Effect of substrate temperature on microstructures and dielectric properties of compositionally graded BST thin films

ZHANG Bai-shun(张柏顺), GUO Tao(郭  涛), ZHANG Tian-jin(章天金),

WANG Jin-zhao(王今朝), QUAN Zu-ci(全祖赐)

School of Physics and Electronic Technology, Hubei University, Wuhan 430062, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Compositionally graded Ba_1-xSr_xTiO₃ (BST) (x = 0-0.3) thin films were prepared on Pt/Ti/SiO₂/Si substrate at different substrate temperatures ranging from 550 ℃ to 650 ℃ by radio-frequency (rf) magnetron sputtering. The effect of substrate temperature on the preferential orientation, microstructures and dielectric properties of compositionally graded BST thin films was investigated by X-ray diffraction, scanning electron microscopy and dielectric frequency spectra, respectively. As the temperature increases, the preferential orientation evolves in the order: randomly orientation→ (111) → highly oriented (111) (α₍₁₁₁₎ = 60.2%). The surface roughness of the graded BST thin films varies with the substrate temperatures. No visible internal interface in the compositionally graded thin films can be observed in the cross-sectional SEM images. The graded BST thin films deposited at 650 ℃ possess the highest dielectric constant and dielectric loss, which are 408 and 0.013, respectively.

Key words: graded Ba_1-xSr_xTiO₃ thin films; substrate temperature; microstructures; dielectric properties

1 Introduction

Ba_1-xSr_xTiO₃ (BST) is one of the great interests for a variety of integrated devices such as dynamic random access memory (DRAM), decoupling capacitors, pyroelectric infrared (IR) sensors and piezoelectric microelectric microactuators [1-6]. In order to tailor electrical properties of BST thin films for better application on electrical devices, the conception of functionally graded materials (FGMs) was presented, which originated from the development of advanced thermal barrier materials used for space aircraft, nuclear fusion reactors, and so on [7]. The main feature of FGMs is that the composition and structure gradually change from one surface of the material to the other, resulting in a corresponding change in the properties of the material. A typical example is compositionally graded Ba_1-xSr_xTiO₃ (BST) thin films, in which a giant effective pyroelectric coefficient can be obtained. To date, only a few experimental and theoretical studies of compositionally graded BST thin films have been reported. And few researches on compositionally graded BST thin films deposited by rf magnetron sputtering has been reported at present. The influence of important parameters, such as substrate temperature and O₂/Ar ratio, on the physical characteristics of compositionally graded BST thin films deposited by rf magnetron sputtering was, however, not well understood yet. In this work, the effect of substrate temperature on the preferential orientation, microstructures and dielectric properties of compositionally graded BST thin films was reported.

2 Experimental

Compositionally graded Ba_1-xSr_xTiO₃ (BST) (x=0-0.3) thin films were deposited on Pt(111)(180 nm)/Ti(70 nm)/SiO₂/Si (100) substrate by rf magnetron sputtering at the substrate temperature ranging from 550 ℃ to 650 ℃. The thickness of the compositionally graded BST thin film was about 580 nm. Experimental details are described in Table 1. The compositionally graded BST thin films were achieved by continuously sputtering four different kinds of ceramic targets layer by layer with compositions of BaTiO₃, Ba_0.9Sr_0.1TiO₃,

Table 1 Sputtering conditions of graded BST thin films

Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃, respectively. These ceramic targets were prepared by traditional ceramic processing. All the films were annealed at 650 ℃ for 1 h. Then they were cooled down to room temperature naturally.

X-ray diffraction (XRD) technique was carried out to identify the crystal structure and preferential orientation of the compositionally graded BST thin films by using a D/max-ⅢC diffractometer (Cu K_α radiation with a working voltage of 35 kV and current of 25 mA) with a scanning rate of 15 °/min. The microstructures of the grade BST thin films were examined by scanning electron microscopy (JOEL JSM 6700F), both from planar and cross-sectional samples. To measure the dielectric properties, Pt (110 nm) top electrodes with diameters of 300 μm were DC sputtered onto the top surfaces of the graded BST thin films. The dielectric constant and dielectric loss, in the 1-1 000 kHz fre- quency range, were measured by an Agilent 4294A impedance analyzer.

3 Results and discussion

3.1 Preferential orientation of films

The graded BST films deposited at different substrate temperatures were examined, as shown in Fig.1. It is evident that all the films crystallized into an obvious perovskite structure, and the diffraction peaks (100), (110), Si (unknown), 20°-55°. When the graded BST thin film is deposited at (111), Pt (111) and (200) are observed in the 2θ range of 550 ℃, the film is randomly

Fig.1 XRD patterns of compositionally graded BST thin films deposited at different substrate temperatures: (a) 550 ℃; (b) 600 ℃; (c) 650 ℃

oriented. As the substrate temperature increases to 600 ℃, the intensity of (111) diffraction peaks increases greatly. This result implies that the preferential orientation of the graded film at 600 ℃ is (111). And the preferential ratio is 47.7%. Further increasing the substrate temperature to 650 ℃, it’s found that the graded film is (111) highly orientated, in which the preferential ratio is 60.2%. These results reveal that the graded BST thin film orientation can be tailored by adjusting the rf magnetron sputtering processing parameters such as the substrate temperature.

The evolution of preferential orientation in polycrystalline films can be analyzed from the viewpoint of energy minimization. The total energy in any film deposited on substrates is the sum of three components: 1) the surface energy of the film; 2) the film-substrate interface energy; 3) the strain energy in the film [7]. Films grow in such a way that the total energy is minimized. In the case of polycrystalline materials grown polycrystalline substrates, the film generally grows with the plane with the lowest surface energy parallel to the surface of the substrate, thus minimizing the surface free energy of the film. In ceramic systems, with unit cells containing metal cations and anions, the determination of the surface free energy must take into consideration both the electrostatic charges in two-dimensional planes and the surface packing densities. The species that are incident on the substrate comprise ionic, atomic, and molecular species. The influence of the substrate temperature on ionic arrangements is weak, while the influence of the substrate temperature on atomic and molecular arrangements becomes strong. Under a certain working gas pressure, as the substrate temperature increases, the nuclei will be preferentially oriented parallel to the thermodynamically stable planes such as the (110) and (111) planes, due to their high packing densities in the BaTiO₃ perovskite structure. It’s easy for the low-energy species, which vibrate weakly, to grow on the (111) plane. This hypothesis is confirmed by the large increase in the intensity of the (111) diffraction peak observed in the graded BST thin films deposited at 650 ℃. Based on the preceding discussion, the evolution of the preferential orientation of the films as a function of substrate temperature is attributed to that the substrate temperature affects strongly on the rearrangements of species deposited on the substrates.

3.2 Microstructures

The microstructures of the graded BST thin films as a function of substrate temperature were examined by planar and cross-sectional SEM images. The surface morphologies of the graded BST thin films deposited at different substrate temperatures are shown in Fig.2. It can be observed that the surface morphologies vary with the substrate temperatures. While deposited at 550 ℃, the film has smooth and densely packed structure. While the substrate temperature increases to 600 ℃, the grains grow largely and the roughness increases. This is due to that the mobility of species adsorbed onto the substrate becomes higher and thus induces a higher speed of the coalescence of grain islands as the substrate temperature increases. Further increasing the substrate temperature up to 650 ℃, the graded BST grains grow largely and the secondary growth is presented. As a result, the roughness of graded BST thin films decreases. Cross-sectional SEM images of the graded BST thin films are shown in Fig.3. No visible internal interface in the compositionally graded thin films can be observed in the cross-sectional SEM images. Three different structures are observed in these grade BST thin films. While deposited at 550 ℃, the films possess a granular structure, whereas at 600 ℃ they exhibit a columnar structure, and at 650 ℃ they exhibit a polygonal structure with polycrystalline grains throughout the film thickness. At low substrate temperature (550 ℃) the absorbed species have insufficient energy to rearrange themselves to the thermodynamically stable plane. Thus, the adatom mobility is very low. It is expected that the graded films deposited at low temperature grow randomly without preferential orientation. This prediction is confirmed by the XRD results. On the other hand, the graded films deposited at higher substrate temperature are expected to possess better crystallization due to the relatively high adatom mobility. This hypothesis is also confirmed by the XRD results.

3.3 Dielectric properties

The dielectric properties of the graded BST thin

Fig.2 SEM images of compositionally graded BST thin films deposited at different substrate temperatures: (a) 550 ℃;(b) 600 ℃; (c) 650 ℃

Fig.3 Cross-sectional SEM images of compositionally graded BST thin films deposited at different substrate temperatures: (a) 550 ℃; (b) 600 ℃; (c) 650 ℃

films was measured by a mental-insulator-metal (MIM) configuration, in which the graded films were sandwiched between bottom and top Pt electrodes. The frequency dependence of dielectric constant of graded BST thin films deposited at different substrate temperatures is shown in Fig.4(a). It’s observed that the dielectric constant of all the samples has tendency to decrease with increasing frequency. The decrease of the dielectric constant in the low-frequency region below 100 kHz is caused by space charge polarization because of the inevitable oxygen vacancies in the graded BST thin films [7, 8]. It is also observed that the graded BST thin films deposited at 650 ℃ have the highest dielectric constant. The dielectric constant deposited at 650 ℃ at 100 kHz is 408, which is lower than that deposited by PLD [9]. The dielectric loss of graded BST thin films deposited at different temperatures, as a function of frequency, is shown in Fig.4(b). It is observed that the dielectric loss of all samples remains essentially constant around 0.01 in the frequency range of 100–400 kHz. The dielectric loss deposited at 650 ℃ at 100 kHz is 0.013, which is lower than that deposited by PLD [9]. As the frequency increases beyond 400 kHz, the dielectric loss increases and shows a dispersion with frequency. A strong increase of dielectric loss is observed at a high

Fig.4 Dielectric properties of compositionally graded BST thin films deposited at different substrate temperatures as function of frequency: (a) Dielectric constant; (b) Dielectric loss

frequency of 1 MHz, which is due to the resonance of the equivalent circuit. At frequencies on the order of 1 MHz, the stray inductance of the contact and the leads may induce L-C resonance (f_r=(LC)^-1/2) [10], where L and C are the inductance and the capacitance of the equivalent circuit, respectively.

4 Conclusions

The XRD results show that the preferential orientation evolves in the order: randomly orientation→ (111) → highly oriented (111) (α₍₁₁₁₎ = 60.2%) while the temperature increases from 550 ℃ to 650 ℃. The surface roughness of the graded BST thin films varies with substrate temperatures. No visible internal interface in the compositionally graded thin films can be observed in the cross-sectional SEM images. While deposited at 550 ℃, the films possess a granular structure, whereas at 600 ℃ they exhibit a columnar structure, and at 650 ℃ they exhibit a polygonal structure with polycrystalline grains throughout the film thickness. The graded BST thin films deposited at 650 ℃ possess the highest dielectric constant and dielectric loss, which are 408 and 0.013, respectively.

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(Edited by LI Xiang-qun)

Foundation item: Project(50372017) supported by the National Natural Science Foundation of China; Project(E0204) supported by the Major Laboratory of Ferro-Piezoelctric Device of Hubei Province, China

Corresponding author: ZHANG Bai-shun; Tel: +86-27-88661682; Fax: +86-27-88663877; E-mail: shichie118@sina.com