Fujitsu Laboratories, Ltd, 10-1 Morinosato Wakamiya, Atsugi, Kanagawa ... θ-2θ X-ray diffraction (XRD) patterns of 550 °C-deposited, 450 °C-deposited and post ...
Mater. Res. Soc. Symp. Proc. Vol. 1199 © 2010 Materials Research Society
1199-F06-33
Process-dependent coercive fields in undoped and Mn-doped BiFeO3 films formed on SrRuO3/Pt(111) electrodes by rf sputtering Jeong Hwan Kim1, Hiroshi Funakubo1, Yoshihiro Sugiyama2, and Hiroshi Ishiwara1 1
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku Yokohama 226-8502, Japan 2
Fujitsu Laboratories, Ltd, 10-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0197, Japan
ABSTRACT We deposited BiFeO3 (BFO) thin films on SrRuO3 (SRO)/Pt bottom electrodes by radiofrequency (RF) sputtering. Some samples were formed at the substrate temperature of 550 °C, and others were formed at 450 °C and post-annealed at 650 °C for crystallization. The coercive field in the post-annealed BFO film was smaller than that in the 550 °C-deposited BFO film. The coercive field in Mn-doped BiFeO3 (BFMO) films which were deposited at 550 °C on SRO/Pt(111) was lower than that in undoped BFO films. Degradation of the remanent polarization was less significant in the post-annealed BFO film.
INTRODUCTION In future high-density one transistor (1T)-one capacitor (1C)-type ferroelectric random access memories (FeRAMs), the area of each memory cell must be reduced. However, if the capacitor area is simply reduced, polarization reversal current decreases and it becomes difficult to read out stored information using the current. In order to secure the polarization charges in the limited cell area, development of new ferroelectric materials with a large remanent polarization is highly expected. BiFeO3 (BFO) is a promising candidate for fabricating high-density FeRAM, because it has a remanent polarization as large as 90 µC/cm2 [1] and low crystallization temperature. However, BFO has significant problems such as low resistivity and high coercive field at room temperature (RT). In order to decrease the operation voltage of FeRAM, ferroelectric materials with low coercive voltage is required. For fabricating reliable BFO films, various kinds of deposition methods have been used. Typical fabrication methods are pulse laser deposition (PLD) [2] radio-frequency (RF) sputtering [3], metal organic chemical vapor deposition (MOCVD) [4], and chemical solution deposition (CSD) on various substrates [5]. However, there is still a problem in polycrystalline BFO films, in that their leakage current and coercive field are high at RT. In our previous paper, we have preliminary reported that leakage current and coercive field in BFO films are reduced by doping Mn atoms [6]. However, various methods or processes for forming good quality of BFO thin films are needed to solve the problems in BFO films. In this paper, we prepare undoped BFO and Mn-doped BFO (BFMO) thin films using rfsputtering on SrRuO3-coated Pt bottom electrodes. It is shown that the coercive field significantly changes by changing the sputtering conditions to form the films.
EXPERIMENTAL SrRuO3 (SRO) thin films were deposited on Pt(111)/TiO2/SiO2/Si substrates using rfsputtering in Ar:O2=70 sccm:30 sccm at 510 °C for 30 minutes. A typical SRO film thickness was 90 nm. Then, BFO and BFMO (5 % Mn-doped BFO) films were formed on SRO/Pt(111)/TiO2/SiO2/Si substrates using rf-sputtering. Two substrate temperatures of 450 and 550 °C were used and in the former case the deposited films were subsequently annealed for crystallization at 650 °C for 10 minutes in O2 flow. Lastly, Pt top electrodes were deposited using electron-beam evaporation. The crystalline structure of the films was investigated using a multipurpose X-ray diffractometer (X’Pert-Pro MPD Philips). The surface of the films was observed using a scanning electron microscope (SEM). The polarization vs electric field (P-E) and current density vs electric field (J-E) characteristics of the BFO and BFMO capacitors were measured at RT using a ferroelectric test system (Toyo FCE-1A/Fop-100V) and an HP4156A precision semiconductor parameter analyzer (Hewlett-Packard), respectively.
RESULTS
BFO 200
SRO 111 Pt 111
BFO 111
Si 002
BFO 100
Intensity [arb.unit]
θ-2θ X-ray diffraction (XRD) patterns of 550 °C-deposited, 450 °C-deposited and postannealed BFO films (340 nm and 390 nm-thick) and a 550 °C-deposited BFMO films (180 nmthick) are shown in figure 1. All films were deposited on the SRO/Pt/Ti/SiO2/Si structures and post-annealing temperature was 650 °C. As shown in the figure, the Pt films are strongly oriented to a direction. It is expected that the SRO films are also oriented to the direction, although the peak cannot be observed because it overlaps with the Pt peak. The XRD peaks of all films are indexed using a rhombohedral structure. In the BFO and BFMO films deposited at 550 °C, strong peaks appear. On the other hand, and peaks appear in the post-annealed BFO (390 nm) film. No secondary phase other than the perovskite phase appears in these three samples. We conclude from these results that preferred -oriented BFO and BFMO films are formed on SRO/Pt electrodes, while a -oriented BFO film can be formed by low temperature sputtering and subsequent crystallization annealing.
(a) (b) (c) 20
30
40
50
2θ θ [deg] Figure 1. XRD patterns of (a) a 550°C-deposited BFO film, (b) a 450°C-deposited and postannealed BFO films and (c) a BFMO film.
(a) (b) (c) Figure 2. Surface SEM images of (a) a 550°C-deposited BFO film, (b) a 450°C-deposited and post-annealed BFO film, and (c) a 550°C-deposited BFMO film.
Polarization ( μC/cm2 )
Surface SEM images of 550 °C-deposited BFO, post-annealed BFO, and 550 °Cdeposited BFMO films are shown in figure 2. In the 550 °C-deposited pure BFO and BFMO films on the SRO/Pt electrodes, grains are packed more densely and the surfaces are rough. On the other hand, the surface of the 450 °C-deposited and post-annealed BFO film seems to be flatter. The grain size in BFO films ((a) 340 nm and (b) 390nm) is larger than that in the BFMO film (185 nm). Grain size tends to become larger as the film thickness increases.
100
R.T 50 100kHz 0
-50
-100
BFO (550℃ ℃-deposited) BFO (450℃→ ℃→650℃ ℃→ ℃ annealing) BFMO -200 -1500-1000-500 0 500 1000 1500 -150
Electric Field ( kV/cm ) Figure 3 P-E hysteresis loops of a 550°C-deposited BFO film, a 450°C-deposited and post annealed BFO film, and a BFMO film. Figure 3 shows P-E hysteresis loops of BFO and BFMO films deposited on SRO/Pt by rf-sputtering. Circles show the loop for a BFO film deposited at 550 °C, squares show that for a BFO film deposited at 450 °C and post-annealed at 650 °C, and triangles show that for the BFMO film. As shown in the figure, the coercive field is lowest in the post-annealed sample, and the shapes of the hysteresis loops are close to rectangular. The remanent polarizations (Pr) of 550°C-deposited and post-annealed BFO films are approximately 90 and 68 µC/cm2, respectively. The coercive field in the post-annealed BFO film (2 Ec = 489 kV/cm) is smaller by approximately 60 % than that in the 550 °C-deposited BFO film (2 Ec = 1246 kV/cm). The decreased coercive field in the post-annealed BFO film is probably due to the preferred
(b)
Current Density ( A/cm2)
(a)
Current Density ( A/cm2)
orientation of the film. There is a general relationship between the polarization and the coercive field [7], and -oriented BFO has larger polarization and coercive field than -oriented BFO film. The XRD result shown in figure 1 reveals that the grains in the post-annealed BFO film have < 100>-preferred orientation. Then, the post-annealed BFO film is expected to have smaller polarization and coercive field than -oriented BFO film. In the 5 % Mn-doped BFO film (triangles), Pr and 2Ec are 46 µC/cm2 and 686 kV/cm, respectively. The coercive voltage (2Vc) of the BFMO film is 12 V for an applied voltage of 17 V. These results are summarized in table I.
100-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 10 100-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 10
BFO (550℃ ℃-deposited) BFO (450℃→ ℃→650℃ ℃→ ℃ annealing) BFMO -600
-300
0
300
600
Electric Field ( kV/cm )
BFO (550℃ ℃-deposited) BFO (450℃→ ℃→650℃ ℃→ ℃ annealing) BFMO -1
0
1
E/Ec Figure 4. (a)J-E and (b)Normalized J-E characteristics of the 550°C-deposited and the 450°Cdeposited and post-annealed BFO films, and the BFMO film. The horizontal axis in (b) is normalized by Ec.
Table I. Pr and Ec values of the 550°C-deposited BFO film, the 450°C-deposited and postannealed BFO film, and the BFMO film.
Pr Ec
BFO(550 °C-deposited) 90µC/cm2 623kV/cm
BFO(450 °C →650 °C annealed) 68µC/cm2 245kV/cm
BFMO(550 °C-deposited) 47µC/cm2 343kV/cm
Figure 4(a) shows J-E characteristics of the same samples as those in figure 3. As shown in the figure, the current density at a certain electric field is much higher in the post-annealed BFO film than that in the 550 °C-deposited BFO films. However, when the applied electric field is normalized by the coercive field as shown in figure 4(b), the current density values become comparable between the two samples. Figure 4(b) also shows that the leakage current density in the BFMO film is lower in the high electric field region than those in the BFO films and that the breakdown field is higher in the BFMO film. The current conduction mechanism and the origin of the different J-E characteristics between BFO and BFMO films have been discussed in our previous work [6].
CONCLUSIONS BFO and Mn-doped BFO films were deposited at substrate temperatures of 550 °C and 450 °C by using rf-sputter. The 450 °C-deposited BFO film was post-annealed at 650 °C for 10 min in O2 flow. It was formed in the 450 °C-deposited and post-annealed BFO film that the coercive field was reduced, comparing to the 550 °C-deposited BFO film. In the BFMO film, the coercive field and leakage current were also reduced by comparing with the undoped film, but the degradation in the remanent polarization was more significant. We conclude from these results that the coercive field in RF-sputtered BFO and BFMO films can be much reduced by optimizing the sputtering conditions as well as the target composition.
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