Typical sputtering conditions for LNO and BTO films
1. Introduction
Ferroelectric thin films have attracted significant attention recently, due to their great potential for practical use in microelectronic and optoelectronic applications, such as non-volatile ferroelectric random access memories (NvFRAM), high-density capacitors, micro-electromechanical system (MEMS) and electro-optic devices etc. [1,2]. Among all the ferroelectric materials, BaTiO3 (BTO) is most widely investigated not only for its simple chemical composition and remarkable properties (high dielectric constant and non-linear optical properties) but also because of their lead-free and environmental-friendly characteristic, which has become increasingly important from a long term point of view [3]. To date, a number of preparation techniques such as vacuum evaporation [4], sputtering [5], pulsed laser deposition (PLD) [6], molecular beam epitaxy (MBE) [7], metalorganic chemical vapor deposition (MOCVD) [8], sol-gel [9], and hydrothermal method [10] have been reported to fabricate BTO thin films.
On the other hand, newer device concepts may become realities if ferroelectricity can be coupled with conventional Si semiconductor technology. For example, ferroelectric field effect transistors, in which no power is required to maintain logic states, enter the realm of the possible if high-quality ferroelectric oxides can be grown epitaxially on Si with the right kinds of interfaces [11,12]. The criterion for successful integration with semiconductors is the minimum critical thickness for ferroelectricity, which must be of the order of a few nm in order to be compatible with very large scale integration. When in intimate contact with a semiconductor such that the polarization vector is normal to the interface, a ferroelectric oxide tends to develop a depolarizing field that suppresses the ferroelectric ground state. This depolarizing field can be eliminated if the ferroelectric oxide is sandwiched by metal electrodes which perfectly screen the polarization field. However, if the ferroelectric is in contact with a semiconductor and the interface state density is zero, there is very little screening of the polarization field, and the ferroelectric critical thickness becomes unacceptably large.
Meanwhile, the current request of downscaling needs for microelectronic devices has highlighted the importance of size effects as well as strain and dislocation influence on the properties of ferroelectric thin films [13-16]. Change of film strain field, either tensile or compressive, will elongate or shorten the corresponding lattice parameter. As a result, film tetragonality, polarization direction, domain configuration as well as dielectric constant along different axis will be subsequently engineered by that change of strain. However, due to large difference of lattice parameters and thermal expansion coefficients (TECs) between BTO and Si, most of the films reported to date are in an in-plane tensile strain state [12]. Although tensile BTO films with in-plane polarization are suitable for electric-optical applications [17], there are many applications in which a compressive strain leading to out-of-plane polarization is desired, e.g. the ferroelectric memory devices and quantum computing architecture [18,19]. An effective approach to control the strain state of BTO films is to choose an appropriate buffer layer by taking lattice constants and TECs into account. For example, Vaithyanathan
It is known that the well-developed LaNiO3 (LNO) thin film serves not only as a crystallographic seed layer but also as a conductive layer. What is more, the lattice parameter of LNO has a good match with that of BTO (along [100] direction) and Si (along [110] direction) and the TEC of LNO (8.2 × 10-6 K-1) is in between the values of BTO (10.4 × 10-6 K-1) and Si (1.4 × 10-6 K-1) [21,22]. Therefore, the strain state for the LNO buffer layer is expected to have a great influence on the strain state in its adjacent BTO layer, and a compressive strain in BTO on Si substrate could be induced by controlling the strain state in LNO. In this chapter, we will show that by insertion of a LNO buffer layer, high quality of BTO films can be integrated with Si(001) substrates and the structure-property correlations of BTO is largely dependent on the LNO buffer layer. A transformation from tensile to compressive strain state for BTO can be tuned by thickness of LNO. The corresponding microstructure and phase transition features of the BTO films will also be discussed.
2. Materials and experimental procedure
Both LNO and BTO films were fabricated on (001) Si substrates by multi-targets radio frequency (rf) co-sputtering method. Because of the insulating nature of the ferroelectric oxides, like BTO used here, the basic dc-sputtering doesn’t work. Charges will build up at the surface of the insulating target and will eventually cancel out the external electric field. One solution to this problem is to apply an alternating electric field instead of a dc electric field. However, if the introduced alternating electric field is of low frequency, both electrons and ions can respond to the applied oscillating field, which will result in the damage of the film. For example, during one half of the cycle, sputtering of the cathode targets happens and during the other half cycle, sputtering of the anode substrates happens, which is known as re-sputtering and is detrimental to film growth. Radio frequency field is a good way to overcome this shortcoming, since the heavy Ar+ ions cannot respond to a rf electric field while the electrons still can. Therefore, sputtering of the insulators is achievable without re-sputtering the deposited film [23-25]. Here, we utilize electromagnetic fields with a frequency of 13.56 MHz to generate the plasma. As shown in Fig. 1(a), the target is connected to the powered electrode as a “cathode”, while the rest of the vacuum chamber, including substrates, is normally grounded as the “anode”. The rf electric field generates plasma, but only the electrons are light enough to respond to the alternating filed at this frequency. The heavy Ar+ ions “see” only the average electric field. The smaller area of the “cathode” results in a higher electron concentration during each half cycle compared to the electron concentration at the “anode”, which will result in a net negative dc bias between the target and the substrate in a few cycles. It is this self-generated negative dc bias at the target that will drive and accelerate the Ar+ ions towers the target, causing the sputtering of the target surface atoms.
Si (100) wafers were first cleaned ultrasonically in acetone and methanol for 10 min each. During the whole deposition process base vacuum was below 3×10-4 Pa and substrate temperature was maintained at 600 °C. The deposition was generally performed either in vacuum or in a mixed atmosphere of oxygen (1.0 Pa) and argon (3.0 Pa) to prevent oxygen loss. After deposition, the as-deposited films were cooled down to room temperature (RT) in the mixed sputtering atmosphere and no additional annealing was applied. Typical puttering parameters are summarized in the following Table 1.
|
|
|
|
Ar+O2 | Ar+O2 |
|
Ar:O2=3:1 | Ar:O2=3:1 |
|
71 | 67 |
|
600 | 600 |
|
60 | 60 |
|
1.0 | 0.8 |
Specifically, LNO target was made by precursors of lanthanum nitrate hexahydrate and nickel acetate in the following method. First, lanthanum nitrate and nickel acetate powders were washed in distilled water, respectively. After drying, both of them with the precise stoichiometric ratio 1:1 were dissolved by acetic acid in a magnetic beater at a constant temperature of 70 °C to give a homogeneous solution until it became steadily clear and transparent. Then, the homogeneous solution formed was dried at 200 °C with 4 h for the elimination of water and acetate, 600 °C with 4 h for the elimination of nitrate. The sintering process was carried out at 800 °C in an open atmosphere for 10 h. This process was repeated twice with intermittent grinding in an agate mortar. Finally the mixture was pelletized to the target shape, added with ethylene glycol (3% v/v) (in order to increase the viscosity of the powder), and sintered at 800 °C for 24 h.
Microstructure and crystallographic orientation of the films were characterized by
and dielectric loss for various BTO films were measured using an impedance analyzer (HP4194A, Hewlett-Packard Ltd.), at a bias voltage of 0.1 V, in the frequency range of 100 Hz–120MHz. Hysteresis loop was measured under an alternative electric field using a ferroelectric test system (TF 2000 analyzer, axiACCT, Germany). The electric properties were measured in a typical plate-capacitor setup. The top electrode layer was LNO, which was fabricated with the same deposition parameters as the bottom LNO and then patterned with a stainless iron mask of 200 mm in thickness and 1mm in diameter.
3. Growth mechanism and crystal structure
Fig. 3(a) shows typical XRD pattern for LNO films deposited on Si (100) substrate. As can beseen LNO film possesses a pseudocubic crystal structure with a (100) preferred orientation. In fact, the lattice parameters for cubic silicon are
Since magnesium oxide (MgO) is also a common buffer layer for the growth of thin film ferroelectric oxide materials and it exhibits superior stable chemical property, we also grow MgO films on Si substrate. Fig. 3(c) presents the XRD pattern of MgO(400nm)/Si. Obviously, MgO also exhibit seemingly high (100)-orientation and a good crystallinity (see inserted SEM image in Fig.3(c)). This can also be understood in term of the lattice matching between MgO and Si. Cubic MgO has a lattice constant of 4.23 Å, the lattice mismatch between MgO and Si in [100] direction is thus ~ 22.1%. This value is still too large to maintain a film with good crystallographic quality. On the other hand, as illustrated in Fig. 3(d), the lattice mismatch between the two materials in [110] direction is decreased to 10.2%, which is an acceptable value for the growth of highly oriented MgO film. However, it should be noted the lattice mismatch of MgO/Si is still significantly larger than that of LNO/Si, which will affect the structural quality of the upper ferroelectric BTO films.
Fig. 4(a) show XRD patterns of BTO films grown on bare Si(001) and MgO/Si(001) substrates, respectively. As can be seen, the BTO film grown directly on Si shows a typical random perovskite phase with no (100) orientation due to the large lattice mismatch between BTO (3.99 Å) and Si (5.43 Å). Insertion of a MgO intermediate layer between BTO and Si improves the (100) texture orientation of BTO film hugely and the BTO film also exhibit a good crystalline structure (Fig. 4(b)). However, a tiny BTO (110) peak still presents in the XRD pattern, indicating that MgO is not an ideal buffer layer material for highly (100) orientation growth of BTO because of its large lattice mismatch with Si substrate, as mentioned above. In contrast, BTO film grown on LNO/Si exhibits a
pure (001)-orientation structure, as indicated in Fig. 4(c). Since lattice mismatch between BTO and LNO is only ~ 3.76%, the pre-deposited LNO film will act as a seed layer for the (001) orientation growth of the BTO layer. An atomic arrangement relationship among the three kinds of lattices (Si substrate, conductive LNO and ferroelectric BTO) is also described in Fig. 4(c). In the film growth process, this little lattice mismatch is very important as it can induce a proper internal stress energy, which will not only maintain the (100) orientation growth but also induce an additional tetragonal distortion and change the ratio of
The structural quality of BTO films on LNO/Si substrates is further demonstrated by in-plane XRD measurement and HRTEM cross-sectional interface observation. Fig. 5(a) is a typical in-plane {110}
Based on above discussions, the LNO film is a more suitable buffer layer for the growth of high quality BTO films. Since LNO films are also used as electrodes, we have investigated their electric properties. When the film thickness is 400 nm, the resistivity for the highly (100)-oriented LNO film at room temperature is 1.2×10-3 Ω cm. Previous reports have shown that the resistivity for those highly (100)-oriented LNO thin films varies the range of 4.5 × 10-4 ~ 1.0 × 10-2 Ω•cm, depending on fabrication processes [26,27]. The variations in the resistivity could be attributed to composition deviation from typical ABO3 chemical stoichiometry especially the oxygen loss, which were influenced by the deposition method and temperature. The lower resistivity of around 1.2 × 10-3 Ω cm obtained in this work indicates that the highly (100)-oriented LNO films prepared by sputtering could be used as both buffer layer and bottom electrode, which is of significance for fabricating highly-oriented ferroelectric thin films.
4. Grain size effect
It is known that both microstructure and properties of ferroelectric films are dependent greatly on fabrication processes [28-31]. The extrinsic parameters are also assumed to be responsible for variations in microstructure dependence of ferroelectricty, and might be the reason for a broad dispersion in some data such as critical size below which ferroelectricity is eradicated. For example, in the last two decades, there had been a variety of different experimental critical thickness for epitaxial ferroelectric thin films [32-37]. However, recent theoretical and experimental studies have implied that there is no intrinsic thickness limit for ferroelectricity in thin films with thickness down to even several unit cells [38,39]. While for ferroelectric polycrystalline films, nano-particles, or nano-ceramics, there is still no unambiguous conclusion that if there exists a critical grain size responsible for the disappearance of the macroscopic ferroelectricity. To investigate the effect of grain size on the ferroelectric properties of BTO films, it is important to control the experiment so that the only different parameter for different BTO films is variable size of grains. The ferroelectric BTO thin films of 200 nm were first deposited onto the LNO buffer layers at room temperature. The as-deposited films were then annealed in an open-air atmosphere with different temperatures ranging from 400 ~ 800 °C for 2 hours in order to obtain different grain size.
Fig. 6 (a) shows the XRD patterns of the BTO films annealed at several different temperatures. When the annealing temperature is 400 °C, only (100) and (200) peaks for LNO can be observed and no any other peaks related with BTO can be identified, indicating that the BTO film remain an amorphous structure at the temperature. When the annealing temperature is increased to 500 °C, a broadened (200) peak ascribed to the BTO start to appear. With further increasing the annealing temperature, the peaks become much sharper and are gradually intensified. From the results, it can be obtained that the BTO films start to crystallize at 500 °C from the amorphous phase and grow into a (100) preferred orientation on the LNO (100) buffered Si substrates as the annealing temperatures increase. The surface morphologies of BTO films annealed at different temperatures had also been analyzed by AFM observations, as shown in inset of Fig. 6(a). The 400 °C -annealed BTO film exhibits an amorphous structure without the formation of any distinct grains on the surface and increasing annealing temperature will increase the BTO grain size, which agrees well with the XRD results.
From the magnified view of XRD patterns in Fig. 6(b), the lattice constants as well as strain states for both BTO and LNO layers can also be extracted. The (002) peaks for all the LNO layers are positioned at the same diffraction angel, indicating the LNO films possess same lattice constant, which is calculated to be 3.91 Ǻ. Compared with that of bulk LNO (3.84 Ǻ), the larger value reveals that LNO films are under tensile strain. On the other hand, compared with the 2
Microstructures for the annealed films were characterized by plan-view TEM observations, as shown in Fig. 7(a) ~ (d). It is clearly seen that the films annealed at temperatures ≥500 °C are crystallized and featured as uniform and cracks-free. The grain size is increased with increasing the annealing temperatures, changing from 14 to 55 nm in diameter. It is noted that the BTO films in this work exhibit much smaller grain sizes as compared to other reported BTO films [42,43]. The difference is considered to be the result of the influence of microstructure of the LNO buffer layer. It is known that grain size for a newly formed crystal is dependent on the nucleation rate and the growth rate, respectively. In this work, the LNO buffer layer with very fine grain size of 20 ~ 30 nm was used as seed layers for the ferroelectric BTO films. The grain boundaries in the LNO buffer layers will act as nucleation sites for the crystallization of the BTO films during the annealing processes. The smaller grain size of the LNO film leads to more nucleation sites for the crystallization of BTO films and, as a result, the BTO films grow into a microstructure characterized by fine and uniform grains.
Room temperature ferroelectric hysteresis loops of the BTO films annealed at different temperatures are displayed in Fig. 8(a). For BTO films annealed at 600 °C and above, obvious hysteretic shape of polarization vs electric field (
strain state on the Si substrate, as had been obtained by the XRD analysis. On the other hand, for BTO film with the finest grain size of 14 nm (annealed at 500°C), it still exhibits some hysteresis characteristics with
To further study the change of the ferroelectricity with different grain size, Fig. 8(b) plots the
5. Strain engineering and phase transition
The microstructure of the film has a great influence on the corresponding physical properties. Recently, the investigation of strain effect has become increasingly important due to its great influence on the ferroelectric phase transition, domain formation, and polarization magnitude for ferroelectric thin films. Haeni
5.1. Strain modeling
For ferroelectric thin films, internal strains are mainly induced by lattice distortion due to the different lattice parameters [56] and the incompatible thermal expansion coefficients (TECs) between the film and substrate (or buffer layers) [57], to the self-induced strain of phase transition during the cooling process [58], and to the inhomogeneous defect-related strains such as impurities or dislocations [41]. However, the contribution from the later two factors can be avoided by selecting suitable materials and exploring advanced film growth techniques.
Schematic Fig. 9 illustrates the formation and evolution of the strain in a typical epitaxy film growth process. At the film growth temperature, when atoms arrive at the surface of the substrate, they will initially adopt the substrate’s in-plane lattice constant to form an epitaxial film [Fig. 9(a)]. As long as the film thickness (
additional thermal strain may also be exerted on film due to the difference of the TECs between the film and substrate [Fig. 9(d)]. Therefore, the temperature dependent misfit strain in a thin film can be modeled simply by taking into account the combined contribution of the temperature dependent lattice strain [
where,
For the convenience of understanding, we define an original misfit lattice strain
Taking into account the thermal expansion, the lattice constant of the film and substrate at
In addition, structural factors such as growth defects, crystallinity, and oxygen vacancies may also contribute to the
By analyzing the first three terms on the right side of equation (5), we can roughly estimate the final strain in the obtained film.
We start from the LNO buffer layer. Fig. 10 (a) shows the XRD patterns for various LNO films with different thickness. It is obvious the LNO (200) peak shifts toward high angles with increasing the film thicknesses, indicating a decrease in the lattice constant. Fig. 10 (b) shows the LNO thickness dependent lattice constant (
For the LNO film directly grown on a Si substrate, using equations (3), we can calculate the origin misfit lattice strain and
where
5.2. Tensile strained BTO
Fig. 11(a) shows the XRD patterns for 200 nm BTO films grown on the 100 nm LNO buffered Si. In order to determine the in-plane lattice alignment and in-plane constant of BTO, samples were placed on a tilted holder with a set azimuth angle of
The obtained
The temperature dependent dielectric permittivity and dielectric loss for the bilayer films were shown in Fig. 12(a). Over the temperature region, two broad but obvious peaks for the dielectric permittivity and dielectric loss are detected at 30 °C and 170 °C, respectively. This indicates that two phase transitions have occurred. The dielectric response can be explained by the misfit strain-temperature phase diagrams theory [66-71] for an epitaxial polydomain ferroelectric film grown on a “tensile” substrate. As shown in Fig. 12(b), the polydomain ferroelectric films have different phase states and domain configurations compared to epitaxial single-domain film or bulk materials. This results in the contribution of an extrinsic response (domain-wall movements) together with the intrinsic response (substrate induced strain) to the dielectric response in a small signal dielectric measurement in the plate-capacitor setup. The temperature dependent misfit strain can be approximated by equation (1). Since BTO film is pretty thick, the contribution of lattice strain can be neglected, and the total strain is subjected solely to the thermal strain. Thus, the misfit strain (
Fig. 13(a) shows the plan-view HRTEM image of elastic domain pattern for the BTO film. The adjacent elastic domain walls form a coherent twin boundary lying along the surface of {110} twin planes for the minimization of in-plane elastic strain energy. Fig. 13(b) shows the cross-sectional TEM image of elastic domains. It can be clearly seen that the domain walls exhibit a blunt fringe contrast, because the polarization vectors in adjacent domains form an angle and they, as a result, are not in the same height with respect to the observation direction [72].
5.3. Compressive strained BTO
Fig. 14(a) and14(b) show the XRD patterns of normal and 45ºtilted
Electrical properties of compressive strained BTO film have been investigated by ferroelectric and dielectric measurements. Hysteresis loop for the compressive BTO, as shown in Fig. 15(a), exhibits a well-defined shape, which is significantly different from those of tensile BTO films. The
In fact, the reduction of
where,
The compressive BTO exhibits very different domain configurations as compared with a tensile BTO, in which twining
5.4. Phase transition
Fig. 17(a) shows the normal XRD pattern for a 300 nm BTO thin film grown on the 600nm LNO-buffered Si substrate. The lattice constants for BTO film are
Fig. 18(a) and (b) show the temperature dependent dielectric constant (
According to Smolensky and Uchino
where
However, recent nuclear magnetic resonance and Raman scattering studies had both evidenced the coexistence of the displacive character of transverse optical soft mode with the order-disorder character of Ti ions [93], especially in the BTO thin films. As the sputtering is proceed in an oxygen deficient atmosphere, thus the oxygen vacancies induced structural disorders and compositional fluctuations in the film may be responsible for the observed relaxor behavior. Similar diffusive transition had also been observed in BTO films on MgO and Pt-coated Si substrates [94,95].
The relaxor nature of the frequency dependent dielectric response of BTO film can also be examined by the Vogel-Fulcher (VF) relation [96],
where
6. Conclusions
High quality ferroelectric BTO thin films with (100)-preferred orientation have been grown on LNO buffered Si substrate by rf sputtering and the corresponding structure-property correlations have been discussed. Using combination of XRD and HRTEM, it is revealed that highly-oriented BTO film could be achieved on the lattice-mismatched Si in a “cube-on-cube” fashion with LNO as both buffer layer and conductive electrode layer. Polarization-switching measurement points out that while obvious ferroelectricity is obtained for BTO films with grain size larger than 22 nm, a weak ferroelectricity is still observed in BTO film of 14 nm grains, indicating that if a critical grain size exists for ferroelectricity it is less than 14 nm for BTO/LNO/Si system. We also demonstrate that due to their unique feature of gradient lattice constant and thermal expansion coefficient values for ferroelectric BTO, conductive LNO, and substrate Si, the BTO/LNO/Si system exhibits very interesting strain states. By choosing appropriate thicknesses for BTO and LNO, strain in ferroelectric BTO layer could be evolved from tensile strain to compressive strain state. The internal strain has a significant influence on the polarization, dielectric phase transition, and domain configuration for BTO film on Si and this can be used as a tool to engineer the properties of BTO films. The present work may have important implications on the future ferroelectric semiconductor devices.
Acknowledgments
This work is supported by the innovation Foundation of BUAA for PhD Graduates and program for New Century Excellent Talents in university (NCET-04-0160) and Innovative Research Team in University (IRT0512).
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