Scanning Probe Microscopy on heterogeneous CaCu3Ti4O12 thin films
© Fiorenza et al; licensee Springer. 2011
Received: 8 September 2010
Accepted: 4 February 2011
Published: 4 February 2011
The conductive atomic force microscopy provided a local characterization of the dielectric heterogeneities in CaCu3Ti4O12 (CCTO) thin films deposited by MOCVD on IrO2 bottom electrode. In particular, both techniques have been employed to clarify the role of the inter- and sub-granular features in terms of conductive and insulating regions. The microstructure and the dielectric properties of CCTO thin films have been studied and the evidence of internal barriers in CCTO thin films has been provided. The role of internal barriers and the possible explanation for the extrinsic origin of the giant dielectric response in CCTO has been evaluated.
The electrical properties of CaCu3Ti4O12 (CCTO) ceramics and single crystals received considerable attention due to the effective huge permittivity (up to 105) measured in the radio frequencies range, furthermore stable in the 100-400 K temperature range [1–3]. In the recent literature, this giant permittivity has been commonly related to extrinsic effects, i.e. not associated to the bulk material property itself. Possible extrinsic mechanisms to account for the colossal permittivity behaviour have been supported by results from impedance spectroscopy (IS) , Raman spectroscopy  and first-principles calculations . In particular, the IS data on CCTO polycrystalline ceramics reported so far, have been modelled considering an equivalent circuit of two elements, each consisting of a parallel resistor-capacitor (RC), connected in series. One RC element (Rgb and Cgb) simulates the grain boundary response, whereas the other (Rb and Cb) simulates the bulk contribution . The model is suitable to simulate, in a first approximation, the measured capacitance (C) vs. frequency (f) curves showing relaxation at high frequencies. Therefore, the origin of the huge permittivity, arising from the capacitive response before the observed relaxation, has been mainly attributed to an internal barrier layer capacitor (IBLC) behaviour associated with insulating grain boundaries and semiconducting grains structure. This explanation has been corroborated imaging the insulating barriers at the grain boundaries of CCTO ceramics by both nanocontact current-voltage measurements  and Scanning Probe Microscopy (SPM) with conductive tips [8, 9] as already demonstrated on other microelectronic investigation [10, 11].
However, for microelectronics applications, CCTO thin films are much more interesting than ceramics, thus for those applications the occurrence and the origin of the high permittivity deserve to be reliable demonstrated and studied specifically in thin films. In this context, it should be noted that the IBLC model cannot be responsible for the giant permittivity observed in CCTO single crystals  as well as in epitaxial columnar thin films , where no grain boundary is crossed between the two planar electrodes parallel to the surface. In fact, the giant response, indeed observed nowadays in thin films, has been explained considering an electrode effect according to the Maxwell-Wagner (MW) model , and this raises the question, to date not definitively studied and discussed, about the CCTO capacitor reliability and the importance of Schottky barriers at the electrode-surface interfaces .
In this paper, we report on CCTO thin films deposited by Metal-Organic Chemical Vapor Deposition (MOCVD) possessing a "bricks wall" (BW) morphology and a giant permittivity. In this case the IBLC effect can be present. Here, we demonstrate its occurrence and we evaluate the necessary conditions for a reproducible achievement of huge capacitive density in CCTO integrated condensers.
The electrical characterization at nanometre scale was performed by a VEECO D3100 atomic force microscope (AFM) equipped with a Nanoscope V controller and the Nanoman head operating in air, in contact mode and in closed loop condition, using the Conductive Atomic Force Microscopy (C-AFM) module. Standard experiments were carried out using Nanoworld boron doped diamond tips [19–22]. Laser off measurements have been also carried out to exclude the influence of the laser on the reported electrical measurements at nanoscale.
The macroscopic capacitances versus frequency (C-f) measurements were carried out on Pt/CCTO/IrO2 capacitors by adopting the Terman method and using a HP 4284A equipment at an AC voltage with a fixed amplitude of 50 mV. The test devices have been fabricated with top electrodes having an area of 104 μm2 obtained by a photolithographic lift-off process of the sputtered platinum layer.
The macroscopic characteristics were collected at different temperatures, in a range from 298 to 473 K.
The present CCTO films possess a BW structure with conducting grains surrounded by insulating grain boundaries, thus prompting to consider the IBLC model as a possible explanation for the observed temperature dependence of the relaxation frequencies.
Previous reports [26, 27] have shown that the microstructure and the electrical properties of CCTO ceramics are strongly dependent on processing conditions. In fact, the grain size increases with increasing the sintering temperature and/or the processing time as well [26, 27]. The presence of the IBLC effect on CCTO ceramics has been also reported and related to the synthesis conditions.
The fabrication of "bricks wall" CCTO thin films encourages the analogy with the ceramics (not possible for columnar films). Both the presence of a temperature relaxation frequency dependence (Figure 2a) and the presence of insulating grain boundaries surrounding semiconducting grains (Figure 3a) urges the use of the IBLC model to explain the giant permittivity response in thin films.
Considering now the dielectric characteristics (Figure 2) when the IBLC is present, the temperature dependent relaxation frequency can be used to study the electrical properties of the grain boundaries. Their barrier height can be determined by measuring the current flowing in a wide temperature range (298-473 K). In fact, the presence of internal barriers can be related to a hopping transport model inducing a thermal activated conductivity . The Arrhenius plot of the measured conductivity allowed to estimate the grain boundary barrier activation energy, it is Ea~0.25 eV. This measured activation energy for the conduction in the CCTO films is lower than found in ceramics [26, 27]; this discrepancy can be essentially explained by the different conducting/insulator volume fraction in the two cases due mainly to the huge difference in the grain size.
Finally, it is noteworthy that remarkable high capacitance density (about 100 nF/mm2) can be achieved at room temperature with a reasonable dispersion factor (tanδ < 1 at 1 MHz) and in a wide frequency range (102-106 Hz) at 473 K.
CCTO thin films presenting a BW structure have been fabricated by MOCVD. In these films the main mechanism has been proposed for the explanation of the extrinsic giant permittivity response. The presence of the IBLC effect was demonstrated. Remarkable high capacitance density (about 100 nF/mm2) can be achieved at room temperature.
The authors wish to thank Mr. Salvatore Di Franco of the CNR-IMM of Catania for assisting in lithographic processes.
This work has been supported by European Union under the project NUOTO (New Materials with Ultrahigh k dielectric constant fOr TOmorrow wireless electronics). NMP3-CT-2006-032644.
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