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Fabrication and ionic conductivity of amorphous lithium meta-silicate thin film
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Solid State Ionics 179 (2008) 536 – 542 www.elsevier.com/locate/ssi
Fabrication and ionic conductivity of amorphous lithium meta-silicate thin film Shin-ichi Furusawaa,⁎, Atsushi Kamiyamaa , Takao Tsuruib a
b
Graduate School of Engineering, Gunma University, Tenjin-Cho Kiryu 376-8515, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Received 21 November 2007; received in revised form 27 March 2008; accepted 28 March 2008
Abstract Lithium meta-silicate (Li2O–SiO2, abbreviated as LSO) thin film was fabricated on SiO2 glass substrate by a pulsed laser deposition (PLD) method. By X-ray diffraction measurement, as-prepared thin film is found to have amorphous structure. Temperature and thickness dependence of the ionic conductivity was measured at the temperature range from 500 to 700 K. The amorphous thin film shows about 1 to 2 orders higher ionic conductivity than that of polycrystalline Li2SiO3. The thickness dependence of the ionic conductivity shows a local maximum at around the thickness 0.15 μm. It is considered that the thickness dependence of ionic conductivity of amorphous LSO thin film would be originated from a hetero-interface effect. © 2008 Elsevier B.V. All rights reserved. Keywords: Lithium meta-silicate; Thin film; Ionic conductivity; Pulsed laser deposition
1. Introduction As an application of ionic conductor, the research of the ionic device such as the micro-battery has been actively carrying out. For the application of the lithium ionic conductor to the microdevice, it is important to study not only the application, but also to study the physical property. Especially, studying of the ionic conduction mechanism at hetero-interface region between the ionic conductor thin film and the substrate is significant for both sides of application and science. In the applied field, this study will give a guideline for the material design and the control of the physical property of the ionic conductor. Academically, the study will contribute to the elucidation of the mechanism of the phenomenon of the ionic conductivity enhancement which is called as the “insulator dispersion effect” or “Liang effect”. Insulator dispersion effect was discovered by C. C. Liang in the LiI–Al2O3 composite system in 1973, and it has been observed in various ionic conductors such as AgCl, AgI, CaF2, etc. [1–14]. The insulator dispersion effect is considered to be ⁎ Corresponding author. Tel.: +81 227 30 1727; fax: +81 227 30 1707. E-mail address: [email protected] (S. Furusawa). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.03.040
due to the forming of “high ionic conduction region” at the hetero-interface region between the dispersion insulator and ionic conductor. However, the formation mechanism of the high ionic conduction region and the conduction mechanism at the interface have not been elucidated. From such a standpoint, we have been studying the ionic conduction of the insulator dispersed ionic conductors and ionic conductor thin films on the insulator substrate [9,10,15–17]. Furthermore, in application of lithium ionic conductor thin film, an inorganic oxide lithium ionic conductor would be one of promising solid electrolytes, since there are many materials having high chemical stability and high mechanical strength. Especially, it is important to study the material without the rareearth metal element, since the increase of the price of the rareearth metal and the depletion of the planet's rare-earth metal is apprehended. Therefore, the study of the physical property of the material which does not contain rare-earth metal element is also important, when the future situation of metal resources is considered. From such a standpoint, we have been studying the ionic conduction mechanism of inorganic oxides lithium ionic conductor thin film such as LiAlSiO4 which was fabricated by a pulsed laser deposition (PLD) method [15,17].
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From the viewpoint, to clarify ionic conduction mechanism at the interface region of the lithium silicate group, there is the significance on the fabrication of lithium meta-silicate (abbreviated as LSO) thin film and measurement of the ionic conductivity. However, there is no study about the fabrication and the ionic conduction of the lithium meta-silicate thin film, in author's knowledge. The purpose of this work is to fabricate lithium metasilicate thin film by a pulsed laser deposition method, and to study the ionic conduction mechanism at the hetero-interface region. It will be important to mention about the glass transition and crystallization temperature of the lithium meta-silicate glass, before discussing the ionic conduction at high temperature. There are some reports on glass transition and crystallization temperature by differential thermal analysis (DTA) [18–21]. P. Pernice et al. reported that Li2Al2xSi(1 − 2x)O3 − x (x = 0.00, 0.05, 0.1) show glass transition at 698 K, and the growth of Li4SiO4 crystals and their conversion in Li2SiO3 crystals occur at temperature range from 854 to 945 K. And the activation energy Δ of electrical conductivity of the series depends on x; Δ decreases with increasing x [20]. 2. Experimental As a PLD target, polycrystalline Li2SiO3 was used. Li2SiO3 was synthesized on the basis of following chemical reaction formula, L2 CO3 þ SiO2 →Li2 SiO3 þ CO2 ↑
ð1Þ
Li2SiO3 target was prepared by the following procedures. (1) High purity Li2CO3 and SiO2 were mechanically mixed at 1:1 mol ratios. (2) The mixture was pressed at 1200 kgf/cm2, and sintered in a SiC furnace at 900 °C for 5 h. (3) The sintered material was ground with ethanol. Afterwards, it was pressed again at 1200 kgf/cm2 as a pellet with the size of 26 mm diameter and 6 mm thickness. (4) The pellet was put on a platinum plate mounted in the SiC furnace, and sintered at 1000 °C for 5 h. The sintered pellet was used as the PLD target. The powder X-ray diffraction pattern of prepared target showed good agreement with the data of “The Powder Diffraction File” of Li2SiO3 (PDF data No.29-0828) (Fig. 1(A)). As a laser beam source, KrF excimer laser (wave length 248 nm, 150 mJ/pulse) and Nd-YAG laser (wave length 266 nm, 180 mJ/pulse) was used. The incident laser beam from the laser beam source was focused on the target mounted in a vacuum chamber through a condenser lens with incidence angle 45°. The rotation of the target was controlled by a variablespeed motor at 0 to 10 r.p.m. The chamber pressure was controlled under 1 × 10− 5 Torr. Optical polished quartz glasses were used as the substrate, and the temperature of the substrate was kept at the room temperature. Thickness of the prepared thin films was controlled by the number of the laser pulse, and measured by a profile micrometer VF-7500 (KEYENCE) with
Fig. 1. X-ray diffraction pattern of (A) Li2SiO3 target, (B) as-prepared LSO thin film, (C) post-annealed LSO thin film.
resolution 0.01 μm. The deposition rate for KrF and Nd:YAG laser was about 0.15 and 0.17 Å/pulse, respectively. In the case of super-thin films whose thickness is over the resolution of VF7500, the thickness was estimated from the deposition rate and number of laser pulse. Local micro-structures were evaluated by cross-sectional transmission electron microscopy (TEM) using a JEOL JEM4000EX microscope operated at 400 kV with a resolution of 0.17 nm. For the TEM specimen preparation, the thin films were sectioned perpendicular to the substrate. The samples were then glued to each other by an epoxy resin, and then mechanically sliced and polished using the conventional machining processes. They were finally ion polished with a 3 kV argon ion beam. For the measurement, a comb type Au electrode was mounted on the thin film by a vacuum deposition method. The sample was mounted in a test fixture in an electric furnace, and the temperature of the sample was monitored by an alumelchromel thermocouple. The impedance measurements of the thin films were carried out by HP4194A Impedance/Gain-Phase Analyzer (HEWLETT PACKARD) in a frequency range from 100 Hz to 10 MHz and in a temperature range from 500 to 700 K. 3. Result and discussion Fig. 2(A) and (B) show the surface and the cross-sectional SEM image of LSO thin film, respectively. The thin film is transparent. By SEM observation of the thinnest thin film, a channel state or a hole state was not observed, and the uniformity of the thin film was confirmed. Although there are small particles with the size of several microns called the debris, it was difficult to avoid the adhesion of the debris.
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which is a peculiar pattern of amorphous, and the electron diffraction patterns also indicate the structure of the both of thin film and the debris would be amorphous. These observations suggest that the structure of as-prepared thin film is the amorphous structure. This result agrees with the result of XRD. Impedance plot of the amorphous lithium meta-silicate (aLSO) thin film with film thickness 0.14 μm at various temperatures is shown in Fig. 5. The plotted data were well fitted by the Debye's relaxation formula; Z 4 ðxÞ ¼ Zl þ
Fig. 2. (A) SEM image of LSO thin film surface (film thickness 0.036 μm). (B) Cross-sectional SEM image of LSO thin film.
In Fig. 3, the number of the debris per 100 μm2 was plotted as a function of the film thickness. As shown in Fig. 3, it was proven that the number of the debris is proportional to the film thickness. This result indicates that the number of the debris per unit volume does not depend on the film thickness. Therefore, it would be considered that the existence of the debris would not contribute for thickness dependence of the physical properties. X-ray diffraction (XRD) pattern of as-prepared thin film is shown in Fig. 1(B). As shown in Fig. 1(B), except for the broad diffraction peak which originated from the substrate (at around 2θ = 22°), only an unknown weak peak (at around 2θ = 31.4°) was observed. Fig. 1(C) shows the diffraction pattern of the post-annealed LSO thin film which was annealed at 500 °C for the 10 h in the air. As shown in Fig. 1(C), the diffraction angle of the observed peaks of post-annealed LSO thin film shows good agreement with that of the Li2SiO3 crystal, and the crystallite size estimated from the Scherrer equation and half-width of the diffraction peak was about 16 nm. This result shows that the amorphous LSO thin film transforms to the Li2SiO3 structure at about 80 K lower than the crystallization temperature (about 854 K) which was reported in the reference [20]. Fig. 4 and the inserted figure show the cross-sectional TEM image and the electron diffraction pattern of LSO thin film, respectively. The TEM image shows a labyrinthine structure
Z0 Zl 1 þ ðixsÞb
ð2Þ
Here, Z⁎(ω), Z0, Z∞, ω and τ denote the complex impedance, DC impedance, impedance at high frequency limit, angular frequency and relaxation time, respectively. β means the width of the relaxation time distribution. DC ionic conductivity σ can be estimated from the arc. The low frequency part which deviated from the circular arc corresponds to the impedance of the electrode. In order to examine the thickness dependence of ionic conductivity, temperature dependence of ionic conductivity of the amorphous thin film was measured for various film thicknesses. In the measurement, the heating and cooling process to be 1 cycle, the measurement was carried out 2 cycles for the reproducibility. And it was confirmed that the experimental results showed good reproducibility after the 1st heating process. The temperature dependence of ionic conductivity in the first cooling process for various film thicknesses is shown in Fig. 6. For the comparison, the temperature dependence of ionic conductivity of polycrystalline Li2SiO3 is also plotted in Fig. 6. It should be noted that the data of polycrystalline Li2SiO3 are in good agreement with the data of Li2SiO3 glass sample heated at 550 °C which were reported by P. Pernice et al. (broken line).[18] Dashed line corresponds to the conductivity of Li2SiO3 quenched glass which was estimated from the data of reference [20]. As shown in Fig. 6, ionic conductivity of the thin films increases exponentially with increasing temperature, and this
Fig. 3. Thickness dependence of the number of debris.
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Fig. 4. Cross-sectional TEM image of LSO thin film. The inserted figures show the electron diffraction pattern of thin film (A) and debris (B).
result suggests that the ionic conduction process is a thermal activation type. The temperature dependence of σ was well fitted by the Nernst–Einstein relationship; D N ðZeÞ2 a2 rT ¼ r0 exp C0 f : ð3Þ ; r0 ¼ kB T kB Here, N is a number density of the carrier, Ze is a charge of the carrier, a is the hopping distance, Γ0 is the attempt frequency, Δ is the activation energy and f is a correlation factor whose value would be about 1. The estimated values of the activation energy Δ of the thin films are shown in the figure. As shown in Fig. 6, although it seems there is no slope change with the structural phase transition such as a glass
Fig. 5. Impedance plot of amorphous LSO thin film with thickness 0.14 μm for various temperatures. ○: 549 K, □: 599 K,◊: 649 K, ×: 700 K.
transition, the activation energy of lithium ion conductivity in high temperature region is larger than that in a relatively low temperature region for LSO films with the thickness below 260 nm. The origin of this slope change has not yet been clarified. Furthermore, the ionic conductivity of the amorphous thin films is 1 or 2 orders higher than that of polycrystalline sample, and the activation energy of the thin films is smaller
Fig. 6. Temperature dependence of the ionic conductivity of amorphous LSO thin films for various film thickness, and polycrystalline Li2SiO3. Dashed line and broken line corresponds to the data of reference [18] and [20], respectively.
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Fig. 7. Thickness dependence of ionic conductivity of amorphous LSO thin film. ●: 600 K, ■: 700 K. The dotted curves are drawn to guide the eye.
than that of the polycrystalline Li2SiO3. This result suggests that the amorphous structure is important to obtain high ionic conductible thin film. Furthermore, it was confirmed that the ionic conductivity of the amorphous thin film depends on the film thickness. This result suggests the existence of a hetero-interface effect for the ionic conduction in a-LSO thin film. In addition, the activation energy of the ionic conduction of the thin film also shows thickness dependence within 0.72 to 0.79 eV. This fact may suggest that the mechanism of the interfacial effect is a mechanism with a spatial distribution of activation energy. Thickness dependence of the ionic conductivity at 600 and 700 K is shown in Fig. 7. Broken line and dashed line in the figure correspond to ionic conductivity of the polycrystalline Li2SiO3 at 600 and 700 K, respectively. As shown in the figure, it was proven that the ionic conductivity of the a-LSO thin film shows a maximum at around film thickness 0.14 μm. This result suggests the ionic conductivity σ would be a function of the distance x from the substrate, and it shows local maximum at around x = 0.14 μm. In order to discuss the thickness dependence of σ, one should assume the appropriate model for each case. In this report, let us consider the experimental result by the simplest
Fig. 9. (A) The thickness dependence of σeff for 3 layer model. (B) The conductivity σ of amorphous LSO thin film was plotted as a function of 1/d.
model. The outline of the model is shown in Fig. 8. For simplicity, the system is simplified in three layers; (1) “Hetero-interface region” (0 b x b λ1) The region in the interface vicinity. The average ionic conductivity is σsub. (2) “Middle region” (λ1 b x b λ1 + λ2) This region separates from the interface, however the ionic conduction receives the effect from the interface. The average ionic conductivity is σλ. Table 1 The parameters used for the fitting of Fig. 9(B)
Fig. 8. A simple model of spatial distribution of ionic conductivity. The thin film with thickness d was simplified in three layers.
Temperature
600 K
700 K
λ1 [μm] λ2 [μm] σ∞ [Ω− 1cm− 1] σλ [Ω− 1cm− 1] σsub [Ω− 1cm− 1]
0.070 0.080 4.9 × 10− 5 1.4 × 10− 3 3.4 × 10− 4
0.066 0.082 4.1 × 10− 5 1.3 × 10− 2 2.4 × 10− 3
S. Furusawa et al. / Solid State Ionics 179 (2008) 536–542
(3) “Bulk region” (λ1 + λ2 b x b d) The region sufficiently separated from the interface, and the ionic conduction does not receive the effect from the interface. The average ionic conductivity is σ∞. From the measurement, the effective ionic conductivity σeff is obtained. One can easily obtain the thickness dependence of σeff as,
reff
8 1 > > > < frsub k1 þ rk k2 rl ðk1 þ k2 Þg d þ rl k1 ¼ ðrsub rk Þ þ rk > > > d : rsub
ðdNk1 þ k2 Þ ðk1 bdbk1 þ k2 Þ
:
ðdbk1 Þ
ð4Þ Eq. (4) means, when σeff is plotted against 1/d, it should fall on straight lines in the correspondent region. The condition in which σeff has a peak is σλ N σsub, σ∞. For example, the thickness dependence of σeff for the case of σ∞ b σsub b σλ is shown in Fig. 9(A). Then, σ at 600 and 700 K were replotted as a function of 1/d (shown in Fig. 9(B)). As shown in Fig. 9(B), the data were well fitted by Eq. (4) with the parameters in Table 1. Here, σ of the thinnest sample (thickness 0.036 μm) was assumed as a value of
541
σsub. The result suggests that ionic conductivity of middle region may be about 6 times higher than that of the interface region. In addition, the similar parameter fitting was carried out at various temperatures, and the temperature dependence of ionic conductivity for each layer was plotted in Fig. 10. The estimated Δ and σ0 were shown in Fig. 10. As is seen from Fig. 10, the activation energy of the middle region and interface region is estimated as 0.75 ± 0.02 and 0.80 ± 0.02 eV, respectively. Considering the standard deviations, the difference seems to be significant. This result may suggest that the activation energy depends on the distance from the interface. It is considered that the spatial distribution of the activation energy would be originated from the structural modulation which originates from the misfit between a substrate and a thin film, since the activation energy for ionic conduction is one of the structural sensitive parameter. The ionic conductivity may show the substrate dependence, if the thickness dependence of the ionic conductivity originates from the hetero-interface. And it would be reported in next report. Finally, as the origin of the thickness dependence of the conductivity, some mechanisms may be considered. 1) Structural modulation or strain of the thin film. It may be presumed that one of the origins of the thickness dependence is attributed to the structural modulation or residual stress of the thin film, since the activation energy depends on the film thickness. That is to say, the structure of the thin film in the hetero-interface may be affected in the structure of the substrate (misfit effect). 2) Spatial distribution of the charge carrier density. The charge carrier density may be a function of the distance from the interface (space charge-effect). In order to show the thickness dependence shown in this report, the thickness of the spatial variation of the density of the charge carrier would be about 150 nm. Although it is difficult to be considered that the spacecharge region has such thickness, it can not be denied. There may be the spatial distribution of the density of lithium ion in some origins. For example, it may be generating the spatial distribution of the effective carrier density by the increase of the conduction path which originates from the structural modulation of the thin film. 4. Conclusions
Fig. 10. Temperature dependence of ionic conductivity for each layer. The straight lines are fitting result by Eq. (3).
(1) Amorphous lithium meta-silicate thin film was fabricated on the SiO2 glass by the PLD method. (2) The ionic conductivity of the amorphous thin film was 1 or 2 orders higher than that of the polycrystalline Li2SiO3. (3) The ionic conductivity of the amorphous thin film depends on the film thickness. (4) The thickness dependence of ionic conductivity of the amorphous thin film was qualitatively explained by simplified model which assumes 3 layers. (5) The thickness dependence of ionic conductivity in amorphous thin film would be due to the hetero-interface effect. Probably, the structure of the thin film in the heterointerface would be affected in the structure of the substrate, and the structural modulation may play an important role in the mechanism of the hetero-interface effect.
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Acknowledgements The present work was supported by the Grant-in-Aid for Scientific Research on Priority Area, “Nanoionics (439)” and Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] C.C. Liang, J. Electrochem. Soc. 120 (1973) 1289. [2] K. Shahi, J.B. Wagner Jr., J. Solid State Chem. 42 (1982) 107. [3] F.W. Poulsen, N.H. Andersen, B. Kindl, J. Schoonman, Soild State Ionics 9/10 (1983) 119. [4] S. Furusawa, S. Miyaoka, Y. Ishibashi, J. Phys. Soc. Jpn. 60 (1991) 1666. [5] M. Nagai, T. Nishino, Solid State Ionics 53–56 (1992) 63. [6] A. Bunde, Solid State Ionics 75 (1995) 147. [7] S. Furusawa, S. Miyaoka, Y. Ishibashi, J. Phys. Soc. Jpn. 62 (1993) 196. [8] T. Ida, K. Kimura, Solid State Ionics 107 (1998) 313.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
S. Furusawa, Y. Sakai, J. Phys. Soc. Jpn. 68 (1999) 976. S. Furusawa, T. Kawaguchi, J. Phys. Soc. Jpn. 70 (2001) 3585. J. Maier, Solid State Ionics 75 (1995) 139. K. Funke, D. Wilmer, T. Lauxtermann, R. Holzgreve, S.M. Bennington, Solid State Ionics 86–88 (1996) 141. T. Jow, J.B. Wagner, J. Electrochem. Soc. 126 (1979) 1963. S. Jiang, J.B. Wagner Jr., J. Phys. Chem. Solids 56 (1995) 1101. S. Furusawa, S. Shimizu, K. Sekine, H. Tabuchi, Solid State Ionics 167 (2004) 325. S. Furusawa, H. Tabuchi, T. Sugiyama, S. Tao, J.T.S. Irvine, Solid State Ionics 176 (2005) 55. S. Furusawa, H. Tabuchi, T. Tsurui, Solid State Ionics 178 (2007) 1033. P. Pernice, A. Aronne, A. Marotta, Solid State Ionics 37 (1989) 79. A. Marotta, A. Aronne, P. Pernice, F. Branda, Thermochim. Acta 140 (1989) 191. P. Pernice, A. Aronne, A. Marotta, Thermochim. Acta 157 (1990) 77. A. Marotta, P. Pernice, A. Aronne, A. Buri, J. Non-Cryst. Solids 127 (1991) 159.
Solid State Ionics 179 (2008) 536 – 542 www.elsevier.com/locate/ssi
Fabrication and ionic conductivity of amorphous lithium meta-silicate thin film Shin-ichi Furusawaa,⁎, Atsushi Kamiyamaa , Takao Tsuruib a
b
Graduate School of Engineering, Gunma University, Tenjin-Cho Kiryu 376-8515, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Received 21 November 2007; received in revised form 27 March 2008; accepted 28 March 2008
Abstract Lithium meta-silicate (Li2O–SiO2, abbreviated as LSO) thin film was fabricated on SiO2 glass substrate by a pulsed laser deposition (PLD) method. By X-ray diffraction measurement, as-prepared thin film is found to have amorphous structure. Temperature and thickness dependence of the ionic conductivity was measured at the temperature range from 500 to 700 K. The amorphous thin film shows about 1 to 2 orders higher ionic conductivity than that of polycrystalline Li2SiO3. The thickness dependence of the ionic conductivity shows a local maximum at around the thickness 0.15 μm. It is considered that the thickness dependence of ionic conductivity of amorphous LSO thin film would be originated from a hetero-interface effect. © 2008 Elsevier B.V. All rights reserved. Keywords: Lithium meta-silicate; Thin film; Ionic conductivity; Pulsed laser deposition
1. Introduction As an application of ionic conductor, the research of the ionic device such as the micro-battery has been actively carrying out. For the application of the lithium ionic conductor to the microdevice, it is important to study not only the application, but also to study the physical property. Especially, studying of the ionic conduction mechanism at hetero-interface region between the ionic conductor thin film and the substrate is significant for both sides of application and science. In the applied field, this study will give a guideline for the material design and the control of the physical property of the ionic conductor. Academically, the study will contribute to the elucidation of the mechanism of the phenomenon of the ionic conductivity enhancement which is called as the “insulator dispersion effect” or “Liang effect”. Insulator dispersion effect was discovered by C. C. Liang in the LiI–Al2O3 composite system in 1973, and it has been observed in various ionic conductors such as AgCl, AgI, CaF2, etc. [1–14]. The insulator dispersion effect is considered to be ⁎ Corresponding author. Tel.: +81 227 30 1727; fax: +81 227 30 1707. E-mail address: [email protected] (S. Furusawa). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.03.040
due to the forming of “high ionic conduction region” at the hetero-interface region between the dispersion insulator and ionic conductor. However, the formation mechanism of the high ionic conduction region and the conduction mechanism at the interface have not been elucidated. From such a standpoint, we have been studying the ionic conduction of the insulator dispersed ionic conductors and ionic conductor thin films on the insulator substrate [9,10,15–17]. Furthermore, in application of lithium ionic conductor thin film, an inorganic oxide lithium ionic conductor would be one of promising solid electrolytes, since there are many materials having high chemical stability and high mechanical strength. Especially, it is important to study the material without the rareearth metal element, since the increase of the price of the rareearth metal and the depletion of the planet's rare-earth metal is apprehended. Therefore, the study of the physical property of the material which does not contain rare-earth metal element is also important, when the future situation of metal resources is considered. From such a standpoint, we have been studying the ionic conduction mechanism of inorganic oxides lithium ionic conductor thin film such as LiAlSiO4 which was fabricated by a pulsed laser deposition (PLD) method [15,17].
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From the viewpoint, to clarify ionic conduction mechanism at the interface region of the lithium silicate group, there is the significance on the fabrication of lithium meta-silicate (abbreviated as LSO) thin film and measurement of the ionic conductivity. However, there is no study about the fabrication and the ionic conduction of the lithium meta-silicate thin film, in author's knowledge. The purpose of this work is to fabricate lithium metasilicate thin film by a pulsed laser deposition method, and to study the ionic conduction mechanism at the hetero-interface region. It will be important to mention about the glass transition and crystallization temperature of the lithium meta-silicate glass, before discussing the ionic conduction at high temperature. There are some reports on glass transition and crystallization temperature by differential thermal analysis (DTA) [18–21]. P. Pernice et al. reported that Li2Al2xSi(1 − 2x)O3 − x (x = 0.00, 0.05, 0.1) show glass transition at 698 K, and the growth of Li4SiO4 crystals and their conversion in Li2SiO3 crystals occur at temperature range from 854 to 945 K. And the activation energy Δ of electrical conductivity of the series depends on x; Δ decreases with increasing x [20]. 2. Experimental As a PLD target, polycrystalline Li2SiO3 was used. Li2SiO3 was synthesized on the basis of following chemical reaction formula, L2 CO3 þ SiO2 →Li2 SiO3 þ CO2 ↑
ð1Þ
Li2SiO3 target was prepared by the following procedures. (1) High purity Li2CO3 and SiO2 were mechanically mixed at 1:1 mol ratios. (2) The mixture was pressed at 1200 kgf/cm2, and sintered in a SiC furnace at 900 °C for 5 h. (3) The sintered material was ground with ethanol. Afterwards, it was pressed again at 1200 kgf/cm2 as a pellet with the size of 26 mm diameter and 6 mm thickness. (4) The pellet was put on a platinum plate mounted in the SiC furnace, and sintered at 1000 °C for 5 h. The sintered pellet was used as the PLD target. The powder X-ray diffraction pattern of prepared target showed good agreement with the data of “The Powder Diffraction File” of Li2SiO3 (PDF data No.29-0828) (Fig. 1(A)). As a laser beam source, KrF excimer laser (wave length 248 nm, 150 mJ/pulse) and Nd-YAG laser (wave length 266 nm, 180 mJ/pulse) was used. The incident laser beam from the laser beam source was focused on the target mounted in a vacuum chamber through a condenser lens with incidence angle 45°. The rotation of the target was controlled by a variablespeed motor at 0 to 10 r.p.m. The chamber pressure was controlled under 1 × 10− 5 Torr. Optical polished quartz glasses were used as the substrate, and the temperature of the substrate was kept at the room temperature. Thickness of the prepared thin films was controlled by the number of the laser pulse, and measured by a profile micrometer VF-7500 (KEYENCE) with
Fig. 1. X-ray diffraction pattern of (A) Li2SiO3 target, (B) as-prepared LSO thin film, (C) post-annealed LSO thin film.
resolution 0.01 μm. The deposition rate for KrF and Nd:YAG laser was about 0.15 and 0.17 Å/pulse, respectively. In the case of super-thin films whose thickness is over the resolution of VF7500, the thickness was estimated from the deposition rate and number of laser pulse. Local micro-structures were evaluated by cross-sectional transmission electron microscopy (TEM) using a JEOL JEM4000EX microscope operated at 400 kV with a resolution of 0.17 nm. For the TEM specimen preparation, the thin films were sectioned perpendicular to the substrate. The samples were then glued to each other by an epoxy resin, and then mechanically sliced and polished using the conventional machining processes. They were finally ion polished with a 3 kV argon ion beam. For the measurement, a comb type Au electrode was mounted on the thin film by a vacuum deposition method. The sample was mounted in a test fixture in an electric furnace, and the temperature of the sample was monitored by an alumelchromel thermocouple. The impedance measurements of the thin films were carried out by HP4194A Impedance/Gain-Phase Analyzer (HEWLETT PACKARD) in a frequency range from 100 Hz to 10 MHz and in a temperature range from 500 to 700 K. 3. Result and discussion Fig. 2(A) and (B) show the surface and the cross-sectional SEM image of LSO thin film, respectively. The thin film is transparent. By SEM observation of the thinnest thin film, a channel state or a hole state was not observed, and the uniformity of the thin film was confirmed. Although there are small particles with the size of several microns called the debris, it was difficult to avoid the adhesion of the debris.
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which is a peculiar pattern of amorphous, and the electron diffraction patterns also indicate the structure of the both of thin film and the debris would be amorphous. These observations suggest that the structure of as-prepared thin film is the amorphous structure. This result agrees with the result of XRD. Impedance plot of the amorphous lithium meta-silicate (aLSO) thin film with film thickness 0.14 μm at various temperatures is shown in Fig. 5. The plotted data were well fitted by the Debye's relaxation formula; Z 4 ðxÞ ¼ Zl þ
Fig. 2. (A) SEM image of LSO thin film surface (film thickness 0.036 μm). (B) Cross-sectional SEM image of LSO thin film.
In Fig. 3, the number of the debris per 100 μm2 was plotted as a function of the film thickness. As shown in Fig. 3, it was proven that the number of the debris is proportional to the film thickness. This result indicates that the number of the debris per unit volume does not depend on the film thickness. Therefore, it would be considered that the existence of the debris would not contribute for thickness dependence of the physical properties. X-ray diffraction (XRD) pattern of as-prepared thin film is shown in Fig. 1(B). As shown in Fig. 1(B), except for the broad diffraction peak which originated from the substrate (at around 2θ = 22°), only an unknown weak peak (at around 2θ = 31.4°) was observed. Fig. 1(C) shows the diffraction pattern of the post-annealed LSO thin film which was annealed at 500 °C for the 10 h in the air. As shown in Fig. 1(C), the diffraction angle of the observed peaks of post-annealed LSO thin film shows good agreement with that of the Li2SiO3 crystal, and the crystallite size estimated from the Scherrer equation and half-width of the diffraction peak was about 16 nm. This result shows that the amorphous LSO thin film transforms to the Li2SiO3 structure at about 80 K lower than the crystallization temperature (about 854 K) which was reported in the reference [20]. Fig. 4 and the inserted figure show the cross-sectional TEM image and the electron diffraction pattern of LSO thin film, respectively. The TEM image shows a labyrinthine structure
Z0 Zl 1 þ ðixsÞb
ð2Þ
Here, Z⁎(ω), Z0, Z∞, ω and τ denote the complex impedance, DC impedance, impedance at high frequency limit, angular frequency and relaxation time, respectively. β means the width of the relaxation time distribution. DC ionic conductivity σ can be estimated from the arc. The low frequency part which deviated from the circular arc corresponds to the impedance of the electrode. In order to examine the thickness dependence of ionic conductivity, temperature dependence of ionic conductivity of the amorphous thin film was measured for various film thicknesses. In the measurement, the heating and cooling process to be 1 cycle, the measurement was carried out 2 cycles for the reproducibility. And it was confirmed that the experimental results showed good reproducibility after the 1st heating process. The temperature dependence of ionic conductivity in the first cooling process for various film thicknesses is shown in Fig. 6. For the comparison, the temperature dependence of ionic conductivity of polycrystalline Li2SiO3 is also plotted in Fig. 6. It should be noted that the data of polycrystalline Li2SiO3 are in good agreement with the data of Li2SiO3 glass sample heated at 550 °C which were reported by P. Pernice et al. (broken line).[18] Dashed line corresponds to the conductivity of Li2SiO3 quenched glass which was estimated from the data of reference [20]. As shown in Fig. 6, ionic conductivity of the thin films increases exponentially with increasing temperature, and this
Fig. 3. Thickness dependence of the number of debris.
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Fig. 4. Cross-sectional TEM image of LSO thin film. The inserted figures show the electron diffraction pattern of thin film (A) and debris (B).
result suggests that the ionic conduction process is a thermal activation type. The temperature dependence of σ was well fitted by the Nernst–Einstein relationship; D N ðZeÞ2 a2 rT ¼ r0 exp C0 f : ð3Þ ; r0 ¼ kB T kB Here, N is a number density of the carrier, Ze is a charge of the carrier, a is the hopping distance, Γ0 is the attempt frequency, Δ is the activation energy and f is a correlation factor whose value would be about 1. The estimated values of the activation energy Δ of the thin films are shown in the figure. As shown in Fig. 6, although it seems there is no slope change with the structural phase transition such as a glass
Fig. 5. Impedance plot of amorphous LSO thin film with thickness 0.14 μm for various temperatures. ○: 549 K, □: 599 K,◊: 649 K, ×: 700 K.
transition, the activation energy of lithium ion conductivity in high temperature region is larger than that in a relatively low temperature region for LSO films with the thickness below 260 nm. The origin of this slope change has not yet been clarified. Furthermore, the ionic conductivity of the amorphous thin films is 1 or 2 orders higher than that of polycrystalline sample, and the activation energy of the thin films is smaller
Fig. 6. Temperature dependence of the ionic conductivity of amorphous LSO thin films for various film thickness, and polycrystalline Li2SiO3. Dashed line and broken line corresponds to the data of reference [18] and [20], respectively.
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Fig. 7. Thickness dependence of ionic conductivity of amorphous LSO thin film. ●: 600 K, ■: 700 K. The dotted curves are drawn to guide the eye.
than that of the polycrystalline Li2SiO3. This result suggests that the amorphous structure is important to obtain high ionic conductible thin film. Furthermore, it was confirmed that the ionic conductivity of the amorphous thin film depends on the film thickness. This result suggests the existence of a hetero-interface effect for the ionic conduction in a-LSO thin film. In addition, the activation energy of the ionic conduction of the thin film also shows thickness dependence within 0.72 to 0.79 eV. This fact may suggest that the mechanism of the interfacial effect is a mechanism with a spatial distribution of activation energy. Thickness dependence of the ionic conductivity at 600 and 700 K is shown in Fig. 7. Broken line and dashed line in the figure correspond to ionic conductivity of the polycrystalline Li2SiO3 at 600 and 700 K, respectively. As shown in the figure, it was proven that the ionic conductivity of the a-LSO thin film shows a maximum at around film thickness 0.14 μm. This result suggests the ionic conductivity σ would be a function of the distance x from the substrate, and it shows local maximum at around x = 0.14 μm. In order to discuss the thickness dependence of σ, one should assume the appropriate model for each case. In this report, let us consider the experimental result by the simplest
Fig. 9. (A) The thickness dependence of σeff for 3 layer model. (B) The conductivity σ of amorphous LSO thin film was plotted as a function of 1/d.
model. The outline of the model is shown in Fig. 8. For simplicity, the system is simplified in three layers; (1) “Hetero-interface region” (0 b x b λ1) The region in the interface vicinity. The average ionic conductivity is σsub. (2) “Middle region” (λ1 b x b λ1 + λ2) This region separates from the interface, however the ionic conduction receives the effect from the interface. The average ionic conductivity is σλ. Table 1 The parameters used for the fitting of Fig. 9(B)
Fig. 8. A simple model of spatial distribution of ionic conductivity. The thin film with thickness d was simplified in three layers.
Temperature
600 K
700 K
λ1 [μm] λ2 [μm] σ∞ [Ω− 1cm− 1] σλ [Ω− 1cm− 1] σsub [Ω− 1cm− 1]
0.070 0.080 4.9 × 10− 5 1.4 × 10− 3 3.4 × 10− 4
0.066 0.082 4.1 × 10− 5 1.3 × 10− 2 2.4 × 10− 3
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(3) “Bulk region” (λ1 + λ2 b x b d) The region sufficiently separated from the interface, and the ionic conduction does not receive the effect from the interface. The average ionic conductivity is σ∞. From the measurement, the effective ionic conductivity σeff is obtained. One can easily obtain the thickness dependence of σeff as,
reff
8 1 > > > < frsub k1 þ rk k2 rl ðk1 þ k2 Þg d þ rl k1 ¼ ðrsub rk Þ þ rk > > > d : rsub
ðdNk1 þ k2 Þ ðk1 bdbk1 þ k2 Þ
:
ðdbk1 Þ
ð4Þ Eq. (4) means, when σeff is plotted against 1/d, it should fall on straight lines in the correspondent region. The condition in which σeff has a peak is σλ N σsub, σ∞. For example, the thickness dependence of σeff for the case of σ∞ b σsub b σλ is shown in Fig. 9(A). Then, σ at 600 and 700 K were replotted as a function of 1/d (shown in Fig. 9(B)). As shown in Fig. 9(B), the data were well fitted by Eq. (4) with the parameters in Table 1. Here, σ of the thinnest sample (thickness 0.036 μm) was assumed as a value of
541
σsub. The result suggests that ionic conductivity of middle region may be about 6 times higher than that of the interface region. In addition, the similar parameter fitting was carried out at various temperatures, and the temperature dependence of ionic conductivity for each layer was plotted in Fig. 10. The estimated Δ and σ0 were shown in Fig. 10. As is seen from Fig. 10, the activation energy of the middle region and interface region is estimated as 0.75 ± 0.02 and 0.80 ± 0.02 eV, respectively. Considering the standard deviations, the difference seems to be significant. This result may suggest that the activation energy depends on the distance from the interface. It is considered that the spatial distribution of the activation energy would be originated from the structural modulation which originates from the misfit between a substrate and a thin film, since the activation energy for ionic conduction is one of the structural sensitive parameter. The ionic conductivity may show the substrate dependence, if the thickness dependence of the ionic conductivity originates from the hetero-interface. And it would be reported in next report. Finally, as the origin of the thickness dependence of the conductivity, some mechanisms may be considered. 1) Structural modulation or strain of the thin film. It may be presumed that one of the origins of the thickness dependence is attributed to the structural modulation or residual stress of the thin film, since the activation energy depends on the film thickness. That is to say, the structure of the thin film in the hetero-interface may be affected in the structure of the substrate (misfit effect). 2) Spatial distribution of the charge carrier density. The charge carrier density may be a function of the distance from the interface (space charge-effect). In order to show the thickness dependence shown in this report, the thickness of the spatial variation of the density of the charge carrier would be about 150 nm. Although it is difficult to be considered that the spacecharge region has such thickness, it can not be denied. There may be the spatial distribution of the density of lithium ion in some origins. For example, it may be generating the spatial distribution of the effective carrier density by the increase of the conduction path which originates from the structural modulation of the thin film. 4. Conclusions
Fig. 10. Temperature dependence of ionic conductivity for each layer. The straight lines are fitting result by Eq. (3).
(1) Amorphous lithium meta-silicate thin film was fabricated on the SiO2 glass by the PLD method. (2) The ionic conductivity of the amorphous thin film was 1 or 2 orders higher than that of the polycrystalline Li2SiO3. (3) The ionic conductivity of the amorphous thin film depends on the film thickness. (4) The thickness dependence of ionic conductivity of the amorphous thin film was qualitatively explained by simplified model which assumes 3 layers. (5) The thickness dependence of ionic conductivity in amorphous thin film would be due to the hetero-interface effect. Probably, the structure of the thin film in the heterointerface would be affected in the structure of the substrate, and the structural modulation may play an important role in the mechanism of the hetero-interface effect.
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Acknowledgements The present work was supported by the Grant-in-Aid for Scientific Research on Priority Area, “Nanoionics (439)” and Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] C.C. Liang, J. Electrochem. Soc. 120 (1973) 1289. [2] K. Shahi, J.B. Wagner Jr., J. Solid State Chem. 42 (1982) 107. [3] F.W. Poulsen, N.H. Andersen, B. Kindl, J. Schoonman, Soild State Ionics 9/10 (1983) 119. [4] S. Furusawa, S. Miyaoka, Y. Ishibashi, J. Phys. Soc. Jpn. 60 (1991) 1666. [5] M. Nagai, T. Nishino, Solid State Ionics 53–56 (1992) 63. [6] A. Bunde, Solid State Ionics 75 (1995) 147. [7] S. Furusawa, S. Miyaoka, Y. Ishibashi, J. Phys. Soc. Jpn. 62 (1993) 196. [8] T. Ida, K. Kimura, Solid State Ionics 107 (1998) 313.
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