Yukio Fukuda

Effects of interfacial roughness on the leakage properties of SrTiO3 thin film capacitors

Abstract Recent reports on the current-voltage (I-V) characteristics of SrTiO3 and (Ba,Sr)TiO3 thin film capacitors with Pt electrode suggest that their leakage properties are controlled by Schottky emission from cathode. However, their dynamic permittivities obtained from the Schottky plot are always much smaller than those expected from optical measurements. The most plausible approach to explain this disagreement is to incorporate the effect of electric field enhancement due to interfacial roughness into the Schottky emission model. Electric field at the interface, for the first order approximation, can be given by [(r+t)/r](V/t), where r, t and V respectively denote radius of curvature representing interfacial roughness, dielectric thickness and applied voltage. This model predicts that I-V characteristics are the functions of r and V for the ultimate case of r ≪ t. We have closely studied the I-V characteristics of sputter-deposited SrTiO3 films and have obtained the results which strongly support ou...

Recent progress in sputtering PZT thin films for ferroelectric memories

Abstract This paper describes amorphous Pb(Zr, Ti)O3 (PZT) thin films deposited by cosputtering Pb(Zr0.5 Ti0.5)O3 and PbO targets. By optimizing the amount of the excess Pb and the deposition temperature, PZT thin films with a single perovskite phase were obtained successfully on Ir substrates and Pt substrates at 520°C. 250-nm-thick PZT films crystallized by rapid thermal annealing (RTA) at 600°C for 20 s exhibited excellent ferroelectric properties: a coercive voltage of 1.0 V, a remanent polarization density of about 40 μC/cm2, and a polarization switching endurance over 1x109 cycles. Although a heat treatment in a reductive ambient causes degradation of ferroelectric properties of PZT thin films, their degraded ferroelectric properties can be easily recovered from by a 1-min RTA in an oxygen at 400°C.

Sputtered Pb(Zr, Ti)O 3 Thin Films for Ferroelectric Capacitors

Sputtered Pb(Zr, Ti)O 3 thin films with superior ferroelectric properties were successfully obtained by controlling the grain structure and the film compositions. We found that amorphous PbTiO 3 buffer layers are effective in forming PZT thin films with fine dense grains. The sputtered PZT thin films with Ti-rich phase showed excellent ferroelectric properties. The polarization retention properties of PZT capacitors with Ti-rich phase are remarkable, and the value of the retained polarization density after 10 years is expected to be larger than 40 μC/cm 2 . Further, 150-nm-thick PZT capacitors with Zr/Ti=30/70 showed 2 P , at 1.5 V of more than 30 μC/cm 2 , and good retention property. These results indicate the potential of the lower voltage operation of sputtered PZT capacitors by optimizing the film composition and thickness.

Effects of post-annealing on the leakage and dielectric properties of sputter-deposited Pt/Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3//Pt thin film capacitor

This paper describes electrical properties of sputter-deposited Pt/Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3//Pt capacitor. Particular attention will be paid to the effects of a post-annealing in an oxygen ambient on the properties. It is shown that leakage and dielectric properties of the capacitor are drastically improved by the post-annealing. These improvements are attributed to decrease in an oxygen vacancy concentration of the dielectric.

A model for describing current-voltage characteristics of SrTiO/sub 3/ capacitors under low electric fields

It is now widely accepted that the well-fabricated SrTiO/sub 3/ (STO) thin-film capacitors with high-work-function metallic electrodes show excellent low leakage characteristics. However, the magnitude of the leakage current in the low electric fields shows strong dependences on the measurement conditions, namely the values of voltage-step /spl Delta/V and delay-time t/sub d/ in the conventional staircase current-voltage ramps. This anomalous effect is attributed to the dielectric relaxation phenomena of the STO capacitor structures. In this paper, we propose a model which describes the relationship between such measurement conditions and observed current-voltage characteristics. The absorption current due to the dielectric relaxation of the Debye-type relaxation species can be formulated as I(V=jx/spl Delta/V)=/spl Delta/V/sup m//spl Sigma//sub i=1//sup n//spl Sigma//sub j=1/(C/sub i///spl tau//sub i/)exp(-jxt/sub d///spl tau//sub i/), where C/sub i/ and /spl tau//sub i/ respectively denote the capacitance and the relaxation time constant of the i-th relaxation species. The unknown parameters in the equation, C/sub i/ and /spl tau//sub i/, can be determined by applying a multiple nonlinear least squares method to the current-time measurements. The model was applied to characterize the sputter-deposited STO capacitors which exhibited apparent leakage currents of less than 10 nA/cm/sup 2/ for the electric field strengths of less than 1 MV/cm. We observed three kinds of relaxation species for such capacitors. The current-voltage curves calculated based on these relaxation species fit very well with the measured values obtained under the various measurement conditions.