Hai Joon Lee

The effect of K and Na excess on the ferroelectric and piezoelectric properties of K0.5Na0.5NbO3 thin films

We have fabricated K0.5Na0.5NbO3 (KNN) thin films on Pt substrates by a chemical solution deposition method and investigated the effect of K and Na excess (0?30?mol%) on ferroelectric and piezoelectric properties of KNN thin film. It was found that with increasing K and Na excess in a precursor solution from 0 to 30?mol%, the leakage current and ferroelectric properties were strongly affected. KNN thin film synthesized by using 20?mol% K and Na excess precursor solution exhibited a low leakage current density and well saturated ferroelectric P?E hysteresis loops. Moreover, the optimized KNN thin film had good fatigue resistance and a piezoelectric constant of 40?pm?V?1, which is comparable to that of polycrystalline PZT thin films.

Ferroelectric and piezoelectric properties of Na0.52K0.48NbO3 thin films prepared by radio frequency magnetron sputtering

Ferroelectric Na0.52K0.48NbO3 (NKN) thin film on Pt/Ti/SiO2/Si substrate was prepared using the radio frequency magnetron sputtering method. The single phase and grain morphologies of NKN were confirmed by x-ray diffraction and atomic force microscopy analysis, respectively. The remnant polarization Pr and coercive electric field Ec of NKN film were 22.5 μC/cm2 and 90 kV/cm, respectively. The NKN film displayed low frequency dielectric dispersion in the temperature range 25–500 °C. The leakage current density of the film was 3.0×10−7 A/cm2 at 100 kV/cm. The piezoelectric constant d33 was estimated to be 45 pm/V using the piezoelectric force microscopy.

Enhanced ferroelectric properties of LiNbO3 substituted Na0.5K0.5NbO3 lead-free thin films grown by chemical solution deposition

We have fabricated environmental friendly lead-free ferroelectric Na0.5K0.5NbO3 (NKN) and 0.95Na0.5K0.5NbO3–0.05LiNbO3 (0.95NKN-0.05LN) thin films by chemical solution deposition using metal-organic compounds, and studied the effects of LN substitution through the dielectric and ferroelectric properties. The small amount of LN substitution for NKN thin film led to a marked improvement in leakage current properties at the high electric field region. Furthermore, the 0.95NKN-0.05LN thin film (350nm) displayed clear ferroelectricity with well saturated P-E hysteresis loop with 2Pr and 2Ec values of 19.5μC∕cm2 and 91kV∕cm, respectively. The 0.95NKN-0.05LN films will be interesting for applications in lead-free ferroelectric and piezoelectric devices.

Raman Spectra Study of K0:5Na0:5NbO3 Ferroelectric Thin Films

We have fabricated K0.5Na0.5NbO3 (KNN) thin films on Pt substrates by a chemical solution deposition method and investigated the effect of K and Na excess (0–30 mol %) for KNN thin film. The KNN film with 20 mol % excess K and Na was an optimized KNN perovskite phase. From the Raman spectra, a change in the scattering mode was observed due to the chemical composition fluctuations of the excess K and Na. The peak of the (v1 + v5) internal vibrational mode in the NbO6 octahedra was split into two vibration modes which were shifted toward lower or higher wavenumbers depending on the K and Na cation deficiencies or redundancies.

Piezoelectric and Ferroelectric Properties of Lead-Free Bi4−xNdxTi2.97V0.03O12 Ceramics

We fabricated lead-free Bi 4−xNdxTi 2.97 V 0.03 O 12 (x = 0.25∼1.00; BNdTV-x) ceramics using the conventional solid-state reaction method and investigated their ferroelectric and piezoelectric properties. BNdTV ceramics exhibited the phase transition from orthorhombic to pseudo-tetragonal structure with increasing Nd concentration. At the morphotropic phase boundary of x = 0.85, BNdTV ceramics showed the highest values of P r (13 μC/cm 2 ), k t (17%) and Q m (2130), which are promising for applications to high frequency resonators.

Polarization Characteristics of High Valence Ion Doped Bismuth Lanthanum Titanate Ceramics

Bi3.25La0.75Ti3−3x Nb3x O12 (BLTN) and Bi3.25La0.75Ti3−3x V3x O12 (BLTV) ceramics (x = 0, 0.01, 0.03, 0.05, 0.07 and 0.1) were prepared by a solid state reaction method. For all BLTN and BLTV compositions, bismuth-layered perovskite structures were confirmed using X-ray diffraction. The remanent polarization increases and reaches to the maximum value at x = 0.05 for BLTN and at x = 0.01–0.03 for BLTV ceramics. With increasing doping concentrations, the Curie temperature of BLTV ceramics increased while that of BLTN ceramics decreased. For the substitutions of high-valence Nb5 + and V5 + ion for Ti4 + ion, effects of ion doping on dielectric and ferroelectric properties were investigated.

The Ferroelectric Properties of (Na0.5K0.5)NbO3 Thin Films Fabricated by rf-Magnetron Sputtering

Fatigue-free Na 0.5 K 0.5 NbO 3 (NKN) thin films were grown on Pt/Ti/SiO 2 /Si substrates by the radio frequency magnetron sputtering method. The addition of enriched Na 2 CO 3 and K 2 CO 3 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 (2P r) and the coercive field (2E c) of the NKN films annealed at 600°C were 32.2 μ C/cm 2 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/cm 2 at 150 kV/cm. The film showed fatigue-free characteristics under a ± 5 V bipolar square pulse of 1 MHz for up to 1.0×10 10 cycles. The NKN film prepared by radio frequency magnetron sputtering is an attractive alternative for applications involving nonvolatile ferroelectric random access memory (FRAM) devices.

Leakage Current Characteristics of Lead-Free K0.5Na0.5NbO3 Ferroelectric Thin Films with (K,Na) Excess and Li Substitution

K0.5Na0.5NbO3 thin films with (K,Na) excess and LiNbO3 substitution were fabricated on a Pt substrate by a chemical solution deposition method. The K0.5Na0.5NbO3 thin films with (K,Na) 10 mol % excess and LiNbO3 5 mol % substitution showed a saturated ferroelectric polarization–electric field (P–E) hysteresis loop and the estimated remanent polarization (Pr) and coercive field (Ec) were 8.6 µC/cm2 and 40 kV/cm at 280 kV/cm, respectively. Compared to pure K0.5Na0.5NbO3 thin films, the ferroelectric polarization of the thin film was improved and leakage current densities were decreased. Moreover, the ferroelectric polarization and leakage current characteristics are stable for higher applied electric fields and elevated temperatures. The conduction behaviours of these films were analyzed by Schottky and Poole–Frenkel emission.

Pyroelectric and Transient Current Properties of Multilayer Pb1.1(Zr0.3Ti0.7)O3/PbTiO3 and Pb1.1 (Zr0.3Ti0.7)O3 Thin Films

Pb 1.1 (Zr 0.30 Ti 0.70 )O 3 (PZT) and Pb 1.1 (Zr 0.30 Ti 0.70 )O 3 /PbTiO 3 (PZT/PT) multi-layered thin films are grown on Pt/Ti/SiO 2 /Si substrate by sol-gel method. The kinetics of current change of pyroelectric and transient response between PZT and PZT/PT ferroelectric thin films is investigated. The shape of peak in the PZT film is sharp, but that of PZT/PT is broadened. An increase in the peak value of switching current and switching time in the PZT/PT film is observed. The change of pyroelectric and transient current between PZT/PT and PZT films is discussed in terms of internal field.

Influence of Bi Excess on Electrical Properties of Bi3.25 + x La0.75Ti3O12+δ Thin Films

The Bi-excess Bi3.25+ x La0.75Ti3O12+δ (x = 0, 0.05, 0.10, 0.15, 0.20, BLT) thin films were prepared by pulsed laser deposition. Single phases of BLT thin films was confirmed by X-ray diffraction. For the small Bi-excess BLT films, ferroelectric properties are improved. One of them, 10 mol% Bi-excess BLT films exhibits high remanent polarization, low leakage current density, and improved switching polarization. Influence of Bi excess on ferroelectric and electrical properties are investigated by ferroelectric P-E hysteresis loops, transient current, and leakage current.