Somu Kumaragurubaran

BaTiO3 based relaxor ferroelectric epitaxial thin-films for high-temperature operational capacitors

The epitaxial growth of 0.6[BaTiO3]–0.4[Bi(Mg2/3Nb1/3)O3] (BT–BMN) relaxor ferroelectric thin-films on (100) Nb doped SrTiO3 substrates has been achieved and the structure is investigated for high-temperature capacitor applications. The post growth annealing decreases the oxygen vacancy and other defects in BT–BMN films, resulting in the enhancement of dielectric constant. An insertion of intermediate SrRuO3 layers as an electrode instead of Pt, sandwiching the film, is found to be more effective in enhancing the dielectric constant. A very high dielectric constant exceeding 400 was achieved from high-temperature annealed film and the film showed an excellent dielectric constant stability of below 11% in the temperature range of 80–400 °C. This will enable smaller, high-temperature tolerant, monolithically integrated thin-film capacitors on power semiconductor devices.

A Comparative Study of Dielectric Relaxation in Sr0.61Ba0.39Nb2O6 and Sr0.75Ba0.25Nb2O6 Single Crystals

Dielectric behaviour of Sr0.61Ba0.39Nb2O6 (61SBN) and Sr0.75Ba0.25Nb2O6 (75SBN) single crystals has been studied. It is observed that 61SBN has relatively low diffuseness as well as smaller relaxation compared to 75SBN. Increase in the configurational entropy imposed by the change in defect structure is suggested to be the origin of this behaviour. Analysis using Vogel–Fulcher expression and modified Curie–Weiss law give a signature of long-range polar order in 61SBN. The value of γ for 61SBN is closer to unity (1.14), where as for 75SBN it is 1.52. The fluctuation (in the orientation) of dipoles near dielectric maxima in SBN system is mediated by a paraelectric state.

Combinatorial synthesis of BaTiO3–Bi(Mg2/3Nb1/3)O3 thin-films for high-temperature capacitors

Combinatorial thin-film of (1−x)[BaTiO3]–x[Bi(Mg2/3Nb1/3)O3] — (BT–BMN) was grown on Pt/Ti/SiO2/Si, using pulse laser deposition (PLD) method, by ablating stoichiometric and Bi-10 wt % enriched targets to optimize the Bi content. X-ray photoelectron spectroscopy analysis revealed a linear Bi composition spread. As-deposited films post-annealed at high-temperatures, under oxygen atmosphere, turned into crystalline state. The crystallinity, characterized by X-ray diffraction, is better towards Bi-enriched region. The dielectric constant showed a strong dependency of Bi composition and saturated over Bi-7 wt %. The scanning nonlinear dielectric microscopic investigation revealed ferroelectric phase distribution is better around Bi-7 wt % region where the measured leakage current is also minimum. Dielectric constant over 240 and dielectric constant stability below 13% (25–400 °C range) were obtained.

Ferroelectric Studies on Sr0.61Ba0.39Nb2O6 Single Crystal Grown by Double-Crucible Stepanov Technique

Sr0.61Ba0.39Nb2O6 crystals grown by a modified double-crucible Stepanov technique have coercive fields independent of thickness. The coercive field is about 250 V/mm as measured from the ferroelectric hysteresis loop. However, the coercive field is found to depend strongly on the ramp rate. Dielectric measurements revealed a diffused phase transition at about 352 K and a Curie constant of about 4.5 ×105 K.

Polarization Switching and Defect Structure in Pure and MgO Doped Near-Stoichiometric LiNbO3 Crystals

Pure and MgO doped (up to 4 mol%) near-stoichiometric LiNbO 3 crystals are grown from Li rich (Li-55 mol%) melt. Significant improvement in the optical properties such as increase of optical damage resistance, accompanied by a shift in the –OH peak position in FTIR spectrum, and transparent range is observed at 1.75 mol% of MgO concentration in the crystal. Also the coercive field required for ferroelectric domain switching is minimum at 1.75 mol% MgO doping. Based on the results, the correlation between the defect structure and ferroelectric domain switching behavior in pure and MgO doped SLN is discussed.

Interface stability of electrode/Bi-containing relaxor ferroelectric oxide for high-temperature operational capacitor

The interface stability between electrodes (Pt, TaC, TiC, and RuO2) and a Bi-containing relaxor ferroelectric oxide, BaTiO3–Bi(Mg2/3Nb1/3)O3 (BT–BMN), applied to a high-temperature operational capacitor was investigated by hard X-ray photoelectron spectroscopy. All the electrodes showed electron filling at the Fermi level after annealing at 400 °C. However, Pt and TaC indicated electrical property degradations due to the thick intermediate layer formation and defect formation of the BT–BMN layer relating to the Bi diffusion into the electrodes. In contrast, TiC inhibited the Bi diffusion and did not show any change in the band alignment after annealing. Furthermore, RuO2 eliminated the defect formation in BT–BMN and showed no change in the band alignment although the Bi diffusion was also observed. These results suggest that the TiC/RuO2/BT–BMN stack structure is a potential candidate for the high-temperature operational capacitor.

Effect of Impressing Rate of Field on Polarization Reversal in Mg Doped Near Stoichiometric Lithium Tantalate Single Crystals

The coercive field measured from ferroelectric hysteresis loop for pure and Mg:SLT (0.5 mol% doped) crystals were found to be independent of ramp rate of voltage, while it depends strongly on ramp rate for Mg:SLT (1 mol% doped) crystals. The measured coercive field decreased monotonously depending on the Mg concentration and low E c value of 6 kV/cm is obtained for Mg(1.0 mol%)SLT crystal. Internal field of about 1 kV/cm present in pure SLT completely gets vanished in Mg(1.0 mol%)SLT. Spontaneous polarization is insensitive to the Mg addition. The origin of the internal field and large changes in switching fields appear largely to be dependent on the [Li]/[Li+Ta] ratio in crystals. On repeated cycling, the coercive field is found to have a marginal variation in pure and Mg doped SLT crystals.