Annealing Effects on Electrical Properties and Defects of CaCu 3 Ti 4 O 12 Thin Films Deposited by Pulsed Laser Depositi
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1256-N02-06
Annealing Effects on Electrical Properties and Defects of CaCu3Ti4O12 Thin Films Deposited by Pulsed Laser Deposition Guochu Deng and Paul Muralt Laboratoire de Céramique, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland. ABSTRACT We present a systematic study of semiconductor and dielectric properties as a function of annealing treatments at CCTO thin films deposited by pulsed laser deposition at 720 °C in 200 mTorr oxygen. The as-deposited thin film samples contain a high concentration of defects that contribute to the semiconductivity in the interior of grains. With increasing annealing temperature, the apparent dielectric constant decreases, and the resistance increases, both at a given temperature (e.g. room temperature). After annealing at 680oC, the semiconductivity was almost completely suppressed and CaCu3Ti4O12 behaved as a dielectric material. Knowing that oxygen vacancies are removed during annealing, one can infer that the dopant states are related to oxygen vacancies. A double plateau behavior in the dielectric constant vs temperature graph indicates that there are at least two defect levels in CaCu3Ti4O12 thin films. This was confirmed by simulating the capacitance response of a Schottky barrier containing two defect levels. Apart of the usual acceptor level, a trap at 500 meV from the valence band was identified. The finally achieved quasi intrinsic material exhibits a negative temperature dependency of the dielectric constant below 120 K. INTRODUCTION The colossal dielectric constant (CDC) phenomenon, which was firstly observed in CaCu3Ti4O12 (CCTO) ceramics,1 remains the subject of much scientific interest due the unsatisfying elucidation of the CDC phenomenon. Intensive investigations were conducted in both experimental and theoretical ways2 at bulk ceramics3, single crystals2, and polycrystalline as well as epitaxial thin films on various substrates4,5. All of these samples showed the characteristic CDC features. Similar high dielectric constants were also observed in further compositions derived from CCTO, like (Bi1/2Na1/2)Cu3Ti4O12.6 These experimental efforts provided abundant results and facts about CDC phenomena and materials. Nevertheless, the complexity of findings has lead to contradicting explanations. An intrinsic origin in the form of a ferroelectric relaxor mechanism was still defended in 2005,7 while others were excluding an intrinsic origin and looking for an extrinsic one, more precisely a barrier layer capacitance effect. In 2002, Sabramanian and Sleight proposed that the numerous twin boundaries may constitute such barrier layers.8 In the same year, D. Sinclair et al. concluded from impedance spectroscopy that CCTO ceramics is electrically heterogeneous, composed of semiconducting grains with insulating grain boundaries, and attribute the CDC phenomenon to an internal barrier layer capacitance (IBLC).3 This explanation proved to be correct for the ceramic CCTO case, however, it encountered difficulties when it was applied to the single crystals
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