Pulsed Laser Deposited Na 0.5 K 0.5 NbO 3 Thin Films
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Department of Condensed Matter Physics, Royal Institute of Technology, SE- 100 44 Stockholm, SWEDEN; • Department of Electrical Engineering, Korea University, Seoul, KOREA.
ABSTRACT Na0 .5K 0.5NbO 3 films have been grown onto polycrystalline Pt80 Ir20 substrates at two different regimes: high (- 400 mTorr) and low (- 10 mTorr) oxygen pressure. Films grown at high oxygen pressure have been found to be single phase and highly c-axis oriented. The concept of discriminatedthermalizationhas been developed to explain the dynamics of the laser ablation process and to find reliable pulsed laser deposition (PLD) processing conditions. The phenomenon of selfassembling of Na0 .5K0.5NbO 3 films along [001] direction has been observed. On the other hand, films grown at low oxygen pressure have been found to be mixed phase of ferroelectric Na 0.5 K0 .5NbO 3 and paraelectric potassium niobates. Superparaelectric behavior has been observed in these films: 5% tunability at electric field of 100 kV/cm, losses as low as 0.003 and excellent stability to the temperature and frequency changes.
INTRODUCTION Recently several considerably interesting characteristics, both from fundamental and technological points of view, of perovskite Nao.5K0.5NbO 3 (hereafter NKN) thin films have been reported. [1,2] Strong electric field dependence of dielectric permittivity with very low
loss at room temperature and high piezoelectric coefficient with moderate dielectric constant promise vast variety of emerging applications. We report on the pulsed laser deposited (PLD) NKN thin films onto Pt 8oIr 20 (here after Pt) substrates at different oxygen ambient gas pressure.
EXPERIMENTAL PROCEDURES A KrF excimer laser (Lambda Physik-300, pulse width of 20 ns) has been used to ablate a stoichiometric NKN ceramic target. Slightly c-axis textured polycrystalline PttoIr 2o substrates have been used. Two different PLD processes have been implemented (see the Table): deposition at high oxygen ambient pressure (hereafter
TABLE. Processing conditions for HP- and LPNa0.5K0.SNbO 3 films PARAMETER LP IPHP 575
Temperature MC)
-
640
-400
Oxygen pressure (mTorr) 2
Laser energy density (J/cm ) Annealing condition (°C, min)
4
5
650, 30
149 Mat. Res. Soc. Symp. Proc. Vol. 603 © 2000 Materials Research Society
670
10 3- 4 400, 30
HP) and low pressure (hereafter LP). To make dielectric characterization, Au upper electrodes with a diameter of 0.55 mm have been thermally evaporated on the top of NKN layer at room temperature. The dielectric, ferroelectric, and crystalline quality characterization techniques used are described elsewhere. [2]
RESULTS AND DISCUSSION Composition and crystalline properties of the fabricated NKN films have been proved to be crucially dependent on ambient gas pressure. Figs. 1 show x-ray diffraction (XRD) 0-20 scans for NKN LP-film (a), HP-film (b), and rocking curves of the NKN-002 and Pt-002 reflections for HP-film (c).
20-
c HP - Na 05
K 0 5 NbO 3/Pt80 1r20
U,1 5
o_ C.)
aCL
>'10 U/) 0)-
C:
C
0 10
20 30 0(deg)
40
2
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