Structural and Electrical Characteristics of Metal-Ferroelectric Pb1.1(Zr0.40Ti0.60)O3-Insulator (ZnO)-Silicon Capacitors for Nonvolatile Applications

In this work metal-ferroelectric-insulator-semiconductor (MFIS) thin-film structures using (PZT) as the ferroelectric layer and zinc oxide (ZnO) as the insulator layer were fabricated on n-type (100) Si substrate. and ZnO thin films were prepared on Si by the sol-gel route and thermal deposition method, respectively. On the optimized PZT (140？nm) and ZnO (40？nm) films were examined by scanning electron microscope (SEM). From AFM data the root mean square (r.m.s.) roughness of the film surface is 13.11？nm. The leakage current density of ZnO/n-Si (MIS) structure was as low as 1.8 × 10？8 A/cm2 at 2.5？V. The capacitance versus voltage (C-V) characteristics of the annealed ZnO/Si (MIS) diode indicated the good interface properties and no hysteresis was observed. Au/PZT (140？nm)/ZnO (40？nm)/Si (100) leakage-current density was about 5.7 × 10？8？A/cm2 at positive bias voltage of 3？V. The large memory window width in C-V (capacitance-voltage) curve of Au/PZT/ZnO/Si capacitor was about 2.9？V under ±12？V which thus possibly enables nonvolatile applications. The memory window as a function of temperature was also discussed. 1. Introduction Ferroelectric materials such as Pb(ZrTi)O3 (PZT) have been extensively studied for dynamic random access memory (DRAM) and nonvolatile memory applications [1, 2]. In particular, the ferroelectric field-effect transistors (ferroelectric FETs), in which the gate with metal/ferroelectric/semiconductor (MFS) structure is controlled by the spontaneous polarization of ferroelectric materials, are expected to be one of the leading candidates for future nonvolatile memory devices [3], because of their fast switching speed, nonvolatility, tolerance against radiation, and high integrated density. In order to realize ferroelectric FETs, preparation of ferroelectric/Si structures with a sharp interface is essential. However, it is very difficult to deposit the ferroelectric PZT films directly on silicon substrates without interfacial reaction [4], because Pb is highly reactive with Si and easily diffuses into the Si substrates. Therefore, an insulating buffer layer preventing interdiffusion of Si and Pb is necessary for PZT film deposition on Si substrates. Thus many kinds of buffer layer such as Y2O3 [5], CeO2 [6], YMnO3 [7], SrTiO3 [8], PbO [9], HfO2 [10], Si3N4 [11], and TiO2 [12] films have been proposed to avoid the diffusion at the interface between ferroelectric materials and Si substrates. But it was found that devices with these buffer layers have large current drops due to the high density of crystalline defects or carrier traps