DOI: 10.1007/s00339-004-3068-1 Materials Science & Processing
x.y. chen1,2,u j. wang2 k.h. wong2 c.l. mak2 g.x. chen1 j.m. liu1 m. wang1 z.g. liu1
Growth of orientation-controlled
Pb ( Mg , Nb ) O 3 − PbTiO 3 thin films on Si(100) by using oriented MgO films as buffers
1National Laboratory of Solid State Microstructures Physics and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P.R. China
2Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P.R. China
Received: 17 June 2004/Accepted: 20 September 2004 Published online: 18 November 2004 • © Springer-Verlag 2004 ABSTRACTThin films of relaxor ferroelectric Pb(Mg,Nb)O3− PbTiO3with different orientations were grown by pulsed-laser deposition on Si(100). By using (111)-, (110)- and (100)- oriented MgO thin film as buffer and the LaNiO3 thin film as a bottom electrode,(110)- and(100)- oriented or preferred and polycrystalline PMN-PT thin films were obtained. The (110)- oriented PMN-PT thin film showed dielectric permittivity of about 1350 and loss factor tanδof<0.07.
PACS68.55.Jk; 81.15.Fg; 77.84.Dy
1 Introduction
Growth of thin films of multicomponent oxides, especially perovskite-structured oxides, e.g., ferroelectrics, colossal magneticresistance oxides, on silicon is potentially important for exploring new microelectronic devices. Many of these oxides are anisotropic, and therefore properties of film of these oxides are depended on the film’s orienta- tion. It is then interesting to be able to control the orien- tation of the films in order to optimize the properties. In this work we demonstrate that films of relaxor ferroelectrics (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN-PT) can be pre- pared with controlled orientation on silicon substrate. PMN- PT are interesting due to their very large piezoelectric re- sponses, unique dielectric behavior and electro-optic prop- erties, especially its large dielectric permittivity and elec- tromechanical constants make these materials very attrac- tive for microelectronic, microelectromechanical and optical applications, such as actuators, capacitors and transducers etc [1–8]. Therefore, thin films of PMN-PT have been pre- pared by a variety of method, such as pulsed-laser deposi- tion (PLD) [1, 3–6, 9–11], sol-gel [12–17], and metalorganic chemical-vapor deposition [18]. It has been showed that the dielectric properties of PMN-PT films were affected by its orientation [5, 9, 14, 16, 17]. The (100)- and (111)-textured PMN-PT were obtained by these works [14, 16, 17], and it showed that a higher volume fraction of(110)-oriented phase
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was favourable to produce a larger dielectric constant [5, 9].
However, there were few reports in literatures about prep- aration of the(110)-textured PMN-PT thin films and their dielectric properties, especially on silicon. Tyunina et al. has reported the growth of PMN-PT thin films with 5∼10%of relative volume of(110)-oriented phase on MgO single crys- tal substrate [5]. In the present work, based on our previous work of orientation-controlled growth of MgO buffer layers and LaNiO3electrode thin films on Si(100)[19, 20], we report on the preparation of three kinds of oriented 0.9PMN-0.1PT thin films on Si(100). Our emphasis is on the orientation con- trol of the PMN-PT films by buffer layer technique. Espe- cially, highly(110)-oriented PMN-PT thin films with>99%
of relative volume of(110)oriented phase were obtained.
2 Experimental
The control of the orientation of the PMN-PT thin films was accomplished by using an oriented-LaNiO3(LNO) thin film, which was also as a bottom electrode, as template.
The detailed description of preparation of the three kinds of oriented MgO and LNO thin films was presented in previ- ous papers [19, 20]. Briefly, highly(100)-,(110)- or(111)- oriented MgO were deposited firstly, followed by deposition of the LNO film without breaking the vacuum. At last the PMN-PT thin films were prepared at 580◦Cunder 180 mTorr of oxygen pressure without also breaking the vacuum. The PMN-PT target used in present work had the 0.9PMN-0.1PT composition with 30% excess of PbO and was prepared by the conventional solid state reaction. After deposition of the PMN-PT thin films, they were post-annealed at 600◦Cunder about 0.5 barof oxygen pressure for 15 min. The orientation of the films was then analyzed with a Rigaku D/Max-RA type X-ray diffractometer (XRD). The surface of PMN-PT was ex- amined by using a Digital Instruments Nanoscope III atomic force microscope (AFM). The cross section was measured by a Phillip XL30 field-emission scanning electron microscopy (FESEM). For measurements of the dielectric properties, cir- cular platinum top electrode with 0.18 mmin diameter was prepared on the PMN-PT thin films at room temperature and in high vacuum by PLD. The dielectric properties were meas- ured on a Hewlett-Packard (HP 4294A) impedance analyzer.
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3 Results and discussions 3.1 XRD patterns
Figure 1 shows theθ–2θXRD patterns of the het- erostructure of PMN-PT/LNO(110)/MgO(111)/Si(100). It shows that the PMN-PT film is almost completely (110)- oriented. The orientation relationship between the film of the PMN-PT, the LNO and the MgO is PMN-PT(110)/
LNO(110)/MgO(111)/Si(100), indicating that the orienta- tion of the PMN-PT film inherits from the orientation of the underlying LNO film. From the Fig. 1 it can be seen that the etching of the Si substrate does not produce important effect on the orientation of the final PMN-PT thin film.
With singly(100)-oriented LNO as a buffer, which was deposited on the(220)-oriented MgO-buffered Si(100), the PMN-PT film became mainly (100)-orientation preferred, though some other orientations also can be observed, as shown in Fig. 2. It indicates again that the orientation of the PMN-PT film is determined by the underlying LNO layer.
The value of I(200)/(I(110)+I(111)+I(200)+I(211)+ I(311)), where I(hkl)is the diffraction intensity of the(hkl) peak of the PMN-PT film, can be qualitatively to estimate the texture degree of the(200)peak. By comparing this value of the Fig. 2a and b, which were 0.49 and 0.36 respectively, it can be inferred that the etching of the Si substrate before deposi- tion of the MgO buffer can improve the texture degree of the (100)-orientated PMN-PT film.
Figure 3 is the θ–2θ XRD patterns of the heterostruc- ture of PMN-PT/LNO/MgO(100)/Si(100). It shows that the PMN-PT in this case is polycrystalline. Recalling that the un- derlying LNO bottom electrode is polycrystalline, this result again indicates dependence of the orientation of the PMN-PT film on the orientation of the underlying bottom layer. Com- parison between Fig. 3a an b shows that the etching affects the orientation to some extent.
FIGURE 1 XRDθ–2θscan patterns of the PMN-PT films on substrate of LNO(110)/MgO(111)/Si(100). (a) and (b) correspond to the Si substrate was etched and not etched before the deposition of the MgO buffer layer, respectively
FIGURE 2 XRDθ–2θscan patterns of the PMN-PT films on substrate of LNO(100)/MgO(110)/Si(100). (a) with Si substrate etched and (b) with Si substrate unetched
FIGURE 3 XRDθ–2θscan patterns of the PMN-PT films on substrate of LNO(polycrystalline)/MgO(100)/Si(100). (a) with Si substrate etched and (b) with Si substrate unetched
These results show that the orientation of the bottom elec- trode layer is critical to the orientation of the final PMN-PT layer. In order to obtain a highly textured PMN-PT film, a high quality of LNO layer is absolutely important.
3.2 Surface morphology
Figures 4–6 are AFM image of the films of the (110)-,(100)- and polycrystalline PMN-PT, respectively. The shape of grains of the PMN-PT thin film is needle-like in the(110)-orientated film (Fig. 4), square or rounded square in the(100)-oriented film (Fig. 5) and irregular in the poly- crystalline film (Fig. 6). The needle-like grains in the(110)-
FIGURE 4 AFM images of the surface of the(110)-oriented PMN-PT thin film. (a) and (b) have the same meaning as that in Fig. 1
oriented film have a high aspect ratio, while the grains in the other two cases are more or less equiaxed, suggesting a high anisotropic in plane growth in the (110)-oriented film and roughly isotropic in plane growth in the(100)-oriented and the polycrystalline films. The phenomena that the grain shape is closely related to the orientation is also observed in other system [21].
By comparing the (a) with the corresponding (b) in Figs. 4–6, it can be seen that the grain size and shape in (a) are different to some extent from the corresponding (b), de- pending on the orientation of the film. This indicates that the etching of the Si before the deposition of the MgO buffer has important effect on the morphology of the final film, though the etching sometimes has no important effect on the orien- tation. It suggests that the etching affects the growth of the buffer layer [22] and then somehow affects the growth of the PMN-PT film. The mechanism to govern the orientation rela- tionship can not be illustrated with the present results.
3.3 Cross section
As an example, Fig. 7 shows an FESEM photo- graph of the cross section of a PMN-PT/LNO/MgO/Sihet- erostructure. Three layers can be distinguished clearly. The PMN-PT thin films have columnar structure from the interface
FIGURE 5 AFM images of the surface of the(100)-oriented PMN-PT thin film. (a) and (b) have the same meaning as that in Fig. 1
with the LNO bottom electrode to top of the thin film. The in- terface between the PMN-PT and the LNO layer is very sharp and smooth. The thickness of MgO, LNO and PMN-PT can be evaluated to be about 70, 250 and 500 nm, respectively.
3.4 Dielectric properties
The room-temperature dielectric properties of the (110)-oriented PMN-PT film were characterized, as shown in Fig. 8. The dielectric permittivity at 1 KHz,ε, was near 1350, which is even larger than the value ofεreported for PMN- PT thin films grown on MgO single crystal substrates with La0.5Sr0.5CoO3 as the bottom electrode [5]. The loss factor, tanδ, in the films did not exceed 0.07between 0.5–100 kHz.
It is noted here that the dielectric permittivity of the bulk 0.9PMN-0.1PTat 1 kHz and room temperature is larger than 104[23, 24], meaning that the dielectric properties of PMN- PTfilm sensitively depends on the microstructures of the film and preparation processes involved. A sharp increase in tanδ was observed at frequency, f , 200–300 kHz. It is noted that the tanδat the frequency below 1 kHz was slightly larger than that at the f between 1–100 kHz, indicating that the PMN- PT thin film was little leaky electrically for direct current.
For the two other oriented PMN-PT films, it was found that the leakage current was too large for a reliable measurement
1148 Applied Physics A – Materials Science & Processing
FIGURE 6 AFM images of the surface of the polycrystalline PMN-PT thin film. (a) and (b) have the same meaning as that in Fig. 1
FIGURE 7 An FESEM photograph of cross section of the PMN- PT/LNO(110)/MgO(111)/Si(100)heterostructure
of impedance spectroscopy. A possible reason was the defi- ciency of Pb in the PMN-PT film [3].
4 Conclusion
(110)-,(100)-oriented and polycrystalline PMN- PT thin films were prepared on Si(100) with MgO as the buffer to control the orientation of the PMN-PT films
FIGURE 8 Room temperature dielectric permittivity, ε, and loss factor, tanδ, as a function of frequency in PMN-PT(110)/LNO(110)/MgO(111)/
Si(100)heterostructure
and with LaNiO3 as the bottom electrode. The relation- ship of the orientation of the three kinds of heterostruc- tures are: PMN-PT(110)/LNO(110)/MgO(111)/Si(100), PMN-PT(100)/LNO(100)/MgO(110)/Si(100)PMN and PMN-PT(polycrystalline)/LNO(polycrystalline)/MgO(100)/
Si(100), respectively. The grain of the(110)-oriented PMN- PT films shows needle-like in shape, while the grain in the other two oriented films is more or less equiaxed. The PMN-PT film is of columnar structure with a sharp interface between the film and the bottom electrode. The dielectric per- mittivity of the (110)-oriented PMN-PT films can be up to 1350at 1 kHz and the loss factor can be less 0.07in the fre- quency range below 100 kHz, which are comparable to those of the PMN-PT thin films prepared on MgO single crystal substrate.
ACKNOWLEDGEMENTS The authors thank Dr. J. Wang, K.M. Yeung, K.S. So, M.N. Yeung and Y. Yeung for their kind help in measurements. This work was supported by the National Natural Scien- tific Foundation of China and the Center for Smart Materials of the Hong Kong Polytechnic University. The technical assistance from The Film Mate- rial Laboratory of Suzhou University and the Analytical Center of Nanjing University is also acknowledged.
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