Статьи в высокорейтинговых журналах за лето 2018 года

Предлагаем вашему вниманию новую подборку статей сотрудников кафедры и отдела микроэлектроники, опубликованных в высокорейтинговых журналах

• A non-destructive, fast evaluation of pvd diffusion barriers deposited on porous low-k dielectrics / W. Yingjie, H. Peng, Z. Jing et al. // Microelectronic Engineering. — 2018. — Vol. 198. — P. 22. [ DOI ]
  Non-destructive and fast evaluation of thin diffusion barriers deposited on top of porous low-k dielectrics by spectroscopic ellipsometry is demonstrated. Studying Physical Vapor Deposited (PVD) CoTa and CoW alloys has shown that 3 nm and 5 nm thick barriers still have holes sufficient for penetration of neutral molecules. This study also detected damage to OSG low-k films which occurs during barrier deposition. VUV light emitted by Ar plasma which is used for metal target sputtering is likely to have caused this damage. For this reason, low-k films were placed under the barriers in order to adsorb moisture during air storage. W atoms also penetrated pores of low-k film during the deposition phase.
• Ambient condition production of high quality reduced graphene oxide / S. A. Evlashin, S. E. Svyakhovskiy, F. S. Fedorov et al. // Advanced Materials Interfaces. — 2018. — P. 1800737. [ DOI ]
  Reduced graphene oxide (GO) becomes one of the most popular materials for applications in various optical, electronic, and sensor devices. Even though many methods are already reported for reduced graphene oxide synthesis, they usually raise issues related to their efficiency, quality, and environmental impact. This work demonstrates a simple, environmental friendly, and effective method for reducing graphene oxide under ambient conditions using nanosecond infrared laser irradiation. As a result, a Raman band intensity ratio of I(G)/I(D) of 4.59 is achieved with an average crystallite size of ≈90 nm. This graphene is of higher quality than what can be achieved with most of the existing methods. Additionally, the demonstrated reduction technique allows the selective reduction of graphene oxide and control the amount of functional groups on the surface of the material. Gas sensors fabricated according to the proposed technique efficiently detect NO₂, NH₃, and H₂S with the sensitivity down to 10 ppm.
• Low-k protection from f radicals and vuv photons by multilayer pore grafting approach / A. Zotovich, A. Rezvanov, R. Chanson et al. // Journal of Physics D - Applied Physics. — 2018. — Vol. 51, no. 32. — P. 325202. [ DOI ]
  Polymer grafting is studied in porous low-k SiCOH films as a protection against plasma damage. Pores of low-k films were covered by plasma damage management (PDM) polymer. A multistep deposition approach was applied to increase polymer layer thickness that helped avoid pore stuffing, nonuniform deposition and polymer overburden. To study polymer protection low-k films were exposed to F radicals and VUV photons, separately and simultaneously at temperatures from -45C to +10C. Effective polymer protection at room temperatures was demonstrated. Lowering the temperature decreases degradation by F radicals while VUV damage, which is temperature independent, becomes dominant. Low-k damage protection was also significant under simultaneous exposure to F radicals and VUV photons. In the tested temperature range damage under simultaneous exposure to F radicals and VUV photons was higher than the sum of separate F and VUV damages due to a synergistic effect. To decrease the material k-value after etching the polymer was removed from the pore walls using UV cure. It is shown that almost complete polymer removal was achieved after UV treatment. The described approach was applied to low-k etching in RF CCP CF4, CF4/Ar plasmas and exposure to Ar plasma. Significant improvement of the film k-value after the plasma treatment was confirmed.
• Magnetic and superconducting phase diagram of nb/gd/nb trilayers / Y. N. Khaydukov, A. S. Vasenko, E. A. Kravtsov et al. // Physical Review B - Condensed Matter and Materials Physics. — 2018. — Vol. 97, no. 14. — P. 144511. [ DOI ]
  We report on a study of the structural, magnetic, and superconducting properties of $\text{Nb}(25\text{ nm})/\text{Gd}(d_f)/\text{Nb}(25\text{ nm}$) hybrid structures of a superconductor/ ferromagnet (S/F) type. The structural characterization of the samples, including careful determination of the layer thickness, was performed using neutron and x-ray scattering with the aid of depth-sensitive mass spectrometry. The magnetization of the samples was determined by superconducting quantum interference device magnetometry and polarized neutron reflectometry, and the presence of magnetic ordering for all samples down to the thinnest Gd(0.8 nm) layer was shown. The analysis of the neutron spin asymmetry allowed us to prove the absence of magnetically dead layers in junctions with Gd interlayer thickness larger than one monolayer. The measured dependence of the superconducting transition temperature $T_c(d_f)$ has a damped oscillatory behavior with well-defined positions of the minimum at $d_f=3\text{ nm}$ and the following maximum at $d_f=4\text{ nm}$, in qualitative agreement with prior work [J. S. Jiang et al., Phys. Rev. B 54, 6119 (1996)]. We use a theoretical approach based on the Usadel equations to analyze the experimental $T_c(d_f)$ dependence. The analysis shows that the observed minimum at $d_f=3\text{ nm}$ can be described by the so-called zero to π phase transitions of highly transparent S/F interfaces with a superconducting correlation length $\xi_f\approx4\text{ nm}$ in Gd. This penetration length is several times higher than for strong ferromagnets like Fe, Co, and Ni, thus simplifying the preparation of S/F structures with $d_f\sim\xi_f$ which are of topical interest in superconducting spintronics.
• N₂ dissociation and kinetics of N(⁴s) atoms in nitrogen dc glow discharge / A. V. Volynets, D. V. Lopaev, T. V. Rakhimova et al. // Journal of Physics D - Applied Physics. — 2018. — Vol. 51, no. 36. — P. 364002.  [ DOI ]
  N₂ dissociation in pure nitrogen plasma has a long history of research. It seems to be a complex process which comprises many reactions involving various electronic and vibrational nitrogen states whose contributions can vary depending on conditions. In this paper, we studied N₂ dissociation in the stationary N₂ discharge both experimentally and theoretically. We used a DC glow discharge in a quartz tube in pure N₂ at moderate pressures (5–50 Torr). The degree of dissociation, atomic nitrogen loss rate and gas temperature were measured by applying optical emission spectroscopy (OES) and as a result an 'effective' rate constant $k^{eff}_{diss}$ for nitrogen dissociation was obtained across a wide range of the reduced field E/N. The analysis of N₂ dissociation was carried out using a specially developed 1D radial self-consistent model which takes into account the spatial inhomogeneities of species concentrations, $E/N$, electron energy distribution function, $T_{gas}$ etc, together with fairly complete plasma-chemical kinetics and all the cross-sections known to date for electron kinetics. The model was successfully validated through the experimental results obtained for electric field, gas temperature and N atom density. Comprehensive analysis of closely coupled processes in nitrogen plasmas—gas heating, VDF formation and N₂ dissociation—was carried out. Simulations reproduced the experimental data on $k^{eff}_{diss} (E/N)$ well and allowed us to evaluate the different contributions of the various dissociation channels considered. It was shown that the nitrogen dissociation mechanism in the stationary N₂ discharge is provided by direct electron impact via the excitation of the pre-dissociative states from the vibrationally excited nitrogen molecules N₂(X, υ). The upper limit for the rate constant of the processes N₂(A)  + N₂(14  ≤  υ  ≤  19)  →  N  +  N  +  N₂ was estimated to be $5\cdot10^{-14}\text{ cm}^3\text{s}^{-1}$.
• Protected 0-$\pi$ states in sisfs junctions for josephson memory and logic / S. V. Bakurskiy, N. V. Klenov, I. I. Soloviev et al. // Applied Physics Letters. — 2018. — Vol. 113, no. 8. — P. 082602–1–082602–5. [ DOI ]
  We study the peculiarities in current-phase relations (CPR) of the SIsFS junction in the region of 0 to $\pi$ transition. These CPR consist of two independent branches corresponding to 0- and p-states of the contact. We have found that depending on the transparency of the SIs tunnel barrier, the decrease in the s-layer thickness leads to transformation of the CPR shape going in the two possible ways: either one of the branches exists only in discrete intervals of the phase difference u or both branches are sinusoidal but differ in the magnitude of their critical currents. We demonstrate that the difference can be as large as 10% under maintaining superconductivity in the s-layer. An applicability of these phenomena for memory and logic application is discussed.

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