The characterization of retention properties of metal–ferroelectric (PbZr0.53Ti0.47O3)–insulator (Dy2O3, Y2O3)–semiconductor devices
Introduction
Ferroelectric field effect transistors (FEMFETs) with a metal–ferroelectric–insulator–silicon (MFIS) structure is a promising candidate for non-volatile random access memory because of its high speed, single-device structure, low power consumption, and non-destructive read-out operation [1], [2]. The purpose of the insulator layer is to prevent the reaction and inter-diffusion between the ferroelectric layer and silicon substrate as well as to improve the retention properties [3], [4]. However, MFIS structures also have some disadvantages such as depolarization field in the ferroelectric film and the increase of operation voltage due to additional voltage across the insulator layer. In order to minimize the voltage across the insulator layer, high dielectric constant insulators are desirable for the MFIS structures. In this work, the electrical properties of MFIS devices with high-k insulator layers were studied. Parameters such as memory window and retention time were characterized. Retention is an important consideration of MFIS devices [5]. The two major causes of short retention time in MFIS devices are depolarization field [6], [7] and gate leakage current [8], [9]. A large conduction band offset at the high-k oxide/Si interface is helpful to reduce leakage current. In this work, Dy2O3 and Y2O3 high dielectric constant films were used as the insulator layers [10]. Dy2O3 has a dielectric constant of 11–13 and Y2O3 a dielectric constant of 12–18, respectively [11], [12]. They also have good thermal stability with silicon [13], [14]. In this work, the variation of the memory window as a function of annealing temperature was studied for Al/PZT/Dy2O3/Si capacitors and the retention times of Al/PZT/Dy2O3/Si and Al/PZT/Y2O3/Si transistors were investigated. In the literature, there were several reports on MFIS properties with Y2O3 insulator layer [14], [15], [16]. Park et al. [15] studied similar Al/PZT/Y2O3/Si structure. However, no transistor characteristic and retention time were reported. Prior work on Al/PZT/Dy2O3/Si MFIS transistors was reported by Chang et al. of our laboratory [11]. More detailed study was made in this work.
Section snippets
Experimental
P-type, (1 0 0) orientation, 4-in. diameter silicon wafers (1–10 Ω cm) were used as the starting substrates. A 500-nm SiO2 film was first grown on the silicon wafers by wet oxidation. The oxide on the backside was then removed. Boron was implanted on the backside (2 × 1015/cm2, 30 keV) to make substrate contact. A rapid thermal annealing (RTA) was performed at 800 °C in N2 for 5 min to activate the boron implant. The Dy2O3 and Y2O3 thin films were deposited by RF magnetron sputtering at room
Results and discussion
Fig. 1a shows the ideal energy band diagram of the Al/PZT/Dy2O3/Si structure. The energy band gap of Dy2O3 is 4.8 eV. The electron and hole barrier heights at the Dy2O3/Si interface are 0.79 eV and 2.91 eV, respectively [11]. Fig. 1b shows the ideal energy band diagram of the Al/PZT/Y2O3/Si MFIS structure. The energy band gap of Y2O3 is 6.0 eV. The electron and hole barrier heights at the Y2O3/Si interface are 2.3 eV and 2.6 eV, respectively [12]. It can be seen that the Y2O3/Si interface has a much
Conclusions
Metal–Ferroelectric–insulator–silicon (MFIS) capacitors and field effect transistors using PZT as the ferroelectric layer and Dy2O3 or Y2O3 as the insulator layer are successfully fabricated. The non-volatile operation of the MFIS transistors is demonstrated. The memory windows of Al/PZT/Dy2O3/Si and Al/PZT/Y2O3/Si capacitors with a sweep voltage of 10 V are 1.03 V and 1.48 V, respectively. The larger conduction band offset at the Y2O3/Si interface is helpful for the reduction of charge injection
Acknowledgement
The authors thank the National Science Council, Taiwan, Republic of China, for supporting this work under Contract No. NSC94-2215-E-007-027.
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