Photovoltaic effect of Sn 2 P 2 S 6 ferroelectric crystal and ceramics

Transcription

1 Photovoltaic effect of Sn 2 P 2 S 6 ferroelectric crystal and ceramics Y.W. Cho and S.K. Choi Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusung-dong Yusung-gu, Taejon , South Korea Yu.M. Vysochanskii Institute of Solid State Physics & Chemistry of Uzhgorod State University, Uzhgorod, Ukraine (Received 8 February 2001; accepted 6 September 2001) To investigate the photovoltaic effect in Sn 2 P 2 S 6 ferroelectrics, Sn 2 P 2 S 6 crystal and ceramic sample that had a high relative density of 95% and a grain size of below 1 m were fabricated. The steady-state photovoltaic currents under infrared light illumination on both Sn 2 P 2 S 6 crystal and ceramic sample were observed for the first time. The photovoltaic currents i pv on Sn 2 P 2 S 6 ceramic sample and crystal were 5 and 25 na/cm, respectively. The difference in the magnitude of the photovoltaic current resulted from the difference in the remanent polarization P r between the crystal and the ceramic sample. The values of the photovoltaic field E pv determined from I V curve for Sn 2 P 2 S 6 ceramic sample and crystal were 0.2 and 6 V/cm, respectively; these values were several orders lower than that observed on the perovskite-type ferroelectric ceramics. The photovoltaic fields observed in this study were discussed with the conductivity of the crystal and the ceramic sample, in consideration of the figure of merit for the photostriction. I. INTRODUCTION In pyroelectric crystals including poled ferroelectric ceramics, photoexcited electrons generate a photovoltaic field larger than a band gap in an open circuit and a photovoltaic current to the poling direction in a closed circuit without an external field. 1 This phenomenon has been known to be the photovoltaic effect in ferroelectrics. Photostriction effect is a phenomenon in which a photoinduced strain appears in poled ferroelectric materials when it is illuminated and is expressed as a superposition phenomenon of the photovoltaic effect and inverse piezoelectric effect. Ferroelectric material, which has the photostriction effect, can be applied to photodriven devices such as optically activated microactuators and robotics. Also, it is especially appropriate for application to micromechanisms, ultrahigh vacuums, and space technology. Photostriction effect is actively but limitedly studied for Pb-based oxide ferroelectric materials because the oxide ferroelectric materials have a high band gap (approximately 3 ev). Consequently, only ultraviolet (UV) must be used for photostriction effect. 2 8 To extend the range of application of the photodriven devices using only UV at present, the photovoltaic phenomena on the ferroelectric material, which has a low band gap and shows reaction in the infrared (IR) range, should be studied. Sn 2 P 2 S 6 (hereafter SPS) belongs to the class of sulfide ferroelectrics and undergoes ferroelectric phase transition of the 2nd order at T c 339 K. The crystal symmetry is monoclinic P c in the ferroelectric phase and P 21/c in the paraelectric phase. 9,10 SPS has a band gap close to 2.3 ev, exhibits a pronounced photoconductivity and photorefractive property in the infrared range, 11 and also shows large pyroelectric and piezoelectric coefficients and interesting electro-optic and nonlinear-properties. 12 Its ferroelectric properties have been investigated with only single crystal. However, photovoltaic and photostriction effects on the SPS single crystal were not observed yet. Furthermore, in the case of applications to photostrictive microactuator technology in the IR spectrum range, it is necessary to fabricate SPS thin film with epitaxial or polycrystalline growth. In this study, SPS crystal and ceramics were fabricated using a chemical vapor transport method and the followed solid-state sintering process, respectively. The photovoltaic current i pv, and field E ph on SPS ceramics and single crystal under illumination were observed for J. Mater. Res., Vol. 16, No. 11, Nov Materials Research Society 3317

2 the first time. The photovoltaic fields were discussed with the conductivities of SPS crystal and ceramics, in consideration of the figure of merit for the photostriction. II. EXPERIMENTAL PROCESS SPS crystals with high quality and a large size of 5 5mm 2 were prepared by a chemical vapor transport technique in a quartz tube using SnJ 4 as a transport agent. Details of the growth procedure of crystals obtained have been published elsewhere. 13 Figure 1 shows the pole figures measured on [002] grown SPS crystal. SPS ceramics were prepared by the sintering process. First, SPS crystals with a small size of 1 1mm 2 were also synthesized by a chemical vapor transport method. 14 The stoichiometric amounts of the constituting elements (sulfur, phosphorus, and tin powder) and about 0.1 g of iodine as transport agent were sealed into a vacuum quartz ampule (length 150 mm, diameter 20 mm). For growing crystals, the ampule was placed in a horizontal furnace that had two temperature zones, a 903 K hot zone and an 873 K cold zone, for 3 days. The crystals grown by this method were crushed with ZrO 2 balls in stainless steel mold for 10 min. The powder was uniaxially pressed to form pellets and then cold isostatically pressed at 300 MPa/cm 2. Before the sintering process, a pellet and a given amount of the atmospheric powder (the weight ratio of sulfur to phosphorus was fixed at 3:1) were sealed into a vacuum quartz ampoule (approximate volume of 15 ml). The atmospheric powder was used to prevent volatilization of sulfur and phosphorus during sintering. The pellets were sintered with various amounts of the atmospheric powder in a vacuum quartz ampule, and the sintering temperature and time were fixed at 773 K and 2 h, respectively. In this study, SPS ceramics had a maximum relative density of 95% when the amount of the atmospheric powder was 0.03 g. As shown in Fig. 2, the x-ray diffraction analysis shows that the sintered ceramics consist of a single phase. Figure 3 shows the microstructure of the SPS ceramics, which has a maximum relative density of 95%. It can be seen from this figure that the grain size is less than 1 m. The ceramics were supplied for measuring the ferroelectric and photovoltaic properties. SPS ceramics were electroded on 2 5mm 2 faces with Au by sputtering. In the case of single crystals, 2 5mm 2 faces that are perpendicular to [002] direction were also electroded with Au by sputtering. The current applied voltage curves for crystal and ceramics were measured in a voltage range of 0 20 V/cm at room temperature to obtain the dark conductivity, d. The real and the imaginary capacitances were measured by a HP4192 Impedance Gain/Phase analyzer in a temperature range of K. The Saywer Tower method was used for the measurement of the hysteresis curve in the crystal and ceramics. To investigate the photovoltaic effect, the crystal and ceramics were poled at 3 kv/cm. After an aging treatment, they were moved into a dark chamber and FIG. 2. XRD patterns of (a) Sn 2 P 2 S 6 powder fabricated by a chemical vapor transport technique and (b) ceramics sintered in a vacuum quartz ampule at 500 C, 2 h, with an atmospheric powder (sulfur and phosphorus). FIG. 1. Pole figures of Sn 2 P 2 S 6 single crystal: (a) [002] direction; (b) [100] direction J. Mater. Res., Vol. 16, No. 11, Nov 2001

3 FIG. 3. Scanning electron micrograph of Sn 2 P 2 S 6 ceramics with a relative density of 95%. Note that the average grain size is less than 1 m. illuminated with the uniform light of a 300-W xenon lamp through a wave-guide and a focusing lens. The photovoltaic current i pv without an external electric field was measured with a Keithley 486 Picoammeter. The photovoltaic field E pv was determined from the current applied voltage (I V ) curve under uniform illumination. The piezoelectric coefficient, d 33, of SPS crystal was measured using the Berlincourt d 33 -meter. III. RESULTS AND DISCUSSION Figure 4 shows the temperature dependence of the relative dielectric constant at various frequencies for the SPS single crystal and the ceramics, respectively. In the case of the single crystal, the sharp anomaly by the phase transition is observed at 337 K, which does not depend on the frequency. The phase transition of the SPS crystal has been reported to be 2nd order. 15 The maximum dielectric constants max of SPS crystal were which were similar to the previous study. 15 However, the ceramic sample shows a large deviation from the normal dielectric anomaly typically observed in the single crystal. The Curie temperature of ceramics is observed at 325 K, which is slightly lower than the single crystal. The maximum dielectric constant of the ceramic sample is much lower ( max ) than the single crystal. Especially, the phase transition of the ceramic sample also deviates largely from the ideal 2nd-order type, comparing with the single crystal system shown in Fig. 4(a). It is considered that these differences between the ceramic and the crystal are due to the grain size effect in the ceramic sample. As already shown in Fig. 3, the average grain size is less than 1 m for the ceramic sample fabricated in this study, while the single crystal is regarded as the infinite-size sample. According to the previous studies, when the grain size of the ferroelectric ceramic decreases, the dielectric anomaly FIG. 4. Variation of dielectric constant of (a) Sn 2 P 2 S 6 crystal and (b) ceramics as a function of frequency and temperature. during the phase transition becomes broad, the maximum dielectric constant at T c decreases, and the transition temperature shifts lower. The dielectric behavior of finegrained ferroelectrics has been understood on the basis of a diffuse phase transition model The diffuseness of a phase transition can be described by a parameter called the diffuseness exponent, which can be determined by fitting the high-temperature dielectric data (for T > T max, where T max is the temperature corresponding to the peak in the dielectric response) to a modified Curie Weiss equation 19 (1/ ) (1/ max ) C 1 (T T max ), (1) where is the relative dielectric constant at a given temperature above T c and C is a Curie Weiss constant. Here 1 corresponds to the conventional Curie Weiss law and 2 to completely diffuse transition, while 1 < < 2 is reported on systems with intermediate degrees of diffuseness. In this study, the diffuseness exponents obtained from Eq. (1) were 1.0 and 1.6 for SPS crystal and ceramics, respectively. These results show that while SPS crystal is a conventional Curie Weiss system, the dielectric property of SPS ceramics becomes more diffuse. That is, the dielectric constant in SPS ceramics J. Mater. Res., Vol. 16, No. 11, Nov

4 is largely dependent on the grain size. Therefore, it is thought that the large difference in the dielectric constant between the crystal and ceramic (Fig. 4) is due to a small grain size in SPS ceramics fabricated in this study. It is interesting that while SPS crystal has another diffuse dielectric anomaly just above T c besides a sharp dielectric anomaly at T c, it is not observed in SPS ceramics. The diffuse dielectric anomaly just above T c in SPS crystal is very similar with the diffuse phase transition observed in relaxor ferroelectrics such as PbMg 1/3 Nb 2/3 O 3 (PMN) and (Pb,La)(Zr,Ti)O 3 (PLZT). 24,25 It has been also observed in various perovskite-type ferroelectric materials such as BaTiO 3, SrTiO 3, and Ladoped PbTiO 3 ceramics at low frequency (<10 Hz) in the temperature range of C above T c. 26,27 It was proposed by Bidault et al. that the diffuse dielectric anomaly above T c observed in various perovskite-type ferroelectrics was related to the space charge dipole relation at two dielectric-electrode interfaces. 27 Detailed examinations on the diffuse dielectric anomaly just above T c in SPS crystal are in progress. Figure 5 shows the ferroelectric hysteresis loops obtained in the SPS crystal and ceramics. The P E curve for SPS crystal clearly shows hysteresis. SPS ceramics, however, show the typical P E curve observed in finegrained ferroelectric ceramics It should be noted that this curve clearly has two remanent polarization states at zero field. The remanent polarization P r in SPS ceramics decreases from 5.5 C/cm 2 in SPS crystal to 1.2 C/cm 2, and the coercive field E c increases from 1.2 to 2.5 kv/cm. This behavior can be understood in grain size effect as well as the presence of the upper limit for the polarization in ferroelectric ceramic because the crystallites which have initially random direction in crystallographic distribution are oriented along the only allowable direction nearest to the poling field 28,29 and also domain switching is additionally suppressed by the defects and internal strains within the crystallites. 30 In this study, the photovoltaic currents i pv in SPS crystal and ceramics were observed for the first time and the results are shown in Fig. 6. The transient current of (+) sign at light on is a pyroelectric current i py related to the reduction of spontaneous polarization resulting from a temperature rise due to photon absorption. 31 Similarly, when the light is turned off, the i py of ( ) sign should flow since spontaneous polarization increases as a result of a decrease in the temperature of the sample. Figure 6 clearly shows that, after i py of (+) sign at light on, the short-circuit i pv flows steady-statically under a light intensity of 2.1 W/cm 2. The steady-state i pv of 25 and 5 na/cm were observed in SPS single crystal and ceramics, respectively. The photovoltaic current density J pv is given by 1 J pv K 1 I, (2) where is the absorption constant, I the light intensity, and K 1 the Glass constant given by K 1 (q/h ) l P 0 + z i l i, (3) where l is the average shift of the electron, P 0 the spontaneous polarization, Z i the charge of ith ion, and l i the displacement of the ith ion, which is proportional to P 0. It can be seen from Eq. (3) that the Glass constant K 1 is dependent on the magnitude of the spontaneous polarization. Since and I in Eq. (2) are a constant in this study, the difference in i pv between SPS crystal and ceramics in Fig. 6 results from the difference in K 1, that is, the difference in the remanent polarization in Fig. 5. FIG. 5. P E hysteresis loops of (a) Sn 2 P 2 S 6 crystal and (b) ceramics. It should be noted that Sn 2 P 2 S 6 ceramics have a two-polarization state at zero field. FIG. 6. Steady-state photovoltaic currents of (a) Sn 2 P 2 S 6 crystal and (b) ceramics under homogeneous illumination. Two transient currents on light-on and -off are the pyrocurrent by heating and cooling of the sample J. Mater. Res., Vol. 16, No. 11, Nov 2001

5 and ceramics, respectively. The calculated ph are and ( cm) 1 for SPS crystal and ceramics, respectively. Grabar 33 also observed directly ph of ( cm) 1 for SPS crystal around room temperature. It is, therefore, concluded that the low photovoltaic fields E ph in SPS crystal and ceramics are due to the high photoconductivity in SPS crystal and ceramics. The product of the photovoltaic field E pv under the illumination and piezoelectric coefficient d 33 estimates the figure of merit for the photostriction. The measured d 33 for the crystal in this study was C/N. Since the obtained E pv for the crystal was 0.2 V/cm, the calculated value of the figure of merit for the crystal is , which is lower by a thousand times than that of PLZT ceramics (3/52/48). 34 FIG. 7. Current density versus voltages under uniform illumination. The intercept of the straight line with the abscissa and the ordinate is the photovoltaic field E ph and the photovoltaic current density j ph, respectively. Figure 7 shows the current density versus applied voltage for (a) SPS crystal and (b) ceramic under uniform illumination. The short-circuit current density J pv can be determined from the intercept with the ordinate, 1 and the obtained J pv is and A/cm 2 for SPS crystal and ceramics, respectively. The open circuit photovoltaic fields E ph that can be also determined from the intercept of the straight line with the abscissa are 0.2 and 6 V/cm for SPS crystal and ceramics, respectively. These values of E ph are lower to the extent of several orders than that observed on the perovskite-type ferroelectric ceramics such as BaTiO 3 and PLZT. 32 Under open circuit conditions, the photovoltaic current charges the sample capacitance generating a macroscopic field E given by 1 J pv K 1 I + E pv, (4) where is the electrical conductivity of the sample during uniform illumination, so that in the steady state the saturation field E sat,pv is given by E sat,pv K 1 I/. (5) The slope of the straight line shown in Fig. 7 leads to calculating the value of in Eq. (5). The obtained values of are and ( cm) 1 for SPS single crystal and ceramics, respectively. Therefore, the value of E pv for the crystal is lower than that of the ceramics because the crystal has a larger than SPS ceramics. Furthermore, the electrical conductivity of the sample during uniform illumination is given by the sum of the dark conductivity d and the photoconductivity ph. In this study, d measured at room temperature was and ( cm) 1 for SPS crystal IV. CONCLUSIONS In this study, to observe photovoltaic effect in Sn 2 P 2 S 6 ferroelectrics and Sn 2 P 2 S 6 crystal and ceramics were fabricated by a chemical vapor transport technique and followed by a solid-state sintering process. The Sn 2 P 2 S 6 ceramic sample that has a high relative density of 95% and grain size less than 1 m exhibited a lower dielectric constant ( max ) at T c in the khz, K region than Sn 2 P 2 S 6 single crystal ( max ). A diffuse dielectric anomaly at T c was also observed in Sn 2 P 2 S 6 ceramics. The peak position of this anomaly was not dependent on the frequency. The remanent polarization observed in P E curve for Sn 2 P 2 S 6 ceramics was P r 1.2 C/cm 2, while Sn 2 P 2 S 6 crystal has a large remnant polarization of 5.5 C/cm 2. The steadystate photovoltaic currents under infrared light illumination on both Sn 2 P 2 S 6 crystal and ceramics were observed for the first time. The photovoltaic currents on Sn 2 P 2 S 6 ceramics and crystal were 5 and 25 na/cm, respectively. The difference in the magnitude of the photovoltaic current was resulted from the difference in P r between the crystal and the ceramic sample. The values of the photovoltaic field determined from I V curve for Sn 2 P 2 S 6 ceramics and crystal were 0.2 and 6 V/cm, respectively; these values were several orders lower than that observed on the perovskite-type ferroelectric ceramics. It was considered that the lower photovoltaic fields observed in this study were due to the high photoconductivity of Sn 2 P 2 S 6 ceramics and crystal. ACKNOWLEDGMENT This work is partially supported by the Brain Korea 2/ project in REFERENCES 1. A.M. Glass, D. von der Linde, D.H. Auston, and T.J. Negran, J. Electron. Mater. 4, 915 (1975). J. Mater. Res., Vol. 16, No. 11, Nov

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