Akitoshi Nishimura

Effects of Titanium Buffer Layer on Lead-Zirconate-Titanate Crystallization Processes in Sol-Gel Deposition Technique

Effects of the Ti buffer layer on crystallization processes of lead-zirconate-titanate (PZT) films by the sol-gel deposition technique were investigated. The Ti buffer layer was deposited on a Pt/TiN/SiO2/Si substrate by electron beam evaporation at the back-pressure of 1×10-11 Torr. Homogeneous crystalline PZT film of perovskite structure with fine grains were successfully obtained on the substrate with a Ti buffer layer. In contrast, a rosette-structure film which consists of large grains surrounded by a second phase was formed on a pure Pt surface. Ti existing at the interface between Pt and PZT acts as nuclei, and affects the growth of fine grains. Furthermore, leakage current density was drastically reduced from 5×10-5 A/cm2 to 6×10-9 A/cm2 at 4 V for 200-nm-thick film formed on a substrate with a buffer layer. Crystalline orientations and microstructures of PZT films were strongly dependent on the equivalent thickness of the buffer layer and there was an optimum thickness. When the equivalent thickness of the buffer layer was 2 nm, PZT showed -preferred orientation and a homogeneous film structure with fine grains on the -oriented Pt/SiO2/Si substrate.

Analysis of the resistance degradation of SrTiO3 and BaxSr(1−x)TiO3 thin films

For bulk SrTiO3 (ST), it is known that the leakage current increases abruptly at a certain time under a constant bias (resistance degradation). We investigated the resistance degradation of ST and Bax Sr(1-x )TiO3 (BST) thin films because it will be a reliability problem when these thin films are applied to DRAM cell capacitors or on-chip capacitors. The 1000-A-thick ST or BST was sputter-deposited in situ on an electron-beam-evaporated Pt bottom electrode. The I-t and I-V characteristics were measured for these films with thermally evaporated Au top electrodes. For positive bias on the top electrode, the films showed resistance degradation similar to that of bulk single-crystal ST. However no degradation was found for negative bias. The I-V characteristics showed that only the electric field enhancement at the bottom interface does not account for this polarity dependence of the resistance degradation. A possible explanation is intrinsic inhomogeneous distribution of oxygen vacancies in the films.

Dielectric Properties of (111) and (100) Lead-Zirconate-Titanate Films Prepared by Sol-Gel Technique

Dielectric properties of - and -preferred lead-zirconate-titanate (PZT) films with thickness of 400 nm prepared by the sol-gel deposition technique have been investigated from ferroelectric random-access memory (FRAM) and dynamic random-access memory (DRAM) application standpoints. Remanent polarization of -preferred film is 23.5 µ C/cm2, which is somewhat higher than that of the -preferred one. In contrast, the dielectric constant of -preferred film is 730 which is larger than that of the -preferred one. Thus, dielectric properties of PZT films are strongly dependent on their crystalline orientations. The -preferred PZT film is appropriate for FRAM application and the -preferred one is better for DRAM.

Current-Voltage Characteristics of Electron-Cyclotron-Resonance Sputter-Deposited SrTiO3 Thin Films

Current-voltage characteristics of SrTiO3 thin films prepared on Pt electrodes by electron-cyclotron-resonance sputtering have been studied. The leakage current characteristics of the films show an ohmiclike conduction for electric field strengths lower than about 1 MV/cm, while the leakage current for higher electric field strengths is limited by Schottky emission. These ohmiclike leakage characteristics in the low-electric-field region show strong dependences on the measurement conditions, namely, the values of voltagestep and measurement delaytime in the conventional stepwise current-voltage ramps. This result is attributed to the absorption current due to the dielectric relaxation phenomena of the SrTiO3 capacitor. The overall current-voltage characteristics can be explained by Schottky emission from the Pt electrode.

Origin of dielectric relaxation observed for Ba0.5Sr0.5TiO3 thin-film capacitor

In order to identify the origin of dielectric relaxation of Pt/Ba1-x Srx TiO3/Pt thin-film capacitors, effects of post-annealing in oxygen ambient on their electrical properties were investigated. From comparison of the electrical properties of as-deposited and post-annealed capacitors, it is concluded that electrons from oxygen vacancies in the interfacial depletion region are the origin of the phenomenon.

Dielectric properties of (Ba, Sr)TiO3 thin films and their correlation with oxygen vacancy density

This paper describes the dielectric properties of (Ba, Sr)TiO3 (BST) thin films deposited by rf magnetron sputtering, focusing on their correlation with the oxygen vacancy density in the films. The dielectric properties specific to the BST films can be explained by considering the influence of the dielectric relaxation phenomenon following a power law dependence on time in the time-domain measurement and on frequency in the frequency-domain measurement. From an electrical comparison of the films with and without post-annealing in oxygen ambient, it is derived that charging/discharging electrons at oxygen vacancies in the interfacial Schottky depletion layer, whose width is modulated by the applied voltage, are responsible for the observed dielectric properties. Preliminary results of the oxygen vacancy density measurement using a gas-solid oxygen isotopic exchange reaction showed that the higher the post-annealing temperature, the lower the oxygen vacancy density.

Electrode Dependences of Switching Endurance Properties of Lead-Zirconate-Titanate Thin-Film Capacitors

Switching endurance properties of lead-zirconate-titanate thin-film capacitors with gold, platinum and iridium top electrodes were investigated using a pulse switching characterization technique. Lead-zirconate-titanate capacitor structure formed with Ir top and bottom electrodes exhibited superior switching endurance to Au/PZT/Ir and Pt/PZT/Ir capacitors. The difference between switched and nonswitched polarizations of an Ir/PZT/Ir capacitor reversed by bipolar pulses was more than 38 µ C/ cm2 after 2×109 switching cycles. Nonswitched polarization of this capacitor for negative read-pulses decreased gradually with increase in the number of switching cycles. When positive and negative unipolar pulses, and DC biases were applied to the top electrodes of Pt/PZT/Ir and Ir/PZT/Ir capacitors, remanent polarizations of each capacitor were not changed significantly. However, nonswitched polarizations for negative read-pulses decreased with increase in the number of negative unipolar pulses applied or DC bias application time. Drastic decrease in remanent polarization of a Pt/PZT/Ir capacitor was caused only by bipolar pulse application. The reduction of nonswitched polarization for negative read-pulses suggested the formation of depletion-layer capacitances at the interfaces between top electrodes and PZT layers.

Preparation of -oriented Lead-Zirconate-Titanate Films by Sol-Gel Technique

A sol-gel technique to control the crystalline orientation of lead-zirconate-titanate (PZT) films on Pt/Ti/SiO2/Si substrates has been studied. It is found that crystalline orientations of sintered films show strong dependence on their thicknesses and -oriented films can be obtained by limiting their thicknesses to 100 nm. It is also found that thicker oriented films can be obtained by stacking up these oriented films. The scanning electron microscopy observation reveals that a well-oriented film surface is smoother than those of non oriented ones. The leakage current of the 240-nm-thick oriented film is about 7 nA/cm2 at 3 V, which is an order of magnitude lower than the best reported lanthanum and iron-doped PZT film.

Ferroelectric Properties of Crystalline-Oriented Lead-Zirconate-Titanates Formed by Sol-Gel Deposition Technique

Ferroelectric properties of lead-zirconate-titanate (PZT) films formed by the sol-gel deposition technique have been discussed from the standpoint of ferroelectric random-access memory (FRAM) and dynamic random-access memory (DRAM) applications. PZT films with -preferred orientation and random orientation were formed on Pt/Ti/SiO 2 /Si substrates by optimizing fabrication conditions. Remanent polarization of the -preferred film was 23.5 μC/cm 2 which is somewhat higher than that of the randomly oriented one. An abrupt saturation of remanent polarization was observed only for the -preferred film. The voltages which provide saturations of remanent polarization densities for the -preferred films are almost constant between 3.5 V to 4 V independent of thickness. The dielectric constant of the randomly oriented film was 730 which is larger than that of the -preferred one and gradually decreased depending on the applied electric field. The -preferred PZT film is appropriate for FRAM application, and the randomly oriented one is preferred for DRAM application

Electrical Comparison of Sol-Gel Derived Lead-Zirconate-Titanate Capacitors with Ir and Pt Electrodes

The electrical properties of sol-gel-derived Ir/lead-zirconate-titanate (PZT)/Ir and Pt/PZT/Pt capacitors were carefully investigated. PZTs with columnar structures were well crystallized on both Ir and Pt substrates. 250-nm-thick PZT films deposited on Ir and Pt showed random and -preferred orientations, respectively. The Ir/PZT/Ir capacitor showed large remanent polarization density of 20.7 µ C/cm2 at applied voltage of 2.5 V even though this PZT is randomly oriented. This value is almost equal to that of -oriented film prepared on Pt. Furthermore, coercive field of the Ir/PZT/Ir capacitor is much lower than that of the Pt/PZT/Pt one. For endurance property for switching, Pr of Pt/PZT/Pt capacitor was degraded to 45% of the initial value by switching of 1×105 cycles. On the other hand, that of Ir/PZT/Ir capacitor was decreased to 45% by 1×107 switching cycles. Degradation of remanent polarization density by switching was reduced considerably by using Ir electrodes.