Imprint model based on thermionic field emission mechanism considering energy distribution of trap levels

Abstract

In recent years, nonvolatile memories with use of new materials have attracted considerable attention. In particular, ferroelectric RAMs (FeRAMs) (Scott and Araujo, 1989) were realized recently and are expected to take the place of DRAMs and other ROMs. The ferroelectric material used for FeRAMs has perovskite crystal structure with bistable states, each of which corresponds to logic states, "0" and "1". However, to use FeRAMs as main memories of computers, we have to overcome a few reliability issues: (a) retention, a decrease in polarization charge after long-term storage (Gruveman and Tanaka, 2000; Nakao et al, 1998), and relaxation, the decrease in polarization charge immediately after applying voltage; (b) imprint, the shift in specific polarized direction in the hysteresis curve (Hase et al, 1998; Nagasawa and Nozawa, 1999; Al-Sharif et al, 1996; Lee and Ramesh, 1995); (c) fatigue, the decrease in polarizability by repeat writing (Mihara et al, 1994; Lee et al, 2000). SBT thin films are currently investigated because of their high fatigue endurance. However, there are other issues, such as imprint. In this paper, we investigated the characteristics of imprint in SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) and Pb(Zr,Ti)O/sub 3/ (PZT) thin films.