Preferentially oriented BaTiO3 thin films deposited on silicon with thin intermediate buffer layers
© George et al.; licensee Springer. 2013
Received: 4 January 2013
Accepted: 26 January 2013
Published: 7 February 2013
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© George et al.; licensee Springer. 2013
Received: 4 January 2013
Accepted: 26 January 2013
Published: 7 February 2013
Barium titanate (BaTiO3) thin films are prepared by conventional 2-methoxy ethanol-based chemical solution deposition. We report highly c-axis-oriented BaTiO3 thin films grown on silicon substrates, coated with a lanthanum oxynitrate buffer layer of 8.9 nm. The influence of the intermediate buffer layer on the crystallization of BaTiO3 film is investigated. The annealing temperature and buffer layer sintering conditions are optimized to obtain good crystal growth. X-ray diffraction measurements show the growth of highly oriented BaTiO3 thin films having a single perovskite phase with tetragonal geometry. The scanning electron microscopy and atomic force microscopy studies indicate the presence of smooth, crack-free, uniform layers, with densely packed crystal grains on the silicon surface. A BaTiO3 film of 150-nm thickness, deposited on a buffer layer of 7.2 nm, shows a dielectric constant of 270, remnant polarization (2Pr) of 5 μC/cm2, and coercive field (Ec) of 60 kV/cm.
Barium titanate (BaTiO3 or BTO) thin films have been extensively studied over the years because of the wide range of applications in thin-film capacitors, non-volatile memories, electro-optical devices, and MEMS devices, owing to their interesting dielectric, ferroelectric, piezoelectric and electro-optical properties. A variety of methods have been demonstrated for the growth of BTO thin films. Chemical solution deposition has gained wide acceptance because of its low capital investment, simplicity in processing, and easy composition control.
The epitaxial deposition of thin films on silicon substrates is a key technology for the development of small photonic and electronic devices, based on the current CMOS fabrication platform. The leakage current and optical scattering are expected to be much smaller for epitaxial thin films compared to polycrystalline thin films. However, the epitaxial growth of ferroelectric thin films on silicon substrates still remains a challenge. It has been reported that the deposition at elevated temperatures causes severe reactions at the thin film/silicon interfaces, resulting in silicate formation and degradation of the quality of the thin films. Interdiffusion of silicon and the constituent elements at high temperature results in intermediate pyrochlore and secondary-phase formation rather than a pure perovskite phase. Different methods have been proposed to use either a seed or buffer layer to promote crystal growth. Single-crystalline substrates as well as oriented thin films of MgO (1 0 0), SrTiO3 (1 0 0), LaAlO3, SRO/CeO2/YSZ, LaNiO3 (1 0 0), and Pt/Ti/ SiO2 have been used to promote the growth of perovskite BaTiO3 thin films. Since the structure and orientation of the buffer layer can influence the subsequent ferroelectric thin-film growth, the deposition conditions and processing parameters play an important role[15, 16].
In the present work, we demonstrate the growth of BaTiO3 thin films on silicon substrates by chemical solution deposition. A lanthanum oxynitrate buffer layer of 6- to 10-nm thickness is used as an intermediate buffer layer to promote the BaTiO3 crystal growth. We have studied the influence of the thickness and the heat treatment of the buffer layers and the deposition conditions of the BaTiO3 on the crystallinity, orientation, and morphology of the BaTiO3 films.
Polyvinyl pyrrolidone (45% in water) dissolved in 2-propanol is spin-coated onto the silicon substrate as an adhesion layer prior to the buffer layer deposition. Buffer layer solutions are prepared by dissolving lanthanum nitrate hydrate in 2-propanol. The solution is spin-coated on the silicon wafers at 3,000 rpm for 45 s and subjected to a heat treatment at 450°C for 5 min. Lanthanum nitrate hydrate (La(NO3)3) decomposes through nine endothermic weight loss processes with increasing temperature. Between 440°C and 570°C, the lanthanum nitrate hydrate is decomposed to the intermediate-phase lanthanum oxynitrate (LaONO3). The thickness of the obtained buffer layers in this work ranges between 6 and 10nm as measured with ellipsometry.
Reagent grade barium acetate Ba(CH3COO)2 and titanium butoxide Ti(C4H9O)4 are used as precursor materials for barium and titanium, and glacial acetic acid and 2-methoxy ethanol are used as the solvents. The molarity of the solution is 0.25 M. The BTO precursor sol is spin-coated at 3,500 rpm for 45 s, followed by pyrolysis on a hot stage at 350°C to burn out the organic components. This leads to a film thickness of about 30 nm. This process is repeated three or four times to obtain a film thickness around 100 nm. Then, the silicon substrate with the BTO amorphous film is subjected to a high-temperature annealing at 600°C to 750°C for 20 min, with a tube annealing furnace in ambient air. The ramping rates for heating and cooling of the specimen in the annealing system are 100°C/min and −50°C/min, respectively. The process cycle (two or three spin coatings and subsequent high-temperature treatment) is repeated several times to obtain an oriented thin film with a thickness of a few 100 nm.
The samples are first cleaned with acetone, isopropanol, and de-ionized water. The measurements are carried out with a D8 Discover diffractometer (Bruker Technologies Ltd., Billerica, MA, USA) with CuKα radiation. The diffractograms are recorded for 2θ angles between 15° and 64°, with a step size of 0.004° and time step of 1.2 s.
The cross-section images of the specimens are prepared by a FEI Nova 600 Nanolab dual-beam focused ion beam system (FIB; FEI Co., Hillsboro, OR, USA) and an associated scanning electron microscope (SEM). It allows simultaneous milling and imaging of the specimens. The SEM column is equipped with a high-performance field-emission gun electron source, whereas the FIB system has a gallium liquid metal ion source.
The surface roughness of the BTO thin films are measured by atomic force microscopy (AFM) analysis. The measurements are carried out with Molecular Imaging's Pico Plus modular scanning probe microscope system (Molecular Imaging Inc., Arizona, USA), in contact mode.
Prior to the measurements, a top electrode is deposited with either chromium (Cr) or indium tin oxide (ITO; area 3.14 mm2, thickness 50 to 100 nm) by RF magnetron sputtering. A thin layer (15 to 30 nm) of ITO is used for the bottom electrode. The capacitance versus voltage (C-V) characteristics are measured with a HP4192 ALF impedance analyzer (Agilent Technologies, Santa Clara, CA, USA). The capacitance is measured for a small alternating current (AC) voltage which is superposed on a direct current (DC) voltage offset.
A Sawyer-Tower circuit is used to measure the hysteresis loop in the polarization-electric field (P-E) diagram of the BTO films. The measurements are carried out at frequencies in the range of 100 Hz to 1 kHz with a sinusoidal AC voltage with an amplitude of 10 V peak-to-peak.
Figure1a represents a comparison between the BTO thin films deposited on silicon (annealed at 700°C) with different thicknesses of the intermediate buffer layer. When the buffer layer thickness is 4.4 nm, the secondary fresnoite phases (Ba2TiSi2O8) are dominant and only few diffraction peaks correspond to crystalline BTO. However, it is found from our experiments that a slightly thicker buffer layer of 7 nm is sufficient to yield well-defined diffraction peaks corresponding to stoichiometric BTO (BaTiO3), with a mixed <100> and <111> orientation. Even though a clear peak split is not observed at 45°, the broadened diffraction peak shows the possibility of a <002> BTO orientation. Any further increase in the buffer layer thickness leads to a stronger diffraction intensity along the <100> orientation. The increase in the buffer layer thickness reduces the strain energy within the BTO film and influences orientation of the film with a better <100> texture. However, the deposition of thicker buffer layer is limited because of the poor adhesion of the lanthanum nitrate buffer layer with the underlying PVP organic film. The X-ray diffraction (XRD) measurements indicate that the films are crystallized into a pure perovskite phase, with a tetragonal geometry.
It is evident from Figure1b that no diffraction peaks are observed for the samples (buffer layer thickness 8.9 nm) annealed at 600°C, whereas it shows well-defined peaks for films annealed at 700°C. The films annealed at 600°C do not show any diffraction peaks of fresnoite or BTO, indicating the amorphous nature of the film. The peak observed around 26° correspond to La2O3. The absence of the fresnoite silicate phases also indicates that no reaction happened at the BTO/buffer layer interface due to the interdiffusion of Si. Figure1c shows the XRD patterns of BTO thin films (annealed at 700°C) deposited on 8.9-nm-thick buffer layers that are heat-treated at 450°C or 600°C. It is obvious from the measurements that crystallization of the BTO films is influenced by the heat treatment of the buffer layer. Since the LaO(NO3) intermediate phase is only present up to 570°C, after which an non-stoichiometric unstable La(O)1.5(NO3)0.5 phase appears, it is clear that the LaO(NO3) phase exhibits superior properties as an intermediate layer. The heat treatment influences the nucleation mechanism of the BTO film and results in different diffraction peaks in the XRD spectrum.
Comparison of the BTO thin films deposited on different buffer layers with the bulk material
Figure3a,b shows the top surface view of BTO films annealed at 700°C, with buffer layers of thickness 6 and 7.2 nm, respectively. The presence of the well-defined polygonal crystal grains is visible, and it shows the complete transformation of the amorphous films into a perovskite phase. The presence of the intercrystal voids in the BTO films (approximately 150 nm) deposited with buffer layers less than 6 nm is visible in Figure3a,c. This increases the chance of electrical short circuit between the bottom ITO and the top evaporated Cr contact as we also experienced in the electrical measurements. However, the present work shows that the density of the intercrystal voids can be decreased to a great extent by increasing the thickness of the buffer layer to 7.2 nm. The films deposited with BTO seeding layers have further improved quality and appear to have a dense structure without the presence of pin holes (Figure3d). It is also found that as the thickness of the BTO film before annealing is increased above 150 nm, the annealing process results in nano-cracks on the film surface.
The SEM cross-section images as shown in Figure3c,d are prepared by cleaving the silicon sample. The cleaving causes rough edges, and the brittle nature of the thin film results in numerous regions without material. However, the presence of the thin buffer layer is evident, and the thickness matches with the data from ellipsometry measurements. The grain sizes of the films deposited at 700°C with a buffer layer of thickness of 7.2 nm are found to be between 30 and 50 nm, which is comparable to the other reported values.
The samples deposited with buffer layers below 6 nm often show electrical short circuit between the top and bottom contacts due to the intercrystal void formation. However, the highly oriented BTO films (150 nm) deposited on a BTO seed layer with buffer layers thicker than 7 nm, followed by layer-by-layer coating and annealing procedure (30 nm each time), show well-defined hysteresis loops. The BTO thin films (150 nm) appear to be stable, without breakdown up to electric fields of 400 kV/cm. The polarization of the films does not reach saturation due to the electrical breakdown at higher voltages. The films deposited with a 7-nm buffer layer show a dielectric constant of 270, remnant polarization of (2Pr) 3 μC/cm2, and coercive field (Ec) of 60 kV/cm, whereas the BTO film deposited on an 8.9-nm buffer layer shows a 2Pr of 5 μC/cm2 and Ec of 100 kV/cm.
Well-crystallized BTO thin films are deposited on silicon substrates, coated with a 6- to 9-nm-thick lanthanum oxynitrate buffer layer. We have demonstrated that the thickness of the buffer layer is important for the crystallization, microstructure, and electrical properties of the subsequently deposited BTO thin film. We have also presented a method to control the orientations of the BTO films either by controlling the thickness of the buffer layers or by modifying the deposition procedure. A buffer layer of 6 nm is found efficient to prevent secondary-phase formation and to allow high-temperature deposition. The problems associated with the formation of the intercrystal voids have been improved by controlling the process as well as buffer layer parameters. The BTO films deposited on the 7.2-nm-thick lanthanum nitrate buffer layer show a relative dielectric constant of 270, a remnant polarization (2Pr) of 5 μC/cm2, and a coercive field (Ec) of 100 kV/cm, which make it a suitable candidate for future electronic and photonic devices. Although the electrical properties are not as good as reported elsewhere, we believe this is the thinnest buffer layer reported up to now which results in preferentially oriented and well-crystallized BTO thin films.
This research was supported by the Interuniversity Attraction Poles program of the Belgian Science Policy Office, under grant IAP P7-35 (Photonics@be).
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