Barrier voltage deformation of ZnO varistors by current pulse E. R. Leite, J. A. Varela, and E. Longo Citation: Journal of Applied Physics 72, 147 (1992); doi: 10.1063/1.352175 View online: http://dx.doi.org/10.1063/1.352175 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/72/1?ver=pdfcov Published by the AIP Publishing [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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Quimica, UFSCar, S Carlos, ,!X P., Brasil (Received 30 September 1991; accepted for publication 16 March 1992) The phenomenon of electrical degradation in ZnO varistors was studied by application of high-intensity current pulses. A wave shape of 8X20 ,US and rectangular waves of 1 and 2 ms were used. The degradation was estimated by reference electric-field variation and by Schottky voltage barrier deformation. The results showed that current pulses reduce both the height and the width of the barrier voltage. It was also observed that the donor density Nd did not change but the surface states density N, decreased with degradation. 1. INTRODUCTION II. EXPERIMENTAL PROCEDURE ZnO varistors are known for having a nonohmic be- havior, low leakage current, and high-energy-absorption capacity. These characteristics allow these materials to be used as surge arresters; however, they can be degraded during use. A typical ZnO varistor microstructure includes semiconducting ZnO grains, BizOs-rich intergranular phases, and Zn,Sb,O,, spine1 phases. The barrier voltage is formed among several types of grain boundary where the most effective are the ZnO-ZnO homojunctions and B&O3 thin layer between ZnO grains. ’ A coherent model to describe the conduction mecha- nism was proposed by Eda’ in which Schottky-type barrier voltages are separated by an isolating film of finite width. The conduction mechanism through Schottky barriers could be due to thermionic emission from the ZnO grain conduction band to the isolating film conduction band. According to this model the current density J and electric field E are related by ZnO varistor samples of 22 mm height and 45 mm diameter were obtained by conventional processing, and their compositions are given in Table I. A current pulse generator (Haefely model E) of 50 kJ for a short width of 20 ps, and rising slope equal to 8 ps, i.e:, an impulse of 8 X20 ps, and a rectangular current pulse Haefely generator for a long-duration shot of 1 ms and amplitude of 75 A (1 ms/75 A) and 2 ms and ampli- tude of 150 A (2 ms/150 A), as shown schematically in Fig. 1, were used in this work. The degradation was esti- mated by measuring the reference electric field at 0.05 mA/cm2 (Eo.os) as a function of the number of applied pulses and of the pulses energy. The Eo.os was measured before and after each pulse cycle in dc bias under forward biasing voltage. A cycle was defined by five applied pulses and between each cycle enough time was allowed to cool down the sample to room temperature. J=JO exp[ - (+-pE’“)/kT], (1) where Jo is a constant, 4 the barrier voltage height (eV>, /3 a constant related to the barrier voltage width W, k is the Boltzman constant, and T is the absolute temperature WI. To verify the effect of current pulses on the barrier voltage, 4 and W were determined before and after four cycles. To determine these parameters it was assumed that: (a) the barrier voltage is of Schottky type separated by an isolating film; (b) the conduction mechanism is by thermi- onic emission. The barrier voltage of ZnO varistors can be electri- cally, chemically, and thermally degraded during use, lead- ing to the reduction of I$ and consequently to the increase of leakage current, which could be catastrophic for surge arresters. The degradation of these barriers has been exten- sively studied3-5 but the effect of high-intensity current pulses on the degradation is not well known. Researches in degradation due to pulses&” do not explain the whole phe- nomenon that is of fundamental importance for ZnO varis- tor technology. In the thermionic emission model, Eq. ( 1 ), the /3 con- stant is given by p= [ ( l/n W) ( 2e3/4reoer) ] *‘2, (2) where n is the number of ZnO grains in series in a sample of width L (cm), W is the barrier voltage width (nm), e is the electron charge, and e. and E, are the electric permit- tivities of the vacuum and material, respectively. Defining G as the, average ZnO grain sizes, This work aims to study the effects of several current pulses with different intensities and shapes on the degrada- tion of ZnO varistors. Barrier voltage height C$ and width W variation of ZnO varistor were analyzed by thermal activation. TABLE I. Composition of ZnO varistors in mol %. ZnO B&O, coo MnO, Sb20, 97.0 1.0 0.50 0.50 1.5 Cr203 0.50 147 J. Appl. Phys. 72 (l), 1 July 1992 0021-8979/92/i 30147-04$04.00 @ 1992 American Institute of Physics 147 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.225 On: Tue, 14 Jan 2014 13:14:14 C i -__ QSi --_ ? = C&&i _ -_____ E b rihq 0.1 i w-8P L Widiw0p* f m i= rrahm cmm+ FIG. 1. Schematic representation of current pulses used in this work. n=L/G, (3) G values were determined by scanning electron microscopy (SEM) using the line intercept method, obtaining 13 A4 pm. A dispersive energy unit (EDS) attached to the SEM was used to analyze the distribution of dopants in the ZnO varistors microstructure. By plotting In J vs E”2 in Bq. ( 1) a straight line is obtained where the slope is P/kT. Therefore the barrier voltage width W can be determined by Bq. (2). The plot of In J vs l/T of Bq. ( 1) is a straight line with a slope equal to (4 - /?E1’2)/k. By using /3 values, 4 is determined. Both 4~and W values were carried out in a range of tem- perature from 60 to 100 “C! in a dc biasing test. III. RESULTS AND DISCUSSIONS A. Variation of reference electric field (E,& The variation of Eo.os with the number of 500 A/cm2 applied pulses of 8 X 20 ps is shown in Fig. 2. It is observed in this figure that there is a gradual reduction of Eo,os indicating that the leakage current is increasing.” The dis- charge voltage at 500 A/cm2 (Esoo) for several applied pulses is shown in Table II. It is observed in this table that there is a little variation of Esoo after four cycles of applied pulses (about 2%). By analyzing the results of Fig. 2 and Table II it is seen that the current pulses are modifying the 10 Ok---.---- ~___-___--~ 1 w I B 60 .z .2 50 3 ‘l-8 La .- 0 Nuker 26 of ap:lied pulses of 500 A2/0& LlG. 2. Variation of E,,,, with the number of 500 A/cm’ applied pulses of 8X20 gs.- TABLE II. Variation of E,, with the shorts of 8x20 PCS of 500 A/cm’. Esoo before Esoo after one cycle’ EsM) after four cycles AE,, max W/cm) (V/cm) (V/cm) (%) 2830 2810 2773 2 barrier voltage localized at grain boundary leading to its degradation, but with negligible modification in the electric properties of ZnO grains. Figure 3 shows the variation of Eo.os with the pulse energy, which is estimated by s t E= i2 dt, (4) 0 where E is the pulse energy per resistence unit in A2 s, i is the electric current, and t is the time (s). Rectangular current pulses of 1 ms/75 A and 2 ms/150 A and impulses of 8 x 20 ,US were used. Analyzing Fig. 3 a linear behavior of E,/Eb with the In of pulses energy can be seen (E, is the Eo.os after the ap- plied cycles and Eb is the Eo.os before the applied cycles). It is also observed in Fig. 3 that there is a limiting energy E. for degradation to start. Pulses with energy lower than that limit value do not cause degradation in the varistor. Thus, from Fig. 3, the empirical equation E,/E,= 1 -p ln(E/Eo) (5) canbe proposed, where p is a constant that can be deter- mined from the slope of the experimental curve of Fig. 3. The value of p obtained in this case (0.044) is related to degradation mechanisms and is a good parameter to verify the varistor degradation resistance. B. Barrier voltage degradation In order to verify the current pulse effect on the barrier voltage $J and W were determined in samples before and after four cycles of shorts of 8 X20 ys of 250 and 500 A/cm’. Table III presents data for 4 and W before and ‘1or----- ~~.. -~._ 1’::: 7‘ 2 g 80- _-- 6 w 50 I-..& . . . . . ..I...... I CLL I LU‘Y I I I 8 If II 1 10 100 Pulse energy (A.8) 1000 10000 FIG. 3. Variation of Eoas with the pulse energy for applied pulses of 8 x 20 /JCLS and rectangular pulses. 148 J. Appl. Phys., Vol. 72, No. 1, 1 July 1992 Leite, Varela, and Longo 148 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.225 On: Tue, 14 Jan 2014 13:14:14 TABLE III. Values of 4 and W before and after application of 20 current pulses. Before applied pulses After 20 pulses of 250 A/cm’ After 20 pulses of 500 A./cm’ 4 (eV) W bun) 0.40 f 0.02 13*1.5 0.40*0.01 10.5 hO.9 0.34 7.9 after degradation. The value of 4 measured is smaller than usually obtained” and it should be due to the low concen- tration of MnOz in the composition (listed in Table I). Pianaro13 shows that the nonohmic characteristic of ZnO varistors increases with the increase of MnO, concentra- tion in a six-component composition. This table shows that (p and W are deformed by current pulses, leading to deg- radation of the ZnO varistor. It is also observed in this table that W is more sensitive to degradation. From this fact it appears that the barrier voltage is deformed in two stages. In the first stage, the deformation is due to a de- crease in the barrier width, and in the second stage it is due to lowering of the barrier height. These results are not in agreement with the-degradation mechanism under dc bias, proposed by Hayashi et aZ.,14 in which the first stage is due to lowering of the barrier height and the second stage is due to a decrease in the barrier width. From the proposed model of Gupta and Carlson,’ in which the barrier voltage of Schottky type is due to for- mation of an atomic defect at the ZnO grain boundary and that the negative charge states in the grain boundary are compensated by positive charges at depletion layer, a bar- rier modelI is proposed in which #= (e2N~V(2qg.Nd), (6) where N, is the surface state density (negative charges) and Nd is the donor density (positive charges). It is seen that the relative modification in N, and Nd can explain the variation in 4. The reduction of 4 can be promoted either by increasing Nd or decreasing N, Nd is related to I$ and W through the following equa- tion: Nd={2~o~,[~-((Ec-EF)]}/e2W2, (7) where EC is the conduction-band minimum energy and EF is the Fermi energy. It is observed in Eq. (7) that only (EC - EF) is not known or measured in the varistor. This parameter was measured for a ZnO-Co0 system17 and its value is 0.30 eV. Figure 4 shows a typical microstructure of a ZnO varistor obtained by SEM. X-ray microanalysis ob- tained in this varistor shows that the main dopant in ZnO grains is cobalt; therefore, it seems to be appropriate to use this value for the ZnO varistor system. Thus, from these considerations, the Nd parameter was calculated for the varistor before and after application of shorts of 8 x 20 ,W of 250 and 500 A/cm2. Table IV presents data of the Nd parameter before and after pulse degradation. It is seen that Nd does not change significantly with the current pulse degradation. The surface state density N, is given by 149 J. Appl. Phys., Vol. 72, No. 1, 1 July 1992 Zn - High concentration sb -High concentration FIG. 4. Typical microstructure of ZnO varistor. The picture shows the points analyzed by x-ray microanalysis (3 100 x ) . N,= Q.Je, (8) where Q, is the total charge trapped at the interface and can be determined by Q,=2 jaw Nd edx, (9) where x is the distance parameter that changes from 0 up to barrier voltage width W. By considering Nd independent of x, Qs=2Nd eW, (10) and then Ns=2NdW. (11) Equation (11) is the electric neutrality condition for the material. As Nd practically does not change, the reduction of the barrier width with degradation leads to a decrease in Nr Thus the phenomena of degradation promoted by cur- rent pulses are due to the decrease of N, i.e., the decrease in negative charges located at the ZnO grain boundary, which can be either zinc vacancies or adsorbed oxygen. The decrease of N, could be promoted by reactions be- tween atomic defects with migration of positive charge TABLE IV. Variation of Nd as function of applied pulses of 8 x20 ps. Nd x lo-l8 (cm-‘) Before applied pulses After 20 pulses of 250 A/cm2 After 20 pulses of 500 A/cm’ 0.65 ( AO.22) 0.76 (*0.23) 0.60 Leite, Varela, and Longo 149 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.225 On: Tue, 14 Jan 2014 13:14:14 IA) intyface (B) o-9 migration of positive charge FIG. 5. This picture shows the bond diagram at the grain-boundary re- gion. (a) Band diagram before pulse degradation. (b) After four cycles of 250 A/cm’. In the first stage the migration of positive charge from de- pletion layer to grain boundary and the decrease in the barrier width can be seen. (c) After four cycles of 500 A/cm?. In the second stage the chemical interaction between positive charge and negative charge at the grain boundary and lowering in the barrier height can be seen. from the depletion layer to the grain boundary, followed by a reaction with negative charge,r5 as shown schematically in Fig. 5. Figure 5(b) represents the first stage of the bar- rier voltage deformation, with a decrease in the barrier width. Figure 5 (c j illustrates the second stage with a low- ering of the barrier height and a continuous decrease in the barrier width. The first stage of barrier deformation can be related to the migration of positive charge in depletion layer and the second stage can be related to the reaction between positive charge and negative charge at the inter- face and a continuous migration of positive charge from the depletion layer to the grain boundary. As shown in Fig. 6 the J-E characteristics of ZnO varistors were altered after four cycles of shorts of 8 x 20 ,ILS of 500 A/cm2. The change of the J-E characteristic measured by forward voltage to the biasing voltage was larger than that measured by the reverse voltage. This re- sult explain the asymmetrical deformation of Schottky bar: riers proposed in Fig. 5 and is in agreement with the liter- ature.*-” IV. CONCLUSIONS ~. ZnO, varistors are degraded by current pulses with higher energy than the limiting value. The state of degra- o.oo,d&. --Le. ~.--_ 800 1000 ELECTRIC FIELD (V/cm) FIG. 6. J-E characteristics of ZnO varistors before and after four cycles of shorts of 8x20 ps of 500 A/cm’. After the degradation asymmetrical characteristics are denoted, respectively, with forward bias ( + ) and with reverse bias ( - ) . dation depends on the number of applied current pulses. The variation of the reference electric field Eo.os due to the pulses energy is given by E,/Eb = 1 - p In E/E,, where E. is the limiting energy. Current pulses only degrade the barrier voltage but not the ZnO grains’ electric properties. The experimental results suggest that the degradation is promoted by the decrease in N,, i.e., by decreasing the negative charges trapped at the ZnO grain boundaries. ACKNOWLEDGMENTS The financial help given by 3M do Bras& FAPESP, and CNPq is acknowledged. The authors also acknowledge 3M do Brasil. for the technical help during the execution of this work and M. T. Jiischik, M. A. M. Carneiro, and P. S. P. Catzo for helpful discussions. ‘E. Olsson and G. L. Dunlop, J. Appl. Phys. 66, 3666 (1989). ‘K. Eda, J. Appl. Phys. 49, 2964 (1978). 3 K. Eda, A. Iya, and M. Matsuoka, J. Appl. Phys. 51, 2678 (1980). 4K. Sato and Y. Teikada, J. Appl. Phys. 53, 8819 (1982). ‘T. K. Gupta and W. G. Carlson, J. Appl. Phys. 52, 4104 (1981). ‘C. G. Shirley and W. M. Paulson, J. Appl. Phys. 50, 5782 (1979). ‘K. Eda, J. Appl. Phys. 56, 2948 (1984). ‘M. Kobayashi, M. Mizuno, T. Aizawa, M. Hayashi, and K. Mitani, IEEE Trans. Power Appar. Syst. PAS-97, 1149 ( 1978). 9k Bui, K. Abdullah, A. LoubieR, M. 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