Hiroshi Ishiwara

Room temperature ferroelectric properties of Mn-substituted BiFeO3 thin films deposited on Pt electrodes using chemical solution deposition

Mn-substituted BiFeO3 (BFO) thin films were fabricated by chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) structures. X-ray diffraction analysis revealed that the BFO lattice was somewhat distorted when 5% of Fe atoms were substituted with Mn atoms, but no secondary phase appeared by Mn substitution up to 20%. The leakage current density at higher electric field than 0.6MV∕cm decreased by Mn substitution of 3%–5%, compared with a pure BFO film. Because of the low leakage current density in the high electric field region, well saturated polarization hysteresis loops with remanent polarization of 100μC∕cm2 were observed in the 5% Mn-substituted BFO films at a measurement frequency of 1kHz.

Theoretical Considerations on Lateral Spread of Implanted Ions

This paper presents a theoretical analysis on the lateral spread of the distribution for implanted ions in amorphous targets. First, it is shown that the solutions of the second order differential equations concerning moments of the ranges are necessary and sufficient to estimate the lateral distribution, if the Gaussian distribution is assumed. The calculated results of , and the lateral spread are presented. Next, it is shown that the actual distribution function along the lateral direction is approximately expressed by the complementary error function at the window edge, in case when ions are implanted through the mask-window. The variation of the lateral spread with incident ion mass is also calculated. From these calculations, it is concluded that the lateral spread can not be neglected in the case of light ions such as B+ incident to Si.

Low-voltage operation of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer capacitors and metal-ferroelectric-insulator-semiconductor diodes

Exceptionally low-voltage operation of organic ferroelectric capacitors and diodes was demonstrated. Ferroelectric polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] thin films were prepared by the solvent-cast method. Metal-ferroelectric-metal capacitors with 60-nm-thick P(VDF-TrFE) films exhibited well-saturated hysteresis curves whose coercive voltage (Vc) and remanent polarization (Pr) were 2.0V and 11.9μC∕cm2, respectively. The authors also fabricated metal-ferroelectric-insulator-semiconductor diodes with 100-nm-thick P(VDF-TrFE) films. Rectangular-shaped capacitance-voltage (C-V) hysteresis loops were obtained with a voltage sweep range narrower than 5V. The maximum memory window of 4.7V was achieved.

Microstructure and frequency dependent electrical properties of Mn-substituted BiFeO3 thin films

Mn-substituted (0–10at.%) BiFeO3 (BFO) thin films were fabricated by chemical solution deposition on Pt∕Ti∕SiO2∕Si substrate. X-ray diffraction analysis revealed that no secondary phase appeared even if Fe atoms were substituted with Mn atoms up to 10at.%. However, in the 10at.% Mn-substituted BFO film, the lattice parameters decreased by approximately 0.6%. Substitution of small amount of Mn atoms such as 3–5at.% in BFO films was effective in reducing the leakage current density at a high electric field. The P‐E (polarization versus electric field) hysteresis loops were measured by changing frequencies of triangular voltage pulses from 1to100kHz. In Mn-substituted BFO films, frequency dependences of the remanent polarization were not pronounced in the range from 10to100kHz and these measurements showed that the remanent polarization increased with increase of the Mn concentration.

Double heteroepitaxy in the Si (111)/CoSi2/Si structure

Epitaxial growth of CoSi2 films on Si substrates and the growth of Si films on the Si(111)/CoSi2 structure are investigated. Solid phase epitaxy is used to grow both CoSi2 and Si films. Molecular beam epitaxy is also used to grow the top Si films in the double heteroepitaxy. It has been found that two dominant factors required to obtain good epitaxial films are substrate cleaning by lamp heating before the film deposition and annealing of the deposited films without exposure to air. Excellent crystalline quality of the CoSi2 films on (111) Si substrates and good quality of the Si films on the Si(111)/CoSi2 structure have been demonstrated by ion channeling and backscattering techniques and reflection electron diffraction analysis. Uniformity of the grown films has also been examined by scanning electron microscopy.

Epitaxial Growth of SrTiO3 Films on Si(100) Substrates Using a Focused Electron Beam Evaporation Method

A novel epitaxial growth method of SrTiO3 films on Si(100) substrates is presented in which, in order to reduce the surface oxide layers of the substrates, thin Sr layers are deposited prior to deposition of SrTiO3 films. It has been shown that the film composition becomes nearly stoichiometric when single-crystalline SrTiO3 sources are evaporated by a focused electron beam and that SrTiO3 films grow epitaxially on Si(100) substrates under the optimum conditions of the Sr layer thickness, deposition temperature, and annealing temperature.

Electrical properties of LaAlO3/Si and Sr0.8Bi2.2Ta2O9/LaAlO3/Si structures

Lanthanum aluminate (LaAlO3) films were deposited on Si(100) substrates by evaporating single-crystal pellets in vacuum using an electron-beam gun. Then, they were annealed in N2 ambience at 700 °C for 10 min using an electric furnace. X-ray diffraction analysis showed that the LaAlO3 films were amorphous even after the annealing process. No hysteretic characteristics were observed in the capacitance–voltage (C–V) measurement and the dielectric constant of the LaAlO3 films was estimated to be 21–25. It was also found that the leakage current density decreased by about three orders of magnitude after the annealing process. On these films, Sr0.8Bi2.2Ta2O9 films with 210 nm thickness were deposited by a sol–gel method. All samples annealed in O2 atmosphere at temperatures ranging from 650 to 750 °C showed hysteretic C–V characteristics, and the memory window width in the sample annealed at 750 °C for 30 min was about 3.0 V for a voltage sweep of ±10 V. It was also found that the capacitance values biased in ...Lanthanum aluminate (LaAlO{sub 3}) films were deposited on Si(100) substrates by evaporating single-crystal pellets in vacuum using an electron-beam gun. Then, they were annealed in N{sub 2} ambience at 700{sup o}C for 10 min using an electric furnace. X-ray diffraction analysis showed that the LaAlO{sub 3} films were amorphous even after the annealing process. No hysteretic characteristics were observed in the capacitance--voltage (C--V) measurement and the dielectric constant of the LaAlO{sub 3} films was estimated to be 21--25. It was also found that the leakage current density decreased by about three orders of magnitude after the annealing process. On these films, Sr{sub 0.8}Bi{sub 2.2}Ta{sub 2}O{sub 9} films with 210 nm thickness were deposited by a sol--gel method. All samples annealed in O{sub 2} atmosphere at temperatures ranging from 650 to 750{sup o}C showed hysteretic C--V characteristics, and the memory window width in the sample annealed at 750{sup o}C for 30 min was about 3.0 V for a voltage sweep of {+-}10 V. It was also found that the capacitance values biased in the hysteresis loop were unchanged over 12 h. {copyright} 2001 American Institute of Physics.

Reduced leakage current in La and Ni codoped BiFeO3 thin films

La (2.5%) and Ni (2.5%) codoped BiFeO3 (BLFNO) thin films were formed by chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) structures. BiFeO3 (BFO), La(5%)-doped BFO, and Ni(5%)-doped BFO films were studied for comparison. X-ray diffraction analysis showed that the crystal structure of all films was single perovskite phase, however, structural modifications were observed in La-doped films. The leakage current density at 500kV∕cm was reduced by approximately three orders of magnitude by codoping La and Ni atoms, compared with BFO films. In BLFNO films, well saturated remanent polarization of 70μC∕cm2 was obtained at room temperature at 10kHz with the reduced coercive field by approximately 30%.

Silicon/insulator heteroepitaxial structures formed by vacuum deposition of CaF2 and Si

Epitaxial growth of CaF2 films onto Si(100) and (111) substrates and the growth of Si films onto the CaF2/Si structure have been investigated. It has been found from ion channeling and backscattering measurements that the optimum growth temperatures at which the crystalline quality of the CaF2 films is excellent range from 600 to 800 °C for Si(111) substrates and from 500 to 600 °C for Si (100). It has also been found that a heteroepitaxial Si/CaF2/Si(111) structure is formed by vacuum deposition of Si onto the heated CaF2/Si(111) structure.

Formation of LaAlO3 films on Si(100) substrates using molecular beam deposition

Lanthanum aluminate (LaAlO3) films were deposited on Si(100) substrates by a molecular beam deposition method with an electron beam gun and annealed typically in N2 atmosphere at 800 °C for 1 min. Reflection high-energy electron diffraction observation as well as x-ray diffraction analysis showed that the crystalline quality of the LaAlO3 films was amorphous, even after annealing at 800 °C. It was also found from x-ray fluorescence measurements that the ratio of La-to-Al for LaAlO3 films was almost 1:1. The dielectric constant of LaAlO3 films was estimated to be 20–25 and the leakage current density was improved by about eight orders of magnitude in maximum after the annealing process.