Characteristics of (Pb,Sr)TiO3 thin films with various Sr content

Characteristics of (Pb,Sr)TiO3 thin films with various Sr content

ARTICLE IN PRESS Journal of Crystal Growth 308 (2007) 213–217 www.elsevier.com/locate/jcrysgro Characteristics of (Pb,Sr)TiO3 thin films with various...

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ARTICLE IN PRESS

Journal of Crystal Growth 308 (2007) 213–217 www.elsevier.com/locate/jcrysgro

Characteristics of (Pb,Sr)TiO3 thin films with various Sr content Hung-Yao Chena, Jenn-Ming Wua,, Hsin-Erh Huangb, Hui-Yun Borb a

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Materials and Electron-Optics Research Division, Chung-Shan Institute of Science and Technology, Taoyuan 325, Taiwan, ROC

b

Received 8 May 2007; accepted 28 July 2007 Communicated by D.P. Norton Available online 3 August 2007

Abstract (Pb1x,Srx)TiO3 (PST) thin films with various Sr content (x ¼ 0.2, 0.4, and 0.6) were fabricated on Pt/Ti/SiO2/Si substrates by radio frequency magnetron sputtering. With the increase of Sr content, the structure of PST films changed from tetragonal to cubic symmetry and the grain size slightly decreased. Dielectric and ferroelectric properties were highly depended on the composition of PST films. PST (x ¼ 0.2) films showed a good polarization-switching characteristic with a relatively lower dielectric constant. PST (x ¼ 0.6) films displayed the excellent tunable dielectric property with a low loss tangent and performed well on the insulating characteristic. These results illustrated that PST thin films with a suitable composition have potential for applications in nonvolatile ferroelectric random access memory, dynamic random access memory and tunable devices. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Cd; 77.80.Dj; 77.55.+f Keywords: A3. Radio frequency magnetron sputtering; B1. Perovskite; B2. Dielectric; B2. Ferroelectric

1. Introduction Ferroelectric thin films have received much attention during the past years for the applications in ferroelectric random access memories (FeRAM), dynamic random access memories (DRAM) and tunable devices due to their large spontaneous polarization and excellent dielectric properties [1–3]. Lead titanate (PbTiO3), which has a large spontaneous polarization and a relative small dielectric constant, is a classic ferroelectric material. However, the large tetragonal strain of PbTiO3 is unfavorable to its applications. Therefore, the compositional modification of PbTiO3 thin film has attracted many interests in decreasing tetragonal strain and enhancing the dielectric and pyroelectric properties. Several elements, such as La [4], Ba [5], Ca [6], and Sr [7–9], have successfully been used in substitution for Pb to obtain modified PbTiO3 thin films by different deposition technologies. Corresponding author. Tel.: +886 3 5715131; fax: +886 3 5722366.

E-mail address: [email protected] (J.-M. Wu). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.07.043

Among them, the (Pb1x,Srx)TiO3 (PST) materials are solid solutions of the PbTiO3–SrTiO3 system which possesses the perovskite structure [10]. With increasing the Sr content, the Curie temperature (Tc) decreases linearly from 490 to 230 1C and the lattice structure changes from tetragonal to cubic symmetry. In other words, the property of PST thin films highly depends on the composition of the Pb/Sr ratio so it can be used in multiple applications by choosing an appropriate composition easily. Recently, the Sr-rich PST thin films have attracted much attention for its large electric-field-dependent dielectric constant [11,12], and the ferroelectric polarization characteristic of Pb-rich PST thin films has also been investigated in the application of the field effect transistor FeRAM (FET-FeRAM) [13,14]. In the present study, PST thin films with various Sr content (x ¼ 0.2, 0.4, and 0.6) were fabricated on Pt/Ti/ SiO2/Si substrates by radio frequency magnetron sputtering. The crystal structure, surface morphology, and electrical properties, which depended on the composition of PST films, were investigated.

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(211)

(200)

(111)

(110)

Intensity (a.u.)

Pt Pt

x=0.2

x=0.4

Si

x=0.6 20

30

40 2θ degree

50

60

a axis c axis

4.10

c/a ratio

1.06

4.15

Lattice constant (Å)

The substrates used were prepared by Si(1 0 0) wafers with a 150 nm silicon dioxide insulating layer. A 50 nm Ti adhesion layer and a 150 nm Pt layer were subsequently evaporated onto the oxide layer. The PST target with a diameter of 2 in was prepared by conventional ceramic powder processing, including powder mixing and drypressing. High-purity PbO, SrCO3 and TiO2 oxide powders were used as raw materials. The pressed target was baked for 1 h at 550 1C to remove organic additives, followed by sintering at 650 1C for 2 h to realize suitable strength. The composition of the sputtering target was (Pb1x,Srx)TiO3 (x ¼ 0.2, 0.4, and 0.6) plus 10 mol% excess PbO to compensate for the loss of Pb at elevated temperature. After the pressure of sputtering chamber was pumped down to a base pressure of 8  105 Torr, a mixture gas of Ar/O2 ¼ 4/1 was introduced and the processing temperature was maintained at 450 1C during the sputtering process. PST films were prepared at a fixed power of 60 W and constant pressure of 5 mTorr. The thickness of PST films was about 290 nm. The crystal structure of the films was investigated using a Rigaku-D X-ray diffractometer (XRD) with Cu Ka radiation for y–2y scanning. A JEOL JSM-6500F field emission scanning electron microscope (FESEM) was utilized to assess surface morphology. For electrical measurements, top electrodes of Pt dots with diameters of 100 mm were patterned by the photolithography and liftoff process. The dielectric property was measured at 50 mV using an HP4284A impedance analyzer. The hysteresis loops of PST films were measured by the RT-66A (Radiant Technology). The Agilent 4155C semiconductor parameter analyzer was applied for the direct current–voltage measurement.

(100)

2. Experimental procedure

1.04 1.02 1.00

4.05 0.2

0.4

0.6

Sr content (at%)

4.00 3.95 3.90 0.2

0.4 Sr content (at%)

0.6

Fig. 1. (a) X-ray diffraction patterns and (b) lattice parameters of (Pb1x,Srx)TiO3 (x ¼ 0.2, 0.4, and 0.6) thin films deposited on Pt/Ti/SiO2/ Si substrates.

3. Results and discussion The crystalline structure of the as-deposited films with variable Sr content, determined by X-ray diffraction, is shown in Fig. 1(a). The X-ray result showed all PST films were polycrystalline perovskite structure with no second phase. According to the origin of low processing temperature in PbTiO3 films, the PST film was well crystallized at such a low temperature (450 1C). As the Sr content increases, the peak of diffraction slightly shifts toward higher angle that indicates the lattice parameter has changed. The lattice parameter of different Sr contents was estimated by the X-ray result and shown in Fig. 1(b). The lattice constant of c-axis decreases obviously and the lattice constant of a-axis changes slightly as the Sr content increases. By the lattice change of c- and a-axis, the change of the tetragonal distortion ratio, c/a, was also defined and shown in Fig. 1(b). The change of c/a ratio indicates the structure transfers from tetragonal to cubic symmetry with the increase of Sr content. The c/a ratio was 1.048 at PST (x ¼ 0.2) and close to 1 at PST (x ¼ 0.6) films.

The surface morphology of PST films with different Sr content was examined by FESEM, as seen in Fig. 2. The granulated grains appeared on all the PST films. The grain sizes of PST (x ¼ 0.2) and PST (x ¼ 0.4) films were similar and slightly decreased in PST (x ¼ 0.6) films. The surface morphology became denser and smoother with the increase of Sr content. For PST (x ¼ 0.2) films, the surface was rough and the structure was not very dense with some small apertures. The PST (x ¼ 0.4) film was denser than the PST (x ¼ 0.2) film without small apertures, but it was still rougher than the PST (x ¼ 0.6) film. The dielectric properties of PST thin films with variable Sr content are shown in Fig. 3. The loss tangent of all films was below 0.04 and the dielectric constant increased with the increase of Sr content. The phenomenon is due to the tendency of decreasing (Tc) towards room temperature as Sr content increases. In addition, PST (x ¼ 0.6) films had the lowest loss tangent thought the loss tangent of PST films with Sr content in x ¼ 0.2 and 0.4 was close. The values of dielectric constant measured at a frequency of

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215 x=0.2 x=0.4 x=0.6

0.12

500 0.08 400

tanδ

dielectric constant (εr)

600

0.04 300 0.00 103

104 105 Frequency (Hz)

106

Polarization (μC/cm2)

Fig. 3. Dielectric constant as a function of frequency for PST (x ¼ 0.2, 0.4, and 0.6) thin films.

30

x=0.2

20

x=0.4 x=0.6

10 0 -10 -20 -30 -300

-200

-100

0

100

200

300

Electric field (kV/cm)

Fig. 4. Hysteresis loops of PST films with variable Sr content.

Table 1 The value of remanent polarization and coercive field of PST films obtained from Fig. 4 Sr content (x)

2

Remanent polarization, 2Pr (mC/cm ) Coercive field, 2Ec (kV/cm)

Fig. 2. FESEM images of the surface morphology: (a) PST (x ¼ 0.2), (b) PST (x ¼ 0.4), and (c) PST (x ¼ 0.6).

1 MHz were 364, 437, and 473 for PST (x ¼ 0.2, 0.4, and 0.6) thin films, respectively. The well-defined hysteresis loops of PST thin films measured at 310.28 kV/cm are depicted in Fig. 4. Like the dielectric property, the characteristic of polarization

0.2

0.4

0.6

20.73 168.62

9.68 112.41

1.62 27.93

depended on the composition of PST films. As the Sr content increased, the remanent polarization and coercive field decreased. It may be due to the gradually increase of Sr content that decreased the tetragonal distortion and the PST film transferred from ferroelectric to paraelectric phase. This result was consistent with the X-ray result shown in Fig. 1(b). The value of remanent polarization and coercive field obtained from Fig. 4 is listed in Table 1.

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0.09

105

450

0.08

103

400

0.07

350

0.06

300

0.05

250

0.04

200

0.03

150

0.02

100

0.01

500

-1000

-500

0

500

1000

Electric field (kV/cm) Fig. 5. The electric-field dependence of the dielectric properties of the PST (x ¼ 0.6) films.

The PST (x ¼ 0.2) film has the largest remanent polarization and coercive field. Although the value of polarization is lower than PZT films, the relatively larger coercive field and lower dielectric constant benefit Pb-rich PST thin films to apply in FET-FeRAM. In addition, the PST (x ¼ 0.6) film shows almost paraelectric characteristic. With its higher dielectric constant and low loss tangent, the Sr-rich PST thin film is suitable for the application in DRAM and tunable devices. The capacitance–voltage (C–V) characteristic of the PST (x ¼ 0.6) film measured at 1 MHz is shown in Fig. 5. In spite of the C–V characteristic presented a butterfly hysteresis loop because of the slightly spontaneous polarization-switching of PST (x ¼ 0.6) thin films, the large electric-field-dependent dielectric constant was observed obviously. To quantify the tunable ability and determine the quality of PST (x ¼ 0.6) thin films, the tunability (T) and the figure of merit (FOM) are defined as [15] T ð%Þ ¼

ð0Þ  ðV Þ ð0Þ

and FOM ¼

T , tan d

where e(0) and e(V) are the dielectric constants at zero and maximum dc bias voltage during the measurement. The tan d is the value of loss tangent at zero bias. By calculation, the T and FOM of PST (x ¼ 0.6) thin films were 74.95% and 29.85, respectively. The high T and low loss tangent provide paraelectric PST films with potential for the application in tunable microwave devices. Fig. 6 displays the leakage current density of PST films against applied electric field. The leakage current reduced remarkably with the increase of Sr content, especially in

Current density (A/cm2)

tanδ

dielectric constant (εr)

x=0.6

101

x=0.2 x=0.4 x=0.6

10-1 10-3 10-5 10-7 10-9 10

100

1000

Electric field (kV/cm) Fig. 6. Leakage current density as a function of applied field for PST (x ¼ 0.2, 0.4, and 0.6) thin films.

PST (x ¼ 0.6) films. The reduction in leakage current density could be referred to the smoother surface and the denser microstructure as Sr content increased. The grain size that determines the magnitude of the grain boundary area also influences the value of leakage current densities [16]. With the decrease of the grain size, the grain boundary area which prohibits the charge carrier from passing through films increases and the leakage current density can be suppressed. In addition, the lead oxide evaporation during the deposition process with a high temperature is believed to produce more oxygen vacancies and the oxygen vacancy is responsible for increasing leakage current densities in the films. Therefore, with the increase of Sr content, not only PbO loss and oxygen vacancies decreased, but the leakage current density was also reduced. In short, the PST (x ¼ 0.6) thin film had the lowest leakage current density and the highest breakdown field. The leakage current densities measured at 1 V (34.48 kV/cm) were 1.04  104 A/cm2, 7.82  108 A/cm2, and 3.95  109 A/cm2 for PST (x ¼ 0.2, 0.4, and 0.6) films, respectively. As well as the characteristic of the leakage current density, PST (x ¼ 0.6) films had a better lifetime characteristic as indicated in Fig. 7. Time-dependent dielectric breakdown (TDDB) is the property used to examine the resistance degradation of dielectric films. The grain boundary model and the reduction model are usually taken for explaining the behavior of resistance degradations in films with the perovskite structure [17,18]. For the grain boundary model, it supposes that a large potential drops across the grain boundary because of its high resistance. Films with a smaller grain size possess more grain boundary areas, which share this large potential drop; therefore, the resistance degradation can be suppressed. Besides, the reduction model suggests oxygen vacancies and injection electrons make films suffer a chemical reduction. It causes the increase of the electronic

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109

10 years lifetime

time to breakdown (sec)

108 107

x=0.2

106

x=0.4 x=0.6

105 104 103 102

217

tunable dielectric property with a high T (74.95%) and FOM (29.85). The PST (x ¼ 0.6) films also showed the lowest leakage current density and the best TDDB property. The improvement of surface morphology and the decrease of grain size may contribute to the improvement of insulating properties. In sum, the result of this study indicated PST thin films with an appropriate composition have potential for multiple applications in electronic devices. Acknowledgment

101 100 100

1000 Electric field (kV/cm)

10000

Fig. 7. Time-dependent dielectric breakdown (TDDB) as a function of field of PST (x ¼ 0.2, 0.4, and 0.6) thin films.

conductivity and the resistance degradation. According to these reasons, films with smoother surfaces and less oxygen vacancies can obtain a longer lifetime. Consequently, the PST (x ¼ 0.6) films had the advantages which are mentioned above, so it exhibited excellent reliability under long-term electrical stress. 4. Conclusion This study investigated the various Sr content strongly influenced the crystal structure and electrical properties of PST thin films. The surface roughness and grain size of PST films decreased as the Sr content increased. The dielectric constant of PST films at 1 MHz were 364, 437, and 473 for PST (x ¼ 0.2, 0.4, and 0.6) films, respectively. The largest remanent polarization (2Pr) and coercive field (2Ec) showed in PST (x ¼ 0.2) films were 20.73 mC/cm2, and 168.62 kV/cm, but the values in PST (x ¼ 0.6) films looked like a paraelectric material. This result was due to the increase of Sr content that decreased the tetragonal distortion. The PST (x ¼ 0.6) films displayed the excellent

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