Pt thin film capacitors

Materials Letters 58 (2004) 1715 – 1721 www.elsevier.com/locate/matlet

Effects of post-annealing on the dielectric properties of Au/BaTiO3/Pt thin film capacitors E.J.H. Lee a, F.M. Pontes a, E.R Leite a,*, E. Longo a, R. Magnani a, P.S. Pizani b, J.A. Varela c a

LIEC, CMDMC, Department of Chemistry, UFSCar, Via Washington Luiz, km 235, CP-676, CEP-13565-905, Sa˜o Carlos, SP, Brazil b Department of Physics, UFSCar, Via Washington Luiz, km 235, CEP-13565-905, Sa˜o Carlos, SP, Brazil c Institute of Chemistry, UNESP, Araraquara, SP, Brazil Received 6 January 2003; received in revised form 17 October 2003; accepted 17 October 2003

Abstract Barium titanate thin films were prepared by the polymeric precursor method and deposited onto Pt/Ti/SiO2/Si substrates. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR) and micro-Raman spectroscopy were used to investigate the formation of the BaTiO3 perovskite phase. Afterwards, the films were submitted to post-annealing treatments in oxygen and nitrogen atmospheres at 300 jC for 2 h, and had their dielectric properties measured. It was observed that the electric properties of the thin films are very sensitive to the nature of the post-annealing atmosphere. This study demonstrates that post-annealing in an oxygen atmosphere increases the dielectric relaxation phenomenon and that post-annealing in a nitrogen atmosphere produces a slight dielectric relaxation. D 2004 Elsevier B.V. All rights reserved. Keywords: Barium titanate; Dielectric properties; Conduction mechanism; Post-annealing

1. Introduction The development of thin film technology has been receiving great investments due to the possibility of achieving a greater miniaturization of integrated electronic circuits used nowadays. Barium titanate (BaTiO3) is one of the most studied perovskite-type materials, especially due to its potentiality in applications such as capacitive and nonvolatile memory cells (DRAMs and FRAMs), electro-optical integrated circuits, pyroelectric devices, etc. [1 –5]. Many techniques have been studied to deposit oxide films, some of which are chemical methods, while others are physical methods. Laser ablation [6], radio-frequency sputtering [7] and pulsed laser deposition [8] are some of the physical methods. These deposition techniques provide highly homogeneous thin films, and a great control on the thickness, but they require complex and expensive equipments. The metallorganic chemical vapor deposition (MOCVD) method [9] has the advantage to be able to deposit thin films in large areas, which is very interesting

* Corresponding author. E-mail address: [email protected] (E.R. Leite). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.10.047

for industrial applications. Other chemical methods, such as sol – gel processes [10], are cheaper, and have an easier processability and ability to control the stoichiometry and microstructure of materials. This control enables the possibility of attaining the desired properties. The polymeric precursor method is a sol – gel derived process, which has the advantage of being a water-based process, and therefore does not require a special control of the atmosphere conditions. A great deal of studies on perovskite-type thin films, obtained by the polymeric precursor method, have been done, with results that demonstrate a great potential for thin film technology applications [11– 13]. The industrial development of a material depends on some parameters, such as easy processability, low heat treatment temperatures, good properties at room temperature and protection against failure occurrences. Due to these requisites, a great deal of studies on the transport of charged species have been realized on BaTiO3 thin films, since charged species can lead to changes in the operational characteristics of devices or functional failures. Charge transport in bulk barium titanate occurs basically due to the oxygen non-stoichiometry. The conduction characteristics are different according to the oxygen activity. Oxygendeficient regions present n-type conduction, while oxygen-

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rich regions present p-type conduction [14]. In thin films, the conduction mechanisms are different and have not been fully elucidated. Pontes et al. [15] studied the dielectric and ferroelectric properties of post-annealed BST thin films treated in different atmospheres. They observed that these properties are considerably sensitive to the post-annealing treatment. Moreover, Yi et al. [16] observed better dielectric properties in Mg-doped (Ba,Ca)(Ti,Zr)O3 thin films postannealed in oxidizing atmospheres. This paper reports the changes in the dielectric properties and conduction mechanism of BaTiO3 thin films when these are submitted to post-annealing treatments in oxygen and nitrogen atmospheres.

2. Experimental procedure In order to obtain the BaTiO3 thin films, water-based solutions containing barium and titanium cations were prepared. The raw materials for the solution synthesis were Ti[OCH(CH3)2]4 (titanium isopropoxide (IV)), ethylene glycol, citric acid and barium carbonate (BaCO3). In the first place, a titanium citrate solution was obtained by the dissolution of titanium isopropoxide (IV) in a water solution of citric acid. Afterwards, barium carbonate was added stoichiometrically to the citrate, in order to chelate barium cations. The complete dissolution of the salt was achieved by the addition of ammonium hydroxide, which prevented the precipitation. Ethylene glycol was then added to the solution to promote polyesterification reactions. The citric acid/ethylene glycol mass ratio used was fixed at 60:40. The viscosity of the solutions was adjusted by controlling the water content, and was fixed at 20 mPa s. The precursor solutions were deposited onto Pt/Ti/SiO2/ Si substrates, by the spin-coating technique. The deposition speed and time were 5000 rpm and 30 s, respectively. After the deposition, the films were dried on a hot plate at approximately 60 jC for 30 min and then were led to a heat treatment. The heat treatment consisted of two stages: (a) firstly, the films were treated at 400 jC during 2 h in order to pyrolyze the organic material, (b) then the films were treated at 700 jC during 2 h in order to promote the crystallization. The heating and cooling rates were fixed at 5 jC/min. The BaTiO3 thin films were structurally characterized by X-ray diffraction (XRD) (Cu Ka radiation) in the mode of 2h –h scan, recorded on a Rigaku diffractometer (D/max2400). The power conditions were 40 kV at 150 mA. The micro-Raman measurements were performed at room temperature in the thin films, using the 514.5 nm line of an argon ion laser as the excitation source. The power was kept at 15 mW and a 100  lens was used. The spectra were recorded using a T-64 Jobin – Yvon triple monochromator coupled to a CCD detector. The infrared spectra were obtained by means of a Equinox/55 (Bruker) Fourier transformed infrared (FTIR) spectrometer equipped with a 30j

specular reflectance accessory. The FTIR reflectance spectra of thin films were recorded at room temperature in the wavenumber range 350 –1200 cm 1. The surface and cross-section morphology of the thin films were examined using a Zeiss DSM940A scanning electron microscope (SEM). The film thickness was measured by observing the cross-section of the films by SEM. Atomic force microscopy (AFM) was used to obtain an accurate analysis of the sample surface and the quantification of very important parameters such as roughness and grain size. A Digital Instruments Multimode Nanoscope IIIa (Santa Barbara, CA) was used. In order to perform the electrical measurements, gold electrodes were sputtered onto the film surfaces to enable the electric measurements, forming parallel-plate capacitors with a metal – ferroelectric – metal configuration (MFM). After deposition of the top electrodes, the films were subjected to different post-annealing treatments in a tube furnace at 300 jC for 2 h. The measurements were carried out at room temperature after each post-annealing treatment. The same sample was used in all post-annealing experiments. Dielectric constant and loss as a function of the frequency were measured by means of a Hewlett Packard HP4194A equipment. The leakage current density – electric field ( J– E) characteristics were measured by a Keithley 237 High Voltage Source Measure Unit.

3. Results and discussion The crystalline nature of the films was studied by X-ray diffraction. Fig. 1 shows the X-ray diffraction pattern of BaTiO3 thin film on a Pt/Ti/SiO2/Si substrate annealed at 700 jC for 2 h. BaTiO3 crystal phase with the perovskite structure was observed in the samples annealed at 700 jC.

Fig. 1. X-ray diffraction pattern of the BaTiO3 thin film annealed in air at 700 jC for 2 h.

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The XRD patterns also show that the films were polycrystalline with no evidence of preferential orientation or secondary phases. Fig. 2 shows the FTIR spectrum of the BaTiO3 film heattreated at 700 jC for 2 h. Absorption peaks at 700, 495, 454 and 400 cm 1 are apparent in the spectrum. These results are in agreement with the ones reported by Berreman [17]. It is commonly found in the literature that these peaks may be ascribed to the resonance with the longitudinal optical (LO) phonon modes and transverse optic (TO) phonon modes associated with the crystallinity of BaTiO3 films. BaTiO3 films are characterized by a transverse optical (TO) mode at around 495 cm 1, attributed to the stretching mode of TiO6 octahedra. At thin films, reflectance minima may also be observed corresponding to the longitudinal optic (LO) modes at around 700 and 453 cm 1. The observation of the LO modes is possible due to the finite thickness of the films in a phenomenon known as the Berreman effect [17]. Fig. 3 shows the room temperature Raman spectrum of BaTiO3 film annealed at 700 jC for 2 h. The appearance of a peak at 305 cm 1 indicates asymmetry within the TiO6 octahedra of BaTiO3 thin films prepared by the polymeric precursor method. Therefore, it can be assumed that the formed BaTiO3 film does not have cubic symmetry. In addition, the higher sensitivity of Raman spectroscopy to prove the local rather than long range structure seems to demonstrate that the BaTiO3 prepared by the polymeric precursor method is tetragonal, rather than cubic. Therefore, the Raman spectroscopy provides unambiguous confirmation of the presence of the tetragonal ferroelectric phase of BaTiO3 through the observation of the 305 and 720 cm 1 peaks [17,18]. The film thickness value was estimated at approximately 635 nm by the examination of cross-section in a scanning electron microscope. Fig. 4 shows a surface image of a

Fig. 2. FTIR spectrum of the BaTiO3 thin film annealed in air at 700 jC for 2 h.

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Fig. 3. Room temperature micro-Raman spectrum of a BaTiO3 thin film annealed in air at 700 jC for 2 h.

BaTiO3 thin film annealed at 700 jC for 2 h, obtained by atomic force microscopy (AFM). The thin films exhibit a granular structure, which indicates their crystalline nature. Average grain size and surface roughness were estimated at 80 – 100 and 4 nm, respectively. Fig. 5 shows the variation of the dielectric constant and dielectric loss as a function of frequency for Au/BaTiO3/Pt capacitors annealed in O2 and in N2 atmospheres at 300 jC for 2 h. The as-deposited capacitors presented good dielectric properties, where the dielectric constant and dielectric loss values at 100 kHz were approximately 380 and 0.07, respectively. The as-deposited capacitors were submitted to four post-annealing processes, as described in Table 1. The dielectric behavior of the capacitors is extremely sensitive to the post-annealing atmosphere. The oxygen post-annealed capacitors presented a strong dielectric relaxation effect at the low frequency range (102 Hz – 104 kHz). It is possible that this phenomenon is related to space charge polarization. The oxygen post-annealing treatment may generate negatively charged oxygen ions that may be adsorbed by the films resulting in polarized regions. This relaxation can be probably related to these polarized nanoregions that can be located at the thin film – electrode interface and/or at the BaTiO3 grain boundaries. This hypothesis seems plausible, since it is unlikely that a low temperature treatment could change the remaining polariztion mechanims. The dielectric properties are related to the polarizability of materials and receive contributions from four mechanisms: electronic polarization, ionic polarization, dipole polarization and space charge polarization. Each one of these mechanisms contributes at different frequency ranges, due to the dependence of the polarization-responsible species on electric field [19]. However, space charge polarization contributes to the increasing of the polarization of dielectric materials at the low frequency range, as observed

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Fig. 4. Surface micrograph of a BaTiO3 thin film heat-treated at 700 jC, carried out by atomic force microscopy.

in the results described. Fig. 5b also shows the influence of post-annealing under different atmospheres on the dielectric loss (tan d) as a function of frequency for capacitors. These curves showed a distinctly visible characteristic peak around the frequency range of 102 –104 Hz when the capacitors were post-annealed in oxygen at the frequency range corresponding to the largest dispersion for dielectric constant (Fig. 5a). These curves demonstrate that the oxygen atmosphere contributes to the relaxation phenomenon in the dielectric behaviour. The curves became flat (Fig. 5b) after

post-annealing in a nitrogen atmosphere. This result strongly suggests that the relaxation phenomenon arises from the post-annealing treatment in an oxygen atmosphere. These results are in agreement with the dielectric constant measurements (Fig. 5a). The subsequent nitrogen post-annealing diminished the low frequency dielectric relaxation effect on the capacitors. This indicates that, after the nitrogen treatment, the space charge polarization has a reduced effect. The dielectric behavior of the nitrogen post-annealed capacitors is very

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Fig. 5. Dielectric constant (a) and dielectric loss (b), as functions of the frequency for BaTiO3 capacitors, after being submitted to different post-annealing atmospheres.

similar to the one observed for the as-deposited capacitors. Probably the nitrogen atmosphere removes the charges accumulated during the oxygen post-annealing. These results Table 1 The post-annealing condition of each step Step

Post-annealing conditions

First treatment

Second treatment

1

2

3

4

300 jC O2 , 2 h

300 jC N2 , 2 h

300 jC O2, 2 h

300 jC N2, 2 h

are in good agreement with the ones obtained by Pontes et al. [15]. On the other hand, the results obtained by Yi et al. [16] are quite different, since the oxygen post-annealing treatments in that study were done at high temperatures (700 jC), which led to the filling of oxygen vacancies, consequently enhancing the dielectric properties of the thin films. When the capacitors were again post-annealed in oxygen, the phenomenon of the space charge polarization appeared and a large dielectric relaxation was once more observed (Fig. 5a), while the nitrogen atmosphere reduces it. This result strongly indicates that the dielectric relaxation ob-

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served is not a bulk-related phenomenon, but an oxygenrelated interface phenomenon. However, the dielectric relaxation still could be observed in the films after the second nitrogen treatment. The most probable reason for this effect is that the second nitrogen treatment was unable to remove all the charged species accumulated during the second oxygen post-annealing. The J –E (leakage current density versus electric field) characteristics can be seen in Fig. 6. The as-deposited capacitors presented an ohmic behavior at low electric fields. However, when a critical electrical field is attained,

the leakage current– electric field relationship deviates from linearity, indicating that the conduction mechanism changes from ohmic to either a Schottky or a Poole – Frenkel emission (Fig. 6a). To obtain more details on the electric conduction mechanism of these films, other measurements such as complex impedance or current– voltage as a function of the temperature would be required. The oxygen post-annealed capacitors present a J – E characteristic strictly nonlinear (Fig. 6b). It is possible that this effect is due to charges accumulated during the oxygen post-annealing. These charges may form a space charge

Fig. 6. Leakage current behavior of the BaTiO3 capacitors after being submitted to different post-annealing atmospheres.

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cloud, which discourages charge injection from the electrode to the thin film, resulting in the observed jump of the J –E curve. Therefore the conduction mechanism is changed to a space-charge-limited (SCL) conduction. The J – E behavior of the nitrogen post-annealed thin films is very similar to the one observed for the as-deposited films. On the other hand, the conduction mechanism of the capacitors after the second nitrogen post-annealing is similar to the ones observed in the oxygen post-annealed capacitors, indicating the presence of space charges. The described results indicate that oxygen post-annealing treatments provoke an increase on the dielectric relaxation phenomenon, probably due to the accumulation of space charges (oxygen ions) in the capacitors. More conclusive information such as the identification and localization of the oxygen ions in the thin films are being currently done. On the other hand, the nitrogen atmosphere removes part of the adsorbed space charges. Pontes et al. [15] observed that the nitrogen post-annealing treatment had a better efficiency in removing space charges of BST thin films. It is possible that the increase in the charged species removal efficiency is due to the less ionic nature of strontium, when compared to barium. To enhance the dielectric properties of BaTiO3 thin films, it would possibly require a post-annealing at higher temperatures and/or pressures. Another possibility would be to dope the BaTiO3 thin films with less ionic metals, such as magnesium, calcium, etc.

4. Conclusions The atmosphere used in the post-annealing treatment considerably affected the dielectric properties. The space charge polarization is inherently related to the non-uniform charge accumulation. These charges can be originated during the post-annealing treatment in an oxygen atmosphere. Post-annealing in oxygen provokes an increase on the relaxation phenomenon, possibly due to the accumulation of negatively charged oxygen ions, by their adsorption on the thin films. This effect results in the degradation of the dielectric properties of the thin films. The oxygen postannealing presented higher dielectric loss and a spacecharge-limited conduction behavior. Post-annealing in nitrogen atmospheres diminished this effect, probably by the removal of part of the charged species adsorbed on the electrode – thin film interface. Since not all space charges are eliminated, the nitrogen post-annealing treatments were not sufficient to restore the original dielectric properties of the

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films. Further studies on the post-annealing atmosphere effects on the dielectric properties of thin films are being currently done. It could be concluded that it is important to observe the nature of the atmosphere used in the annealing and postannealing of thin films to be applied in electronics, since this could lead to the degradation or failure of the material.

Acknowledgements The authors gratefully acknowledge the financial support from the Brazilian financing agencies—FAPESP, CNPq, PRONEX and Capes.

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