Page 22

janfeb13

In situ X-ray characterization of piezoelectric ceramic thin films probe piezoelectric- variation of the lattice constant during ity. For example, the pulse. Figure 1 shows the Systematic measurements of the structural changes piezoelectric properties of thin-film induced in an epi- capacitors can be made by either apply- taxial thin film ing voltage pulses of various magnitudes of BiFeO3 by an or by sweeping the voltage and record- electric field pulse ing the diffraction pattern as a function lasting 12 nanosec- of time. The latter approach is shown (Credit: Do, et al., Taylor & Francis Ltd. Reprinted with permission.) onds.11 The piezo- in Figures 2(a) and (b) and shows the electric expansion distortion resulting from positive and during the electric- negative pulses applied to the bot- field pulse—a strain tom electrode of a Pb(Zr,Ti)O3 (PZT) of approximately thin-film capacitor.12 The measure- 0.5 percent—shifts ments required a series of thousands of the diffraction peak electric-field pulses to allow acquisition to a smaller wave- of the diffraction pattern over the full vector, qz. For this range of relevant angles. In this case, measurement, the positive and negative pulses produce electric fields were piezoelectric expansion because the first Figure 2. Shift in the 002 Bragg reflection of a Pb(Zr,Ti)O thin film in which the top electrode is grounded and (a) positive or synchronized with few pulses are enough to switch the sign3 (b) negative polarity voltage pulses are applied to the bottom X-rays generated by of the polarization of the PZT capaci- electrode. The reflection shifts to smaller angles, corresponding individual bunches tor. Combining the strains measured to larger lattice constants, in both cases because the measure- of stored electrons from the shift of the diffraction pattern ments require many electric-field pulses and the remnant polar- at the Advanced with the time dependence of the volt- ization rapidly switches to the direction favored by the sign of Photon Source age leads to the plots of strain as a func- the applied field. (c) Field-dependent strain measured from (a) facility (Argonne tion of voltage shown in Figure 2(c). and (b), plotted as a function of the applied voltage. The strain National Laboratory, The slopes of these lines give piezoelec- is proportional to the voltage in both cases, with piezoelectric Argonne, Ill.). Thus, tric coefficients that are consistent with coefficients of 53 pm/V and 54 pm/V for positive and nega- tive voltage pulses, respectively.12 the time resolu- previous measurements in the same tion is limited only material.12 tion angle are related through the Bragg by the duration of the X-ray bunches Alternating the sign of applied equation λ = 2dsin θ. Reciprocal space and by the electrical bandwidth of voltage pulses switches the capacitor is spanned by wavevectors so that the the equipment generating the voltage between two polarization states in each Bragg reflections occur at wavevectors pulses. The characteristic rise-and-fall repetition of the pulse sequence. The with magnitude q = 2π/d. The intensity times of the shift in the diffraction peak diffraction patterns and strain observed of X-ray reflections depends on the shown in Figure 1 are 1.4 nanoseconds in this case are shown in Figures 3(a) direction of the polarization, an effect and correspond to the charging time and (b). As was the case in Figure 2, that can be combined with nanofocused constant of the capacitor. In addi- large pulses of either sign lead to large X-ray beams to produce maps of the tion, the shift of the diffraction peak piezoelectric expansions. When the direction of the remnant polarization.10 provides a quantitative measure of the voltages switch signs, however, the Because X-ray diffractometry allows fields. Consequently, in situ X-ray dif- (Credit: Do, et al., Taylor & Francis Ltd. Reprinted with permission.)- for precise measurement of lattice parameters, the piezoelectric coef ficients can be determined in situ, that is, while the sample is subject to either constant or varying electric fractometry provides a means to begin understanding the fundamental source of piezoelectric phenomena. Measuring lattice constants of Figure 3. (a) Piezoelectricity-induced angular shift of the 002 Bragg X-ray reflection of piezoelectric thin films a Pb(Zr,Ti)O3 thin film in a bipolar applied electric field. (b) Piezoelectric hysteresis loop Epitaxial thin-film capacitors are an derived from (a). These measurements allow the local coercive electric field and piezeo- excellent system for testing new ways to electric coefficients to be measured.12 20 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 1


janfeb13
To see the actual publication please follow the link above