Sung Ho Choh

Comparison of memory effect between YMnO3 and SrBi2Ta2O9 ferroelectric thin films deposited on Si substrates

For the ferroelectric gate-type capacitors, we have fabricated Pt/YMnO3(YMO)/Si and Pt/SrBi2Ta2O9(SBT)/Si structures. We have used the highly c-axis oriented hexagonal YMO thin films (er≈19) and the polycrystalline SBT thin films (er≈150) with a dominant (115) orientation, respectively. The memory effect resulting from the ferroelectric switching properties is investigated as a function of the dielectric constant of ferroelectric thin films with 150 nm in thickness. About 3 times wider memory window is obtained by using a relatively low dielectric constant of YMO than that using a relatively high dielectric constant of SBT. Typical memory windows of the Pt/YMO/Si and the Pt/SBT/Si capacitors are 1.24 and 0.34 V, respectively, at a gate voltage of 5 V.

Characteristics of Metal/Ferroelectric/Insulator/Semiconductor Field Effect Transistors Using a Pt/SrBi2Ta2O9/Y2O3/Si Structure

For the fabrication of metal/ferroelectric/insulator/semiconductor field effect transistors (MEFISFETs), SrBi2Ta2O9 (SBT) film was formed onto Y2O3 layer using the metal organic deposition (MOD) method. Memory windows of MEFISFET were in the range of 0.96–1.38 V when the gate voltage varied from 5 to 7 V. Current-voltage characteristic and transconductance curve of the MEFISFET show the effective gate modulation and the stable memory effect induced by the ferroelectric properties.

Effects of excess oxygen on the properties of reactively sputtered RuOx thin films

Conducting RuOx thin films are fabricated by reactive sputtering with Ar+O2 plasma for an application as the bottom electrode in a ferroelectric thin film capacitor. The structure and composition of RuOx should be controlled to promote the required characteristics in the deposited ferroelectric film. The stoichiometry and resistivity of RuOx are easily controlled by varying the relative O2 partial pressure in the sputtering ambient between 10% and 50%, where the oxygen composition is varied from 2.0 to 2.4 and the resistivity of 100‐nm‐thick RuO2 film is changed from 145 to 280 μΩ cm corresponding to the variation of oxygen concentration. The films deposited in O2 pressures lower than 30% show preferential (110) RuO2 grain growth, whereas they show (101) in higher O2 pressures. The films deposited at high O2 pressures show some decomposition of the film during a high temperature annealing in N2 ambient.

Cr3+ centres in LiNbO3: Experimental and theoretical investigation of spin hamiltonian parameters

Abstract X-band EPR spectra of Cr3+ ions in ferroelectric LiNbO3 single crystal at room temperature are analyzed assuming Cr3+ enters either of the three possible sites: Li, Nb and structural vacancy (SV) site. The observed spectra can be resolved into (I) the main (Cr13+) and (II) the weak (CrII3+) spectra. The axial zero-field splitting parameter for a Cr3+ centre at Li site (DLi) as well as Nb site (DNb) is calculated using the superposition model. The values DLi and DNb agree well with the experimental D value for CrI3+ and CrII3+, respectively. This confirms that the main spectra can be attributed to CrI3+ at the Li site and the weak spectra to CrII3+ at the Nb site. For Cr3+ ion at the structural vacancy site the parameter DSV is calculated as a function of displacement of Cr3+ ion from the centre of the oxygen octahedron along the [1 1 1] direction. Other weak and unresolve spectra also observed by us can possibly be due to Cr3+ centres at vacancy site.

Deposition and Properties of Reactively Sputtered Ruthenium Dioxide Thin Films as an Electrode for Ferroelectric Capacitors

Ruthenium dioxide (RuO2) films are studied for use as a bottom electrode of the (Ba, Sr)TiO3 thin-film capacitor. RuO2 films have been deposited by reactive DC magnetron sputtering of ruthenium at a relatively low sputtering power. Stoichiometric RuO2 films are obtained at oxygen partial pressures as low as 0.6 mTorr. The properties of the films have been investigated using techniques such as Rutherford backscattering spectrometry, Auger electron spectrometry, X-ray diffraction, and scanning electron microscopy. The oxygen composition in as-deposited RuOx films increases from 2.0 to 2.4 with the increase of initial O2 partial pressure from 1.2 to 5.6 mTorr at a sputtering power of 200 W. The films deposited under low oxygen partial pressures followed by annealing show preferential crystal growth in the [110] direction, whereas those deposited in high oxygen partial pressures show growth in the [101] direction. A resistivity of 65 µΩcm is obtained after annealing at 800°C. Even after high-temperature deposition and subsequent annealing processes, clear interfaces between (Ba, Sr)TiO3 and RuO2 films are obtained.

A theoretical investigation of the zero-field splitting parameters for an Mn2+ centre in a BiVO4 single crystal

The zero-field splitting (ZFS) parameters D, E and F for an Mn2+ centre in a BiVO4 single crystal are calculated using the point-charge electrostatic model and the superposition model. The calculated ZFS parameters at the Bi and V sites are compared with the experimental values for Mn2+. The calculated second-order axial and rhombic ZFS parameters at the Bi site turn out to be similar to those from experiment. The superposition model and the point-charge model give similar results. This supports the notion that the Mn2+ impurity substitutes for the Bi3+ ion in BiVO4.

Domain wall of ferroelastic BiVO4 studied by transmission electron microscopy

Abstract The domain structure of the ferroelastic BiVO4 has been studied by transmission electron microscopy and electron diffraction methods. The measured angle of 44±1° between the [100] axis and the domain boundaries is in good agreement with 44.5° determined by the nuclear magnetic resonance and X-ray diffraction methods in our preceding papers. Therefore, we have confirmed that the domain boundary in BiVO4 is the prominent (101) W plane.

Electron paramagnetic resonance studies of Co2+ ions in congruent and nearly stoichiometric LiNbO3 single crystals

Congruent and nearly stoichiometric LiNbO3 single crystals doped with Co 2+ ions have been investigated by the electron paramagnetic resonance technique. The spin-Hamiltonian parameters, gk D 2:671.2/, g? D 5:052.2/, Ak D 40.2/ 10 4 cm 1 , and A? D 154.1/ 10 4 cm 1 , are obtained without including the nuclear Zeeman and quadrupole interactions. The factor,k, of reduction due to the covalency is estimated as 0:926k6 1 using the aboveg-values. On the basis of the temperature dependence of the linewidth, the spin-lattice relaxation and the spin-spin relaxation mechanisms are investigated. The former is a sum of a weak direct process with a phonon bottleneck .T 2:4/ and an Orbach process with 1 156 cm 1 (or 225 K), and the latter is a sum of a typical temperature-independent process and an anomalous process with a temperature dependence/T 3:6 . This anomaly is considered as a shortening of T2 due to the increase of the correlation length of the spin-spin interaction as the temperature decreases. The integrated signal intensity also showing an anomaly is a consequence of the linewidth anomaly.

ESR study of rare-earth ions with the effective spin of 1/2 in LiNbO3 crystals

Rare-earth ions of Nd3+ and Er3+ in nearly stoichiometric and MgO-doped LiNbO3 crystals, respectively, have been investigated by employing an X-band electron spin resonance (ESR) spectrometer. The grown crystal was heated in Li-rich powder at 1100°C in order to make it nearly stoichiometric by the vapor transport equilibrium technique. Due to the fact that the ESR linewidth is much narrower in the stoichiometric crystal than in the congruent LiNbO3, we were able to determine the hyperfine constants of143Nd and145Nd at 4 K. By codoping MgO into LiNbO3, a new Er3+ center has been observed with a differentg-tensor. We propose that the new Er3+ center in Mg-doped LiNbO3 occupies the niobium site due to the local excessive Mg2+ ion at the lithium site, whereas Nd3+ and Er3+ in congruent crystals reside at the lithium site. The proposal is consistent with theg-value anisotropy.

The domain structure of ferroelastic BiVO4 studied by magnetic resonances

The domain structure of the ferroelastic BiVO4 single crystal has been investigated using the nuclear magnetic resonance (NMR) of 51V and by the electron paramagnetic resonance (EPR) of the Mn2+ ions contained in the crystal as an impurity. The 14 resonance lines of 51V (I=7/2) in a BiVO4 crystal with the twin domain were measured in the crystallographic a-b (or c-b) and c-a (or a-c) planes with an FT NMR spectrometer. Two sets of Mn2+ EPR signals were also obtained in the c-a plane. These two sets of NMR and EPR signals originate from the twin-domain structure. From these two sets of experimental data from 51V NMR and Mn2+ EPR, it is confirmed that the BiVO4 single crystal has the prominent (W-plane) domain wall reported previously. The investigated domain structure is found to be stable with time in contrast with a previous report by Baran et al. The previous model of the twinning mechanism derived from NMR and X-ray diffraction data has been improved by employing the EPR results. The observed W plane of the domain wall in BiVO4 can be explained by the ferroelastic species 4/mmm F 2/m instead of 4/m F 2/m.