar/chf3 ccp discharges / O. Proshina, T. Rakhimova, A. Zotovich et al. // Plasma Sources Science and Technology. — 2017. [ DOI ]
  Low-pressure RF plasma in fluorohydrocarbon gas mixtures is widely used in the modern microelectronics, e. g. in etching of materials with low dielectric constant (low-k materials). Multifold experimental and theoretical study of radio frequency capacitively coupled plasma at 81 MHz in Ar/CF₄/CHF₃ has been carried out at 50 mTorr and 150 mTorr gas pressures. The wide set of experimental diagnostics together with the hybrid PIC MC model calculations was applied to the detailed study of the plasmas. Measurements of F atoms, HF molecules and CF_x radicals, electron density, electronegativity and positive ion composition were performed. Absolutely calibrated VUV spectrometry was carried out to measure VUV photon fluence towards the electrode. This combined experimental and model approach allowed to establish fundamental mechanisms of charged and neutral species elementary reactions. Dissociative charge transfer reactions and fluoride transfer reactions influence a lot the main ion (CF₃⁺, CHF₂⁺) composition in Ar/CF₄/CHF₃ plasma. The mechanisms of heavy ions formation in Ar/CHF₃ are also discussed. The important role of additional attachment mechanisms (besides dissociative attachment to feedstock gases, CF₄ , CHF₃) was analyzed. The catalytic chain mechanism including HF molecules that defines CF_x kinetics in Ar/CHF₃ plasma was validated.This multifold approach enabled us to determine the complicated plasma chemical composition of active species and the fuxes of VUV photons to the surface of the processed material that is important for understanding of low-k damage.
• Observability of surface currents in p-wave superconductors / S. V. Bakurskiy, N. V. Klenov, I. I. Soloviev et al. // Superconductor Science and Technology. — 2017. — Vol. 30, no. 4. — P. 044005. [ DOI ]
  A general approach is formulated to describe spontaneous surface current distribution in a chiral p-wave superconductor. We use the quasiclassical Eilenberger formalism in the Ricatti parametrization to describe various types of the superconductor surface, including arbitrary roughness and metallic behaviour of the surface layer. We calculate angle resolved distributions of the spontaneous surface currents and formulate the conditions of their observability. We argue that local measurements of these currents by muon spin rotation technique may provide an information on the underlying pairing symmetry in the bulk superconductor.
• Photoabsorption and damage of osg low-k films by vuv emission at 140-160 nm / D. V. Lopaev, V. V. Rakhlinsky, S. M. Zyryanov et al. // Plasma Processes and Polymers. — 2017. — P. e1700166. [ DOI ]
  Vacuum Ultraviolet (VUV) absorption and damage of porous OSG low-k dielectrics in the wavelength range 140-160 nm was measured. The measurements were done for two OSG films with different porosity. VUV absorption by OSG dielectrics very slowly decreases with increasing wavelength compared to SiO2 which absorption sharply drops reaching the absorption edge. As the analysis has shown the absorption cross-sections reduced to the Si atom density of these films are very close that indicates the same mechanisms of absorption and damage. The possible absorption and damage mechanisms are briefly discussed.
• Bogatskaya A. V., Volkova E. A., Popov A. M. Prospects of odd and even harmonic generation by an atom in a high-intensity, laser field // Laser Physics Letters. — 2017. — Vol. 14, no. 5. — P. 055301. [ DOI ]
  A new approach for studying the spontaneous emission of an atomic system in the presence of a high-intensity laser field is used to study the process of harmonic generation. The analysis is based on consideration of quantum system interaction, with the quantized field modes being in the vacuum state, while the intense laser field is considered to be classically beyond perturbation theory. The numerical analysis of the emission from the single one-electron 1D atom irradiated by the femtosecond laser pulse of a Ti:Sa laser is discussed. It is demonstrated that not only odd, but also even harmonics can be emitted if the laser field is strong enough. The origin of the appearance of even harmonics is studied. The obtained results are compared with those found in the framework of the semiclassical approach that is widely used to study harmonic generation. It is found that the semiclassical approach is inapplicable in the strong-field limit.
• Sequential reduction of the silicon single-electron transistor structure to atomic scale / S. A. Dagesyan, V. V. Shorokhov, D. E. Presnov et al. // Nanotechnology. — 2017. — Vol. 28. — P. 225304. [ DOI ]
  Here we present an original CMOS compatible fabrication method of a single-electron transistor structure with extremely small islands, formed by solitary phosphorus dopants in the silicon nanobridge. Its key feature is the controllable size reduction of the nanobridge in sequential cycles of low energy isotropic reactive ion etching that results in a decreased number of active charge centers (dopants) in the nanobridge from hundreds to a single one. Electron transport through the individual phosphorous dopants in the silicon lattice was studied. The final transistor structure demonstrates a Coulomb blockade voltage of ∼30 mV and nanobridge size estimated as 15 x 20 x 20 nm^3. Analysis of current stability diagrams shows that electron transport in samples after the final etching stage had a single-electron nature and was carried through three phosphorus atoms. The fabrication method of the demonstrated structure allows it to be modified further by various impurities in additional etching and implantation cycles.
• Silicon dioxide and low-k material sputtering in dual frequency inductive discharge by argon ions with energies from 16 to 200 ev / D. V. Lopaev, T. V. Rakhimova, A. T. Rakhimov et al. // Journal of Physics D - Applied Physics. — 2017. [ DOI ]
  Thermal and PECVD deposited silicon dioxide and organosilicate (OSG) low-k materials with porosity from 24 to 44 % and corresponding k values from 2.5 to 2.0 were sputtered in dual frequency inductive discharge by argon ions with energies from 16 to 200 eV. The film thickness was measured in process by laser ellipsometry and controlled before and after plasma treatment by spectroscopic ellipsometry. Sputtering rate and yield dependencies on ion energy were measured. It was shown that SiO2 sputtering threshold in plasma is significantly lower (< 10 eV) than the threshold measured in ion beam sputtering experiments. This is attributed to the fact that in ion beam sputtering there can be significant charging of the material surface leading to the reduction of ion energy. For low-k sputtering it is shown that the sputtering yield can be significantly higher than that of SiO2 and also –CH3 group removal by vacuum ultraviolet photons from the discharge significantly influences material sputtering rate.
• Single-electron tunneling through an individual arsenic dopant in silicon / V. V. Shorokhov, D. E. Presnov, S. V. Amitonov et al. // Nanoscale. — 2017. — Vol. 9. — P. 613–620. [ DOI ]
  We report the single-electron tunneling behaviour of a silicon nanobridge where the effective island is a single As dopant atom. The device is a gated silicon nanobridge with a thickness and width of ∼20 nm, fabricated from a commercially available silicon-on-insulator wafer, which was first doped with As atoms and then patterned using a unique CMOS-compatible technique. Transport measurements reveal characteristic Coulomb diamonds whose size decreases with gate voltage. Such a dependence indicates that the island of the single-electron transistor created is an individual arsenic dopant atom embedded in the silicon lattice between the source and drain electrodes, and furthermore, can be explained by the increase of the localisation region of the electron wavefunction when the higher energy levels of the dopant As atom become occupied. The charge stability diagram of the device shows features which can be attributed to adjacent dopants, localised in the nanobridge, acting as charge traps. From the measured device transport, we have evaluated the tunnel barrier properties and obtained characteristic device capacitances. The fabrication, control and understanding of such “single-atom” devices marks a further step towards the implementation of single-atom electronics.
• Bogatskaya A. V., Volkova E. A., Popov A. Spontaneous emission from the atom stabilized by a strong high-frequency laser field // Laser Physics. — 2017. — Vol. 27, no. 9. — P. 095302. [ DOI ]
  The spontaneous emission of a quantum system driven by a high-intensity, high-frequency classical laser field is analyzed. The study is based on the accurate consideration of the quantum system interacting with vacuum-quantized field modes in the first order of perturbation theory, while the intense laser field is considered classically beyond this theory. It is demonstrated that the spectrum of the spontaneous emission can be used for analyzing the strong-field dynamics and the structure of the energy spectrum of an atomic system. In particular, it is found that in high-frequency fields (where the energy of the laser quanta is greater than the ionization potential) atoms manifest the features of the Kramers–Henneberger atom. It is also found that in the stabilization regime, the atom emits both odd and even laser radiation harmonics.

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