Elsevier

Physics Letters A

Volume 373, Issues 27–28, 22 June 2009, Pages 2389-2392
Physics Letters A

Effect of thickness on the dielectric property and nonlinear current–voltage behavior of CaCu3Ti4O12 thin films

https://doi.org/10.1016/j.physleta.2009.05.001Get rights and content

Abstract

Dielectric properties and leakage current behavior in CaCu3Ti4O12 (CCTO) films with different thickness are studied. It is found that the permittivity is affected simultaneously by the electrode interface and the microstructure of the film. The influence of the interfacial layer reduces and the ac electric process transfers from grain boundary to bulk conductivity relaxation with the film thickness increasing. The Poole–Frenkel emission is dominated in thicker CCTO film. Interface-related conductive process coexists with the bulk-limited process in thinner film.

Introduction

As a complex cubic perovskite-like oxide, CaCu3Ti4O12 (CCTO) has attracted considerable attention in recent years due to its remarkable properties [1], [2], [3], [4], [5], [6], [7], [8]. CCTO exhibits a colossal dielectric constant (CDC) at room temperature and weak temperature dependence between 100 and 400 K, which indicates that CCTO has potential application in devices based on capacitive elements, for example the dynamic random access memories (DRAM) [1], [2], [3], [4]. Another intriguing aspect of CCTO is the nonlinear current–voltage (IV) behavior observed by Chung et al. in 2004 [1]. The nonlinear coefficient reaches the maximum about 900, which suggests that CCTO is a promising candidate for varistor. Up to now, many explanations about the origin of the CDC in CCTO have been proposed [2], [3], [4], [5], [6]. Some researchers attributed the CDC to the intrinsic property, for example, the off-center displacement of Ti ions [2]. Others researchers ascribed the CDC to the extrinsic effects, e.g. the Maxwell–Wagner effect [3], [4], [5], [6]. Furthermore, studies on the nonlinear electric behaviors in CCTO are also carried out. Chung et al. and Cai et al. investigated the nonlinear behaviors in Nb/Ta-doped and Mn-doped CCTO ceramic, respectively [7], [8].

Compared to the studies on CCTO crystal and ceramic, many work focus on the CCTO film for the integration of CCTO into the microelectronic devices based on silicon [9], [10], [11], [12], [13]. CCTO films have been prepared by many methods including pulsed laser deposition (PLD), sol-gel. Some technologies have also been employed to depress the dielectric loss in CCTO films [12], [13]. For dielectric films, it has been proven that the thickness has great influence on the electric properties [14]. With a view to the applications of CCTO film, detailed investigations about the effect of the thickness on the properties of CCTO are necessary. In this Letter, we reported the researches about the influence of thickness on the dielectric properties and IV behaviors of CCTO films prepared by sol-gel method. The results are significative for the applications of CCTO thin film in microelectronic devices.

Section snippets

Experimental procedures

We have found that the LaNiO3 (LNO) bottom electrode can be used to reduce the dielectric loss of CCTO [15]. Therefore, the CCTO thin films studied in this work are fabricated by sol-gel on LNO coated silicon wafer. The precursor of CCTO was spin coated on substrate and the wet films were annealed in a rapid thermal process (RTP) furnace. The spin-coating and annealing were repeated several times until the film reached expected thickness. Details about the film preparation have been published

Dielectric properties

Fig. 1 shows the frequency dependence of the relative complex permittivity (ε) of the three samples. The real parts (ε) decreases slightly in the range between 100 Hz and 100 kHz and decreases rapidly in the range between 100 kHz and 1 MHz with the frequency increasing. The imaginary part (ε) reaches the minimum nearby 10 kHz and exhibits a relaxation peak at 600 kHz. All of these features are similar to the reported results [8], [17], [18]. The positions of the relaxation peaks in the

Conclusions

In summary, the effect of thickness on CCTO film has been discussed based on the experimental results of dielectric and IV measurements. It is indicated that the electric properties of the films with thickness less than 100 nm are affected not only by the interfacial layer between the metal electrode and CCTO film, but also by the microstructure of the films. The effect of interfacial layer is evident in film with thickness less than 50 nm, and reduces as the film thickness increasing. The

Acknowledgements

This work was sponsored by Major State Basic Research Development Program of China (2007CB924900), China Postdoctoral Science Foundation (20070420633), Shanghai Municipal Commission of Science and Technology Project (07DZ22943, 07JC14018, 08JC1409000, 08520706100), Shanghai Leading Academic Discipline Project (B411) Shanghai Pujiang Program (07PJ14034), and Shanghai Postdoctoral Scientific Program (07R214118).

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