Nanostructures with fullerene C₆₀ were obtained using vacuum sublimation thermal C₆₀ fullerene powder onto unheated substrates made of silicon, mica, silica and coverslip glass. The effect of the structure, composition and mechanical stresses in the films on fundamental absorption, density-of-states tails in them were investigated by Ramаn spectroscopy, atomic force microscopy, light absorption, electroreflectance modulation spectroscopy and measuring the bend of heterosystems. Ascertained in this work has been the origin of variation observed in literature data concerning the width of the band gap Eg between 1.48 to 2.35 eV and the nature of the fundamental absorption edge in solid C₆₀. This variation is related with decomposition of fullerene molecules caused by the increase in temperature of sublimation. It has been found that C₆₀ in the crystalline state is direct band-gap semiconductor with Eg close to 1.6 eV in the singular point X of the Brillouin zone. The electroreflectance spectra of films and heterosystems bending were used to calculate the Eg dependence on the internal mechanical stresses. The respective coefficient value is equal to –2.8 10⁻¹⁰ eV/Pa

Kolyadina, E.Yu.

Matveeva, L.A.

Neluba, P.L.

Venger, E.F.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

 Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 349-353. doi: 10.15407/spqeo18.03.349  © 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  349 PACS 78.40.Ri Analysis of the fundamental absorption edge of the films  obtained from the C60 fullerene molecular beam in vacuum and effect of internal mechanical stresses on it E.Yu. Kolyadina, L.A. Matveeva, P.L. Neluba, E.F. Venger V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,  45, prospect Nauky, 03028 Kyiv, Ukraine Phone: 38 (044) 525-83-17, e-mail: matveeva@isp.kiev.ua    Abstract. Nanostructures with fullerene C60 were obtained using vacuum sublimation thermal C60 fullerene powder onto unheated substrates made of silicon, mica, silica and coverslip glass. The effect of the structure, composition and mechanical stresses in the films on fundamental absorption, density-of-states tails in them were investigated by Ramаn spectroscopy, atomic force microscopy, light absorption, electroreflectance modulation spectroscopy and measuring the bend of heterosystems. Ascertained in this work has been the origin of variation observed in literature data concerning the width of the band gap Eg between 1.48 to 2.35 eV and the nature of the fundamental absorption edge in solid C60. This variation is related with decomposition of fullerene molecules caused by the increase in temperature of sublimation. It has been found that C60 in the crystalline state is direct band-gap semiconductor with Eg close to 1.6 eV in the singular point X of the Brillouin zone. The electroreflectance spectra of films and heterosystems bending were used to calculate the Eg dependence on the internal mechanical stresses. The respective coefficient value is equal to –2.8 10–10 eV/Pa.  Keywords: C60 fullerene film, fundamental absorption, electroreflectance, mechanical stresses.  Manuscript received 03.04.15; revised version received 24.07.15; accepted for publication 03.09.15; published online 30.09.15.           1. Introduction Heteroepitaxial deposition of thin solid films is widely used in modern semiconductor technique and physical researches. Therefore, to find new manufacturing technologies for nanostructures and to identify their physical properties interesting for practical use are rather topical. Due to the unique properties inherent to multi-atomic nanostructures with fullerenes, they attract great attention. Also, they allow modification both during film deposition [1] and due to external effects [2, 3]. Among famous fullerene molecules, C60 is the most symmetrical and stable one. The carbon atoms in it have the covalent type of bonding, while the molecules in respective crystal exhibit a weak bond corresponding to van der Waals interactions. Development of technologies for the production of fullerenes in the form of film material on various substrates extends the use of nanostructures with fullerenes to various fields of science and nanotech-nology, as well as for practical use. Heterostructures based on them are used for solar cells [4, 5] and sensors of physical quantities [6]. Films with C60 fullerenes are promising for applications in biomedicine, robotics, and as protective coatings [7]. Electronic and optical properties of semiconductor are determined by its band structure (a band gap Eg and the type of electronic transitions in it). Up to date, it is not practically ascertained what is the real width of the  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 349-353. doi: 10.15407/spqeo18.03.349  © 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  350 band gap in crystalline C60. The data obtained using various methods (optical absorbance, ellipsometry, photoconductivity, microwave conductivity, electron energy loss spectroscopy, surface photovoltaic spectro-scopy and photoemission) show a large spread of its value from 1.5 up to 2.35 eV [8], for instance, shown in [9] is the value Eg = 1.47 eV. The discriminating feature of the heterosystems is the presence of internal stresses in them. They arise from the mismatch of lattice parameters and thermal expansion coefficients of the film and substrate, give rise to technological defects at the interface, and change the band structure of semiconductor. Relaxation of internal mechanical stresses in devices reduces their stability [10]. The objective of this work was to study the fun-damental absorption of C60 films with different methods and the effect of internal mechanical stresses on it. 2. Experiment Low temperature sublimation C60 (350 °C) simplifies the process of manufacturing the solid-state fullerene heterosystems. The fullerene films were deposited on various non-heated substrates (silicon, mica, silica, glass coverslip) by evaporation of microcrystalline C60 powder (purity 99.9 %) in vacuum at the pressure of 10–4 Pa from an effusion tantalum cell at the temperature of 650 to 800 K [1]. The thickness of films was 0.1 to 2 μm. All substrates had a smooth and flat surface. Substrate of silicon was prepared by standard industrial processing with finish chemical polishing. Mica substrates with natural surface were prepared by cleavage. The surface of the silica substrate was mechanically polished. The substrate and film surfaces nanomorphology were studied using atomic force microscopy with a Nanoscope III-a operating in the periodical mode. It is known that even for films of atomic semiconductors the growth rate strongly influences on their structure, internal stresses, optical and electronic properties [11]. Stepped disposition of substrates in vacuum camera enabled us to obtain a single technological cycle of the film at different rates of growth (10 to 500 nm/min) and change their physical properties [7]. To analyze the film composition and fundamental absorption, as well as internal mechanical stresses in the films, we used a complex of experimental methods. It included measurements of Raman scattering, trans-mission and reflection of light, modulation spectroscopy electroreflectance and the bending radius of heterosystems. To excite Raman spectra, an Ar+-laser with the emission wavelengths 514.5 nm was used.  Study of the electron band structure by using the modulation spectroscopy method should help in under-standing the nature and mechanism of light absorption of solid C60. A feature of the electroreflectance method is its high resolution, appearance of the signal only at the direct transitions in critical points of the Brillouin zone and its disappearance at some distance from the critical point. The method provides information on the band structure of semiconductor, its electronic and optical properties, structural perfection and internal mechanical stresses in films and interfaces film-substrate. Note that there is no data in the literature on electroreflectance of fullerenes. To study heterosystems with fullerenes, this modulation method was used for the first time. The measurements were performed in silica electrolytic cell with 0.1N KCl water solution at room temperature. The band gap Eg and parameter of spectral broadening Г were determined using the three-point method by Aspnes [12]. The value of Г characterizes the quality of the film surface. The magnitude and sign of the internal stress in the films were studied by measuring the heterosystem bending recorded on the Talesurf-4 profilograph by using the Stoney formula [13]: σ = Ed2/[6Rt (1 – ν)],       (1) where R is the radius of heterostructure curvature, E and ν – the Young modulus and Poisson’s ratio of the substrate, d and t are the substrate and film thicknesses, respectively. 3. Results and discussion Depending on the conditions for condensation of the C60 molecular beam onto a substrate, the film was prepared with a different crystal structure and composition: poly-crystalline fullerite C60, composite (C60:a-C, a-C:C60) and amorphous a-C film not containing fullerenes. The surface of fullerite films was smooth, with uniform arrangement of the molecules on it (Fig. 1a). Raman spectra contain vibration modes with symmetry Hg, Ag, and Hg at frequencies 1425, 1470 and 1575 cm–1, respectively (Fig. 1b). According to [14], C60 carbon molecules form a molecular crystal, and the new form of C60 carbon crystal has a new semiconductor material with a direct band gap. Eg = 1.6 eV within the region E0 (hg – t1u) transition and near 2 eV higher energy transition  (hg – t2u). The top of the valence band and bottom of the conduction one are located at the same point X of the Brillouin zone. The signal of electroreflectance is also present only for direct transitions. From the electroreflectance spectra of the C60 film on a silicon substrate (Fig. 2a), we have Eg = 1.65 eV for (hg – t1u) transition and about 2.1 eV for (hg – t1u) transition. These data coincide with the value Eg obtained from absorption spectra of the films on transparent substrates. The C60 fullerene band gap depends on the presence of mechanical stresses. Regardless of the conditions for obtaining the films, they were always present on the convex side of the substrate. It means that in films the compression stresses occur and in the substrate – stretching ones [13]. Fig. 2b shows the dependence Eg in films C60 determined from the spectra of electro-reflectance and fundamental absorption edge for the E0 transition on the stresses σ: dE0/dσ = – 2.8⋅10–10 eV/Pa.  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 349-353. doi: 10.15407/spqeo18.03.349  © 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  351  Fig. 1. Image of the surface (a) and Raman spectra (b) of the fullerene C60 films on silicon.   A large spectral broadening in the electro-reflectance spectra of C60 films (Г = 100…150 meV) corresponds to their polycrystalline structure. The value of Г characterized the scattering of light excited charge carriers on the structural defects, which leads to a broadening of the fundamental absorption edge due to the presence of tails in the density of states within the band gap of the film. In classical spectroscopy, they arise in the form of an exponential dependence at the fundamental absorption edge when the photon energy is less than Eg and determined by the value of the characteristic energy Δ = dE/dlnk, where k is the coefficient of light absorption. In polycrystalline C60 films, the Δ value is tens of millielectronvolts, and it is independent of their thickness 0.1 to 2 µm, which coincides with the data of [15] to C60 polycrystalline films with a thickness 1.3 to 8.5 µm, obtained using the similar method. It means that the fundamental absorption has a bulk property of C60 films without surface or interface. The authors of [15] believe that the fundamental absorption edge of polycrystalline films of C60 is indirect, and the absorption edge is identical to that of the single-crystal C60 sample with the size of 1 mm3 (Δ = 43 meV). The spectral dependences of k2(E)   Fig. 2. (a) Spectrum of electroreflectance in C60 films on silicon, (b) dependence of the band gap Eg on internal mechanical stresses σ in C60 films on silicon (S) and glass (z) substrates for E0 (hg – t1u) transition.   and lnk(E) for C60 films in the range of the following direct transition (hg – t1u) are shown in Fig. 3. The films have Eg = 2.24 eV (Fig. 3a) and the parameter Δ = 82 eV (Fig. 3b). With increasing the growth rate (sublimation temperature of fullerenes) morphology, composition and crystal structure of the films are changed. The surface of the film was not smooth and inhomogeneous (Fig. 4a). In the Raman spectrum, instead of a strong signal from the pentagons at 1470 cm–1, a very weak signal appeared on broad background photoluminescence (Fig. 4b). Instead of narrow lines at 1425 and 1575 cm–1, recorded were broad bands with the maxima at 1380 and 1600 cm–1. This fact indicates a change in composition and structural perfection of the films with occuring diamond and graphite-amorphous phases in them. Their appearance is caused by the collapse of the C60 molecules on the surface of the substrate [1]. Significant changes have also occurred at the edge of the fundamental absorption inherent to composite carbon films. The signal of electrotreflectance in them is absent, which is typical for amorphous material [1]. Spectral  distribution of the light absorption coefficient and its logarithmic dependence are also changed. The films on   Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 349-353. doi: 10.15407/spqeo18.03.349  © 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  352   Fig. 3. Spectral dependence of k2 (a) and ln k (b) for a-C:C60 films on mica in the range of the (hg – t2u) transition.    Fig. 4. Image of the surface (a) and Raman spectra (b) of the composite a-C:C60 film on silicon.   Fig. 5. Spectral dependence of k1/2 (a) and lnk (b) for a-C:C60 films on silica in the range of (hg–t2u) transition.   the polished silica have Eg = 2.1 eV (Fig. 5a) and the parameter Δ = 135 meV (Fig. 5b). Since the maximum value of Δ for any amorphous material does not exceed 140 meV [16], a large Δ parameter for our films confirms their amorphous structure. For the individual C60 molecule, there is an internal transition at the energy 1.6 eV. It is forbidden for C60 in the solid state [17]. Due to interaction of the molecules with each other, in fullerene crystals and films this prohibition is removed, and in that spectral region the C60 films also absorb light. The authors [17] explain the fundamental absorption edge in molecular crystals by indirect Frenkel excitons within the range 1.5 to 2 eV, while the authors of [9] – by an internal transition in C60, calling it as the mobility edge and defining it as the indirect one. The edge of the intrinsic light absorption in C60 crystal and films the authors of [14] also determine it as indirect. 4. Conclusion The results of comprehensive experimental investigation of the films prepared using the molecular beam C60 falling in vacuum onto various substrates showed strong influence of the preparation process conditions on com-position, structure, optical properties, internal mecha-nical stresses of films and enabled to draw the following conclusions. Uniquely set are causes of variation in the  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 349-353. doi: 10.15407/spqeo18.03.349  © 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  353 literature data on the band gap magnitude in C60 films and the nature of fundamental absorption edge in them. The polycrystalline semiconductor films of C60 have a direct edge of the fundamental absorption in the spectral region near 1.65 eV. Their band gap increases with stress by a factor dEg /dP = –2.8·10–10 eV/Pa. The mechanical stress in amorphous films decreases, and their absorption edge is near 2.2 eV. High-energy direct transitions in C60 polycrystalline films have been also registered in this spectral region. References 1. P.L. Neluba, Peculiarities of fullerenes conden-sation from molecular beam in vacuum // Techno-logia Konstruirovanie Electronnoi Apparature, 6, p. 35-39 (2011), in Russian. 2. E.F. Venger, L.A. Matveeva, P.L. Neluba, Radia-tion-stimulated effects in heterostructures with fullerenes // Optoelektronika i poluprovod. tekhnika, 46, p. 43-48 (2011), in Russian. 3. E.F. Venger, E.Yu. Kolyadina, L.A. Matveeva et al., Increase of thermal and radiation stability of solid heterosystems with C60 fullerene // Sbornik nauchnykh statey “Nanostructury v kondensirovan-nykh sredakh”, Minsk, NAS of Belarus, A.V. Luikov Heat and Mass Transfer Institute, p. 183-189 (2014), in Russian. 4. L.K. Narajanan, M. Jamaguchi, Photovoltaic effects of a:C/C60 /Si (p-i-n) solar cell structures // Solar energy materials and solar cells, 75, p. 345-350 (2003). 5. N.L. Dmitruk, O.Yu. Borkovskaya, L.A. Matveeva et al., Photoelectric properties of microrelief metal-semiconductor heterojunction with an intermediate layer of C60 fullerene // Sbornik nauchnykh statey “Nanostructury v kondensirovannykh sredakh”, Minsk, NAS of Belarus, p.  3-9 (2008), in Russian. 6. E.F. Venger, T.I. Gorbanyuk, L.A. Matveeva, Fullerenes and sensors // 5th Intern. Sci. and Techn. Conf. “Sensors electronics and microsystems technology” (SEMST-5). 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Venger, Effect of film growth rate and thickness on properties of Ge/GaAs (100) // Thin Solid Films, 550(1), p. 715-724 (2014). 12. D.E. Aspnes, Third derivative modulation spectroscopy with low-field electroreflectance // Surf. Sci. 37. p. 418-442 (1973). 13. R.V. Hoffman, Mechanical Properties of Thin Condense Films, Physics of Thin Films. Moscow, Mir, 3, p. 235-243. 14. T. Gotoh, S. Nonomura, H. Watanabe, S. Nitta, Temperature dependence of the optical-absorption edge in C60 films // Phys. Rev. B, 58(15), p. 10 060-10 063 (1998). 15. S. Saito, A. Oshiyama, Cohesive mechanism and energy band of solid C60 // Phys. Rev. Lett. 66(20), p. 2637-2640 (1991). 16. R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman, G.A. Sawatzky, Band gap, excitons, and Coulomb interaction in solid C60 // Phys. Rev. Lett. 68(26), p. 3924-3927 (1992). 17. M. Laštovičková, Some different in the exponential tail behaviour of the fundamental absorption edge in amorphous and crystalline materials // Czech. 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Analysis of the fundamental absorption edge of the films obtained from the C₆₀ fullerene molecular beam in vacuum and effect of internal mechanical stresses on it

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Analysis of the fundamental absorption edge of the films obtained from the C₆₀ fullerene molecular beam in vacuum and effect of internal mechanical stresses on it

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)