Growth and characterization of high temperature La3Nb0.5Ga5.3Al0.2O14 (LNGA) and La3Ta0.5Ga5.3Al0.2O14 (LTGA) piezoelectric single crystals

Solid State Communications 148 (2008) 213–216

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Growth and characterization of high temperature La3 Nb0.5 Ga5.3 Al0.2 O14 (LNGA) and La3 Ta0.5 Ga5.3 Al0.2 O14 (LTGA) piezoelectric single crystals Shujun Zhang a,∗ , Akira Yoshikawa b , Kei Kamada b , Eric Frantz c , Ru Xia a , David W. Snyder c , Tsuguo Fukuda b , Thomas R. Shrout a a

Material Research Institute, Pennsylvania State University, University Park, PA 16802, USA

b

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

c

Electro-Optics Center, Pennsylvania State University, Freeport, PA 16229, USA

article

info

Article history: Received 30 May 2008 Received in revised form 12 July 2008 Accepted 14 August 2008 by X.C. Shen Available online 20 August 2008 PACS: 81.10.Fq 77.84.-s 84.37.+q 78.20. Hp

a b s t r a c t La3 Nb0.5 Ga5.3 Al0.2 O14 (LNGA) and La3 Ta0.5 Ga5.3 Al0.2 O14 (LTGA) single crystals were grown using the Czochralski method. Piezoelectric properties of LNGA and LTGA single crystals were measured and compared to La3 Ga5 SiO14 (LGS) crystals, where the piezoelectric coefficient d11 and electromechanical coupling factor k12 were found to be on the order of 6–7 pC/N and ∼16%, respectively. The resistivity of LNGA was found to be 1.1 × 108  cm at 500 ◦ C, much higher than those values of LTGA and LGS (∼2.2 × 107  cm for LTGA and ∼9 × 106  cm for LGS). The RC time constant of LNGA crystal was found to be 224 µs at 500 ◦ C, while the values were 49 µs and 18 µs at the same temperature for LTGA and LGS, respectively. The good piezoelectric property, together with its high resistivity, exhibit LNGA single crystals a good candidate for piezoelectric applications at elevated temperature. © 2008 Elsevier Ltd. All rights reserved.

Keywords: B. Crystal growth D. Piezoelectricity D. Dielectric response

1. Introduction High-temperature electronics is an area of research offering interesting materials and design challenges and one of the significant industrial important. Just as in the aerospace and aircraft industries, electronic controls are to be placed directly inside jet engines because of reliability and noise requirements, so sensors need to be built that can withstand temperature of 500–1000 ◦ C while allowing mission lifetimes up to 100,000 h. Piezoelectric materials used in electronic devices should possess not only high piezoelectric behavior, but also high resistivity and time constant (τ = RC = K ε0 ρ , where R is the device resistance, C is the capacitance, while K is the dielectric constant and ρ is the resistivity). High electrical resistivity is necessary because the devices must not only develop a charge for an applied stress or strain, but must also maintain the charge for a time long enough to be detected by the electronic system. The length of time that the charge maintained is proportional to the RC time constant and

∗

Corresponding author. Tel.: +1 814 863 2639. E-mail address: [email protected] (S. Zhang).

0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.08.013

thus, a large RC time constant is desirable for many applications, especially for static and low frequency measurements [1–3]. Among the piezoelectric materials, α -quartz is widely used in electronic devices. However, the piezoelectric coefficient of α -quartz is relatively small (∼1–2 pC/N) and thus the amount of electric charge that can be generated is low. Although piezoelectric α -quartz has a transition temperature of 573 ◦ C, its usage temperature range is normally limited to 350 ◦ C, above which, the crystal structure is subject to twinning, destroying its piezoelectric properties [1]. For the last decade, the growth of piezoelectric crystals by Czochralski technique has yielded langasite-type crystals with high thermal stability of frequency and large electromechanical coupling factor. There are several reports concerning the crystal growth and the detailed piezoelectric properties of La3 Ga5 SiO14 (LGS) [3–7], La3 Nb0.5 Ga5.5 O14 (LNG) and La3 Ta0.5 Ga5.5 O14 (LTG) [8–15] single crystals. The langasite family structure is represented by general chemical formula A3 BC3 D2 O14 and belong to the trigonal symmetry, the point group 32 and space group P321, the same acentric crystal class 32 as quartz, but langasite type crystals display relatively large electromechanical coupling factors and piezoelectric coefficients, being on the order of 16% and 6–7 pC/N, respectively. The absence of structural phase

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S. Zhang et al. / Solid State Communications 148 (2008) 213–216

Fig. 1. (a) As grown LNGA single crystal (Dr. Fukuda group, Tohoku University). (b) Cross-section of the as-grown LTGA and LNGA single crystals perpendicular to their growth direction showing the habitual faces and high transparency.

transformation between room temperature and the melting point (∼1500 ◦ C) makes langasite-type crystals promising candidates for electronic applications at elevated temperature [14]. For the LGS, LTG and LNG single crystals, however, the resistivity and time constant at high temperature were found to be low and so, modified langasite-type single crystals with high resistivity are desirable. In this work, aluminum modified langasite-type single crystals La3 Nb0.5 Ga5.3 Al0.2 O14 (LNGA) and La3 Ta0.5 Ga5.3 Al0.2 O14 (LTGA) were grown and the detailed electrical properties were characterized and compared to LGS, LTG and LNG crystals. 2. Experimental procedure LNGA and LTGA single crystals were grown using a conventional RF heating Czochralski method in 98%Ar–2%O2 atmosphere, with a platinum or iridium crucible without an afterheater, the vertical temperature gradient was controlled at 20–30 ◦ C/cm. Starting materials were prepared by mixing and calcination of 4N purity La2 O3 , Ga2 O3 , Al2 O3 , Nb2 O5 or Ta2 O5 powders with nominal compositions. The seed orientation is [001] (crystallo-physical axis Z ) and the crystal pulling rate is 0.5–1.5 mm/h with a rotation rate at 15–25 rpm. In order to reduce the evaporation of Ga2 O3 during growth, besides the low oxygen partial pressure, the crucible temperature was carefully controlled to just above the material melting temperature, and only 60% of the melt was grown into single crystal. The remained melt was found to be lack of Ga2 O3 , due to the volatilization, however, the obtained crystal was found to be uniform by using scanning electron microscopy (SEMEDS) compositional analysis and the composition was determined to be La3.05 Ga5.25 Nb(Ta)0.5 Al0.2 O14 , very close to the nominal composition. The as-grown crystals were amber in color with 25 mm in diameter and 70 mm in length, it exhibited habitual ¯ } facets on the shoulder part growth behaviors, showing six {1010 and cylindrical part, as shown in Fig. 1(a), the diameter of the lower part of the crystal was enlarged because of the thermal field disturbance during growth. The main macroscopic defect inside the crystals was found to be bubbles, which were located at the conical part of the crystal and the lower part with enlarged diameter. Fig. 1(b) presents polished cross sections of the middle part of LNGA and LTGA crystals, showing very good transparency and quality. Crystal bars with dimension 5 × 2 × 0.5 mm3 were cut from the as-grown crystals with edges parallel to the crystallo-physical X , Y and Z axes and electroded using vacuum-sputtered platinum thin film. Capacitance measurements were performed at frequency of 10 kHz, using an HP4284A LCR meter on X - and Z -cut samples. Resonance and antiresonance frequencies were measured using HP4194A Impedance Analyzer. The electromechanical coupling factors and mechanical quality factors were calculated according

to IEEE standard [16]. High temperature measurements were performed in a computer controlled modified furnace with a builtin high temperature sample holder. Resistivity was measured using Keithley model 2410-C source-meter when applying 100 V field on the X -cut samples. In order to avoid the difficulties usually encountered in making resistance measurements in the T  range, readings began from 300 ◦ C and were taken at 50 ◦ C intervals, both with increasing and decreasing temperature. At each interval, the temperature was held until both the resistance and temperature reached a reasonable degree of stability. At the time of the actual reading, the furnace controller was switched off to eliminate electromagnetic radiation interference originating from the triac circuitry in the controller. 3. Results and discussion T can be calculated from the measured The dielectric constant K11 capacitance on X -cut samples and found to be on the order of 21 and 19.5 at room temperature for LTGA and LNGA crystals, T were found to respectively, while the dielectric constants K33 be 79 and 76 from Z -cut samples. The dielectric behavior of X cut samples for LTGA and LNGA single crystals as a function of temperature were presented in Fig. 2(a) (variation of dielectric constant) and Fig. 2(b) (dielectric loss) and compared to LGS single crystals. Because there is no phase transformation occurs prior to the melting temperature around 1500 ◦ C, the dielectric constant increased with increasing temperature and found to be 23.7 and 25.2 at 500 ◦ C for LNGA and LTGA crystals, respectively. The flatter dielectric-temperature behavior of LNGA single crystal, shown in Fig. 2, exhibited that the LNGA possesses much higher temperature stability of dielectric constant and loss, when compared to LGS and LTGA crystals. For investigating the piezoelectric properties of LNGA and LTGA crystals, different cut resonators were made in the form of bars with dimension of 5 × 2 × 0.5 mm3 . The electromechanical coupling factor and piezoelectric coefficients were evaluated by measuring the dielectric constant and the resonance and antiresonance frequencies of the bar. Fig. 3 shows the resonance and anti-resonance characteristics of impedance and phase for LNGA crystals, at room temperature and 500 ◦ C, respectively. Both the resonance and anti-resonance frequencies were found to shift to lower frequencies with temperature increasing. The phase value was found to be on the order of 87.5 ◦ at room temperature, while the value decreased to 76◦ at 500 ◦ C, the indicative of a lower mechanical quality factor at elevated temperature. The mechanical quality factor Q was calculated to be on the order of 4500 at room temperature, decreased with increasing temperature, found to be only ∼900 at elevated temperature of 500 ◦ C. The electromechanical coupling factor k12 was calculated and found to be 16.3% at room temperature and decreased to 15.8% at

S. Zhang et al. / Solid State Communications 148 (2008) 213–216

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Fig. 2. (a) Dielectric constant temperature coefficient ∆K /K as a function of temperature for single crystals LGS, LTGA and LNGA. (K is the dielectric constant at room temperature and ∆K is the different of dielectric constant values at high temperature and room temperature). (b) Dielectric loss as a function of temperature.

Fig. 3. Resonance and anti-resonance frequencies characteristics of impedance and phase for LNGA single crystal at room temperature and 500 ◦ C.

650 ◦ C for LNGA crystal, while for LGS and LTGA crystals, the values of k12 were found to be about 16% at room temperature. The piezoelectric coefficients were found to be 6–7 pC/N for all the crystals with the highest value observed for LNGA crystal around 7.2 pC/N. Fig. 4(a) and (b) present the variations of coupling and piezoelectric coefficients for LGS, LTGA and LNGA crystals, respectively, indicating LNGA crystal possess much better temperature stability. The temperature dependences of the resistivity ρ in crystalline insulators with a certain defect density are caused by localized

Fig. 5. Resistivity as a function of temperature for LGS, LTGA and LNGA single crystals.

electronic levels and can be expressed by Arrhenius law:

ρ = ρ0 exp (Ea /kB T ) where ρ0 is the resistivity at an infinite temperature, Ea is the activation energy, kB is the Boltzmann’s constant and T is the absolute temperature. The plots of ln ρ versus 1/T for LGS, LNGA and LTGA crystals show linear behavior as presented in Fig. 5. From

Fig. 4. (a) Variation of electromechanical coupling factor ∆k/k as a function of temperature for LGS, LTGA and LNGA crystals. (b) Variation of piezoelectric coefficient ∆d/d as a function of temperature for LGS, LTGA and LNGA crystals.

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Table 1 Properties of LGS, LTGA and LNGA single crystals Crystal

k12

d11 (pC/N)

T K11

T K33

Loss

ρ0 ( cm)

Ea (eV)

ρ (RT) ( cm)

ρ (500 ◦ C) ( cm)

RC (500 ◦ C) (µs)

LGSb LGSa LTGa LNGa LTGAb LNGAb

16% / / / 16% 16%

6.2 6.15 7.1 7.4 6.6 7.2

18 19.2 19.6 20.7 21 19.5

50 50.7 76.5 79 79 76

0.0003 / / / 0.0003 0.0002

14 0.0014 0.028 0.022 219 247

0.9 1.1 0.9 1 0.77 0.86

1.1 × 1016 3.9 × 1015 3.5×1013 1.3×1015 1.1×1015 1.6×1016

9 × 106 2 × 104 2×104 7×104 2.2×107 1.1×108

18 0.11 0.17 0.59 49 224

a b

See Ref. [15] and resistivity data calculated from Fig. 5 in Ref. [14] using Z -cut samples. This work data and resistivity measured using X -cut samples.

the fitted lines, the activation energy Ea can be calculated from the slope of the line and found to be 0.9 eV, 0.77 eV and 0.86 eV for LGS, LTGA and LNGA, respectively. The resistivity at room temperature for the crystals was found to be similar while the values at 500 ◦ C very different. The values of resistivity for both LTGA and LNGA crystals were found to be higher than that of LGS crystals, in which LNGA crystal possess the highest value around 1.1 × 108  cm, as compared to the value of LGS crystal (∼9 × 106  cm) and LTGA crystal (∼2.2 × 107  cm). The main properties of the LNGA and LTGA single crystals were listed in Table 1 and compared to LGS, LTG and LNG crystals. Compared to LGS crystal, the LNGA and LTGA crystals possess higher piezoelectric coefficient and much higher resistivity at elevated temperature because the impurities such as Nb2 O5 or Ta2 O5 inside the crystal act as donor, which decrease the number of charge carrier resulting from electron–hole compensation [17, 18]. This will increase the internal friction and as a consequence, the resistivity will increase, especially at elevated temperature. This phenomenon will also be confirmed by the properties of LNG compared to LGS crystals, which listed in Table 1 [14]. Compared to LTG and LNG crystals, LTGA and LNGA exhibited higher resistivity in high temperature range because of the small amount of aluminum ions substituting the gallium ions inside the crystals, which occupy both octahedral and two tetrahedral Ga sites, resulting in the decreasing of Ga–O and increasing of La–O distances [19,20]. On the other hand, the oxygen vacancy concentration was reduced due to the introducing of Al with smaller ion radius (rAl < rGa ). The resistivity was determined not only by the concentration of localized polarons with the participation of oxygen vacancies, but also their mobility, which depends on the structural environment of moving polarons [21]. Thus, it was expected that the Al modified LNG and LTG will possess higher resistivity when compared to the pure counterparts, which is actually the case in this study, as shown in Table 1. Of particular interest for LNGA crystal is its high RC time constant, being on the order of 224 µs at 500 ◦ C, twelve times as the value of LGS crystal (∼18 µs) and four times higher than LTGA crystal (∼49 µs), demonstrating that LNGA crystal is promising for applications at high temperature and low frequency range. It is should be noticed from Table 1 that the resistivity values obtained in this work for LGS crystal was very different from Ref. [14], but close to the values reported in Ref. [22] because the resistivity in

Ref. [14] was measured using a Z -cut sample while in this work and Ref. [22], an X -cut sample was used, due to the quasi-layered structure of langasite in the h0001i direction and anisotropic carrier mobility [21]. 4. Conclusion Large size and high quality LNGA and LTGA single crystals can be readily grown using a conventional Czochralski pulling technique. Both LNGA and LTGA crystal exhibited improved piezoelectric properties compared to LGS crystals and three times larger than the value of quartz. High resistivity and RC time constants were observed for LNGA crystal, together with its non-phasetransformation prior to the melting point, indicating that the aluminum modified langanite crystal promising candidate for high performance applications at elevated temperature. References [1] T.R. Shrout, R. Eitel, C.A. Randall, in: N. Setter (Ed.), Piezoelectric Materials in Devices, EPFL Swiss Federal Institute of Technology, Switzerland, 2002, p. 413. [2] R. Turner, P. Fuierer, R.E. Newnham, T.R. Shrout, Appl. Acoust. 41 (1994) 299. [3] D. Damjanovic, Curr. Opin. Solid State Mater. Sci. 3 (1998) 469. [4] K. Shimamura, H. Takeda, T. Kohno, T. Fukuda, J. Cryst. Growth 163 (1996) 388. [5] J. Detaint, J. Schwartzel, A. Zarka, B. Capelle, J.P. Denis, E. Philippot, Proc. 1994 IEEE Int. Frequency Control Symp., p. 58. [6] M. Adachi, T. Karake, W. Miyamoto, Jpn. J. Appl. Phys. 38 (1999) 3283. [7] B.V. Mill, Y.V. Pisarevsky, 2000 IEEE/EIA Int. Frequency Control Symp., p. 133. [8] Y.V. Pisarevsky, P.A. Senushencov, P.A. Popov, B.V. Mill, Proc. 1995 IEEE Int. Frequency Control Symp., p. 653. [9] H. Takeda, K. Shimamura, T. Kohno, T. Fukuda, J. Cryst. Growth 169 (1996) 503. [10] H. Kawanaka, H. Takeda, K. Shimamura, T. Fukuda, J. Cryst. Growth 183 (1998) 274. [11] I.H. Jung, J. Ko, K. Shim, T. Fukuda, K. Auh, J. Cryst. Growth 220 (2000) 275. [12] I.H. Jung, W. Yang, A. Yoshikawa, T. Fukuda, K. Auh, J. Cryst. Growth 262 (2004) 40. [13] J. Stade, L. Bohaty, M. Hengst, R. Heimann, Cryst. Res. Technol. 37 (2002) 1113. [14] J. Bohm, R.B. Heimann, M. Hengst, R. Roewer, J. Schindler, J. Cryst. Growth 204 (1999) 128. [15] J. Bohm, E. Chilla, C. Flannery, H.J. Frohlich, T. Hauke, R.B. Heimann, M. Hergst, U. Straube, J. Cryst. Growth 216 (2000) 293. [16] IEEE Standard on Piezoelectricity, ANSI/IEEE Standard 176-1987, NY. [17] M. Takahashi, Jpn. J. Appl. Phys. 10 (1971) 643. [18] R. Gerson, H. Jaffe, J. Phys. Chem. Solids 24 (1963) 979. [19] H. Takeda, M. Kumatoriya, T. Shiosaki, Appl. Phys. Lett. 79 (2001) 4201. [20] H. Takeda, S. Tanaka, H. Shimizu, T. Nishida, T. Shiosaki, Key Eng. Mater. 320 (2006) 239. [21] E.N. Domoroshchina, A.B. Dubovskii, G.M. Kuz’micheva, G.V. Semenkovich, Inorganic Mater. 41 (2005) 1218. [22] S. Ganschow, C. Cavalloni, P. Reiche, R. Uecker, Proc. SPIE 2373 (1995) 55.