Effect of La Dopant on Electrical Properties of Pb(Zr,Ti)O 3 Thin Film Capacitors
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Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
were annealed at 700'C for 10 minutes. Top iridium and iridium oxide electrodes were deposited by sputtering to improve fatigue and then patterned using high density plasma etcher. The crystal structure of the PZT and La-doped PZT films was analyzed by x-ray diffraction (XRD) and the surface morphology was observed by scanning electron microscopy (SEM). The hysteresis loops were measured under applied voltages of ±5 V using an RT66A system. The fatigue measurements were carried out by applying a 1 MHz square wave with ±5 V. For evaluation of retention properties, the voltage pulse train shown in Figure 1 was applied to ferroelectric capacitors. The purpose of this test was to evaluate the changes in switching and non-switching charge when the capacitors were left for a long time after a "0" or "I" was written to the memory cells. A positive or negative pulse was applied to the PZT and La-doped PZT
ferroelectric capacitors, after which the capacitors were baked at 150'C. After baking, a write/read pulse-pair was applied to the capacitors, and then, the change of switching and nonswitching charge was measured at room temperature.
(1)
(3)
(2)
+3V
Bake at 150 °_C
-3V
II
liii
80
120,120ps
Fig. 1. The voltage pulse train used for retention evaluation. RESULTS AND DISCUSSION Figure 2 shows the XRD spectra of PZT and La-doped PZT thin films with La contents. The films were prepared with 12 mol% excess Pb precursor and then annealed at 700'C for 10 min. All films show only diffraction peaks of the perovskite phase without the presence of any second phase, at least within the detection limits of x-ray diffraction. The PZT (40/60) film shows highly (11l) preferred orientation, as indicated in Fig. 2(a). However, the PLZT (0.5/40/60) and PLZT(1/40/60) exhibit lower (111) preferred orientation. This difference may imply that La was well dissolved into PZT thin film. The microstructures of the PZT and PLZT films with La content are shown in Figure 3. The PZT and PLZT films have a smooth surface and a thickness of about 200nm. The PZT (40/60) and PLZT (0.5/40/60) films are single phase perovskite without any evidence of second phases. In contrast, the PLZT(1/40/60) film (as shown in Fig. 3(c)) exhibits two microstructure features. One is composed of large perovskite grain and the other is small grain, which may be a pyrochlore phase. The appearance of the
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(b) (U 'I, C
Ua
a C
(a)
\
0• 20
25
30
35
40
45
50
55
rt720.-I
60
20
Fig. 2. X-ray diffraction of (a) PZT(40/60),
Fig. 3. SEM images of (a) PZT(40/60),
(b) PLZT (0.5/40/60) and (c) PLZT (1/40/60) thin films on PtIK)2/SiO 2/Si substrates.
(b) PLZT (0.5/40/60) and (c) PLZT (1/40/60)
thin films on Pt/IrO 2/SiO 2/Si substrates.
second phase may be attributed to Pb deficiency in the PLZT (1/40/60) solution since the amount of 1 mol% La is substituted for I mol% Pb from the total 12 mol% excess Pb content. Figure 4 shows hysteresis loops of PZT (40/60), PLZT (
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