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Crystallographic control of the barrier structure in zinc oxide varistors
International Journal of Inorganic Materials 3 (2001) 1117–1119
Crystallographic control of the barrier structure in zinc oxide varistors C. Leach* Manchester Materials Science Centre, University of Manchester and UMIST, Grosvenor Street, Manchester M1 7 HS, UK
Abstract Scanning electron microscope (SEM) based remote electron beam induced current (REBIC) microscopy has been used to investigate the electrical characteristics of individual grain boundaries in a zinc oxide based varistor. Although some grain boundaries showed the expected ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces are electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias to the grain boundary of several ten’s of millivolts is necessary to restore a symmetrical barrier structure. The orientations of grains on either side of each grain boundary were determined using electron backscattered pattern (EBSP) analysis and the grain boundary plane orientation was calculated by observing the lateral shift of the grain boundary EBIC contrast peak with SEM beam voltage and, hence, penetration depth. It was found that the electrical asymmetry is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the barrier structure. 2001 Elsevier Science Ltd. All rights reserved. Keywords: SEM; EBIC; Varistor; Grain boundary
1. Introduction The special properties of many electroceramics arise from the presence of charged grain boundary planes and the associated space–charge regions [1]. There is now growing evidence that crystallographic aspects of interfacial structure are important in determining details of the electrical behaviour of individual grain boundaries [2–4]. Such factors are particularly relevant in anisotropic crystal systems, such as hexagonal zinc oxide, where differing crystal faces presented on opposite sides of a grain boundary may lead to abrupt changes in the density of states, dielectric constant or band structure and affect the electrical behaviour. REBIC microscopy is a form of the conductive mode of operation of the SEM that is widely used to study electrically active grain boundaries in polycrystalline semiconductors and electroceramics [5–8]. Fig. 1 shows the experimental configuration. Two current collecting electrodes, positioned using a micromanipulator, form ohmic contacts on either side of the grain boundary of interest. Under primary beam irradiation, electron–hole pairs that are generated within the space–charge regions of the grain
*Tel.: 144-161-200-3561; fax: 144-161-200-3586. E-mail address: [email protected] (C. Leach).
boundary are swept up by the electric field, giving rise to an EBIC signal. In the case of a symmetrical, charged grain boundary the opposed fields on either side of the interface generate EBIC currents in opposite directions, and so the EBIC contrast appears bright–dark (termed type I in Ref. [6]). However, whilst such structures are commonly observed in cubic materials with electrically active grain boundaries, such as silicon [8], it is frequently the case with non-cubic crystal systems, such as zinc oxide, that the EBIC signal is suppressed on one side of the interface and only a single bright or single dark line is present (type II contrast [6]), suggesting that the local microstructure may affect details of the electrical behaviour. In this study the relationship between grain boundary structure and the observed EBIC contrast is explored.
Fig. 1. The sample configuration used for conductive mode microscopy.
1466-6049 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00109-X
1118
C. Leach / International Journal of Inorganic Materials 3 (2001) 1117 – 1119
2. Method Zinc oxide powder, doped with 1.0 wt.% bismuth oxide and 0.5 wt.% antimony oxide, was prepared by a mixed oxide route, compacted and sintered at 11008C for 2 h. One face of the sintered pellet was polished and the sample mounted onto an electrically isolated stub for conductive mode imaging in a Phillips 525 SEM using a beam energy of 15 keV and a beam current of 2 nA. Electron backscattered pattern (EBSP) analysis was carried out using a JEOL JSM 6300 fitted with a Nordif CCD camera and HKL Channel 1 software.
3. Results and discussion Fig. 2a is a zero bias REBIC image of a varistor grain boundary that shows characteristic bright–dark type I contrast. When a voltage bias is applied to the grain boundary the electrostatic barrier height is increased on one side of the grain boundary and decreased on the other, modifying the EBIC contrast. Applying 140 mV to the right hand electrode is sufficient to flatten the electronic bands on the left side of the interface. As a result the dark EBIC signal is lost and the grain boundary now shows
bright type II contrast (Fig. 2b). Similarly, applying a voltage bias of 240 mV results in the grain boundary showing dark type II contrast (Fig. 2c). Fig. 2d is of a grain boundary showing bright type II contrast under zero applied bias. If a bias of 290 mV is applied, the symmetrical barrier structure is restored and the grain boundary shows type I contrast (Fig. 2e). Fig. 2f is a zero bias REBIC image of a varistor grain boundary showing dark type II contrast. In this case a positive bias of 150 mV is necessary to restore the type I contrast. In order to establish whether these differences are due to the structure of the varistor grain boundaries, detailed crystallographic measurements were made. The orientations of the grains on either side of each grain boundary were established using EBSP analysis, and the orientation of the grain boundary plane calculated using depth resolved EBIC microscopy. This latter technique uses a range of SEM beam energies to excite the grain boundary progressively at different depths below the sample surface, noting any corresponding offset in the EBIC signal due to grain boundary slope [9]. The pole figures in Fig. 3 show the crystallographic orientations of the left and right hand grain boundary planes of each of the grain boundaries shown in Fig. 2. In the case of the grain boundary showing type I EBIC contrast both grain boundary planes lie
Fig. 2. EBIC images of varistor grain boundaries. (a–c) type I contrast with zero, 140 mV and 240 mV bias applied, (d–e) bright type II contrast with zero and 290 mV applied, (f–g) dark type II contrast with zero and 150 mV applied. (Scale bars510 mm).
C. Leach / International Journal of Inorganic Materials 3 (2001) 1117 – 1119
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Fig. 3. Pole figures showing the orientations of the crystal faces forming the right and left sides of the grain boundaries studied here with (a) type I, (b) bright type II and (c) dark type II (c) contrast.
midway between the c-axis and the basal plane (Fig. 3a). For the interface showing bright type II contrast, the orientation of the grain boundary plane on the left hand side of the interface lies closer to the c-axis than the right hand plane (Fig. 3b) whilst for the interface showing dark type II contrast the opposite is true (Fig. 3c). Thus there is a strong correlation between the orientation of the crystal planes presented on either side of the grain boundary and the observed EBIC contrast; similarly oriented planes give rise to type I contrast whilst grain boundary planes with significantly different orientations show bright or dark type II contrast according to which side of the interface the plane closest to the (00.1) pole lies.
4. Conclusions A relationship between the crystallography of a grain boundary and its electrical structure has been observed for the first time. An asymmetry in the EBIC signal was found
to be related to the orientations of the crystal faces presented on either side of the grain boundary. This behaviour is not taken into account in current grain boundary models for varistor performance.
References [1] Moulson AJ, Herbert JM. Electroceramics, London: Chapman and Hall, 1997. ¨ [2] Ernst F, Kienzle O, Ruhle M. J Eur Ceram Soc 1999;19:665. [3] Kataoka N, Hayashi K, Yamamoto T, Sugawara Y, Ikuhara Y, Sakuma T. J Am Ceram Soc 1998;81:1961. [4] Russell JD, Halls DC, Leach C. J Mater Sci 1997;32:4585. [5] Russell JD, Leach C. J Eur Ceram Soc 1995;15:617. [6] Russell JD, Halls DC, Leach C. Acta Mater 1996;44:2431. [7] Russell GJ, Robertson MJ, Vincent B, Woods J. J Mater Sci 1980;15:939. [8] Palm J, Alexander H. J Phys IV, Colloq C6, supplement to J Phys III 1991;1:101. [9] Leach C. Scripta Mater 2000;43:529–34.
Crystallographic control of the barrier structure in zinc oxide varistors C. Leach* Manchester Materials Science Centre, University of Manchester and UMIST, Grosvenor Street, Manchester M1 7 HS, UK
Abstract Scanning electron microscope (SEM) based remote electron beam induced current (REBIC) microscopy has been used to investigate the electrical characteristics of individual grain boundaries in a zinc oxide based varistor. Although some grain boundaries showed the expected ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces are electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias to the grain boundary of several ten’s of millivolts is necessary to restore a symmetrical barrier structure. The orientations of grains on either side of each grain boundary were determined using electron backscattered pattern (EBSP) analysis and the grain boundary plane orientation was calculated by observing the lateral shift of the grain boundary EBIC contrast peak with SEM beam voltage and, hence, penetration depth. It was found that the electrical asymmetry is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the barrier structure. 2001 Elsevier Science Ltd. All rights reserved. Keywords: SEM; EBIC; Varistor; Grain boundary
1. Introduction The special properties of many electroceramics arise from the presence of charged grain boundary planes and the associated space–charge regions [1]. There is now growing evidence that crystallographic aspects of interfacial structure are important in determining details of the electrical behaviour of individual grain boundaries [2–4]. Such factors are particularly relevant in anisotropic crystal systems, such as hexagonal zinc oxide, where differing crystal faces presented on opposite sides of a grain boundary may lead to abrupt changes in the density of states, dielectric constant or band structure and affect the electrical behaviour. REBIC microscopy is a form of the conductive mode of operation of the SEM that is widely used to study electrically active grain boundaries in polycrystalline semiconductors and electroceramics [5–8]. Fig. 1 shows the experimental configuration. Two current collecting electrodes, positioned using a micromanipulator, form ohmic contacts on either side of the grain boundary of interest. Under primary beam irradiation, electron–hole pairs that are generated within the space–charge regions of the grain
*Tel.: 144-161-200-3561; fax: 144-161-200-3586. E-mail address: [email protected] (C. Leach).
boundary are swept up by the electric field, giving rise to an EBIC signal. In the case of a symmetrical, charged grain boundary the opposed fields on either side of the interface generate EBIC currents in opposite directions, and so the EBIC contrast appears bright–dark (termed type I in Ref. [6]). However, whilst such structures are commonly observed in cubic materials with electrically active grain boundaries, such as silicon [8], it is frequently the case with non-cubic crystal systems, such as zinc oxide, that the EBIC signal is suppressed on one side of the interface and only a single bright or single dark line is present (type II contrast [6]), suggesting that the local microstructure may affect details of the electrical behaviour. In this study the relationship between grain boundary structure and the observed EBIC contrast is explored.
Fig. 1. The sample configuration used for conductive mode microscopy.
1466-6049 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00109-X
1118
C. Leach / International Journal of Inorganic Materials 3 (2001) 1117 – 1119
2. Method Zinc oxide powder, doped with 1.0 wt.% bismuth oxide and 0.5 wt.% antimony oxide, was prepared by a mixed oxide route, compacted and sintered at 11008C for 2 h. One face of the sintered pellet was polished and the sample mounted onto an electrically isolated stub for conductive mode imaging in a Phillips 525 SEM using a beam energy of 15 keV and a beam current of 2 nA. Electron backscattered pattern (EBSP) analysis was carried out using a JEOL JSM 6300 fitted with a Nordif CCD camera and HKL Channel 1 software.
3. Results and discussion Fig. 2a is a zero bias REBIC image of a varistor grain boundary that shows characteristic bright–dark type I contrast. When a voltage bias is applied to the grain boundary the electrostatic barrier height is increased on one side of the grain boundary and decreased on the other, modifying the EBIC contrast. Applying 140 mV to the right hand electrode is sufficient to flatten the electronic bands on the left side of the interface. As a result the dark EBIC signal is lost and the grain boundary now shows
bright type II contrast (Fig. 2b). Similarly, applying a voltage bias of 240 mV results in the grain boundary showing dark type II contrast (Fig. 2c). Fig. 2d is of a grain boundary showing bright type II contrast under zero applied bias. If a bias of 290 mV is applied, the symmetrical barrier structure is restored and the grain boundary shows type I contrast (Fig. 2e). Fig. 2f is a zero bias REBIC image of a varistor grain boundary showing dark type II contrast. In this case a positive bias of 150 mV is necessary to restore the type I contrast. In order to establish whether these differences are due to the structure of the varistor grain boundaries, detailed crystallographic measurements were made. The orientations of the grains on either side of each grain boundary were established using EBSP analysis, and the orientation of the grain boundary plane calculated using depth resolved EBIC microscopy. This latter technique uses a range of SEM beam energies to excite the grain boundary progressively at different depths below the sample surface, noting any corresponding offset in the EBIC signal due to grain boundary slope [9]. The pole figures in Fig. 3 show the crystallographic orientations of the left and right hand grain boundary planes of each of the grain boundaries shown in Fig. 2. In the case of the grain boundary showing type I EBIC contrast both grain boundary planes lie
Fig. 2. EBIC images of varistor grain boundaries. (a–c) type I contrast with zero, 140 mV and 240 mV bias applied, (d–e) bright type II contrast with zero and 290 mV applied, (f–g) dark type II contrast with zero and 150 mV applied. (Scale bars510 mm).
C. Leach / International Journal of Inorganic Materials 3 (2001) 1117 – 1119
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Fig. 3. Pole figures showing the orientations of the crystal faces forming the right and left sides of the grain boundaries studied here with (a) type I, (b) bright type II and (c) dark type II (c) contrast.
midway between the c-axis and the basal plane (Fig. 3a). For the interface showing bright type II contrast, the orientation of the grain boundary plane on the left hand side of the interface lies closer to the c-axis than the right hand plane (Fig. 3b) whilst for the interface showing dark type II contrast the opposite is true (Fig. 3c). Thus there is a strong correlation between the orientation of the crystal planes presented on either side of the grain boundary and the observed EBIC contrast; similarly oriented planes give rise to type I contrast whilst grain boundary planes with significantly different orientations show bright or dark type II contrast according to which side of the interface the plane closest to the (00.1) pole lies.
4. Conclusions A relationship between the crystallography of a grain boundary and its electrical structure has been observed for the first time. An asymmetry in the EBIC signal was found
to be related to the orientations of the crystal faces presented on either side of the grain boundary. This behaviour is not taken into account in current grain boundary models for varistor performance.
References [1] Moulson AJ, Herbert JM. Electroceramics, London: Chapman and Hall, 1997. ¨ [2] Ernst F, Kienzle O, Ruhle M. J Eur Ceram Soc 1999;19:665. [3] Kataoka N, Hayashi K, Yamamoto T, Sugawara Y, Ikuhara Y, Sakuma T. J Am Ceram Soc 1998;81:1961. [4] Russell JD, Halls DC, Leach C. J Mater Sci 1997;32:4585. [5] Russell JD, Leach C. J Eur Ceram Soc 1995;15:617. [6] Russell JD, Halls DC, Leach C. Acta Mater 1996;44:2431. [7] Russell GJ, Robertson MJ, Vincent B, Woods J. J Mater Sci 1980;15:939. [8] Palm J, Alexander H. J Phys IV, Colloq C6, supplement to J Phys III 1991;1:101. [9] Leach C. Scripta Mater 2000;43:529–34.