X-ray diffraction analysis
shows different X-ray diffractograms of BaTiO3 thin films deposited on bare silicon substrates and subjected to an annealing treatment at 600°C or 700°C. The thicknesses of the BTO films are determined as 150 ± 3 nm from spectroscopic (wavelength range approximately 300 to 1,500 nm) ellipsometry measurements. To analyze the films, we have used a multilayer system, where the buffer layer and BTO film (extraordinary and ordinary optical constants) are modeled with corresponding cauchy parameters. It is evident from Figure
that a minimum thickness of the buffer layer is necessary to prevent silicate formation at the Si-BTO interface and to promote crystal growth with a desired orientation.
Figure 1 XRD patterns obtained for the BTO thin films. (a) BTO annealed at 700°C, with buffer layers of different thickness. (b) BTO annealed at different temperatures, with a 8.9-nm buffer layer. (c) BTO annealed at 700°C, with a 8.9-nm buffer (more ...)
a 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 Figure
b 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. Figure
c 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.
Crystal orientation of BTO thin film
The dielectric, piezoelectric, and electro-optical properties of the thin films depend strongly on the crystal orientation. Highly c
-axis-oriented BTO thin films reported before are grown on either a single-crystalline oxide substrate or with a preferentially oriented thick (>100 nm) conductive or dielectric intermediate buffer layer
]. The use of a thick buffer layer limits the performance of the ferroelectric films for certain applications (e.g., electro-optical devices). The results shown in Figure
indicate that we can grow highly c
-axis textured BTO films with LaO(NO3
) buffer layers (keeping the buffer layer thickness as 8.9 nm) by adding the number of annealing steps.
Figure 2 XRD patterns obtained for BTO thin films. The films were deposited on a buffer layer with a thickness of 8.9 nm and a BTO seed layer of 30 nm (a) annealing after each 30-nm BTO layer deposition at different temperatures and (b) annealing at 700°C (more ...)
shows the XRD pattern of BTO films grown on a BTO seed layer. The seed layer is prepared by depositing a thin layer (30 nm) of BTO film on the buffer layer (8.9 nm), followed by pyrolysis (350°C) and annealing (700°C). After the seed layer, either the normal procedure is followed (annealing after 120 nm of BTO is deposited) or layer-by-layer annealing is used (after each 30-nm deposition). It is clear from Figure
that the BTO film grown over a BTO seeding layer has different crystallization properties, compared to the results mentioned in Figure
. The BTO thin films grown with layer-by-layer annealing method show a preferential <100> orientation. The films annealed at both 650°C and 700°C show strong diffraction peaks along the <100> and <200> directions, with no sign of the secondary-phase silicate formation. It is evident from Figure
b that the BTO films that are annealed after deposition of 120 nm of BTO (prepared by two to three spin coating and pyrolysis steps) show a stronger diffraction peak along the <110> direction (compared to the <100> direction). A comparison of the lattice parameters of the BTO film deposited on different buffer layers with bulk BTO crystal is mentioned in Table
Comparison of the BTO thin films deposited on different buffer layers with the bulk material
Microstructure and roughness measurements
The SEM images of BTO thin films grown on silicon <100> substrates with different thicknesses of the lanthanum oxynitrate buffer layer are presented in Figure
. The films annealed at 600°C (not shown) with buffer layers of different thickness are amorphous, and no distinct crystal grains are visible from the SEM measurements.
Figure 3 SEM top view and cross-section images of BTO thin films. SEM top view of BTO films annealed at 700°C, with buffer layers of (a) 6 nm and (b) 7.2 nm. Cross-section images of the BTO film deposited at 700°C (c) deposited with a buffer layer (more ...)
a,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 Figure
a,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 (Figure
d). 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 Figure
c,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
AFM measurements are carried out to estimate the roughness properties of the BTO films. The AFM images of the 150-nm-thick BTO films deposited at 700°C for different thicknesses of the buffer layers are shown in Figure
a,b. The film deposited with the 4.4-nm buffer layer shows a roughness less than 10 nm, whereas the films deposited with buffer layers greater than 6 nm, show a larger roughness (10 to 15 nm) because of larger grain sizes.
AFM images of BTO thin films deposited at 700°C for different thicknesses of intermediate buffer layers. (a) 6 nm and (b) 7.2 nm.
Dielectric and ferroelectric properties
The dielectric and ferroelectric properties of BTO thin films (thickness 150 nm, annealing temperature 700°C) grown on lanthanum oxynitrate buffer layers (thickness 7.2 nm or 8.9 nm, heat treatment 450°C) are estimated with C-V and P-E measurements. The C-V measurement shows the small signal capacitance as a function of a bias DC voltage (see Figure
a). The butterfly shape indicates the ferroelectric hysteresis nature of the BTO tetragonal films. Two maxima for the dielectric constants are observed depending on an increase or decrease in the bias electric field.
Figure 5 AC dielectric constant and P-E hysteresis loop. (a) AC dielectric constant as a function of the DC bias voltage for a BTO thin film (150 nm) annealed at 700°C with a 7.2-nm-thick buffer layer. (b) P-E hysteresis loop measured at 1 KHz with an (more ...)
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.