Dose Rate Dependence of Electrical Characteristics of Lead Zirconate Titanate Capacitors Guoqiang ZHANG 1;2 , Ping SUN 1 , Q. ZOU 1 , X. MEI 1 , Harry E. RUDA 1 , Qi GUO 3 , Xuefeng YU 3 , Diyuan REN 3 and Rongliang YAN 3 1 Center for Advanced Nanotechnology, University of Toronto, 170 College Street, Toronto M5S 3E4, Canada 2 Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China 3 Xinjiang Institute of Physics, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China (Received September 30, 2002; revised March 17, 2003; accepted April 15, 2003; published October 9, 2003) The influence of  -radiation dose rate on the electrical properties of lead zirconate titanate capacitors was investigated. More severe degradations in dielectric constant, coercive field, remanent polarization and capacitance–voltage (C–V ) curves occurred with increasing radiation dose at lower dose rates. The electrical properties exhibited distinct radiation dose rate dependence and the worst-case degradation occurred at the lowest dose rate. The radiation-induced degradation of parameters such as the coercive field drift and distortion of the C–V curve can be recovered partly through post-irradiation annealing. [DOI: 10.1143/JJAP.42.6491] KEYWORDS: ionizing radiation, dose rate, PZT, dielectric constant, coercive field, C–V curves, remanent polarization 1. Introduction Lead zirconate titanate, PbZr x Ti 1x O 3 (PZT) ferroelectric (Fe) thin films have attracted widespread interest for use as a storage element in nonvolatile random access memories (RAMs). PZT also has a great potential use in radiation environments because of its advanced properties such as high speed, big volume and non-volatile characteristics. Recent literature indicates that the electrical and dielectric properties of PZT thin films degrade with increasing radiation dose. 1–7) In these studies, the radiation dose rate was fixed, and there have been no reports on changing the radiation dose rate. Radiation dose rate effects have been reported on Comple- mentary Metal–Oxide–Semiconductor (CMOS) devices, bi- polar devices, GaAs FET devices, and SiGe heterojunction devices. 8–11) An important finding was that for CMOS devices there was no significant radiation dose rate sensi- tivity, 8) whereas gain degradation occurred at a low dose rate in bipolar devices. 9) Moreover, almost no low-dose-rate degradation was observed in SiGe heterojunction bipolar transistors. 11) In this study, we investigate the electrical properties of PZT capacitors  -ray irradiated at different dose rates. The results show distinct radiation dose rate depend- ence. The radiation-induced degradation can be partially recovered by post-irradiation annealing. The presented data are likely to be an important reference for the application of PZT devices in space. 2. Experimental The sol–gel technique has been used to prepare PZT thin films. 500-nm-thick PZT films (with a Zr/Ti ratio of 52/48) were spin-coated on Pt/Ti/SiO 2 /Si substrates. A crystalli- zation temperature of 600  C was selected. Evaporated Au top electrodes with an area of 7:85  10 3 cm 2 were used to measure the dielectric properties of the PZT films. The PZT capacitors were stressed with a floating bias (no bias) during Co-60  -irradiation. Choosing the floating irradiation bias is a means of investigating the worst radiation degradation of PZT capacitors. 2) The total dose was varied in the range from 0 to 5  10 4 Gy(Si), and dose rates of 0.5, 2.5 and 25 kGy(Si)/h were studied. Capacitance–voltage (C–V ) measurements were performed using a 10 kHz HP4194 Material Impedance/Gain-Phase Analyzer and by scanning the applied voltage on the Au top electrode from 15 V to +15 V to 15 V before and after irradiation. Hysteresis curves were obtained using a Sawyer–Tower circuit as shown in Figure 1. 1,3) During the measurement of the output signal (V out ), a 10 kHz, 10 to +10 V triangular wave was used as the input signal (V in ). The integrating capacitor (C int ) was 1 mF, chosen to greatly exceed the capacitance of the ferroelectric capacitor. Assuming that resistance losses are negligible, the polarization charge (per unit area) of the PZT capacitor, P, is related to output voltage by P ¼ðV out  C int Þ=A, where A is the area of the PZT capacitor and C int is integrating capacitance. 1) Remanent polarization (2  P r ) was determined from the hysteresis loop by extracting the zero- field intercept of the top and bottom traces of the loop: that is, the difference between the polarizations when the sample is switched positive and the bias is removed, and when the sample is switched negative and the bias is removed. All samples were stored in air for 290 h with floating bias after total radiation dose of 5  10 4 Gy(Si), and then, their C–V curves and hysteresis loops were measured. The dielectric constant value " was calculated using " ¼ dC=ð" 0 A), where d is the thickness of the PZT film, C is the capacitance from the measured C–V data, " 0 ¼ 8:85  10 14 F/cm and A is the area of the PZT capacitor. 3. Results and Discussion 3.1 C–V curves during irradiation Figure 2 shows the C–V curves of a PZT capacitor irradiated with different total doses at a dose rate of 0.5 kGy/h. First, the experimental data have shown an asym- metrical behavior of C–V curves before irradiation, which is Ferroelectric Capacitor C int V out V in Fig. 1. Schematic of Sawyer–Tower circuit. Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 6491–6495 Part 1, No. 10, October 2003 #2003 The Japan Society of Applied Physics 6491