Dielectric and optical properties of CaCu 3 Ti 4 O 12 thin films containing Ag nanoparticles P. Kumar ⁎, D.C. Agrawal Materials Science Programme, Indian Institute of Technology, Kanpur 208016, India abstract article info Article history: Received 24 September 2009 Accepted 5 November 2009 Available online 10 November 2009 Keywords: CCTO Dielectrics Sol–gel preparation Thin films Nanoparticles Simple sol–gel techniques are used to prepare thin films of a high dielectric constant perovskite CaCu 3 Ti 4 O 12 , containing different amounts of metallic silver nanoparticles. The formations of the silver nanoparticles are verified by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and optical absorption studies. The dielectric properties are found to be significantly affected by the presence of the silver nanoparticles. A maximum in the dielectric constant is observed at an intermediate metal particle concentration. This is explained in terms of the polarization at the particle–dielectric interface and the internal barrier layer capacitor effect. The optical absorption spectrum is compared with Mie theory in electrodynamics for the optical absorption of small particles to extract the particle size of the silver particle. Non-uniform distributions of Ag particles through the thickness of the thin films are reported. © 2009 Elsevier B.V. All rights reserved. 1. Introduction High dielectric constant (ε r ) materials are very important for technological applications including capacitive memory devices. The cubic perovskite CaCu 3 Ti 4 O 12 (CCTO) have attracted ever-increasing attention due to very high ε r ∼ 10 4 , a promising material for minia- turization of the capacitors [1–7,10,14]. Generally, ε r is nearly tem- perature and frequency independent in the temperature range 100– 600 K from dc to 1 MHz and no structural phase transition is observed implying that CCTO is not a ferroelectric [1–3]. The origin of the high dielectric constant in CCTO is still not certain, though the most agreed view is the internal barrier layer capacitance (IBLC) model; CCTO consists of semiconducting grains and highly insulating grain boundaries which act as blocking layers to the conductivity [4–7]. Most of the works on CCTO were done on bulk materials. However, thin films have advantage over bulk material for capacitor application in microelectronics due to their smaller size and high dielectric constant. The conducting metal particles dispersed in a dielectric matrix are known to increase the effective dielectric constant of the medium which is consistent with the percolation law [8,9]. We have studied the effect of silver nanoparticles taking into account the different atmosphere and/or thermal conditions on the dielectric and optical properties of CCTO thin films. 2. Preparations and experimental details Sol–gel prepared silver doped CCTO thin films of composition Ag/Ti molar ratio 0.00, 0.03, 0.06 and 0.09 were deposited on platinized silicon (Pt/Ti/SiO 2 /Si) and soda lime glass. CCTO with slight excess of Calcium (Ca 1.07 Cu 3 Ti 4 O 12 ) was used because the increase in the calcium content up to a certain limit increases the dielectric constant without any increase in dielectric loss [10]. A sol was prepared by dissolving required amounts of titanium butoxide 97% [Ti (OC 3 H 7 ) 4 ] in 2-Ethyl-Hexanoic acid. Mono hydrated calcium acetate 98% [Ca (C 2 HO 2 ) 2 .H 2 O] and mono hydrated copper acetate 98% [Cu (OOCCH 3 ) 2 .H 2 O], were then mixed into the solution at room temperature and solution was refluxed at 120 °C for 30 min. Another sol was prepared by dissolving 99.9% AgNO 3 in deionized water and then added to 2-Propanol. This solution was added to titanium butoxide solution at room temperature with constant stirring. A bluish green solution after filtration was used for thin film preparation. The thin films of thickness ∼ 0.6 μm were deposited on platinized Si (111) substrates by spin coating (25 coatings) at 5 krpm for 30 s. After each coating thin films were annealed at intermediate temperatures 500 °C for 5 min followed by 750 °C for 10 min. The final treatment was done for 2 h at 750 °C in air. Same thin films were annealed in H 2 at 450 °C for 2 h. To measure the dielectric properties, circular Pt electrodes of 0.2 mm diameter, were deposited on these thin films by dc magnetron sputtering. The dielectric properties were studied in metal-insulator-metal configuration before and after H 2 annealing using an impedance analyzer (HP 4192 A) in the frequency range 100 kHz to 1 MHz. X-rays diffraction (ARL X'TRA X-ray diffractometer) was used for phase identification. Scanning electron microscope (FEI Quanta 200HV), transmission electron microscope (JEM-2000 FX II JEOL) were used to observe microstructure, morphology and the formation of silver particles. For UV–VIS spectroscopic and electron microscopy studies thin films were deposited on soda lime glass and NaCl substrates with intermediate annealing temperature at 400 °C for 15 min and followed by annealing at 400 °C for 2 h in air. Same thin films were annealed in H 2 at 450 °C for 2 h. A HITACHI, U-3310 spectrophotometer was used Materials Letters 64 (2010) 350–352 ⁎ Corresponding author. Present address: Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221-0011, USA. Tel.: +1 5135564577. E-mail address: kumarpv@email.uc.edu (P. Kumar). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.11.012 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet