Long Time Data Retention and A Mechanism in Ferroelectric-Gate Field Effect Transistors with HfO 2 Buffer Layer
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Long Time Data Retention and A Mechanism in Ferroelectric-Gate Field Effect Transistors with HfO2 Buffer Layer Koji Aizawa1, Yoshihito Kawashima2 and Hiroshi Ishiwara2 1 Precision & Intelligence Laboratory, Tokyo Institute of Technology, 4259-R2-19 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan 2 Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-S2-9 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
ABSTRACT A mechanism of the long-time data retention in the p-channel MFIS FETs with Pt/SBT/HfO2/Si gate structures was proposed. The MFIS FETs used in this study exhibited the drain current on/off ratio of approximately 6x103 even after 30 days had elapsed at room temperature. From the leakage current characteristics of the MFIS diode, the bulk leakage current density lower than 10-12 A/cm2 was presumed for 30-days data retention. On the other hand, we showed that the decrease of on-state drain current in the retention characteristics was explained by the flat-band voltage shift of approximately -0.3V for 30 days toward negative voltage direction. Therefore, it was also found that the trapped charge density as low as 1011 cm-2 was needed for obtaining the data retention of 30 days. INTRODUCTION Ferroelectric-gate field-effect transistors (FETs), in which a ferroelectric film is used as a gate insulator, have attracted much attention in applications to high-density ferroelectric random access memories. Up to now, metal-ferroelectric-insulator-semiconductor (MFIS) structures used as a gate stack of the ferroelectric-gate FETs have been investigated [1-9], in which an insulating buffer layer acts as diffusion barrier and control of interface properties between the ferroelectric film and semiconductor substrate. However, their several structures still have such problem that the stored data are lost in short time. To resolve this problem, the buffer layers with high thermal stability and high dielectric constants are required. Recently, longer data retention than 3 days was showed in the MFIS FETs using Si3N4 [10] or HfAlO2 [11] films as a buffer layer. Particularly, we showed 15.9-days data retention in the MFIS FETs with HfO2 buffer layer [12]. However, the reasons why the retention period is prolonged are not clarified yet. In this study, we investigate the time dependence of the leakage current density and the analysis of the flat-band voltage shift in the MFIS diodes with a Pt/SrBi2Ta2O9 (SBT)/HfO2/Si structure in order to clarify the mechanisms of the long data retention in the MFIS FETs with HfO2 buffer layer.
EXPERIMANTAL PROCEDURE In this study, we fabricated the MFIS diodes and FETs with a Pt/SBT/HfO2/Si structure. ntype Si(100) were used as substrate, because excellent retention characteristics of MFIS diodes with HfO2 buffer layer have been obtained on n-Si(100) substrates [13]. An HfO2 film of
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Cross section
Top View Source Gate Drain
Ferroelectric film (400nm)
Pt TE(50µm×45µm) Al electrode
W p+
p+
n-sub HfO2 L
W/L=50µm/5µm Figure 1. Illustra
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