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Correlation between microstructure, particle size, dielectric constant, and electrical resistivity of nano-size amorphous SiO2 powder
NanoStructured Materials, Vol. 11, No. 8, pp. 1081–1089, 1999 Elsevier Science Ltd Copyright © 2000 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter
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PII S0965-9773(99)00398-0
CORRELATION BETWEEN MICROSTRUCTURE, PARTICLE SIZE, DIELECTRIC CONSTANT, AND ELECTRICAL RESISTIVITY OF NANO-SIZE AMORPHOUS SiO2 POWDER T. Tepper and S. Berger Department of Materials Engineering, Technion, Haifa 32000 Israel (Received April 27, 1999) (Accepted June 9, 1999) Abstract—Pure amorphous SiO2 powder with nanometer size particles was exposed to various heat treatments up to 1200°C. The microstructure, particle size, dielectric constant and electrical resistivity of the powder were characterized after each heat treatment. It was found that the dielectric constant of the powder is higher compared to that of amorphous SiO2 thin films. This enhancement is correlated with higher density of Si dangling bonds, which contribute to the polarization of the material. A major decrease in the dielectric constant takes place during heating up to 600°C where neither growth nor crystallization of the particles occur but only pronounced reduction in the density of the Si dangling bonds is observed. Pronounced growth and initial crystallization to a cristobalite phase of the powder particles occur at about 1100°C and have a minor effect on the dielectric constant. The Si dangling bonds also serve as electrical conducting centers in the powder and their annihilation due to the heat treatments is well observed as an increase in the electrical resistivity of the powder. ©2000 Acta Metallurgica Inc.
Introduction Nanocrystalline materials are characterized by a high density of interfaces between particles that may reach 50% of the total volume of the material. These interfaces contain point defects such as dangling bonds that can function as space charge traps and may induce interface polarization. Enhancement of the dielectric constant is reported in many nanophase materials such as in Al2O3 [1], Si [1], TiO2 [1–2], Si3N4 [3– 6], Fe2O3 [7], and ZnO [8]. For example [6], the dielectric constant of amorphous silicon nitride material made of nanometer size particles (NASN) is higher than that of micron size particles (380 compared to 8 at 10Hz). ESR measurements show that the density of dangling bonds in the NASN is higher by 3 orders of magnitude compared to micron size particles. The contribution of the dangling bonds to the dielectric constant is demonstrated in NASN specimens prepared at different compaction pressures. Larger compaction pressures enhance the dielectric constant. It is claimed [6] that the increase in compaction pressure increases the density of dangling bonds due to distortion of the interfaces between the particles. In this paper we report on the contribution of Si dangling bonds, growth and crystallization of nano-size amorphous SiO2 powder particles to their dielectric constant and electrical resistivity. The importance of the study is not only in enriching the knowledge on these powders but also in advancing the use of them as enhanced dielectric media in capacitors made of nano-composites. Experimental Pure amorphous SiO2 powder with an average particle size of 20nm was purchased from Aldrich Chemical Company, Inc. Green compacts with a disc shape (10mm in diameter and 1–2.5mm in 1081
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Figure 1. The effect of compaction pressure on relative density of the SiO2 green compacts.
thickness) were prepared by uni-axial compression of the powders under pressures of 100 – 400MPa using an Instron 1195. The green compacts were heat treated at temperatures between 200°C and 1200°C for 1 hour under vacuum of about 10⫺6 Torr. Prior to and after the heat treatments, the densities of the specimens were measured by the Archimedes method. The relative density was determined by comparing the measured density with that of a cristobalite phase. The shrinkage of the green compacts during heat treatments up to 1500°C in vacuum of about 10⫺3 Torr was studied in a dilatometer. The microstructure and composition of the specimens were studied by using XRD and TEM combined with EDS. The XRD measurements were done with a PW1830 Philips diffractometer operated at 40kV and 40mA. Each scan was done between 5° and 95° with a step size of 0.01° and scan rate of 0.18°/min. TEM study was done with a JEOL 2000fx analytical instrument operated at 200kV. TEM specimens were prepared by grinding the compacts into powder, mixing the powder with ethanol and finally dipping amorphous carbon coated grids in the suspension, thus forming a thin coating of the powder particles on the grids. ESR measurements were performed using a Bruker x-band instrument operated
Figure 2. Relative densities of SiO2 specimens heat-treated in a dilatometer and maintained for 3 hours at various temperatures.
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Figure 3. Dilatometric profiles of the SiO2 powders. (a) Relative shrinkage vs. temperature. (b) Shrinkage rate vs. temperature.
at a frequency of 9.766GHz and power of 100.6mW. Specimens for the ESR measurements were prepared by grinding the green compacts into powder prior to and after the heat treatments and then inserting the powder into quartz tubes. Electrical capacitance and resistivity measurements were performed on the green compacts before and after the heat treatments. The measurements were done with a HIOKI 3531 instrument operated under ac conditions in the frequency range between 100Hz and 1MHz and applied voltages of 0 –5 Volts. The measured specimens consist of disc-shaped compacts with a diameter of about 10mm and a thickness of about 2mm. On both faces of each specimen, circular contacts of Al film (8mm diameter and 100nm thickness) were made by evaporation from a resistive boat through a mask. Copper wires were connected to the Al contacts by a silver paste. Finally, the specimens with the copper wires were encapsulated by dipping them into a bath of low-density polyethylene.
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Figure 4. XRD spectra of the SiO2 powders. (a) As-received. (b) After 3 hours at 1300°C. (c) After 3 hours at 1500°C.
Results Archimedes measurements of the green compacts show an increase in density with an increase of the compaction pressure from 100MPa to 300MPa (Fig. 1). At pressures higher than 300MPa the density is constant and cracks appear on the surface of the specimens. A pressure of 250MPa was selected as the preparation pressure of the studied specimens in order to prevent formation of cracks. The density of the green compacts prepared at 250MPa sharply increases by a factor of 2 with increasing temperature from 1100°C to 1300°C (Fig. 2). Dilatometry measurements show that the green compacts start to shrink above 900°C, and the shrinkage increases with increasing temperature (Fig. 3.a). The shrinkage profile is characterized by two major shrinkage rate peaks (Fig. 3.b) which are centered at 1230°C and 1450°C, respectively. XRD measurements of the as-received powder (Fig. 4.a) show only broad peaks that correspond to amorphous SiO2. After the heat treatment at 1300°C for 3 hours broad and narrow peaks are observed in the same spectra (Fig. 4.b) which correspond to the amorphous and crystalline cristobalite phases respectively. After the heat treatment at 1500°C for 3 hours only narrow peaks of the cristobalite phase are observed (Fig. 4.c). TEM study of the as-received powder shows amorphous spherical particles with an average size of about 20nm (Fig. 5.a). The particles remain amorphous with the same average diameter after the heat treatment at 950°C for 1 hour. Only few particles grow and change their shape from spheres to needles due to this heat treatment. Pronounced particle growth up to 2 orders of magnitude and initial crystallization of the amorphous phase to a cristobalite phase were observed after the heat treatment at 1100°C for 1 hour (Fig. 5.b). Sub-micron crystals of the cristobalite phase are obtained after 3 hours of annealing at 1500°C (Fig. 5.c) and in few of them defects such as twins are observed. ESR measurements of the as-received powder show peaks at several magnetic fields such as at 3240G and 3620G (Fig. 6). The lande` factor, g, of these peaks is 2.156 and 1.929, respectively. The intensity of the peaks decreases with increasing the heat treatment temperature and are hardly noticeable after 600°C. The dielectric constant of the green compacts (Fig. 7) decreases with increasing frequency from 100Hz to about 60KHz and remains constant at higher frequencies up to 1MHz. At all frequencies the dielectric constant increases with increasing the compaction pressure of the specimens (Fig. 7). The effect of the heat treatments on the dielectric constant can be divided into three temperature ranges (Fig. 8): pronounced decrease up to 600°C, negligible change between 600°C and 900°C,
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Figure 5. TEM bright field images of the SiO2 powders. (a) As-received powder. (b) Mixture of small crystalline cristobalite particles and sub-micron amorphous particles after 1 hour at 1100°C. (c) Sub-micron crystalline cristobalite particles after 3 hours at 1500°C.
and minor decrease between 900°C and 1100°C. The electrical resistivity of the green compacts is in the range of 105–108 (⍀cm). It decreases by either increasing the frequency from 100Hz to 1MHz or by increasing the compaction pressure (Fig. 9). The effect of the heat treatment
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Figure 6. ESR spectra obtained from as-received and heat treated SiO2 green compacts at temperatures between 200°C and 1100°C for 1 hour.
temperature on the electrical resistivity of the SiO2 green compacts can be divided to two regions (Fig. 10). In the first region the electrical resistivity increases with increasing temperature up to 600°C. In the second region the electrical resistivity remains almost constant at temperatures between 600°C and 1200°C. Discussion The effect of the heat treatment temperature on densification of the green compacts can be explained on the basis of the experimental data obtained by the dilatometry, TEM, and XRD characterization techniques. The density of the green compacts starts to increase at about 950°C due to initial growth of the amorphous SiO2 particles. A sharp increase in density and shrinkage rate is observed at temperatures between 1150°C and 1250°C where pronounced growth of the amorphous particles occurs. Above 1250°C the moderate increase in density and the shrinkage rate peak centered at 1450°C can be attributed to crystallization of the amorphous particles to a cristobalite phase. Crystallization of nanometer size amorphous SiO2 powder to a cristobalite phase
Figure 7. The dielectric constant of the as-received SiO2 green compacts vs. ac applied field frequency at different compaction pressures.
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Figure 8. The dielectric constant at 1kHz of the SiO2 green compacts after the heat treatments for 1 hour at temperatures up to 1100°C.
at about 950°C was previously reported by Gunter et. al. [9]. The ESR measurements show peaks at various magnetic fields such as at 3240G and 3620G. These ESR peaks can be attributed to Si dangling bonds [10 –14]. The ESR peaks presented in Fig.6 are the first derivative of the absorption lines. The area under the integral of these lines is proportional to the total number of Si dangling bonds with unpaired spins. The intensity of the peaks decreases with increasing the heat treatment temperature and the peaks are not observed after 800°C for 1 hour. This result indicates on annihilation of the Si dangling bonds with increasing temperature up to 800°C. The annihilation process takes place at temperatures where neither growth nor crystallization of the SiO2 powder particles are observed. In addition, there are no indications from the TEM study on distorted surfaces or interfaces of the particles due to the heat treatments up to 800°C that may change the density of the Si dangling bonds. Up to 950°C the particles preserve their spherical shape. The dielectric constant of the green compacts is higher than that of a thermally grown continuous
Figure 9. The electrical resistivity of the as-received SiO2 green compacts vs. ac applied field frequency at different compaction pressures.
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Figure 10. The electrical resistivity at 1kHz of the SiO2 green compacts after the heat treatments for 1 hour at temperatures up to 1100°C.
amorphous SiO2 films (3.9) [15]. This comparison indicates that the green compacts contain more defects either inside or on the surface of the amorphous SiO2 particles that contribute to the polarization of the material. The effect of the heat treatment temperature on the dielectric constant and electrical resistivity is well correlated with the changes in the density of the Si dangling bonds and in the size of the amorphous SiO2 particles. The major decrease in the dielectric constant and increase in the electrical resistivity occur during heating up to 600°C. In this temperature range the ESR results show that the density of the Si dangling bonds in the amorphous SiO2 particles decreases with increasing temperature. These dangling bonds contribute to the polarization of the material [16] and also serve as electrical conducting centers. Above 600°C and up to 900°C the dielectric constant and the electrical resistivity remain constant, which can be attributed to the result that no changes are observed in the density of the Si dangling bonds, in the microstructure and in the size of the SiO2 particles. Above 900°C minor decrease in the dielectric constant and increase in the electrical resistivity occur which can be attributed to the growth of the amorphous SiO2 particles. The particle growth is associated with a decrease in density of interfaces between the SiO2 particles and consequently results in a decrease in the density of interface states. These interface states can contribute to interface polarization and also can serve as electrical conducting centers. The effect of the compaction pressure on the dielectric constant and electrical resistivity can be correlated with the change in the relative density of the green compacts. The increase in the compaction pressure from 100MPa to 400MPa increases the relative density of the green compacts by about 8%. Consequently, more interfaces between the SiO2 particles are formed. These interfaces contain point defects that contribute to the polarization and electrical conductivity of the material. The dielectric constant decreases with increasing frequency from 100Hz to about 100kHz and remains almost constant at frequencies between 100kHz and 1MHz. This behavior can be understood from the Debye equations [17] or from other models that refer to various sources for polarization with multiple relaxation times [18]. The electrical resistivity decreases with increasing frequency. This behavior characterizes electrical conduction in non-crystalline phases, which is based on hopping of charge carriers through structural defects that serve as conducting centers near the Fermi level [18]. At about 100kHz there is a change in the slope of the curve of the resistivity vs. frequency which is expected according to the theory [19].
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Summary The dielectric constant of amorphous SiO2 powder with nanometer size particles is higher compared to amorphous SiO2 thin films. This enhancement is attributed to higher density of Si dangling bonds. The major decrease in the dielectric constant takes place during heating up to 600°C where neither growth nor crystallization of the powder particles are observed but only pronounced reduction in the density of the Si dangling bonds is measured. Minor decrease in the dielectric constant is observed after pronounced growth and initial crystallization to a cristobalite phase of the powder particles at 1100°C. The Si dangling bonds also serve as electrical conducting centers and their annihilation due to the heat treatments is well observed as an increase in the electrical resistivity of the powder. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
C. M. Mo, L. Zhang, and G. Wang, Nanostruct. Mater. 6, 823 (1995). D. Zhang, H. F. Zhang, G. Z. Wang, C. M. Mo, and Y. Zhang, Phys. Stat. Sol. A. 157, 483 (1996). T. Wang, L. Zhang, and J. Mou, Chin. Phys. Lett. 10, 676 (1993). L. Zhang, C.M. Mo, T. Wang, and C. Xie, Mater. Res. Soc. Symp. Proc. 286, 107 (1993). L. Zhang, C. Mo, T. Wang, S. Cai, and C. Xie, Phys. Stat. Sol. A. 136, 291 (1993). T. Wang and L. Zhang, Chin. Sci. Bull. 39, 1511 (1994). H. Fang, B. Chen, K. Jiang, J. Sha, Z. Jiao, Q. Zhang, and L. Zhang, Phys. Stat. Sol. B. 192 (1995). J. Lee, J. H. Hwang, J. J. Mashek, T. O. Mason, A. E. Miller, and R. W. Siegel, J. Mater. Res. 10, 2295 (1995). B. Gunther and A. Kumpmann, Nanostruct. Mater. 1, 27 (1992). M. Stapelbroek, D. L. Griscom, E. J. Friebele, and G. H. Sigel, Jr., J. Non-Crystall. Sol. 32, 313 (1979). T. Inokuma, L. He, Y. Kurata, and S. Hasehawa, J. Electrochem. Soc. 142, 2346 (1995). H. Nishikawa, R. Nakamura, Y. Ohki, and Y. Hama, Phys. Rev. B. 48, 15584 (1993). W. L. Warren, E. H. Poindexter, M. Offenberg, and W. Muller-Warmuth, J. Electrochem. Soc. 139, 872 (1992). D. L. Griscom, E. J. Friebele, and G. H. Sigle, Jr., Sol. Stat. Commun. 15, 479 (1974). S. M. Sze, Physics of Semiconductor Devices, 2nd edn., Chap. 7, John Wiley & Sons, New YOrk (1981). K. M. Chang, S. W. Wang, T. H. Yeh, C. H. Li, and J.J. Luo, J. Electrochem. Soc. 144, 1754 (1997). L. L. Hench and J. K. West, Principles of Electronic Ceramics“ John Wiley & Sons, New York (1990). R. W. Sillars, Proc. Inst. Elect. Eng. 80, 378 (1937). N. F. Mott and E. A. Davis, Electronic Processes In Non-Crystalline Materials, Chap. 3, pp. 223–239, Clarendon Press, Oxford, UK (1979).
Pergamon
PII S0965-9773(99)00398-0
CORRELATION BETWEEN MICROSTRUCTURE, PARTICLE SIZE, DIELECTRIC CONSTANT, AND ELECTRICAL RESISTIVITY OF NANO-SIZE AMORPHOUS SiO2 POWDER T. Tepper and S. Berger Department of Materials Engineering, Technion, Haifa 32000 Israel (Received April 27, 1999) (Accepted June 9, 1999) Abstract—Pure amorphous SiO2 powder with nanometer size particles was exposed to various heat treatments up to 1200°C. The microstructure, particle size, dielectric constant and electrical resistivity of the powder were characterized after each heat treatment. It was found that the dielectric constant of the powder is higher compared to that of amorphous SiO2 thin films. This enhancement is correlated with higher density of Si dangling bonds, which contribute to the polarization of the material. A major decrease in the dielectric constant takes place during heating up to 600°C where neither growth nor crystallization of the particles occur but only pronounced reduction in the density of the Si dangling bonds is observed. Pronounced growth and initial crystallization to a cristobalite phase of the powder particles occur at about 1100°C and have a minor effect on the dielectric constant. The Si dangling bonds also serve as electrical conducting centers in the powder and their annihilation due to the heat treatments is well observed as an increase in the electrical resistivity of the powder. ©2000 Acta Metallurgica Inc.
Introduction Nanocrystalline materials are characterized by a high density of interfaces between particles that may reach 50% of the total volume of the material. These interfaces contain point defects such as dangling bonds that can function as space charge traps and may induce interface polarization. Enhancement of the dielectric constant is reported in many nanophase materials such as in Al2O3 [1], Si [1], TiO2 [1–2], Si3N4 [3– 6], Fe2O3 [7], and ZnO [8]. For example [6], the dielectric constant of amorphous silicon nitride material made of nanometer size particles (NASN) is higher than that of micron size particles (380 compared to 8 at 10Hz). ESR measurements show that the density of dangling bonds in the NASN is higher by 3 orders of magnitude compared to micron size particles. The contribution of the dangling bonds to the dielectric constant is demonstrated in NASN specimens prepared at different compaction pressures. Larger compaction pressures enhance the dielectric constant. It is claimed [6] that the increase in compaction pressure increases the density of dangling bonds due to distortion of the interfaces between the particles. In this paper we report on the contribution of Si dangling bonds, growth and crystallization of nano-size amorphous SiO2 powder particles to their dielectric constant and electrical resistivity. The importance of the study is not only in enriching the knowledge on these powders but also in advancing the use of them as enhanced dielectric media in capacitors made of nano-composites. Experimental Pure amorphous SiO2 powder with an average particle size of 20nm was purchased from Aldrich Chemical Company, Inc. Green compacts with a disc shape (10mm in diameter and 1–2.5mm in 1081
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Figure 1. The effect of compaction pressure on relative density of the SiO2 green compacts.
thickness) were prepared by uni-axial compression of the powders under pressures of 100 – 400MPa using an Instron 1195. The green compacts were heat treated at temperatures between 200°C and 1200°C for 1 hour under vacuum of about 10⫺6 Torr. Prior to and after the heat treatments, the densities of the specimens were measured by the Archimedes method. The relative density was determined by comparing the measured density with that of a cristobalite phase. The shrinkage of the green compacts during heat treatments up to 1500°C in vacuum of about 10⫺3 Torr was studied in a dilatometer. The microstructure and composition of the specimens were studied by using XRD and TEM combined with EDS. The XRD measurements were done with a PW1830 Philips diffractometer operated at 40kV and 40mA. Each scan was done between 5° and 95° with a step size of 0.01° and scan rate of 0.18°/min. TEM study was done with a JEOL 2000fx analytical instrument operated at 200kV. TEM specimens were prepared by grinding the compacts into powder, mixing the powder with ethanol and finally dipping amorphous carbon coated grids in the suspension, thus forming a thin coating of the powder particles on the grids. ESR measurements were performed using a Bruker x-band instrument operated
Figure 2. Relative densities of SiO2 specimens heat-treated in a dilatometer and maintained for 3 hours at various temperatures.
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Figure 3. Dilatometric profiles of the SiO2 powders. (a) Relative shrinkage vs. temperature. (b) Shrinkage rate vs. temperature.
at a frequency of 9.766GHz and power of 100.6mW. Specimens for the ESR measurements were prepared by grinding the green compacts into powder prior to and after the heat treatments and then inserting the powder into quartz tubes. Electrical capacitance and resistivity measurements were performed on the green compacts before and after the heat treatments. The measurements were done with a HIOKI 3531 instrument operated under ac conditions in the frequency range between 100Hz and 1MHz and applied voltages of 0 –5 Volts. The measured specimens consist of disc-shaped compacts with a diameter of about 10mm and a thickness of about 2mm. On both faces of each specimen, circular contacts of Al film (8mm diameter and 100nm thickness) were made by evaporation from a resistive boat through a mask. Copper wires were connected to the Al contacts by a silver paste. Finally, the specimens with the copper wires were encapsulated by dipping them into a bath of low-density polyethylene.
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Figure 4. XRD spectra of the SiO2 powders. (a) As-received. (b) After 3 hours at 1300°C. (c) After 3 hours at 1500°C.
Results Archimedes measurements of the green compacts show an increase in density with an increase of the compaction pressure from 100MPa to 300MPa (Fig. 1). At pressures higher than 300MPa the density is constant and cracks appear on the surface of the specimens. A pressure of 250MPa was selected as the preparation pressure of the studied specimens in order to prevent formation of cracks. The density of the green compacts prepared at 250MPa sharply increases by a factor of 2 with increasing temperature from 1100°C to 1300°C (Fig. 2). Dilatometry measurements show that the green compacts start to shrink above 900°C, and the shrinkage increases with increasing temperature (Fig. 3.a). The shrinkage profile is characterized by two major shrinkage rate peaks (Fig. 3.b) which are centered at 1230°C and 1450°C, respectively. XRD measurements of the as-received powder (Fig. 4.a) show only broad peaks that correspond to amorphous SiO2. After the heat treatment at 1300°C for 3 hours broad and narrow peaks are observed in the same spectra (Fig. 4.b) which correspond to the amorphous and crystalline cristobalite phases respectively. After the heat treatment at 1500°C for 3 hours only narrow peaks of the cristobalite phase are observed (Fig. 4.c). TEM study of the as-received powder shows amorphous spherical particles with an average size of about 20nm (Fig. 5.a). The particles remain amorphous with the same average diameter after the heat treatment at 950°C for 1 hour. Only few particles grow and change their shape from spheres to needles due to this heat treatment. Pronounced particle growth up to 2 orders of magnitude and initial crystallization of the amorphous phase to a cristobalite phase were observed after the heat treatment at 1100°C for 1 hour (Fig. 5.b). Sub-micron crystals of the cristobalite phase are obtained after 3 hours of annealing at 1500°C (Fig. 5.c) and in few of them defects such as twins are observed. ESR measurements of the as-received powder show peaks at several magnetic fields such as at 3240G and 3620G (Fig. 6). The lande` factor, g, of these peaks is 2.156 and 1.929, respectively. The intensity of the peaks decreases with increasing the heat treatment temperature and are hardly noticeable after 600°C. The dielectric constant of the green compacts (Fig. 7) decreases with increasing frequency from 100Hz to about 60KHz and remains constant at higher frequencies up to 1MHz. At all frequencies the dielectric constant increases with increasing the compaction pressure of the specimens (Fig. 7). The effect of the heat treatments on the dielectric constant can be divided into three temperature ranges (Fig. 8): pronounced decrease up to 600°C, negligible change between 600°C and 900°C,
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Figure 5. TEM bright field images of the SiO2 powders. (a) As-received powder. (b) Mixture of small crystalline cristobalite particles and sub-micron amorphous particles after 1 hour at 1100°C. (c) Sub-micron crystalline cristobalite particles after 3 hours at 1500°C.
and minor decrease between 900°C and 1100°C. The electrical resistivity of the green compacts is in the range of 105–108 (⍀cm). It decreases by either increasing the frequency from 100Hz to 1MHz or by increasing the compaction pressure (Fig. 9). The effect of the heat treatment
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Figure 6. ESR spectra obtained from as-received and heat treated SiO2 green compacts at temperatures between 200°C and 1100°C for 1 hour.
temperature on the electrical resistivity of the SiO2 green compacts can be divided to two regions (Fig. 10). In the first region the electrical resistivity increases with increasing temperature up to 600°C. In the second region the electrical resistivity remains almost constant at temperatures between 600°C and 1200°C. Discussion The effect of the heat treatment temperature on densification of the green compacts can be explained on the basis of the experimental data obtained by the dilatometry, TEM, and XRD characterization techniques. The density of the green compacts starts to increase at about 950°C due to initial growth of the amorphous SiO2 particles. A sharp increase in density and shrinkage rate is observed at temperatures between 1150°C and 1250°C where pronounced growth of the amorphous particles occurs. Above 1250°C the moderate increase in density and the shrinkage rate peak centered at 1450°C can be attributed to crystallization of the amorphous particles to a cristobalite phase. Crystallization of nanometer size amorphous SiO2 powder to a cristobalite phase
Figure 7. The dielectric constant of the as-received SiO2 green compacts vs. ac applied field frequency at different compaction pressures.
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Figure 8. The dielectric constant at 1kHz of the SiO2 green compacts after the heat treatments for 1 hour at temperatures up to 1100°C.
at about 950°C was previously reported by Gunter et. al. [9]. The ESR measurements show peaks at various magnetic fields such as at 3240G and 3620G. These ESR peaks can be attributed to Si dangling bonds [10 –14]. The ESR peaks presented in Fig.6 are the first derivative of the absorption lines. The area under the integral of these lines is proportional to the total number of Si dangling bonds with unpaired spins. The intensity of the peaks decreases with increasing the heat treatment temperature and the peaks are not observed after 800°C for 1 hour. This result indicates on annihilation of the Si dangling bonds with increasing temperature up to 800°C. The annihilation process takes place at temperatures where neither growth nor crystallization of the SiO2 powder particles are observed. In addition, there are no indications from the TEM study on distorted surfaces or interfaces of the particles due to the heat treatments up to 800°C that may change the density of the Si dangling bonds. Up to 950°C the particles preserve their spherical shape. The dielectric constant of the green compacts is higher than that of a thermally grown continuous
Figure 9. The electrical resistivity of the as-received SiO2 green compacts vs. ac applied field frequency at different compaction pressures.
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Figure 10. The electrical resistivity at 1kHz of the SiO2 green compacts after the heat treatments for 1 hour at temperatures up to 1100°C.
amorphous SiO2 films (3.9) [15]. This comparison indicates that the green compacts contain more defects either inside or on the surface of the amorphous SiO2 particles that contribute to the polarization of the material. The effect of the heat treatment temperature on the dielectric constant and electrical resistivity is well correlated with the changes in the density of the Si dangling bonds and in the size of the amorphous SiO2 particles. The major decrease in the dielectric constant and increase in the electrical resistivity occur during heating up to 600°C. In this temperature range the ESR results show that the density of the Si dangling bonds in the amorphous SiO2 particles decreases with increasing temperature. These dangling bonds contribute to the polarization of the material [16] and also serve as electrical conducting centers. Above 600°C and up to 900°C the dielectric constant and the electrical resistivity remain constant, which can be attributed to the result that no changes are observed in the density of the Si dangling bonds, in the microstructure and in the size of the SiO2 particles. Above 900°C minor decrease in the dielectric constant and increase in the electrical resistivity occur which can be attributed to the growth of the amorphous SiO2 particles. The particle growth is associated with a decrease in density of interfaces between the SiO2 particles and consequently results in a decrease in the density of interface states. These interface states can contribute to interface polarization and also can serve as electrical conducting centers. The effect of the compaction pressure on the dielectric constant and electrical resistivity can be correlated with the change in the relative density of the green compacts. The increase in the compaction pressure from 100MPa to 400MPa increases the relative density of the green compacts by about 8%. Consequently, more interfaces between the SiO2 particles are formed. These interfaces contain point defects that contribute to the polarization and electrical conductivity of the material. The dielectric constant decreases with increasing frequency from 100Hz to about 100kHz and remains almost constant at frequencies between 100kHz and 1MHz. This behavior can be understood from the Debye equations [17] or from other models that refer to various sources for polarization with multiple relaxation times [18]. The electrical resistivity decreases with increasing frequency. This behavior characterizes electrical conduction in non-crystalline phases, which is based on hopping of charge carriers through structural defects that serve as conducting centers near the Fermi level [18]. At about 100kHz there is a change in the slope of the curve of the resistivity vs. frequency which is expected according to the theory [19].
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Summary The dielectric constant of amorphous SiO2 powder with nanometer size particles is higher compared to amorphous SiO2 thin films. This enhancement is attributed to higher density of Si dangling bonds. The major decrease in the dielectric constant takes place during heating up to 600°C where neither growth nor crystallization of the powder particles are observed but only pronounced reduction in the density of the Si dangling bonds is measured. Minor decrease in the dielectric constant is observed after pronounced growth and initial crystallization to a cristobalite phase of the powder particles at 1100°C. The Si dangling bonds also serve as electrical conducting centers and their annihilation due to the heat treatments is well observed as an increase in the electrical resistivity of the powder. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
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