Optical and electrical characterization of chemically and photopolymerized C60 thin films on silicon substrates

Thin Solid Films 515 (2007) 7716 – 7720 www.elsevier.com/locate/tsf

Optical and electrical characterization of chemically and photopolymerized C60 thin films on silicon substrates N.L. Dmitruk a,⁎, O.Yu. Borkovskaya a , I.B. Mamontova a , O.S. Kondratenko a , D.O. Naumenko a , E.V. Basiuk (Golovataya-Dzhymbeeva) b , E. Alvarez-Zauco b b

a Institute for Physics of Semiconductors, National Academy of Science of Ukraine, Kyiv, 03028, Ukraine Laboratorio de Materiales y Sensores, CCADET, Universidad Nacional Autónoma de México Cd. Universitaria, Coyoacan, 04510, México D.F., México

Available online 10 January 2007

Abstract A comparative characterization of C60 thin films grown on silicon substrate by Physical Vapor Deposition and polymerized by chemical reaction with 1,8-octanediamine vapor or UV Pulsed laser irradiation has been carried out by means of Atomic Force Microscopy, and optical reflectance, transmittance and photoluminescence spectroscopies. The photovoltaic response and electrical characteristics of Au/C60/Si diode structures have been investigated. The greatest photoluminescence efficiency and light transmittance, and at the same time the least photocurrent of diode structure were observed for chemically polymerized C60. Found differences in morphology, optical, photoelectric and electrical properties of C60 films polymerized by two methods indicate a difference in their composition. © 2006 Elsevier B.V. All rights reserved. Keywords: Fullerene; Thin films; Polymerization; Optical spectroscopy; Photoelectric properties

1. Introduction Polymerized fullerene C60 films, whose properties can be varied depending on the methods and conditions of polymerization, have considerable potential as new materials for electronics. So, both methods of the fullerene solid phase modification, and comprehensive characterization of polymerized fullerene films have attracted much attention [1–4]. In this work we present a comparative study of optical, photoelectric and electrical characteristics of C60 thin films on silicon substrate both initial and polymerized by two methods: chemical reaction with 1,8octanediamine vapor, or UV Pulsed laser irradiation. The morphology of the films was investigated by atomic force microscopy (AFM). The measurements of transmittance spectra in the λ = 400 ÷ 1200 nm for films on glass substrates, the spectra of photoluminescence and reflectance of C60/Si structures and multiangle-of-incidence ellipsometry (MAI), were performed for their optical characterization. The current-voltage (IV) characteristics and spectra of the short-circuit photocurrent of barrier structures,

⁎ Corresponding author. Tel.: +380 44 525 64 86; fax: +380 44 525 83 42. E-mail address: [email protected] (N.L. Dmitruk). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.103

obtained on C60/Si by evaporation of gold layer over C60, were investigated to determine the polymerized C60 influence on photoelectric and electrical properties of photodiode structures. 2. Experimental details Fullerene films with thickness of 120 nm and areas larger than 1 cm2 were grown on (100) n-Si substrate by sublimation of C60 powder (Mer Corp., 99.5% purity) in a vacuum of 8.2 · 10− 4 Pa without heating the substrates. Chemical polymerization was fulfilled by the gas-phase reaction of C60 film with 1,8diaminooctane heated at about 150 °C for 3 h [5]. Photopolymerization of C60 was performed by irradiation with a pulsed Kr–F excimer laser, operated at λ = 248 nm, with a 30 ns pulse width and pulse repetition rate of 10 Hz at optimal conditions: fluence of 25 mJ/cm2 and about 5 s time of irradiation [6]. The C60 surface microrelief morphology investigation was performed by AFM technique (Nanoscope IIIA, Digital Instruments, CA, USA) in the tapping mode with a Si3N4 tip and Dimension 3000 software. The thickness and optical parameters of C60, the refractive index (n) and extinction coefficient (k), have been estimated by the MAI using LEF-3M ellipsometer with He–Ne-laser (λ = 632.8 nm). Spectra of C60

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photoluminescence (PL) and transmittance of C60 films on glass substrate (satellite) have been measured using grating spectrometer MDR-3 with FEU-100 photomultiplier detector. The PL excitation was performed with the λ = 496.5 nm line of an Argon ion laser. Photodiode structures have been fabricated by evaporation of semitransparent Au layer of 20 nm thickness with intermediate thin Cr layer of about 2 nm thickness (for better adhesion) on C60/Si substrate at room temperature through an opaque mask with opening diameter of 1.3 mm. Also, the structures with thicker Au layer (77 nm) were fabricated on C60/Si and Si substrates simultaneously. The ohmic contact was made all over the rear side of Si plate. All the studies have been carried out at room temperature on the samples that were exposed to air. 3. Results and discussion The surface morphologies of initial (K1) and chemically (K2) or photopolymerized (K3) C60 films on Si substrate are shown in Fig. 1. Films have granular (microcrystallite) structure

Fig. 2. The spectra of reflectance for the C60/Si structures (a) and transmittance for C60/glass structures (b) with initial (1) and chemically (2) or photopolymerized (3) C60 films. The points, ♦ at λ = 0.632 μm in (b) are the results of calculation based on parameters (n, k, d ), obtained with ellipsometry measurements.

Fig. 1. AFM images of the C60 thin films on silicon (100) substrate: initial (a) and chemically (b) or photopolymerized (c); z = 20.0 nm/div.

with average grain diameter of 50 nm for K1, which is typical for C60 films grown on different substrates at room temperature [7]. For modified films the grains become greater, but their roughness changes in different ways: root mean square roughness: Rq = 1.22 nm (K1), 1.08 nm (K2), 2.54 nm (K3). Both treatments reduce the solubility of C60 films in toluene, indicating the transformation of C60 into a different solid phase of polymeric nature. Ellipsometric measurements show only a slight difference of optical parameters (refractive index n and extinction coefficient k) of the investigated films at λ = 632.8 nm: n = 2.028 (K1), 1.957 (K2), 1.985 (K3); k = 0.085 (K1), 0.088 (K2), 0.112 (K3), at complex refractive index ñ = n − ik. So, the calculated transmittances of these films on glass substrates are practically the same at λ = 632.8 nm and agree well with the measured ones (Fig. 2). At the same time, both the light transmittance spectra for fullerene films on the glass substrates, and reflectance spectra for C60/Si structures (Fig. 2a,b) show evidence for a certain decrease of light absorption for polymerized C60 in the wave length ranges of λ ≤ 0.55 μm and λ ≥ 0.65 μm. The room temperature photoluminescence spectra for the investigated C60 films are presented in Fig. 3a. The spectra consist of the asymmetric band with a main maximum at 1.68 ÷ 1.69 eV, that has been attributed [8] to the emission from bulk C60 molecules located in different crystalline environments (X traps). The

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presented in Fig. 4b as normalized to maximal value. The calculated light transmittance spectra into Si is also shown there. It is seen, that carrier photogeneration outside the edges of the diode gives a considerable contribution in Iph affecting not only the Iph value but a character of its spectral dependency. To determine the contribution of C60 layer into the photocurrent of barrier structure the spectra of internal quantum efficiency in arbitrary units (Iph divided by transmittance of Au/C60 or Au layers) for Au/C60/Si and Au/Si structures, respectively are shown in Fig. 5. The Iph/T decrease with λ decrease is observed for both types of structures due to recombination on Si interfaces, but the Iph/T increase for λ b 0.55 μm in the former ones is due to C60 layers, that correlates with their spectra of absorption. In spite of some difference in the characteristics of separate diodes the certain distinguished features should be noted: 1) The photocurrent spectra are completely determined by photogeneration of current carriers in Si and by the light transmission through the metal/C60 layers. The same spectral position of Iph maximum for structure with initial and polymerized C60 films and greater Iph out of maximum for

Fig. 3. Photoluminescence spectra for C60 films at a room temperature (a) and fitting of them with a sum of several Gaussian functions (b): initial (1) and chemically (2) or photopolymerized (3) C60 films on Si substrate.

greatest increase in PL intensity is observed for chemically polymerized C60 film. To analyze the form of PL spectra they were fitted with a sum of several Gaussian functions (Fig. 3b). The redshift of main Gaussian peak for K2 and K3 samples with respect to K1 is seen that is characteristic for polymerized C60 film [1]. At the same time, the number of peaks ensuring the best fit to the experimental curve is different for K2 and K3 indicating on their different morphology, and perhaps the different composition of carbon phases of polymeric nature. Really, the results of IR transmittance and Raman spectra studies for C60 films, both pristine and transformed by the same treatments indicated lowering of fullerene molecule symmetry by conversion of some sp2 carbon atoms into sp3 carbon atoms due to chemical or photopolymerization [5,6]. The short-circuit photocurrent (Iph) spectra for Au/Cr/C60/Si barrier structures (Fig. 4) have been measured under conditions when either the whole surface of structure (Fig. 4a) or only the central part of diode were illuminated. In the latter case the maximal Iph value was observed for initial (K1) structure. Some difference in the Iph value for diodes of the same structure (especially for K3) was also observed but the character of their spectra was identical. To demonstrate this the spectra of Iph are

Fig. 4. Experimental spectral characteristics of the external quantum efficiency (a) and spectral characteristics of photocurrent, normalized to its maximal value (b) for the Au/Cr/C60/Si barrier structures, measured under conditions when either the whole surface of structure (a) or only the central part of diode (b) were illuminated, with initial (1) and chemically (2) or photopolymerized (3) C60 films. The calculated spectra of the light transmittance into Si (4); dAu = 20 nm, dCr ∼ 2 nm.

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polymerized C60 (that correlates with their greater transparency) confirm this affirmation. 2) The structures with initial C60 layer have the greatest photoresponse, which is even greater (for some diodes) than the photoresponse of Au/Si structures, fabricated simultaneously on the same Si substrate. 3) The least photoresponse is observed for structures with chemically polymerized C60. This may be caused by the greatest recombination velocity at the interfaces or inside the C 60 film and, besides, by the change of electrical characteristics of barrier structure. An example of dark forward current-voltage characteristics for Au/C60/Si and Au/Cr/C60/Si barrier structures is presented in Fig. 6. Their analysis has been made in the framework of model taking into account emission, recombination, tunnel and ohmic components of the current flow. It shows that the main mechanisms of the current flow in a small voltage region for all the structures are (i) tunneling with characteristic energy ε, and (ii) a parallel ohmic shunting current which value is determined by shunt resistance Rsh. The tunneling component is less than or equal to ohmic one for structure with K1 layer and Au/Si but is more than ohmic one for K2 and K3 structures. At the same time, ε and Rsh are maximal for K2 structure: ε = 0.12 ÷ 0.14 eV, Rsh ∼ 3·107 Ω for K2; ε = 0.07 ÷ 0.11 eV,

Fig. 6. The dark forward current-voltage characteristics for Au/C60/Si (a) and Au/Cr/C60/Si (b) barrier structures with initial (1) and chemically (2) or photopolymerized (3) C60 films. (4)–for Au/Si barrier structure with an intermediate thin film of natural SiO2 oxide.

Rsh ∼ 3·106 Ω for K1 and K3; ε = 0.08 eV, Rsh ∼ 9·105 Ω for Au/Si. At greater voltage the current value is limited by series resistance Rs, which is maximal for K2 and minimal for K3. Thin (island) Cr intermediate layer influence on electrical characteristics of Au/C60/Si is faint and mainly leads to increase of barrier resistance. So, electrical characteristics of barrier promote diminishing Iph for structures with chemically polymerized C60. At the same time, a character of their Iph spectra with a contribution of edge photogeneration testify decrease of this photocurrent component indicating an increase of interface recombination velocity at the C60/Si interface in these structures with respect to others. 4. Conclusions

Fig. 5. The short-circuit photocurrent spectra reduced to the same number of quanta transmitted through the Au/C60 or Au layers for Au/C60/Si (a) and Au/Si (b) barrier structures with initial (1), chemically (2) or photopolymerized (3) C60 films (a) or without them on the same Si substrates (b).

The comprehensive characterization of pristine (initial) and polymerized by two methods thin C60 films on Si showed that polymerized films have both similar properties (specifically an increased transmittance of light in visible and near UV and IR ranges) and different ones. These are: 1) increased rms roughness of a surface morphology for photopolymerized K3 film; 2) different recombination properties which manifest themselves both in the photoluminescence intensity, maximal

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for chemically polymerized K2 film, and in the minimal photocurrent of a metal/C60/Si barrier structure containing the same K2 film; 3) an increase of series resistance for barrier structures with K2 and its decrease in the case of K3 with respect to K1 (at nearly the same proportion of the tunnel and ohmic components of the current flow for K2 and K3 barrier structures). This indicates on different morphology and composition of polymeric C60 phases for chemically and photopolymerized C60 thin films, specifically on the possible appearance of the amorphous carbon phase for photopolymerized C60 films [5]. References [1] Ying Wang, J.M. Holden, A.M. Rao, P.C. Eklund, U.D. Venkateswaran, DeLyle Eastwood, R.L Lidberg, M.S. Dresselhaus, Phys. Rev., B 51 (1995) 4547.

[2] M.S. Dresselhaus, G. Dresselhaus, P.C. Elkund, Science of Fullerenes and Carbon Nanotubes, Academic Press, 1996. [3] U.D. Venkateswaran, D. Sanzi, A.M. Rao, P.C. Eklund, L. Marques, J.-L. Hodeau, M. Nunez-Regueiro, Phys. Rev., B 57 (1998) R3193. [4] T.L. Makarova, Semiconductors 35 (2001) 243. [5] E.V. Basiuk (Golovataya-Dzhymbeeva), E. Alvarez-Zauco, V.A. Basiuk, in: N.P. Mahalik (Ed.), Chemical Cross-linking in C60 Thin Films/Micromanufacturing and Nanotechnology, Springer-Verlag Publisher, Germany, 2005, pp. 453–461. [6] E. Alvarez-Zauco, H. Sobral, E.V. Basiuk, J.M. Saniger-Blesa, M. Villagran-Muniz, Appl. Surf. Sci. 248 (2005) 243. [7] A. Richter, R. Ries, K. Szulzewsky, B. Pietzak, R. Smith, Surf. Sci. 394 (1997) 201. [8] W. Guss, J. Feldmann, E.O. Gobel, C. Taliani, H. Mohn, W. Muller, P. Haussler, H.-U. ter Meer, Phys. Rev. Lett. 72 (1994) 2644.