Fabrication of Polycrystalline Ferroelectric Gate FET Memory with an Intermediate Electrode
We fabricated a new ferroelectric gate field-effect transistor memory (F-FET) with an intermediate electrode inserted between the ferroelectric capacitor (Cf) and the metal-oxide-semiconductor field-effect transistor (MOSFET) for writing data. The structure was a RuO2 top electrode/polycrystalline ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT)/(Pt/RuO2) intermediate electrode/SiO2/Si substrate. The basic operation of the new memory was confirmed by a discrete circuit that the Cf was connected serially with the MOSFET. For data writing, two positive or negative square pulses applied between the top electrode and the intermediate electrode induced the positive (Pr+) and the negative (Pr-) remanent polarizations, respectively, for coding memory states. For data reading, an unipolar square pulse train was applied between the top electrode and the source of the MOSFET, and the drain of the MOSFET was biased by a DC voltage. When the memory state corresponded to the Pr+ or the P-, the drain current (ID) was small or large, respectively. In this study, we used a polycrystalline PZT film to deal with the difficult problems of an epitaxial PZT film such as complicated techniques and high cost of products for commercialization goal.
Analysis and Improvement of Retention Time of Memorized State of Metal-Ferroelectric-Insulator-Semiconductor Structure for Ferroelectric Gate FET Memory
Retention characteristics of metal-ferroelectric-insulator-semiconductor (MFIS) structures have been studied theoretically by considering effects of charge injections derived from the difference between leakage current densities in the ferroelectric and insulator layers. The calculated curves for time-dependent capacitance have shown good agreements with experimental results. The numerical results for the MFIS structure have indicated that excess current over a certain value through the ferroelectric and the insulator layers causes the retention time to rapidly degrade. An idea of inserting an insulator film between the metal and the ferroelectric layers in an MFIS has also been examined in order to cut down the currents through the ferroelectric layer. The calculations based on our model have found this metal-insulator-ferroelectric-insulator-semiconductor (M-I-FIS) structure to exhibit much longer retention time than the conventional MFIS.
Fabrication of PbZrxTi1-xO3 Films on Si Structures Using Y2O3 Buffer Layers
Lead-zirconate-titanate (PbZrxTi1-xO3:PZT) films were epitaxially grown on Si(111) substrates using Y2O3 buffer layers. Y2O3 layers were prepared in a molecular beam epitaxy (MBE) apparatus with a single electron beam gun, while PZT films were prepared in a vacuum evaporation chamber with both a crucible and an electron beam gun. It was found from in situ RHEED observation that Y2O3 layers grew epitaxially on Si(111) substrates. It was also found from X-ray diffraction analysis that strongly (101)-oriented PZT films grew on the Y2O3/Si(111) structures at a substrate temperature of 700°C. Metal-ferroelectric-insulator-semiconductor(MFIS) capacitors were fabricated using this structure, and capacitance-voltage (C–V) curves showed a memory window of about 2.6 V, which is considered to be due to the ferroelectric nature of the PZT film.