Ferroelectrics, 335:227–232, 2006 Copyright © Taylor & Francis Group, LLC ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150190600691460
The Ferroelectric Properties of (Na0.5 K0.5 )NbO3 Thin Films Fabricated by rf-Magnetron Sputtering HAI JOON LEE,1 CHANG WON AHN,1 SUN HEE KANG,1 ILL WON KIM,1,∗ JAE SHIN LEE,2 AND BYUNG MOON JIN3 1
Department of Physics, University of Ulsan, Ulsan 680-749, Korea School of Materials Science and Engineering, University of Ulsan, 680-749, Korea 3 Department of Physics, Dong-Eui University, Busan, 614-714, Korea 2
Fatigue-free Na0.5 K0.5 NbO3 (NKN) thin films were grown on Pt/Ti/SiO2 /Si substrates by the radio frequency magnetron sputtering method. The addition of enriched Na2 CO3 and K2 CO3 into the NKN ceramic target was necessary in order to compensate for a deficiency in the Na and K concentrations in the film. The remnant polarization (2Pr ) and the coercive field (2E c ) of the NKN films annealed at 600◦ C were 32.2 µC/cm2 and 90 kV/cm at an applied field of 200 kV/cm, respectively. A leakage current density was obtained around 7.3×10− 8 A/cm2 at 150 kV/cm. The film showed fatigue-free characteristics under a ± 5 V bipolar square pulse of 1 MHz for up to 1.0×1010 cycles. The NKN film prepared by radio frequency magnetron sputtering is an attractive alternative for applications involving nonvolatile ferroelectric random access memory (FRAM) devices. Keywords (Na,K)NbO3 thin film; ferroelectricity; FRAM
Introduction Ferroelectric thin films have been attracting attention for possible use in nonvolatile ferroelectric random access memory (FRAM) applications. FRAMs have many advantages, including non-volatility, low power consumption, radiation hardness, and high access speed [1, 2]. Many of the candidate materials for FRAM devices, such as Pb(Zr,Ti)O3 (PZT) and SrBi2 Ti2 O9 (SBT) films have been extensively studied [3–8]. Although the PZT films exhibit excellent ferroelectric properties, they have some serious problems such as undesirable fatigue, retention and imprint chracteristics. The SBT films have fatigue-free characteristics, but they require a processing temperature of greater than 800◦ C and have a low remnant polarization. Recently, it was reported that lanthanide doped BLT films, such as Bi3.25 La0.75 Ti3 O12 , show better fatigue endurance and larger ferroelectric properties compare to those of the SBT films [9, 10]. They also have a high processing temperature (>700◦ C) and a low remnant polarization (∼10 µC/cm2 ). Paper originally presented at IMF-11, Iguassu Falls, Brazil, September 5–9, 2005; received for publication January 26, 2006. ∗ Corresponding author. E-mail:
[email protected]
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The NKN ceramics have been studied extensively, because of their good piezoelectric and ferroelectric properties, but few studies have reported on thin films. M. Blomqvist et al. [11] and X. Wang et al. [12] have grown c-axis oriented Na0.5 K0.5 NbO3 (NKN) films on LaAlO3 substrates by rf magnetron sputtering and obtained high remnant polarization and a large tunability of 35% at room temperature. S. Abadei et al. [13] have deposited NKN films by pulsed laser deposition (PLD) and studied the dielectric properties up to 40 GHz for use in electrical tunable millimeter-wave device applications. S. I. Khartsev et al. [14] also have fabricated NKN films by rf magnetron sputtering and PLD methods, and have investigated the possibility of applications in electro-optical cells. V. M. Kugler et al. [15] have also grown the NKN films using the reactive rf magnetron sputtering method on polycrystalline Pt0.8 Ir0.2 substrates, with a low substrate temperature of 300–450◦ C, and investigated the dielectric properties. Cho et al. [16, 17] have grown NKN films using rf sputtering, PLD, metalorganic chemical vapor composition (MOCVD) and studied the dielectric and currentvoltage (I -V ) properties. But, a detailing of the ferroelectric properties of NKN films such as the polarization fatigue and leakage current characteristics, which are necessary evaluations for FRAM applications, have yet not been reported. In this study, we have deposited NKN thin films onto Pt/Ti/SiO2 /Si substrates by rf magnetron sputtering and investigated the P-E hysteresis loop, polarization fatigue, capacitance-voltage curves, and leakage current behavior.
Experiment The ceramic targets were made from the appropriate amounts of Na2 CO3 , K2 CO3 , and Nb2 O5 powders, with a standard milling and sintering procedure. The addition of enriched Na2 CO3 and K2 CO3 into the target was necessary in order to compensate for the lack of Na and K in the NKN film. The sintering process was performed at 650◦ C for 2 hrs. Rf magnetron sputtering in a high vacuum system (base pressure of 5 × 10−5 Torr) was performed with a target of 51 mm in diameter and a thickness of 5 mm. A final target density of 2.64 g/cm3 was achieved for a Na:K:Nb with a composition ratio of 1.5:1.5:1. Excess Na and K in the target were added to compensate for the loss of these constituents during the deposition. The Pt/Ti/SiO2 /Si substrates were placed in an on-axis position 3 cm apart from the target surface plane. During the deposition, a gas mixture of Ar:O2 (2.5:1.0) was used at a total pressure of 60 mTorr. The growth temperature was 600◦ C and the deposition rate achieved was 1.5 nm/min. After each deposition, the chamber was filled with oxygen (600 Torr) and the film was cooled down to room temperature at a rate of approximately 4◦ C/min. The phase and orientation of the NKN thin films were determined by glancing angle X-ray diffraction analysis (XRD RAD3, Rigaku, Japan). In order to study the electrical properties, Pt top electrodes with a contact area of 2.5 × 10−4 cm2 were deposited by a dc sputter coater (Cressington 108, Cressington, Inc., U.S.A) on the surface of the thin films through a shadow mask. After the Pt top electrode deposition, a MFM (metal/ferroelectric/metal) capacitor was built up. Polarization-field (P-E) and polarization fatigue measurements of the thin film using RT66A (Ferroelectric tester, Radiant Technology, U.S.A) were attained with a virtual ground mode. The dielectric and capacitance-voltage (C-V ) measurements were conducted through an HP4192A LF Impedance Analyzer (Hewlett Packard, Inc., U.S.A) controlled by a computer. The current-voltage (I -V ) characteristics were studied with a Keithley-237 electrometer (Keithley Instruments, Inc., U.S.A) which was also controlled by a computer.
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Figure 1. The glancing angle X-ray diffraction pattern of an NKN film sputtered at 600◦ C.
Results and Discussions Based on the JCPDS file, the NKN has an orthorhombic phase at room temperature with peaks of (001), (110), (002), (012), and (112). Figure 1 shows the glancing angle XRD pattern of the NKN film deposited at 600◦ C on the Pt/Ti/SiO2 /Si substrate. The indexing of the diffraction patterns was done somewhat tentatively and based on the assumption that the symmetry was orthorhombic. Four peaks were observed and they can be indexed as (001), (110), (012), and (112). The obtained NKN film exhibited a polycrystalline structure and the pyrochlore phase was not observed. The polarization-electric field loops of the NKN film are shown in Fig. 2, where the loops display slim shapes. A coercive field of about 46 kV/cm was observed at an applied
Figure 2. The P-E hysteresis loops of the NKN films at various applied voltages between 1 and 4 V.
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Figure 3. (a) The polarization fatigue behavior of the NKN film and (b) P-E hysteresis curves before and after 1.0 × 1010 cycles of polarization switching. This test was performed at 1 MHz.
electric field of 200 kV/cm. The remnant polarization of the NKN film was 32.2 µC/cm2 . This result is comparable with a value of 12.5 µC/cm2 which was obtained from a highly a-b axis oriented SBT thin film [8]. The fatigue of the ferroelectric thin films is defined as the loss of switchable polarization with repeated switching cycles. This loss of switchable polarization limits the duration period of ferroelectric devices. Fig. 3 (a), and (b) show the fatigue behavior of the NKN film and the P-E hysteresis loops of the NKN film before and after 1.0 × 1010 cycles of polarization switching using 1 MHz bipolar pulses. The Pt/NKN/Pt ferroelectric capacitor shows virtually no polarization degradation in switching tests when subjected to symmetric bipolar voltage pulses (peak-to peak 5 V, 50% duty cycle) in the 1 MHz range. These results sufficiently satisfy the requirements for nonvolatile memory applications. Figure 4 shows the voltage dependence of the small signal capacitance of the Pt/NKN/Pt capacitor at a frequency of 100 kHz. The small signal capacitance was measured when the electric voltage was swept from a positive to a negative bias and back again. As expected in ferroelectric capacitors, a butterfly-shape hysteresis appeared. The small applied ac signal excites the vibration of non-180◦ domain walls which carry charges due to polarization discontinuity. In the vicinity of coercive voltages, the polarization was nearly zero, due to randomly oriented domains and a large domain wall density. The two peaks in the C-V curve were located around ±32 kV/cm, which corresponded to the coercive voltage. These values were much less than the apparent coercive voltage (±45 kV/cm) obtained from the hysteresis loops. This was due to the fact that the dc bias voltage ramp rate was much slower than that of the stimulus voltage in the P-E hysteresis measurement. Figure 5 shows the voltage dependent leakage current characteristics of the Pt/NKN/Pt capacitors in a voltage range from 0 V to the 20 V. The leakage current was measured with a voltage step of 0.2 V and delay time of 20 s. The leakage current density was very low (10−9 A/cm2 ) at room temperature. The leakage current density increased gradually up to 10−6 A/cm2 with an increase of applied voltages below 8.6 V (430 kV/cm). The ohmic region of the NKN film is held for an applied field of 430 kV/cm which is a high applied
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Figure 4. The small signal capacitance-voltage (C-V ) curve of the Pt/NKN/Pt capacitor at a frequency of 100 kHz.
field comparable to other ferroelectric thin films such as PZT and BLT [8]. Above 8.6 V, the leakage current density jumped abruptly to ∼10−4 A/cm2 . Although this generally indicates a dielectric breakdown, through repeated measurements on the same device it was ascertained that the large increase in current density was not due to dielectric breakdown. It is believed that this curve provides clues towards the understanding of the conduction mechanism [18, 19]. The low voltage regime of the J -V curve can be interpreted into the Schottky emission current. A detailed leakage current density study as a function of temperature is underway to determine the carrier transport mechanism and the barrier height at the interface. Based on the present results, the NKN film is one of the more desirable candidate materials for FRAM devices, because it has a high remnant polarization, a low coercive, a low processing temperature and fatigue-free characteristics. Especially, the structure of the NKN is simple and the price of the raw materials, such as K2 CO3 , Na2 CO3 , and Nb2 O5 are cheap compared to those required for PZT and SBT.
Figure 5. The leakage current characteristics of the NKN films as a function of applied voltage.
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Conclusions Lead-free Na0.5 K0.5 NbO3 (NKN) thin films were successfully grown on Pt/Ti/SiO2 /Si substrates by radio frequency magnetron sputtering. The obtained NKN film exhibited a polycrystalline structure and the pyrochlore phase was not observed. A coercive field of 46 kV/cm was observed at an applied electric field of 200 kV/cm. The NKN thin film annealed at 600◦ C shows a remnant polarization value of 32.2 µC/cm2 . The Pt/NKN/Pt ferroelectric capacitor shows virtually no polarization degradation in switching test when subjected to symmetric bipolar voltage pulse (peak-to peak 5 V, 50% duty cycle) in the 1 MHz range. The leakage current density generally increases gradually at low voltages in the order of 10−9 –10−6 A/cm2 below 430 kV/cm. The ohmic region of NKN film is held for 430 kV/cm. The NKN film is a promising candidate material for FRAM device, because it has high remnant polarization, low processing temperature and fatigue-free characteristics.
Acknowledgments This work was supported by the University of Ulsan Research Fund of 2005.
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