Structural and transport features of sp-carbon films, synthesized by impulse plasma deposition on metal surface

Ivanenko, I.P.; Krasnoshekov, S.V.; Khvostov, V.V.; Dvoryak, S.V.; Pavlikov, A.V.

Авторы: Ivanenko I.P., Krasnoshekov S.V., Khvostov V.V., Dvoryak S.V., Pavlikov A.V.
Сборник: Труды XXIV международной конференции "Взаимодействие ионов с поверхностью" (ВИП-2019)
Том: 2
Тезисы
Год издания: 2019
Место издания: Москва
Первая страница: 163
Последняя страница: 166
Аннотация: Thin carbon films have many unique properties that are widely used from electronics tomedicine [1]. Nowadays, there is a significant progress in an improvement of the technology ofthe films’ synthesis in order to create films with predetermined properties. In the present workthe low-temperature pulsed-plasma method with ion assistance was used for the deposition ofcarbon films [1]. The advantages of this method include are low substrate temperature (about50°C), high sp1 phase content, precise control of deposited thickness, high deposition rate, highadhesion to the substrate surface, simultaneous use of ion assistance, which allow generating ahigh density of sp1 bonds. The obtained films consist of carbon atom chains orientedperpendicular to the surface of the metal substrate. The structure and electrical properties ofthese films were studied earlier in [2]. The aim of this work was to study and analyze the conductivity and emission mechanismof carbon films, as well as the analysis and study of nanostructures by comparing the measuredand calculated spectra of Raman scattering (RS). For predicting the properties of the synthesizedcarbon structures, a wide range of techniques was used for the first time, including: tunnelingspectroscopy (TS), Raman and IR spectroscopy, quantum chemical calculations, X-ray electronspectroscopy (XPS) and Auger spectroscopy. The following metals were chosen as the substrate materials: Cu, Al, Mo, W, Re, Ta, Ag,Pt. This choice of these metal substrates is caused by a necessity providing a wide range of thework function of the substrate material. The minimum work function for tantalum is 4.2 eV, andthe maximum for rhenium is 5.59 eV [3]. The second important factor is the ability to formchemical compounds (carbides) of a carbon film with substrate materials: Re, Ta, Pt, Mo, W.Studying the conductivity of a carbon film on metal substrates with different work functions canprovide information about the influence of the metal – carbyn-like films interface on thestructure of synthesized film in order to control the growth of the film by varying the frontier. 1. Structural featuresAnalysis of the structural features was carried out by IR and Raman spectroscopymethods by comparing the experimental and calculated spectra. The influence of impurities ofoxygen and nitrogen on the shifts of the main peaks was studied as well, which was notpreviously studied in a recent work [4]. Experimental IR and Raman spectra of carbyne-like carbon films were obtained andanalyzed. Theoretical modeling of the spectra of harmonic vibrations of polyine (–C≡C–)n andcumulene (= C =)n bonds between carbon atoms was carried out. The group characteristicfrequencies were found out, and modeling with taking into account the nitrogen and oxygenatoms. The type of studied molecules is presented in Fig. 1. Figure 1. The model of the molecules under study, the rightmost atom-nitrogen. The terminal groups, as well as impurity atoms, were simulated by hydrogen atoms [2],afterwards they were partially replaced by oxygen and nitrogen, taking into account theexperimental data obtained by using XPS. For obtaining the data on the conductivity of the hetero structures we used the TStechnique implemented in a scanning tunneling microscope (STM) from the Femtoscan company[5], which makes it possible to obtain the I-V characteristics of the tunnel contact probe – studiedstructure. The total resistance of the metal-nanocarbon film-probe structure under study can berepresented as follows: Rsumm= Rfilm+ Rtunnel +Rbonder , (1) where Rfilm is the resistance of the film, and Rtunnel is the resistance of the tunnel junction, and Rboundary is the resistance of the metal-carbon interface. The differentiation of the I-Vcharacteristic of the metal in the absence of a carbon film was carried out on the basis of theapproach described by us earlier [2], which gives the appearance of a smooth symmetricparabola without oscillations. This corresponds to the Simmons formula for the case when thevoltage across the gap is comparable with the work function of the material under study [6].Oscillations observed in [2], which can be understood by the formation of charge density waves,are observed on the structure under study. Such oscillations allows to calculate the length of thelinear fragments. Thus, analyzing the I-V characteristics of heterostructures on different substrates, it isnecessary to take into account the influence of the interface / substrate interface between thecarbyn-like film and the tunnel emission between the probe/carbyn-like film. In general, theconductivity of the heterostructure metal substrate, the carbon structure and the probe consist ofthree components: first, the injection of electrons of the metal substrate into the carbon film;second, the conductivity of the carbon film; and, third, the tunneling of electrons into the probe. To identify the determining conductivity mechanism of the hetero structure, all obtained I-Vwere constructed in Schottky and Fowler-Nordheim coordinates, which is shown in Fig. 2. Figure 2. a, b - in Schottky and Fowler-Nordheim coordinates on Сu, c, d - in Schottky and Fowler-Nordheim coordinates on Re. A potential barrier arised at the metal – carbon frontier is determined by the workfunction of the metal substrate and the Fermi level of the carbon film. The film thickness is about200 nm, so the analysis of the influence of the resistance of the film itself on the conductivity ofthe entire hetero structure is hardly informative. Another conduction site is the tunnel barrierbetween the film surface and the microscope probe. In this case, a tunneling emissionmechanism is possible, for its identification it is necessary to construct the current – voltagecharacteristic in the Fowler – Nordheim coordinates, which is shown in Fig. 2 in the rightcolumn. On all used substrates, the above-barrier emission is observed taking into account theSchottky effect, since the straightening takes place only in these coordinates, which indicates theprevailing emission mechanism. In absence of substrate, the tunnel effect in the Fowler-Nordheim coordinates was detected, although the construction was carried out in the same rangeof applied voltages. This suggests that the determining mechanism of conductivity is theinjection of electrons from a metal substrate. Thus, the main role in the conduction mechanism is played by the substrate – carbon film frontier. The absence of regions with different conductivitymechanisms means that the analyzed carbon film contains chain structures with highconductivity. A deviation from a clear linear dependence in the Schottky emission mechanism isobserved in the range of small applied voltages. For films deposited on Al, Mo and Re, aSchottky emission mechanism is observed over the entire range of applied voltages, and for Cuin the range from 0.5 to 1.8 V, for Pt from 170 to 700 mV and for Ta from 320 to 1300 mV. Thiseffect can be explained by the fact that an intermediate layer is formed between the substrate andthe carbon film. If its potential takes the minimum values, then the contact can be considered asan ideal, but if the thickness of the transition layer is large enough a deviation from the lineardependence takes place, which was observed on the three substrates described above. The minimum barrier height is observed for Re, and the maximum for Al (see Table 1).The obtained values of the barrier heights cannot be described on the basis of the relationshipbetween the areas of the substrate and the nanocarbon film. It is known that the aluminumsubstrate does not form carbides, unlike molybdenum. As a result, leveling occurs, resulting inneutral contact.substrate material substrate work function, eV barrier height, eVRe2000 5,59 0,46Cu2000 4,4 0,47Ta2000 4,94 0,48Pt 5,3 0,53Mo2000 4,2 0,54Al2000 4,33 0,56Table 1. Experimental values of the potential barrier obtained by analyzing the IV characteristics constructed in Schottky coordinates.Bibliography:[1] Guseva M.B., Babaev V.G., Novikov N.D., Alexandrov A.F., Khvostov V.V., SavchenkoN.F., Bystrova N.A., New carbon materials for medical and ecological applications. Journal ofWide Bandgap Materials, 2002; 9(4): 273-2912.[2] Khvostov V.V., Ivanenko I.P., Streletskii O.A., Novikov N.D.,Yakunin V.G., SavchenkoN.F., JETP Letters, №4, Vol. 97, pp.205-208, 2013[3] Fomenko V.S., “Emission properties of materials”, Kiev, Ed. "Naukova Dumka", 1981[4] Babaev V.G., Guseva M.B., Savchenko N.F., Novikov N.D., Khvostov V.V., Flood P. //Surface. X-ray, synchrotron and neutron studies, 2004, No. 3, p. 16-27.[5] http://www.nanoscopy.ru[6] John G. Simmons. J. Appl. Phys. - 1963. - V. 34 238.
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